沙漠研究 19-1, 5- 8 (2009 )Journal of Arid Land Studies“Desert Technology IX ”
Refereed Paper

Self Cooling Mechanism in Photovoltaic Cells and Its Impact on Heat Island Effectfrom Very Large Scale PV Systems in Deserts
Keiko SATO1,3, S. SINHA2,3, Birendra KUMAR3,4, T. KOJIMA1
Abstract: The share of Photovoltaic power generation is likely to increase many folds in coming
decades as per projection of world energy demand by various study groups throughout the world.
Deserts and arid areas have high insolation and sufficient space to house Very Large ScalePhotovoltaic (VLSPV) systems. However, albedo and land cover changes are supposed to
substantially increase sensible heat flux. Increase in surface heat flux, may cause local, regionaland global climate changes. It is concluded on the basis of experimental observations that selfcooling mechanism initiates in Photovoltaic Cell during off-sunshine hours, which can offset ‘Heat
Island Effect’ due to high cell temperature during sunshine hours. It is also observed that the load
has no effect on self-cooling mechanism.
Keywords: Climate change, Energy from desert, Heat Island Effect, Self cooling, VLSPV

1. Introduction:
New technology for economical recovery of fossil fuel resources, new reserves and nuclear energy
may fulfill the energy demand in immediate future, geographical imbalances between supply and demand,
as well as temporal and local supply problems would certainly worsen. Besides, the sustainable prosperityof human beings can no longer be expected if global environmental issues due to fossil fuel use areignored. The share of electrical energy is rising more and more as secondary energy form. PV systems
have unique additional values in environmental, electrical, architectural and socio-economic domain,
besides main function, power generation, for example, avoidance of marginal cost of system-wide energy
production and system-wide generation capacity; deferral of distribution capacity, sub-transmission
capacity, transmission capacity investment; reduction of losses, value of reduced fossil emission, fuel pricerisk mitigation etc. There are several PV deployment projections from USA, Europe and Japan. The US
Photovoltaic Industry Roadmap Steering Committee, which consists of Astropower, Idaho Power, Avista
Labs, Siemens, Solar Industries, Solarex, SEIA, MIT, Spire Corporation, Trace Engineering, Purdue
University, BP Solar and Stella Group, in its report aims to generate 10% of domestic peak power
generation capacity by 2030 using PV technology.
Very Large Scale Photovoltaic Technology (VLSPV) is likely to be as main source of electricity
generation as proposed by Kurokawa (2003). The option has become economically viable at the present
state of technical development. Environmental impact analysis of atmospheric circulations due to land
surface interaction and partitioning of the available energy at the surface between latent and sensible heat
fluxes indicate that daily global mean temperature due to this land-use change (VLSPV installation) inSahara, Gobi and Thar, is approximately 0.001˚C in one year as shown by Sinha et al, (2006). However,there are possibilities that changes in soil temperature due to higher module temperature may act as Heat
Island Effect, adversely affecting the global climate if results obtained from Community Land Model(CLM) developed by Pielke (2002) are to be relied upon. These studies are based upon computation oflarge-scale synoptic patterns and mesoscale atmospheric structures. These must be offset by creating green
patches - an expensive alternative.
An interesting outcome, however, of the work has been unexpected cooling effect from PV modules,
which is visible during off-sunshine hours. The temperature of PV modules is 2-4˚C less than the ambient
temperature when direct sun light is not falling on the modules. This is likely to reduce the heat islandeffect as compared to earlier computed values.

1Department of Materials and Life Science, Faculty of Engi., Seikei University, Tokyo 180-8633, Japan
Centre for Appropriate Technology, Chandragupta Institute of Management, Patna - 800001, Bihar, India
3
Centre for Renewable Energy & Environmental Research, PO Box - 5, Muzaffarpur - 842001, Bihar, India; Ph. - +919431239820, E-mail: creer@sancharnet.in
4
Dept. of Electronic Science, BRABU, Muzaffarpur-842001, Bihar, India
2

