A review of solar collectors and thermal energy storage in solar thermal applicationsomani
Y.Tian a ,C.Y.Zhao b ,⇑
a School of Engineering,University of Warwick,CV47AL Coventry,United Kingdom
b
School of Mechanical Engineering,Shanghai Jiaotong University,200240Shanghai,China
h i g h l i g h t s
"The latest developments in solar thermal applications are reviewed."Various types of solar collectors are summarid.
"Thermal energy storage approaches and systems are discusd."The current status of existing solar power stations is reviewed.
advice的用法a r t i c l e i n f o Article history:
Received 24July 2012
Received in revid form 18November 2012Accepted 20November 2012
Available online 23December 2012Keywords:
Solar collectors
Thermal energy storage Heat transfer enhancement Metal foam
Solar power stations PCM
indifferenta b s t r a c t
Thermal applications are drawing increasing attention in the solar energy rearch field,due to their high performance in energy storage density and energy conversion efficiency.In the applications,solar col-lectors and thermal energy storage systems are the two core components.This paper focus on the latest developments and advances in solar thermal applications,providing a review of solar collectors and ther-mal energy storage systems.Various types of solar collectors are reviewed and discusd,including both non-concentrating collectors (low temperature applications)and concentrating collectors (high temper-ature applications).The are studied in terms of optical optimisation,heat loss reduction,heat recuper-ation enhancement and different sun-tracking mechanisms.Various types of thermal energy storage systems are also reviewed and discusd,including nsible heat storage,latent heat storage,chemical storage and cascaded storage.They are studied in terms of design criteria,material lection and different heat transfer enhancement technologies.Last but not least,existing and future solar power stations are overviewed.
Ó2012Elvier Ltd.All rights rerved.
1.Introduction
CO 2-induced global warming has become a pressing issue,and needs to be tackled.Efficient utilisation of renewable energy re-sources,especially solar energy,is increasingly being considered as a promising solution to global warming and a means of achiev-ing a sustainable development for human beings.The Sun releas an enormous amount of radiation energy to its surroundings:174PW (1PW =1015W)at the upper atmosphere of the Earth [1].When the energy arrives at the surface of the Earth,it has been attenuated twice by both the atmosphere (6%by reflection and 16%by absorption [1])and the clouds (20%by reflection and 3%by absorption [1]),as shown in Fig.1[2].Another 51%(89PW)of the total incoming solar radiation reaches the land and the oceans [1].It is evident that,despite the attenuation,the total amount of solar energy available on the Earth is still of an enormous amount,but becau it is of low-density and intermittency,it needs to be collected and stored efficiently.
Solar collectors and thermal energy storage components are the two kernel subsystems in solar thermal applications.Solar collec-tors need to have good optical performance (absorbing as much heat as possible)[3],whilst the thermal storage subsystems require high thermal storage density (small volume and low con-struction cost),excellent heat transfer rate (absorb and relea heat at the required speed)and good long-term durability [4,5].In 2004,Kalogirou [6]reviewed veral differen
t types of solar thermal collectors that were in common u,and provided relative thermal analys and practical applications of each type.However,the technologies involved in solar collectors have been much im-proved since that review was published,so that some of the latest collectors,such as PVT (Photovoltaic-Thermal)collectors,were not available in time for inclusion in [6].The latest technologies are described in Section 2of the prent paper.In addition,most of
0306-2619/$-e front matter Ó2012Elvier Ltd.All rights rerved.dx.doi/10.1016/j.apenergy.2012.11.051
Corresponding author.Tel./fax:+862134204541.
E-mail address:Changying.zhao@sjtu.edu (C.Y.Zhao).
existing review-type literature on thermal
mainly restricted to low-temperature
are only a few papers addressing
storage applications.The include
group of potential pha change
120°C to1000°C,and provided their
et al.[11],who reviewed the
systems especially for power generation; materials and thermal models that can be latest developments in high-temperature ogies are given in Section3of the prent This paper provides a review of thermal storage methods,and is organid Solar collectors:non-concentrating
collectors.
