Levesque, Jennifer

From: Dennis Evans Sent: Sunday, December 20, 2020 6:52 PM To: cityclerk Subject: Oral Communications Attachments: Archimedies Screw Turbines.pdf

Recognizing Aqueducts as Green infrastructure

Underappreciated and Underutilized Constructed wetlands function like natural tubs or sponges absorbing and slowly releasing water into an engineered process. Which reduces its momentum and erosive potential, reduces flood heights, and allows for groundwater recharge.

Constructed Wetland cells store enormous amounts of water. With an opportunityn to desig habitat for endangered species like juvenile salmon or delta smelt.

Layered substrate, plants and associated microbes reduce the toxic effects of contaminants in our environment. The systems Archimedean screw turbine converts the flow of discharge water into 500 MWh of power, saving more than 210 tons of CO2 emissions per year.

Opening up a door for Proposition 68 grant opportunities for constructed wetland projects that provide groundwater recharge using secondary effluent, stormwater runoff or graywater. A conjunctive multi‐use enhancement of existing projects or used to clean up contamination of groundwater that serves as a source of drinking water for disadvantaged communities.

To implement Managed Aquifer Recharge water managers must:

1. Recognize aquifers as green infrastructure and environmental assets, and their replenishment as a public benefit

2. Engage in strategic, integrated, and multi‐disciplinary water management planning

3. Align water sectors, institutions, and regulations

4. Harness innovation, research, and data

5. Provide sufficient and stable funding

Because a picture is worth a 1000 words let's examine cited research (w) case studies on page 18 of 34 during the meeting.

Respectfully callback information below

Dennis J Evans Project Facilitator TOSs Business Solution

1 [NOTICE: This message originated outside of City of Merced ‐‐ DO NOT CLICK on links or open attachments unless you are sure the content is safe.]

2 Levesque, Jennifer

From: Dennis Evans Sent: Monday, December 21, 2020 12:37 PM To: cityclerk Subject: Oral Communications Recognizing Managed Aquifer Recharge-MAR as Green Infrastructure Attachments: Archimedies Screw Turbines.pdf

Each engineered wetland cell stores enormous amounts of water. With an opportunity to design habitat for endangered species like juvenile salmon or delta smelt.

I would like to examine cited research (w) case studies on page 18 of 35 during the City Council Meeting please review.

Respectfully

Dennis J Evans Project Facilitator TOSs Business Solution

[NOTICE: This message originated outside of City of Merced ‐‐ DO NOT CLICK on links or open attachments unless you are sure the content is safe.]

1 sustainability

Review Archimedes Screw Turbines: A Sustainable Development Solution for Green and Renewable Energy Generation—A Review of Potential and Design Procedures

Arash YoosefDoost and William David Lubitz *

School of Engineering, University of Guelph, Guelph, ON N1G 2W1, Canada; [email protected] * Correspondence: [email protected]

Received: 7 August 2020; Accepted: 4 September 2020; Published: 8 September 2020

Abstract: Archimedes Screws Turbines (ASTs) are a new form of small hydroelectric powerplant that can be applied even in low head sites. ASTs offer a clean and renewable source of energy and are safer for wildlife and especially fish than other hydro generation options. As with other energy solutions, ASTs are not a global solution for all situations. However, in terms of sustainable development, ASTs can offer many economic, social, and environmental advantages that make them an important option for providing sustainable hydropower development. Archimedes screws can operate in low water heads (less than about 5 m) and a range of flow rates with practical efficiencies of 60% to 80% and can generate up to 355 kW of power. ASTs increase the number of suitable sites where it is possible to develop sustainable hydropower, including in undeveloped, hard to access regions and small communities. At many low head sites, ASTs may be more cost-effective, with lower installation and operating costs than alternative hydropower systems. An AST may also reduce the disturbance of natural sedimentation and erosion processes and have smaller impacts on fish and other fauna. ASTs can often be retrofit to existing unpowered dams or weirs, providing new hydropower capacity for very little marginal environmental impact. This review outlines the characteristics of ASTs, then discusses and analyzes how they could benefit the sustainability of hydropower development.

Keywords: sustainable development; Archimedes screw turbine/generator; small/micro/pico hydropower plant; run of river powerplant; ﬁsh friendly turbine; low head hydropower

1. Introduction

1.1. Sustainable Development Sustainable development is described as “the organizing principle for the achievement of human development objectives while at the same time preserving the capacity of natural systems to provide the natural resources and ecological services on which the economy and community rely” [1]. Sustainable development often implies “a development that addresses current needs without influencing potential generations’ capacity to fulfill their own needs” [2,3]. In 1979 René Passet proposed a three-sphere framework for sustainable development projects [4] (Figure1) According to this framework, development can be considered as sustainable only if it simultaneously has positive social, environmental, and economic impacts. If a project satisfies the economic and social aspects but fails to satisfy the environmental aspects, it is categorized as equitable. If a development project can satisfy the environmental aspects but fails one of the social or economic aspects, the development could still be bearable or viable, respectively. If a development cannot satisfy at least two of the three mentioned aspects, it cannot be categorized

Sustainability 2020, 12, 7352; doi:10.3390/su12187352 www.mdpi.com/journal/sustainability Sustainability 2020, 12, 7352 2 of 34 Sustainability 2020, 12, x FOR PEER REVIEW 2 of 34 anywherecannot satisfy in thisat least deﬁnition. two of the Some three mentioned authors consider aspects, a it fourthcannot sustainabilitybe categorized pillar anywhere of culture, in this institutions,definition. Some or governance authors consider [5], or a reconﬁgurefourth sustainability the four pillar domains of culture, to be social-ecology,institutions, or governance economics, politics,[5], or reconfigure and culture the [four6]. domains Overall, theto be focus social of‐ecology, the modern economics, sustainable politics, development and culture [6]. concept Overall, is simultaneousthe focus of the economic modern development, sustainable development social progress, concept and environmental is simultaneous protection economic for development, current and futuresocial progress, generations and [7 environmental]. protection for current and future generations [7].

Figure 1. Sustainable development as the confluence of three constituent parts. Adapted from [8]. Figure 1. Sustainable development as the confluence of three constituent parts. Adapted from [8]. 1.2. Renewable Energy 1.2. Renewable Energy Renewable energy is defined as energy that is obtained from resources that are fully replenished on a humanRenewable time energy scale [9 is]. defined According as energy to REN21’s that is year obtained 2019 report, from resources renewable that resources are fully provided replenished 2378 GWon a of human power time capacity, scale which [9]. According is more than to 33% REN21’s of the year world’s 2019 total report, electrical renewable generating resources capacity. provided In this list,2378 hydropower GW of power capacity capacity, (excluding which pure is more pumped than storage 33% of capacity) the world’s is 1132 total GW, electrical which is generating about half ofcapacity. total renewable In this list, energy. hydropower It is worth capacity mentioning (excluding that in pure 2018, pumped annual new storage investment capacity) in hydropoweris 1132 GW, grewwhich 11% is about in comparison half of total to the renewable previous yearenergy. [10]. It The is majorityworth mentioning of hydropower that investmentin 2018, annual is in largenew damsinvestment and associated in hydropower generating grew stations 11% thatin comparison inherently include to the complexprevious networks year [10]. of social,The majority economic, of andhydropower ecological investment effects, maybe is in more large than dams any and other associated large infrastructure generating stations project that [11]. inherently include complex networks of social, economic, and ecological effects, maybe more than any other large 1.3.infrastructure Hydropower project [11]. Hydropower is one of the most efficient and confident sources of renewable energy [12] and has1.3. considerableHydropower value for a sustainable future [13]. By the end of 1999, around 2650 Terawatt hours (TWh)Hydropower (19%) of the is world’s one of the total most electricity efficient reliedand confident on hydropower sources of [14 renewable]. It rose toenergy about [12] 3100 and TWh has untilconsiderable 2009, and value it is estimatedfor a sustainable that it reachesfuture [13]. to 3606 By TWhthe end in 2020 of 1999, [15]. around Dams are2650 essential Terawatt tools hours for controlling,(TWh) (19%) storing, of the world’s managing, total and electricity operating relied water on for hydropower humankind. [14]. Large It rose dams to about serve various3100 TWh specific until purposes2009, and for it ouris estimated civilization, that including it reaches water to supply,3606 TWh flood in control, 2020 [15]. navigation, Dams are sedimentation essential tools control, for andcontrolling, hydropower storing, [16 ].managing, However, theyand operating also come withwater disadvantages, for humankind. including Large dams flooding serve large various areas ofspecific land, impedingpurposes fish for migration, our civilization, and affecting including the physical water characteristics supply, flood of the control, dam’s downstreamnavigation, riversedimentation [17]. Construction control, of and large hydropower dams needs significant[16]. However, capital, they so many also large come dam with projects disadvantages, are national (orincluding even international) flooding large in scope. areas Currently,of land, impeding most new largefish migration, dams are being and constructedaffecting the to physical provide combinationscharacteristics ofof energy, the dam’s irrigation, downstream and flood river control [17]. Construction in developing of countries.large dams At needs the samesignificant time, damcapital, decommissioning so many large dam is an projects increasing are trend national in developed (or even international) countries because in scope. of environmental Currently, most impacts new andlarge the dams economic are being costs constructed of maintaining to provide aging structurescombinations [11]. of energy, irrigation, and flood control in developingHydropower countries. plants At can the be same classified time, baseddam decommissioning on their installed electrical is an increasing generating trend capacity. in developed Typical categoriescountries because and associated of environmental capacities are:impacts large and hydro the (economic>10 MW), costs small of hydro maintaining (<10 MW), aging mini-hydro structures ([11].<1 MW), micro-hydro (<100 kW), and pico-hydro (<5 kW) [18]. It is estimated that about 10% of globalHydropower hydropower plants is generated can be from classified powerplants based with on their less than installed 10 MW electrical of capacity generating [18]. Micro-hydro capacity. plantsTypical often categories utilize and the associated natural flow capacities of water are: in a large run-of-river hydro (> (ROR) 10 MW), configuration small hydro [ 19(<10]. RORMW), plants mini‐ hydro (<1 MW), micro‐hydro (<100 kW), and pico‐hydro (<5 kW) [18]. It is estimated that about 10% of global hydropower is generated from powerplants with less than 10 MW of capacity [18]. Micro‐ Sustainability 2020, 12, 7352 3 of 34 Sustainability 2020, 12, x FOR PEER REVIEW 3 of 34 hydroinclude plants little oroften no controlledutilize the natural water storage, flow of meaning water in thata run ROR‐of‐river typically (ROR) has configuration small or no reservoirs.[19]. ROR plantsThe lack include of a large little reservoir or no controlled formed by water a dam, storage, or significant meaning control that of ROR river typically flow, avoids has or small minimizes or no reservoirs.the disadvantages The lack associated of a large with reservoir large reservoirs,formed by at a adam, cost or of havingsignificant to accept control more of river variable flow, or avoids poorly ortimed minimizes power generation.the disadvantages associated with large reservoirs, at a cost of having to accept more variableMicro or poorly hydropower timed power plants generation. can often be considered as a sustainable development option for generatingMicro hydropower electricity in plants both developingcan often be and considered developed as countries.a sustainable There development is often no option need for to generatingbuild expensive electricity dams in and both flood developing massive and areas developed for the countries. reservoir. There This minimizesis often no landneed andto build soil expensivedestruction, dams threats and to flood wildlife, massive climate areas change for the eff reservoir.ects, and other This environmentalminimizes land impacts, and soil especially destruction, on threatsecosystems to wildlife, [20] as climate well as change the social effects, impacts and other of ROR environmental hydropower impacts, plants. especially New ROR on hydropower ecosystems [20]technologies as well as such the social as Archimedes impacts of ScrewsROR hydropower Turbines (ASTs) plants. can New be ROR particularly hydropower advantageous technologies in suchthese as regards. Archimedes Screws Turbines (ASTs) can be particularly advantageous in these regards.

1.4. Archimedes Archimedes Screws The Archimedes screw is considered to be oneone ofof thethe earliestearliest hydraulichydraulic machinesmachines [[21].21]. It is composed of a helical array of simple blades that are wrapped around a central cylinder, like a woodscrew [[22].22]. This This screw screw is is supported within a surrounding ﬁxedfixed trough. There There is is small gap between the trough and screw that allows the screw to rotate freely while allowing only a small amount of water to leak past the blade edges. It is believed that the Archimedes screw was invented by Archimedes ofof Syracuse Syracuse (circa (circa 287-212 287‐212 BCE), BCE), the the Greek Greek physicist, physicist, mathematician, mathematician, and and inventor inventor [23]. [23].However, However, there there is evidence is evidence suggesting suggesting the invention the invention and use and of theuse screwof the technologyscrew technology may date may back date to backover threeto over centuries three centuries before Archimedes before Archimedes under the under reign the of King reign Sennacherib of King Sennacherib (704-681 BCE) (704‐ in681 the BCE) 7th incentury the 7th BCE century in the BCE Assyrian in the EmpireAssyrian [24 Empire]. [24].

1.4.1. Archimedes Archimedes Screw Pump Archimedes screws have been used as water pumps for irrigation and de de-watering‐watering for a long time [23]. [23]. Some Some historical historical sources sources claim claim that that Archimedes Archimedes used used the the device device to to launch launch a a ship ship [23]. [23]. Archimedes screwsscrews areare commonly commonly used used today today as as high-volume high‐volume pumps pumps and and are are particularly particularly adapted adapted to wastewaterto wastewater treatment treatment facilities facilities since since debris debris and and obstructions obstructions in thein the water water usually usually have have minimal minimal or noor noeﬀ ecteffect on theon the operating operating screw screw [25 ].[25]. Figure Figure2 shows 2 shows that that the the Archimedes Archimedes screw screw can can be conﬁguredbe configured as aseither either a pump a pump or or a generator a generator [26 [26].].

