energies
Review Thermal Performance through Heat Retention in Integrated Collector-Storage Solar Water Heaters: A Review
Ruth M. Saint 1,* ID ,Céline Garnier 1, Francesco Pomponi 2 ID and John Currie 3 1 School of Engineering & the Built Environment, Edinburgh Napier University, Edinburgh EH10 5DT, UK; [email protected] 2 Resource Efficient Built Environment Lab (REBEL), Edinburgh Napier University, Edinburgh EH10 5DT, UK; [email protected] 3 Scottish Energy Centre, Edinburgh Napier University, Edinburgh EH10 5DT, UK; [email protected] * Correspondence: [email protected]; Tel.: +44-(0)131-455-2837
Received: 16 May 2018; Accepted: 19 June 2018; Published: 20 June 2018
Abstract: Solar thermal systems are a long-standing technology that is receiving increased attention, in terms of research and development, due to ambitious climate change targets and the need for renewable energy solutions. Integrated collector-storage solar water heaters (ICSSWHs) are a potential contributing solution and numerous studies have focussed on the optimisation of their thermal performance and efficiency. A major drawback of these systems is the heavy heat losses experienced during non-collection periods. To combat this, various heat retention strategies have been proposed and evaluated, including baffles plates, additional insulation, multiple glazing layers, selective coatings, and phase change materials. This paper aims to bring together these studies through a systematic review of the existing literature surrounding the performance of ICSSWH systems, focusing on heat retention. This review provides a comprehensive and up-to-date point of reference on relevant research and developments for researchers in this field.
Keywords: integrated collector-storage solar water heater (ICSSWH); thermal performance; technological development; heat retention; solar energy
1. Introduction The current global situation sees ever increasing fossil fuel use with a subsequent increase in climate impact. Given the commitments made to reduce greenhouse gas emissions and mitigate climate change [1], the move towards renewable energy sources need to be accelerated. Solar energy has great potential to contribute a near zero-carbon solution to the world’s energy demand [2]. In Scotland the consumption of heat accounts for 53% of the energy consumed by homes and businesses and the nature of the Scottish climate requires a large proportion of this demand to be dedicated to space and water heating. Domestic hot water use is the second most significant energy use in the home at approximately 13% of total Scottish heat energy demand [3]. On average, Scotland has a 10% greater number of degree days than the UK as a whole, thus necessitating additional space heating. Therefore, solar hot water systems present a promising alternative to fossil fuel powered heating. Given the high specific heat capacity of water there is great potential for energy gain and storage in these systems, often yielding efficiencies of up to 70% [4]. There are several types of solar thermal systems; the most common, commercially available ones being the flat-plate and evacuated tube collectors. A hybrid flat-plate system, the integrated collector-storage solar water heater (ICSSWH) has certain advantages over its more prevalent opponents. The absorber plate and storage tank of the ICSSWH are combined into one compact
Energies 2018, 11, 1615; doi:10.3390/en11061615 www.mdpi.com/journal/energies Energies 2018, 11, 1615 2 of 26 Energies 2018, 11, x FOR PEER REVIEW 2 of 26 unitspace offering saving great capabilities space saving unlike capabilities other flat-plate unlike and other evacuated flat-plate tube and collectors evacuated; which tube collectors; require a whichbulky requireexternal a water bulky storage external tank water and, storage consequently, tank and, additiona consequently,l piping additional, pumps, piping, and valves pumps, which and increase valves whichthe heat increase loss and the heatfailure loss points and failure within points the system. within theHeat system. loss is Heat a major loss isdrawback a major drawback for solar forthermal solar thermalsystems, systems, especially especially during during non-collection non-collection periods periods and and overcast overcast days. days. To To combat combat this, this, numerous recentrecent studiesstudies havehave focusedfocused onon improvingimproving heatheat retentionretention andand systemsystem efficiency.efficiency. ThisThis paperpaper aimsaims toto criticallycritically reviewreview suchsuch bodiesbodies ofof knowledge,knowledge, identifyidentify existingexisting trends,trends, challenges,challenges, andand opportunities,opportunities, andand single-outsingle-out areasareas forfor furtherfurther research.research.
2.2. Background TheThe evolution evolution of of solar solar water water heaters heaters (SWHs) (SWHs) has has been been an an impressive impressive journey journey beginning beginning in in AmericaAmerica withwith thethe firstfirst ofof itsits kind,kind, ‘The‘The ClimaxClimax Solar-WaterSolar-Water Heater’,Heater’, patentedpatented in 18911891 [[5]5].. However, onceonce extensiveextensive oiloil andand gasgas reservesreserves werewere discovereddiscovered researchresearch intointo thesethese systemssystems haltedhalted untiluntil thethe earlyearly 1970s1970s [[6]6].. Presently,Presently, interestinterest inin solarsolar waterwater heatingheating hashas beenbeen revivedrevived inin responseresponse toto globalglobal initiativesinitiatives aimedaimed atat thethe promotion ofof renewable energyenergy technologiestechnologies inin order to mitigate greenhouse gas (GHG) emissionsemissions [[77,,88].]. There are numerous different models of SWHSWH systemssystems but theythey allall havehave thethe samesame goal—togoal—to deliverdeliver hothot waterwaterto tothe theend-user end-user when when required, required, efficiently, efficiently, and and at at a reasonablea reasonable cost. cost. Figure Figure1 shows1 shows an an ICSSWH ICSSWH and and illustrates illustrates the generalthe general key componentskey components of a SWH: of a SWH an absorber: an absorber surface surface to collect to incidentcollect incident solar radiation, solar radiation, a storage a tankstorage (either tank combined (either combined with the with collector the collector or a distributed or a distribute system),d glazingsystem), with glazing high with transmission, high transmission, insulated housinginsulated for housing the collector/storage for the collector/storage unit, and cold unit, water and inletcold andwater hot inlet water and outlet hot water pipes. outlet pipes. TheThe threethree most most common common SWH SWH designs designs are are concentrating concentrating collectors, collectors, evacuated evacuated tube tube collectors, collectors, and flat-plateand flat- collectors.plate collectors. The choice The of choice which of design which to employ design canto employ be determined can be by d theetermined environmental by the conditionsenvironmental and theconditions heating and requirements the heating of requirements the end-user. Flat-plateof the end collectors-user. Flat have-plate varying collectors designs have (serpentine,varying designs parallel (serpentine, tubes or ICSSWH) parallel tubes and, asor the ICSSWH) cheapest and, of the as threethe cheapest options, of are the used three extensively options, ar fore domesticused extensively water heating for domestic applications water due hea toting the applications performance due advantages to the performance relative tocost advantages [9]. They relative collect bothto cost direct [9]. and They diffuse collect radiation both directwhich isand advantageous diffuse radiation in overcast which conditions, is advantageous also solar in tracking overcast is notconditions, required also thus solar lowering tracking system is not costrequired and complexity.thus lowering The system majority cost ofand studies complexity. surrounding The majori thesety systemsof studies aim surrounding to evaluate and these optimise systems their aim performance, to evaluate including and optimise optical their efficiency, performance, thermal efficiency,including andoptical heat efficiency, retention. thermal There efficiency, have been and a number heat retention. of papers There (see have Table been1) over a number the last of decade papers aiming (see Table to review1) over thesethe last heat decade loss reductionaiming to review strategies these and heat to appraiseloss reduction the work strategies being and conducted to appraise in the the area work of thermalbeing conducted performance. in the area of thermal performance.
FigureFigure 1.1. Diagrammatic representation of the key key componentscomponents of of aa SWH, SWH, shownshown as as anan integratedintegrated collector-storagecollector-storage (ICS)(ICS) type.type.
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Table 1. Summary of papers reviewing the current research and outcomes surrounding design factors of solar thermal systems.
Reference Year Title System(s) Reviewed Design Aspects Reviewed Additional insulation; Baffles; Cavity Integrated collector storage solar evacuation; Collector material; Glazing; [6] 2006 ICSSWH water heaters phase change material (PCM); Reflectors; Selective absorber surfaces A comprehensive review on solar Additional insulation; Collector material; [4] 2011 Thermo-syphon water heaters Glazing; Selective absorber surfaces Review of solar water heaters Additional insulation; Baffle Plates; [10] 2013 with integrated collector-storage ICSSWH Glazing; Inlet valve configuration; PCM; units Reflectors; Selective absorber surfaces Solar water heating systems and ICSSWH; Thermo-syphon; Additional insulation; Baffle plates; PCM; [11] 2013 their market trends Direct/indirect circulation; Air Reflectors Flat-plate; Evacuated tube; Additional insulation; Baffle plates; Recent advances in the solar water [12] 2013 compound parabolic concentrator Collector material; Glazing; Reflectors; heating systems: A review (CPC) Selective absorber surfaces A review of solar collectors and Non-concentrating (inc. flat-plate); General overview of current technologies; [13] 2013 thermal energy storage in solar concentrating collectors Fins; PCM thermal applications Review of water-heating systems: General selection approach based ICSSWH; Thermo-syphon; Active; Fins; PCM; Reflectors; Selective absorber [14] 2014 on energy and environmental PV/T; PCM surfaces aspects Building integrated solar thermal [15] 2015 PV/T & solar thermal General overview of current technologies collectors—A review Integrated collector storage solar Additional insulation; Baffle plate/inner [16] 2015 water heaters: Survey and recent ICSSWH sleeve; PCM; Thermal diodes developments Performance enhancement of Flat-plate; PV/T; Evacuated tube; [17] 2015 Nanofluids; Selective absorber surfaces solar collectors–A review Compound parabolic Innovation in flat solar thermal Additional insulation; Collector material; [18] 2016 collectors: A review of the last ten Flat-plate; ICSSWH; PV/T Fins; Nanofluids; Selective absorber years experimental results surfaces A review of water heating system Flat-plate; Evacuated tube; [19] 2016 Auxiliary immersion heating; Nanofluids for solar energy applications Concentrating; ICSSWH Recent developments in Additional insulation; Baffle plate/inner [20] 2016 integrated collector storage (ICS) ICSSWH sleeve; Thermal diodes solar water heaters: A review
Table1 provides a summary of recent review papers in the field of solar thermal systems, detailing the types of systems and the design aspects reviewed. Of the review papers listed, only a handful evaluate the array of thermal optimisation methods available in one article. Smyth et al. [6] present the most comprehensive review of the technical aspects of ICSSWH systems. Kumar and Rosen [10] also review a number of optimisation strategies, collating key literature. However, the main problem is the distributed nature of the information with regards to heat retention strategies and the need for that information to be updated. Therefore, the aim of this paper is to present an up-to-date, comprehensive review of the current research pertaining to the improvement in thermal performance and heat retention with a particular focus on ICSSWH systems.
3. Heat Gain and Retention Strategies ICSSWH systems are the simplest of the SWH designs, combining the collector and storage tank into one compact unit. This creates direct contact between the working fluid (water, in direct systems, as it must be potable) and the absorber plate. As a result, ICSSWH are subject to heavy heat losses, especially during overcast and non-collection periods [21]. Figure2 illustrates the heat transfer mechanisms in a typical ICSSWH system showing the number of ways heat can be lost. The collector surface absorbs solar radiation and converts it to heat which then gets transferred directly to the water through convection. Due to the buoyancy effect, thermal stratification builds up in the water body with the hottest water at the top of the tank where the majority of heat is lost through convection, conduction and radiation. Convective heat losses occur between the absorber plate and the glazing Energies 2018, 11, 1615 4 of 26 and the air cavity needs to be optimised to act as an insulating gap, promoting convective heat transfer to the collectorEnergies 2018 whilst, 11, x FOR also PEER minimising REVIEW losses when ambient temperatures drop. Conductive4 of 26 heat loss from the glazing back andand the side air cavity walls, need ass well to be optimised as the absorber to act as an surface, insulating is gap, a major promoting problem convective for ICSSWH systems andheat they transfer need to theto be collector heavily whilst insulated. also minimising Long-wave losses when radiative ambient heat temperatures losses have drop. the largest impact andConductive occur between heat loss the from absorber the back and plate side and walls, the as glazing,well as the suppressed absorber surface, to anis a extentmajor probl by theem air cavity. for ICSSWH systems and they need to be heavily insulated. Long-wave radiative heat losses have the The glazing needs to have a very high transmissivity as well as optical efficiency to ensure as much largest impact and occur between the absorber plate and the glazing, suppressed to an extent by the incident solarair cavity. radiation The glazing can reachneeds to the have collector a very high surface transmissivity as possible as well whilst as optical again efficiency minimising to ensure radiative losses. Numerousas much incident strategies solar radiation exist to can overcome reach the thesecollector losses surface and as possible to promote whilst heatagain gain.minimising The literature surroundingradiative these losses. methods Numerous of optimising strategies exist thermal to overcome performance these losses will and beto promote reviewed heat in gain. greater The detail in literature surrounding these methods of optimising thermal performance will be reviewed in greater the followingdetail segments. in the following segments.