High-temperature thermal energy
materials,heat transfer enhancement美国运筹学专业排名
An overview of existing and future solar
2.Solar collectors
A solar collector,the special energy exchanger,converts solar irradiation energy either to the thermal
energy of the workingfluid in solar thermal applications,or to the electric energy directly in PV(Photovoltaic)applications.For solar thermal applications,solar irradiation is absorbed by a solar collector as heat which is then transferred to its workingfluid(air,water or oil).The heat carried by the workingfluid can be ud to either provide domestic hot water/heating,or to charge a thermal energy storage tank from which the heat can be drawn for u later(at night or cloudy days). For PV applications,a PV module not only converts solar irradiation directly into electric energy(usually with rather low efficiency), but it also produces plenty of waste heat,which can be recovered for thermal u by attaching PV board with recuperating tubes filled with carrierfluids.
Solar collectors are usually classified into two categories according to concentration ratios[3]:non-concentrating collectors and concentrating collectors.A non-concentrating collector has the same intercepting area as its absorbing area,whilst a sun-tracking concentrating solar collector usually has
faces to intercept and focus the solar
receiving area,resulting in an incread heat
modynamic cycle can achieve higher Carnot
ing under higher temperatures.
2.1.Non-concentrating collectors
2.1.1.Flat-plate collectors
Flat-plate solar collectors are usually
哒哒英语收费tion,and therefore need to be oriented
plate solar collector usually consists of
plates,insulation layers,recuperating tubes
ferfluids)and other auxiliaries.Glazing is
ple sheets of glass or other materials with
short-wave radiation and low transmissivity
tion.It not only reduces convection loss from
but also reduces irradiation loss from the
favorable
greenhou effect.Low-iron glass[12,13]is
glazing material due to its relatively high
radiation(approximately0.85–0.87)[13]and
transmittance for the long-wave thermal
公帑
50l m).Hellstrom et al.[14]studied the impact
mal properties on the performance offlat-plate
found that adding a Teflonfilm as cond
performance by5.6%at50°C,whilst installing a to reduce convection loss incread overall performance by12.1%. Further,antireflection treatment of the glazing cover incread the output by6.5%at50°C operating temperature.
The absorber plate is usually coated with blackened surface in order to absorb as much heat as possible;however various colour coatings have also been propod in the literatures[15–17].Desir-ab
le lective surfaces usually consist of a thin upper layer,which is highly absorbent to shortwave solar radiation but relatively transparent to long-wave thermal radiation,and a thin lower layer that has a high reflectance and a low emittance for long-wave radi-ation.Such lective surfaces with a desirable optical performance usually have a high manufacturing cost,but veral low-cost man-ufacturing ideas have also been propod[18].In addition,to fur-ther improve the thermal performance of a collector,heat loss from the absorber also needs to be reduced.Francia[19]found that a honeycomb inrtion,which is made of transparent material and placed in the airspace between the glazing and the absorber,was beneficial to heat loss reduction.
The heat absorbed by the absorber plate needs to be transferred to workingfluids rapidly to prevent system overheating[20].
Fig.1.The Earth’s energy budget([2],from NASA sources).
Fig.2.Schematic of the double-passage solar collector with porous media in cond channel[25].
Ackermann et al.[24]conducted a computational investigation of the effects of internalfins on solar collector panels,concluding that heat transfer performance was incread byfins,and can be even further improved by decreasing thefin pitch and increasing ther-mal conductivities offin materials.The
study conducted by Sopian et al.[25]showed that the inrtion of porous media in the cond channel,as shown in Fig.2,incread the outlet temperature, thereby increasing the thermal efficiency of the systems.In Fig.2,d1is the upper channel depth and d2is the lower channel depth,both of which were varied in their study.Martinopoulos et al.[26]employed polycarbonate honeycombs to enhance heat transfer in solar collectors.Metal foams[27–29],which have high thermal conductivities and large specific surface area,were con-firmed by many rearchers to have abilities to significantly en-hance heat transfer for pha change materials(PCMs).However, as far as the authors are aware,metal foams have not so far been examined for their potential capability to enhance heat transfer in recuperating tubes.
Relevant thermal analys and numerical modelling for solar collectors have also been undertaken.Saha and Mahanta[30] investigated the thermodynamic optimisation offlat-plate solar collectors,with their model focusing on minimising all factors affecting entropy generation.Their study showed that an optimum operating regime existed.Farahat et al.[31]also conducted an optimisation analysis of combined energy and exergy forflat-plate solar collectors.They concluded that exergy efficiency incread when increasing optical efficiency and incident sunlightflux,but it decread rapidly when increasing ambient temperature and wind speed.They also identified an optimum point
forfluid inlet temperature.Further,pipe diameter was found to have only a min-or effect on exergy efficiency.In addition,Selmi et al.[32]simu-lated heat transfer phenomena inflat-plate solar collectors using commercial CFD codes by considering the mixed heat transfer modes of conduction,convection and radiation between tube sur-face,glass cover,side walls and insulating ba of the collector,and their results achieved good agreement with test
data. Fig.3.PV/Tflow-passage models[43].
