pv combi systems

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PV COMBI SYSTEMS AND SPECIAL DESIGNS FOR BUILDINGS

YIANNIS TRIPANAGNOSTOPOULOS

Physics Department

University of Patras

Physics Department, University of Patras, Patra 26500

GREECE

Tel/Fax: +31 2610 997472 e-mail: [email protected]

INTRODUCTION

Various solar energy systems with optimized energy output, cost and aesthetic integration,

have to be applied in buildings and contribute in the energy supply combined withenvironmental protection. Undoubtedly, the most important issue related with energy and

buildings is bioclimatic architecture which plays an important role in obtaining the

optimized physical lighting and the biggest possible limitation of heating needs in the

winter and cooling needs in the summer. The energetically passive function of the building

must be combined with the study of energy systems installation, systems that can cover

hot water and electricity needs and contribute to the other energy needs of the building. As

far as new buildings are concerned, their design must provide them with the possibility of

passive and active energy systems combination, for the avoidance of augmented cost and

wrong integration of the systems in buildings architecture.

Solar energy systems appear to be the most interesting among renewable energy sources

(RES) for the built environment. The facades and the horizontal or inclined roofs of

buildings are appropriate surfaces for the application of solar thermal collectors and

photovoltaic panels for heat and electricity production respectively. Solar thermal

collectors can be considered practical devices to cover thermal energy needs in DHW,

space heating and cooling, etc. Although they can cover satisfactorily the thermal load of

several buildings in countries with favorable weather conditions, they are not widely

applied. Despite of the achieved system cost improvements, the cost payback time of the

solar thermal collectors is not sufficiently low, considering that these systems have low or

no subsidies. Combi-solar thermal systems have been suggested to maximize the use of

thermal output, providing thermal energy for space heating during winter, space cooling

during summer and hot water for domestic use all year round. New types of solar energy

devices that were developed in Solar Energy Laboratory at University of Patras can lead to

improved solar collectors towards to a greater application of solar energy systems.

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2. GENERAL CONSIDERATIONS

Solar Energy Systems can be applied in a very harmonic way on buildings for covering the

needs of heating, cooling, electricity and lighting. The facades and the horizontal or

inclined roofs of houses, hotels, athletic centers and buildings of other various types

constitute appropriate surfaces for an expanded use of photovoltaic panels and thermalsolar collectors. Every type of building can be designed according to the principles of

bioclimatic architecture for the minimization of the energy needs and the environmental

impact of the building. Apart from design, its equally important to use new heat-

insulating materials and special glasses, that reduce effectively thermal losses of buildings

during the winter and the energy consumption for cooling during the summer and that are

already considered as necessary structural materials for the improvement of energy

behavior of buildings, giving at the same time a new visage to them. In such an aspect, the

prospective energy savings in the buildings sector (especially in new buildings) can be

more than 50% of the energy consumption of standard buildings and become a regularprocedure for built environment construction. The installation of devices and active solar

energy units is related with their cost increase and their harmonization with buildings

architecture and the environment. In the Physics Department of University of Patras, some

new types of solar energy systems with efficient energy output were developed, which are

referred to thermal collectors and hybrid photovoltaic/thermal systems. In both systems we

used booster reflectors to increase the energy output from them.

3. BOOSTER REFLECTORS

Buildings that are placed in low latitude locations usually have horizontal roofs where the

installation of units of solar energy utilization, like solar collectors and photovoltaic

panels, is easy. The installation of solar devices on the rooftops of buildings differs from

the installation on the inclined roofs or the facades of buildings because, even if we can

succeed better energy orientation (south), the sun height difference between seasons

determines their placement in a certain distance to avoid shading. These booster reflectors

can contribute to the increase of the thermal energy output, considering a specific

temperature level of collector operation or to increase the temperature level for the

efficient collector operation. This improvement is correlated to the additionally absorbed

solar radiation, substituting partially the thermal losses of the collectors. Theseinstallations can be effectively applied to residential multiflat buildings, office buildings,

hotels, hospitals, athletic centers and industrial buildings. The additional solar input to the

collector aperture surfaces from spring to fall makes the suggested installation suitable for

effective collector operation in higher temperatures, adapting therefore the temperature

level for the cooling requirements. The shading of a series of solar devices reduces the

output of solar thermal collectors but mainly, the output of photovoltaic panels. On

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horizontal roofs, PV have both of their surfaces exposed to the environment, while PV

placed on inclined roofs or facades have only one since their back surface is covered. The

positive result of parallel series of PV is their conservation in a relatively low temperature

that leads to a better electrical output.

Literature for booster reflectors

The combination of flat reflectors to flat plate collectors has been introduced by Shuman

in 1911 [1] to improve the operating temperature of the system. The next work came half a

century later by Tabor in 1966 [2], who studied the use of booster reflectors placed at

collector eastern and western side. Ten years later McDaniels et al. [3] and Seitel [4]

presented calculations for the increase of solar radiation on the aperture of solar collectors

due to booster reflectors, while Bannerot and Howell [5] gave results for grooved

collectors and Grassie and Sheridan [6] published simulation results for performance

improvement by using booster reflectors.