Figure 2. Archimedes screw pump (left) and an Archimedes screw hydropower plant (right). Figure 2. Archimedes screw pump (left) and an Archimedes screw hydropower plant (right). 1.4.2. Archimedes Screw Turbine 1.4.2.Archimedes Archimedes screws Screw canTurbine be also used to produce power if they are driven by flowing fluid instead of liftingArchimedes fluid. Water screws transiting can be also the screw used to from produce high topower low elevationif they are generates driven by a flowing torque on fluid the instead helical ofplane lifting surfaces, fluid. Water causing transiting the screw the to screw rotate. from This high mechanical to low elevation rotation can generates be used a totorque produce on the electricity helical planeby attaching surfaces, a causing generator the [ 27screw]. In to this rotate. way, This the mechanical AST is a variation rotation of can the be ancient used to Archimedesproduce electricity screw bypump. attaching However, a generator ASTs have [27]. only In this been way, in use the since AST the is a 1990s variation [28]. of ASTs the o ancientffer a clean Archimedes and renewable screw pump.source ofHowever, energy and ASTs can have be safer only than been other in use types since of the hydroelectric 1990s [28]. turbinesASTs offer for a wildlife clean and and renewable especially sourcefish [29 of]. energy The first and AST can was be safer installed than inother the types 1990s of [25 hydroelectric]. Since then, turbines several for hundred wildlife ASTs and haveespecially been fish [29]. The first AST was installed in the 1990s [25]. Since then, several hundred ASTs have been installed to generate electricity [28]. Almost all of these have been built in Europe. There are only two Sustainability 2020, 12, 7352 4 of 34 Sustainability 2020, 12, x FOR PEER REVIEW 4 of 34 operationalinstalled to generateASTs connected electricity to [the28]. grid Almost in North all of theseAmerica, have the been first built of which in Europe. was Thereinstalled are near only Waterford,two operational Ontario, ASTs Canada, connected in 2013 to the[30]. grid in North America, the first of which was installed near Waterford,Generally, Ontario, there Canada, are two in 2013overall [30 ].categories of modern hydropower turbines: impulse and reaction.Generally, However, there work are two is done overall by categories ASTs due of to modern pressure hydropower differences turbines: across the impulse blades and created reaction. by theHowever, weight work of the is water, done byso ASTsthey do due not to categorize pressure di asfferences using either across an the impulse blades createdor reaction by the mechanism. weight of ASTsthe water, constitute so they a third do not category categorize of hydropower as using either converter an impulse that oris driven reaction by mechanism. the weight of ASTs water, constitute which woulda third also category include of water hydropower wheels. converter These machines that is can driven be considered by the weight quasi of‐static water, pressure which would machines. also includeA water water wheel wheels. is generally These machines a circular can rotor be considered with some quasi-staticform of buckets pressure around machines. the circumference. Most Aturn water about wheel a horizontal is generally axis, a circular but there rotor are with several some different form of configurations buckets around (Figure the circumference. 3). Water at aMost higher turn elevation about a horizontalfills buckets, axis, which but thereempty are at several a lower di pointfferent as configurations the wheel turns (Figure [31].3 Horizontal). Water at waterwheelsa higher elevation have a fills vertical buckets, rotation which axis empty and vertical at a lower ones point have ashorizontal the wheel rotation turns axes [31]. [31]. Horizontal waterwheels have a vertical rotation axis and vertical ones have horizontal rotation axes [31].

(b‐2)

(b‐1) (a)

(b‐3)

Figure 3. (a) Horizontally and (b) Vertically Oriented Water Wheels. (b-1) Undershot, (b-2) Overshot Figure 3. (a) Horizontally and (b) Vertically Oriented Water Wheels. (b‐1) Undershot, (b‐2) Overshot (b-3) Centershot. From [32]. (b‐3) Centershot. From [32]. The energy transfer mechanism in an Archimedes screw is similar to a water wheel, although the The energy transfer mechanism in an Archimedes screw is similar to a water wheel, although configuration is different. In an AST, a water bucket is defined as the volume of water entrapped the configuration is different. In an AST, a water bucket is defined as the volume of water entrapped between two adjacent helical plane surfaces. between two adjacent helical plane surfaces. 2. Advantages of Archimedes Screw Generators 2. Advantages of Archimedes Screw Generators 2.1. Technical Advantages 2.1. Technical Advantages The typical water-to-wire efficiency of ASTs is as high as 60% to 80% [33]. Depending on the screwThe design, typical installation, water‐to‐wire operation efficiency condition, of ASTs as well is as as high the fillas height60% to of 80% the [33]. screw, Depending its minimum on andthe screwmaximum design, effi ciencyinstallation, may di operationffer or happen condition, in different as well conditions as the fill (i.e., height fill heights of the and screw,/or rotation its minimum speeds) andfor dimaximumfferent designs. efficiency For may example, differ or measurements happen in different of several conditions ASTs indicates(i.e., fill heights hydraulic and/or effi rotationciencies speeds)higher than for 80%different full-load designs. and asFor high example, as 94% measurements in partial-load andof several conditions ASTs [34 indicates]. Table1 compareshydraulic efficienciesthe operating higher conditions than 80% for full the‐load most and common as high energy as 94% converters in partial‐load used and in hydropower conditions [34]. generation. Table 1 comparesThese materials the operating indicate thatconditions efficient for electricity the most generation common withenergy just converters a few meters used of in water hydropower head and generation.a wide range These of supported materials flow indicate rates that is a efficient particular electricity advantage generation of ASTs [22with] that just could a few increase meters theof waternumber head of potentiallyand a wide suitablerange of sites supported available flow for rates micro is a hydropower particular advantage development. of ASTs [22] that could increaseTo decide the number about of the potentially best option suitable depending sites on available the site for properties, micro hydropower there are charts development. for each turbine operationTo decide condition. about the Most best of option them, depending even those on in the general site properties, literature, there ultimately are charts are for made each based turbine on operationcompanies condition. who usually Most mark of them, the areas even of theirthose products. in general For literature, example, ultimately Figure4 represents are made the based related on companieswater head who and flowusually rate mark for di thefferent areas power of their ranges products. of ASTs For made example, by ANDRITZ Figure 4 company.represents To the deal related with waterthis issue, head several and flow charts rate could for different be combined, power such ranges as Figure of ASTs5[ made35], which by ANDRITZ represents company. the range To of headdeal withand flow this rateissue, corresponding several charts to could the generated be combined, power such of a Pelton, as Figure Francis, 5 [35], Cross-flow, which represents and Kaplan the asrange well of head and flow rate corresponding to the generated power of a Pelton, Francis, Cross‐flow, and Kaplan as well as Water wheels and Archimedes Screws. It is important to note that for ASTs, this Sustainability 2020, 12, x FOR PEER REVIEW 5 of 35 Sustainability 2020, 12, x FOR PEER REVIEW 5 of 35 SustainabilitySustainability2020 2020, 12, ,12 7352, x FOR PEER REVIEW 5 of 355 of 34 SustainabilitychartSustainability only 20202020includes,, 1212,, xx FORFOR industrial PEERPEER REVIEWREVIEW sizes. As it is mentioned in this study, recently there are many55 ofof 3535 chartinvestigations only includes to make industrial use of ASTs sizes. in micro As it and is mentionedeven pico scales. in this This study, includes recently ASTs theresuch asare PicoPica many chart only includes industrial sizes. As it is mentioned in this study, recently there are many investigations10chart and only PicoPica500, includes to make which industrial use ofchange ASTs sizes. thein micro operationAs it and is even mentionedflow picorate scales.and in headthis This ofstudy, includes ASTs recently from ASTs 0.01 suchthere to 10 as are mPicoPica3 /smany and as Waterinvestigations wheels andto make Archimedes use of ASTs Screws. in micro It isand important even pico to scales. note This that includes for ASTs, ASTs this such chart as only PicoPica includes investigationsinvestigations toto makemake useuse ofof ASTsASTs inin micromicro andand eveneven picopico scales.scales. ThisThis includesincludes ASTsASTs suchsuch asas PicoPicaPicoPica3 100.1 and to 10 PicoPica500, m, respectively. which change the operation flow rate and head of ASTs from 0.01 to 10 m3/s and industrial10 and PicoPica500, sizes. As it iswhich mentioned change inthe this operation study, recentlyflow rate thereand head are manyof ASTs investigations from 0.01 to 10 to m make33/s and use of 0.110 andto 10 PicoPica500, m, respectively. which change the operation flow rate and head of ASTs from 0.01 to 10 m /s and ASTs0.1 into micro10 m, respectively. and even pico scales. This includes ASTs such as PicoPica 10 and PicoPica500, which 0.10.1 toto 1010 m,m, respectively.respectively.Table 1. Operating conditions of common hydropower technologies. 3 change the operationTable flow 1. rate Operating and head conditions of ASTs of common from 0.01 hydropower to 10 m /technologies.s and 0.1 to 10 m, respectively. Table 1. OperatingHead conditions Flow of common Rate hydropower technologies. Type TurbineTable 1. Operating conditions of common hydropower technologies.Notes Table 1. OperatingHead(m) conditions Flow of(m common3 Rate/s) hydropower technologies. Type Turbine Head Flow Rate Notes Type PeltonTurbine [36] Head(m) Flow(m3 /s)Rate Notes Type Turbine (m) (m3/s) Notes Type Turbine Head(m)(m) (m) (mFlow(m3/s)/s) Rate (m3/s) Notes Pelton [36] A high-pressure jet of water is directed at PeltonPelton [36][36] Pelton [36] 50 *–1000 <50 Abucket-shaped high-pressure blades jet of andwater nearly is directed all of the at A high-pressure jet of water is directed at AA high-pressurehigh-pressure jetjet ofof waterwater isis directeddirected atat 50 *–1000[12] [35]<50 bucket-shapedflow energyA high-pressure to bladesbe converted and jet ofnearly waterto rotational all is of directed the 50 *–1000 <50 bucket-shaped blades and nearly all of the 50[12]*–1000 [35]<50 <50 flowmechanicalbucket-shaped energyat bucket-shaped energy to bladesbe [37].converted and blades nearly to and rotational all nearly of the all 50[12] *–1000 [12] [35] flow energy to be converted to rotational [12] [35] [35] mechanicalflow energyof the energy flowto be energy [37].converted to be to converted rotational to mechanicalrotational energy mechanical [37]. energy [37]. Impulse mechanical energy [37]. Turgo [38] Impulse A modification of a Pelton that incoming Impulse Turgo [38] ImpulseImpulse Turgo [38] Awater modification jet is directed of a Pelton at an that angle incoming of 20 Turgo [[38]38] A modification of a Pelton that incoming 50 **–250 <10 waterdegreesA modification jetA to modification isthe directed buckets. of a Pelton of atA a largeran Pelton that angle jetincoming that streamof 20 water jetincoming is directed water at jet isan directed angle atof an20 50 [39]**–250 [40]<10 degreesofwater water jet torelative theis directed buckets. to the turbine atA largeran diameterangle jet stream of for20 angle of 20 degrees to the buckets. 5050**–250 **–250 <10<10 <10 degreesdegrees toto thethe buckets.buckets. AA largerlarger jetjet streamstream 50[39] **–250 [39] [40] ofthe water sameA relative largerflow jet andto streamthe head turbine ofproduces water diameter relative more for to [39][39] [40][40] [40] ofof waterwater relativerelative toto thethe turbineturbine diameterdiameter forfor thepower same orthe canflow turbine be andconstructed diameter head produces forsmaller the same [39].more flow the same flow and head produces more powerthe same orand can flow head be constructedand produces head moreproduces smaller power [39]. more or can powerFlow enters or can thebe constructed turbine radially smaller then [39]. is powerpower orbeor cancan constructed bebe constructedconstructed smaller smallersmaller [39]. [39].[39]. Francis [41] Flowredirected enters in thea directionturbine radiallyalong the then axial is Flow enters the turbine radially then is Francis [41] redirectedlengthFlow entersFlowof in entersthe thea direction turbine.turbine the turbine radiallyalongThe radially thepressure then axial then is is Francis [41] 0.2–1000 redirectedredirected in a direction in a direction along along the theaxial axial FrancisFrancis [[41]41] 40–600 lengthdifferenceredirected of inthe acrossa directionturbine. the alongTheblades thepressure axialis length of the turbine. The pressure 0.2–1000[35] lengthlength ofof thethe turbine.turbine. TheThe pressurepressure 40–600[39] 0.2–10000.2–1000 differenceaccomplisheddiff erence acrossby acrosschanging the the bladesflowblades direction is is 40–600 (Specific0.2–1000[35] Flow) difference across the blades is 40–600[39] [39] [35] [35] accomplishedanddifference canaccomplished be 90%acrossby changing to by 95%the changing flowefficientblades direction flow [37].is [39] (Specific[35] Flow) accomplished by changing flow direction [39] (Specific(Specific Flow) Flow)andEfficiencyaccomplished candirection bereduces 90%by and changing todramatically can 95% be 90%flowefficient to directionin 95% flow[37]. (Specific Flow) and can be 90% to 95% efficient [37]. Efficiencyratesand belowcaneffi cientbereduces 75% 90% [ 37of ]. thetodramatically Effi design95%ciency efficient flow reduces in[37] flow[37].

Reaction EfficiencyEfficiencydramatically reducesreduces indramaticallydramatically flow rates below inin flowflow 75% of Kaplan [42] ratesFlowing below water 75% pushing of the design past theflow propeller [37] ReactionReaction ratesrates belowbelowthe design 75%75% ofof flow thethe design [design37] flowflow [37][37] ReactionReaction Kaplan [42] Flowingblades creates water apushing pressure past difference the propeller [37]. Kaplan [42] FlowingFlowing water pushing water pushing past the past propeller the KaplanKaplan [42][42] bladesBestFlowing suited creates water for headsless apushing pressure pastthan difference theimpulse propeller [37].and blades propellercreates a bladespressure creates difference a pressure [37]. <50 0.5–1000 BestFrancisblades suited turbinescreates for headslessa for pressure the same than difference flow,impulse and [37].and for difference [37]. Best suited for headsless BestBest suitedsuited forfor headslessheadsless thanthan impulseimpulse andand [43]<50< 50 0.5–1000[35]0.5–1000 Francisflow rates thanturbines higher impulse for than the and Francissame Francis flow, and turbines and Pelton for for <50 0.5–1000 Francis turbines for the same flow, and for [43]<50[43 ] 0.5–1000[35] [ 35] flowturbinesFrancis ratesthe turbines for samehigher the flow,samefor than the andhead. Francis same for The flowflow, and propeller rates andPelton higherfor [43] [35] flow rates higher than Francis and Pelton [43] [35] turbinesangleflow ratesthancan for higher Francisthevary same than andto head. Francis Peltonoptimize The turbines and propeller energyPelton for the turbines for the same head. The propeller angleextraction.turbines samecan for head.thevary same Theto head. propelleroptimize The angle propellerenergy can angleangle varycancan tovaryvary optimize toto energyoptimizeoptimize extraction. energyenergy extraction.Vertical axis Water Wheels rotate due to the extraction. Verticalforceextraction. exertedVertical axis Water axisto Wheels Waterthe paddles Wheels rotate due rotateby to the due to Water wheel [32] Verticalthe axis force Water exerted Wheels to rotate the paddles due to by the the Water wheel [32] forcemomentumVertical exerted axis Waterof theto Wheelstheflow. paddles Horizontalrotate dueby to axis thethe Water wheel [32] force exertedmomentum to ofthe the paddles flow. Horizontal by the axis momentumWaterforce exertedWheels of thetorotate flow.the becausepaddlesHorizontal byof axisthethe WaterWater wheelwheel [32][32] Water Wheels rotate because of the momentummomentum ofof thethe flow.flow. HorizontalHorizontal axisaxis Watermomentum momentumWheels of therotate ofwater, the because water, as well as well ofas the as the Water Wheels rotate because of the momentumweightWater weightofWheels water of of theon waterrotate thewater, small on because theas water smallwell buckets of wateras thethe <10 <5 momentum of the water, as well as the <10 <5 weightformedmomentum buckets ofbetween water of formed on thethe the paddles.water, small between as water So, well the they buckets paddles.as are the a Quasi- <10 <5 weightweight So, ofof waterwater they are onon theathe bit smallsmall more waterwater efficient bucketsbuckets than <10<10 <5<5 formedbit more between efficient the paddles. than So,conventional they are a Quasi-static formedconventional between the paddles. horizontal So, ones they [31 are]. a Quasi-staticQuasi- bithorizontalformed more between onesefficient [31]. the Thepaddles.than efficiency conventionalSo, they of well-are a PressureQuasi-static bit moreThe effiefficientciency ofthan well-constructed conventional WWs Pressure bitbit moremore efficientefficient thanthan conventionalconventional staticstatic horizontalconstructedis 50%–70% ones WWs [31]. [is31 The ].50%–70% Theefficiency hydraulic [31]. of well-The Pressure horizontalhorizontal onesones [31].[31]. TheThe efficiencyefficiency ofof well-well- PressurePressure constructedhydrauliceffi ciencyefficiency WWs of Verticalis 50%–70%of Vertical Water [31]. Wheels Water The is constructedconstructed WWsWWs isis 50%–70%50%–70% [31].[31]. TheThe Sustainability 2020, 12, x FOR PEER REVIEW hydraulicWheelsreported is reportedefficiency up up to 85%toof 85% [Vertical44 ].[44]. Water6 of 35 hydraulic efficiency of Vertical Water WheelsEfficiencyhydraulicE isffi reportedrangeciencyefficiency between range up toof between 85% 60%Vertical [44].and 60% 80% Water and and 80% Archimedes Screw Wheels is reported up to 85% [44]. Archimedes <10 <10 EfficiencycombinedremainsWheelsand is highrangelowreported remains head evenbetween up highand toas low even60%85%available flow and[44]. as available 80% head and Efficiency range between 60% and 80% and ArchimedesScrew [33,40]<10< 10 [33,45]<10 <10 remainsapproachesEfficiencyhead high range zero. approaches evenbetween Produce as zero. 60% availableup Produceand to 80%355 head upkWand to Archimedes <10[33 , 40] <10 [33,45] remains355 high kW [46even] of poweras available [28]. Practical head ArchimedesScrew [33,40]<10 [33,45]<10 approaches[46]remains of powerhigh zero. even [28].Produce asPractical availableup to even355 head kWin Screw [33,40] [33,45] approacheseven zero. in combined Produce low up head to 355 and kW Screw [33,40] [33,45] [46]approaches of power zero. [28]. Produce Practical up to even355 kWin [46] oflow power flow [28]. Practical even in Notes: * A Pelton turbine with flow 50 m3/s is a very extreme[46][46] case; of of** Micro-hydropowerpower [28].[28]. impulse PracticalPractical turbines eveneven inin Notes: * A Pelton turbine with flow 50 m3/s is a very extreme case; ** Micro-hydro impulse turbines sometimes sometimes operate with less than 50 m of head. operate with less than 50 m of head.