Figure 2. Illustration of the heat transfer processes in an ICSSWH. Adapted from Smyth et al. [6]. Figure 2. Illustration of the heat transfer processes in an ICSSWH. Adapted from Smyth et al. [6]. 3.1. Additional Insulation 3.1. AdditionalInsulation Insulation is a simple and effective way of retaining heat, particularly at night. The two main types of insulation are opaque and transparent. Opaque insulation is primarily used around the back Insulation is a simple and effective way of retaining heat, particularly at night. The two main and sides of the systems where ambient heat losses through conduction and convection are high but types of insulationthere is no need are opaque for transmittance and transparent. of solar radiation Opaque [22,23] insulation. A large percentage is primarily of heat usedis lost through around the back and sides ofthethe glazing systems as this whereneeds to ambient be exposed heat to gain losses solar throughirradiance. conduction This can potentially and convection be alleviated areby high but there is nopartially need for insulating transmittance the air cavity, of solar using radiationtransparent [ insulated22,23]. A glazing large materials, percentage and/or of by heat applying is lost through an insulated cover during the night and non-collection periods. the glazing as this needs to be exposed to gain solar irradiance. This can potentially be alleviated by partially insulating3.1.1. Transparent the air Insulation cavity, Materials using transparent (TIMs) insulated glazing materials, and/or by applying an insulated coverTIMs represent during thea new night class andof thermal non-collection insulation used periods. to reduce unwanted heat losses from air gaps and evacuated spaces. TIMs are transparent to solar irradiation yet they can provide good 3.1.1. Transparentthermal insulation. Insulation Early Materials attempts to (TIMs) use these materials, such as experimental work by McCracken [23], led to overall decreases in system efficiency, by ~20% during daytime collection, due to their TIMspoor represent solar transmittance a new class (0.85 of in thermal this case). insulation However, research used to and reduce development unwanted of these heat materials losses from air gaps and evacuatedhas led to greater spaces. collector TIMs performances are transparent and they to solarcould irradiationbe greatly beneficial yet they in increasing can provide the solar good thermal insulation. Early attempts to use these materials, such as experimental work by McCracken [23], led to overall decreases in system efficiency, by ~20% during daytime collection, due to their poor solar transmittance (0.85 in this case). However, research and development of these materials has led to greater collector performances and they could be greatly beneficial in increasing the solar gain of thermal energy systems. TIMs include organic-based transparent foams, honeycomb or capillary Energies 2018, 11, 1615 5 of 26 Energies 2018, 11, x FOR PEER REVIEW 5 of 26
gain of thermal energy systems. TIMs include organic-based transparent foams, honeycomb or structures, and inorganic glass foams (e.g., silica aerogel). There are numerous configurations available capillary structures, and inorganic glass foams (e.g., silica aerogel). There are numerous in terms ofconfigurations cellular geometry, available in as terms illustrated of cellular in geome Figuretry,3 [as24 illustrated]. in Figure 3 [24].
Figure 3. Diagrammatic representation of different cellular geometries. (A) Absorber-parallel; (B) Figure 3. Diagrammatic representation of different cellular geometries. (A) Absorber-parallel; Absorber-perpendicular; (C) Mixed configuration; (D) Cavity structures; (E) Homogeneous. The (B) Absorber-perpendicular;arrows indicate the absorptance (C) Mixed and reflectance configuration; of the different (D) Cavity TIMs in structures; terms of incident (E) Homogeneous. solar The arrowsirradiance. indicate Adapted the absorptance from Kaushika and and reflectanceSumathy [24]. of the different TIMs in terms of incident solar irradiance. Adapted from Kaushika and Sumathy [24]. Kaushika and Sumathy [24] conducted a review of solar TIMs, presented in Table 2. It was found that a honeycomb material had the highest efficiencies when compared with a single and double Kaushikaglazed and system Sumathy and a transparent [24] conducted slab of a methyl review methacrylate of solar TIMs, (MMA). presented It can also in Tablebe seen2. that It was found that a honeycombefficiency increases material with had TIM the thickne highestss. efficiencies when compared with a single and double glazed systemTable and 2. a Efficiency transparent comparison slab of single methyl glazing, methacrylate double glazing, (MMA). MMA slab, It and can honeycomb also be seencover that efficiency increases withsystem. TIM thickness.Data taken from Kaushika and Sumathy [24].
Efficiency (%) Table 2. EfficiencyTIM Thickness comparison (m) of single glazing, double glazing, MMA slab, and honeycomb cover Single Glazing Double Glazing MMA Slab Honeycomb system. Data taken from0.025 Kaushika and0.106 Sumathy [24].0.32 0.299 0.328 0.05 0.11 0.329 0.374 0.41 0.075 0.119 0.346Efficiency 0.396 (%) 0.439 TIM Thickness (m) 0.1 0.128 0.357 0.404 0.45 Single Glazing Double Glazing MMA Slab Honeycomb 0.025Schmidt and Goetzberger 0.106 [25] determined that, 0.32 for a single-tube 0.299 absorber mounted in0.328 an involute reflector, the ICS performance was almost constant for a TIM (polycarbonate honeycomb 0.05 0.11 0.329 0.374 0.41 structure) thickness ranging from 50–150 mm. Chaurasia and Twidell [26] compared two identical 0.075 0.119 0.346 0.396 0.439 ICS units, one with TIM and the other without. The system with TIM had a total heat loss factor of 0.1 0.128 0.357 0.404 0.45 1.03 W/m2K, with hot water at temperatures 8.5–9.5 °C higher the next morning, compared with 7.06 W/m2K for the glass only system. The study found that, by using TIM, storage efficiency was 24.7% Schmidthigher; and 39.8% Goetzberger compared to [25 15.1%] determined without the that,TIM. forReddy a single-tube and Kaushika absorber [27] conducted mounted a series in of an involute comparative studies on the effect of different TIM configurations for a rectangular ICSSWH. It was reflector, the ICS performance was almost constant for a TIM (polycarbonate honeycomb structure) reported, and later corroborated [28,29], that an absorber-perpendicular configuration (transparent thickness ranginghoneycomb from structure 50–150 immersed mm. Chaurasiain an air layer) and exhibited Twidell superior [26] compared efficiencies two over identical corresponding ICS units, one with TIM andabsorber the-parallel other without. configurations The system(multiple withcovers TIM of glass/plastic had a total films). heat Reddyloss factor and Kaushika of 1.03 W/m[27] 2K, with hot waterconcluded at temperatures that the 8.5–9.5 double-wall◦C higher structured the polycarbonate next morning, TIM compared sheet was thewith most 7.06 effective W/m 2K for the configuration and a system with a thickness of 10 cm can achieve average solar collection and storage glass onlyefficiencies system. in The the studyrange of found 20–40% that, and bulk by usingwater temperatures TIM, storage between efficiency 40 and 50 was °C. 24.7% higher; 39.8% compared to 15.1% without the TIM. Reddy and Kaushika [27] conducted a series of comparative studies on3.1.2. the Opaque effectof Insulation different TIM configurations for a rectangular ICSSWH. It was reported, and later corroboratedA number [28 ,of29 studies], that have an absorber-perpendicularexperimentally reviewed the collector configuration performance (transparent of fully-versus honeycomb structure immersedpartially-exposed in an designs. air layer) Tripanagnostopoulos exhibited superior et al. efficiencies[30] evaluated over the performance corresponding of two absorber-paralleldouble- vessel systems utilising asymmetric compound parabolic concentrator (CPC) reflectors where one configurations (multiple covers of glass/plastic films). Reddy and Kaushika [27] concluded that the had both tanks fully exposed while the other was partially exposed. The latter showed improved heat double-wallretention structured during polycarbonate non-collection periods TIM but sheet the wasfully exposedthe most design effective exhibited configuration greater collection and a system with a thicknessefficiency. of Smyth 10 cm et can al. achieve[31] investigated average two solar ICSSWH collection systems and enclosed storage within efficiencies a concentrating in the range of 20–40% andcollector. bulk One water system temperatures was a fully exposed, between 1.0 m 40 long and cylindrical 50 ◦C. tank and the other a 1.5 m long tank
3.1.2. Opaque Insulation A number of studies have experimentally reviewed the collector performance of fully-versus partially-exposed designs. Tripanagnostopoulos et al. [30] evaluated the performance of two double-vessel systems utilising asymmetric compound parabolic concentrator (CPC) reflectors where one had both tanks fully exposed while the other was partially exposed. The latter showed improved heat retention during non-collection periods but the fully exposed design exhibited greater collection efficiency. Smyth et al. [31] investigated two ICSSWH systems enclosed within a concentrating collector. One system was a fully exposed, 1.0 m long cylindrical tank and the other a 1.5 m long tank with the Energies 2018, 11, 1615 6 of 26 top third heavily insulated. Unlike Tripanagnostopoulos et al. [30], the insulated system exhibited a 13% increase in collection efficiency and increased thermal retention of up to 37% in the upper insulated section due to stratification. However, these results are perhaps reflecting the greater storage capacity and lower average water storage temperatures of the 1.5 m tank. Chaabane et al. [32] also found, through a numerical CFD study, that a partially insulated tank effectively retained heat during non-collecting periods. For the studied CPC mounted ICS system the authors found that the additional insulation did affect the optical efficiency, with slightly lower water temperatures observed–up to ◦ 7.5 C differenceEnergies 2018 between, 11, x FOR PEER the REVIEW insulated and non-insulated systems. However, this6 of 26 trade-off was worthwhile with higher temperatures (up to 7 ◦C) at the start of the collection period and a ~25% reduction inwith thermal the top losses. third heavily insulated. Unlike Tripanagnostopoulos et al. [30], the insulated system exhibited a 13% increase in collection efficiency and increased thermal retention of up to 37% in the Using anupper experimental insulated section set-up, due to stratification. Souza et al. However, [33] and these Swiatek results are etperhaps al. [34 reflecting] studied the gre aater parallelepiped ICSSWH systemstorage which capacity was and partially lower average heated. water storage Souza temperat et al. [33ures] studiedof the 1.5 m the tank. effect Chaabane on thermal et al. [32] stratification also found, through a numerical CFD study, that a partially insulated tank effectively retained heat within a cavity with a heat flux applied only to bottom 0.2 m while Swiatek et al. [34] built on this and during non-collecting periods. For the studied CPC mounted ICS system the authors found that the applied the heatadditional flux toinsulation a 0.2 m did section affect the in theoptical middle efficiency, of the with cavity. slightly Beyond lower water the temperatures heated zone, the front plate of the cavityobserved in–up both to 7.5 studies °C difference was between entirely the insulated insulated and with non extruded-insulated systems. polystyrene. However, Souzathis et al. [33] trade-off was worthwhile with higher temperatures (up to 7 °C) at the start of the collection period found that by only heating a short section at the bottom of the cavity the evolution of thermal and a ~25% reduction in thermal losses. ◦ stratification withinUsing the an tankexperimental was unsatisfactory set-up, Souza et al. with [33] aand maximum Swiatek et al. temperature [34] studied a differenceparallelepiped of only 4.2 C being reported.ICSSWH The adaptations system which wasmade partially in the heated. later study Souza et yielded al. [33] enhancedstudied the levels effect on of thermal stratification, with stratification within a cavity with a heat flux applied only to bottom 0.2 m while Swiatek et al. [34] a maximum temperature different of 20.1 ◦C[34]. built on this and applied the heat flux to a 0.2 m section in the middle of the cavity. Beyond the heated zone, the front plate of the cavity in both studies was entirely insulated with extruded polystyrene. 3.1.3. Night CoverSouza et al. [33] found that by only heating a short section at the bottom of the cavity the evolution of thermal stratification within the tank was unsatisfactory with a maximum temperature difference As wellof as only insulating 4.2 °C being the reported. air cavity, The adaptations an insulating made nightin the later cover study can yielded be utilised enhanced to levels reduce of the issue of night-time heatstratification, losses. Kumarwith a maximum and Rosen temperature [35] undertookdifferent of 20.1 a °C comparative [34]. performance investigation of rectangular ICSSWHs, assessing various heat loss reduction strategies. The following five cases were 3.1.3. Night Cover assessed: (1) single glass cover without night insulation; (2) single glass cover with night insulation As well as insulating the air cavity, an insulating night cover can be utilised to reduce the issue cover; (3) doubleof night glass-time heat cover losses. without Kumar and night Rosen insulation [35] undertook cover; a comparative (4) transparent performance insulationinvestigation with single glass cover; andof rectangular (5) insulating ICSSWHs, baffle assessing plate various with heat single loss reduction glass cover. strategies. They The foundfollowing that five thecases TIM covered system reachedwere lower assessed: absorber (1) single plate glass cover temperatures without night than insulation; single (2) and single double glass cover glazed with night cases due to the insulation cover; (3) double glass cover without night insulation cover; (4) transparent insulation with system receivingsingle 10–15%glass cover; less and solar (5) insulating radiation. baffle However, plate with single the TIM glass layercover. They did helpfound retainthat the heat TIM in the water store duringcovered the night, system more reached so thanlower absorber the double plate temperatures glazing and than significantly single and double more glazed than cases the due single glazing. The single andto the double system glazing receiving perform 10–15% less in solar a similar radiation. manner However, during the TIM periods layer did of help insolation retain heat but in at night the the water store during the night, more so than the double glazing and significantly more than the double glazedsing systemsle glazing. exhibits The single greater and double heat retention. glazing perform Figure in a4 similarillustrates manner the during bulk periods water of temperatures of the ICSSWHinsolation systems but atover night the a 24 double h period glazed systems showing exhibits that greater all five heat casesretention. perform Figure 4 well.illustrates However, it is clear that Casesthe bulk 2 and water 3 maintain temperatures the of highestthe ICSSWH temperatures systems over aand 24 h collectorperiod showing efficiencies, that all five ranging cases from 57.1 perform well. However, it is clear that Cases 2 and 3 maintain the highest temperatures and collector to 79.4% andefficiencies, 57.6 to 81.2%, ranging respectively from 57.1 to 79.4% (Figure and 57.64). to 81.2%, respectively (Figure 4).