In the next years, Grimmer et al [7], Baker et al [8] and Mannan and Bannerot [9] studied

several parameters on the booster reflector concept. Following them, Rudloff et al [10]

calculated the performance increase of booster reflectors and showed that it corresponds to

the additional cost from the reflectors and Larson [11,12] presented results for the effect of

reflector slope to the achieved concentration. In the same period Taha and Eldighidy [13]

studied the effect of the optimized orientation of solar collectors and the contribution of

the reflectors. In the following years Chiam [14,15] studied the effect of reflectors on the

horizontal collector sides and Jones [16] on the reflectors between parallel rows of solar

collectors. Garg and Hrishikesan [17] presented analytical formulas for the prediction of

reflected solar radiation at three locations and Faiman with Zemel [18] combined flat

reflector with a flat plate thermosiphonic device.

The same period, the experiments of Perers et al [19] and Brunstrom with Karlsson [20]

showed that booster reflectors increase the annual thermal output by 30% (for system

operation at about 50C). In addition, Perers and Karlsson [21] studied 2D and 3D

collector-reflector geometries and their results confirmed that the use of reflectors results

again to a 30% annual performance increase. Narashima Rao et al [22] presented

algorithm for the calculation of the additional solar radiation on collector aperture area.Perers et al [23] gave again theoretical and experimental results for the effective use of

several reflectors. Kumar et al [24] provided results for systems with reflectors mounted

on all sides of flat collectors, while Ballentin and Wilk [25] for the lower horizontal

collector side. Recently, Ronnelid and Karlsson [26] calculated the additional solar

radiation from V-corrugated reflectors, improving the system performance by 10% and

Hussein et al [27] presented similar results regarding several design parameters.

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Experiments with booster reflectors

In our laboratory we have studied the effect of booster reflectors to solar collectors,

starting with the combination of flat reflectors with flat plate thermosiphonic systems [28]

and later to flat plate collectors with black and colored absorbers [29]. Aiming to

determine the effect of stationary reflectors to flat plate collectors, we tested one collectorunit operating at maximum thermal output (ambient temperature level) and a second unit

at stagnation (no fluid flow). We used also two other systems of similar collectors, which

were combined with flat stationary reflectors [30].

Fig. 1 Cross section of the installation of the collectors with the stationary booster

reflector.

The positive effect of flat reflectors of increasing the thermal output was experimentally

determined by using four flat plate collectors properly installed in order to get comparative

results. Two collectors were combined with a stationary flat reflector in front of them to

get additional solar radiation by reflection. The other two of them were of same type andwere operating as reference collectors. The stationary collector-reflector system was tested

daily during the year in low water input temperature and also in stagnation operation. The

results showed that the increase of thermal output is satisfactory from spring to fall in low

temperatures operation and can be significant for operation in high temperatures. The

results show that the booster reflectors can increase the thermal output of solar collectors,

which is low during winter due to the low sun altitude, but sufficiently high the rest

seasons. The cost of the booster reflectors is overcome by the additional amount of the

thermal energy output, resulting therefore to a reduction of the system payback time and

making these systems more attractive for application.

We installed two rows of flat plate collectors on the roof of Physics Department building,

(facing South and the one row behind the other), with the first row of collectors without

and the second row with booster reflector. In Fig.1 we show the cross section of the

installation, presenting the geometry, the incoming and the reflecting solar rays, the slope

of the collector, and of the reflector and the angle between collector and reflector. Inaddition, we include the collector width L, the reflector width R and the distance D

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between the parallel collector rows. The collectors have high emissivity absorber (black

paint, =0.95 and =0.9), one glass cover and thermal insulation. Thermocouples were

placed at several positions (water input and output, four positions on the absorber). We

used a typical glass mirror as flat reflector and during the experimental tests we circulated

cold water in the two collectors, leaving the other two to operate without water flow. Thus,

we achieved the collector operation at low temperature and also at stagnation condition.

We selected collector slope =38.25, equal to the latitude of Patras and reflector slope

=28.3, equal to the maximum sun altitude for Summer solstice at Patras. The collector

width was L=0.6 m, the reflector width R=0.78 m, the angle between the collector and

reflector=113.45 and the distance between the two collectorsD=1.16 m.

We measured water input temperature i (C), water output temperature o (C), ambient

temperature a (C), absorber temperatures S1, S2, S3, S4 (inoC, from the bottom to the

top), solar radiation G (Wm-2), wind speed Vw (ms-1) and mass flow rate m . The four

temperatures of the absorber surface of each collector at absence of water flow were usedto determine the stagnation temperature of the collectors. For the collected data we

calculated the mean daily efficiency d as function of the ratio mm G/T ( mT = miT , -mT , and miT , , mT , , mG , mean daily values). The mean daily performance was

calculated by the relation sdd QQ /= , with dQ the daily thermal output of the collector

and sQ the daily incoming solar radiation on the collector aperture. For the calculation of

d the reflected radiation was not considered in the calculation of sQ , aiming to make

clearer the effect of the booster reflector to the system performance. For the stagnation

operation we have d =0 and the calculation of mm G/T is based on the formula

mamSm TTT ,, = with 44321 /,,,,, mSmSmSmSmS TTTTT +++= . The pairs of d and mm G/T

were used to define the linear correlation of the collector daily performance.

In Fig.2 we present the diagrams [30] of the mean daily performance of the tested flat

collectors for the winter (14 Dec), Spring (16 Mar), Summer (14 Jul) and Fall (18 Oct).

