Scholarly article on topic 'A review of PV/T technologies: Effects of control parameters'

A review of PV/T technologies: Effects of control parameters Academic research paper on "Materials engineering"

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Abstract of research paper on Materials engineering, author of scientific article — Kamran Moradi, M. Ali Ebadian, Cheng-Xian Lin

Abstract Both air and water cooled PV/T collectors have enjoyed growing attentions in recent years. Investigators have reported PV/T research data within a wide range of control parameters. In this paper, the effects of the major control parameters on the thermal/electrical performance of PV/T collectors are compiled and reviewed. Figures and tables are provided to give an overall picture about how PV/T performance could be improved in terms of these parameters. Although investigators understand the effects of different parameters, the improvement of PV/T performance by optimizing these parameters has not been fully realized.

Academic research paper on topic "A review of PV/T technologies: Effects of control parameters"

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International Journal of Heat and Mass Transfer

journal homepage: www.elsevier.com/locate/ijhmt

A review of PV/T technologies: Effects of control parameters

i CrossMark

Kamran Moradi, M. Ali Ebadian *, Cheng-Xian Lin

Department of Mechanical and Materials Engineering, 10555 W. Flagler Street, EC 3444, Florida International University, Miami, FL 33174, USA

ARTICLE INFO

Article history: Received 10 April 2013 Accepted 16 April 2013 Available online 25 May 2013

Keywords:

Photovoltaic/thermal PV/T

Hybrid collectors Thermal efficiency Electrical efficiency

ABSTRACT

Both air and water cooled PV/T collectors have enjoyed growing attentions in recent years. Investigators have reported PV/T research data within a wide range of control parameters. In this paper, the effects of the major control parameters on the thermal/electrical performance of PV/T collectors are compiled and reviewed. Figures and tables are provided to give an overall picture about how PV/T performance could be improved in terms of these parameters. Although investigators understand the effects of different parameters, the improvement of PV/T performance by optimizing these parameters has not been fully realized.

© 2013 Elsevier Ltd. All rights reserved.

1. Introduction

Solar energy is usually utilized after it is converted into certain energy forms suitable for certain applications. These forms include thermal energy and electrical energy, which can be produced by photovoltaic (PV) systems. In a hybrid system, such as the photovoltaic/thermal (PV/T) energy system [1], both thermal energy and photovoltaic solar energy (electrical) are produced at the same time. This is achieved when PV panel or laminate, which convert solar radiation into electricity, also functions as the absorber of a thermal collector. The materials used for PV cells are mostly very sensitive to temperature. If the temperature increases, the electrical efficiency will drop. However, if the thermal energy that causes the increment in temperature in solar cells is removed and used in a proper way, it prevents the temperature increase in PV cells (as they are cooled), and increases the overall efficiency of the system at the same time.

During the last few years, PV/T technology has received great attentions worldwide. In the reported PV/T systems, both air and water have been used in the thermal part of the system. Highly specialized solar cells have evolved that capture the light and transform it to electricity current effectively. A number of design principles have been developed. Many new features, both in structure and energy conversion processes, have been invented. The general progress in this field has been reviewed by several

q This is an open-access article distributed under the terms of the Creative Commons Attribution-NonCommercial-No Derivative Works License, which permits non-commercial use, distribution, and reproduction in any medium, provided the original author and source are credited.

* Corresponding author. Tel.: +786 325 4577. E-mail address: ebadian@fiu.edu (M. Ali Ebadian).

0017-9310/$ - see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.ijheatmasstransfer.2013.04.044

investigators, including Charalambous et al. [2], Xondag [3], Hasan and Sumathy [4], Chaar et al. [5], Tiwari et al. [6], Tyagi et al. [7].

In this paper, we reviewed the recent development in the PV/T technologies, with an emphasis on the encountered thermal/fluid management and combined thermal-electrical issues, particularly the efficiencies, which have been confirmed by various investigators. Some common control parameters from different original research works that are found to affect the PV/T performance are compiled, compared and analyzed in details. The reported data and findings are also tabulated to provide an overall picture of the influencing parameters and to identify the developing trend, which we hope will help engineering designs as well as future research efforts.

In the following sections, we will first give an overall review of some of the related literature, then examine the key parameters that affect PV/T performance, PV cell materials, and collector geometries. Although we focus on the recent development in PV/ T technologies, we have to acknowledge that not all of the published papers, including some excellent papers, are included in this review. As our objectives are to evaluate the control parameters, we have primarily selected those papers that have examined the effects of certain parameters related to PV/T performance based on our own judgment.

2. Selected literature

Among the air-based PV/T systems, Hegazy [8] proposed and investigated four basic modes, shown in Fig. 1, to study their performance as collectors. He performed theoretical analysis, and found that mode I, in which air is flowing over the absorber, has

Nomenclature

Ac collector area, m2 OCSHT one cover sheet and tube model

Aa aperture surface area, m2 OT operating temperature, K

APL absorber plate length, m OWT outlet water temperature, K

BST back surface temperature, K PF packing factor

CD channel depth, m Qu useful energy output of a collector, W

CHAO channel above opaque PV RL resistant load

CHBO channel below opaque PV RD reduced temperature

CHBT channel below transparent PV model S solar energy absorbed by a collector, W/m2

CL collector length, m SCT solar cell temperature, K

CT crop temperature, K ST storage temperature, K

DL duct length, m SCRN solar cell row numbers

EA exit area, m2 TA collector tilt angle, degree

FF free flow model TAI two absorber insulated

FT fluid temperature, K TANI two absorber non-insulated

FN fan numbers Ta ambient temperature, K

G solar radiation intensity, W/m2 Tp,m mean absorber plate temperature, K

GL glazed TCSHT two cover sheet and tube model

GWT glazed without tedlar UCSHT uncovered sheet and tube model

Gf incident solar energy, W/m2 UGL unglazed

GRE grade of thermal energy UGWT unglazed without tedlar

HLC heat loss coefficient, W K-1m-2 UL heat transfer coefficient

Im PV current at maximum power point, A Vm PV voltage at maximum power point, V

IT inlet temperature, K VW vent width, m

IV inlet velocity, m/s WF with fin

IWT inlet water temperature, K WOF without fin

MOW mass of water in storage tank, kg WS wind speed, m/s

MFR mass flow rate, kg/s WSTT water storage tank temperature, K

OAT outlet air temperature, K

the lowest performance. The mode I gives an overall (electrical and thermal) efficiency of about 55% at a flow rate of 0.04 kg/s per unit area. The amount of energy lost due to fan for creating forced convection was calculated and was found the least for mode III. Mass flow rate was considered as one of the important parameters for the system.

Tonui and Tripanagnostopoulos [9] investigated an air system in their experiment. They did two modifications in the channel to extract much more thermal energy and make the PV much cooler to get higher efficiency. They suggested the use of thin flat metal sheet suspended at the middle or finned back wall to improve performance in the air system. Tonui and Tripanagnostopoulos [10] studied two low cost modification techniques to enhance heat transfer to air stream in the air channel. They used both glazed and unglazed models. They used flat plate collector and their thermal model was based on natural ventilation. They developed a mathematical model to investigate the induced air flow rate and PV/T system temperatures. They analyzed the effect of some important parameters like mass flow rate, tilt angle and ambient (inlet) temperature on efficiency. They found an optimum point for channel depth and mass flow rate, after which point the PV/T behavior is reversed.