Figure 4. An example of commercial Archimedes Screws operating range [47,48].

Sustainability 2020, 12, x FOR PEER REVIEW 6 of 34

SustainabilityNotes: * A 2020 Pelton, 12, x turbine FOR PEER with REVIEW flow 50 m3/s is a very extreme case; ** Micro‐hydro impulse turbines 6 of 34 sometimes operate with less than 50 m of head. Notes: * A Pelton turbine with flow 50 m3/s is a very extreme case; ** Micro‐hydro impulse turbines Sustainabilitysometimes2020, 12 ,operate 7352 with less than 50 m of head. 6 of 34

Figure 4. An example of commercial Archimedes Screws operating range [47,48]. Figure 4. An example of commercial Archimedes Screws operating range [47,48]. Figure 4. An example of commercial Archimedes Screws operating range [47,48].

Figure 5. The range of head and ﬂow rate corresponding to the generated power of diﬀerent turbines Figure 5. The range of head and flow rate corresponding to the generated power of different turbines (Adapted from [[35]).35]). Figure 5. The range of head and flow rate corresponding to the generated power of different turbines 2.2. Economic(Adapted Aspects from [35]). of ASTs

2.2.1. Capital Costs

Depending on site specifications, an Archimedes screw may offer cost advantages, especially in terms of capital costs. For example, a case study for a specific site in Yorkshire UK found that the AST Sustainability 2020, 12, 7352 7 of 34 cost would be about 10% less than a Kaplan turbine, while the generated energy was estimated to be about 15% more. For this case, in terms of capital cost per MWh per year, the AST was projected to be 22% cheaper than a Kaplan turbine [49].

2.2.2. Operational and Maintenance Costs Overall operation and maintenance costs of ASTs are expected to be lower than other turbines [50]. Regular AST maintenance includes checking fluid levels and replacing grease cartridges at the upper bearing and gearbox. The bottom bearings are usually designed to operate without any maintenance until replacement [51]. ASTs have few wear points, and their operational speed is low, reducing wear and scouring issues. The common types of physical and chemical erosion occur only in trough and blade flights. Major maintenance of an AST is typically required after 20 to 30 years for the lower bearing [52]. The screw flights would typically be refurbished when the lower bearing and the trough is replaced [50]. Since maintenance and operating costs of ASTs are lower than other micro-hydro turbines [53], they can be considered as one of the best options for undeveloped regions and areas with no easy access like high elevations of mountains, or small communities that are far from facilities and infrastructure. This is an important advantage for small communities where connecting to central grids is not easy nor cost-effective.

2.3. Environmental and Social Advantages

Wildlife and Social Advantages Dams block the natural connection of a river between upstream and downstream regions. Therefore, river ecosystems may lose their access which causes negative environmental impacts such as wide-ranging species extinctions. This has been documented in the temperate areas of the Americas due to large hydropower dams [54]. For example, the extinction of 38 species, and the near-extinction of other 71 species occurred due to dams in the Mobile River basin, Alabama, USA [55]. It is important to note that negative effects on ecosystems are not limited only to large dams. Any disturbance in natural processes, including the construction of hydraulic structures, such as small weirs or diversions, can also have side effects. Studies indicate that even low-head hydropower facilities are not risk-free especially for fish that migrate downstream [56]. A novel study was conducted by Boys et al. [56] on the physical stresses that would be experienced by fish species passing through a very low head (VLH) turbine, Archimedes screw, and horizontal Kaplan turbines. A novel fish-simulating sensor package called Sensor Fish was passed through operating turbines and recorded conditions in order to assess potential injury mechanisms reported in studies on live-fish [56]. This study found that rapid decompression is not significant in VLH and ASTs. However, in the low- head horizontal Kaplan turbine, it was so serious that impacts on fish could be expected to be comparable to high-head vertical axis Kaplan turbines. It was also mentioned that physical strikes are a potential fish injury source in all turbine types. The authors suggested that minimizing structures near the tailrace of screw turbines and operating VLH turbines at higher powers with greater runner blade openings could reduce the possibility of strikes [56]. Several studies have found that using ASTs could limit impacts on wildlife and aquatic species. Archimedes screws are even used as pumps to move fish between holding pens in some fish farms with between 98% and 99% of fish unharmed during passage [57]. The results could be similar in ASTs since both systems have slow rotational speeds and large openings that allow safe passage of small objects. A British study [58] showed that eels and juvenile salmon passed through an AST safely with a low mortality rate. Further investigations found that applying a rubber bumper to the leading edge of the screw blades further reduced injury potential: with this intervention, no salmon sustained injuries of any sort and just minor recoverable damage was reported for less than 1% of eels [58]. Another study indicates that fish lighter than 1 kg can safely pass through an AST that is rotating at typical Sustainability 2020, 12, 7352 8 of 34 operating speeds [59]. Applying rubber bumpers on the leading edges can increase this mass up to 4 kg without being harmed [59]. According to the UK Environment Agency Hydropower Good Practice Guidelines, using the compressible rubber bumps is recommended for ASTs with a typical speed higher than 3.5 m/s. This guideline also suggests using trash-racks to screen larger debris before entering the turbine for a range of AST diameters in order to block the passage of fish and debris that are larger than the AST’s buckets and diverting them into a spillway or parallel stream with no harm [60]. A two-year study on the possibility of second-order effects of ASTs tried to answer additional questions about fish behavior alternation before and after passage as well as long-term survival and fitness aftereffects [61]. Movement patterns and behavior analysis indicated that adult eels could descend the AST. However, under high flows, they preferred the faster passage of the parallel overshot weir. Passing through the screw caused no immediate eel mortality and no effect on their migration behavior. This study mentioned that adult eels milled about at the entrance to the screw outlet and then rejected it as a migration path. Other fish exhibited startle response on the turbine start-up. However, ASTs can be considered a potential downstream passage route for both eels and Potadromous species [61]. It is also notable that compared to other turbine types, ASTs allow greater downstream passage of sediment, floating debris, and other material. This capacity to pass larger materials means that trash racks and upstream screens can be coarser, and therefore less expensive, with less head loss, than those necessary for other turbine types.

3. Disadvantages of Archimedes Screw Generators

3.1. A Relatively New Technology Looking back to the history of using Archimedes screws as generators shows that this is a relatively new technology, and there are many not well-known things about ASTs in comparison to other hydropower technologies. Fortunately, ASTs have become increasingly popular in Europe during the past decade because of their robustness, simplicity, and ﬁsh friendliness [62], and there is an increasing rate of research interest to these machines.

3.2. Insufficient General Design Guideline Currently, there is no perfect theory or general standards or rules for the optimal hydraulic design of ASTs, and their hydro powerplant and designs are highly dependent on the experience of the designer engineer [62]. Presently, the Rorres [26] and Nuernbergk and Rorres [62] are well-known optimum AST design proposed procedures. However, they are not very easy to understand and implement. It seems that there is not sufficient English guidance for the optimum design of ASTs for different sites and flow conditions [30]. Even the non-English literature, including Brada (1996) [63], Aigner (2008) [64], Schmalz (2010) [65], Lashofer et al. (2011) [66], and the new version of “Hydro-power screws—Calculation and Design of Archimedes Screws” [67], which is available only in the German language. However, they do not provide a comprehensive physics-based design model for ASTs [28,30]. The design of AST plants is discussed further in Section5.

3.3. Technical Limitations For very high ﬂow rates or water heads, a single screw may not take advantage of all available potential due to material, structural, technical, and physical limitations: bending could be a serious issue for very long structures. Increasing the inner diameter of the screw could help to increase the AST length with the cost of making the screw larger or reducing the eﬀective area. However, because of the weight of the screw, the bearing limitations is one of the most critical technical constraints. The idea of multi-AST powerplants (parallel or in series) could be considered as a solution. However, according to Table1, there are other turbines developed to take advantage of such conditions. Sustainability 2020, 12, 7352 9 of 34

Archimedes screws operate at low rotation speeds relative to other hydropower turbines. This provides environmental advantages such as good ecological behavior and reduced noise. However, a gearbox is required to convert this rotation speed to the required speed of the generator. Although this could not be considered as an important issue the losses in the gearbox, as well as the generator may affect the overall efficiency of the system. ASTs can handle flow rates even of up to 20% more than optimal filling without a significant loss in efficiency [63]. However, running screws in non-optimal conditions such as high filling heights or rotation speeds higher than the Muysken maximum rotation speed (Equation (9)) may lead to significant losses due to physical and hydraulic limitations. Fortunately, there are some solutions to deal with some of these limitations: when the conditions are not perfect for a single screw, installing more than one screw, and utilizing variable-speed ASTs, allows developers to fully utilize available flow at a wider range of sites, including those with high seasonal variability.

4. ASTs: A Conceptual Approach Based on the discussed advantages of ASTs, they could provide a wide range of opportunities and options for developments. In terms of sustainable development, ASTs oﬀer economic, social, and environmental advantages and could address or compensate some of the sustainable development components in some of the current water industry developments. This section will propose and analyze some of the ASTs applications that could improve the sustainability of water industry developments.

4.1. Site Considerations While large dam sites require very special conditions to construct, this is less of an issue in site selection for AST-based run-of-river (ROR) hydropower plants. Even for ROR plants with a reservoir, the size of the reservoir is much smaller than traditional hydropower plants. By utilizing ASTs, ROR hydropower plants could utilize relatively small heads and flows. The size of an AST can be scaled based on the available volume of flow and site specifications. Conventional ASTs can operate with heads between 1 m and 6 m, and flow rates between 0.1 m3/s and 6 m3/s, producing 1 kW [28] to 300 kW [47,48] of power generation. However, recent development shifted this range dramatically. Currently, there are Pico ASTs that can generate around 10 watts of power with 0.01 m3/s of flow rate and 0.1 m of water head [45]. The largest installed AST could generate up to 355 kW of power with 3 m of water head and has 14.5 m3/s of flow rate capacity [46]. According to Table1, currently ASTs are suitable for sites with head up to 10 m [ 40,45,68] and flow rates up to 10 m3/s [33,35,45]. Recently, manufacturers have been announced larger screws that can pass flow rates as high as 15 m3/s and generate up to 800 kW of power [34]. Surveying operating ASTs, the length of the operating ASTs varies from about 1 m to 17 m. Figure6 shows Cragside (UK) screw, which is a uniquely 17 m long and steep screw plant that generated about 21 MWh in 2019 [69]. Figure7 shows several Pico hydro-scale ASTs in series, each about 1 m long, being used in Japan [ 70]. Sustainability 2020, 12, x FOR PEER REVIEW 10 of 34

FigureFigure 6. 6.Cragside Cragside AST, Northumberland, UK UK (modified (modiﬁed from from [71,72]). [71,72]).

ASTs can operate efficiently even at very low heads and flows [24]. Therefore, ASTs could help to generate electricity for hard to access or undeveloped regions where connecting them to the electricity network is difficult, expensive, or even impossible. Ideally, ASTs could make hydropower generation possible almost everywhere flowing rivers are available. In areas of low flow but the higher head, this idea could be expanded to a theoretical chain of hydropower plants such as what is proposed in Figure 7a. Figure 7b shows an implementation of such a chain of ASTs in a roadside irrigation channel. The power output was used to power adjacent street lights in Japan [70]. The Yorkshire Water’s Esholt wastewater treatment (in Section 4.3) could be considered as another example of using several ASTs in series to take advantage of available water head alongside a relatively long (here around 30 m) pathway [73]. The idea of installing ASTs in series in chains of small hydropower plants could further increase the potential of green and renewable hydroelectricity generation in locations where the flow is low and the head is somewhat higher than would be appropriate for a single AST. In remote areas, the modular nature of multiple small ASTs could be a logistical advantage. In appropriate locations, installing chains of ASTs in series could offer several advantages in addition to reducing fossil fuels usage and greenhouse gas (GHG) emissions by: ‐ Allowing hydroelectricity generation in regions where construction of large dams is not reasonable or feasible. For example, in relatively smooth plains where suitable conditions do not exist to support the construction of large dams. ‐ Maximizing the hydroelectricity generation even in regions where a large dam exists by extracting energy alongside the river downstream. ‐ Reducing the electricity power loss alongside the distribution network by generating power near where it is required and consumed, which could reduce the length and cost of the distribution network (such as illustrated in Figure 7b). ‐ Generating baseload power with small hydropower plants. Currently, in many locations, the majority of electricity baseload is mainly generated by fossil fuel and nuclear powerplants and hydropower is used to meet peak demands. However, the proposed theoretical chain of ROR powerplants could generate baseload since most of the ROR plants do not store water. Sustainability 2020, 12, 7352 10 of 34

ASTs can operate efficiently even at very low heads and flows [24]. Therefore, ASTs could help to generate electricity for hard to access or undeveloped regions where connecting them to the electricity network is difficult, expensive, or even impossible. Ideally, ASTs could make hydropower generation possible almost everywhere flowing rivers are available. In areas of low flow but the higher head, this idea could be expanded to a theoretical chain of hydropower plants such as what is proposed in Figure7a. Figure7b shows an implementation of such a chain of ASTs in a roadside irrigation channel. The power output was used to power adjacent street lights in Japan [70]. The Yorkshire Water’s Esholt wastewater treatment (in Section 4.3) could be considered as another example of using several ASTs in series to take advantage of available water head alongside a relatively long (here around 30 m) Sustainabilitypathway [ 732020]., 12, x FOR PEER REVIEW 11 of 34

(a) (b)

FigureFigure 7. 7. ((aa)) A A theoretical theoretical chain chain of small hydropower plantsplants ( (bb)) Application Application of of a a chain chain of of small small ASTs ASTs in inan an irrigation irrigation channel channel to to power power street street lights lights in in Japan Japan [70 [70].].

