Integration of photovoltaic and thermoelectric

system to harvest wide spectrum of solar radiation

Gaurav Kumara

a Department of Mechanical

Engineering, 2181

Glenn L. Martin Hall, Building 088

University of Maryland, College Park, MD 20742, USA

Abstract

The most advanced modern day Photovoltaic (PV)

cells can use only about 50% of the solar radiation 17 since only low

wavelength solar spectrum is used by PV cells to generate electricity, high

wavelength infrared radiation and higher are not used by PV cells. To make

things worse, infrared radiation heats up the PV cells increasing its operating

temperature and thereby reducing its efficiency. This study reviews the integration

of photovoltaic (PV) and thermoelectric (TE) technologies for harvesting wide range

of solar spectrum to generate electricity. One of the common techniques

employed to do so is ‘spectrum beam splitting’ in which short wavelength

spectrum is directly used by PV devices to generate electricity and high

wavelength solar radiation is used to generate electricity using TE devices 1.

With integrated PV and TE system, full solar spectrum can be exploited.

Keywords: Photovoltaic cell; Thermoelectric device; Spectrum

beam splitting; Integrated device.

1. Introduction

European Union and China have planned to

harvest 20% and 15% respectively, of their total power generation from

renewable energy sources by 2020 and globally there has been an equally strong

drive towards renewable energy sources 1. The amount of solar radiation incident

on earth’s surface is approximately 1.2 × 105 Tera Watts whereas the

present energy consumption of the planet is roughly three orders of magnitude

lower 2 3. Artificially solar power can be harvested either in forms of electricity

(solar photovoltaic PV) or heat energy (solar thermal). Theoretically, the

upper limit of conversion efficiency of solar PV panels is estimated to be approximately

30% 4 5 6 7. However, practically the efficiency of these devices are

affected by multiple parameters and typically it is 40–50% below the upper

limit mentioned above 8. Due to such limitations on efficiency of these

devices it is practically impossible to harvest wide spectrum of solar

radiation using a single semi-conductor material 9. Although a combination of

various specialized materials with intermediate band gaps can be used to

overcome this problem, but it would expensive and hence would not be feasible

10 11. On the other hand, within the last decade there has been intense research

on thermoelectric materials which can convert waste heat directly into

electricity, to increase the efficiency of energy consumption. This opens up a

door to an alternative option which is to integrate PV cells with thermo-electric

generator which can convert solar thermal energy into electricity. However, low

efficiency of TE devices and elevated operating temperature of PV cells which

reduces its efficiency are some of the major obstacles in this technique. In

order to alleviate these problem, spectrum beam splitting technique has drawn

attention in which PV cells and TE devices would run in parallel, and the beam

splitter would allow only low wavelength solar radiation to reach the PV cells,

thereby reducing its operating temperature as well as the heat load 1213. Fig.

1 depicts the fraction of solar spectrum which can be used for PV cells and TE

devices respectively in an integrated system 17. In order to achieve good

performance for these systems it is important to create a sharp spectral split

between the photovoltaic (low wavelength) and thermal (high wavelength) components

of the system and then convert the solar irradiation into electron–hole pairs

or heat, respectively. Spectral beam splitting technique has multiple

advantages like eliminating the current-matching condition for stacked multi-junctions,

eliminating the requirement for lattice matching between neighboring cells, the

system is reasonably insensitive to spectral conditions like fluctuations in

humidity etc., and most importantly it minimizes photon entropy which has been

shown to increase the theoretical maximum efficiency by approximately 5% 26.

Fig.1 Fractions of solar spectrum used for PV and TE devices 17

2.

Related work

The basic idea for a hybrid of PV and TE

devices had been published in 2008 by Tritt 19. Since then many papers have

been published on combining PV and TE devices, Baranowski et al.

(2012) 20 claimed efficiency of 15.9 % for concentrated solar thermoelectric

generators (STEG). A substantial amount of work has been dedicated towards

concentrating solar power on to thermoelectric generators, Chávez Urbiola and

Vorobiev (2013) 21 designed a hybrid system in which hot water produced from

co-generation was used as a coolant for hot side of TE generator and achieved

approximately 5% electrical efficiency. Leon et al. (2012) 22 and

Lertsatitthanakorn et al. (2013) 23 24 used different strategies for

designing TEG and cooling technique to evaluate the effectiveness of

concentrated solar power on hybrid systems. McEnany et al. (2011) 25 claimed

that under high operating temperature and optical irradiance, more than 10%

efficiency can be achieved by cascading TEGs.

3. Description of integrated system

Physically

there is Shockley–Queisser limit of 31% on solar energy conversion efficiency by

photovoltaic cells 18 which in turn puts a limit on the power density for

space constrained applications. There are three major factors responsible for this

loss in solar conversion efficiency of a semi-conductor device with single

absorption threshold: (1) loss S1, energy barrier where photons below threshold

energy Eg are not absorbed; (2) loss S2, thermalization where

photons with energy higher than Eg can generate electron-hole pairs and

lose energy in the form of heat; and (3) loss S3, small fraction of excited

states recombine with the ground state 14.