These results contribute to a maximum ( mm G/T = 0 KW-1m2) increase of 24% for Spring,

38% for Summer, 31% for Fall and 11% for Winter, resulting to a 25% mean annual

value. For operation in higher temperatures ( iT >40C) the improvement is significant,

confirming the results of other authors [19-21]. For operation at 050./ = mm GT KW-1,

which could correspond to iT =50C, T =25 C and mG =500 Wm-2 1 (dotted lines in

Fig.2) the performance increase is much more higher, being above 30% for winter, 80%for Spring, 50% for Summer and 75% for Fall (60% as mean annual increase). These

results show the positive effect of booster reflectors and considering an additional cost of

about 20% for the reflectors, we have a net performance gain of 40%.

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WINTER

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FLAT + REF.

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SUMMER

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AUTUMN

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FLAT + REF.

Fig.2 Experimental results of mean daily efficiency of the studied systems (FLAT and

FLAT+REF) for the four seasons [30].

The results show that the combination of flat plate collectors with booster reflectors is

satisfactory regarding thermal output, increasing by ~25% the annual yield for system

operation at ambient temperature level and doubling it (~50%) for operation at about 50oC. This performance is due to the fact that the increase of solar input on collector aperture

provides an additional energy amount that can replace the thermal losses from the

collector to the ambient, mainly when the system is operating at higher temperatures.

Considering the stationary collector-reflector installation, the contribution of booster

reflector is low during winter because of the low sun altitude, but it is high during the restseasons.

We estimate that the addition of booster reflectors to solar collector installations increase

system cost by about 20%, in case of typical collectors, which is relatively lower if

improved performance collectors (selective absorber, double glazing, TIM thermal

protection, vacuum tubes, CPC type collectors, etc) are considered. This additional cost is

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not high and can be overcome by the increase of thermal output for system operation

higher than 10 oC above ambient. The performance improvement is significant enough for

higher operating temperature of the collector, as for 50 oC or higher, contributing therefore

to become more attractive and cost effective for several solar energy applications (space

heating and cooling, desalination, industrial heat, etc). Finally, the application of booster

reflectors can be considered interesting for the buildings with horizontal roof, mainly in

southern countries, as the residential multiflat buildings, hotels, hospitals, athletic centers,

industries, etc.

References for booster reflectors

[1] Meinel .. and Meinel M.P. Applied Solar Energy. AddisonWesley Publ. Co

(1997).

[2] Tabor H. Mirror boosters for solar collectors. Solar Energy 10, pp. 111-118 (1966).

[3] McDaniels D.K., Lowndes D.H., Mathew H., Reynolds J. and Grey R. Enhancedsolar energy collection using reflector-solar thermal collector combinations. Solar

Energy 17, pp. 277-283 (1975).

[4] Seitel S. Collector performance enhancement with flat reflectors. Solar Energy 17,

pp. 291-295 (1975).

[5] Bannerot R.B. and Howell J.R. Moderately concentrating flat plate solar energy

collectors. ASME 75, pp. 2-11 (1975).

[6] Grassie S.L. and Sheridan N.R. The use of planar reflectors for increasing the energy

yield of flat plate collectors. Solar Energy 19, pp. 663-668 (1977).

[7] Grimmer D.P., Zinn K.G., Herr K.C. and Wood B.E. Augmented solar energy

collection using different types of planar reflective surfaces. Solar Energy 31, pp.

497-501 (1978).

[8] Baker S. McDaniels D.K., Kaehn H.D. and Lowndes D.H. Time integrated

calculation of the insolation collected by reflector-collector system. Solar Energy 20,

pp. 415-417 (1978

[9] Mannan K.D and Bannerot R.B Optimal geometries for one and two faced symmetric

side wall booster mirrors. Solar Energy 21, pp. 385-391 (1978).

[10] Rudloff F.A., Swanson S.R., Boehm R.F. Computer simulation results for planar

reflectors and flat plate solar collectors. ASME 79 WA/Sol - 37, pp. 1-8 (1979).

[11] Larson D.C. Concentration ratios for flat plate solar collectors with adjustable flatmirrors. Energy 4, pp. 170-175 (1980).

[12] Larson D.C. Optimization of flat plate collector-flat mirror systems. Solar Energy 24,

pp. 203-207 (1980).

[13] Taha I.S. and Eldighidy S.M. Effect of off-south orientation on optimum conditions

for maximum solar energy absorbed by flat plate collector augmented by plane

reflector. Solar Energy 25, pp. 373-379 (1980).

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[14] Chiam H.F. Planar concentrators for flat plate collectors. Solar Energy 26, pp. 503-

509 (1981).

[15] Chiam H.F. Stationary reflector-augmented flat plate collectors. Solar Energy 29, pp.

65-69 (1982).

[16] Jones JR R.E. Radiation on reflector augmented flat plate collectors. Solar Energy 23,

pp. 527-531 (1984).

[17] Garg H.P. and Hrishikesan D.S. Enhancement of solar energy on flat plate collector

by plane booster mirrors. Solar Energy 40, pp. 295-307 (1988).

[18] Faiman D. and Zemel A. Low profile solar water heaters: the mirror booster problem

revisited. Solar Energy 40, pp. 385-390 (1988).

[19] Perers B., Halletun H., Karlsson B. and Brunstrom C. Field testing of high

performance flat plate collectors with external reflectors. Proc. Int. Conference

NORTHSUN 88, pp. 445-450 (1988).