2.1.2.Hybrid PVT collectors
Hybrid photovoltaic/thermal(PVT)
neously convert solar energy into electricity
PVT collector consists of a PV module with
the range of5%–20%and an absorber plate
removal device)attached on the back of the
removal plate cools the PV module down to a
for better electrical performance,and at the
the waste heat,which can then be utilid
applications,such as domestic hot water
and washing)and adsorption cooling systems
Most of the significant amount of recent
tors has been related toflat-plate collectors,
tion focusing on absorber plate and tube
flow rates[36,37],tank size[38],PV cell
of amorphous silicon[40,41],u of metal
ple-passage configurations[43](shown in Fig.
air collectors[44,45].The u of low
optics with PVT has also received some
shows a comparison between four different PVT collectors(Hegazy
[43]).It was found that under similar operational conditions,the Model I collector had the lowest performance and the Model III collector demanded the least fan power.
Performance comparisons between hybrid PVT collectors and conventional PV-only systems have also been conducted.All the re-sults indicated that hybrid PVT systems can achieve incread en-ergy conversion efficiency with potential cost benefits[48,39,49]. With detailed theoretical models for
PVT collectors being devel-oped,the complicated balance between thermal outputs and elec-trical outputs has been investigated[35,50,37,51].In addition,the exergy analysis of PVT collectors,bad on the cond Law of Ther-modynamics,has been reported by Joshi and Tiwari[52].
2.1.
3.Enhanced hybrid PVT collectors–Bifacial PVT
Hybrid PVT collectors can be classified into tho that u water as the heat removal medium,and tho that u air.Water is a desirable workingfluid in hybrid PVT collectors,becau of its high heat capacity and excellent optical properties.Tina et al.[53]irradiation can be utilid by PV modules to produce electricity. Other rearchers[54–56]have also confirmed such a natural com-patibility of water to PV modules.
Fig.4shows the optical transmission spectrum of a water layer with a thickness of1.5cm[54,55],as well as the absorption spec-trum of a mono-crystalline layer of a PV solar cell with a thickness of50l m[55,56].Fig.4shows that water absorption only slightly affects the working region of a silicon PV cell(water transmissivity decreas at around950nm),but it strongly absorbs the sunlight with the wavelengths above1100nm.Therefore,the combination of a water-filled solar collector with silicon bif
acial PVT hybrid module appears to be very promising.
Robles et al.[55]made a bifacial PV module covered by water, which can absorb long wavelength rays to produce heat and trans-mit short wavelength rays to PV module to produce electricity.The data for short-circuit is shown in Fig.5.The lowest curve repre-nts the short circuit current I sc of the rear face alone at different time in a day;the middle curve reprents I sc for the front panel; the highest curve gives the total I sc for both faces of the PV module. The highest total value of I sc is7.1A,and the corresponding values for the front and real face are5.1A and2A,respectively.They dem-onstrated that the bifacial PV module produced approximately40% more electric energy than a conventional PVT system,with no noticeable increa in the system cost.
weather是什么意思However,the system efficiency in a bifacial PVT module can be further improved if the waste heat can be recovered to produce domestic hot water.To achieve higher efficiency,the optimisation of relevantflow passage design and heat transfer characteristics needs to be studied.The suggestion is that the double-flow passage (e Fig.3)can be ud in the bifacial PV module for further enhancement of the system efficiency.The double-flow passage not only removes excess heat more efficiently,but also saves the pump in the system which gives an even higher electricity output. Another problem for such a water-type PVT system is its difficulty to be ud in extremely cold regio
ns becau freezing can easily break up the collectors[57].A heat pipe-type PVT system was re-cently propod by Pei et al.[57],and it was claimed that their sys-tem allowed heat transport almost without any temperature drop, and that corrosion can also be reduced.