Figure 4. Daily variation in the bulk water temperature (left) and collector efficiencies for the five Figure 4. Dailyreviewed variation cases (right) in the. (1) single bulk glass water cover temperature without night insulation; (left) and (2) single collector glass cover efficiencies with night for the five reviewed casesinsulation (right cover;). (1) (3) single double glass glass cover cover without without night nightinsulation insulation; cover; (4) transparent (2) single insulation glass coverwith with night insulation cover; (3) double glass cover without night insulation cover; (4) transparent insulation with single glass cover; and (5) insulating baffle plate with single glass cover. Adapted from Kumar and Rosen [35]. Energies 2018, 11, x FOR PEER REVIEW 7 of 26
single glass cover; and (5) insulating baffle plate with single glass cover. Adapted from Kumar and Energies 2018, 11, 1615 7 of 26 Rosen [35].
3.2.3.2. AuxiliaryAuxiliary HeatingHeating Currently,Currently, averageaverage totaltotal hothot waterwater demanddemand cannotcannot bebe metmet byby SWHsSWHs alonealone duedue toto theirtheir diurnaldiurnal naturenature andand thisthis isis aa problemproblem forfor allall solarsolar thermal technologies,technologies, particularlyparticularly inin temperatetemperate climatesclimates suchsuch asas Scotland.Scotland. GarnierGarnier [ 36[36] tested] tested the the thermal thermal performance performance of anof ICSSWHan ICSSWH with with a 1 ma 21absorber m2 absorber area area and 50and L capacity. 50 L capacity. They discovered They discovered that the that amount the amountof energy of required energy to required supply ato three-person supply a three household-person withhousehold a domestic with hota domestic water demand hot water of 50 demand L/person/day of 50 L/person/day at 55 ◦C is at approximately 55 °C is approximately 7.5 kWh/day, 7.5 2747kWh kWh/year./day, 2747 kWh The/ amountyear. The of amount energy of that energy the studied that the ICSSWH studied couldICSSWH contribute could contribute to this demand to this isdemand shown in is Figure shown5 ; in 1107 Figure kWh, 5; or 1107 40% kWh, of the or yearly 40% energy of the demand. yearly energy Therefore, demand. the remaining Therefore, 60% the wouldremaining need 60% to be would supplied need by to an be auxiliary suppliedheater by an forauxiliar diurnaly heater demand. for diurnal demand.
FigureFigure 5.5. Domestic hot water water energy energy requirement requirement and and ICSSWH ICSSWH contribution contribution for for a a three-person three-person householdhousehold inin 2007.2007. AdaptedAdapted fromfrom GarnierGarnier [[36]36]..
AA solarsolar combi combi system system (SCS) (SCS) is oneis one way way to provideto provide this this additional additional heat heat requirement, requirement, if multiple if multiple solar thermalsolar thermal panels arepanels unfeasible. are unfeasible. Sarbu and Sarbu Sebarchievici and Sebarchievici [37] summarise [37] thesummaris major studiese the major surrounding studies SCSsurrounding in the literature. SCS in the Drück literature. and Hahne Drück [38 and] conducted Hahne [38] a comparative conducted a studycomparative of four study hot water of four stores, hot 2 eachwater connected stores, each to aconnected 10 m2 flat-plate to a 10 m collector flat-plate area, collector where area, the back-up where the heat back method-up heat is an method immersed is an heatimmersed exchanger. heat exchanger. The authors The found authors that, found for a wellthat, performing for a well performing SCS, the most SCS, important the most factors important are goodfactors thermal are good insulation thermal and insulation the optimal and configuration the optimal configuration of the hot water of the and hot auxiliary water heating and auxiliary loops. Byheating using loops. an SCS By over using a conventionalan SCS over a boiler, conventional fractional boiler, energy fractional savings energy of up to saving 21% cans of beup realised.to 21% can be realised.A study in Jordan looked at the techno-economic feasibility of a SWH system with a built-in electricA study coil as in an Jordan auxiliary looked heater at th comparede techno-economic to a gas geyserfeasibility system of a [39SWH]. The system author with found a built that-in theelectric SWH coil with as thean auxiliary integrated heater auxiliary compared heater to was a gas more geyser economic system due [39] to. theThe longer author operational found that lifethe expectancySWH with and the the integrated number auxiliary of days where heater the was use more of the economic electric coil due is not to the required. longer If operational the number life of daysexpectancy that electricity and the isnumber used to of provide days where the full the daily use hotof the water electric needs coil of is a not family required. is 120 orIf lessthe number the actual of costdays of that the electricity SWH system is used is less to than provide the gas the system, full daily over hot the water operational needs of life. a family is 120 or less the actual cost of the SWH system is less than the gas system, over the operational life. Legionella/Freezing Legionella/Freezing Due to bacterial growth and freezing potential, most ICSSWHs are currently connected to a secondaryDue to auxiliary bacterial heating growth system. and freezing Hot water potential, consumed most by ICSSWHs the end-user are must currently be at connected a temperature to a thatsecondary is high auxiliary enough to heating kill any system. bacteria Hot that water may cons haveumed formed by inthe a end system,-user themust biggest be at concerna temperature being Legionellathat is high pneumophila enough to. Akill review any bacteria on behalf that of may the World have formed Health Organisationin a system, the [40 biggest] states thatconcernLegionella being thrivesLegionella at 36pneumophila◦C and within. A review a range on ofbehalf 25–50 of◦ theC. WaterWorld temperatures Health Organisation are required [40] states to exceed that 60Legionella◦C for severalthrives minutes,at 36 °C and at least within once a arange day, toof kill25– off50 °C. any Water bacteria. temperatures Alternatively, are drawrequired off temperatures to exceed 60 could°C for beseveral increased minutes, to 70 at◦C least which once destroys a day, the to bacteriakill off any instantly. bacteria. This Alternatively, is potentially draw unattainable off temperatures through solarcould insolation be increased ina to temperate 70 °C which climate destroys such theas Scotland,bacteria instantly. especially This in winter is potentially months. unattainable Therefore, methods of back-up heating that work in conjunction with SWHs need to be incorporated. Energies 2018, 11, x FOR PEER REVIEW 8 of 26
through solar insolation in a temperate climate such as Scotland, especially in winter months. Energies 2018, 11, 1615 8 of 26 Therefore, methods of back-up heating that work in conjunction with SWHs need to be incorporated. Freezing is also an issue for ICSSWHs due to their high level of exposure to the elements. Smyth et al.Freezing [6] suggested is also that an systems issue forless ICSSWHs than 100 mm due deep to their were high at greater level risk of exposure of freezing to and the to elements. prevent Smythdamage et toal. [the6] suggested collector thatand systems storage less tank than the 100 depth mm should deep were be greaterat greater than risk this. of freezing Schmidt and and to preventGoetzberger damage [25] to predicted the collector that, andfor Northern storage tank European the depth climates, should the be risk greater of freez thaning this. only Schmidt occurs and for GoetzbergerICS systems [with25] predicted a specific that, collector for Northern volume lower European than climates, ~70 L/m2 the and risk that of damage freezing due only to occurs freezing for ICSonly systems occurs if with more a specificthan 20% collector of the ICS volume water lower content than is ~70frozen. L/m 2 and that damage due to freezing only occurs if more than 20% of the ICS water content is frozen. 3.3. Baffle Plate/Inner Sleeve 3.3. Baffle Plate/Inner Sleeve Baffle plates have been used successfully in numerous ICSSWH designs as a method of improvedBaffle plates thermal have performance been used successfully and heat in retention. numerous Various ICSSWH studies designs [4 as1– a43 method] have ofutilised improved an thermalinsulating performance plate, inserted and parallel heat retention. to the absorber Various plate, studies which [41– 43creates] have a utilisednarrow channel an insulating with a plate, thin insertedlayer of parallelwater which to the can absorber reach plate, much which higher creates temperatures a narrow than channel the withmain a water thin layer body. of waterThis system which canseparates reach much the downward higher temperatures and upward than the flow main, thus water decreasing body. This mixing system in separates the storage the downward tank and andpromoting upward stratification. flow, thus decreasing Once the water mixing is heated in the storage it is deposited tank and at promotingthe top of the stratification. tank, allowing Once colder the waterwater is to heated be drawn it is deposited up into the at channel.the top of Souza the tank, et al. allowing [33] studied colder a watern ICS tosystem be drawn incorporating up into the a channel.stratification Souza plate et al.running [33] studied parallel an to ICS the system absorber incorporating surface, which a stratification was only heated plate runningover the parallelbottom to0.2 the m. absorberIt was found surface, that which there waswas onlynot a heated significant over theimprovement bottom 0.2 in m. thermal It was foundstratification, that there with was a notmaximum a significant temperature improvement difference in thermal of 4.2 °C stratification, being achieved. with A a follow maximum-up study temperature by Swiatek difference et al. [34] of 4.2found◦C beingthat a achieved.shorter stratification A follow-up plate study (0.75 by Swiatek m compared et al. [ 34to] 0.9 found m) in that the a shorter1.3 m rectangular stratification system plate (0.75improved m compared thermal tostratification, 0.9 m) in the with 1.3 ma maximum rectangular temperature system improved difference thermal of 20.1 stratification, °C. In contrast with to a maximumSouza et al. temperature [33], Swiatek difference et al. [34] of heated 20.1 ◦C. a In0.2 contrast m section to Souzain the etmiddle al. [33 ],of Swiatek the tank et and al. [34reduced] heated the a 0.2channel m section gap from in the 8 middlemm to 5 of mm. the tank and reduced the channel gap from 8 mm to 5 mm. ZiapourZiapour and AghamiriAghamiri [[44]44] lookedlooked atat thethe effecteffect ofof thethe channelchannel widthwidth onon collectorcollector efficiencyefficiency inin aa twotwo trapezoidtrapezoid ICSSWHICSSWH systemsystem andand foundfound thatthat aa gapgap ofof 66 mmmm hadhad thethe highesthighest performance.performance. KumarKumar and Rosen [35] [35] studied studied a arectangular rectangular ICSSWH ICSSWH with with an insulating an insulating baffle baffle plate plate and single and single glazing. glazing. They Theyfound found that absorber that absorber plate temperatures plate temperatures peaked peaked at 63 °C at 63during◦C during the day the then day suffered then suffered heavy heavylosses lossesduring during the night, the night, dropping dropping by approximately by approximately 30 30°C.◦ C. However, However, in in the the cylindrical ICSSWH ICSSWH configurationconfiguration presented by by Smyth Smyth et et al. al. [3 [311,45,,4546,46] ]it it was was found found that that by by perforating perforating the the baffle baffle plate, plate, to toavoid avoid the the heat heat build build-up-up at the at the top top of ofthe the channel channel that that hinders hinders flow, flow, night night-time-time heat heat losses losses could could be bereduced reduced by by20% 20% as asthe the plate plate prevents prevents reverse reverse flow flow in in the the absence absence of of a a heat source. The operatingoperating principleprinciple ofof thisthis perforatedperforated bafflebaffle plateplate isis illustratedillustrated inin FigureFigure6 .6.