Areas S1, S2, and S3 as shown in Fig. 2 are

representatives of these three losses respectively and only S4 can be delivered

to the load. As can be seen, when distance between the outer curve and inner

curve reduces, type-3 loss also reduces. The actual work W by each absorbed

photon can be calculated using the equation below 10:

where K, e, h, c, T, C,

are Boltzmann constant, charge of an electron,

Planck’s constant, speed of light, operating temperature, concentration ratio

and solar flux in photons respectively. From above equation it can be derived

that voltage at maximum power is given by:

Fig.2

Graphical analysis of the efficiency of a single band gap PV cell 10

The effect of operating temperature on PV cells

can be derived from the above equations, and as can be seen in Fig. 3,

efficiency of PV cell reduces as operating temperature increases, which

indicates that cooling/ super-cooling of PV cell is instrumental in affecting

performance of PV cells. Similarly, effect of concentration ratio on PV cell

performance has been studied and as can been from Fig. 4, concentration ratio

has much smaller effect on cell performance as compared to operating

temperature, however concentrating devices has cost benefits i.e. by reducing

the amount of area required to convert a fixed amount of solar energy 13.

However increasing concentration ratio has indirect negative effects too as it

can increasing the PV cell temperature or cooling load per unit cell.

Fig. 3

Effect of operating temperature on efficiency of PV cell Fig.4

Effect of concentration ratio on performance of PV cell

For semi-conductors with wide band gaps, a

large amount of solar energy is not absorbed since energy of photons is less

than Eg. Hence if this portion of solar spectrum can be removed using

a spectrum beam splitter, it would not only reduce the cooling load, but this

energy can also be used by TE generators for energy recovery 15.

Just like PV cells, the performance of

thermoelectric generators also depends on several factors which are the hot

side temperature Th, cold side temperature Tc and

thermoelectric constant ZT of the material used for TE device:

where

k,

and S are material properties represent the thermal

conductivity, electrical conductivity and Seebeck coefficient respectively.

Typically thermoelectric constant ZT= 1 for modern thermoelectrics, ZT= 2 for

nano-scale microstructures produced in lab and ZT= 4 for quantum tunneling

thermionic converters 9. Effect of hot side and cold side temperatures on

efficiency of TE device are shown in Fig. 5 and Fig. 6 respectively. Mathematically

it is evident that increasing the hot side temperature and reducing the cold

side temperature would result in better efficiency of TE devices, however reducing

cold side temperature is more prominent when it comes to TE devices with small

thermoelectric constant. Using a TE device to harvest long wavelength solar

spectrum in combination with a PV cell can boost the overall performance of the

system by approx. 10% of the PV 17. Also, it can be seen that as value of ZT

for the material increases the theoretical efficiency approaches Carnot

efficiency. Thus using a TE device to harvest long wavelength solar spectrum in

combination with a PV cell can boost the overall performance of the system by

approx. 10% of the PV 17

Fig. 5

Effect of hot side temperature on efficiency of TE device Fig. 6 Effect of cold side temperature on

efficiency of TE device

As can be seen in Fig. 7 1, in order to

ensure low operating temperature for PV cell and low cold side temperature for

TE device, the PV cell and TE generator are constructed on different sides of

the cooling chamber. The beam splitter directs the short wavelength solar

radiation towards the PV cell and long wavelength radiation is used to generate

high temperature thermal energy which is used for hot side of the TE generator.

During off-peak times cooling chamber is cooled using ambient air, and excess electricity

is stored in a deep freezer in form of high grade cold which can be used by

cooling chamber during peak times to enhance the power output of both PV cell

and TE device.

Fig. 7

Schematic of integrated PV-TE device 1

4. System performance

The overall efficiency of the integrated system

is defined as:

where

and

represent the operational efficiencies of PV

and TE system respectively which are ratios of actual power output to the ideal

power output for PV and TE systems respectively. Operational efficiency takes

into consideration the heat losses too that occur in heat storage process for

TE device. Typically operational efficiencies for advanced PV and TE devices

are ~0.8, hence

would be a reasonable approximation 9. Total

conversion efficiency can be written as a function of

,

C, ZT,

and

:

An optimal band gap

can be determined by maximizing the above

mentioned

depending on PV operating temperature,

concentration ratio, and hot and cold side temperatures. Fig. 8 1 shows that

optimal band gap does not vary much with change in operating temperature, and

optimal band gap reduces with increase in concentration ratio. With optimal

band gap, contour of constant total efficiencies has been plotted with

operating temperature and concentration ratio as shown in Fig. 9 1 which

reveals that for same operating temperature, higher concentration ratio gives

higher total efficiency. An increase of concentration ratio by 10 times

increases the total efficiency by approx. 0.8%, which is equivalent to reducing

the operating temperature by 14K. Also regardless of the concentration ratio,

the total efficiency can be improved by approx. 30%, by reducing the operating

temperature to 160K 1. However increasing the concentration ratio increases

the cooling power load due to which the concentration ratio must be kept below

100 for all applications 16.

Fig. 8 Change in optimal band gap with Tc

and C 1 Fig. 9 Contour

of total efficiency with Tc and C 1

Total efficiency of the integrated system also

increases with increase in hot side temperature as can be seen in Fig. 10 1,

however the rate of increase of total efficiency reduces with increase in

.

As compared to

,

doubling the thermoelectric constant ZT results in approx. 2% increase in total

efficiency.

Fig. 10 Total efficiency with Th and

ZT 1

Conclusion

This paper reviews the concept of a hybrid

PV-TE device for efficiently harvesting wide solar spectrum. An optimal band

gap material would perform well at both peak and off-peak times over wide range

of operating temperatures and increasing the hot side temperature,

concentration ratio and thermoelectric constant will improve the overall

efficiency of the integrated system. The simple structure also ensures that

this system can potentially be used as domestic power generator.

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