4.2. ReducingThe idea Erosion of installing and Disturbance ASTs in series of Natural in chains Sedimentation of small hydropower Processes plants could further increase the potential of green and renewable hydroelectricity generation in locations where the flow is low Suspended sediments can cause damage passing through traditional turbines i.e., by eroding and the head is somewhat higher than would be appropriate for a single AST. In remote areas, the component surfaces, especially when sediments are composed of hard materials like quartz and the modular nature of multiple small ASTs could be a logistical advantage. In appropriate locations, feldspar [74]. To prevent sediment related problems in small hydropower plants, the construction of installing chains of ASTs in series could offer several advantages in addition to reducing fossil fuels settling basins to remove sediments is possible [75] but it is often uneconomical [76]. In addition, even usage and greenhouse gas (GHG) emissions by: if construction and operating costs are ignored, there is no guarantee of complete removal of suspended- Allowing sediment hydroelectricity before the generation water enters in regions the whereplant construction[76]. Unlike of many large damstraditional is not reasonableturbines, suspendedor feasible. sediments For example, could pass in relativelythrough ASTs smooth with plains minimal where effect suitable because conditions of their do lack not of exist tight to tolerancessupport and the their construction low operational of large speeds dams. [50]. Letting the sediments pass a hydropower plant instead- Maximizing of depositing the hydroelectricityupstream in reservoirs generation or settling even in basins regions could where offer a large environmental dam exists by advantages extracting by reducingenergy alongsidethe disturbance the river of downstream. the natural erosion/sedimentation process while also reducing operating- Reducing costs theassociated electricity with power settling loss basin alongside maintenance. the distribution Passing network sediment by generatingcould lead powerto erosion near preventionwhere and it is soil required conservation and consumed, since according which could to Lane’s reduce law the water length transporting and cost ofsediment the distribution has less capacitynetwork for eroding (such asbanks illustrated and natural in Figure features7b). [77]. Passing sediment through the plant could also offer- Generatingeconomic advantages baseload power since reduced with small upstream hydropower sediment plants. deposition Currently, increases in many the locations,effective servicethe life majority of reservoirs of electricity and dams. baseload is mainly generated by fossil fuel and nuclear powerplants and hydropower is used to meet peak demands. However, the proposed theoretical chain of ROR 4.3. Powerpowerplants Generation could from generate Unconventional baseload Water since Resources most of the ROR plants do not store water. Considering the advantage of power generation even with a highly sediment contaminated flow with4.2. Reducingmuch lower Erosion water and levels, Disturbance ASTs could of Natural utilize Sedimentation unconventional Processes water resources to generate power. A recentSuspended study on sediments micro hydropower can cause damageplants using passing sewage through water traditional flows indicates turbines that i.e., a single by eroding AST withthe component a sewage line surfaces, with a especially flow rate whenof about sediments 0.24 m3/sec are composedand 1.5 m of of the hard head materials could likegenerate quartz up and to 1963 watts of power [78]. This experiment was physically implemented at the sewage treatment plant for municipal wastewater of Hayatabad Peshawar, Pakistan. It was found that in the worst scenario a coarse screen can be added to prevent the garbage from entering the screw. In addition, increasing the number of screws, flow rate, and head lead to a linear increase in generated power [78]. Moreover, theoretically, sediments could be released continuously from reservoirs, and this flow used for power generation by ASTs. Another example is the Yorkshire Water’s Esholt wastewater treatment upgrade in 2007 that enables it to generate hydropower from untreated sewage by installing two Archimedean screw generators installed in series (Figure 8) between the inlet works and the new primary settlement tanks [79]. This hydropower plant has been switched on since 2009 [73] and could make use of up to 3.24 m3/s of wastewater with about 10 m of the head passing through a 1.8 m diameter pipe when it flows sequentially through the two screws [79]. The diameter and length of each screw are 2.6 m and 14 m, Sustainability 2020, 12, 7352 11 of 34 feldspar [74]. To prevent sediment related problems in small hydropower plants, the construction of settling basins to remove sediments is possible [75] but it is often uneconomical [76]. In addition, even if construction and operating costs are ignored, there is no guarantee of complete removal of suspended sediment before the water enters the plant [76]. Unlike many traditional turbines, suspended sediments could pass through ASTs with minimal effect because of their lack of tight tolerances and their low operational speeds [50]. Letting the sediments pass a hydropower plant instead of depositing upstream in reservoirs or settling basins could offer environmental advantages by reducing the disturbance of the natural erosion/sedimentation process while also reducing operating costs associated with settling basin maintenance. Passing sediment could lead to erosion prevention and soil conservation since according to Lane’s law water transporting sediment has less capacity for eroding banks and natural features [77]. Passing sediment through the plant could also offer economic advantages since reduced upstream sediment deposition increases the effective service life of reservoirs and dams.

4.3. Power Generation from Unconventional Water Resources Considering the advantage of power generation even with a highly sediment contaminated flow with much lower water levels, ASTs could utilize unconventional water resources to generate power. A recent study on micro hydropower plants using sewage water flows indicates that a single AST with a sewage line with a flow rate of about 0.24 m3/s and 1.5 m of the head could generate up to 1963 watts of power [78]. This experiment was physically implemented at the sewage treatment plant for municipal wastewater of Hayatabad Peshawar, Pakistan. It was found that in the worst scenario a coarse screen can be added to prevent the garbage from entering the screw. In addition, increasing the number of screws, flow rate, and head lead to a linear increase in generated power [78]. Moreover, theoretically, sediments could be released continuously from reservoirs, and this flow used for power generation by ASTs. Another example is the Yorkshire Water’s Esholt wastewater treatment upgrade in 2007 that enables it to generate hydropower from untreated sewage by installing two Archimedean screw generators installed in series (Figure8) between the inlet works and the new primary settlement tanks [79]. This hydropower plant has been switched on since 2009 [73] and could make use of up to 3.24 m3/s of wastewater with about 10 m of the head passing through a 1.8 m diameter pipe when it Sustainability 2020, 12, x FOR PEER REVIEW 12 of 34 flows sequentially through the two screws [79]. The diameter and length of each screw are 2.6 m and 14respectively m, respectively [73]. This [73]. Thisupgrade upgrade could could provide provide more more than than 180 180 kW kW of of treatment treatment process’ process’ required required power [[79]79] of the UK’s firstfirst energy self-suself‐sufficientfficient (neutral)(neutral) urbanurban sewagesewage plantplant [[80].80].

FigureFigure 8. 8. YorkshireYorkshire Water’s Esholt wastewater treatment AST hydropower plant [[73].73].

4.4. Conservation and Improvement of Resources ASTs could offer a considerable value add‐on for some big projects whose main purpose is not power generation [79] for improvement or conservation of current resources. The Yorkshire Water’s Esholt wastewater treatment upgrade, which is mentioned in Section 4.3, is one of the examples. In addition, the wide range of AST operating feasibility makes it possible to use them as an upgrade or retrofitting solution at existing dams. Since the civil work associated with the dam already exists, this can reduce capital costs of installing an AST hydropower plant. Many ASTs have been installed at sites with existing dams. In some cases, the repair costs of a dam could be three times higher than dam removal. Therefore, dam deconstruction may be economically more reasonable than keeping a dam in place, because of maintenance and renovation costs [81]. However, dam removal expenses are usually too high to be covered just by one financer [82]. In addition, many existing dams have been in place for a very long time, and it is not easy to balance the ecosystem and cultural‐historical values of such old and historically important structures in communities [83]. The ecosystem and social changes resulting from dam construction usually lead to a new balance over several decades of dam operation [84]. In addition, the deconstruction of old dams provides a large area of stored sediment that is highly suitable to plant colonization. So, native species may fail to survive in competition with aggressive plant colonists [85–87]. Few investigations have focused on the dam deconstruction effects on contaminant distribution, especially in the case of small dams with heights less than 2 m [88]. However, some studies about the evaluation of the extent and magnitude of biological and chemical changes caused by dam removal documented potential changes especially in contaminant inventories [89]. As an example, after the Ft. Edwards dam removal in 1973, analysis of the displaced sediment downstream during 1977–1978 indicated highly contaminated polychlorinated biphenyls (PCBs) [90]. Therefore, sometimes the deconstruction of old dams may not be the most effective solution. Each dam deconstruction should be evaluated for the potential sedimentary contaminant redistribution [89] and other possible consequences. Figure 9 is an example of the shock load of releasing sediments deposited in a reservoir during the storage restoration process of Manjil Dam, Iran, in 1982–1983. This sediment shock load caused massive extinction of downstream aquatic life. Sustainability 2020, 12, 7352 12 of 34

4.4. Conservation and Improvement of Resources ASTs could offer a considerable value add-on for some big projects whose main purpose is not power generation [79] for improvement or conservation of current resources. The Yorkshire Water’s Esholt wastewater treatment upgrade, which is mentioned in Section 4.3, is one of the examples. In addition, the wide range of AST operating feasibility makes it possible to use them as an upgrade or retrofitting solution at existing dams. Since the civil work associated with the dam already exists, this can reduce capital costs of installing an AST hydropower plant. Many ASTs have been installed at sites with existing dams. In some cases, the repair costs of a dam could be three times higher than dam removal. Therefore, dam deconstruction may be economically more reasonable than keeping a dam in place, because of maintenance and renovation costs [81]. However, dam removal expenses are usually too high to be covered just by one financer [82]. In addition, many existing dams have been in place for a very long time, and it is not easy to balance the ecosystem and cultural-historical values of such old and historically important structures in communities [83]. The ecosystem and social changes resulting from dam construction usually lead to a new balance over several decades of dam operation [84]. In addition, the deconstruction of old dams provides a large area of stored sediment that is highly suitable to plant colonization. So, native species may fail to survive in competition with aggressive plant colonists [85–87]. Few investigations have focused on the dam deconstruction effects on contaminant distribution, especially in the case of small dams with heights less than 2 m [88]. However, some studies about the evaluation of the extent and magnitude of biological and chemical changes caused by dam removal documented potential changes especially in contaminant inventories [89]. As an example, after the Ft. Edwards dam removal in 1973, analysis of the displaced sediment downstream during 1977–1978 indicated highly contaminated polychlorinated biphenyls (PCBs) [90]. Therefore, sometimes the deconstruction of old dams may not be the most effective solution. Each dam deconstruction should be evaluated for the potential sedimentary contaminant redistribution [89] and other possible consequences. Figure9 is an example of the shock load of releasing sediments deposited in a reservoir during the storage restoration process of Manjil Dam, Iran, in 1982–1983. This sediment shock load causedSustainability massive 2020, 12 extinction, x FOR PEER of REVIEW downstream aquatic life. 13 of 34

Figure 9. SedimentSediment shock shock load load released released from Manjil Dam, Iran, during the storage restoration process in 1982–1983,1982‐1983, which which caused caused ecological ecological disaster and a massive extinction of aquatic life downstream.

Installing an AST at an existing dam or weir can offer economic and environmental benefits by Installing an AST at an existing dam or weir can offer economic and environmental benefits by generating renewable hydroelectricity at a previously unutilized site with a minimum of changes generating renewable hydroelectricity at a previously unutilized site with a minimum of changes to to the overall site [20]. An AST installation can also provide a reason for performing deferred and the overall site [20]. An AST installation can also provide a reason for performing deferred and ongoing maintenance on an otherwise neglected dam, improving the structural stability of dam ongoing maintenance on an otherwise neglected dam, improving the structural stability of dam infrastructure [27]. Considering ASTs as a retrofit option could offer environmental advantages, infrastructure [27]. Considering ASTs as a retrofit option could offer environmental advantages, especially if the deconstruction of an old dam may threaten aquatic and wildlife in the downstream. especially if the deconstruction of an old dam may threaten aquatic and wildlife in the downstream. Moreover, as detailed in Section 3.2, such upgrades could help reduce the disturbance of sediment transport processes and realize environmental and economic advantages by implementing renewable hydroelectricity generation. Many sites such as small weirs or dams without existing hydropower plants are suitable for installing or using ASTs as an upgrade. At existing sites, the dam construction has already been completed, return on investment has been satisfied, and the environmental impacts incurred for many years [52]. The minimum construction requirements of ASTs and their small scale makes the environmental impacts and GHG emission of this upgrade very low [91]. Additionally, such upgrades provide green electricity, which could help to reduce fossil fuel usage and continuous greenhouse gas emissions. Therefore, upgrading dams for hydroelectricity can not only be a reasonable economic decision but it could also partly remediate some prior environmental or social impacts. For example, within the province of Ontario, Canada, there are approximately 2600, mostly small, dams [92] that were primarily constructed for flood control and milling. About 70% of these dams were constructed prior to 1970. Since the expected service life of a typical small dam ranges between 50 and 70 years, many of these dams will require major restorations or structural repairs within 10 to 15 years because of safety and structural quality concerns [93]. Additionally, in the United States, the Army Corps of Engineers national inventory estimates there are 18,140 dams with heights under 4.6 m that are in need of structural repair due to improper or deferred maintenance [94]. It is also worth mentioning that in the province of Ontario, Canada, nearly one‐quarter of all energy production is provided through hydro resources, which are about 90% of all the province’s renewable energy supplies [95]. This represents about 8100 MW of hydroelectricity capacity from 240 dam sites across 24 river systems [96]. Since the focus of increases in hydropower is largely for sites with generating capacity greater than 1 MW [97], sites under 1 MW power generating potential were ignored in this study. However, ASTs could be considered as a reasonable solution for the utilization of these sub‐1 MW sites [27]. According to Ontario Hydro and the Ministry of Natural Resources Water Potential Site database, there are approximately 280 sites within Ontario (Figure 10) with less than 200 kW power generating capacity and head less than 5 m [98]. Considering these statistics and typical AST efficiency, the total power generating capacity of ASTs just within Ontario is approximately 16 MW Sustainability 2020, 12, 7352 13 of 34