FigureFigure 6. OpeOperatingrating principle principle ofof the the heat heat retaining retaining ICSSWH ICSSWH design design during during collection collection (left) ( leftand) non and- non-collectioncollection (right) (right periods) periods.. Adapted Adapted from from Smyth Smyth et al. et [45] al.. [45].
Energies 2018, 11, 1615 9 of 26
Sokolov and Vaxman [47] and, in a follow-up study, Vaxman and Sokolov [48] assessed a triangular Energies 2018, 11, x FOR PEER REVIEW 9 of 26 system with a baffle plate and achieved an efficiency of 53%. Smyth et al. [31] presented results on an experimentalSokolov analysis and Vaxman of a 1.5[47] m and, cylindrical in a follow ICS-up vessel, study, mounted Vaxman within and Sokolov a reflector [48] cavity,assessed with a the uppertriangular third heavily system insulated with a baffle and plate a perforated and achieved inner an sleeve efficiency acting of 53%. as a Smyth baffle. et They al. [31] showed presented that 60% of theresults thermal on an energy experimental stored within analysis the of vessela 1.5 m could cylindrical be retained ICS vessel, over mounted a 16 h non-collection within a reflector period. The mostcavity, efficient with the configuration upper third heavily was the insulated 1.5 m vessel,and a perforated with the topinner third sleeve insulated, acting as no a baffle. insulated They cover duringshowed the non-charging that 60% of the phase, thermal and energy a perforated stored within inner sleeve.the vessel It outperformedcould be retained the over same a 16 configuration h non- withoutcollection the inner period. sleeve The most by 1.3% efficient and configuration the 1 m vessel waswith the 1.5 no m insulated vessel, with upper the top section third insulated, and no inner no insulated cover during the non-charging phase, and a perforated inner sleeve. It outperformed the sleeve by almost 7%. same configuration without the inner sleeve by 1.3% and the 1 m vessel with no insulated upper El-Sebaii [49] studied the thermal performance of a shallow solar-pond, a batch system similar section and no inner sleeve by almost 7%. to ICSSWH,El-Sebaii with [49] an integratedstudied the bafflethermal plate. performance Thermal of a performance shallow solar was-pond, found a batch to system improve similar with the inclusionto ICSSWH, of the bafflewith an plate. integrated The highest baffle plate. daily Thermal efficiencies, performance up to 64.3%, was found were to achieved improve usingwith the a plate withoutinclusion vents of and the positioned baffle plate. at The approximately highest daily two-thirdsefficiencies, ofup the to 64.3%, total height were achieved of the collector. using a pl However,ate whenwithout a baffle vents plate and with positioned vents was at used approximately the performance two-thirds is less of dependent the total height on its of position the collector. within the pond.However, It was also when shown a baffle that plate the with baffle vents plate was material used the hadperformance a negligible is less impact dependent on water on its position temperature throughoutwithin the the pond. pond, It ranging was also from shown 58 that◦C to the 59.1 baffle◦C toplate 60.3 material◦C for had mica, a negligible aluminium impact and stainlesson water steel, respectively.temperature These throughout results conformedthe pond, ranging to those from previously 58 °C to 59.1 reported °C to 60.3 by °C Kaushik for mica, et aluminium al. [41]. The and baffle stainless steel, respectively. These results conformed to those previously reported by Kaushik et al. plate also allowed the pond to retain hot water overnight, reaching temperatures of 71 ◦C in the late [41]. The baffle plate also allowed the pond to retain hot◦ water overnight, reaching temperatures of afternoon71 °C andin the providing late afternoon water and at providing temperatures water of at 43 temperaturesC in the early of 43 morning °C in the the early following morning day. the following day. 3.4. Fins In3.4. 1907, Fins Charles L. Haskell [50] patented a solar heater design incorporating ‘struts’ for improved heat conductionIn 1907, asCharles well asL. Haskell structural [50]stability patented (Figurea solar heater7). Since design then, incorporating ‘struts’, or ‘struts’ fins/extended for improved surfaces, are commonlyheat conduction used as in well commercial as structural evacuated stability (Fig tubeure and 7). Since flat-plate then, ‘struts’, collectors, or fins/extended protruding surfaces, out from the collectorare commonly pipes to enhanceused in commercial heat transfer evacuated to the tube working and flat fluid.-plate In collectors, ICSSWH protruding systems, theseout from fins the can be placedcollector directly pipes into to the enhance water heat body, transfer i.e., storage to the working tank, and fluid. extend In ICSSWH throughout systems, the these full depthfins can allowing be heatplaced to be transferred directly into throughthe water conductionbody, i.e., stor toage the tank, areas and that extend are throughout not in close the contact full depth to the allowing absorbing heat to be transferred through conduction to the areas that are not in close contact to the absorbing surface. Youcef-Ali [51] conducted an optimisation study evaluating the impact of fin length and surface. Youcef-Ali [51] conducted an optimisation study evaluating the impact of fin length and various glazing types on thermal performance. The author studied a solar air collector with offset various glazing types on thermal performance. The author studied a solar air collector with offset rectangularrectangular plate plate fin absorbers fin absorbers and and found found that that they they generated generated a a higher higher heater heater transfer transfer versusversus a flat flat-plate- alone.plate The alone. flat-plate The flat collector,-plate collector, with double with double glazing, glazing, produced produced an efficiency an efficiency of 38% of 38% against against 64% 64% for the offsetfor absorber the offset plate absorber with plate 50 mm with fins. 50 mm fins.
FigureFigure 7. Solar 7. Solar water water heater heater incorporating incorporating finsfins patentedpatented in in 1907 1907 by by Charles Charles L. Haskell. L. Haskell. Illustrated Illustrated is is the mountedthe mounted collector collector and and a crossa cross section section (A-B)(A-B) to show the the internal internal configuration. configuration. Adapted Adapted from from HaskellHaskell [50]. [50].
Energies 2018, 11, x FOR PEER REVIEW 10 of 26 Energies 2018, 11, 1615 10 of 26 Gertzos and Caouris [52] optimised the arrangement of structural and functional parts in a flat- plateGertzos ICSSWH and experimentally Caouris [52 ]and optimised numerically. the arrangementTheir aim was of to structuraleliminate the and stagnation functional area parts near in the a flat-plategeometrical ICSSWH centre of experimentally the stored water and body numerically. and provide Their structural aim was stability. to eliminate The authors the stagnation found that area the nearpresence the geometricalof fins had no centre effect of on the the stored outlet waterwater bodytemperature and provide as the structuralincrease in stability.heat transfer The rate authors was foundinsignificant that the due presence to their ofsmall fins area, had which no effect was on dwarfed the outlet by waterthat of temperaturethe tank. Li and as theWu increase [53] studied in heat the transfereffect of rateextended was insignificant fins on improving due to heat their pe smallrformance area, whichin shell was-tube dwarfed thermal by energy that of storage the tank. units Li with and Wuphase [53 change] studied materials the effect (PCMs). of extended They found fins on that improving by incorporating heat performance fins into inthe shell-tube tube design thermal the chargi energyng storagetime could units be with shortened phase by change up to materials 20%, depending (PCMs). on They the foundPCM material that by incorporating used. fins into the tube designW theork charging conducted time by could Mun beeer shortened et al. [22] by, which up to 20%, was depending later ratified on by the Junaidi PCM material [54], led used. to the inclusionWork and conducted optimisation by Muneer of elongated et al. [22], fins which inside was later a box ratified-type ICSSWH by Junaidi to [ 54 improve], led to heatthe inclusion transfer andthroughout optimisation the water of elongated body. Junaidi fins inside [54] carried a box-type out simulations ICSSWH to improveon both a heat finned transfer and un throughout-finned SWH the waterand determined body. Junaidi that [54 the] carried finned outcollector simulations performed on both significantly a finned and better. un-finned Currie SWHet al. and[9] analysed determined the thatoptimisation the finned of collector fin length performed and thickness significantly for an better. even Currieheat distribution et al. [9] analysed throughout the optimisation the storage oftank fin lengthwithout and disrupting thickness flow for patterns. an even heat The distributionprototype subsequently throughout tested the storage by Garnier tank without [55] utilised disrupting four 3 flowmm-thick patterns. aluminium The prototype fins mounted subsequently vertically tested in by a Garnier square [ shaped55] utilised collector. four 3 These mm-thick were aluminium found to finsimprove mounted both vertically the thermal in a performance square shaped and collector. structural These strength were found of the to ICSSWH improve (Fig bothure the 8). thermal A 13% performanceincrease in heat and transfer structural was strength found, as of a the direct ICSSWH result (Figure of the fin8). Ainstallation. 13% increase Mohsen in heat and transfer Akash was [56] found,also studied as a direct the performance result of the fin of installation.a box-type ICSSWH Mohsen and with Akash extended [56] alsoheat studied transfer the fins. performance They found of athat box-type the use ICSSWH of fins can with improve extended the heat cumulative transfer fins.efficiency They of found the system that the by use 9%, of during fins can the improve month the of cumulativeNovember in efficiency Jordan. of the system by 9%, during the month of November in Jordan. FollowingFollowing a numericalnumerical study by Chaabane et al.al. [[57]57],, thethe thermalthermal performanceperformance ofof aa cylindricalcylindrical ICSSWH,ICSSWH, mountedmounted in in a a CPC, CPC, was was improved improved by by the the inclusion inclusion of rectangularof rectangular radial radial fins. fins. This This work work was basedwas based on, and on, validatedand validated by, the by, experimental the experimental data data presented presented by Chaouachi by Chaouachi and Gabsiand Gabsi [58]. [58] The. finsThe createfins create a modified a modified surface surface that enhanced that enhanced the convective the convective heat transfer heat due transfer to their due higher to characteristic their higher length.characteristic This resulted length. inThis a double-edged resulted in a sword double with-edged higher sword water with temperatures higher water and temperatures reduced thermal and lossesreduced during thermal the charginglosses during period the but charging higher thermal period lossesbut higher during thermal non-collection losses during periods. non The-collection authors alsoperiods. noted The a significant authors also correlation noted a between significant fin correlation length and betweenaverage water fin length temperature, and average convective water heattemperature, transfer, andconvective heat retention. heat transfer, Three finand lengths heat retention. were modelled, Three fin 15, lengths 30 and 45were mm, modelled, and in all 15, cases 30 theand 45 45 mm mm, fins and produced in all cases the the best 45 resultsmm fins during produced day-time the best operation. results during As the day fin- depthtime operation. increases As so doesthe fin the depth availability increases of so hot does water; the temperaturesavailability of overhot water; 50 ◦C temperatures were maintained over for 50 6.5°C were h and maintained more than 10for h 6.5 for h a and finlength more t ofhan 15 10 mm h andfor a 45 fin mm, length respectively. of 15 mm During and 45 night-timemm, respectively. operation, During the thermal night-time loss coefficientoperation, isthe higher thermal for theloss casescoefficient with fins, is higher with afor slight the increasecases with in thefins, coefficient with a slight with increase increasing in the fin depth.coefficient However, with increasing the higher fin temperatures depth. However, and longer the higher heat retention temperatures gained and during longer the heat day retention means a bettergained overall during performance the day means for thea better finned overall ICS over performance un-finned. for The the author finned suggests ICS over a night un-finned. insulation The coverauthor to suggests combat thea night radiative insulation heat losses.cover to combat the radiative heat losses.
Figure 8.8. Exploded view of the ICSSWH studied. Adapted from Garnier etet al.al. [[5555].].