62. Materials and Methods
The open circuit voltage , Voc, of a p-n junction is related to the band gap, Eg , and is generally expressed interms of short circuit current, Isc, the dark current Io, and the junction perfection factor Ao by;⎤⎡ I(1)
Voc = Ao ⎛⎜ KT ⎞⎟ ln ⎢⎛⎜ sc ⎞⎟ + 1⎥q ⎠ ⎣⎝ I o ⎠ ⎦⎝In the absence of light, the relationship between the flow of junction current Ij and imposed voltage Vin a p-n junction is given by;⎞⎟ − 1⎤ and I = qDh pno + qDe n po(2)I j = I o ⎡⎢exp⎛⎜ qVoKT⎠ ⎥⎦LhLe⎣ ⎝Where, K is Boltzmann constant q is the electronic charge and Io is the saturation current; Dh and De are
diffusion constants of holes and electrons at a particular temperature (T) respectively; npo and pno are thethermal equilibrium densities of electrons on p-side and of holes on the n-side of the junction respectively;
Le and Lh are the electron diffusion length on p-side and hole diffusion length on n-side respectively. Io isalso called dark current.
The increase in cell temperature during day time in all seasons is observed (Fig.1a). It is due torecombination of hole-electron pair in presence of higher frequency of electromagnetic waves (hυ > Eg).
As it has been expected and can be explained by the above equation. The increase in cell temperature caninfluence the local climate. Pielke (2001) analyzed that the changes in land cover influence the climate andmore specifically hydrological cycle through the partitioning of the incoming solar radiation into turbulent
sensible and latent heat fluxes. The energy budgets at the surface can be written as follows,
RN = QG + H + L (E + T)
(3)
(4)
RN = QS (1 – A) + QLW (↓) - QLW ( ↑)P = E + T + RO + I(5)
Where RN is net radiative fluxes; P is precipitation; E is evaporation (conversion of liquid water intowater vapor from the soil surface); T is transpiration (phase conversion to water vapor, through stoma onplants); QG is soil heat flux; H is turbulent sensible heat flux; L(E + T) is turbulent latent heat flux; L islatent heat of vaporization; RO is runoff; I is infiltration; Qs is insolation; A is albedo; QLW (↓) is down
welling long-wave radiation; QLW ( ↑) is upwelling long wave radiation, which is,(6)QLW ( ↑) = (1 - ε ) QLW (↓) + ε σ Ts4Where, ε is the surface emissivity; σ is Stefan’s constant and Ts is surface temperature. Sinha et al.
(2005) assumed that the PV system covers the land surface horizontally not inclined to any angle to thewhole area of Sahara, gobi and Thar deserts. The net energy flux at the ground surface is sum of beam &diffuse solar radiation and long wavelength radiation from earth and atmosphere (Eq. 3). The sensible heat
flux and latent heat flux was calculated by the following equations,(7)H = ρatm Cp (θatm – Tg)/ rahWhere, ρatm - air density, Cp - specific heat of air, θatm – potential temperature, Tg - ground temperature,rah - aerodynamic resistance to sensible heat flux and water vapor transfer between atmosphere at height z1.
Bonan et al. (2002) developed CLM3.0 model which was run for one year in two cases of the assumed
albedo for the whole area of Gobi desert (162.8×104 km2), Sahara desert (743×104 km2) and Thar desert(44.3x104 km2). The following two cases were considered, (A) visible range radiation = 0.20, infraredradiation = 0.20 and (B) visible range radiation = 0.10, infrared radiation = 0.20. The input global data was
obtained from CCM3.0 web site to run the model in offline mode for one year projections. The projected
increase in daily global mean temperature due to this land-use change in Sahara, Gobi and Thar, isapproximately 0.001˚C in one year. Global mean temperature rise of 0.1˚C in one century can be utmost
concluded without green area patches around VLSPV (Sinha et al., 2006).
During sunshine hours, absorption of solar energy reduces surface albedo. The temperature of themodule increases and heat dissipation from the back-sheet of the PV module is expected to increase thesensible heat flux and the ground temperature and affect land-surface interaction in long run. PVtemperature (cell temperature and back sheet) is therefore monitored along with ambient temperature in all7AugustDecember

100
1-10

2345678910 11 1213 14 1516 17 18 1920 21 2223 24HoursFig. 1b. Hourly variation of temp. diff. between PV Celland ambient in the month of January, May,
August and December.
0Temp Diff between PV and Am b

the seasons in Japan and India. Monocrystalline and multi-crystalline solar cellsof 3 W, 5 W, 10 W, 20 W, 40 W and 80
W modules from TATA BP, Kyocera andToshiba were used for collecting data.

May20

Temperature between(deg PVC)and Ambient (oC)Fig. 1a. Experimental set up atCREER, Bihar, India.

