inventions

Review Review of Tidal Lagoon Technology and Opportunities for Integration within the UK Energy System

Grazia Todeschini

College of Engineering, Swansea University, Swansea SA1 1EP, UK; [email protected]; Tel.: +44-01792-606675

Received: 15 May 2017; Accepted: 15 July 2017; Published: 28 July 2017

Abstract: The number of distributed resources for renewable energy installed worldwide has been increasing rapidly in the last decade, and the great majority of these installations consist of solar panels and wind turbines. Other renewable sources of energy are not exploited to the same level: for instance, tidal energy is still a minute portion of the global energy capacity, in spite of the large amount of potential energy stored in tidal waves, and of the successful experience of the few existing plants. The world’s second largest tidal range occurs in the UK but at the moment tidal installations in this country are limited to a few prototypes. More recently, there has been a renewed interest in harnessing tidal energy in the UK, and a few tidal lagoon projects have been evaluated by the UK government. This paper provides an overview of the historical and current developments of tidal plants, a description of operation of tidal lagoons, challenges and opportunities for their integration within the UK energy systems and solutions to improve the dispatchability of tidal energy. The concepts described in the paper are applied to a tidal project proposed for South Wales.

Keywords: capacity factor; dispatchability; energy conversion; energy mix; renewable energy; ocean energy; spinning reserve; tidal energy; tidal lagoon

1. Introduction In the last decades, most of the western countries have been changing the nature of the energy generation mix, moving from traditional to renewable energy sources. Among the latter, wind plants and photovoltaic panels are the technologies which have been growing more rapidly, both in terms of number of installations and of technological development. For example, in the US, the total amount of solar and wind energy installed in 2013 was 44% of the new energy capacity, while in 2014 was 53% [1]. At the end of 2015, the combined solar and wind power installed worldwide was 433 GW and 230 GW, respectively [2]. The oceans are a source of energy which has not been exploited to the same level [3]. According to [4], the world’s theoretical wave power resource is estimated to be 2 TW, while [5] reports that the harvestable power is approximately 1 TW. Similar estimates can be found in [6–8]. In spite of this potential, ocean energy contributes only to a small percentage of worldwide power generation: at the end of 2015, the total tidal power installed was only 530 MW (not including pilot and demonstration projects) [9]. Figure1 shows the contribution of wind, solar and ocean energy to the total renewable energy capacity installed worldwide in 2015, and indicates clearly that the percentage of ocean energy is negligible.

Inventions 2017, 2, 14; doi:10.3390/inventions2030014 www.mdpi.com/journal/inventions Inventions 2017, 2, 14 2 of 21 Inventions 2017, 2, 14 2 of 21

Inventions 2017, 2, 14 2 of 21

FigureFigure 1. Contribution 1. Contribution of wind, of wind, solar solar and and ocean ocean energy energy to the to totalthe total renewable renewable energy energy capacity capacity installed installed worldwide at the end of 2015 [2,9]. worldwideFigure at1. theContribution end of 2015 of [wind,2,9]. solar and ocean energy to the total renewable energy capacity Theinstalled main worldwide challenges at tothe the end development of 2015 [2,9]. of technologies to harness ocean energy include: the highThe upfront main challenges cost, the requirement to the development of high maintena of technologiesnce, the concerns to harness related ocean to the energy impact include:on the the highmarine upfrontThe environment main cost, challenges the requirementand the to availabilitythe development of high of cheapest maintenance, of technologies energy thesources to concerns harness [5,10–17]. relatedocean energy to the include: impact the on the marinehigh environmentIn upfront the UK, cost, the and the trend therequirement availabilityis similar of tohigh of other cheapest maintena Western energynce, countries:the sources concerns the [5 related ,10government–17 ].to the impacthas set on very the marine environment and the availability of cheapest energy sources [5,10–17]. ambitiousIn the UK, targets the trend of reducing is similar CO to2 emissions other Western by 80% countries: of 1990 levels the governmentby 2050, and therefore has set very has ambitiousbeen promotingIn the the UK, growth the trendof renewable is similar energy to otherinstallation Westerns, in countries:particular solarthe government energy and windhas setenergy. very targets of reducing CO2 emissions by 80% of 1990 levels by 2050, and therefore has been promoting ambitious targets of reducing CO2 emissions by 80% of 1990 levels by 2050, and therefore has been the growthAccording of renewableto [18], tidal energyenergy could installations, supply up in to particular 29% of the solar UK demand. energy andIn spite wind of this, energy. at the According end ofpromoting 2016, ocean the energy growth contributed of renewable only energy 0.05% installation to the installeds, in particular capacity [19],solar as energy shown and in Figurewind energy. 2. to [18According], tidal energy to [18], could tidal energy supply could up tosupply 29% up of theto 29% UK of demand. the UK demand. In spite In ofspite this, of atthis, the at endthe end of 2016, oceanof energy 2016, ocean contributed energy contributed only 0.05% only to the0.05% installed to the installed capacity capacity [19], as [19], shown as shown in Figure in Figure2. 2.

Figure 2. Contribution of different energy sources to the total capacity installed in the UK at the end of 2016 [19]. Figure 2. Contribution of different energy sources to the total capacity installed in the UK at the end Figure 2. Contribution of different energy sources to the total capacity installed in the UK at the end of Inof the2016 last [19]. decade, there has been a renewed interest in harnessing the tidal resource in the UK, 2016 [19]. and a few pioneering projects have been successfully deployed [20]. Large projects (in the MW or GW ratedIn the power last decade, range), thereconsisting has been mostly a renewed of “tidal intere lagoons”,st in harnessingare at the moment the tidal under resource consideration in the UK, byInand the the a UKfew last government pioneering decade, there projects[21]. has have been been a renewed successfully interest deployed in harnessing [20]. Large the projects tidal resource (in the MW in theor UK, and aGW few Thisrated pioneering paper power will range), projects focus consisting on have the beendescription mostly successfully of “tidalof tidal lagoons”, deployed lagoons, are because [ 20at ].the Large momentof their projects impactunder (in consideration on the the MW local or GW ratedenergyby power the UKsystem. range), government Other consisting technologies [21]. mostly for of harnessing “tidal lagoons”, tidal energy, are at thesuch moment as tidalunder stream, consideration are at the by the UKmoment governmentThis at paper a prototype will [21]. focus stage, on and the theydescription won’t play of tidal a significant lagoons, role because in the of energy their impactmix, at onleast the in local the energy system. Other technologies for harnessing tidal energy, such as tidal stream, are at the nearThis future. paper The will paper focus is ondivided the descriptionin two parts: of tidal lagoons, because of their impact on the local moment at a prototype stage, and they won’t play a significant role in the energy mix, at least in the energy system. Other technologies for harnessing tidal energy, such as tidal stream, are at the moment (1)near future.The first The partpaper provides is divided a description in two parts: of the mechanism behind the generation of tides, an at a prototypeoverview stage, andof tidal they plants won’t playcurrently a significant in service role and in thea description energy mix, of at tidal least lagoons. in the near The future.

The paper(1) ischaracteristicsThe divided first part in two ofprovides energy parts: agenerated description by oftidal the la megoonschanism (variability behind and the intermittency)generation of aretides, then an overview of tidal plants currently in service and a description of tidal lagoons. The (1) The firstcharacteristics part provides of aenergy description generated of the by mechanismtidal lagoons behind (variability the generation and intermittency) of tides, are an overviewthen of tidal plants currently in service and a description of tidal lagoons. The characteristics of energy generated by tidal lagoons (variability and intermittency) are then described. Methodologies to reduce both variability and intermittency of energy generated are discussed. Inventions 2017, 2, 14 3 of 21 Inventions 2017, 2, 14 3 of 21

(2) Thedescribed. second part Methodologies addresses the to integration reduce both of tidalvariab energyility and within intermittency the UK energy of energy mix. This generated section startsare with discussed. a comparison between the characteristics of solar, wind and tidal energy, and then (2) showsThe the second current part levels addresses of energy the generation integration in of the tidal UK, energy divided within by energy the UK source. energy A tidal mix. project This proposedsection for starts South with Wales a comparison is used as between an example the toch quantifyaracteristics the of impact solar, of wind this formand tidal of energy energy, on theand UK energythen shows system. the current levels of energy generation in the UK, divided by energy source. A tidal project proposed for South Wales is used as an example to quantify the impact of this 2. Terminologyform of energy on the UK energy system.