3.5. Glazing Solar water heaters generally include a glazed aperture over the absorber area with an optimised air gap to suppress upward convection and minimise heat losses, whilst maintaining a high
Energies 2018, 11, 1615 11 of 26
3.5. Glazing Solar water heaters generally include a glazed aperture over the absorber area with an optimised air gap to suppress upward convection and minimise heat losses, whilst maintaining a high transmissivity [59]. Glass is the most commonly used material as it is resistant to degradation from ultraviolet (UV) radiation and it has a very good transmittance to solar radiation (up to 90%), as well as a low transmittance to the thermal radiation emitted by the absorber [10,60]. Still, approximately 60% of heat loss in residential buildings can be attributed to the glazed areas [61] and this is no less important in an ICSSWH system. There are numerous ways to improve thermal performance and heat retention through the type and configuration of the glazed aperture including multilayer, vacuum, aerogel, and PCM glazing as well as low-emittance coatings. The most common options are reviewed below.
3.5.1. Glazing Layers The study by Kumar and Rosen [35], reviewed in Section 3.1.3, looked at various heat retention strategies for a rectangular ICSSWH system. They found that a double glazed system maintained the highest temperatures and collector efficiencies which shows that thermal retention can be improved with multiple glazing layers. However, the amount of insolation transmitted to the absorber plate is reduced. In spite of this, Bishop [62] studied a high volume ICS water heater (two 170 L tanks), designed for use in freezing climates, which incorporated six glazing layers—one top sheet of low-iron glass and five sheets of high transmission polyester film with an overall solar transmittance of 70%. It was concluded that a twin-tank system with a total area of 2.88 m2 produced enough water at 50 ◦C, in January, for a family of four in the climate of Denver, Colorado, USA. This suggests that the reduced heat gain due to lower transmittance is, at least, balanced by the heat retained due to the greater thermal resistance of the multiple layers. In freezing climates, where solar insolation is presumably weak, this is highly beneficial as heat loss to the ambient environment will cripple any ICSSWH system. However, in locations where temperatures remain above freezing, with sufficient solar insolation, multiple glazing layers may reduce collector efficiency as well as significantly increase the cost of the unit. Youcef-Ali [51] also advocated for a multi-glazed system in a study of a solar air collector. The author experimentally compared two types of transparent cover; double and triple glazed. The triple glazed system produced a better thermal performance reaching efficiencies up to 68% while the double glazed system peaked at 64%. The triple glazing shows a reduced transmission of solar energy but this is outweighed by the greater heat retention. AL-Khaffajy and Mossad [63] reviewed the optimisation of air cavities between glazing layers for a solar water heater with double glazing. This air gap has a strong influence on heat loss as it determines the level of convective motion between the absorber plate and subsequent layers of glazing. In the simulation, the air gap spacing between the absorber and the lower glass cover (L1) and between the lower and top glass cover (L2) ranged from 15–50 mm. It was found that the lowest heat loss was achieved with the combination of L1 = 40 mm and L2 = 25 mm. Another addition to multiple layers is vacuum glazing which offers a low heat loss, high optical transmittance solution. A double-glazed system with a vacuum gap between the glass sheets aims to eliminate conductive and convective heat transfer [61]. The issue of the glass layers collapsing in on themselves due to the high pressure of the vacuum can be combatted by inserting support pillars that have a minimal impact on optical performance [64]. Han et al. [65] found an overall heat transfer coefficient of 2.55 W/m2K for a 1 m2 double-glazed sample with a vacuum gap. The experiments considered the heat conduction through the support pillars and edge seal and the radiation between two glass sheets. As reported by Jelle et al. [66], SPACIA-21 a product from the Pilkington Company (Tokyo, Japan) (Figure9), shows excellent promise with an overall heat loss coefficient of 0.70 W/m 2K and a small external thickness of 21 mm, as opposed to a comparable multilayer glazing configuration at 40 mm. Cuce and Riffat [67] included translucent aerogel support pillars into a vacuum glazing Energies 2018, 11, 1615 12 of 26 design and modelled their impact on thermal performance and found a 44% reduction in the panels U-value. The authors indicated that a lower U-value could be reached if the number and distribution of theEnergies support 2018,pillars 11, x FOR is PEER optimised. REVIEW 12 of 26
FigureFigure 9. Schematic 9. Schematic of aof vacuum a vacuum glazing glazing panel panel detailingdetailing the the edge edge seal seal and and support support pillars. pillars. Adapted Adapted fromfrom NSG NSG Group Group [64 ].[64].
Vacuum glazing appears to be a promising option, however, there is an issue with conductive Vacuumheat loss through glazing the appears contiguous to be seal a promising enclosing the option, glass sheets. however, Fang thereet al. [68] is an conducted issue with a study conductive on heat losstriple through vacuumthe glazing contiguous and the sealimpact enclosing of the edge the seal glass on sheets. overall Fangthermal et al.transmission. [68] conducted They afound study on triplethat vacuum with smaller glazing glazing and the size impact there is of a larger the edge ratio seal of heat on overallconduction thermal through transmission. the edge seal They to that found that withof the smaller total glazing glazing area. size A there1 m2 sample is a larger demonstrated ratio of heat a total conduction thermal transmission through the of edge0.49 W/m seal2 toK that of thecompared total glazing to 0.65 area. W/m A2K 1 for m2 0.5sample m2, equating demonstrated to a 24.6% a totaldecrease. thermal The width transmission of the edge of 0.49seal also W/m 2K comparedmade a to significant 0.65 W/m impact2K for on 0.5 heat m transmission.2, equating toIncreasing a 24.6% it decrease. from 3 mm The to 10 width mm resulted of the edgein a 24.7% seal also and 25% increase in thermal transmission for indium and solder glass edge seals (with no frame made a significant impact on heat transmission. Increasing it from 3 mm to 10 mm resulted in a rebate), respectively. 24.7% and 25% increase in thermal transmission for indium and solder glass edge seals (with no frame rebate),3.5. respectively.2. Selective Coatings/Glazing Material
3.5.2. SelectiveA number Coatings/Glazing of studies investigated Material the use of selective coatings on glass apertures [69–73]. A low- emissivity selective coating is designed to suppress infrared radiation exchange and acts as a selective Areflector. number Commonly of studies used investigated materials for thethese use coatings of selective have a transmission coatings on near glass zero apertures for longwave [69 –73]. A low-emissivityinfrared (i.e., energy selective radiated coating from is warm designed objects) to an suppressd low reflectivity infrared for radiation shortwave exchange infrared (i.e. and, acts as a selectivesolar energy). reflector. Therefore, Commonly solar energy used will materials be able to forpenetrate these due coatings to the low have reflectance a transmission of the coating near zero for longwavewhilst heat infrared emanating (i.e., from energy a solar radiated collector will from be warm blocked objects) by its low and transmissivity. low reflectivity Bainbridge for shortwave [72] infraredfound (i.e., that solar double energy). glazing Therefore, with a selective solar energy transmission will be ablefilm workedto penetrate as well due as to a the night low cover reflectance in of thereducing coating night whilst-time heat heat emanating losses. How fromever, a solar infrared collector reflective will coatings be blocked reduce by the its transmission low transmissivity. of solar radiation and Ahmadzadeh and Gascoigne [69] suggested that, for most operating conditions, Bainbridge [72] found that double glazing with a selective transmission film worked as well as the overall performance is better with plain glazing. Although, Muneer et al. [74] state that a low- a night cover in reducing night-time heat losses. However, infrared reflective coatings reduce the emissivity coating has little effect on daylight transmissivity with a 5% reduction, from 80% to 75% transmissiontransmission, of solar when radiation one layer and is added Ahmadzadeh to a double and-glazed Gascoigne window. [69] This suggested may seem that, like for a mostsignificant operating conditions,decrease the but overall when compared performance to the isalmost better 50% with reduction plain glazing. in the U-value Although, it may Muneerbe a fair trade. et al. [74] state that a low-emissivityThese selective coating coatings has can little aid effect transmission on daylight and transmissivityretention of solar with radiation, a 5% reduction, however, from they 80% to 75%come transmission, with a high when production one layer cost and is added the cost to-benefit a double-glazed ratio is low window. [61]. Also, This the may coatings seem are like a significantsusceptible decrease to abrasion but when and dust compared accumulation to the over almost time, 50% thus reduction reducing in solar the radiationU-value gain it may and be a fair trade.overall system performance, as well as increasing maintenance [10]. It is possible to use plastic films Theseor sheets selective instead coatingsof glass for can the aid glazing transmission material andas they retention have high of solartransmittance radiation, and however, low emittanc theye come of solar radiation. However, they are prone to deform at high temperatures and become opaque due with a high production cost and the cost-benefit ratio is low [61]. Also, the coatings are susceptible to yellowing from UV radiation [60]. A polycarbonate sheet can also be used as it is weather-proof to abrasionand UV and-resistant dust [75] accumulation. A review by over Kumar time, and thus Rosen reducing [35] shows solar that radiation either a double gain and-glazed overall system system performance,or a single as-glazed well as system increasing with an maintenance insulated night [10-].time It is cover possible are the to usemost plastic advantageous films or options sheets insteadin of glassterms for of the heat glazing gain and material retention as as they we havell as economic high transmittance feasibility. and low emittance of solar radiation. However, they are prone to deform at high temperatures and become opaque due to yellowing from
Energies 2018, 11, 1615 13 of 26
UV radiation [60]. A polycarbonate sheet can also be used as it is weather-proof and UV-resistant [75]. A review by Kumar and Rosen [35] shows that either a double-glazed system or a single-glazed system with an insulated night-time cover are the most advantageous options in terms of heat gain and retention as well as economic feasibility. Energies 2018, 11, x FOR PEER REVIEW 13 of 26 Energies 2018, 11, x FOR PEER REVIEW 13 of 26 3.6. Inlet3.6. PipeInlet Pipe Configuration Configuration 3.6. Inlet Pipe Configuration ThermalThermal stratification stratification within within a watera water store store is is anan essential contributor contributor to tooverall overall efficiency. efficiency. A ful Aly fully mixedmixed waterThermal water body stratificationbody not not only only has within has a lowera a lower water maximum maximumstore is an essential temperature contributor but but also also to less overall less capacity capacity efficiency. for heat for A heatfulgain.ly gain. The levelmixedThe oflevel water stratification of stratificationbody not withinonly within has a storagea alower storage maximum tank tank depends depends temperature on on the the charging butcharging also less and and capacity dischargingdischarging for heat cycles cycles gain. of of the The level of stratification within a storage tank depends on the charging and discharging cycles of SWH;the the SWH; flow therates flow and rates water and velocities; water velocities; the size the and size shape and shape of the of system the system and; theand; size the andsize locationand the SWH; the flow rates and water velocities; the size and shape of the system and; the size and of thelocation inlet and of outletthe inlet pipes and outlet [6]. The pipes design [6]. The and design configuration and configuration of the inlet of the pipe inlet can pipe have can a have significant a location of the inlet and outlet pipes [6]. The design and configuration of the inlet pipe can have a impactsignificant on thermal impact stratification on thermal asstratification it can be usedas it can to controlbe used to flow control velocities flow velocities and thus and reduce thus reduce turbulent significantturbulent impact mixing on[7 6thermal–78]. Diffuser stratification designs as itdiffer can be greatly used to and control includ flowe velocitiesslotted tubes, and thus pipes reduce with mixing [76–78]. Diffuser designs differ greatly and include slotted tubes, pipes with inverted cups, turbulentinverted cups, mixing solid [76 baffle–78]. Diffuserplates, perforated designs differ circular greatly plates, and radial includ-flowe slotteddisks, distributed tubes, pipes nozzles, with solidinvertedand baffle perforated plates, cups, solid perforatedtubes baffle [79]. plates,Hegazy circular perforated [80] plates, experimentally radial-flowcircular plates, tested disks, radial three distributed- flowinlet disks,geometries nozzles, distributed (Fig andure nozzles, 1 perforated0) and tubesandfound [79 ].perforated Hegazy that all tubes [ 80 the] experimentally [79] designs. Hegazy promoted [80] tested experimentally good three thermal inlet tested geometries stratification. three inlet (Figure geometries The 10 slotted) and (Fig found inleture 1 slightly0 that) and all the designsfoundoutperformed promoted that all good the the perforat thermal designsed promotedstratification. and wedged good pipes The thermal slotted and the stratification. inlet design slightly is also The outperformed simpler slotted and inlet cheaperthe slightly perforated to and wedgedoutperformedmanufacture. pipes the and perforat the designed and is also wedged simpler pipes and and cheaper the design to manufacture. is also simpler and cheaper to manufacture.