Sarhaddi et al. [11] did an investigation on flat plate PV/T air collector's electrical and thermal efficiencies and also evaluated the overall exergic performance. Their research showed the thermal, electrical and overall energy efficiency of PV/T air collector is about 17.18%, 10.01% and 45%, respectively for a sample of climate, operating and design parameters. They claimed that when inlet air temperature or wind speed or duct length increases, the overall energy efficiency and thermal efficiency of the PV/T air collector decrease, and also, if the inlet air velocity is increased, the overall energy efficiency and thermal efficiency of the PV/T air collector increase. There is an optimum value for solar irradiation amount, which was also identified.

Shahsavar and Ameri [12] used a thin aluminum sheet suspended at the middle of air channel to increase the heat transfer surface. Meanwhile they tested two, four and eight fans operating in their model, and validated a theoretical method. They figured out that an optimum number of fans is needed to get the maximum electrical efficiency. They confirmed that the unglazed system has a lower overall efficiency than glazed system.

Sopian et al. [13] compared the single pass and double pass PV/ T collectors. They found that double pass model has higher efficiency than single pass one. They confirmed that with the increase of mass flow rate the electrical efficiency will increase. However, when it is exceeding the optimum mass flow rate the thermal efficiency actually decreases.

Tiwari and Sodha [14] developed a thermal model of an integrated photovoltaic and thermal solar water/air heating system. They studied the parameters of this system numerically and analytically. Their configurations are shown in Fig. 2. They believe that the climate condition has very important role which affects the efficiency. They found out the overall thermal efficiency of their system in winter and summer being 77% and 65% respectively. They validated their results with experimental results by Huang et al. [15]. Their numerical simulation predicted a daily thermal efficiency of around 58%, which is very close to the experiment value 61.3% obtained by Huang et al. [15].

Tiwari and Sodha [16] carried out research on the evaluation of PV/T air collectors, with different configurations, glazed, unglazed, with tedlar, and without tedlar, in their experiments. They derived an analytical expression similar to Hottel-Whiller-Bliss (HWB) equation for flat plate collectors in terms of climate and design parameters. They found the glazed hybrid PV/T without tedlar has the best performance. Their models are shown in Fig. 3.

Joshi et al. [17] evaluated the air type collector system's performance. They compared two configurations for PV models, with glass-to-tedlar and glass-to-glass. In terms of overall thermal per-

Fig. 1. Four configurations that Hegazy [8] used. (a) mode I: air flow over the absorber, (b) mode II: air flow under the absorber, (c) mode III: air passes both sides of absorber in single pass, (d) mode IV: air passes both sides of absorber in double pass.

formance, they found that PV module glass-to-glass gives better results. Their study of the different effects of control parameters confirmed that increasing the length of the duct in both cases decreases overall thermal efficiency.

Kamthania et al. [18] developed a thermal model by using the energy balance equations of the proposed hybrid photovoltaic thermal double pass façade under quasi-steady state condition. An analysis has been carried out to calculate annual energy and exergy gain for the hybrid system.

Solanki et al. [19] investigated the indoor simulation and testing of PV/T air collectors. While the irradiation rate and inlet temperature was 600 W/m2 and 38 °C, respectively, they got the electrical and thermal efficiencies of 8.4% and 42%, respectively.

Kumar and Rosen [20] modified the thermal part of a double pass PV/T system by adding vertical extension surfaces (fins) which affected heat transfer rate and efficiency. They evaluated the effects of design, climate and operating parameters on the factors that can affect the efficiency directly. They believe the extended surface reduces the solar cell temperature considerably from 82 °C to 66 °C. They found the packing factor is one of the important parameters in designing PV/T collectors.

Among the water-based PV/T systems, Garg et al. [21] analyzed a system that is basically a conventional forced circulation type water heater. It is confirmed in their research that there is an optimum flow rate for which the collector efficiency is at the maxi-

mum. They examined pump-on and -off effects on the total efficiency. They believe a normal domestic solar water heater of about 2 m2 generates sufficient electrical energy to run two tube lights of 20 W each for 5 h and one television of 30 W for 4 h.

Chow [22] did research on performance analysis of PV/T collector by explicit dynamic model. He believes that the operation of a PV/T collector is inherently dynamic and steady state model is useless. He did his research theoretically and numerically and showed some parameters like mass flow rate on electrical, thermal and total efficiencies. An aluminum-alloy flat-box type hybrid solar collector functioned as a thermosyphon system was constructed by He et al. [23]. While the system efficiencies did vary with the operating conditions, the test results indicated that the daily thermal efficiency could reach around 40% when the initial water-temperature in the system is the same as the daily mean ambient temperature.

Zondag et al. [24] has carried out comprehensive research on water type PV/T collectors. He evaluated nine different designs and found the channel-below-transparent-PV design gives the best efficiency. He suggested that sheet-and-tube PV/T is a good design as its efficiency is just 2% worse but the manufacturing process is much easier compared to other designs.

Wu et al. [25] and Pei et al. [26] investigated heat pipe PV/T hybrid systems. Wu et al. [25] found the electrical, thermal and exer-gy efficiencies of 8.45%, 63.65% and 10.26% respectively for their

Fig. 2. Hourly variation of the thermal, electrical and exergy efficiencies with the local time at different packing factors (0.7, 0.8, 0.9) where the mass flow rate is 0.05 kg/s, the ambient temperature is 37 "C and the incident radiation is 8.6 W/m2 [25].

system. They also confirmed that the thermal efficiency can be improved by increasing the mass flow rate and lowering the inlet water temperature, packing factor of solar cell and heat loss coefficient in the PV/T system.

3. Parametric studies of PV/T

3.1. Packing factor

One of the important parameters in designing and studying a PV/T system is packing factor, which generally means the fraction of absorber plate area covered by the solar cells. In specific applications such as buildings, Vats et al. [27] studied the effects of packing factor on energy and performed exergy analysis of a PV/ T system with air duct flow. Fig. 4 demonstrates the efficiency behaviors of different PV cell materials due to change in packing factor. For example, Fig. 1a shows the thermal and electrical annual energy variations caused by changing of packing factor in each PV cell modules. The overall annual thermal energy and exergy variations are shown in Fig. 1b and c respectively at two different packing factors in each PV cells. The increase of packing factor doesn't always increase the annual energy gain or electrical efficiency. In the figure above, the effect of higher packing factor on the annual thermal efficiency and annual exergy analysis is also shown. If the packing factor is raised too much the thermal exit temperature will get higher due to absorbing high amount of thermal energy so it will increase the cell temperature, which causes the decrease in

electrical efficiency. Meanwhile decreasing the packing factor too much will decrease the electrical efficiency because the radiation absorber area is less.

In the study of Wu et al. [25] on PV/T hybrid system, the exergy analysis showed that the exergy efficiency behaves quite irregularly. For example, according to the Fig. 2, the lower exergy efficiency happens in packing factor equal to 0.8 in the experiment when they had three packing factors as 0.7, 0.8 and 0.9. The higher exergy efficiency is related to the packing factor equals to 0.9.

In Fig. 5, we show the packing factors that different researchers have been used. It is obvious that most of the researchers chose the packing factors higher than 50% and less than 90%.

Generally speaking, a comprehensive knowledge about the variation of packing factor and its effects with different fluids in different PV/T systems still does not exist. This also opened the door for optimization of the system design.