2. TerminologyA summary of the elementary terminology that will be used in the paper is as follows: • EbbA summary tide: The of period the elementary between terminology high tide and that low will tide, be used during in the which paper water is as follows: flows away from the shore.  Ebb tide: The period between high tide and low tide, during which water flows away from the • Flow tide: The period between low tide and high tide, during which water flows to the shore. shore. • Spring tide: maximum tidal range, obtained at full moon or new moon (see Section4 for more  Flow tide: The period between low tide and high tide, during which water flows to the shore. details on this definition).  Spring tide: maximum tidal range, obtained at full moon or new moon (see Section 4 for more • Neapdetails tide: on minimumthis definition). tidal range, obtained at first moon quarter or last moon quarter (see Section4  forNeap more tide: details minimum on this tidal definition). range, obtained at first moon quarter or last moon quarter (see • LunarSection day: 4 for the more interval details of on time this between definition). two successive crossings of the meridian by the moon  (approximatelyLunar day: the 24interval h and of 50 time min). between two successive crossings of the meridian by the moon • Tidal(approximately barrage: it is24 the h and most 50 common min). method used to harness tidal energy, and consists in creating  aTidal barrage barrage: across it an is existingthe most basin, common such method as the estuary used to of harness a river. tidal This energy, system and is schematically consists in showncreating in Figurea barrage3a across an existing basin, such as the estuary of a river. This system is • Tidalschematically lagoon: a shown lagoon in differs Figure from 3a a barrage because instead of using an already existing basin,  seawallsTidal lagoon: are built a lagoon to create differs a new from artificial a barrage structure. because A instead tidal lagoon of using may an be already “onshore” existing (Figure basin,3b) orseawalls “offshore” are (Figurebuilt to3 createc). a new artificial structure. A tidal lagoon may be “onshore” (Figure • Tidal3b) or stream: “offshore” this (Figure technology 3c). consists of installing turbines inside the water and allowing  generationTidal stream: of energythis technology by the tidal consists wave. of This installing type ofturbines generation inside is stillthe atwater the researchand allowing stage andgeneration is not addressed of energy in by this the paper. tidal wave. One limitation This type ofof thisgeneration technology is still is theat the low research energy stage density and of theis not tidal addressed stream. in this paper. One limitation of this technology is the low energy density of the tidal stream.

Figure 3. Different systems used for tidal generation; (a) tidal barrage; (b) tidal lagoon; (c) offshore Figure 3. Different systems used for tidal generation; (a) tidal barrage; (b) tidal lagoon; (c) offshore lagoon. lagoon.

3.3. The The Generation Generation ofof TidesTides andand thethe TidalTidal RangeRange VariabilityVariability TidesTides are are generated generated by by the the combined combined effect effect of of gravity gravity and and of waterof water inertia inertia [22 –[22–24].24]. Gravity Gravity is due is todue the to attractive the attractive force force of the of moon the moon and theand sun, the andsun, theand moon the moon has the has largest the largest effect effect on tides on tides due todue its proximityto its proximity to the to Earth. the Earth. According According to [25 to], the[25], tidal the tidal force force due due to the to sunthe sun is approximately is approximately half half of theof forcethe force caused caused by the by moon. the moon. TheThe side side of of the the Earth Earth which which is closer is closer to the to moon the moon experiences experiences the highest the forcehighest of gravity.force of Therefore, gravity. theTherefore, water on the this water side ison attracted this side to is the attracted moon, thus to the causing moon, a risethus on causing the sea a level, rise ason shownthe sea in level, Figure as4 . Onshown the oppositein Figure side 4. On of the opposite Earth’s surface, side of the effectEarth’s of surface, gravity the is smaller, effect of and gravity inertia is smaller, prevails: and the waterinertia tends prevails: to go the in awater ‘straight’ tends line, to go and in thisa ‘straight’ effect causes line, and a second this effect rise causes in water a second levels. rise in water levels. Inventions 2017, 2, 14 4 of 21 Inventions 2017, 2, 14 4 of 21 Inventions 2017, 2, 14 4 of 21

Figure 4. Effect of gravity and inertia on the sea level of Earth. Figure 4. EffectEffect of of gravity gravity and inertia on the sea level of Earth. The position of the “rises” changes during the day, due to the combined effect of the rotation of The position of the “rises” changes changes during during the the day, day, due to the combined effect of the rotation of the Earth around itself and of the rotation of the moon around the Earth. When the effects of those thethe Earth around itself and of the rotation of the moon around the Earth. When the effects of those two rotations are combined, each point on Earth takes approximately 24 h and 50 min to return to twotwo rotations areare combined,combined, eacheach point point on on Earth Earth takes takes approximately approximately 24 24 h andh and 50 50 min min to returnto return to the to the same position under the moon: the “lunar day” is 50 min longer than the solar day because the thesame same position position under under the moon:the moon: the “lunarthe “lunar day” day” is 50 is min 50 longermin longer than thethan solar the daysolar because day because the moon the moon revolves around the Earth in the same direction as the Earth rotates around its axis. The moonrevolves revolves around around the Earth the in Earth the same in the direction same dire as thection Earth as rotatesthe Earth around rotates its axis.around The its comparison axis. The comparison between the length of the lunar day and the length of the solar day is shown in Figure 5. comparisonbetween the between length of the the length lunar of day the and lunar the day length andof the the length solar of day the is solar shown day in is Figure shown5. in The Figurex-axis 5. The x-axis shows time, while the y-axis shows the water level; the blue curve indicates the variation Theshows x-axis time, shows while time, the ywhile-axis showsthe y-axis the shows water level;the water the bluelevel; curve the blue indicates curve theindicates variation the ofvariation the sea of the sea water level for a lunar day. This curve illustrates that each point on Earth experiences two ofwater the levelsea water for a level lunar for day. a lunar This curveday. This illustrates curve thatillustrates each point that each on Earth point experiences on Earth experiences two high tides two high tides and two low tides every lunar day. highand twotides low and tides two everylow tides lunar every day. lunar day.

Figure 5. Comparison of the length of solar day and of lunar day, and typical variation of water Figure 5. ComparisonComparison ofof the the length length of of solar solar day day and and of lunarof lunar day, day, and typicaland typical variation variation of water of water levels levels at one location on Earth. levelsat one at location one location on Earth. on Earth. The tidal current accompanies the rising and falling of the tide. The incoming tide along the The tidal current accompanies the rising and falling of the tide. The incoming tide along the coastThe and tidal into current the bays accompanies and estuaries the rising is called and fallinga flood of thecurrent; tide. Thethe incomingoutgoing tidetide alongis called the coastebb coast and into the bays and estuaries is called a flood current; the outgoing tide is called ebb current.and into the bays and estuaries is called a flood current; the outgoing tide is called ebb current. current. The difference in height between the high tide and the low tide is called “tidal range”. The tidal The difference in height between the high tide and the low tide is called “tidal range”. The tidal range changes daily due to two main effects: the the chan changingging relative position of the sun, the moon and range changes daily due to two main effects: the changing relative position of the sun, the moon and thethe Earth, and the elliptical orbits of the Earth around the sun and of the moonmoon aroundaround thethe Earth.Earth. the Earth, and the elliptical orbits of the Earth around the sun and of the moon around the Earth. At full moon and new moon, when the sun, the Earth and the moon are aligned, the effect of At full moon and new moon, when the sun, the Earth and the moon are aligned, the effect of gravity is higher than usual, and results in above-average tidal ranges—referred to as “spring tides”. gravity is higher than usual, and results in above-average tidal ranges—referred to as “spring tides”. At first first quarter and third quarter, the effect of su sunn and moon gravity cancel each other, and the tidal At first quarter and third quarter, the effect of sun and moon gravity cancel each other, and the tidal ranges are less than average—thus resulting in “neap tides”. ranges are less than average—thus resulting in “neap tides”. The above effects are further modulated by the ellip ellipticaltical orbits: when the moon is closest to the The above effects are further modulated by the elliptical orbits: when the moon is closest to the Earth (at(at perigee),perigee), the the gravitational gravitational forces forces are are higher. higher. About About two weekstwo weeks later, whenlater, thewhen moon the ismoon farthest is Earth (at perigee), the gravitational forces are higher. About two weeks later, when the moon is farthestfrom the from Earth the (at Earth apogee), (at apogee), the gravitational the gravitational forces are forces smaller. are smaller. In a similar In a way,similar when way, the when Earth the is farthest from the Earth (at apogee), the gravitational forces are smaller. In a similar way, when the Earthclosest is to closest the sun to (perihelion), the sun (perihelion), the tidal ranges the tidal are enhanced.ranges are Whenenhanced. the Earth When is furthestthe Earth from is furthest the sun Earth is closest to the sun (perihelion), the tidal ranges are enhanced. When the Earth is furthest from(aphelion), the sun the (aphelion), tidal ranges the are tidal reduced ranges [25 ].are Wind reduced and atmospheric [25]. Wind pressureand atmospheric also cause pressure variations also in from the sun (aphelion), the tidal ranges are reduced [25]. Wind and atmospheric pressure also causethe range variations of the tides,in the uprange to 2 of m the in regionstides, up which to 2 m are in prone regions to thiswhich effect are [prone26]. to this effect [26]. cause variations in the range of the tides, up to 2 m in regions which are prone to this effect [26]. InIn additionaddition to to the the phenomena phenomena described described above, abov the amplitudee, the amplitude of the tidal of rangethe tidal varies range significantly varies In addition to the phenomena described above, the amplitude of the tidal range varies significantlybetween different between points different on Earth points due toon the Earth shape due of to the the oceans shape and of the of theoceans coasts and [27 of]. the Figure coasts6 shows [27]. significantly between different points on Earth due to the shape of the oceans and of the coasts [27]. Figurethe variation 6 shows of the tidal variation range on of thetidal surface range ofon thethe oceans:surface theof the blue oceans: colour the corresponds blue colour to corresponds the smaller Figure 6 shows the variation of tidal range on the surface of the oceans: the blue colour corresponds totidal the range, smaller and tidal the range, red colour and the to the red larger colour tidal to the range larger [28 tidal]. range [28]. to the smaller tidal range, and the red colour to the larger tidal range [28]. Inventions 2017, 2, 14 5 of 21 Inventions 2017, 2, 14 5 of 21 Inventions 2017, 2, 14 5 of 21