FigureFigure 10. 10.Examples Examples of of inlet inlet designs. designs. Adapted from from Hegazy Hegazy [80] [80. ]. Figure 10. Examples of inlet designs. Adapted from Hegazy [80]. Chung et al. [81] analysed the impact of diffuser configuration on the thermal stratification ChungwithinChung et a rectangularal. et [ 81 al.] analysed[81] wateranalysed the storage impact the impact tank. of diffuserThree of diffuser diffuser configuration configuration types were on theonstudied the thermal thermal—radial stratification stratification plate, radial within a rectangularwithinadjusted a rectangularplate water, and storage H water-beam. tank. storageThe Three results tank. diffuser ratified Three thediffuser types knowledge were types studied—radial thatwere diffuser studied shape—radial plate, has plate,a radial significant radial adjusted plate,adjustedimpact and H-beam. onplate thermal, and The H performance- resultsbeam. The ratified results and that the ratified knowledge the design the knowledge should thatdiffuser be that tailored diffuser shape to shape the has aspect ahas significant a ratiosignificant of the impact on thermalimpactstorage performance onta nk. thermal In this performance case, and the that H the- beam and design thatproduced the should design a thicker be should tailored thermocline beto tailored the (and aspect to therefore the ratio aspect ofhigher the ratio storage thermal of the tank. storage tank. In this case, the H-beam produced a thicker thermocline (and therefore higher thermal In thisstratification) case, the H-beam than the produced radial diffusers. a thicker The thermocline difference (and between therefore the radial higher plates thermal was stratification) minimal. stratification) than the radial diffusers. The difference between the radial plates was minimal. than theDragsted radial et diffusers. al. [82] compar The differenceed a new inlet between diffuser the design radial with plates well was-established minimal. ones, Dragsted illustrated et al.in [82] DragstedFigure 11 .et al. [82] compared a new inlet diffuser design with well-established ones, illustrated in comparedFigure a1 new1. inlet diffuser design with well-established ones, illustrated in Figure 11.
Figure 11. Different inlet diffusers tested. (A) a simple rigid polymer pipe; (B,C) a polymer pipe with Figure 11. Different inlet diffusers tested. (A) a simple rigid polymer pipe; (B,C) a polymer pipe with Figurethree 11. “nonDifferent-return” inlet valves diffusers ((B) open tested.-ended, (A )(C a) simple with a T rigid-piece polymer end-cap); pipe; (D) a (flexibleB,C) a perforated polymer pipe pipe. with three “non-return” valves ((B) open-ended, (C) with a T-piece end-cap); (D) a flexible perforated pipe. threeAdapted “non-return” from Dragsted valves (( Bet) al. open-ended, [82]. (C) with a T-piece end-cap); (D) a flexible perforated pipe. Adapted from Dragsted et al. [82]. Adapted from Dragsted et al. [82]. The results found that performance depended on flow rate. At higher flow rates (4 L/min) designsThe B results and C performed found that the performance best while design depended D worked on flow better rate. at Atlower higher flow flowrates rate(1 ands (4 2 L/min)L/min). designs B and C performed the best while design D worked better at lower flow rates (1 and 2 L/min).
Energies 2018, 11, 1615 14 of 26 Energies 2018, 11, x FOR PEER REVIEW 14 of 26
During charging periods design D showed the best performance due to the greater number of The results found that performance depended on flow rate. At higher flow rates (4 L/min) designs perforations. B and C performed the best while design D worked better at lower flow rates (1 and 2 L/min). During charging periods design D showed the best performance due to the greater number of perforations. 3.7. Phase Change Materials (PCM) 3.7. PhaseLatent Change heat storage Materials materials, (PCM) also known as phase change materials (PCMs), are an increasingly popularLatent method heat storage of extended materials, heat also energy known storageas phase and change can materials be applied (PCMs), in many are an solar increasingly energy popularapplications method [83–8 of5]. During extended daytime heat energycharging storage the PCM and store can excess be applied thermal inenergy many as solarlatent energyheat by applicationschanging phase. [83–85 During]. During non daytime-collection charging periods, the as PCM hot storewater excess is withdrawn thermal energyfor domestic as latent use heat fresh, by changingcold, water phase. replaces During it non-collection with no solar periods, energy asfor hot heat water gain. is withdrawn Therefore, forthe domestic stored latent use fresh, energy cold, is waterreleased replaces as the itPCM with change no solar phase energy and for revert heat back gain. to Therefore, their original the storedstate. There latent are energy different is released classes asof thePCM PCM including change organic phase and compounds, revert back inorganic to their originalmaterials state., and There eutectic are mixtures different [86] classes. One of of PCM the includingbenefits of organic PCM is compounds, that, by storing inorganic excess materials,energy, the and heat eutectic losses mixturesthat occur [86 when]. One the of collector the benefits is at ofits PCMhighest is that,temperature by storing are excessreduced energy,, thus theenhancing heat losses system that efficiency occur when [87] the. collector is at its highest temperatureWith regards are reduced, to PCM thus integration enhancing into system ICSSWH, efficiency a few studies [87]. have been conducted to determine the With thermal regards performance to PCM integration [18,88,89].into Tarhan ICSSWH, et al. a few[90] studiesinvestigated have been the conducted effect of PCMto determine on the thetemperature thermal performance distribution [18 within,88,89 the]. Tarhan water et tank. al. [90 Three] investigated trapezoidal the ICSSWH effect of PCM systems on the were temperature analysed, distributionone without withinPCM to the act water as a reference tank. Three heater trapezoidal and two ICSSWH with different systems configurations were analysed, of organic one without PCM PCM(Figure to act12). as a reference heater and two with different configurations of organic PCM (Figure 12).
FigureFigure 12.12. CrossCross sectionalsectional view view of of thethe three three trapezoidal trapezoidal ICSSWH ICSSWH systems systems studied. studied. ((AA)—reference)—reference collector;collector; ((BB)—PCM)—PCM storagestorage tanktank filledfilled with with myristic myristic acid, acid, actingacting as as an an absorber absorber plate;plate; ((CC)—PCM)—PCM storagestorage tanktank filledfilled with with lauric lauric acid, acid, acting acting as as a a baffle baffle plate. plate. Adapted Adapted from from Tarhan Tarhan et et al. al. [ 90[9].0].
The first set-up utilised myristic acid, in a PCM storage tank, as an absorber plate (Figure 12B) The first set-up utilised myristic acid, in a PCM storage tank, as an absorber plate (Figure 12B) and in the second set-up lauric acid in the PCM storage tank was used as a baffle plate (Figure 12C). and in the second set-up lauric acid in the PCM storage tank was used as a baffle plate (Figure 12C). The configuration with myristic acid proved to have the highest overall performance with water The configuration with myristic acid proved to have the highest overall performance with water temperature differences up to 4 °C after the cooling period. This is due to the solidification temperature differences up to 4 ◦C after the cooling period. This is due to the solidification temperature temperature of the myristic acid, it solidifies at 51–52 °C and acts as a thermal barrier so the absorber of the myristic acid, it solidifies at 51–52 ◦C and acts as a thermal barrier so the absorber plate essentially plate essentially becomes a night cover. The reference collector, with no PCM, reached higher peak becomes a night cover. The reference collector, with no PCM, reached higher peak temperatures during temperatures during the collection period but this heat was lost during the night. the collection period but this heat was lost during the night. Chaabane et al. [89] carried out a numerical study of a CPC mounted ICSSWH system with two different PCM—myristic acid and RT42-graphite. The authors also found myristic acid to be the most beneficial as it reaches higher maximum temperatures during the day and allows better heat
Energies 2018, 11, 1615 15 of 26
Chaabane et al. [89] carried out a numerical study of a CPC mounted ICSSWH system with two different PCM—myristic acid and RT42-graphite. The authors also found myristic acid to be the most beneficial as it reaches higher maximum temperatures during the day and allows better heat preservation during the night, with water temperatures almost 30 ◦C higher than a non-PCM system after 18 h of operation. The thermal efficiency of the system without PCM is only slightly higher during the collection period but drops below the PCM configurations overnight. Hamed et al. [86] also did a numerical study, analysing the charging and discharging performance of a rectangular ICSSWH. The type of PCM used in the calculations was not stated, only the thermophysical properties. As with the other studies the inclusion of a PCM resulted in a longer charging time, reaching lower peak temperatures, and higher available temperatures during the non-collection period with a maximum temperature difference of almost 9 ◦C. However, the use of PCMs in thermal energy storage is limited and often unavailable on a commercial market due to economic and environmental constraints [85]. Choosing the right type of PCM and the position within an ICS system is crucial to its effective function [89]. Also, the evaporation associated with certain types of PCM requires enormous changes in volume of the storage materials making storage complex and impractical [86]. Many organic PCMs have a low rate of heat transfer as well as low thermal conductivity and for small scale applications, such as domestic solar water heating, the cost of this technology is still too great [85].
3.8. Reflectors ICSSWH systems can absorb both diffuse and direct solar radiation and an easy way to enhance the level of diffuse radiation collected is to apply strategically positioned reflective surfaces. In terms of ICS systems, the most common configuration of collector and reflector is the compound parabolic concentrating (CPC) collector. A cylindrical water store is set into a reflective trough which reflects incident radiation to the absorber. Devanarayanan and Kalidasa Murugavel [91] conducted a comprehensive review of the development and progress surrounding ICSSWH with integrated CPC. The authors classified the different configurations of integrated compound parabolic concentrator storage solar water heater (ICPCSSWH) systems based on the associated absorber. The main conclusions from this review were that ICPCSSWH systems are cost-effective and simple to construct with a satisfactory thermal efficiency that can compete with flat-plate thermosyphonic units (FPTU). Also, they have a short response time across discharging and recharging. Drawbacks, however, include their lower optical efficiency, especially at lower sun angles (i.e., winter), high heat losses, particularly during non-collection periods, and only moderate thermal stratification within the water store. Souliotis et al. [92,93] carried out research studies on ICSSWH systems mounted in CPC reflector troughs with the aim of assessing the thermal performance and enhancing the night-time heat retention. The authors found that seasonal variation has little impact on optical efficiency for the experimental location (Patras, Greece) with values ranging from 77% in winter to 79% in summer [92]. When compared with a FPTU, the ICS system has poorer heat retention with the bulk water temperature, after 24 h, being 7 ◦C lower. However, during the charging period the ICPCSSWH has a greater system efficiency and thermal stratification due, in part, to the partial vacuum in the annulus between the absorber and storage of the ICS unit. This added thermal diode improves the operation of the system but high thermal losses, poor stratification, and optical efficiency are still observed. These results were reiterated in a later study with a bulk water temperature difference of 12 ◦C between the FPTU and ICS models, after 24 h [93]. However, thermal losses can be improved by incorporating double glazing into the design. Varghese et al. [94] carried out a study on a cylindrical ICPCSSWH in Delhi, India, with the aim of improving heat retention by including an air gap in the side walls (the arms of the CPC). They found maximum experimental efficiencies of 38% and water temperatures of 53 ◦C. By introducing an air gap the maximum outlet temperature can be enhanced and continues to rise even after collector efficiencies Energies 2018, 11, 1615 16 of 26 drop. This is due to the thermal barrier, created by the air gap, between the absorber and the ambient air which decreases the thermal loss coefficient by up to 52.5%, depending on water temperature. Not all reflector mounted ICS systems utilise cylindrical collectors, however. Ziapour et al. [95] analysedEnergies 2018 the, 11 performance, x FOR PEER REVIEW improvement of a rectangular PV/T system which incorporated reflectors16 of 26 that acted as removable insulation covers (Figure 13). The findings of the numerical model showed thatthat thethe reflectorsreflectors effectivelyeffectively reducedreduced thethe night-timenight-time heatheat losseslosses whilstwhilst increasingincreasing thethe levellevel ofof solarsolar radiationradiation incidentincident on the the absorber absorber plate. plate. With With reflectors, reflectors, peak peak diurnal diurnal temperatures temperatures reach reach 69.2 69.2 °C;◦ 9.5C; 9.5°C ◦highlyC highly than than a system a system without without reflectors. reflectors. Water Water temperatures temperatures in the in morning the morning (6 a.m (6.) a.m.)are also are much also muchhigher higher with values with values up to up 54 to °C 54 compared◦C compared to 31 to °C, 31 ◦thusC, thus offering offering a 43% a 43% improvement. improvement. The The angle angle at atwhich which the the reflectors reflectors are are mounted mounted has has a a strong strong influence influence on on the the total solar radiation incidentincident onon thethe absorberabsorber plateplate andand aa modelmodel optimisingoptimising thesetheseangles angleswas waspresented. presented.