3.2. Mass flow rate

Mass flow rate is another important parameter in designing a PV/T system. The convection heat transfer coefficient is sensitive to mass flow rate variations. The higher the convection heat transfer coefficient is, the higher the heat transfer rate and the lower the exit temperature. This will result in higher thermal efficiency as well as electrical efficiency. Fluid material (gas or liquid), velocity magnitude, and the geometry of PV/T thermal system are the parameters to control the mass flow rate.

In general, at high mass flow rate, more heat can be removed, resulting in lower absorber plate temperature. However, if flow residence time in the channel is too short due to increase of velocity, the absorber plate temperature might not be reduced linearly as much as expected. It is believed that an optimal mass flow rate exists which allow a PV/T system to produce the highest thermal and overall efficiencies.

The ranges of mass flow rate that different researchers have investigated are compared and shown in Fig. 6. Both the minimum and the maximum mass flow rate values are labeled on the chart. Most researchers have reported the mass flow rates with absolute values. From heat transfer stand points, it might be of importance to use some non-dimensional numbers, such as Reynolds number. The authors believe this is necessary for future research to make the reported data more generalized and useful for similar designs.

It should be noted here that the hybrid systems for solar energy utilization have been an interesting common subject among different engineering majors. Material science advances produce state of the art solar cells nowadays, meanwhile mechanical thermal designs and their optimizations enable the adsorbing much more thermal energy from sun irradiation and cooling solar cells to produce more electricity. As many control parameters could come into play, the generation of some tables considering the effects of different parameters on the efficiency of PV/T becomes one of the main objectives of this paper. These tables and figures lead the new researchers to find their way fast among a lot of published papers to identify the effects of different parameters, such as mass flow rate. Again, the selected references are those which have studied relatively the most of the effecting parameters.

3.3. Efficiency

A literature review is ready in the tables below which show the research works of each research team on different PV/T models. As to some contradictions in the conclusions of different research works, the most probable reason for that is the different experiment conditions. But there are some other facts that are true sometimes, which need to be further examined of course.

Fig. 3. (a) Cross sectional view of unglazed PV/thermal air (i) with tedlar [14] (ii) without tedlar. (b) cross section view of glazed PV/thermal air (i) with tedlar (ii) without tedlar [16].

Fig. 4. (a) Annual electrical and thermal energy, (b) overall annual thermal energy, (c) overall annual thermal exergy, for different types of PV modules at packing factors of 0.62 and 0.83 [27].

100 90 80 70 60 50 40 30 20 10 0

-n^ec 89*88 90 A

83 ■ 81.81 ■ 83 ■

♦ 70

62.8 ■

so 50 48.01

■ 38

■ 20

Fig. 5. The packing factors which different researcher have been used.

Fig. 6. Mass flow rate ranges that different researchers have been experimented.

The steady state thermal efficiency of a basic flat plate solar collector is calculated by Duffie and Beckman [28]:

J'Quds Ac R GTds

The useful energy output of a collector is then the difference between the absorbed solar radiation and the thermal loss:

Qu = Ac [S - UL(TPm - Ta

where, S is the solar energy absorbed by a collector, GT is the incident solar energy, UL is the heat transfer coefficient, Tpm is the mean

absorber plate temperature, Ta is ambient temperature, and Ac is the collector area.

The mean absorber plate temperature is depending on some parameters like geometry and fluid condition, which makes it very hard to calculate. In order to solve this problem, they proposed some correction coefficients which are depending on the geometry and fluid characteristics. Tripanagnostopoulos et al. [29] proposed a formula to calculate the electrical efficiency by the following:

Fig. 7. Thermal, electrical and overall efficiencies reported by different researchers.

Table 1

Some selected parameters that different researchers have reported.

Author Collector Aperture area Tilt Inlet Outlet Tout — Tin (°C) Channel depth Absorber plate Solar Fan or pump energy/power Geometry note

type (m2) angle temp. temp. (K) (m) length (m) irradiation (KWh/d)

(K) (W/m2)

Hegazy (2000) Air 9 40 13-58 9 0-980 0.005-10 Flat plate, module dimension [98.2X43.6X3.85] cm

Tonui(2008) Air 0.4 40 30-80 (°C) 0.05- 0.5 1 0-1000 Flat plate, rectangular duct,

Tonui(2007) Air 0.4 40 293 ( C) 20-70 0.05- -0.5, test 1 800 0-0.5 (W) flat plate, rectangular duct, fin thickness = 0.5 mm

(°C) value :: 0.15

Sarhaddi et al. Air 298.15 308.76 0.05 L1 =1.2, 400-1000 flat plate, PV/T width = 0.45 m, air duct length: 1.2 m

(2010) L2 = 0.527

Shahsavar and Air 30 32-48 97.7 x 46.2 x 1.1 600-800 Flat plate, module dimension [196X54X35] cm

Ameri (2010) (°C)

Sopian et al. Air 15-38 30 < average 0-850 Flat plate, 0.5 m < L < 2 m and 0.1 cm < b < 10 cm

(1996) temp. < 55

Osthuizen Air 0.75 45 25-43 1.5 flat plate, channel type

(1996)

Tiwari and Sodha Air 0.61 30 34-39 36-54 2-20 1-8 167-710 2 fans are each 12 W at Flat plate, duct flow

(2007) (°C) inlet

Tiwari and Sodha Air/water 0.64 35-48 1-8 100-700 Flat plate-duct flow

(2006a) (°C)

Tiwari and Sodha Air/water 0.516 30-50 1-5 480-852 Flat plate-duct flow

(2006b)

Joshi and Tiwari Air 0.605 31-44 33-46 1-7.2 100-658 Flat-plate-duct flow

(2009)

Deepali and Air 0.15-0.76 (Test 7-30 0.3-1.5 (test Flat plate-duct flow

Tiwari (2011) value = 0.6) value = 1.2)

Kumar and Air 1 25 °C 5.6-7.4 0.03- 0.11 m 1 500-1000 Flat plate, channel flow, fiberglass insulation in

Rosen (2011) downlayer 5 cm thickness

Wu et al. (2011) Water 33- 38.85- 0.92 300-850 Heat pump flat plate

41 °C 40.36 °C

Garget al. (1994) Water 2 52-76 °C 0.1 Pump energy use in 8 h Flat plate-duct flow

experiment 0.52 Wh

Chow (2003) Water 2 45 800 Flat plate-tube flow-diameter of tubes: 0.01 m, width

of spacing between tubes = 0.2 m

Zondag et al. Air/water 45 1.776 800 Flat plate, channel flow and shell-and-tube flow, tube

(2003) diameter = 0.01, tube spacing = 0.095

Dubey et al. Water 2 30-85 °C 300-900 Flat plate, D = 0.0125 and W = 0.125

(2009)

Robles-Ocampo Bifacial 0.649 0-40 24- 34-50 °C 3 590-954 Flat plate-transparent solar plane used

(2007) water 30 °C

Dubey et al. Water 2.16 30 35-85 °C 15.9-17.5 W Flat plate-tube in plate type

(2008)

Dubey et al. Air 0.605 30 1 200-800 Flat plate-with/without duct-with/without tedlar

(2009)

He et al. (2006) Water 1.64 35 13- 30-60 °C 14-37 1.38 150-850 Flat plate-duct flow

Silva and Water 15- 35 C Sheet and tube flat collector

Fernandes (2010)

Table 2

Literature of efficiency research works by different researchers.