Moreover, as detailed in Section 3.2, such upgrades could help reduce the disturbance of sediment transport processes and realize environmental and economic advantages by implementing renewable hydroelectricity generation. Many sites such as small weirs or dams without existing hydropower plants are suitable for installing or using ASTs as an upgrade. At existing sites, the dam construction has already been completed, return on investment has been satisfied, and the environmental impacts incurred for many years [52]. The minimum construction requirements of ASTs and their small scale makes the environmental impacts and GHG emission of this upgrade very low [91]. Additionally, such upgrades provide green electricity, which could help to reduce fossil fuel usage and continuous greenhouse gas emissions. Therefore, upgrading dams for hydroelectricity can not only be a reasonable economic decision but it could also partly remediate some prior environmental or social impacts. For example, within the province of Ontario, Canada, there are approximately 2600, mostly small, dams [92] that were primarily constructed for flood control and milling. About 70% of these dams were constructed prior to 1970. Since the expected service life of a typical small dam ranges between 50 and 70 years, many of these dams will require major restorations or structural repairs within 10 to 15 years because of safety and structural quality concerns [93]. Additionally, in the United States, the Army Corps of Engineers national inventory estimates there are 18,140 dams with heights under 4.6 m that are in need of structural repair due to improper or deferred maintenance [94]. It is also worth mentioning that in the province of Ontario, Canada, nearly one-quarter of all energy production is provided through hydro resources, which are about 90% of all the province’s renewable energy supplies [95]. This represents about 8100 MW of hydroelectricity capacity from 240 dam sites across 24 river systems [96]. Since the focus of increases in hydropower is largely for sites with generating capacity greater than 1 MW [97], sites under 1 MW power generating potential were ignored in this study. However, ASTs could be considered as a reasonable solution for the utilization of these sub-1 MW sites [27]. According to Ontario Hydro and the Ministry of Natural Resources Water Potential Site database, thereSustainability are approximately 2020, 12, x FOR PEER 280 sitesREVIEW within Ontario (Figure 10) with less than 200 kW power generating14 of 34 capacity and head less than 5 m [98]. Considering these statistics and typical AST efficiency, the total power[99]. Therefore, generating such capacity a retrofitting of ASTs and just upgrade within Ontario approach is approximately could be considered 16 MW as [99 a]. low Therefore, impact suchdevelopment a retrofitting that andcan upgradeprovide a approach considerable could amount be considered of energy, as a especially low impact at development the local community that can providelevel [27]. a considerable amount of energy, especially at the local community level [27].

(a) (b)

Figure 10.10. (a) Current Hydroelectric Plants in Ontario, Canada (Blue);(Blue); (b) Sites withwith HydropowerHydropower Generation PotentialPotential (Red),(Red), ASTAST SuitableSuitable SitesSites (Green)(Green) [[99].99].

4.5. AST Plant Configurations Most early ASTs were designed to operate at one fixed rotation speed [88]. However, variable speed operation is also possible by adding additional electrical equipment and accepting an efficiency reduction. Variable‐speed ASTs are a practical solution to deal with large seasonal flow fluctuations. Increasing the AST rotation speed increases the volume of flow that can pass through the AST. However, AST efficiency is reduced when rotation speeds exceed the maximum rotation speed suggested by Muysken relationship (Equation (9)) [100]. In addition, noise becomes a more significant issue at higher rotation speeds. When the volume of flow in a river is so high that an excessively large diameter (exceeding 5 m) or high rotation speed AST (exceeding Equation (10)) would be required to utilize the available flow, a multiple‐screw powerplant can be considered. In such cases, two or more ASTs can be installed (typically side by side) instead of a single big screw. When flow at the site is large, all ASTs could be turned on to generate power in their most efficient condition. The extra AST(s) could be turned off at times when the flow is reduced, allowing the remaining AST(s) to operate at closer to optimum conditions. Therefore, at sites where a considerable fluctuation in river flow is anticipated, plant designers may consider variable‐speed ASTs and multi‐AST plants in order to effectively utilize available flow at both low flow and high conditions. Using two or more ASTs in a single site could also provide other benefits including easier maintenance, more flexible operation plans, and even some savings in major costs. Currently, AST design is usually based on site specifications, regulatory limitations, and characteristics of available flow. One possibility is that multi‐AST powerplants could lead to significant capital cost savings by allowing mass production of highly optimized ASTs in several specific sizes. Figure 11 shows a crowd‐sourced map maintained by Alois Lashofer of the locations of many AST powerplants across the world [101] as of July 2020. The majority of the mapped AST powerplants are single screw plants. However, there are several plants that benefit from more than one screw installation. While most multi‐screw installations are in a parallel configuration, there are examples of installations of two plants in series to take advantage of higher head [79] such as the Yorkshire Water’s Esholt wastewater treatment multi‐AST hydropower, which is mentioned in Section 4.3. Sustainability 2020, 12, 7352 14 of 34

4.5. AST Plant Configurations Most early ASTs were designed to operate at one fixed rotation speed [88]. However, variable speed operation is also possible by adding additional electrical equipment and accepting an efficiency reduction. Variable-speed ASTs are a practical solution to deal with large seasonal flow fluctuations. Increasing the AST rotation speed increases the volume of flow that can pass through the AST. However, AST efficiency is reduced when rotation speeds exceed the maximum rotation speed suggested by Muysken relationship (Equation (9)) [100]. In addition, noise becomes a more significant issue at higher rotation speeds. When the volume of flow in a river is so high that an excessively large diameter (exceeding 5 m) or high rotation speed AST (exceeding Equation (10)) would be required to utilize the available flow, a multiple-screw powerplant can be considered. In such cases, two or more ASTs can be installed (typically side by side) instead of a single big screw. When flow at the site is large, all ASTs could be turned on to generate power in their most efficient condition. The extra AST(s) could be turned off at times when the flow is reduced, allowing the remaining AST(s) to operate at closer to optimum conditions. Therefore, at sites where a considerable fluctuation in river flow is anticipated, plant designers may consider variable-speed ASTs and multi-AST plants in order to effectively utilize available flow at both low flow and high conditions. Using two or more ASTs in a single site could also provide other benefits including easier maintenance, more flexible operation plans, and even some savings in major costs. Currently, AST design is usually based on site specifications, regulatory limitations, and characteristics of available flow. One possibility is that multi-AST powerplants could lead to significant capital cost savings by allowing mass production of highly optimized ASTs in several specific sizes. Figure 11 shows a crowd-sourced map maintained by Alois Lashofer of the locations of many AST powerplants across the world [101] as of July 2020. The majority of the mapped AST powerplants are single screw plants. However, there are several plants that benefit from more than one screw installation. While most multi-screw installations are in a parallel configuration, there are examples of installations of two plants in series to take advantage of higher head [79] such as the Yorkshire Water’s Esholt wastewater treatment multi-AST hydropower, which is mentioned in Section 4.3.

Figure 11. The map of AST powerplants across the world [79].

Sustainability 2020, 12, x FOR PEER REVIEW 15 of 34

Sustainability 2020, 12, 7352 15 of 34 Figure 11. The map of AST powerplants across the world [79].

Figure 1212 shows shows some some representative representative multi-AST multi powerplants‐AST powerplants in the United in the Kingdom. United MonmouthKingdom. NewMonmouth Hydro New (Figure Hydro 12a) is(Figure constructed 12a) is on constructed the site of the on Oldthe Monmouthsite of the Old Hydro Monmouth Station, whichHydro operated Station, fromwhich 1899 operated to 1953. from The 1899 new to hydro 1953. schemeThe new uses hydro two scheme ASTs and uses has two been ASTs operating and has since been 2009operating [102]. Thissince multi-AST 2009 [102]. powerplantThis multi‐AST on River powerplant Monnow on couldRiver Monnow generate 75could kilowatts generate of 75 power kilowatts by each of power screw, whichby each rotates screw, at which only 20 rotates RPM and at only utilizes 20 RPM 3.4 m and of water utilizes head 3.4 [103 m ].of Radyr water Hydro head [103]. Scheme Radyr (Figure Hydro 12b) appliesScheme two(Figure 200 kW12b) ASTs applies with two a 3.5 200 m kW outer ASTs diameter with a and3.5 m 10 outer m length diameter in order and to 10 utilize m length a site in on order the 3 Riverto utilize Taff awith site 3.5on mthe head River and Taff a mean with flow3.5 m of head 22 m and/s. Thisa mean powerplant flow of 22 can m generate3/s. This powerplant 1.8 million kWh can ofgenerate energy 1.8 annually, million whichkWh of reduces energy 785 annually, tons of which CO2 emission reduces 785 [104 tons]. Linton of CO Falls2 emission Hydro [104]. powerplant Linton (FigureFalls Hydro 12c) powerplant operated from (Figure 1909 12c) to 1948. operated Since from 2012 1909 it has to 1948. been Since retrofitted 2012 it by has two been ASTs retrofitted that have by 3 2.4two m ASTs diameter that have and 2.72.4 mm ofdiameter head. The and maximum 2.7 m of head. flow rateThe ismaximum 4.5 m /s [flow79] and rate each is 4.5 screw m3/s can[79] passand 3 2.6each m screw/s, which can pass results 2.6 m in3/s, a combinedwhich results 100kW in a combined of output 100kW power of at output their full power capacity at their [105 full]. capacity With an annual[105]. With production an annual of about production 500 MWh, of about this powerplant 500 MWh, this could powerplant save more could than 210 save tons more of CO than2 emissions 210 tons perof CO year2 emissions [105]. per year [105].

(a) (b) (c)

Figure 12.12. Multi-ASTMulti‐AST hydropower hydropower plants plants in in the the UK: UK: (a) Monmouth(a) Monmouth New New Hydro Hydro Scheme, Scheme, Osbaston Osbaston [103]; Sustainability([103];b) Radyr (2020b) Radyr weir, 12, x hydro, FORweir PEER hydro, Radyr REVIEW [Radyr106]; ( c[106];) Linton (c) FallsLinton Hydroelectric Falls Hydroelectric Power Station, Power Threshfield Station, Threshfield [107]. 16 of 34 [107]. One of thethe mostmost recentrecent multi-AST multi‐AST powerplants powerplants (Q2, (Q2, 2019) 2019) is is the the Solvay Solvay industrial industrial plant, plant, which which is installedis installed as as a retrofita retrofit/upgrade/upgrade on on the the small small existing existing weir weir in in Torrevieja, Torrevieja, Spain Spain (Figure (Figure 13 13).). TheThe entire project was constructed in just 4 months.months. This plant diverts part of the Saja River and applies two parallel ASTs thatthat areare ableable toto produceproduce aa totaltotal ofof 7070 kW of power (35 kW each) with 2 m of water head 3 and 5 m3/s/s of flow flow rate for both turbines. The generated energy will be sold to the electrical grid and the return on investmentinvestment will be recoveredrecovered aboutabout 77 toto 88 yearsyears afterafter itsits start-upstart‐up datedate [[108].108].

Figure 13. Solvay multi multi-AST‐AST powerplant project at a glance [[108].108].

Even currentcurrent AST AST powerplants powerplants could could upgrade upgrade to multi-AST to multi hydropower.‐AST hydropower. Linton Lock Linton powerplant Lock ispowerplant one of the is pioneers one of the in such pioneers an approach in such thatan approach benefits fromthat twobenefits screws from with two two screws different with sizes. two Thedifferent first screwsizes. The was first installed screw in was 2012 installed with a in diameter 2012 with and a diameter length of and 3 m length and 8.5 of m,3 m respectively. and 8.5 m, Withrespectively. 3.2 m of With water 3.2 head m of and water 4.5 m head3/s capacity, and 4.5 thism3/s screw capacity, could this off screwer 101 kWcould of offer power 101 output. kW of In power 2017, theoutput. second In 2017, screw the with second a 5 m screw of diameter with a was 5 m installed of diameter beside was it, installed which could beside be it, considered which could as one be ofconsidered the largest as screwsone of the in the largest world. screws With in 3 the m of world. water With head 3 andm of 14.5 water m3 head/s capacity, and 14.5 this m screw3/s capacity, could providethis screw 355 could kW ofprovide power 355 output kW of [46 power]. Figure output 14 shows [46]. Figure this powerplant. 14 shows this The powerplant. bigger screw The is bigger called “Widdingtonscrew is called Plant” “Widdington and the smaller Plant” and one the is called smaller the one “Linton is called Plant.” the “Linton Plant.”

Figure 14. Linton Lock multi‐AST powerplant.

Considering the number of installed screws, the Marengo powerplant in Goito, Italy, could be considered as one of the multi‐AST hydropower world records since it applies six parallel ASTs. Figure 15 shows this powerplant which is installed on the Mincio River through the Pozzolo‐Maglio drain channel and has an average potential of 222.7 kW (up to 306 kW [109]) with 1.6 m of water head and 1.4 and 22.2 m3/s average and maximum flow rates, respectively [110]. Each screw is 3 m in diameter and 4 m in flighted length, and can pass 3.7 m3/s of flow rate with a power of up to 54 kW [109]. Sustainability 2020, 12, x FOR PEER REVIEW 16 of 34

One of the most recent multi‐AST powerplants (Q2, 2019) is the Solvay industrial plant, which is installed as a retrofit/upgrade on the small existing weir in Torrevieja, Spain (Figure 13). The entire project was constructed in just 4 months. This plant diverts part of the Saja River and applies two parallel ASTs that are able to produce a total of 70 kW of power (35 kW each) with 2 m of water head and 5 m3/s of flow rate for both turbines. The generated energy will be sold to the electrical grid and the return on investment will be recovered about 7 to 8 years after its start‐up date [108].

Figure 13. Solvay multi‐AST powerplant project at a glance [108].

Even current AST powerplants could upgrade to multi‐AST hydropower. Linton Lock powerplant is one of the pioneers in such an approach that benefits from two screws with two different sizes. The first screw was installed in 2012 with a diameter and length of 3 m and 8.5 m, respectively. With 3.2 m of water head and 4.5 m3/s capacity, this screw could offer 101 kW of power output. In 2017, the second screw with a 5 m of diameter was installed beside it, which could be considered as one of the largest screws in the world. With 3 m of water head and 14.5 m3/s capacity, thisSustainability screw could2020, 12 provide, 7352 355 kW of power output [46]. Figure 14 shows this powerplant. The bigger16 of 34 screw is called “Widdington Plant” and the smaller one is called the “Linton Plant.”

Figure 14. LintonLinton Lock multi multi-AST‐AST powerplant.