Figure 6. Distribution of tidal range within the oceans. Retrieved from [28]. FigureFigure 6. 6.Distribution Distributionof of tidaltidal rangerange within the the oceans. oceans. Retrieved Retrieved from from [28]. [28 ]. The points on Earth with zero tidal range are called “Amphidromic” and they are typically The points on Earth with zero tidal range are called “Amphidromic” and they are typically locatedThe points in the onmiddle Earth of with the oceans, zero tidal while range higher are called tidal ranges “Amphidromic” are found on and the they coasts are [29]. typically located located in the middle of the oceans, while higher tidal ranges are found on the coasts [29]. in the middleOne factor of the that oceans, contributes while to higher higher tidal tidal ranges range areon the found coast on is the related coasts to [the29]. reduced speed of One factor that contributes to higher tidal range on the coast is related to the reduced speed of tidalOne waves. factor Further that contributes enhancement to higher of the tidal tidal range range on may the coast occur is for related two toreasons: the reduced (1) resonant speed of tidal waves. Further enhancement of the tidal range may occur for two reasons: (1) resonant tidalcoupling waves. between Further enhancementthe natural frequency of the tidal of the range area may and occur the tidal for twofrequency reasons: [30] (1) and resonant (2) funneling coupling coupling between the natural frequency of the area and the tidal frequency [30] and (2) funneling between[31,32]. the natural frequency of the area and the tidal frequency [30] and (2) funneling [31,32]. [31,32]. TheThe effect effect of of resonance resonance can be be explained explained by by referring referring to the to thecase case of Severn of Severn Estuary Estuary (in the (inUK) the The effect of resonance can be explained by referring to the case of Severn Estuary (in the UK) UK)[33]: [33 the]: the natural natural frequency frequency of the of theestuary estuary is 36,400 is 36,400 m1/2 m, while1/2, while the tidal the tidalfrequency frequency is 36,000 is 36,000 m1/2. The m1/2 . [33]: the natural frequency of the estuary is 36,400 m1/2, while the tidal frequency is 36,000 m1/2. The Thesimilarity similarity of ofthese these two two numbers numbers explains explains the thehigh high tidal tidal ranges ranges in this in area. this area. similarity of these two numbers explains the high tidal ranges in this area. TheThe effect effect of of funnelling funnelling is due due to to the the upward upward gradient gradient of the of sea the bed sea within bed within estuaries estuaries [33]. As [a33 ]. The effect of funnelling is due to the upward gradient of the sea bed within estuaries [33]. As a Asresult a result of offunnelling, funnelling, thethe height height of the of thesea sealevel level inside inside the estuaries the estuaries may mayincrease increase by several by several meters, meters, as result of funnelling, the height of the sea level inside the estuaries may increase by several meters, as it is being pushed forward by the incoming sea water. This principle is illustrated in Figure 7. asit is is being being pushed pushed forward forward by by the the incoming incoming sea sea water. water. This This principle principle is illustrated is illustrated in Figure in Figure 7. 7.

Figure 7. Effect of funnelling in an Estuary: the water levels inside the estuary rise due to the force of Figure 7. Effect of funnelling in an Estuary: the water levels inside the estuary rise due to the force of Figureincoming 7. Effect water. of funnelling in an Estuary: the water levels inside the estuary rise due to the force of incoming water. incoming water. 4. Overview of Existing Tidal Plants and of the Projects Proposed for the Bristol Channel 4. Overview of Existing Tidal Plants and of the Projects Proposed for the Bristol Channel 4. OverviewThe first, of Existing most famous Tidal (and Plants for and many of thedecades, Projects the Proposedonly) tidal forplant the is Bristol La Rance Channel Tidal Power The first, most famous (and for many decades, the only) tidal plant is La Rance Tidal Power Station, located in Brittany, northwestern France [20]. La Rance Tidal Power Station was Station,The first, located most in famous Brittany, (and northwestern for many decades, France the[20]. only) La tidalRance plant Tidal is LaPower Rance Station Tidal Powerwas commissioned in 1967 after three years of constructions, and it took almost 20 years to recover the Station,commissioned located in in Brittany, 1967 after northwestern three years Franceof constructi [20]. Laons, Rance and it Tidal took Power almost Station 20 years was to commissionedrecover the Inventions 2017, 2, 14 6 of 21 Inventions 2017, 2, 14 6 of 21 in 1967 after three years of constructions, and it took almost 20 years to recover the initial capital cost. initial capital cost. This plant required building a 700 m dam which houses 24 turbines, for a total This plant required building a 700 m dam which houses 24 turbines, for a total installed capacity of installed capacity of 240 MW. The annual average electricity generation is approximately 540 GWh, 240 MW. The annual average electricity generation is approximately 540 GWh, and the cost of energy and the cost of energy generated by this plant is currently 1.5 p/kWh. generated by this plant is currently 1.5 p/kWh. In 2011, the Sihwa Lake Tidal Power Station was commissioned in South Korea along the In 2011, the Sihwa Lake Tidal Power Station was commissioned in South Korea along the northwest coast near Seoul [5]. The total installed capacity is 254 MW and, at the moment, this is the northwest coast near Seoul [5]. The total installed capacity is 254 MW and, at the moment, this largest existing tidal plant. Construction time was 8 years (in spite of using an existing dam), and the is the largest existing tidal plant. Construction time was 8 years (in spite of using an existing dam), energy production cost is currently 0.6 p/kWh. The annual electricity generation is 552 GWh: this and the energy production cost is currently 0.6 p/kWh. The annual electricity generation is 552 GWh: figure is quite close to the value reported for La Rance, indicating that the efficiency of the two plants this figure is quite close to the value reported for La Rance, indicating that the efficiency of the two is similar, in spite of the time lapse between the two developments. Due to the large success of this plants is similar, in spite of the time lapse between the two developments. Due to the large success of project, South Korea is planning for additional barrages. this project, South Korea is planning for additional barrages. The sum of the total installed power at La Rance Tidal Power Station and Sihwa Lake Tidal PowerThe Station sum is of 494 the MW, total which installed is close power to the at Latotal Rance installed Tidal power Power of Station530 MW and reported Sihwa in Lake Section Tidal 2. PowerThis implies Station that is 494 the MW, other which existing is close tidal to barrages the total are installed significantly power ofsmaller: 530 MW the reported rated power in Section of the2. TAnnapolishis implies Royal that theGenerating other existing Station tidal (Canada, barrages 1984) are significantlyis 20 MW, and smaller: the average the rated annual power energy of the Annapolisgenerated Royalis 50 GeneratingGWh. Kislaya Station Guba (Canada, Tidal 1984)Power is 20Station MW, and(Russia) the average is rated annual 1.7 MW. energy The generated UK is ischaracterized 50 GWh. Kislaya by a Gubalarge Tidaltidal Powerrange Station[34,35], (Russia)as shown is ratedin Figure 1.7 MW. 6, Thebut UKall of is characterizedthe existing tidal by a largeinstallations tidal range are [at34 the,35], prototype as shown stage in Figure [36].6, In but 2016, all of two the 100 existing kW tidal tidal turbines installations were are commissioned at the prototype in stageShetland, [36]. Scotland, In 2016, two and 100 they kW will tidal be turbines part of werea larger commissioned tidal array in[37]. Shetland, Similar Scotland, pilot projects and they are willunder be partconstructions of a larger or tidal testing array [38,39]. [37]. Similar pilot projects are under constructions or testing [38,39]. The Severn Estuary is one of the largest estuaries in Britain, and is well known for having the second largest tidal tidal range range in in the the world world [40,41]. [40,41]. Figu Figurere 88 shows shows the the location location of of the the Severn Severn Estuary Estuary in inSouth South West West UK, UK, between between England England and and Wales. Wales. Ther Theree is is no no defined defined boundary boundary between the Severn Estuary and the Bristol Channel, and the two namesnames areare sometimessometimes usedused interchangeably.interchangeably.