Thermal

efficiency

Electrical

efficiency

Overall

efficiency

Thermal efficiency

Increases vs. mass flow rate increment (25-58%)

Increases vs. mass flow rate increment (6.7-8.1%) Air pass over the absorber

(Forced convection) MFR < 0.02 ->rank = 2 (Increasing vs. MFR increment) MFR > 0.02 ->rank = 4 (Decreasing vs. MFR increment)

Air pass under the absorber

(Forced convection) MFR < 0.02 -> rank = 1 (Increasing vs. MFR increment)

MFR > 0.02 -> rank = 3 (Decreasing vs. MFR increment)

- 20% at optimum point of CD

- Increases vs. CL increment (more heating -> more outlet temp.)

- Increases vs. EA increment and tends to remain constant in larger EA

- Glazed > unglazed

- Fin > TMS

- Decreases with ambient temp. increment

- Increases within increment of TA

- Glazed + fined = 52%

Air pass both sides of the absorber, one

pass fashion

(Forced convection)

MFR <0.02 -> rank = 1

(Increasing vs. MFR increment)

MFR > 0.02 -> rank = 1 (Decreasing vs. MFR increment)

Air pass both sides of the absorber, double

pass fashion

(Forced convection)

MFR <0.02 -> rank = 1

(Increasing vs. MFR increment)

MFR > 0.02 -> rank = 2 (Decreasing vs. MFR increment)

Electrical efficiency

Decreases vs. CL decrement (more heating -> high PV temp.)

Overall efficiency

Thermal efficiency

If electrical efficiency assumed be constant condition, the effects of different parameters on thermal efficiency is the same on overall efficiency

Glazed + fined = 61-62%

Decreases vs. IT increment (17.2-9.2%) Increases vs. IV increment (0-41%) Almost constant with G increment (18%) Decreases vs. WS increment (20.7-6.7%) Decreases vs. DL increment (17-10%)

Electrical - Almost constant with IT increment (10%)

efficiency - Almost constant with IV increment (10%)

- Initially increases (6.5-11%) vs. G increment between 5 and 160 W/m2 then decreases (11-9.3%) vs. increment of G (160-1000 W/m2)

- Increases vs. WS increment (9.5-11%)

- Almost constant with DL increment (10.2%)

Overall - Decreases vs. IT increment (45-36.4%)

efficiency - Increases vs. IV increment (26-71%)

- Initially increases (37-48.4%) vs. G increment between 5 and 160 W/m2 then decreases (48.4-42.7%) vs. increment of G (160-1000 W/m2)

- Decreases vs. WS increment (47.5-37.5%)

- Decreases vs. DL increment (46-38.5%)

Thermal efficiency

GL > UGL

FN increment, MFR increment ->thermal Eff. Increment Refs. [13,14,9,10] Increases vs. decrement of RL

Electrical efficiency

GL < UGL

FN increment causes electrical loss increment

FN increment ->more cooling ->lower PV temp.->higher electrical efficiency An optimum point is existed between FN vs. electrical efficiency Optimum FN for highest electrical efficiency is FN = 2 When Fn = 0 > when FN = 4 or 8

Overall efficiency

GL > UGL

When FN > 0 ->almost constant (cause thermal increases via FN increment and electric decreases mostly via FN increment)

Thermal efficiency

Thermal efficiency increases ->mean PV temp. decrement Increases vs. MFR increases Slightly decreases vs. PF increment (Tout decreases) Thermal efficiency of double pass is higher than single pass Increases vs. Pf decreases

Electrical - Increases vs. mean PV temp. decrement

efficiency - Decreases vs. APL increment (average absorber plate temp. increases)

- Increases vs. MFR increment

- Electric efficiency of double pass (DP) is higher than single pass (SP) (APL = 1 m, MFR =100 kg/h, PF = 1 ->electric eff. of DP = 7.5% which is higher than electric eff. of SP = 6.7%)

- Slightly decreases vs. PF decrement

Overall efficiency

Increases vs. mean PV temp. decrement (because both thermal and electric efficiencies are increasing) Increases vs. MFR increment Decreases vs. APL increment Increases vs. PF decrement

Thermal efficiency

Electrical efficiency

Overall efficiency

Thermal efficiency

Reverse flat plate efficiency is higher than conventional flat plate dryer

Decreases vs. load (mass of crops) increment GL > UGL

With tedlar > without tedlar

Electrical efficiency

Increases vs. MFR increment (cause SCT is decreasing vs. MFR increment)

Decreases vs. length of hybrid PV/T system increment (increment of operation temperature range)

Overall efficiency

Increases vs. MFR increment

Decreases vs. length of hybrid PV/T system increment (increment of operation temperature range)

Significant increase in the hybrid PV/T system if more small modules are connected in series for a given length of the system

8 Thermal - In integrated PV/T system (IPVTS) for summer and winter conditions is about 65% and 77%, respectively

efficiency - In IPVTS decreases with APL increment (more heat losses at higher length)

- In summer is lower vs. winter (due to lower heat losses in the winter (e.g. from the storage tank in winter))

- 29% when Average SCT is 27.88 "C

Electrical - Can be increased either by PF increment or SCT decrement

efficiency - Model II, GT, is lowest,(SCT is highest)

- 11.83% when Average SCT is 27.88 "C

Overall efficiency

IPVTS with water is higher than with air for all configuration except mode IV Increases vs. MFR increment of water

IPVTS with water is higher than with air except model IV (GWT)

Inlet temperature in water is lower than inlet air, so water system works with good efficiencies

77.25% (if APL = 1.2 m, thermal efficiency = 48.25-in winter)

60-65% (if APL = 1.2 m, thermal efficiency = 48.25-in summer)

Model III (UGWT) has better performance at lower OT

Model II (GT) has better performance at high OT

9 Thermal efficiency

■ Higher in WF (higher heat transfer rate) Addition of fin increases to 15.5%

Variation with solar irradiance is not significant in low MFRs Slightly increasing vs. solar irradiance increment in higher MFRs

Electrical efficiency

Overall efficiency

Higher packing factors are beneficial as they lead to the production of more electrical output per unit collector area and help the controlling of the SCT

Higher in WF (higher heat transfer rate hence the lower SCT) Addition of fin increases to 10.5%

Decreases vs. solar irradiance increment (SCT is increasing) SCT is increasing linearly vs. solar Irradiance increment 20% when MFR is increasing from 0.03-0.15 kg/s

Increases vs. MFR increment (at higher solar irradiation levels)

Increases 17% vs. increment of PF from 0.38 to 0.98 (most of this increase is electrical)

10 Thermal efficiency

63.65%

Decreases vs. IWT increment

Increases vs. MFR increment (OWT decreases but rate of increment in MFR is higher than rate of decrease in OWT) Decreases vs. PF increment

Increasing of HLC is an disadvantageous for thermal performance Decrease vs. HLC increment (heat gain decreases)

Electrical efficiency

Decreases vs. IWT increment (great effect - because increase in IWT causes SCT increment) Increases vs. MFR increment (SCT decreases vs. heat removal increment) Increases vs. PF increment

Increasing of HLC is beneficial for output electricity

Overall efficiency

10.26%

Increases vs. IWT increment

Increases vs. MFR increment (GTE is decreasing vs. MFR increment)

(continued on next page)

11 Thermal efficiency

Influence of MFR increment on thermal eff. and electrical eff. is higher than its influence on overall exergy eff. Increases vs. PF increment (increase-rate of electrical output is higher than decrease-rate of thermal exergy) Decreases vs. thermal exergy and GTE decrease Weaker decrease in overall exergy vs. HLC increase