Considering the number of installed screws, the Marengo powerplant in in Goito, Goito, Italy, Italy, could be considered as one of the multi multi-AST‐AST hydropower hydropower world world records records since since it it applies applies six six parallel parallel ASTs. ASTs. Figure 15 shows this powerplant which is installed on the Mincio River through the Pozzolo Pozzolo-Maglio‐Maglio drain channel channel and and has has an an average average potential potential of of222.7 222.7 kW kW (up (up to 306 to 306kW kW[109]) [109 with]) with 1.6 m 1.6 of mwater of water head andhead 1.4 and and 1.4 22.2 and m 22.23/s maverage3/s average and andmaximum maximum flow flow rates, rates, respectively respectively [110]. [110 Each]. Each screw screw is 3 is m 3 in m diameterin diameter and and 4 m 4 in m flighted in flighted length, length, and andcan pass can pass3.7 m 3.73/s of m 3flow/s of rate flow with rate a with power a power of up to of 54 up kW to

Sustainability[109].54 kW [109 2020]. , 12, x FOR PEER REVIEW 17 of 34

FigureFigure 15. MarengoMarengo multi multi-AST‐AST hydropower hydropower plant in Goito, Italy [109]. [109].

4.6. Future Future Sustainability Sustainability According to the United Nations Development Program (UNDP), 1.4 billion individuals had no access to electricity in in the the past past decade decade [111], [111], especially especially in in poor poor regions regions of of developing developing countries countries [112]. [112]. Geographical limitations limitations [113] [113] and uneven population distribution distribution are are two two important issues issues in effectivenesseﬀectiveness ofof the the electrical electrical distribution distribution infrastructure infrastructure [114]. The[114]. structural The structural simplicity, simplicity, low operational low operationaldemands, and demands, modest costs and make modest ASTs costs an environment-friendly make ASTs an environment and sustainable‐friendly solution, and especiallysustainable in solution,developing especially countries. in developing ASTs could countries. be used to ASTs generate could electricity be used evento generate with unconventional electricity even water with unconventionalresources such as water the about resources 2 kW such AST as powerplant the about that2 kW runs AST by powerplant sewage water that in runs Hayatabad by sewage Peshawar, water inPakistan Hayatabad [78]. Peshawar, There have Pakistan been several [78]. investigationsThere have been proposing several investigations pico-hydro ASTs proposing as an electriﬁcation pico‐hydro ASTs as an electrification solution [115,116] for the 2519 villages in Indonesia that do not have access to electricity because of inaccessible locations [116]. However, only around 1.8% of micro‐hydro potential (out of 400 MW potential capacity) is currently exploited in Indonesia as a developing country [117]. Recent studies have examined applying ASTs in small, micro, and even pico scales, and also considered methods to simplify the application of these ASTs. A recent study in Aceh,

Indonesia found that even a very simple AST with just one blade (𝑁1) with 𝐷 𝑆 0.26 m, 𝐷 0.14 m, 𝐿 1 m and 𝛽 30° could generate about 116 Watts of power with 1 m of head at a flowrate of 0.02 m3/s and 24.75 rad/s rotation speed [118]. A commercial version of this idea has been developed: currently, the smallest all‐in‐one portable AST generator from this project weighs just 17.5 kg, and is approximately 1 m long and 0.28 m wide [70]. It could be installed in very shallow waters and can generate around 10 watts of power in a flow of 0.01 m3/s and a head of 0.1 m [45]. Figure 16a shows a diagram of this all‐in‐one AST generator. This generator could be installed easily, and a 0.15 m tall metal plate helps to provide enough water head to generate power from even a very shallow water. It provides a wide range of new suitable sites to generate electricity. The turbines have been tested in undeveloped regions of Myanmar (Figure 16b) and in urban flows (Figure 16c). The generated power of each unit is very low in comparison to larger plants; however, units could be used in series (Figure 7b) or in parallel (Figure 16d,e) configured as a multi‐AST powerplant almost everywhere alongside any flow. Sustainability 2020, 12, 7352 17 of 34 solution [115,116] for the 2519 villages in Indonesia that do not have access to electricity because of inaccessible locations [116]. However, only around 1.8% of micro-hydro potential (out of 400 MW potential capacity) is currently exploited in Indonesia as a developing country [117]. Recent studies have examined applying ASTs in small, micro, and even pico scales, and also considered methods to simplify the application of these ASTs. A recent study in Aceh, Indonesia found that even a very simple AST with just one blade (N = 1) with DO = S = 0.26 m, Di = 0.14 m, L = 1 m and β = 30◦ could generate about 116 Watts of power with 1 m of head at a flowrate of 0.02 m3/s and 24.75 rad/s rotation speed [118]. A commercial version of this idea has been developed: currently, the smallest all-in-one portable AST generator from this project weighs just 17.5 kg, and is approximately 1 m long and 0.28 m wide [70]. It could be installed in very shallow waters and can generate around 10 watts of power in a flow of 0.01 m3/s and a head of 0.1 m [45]. Figure 16a shows a diagram of this all-in-one AST generator. This generator could be installed easily, and a 0.15 m tall metal plate helps to provide enough water head to generate power from even a very shallow water. It provides a wide range of new suitable sites to generate electricity. The turbines have been tested in undeveloped regions of Myanmar (Figure 16b) and in urban flows (Figure 16c). The generated power of each unit is very low in comparison to larger plants; however, units could be used in series (Figure7b) or in parallel Sustainability 2020, 12, x FOR PEER REVIEW 18 of 34 (Figure 16d,e) configured as a multi-AST powerplant almost everywhere alongside any flow.

(a)

(b) (c) (d) (e)

Figure 16. Ultra-smallUltra‐small all-in-oneall‐in‐one Archimedes screw generator (PicoPica 10): (a) PicoPica 10’s system Schematic [[70];70]; (b) operational test inin MyanmarMyanmar [[119];119]; ((cc)) powerpower GenerationGeneration fromfrom unconventionalunconventional water resources [[120];120]; ((d)) Multi-ASTMulti‐AST (two(two units) running since 2012 in Ibi-Cho,Ibi‐Cho, Gifu, Japan, to supply electricity for the security lights and the electric fence [[121];121]; ((e)) aa multi-ASTmulti‐AST application for shallow water resources [[122].122].

Figure 1717 representsrepresents the the fixed fixed and and larger larger version version of of this this small small commercial commercial AST AST generator generator that that has beenhas been installed installed in Nikko in Nikko City, Japan,City, Japan, since 2017since [ 1232017]. It[123]. can generateIt can generate about 500 about W with 500 W a flow with of a 0.1 flow m3 of/s and0.1 m 0.73/s m and of head0.7 m [ 45of ].head This [45]. AST This has aAST net has weight a net of weight 250 kg andof 250 it couldkg and generate it could enough generate power enough for nighttimepower for lighting, nighttime refrigerators lighting, refrigerators for preserving for food preserving and medicine, food telecommunicationand medicine, telecommunication equipment such asequipment cellular phones, such as and cellular televisions phones, [45 and]. Systems televisions like these[45]. Systems could feasibly like these provide could electricity feasibly forprovide poor communitieselectricity for topoor provide communities night-time to provide lighting night and charging‐time lighting their laptopsand charging or mobile their phones. laptops Pico-scaleor mobile ASTsphones. present Pico‐ ascale flexible ASTs and present reasonably a flexible practical and option reasonably for improving practical local option economies for improving and welfare local in remote,economies developing and welfare communities. in remote, developing communities.

Figure 17. Small commercial all‐in‐one AST generator (PicoPica 500), Nikko City, Japan [45,123].

5. Design Archimedes Screw Hydro Powerplants Principles Figure 18 shows the relationships between AST and the other parts of a simplified AST‐based ROR hydropower system. This figure indicates that an Archimedes Screw Turbine (AST) run‐of‐river (ROR) hydroelectricity powerplant can be considered as a system with three major components: a reservoir, a weir, and the AST (which is connected to the system by a control gate and trash rack). At most real AST locations, the incoming flow must be divided between the AST and a parallel weir. Typically, a minimum flow over the weir is mandated for the protection of the local environment. Therefore, other outlets as well as a fish ladder could be considered as the other components of this Sustainability 2020, 12, x FOR PEER REVIEW 18 of 34

(a)

(b) (c) (d) (e)

Figure 16. Ultra‐small all‐in‐one Archimedes screw generator (PicoPica 10): (a) PicoPica 10’s system Schematic [70]; (b) operational test in Myanmar [119]; (c) power Generation from unconventional water resources [120]; (d) Multi‐AST (two units) running since 2012 in Ibi‐Cho, Gifu, Japan, to supply electricity for the security lights and the electric fence [121]; (e) a multi‐AST application for shallow water resources [122].

Figure 17 represents the fixed and larger version of this small commercial AST generator that has been installed in Nikko City, Japan, since 2017 [123]. It can generate about 500 W with a flow of 0.1 m3/s and 0.7 m of head [45]. This AST has a net weight of 250 kg and it could generate enough power for nighttime lighting, refrigerators for preserving food and medicine, telecommunication equipment such as cellular phones, and televisions [45]. Systems like these could feasibly provide electricity for poor communities to provide night‐time lighting and charging their laptops or mobile phones.Sustainability Pico2020‐scale, 12, 7352 ASTs present a flexible and reasonably practical option for improving 18local of 34 economies and welfare in remote, developing communities.

FigureFigure 17. 17. SmallSmall commercial commercial all all-in-one‐in‐one AST AST generator generator (PicoPica (PicoPica 500), 500), Nikko Nikko City, City, Japan Japan [45,123]. [45,123].

5.5. Design Design Archimedes Archimedes Screw Screw Hydro Hydro Powerplants Powerplants Principles Principles FigureFigure 1818 shows thethe relationshipsrelationships between between AST AST and and the the other other parts parts of aof simplified a simplified AST-based AST‐based ROR RORhydropower hydropower system. system. This This figure figure indicates indicates that anthat Archimedes an Archimedes Screw Screw Turbine Turbine (AST) (AST) run-of-river run‐of‐ (ROR)river (ROR)hydroelectricity hydroelectricity powerplant powerplant can be can considered be considered as a system as a withsystem three with major three components: major components: a reservoir, a reservoir,a weir, and a weir, the AST and (which the AST is (which connected is connected to the system to the by system a control by a gate control and trashgate and rack). trash At rack). most realAt mostAST real locations, AST locations, the incoming the incoming flow must flow be dividedmust be between divided thebetween AST andthe AST a parallel and a weir. parallel Typically, weir. Typically,aSustainability minimum a 2020 minimum flow, 12, overx FOR flow the PEER weir over REVIEW is the mandated weir is mandated for the protection for the protection of the local of environment. the local environment. Therefore,19 of 34 Therefore,other outlets other as outlets well as as a well fish ladderas a fish could ladder be could considered be considered as the other as the components other components of this system.of this Therefore,system. Therefore, in an AST in hydropower an AST hydropower plant system, plant incoming system, river incoming flow is river the input, flow and is the the input, weir overflow, and the ASTweir outletoverflow, flow, AST and outlet other flow, outlets and (i.e., other fish outlets ladder (i.e., flow) fish are ladder the outputs. flow) are the outputs.

Figure 18. A Simple Model of an AST AST-based‐based Run-of-RiverRun‐of‐River (ROR) System.

5.1. Archimedes Screw Hydro Powerplants Design Assessments In order toto designdesign an an Archimedes Archimedes screw screw hydro hydro powerplant, powerplant, three three important important assessments assessments should should be done:be done: (1) electricity(1) electricity needs needs assessment assessment and (2) siteand assessments. (2) site assessments. Then, the type Then, of screwthe type (ﬁxed of/variable screw speed)(fixed/variable should be speed) determined should inbe (3) determined plant design in (3) assessment. plant design assessment. For the electricity needs assessment, itit isis recommendedrecommended to:to: ‐- Estimate the required power as well as predict thethe possiblepossible futurefuture changeschanges inin demand.demand. ‐ Determine whether the powerplant will be connected to the grid or will be off‐grid. ‐ If the AST powerplant is designed to be off‐grid, it would be important to: ‐ Consider the comparison of the value of reliable baseload power versus maximizing the annual production of the powerplant in making decisions for the design. ‐ Define and consider the needed voltages and/or currents. For the site assessment: ‐ Permitting requirements, restrictions on access or water use should be determined. ‐ The flow duration curve of the river should be used to determine the baseload and other options for the operation of the powerplant. This information also helps to make decisions in the plant design step. ‐ Ecological assessments should be done, and the water needs of other users or ecological functions should be determined. For example, the required flow for fish passage or installing a fish ladder should be determined. ‐ The potential plant location should be selected based on the following considerations: ‐ Accessibility for construction, operation, and maintenance. This is important for civil works and installation of the screw, especially for larger powerplants that have big and bulky screw(s) that need cranes for installation ‐ Geotechnical concerns, particularly stability and suitability for plant foundations ‐ Supply channel and outlet channel routing ‐ Conflicts with other site use considerations. In plant design assessment, the type of plant should be selected based on previous assessments: ‐ For steady flow, a single‐speed plant could be considered as a simpler and more efficient option. On average, the cost per watt of these systems is less than the variable‐speed ones. Even if the Sustainability 2020, 12, 7352 19 of 34

- Determine whether the powerplant will be connected to the grid or will be oﬀ-grid. - If the AST powerplant is designed to be oﬀ-grid, it would be important to:

- Consider the comparison of the value of reliable baseload power versus maximizing the annual production of the powerplant in making decisions for the design. - Deﬁne and consider the needed voltages and/or currents.

For the site assessment:

- Permitting requirements, restrictions on access or water use should be determined. - The flow duration curve of the river should be used to determine the baseload and other options for the operation of the powerplant. This information also helps to make decisions in the plant design step. - Ecological assessments should be done, and the water needs of other users or ecological functions should be determined. For example, the required flow for fish passage or installing a fish ladder should be determined. - The potential plant location should be selected based on the following considerations:

- Accessibility for construction, operation, and maintenance. This is important for civil works and installation of the screw, especially for larger powerplants that have big and bulky screw(s) that need cranes for installation - Geotechnical concerns, particularly stability and suitability for plant foundations - Supply channel and outlet channel routing - Conﬂicts with other site use considerations.

In plant design assessment, the type of plant should be selected based on previous assessments:

- For steady flow, a single-speed plant could be considered as a simpler and more efficient option. On average, the cost per watt of these systems is less than the variable-speed ones. Even if the available flow is not sufficient to fill the screw at its operating speed, fixed speed screws can still generate power in a partially full condition [30]. - For large surplus flow, variable-speed screws could be recommended to generate partial power at low flow times. It would be particularly important if the powerplant will be off-grid, or other power sources (like diesel generators) are not available or are expensive. Generally, variable-speed screws are recommended when there is an excess flow that could be used to generate more power from an available flow of when the flow varies. The main reason is that although operating ASTs at the full capacity may be the most mechanically efficient operating condition, it will not definitely lead to the highest overall energy generation. Therefore, it may not be the most economically efficient operating condition [30].