Figure 8. Area surrounding the Severn Estuary and the Bristol Channel in south-west UK. Figure 8. Area surrounding the Severn Estuary and the Bristol Channel in south-west UK. Given its high tidal ranges, this area has been the subject of many feasibility studies for tidal projects,Given the its first high dating tidal ranges,back to this1849 area [42]. has Du beene to thea combination subject of many of environmental feasibility studies concerns for tidaland projects,availability the of first more dating economical back to energy 1849 [ 42sources]. Due (mostl to a combinationy coal), all projects of environmental have been abandoned. concerns and In availability2008 and 2009, of more a number economical of exploratory energy sources studies (mostly were coal), undertaken all projects regarding have been a tidal abandoned. barrage In in 2008 the andSevern 2009, Estuary. a number In 2010, of exploratory the UK government studies were deci undertakended that the regarding barrage a would tidal barrage be too inrisky the Severndue to Estuary.high costs In and 2010, environmental the UK government issues [5]. decided that the barrage would be too risky due to high costs and environmentalMore recently, issues a few [5]. projects of “tidal lagoons” have been proposed for the Severn Estuary/BristolMore recently, Channel a few region projects [20]. of “tidal While lagoons” the tidal have barrage been is proposed a structure for which the Severn blocks Estuary/Bristol the estuary of Channela river, a regiontidal lagoon [20]. While is confining the tidal a part barrage of the is sea a structure by means which of seawalls, blocks the as shown estuary in of Figure a river, 3. a tidal lagoonIn isFebruary confining 2016, a part the of UK the government sea by means commission of seawalls,ed asan shown independent in Figure review3. to assess the case for tidalIn February lagoons 2016,and whether the UK government they could commissionedplay a cost-effective an independent role as part review of UK to assessenergy the mix. case The for tidalresult lagoons of the review and whether was published they could in January play a cost-effective 2017, and one role of the as part main of conclusions UK energy mix.is that The “there result is ofa very the reviewstrong case was for published a small scale in January pathfinder 2017, project and one (less of than the main 500 MW) conclusions as a soon is as that is reasonably “there is a practicable” [21]. In the same document, the Swansea Bay Tidal Lagoon is indicated as the most likely pathfinder, due to the small size (320 MW) and the advanced state of development. According Inventions 2017, 2, 14 7 of 21

Inventions 2017, 2, 14 7 of 21 very strong case for a small scale pathfinder project (less than 500 MW) as a soon as is reasonably practicable”to the review, [21]. other In the larger same document,projects should the Swansea be considered Bay Tidal Lagoononly after is indicated the pathfinder as the most will likely be pathfinder,commissioned due and to the operational small size for (320 a reasonable MW) and theperiod. advanced state of development. According to the review,In otherspite of larger the projectsconclusions should reported be considered above, ther onlye afterare still the pathfinderconcerns regarding will be commissioned the tidal lagoon and operationalprojects. The for first a reasonable major concern period. is related to environmental impact on habitats, fish movements, birdlifeIn spiteand deposit of the conclusions of silt [43,44]. reported The environm above, thereental are concern still concerns has been regarding in part mitigated the tidal lagoonby the projects.comprehensive The first assessments major concern which is related are part to environmental of the development impact on consent habitats, process fish movements, [21], and birdlifeby the andadjustments deposit of which silt [43 have,44]. been The environmentaldone to the design concern to minimize has been inthe part im mitigatedpact on migrating by the comprehensive birds and on assessmentsfishes [43]. Under which arethe part UK of “Conservation the development of consentHabitats process and Species [21], and Regulations” by the adjustments [45], whichthe lagoon have beendevelopers done toare the obliged design toto minimizeimplement the measures impact on to migrating minimize birds disruption and onfishes during [43 construction,]. Under the UK to “Conservationcontinuously monitor of Habitats the and environmental Species Regulations” impact, [an45],d theto provide lagoon developers compensatory are obliged habitats to implementto replace measuresdesignated to features. minimize In disruptionspite of these during efforts, construction, the developers tocontinuously are still waiting monitor for a marine the environmental license from impact,environment and to body provide Natural compensatory Resources habitatsWales. to replace designated features. In spite of these efforts, the developersA second concern are still waitingis the cost for of a the marine structure license an fromd the environment amount of government body Natural funding Resources required Wales. to makeA the second project concern commercially is the cost viable of the [46]. structure At the andmoment the amount of this writing of government (July 2017), funding no decision required has to makebeen made the project by the commercially UK government viable on [the46]. funding At the moment agreement. of this writing (July 2017), no decision has been made by the UK government on the funding agreement. 5. Tidal Lagoon Operation 5. Tidal Lagoon Operation 5.1. Single-Basin Operation 5.1. Single-Basin Operation Tidal lagoons create a confined space where a mass of water is stored and released in a controlledTidal lagoonsmanner. create In the a simplest confined forms, space where tidal lagoons a mass ofconsist water of is storeda single and basin released that is in created a controlled by a manner.seawall, Inas theshown simplest in Figure forms, 3b. tidal The lagoons turbines consist are located of a single in water basin passages that is created and are by adesigned seawall, asto shownconvert in the Figure potential3b. The energy turbines of the are locatedwater into in water rotati passagesonal energy and arefirst designed (by the toblades), convert and the then potential into energyelectric ofenergy the water (by the into generators). rotational energyThe turbines first (by are the located blades), in andopenings then intowithin electric the seawall, energy (bywhich the generators).are fitted with The control turbines gates, are locatedreferred in to openings as “sluice within gates”, the seawall,used to regulate which are the fitted flow with of water control in gates, and referredout of the to lagoon as “sluice [47]. gates”, used to regulate the flow of water in and out of the lagoon [47]. Figure 99 illustrates illustrates thethe principleprinciple ofof operationoperation ofof single-basin single-basin tidaltidal lagoons.lagoons. InIn thisthis figure,figure, thethe turbines are indicated by the red circle.circle. Based on thethe water level, the position of the sluice gates and the spinning direction ofof thethe turbines,turbines, fourfour operationoperation modesmodes cancan bebe identified.identified.

Figure 9.9. IllustrationIllustration of of steps steps in in a single-basin a single-basin tidal tida lagoonl lagoon operation: operation: the turbines the turbines are installed are installed within thewithin lagoon the walls,lagoon and walls, water and flow water is regulated flow is by regulated means of sluiceby means gates. of (a )sluice flow tide gates. generation; (a) flow (b )tide no generationgeneration; at(b high); no tide;generation (c) ebb at tide high generation; tide (c); ebb (d) tide no generation generation at (d low) no tide.generation at low tide. Inventions 2017, 2, 14 8 of 21

Figure9a: At high tide, the sluice gates are open and water is allowed to flow inside the lagoon, thus spinning the turbines clockwise and generating electric power. This process is referred to as “flow tide generation”. Figure9b: When the maximum water level inside the lagoon is reached, the sluice gates are closed and water flow is interrupted. Figure9c: At low tide, the sluice gates are open, and water is allowed to flow outside the lagoon, thus spinning the turbines counterclockwise. This process is referred to as “ebb tide generation”. Figure9d: When the minimum water level inside the lagoon is reached, the gates are closed, until the next tide, which will cause the generation cycle to start again. The potential energy stored in the lagoon is a function of the mass of water, and can be expressed as follows [48]: 1 E = Aρgh2 (1) 2 where A is the area of the surface enclosed by the walls, ρ is the water density, g is gravitational acceleration (9.8 m/s2) and h is the differential height between the water level inside the lagoon and the sea water level. The average electric power generated by the lagoon depends on the generation time T and of the efficiency of the energy conversion process eg:

E 1 Aρgh2 P = e = e (2) av g T 2 g T

The efficiency eg depends on several factors, including energy losses and the operating point of the turbines [49–51]. According to [52], Kaplan turbines will be deployed for the tidal lagoon projects: this technology is the same used at other tidal plants, with modifications such as “triple regulation” that allows a more optimized efficiency curve [46]. The capacity factor (CF) is a coefficient used to quantify the efficiency of energy sources. The CF is defined as the ratio between the electricity generated for a certain time and the energy that could be generated at continuous full-power operation during the same period:

Egenerated CF = (3) Emaximum

The CF of tidal lagoons has been estimated in publications such as [44,53]. In spite of applying different models and assumptions, all references indicate that a CF around 0.2 (20%) is realistic for tidal lagoons. This value is similar to the one obtained at existing tidal plants, such as La Range (All references agree that generating energy in both directions yields overall less efficiency than generating only on ebb tide [53]). Even if this statement may seem counterintuitive, it has been confirmed by the operational experience at La Range, where initially generation was bi-directional, and later on the scheme was modified to generate energy only on the ebb tide [54]). One phenomenon which reduces the CF is the variation of differential height h while the basin is filled or emptied. The highest power generation is theoretically obtained at maximum differential height, and this would require emptying the basin instantaneously. In the practice, the basin would be emptied across a few hours, and the amount of energy generated would depend on the instantaneous differential height. This concept is visualized in Figure 10. The top graph shows the sea water level (blue curve) and the lagoon water level (red curve) over time (hour). The green shades indicate the time ranges when electrical energy generation takes place. The letters at the top of the graph correspond to the operating modes shown in Figure9. The lower graph plots the power generated versus time, for a 60 MW rated lagoon. Power generation is both intermittent (as there are times where the differential height is too low to be used for generating electricity) and variable (as the power output changes with time). Based on the above, there is a small window of time which allows maximum power generation, and the opening and closing of the gates cannot be adjusted based on the demand and generation Inventions 2017, 2, 14 9 of 21 Inventions 2017, 2, 14 9 of 21 Inventions 2017, 2, 14 9 of 21 levels,levels, but but rather, rather, on on the the water water levels. levels. Additionally, Additiona thelly, lagoonthe lagoon operation operation needs needs to resemble to resemble the natural the flownaturallevels, of water butflow rather, toof water limit on theto the limit impact water the onimpactlevels. the Additiona localon the ecosystem, locallly, ecosystem, the therefore,lagoon therefore, operation the emptying the needs emptying to and resemble and filling filling the cycle cannotcyclenatural becannot alteredflow be of altered significantlywater to si gnificantlylimit [ 10the,40 impact ].[10,40]. on the local ecosystem, therefore, the emptying and filling cycle cannot be altered significantly [10,40].