Slightly decreases vs. MOW decrement Increases vs. PF increment Increases vs. MFR increment

Electrical - Increases vs. MOW increment

efficiency - Increases vs. PF increment

- Increases vs. MFR increment until optimum MFR

- Decreases vs. MFR increment greater than optimum MFR

Overall - Increases vs. MOW increment

efficiency - Increases vs. PF increment

- Increases vs. MOW increment

- Increases vs. MFR increment until optimum MFR

- Decreases vs. MFR increment greater than Optimum MFR

12 Thermal efficiency

Electrical efficiency

Overall efficiency

13 Thermal efficiency

Electrical efficiency

Increases vs. MFR increment

- Increases vs. MFR increment

- Under the given condition the maximum combined efficiency is 70% (electric efficiency + thermal efficiency)

UCSHT (0.24) < FF (0.34) < CHBO = OCSHT (0.35) < CHBT = TANI (0.37) < CHAO = TCSHT (0.38) < TAI (0.39)Decreases vs. inlet temp. increment (in all models: UCSHT - uncovered sheet and tube, FF - free flow, CHBO - channel below opaque PV, OCSHT - one cover sheet and tube, CHBT -channel below transparent PV, TANI - two-absorber non-insulated, CHAO - channel above PV, TCSHT - two covers sheet and tube, TAI - two-absorber insulated.)

- TCSHT (0.058) < CHAO = TAI = TANI (0.061) < FF (0.063) < CHBT (0.065) < OCSHT (0.066) < CHBO (0.067) < UCSHT (0.076)

- Decreases vs. inlet temp. increment (in all models)

Overall - CHBT is the best option from performance point of view

efficiency - OCSHT is an alternative for CHBT because its efficiency is only 2% less

- OCSHT is easy in manufacturing

- OSSHT is the most promising model for domestic hot water production

- For lower temperature applications UCSHT is better (since the reflection losses at the cover is foregone and heat loss will remain low because of the lower temp. level) - there is a good perspective to use UCSHT with a heat pump

where, Aa is the aperture surface area, Im is PV current at maximum power point, Vm is the PV voltage at maximum power point, and G is the solar radiation intensity. And they also believe that the total efficiency of hybrid PV/T systems is corresponding to the sum of the electrical and thermal efficiency of the system for certain operation conditions. But different researchers have different ideas to calculate the overall efficiency. Another way to find the overall efficiency is to find the availability of system output as in Chow et al. [30], Joshi et al. [31] and Dubey et al. [32].

Different researchers in different period of times have reported thermal, electrical and overall efficiencies, which are seen in Fig. 7. As seen in the figure, for thermal efficiency, as high as 83% has been achieved, while for overall efficiency, 80% has been reported. Efficiency has been considered as the most important parameter for a PV/T system. As can be seen from the figure, the improvement of the efficiencies is not dramatic over the examined years. Other selected parameters and their ranges that are examined by the same researchers are tabulated in Table 1, which clearly indicates that investigators have not adopted a universal practice to report the related parameters in a way that they can be compared. While some investigators reported more parameters, some others choose to report only those they believe are necessary.

To show what parameters affect the efficiencies, some additional details of selected research works are listed in the Table 2. The table depicts what are the different individual researchers' findings about the parametric effects on thermal, electrical and overall efficiency. It provides researchers with an overview of the key references that might be of interests to their future research.

The researchers are listed by identification numbers as below:

(I) Hegazy [8], (2) Tonui and Tripanagnoustopoulous [9-10], (3) Sarhaddi et al. [11], (4) Shahsavar et al. [12], (5) Sopian et al. [13], (6) Goyal et al. [33], (7) Tiwari and Sodha [16], (8) Tiwari and Sodha [14], (9) Kumar and Rosen [20], (10) Wu et al. [25],

(II) Garg and Agarwal [21], (12) Chow [22], (13) Zondag et al. [24]. Table 3 provides the facts about how the efficiencies will change

with the change of different parameters. What are listed in the table are the important factors that have been very well studied and reported in the literature for the design of an effective PV/T system. Other factors that are recommended to be considered include:

• For overall efficiency of PV/T glazed configuration is mostly recommended rather than unglazed.

• Using the fans is good in producing higher air mass flow rate and based on this higher efficiency will be gained. However the number of fans is very important as there must be an optimum number of fans in this system.

• Water is found to be better than air primarily from thermal point of view regardless of the cost and difficulties, because of water's high heat capacity and conductivity.

• Solar cell temperature variation versus solar radiation intensity is linear, which can be explored to our benefits with innovative designs.

• One cover sheet and tube model will perhaps remains as the most popular water type PV/T when both performance and cost are considered at least in the near future.

Table 3

Efficiency fact table.

Decreases vs.

Increases vs.

Constant

Thermal efficiency

Electrical efficiency

Overall efficiency

Mass flow rate increment after optimum mass flow rate

Ambient temperature increment

Inlet temperature increment

Duct length increment

Packing factor increment

Heat loss coefficient increment

Mass of water decrement in the storage tank slightly (if PF and MFR is constant)

Collector length decrement

In higher solar radiation intensities ranges increments Absorber plate length increment Inlet water temperature increment Inlet temperature increment

Inlet temperature increment

In higher solar radiation intensities ranges increments Wind speed increment Duct length increment Absorber plate length increment

Mass flow rate increment until optimum mass flow rate

Collector length increment

Exit area increment but after a range remains

constant

Tilt angle increment Inlet velocity increment Fan number increment (because of 2) Using fin

Packing factor increment (if MFR and MOW are constant)

Solar cell temperature decrement

Packing factor increment

Mass flow rate increment

Initial solar radiation intensity increment until

specific range

Wind speed increment

Mean PV temp. decrement

Using fin

Mass flow rate increment Inlet velocity increment

Initially solar radiation intensity increment until specific range

Mean PV temperature decrement Packing factor increment

Inlet velocity variations Duct length variations

Fan number variations

3.4. Other important parameters

The other important parameters used by the selected researchers are listed in Table 4. These include the type of thermal collector systems, experiment details, and the working fluids. Some of their observations, particularly those about the temperature differences between the inlets and outlets, are included in the table as well.

4. PV solar cell materials

Chapin et al. [34] was the first who published a research paper regarding generating power using photovoltaic phenomenon using crystalline silicon, although the phenomenon's age is older than this research work. It was a big research subject opening for the energy field researchers to work on. Since then, a lot of research has been carried out, some of them have been reviewed by Green [35] and Partain [36].

In the paper by Gratzel [37], he believes photovoltaics has been dominated by solid state junction devices, often made of silicon. According to his report, this dominance is now being challenged by the emergence of a new generation of photovoltaic cells, based on for example, nanocrystalline materials and conducting polymer films which have attractive features like cheap fabrication and high flexibility. He provides a comparison in performance among new photoelectrochemical systems and conventional devices which is shown in Table 5. He believes photoelectrochemical systems can be produced more cheaply and at less cost in energy than silicon cells, for which 5 GJ have to be spent to make 1 m2 of collector area. Unlike silicon, their efficiency increases with temperature, narrowing the efficiency gap under normal operating conditions. They usually have a bifacial configuration, allowing them to capture light from all angles.

Miles et al. [38] published a paper regarding the overview of the materials and methods used for fabricating photovoltaic solar cell devices. They categorized different type of materials used in this industry and discussed the important environmental and energy issues with regard to the manufacturer, use and disposal of the solar cells and modules. The tabulated best efficiencies that reported for different type of solar cells are shown in Table 6.