2.2.Concentrating collectors
2.2.1.Heliostatfield collectors
Concentrating collectors(usually equipped with sun-tracking techniques)have much higher concentration ratio than non-concentrating collectors.They can achieve higher temperatures
4.Optical spectra of water and Silicon parameters[55]:(1)transmission characteristics of a water layer with thickness of1.5cm,(2)absorption character-istics of a typical c-Si layer with thickness of50l m.
5.Hourly variations of the short-circuit currents of the PV module:the rear face alone(curve1),the front face(curve2)and the total(curve3)[55].
of working fluids,meaning that it is possible to achieve a higher thermodynamic efficiency.The Heliostat Field Collector,also called the Central Receiver Collector,consists of a number of flat mirrors/heliostats.Due to the position change of the sun during the day,the whole array of mirrors/heli
ostats needs to have preci orientation to reflect incident solar lights to a common tower.The orientation of every individual heliostat is controlled by an automatic control system powered by altazimuth tracking technology.In addition,to place the heliostats with a higher overall optical efficiency,an optimid field layout design is needed.Wei et al.[58]propod a technique which they called ‘YNES’to design the optimid field layout.
An optimid field layout of heliostats can efficiently reflect so-lar light to the central tower,where a steam generator is located to absorb thermal energy and heat up water into the high-temperature and high-pressure steam (to drive turbine generators).The heat transfer fluid inside the steam generator can either be water/steam,liquid sodium,or molten salts (usually sodium nitrates or potassium nitrates),whilst the thermal storage media can be high temperature synthetic oil mixed with crushed rock,molten nitrate salt,or liquid sodium [11,59].
Central tower solar collectors can be classified into external-type and cavity-type,depending on which kind of central receiver is ud.The receiver ud at the Solar One (Barstow,California,USA)is of the external type and as shown in Fig.6a,it is located at the top of the central tower;it compris 24panels (receiver diameter:7m),six of which are for preheating water and eighteen design is shown in Fig.6b.The flux from the heliostat field is re-flected through an aperture (about one third to one half
of the internal absorbing surface area [60])onto the absorbing surfaces which form the walls of the cavity.The aperture size is minimid to reduce convection and radiation loss without blocking out too much of the solar flux arriving at the receiver.
The primary limitation on receiver design is the heat flux that can be absorbed through the receiver surface and transferred into the heat transfer fluid,without overheating the receiver walls and the heat transfer fluid within them.A survey of typical design peak values is given in Table 1[60].The average flux over the entire ab-sorber wall is typically one half to one third of the peak values.Two other important considerations when designing heat flux are (1)limiting the temperature gradients along the receiver panels and (2)the daily heat cycling of the receiver tubes.
Fig.6.Two types of solar towers [60]:(a)external receiver and (b)cavity receiver.
Table 1
Typical design values of receiver peak flux.Heat transfer fluid Configuration Peak flux (MW/m 2)Liquid sodium In tubes
1.50Liquid sodium
In heat pipes (transferring to air) 1.20Molten nitrate salt In tubes 0.70Liquid water In tubes 0.70Steam vapor In tubes
0.50Air
In
metal tubes
0.22
542Y.Tian,C.Y.Zhao /Applied Energy 104(2013)538–553
modularity which can be easily scaled up to meet the power needs in remote area,where centralid power supply is too expensive.
Such parabolic dish-engine technologies have been successfully demonstrated in a number of applications,typical of which was the STEP(The Solar Total Energy Project)project in USA[65].The STEP was a large solar parabolic dish system that operated between 1982and1989in Shenandoah,Georgia,consisting of114dishes (each one being7m in diameter).The system produced
high-pressure steam for electricity generation,medium-pressure steam for knitwear pressing,and low-pressure steam to run the air con-ditioning system for a knitwear factory nearby.
2.2.
3.Parabolic trough collectors
Parabolic trough collectors can concentrate sunlight with a con-centration rate of around40,depending on the trough size.The fo-cal line temperature can be as high as350°C to400°C.The key component of such collectors is a t of parabolic mirrors,each of which has the capability to reflect the sunlight that is parallel to its symmetrical axis to its common focal line.At the focal line, a black metal receiver(covered by a glass tube to reduce heat loss) is placed to absorb collected heat.