FigureFigure 10. 10.Sea Sea water water level, level, lagoon lagoon waterwater level and energy generation generation for for one one tidal tidal period period (half (half tidal tidal day),day),Figure and and 10. for for Sea a a 60 60water MW MW lagoon.level, lagoon. lagoon AdaptedAdapted water from level [[44].44 and]. energy generation for one tidal period (half tidal day), and for a 60 MW lagoon. Adapted from [44]. Due to the effect of neap and spring tides, power generation peaks vary during a lunar month. Due to the effect of neap and spring tides, power generation peaks vary during a lunar month. FigureDue 11 showsto the effectthe daily of neap energy and generation spring tides, over po tidalwer periodgeneration for a peaks 25 kW vary rated during plant. a lunar month. FigureFigure 11 11shows shows the the daily daily energy energy generation generation over over tidaltidal periodperiod for a 25 kW kW rated rated plant. plant.

Figure 11. Estimated power generation for a 25 kW-rated tidal plant in [55]. Figure 11. Estimated power generation for a 25 kW-rated tidal plant in [55]. Figure 11. Estimated power generation for a 25 kW-rated tidal plant in [55]. The power generation cycles shown in Figures 10 and 11 illustrate the main limitations of the single-basinThe power system: generation cycles shown in Figures 10 and 11 illustrate the main limitations of the single-basinThe power system: generation cycles shown in Figures 10 and 11 illustrate the main limitations of the single-basin- Generation system: is intermittent, and takes place for approximately 14 h a day [26,52]. -- TheGeneration optimal istime intermittent, for power and generation takes place is depende for approximatelynt on the tidal 14 hcycle, a day and [26,52]. cannot be adjusted - - GenerationtoThe follow optimal power is intermittent,time demand for power levels. and generation takes place is depende for approximatelynt on the tidal 14 hcycle, a day and [26 cannot,52]. be adjusted - - TheTheto optimalfollow amount power time of power fordemand power generated levels. generation is at peak is dependent only when on maximum the tidal differential cycle, and cannotheight is be obtained. adjusted to -- followTheThe amountamount power demandof power levels.generatedgenerated isis atdependent peak only onwhen the maximummaximum differential height reached height by is the obtained. water

- - TheandThe amount onamount the oftide of power powerlevel, generated therefore, generated isthe is at dependentsame peak basin only whenonwi llthe yield maximummaximum more energy differentialheight atreached spring height bytide isthe than obtained. water at and on the tide level, therefore, the same basin will yield more energy at spring tide than at - Theneap amount tide. of power generated is dependent on the maximum height reached by the water and neap tide. on the tide level, therefore, the same basin will yield more energy at spring tide than at neap tide. Inventions 2017, 2, 14 10 of 21 Inventions 2017, 2, 14 10 of 21

To overcome the above-mentioned limitations, a few mitigating solutions will be described in the To overcome the above-mentioned limitations, a few mitigating solutions will be described in next sections. the next sections. 5.2. Solutions to Improve Energy Generation 5.2. Solutions to Improve Energy Generation One solution to mitigate the lower generation levels at neap tide consists in pumping additional One solution to mitigate the lower generation levels at neap tide consists in pumping additional water inside the basin when the water level is below the maximum. In this way, the highest power water inside the basin when the water level is below the maximum. In this way, the highest power generation can be achieved consistently through the month [56]. This solution is implemented at La generation can be achieved consistently through the month [56]. This solution is implemented at La Rance power station, and has proved to yield an additional 10% power [54]. Rance power station, and has proved to yield an additional 10% power [54]. The effectiveness of this mechanism can be demonstrated by modifying equation (1) [57]. It is The effectiveness of this mechanism can be demonstrated by modifying equation (1) [57]. It is assumed that pumping allows increasing the water level inside the lagoon by the height “b”. Since assumed that pumping allows increasing the water level inside the lagoon by the height “b”. Since pumping water inside the lagoon requires a negative input power, (1) is modified as follows: pumping water inside the lagoon requires a negative input power, (1) is modified as follows:  2  11 2 b P = Aρg e (h + b) − (4) av =2 gℎ e (4) 2 p wherewhere epis is the the efficiency efficiency of of pumping. pumping. InIn [[57],57], itit isis statedstated thatthat forfor aa tidaltidal range of 4 m, with with ep == 0.850.85 andand eg= =0.9, 0.9,the theoptimal optimal heightheight ““bb”” isis aroundaround 2626 m,m, whichwhich isis unfeasible,unfeasible, duedue toto thethe costcost ofof buildingbuilding suchsuch aa highhigh wallwall andand thethe visualvisual impactimpact ofof thisthis structure.structure. EvenEven ifif the optimal height is not practical, (4) cancan be used to demonstrate the effect of increasing thethe waterwater heightheight insideinside thethe lagoonlagoon atat neapneap tide.tide. TheThe normalizednormalized powerpower generatedgenerated is expressedexpressed asas aa functionfunction ofof ““b” in Figure 1212.. This figure figure is obtained by calculating Pav according according toto (4)(4) andand withwith thethe parametersparameters listedlisted aboveabove ((hℎ= = 4 4 m, m,e p= =0.85 0.85 and ande g= 0.9), = 0.9), and and by by normalizing normalizing the the result result with with respect respect to theto the ones ones obtained obtained from from (1). (1).

Figure 12. Average tidal power generation as a function of the water height “b”: the y-axis shows the Figure 12. Average tidal power generation as a function of the water height “b”: the y-axis shows the value of obtained from (4) normalized with respect to the value of obtained from (1). value of Pav obtained from (4) normalized with respect to the value of Pav obtained from (1). The first region of the plot is of practical interest since it reflects the increased power obtained at neapThe tide, first when region the ofwater the levels plot is are of practicala few mete interestrs below since the it maximum. reflects the For increased example, power assuming obtained b = at0.5 neap m yields tide, whena power the boost water of levels about are 10%. a few The meters expression below the(4) maximum.assumes that For the example, cost of assumingenergy at bpumping= 0.5 m yields and at a generating power boost is ofthe about same. 10%. Clearly, The if expression pumping (4)is done assumes at a thatlower the cost cost (for of energyexample, at pumpingwhen there and is at an generating excess of is generation the same. Clearly,from rene if pumpingwable sources), is done atthe a lowersolution cost above (for example, is even whenmore thereattractive. is an excess of generation from renewable sources), the solution above is even more attractive. AA secondsecond andand moremore sophisticatedsophisticated solutionsolution thatthat allowsallows constantconstant powerpower generationgeneration consistsconsists inin buildingbuilding aa two-basintwo-basin system.system. TheThe operationoperation ofof aa two-basintwo-basin systemsystem hashas beenbeen describeddescribed inin earlyearly publicationspublications referringreferring toto tidaltidal energyenergy generationgeneration [[26],26], butbut toto thethe knowledgeknowledge of thethe Author, itit hashas beenbeen implementedimplemented onlyonly inin aa smallsmall tidaltidal plantplant inin China China (Haisan (Haisan Tidal Tidal Power Power Plant), Plant), where where the the rated rated installed installed power power is 250 is kW. 250 As kW. a result As a result of the two-basin design, the plant generates electricity continuously, with an average output power equal to 39 kW [56]. Inventions 2017, 2, 14 11 of 21

Two configurations are possible, as shown in Figure 13 [26,47,57]. In both cases, the two basins areof the separated two-basin by design,a wall, theand plant one is generates used as electricity“high-level continuously, basin”, the other with as an “low-level average output basin”. power The Inventions 2017, 2, 14 11 of 21 mainequal difference to 39 kW [between56]. the two solutions consists in allowing water to flow between the two basins Two configurations are possible, as shown in Figure 13[ 26,47,57]. In both cases, the two basins (Figure 13a),Two or configurations in allowing water are possible, to flow as be showntween in each Figure basin 13 [26,47,57]. and the seaIn both only cases, (Figure the two13b). basins are separatedare separated by a wall,by a wall, and oneand isone used is used as “high-level as “high-level basin”, basin”, the the other other as “low-levelas “low-level basin”. basin”. The The main differencemain difference between thebetween two the solutions two solutions consists consists in allowing in allowing water water to to flowflow between between the the two two basins basins (Figure(Figure 13a), 13a), or in or allowing in allowing water water to flowto flow between between each each basinbasin and the the sea sea only only (Figure (Figure 13b). 13 b).