JanuaryC)

Temperature
betweenandAmbient(oC)
Temp diff betweenPVPVandAmbient(deg30Temp diff in Cloudy day-0.5-1

3. Results and Discussion-1.5Fig. 1 (b) shows hourly difference
variation of PV Cell Temperature and-2
ambient temperature in the month ofJanuary, May, August and December on-2.5
typical sunshine days. It clearly shows Fig. 1c. 1Hourly4 variation
71316
19 between
22of 10temperature
difference
PV
Hours
that during sunshine hours, cell
Cell and ambient temperature in a typical cloudy day.
temperature is more than the ambient
temperature. The difference is as large as 23 degree Centigrade during peak sunshine hour. However,during off-sunshine hours, i.e., when direct beam is not incident, cell temperature is less than the ambienttemperature. The difference is as large as 4 degree centigrade after cooling of the module stabilizes. Fig. 1(c) similarly shows the same phenomenon in case of cloudy day. This indicates that diffused radiation hasalso cooling effect. Diffused radiation on the other hand produces electromotive force (emf) from themodule but with less efficiency. Rise in cell temperature due to recombination process during day time canbe explained theoretically by Eq. (1) but the cooling of cell during off sunshine hours can not be explainedby this equation.
The two results collectively indicate that emf or load has no effect on cell temperature. It is direct
sunlight, which increases the cell temperature due to high rate of production of electron-hole pair and theirrecombination consequently. The effect of cooling during off-sunshine hours can be compared toevaporation from water surface, where ambient temperature is used for creation of electron-hole pair but
recombination does not take lace due to internal circuit formation in PV module. Therefore, cooling effectis observed in all types of PV modules.
Fig. 2 shows the variation of back sheet temperature of PV module, cell temperature, ambient
temperature and solar intensity in the month of October during cloudy day. Fig. 3 shows the corresponding
variation of open circuit voltage of PV module and Solar Intensity. The results are identical in all cases.
This shows that PV power and type has almost no effect on cooling mechanism. Results further indicatethat climate has no effect on this interesting characteristic of PV panels since experiments were carried outin Tokyo and Muzaffarpur (India) had same results. The cooling mechanism can be best explained on thebasis of internal loop formation which accounts for very small current and therefore no recombination
takes place between thermally generated electron-hole pairs from ambient temperature.
Since deserts are quite often enveloped by dust storms, this result is of critical importance. VLSPV in
diffused light would decrease overall temperature of the ambient.

830100
Solar Intensity

Temperature (oC)8070206050401025903020100Open Circuit Voltage

Solar Intensity

2080701560504010305201000678910 11 12 13 14 15Hours

16 17 18 19Fig. 2. Variation of Back sheet, Cell, Ambient
Temperature and Solar Intensity in a typicalcloudy day.

90
Solar Intensity (W/m2)

Ambient Temperature

Voltage (open circuit)

Cell Temperature

Solar Intensity (W/m2)

Backsheet Temp of PV

1000678910 11 12 13 14 15 16 17 18 19
Hours

Fig. 3. Variation of Open Circuit Voltage of PV
module and Solar Intensity in a typicalcloudy day.

4. Conclusions
Experimental investigations and the Physics of Photovoltaic system reveals that during off-sunshine
hours (absence of direct beam radiation), even if diffused sunlight is adequately available, moduletemperature will be always below atmospheric temperature. This phenomenon is likely to reduce Heat
Island Effect from Very Large Scale Solar Photovoltaic (VLSPV) Systems if installed in major deserts.
Acknowledgement
Authors are grateful to Nalanda Open University, Binar for partial financial for support in carrying out
the research work.
References
Bonan G.B., Oleson K.W., Vertenstein M., Levis S., Zeng X., Dai Y., Dickinson R.E., Yang Z.L. (2002): The land surface
climatology of the Community Land Model coupled to the NCAR Community Climate Model. J. Climate, 15: 3123–3149.
Kurokawa, K. (2003): Energy from the Desert: Feasibility of Very Large scale Photovoltaic Generation (VLSPV) systems, James &James, Sc. Publishers Ltd, London.
Pielke R.A. (2001): Influence of the spatial distribution of vegetation and soils on the prediction of cumulus convective rainfall. Rev.
Geophys., 39: 151-177.
Pielke R.A. (2002): Mesoscale meteorological modeling. 2nd Edition, Academic Press, San Diego, CA, USA.
Sinha S., Sanjay Kumar, Kurokawa K., Nishimura T., Kato M. (2005): Impact assessment of VLSPV on global climate, IEA Task 8internal assessment report.
S.Sinha, Sanjay Kumar, K. Kurokawa, M. Kato, T. Nishimura (2006): Global Climate Impact Study of VLSPV installation in Deserts,
Journal of Arid Land Studies, 15(4): 291-296.
Stohlgren T.J., Chase T.N., Pielke R.A., Kittel T.G.F., Baron J. (1998): Evidence that local land use practices influence regionalclimate and vegetation patterns in adjacent natural areas. Global Change Biology, 4: 495-504.