In the following, we categorize the different kinds of materials which are used in the PV/T industry, and compare their advantages and disadvantages with each other.

4.1. Silicon solar cells

4.1.1. Single crystalline silicon

Single crystalline silicon solar cells are the most widely used semiconductors. Their ideal efficiency is 24.7% and commercial module efficiency is 18%. In research and development sector, the single crystal ribbon silicon is the most favorite. The novel ''Sliver cell'' and module design offers the potential for a 10-20 times reduction in silicon consumption for the same sized solar module, while also having the added benefit, in an industrial production environment, of requiring 20-40 times fewer wafer starts per MW than for conventional wafer-based technologies as reported by Franklin et al. [39]. The most advantages of this material are their low cost and high quality.

4.1.2. Polycrystalline silicon

The energy conversion efficiency for a commercial module made of polycrystalline silicon ranges between 10% and 14% as stated by Hermann in 1998 [40]. Tyagi et al. [7] mentioned two advanced approaches in producing polycrystalline silicon PV cells. The common one is to slice thin wafers from blocks of cast and the other one is the ''Ribbon growth,'' in which silicon is grown directly as thin ribbon or sheets with the appropriate thickness for PV cells. Edge defined film fed (EFG) growth is among the most commercially developed ones. Some of the advantages of polycrys-talline silicon are:

• Stronger than single crystalline.

• Can be cut into one-third the thickness of single crystal.

• EFG has slightly lower wafer cost.

• EFG has less strict growth requirement.

• Lower costs comparing to single crystal manufacturing costs.

• Electrical production is higher than amorphous ones [24].

• Improved efficiency when compared to amorphous silicon while still using only a small amount of material [41].

Table 4

Other parameters in the literature by different researchers.

1 Thermal system technology

Working fluid

Tout Tin

Important parameter Collector

Thermal system technology Working fluid

Tout Tin

Important parameter Collector

Thermal system technology Working fluid

Tout Tin

Important parameter

Collector

Thermal system technology

Working fluid

Important parameter

Collector

Thermal system technology

Working fluid

Important parameter Collector

6 Thermal system technology Working fluid

Tout Tin

Important parameter Collector

7 Thermal system technology

Working fluid

Important parameter Collector

Thermal system technology

Working fluid

Tout Tin

Important Parameter Collector

Thermal system technology Working fluid

Air pass over the absorber (forced convection) Air pass under the absorber (forced convection)

Air pass both sides of the absorber one pass fashion (forced convection) Air pass both sides of the absorber double pass fashion (forced convection) Air

Decreases vs. increment of mass flow rate (58-13 K) MFR = (0.005-0.04 kg/s m2) Flat plate

Fin - natural convection - TMS (thin metal sheet) - natural convection Air

Higher if CL is increasing

- CD = (5-10 cm) Flat plate

- Numerical and experimental analysis

- Air passes by forced convection under the PV panel Air

When inlet temperature (or wind speed or duct length) is increased the thermal and overall efficiency decreases

- IT = (test range 300-305 K)

- IV = (test range 0.001-10 m/s)

- G = (test range 5-1000 W/m2) - optimum point is 160 W/m2)

- WS = (0-10 m/s)

- DL = (1.5-6 m) Flat Plate

- Numerical and experimental investigations

- Glazed and unglazed

- Natural and forced convection considered Air

- FN = (varies 0, 2, 4, 8)

- MFR = (test range 0-0.4 kg/s) Flat plate

- SP = single pass geometry

- DP = double pass geometry

- Theoretical and experimental analysis Air

- Decreases vs. MFR increases (Tout decreases)

- decreases vs. PF increment N/A

Flat plate

- Reverse flat plate dryer

- Flat plate dryer Air

- CT in RFP is higher than conventional dryer N/A

RFP = reverse flat plate

- Mode I: unglazed PV/T with tedlar

- Mode II: unglazed PV/T without tedlar

- Mode III: glazed PV/T with tedlar

- Mode IV: glazed PV/T without tedlar Air

- OAT, BST and SCT in GL modules are significantly higher than UGL ones (due to reduction in top loss coefficient)

- OAT, BST and SCT in IV are slightly higher than III N/A

Flat plate

- Theoretical and experimental

- Integrated photovoltaic and thermal solar (IPVTS) (water/air)

- Mode I: unglazed PV/T with tedlar (UGT)

- Mode II: glazed PV/T with tedlar (GT)

- Mode III: unglazed PV/T without tedlar (UGWT)

- Mode IV: glazed PV/T without tedlar (GWT)

- Optimum MFR in their experiments = 0.02 kg/s Air/water

- Model III (UGWT) water temp. is the highest (summer and winter - because max thermal energy gain is responsible for higher water temp. due to absence of the tedlar)

- WSTT variation increases vs. the mass of the water in tank decrement N/A

Flat plate

- Theoretical

- Optimum MFR in their experiments = 0.12 kg/s Air

- Increase in OAT is linear with solar Irradiation increment and vs. MFR decrement

Important Parameter - PF = packing factor (=0.5, the fraction of absorber area occupied by photovoltaic cells)

Collector Flat plate

10 Thermal system - Theoretical

technology - e-NTU model

- Heat pipe model proposed for cooling solar cells (it can do uniform cooling and has a control on solar cell temp.)

Working fluid Water

Tout Tin - SCT increases (maximum 2.5 "C) vs. SCRN increment (the variation is slightly so SCT can be considered uniform)

- SCT decreases vs. MFR increment

- SCT decreases vs. PF increment (0.7-0.9)

- OWT decreases vs. PF increment

- SCT decreases vs. HLC increment

Important parameter N/A

Collector Flat plate

11 Thermal system - Numerical and theoretical analysis

technology

Working fluid Water

Tout Tin - ST decreases vs. MOW increment

- ST decreases vs. PF increment

- ST increases vs. MFR increment until Optimum MFR

- Decreases vs. MFR increment greater than Optimum MFR

Important parameter - PF: Acell/Acollector

Collector Flat plate

12 Thermal system - Based on the control volume finite difference approach, an explicit dynamic model

technology - Transport Delay Fluid flow model

- the operation of a PV/T collector is inherently dynamic

- Two key manufacturing defects identified in PV/T collectors: (1) the imperfect adhesion between PV plate and the absorber plate (2)

the imperfect bounding between the absorber plate and the metallic tubes

Working fluid Water

Tout Tin - In this explicit model, the x-direction temperature gradient is not affecting the temp. in y-direction

- The variation in y-direction is nonlinear

Important parameter N/A

Collector Flat Plate

13 Thermal system - Numerical and experimental UCSHT,OCSHT,TCSHT,CHAO, CHBO,CHBT,FF, TAI, and TANI configurations

technology - At higher RT water is not a good choice for a FF model

- condensation on the top glass is an disadvantageous that was not considered in this research - which reduces the efficiency in high RTs

- FF model is intrinsic prevention against overheating (because of strong increase in rate of evaporation towards higher RTs)

Working fluid Water

Tout Tin N/A

Important parameter - RT: (Tin - Ta)/G

Collector Flat plate

Its disadvantages as compared to other silicon based PV solar cells include being less energy efficient versus single crystal silicon, or having lower thermal contribution output comparing to amorphous silicon.