For all plant designs:

- A sluice gate to control flow to the plant, and trash racks to prevent large debris from entering the plant, must be planned upstream of the screw. Ease of trash rack cleaning is important. - Provision to dry out the top and bottom of the screw for bearing inspection, maintenance should be included in the plant design. The sluice gate should be able to shut off all flow. At the outlet, build vertical grooves to hold stop logs to allow drying out of the screw outlet. - Be sure to plan for flooding water levels, and be sure to protect electrical components from water damage. This could be done by elevating the generator, sealing the powerhouse, and/or putting control equipment on the bank at a higher level. Sustainability 2020, 12, x FOR PEER REVIEW 20 of 34

available flow is not sufficient to fill the screw at its operating speed, fixed speed screws can still generate power in a partially full condition [30]. ‐ For large surplus flow, variable‐speed screws could be recommended to generate partial power at low flow times. It would be particularly important if the powerplant will be off‐grid, or other power sources (like diesel generators) are not available or are expensive. Generally, variable‐ speed screws are recommended when there is an excess flow that could be used to generate more power from an available flow of when the flow varies. The main reason is that although operating ASTs at the full capacity may be the most mechanically efficient operating condition, it will not definitely lead to the highest overall energy generation. Therefore, it may not be the most economically efficient operating condition [30]. For all plant designs: ‐ A sluice gate to control flow to the plant, and trash racks to prevent large debris from entering the plant, must be planned upstream of the screw. Ease of trash rack cleaning is important. ‐ Provision to dry out the top and bottom of the screw for bearing inspection, maintenance should be included in the plant design. The sluice gate should be able to shut off all flow. At the outlet, build vertical grooves to hold stop logs to allow drying out of the screw outlet. ‐ Be sure to plan for flooding water levels, and be sure to protect electrical components from water damage. This could be done by elevating the generator, sealing the powerhouse, and/or putting Sustainability 2020, 12, 7352 20 of 34 control equipment on the bank at a higher level.

5.2. Design of Archimedes Screws The Archimedes screw itself is a central component in an an Archimedes Archimedes Screw Screw Hydropower Hydropower Plant. Plant. Figure 1919 shows shows a typicala typical Archimedes Archimedes screw screw conﬁgured configured as a hydropower as a hydropower plant and plant the most and importantthe most dimensionsimportant dimensions and parameters and parameters required to deﬁnerequired the to Archimedes define the screw.Archimedes These are:[labelscrew. These=,labelsep are: =0mm] Dii: Inner Inner diameter diameter DO:: Outer Outer diameter diameter L:: Total lengthlength ofof thethe screwscrew β:: Inclination Angle of the Screw N: Number of helical planed surfacessurfaces S:: Screw pitchpitch (Distance(Distance alongalong thethe screwscrew axisaxis forfor oneone completecomplete helicalhelical planeplane turn)turn) ff:: Fill Fill Height Height of of the the bucket bucket [30] [30] Gw:: The The gap gap between between the the trough trough and and screw screw hu:: Upper Upper (inlet) (inlet) water water level level hL:: Lower Lower (outlet) (outlet) water water level level

Figure 19. Archimedes Screw Geometry and Parameters.

The geometry of an Archimedes screwscrew isis determineddetermined byby externalexternal ((DDOO, L, and β) and internal (Di,, N,, and S)) parameters. The external parameters are generally determines based on the location of thethe screw and thethe passingpassing flowflow rate.rate. The internal parameters could be selected in a wayway thatthat optimizesoptimizes the performance of the screw [26]. Typically, the screw manufacturer should be involved in detailed design. The following process may be useful for initial planning and preliminary design of an AST site. First, determine the overall size and inclination angle of the screw. The inclination angle should be determined based on the site slope. If there are minimal constraints on angle (and installation space), a value of β = 22◦ could be considered since many current AST powerplants are installed at a similar inclination angle unless there is a need for steeper slope. Be careful if considering β values in excess of about 30◦ since screw capacity will decrease markedly, or less than about 20◦ due to longer screw length. Determine the length (L) of the screw based on site specifications and technical limitations. Use information from existing plants and Equations (10) or Equations (11) to select an overall diameter Do to accommodate the available flow. Check L/Do. If L/Do is less than about 2, expect efficiency to be somewhat reduced. If L/Do is less than about 1.25, the screw will likely be too short for its diameter. Consider two or more smaller diameter screws, particularly as L/Do gets smaller. Use Equation (10) for screw rotation speed ω, let pitch S = Do and use Di = Do/2. These values will be reasonable for preliminary planning, but may not be optimum values for the final design. After determination of the geometry of the screw, the algorithm presented by Nuernbergk and Rorres [62] can be used to determine the inlet water head required for optimal operation of the screw to fill it to its optimum volume capacity without occurring overflow. This is needed to vertically position the screw relative to the dam crest or expected reservoir level. The screw must be low enough to ensure Sustainability 2020, 12, 7352 21 of 34 it fills completely, but not so low that available head is not utilized. The Archimedes screw power generation model that is introduced in Section 5.3 could be used to estimate the generated power of the designed powerplant. Finally, the net generated power of the Archimedes screw is the difference of the estimated generated power and the power losses introduced in Section 5.4. Finally, the reader is cautioned that the process above, and in the following two sections, should only be used to check feasibility of an AST at a site. Additional design work will be needed to properly optimize the screw plant for a site.

5.3. Estimating the Generated Power of the Archimedes Screws Despite the literature of using Archimedes screws as pumps, currently, there is little English documentation in using them for extracting energy from flow, and a significant portion of it is about the case studies of installations, many of which are qualitative [30]. Several researchers have worked on developing mathematical models to predict the power output of an Archimedes screw. Early AST power models assumed that the screw was driven by the weight of the water enclosed by the screw blades [25,124]. Essentially, water contained within the buckets of the rotating screw produces a static pressure distribution on all submerged surfaces, and this distribution of pressures results in a net force in the direction of rotation. Müller and Senior (2009) offered a model based on the hydrostatic pressure difference across the screw surfaces. To consider the effect of gap leakage, they used Nagel’s (1968) empirical equation. However, their model simplifies the screw geometry in a level that they concluded that, by ignoring the bearing and friction losses, theoretically, there is no dependency between the rotation speed and the efficiency of an AST [21,30]. The main assumption for almost all Archimedes screws models is that the water level in each bucket is the highest level at which no water flows to the next bucket over the top of the inner cylinder. There is little theory or data for when ASTs run at partially full conditions [30]. Lubitz et al. [30] proposed a model to estimate the efficiency of screws for all range of possible fill levels. Based on the idea of analyzing a single water bucket, Lubitz et al. [30] proposed several mathematical models to estimate the flow and power of an ideal screw in a steady flow regime. This quasi-static model calculates the volumes of water buckets and the resulting torque on the screw by assuming the screw is not rotating and experiencing no internal water flows [30]. For an ideal screw operating under steady-state conditions (steady flow, constant rotational speed), all the buckets will have the same shape and volumetric size. The shape and size of a bucket are determined by the geometry of the screw, the screw rotation speed ω, and the volume flow rate of water through the screw Q [30]. The model determines the forces and flows operating within a single bucket for an idealized infinitely long screw. It is assumed that all buckets within the screw effectively function identically to this idealized bucket. Forces, torques, and power then can be scaled up based on the total length of the screw (L) to calculate total screw power [30].

5.3.1. Bucket Volume Theory The Lubitz et al. [30] model requires defining the general positions on the helical plane surfaces in cylindrical coordinates (Figure 20). A ‘w’ axis is aligned with the rotational axis of the central cylindrical shaft and a vertically oriented Cartesian axis ‘z’ is also defined with positive z vertically upwards. This vertical axis is used to calculate local water depths by projecting physical locations on the helical plane surfaces onto the vertical axis. It is assumed that the first leading helical plane edge is vertically oriented at the top of the screw. So, for any position along the w axis, the radial positions (r(ω)) and angular positions (θ(ω)) on the leading plane are described by the geometry of a helicoid of pitch length S. For any given position along the ‘w’ axis [30]:

r(w) = r (1) Sustainability 2020, 12, x FOR PEER REVIEW 22 of 34

are determined by the geometry of the screw, the screw rotation speed , and the volume flow rate of water through the screw Q [30]. The model determines the forces and flows operating within a single bucket for an idealized infinitely long screw. It is assumed that all buckets within the screw Sustainability 2020, 12, 7352 22 of 34 effectively function identically to this idealized bucket. Forces, torques, and power then can be scaled up based on the total length of the screw (L) to calculate total screw power [30]. W θ(w) = 2π (2) 5.3.1. Bucket Volume Theory S whereTher is Lubitz the radial et al. position, [30] model and requiresθ is the angular defining position the general (Figure positions 20). For on a screwthe helical with plane the number surfaces of bladesin cylindricalN, the vertical coordinates position (Figure on the 20). leading A ‘w’ helical axis is plane aligned surface with (Z the1) and rotational the upstream axis of helical the central plane ( ) (cylindricalZ2) at any pointshaft likeand Xa verticallyr, θ could oriented be deﬁned Cartesian by [30 ]:axis ‘z’ is also defined with positive z vertically upwards. This vertical axis is used to calculate local waterS θdepths by projecting physical locations on Z = r Cos(θ) Cos(β) Sin(β) (3) the helical plane surfaces onto the vertical1 axis. It is assumed− 2π that the first leading helical plane edge is vertically oriented at the top of the screw. So, for any position along the w axis, the radial positions (𝑟𝜔) and angular positions (𝛳𝜔) on the leading planeSθ are Sdescribed by the geometry of a helicoid Z2 = r Cos(θ) Cos(β) Sin(β) (4) of pitch length S. For any given position along the ‘w−’ axis2π −[30]:N

FigureFigure 20.20. TheThe relationshiprelationship between between the the angular angular and and radial radial positions positions within within the the screw screw in in the the Lubitz Lubitz et al.et al. (2014) (2014) Archimedes Archimedes Screw Screw model model coordinate coordinate system. system.

The minimum fill height can be approximated to occur at θ = π and r = Do/2 and the maximum 𝑟𝑤 𝑟 (1) (100%) fill height occurs at approximately θ = 2π and r = Di/2. Therefore, the minimum bucket water depth Z , maximum bucket depth without overflowing𝑊 Z and the actual water depth within the min 𝜃𝑤 2𝜋 Max (2) 𝑆 bucket Zwl can be defined and related to the nondimensional water fill height ( f ): where r is the radial position, and  is the angular position (Figure 20). For a screw with the number Do S of blades 𝑁, the vertical position onZ the= leadingCos helical(β) planeSin( βsurface) (𝑍) and the upstream helical(5) min − 2 − 2 plane (𝑍) at any point like 𝑋𝑟, 𝜃 could be defined by [30]: D = i ( ) 𝑆𝜃( ) 𝑍 ZMax 𝑟 𝐶𝑜𝑠𝜃Cos β 𝐶𝑜𝑠𝛽S.Sin β𝑆𝑖𝑛𝛽 (3)(6) 2 − 2𝜋 Zwl Zmin 𝑆𝜃 𝑆 Zwl = Zmin + − (ZMax Zmin) = Zmin + f .(ZMax Zmin) (7) 𝑍 Z max 𝑟 𝐶𝑜𝑠𝜃Z −𝐶𝑜𝑠𝛽 – 𝑆𝑖𝑛𝛽 − (4) − min 2𝜋 𝑁 An infinitesimal, cylindrical volume element (dV) can be defined parallel to the ‘w’ axis connecting The minimum fill height can be approximated to occur at θ = π and r = Do/2 and the maximum adjacent points on the helical planes on the upstream and downstream of the bucket. If only the portion (100%) fill height occurs at approximately θ = 2π and r = Di/2. Therefore, the minimum bucket water of this elemental volume that is submerged below the water line is considered part of the overall water depth 𝑍, maximum bucket depth without overflowing 𝑍 and the actual water depth within bucket volume, the overall volume of a bucket V can be determined as [30]: the bucket 𝑍 can be defined and related to the nondimensional water fill height (𝑓):  𝐷 𝑆  0 Zl > Zwl and Zwl < Z2  𝑍 𝐶𝑜𝑠𝛽 𝑆𝑖𝑛𝛽 (5)  Zwl Z1 S 2 2 dV =  Z −Z N r dr dθ Z1 Z2 and Zwl Z2  2− 1 ≤ ≤  S 𝐷 (8)  N r dr dθ Zl > Zwl and Zwl < Z2 𝑍 𝐶𝑜𝑠D 𝛽 𝑆.𝑆𝑖𝑛𝛽 (6) R2 r= 0 R θ=2 π V = 2 dV 𝑍 𝑍 Di θ=0 r= 2 𝑍 𝑍 𝑍 𝑍 𝑍 𝑓.𝑍– 𝑍 (7) 𝑍 𝑍 5.3.2. Flow Rate and Leakage Models Knowledge of how much water will enter an AST depending on the depth of the water at the inlet is important, since to first order, the amount of power generated by an AST is proportional to the volume flow rate of water through it. Developing a general relationship for the volume of flow passing through an AST as a function of the inlet water level for all screw sizes is challenging because while most water Sustainability 2020, 12, 7352 23 of 34

flows through the screw within the buckets formed by the helical array of blades, there is a small gap between the trough and screw which could be considered as free flow. Screw geometry and rotation speed are also important factors that intensify the difficulties. Nuernbergk and Rorres [62] proposed an analytical model for the optimal design of full-scale screws based on the water-inflow conditions for screws running at a fixed speed near to the Muysken’s maximum recommended rotation speed (ωM) for Archimedes screws (Equation (9)) [100]. Introducing the concept of effective cross-sectional water area within the screw (AE)[125] and axial transport velocity (VT = Sω/2π), it can be shown that

5π ω = (9) M 2/3 3Do

Q = AEVT (10)

Based on a relatively similar concept and by defining the concept of effective area (AE), YoosefDoost and Lubitz [125] developed a new equation for the volume of flow passing through an AST. This nondimensional equation could estimate the total flow rate for different rotation speeds and inlet water levels of the studied lab-scale (small) and full-scale ASTs [125].