[20] Brunstrom C. and Karlsson B. External reflectors for large area flat plate collectors.

Proc. Int. Conf. NORTHSUN 88, pp. 363-367 (1988).[21] Perers B., Karlsson B. External reflectors for large solar collector arrays, simulation

model and experimental results. Solar Energy 51, pp. 327-337 (1993).

[22] Narashima Rao A.V., Chalam R.V., Subramanyam S. and Sithararama Rao T.L.

Energy contribution by booster mirrors. Energy Convers. Mgmnt 34, pp. 309-326

(1993).

[23] Perers B., Karlsson B.and Bergkvist M. Intensity distribution in the collector plane

from structured booster reflectors with rolling grooves and corrugations. Solar

Energy 53, pp 215-226 (1994).

[24] Kumar R., Kaushik S.C. and Garg H.P. Analytical study of collector solar gain

enhancement by multiple reflections. Energy 20, pp 511-522 (1995).

[25] Bollentin J.W. and Wilk R.D. Modeling the solar irradiation on flat plate collectors

augmented with planar reflectors. Solar Energy 55, pp 343-354 (1995).

[26] Ronnelid M. and Karlsson B. The use of corrugated booster reflectors for solar

collector fields. Solar Energy 65, pp 343-351 (1999).

[27] Hussein H.M.S., Ahmad G.E., Mohamad M.A. Optimization of operational and

design parameters of plane reflector tilted flat plate solar collector systems. Energy

25, pp 529-542 (2000).

[28] Tripanagnostopoulos Y. and Yianoulis P. Solar water heaters with booster mirrors.

Proc. Int. Conference WREC III, pp. 1908-1910 (1994).[29] Tripanagnostopoulos Y., Souliotis M. and Nousia Th. Solar collectors with colored

absorbers. Solar Energy 68, pp. 343-356, (2000).

[30] Tripanagnostopoulos Y. and Souliotis M. Booster reflector contribution to

performance improvement of solar collectors. Proc (CD-ROM) WREC2005 ,

Aberdeen 22-27 May (2005).

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4 SOLAR COLLECTORS WITH COLORED ABSORBERS

The issue of aesthetic integration of solar collectors in building architecture and the

environment is important and constitutes a reason for the limited application of these

devices in the built environment. In an extended use of solar energy, the majority of the

exterior surfaces of buildings will be covered with absorbing surfaces of solar collectorsand their color (the monotony of black color) is a basic factor that has to be taken under

consideration, especially in the case of traditional communities. As an alternative solution

and instead of black color, collectors with different colors have been proposed (blue, red-

brown, green etc) and although their absorption ability is a little lower (20%), their use

offers an interesting colorful aspect to the exterior of buildings (Tripanagnostopoulos et al,

2000, Kalogirou et al, 2005) Colored solar absorbers can contribute in the expansion of

solar energy systems utilization and sensitize or even motivate architectures to include

these systems in their designs. Although the developed in our laboratory solar collectors

with colored absorber are obviously of lower efficiency than the collectors with blackabsorbers, they could be considered suitable for aesthetically sensitive applications,

providing interesting solutions to the architects to overcome the monotony of black

collectors.

Study of colored collectors

Flat plate collectors with black, blue and red-brown absorber were constructed and tested

in our laboratory and theoretical results regarding colored collectors with absorbers of

variable absorptivity and emissivity showed the effective use of booster reflectors to the

increase of thermal efficiency. In the second work, additionally to the collectors of typicalform, unglazed collectors with colored absorber were also studied giving interesting

results. Flat plate unglazed colored collectors are cheaper than typical glazed collectors,

but the increased absorber thermal losses limit their effective use in low temperature

applications, as water preheating for domestic or industrial use, water heating of

swimming pools, space heating, etc.

In Fig. 3 the yearly efficiency of typical form flat plate solar collectors using glazing and

thermal insulation (GL/INSUL) and of flat plate unglazed solar collectors with back

thermal insulation (UNGL/INSUL) are presented. These results are calculated for colouredcollectors with construction same to that of the experimental models of flat collectors that

were tested in our laboratory, regarding collector inclination = 40 and operation in

Patras (latitude 38N). The yearly efficiencies are given for collectors with absorbers of

absorptivity = 0.95, = 0.85 and = 0.75 and being all of emissivity = 0.9, asfunction of the water input temperature iT .

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The comparison of the yearly efficiency of typical form collectors (GL/INSUL) shows that

for a usual input temperature, as it is iT = 40C, the colored collectors with =0.85 (dark

colored absorbers) and =0.75 (light colored absorbers) have a 20% and 40% lower

yearly efficiency, respectively to that of the collector with black absorber (=0.95).

Fig. 3 Yearly efficiency of colored glazed and unglazed solar collectors

For the unglazed type collectors (UNGL/INSUL) and for a temperature 20C CTi 30

the colored collectors have about 15% (dark color = 0.85) and 25% (light color, =0.75) lower yearly efficiency compared to that of the unglazed black collector (= 0.95).

In addition, for low values of iT the unglazed collectors have a higher yearly efficiency

compared to glazed collectors of the same value of .

In Fig.4 the calculated monthly efficiencies of the above collector types, for inclination

= 40 and operation in Patras considering input water temperature iT = 20C, are

presented. From these results we can see that the unglazed collectors are of higher

efficiency than typical form collectors during the period from April to October, due to the

lower optical losses from the absence of glazing and the positive convention effect from

the ambient air to the absorber surface. Considering the effect of using colored absorbers

instead of black absorber, dark tone absorber (= 0.85) gives satisfactory results in both

glazed (GL/INSUL) and unglazed (UNGL/INSUL) solar collectors, for all months.