4.1.3. Amorphous silicon

Amorphous silicon was first discovered at 1974. This material is a non-crystalline form of silicon, which has disordered atomic structure. It is less sensitive to temperature [24]. Amorphous silicon is the most popular thin film technology with cell efficiencies of 5-7% and double- and triple-junction designs raising it to 8-10% [41]. Stable efficiency is important in a PV module, which is controlled by different environmental parameters. Different researchers proposed many techniques to stabilize the efficiency [42-43]. Some of the advantages of amorphous silicon PV solar cells are as below:

Table 5

Performance of photovoltaic and photoelectrochemical solar cells [37].

Type of cell Efficiency (%)* Research and technology needs

Cell Module

Crystalline silicon 24 10-15 Higher production yields, lowering of cost and energy content

Multicrystalline silicon 18 9-12 Lower manufacturing costs and complexity

Amorphous silicon 13 7 Lower production costs, increase production volume and stability, improve efficiency

CuInSe2 19 12 Replace indium (too expensive and limited supply), replace CdS window layer, scale up production

Dye-sensitized 10-11 7 Improve efficiency and high nanostructured materials temperature stability, scale up production

Bipolar AlGaAs/Si 19-20 - Reduce materials cost, scale up photo electrochemical cells

Organic solar cells 2-3 - Improve stability and efficiency

* Efficiency defined as conversion efficiency from solar to electrical power.

• High sun light absorptivity (40 times higher than single crystal).

• Lower manufacturing costs comparing to other silicon based PV solar cells.

• Can be deposited on the low cost substrates (steel, glass, plastic).

• Higher manufacturing temperature is not needed, so energy used for manufacturing is less comparing to other PV solar cell materials.

4.2. III-V group solar cells

4.2.1. Gallium arsenide (GaAs)

Gallium arsenide is chemically made of two major elements Gallium and Arsenic. Its crystal structure looks alike silicon. Photovoltaic conversion efficiency over 25% has been achieved on single-junction solar cells fabricated in epitaxially grown GaAs on a single

Table 6

Best efficiencies reported for the different types of solar cell [38].

Cell type

Highest reported efficiency for small area cells produced in the laboratory

Highest reported module efficiency

x-Si (crystalline Si) Multi-c-Si

aSi:H, amorphous Si ic-Si/aSi:H (micro-

morph cell) HIT cell GaAs cell InP cell

GaInP/GaAs/Ge

multijunction CdTe

Dye sensitized cell

24.7% (UNSW,PERL) 20.3% (FhG-ISE)

10.1% (Kaneka), N.B. Single junction 11.7% (Kaneka), N.B. Minimodule

21% (Sanyo) 25.1% (Kopin) 21.9% (Spire)

32% (Spectolab), N.B. 37.3% under construction 16.5% (NREL) 8.2% (ECN)

22.7% (UNSW/Gochermann 15.3% (Sandia/HEM)

Triple junction. Stabilized efficiency = 10.4% 11.7% (Kaneka), N.B. Minimodule

18.4% (Sanyo) Not relevant Not relevant Not relevant

10.7% (BP Solarex) 13.4% (Showa shell), N.B. for copper Gallium indium

sulphur selenide

4.7% sub-module (INAP)

External current load

Fig. 8. Schematic diagram of the dye-sensitized solar cell [56].

crystal substrate [44]. One of the most popular applications with GaAs based solar cells is space application, because it is very resistant in front of solar radiations and it saves the solar devices against huge amount of radiations. Some of the advantages of this type of materials are as below:

• High level of absorptivity.

• Low thickness is needed to absorb the sunlight (so less material is needed as comparing to other solar cell materials).

• High heat resistance.

The single-crystal substrate that is used to grow GaAs on is very expensive which is supposed to be a disadvantage. According to

Fig. 9. Cross section of flat plate collectors.

this fact it is mostly preferred to use less amount of material, so the application is limited to concentric PV/T systems which need less area hence material. Although the flat plate collector PV/T system is the most popular one, it is not affordable to use this material because of the cost.

4.2.2. Indium phosphide (InP)

The first high efficiency InP based devices were produced in 1970s and the first InP solar cell were homo junction devices produced by thermal diffusion. InP solar cells have long been demonstrated to degrade less under irradiation than GaAs and Si [4549]. It has a broad application in space industry, because the room temperature annealing and majority-carrier injection-enhanced annealing are responsible for the recovery of photovoltaic properties of degraded cells [47,49]. Li et al. [49] believe that with a simple structure that they found, the end of life efficiency of InP based solar cells is about 10% (AM0, 1Sun); its highest power/weight ratio is about 130 W/g (only the weight of epitaxial layer is considered). But being so much pricy in production, easily cleaved, mechanically weaker than silicon and having higher density from weight point of view might be considered disadvantages for certain applications.

4.3. Thin film solar cells

A thin film solar cell is made by depositing one or more thin layers of photovoltaic material on a substrate, such as glass, metal or plastic foil. The thickness range of such a layer is wide and varies from a few nanometers to tens of micrometers. Thin-film PV technologies based on inorganic materials are being developed rapidly, both in the laboratory and in industry. Aberle [50] believes that globally, more than a dozen thin-film silicon PV lines are presently being commissioned or planned for amorphous and/or microm-orph solar cells. Significant thin-film PV production levels are presently also being set up for CIS and CdTe. In his comprehensive review, he categorized the thin-film solar cell technologies into:

• Amorphous silicon solar cells.

• Microcrystalline silicon solar cells.

• Micromorph tandem silicon solar cells.

• Polycrystalline silicon solar cells.

• Cadmium telluride solar cells.

• Copper indium diselenide solar cells.

Gallium arsenide (GaAs), copper, cadmium telluride (CdTe), indium diselenide (CuInSe2), and titanium dioxide (TiO2) are materials that have been mostly used for thin film PV cells [41]. Aberle [50] believes that the c-Si thin-film PV approaches that have evolved during the last 10 years can broadly be classified as fol-

Table 7

Fluid point of view comparison in PV/T collectors.

Fluid Sub- Advantages Disadvantages

Based division

Air • More adopted for building project application based on European and north American markets [61] • No freezing and no boiling of the collector fluid • No damage if leakages occur [61] • the most popular PV/T collector [62] • Minimal usage of material and low operating cost • Low heat capacity and low heat conductivity, which result in a low heat transfer • Low density, which results in a high volume transfer • High heat losses through air leakage • Possible noise • They have less applications compared to the water collectors [62] • They have relatively slow heat transfer rate due to lower thermal conductivity [61] • Low specific heat capacity of air necessities greater volume of air per unit collector area for storage of a given unit of thermal energy [20]

Water S-A-T* Simplest way to construct PV/T 2% less efficient compared to other types of collector

Channel In first case**: more strength for resisting against water pressure because the backside could be strengthened with metal back. Backside of PV laminate is completely watertight In second Case***: can be expected for high efficiency If a wide channel is used which is covered by one large glass plate, very thick glass may be necessary to withstand the water pressure, resulting in a heavy but fragile construction [63] In second Case**: transparent PV laminates are so expensive.

Free Comparing to the channel case: one glass layer is going to be Comparing to the channel case: the increased heat loss due to

flow eliminated, accordingly reflections and material costs will decrease evaporation. Because the evaporation pressure is not low, the evaporation will cause some problems at high temperatures.