Figure 13. Two possible configurations for a two-basin tidal power generating system; (a) water is allowedFigure to flow 13. Twobetween possible the configurationstwo basins; (b for) water a two-basin is allowed tidal to power flow onlygenerating between system; each ( abasin) water and is the Figure 13. Two possible configurations for a two-basin tidal power generating system; (a) water is allowed sea. allowed to flow between the two basins; (b) water is allowed to flow only between each basin and the to flow between the two basins; (b) water is allowed to flow only between each basin and the sea. sea. Figure 14 illustrates different operational modes for the system shown in Figure 13a; the notationFigure isFigure 14similar illustrates 14 illustratesto the different onedifferent operationalused operational in Figure modes mo 9. fordes In the forthis systemthe case, system shown the shown turbines in Figure in Figure generate13a; the13a; notation thepower notation is similar to the one used in Figure 9. In this case, the turbines generate power continuouslyis similar to the and one rotate used always in Figure in9 the. In same this case, direction, the turbines from the generate “high” power basin continuouslyto the “low” andbasin. rotate The alwayscontinuously in the same and direction, rotate always from in thethe “high”same direction, basin tofrom the the “low” “high” basin. basin Theto the dotted “low” greenbasin. The arrows dotteddotted green green arrows arrows indicate indicate pumping, pumping, while while the the co continuousntinuous blueblue arrows arrows indicate indicate the the direction direction of of indicate pumping, while the continuous blue arrows indicate the direction of water flow. water waterflow. flow.

Figure 14. Tidal energy generation mechanism for the system shown in Figure 13a. The continuous blue arrows indicate the direction of gravity-driven water flow, the dotted green arrows indicate the Figuredirection 14. Tidal of energy energypumped generation generation water flow. mechanism mechanism The turbines for for gene the the systemrate sy powerstem shown shown at all in stages.Figure in Figure Adapted 13a. 13a. The fromThe continuous continuous[26]. (a) blue bluearrows arrowsgeneration indicate indicate theat high direction the tide; direction ( ofb) gravity-drivengeneration of gravity-driven at ebb water tide; flow, (waterc) generation the flow, dotted the at green lowdotted tide; arrows green (d) generation indicate arrows the indicateat flood direction the directionof pumpedtide of waterpumped flow. water The turbinesflow. The generate turbines power gene atrate all power stages. at Adapted all stages. from Adapted [26]. (a) from generation [26]. (a at) generationhigh tide; ( bat) generationhigh tide; ( atb) ebbgeneration tide; (c) at generation ebb tide; at(c) low generation tide; (d) at generation low tide; at(d flood) generation tide at flood tide Inventions 2017, 2, 14 12 of 21 Inventions 2017, 2, 14 12 of 21 The calculations in [57] show that a two-lagoon system in a location with a tidal range of 4 m 2 2 can deliverThe calculations a constant in power [57] show of 4.5 that W/m a two-lagoon. The power system density in of a location4.5 W/m with is 50% a tidal larger range than of 4the m 2 maximumcan deliver possible a constant average power power of 4.5 density W/m2 of. Thea single power-basin density system of 4.5in the W/m same2 is location 50% larger (3 W/m than). the maximumThe capital possible cost average of a two-basin power density scheme of a is single-basin higher because system of in the the sameneed locationfor extra (3 walls; W/m 2the). advantageThe capital of this cost solution of a two-basin is that power scheme generation is higher becauseis fully ofdispatchable the need for (A extra power walls; plant the capable advantage of providingof this solution required is that amounts power generationof power on is fullydemand dispatchable at the request (A power of power plant capablegrid operators, of providing in a mannerrequired similar amounts to ofa pumped power on hydro-system). demand at the The request stea ofdy power power grid generation operators, of inthe a two-basin manner similar system to isa pumpedmore valuable hydro-system). than the The intermittent steady power andgeneration less-flexible of thepower two-basin from the system single-basin is more valuable solution. than A cost-benefitthe intermittent analysis and less-flexiblefor each site power allows from determining the single-basin if the highest solution. initial A cost-benefit costs are justified. analysis for each site allowsThe use determining of energy ifstorage the highest to shift initial power costs generation are justified. to match demand seems to be at the momentThe useunrealistic, of energy since storage currently to shift energy power generationstorage is toused match to demandprovide seemsmostly to ancillary be at the services moment [1,19,58,59].unrealistic, since currently energy storage is used to provide mostly ancillary services [1,19,58,59].

6.6. Integration Integration of of Tidal Tidal Energy Energy in in the UK Energy Generation Mix

6.1.6.1. Characteristics Characteristics of of Solar, Solar, Wind and Tidal Energy EnergyEnergy generation generation from from renewable renewable sources sources depends on the availability availability of of natural natural elements, elements, and and thereforetherefore it it is is in in general general not dispatchable [9,39]. [9,39]. ThisThis section section will will compare compare the the characteristics characteristics of of three three forms forms of of renewable renewable energy: energy: solar solar energy, energy, wind energy andand tidaltidal lagoonlagoon generation. generation. Other Other sources sources of of renewable renewable energy energy are are not not discussed discussed in this in thissection section since since they they represent represent a small a small portion portion of installed of installed capacity, capacity, as shown as shown in Figure in Figure2. 2. TheThe variability variability of of energy energy generated generated from from rene renewablewable sources sources depends depends not not only only on on the the natural elementelement under considerationconsideration (wind,(wind, sun sun or or tide), tide), but but also also on on the the location location and and on the ontechnology the technology used. used.In spite In ofspite this, of some this, some common common trends trends can be can observed. be observed. FigureFigure 1515 showsshows the the typical typical pattern pattern of powerof power generated generated by a solarby a panelsolar inpanel the UKin onthe a UK cloudless on a cloudlessday [60]. Theday peak[60]. powerThe peak generated power ingenerated January in is approximatelyJanuary is approximately 60% of the peak 60% powerof the generatedpeak power in generatedJuly and April. in July In and the winter,April. In power the winter, generation power takes generation place for takes 8 h a day,place while for 8 inh thea day, summer while it in takes the summerplace up it to takes 16 h aplace day. up As to a result,16 h a theday. energy As a result generated, the energy in the wintergenerated months in the is onlywinter 20% months of the is energy only 20%generated of the in energy the summer generated months, in the as shownsummer in months, Figure 16 as. In shown April andin Figure July the 16. power In April generation and July curve the power(Figure generation 15) is very curve similar, (Figure and 15) this is can very be similar, explained and by this the can higher be explained efficiency by ofthe the higher solar efficiency panels at oflower the solar temperatures panels at in lower April, temperatures although irradiance in April, is although higher in irradiance July. is higher in July.

FigureFigure 15. TypicalTypical power generation curves curves for for a a solar panel in in the UK on a cloudless day; adapted fromfrom [60]. [60]. Inventions 2017, 2, 14 13 of 21 Inventions 2017, 2, 14 13 of 21 Inventions 2017, 2, 14 13 of 21

FigureFigure 16. 16. TypicalTypical Typical monthly monthly energy energy generationgeneration for for a a solar solar panel panel in in the the UK; UK; adapted adapted from from [60]. [[60].60 ]. Figure 16. Typical monthly energy generation for a solar panel in the UK; adapted from [60].

Figure 17 shows wind power generation in the UK for three sample days in April 2017. These FigureFigure 1717 17 showsshows shows wind windwind power power power generation generation generation inin the inthe theUK UKfor threethree for three samplesample sample days days in in days April April in 2017. 2017. April These These 2017. graphs indicate that wind generation is quite difficult to forecast, compared to solar generation, due Thesegraphsgraphs graphs indicate indicate indicate that that wind thatwind generation wind generation generation is is quite quite is quitediffdifficulticult difficult to forecast, to forecast, comparedcompared compared to to solar solar to generation, solargeneration, generation, due due to the high variability of wind speed in consecutive days. dueto theto to the high the high high variability variability variability of of wind wind of wind speed speed speed in in consecutive consecutive in consecutive days.days. days.

Figure 17. Wind power generated on three consecutive days in the UK. Data retrieved from [61]. Figure 17. Wind power generated on three consecutive days in the UK. Data retrieved from [61]. Figure 17. WindWind power generated on three consecutive days in the UK. Data retrieved from [[61].61]. Figure 18 shows monthly wind energy generation in the UK between January 2013 and April Figure 18 shows monthly wind energy generation in the UK between January 2013 and April 2017.Figure The generation18 shows monthlycurve shows wind that energy peak valuesgeneration are reached in the inUK January, between therefore January complementing 2013 and April 2017.Figure The 18 generation shows monthly curve shows wind that energy peak generation values are in reached the UK in between January, January therefore 2013 complementing and April 2017. 2017.energy The generation fromcurve solar shows panels. that peakIt is alsovalues possible are reached that wind in January, generation therefore is curtailed complementing in other Theenergy generation generation curve from shows solar that panels. peak values It is also are reachedpossible inthat January, wind generation therefore complementing is curtailed in other energy energymonths generation due to lower from demand. solar panels. It is also possible that wind generation is curtailed in other generationmonths due from to solarlower panels. demand. It is also possible that wind generation is curtailed in other months due tomonths lower due demand. to lower demand.