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Fig. 4 Monthly efficiency of colored glazed and unglazed solar collectors

Regarding cost, we estimate that for applications with priority in aesthetic view, colored

collectors could be more interesting and an additional cost of about 20% (to increase the

collector area) can overcome their lower thermal output compared to same type collectors

with black absorber. The use of blue collectors in white buildings close to the sea, of red

brown collectors for buildings with inclined roof and traditional architecture and of other

colored collectors (green, yellow, violet, mixed colors, etc) on modern buildings could

change the view of them, resulting to a wider use of solar thermal collectors.

Regarding optical annoyance from reflected light, the use of glass coatings that diffuse

reflected light is the most appropriate solution, while we can consider worthy the use of

uncovered (UNGLAZED) collectors for low temperature applications (swimming pools,

preheating of water, etc). In this perspective, uncovered collectors can be combined with

colored surface for an even more interesting integration of solar collectors in facades and

inclined roofs of buildings. In the horizontal roof applications, it is effective to use flat

booster reflectors between the rows of collectors with colored absorbers and increase the

thermal output of them (Tripanagnostopoulos et al, 2000,).

References for colored collectors

Tripanagnostopoulos Y., Souliotis M. and Nousia Th. Solar collectors with colored

absorbers. Solar Energy 68, pp. 343-356, (2000).

Kalogirou S, Tripanagnostopoulos Y., Souliotis M. Performance of solar systems

employing collectors with colored absorber. Energy and Building 37, 824-835, (2005).

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5 HYBRID PHOTOVOLTAIC/THERMAL SOLAR SYSTEMS

Photovoltaic (PV) convert a small percentage of solar radiation into electricity, 5%-15%

depending on the type of PV, with the greater percentage converted into heat. The solar

radiation increases the temperature of PV modules, resulting in a drop of their electrical

efficiency, but their installation on horizontal roofs of buildings permits their naturalcooling. In the facades and inclined roofs, cooling of PV rear surface is under research,

which will also have positive result in protecting building overheating during summer.

The temperature of PV modules increases by the absorbed solar radiation that is not

converted into electricity causing a decrease in their efficiency. This undesirable effect can

be partially avoided by heat extraction with a fluid circulation. In hybrid PV/T solar

systems the reduction of PV module temperature can be combined with a useful fluid

heating. Hybrid PV/T systems can provide electrical and thermal energy, thus achieving a

higher energy conversion rate of the absorbed solar radiation. They consist of PV modulescoupled to heat extraction devices, in which air or water of lower temperature than that of

PV modules is heated whilst at the same time the PV module temperature is reduced.

In PV/T system applications the production of electricity is the main priority, therefore it

is necessary to operate the PV modules at low temperature in order to keep PV cell

electrical efficiency at a sufficient level. This requirement limits the effective operation

range of the PV/T thermal unit for low temperatures, thus, the extracted heat can be used

mainly for low temperature applications such as space heating and natural ventilation of

buildings, and air or water preheating. Air-cooled PV/T systems have been recently

applied in buildings, integrated usually on their inclined roofs or faades. By these systemsbuilding space heating needs during winter can be covered and also building overheating

during summer is avoided. Water-cooled PV/T systems are practical systems for water

heating, but they are not yet improved enough for widely commercial applications.

Natural or forced air circulation is a simple and low cost method to remove heat from PV

modules, but it is less effective at low latitudes where ambient air temperature is over 20oC

for many months during the year. In BIPV applications, unless special precautions are

taken, the increase of PV module temperature can result to the reduction of PV efficiency

and to the increase of undesirable heat transfer to the building, mainly during summer. Inair cooled hybrid PV/T systems an air channel is usually mounted at the back of the PV

modules. Air of lower temperature than that of PV modules, usually ambient air, is

circulating in the channel and thus both PV cooling and thermal energy collection can be

achieved. Therefore, by air cooling the PV electrical efficiency is kept at a sufficient level

and the thermal energy collected can be used for the building thermal needs.

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Regarding water heat extraction, the water can circulate through pipes in contact with a

flat sheet, placed in thermal contact with the PV module rear surface. In PV/T systems the

thermal unit for air or water heat extraction, the necessary fan or pump and the external

ducts or pipes for fluid circulation constitute the complete system. To increase the system

operating temperature, an additional glazing is used, but this results in a decrease of the

PV module electrical output because an amount of solar radiation is reflected away,

depending on the angle of incidence.

Hybrid PV/T systems can be applied mainly in buildings for the production of electricity

and heat and are suitable for PV applications under high values of solar radiation and

ambient temperature. In these devices, water or air is circulated in thermal contact with the

PV, exchanging heat (Fig. 5). When air is used, the contact with PV panels is direct, while

in the case of use of fluids, the contact is made through a heat exchanger. These devices

are not yet applied in an international scale and their applications until now are mostly for

demonstration reasons and mainly have to do with air hybrid PV/T on the facades ofbuildings.