Two- High thermal efficiency [24] Heavy channel cover-even more stronger than channel case [63]

absorber

* =S_A_T: Sheet and Tube. ** =Fig. 11,1st one in channel type. *** =Fig. 11, 2nd.

lows: (i) fabrication of thin, long stripes of c-Si material from thick single crystalline Si wafers (''ultrathin slicing''); (ii) growth of c-Si thin-films on native or foreign supporting materials. He also reviewed the most promising thin-film c-Si PV technologies that have emerged during the last 10 years and found that three different thin-film c-Si PV technologies (SLIVER, hybrid, CSG) can be transferred to industrial production, pointing out some special features of thin-film technology as advantages:

• The deposition spray technique for deposition on glass or metal is cheaper.

• The manufacturing process is faster using up less energy.

• Most promising for next generation of solar cells.

• Reduce the amount of semiconductor material required for each cell when compared to silicon wafers and hence lower the cost of production of photovoltaic cells [41].

Barnett et al. [51] reported that solar cells utilizing thin-film polycrystalline silicon can achieve photovoltaic power conversion efficiencies greater than 19% as a result of light trapping and back surface passivation with optimum silicon thickness. Powalla and Dimmler [52] assessed that all existing thin-film PV technologies, especially the Cu(In, Ga)Se2 (CIGS)-based technology, have a high cost reduction potential at high production volumes projecting futuristic challenges to combine high production volumes with high throughput, sufficient yield and superior quality to achieve efficiencies of above 11% and a maximum of 12.7%.

4.4. Dye sensitized solar cells

Dye solar cells, which were published by O'Regan and Grätzel [53] for the first time, have promised to provide a 'leapfrog' in solar cell cost effectiveness and the field has attracted an increasing

Fig. 10. A classification of liquid based collectors [25].

Fig. 11. Four water based collectors: classification and their sub-classifications, extracted from [24].

number of academic and industrial research teams [54-55], especially in the past 5 years since O'Regan and Grätzel and his team were able to demonstrate the first 10% efficient cell. Dye-sensitized solar cell (DSSC) is a semiconductor photovoltaic device that directly converts solar radiation into electric current. The operational principle of DSSC is illustrated in Fig. 8.

Gong et al. [56] gave a comprehensive review about dye sensitized solar cell system, which consists of the following:

• A transparent anode made up of a glass sheet treated with a transparent conductive oxide layer;

• A mesoporous oxide layer (typically,TiO2) deposited on the anode to activate electronic conduction;

• A monolayer charge transfer dye covalently bonded to the surface of the mesoporous oxide layer to enhance light absorption;

• An electrolyte containing redox mediator in an organic solvent effecting dye regenerating;

• A cathode made of a glass sheet coated with a catalyst (typically, platinum) to facilitate electron collection.

The semiconductor film is usually TiO2 and sensitized onto the surface of the semiconductor. Electrolyte contains a redox mediator. Counter electrode is capable of regenerating the redox mediator, like palatine. Their simple structure, low weight, flexibility, and low manufacturing costs are counting as advantages over other solar cells. They are transparent and can be found in different colors which make them look interesting from architectural point of view. They are working much better than silicon based solar cells in darkness. It is believed that with consistent efforts, DSSCs will be a reliable electrical power supplier in the future [56].

5. Collectors geometries

The geometry of collectors is very important in the hybrid system designs and also in their applications. They are generally categorized as flat plate type and concentric type (not reviewed in this paper). Each of them has advantages and disadvantages.

5.1. Flat plate

The most conventional collector for low temperature applications (less than 60 °C) or medium temperature (less than 100 °C) is the flat plate type [7]. The geometry is like what is shown in Fig. 9, containing several layers. Some of the most important features of flat plate collectors are listed below including advantages and disadvantages.

5.1.1. Advantages

Flat plate collectors have a wide view for absorbing the sun light energy comparing to concentric ones. Because of their geometry, both beam and diffuse solar irradiance are encountering the collector. Based on this type of collector design, tracking of the sun is not a case. Even if the tracking system added to the flat plate collectors will increase the efficiency, the reduction of cost by eliminating the tracking system is much cost effective [57].

5.1.2. Disadvantages

When flat plate collectors are not supposed to be tracked they have great amount of cosine losses, therefore less total energy falls on a rigid surface during the day.

5.2. PV/T fluids

Organs in our body are doing their specific job individually as the parts of our body cycle, but the blood is the fluid that makes the body cycle works as a mechanism. Fluids in the PV/T are like blood in the body cycle and divided into most popularly water and air. Each of them has specific applications with advantages and disadvantages.

5.2.1. Air based collectors

Air type collectors do not have specific classification, but how the air as the thermal fluid is used, defines the collector's types, such as above the absorber, below the absorber and both sides of the absorber in single or double pass ways. Some of the general advantages and disadvantages of air based collectors are provided in Table 7.

5.2.2. Liquid based collectors

Beside air, water is also an available, clean and affordable (less than air) fluid that was used by designers and researchers in PV/T systems. Daghigh et al. [58] believe the most common working fluid in liquid based PV/T collectors are water, water/air, and most recently refrigerant. Chow et al. [59] did an experimental study on integrated PV/T water heating systems and found the thermal efficiency as 38.9% and electric efficiency as 8.56% in a local experimental condition in Hong-Kong. Tiwari et al. [60] performed an analytical investigation of the prediction of water temperature in a constant mass flow rate condition for integrated PV/T solar water heater. In Fig. 10, Daghigh et al. [58] provided a diagram to show the types of the liquid based PV/T collectors and their suggested application fields.

There are generally four categories for water based PV/T collectors in terms of the heat transfer techniques, i.e. sheet and tube, channel, free flow, and two absorbers, which are shown in Fig. 11, which is extracted from the paper by Zondag et al. [24].

The comparison between water type PV/T collectors, including the sub-categories in concern, are provided in Table 7. Also provided in this Table are the properties about air type collectors for comparison purpose. It must be notified that the application type is the most important criteria to select the fluid type for PV/T design. Generally each type has some advantages and disadvantages which are listed in the Table. Water based PV/T collectors are generally divided in four sub category with each of them comparing to each other by some advantages and some disadvantages. It seems the sheet and tube type among the water based PV/T collectors is the most promising for practical applications with both cost and effectiveness considered.

6. Conclusions

This paper reviewed the recent development in various PV/T systems, with attentions paid to the effects of different control parameters as well as advantages and disadvantages of the different designs and thermal/fluid management schemes. The efficiency is the most important parameter which must be considered in PV/T technologies. An efficiency fact table was introduced in this paper to give a general picture for designers and researchers. During the last decades, improvement of efficiencies is steady but not dramatic.

The material of the photovoltaic cells is playing a big role in the electrical efficiency. A brief review of the most promising materials used in PV/T industry was reported in this paper. Each technology on the material side has its own advantages and disadvantages that are summarized. There are obviously still a lot to do to develop the new materials with new technologies for PV/T applications.

For different applications we need to use proper collector geometry. Based on what has been done in this research, the most promising PV/T application in residential applications is mostly the flat plate geometry in collector design. For big scale applications like power plants, concentric collector geometry is preferred, which is not reviewed in this paper.

The fluid on the thermal side of PV/T is generally gas, liquid or both of them. The most conventional one in gas system is air, unless the application demands specific gas like CFC's. Air is affordable, clean and available for nearly any application on the earth. Among the different liquids, water is also affordable (less than air), clean and available. The maintenance costs for air based system is cheaper than the water based. But for specific application, using of other liquids may be needed in which special configuration or design must be applied. The Sheet and Tube geometry is considered as the one with high efficient and less expensive in water based PV/T's for practical applications, such as building integrated systems.

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