Figure 18. Wind energy generated monthly in the UK between January 2013 and April 2017. Data Figure 18. Wind energy generated monthly in the UK between January 2013 and April 2017. Data retrieved from [61]. retrieved from [61]. Figure 18. WindWind energy energy generated generated monthly monthly in the in theUK UKbetween between January January 2013 2013and April and April2017. 2017.Data Dataretrieved retrieved from from[61]. [61]. Inventions 2017, 2, 14 14 of 21

Inventions 2017, 2, 14 14 of 21 The daily and monthly power generation pattern for a tidal lagoon is shown in Figures 10 and 11. As discussedThe daily in Section and monthly6, power power generation generation is variable, pattern however for a tidal it islagoon has two is shown distinctive in Figures characteristics: 10 and high11. long-termAs discussed predictability in Section and 6, power low variability generation through is variable, the year however [10]. it is has two distinctive characteristics:A summary high of the long-term variability predictabilit of differenty and renewable low variability energy through sources the isyear shown [10]. in Table1[ 62]. This tableA summary suggests of that the the variability use of different of different energy renewa sourcesble energy allows sources reducing is theshown overall in Table variability 1 [62]. of energyThis table generation. suggests This that argument the use of isdifferent presented energy to support sources the allows use reducing of tidal lagoons the overall in [ 21variability]. of energy generation. This argument is presented to support the use of tidal lagoons in [21]. Table 1. Variability of different renewable energy sources; adapted from [62]. Table 1. Variability of different renewable energy sources; adapted from [62]. Resource Annual Hourly Resource Annual Hourly Wind Low High WindSolar HighLow MediumHigh SolarTidal High Low Medium High Tidal Low High 6.2. The Role of Tidal Lagoons 6.2. The Role of Tidal Lagoons The developers of the tidal lagoons claim that a set of projects distributed across the UK coast may beThe able developers to provide of a the constant tidal lagoons “base load”, claim duethat toa set the of different projects timesdistributed the tides across take the place UK atcoast these locationsmay be [able63,64 to]. provide However, a constant no supporting “base load”, evidence due to ofthe this different claim times has been the tides provided. take place The at analysis these carriedlocations out in[63,64]. [65] shows However, instead no supporting that base-load evidence generation of this is claim difficult hasto been achieve, provided. given The the analysis proximity ofcarried these projects out in [65] and shows the lack instead of sufficient that base-load spatial gene diversity.ration is The difficult analysis to achieve, carried given out in the [21 proximity] reinforces of these projects and the lack of sufficient spatial diversity. The analysis carried out in [21] reinforces this conclusion, stating that “it is not realistic” for tidal lagoons to provide base load. this conclusion, stating that “it is not realistic” for tidal lagoons to provide base load. Rather than geographical diversity, high predictability seems to be the most profitable Rather than geographical diversity, high predictability seems to be the most profitable characteristic of tidal lagoons. Since it is possible to predict accurately and in advance the times and characteristic of tidal lagoons. Since it is possible to predict accurately and in advance the times and levels of tidal generation, this source of energy can be accurately integrated in the forecast; therefore, levels of tidal generation, this source of energy can be accurately integrated in the forecast; therefore, ratherrather than than being being used used as as ‘base-load’, ‘base-load’, it it can can be be integrated integrated moremore inin thethe form of “spinning “spinning reserve”. reserve”. InIn the the UK, UK, the transmissionthe transmission system system operator operator keeps keeps a number a number of power of power stations stations (mainly (mainly gas-fired) runninggas-fired) at less running than theirat less full than capacity, their full thus capacity providing, thus about providing 1 GW about of spinning 1 GW of reserve: spinning this reserve: capacity canthis automatically capacity can respondautomatically to any respond shortfall to inany generation shortfall in within generation seconds within [66]. seconds [66]. TheThe use use of of spinning spinning reserves reserves is is exemplifiedexemplified in Figure 19,19, which which shows shows daily daily energy energy generation generation inin the the UK UK for for April April 2017, 2017, divideddivided per per energy energy source source.. Base-load Base-load generation generation is currently is currently provided provided by bynuclear nuclear energy, energy, while while gas gas generation generation is variable, is variable, and seems and seems to compensate to compensate for the forvariability the variability of wind of windgeneration, generation, as the as thetwo two bars bars vary vary asynchronously asynchronously (where (where wind wind energy energy generation generation is larger, is larger, gas gas generationgeneration is is lower). lower).

Figure 19. Daily energy generation in the UK by source, April 2017. Data retrieved from [61]. Figure 19. Daily energy generation in the UK by source, April 2017. Data retrieved from [61]. Inventions 2017, 2, 14 15 of 21

The use of gas generation to provide spinning reserve is expensive and polluting, since operating gas turbines intermittently is an inefficient process which generates high levels of CO2. Since tidal lagoons are going to be connected to the transmission system, and their output power can be easily predicted, it can be argued that they may be used for providing spinning reserve. This possibility can be verified only by means of accurate integration studies, and may provide justification to improve the generation pattern of tidal plants and reduce output power fluctuations. Section 7.2 will show a modified version of the graph shown in Figure 19 when tidal lagoons are integrated within the energy system. In order to address this topic, it is first necessary to quantify the generation pattern expected from tidal lagoons.

7. Quantitative Analysis This section will consider one of the proposed tidal projects in South Wales as an application example. Some of the formulae shown in Section5 will be applied to a real project, thus quantifying the expected power generation, based on available data.

7.1. Estimated Annual Energy Generation The Swansea Tidal Lagoon is considered a “pathfinder” project, due to its small size and advanced state of development (Section3). This project consists of a single-basin system, with 16 bi-directional 20 MW hydro turbines for a total 320 MW power rating. The seawall is 9.5 km long, and the total area enclosed in the lagoon is 11.5 km2 [52]. The Swansea Tidal Lagoon will be connected to the 275 kV Baglan substation, which is a part of the National Grid system [21]. To estimate the total energy by this project over a year, the tidal data for Swansea Bay is required: this information can be retrieved from online databases, such as [67]. The 2017 tidal data indicate a maximum tidal range between 2.1 m (average range neap tide) and 8.5 m (average range spring tide). The average tidal range for 2017 is expected to be 5.3 m. The potential energy stored inside the lagoon can be calculated as follows by applying (1), where the average tidal range as differential height:

1 1 E = Aρgh2 = × 11.5 × 106 × 1025.18 × 9.81 × 5.32 = 1624 × 109 J = 451 MWh (5) 2 2 Since their basin is filled twice a day and emptied twice a day, the annual potential energy can be calculated as follows: Eyear = E × 4 × 365 = 658.46 GWh (6) As discussed in Section 5.1, the annual energy generated by the lagoon will be significantly lower than the energy potential stored in the water. Since tidal lagoons are generally only at the design stage, the annual yearly energy generation can be only estimated. The developers [52] expect an annual energy generation of approximately 530 GWh. An independent study carried out in [44] estimates the annual energy generation for different operating modes. For the dual-mode energy generation scheme with no pumping (which is the operating mode currently under consideration for Swansea Tidal Lagoon), the calculated energy generation is approximately 480 GWh, which is 9.4% less than the value reported by the lagoon developers (This is the lowest amount of energy production reported in the paper: both ebb-generation and pumping would lead to significantly higher energy generation. La Range and Sihwa use ebb-generation only [5]). The calculation of the CF gives some further insights on the validity of the energy generation levels estimated by the developers. By substituting this value in (3), the following is obtained:

9 Egenerated 530 × 10 = = = CF 6 0.189 (7) Emaximum 320 × 10 × 365 × 24 Inventions 2017, 2, 14 16 of 21

The above capacity factor is aligned with the values reported in the literature for tidal plants (Section 5.1), which is approximately 0.2. Assuming that generation takes place 14 h a day, the average power generation can be estimated as follows: E 530 × 106 P = = = 103.7 MW (8) Inventions 2017, 2, 14 av T 14 × 365 16 of 21 Other sources report a lower value; for example, [68] estimates the output power to be 530 10 = = = 103.7 MW (8) approximately 1/3 of the figure above (36 MW).14 365 Only the operational experience will be able to quantify figures such as capacity factor and output average power, however, the figures above allow Other sources report a lower value; for example, [68] estimates the output power to be assessingapproximately the impact 1/3 of thisof the project figure onabove the (36 local MW). energy Only system.the operational experience will be able to Relatingquantify thefigures average such as powercapacity factor generation and output to theaverage load power, consumption however, the in figures South above Wales allow helps in understandingassessing the impact impact of ofthis the project proposed on the local project energy in system. the region. According to the local distribution system operatorRelating (Western the average Power power Distribution), generation to the the maximum load consumption load in in South South Wales Wales is helps expected in to be approximatelyunderstanding 2200 MWthe impact for 2017, of the and proposed remain project quite in constant the region. for According a few years to the [69 local]—this distribution trend is similar system operator (Western Power Distribution), the maximum load in South Wales is expected to be to the rest of the UK. The minimum load is projected to be around 1000 MW. approximately 2200 MW for 2017, and remain quite constant for a few years [69]—this trend is If thesimilar highest to the averagerest of the power UK. The generation minimum load value is projected (103.7 MW) to be is around assumed, 1000 MW. the Swansea Tidal Lagoon could generateIf the about highest 5% average of the maximumpower generation load demand value (103.7 and MW) 10% ofis theassumed, minimum the Swansea load demand. Tidal If the more conservativeLagoon could estimategenerate about is used, 5% of the the contribution maximum load to demand the local and demand 10% of the will minimum be less thanload 2% for maximumdemand. load If and the more less thanconservative 3% at minimum estimate is used load., the contribution to the local demand will be less than 2% for maximum load and less than 3% at minimum load. 7.2. Integration of Tidal Lagoons within the UK Energy Mix 7.2. Integration of Tidal Lagoons within the UK Energy Mix The dailyThe daily energy energy generation generation pattern pattern forfor the Sw Swanseaansea Tidal Tidal Lagoon Lagoon can be can estimated be estimated using the using the tidal datatidal provided data provided in [67 in]. [67]. Figure Figure 20 20 shows shows estimatedestimated daily daily energy energy generation generation for 2017. for 2017.

Figure 20. Estimated daily energy generation for the Swansea Tidal Lagoon, based on 2017 tidal data Figure 20. Estimated daily energy generation for the Swansea Tidal Lagoon, based on 2017 tidal data reported in [67]. reported in [67]. Since generation data for South Wales divided by energy source is not available, it is not Sincepossible generation to quantify data the for impact South of the Wales Swansea divided Tidal byLagoon energy for this source region. is not Therefore, available, the analysis it is not is possible extended to the UK, using the data available in [61]. It is assumed that the total tidal power installed to quantifyin the the UK impact is 10 GW, of based the Swansea on the total Tidal size Lagoonof the projects for this proposed region. in Therefore, [52,65]. The theenergy analysis generated is extended to the UK,by Swansea using theTidal data Lagoon available (Figure 20) in [is61 scaled]. It isaccord assumedingly, since that it the has totalbeen discussed tidal power in Section installed 6.2 in the UK is 10that GW, geographic based ondiversity the total won’t size result of in the a signif projectsicantproposed smoothing inof [power52,65 ].output The of energy the different generated by Swanseaproposed Tidal Lagoon projects. (FigureFigure 21 20 is) a is modified scaled accordingly,version of Figure since 19, where it has tidal been energy discussed is included in Section while 6.2 that geographickeeping diversity the same won’t total resultenergy ingeneration. a significant This smoothingis accomplished ofpower by reducing output the of amount the different of energy proposed generated by gas-fired power plants. The amount of tidal energy varies significantly through the projects. Figure 21 is a modified version of Figure 19, where tidal energy is included while keeping the month; however, this variation is predictable and therefore allows for planning the use of other same totalenergy energy sources. generation. This is accomplished by reducing the amount of energy generated by gas-fired power plants. The amount of tidal energy varies significantly through the month; however, this variation is predictable and therefore allows for planning the use of other energy sources. InventionsInventions 2017,, 2,, 14 14 1717 of 21 Inventions 2017, 2, 14 17 of 21

Figure 21. Modification of the energy generation pattern in the UK by source, taking into account Figure 21. Modification of the energy generation pattern in the UK by source, taking into account tidalFigure generation. 21. Modification It is assumed of the thatenergy all generationpower gene patternrated by in the the tidal UK lagoonsby source, can taking be dispatched into account (no tidal generation. It is assumed that all power generated by the tidal lagoons can be dispatched curtailmenttidal generation. is applied). It is assumed that all power generated by the tidal lagoons can be dispatched (no curtailment(no curtailment is applied). is applied). The role of tidal lagoons may become even more important in the next decade. The UK governmentThe role has of set tidaltidal as an lagoonslagoons objective maymay to becomephasebecome out eveneven all coal moremore plants importantimportant by 2025 in in[18,70], thethe nextnext due decade.todecade. environmental The UK government has set as an objective to phase out all coal plants by 2025 [18,70], due to environmental concerns,government CO has2 emission set as an targets, objective and to the phase increasing out allcoal cost plants of running by 2025 coal [18 plants,,70], due when to environmental compared to 2 otherconcerns, technologies. CO 2 emissionemission In the targets, targets, last few and and years, the the severalincreasing increasing coal cost costplants of running have already coal plants, been closed, when and compared the trend to willother continue. technologies. Figure In In 22 the shows last few the years,installed several capaci co coaltyal per plants technology have already for the been UK closed, transmission and the system. trend Whilewill continue. at the end Figure of 2016 22 shows the installed the installed installed coal generation capaci capacityty per per was technology technology approximately for for the the 18 UK UK GW, transmission transmission it is scheduled system. system. to decreaseWhile at theto 0 endGW ofby 2016 the end the installedof 2025 [71]. coal Renewable generation generationwas approximately is projected 18 GW, to grow it is from scheduled 31 to 56to GW,decrease and totonuclear 00 GWGW byfromby the the 9 end toend 16 of of GW. 2025 2025 [Energy71 [71].]. Renewable Renewable demand generationis generationexpected is to projectedis remain projected toconstant growto grow from [19]. from 31 The to 31 56trends to GW, 56 shownGW,and nuclearand in nuclearFigure from 22 9from toseem 16 9 GW. toto 16indicate Energy GW. Energy demanda discrepancy demand is expected between is expected to remain the increase to constant remain in installedconstant [19]. The [19].generation, trends The shown trends and in theshownFigure constant 22in seemFigure value to 22 indicate of seem demand. to a discrepancyindicate However, a discrepancy between renewable the between increasesources thedo in installedincreasenot generate generation,in installed continuously andgeneration, the and constant their and outputthevalue constant of power demand. value is most However, of demand.of the renewabletime Ho wellwever, below sources renewable the do rated not sources generatevalue do[33,72,73]. continuouslynot generate The monthly andcontinuously their CF output used and by power their the UKoutputis most government power of the timeis mostto wellestimate of below the timeFeed the well ratedin Tariff below value genera the [33, 72ratedtion,73 ]. rangesvalue The monthly[33,72,73]. betweenCF 1.4%Theused monthlyto by 17% the for UKCF solar governmentused energy by the andUKto estimate governmentbetween Feed 15.2% into Tariffestimate and generation45.3% Feed for in wind rangesTariff energy genera between [74].tion 1.4% rangesThis to means 17% between for that solar 1.4% in energy order to 17% andto forreplace between solar existing energy 15.2% capacityand 45.3%between from for 15.2% windfossil energyandfuels, 45.3% a [74 larger]. for This amountwind means energy thatof generation in [74]. order This to based replacemeans on existingthat renewable in order capacity sources to fromreplace needs fossil existing to fuels, be installed.capacitya larger amount from fossil of generation fuels, a larger based amount on renewable of generation sources needsbased toon be renewable installed. sources needs to be installed.

Figure 22. Current and projected installed power output (in (in GW) per generation technology, technology, for the Figure 22. Current and projected installed power output (in GW) per generation technology, for the UK energy transmission system. Data extracted from [19,71]. [19,71]. UK energy transmission system. Data extracted from [19,71]. 8. Conclusions 8. Conclusions This paper provided a review of the tidal lagoon technology and discussed the integration of this renewableThis paper sourceprovided within a review the UKof the energy tidal mix.lagoon A technologysummary ofand existing discussed tidal the projects integration and of this renewable source within the UK energy mix. A summary of existing tidal projects and of Inventions 2017, 2, 14 18 of 21

8. Conclusions This paper provided a review of the tidal lagoon technology and discussed the integration of this renewable source within the UK energy mix. A summary of existing tidal projects and of projects proposed for the UK has been presented. The principles of operation of tidal lagoons have been described, and the example of the Swansea Tidal Lagoon has been used to quantify the possible impact of a tidal lagoon project on the local energy system. The paper ended with a discussion for the potential integration of tidal lagoons within the UK energy mix. Given the current plans to retire coal plants in the UK, an energy deficit is expected in the next decade, and tidal energy may help in filling this gap, particularly given the predictability of this energy source. At the moment, capital cost for these plants is still a major hurdle, and substantial government incentives are required to make these projects financially viable. The environmental impact of these infrastructures is also of concern. Implementing measures to mitigate the variability will make tidal lagoons more valuable, but will cause a further increase of the initial capital cost. More studies are required to fully understand the interaction of tidal lagoons with the local power system [75–77] and with the marine environment; however, it seems that there is an opportunity for the integration of this technology given the expected deficit in energy generation in the UK in 2025.

Acknowledgments: The authors would like to thank the European Regional Development Fund (ERDF) and the Welsh Government for funding this research through FLEXIS project. Conflicts of Interest: The authors declare no conflict of interest.

References

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