Fig. 5 Cross section of the basic PV/T experimental models for water and air heating,

without and with extra transparent cover. a. PV/WATER type,

b. PV/WATER + GL type, c. PV/AIR type, d. PV/AIR + GL type

To the direction of developing hybrid PV/T systems, experimental models have beenconstructed, using water or air as the heat removal fluid (Tripanagnostopoulos et al, 2000,

2001a, 2001b, 2002a, 2002b). Hybrid PV/T using air can be applied in our country for

space heating of building during winter and for cooling during summer by the creation of

strong upward air stream. Water hybrid PV/T can be used in our country during the whole

year, for the pre-heating of water, since the temperature of water in the water supply

network is up to 20 C, even during summer months.

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Hybrid PV/T systems are appropriate for installation in buildings with thermal and

electricity needs, like houses, apartment buildings, hotels, hospitals, athletic centers and

industries. They can be placed on the facades, inclined or horizontal roofs of buildings -

instead of separate PV panels and solar thermal collectors - in a more practical utilization

of the existing surfaces.

Literature for PV/T systems

Theoretical and experimental studies are referred to hybrid PV/T systems with air or/and

water heat extraction from the PV modules. Kern and Russell (1978) present the design

and performance of water and air cooled PV/T systems, while Hendrie (1979) and

Florschuetz (1979) include PV/T modeling in their works. Numerical methods predicting

PV/T system performance are developed by Raghuraman (1981), computer simulations

are studied by Cox and Raghuraman (1985), a low cost PV/T system with transparent type

a-Si cells is proposed by Lalovic et al (1986-87) and results from an applied air type PV/T

system are given by Loferski et al (1988). Next years, Bhargava et al (1991), Prakash

(1994), Garg and Agarwal (1995) present same aspects of a water type PV/T system and

Sopian et al (1996) and Garg and Adhikari (1997) present a plenty of results regarding the

effect of design and operation parameters on the performance of air type PV/T systems.

Because of their easier construction and operation, hybrid PV/T systems with air heat

extraction are more extensively studied, mainly as an alternative and cost effective

solution to building integrated PV systems (BIPV). Following the above referred studies,

test results from PV/T systems with improved air heat extraction are given by Ricaud andRoubeau (1994) and from roof integrated air-cooled PV modules by Yang et al (1994).

Regarding building integrated PV/T systems Posnansky et al (1994), Ossenbrink et al

(1994) and Moshfegh et al (1995) include in their works considerations and results on

these systems. Later, Brinkworth et al (1997), Moshfegh and Sandberg (1998), Schroer et

al (1998), Brinkworth (2000), and also Brinkworth et al (2000) present design and

performance studies regarding air type building integrated hybrid PV/T systems. We also

could refer the work of Eicker et al (2000) who give monitoring results from a BIPV PV/T

system that operates during winter for space heating and during summer for active cooling

and of Bazilian et al (2001a), who evaluate the practical use of several PV/T systems with

air heat extraction in the built environment. The building integrated photovoltaics is going

to be a sector of a wider PV module application and the works of Hegazy (1999), Lee et al

(2001), Bazilian et al (2001b), Chow (2003) and Ito with Miura (2003) give interesting

modeling results on air cooled PV modules.

Water heat extraction is more expensive than air, but as water from mains remains usually

under 20C in low latitude countries, which have high air temperatures during summer,

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the water heating can be used during all seasons. The liquid type hybrid PV/T systems are

less studied than air type systems and the works that follow the first period of PV/T

system development are the study of Bergene and Lovvik (1995) who give a detailed

analysis on liquid type PV/T systems, of Hausler and Rogash (2000) with the study of a

latent heat storage PV/T system, Elazari (1998), presenting the performance and economic

aspects of a commercial type PV/T water heater and Kalogirou (2001), studying a PV/T

water heater for the conditions of Cyprus. Later, Huang et al (2001) present an integrated

PV/T system with hot water storage and Sandness and Rekstad (2002) give results for

PV/T collectors with polymer absorber. Finally, the dynamic 3D and steady 3D, 2D and

1D models for PV/T prototypes with water heat extraction by Zondag et al (2002), the

systems with water circulation in channels attached to PV modules, also by Zondag et al

(2003), the modelling results by Chow (2003) and the study on domestic PV/T systems by

Coventry and Lovegrove (2003), are some of the recent works on water cooled PV/T

systems.

The electrical and thermal output of hybrid PV/T systems can be increased by using

reflectors of low concentration, either of flat type as presented by Sharan et al (1985), Al-

Baali (1986), and Garg et al (1991) or of CPC type as proposed by Garg and Adhikari

(1999), Brogren et al (2000), Karlsson et al (2001) and Brogren et al (2002). Economic

aspects on PV/T systems are given by Leenders et al (2000), while consideration of the

environmental impact of PV modules by using Life Cycle Assessment (LCA)

methodology has been been extensively used at University of Rome La Sapienza. Apart

of their studies on photovoltaic systems, Frankl et al (2000) presented LCA results on the

comparison of PV/T systems with standard PV and thermal systems, thus confirming the

environmental advantage of PV/T systems.