!b !c Q A ω = a E (11) QMax AMax ωM = 2 = where AMax πDO/4 , QMax is determined from Equation (11) by setting AE AMax. The constants a, b, and c are related to the screw properties and optimum value may be different for each screw. Preliminary investigations suggest that a = 1.242, b = 1.311, and c = 0.822 give reasonable predictions of Q for a range of AST sizes [125]. In addition to the main flow within the buckets, and leakage flow through the gaps between blade and trough, there are several other paths for flow through an AST. The total flow (Q) of an AST can be divided into the following five components: (1) Main flow that is contained with the buckets and causes torque generation (QM), (2) Gap leakage flow (QG), (3) Over filling flow leakage (QO), when water levels within a bucket are so high that some water spills over the top of the central cylinder, (4) Friction-leakage (QF), when water adheres to the flights and is flung out of the screw, and (5) No guiding plate leakage (QP), which occurs when water levels are high enough that water spills out of the top edge of the trough [62]. Total flow Q is the sum of all five of these flow components:

Q = QM + QG + QP + QO + QF (12)

It can be assumed that generally, only QM contributes to meaningful power generation, while the other flow components do not contribute useful power and so are ideally minimized or eliminated. In modern screws, QP is usually eliminated by using a guiding plate to extend the trough to enclose more of the rotating screw. For screws running up to the optimal flow rate, QO is zero since overflow only happens above the optimum flow rates. Finally, the effect of QF is usually negligible in ASTs [62]. The gap flow QG is of particular interest, since it is necessary to include a gap between the blades and trough, and so it is not possible to eliminate this component of lost flow. Nagel [126] presented an empirical equation for gap leakage flow in Archimedes screw pumps (not turbines), for the case of full buckets (f = 1) at normal rotational speeds:

1.5 QG = 2.5 GW Do (13) where the GW gap width (in meters) and the diameter (Do) must be entered in units of meters, to get a 3 resulting QG in m /s. The gap width is not easy to measure in full-scale screws. Nagel also provided an empirical relation to estimate GW based on the size of the screw: Sustainability 2020, 12, x FOR PEER REVIEW 24 of 34

It can be assumed that generally, only QM contributes to meaningful power generation, while the other flow components do not contribute useful power and so are ideally minimized or eliminated. In modern screws, 𝑄 is usually eliminated by using a guiding plate to extend the trough to enclose more of the rotating screw. For screws running up to the optimal flow rate, 𝑄 is zero since overflow only happens above the optimum flow rates. Finally, the effect of 𝑄 is usually negligible in ASTs [62]. The gap flow QG is of particular interest, since it is necessary to include a gap between the blades and trough, and so it is not possible to eliminate this component of lost flow. Nagel [126] presented an empirical equation for gap leakage flow in Archimedes screw pumps (not turbines), for the case of full buckets (f = 1) at normal rotational speeds:

. 𝑄 2.5 𝐺 𝐷 (13)

Sustainabilitywhere the 2020𝐺 , gap12, 7352 width (in meters) and the diameter (Do) must be entered in units of meters, 24to of get 34 a resulting QG in m3/s. The gap width is not easy to measure in full‐scale screws. Nagel also provided an empirical relation to estimate 𝐺 based on the sizep of the screw: GW = 0.0045 DO (14) 𝐺 0.0045𝐷 (14) Nagel’s modelmodel is is necessarily necessarily an an estimate estimate only, only, as allas physicalall physical and dynamicand dynamic properties properties of the of actual the flowactual regime, flow regime, rotational rotational mechanics mechanics of the screw, of andthe screw, the fluid and mechanics the fluid of mechanics the flow are of neglected the flow [126 are]. Neurnbergkneglected [126]. and Neurnbergk Rorres proposed and Rorres a more proposed complex a equation more complex attributed equation to Muysken attributed to estimate to Muysken gap flowto estimate leakage gap by flow including leakage some by additionalincluding some parameters additional [62]: parameters [62]: s !2 D𝐷O G𝐺W 𝑆S 22 p QG𝑄= 𝜇µAG𝐺W (11 + )11 + ( 𝛼(α 3𝛼+ α5)𝛼+ α4) 2𝑔𝛥ℎ2g∆ h (15)(15) 22 D𝐷O 𝜋𝐷πD O 33 where µ𝜇A isis the the contraction contraction discharge discharge coecoefficientfficient whichwhich isis dependentdependent onon thethe shapeshape ofof thethe edge of the blade and is typically in the range of between 0.65 and 1 for the minimum and maximum leakage, respectively. The head head difference difference Δh∆h = 𝑆/𝑁Sinβ(S/N)Sinβ) )and and 𝛼α3,, 𝛼α4, and α𝛼5 areare wetted wetted anglesangles aroundaround the gap (in radians) that can be determined from thethe algorithmalgorithm proposedproposed inin RorresRorres (2000)(2000) [[26,62].26,62]. Lubitz et al. [[30]30] presentedpresented anan equationequation forfor QQGG that is functionallyfunctionally equivalent to thethe Muysken (1932) [[100]100] and Nuernbergk and Rorres [62] [62] leakage leakage models, models, but but cast cast in in different different geometric geometric variables. variables. It assumed thatthat thethe entireentire gap gap leakage leakage is is driven driven by by the the static static pressure pressure di differencefference across across the the gap, gap, which which is theis the result result of of the the water water height height di ffdifferenceerence between between adjacent adjacent buckets. buckets.

𝑙 r2𝑔𝑆 le 2gS (16) Q𝑄=𝐶𝐺𝑤CGw (l 𝑙 + ) 𝑠𝑖𝑛sin β 𝛽 (16) G w 1.5 𝑁N where C isis a minor lossloss coecoefficientﬃcient that is less than or equal to 1 and previously taken to be 0.89; g is 22 the gravitational constantconstant (9.81(9.81 m m/s/s );); and andl w𝑙and andle are𝑙 are wetted wetted lengths lengths along along with with a single a single turn turn of one of ﬂight,one flight, with withlw being 𝑙 being the length the length of the of gap the thatgap that is submerged is submerged on both on both sides, sides, and andle being le being the the length length of theof the gap gap that that is submergedis submerged on on one one side side and and exposed exposed to airto air on on the the other other (Figure (Figure 21). 21).

Figure 21.21. Lubitz et al. gap leakage flow ﬂow model parameters. From [30]. [30].

When bucket fill level exceeds 100% (f > 1), rising the water above the center cylinder causes a secondary flow that lets the water pour over the top of the center cylinder into the downstream bucket. Aigner [62] presented a leakage model based on assuming that the overflow could be approximated as weir flow through a triangular spillway [64]. The weir is approximated as a simple angled, V-notch weir since this is approximately the shape that the central shaft and the planes make at the overflow point [27]. 4 p 1 q Q = µ 2g (tan β + ) h5 (17) O 15 tan β ue where µ is a loss coefficient and hue is the overflow head, calculated as [27]

hue = Z Zmax Z > Zmax (18) wl − wl Sustainability 2020, 12, 7352 25 of 34

At optimum ﬁll height and below, no spill across the central tube happens, and hue and also QO will be zero.

5.3.3. Torque and Power Models The torque on the screw is the result of water pressure on the helical planes. The Lubitz et al. model determines the hydrostatic pressure p at any point on the plane surfaces at a depth z below the water level by assuming static conditions within the buckets [30]: ( ρg(Z Z) Z < Z p = wl − wl (19) 0 Z Z ≥ wl The net pressure on the helical plane surfaces is the diﬀerence between the pressure on the up and downstream surfaces of the blade. Therefore, if p1 and p2 are assumed as the pressures on each side of the plane surface, the net torque on each element area of the helical plane surface (dT) and the total torque from a single bucket (T) can be calculated by integrating torque over the entirety of the submerged surfaces: Sθ dT = (p p ) r dr dθ (20) 1 − 2 2π

D0 Z r= 2 Z θ=2 π T = dT (21) Di r= 2 θ=0 The total torque in full length of screw is related to the total number of buckets and can be calculated by: NL T = T( ) (22) Total S Then, the total power will be: Pout = TTotalω (23)

5.4. Archimedes Screw Power Loss Models The discussion above has described the performance of an ideal screw, in which many loss mechanisms are neglected. While overflow and gap leakage flow reduce the overall efficiency of an AST [26,127] and power output can be limited by the amount of water that can enter the screw inlet [62], a complete AST power loss model should consider all possible known head losses [27], which include:

- Inlet losses due to head loss through the screw entrance - Internal hydraulic friction between water and moving screw surfaces - Outlet losses due to exit eﬀects, geometric head losses, and additional drag torque - Friction of bearings - Additional mechanical and electrical losses in gearboxes, generators, and electrical controls

The screw in an AST is supported at both ends by a bearing. Friction losses in bearings reduce torque available at the AST shaft. The magnitude of these losses depends on mechanical properties of the bearing, which may vary from one AST installation to another, and because bearing losses are both relatively low in full-scale ASTs and diﬃcult to predict a priori, there is little guidance in the literature on this loss mechanism. While there are equations in the literature for predicting inlet and hydraulic frictional power losses, most of the outlet power loss calculation attempts are based on adapting equations from related problems [128,129] such as the Borda-Carnot equation for culvert outlet exit power losses. Notably, Nuernbergk presents equations for non-optimal outlet water level loss [28] and Kozyn and Lubitz developed an empirical equation for outlet drag torque power loss [130]. Methods of accurately calculating all of these power losses is a current area of AST research. Sustainability 2020, 12, 7352 26 of 34

6. Conclusions Archimedes Screws Turbines (ASTs) are a new form of turbines for small hydroelectric powerplants that could be applied even in low head sites. ASTs offer a clean and renewable source of energy. They are safer for wildlife and especially fish. The low rotation speed of ASTs reduces negative impacts on aquatic life and fish. It is important to note that ASTs are not a uniquely global solution for all energy generation needs. ASTs have their own drawbacks just like any other technologies: using Archimedes screws as generators is a relatively new technology, and in comparison with other hydropower technologies, there are many not well-known things about ASTs. Currently, there are no standards for the design of ASTs, and AST hydro powerplant designs are highly dependent on the experience of the engineer who designs them. For very high flow rates or water heads, a single screw may not take advantage of all available potential due to material, structural, technical, and physical limitations. However, the increasing interest in ASTs, new advancements, and ideas such as multi-AST powerplants offer some solutions to extend AST usability. ASTs provide a range of practical advantages for generating electrical energy at suitable locations. For supporting sustainable development, ASTs offer economic, social, and environmental advantages. Considering the flexibility and advantages of ASTs, they could be considered as one of the most practical options for a more sustainable electricity generation:

1. To increase the number of suitable sites for power generation even in sites with very low flow rates and/or water head. ASTs can be designed to operate in a wide range of flow rates (currently from 0.01–10 m3/s) and water heads (currently from 0.1–10 m), including at sites where other types of turbines may not be feasible. This increases the number of potentially suitable sites for hydropower. 2. To maximize hydropower generation even in rivers with high flow rate fluctuations. ASTs can handle flow rates even of up to 20% more than optimal filling without a significant loss in efficiency [63]. Even when the conditions are not perfect for a single screw, installing more than one screw, and utilizing variable-speed ASTs, allows developers to fully utilize available flow at a wider range of sites, including those with high seasonal variability. 3. To retrofit old dams or upgrade current dams or mills to make them economically (power generation) and environmentally (renewable energy) reasonable. Using ASTs as an upgrade for retrofitting old dams or upgrading operational dams makes it possible to add electrical generation with extremely low incremental environmental impact, at reasonable costs and with good potential for low social impacts while providing an incentive to maintain aging dams and infrastructure. ASTs utilized in this manner could help to reduce fossil fuel usage and greenhouse gas emissions by displacing electricity generated by more polluting methods. 4. To reduce the hydroelectricity major operational and/or maintenance costs: In addition to retrofit/upgrade current dams advantages, at appropriate sites, the capital costs of AST hydropower can be less than other hydropower technologies. The overall maintenance demands and costs of ASTs are often lower than other turbines. Major maintenance is required after the 20 to 30 years. 5. To reduce the disturbance of natural erosion and sedimentation processes which could lead to soil and land conservation. 6. To make hydropower generation safer for aquatic wildlife, especially for fish. 7. To generate electricity for small communities or regions that are hard to access or connect to the power grid, especially because of the low operation and maintenance demands and costs of ASTs. These characteristics make ASTs a potential candidate for providing electrical power in undeveloped, remote regions, and small communities that currently lack energy infrastructure. 8. To improve the welfare of the developing countries and regions with limited access to the power grid or other infrastructures. Despite many other technologies, ASTs do not require high manufacturing capabilities and hi-tech technologies to design, implement, operate, Sustainability 2020, 12, 7352 27 of 34

or maintain. Simplicity, low operational demands, and moderate costs make ASTs a practical environment-friendly and sustainable solution for supplying energy, especially in developing countries. At remote locations with a low head water supply, ASTs may provide a possible means of providing electricity that would otherwise be impractical in developing communities. Improving the economy and welfare of such communities is a win-win futuristic sustainable development approach that could be facilitated by using AST hydroelectric plants.

Author Contributions: Conceptualization, A.Y. and W.D.L.; writing—original draft preparation, A.Y.; writing—review and editing, A.Y. and W.D.L. All authors have read and agreed to the published version of the manuscript. Funding: This research was funded by the Natural Sciences and Engineering Research Council (NSERC) of Canada, grant number CRDPJ 513923-17. Acknowledgments: The authors would like to thank Tony Bouk and Brian Weber of Greenbug Energy Inc. for their ongoing support, and Scott Simmons for sharing information and images of his research about the Archimedes screws. Conﬂicts of Interest: The authors declare no conﬂict of interest.

Nomenclature The following symbols are used in this paper:

2 AE Effective cross-sectional water area at the screw’s inlet (m ) 2 AMax Maximum cross-sectional water area at the screw’s inlet (m ) a Coefficient of dimensionless flow rate (-) b Coefficient of dimensionless area constant (-) c Coefficient of dimensionless rotation speed constant (-) Di The inner diameter of the Archimedes screw (m) DO The outer diameter of the Archimedes screw (m) f Fill height of water in a bucket of screw (-) g Gravitational constant (9.81 m/s2) hu Upper (inlet) water level of the screw (m) hL Lower (outlet) water level of the screw (m) Gw Gap width (The gap between the trough and screw) (m) hue Overflow head (m) L The total length of the screw (m) The wetted gap length along with a single turn of one flight that is submerged on one le (m) side and exposed to air on the other The wetted gap length along with a single turn of one flight that is submerged on lw (m) both sides N Number of helical planed surfaces (-) The hydrostatic pressure at any point on the plane surfaces at a depth z below the p (Pa) water level POut Output power/shaft power of screw (W) Q Total flow rate passing through the screw (m3/s) 3 QF Friction-leakage (m /s) 3 QG Gap leakage flow (m /s) 3 QM The main flow that is contained with the buckets and causes torque generation (m /s) 3 QMax The maximum flow rate that could pass through a screw when ω = ωM and AE = AMax (m /s) 3 QO Overfilling flow leakage (m /s) 3 QP No guiding plate leakage (m /s) r Radial position (m) S Pitch of the screw (Distance along the screw axis for one complete helical plane turn) (m) T The torque of a single bucket (Nm) V The overall volume of a bucket (m3) VT Axial transport velocity (m/s) TTotal Total torque of the entire screw (Nm) Sustainability 2020, 12, 7352 28 of 34 w The location along screw centerline (m) z Vertical location (m) Zmin Minimum bucket water depth of the screw (m) ZMax Maximum bucket depth of screw without overflowing (m) Zwl The actual water depth within the bucket (m) α3, α4, α5 Wetted angles around the gap (rad) β The inclination angle of the screw (rad) ∆h The head difference (m) θ Angular position (rad) µ Coefficient of the loss µA Contraction discharge coefficient ρ Density of water (1000 kg/m3) ω The rotation speed of the screw (rad/s) ωM The maximum rotation speed of the screw (Muysken limit) (rad/s) Subscripts i inner min minimum Max Maximum O Outer Total Total wl Water surface level 1 Surface 1 (downstream side of bucket) 2 Surface 2 (upstream side of bucket)

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