FigureFigure13. 13. SchematicSchematic representationrepresentation of of the the passive passive PV/T PV/T systemsystem studied.studied. I(t)I(t) denotesdenotes thethe incidentincident solarsolar radiation.radiation. AdaptedAdapted fromfrom ZiapourZiapour etet al.al. [[9955].].
AlthoughAlthough reflectors reflectors have have potential, potential, their their shape shape and andsize are size not are practical not practical for mounting for mounting on a pitched on a roof.pitched The roof. impact The on impact the aesthetics on the aesthetics of the structure of the might structure be off-putting might be for off- prospectiveputting for consumers.prospective Theconsumers. issue of windThe issue loading of wind is also loading magnified is also with magnified these ICS with configurations these ICS asconfigurations their bulky and as protrudingtheir bulky natureand protruding makes them nature more makes susceptible them more to higher susceptible wind loads. to higher This wouldwind loads. be especially This would prominent be espe incially the caseprominent of the latterin the study case of as the the latter reflectors, study which as the are reflectors, open during which the are day, open could during be damagedthe day, could in high be winds.damaged Ziapour in high et winds. al. [95] Ziapour made no et mention al. [95] made of the no impact mention of wind of the in impact their numerical of wind in study, their numerical however, thisstudy, would however, be an importantthis would consideration be an important under consideration practical application. under practical application.
3.9.3.9. SelectiveSelective AbsorberAbsorber SurfacesSurfaces AnAn absorbentabsorbent coatingcoating isis oftenoften usedused toto enhanceenhance thethe absorptionabsorption ofof solarsolar radiation.radiation. BlackBlack paintpaint isis commonlycommonly used used as as it is it a is good a good absorber absorber of radiation of radiation within within the visible the and visible infrared and partinfrared of the part spectrum of the andspectrum therefore and absorbstherefore the absorbs most heat.the most Due heat. to the Due low to emittancethe low emittance of a black of a object black it object can absorb it can absorb up to 96%up to of 96% incident of incident insolation insolation [60,96]. [ For60,96 greater]. For absorptivitygreater absorptivity and lower and emissivity, lower emissivity, a spectrally a spectrally selective absorberselective surfaceabsorber can surface be employed. can be employed. Smyth et al. Smyth [6] conducted et al. [6] aconducted thorough reviewa thorough of several review studies of several that havestudies verified that have the benefit verified of the selective benefit absorber of selective coatings absorber [72,97 coatings–101]. The [72 reviewed,97–101]. studiesThe reviewed demonstrated studies improveddemonstrated water improved temperatures water but temperatures poor heat retention but poor during heat retention non-collection during periods,non-collection compared periods, to a plaincompared design to [99 a ,101plain]. Cummingsdesign [99, and101] Clark. Cummings [100] showed and Clark that the[10 average0] showed annual that deliveredthe average energy annual for adelivered single-glazed energy selective for a single absorber-glazed design selective would absorber increase design in the range would of increase 26–44% comparedin the range with of 26 a basic–44% design.compared This with indicates a basic the design. potential This of indicates selective the surfaces potential and of optimising selective thesurf emittanceaces and optimising of the coating the couldemittance make of up the the coating small could temperature make up differences the small observed temperature [99, 102difference]. s observed [99,102]. Teixeira et al. [73] presented a numerical model that allows the selectivity of absorber coatings to be correlated with the collector efficiency. The analysed composite cermet coatings have high spectral selectivity, with absorption in the range of 0.88 to 0.94 and emissivity ranging from 0.15 to 0.04, and were subject to three different sputtering conditions—single layer, multilayers and gradient coatings. The study showed that a graded coating had much higher absorptivity, due to reduced reflectance, than a pure cermet film. Layered coatings also had much lower reflectance, and therefore
Energies 2018, 11, 1615 17 of 26
Teixeira et al. [73] presented a numerical model that allows the selectivity of absorber coatings to be correlated with the collector efficiency. The analysed composite cermet coatings have high spectral selectivity, with absorption in the range of 0.88 to 0.94 and emissivity ranging from 0.15 to 0.04, and were subject to three different sputtering conditions—single layer, multilayers and gradient coatings. The study showed that a graded coating had much higher absorptivity, due to reduced reflectance, than a pure cermet film. Layered coatings also had much lower reflectance, and therefore greater absorptivity, for aluminium and copper surfaces, although glass did not follow this behaviour. For both cases reflectance is less than 10%, over the visible range of 380–780 nm associated with the luminance of daylight [103]. The type of coating used for the absorber surface is not the only consideration but also the profile; for example, whether it is planar or corrugated. Kumar and Rosen [104] reported on the thermal performance of an ICSSWH with a corrugated absorber surface. This surface has a higher characteristic length and, therefore, a higher surface area exposed to solar radiation. It was concluded that the corrugated surface had higher operating temperatures for a greater period than the plane surface but a marginally reduced system efficiency, as a result. As the corrugation depth increases from 0.4 mm to 1 mm, the maximum temperature of the water increases from 53 to 64 ◦C while the efficiency decreases from 46.8% to 42.4% (with night-time insulation) and 40% to 35% (without night insulation). At a corrugation depth of 1 mm average water temperatures are 5 to 10 ◦C higher than with a plane surface during collecting periods.
3.10. Storage Tank/Collector Material
3.10.1. Metals Traditionally, SWHs have been constructed from metal due to their strength and durability. The most common are copper, stainless steel, and aluminium. Copper has the highest thermal conductivity at 385 W/m·K but it is also the most expensive. Therefore, in the interests of developing a cost-effective solution stainless steel or aluminium are more often utilised [105]. In terms of structural strength, stainless steel outperforms aluminium which is a ‘soft’ metal and can deform at high temperature and pressure. Aluminium can also suffer galvanic corrosion when connected to conventional copper pipework whereas stainless steel is resistant to corrosion [105]. Aluminium does, however, have a much higher thermal conductivity at 237 W/m·K compared to 14.9 W/m·K for stainless steel [96]. Heat transfer and the thermal conductivity of the vessel material have a significant impact on thermal stratification within the water store. Vertical conduction in the tank walls, coupled with losses to the ambient environment, induces convective currents that rapidly degrade thermal stratification [6]. This suggests that a material with a lower thermal conductivity could be beneficial in reducing the convective heat motion, thus enhancing stratification. However, there is the trade-off of reduced transfer of absorbed heat to the water body. Ziapour et al. [106] simulated and compared four different types of absorber for passive PV/T systems. Here, PV panels were mounted onto different absorber plate types of an ICSSWH—an aluminium plate with fins; aluminium without fins; Tedlar (a highly versatile polyvinyl fluoride polymer material) and; black painted glazing. The simulation results showed that the aluminium absorber plate with fins had the highest electrical and thermal efficiencies, with combined PV/T efficiencies up to 88% [106]. Garnier [36] modelled the impact of stainless steel versus aluminium on heat transfer, comparing aluminium thicknesses of 3 mm and 1.5 mm and 1.5 mm thick stainless steel, and found heat transfer rates of 67.3%, 63% and 25.3%, respectively. These computational results were found to be in close agreement with experimental data. Garnier [36] also conducted a monetary analysis and life cycle assessment of stainless steel versus aluminium systems. ICS systems strive to be a “green” technology, therefore, the embodied energy, embodied carbon, and recyclability must be taken into consideration. The author found that the embodied energy of stainless steel was 66% less than aluminium and, likewise, the embodied carbon was 26% lower with stainless steel over Energies 2018, 11, 1615 18 of 26 aluminium. This difference could be reduced if the percentage of recycled material in the aluminium is increased.
3.10.2. Polymer and Composite Materials The use of solar thermal energy systems has increased dramatically in last decade yet the metal based collectors, despite a high thermal performance, are still relatively expensive to buy and install [17]. More recently, research has been undertaken on the use of polymer and composite materials for the ICSSWH components [75,107,108]. These have the potential to simplify ICSSWH construction as well Energies 2018, 11, x FOR PEER REVIEW 18 of 26 as decrease the cost of the unit as a whole [75]. Polymers are light-weight and non-corrosive, which cannotwell be as said decrease for metal the basedcost of materialsthe unit as [14 a ].whole The manufacture[75]. Polymers of are the light system-weight would and non need-corrosive, to be altered in termswhich of cannot welding, be said soldering, for metal mechanical based materials treatment, [14]. The and manufacture assembly of (moulding the system would and gluing need to of the polymerbe altered composite). in terms Fridof welding, et al. [soldering,75] looked mechanical at the use treatment, of polymer and assembly composite (moulding materials and in gluing ICSSWH construction,of the polymer incorporating composite). only Frid three et components—glazing, al. [75] looked at the use absorber of polymer plate, composite and storage materials tank/SWH in ICSSWH construction, incorporating only three components—glazing, absorber plate, and storage casing (Figure 14). The glazing is weather-proof, UV-resistant, plate-type polycarbonate and the tank/SWH casing (Figure 14). The glazing is weather-proof, UV-resistant, plate-type polycarbonate absorber plate is manufactured from a fibre-glass or carbon-filled plastic and coated with a selective and the absorber plate is manufactured from a fibre-glass or carbon-filled plastic and coated with a absorberselective coating. absorber The coating. storage The tank storage is aseries tank isof a series troughs of troughs and is and integrated is integrated with with the the outer outer wall wall of the systemof (Figurethe system 14 ),(Fig withure the14), area with between the area between the two the filled two with filled thermal with thermal insulation, insulation, to reduce to reduce the numberthe of partsnumber in the of unit. parts in the unit.
FigureFigure 14. Schematic 14. Schematic of experimentalof experimental polymer polymer compositecomposite ICSSWH ICSSWH design design with with only only 3 components. 3 components. Adapted from Frid et al. [75]. Adapted from Frid et al. [75]. This prototype demonstrated its in-situ operability, however, the engineering solutions applied Thisin itsprototype construction demonstrated are suitable only its in-situfor lowoperability, volume production, however, and the thermal engineering vacuum solutions moulding applied of in itsthe construction polycarbonate are glazing suitable gave only no for guaranteed low volume result production, [107]. This brings and thermal into question vacuum the moulding mass of thereproducibility polycarbonate of glazingthe unit as gave well no as guaranteedits structural resultintegrity. [107 Also,]. This the bringsestimated into minimum question service the mass reproducibilitylifetime for the of thecasing, unit absorbing as well aspanel, its structuraland glue joints integrity. was 7 years Also, at the a cost estimated of $70‒90 minimum/m2 (receiving service lifetimesurface), for the an casing, average absorbing or maximum panel, expectedand glue lifetimejoints was was 7 years not mentioned at a cost of [75] $70–90/m. While 2 this(receiving is approximately half the cost of traditional materials, which are in the range of $150‒200/m2 (prices surface), an average or maximum expected lifetime was not mentioned [75]. While this is approximately commonly fluctuate), the unit also has half the lifetime with stainless steel and aluminium systems 2 half theoffering cost satisfactory of traditional performance materials, for whichup to 20 are years. in theThere range is an ofinternational $150–200/m effort (pricesto advance commonly the fluctuate),use of thepolymer unit m alsoaterials has in half SWH the systems lifetime in order with to stainless lower the steel initial and cost. aluminium However, more systems research offering satisfactoryneeds to performance be done in terms for of up the to structural 20 years. integrity There of is the an unit, international their ability effort to resist to UV advance degrad theation, use of polymerand materialsoverheat protection in SWH of systems the absorber in order to prevent to lower overrunning the initial the cost. maximum However, allowable more temperature research needs to beof done the in polymer terms [6,17 of the]. Despitestructural the integrity flexibility of and the freeze unit, tolerance their ability of polymers to resist theyUV degradation, have a lower and overheatthermal protection conductivity of the than absorber metals toand prevent a much overrunningshorter lifetime the so maximumin terms of the allowable cost to benefit temperature ratio of the polymermetals still [6, 17have]. Despite the upper the hand flexibility [14]. and freeze tolerance of polymers they have a lower thermal conductivity than metals and a much shorter lifetime so in terms of the cost to benefit ratio metals still 3.11. Thermal Diodes have the upper hand [14]. A thermal diode is a device which causes heat to flow preferentially in one direction and is a 3.11. Thermalmethod of Diodes heat retention during the night and non-collecting periods, offering improved efficiencies and higher temperatures. Mohamad [42] introduced a simple thermal diode into triangular ICS Asystems thermal with diode incorporated is a device baffle which plates. causes The thermal heat diode to flow design preferentially is located at in the one base direction of the vess andel, is a methodat the of heatentry retentionto the baffle during channel, the and night consists and non-collecting of a light weight periods, plastic offering‘gate’ that improved prohibits reverse efficiencies flow (Figure 15). Mohamad [42] showed that reverse circulation at night-time is prevented, particularly when storage temperatures are high, thus producing storage efficiencies of 68.6% and 53.3% with and without the diode, respectively.