Parabolic trough collectors can be orientated either in an east–west direction,tracking the sun from north to south,or a north–south direction,tracking the sun from east to west.An experimental study was performed by Bakos[66]to investigate the effect of the two-axis tracking of parabolic trough on the sun-light collected,and they made a comparison with the ca which ud afixed surface orientation(tilted at40°towards south).Their results indicated that the measured collected solar energ
y on the tracking surface was significantly larger(up to46.46%)compared with thefixed surface.Abdallah[67]experimentally examined the effect of using different types of sun tracking systems on the voltage–current characteristics and electrical power forflat-plate photovoltaics(FPPV),by comparing four types of electromechani-cal sun-tracking systems:two axes,one axis vertical,one axis east–west,and one axis north–south.His results indicated that the volt–ampere characteristics on the tracking surfaces were sig-nificantly greater than that on afixed surface,with the incread electrical power gain up to43.87%,37.53%,34.43%and15.69% for the four types.In addition,Kacira et al.[68]found that the optimum tilt angle varied from13°in summer to61°in winter (experiment location:latitude37°N and longitude38°E).Mondol et al.[69]found that the monthly optimum collection angle for a south-facing surface varied from20°in summer to60°in winter (location:latitude55°N and longitude6°W).
Parabolic trough collectors have multiple distinctive features and advantages over other types of solar systems.Firstly,they are scalable,in that their trough mirror elements can be installed along the common focal line.Secondly,they only need two-dimensional tracking(dish-engine collectors need three-dimensional tracking,making systems more complicated),so they can achieve higher tracking accuracy than dish-engine collectors.
apartfrom
3.Solar thermal energy storage
After the thermal energy is collected by solar collectors,it needs to be efficiently stored when later needed for a relea.Thus,it be-comes of great importance to design an efficient energy storage system.Section3of the prent paper focus on the solar thermal energy storage,discussing its design criteria,desirable materials and emerging technologies for heat transfer enhancement.
3.1.Criteria for design
There are three main aspects that need to be considered in the design of a solar thermal energy storage system:technical proper-ties,cost effectiveness and environmental impact.
Excellent technical properties are the key factors to ensure the technical feasibility of a solar thermal energy storage system. Firstly,a high thermal storage capacity(nsible heat,latent heat or chemical energy)is esntial to reduce the system volume and increa the system efficiency.Secondly,a good heat transfer rate must be maintained between the heat storage material and heat transferfluid,to ensure that thermal energy can be relead/ab-sorbed at the required speed.Thirdly,the storage material needs to have good stability to avoid chemical and mechanical degrada-tion after a certain number of thermal cycles.The other technical properties,such as compatibility and heat loss,are liste
d in Table2.
Cost effectiveness determines the payoff period of the invest-ment,and therefore is very important.The cost of a solar thermal energy storage system mainly consists of three parts[11]:storage material,heat exchanger and land cost.Cost effectiveness is usu-ally connected with the aforementioned technical properties,be-cau high thermal storage capacity and excellent heat transfer performance can significantly reduce the system volume.
Apart from technical properties and cost effectiveness,there are other criteria to be considered,such as operation strategy and inte-gration to a specific power plant,which are listed in Table2.
3.2.Materials
The materials ud for solar thermal energy storage are classi-fied into three main categories according to different storage mechanisms:nsible heat storage,latent heat storage and chem-ical heat storage(with their storage capacity in ascending order). Sensible heat storage is the most developed technology and there are a large number of low-cost materials available[70–72],but it has the lowest storage capacity which significantly increas the system size.Latent heat storage has much higher storage capacity, but poor heat transfer usually accompanies if not employing heat tranmistakes是什么意思
sfer enhancement.Chemical storage has the highest storage capacity,but the following problems restrict its application:com-plicated reactors needed for specific chemical reactions,weak long-term durability(reversibility)and chemical stability.
Table2
Design criteria of a solar thermal energy storage system.
Criteria Influencing factors
Technical criteria 1.High thermal energy storage capacity(the most
important)
2.Efficient heat transfer rate between HTF and storage
material
3.Good mechanical and chemical stability of storage
material
4.Compatibility between HTF,heat exchanger and/or
storage material
5.Complete reversibility of a large number of charging
and discharging cycles
6.Low thermal loss and ea of control
Cost-effectiveness criteria 1.The cost of thermal energy storage materials
2.The cost of the heat exchanger
3.The cost of the space and/or enclosure for the thermal energy storage
Environmental criteria 1.Operation strategy
2.Maximum load
3.Nominal temperature and specific enthalpy drop in
load
4.Integration to the power plant
Y.Tian,C.Y.Zhao/Applied Energy104(2013)538–553543