Study of PV/T systems

Design and performance improvements of hybrid PV/T systems with water or air as heat

removal fluid, have been carried out at the Physics Department of the University of Patras,

Greece . The investigated models include a number of modifications that contribute to the

increase of thermal efficiency, to the decrease of PV module temperature and to the

improvement of the total energy output of the PV/T system. Design concepts, prototype

construction and test results for water and air-cooled PV/T systems (Tripanagnostopouloset al 2002a) are referred to PV/T systems with and without additional glass cover. A PV-

ICS system to heat and store water (1998) and the dual type PV/T system operating either

with water or air heat extraction (2001a), have been suggested as alternative PV/T

systems. Regarding air type PV/T systems, experimental results with modified designs are

presented in several works (2000, 2001b, 2002b). Also, results from a life cycle analysis

are given (2003), where water cooled PV/T systems are compared with standard PV

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modules and give an idea about the environmental impact of the studied systems. In

addition, economic aspects and performance results for water-cooled PV/T systems that

could be applied in houses, multiflat residential buildings, hotels, etc, are included in the

work of Tselepis and Tripanagnostopoulos (2002).

A low cost modification was investigated in University of Patras, by which satisfactory air

heating, reduced PV module temperature and low increase of the opposite channel wall

temperature can be achieved (Tripanagnostopoulos et al, 2000). This modification consists

of a thin flat metallic sheet (TFMS) placed inside the system air channel and along the air

flow, which doubles the heat exchanging surface area and reduces the heat transfer to the

back side of the PV/T system. Considering PV/T solar systems installed on horizontal

building roof, the parallel rows keep a distance from one to the other in order to avoid PV

module shading. University of Patras investigated the use of stationary flat diffuse

reflectors (Fig.6) placed between the PV modules from the higher part of the one row of

them to the lower part of the next row (2001a, 2002a). This installation increases solarinput on PV modules almost all year, resulting to an increase of electrical and thermal

output of the PV/T systems. The diffuse reflectors differ from the specular reflectors, as

they avoid the illumination differences on module surface and the reduction of the

electrical efficiency, because they provide an almost uniform distribution of reflected solar

radiation on PV module surface.

Fig. 6 PV/T systems with booster diffuse reflectors:

a. Horizontal building roof system installation

b. PV/T + REF experimental system with indication of diffuse reflected solar rays

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We tested the PV/T models regarding the effective electrical efficiency el of it as a

function of concentration ratio .C Actually the obtained value of el is rather the system

performance than the system efficiency. In the steady state tests of PV/T systems with

booster diffuse reflector we can use in all experiments the proper collector - reflector

geometry to achieve the same effective concentration factor tC with homogeneous

illuminance of PV module surface (we take tCC= ). The angle between the reflector and

the PV module plane and the diffuse reflector must be adjusted in order to measure the

total incoming solar radiation that gives a value of C (for homogeneous additional solar

radiation) of about 1.35, for a net solar radiation intensity G 850 Wm-2 on the plane of

the PV module. The value C 1.35 can be considered as an approximation of a meanvalue during Summer, when the effect of the diffuse reflector on thermal and electrical

performance of the PV/T system is more important and used in following tests.

Steady state experiments were performed during noon ( 2 hours), with systems oriented

to the sun in order to ensure constant value of the incoming solar radiation intensity and

using in calculations the collected data extracted from the outdoor tests under almost

constant conditions. Solar radiation intensity variation must not exceed 20 Wm-2, with

diffuse solar radiation up to about 25% of the total incident solar radiation, ambient air

temperature variation 1 K and wind speed variation 0.5 ms-1 in the range of 1-2 ms-1.

The other parameters are the incoming solar radiation G on aperture area, the aperture

area A of the tested systems, the fluid mass flow rate dt/dmm = (0.02 Kgs-1), the fluid

temperature rise ( io TT ) and the fluid specific heat pc (4180 Jkg-1K-1 for water, 1007

Jkg-1K-1 for air). The steady state thermal efficiency th of the hybrid PV/T systems iscalculated by the relation: GA/)TT(cm iopth = .

The variation of thermal efficiency th relative to the fluid input temperature iT , the

ambient temperature T and the incoming solar radiation G is determinedexperimentally as a function of the ratio GT / with TTT i . The function

)/( GTfth = is used for the performance determination of thermal collectors. It can bealso used for the hybrid PV/T systems, as the thermal part of them corresponds

approximately to a thermal collector. In stagnation operation ( 0th )we used the water

heat exchanger temperature (wheT

) or air channel temperature (airT

), as the systems wereoperating without fluid flow. These temperatures were used to determine the

corresponding ratio GT / , with awhe TTT or aair TTT . During the tests the PV

electrical output was connected to a load, simulating real system operation. With the tested

PV/T systems at thermal equilibrium under ambient conditions, we determine the values

of current mI (in A) and voltage mV (in V) at maximum power point of PV module

operation from the collected I (in A), V (in V) data. The values of mI and mV and the

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net incoming solar radiation G are used to find the PV module electrical efficiency el

for system aperture area aA using the relation: GAVI mmel / . In hybrid PV/T systemswe can consider the total efficiency tot , which corresponds to the sum of the electrical

efficiency el and the thermal efficiency th of the system, for certain operating

conditions.

Studying the effect of booster diffuse reflectors on the hybrid experimental models we

calculated thermal and electrical efficiency from the net incoming solar radiation G on

the PV module surface (without the additional solar input from the diffuse reflector). In

concentrating solar devices the concentration ratio C is determined by the ratio of the

system aperture area to the absorber surface area. In our work the calculation of thermal (

th ) and electrical ( el ) efficiencies is based on G and not on the amount GC , in order

to get the effective values of the corresponding efficiencies, considering that the

additional solar input from the reflector affects th and el rising their values by the

increase of fluid output temperature oT (or wheT , airT ) and mI , mV respectively. Theincrease of system electrical and thermal output (included in the effective values of th

and el ), provides a clearer comparison of the results from the tested systems with and

without diffuse reflectors.