Energies 2018, 11, 1615 19 of 26 and higher temperatures. Mohamad [42] introduced a simple thermal diode into triangular ICS systems with incorporated baffle plates. The thermal diode design is located at the base of the vessel, at the entry to the baffle channel, and consists of a light weight plastic ‘gate’ that prohibits reverse flow (Figure 15). Mohamad [42] showed that reverse circulation at night-time is prevented, particularly when storage temperatures are high, thus producing storage efficiencies of 68.6% and 53.3% with and withoutEnergies the 2018 diode,, 11, x respectively.FOR PEER REVIEW 19 of 26
Energies 2018, 11, x FOR PEER REVIEW 19 of 26
FigureFigure 15. Schematic 15. Schematic detail detail of theof the ICSSWH ICSSWH with with a a bafflebaffle plate and and a a thermal thermal diode. diode. Ta— Taambient—ambient air air
temperature;temperature; Tw—bulk Tw—bulk water water temperature. temperature. Adapted Adapted from Mohamad Mohamad [42] [42. ]. Figure 15. Schematic detail of the ICSSWH with a baffle plate and a thermal diode. Ta—ambient air temperature;Sopian et al. T [10w—9bulk] introduced water temperature. a thermal Adapted diode into from their Mohamad ICS design [42]. and the temperature drop in Sopian et al. [109] introduced a thermal diode into their ICS design and the temperature drop in the storage tank overnight was reduced from 20 °C without a thermal diode to 10 °C with a diode. the storage tank overnight was reduced from 20 ◦C without a thermal diode to 10 ◦C with a diode. CreatingSopian an etevacuated al. [109] introducedlayer between a thermal the absorber diode intoplate their and ICSwater design cavity and is anotherthe temperature form of thedroprmal in Creatingthediode. storage an Souliotis evacuated tank etovernight al. layer [92] betweenstudied was reduced an the ICS absorber fromvessel 20 design °C plate without mounted and a water thermal in acavity CPC diode reflector is to another 10 trough°C with form where a ofdiode. thermal an diode.Creatingannulus Souliotis betweenan etevacuated al. the [92 ]cylinders studiedlayer between is an partially ICS the vessel absorber evacuated design plate and mountedand contains water incavity a asmall CPC is amountanother reflector formof troughwater, of the which wherermal an annulusdiode.changes between Souliotis phase the at et low cylindersal. [9temperature2] studied is partially an and ICS produces vessel evacuated design vapour.and mounted This contains phase in a changeCPC a small reflector creates amount trough a thermal of water,where diode an which changesannulustransfer phase mechanismbetween at low the temperature from cylinders the outer is and partially absorbing produces evacuated surface vapour. toand the Thiscontains inner phase storage a small change tank amount surf createsace. of Experimentalwater, a thermal which diode changes phase at low temperature and produces vapour. This phase change creates a thermal diode transferresults mechanism showed thatfrom the the systems outer absorbing performance, surface when to compared the inner with storage a FPTU, tank issurface. as effective Experimental both transfer mechanism from the outer absorbing surface to the inner storage tank surface. Experimental resultsduring showed day that and thenight systems-time operation. performance, More whenrecently, compared Souliotis with et al. a [11 FPTU,0] did is a as follow effective-up study both on during results showed that the systems performance, when compared with a FPTU, is as effective both day andthis night-timeCPC mounted operation. ICSSWH Moresystem recently, where extensive Souliotis experimental et al. [110] data did was a follow-up collected studyover more on thisthan CPC duringtwo years. day Theand night authors-time found operation. that the More PCM recently, vapour Souliotis pressure et wasal. [11 a 0 crucial] did a follow parameter-up studyand theon mounted ICSSWH system where extensive experimental data was collected over more than two years. thistemperature CPC mounted increase ICS duringSWH system diurnal where collection extensive periods, experimental at the optimal data pressurewas collected, reaches over 39 more °C while than The authorstwo years. found The that authors the PCM found vapour that the pressure PCM wasvapour a crucial pressure parameter was a crucial and the parameter temperature and increasethe the maximum heat loss during night-time operation is 13 °C with a thermal◦ loss coefficient between duringt1.60emperature diurnal and 1.62 collection increase W/K. This during periods, performance diurnal at the collectionwas optimal shown pressure,periods, to be better at reachesthe than optimal a 39 commercial pressureC while, theFPTU.reaches maximum 39 °C while heat loss duringthe night-time maximum operationheat loss during is 13 ◦nightC with-time a thermaloperation loss is 13 coefficient °C with a thermal between loss 1.60 coefficient and 1.62 between W/K. This performance1.60 and was1.62 W/K. shown This to performance be better than was a shown commercial to be better FPTU. than a commercial FPTU.
Figure 16. Conceptual design illustrating the three concentric cylinders with the thermal diode mechanism working between the inner and outer vessels. Adapted from Smyth et al. [111]. FigureFigure 16. Conceptual16. Conceptual design design illustrating illustrating the the three three concentric cylinders cylinders with with the the thermal thermal diode diode mechanismmechanismSmyth working et al.working [11 between1] betweenalso conducted the the inner innerand anand experimentalouter outer vessels.vessels. Adapted evaluation from from of Smyth a Smyth novel et al.et thermal al.[111 [111]. diode]. in an ICSSWH to be used as a pre-heater. The collector was tested using a solar simulation facility and Smyth et al. [111] also conducted an experimental evaluation of a novel thermal diode in an Smythconsisted et of al. three [111 concentric] also conducted cylinders— anthe experimental outer glazing, evaluationthe middle absorbing of a novel surface thermal and the diode inner in an ICSSWH to be used as a pre-heater. The collector was tested using a solar simulation facility and ICSSWHstorage to betank used (Figure as a16). pre-heater. As in the latter The study, collector the annular was tested cavity using between a solar the absorbing simulation surface facility and and consistedthe storage of tank three is concentric partially evacuated cylinders —andthe contains outer glazing, a small the amount middle of absorbing liquid/vapour surface PCM and which the inner acts storageas a ther tankmal (Figdiode.ure The 16). importanceAs in the latter of a study, transparent the annular aperture cavity cover between and cavity the absorbing back insulation surface were and thehighlighted storage tank as they is partially are crucial evacuated in achieving and contains the saturation a small temperature amount of liquid/vapour which promotes PCM heat which transfer acts asthrough a ther mal convection diode. The as importance opposed to of radiation a transparent only. aperture For the cover thermal and diodecavity mechanismback insulation to workwere highlighted as they are crucial in achieving the saturation temperature which promotes heat transfer
through convection as opposed to radiation only. For the thermal diode mechanism to work
Energies 2018, 11, 1615 20 of 26 consisted of three concentric cylinders—the outer glazing, the middle absorbing surface and the inner storage tank (Figure 16). As in the latter study, the annular cavity between the absorbing surface and the storage tank is partially evacuated and contains a small amount of liquid/vapour PCM which acts as a thermal diode. The importance of a transparent aperture cover and cavity back insulation were highlighted as they are crucial in achieving the saturation temperature which promotes heat transfer through convection as opposed to radiation only. For the thermal diode mechanism to work effectively certain temperatures need to be reached and maintained to facilitate the evaporation-condensation cycle. Overall efficiencies were relatively poor, reaching a maximum of ~36%, but the impact on heat retention was considerable with a reduction in thermal losses of approximately 40%, compared with conventional ICS systems.
4. Conclusions ICSSWH systems have a number of benefits over other solar thermal systems, namely that they do not require an additional water storage tank, are simple to construct and install and with fewer associated costs. A consequence of their more exposed nature is the increased level of heat loss, especially during non-collection periods. This review has presented the numerous innovations and developments in current research surrounding the improvement in the overall performance of these ICS systems, amongst others. We have reviewed and discussed the various heat retention strategies and the impact on thermal performance and efficiency, the associated heat loss mechanisms are summarised in Table3. Some methods, such as selective absorber coatings, can have a positive effect on heat gain whilst the effect on heat losses is negligible and the same level of performance can be achieved by simpler and cheaper means, such as a night-time cover or double-glazing. Reflectors can aid the absorption of diffuse solar radiation during collection periods but have a limited impact on heat retention during the night. Also, their bulky size and appearance would make their installation difficult and unpleasant, especially for domestic consumers. Research is placing a heavy focus on PCM for thermal energy storage and they have shown some promise. However, their performance greatly depends on the chosen materials and their design and thermochemical properties. Furthermore, their cost and relative complexity to incorporate into ICSSWHs makes them an impractical solution for domestic hot water systems at present.
Table 3. Summary of the heat retention methods reviewed and the associated heat loss mechanisms.
Heat Gain/Retention Strategy Heat Loss Mechanism Impacted Additional insulation Reduces convective, conductive, and radiative heat losses Auxiliary heating Provides additional heat to meet demand/combat bacteria and freezing Baffle plate/inner sleeve Reduces convective heat losses and promotes thermal stratification Fins Promotes heat transfer to the bulk water body through conduction Impacts on radiative heat losses and transmissivity, creates an air cavity which supresses Glazing internal convection Inlet pipe configuration Impacts on thermal stratification and therefore heat gain Can impact on conductive, convective and radiative heat losses depending on their use. Phase change materials Provides stored heat during non-collection periods Reflectors Impacts on the level of incident radiation, can reflect radiative heat losses Selective absorber surfaces Enhances the absorption and reduces the emission of solar radiation Storage tank/collector material Impacts on conductive heat losses Thermal diodes Reduces convective heat losses
Potential methods to improve thermal performance that do not add significant cost and complexity to ICS systems include baffles, multiple glazing layers, additional insulation and fins. Baffles and fins promote the convection and conduction transfer of heat from the absorber to the water store while the glazing and insulation prevent convective and radiative heat losses. The combination of different strategies would be beneficial for the overall thermal performance of the system without Energies 2018, 11, 1615 21 of 26 significantly impacting the efficiency. This paper has gathered together the relevant and up-to-date research and developments surrounding the performance of ICSSWH systems, focusing on heat retention, with the aim to provide a concise summary of the current literature for researchers in this field. PCM are favoured as the saving grace for solar thermal storage. However, they are currently unfeasible for small-scale, domestic applications. So far, the benefits do not outweigh their cost, making them unsustainable. Promising avenues for further research that have emerged from this critical review include enhanced glazing systems, adding additional insulation to portions of the system, and incorporating baffle plates. Although not considered in this paper, a number of the reviewed studies noted the importance of flow rate and draw-off frequency on system efficiency. As such, this is an important consideration for the optimisation of any ICSSWH system and an interesting avenue for further research.
Author Contributions: R.S. conducted the research behind this review article and wrote the paper with substantial contributions from C.G. during the initial drafts. F.P. and J.C. made significant contributions which included comments, guidance, and help to complete the final draft. Acknowledgments: The Resource Efficient Built Environment Lab (REBEL) kindly covered the costs to publish open access. Conflicts of Interest: The authors declare no conflict of interest.
Nomenclature
CFD computational fluid dynamics CPC compound parabolic concentrator FPTU flat plate thermosyphonic units GHG greenhouse gas ICPCSSWH integrated compound parabolic concentrator storage solar water heater ICS integrated collector-storage ICSSWH integrated collector-storage solar water heater PCM phase change materials PV Photovoltaic PV/T photovoltaic-thermal SCS solar combi system SWH solar water heater TIMs transparent insulation materials
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