The study of the experimental PV/T model (PV/WATER) with diffuse reflector includes

tests with variable percentage of the additional solar radiation from the diffuse reflector,

with respect to its electrical efficiency as a function of the pc-Si PV module temperature.

We consider a concentration factor C that corresponds to a homogeneous increase of the

incoming solar radiation on PV module surface, with value C=1 for net solar input

(without additional solar radiation from the diffuse reflector) and C=1.1 for an effective

10% additional solar input, etc, up to C=1.5 for 50 % additional solar input on PV

module. Higher values of factorC are not usually achieved in practice, because of the use

of diffuse reflector in the collector - reflector geometry of Fig.6.

Thermal performance of hybrid PV/T systems with booster diffuse reflector depends on

the fluid input temperature, ambient temperature, incident net solar radiation and the

concentration factorC . The aim of this work is to give comparative results, so the hybrid

PV/T systems were tested under almost same incident net solar radiation G and

concentration factorC , in a small range of ambient temperatures T and wind speed wV. In practice the additional solar radiation is not uniformly distributed on PV module

surface and we consider an effective concentration factor tC that corresponds to solar

radiation profile from bottom to top of PV module surface. We can calculate el for0.1C to C=1.5 (with step 0.1). In Fig. 7 the results from these tests, using fitting lines

for the values of el of the corresponding concentration factorC are presented. These

results correspond to PV module of pc-Si type. The solar radiation from the diffuse

18


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reflector affects the PV module electrical performance but in calculations we use the net

solar radiation on PV module (effective eln ) to get the electrical output from the

proposed combination in comparison with that without using diffuse reflector (results for1C ).

0.09

0.10

0.11

0.12

0.13

0.14

0.15

0.16

0.17

0.18

35 40 45 50 55 60 65 70

PV Temperature (C)

E

lectrical

efficiency

el

C=1.0 C=1.1

C=1.2 C=1.3

C=1.4 C=1.5

Fig. 7 Results of PV/T system electrical efficiency el for the diffuse reflector

concentration factors 1=C to =C 1.5 (step 0.1) and for variable pc-PV module

operating temperature.

The tests can be performed with concentration factor up to =C 1.5, where the obtainedmaximum value was an upper limit for the used type of diffuse reflector. The results of

Fig. 7 show that an electrical efficiency increase of about 25% for PV operating

temperature 40pvT C to about 35% for 70pvT C is achieved, comparing the effect of

using stationary booster diffuse reflector with its practical possible maximum value of

concentration factor ( =C 1.5) to the plain PV system ( =C 1.0). From the results of Fig. 7

we also calculate a mean electrical efficiency drop of 0.08 % per K of PV temperature

increase. In Figs 8 and 9 the thermal efficiency results and in Figs 9 and 10 the electrical

efficiency results give an idea about the performance of the studied hybrid pc-Si PV/T

systems with water or air heat extraction and regarding the combination with additionalgrazing or/and diffuse reflector.

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0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

-0.02 -0.01 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10

/ G(KW

-1m

2)

Thermalefficiencyn

th

PV / WATER

PV / WATER +GL

PV / WATER +REF

PV / WATER +GL +REF

Fig. 8 Thermal efficiency th results of systems PV/WATER, PV/WATER + GL,

PV/WATER + REF and PV/WATER + GL + REF, as function of GT/ operating

values.

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0.9

1.0

0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10

/ G(KW

-1m

2)

Thermalefficiency

th

PV / AIR

PV / AIR +GL

PV / AIR +REF

PV / AIR +GL +REF

Fig.9 Thermal efficiency th results of systems PV/AIR, PV/AIR + GL, PV/AIR + REF

and PV/AIR + GL + REF, as function of GT/ operating values.

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0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

-0.02 -0.01 0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10

/ G (KW-1m2)

Electrical

efficie

ncy

el

PV / WATER

PV / WATER +GL

PV / WATER +REF

PV / WATER +GL +REF

Fig.10 Electrical efficiency el results of systems PV/WATER, PV/WATER + GL,

PV/WATER + REF and PV/WATER + GL + REF, for the corresponding GT/

values of Fig. 9 experiments.

0.00

0.02

0.04

0.06

0.08

0.10

0.12

0.14

0.16

0.18

0.00 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.10

T / G(KW

-1m

2)

Electricalefficie

ncy

el

PV / AIR

PV / AIR +GL

PV / AIR +REF

PV / AIR +GL +REF

Fig. 11 Electrical efficiency el results of systems PV/AIR, PV/AIR + GL, PV/AIR +

REF and PV/AIR + GL + REF, for the corresponding GT / values of Fig.

11 experiments.

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The use of PV/T systems with additional glazing is interesting mainly for the increase of

system thermal output, because the PV electrical efficiency is reduced or remains the same

in case of using both glazing and booster diffuse reflector. The use of stationary booster

diffuse reflectors with PV/T systems on horizontal building roof installations is an

effective application. This combination increases PV/T system cost by 4%-5%, but results

to an improvement of the electrical output (for =C 1.35) by 15%-16%. The results depend

on the location of the installation and the achieved concentration factor during winter (for

Patras) is low (


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