Spectrum splitting for efficient utilization of solar radiation: a novel photovoltaic–thermoelectric power generation system
© Elsarrag et al. 2015
Received: 19 August 2015
Accepted: 23 October 2015
Published: 4 November 2015
Standard photovoltaic solar cells (PV cells) use only about half of the light spectrum provided by the sun. The infrared part is not utilized to produce electricity. Instead, the infrared light heats up the PV cells and thereby decreases the efficiency of the cell. Within this research project, a hybrid solar cell made of a standard PV cell and a thermally driven thermoelectric generator (TEG) is being developed. The light of the sun splits at about 800 nm. The visible and ultraviolet part is transferred to the PV cell; the infrared part illuminates the thermal TEG cell. With the hybrid solar cell, the full solar spectrum is exploited. In this paper, theoretical and experimental results for improving the performance of thermoelectric elements coupled with photovoltaic modules have been presented. The proposed concepts and the experimental results have provided a key input to develop a large scale of a hybrid PV-TE system.
The basic idea of PV-TE was introduced by Tritt (2008) and Kraemer et al. (2011) who studied the utilization of both ultraviolet (UV) and infrared (IR) parts. Various papers have been published dealing with the combined use of thermoelectric and PV or solar thermal systems. Baranowski et al. (2012) claimed efficiencies of 15.9 % for concentrated solar thermoelectric generators (STEG) by developing a balance model and analyzing the present day materials under ideal conditions. A number of works on the STEG hybrids are based on concentrating solar power on to TEGs. Chávez Urbiola and Vorobiev (2013) designed and tested such a system with co-generation of hot water which was used as the coolant for the TEG hotside and achieving 5 % electrical efficiency. The studies conducted by Eswaramoorthy and Sanmugam (2013) and Kalogirou (2013) on the use of such systems in specific geographic locations gave more insight into the feasibility and possibility of large scale deployment of the systems. Leon et al. (2012) and Lertsatitthanakorn et al. (2013a, 2013b) evaluated the possibilities of concentrated solar power on hybrid systems using different strategies for TEG design and the cooling technique. Lippong et al. (2012) successfully implemented a cooling mechanism for solar TEG hybrid using phase change material and implied the possibility of using it as a sustainable system for independent operation. McEnany et al. (2011) developed an analysis model and denotes that, with the presently available materials and technology, efficiencies of more than 10 % can be achieved using solar TEG hybrid systems by the cascading of TEGs and under high temperature and optical irradiance operation. Meir et al. (2013) suggested controlled shaping of electric potential distribution in the thermoelectric converters for more efficient generation of thermoelectric energy, in theory. Mizoshiri et al. (2012) tested a hybrid system by implementing spectrum splitting on a thin-film TEG and focusing the near infra-red (NIR) radiation onto the TEG while the PV received the rest of the spectrum. The use of thin-film selective absorber coating for TEGs in the performance of hybrid systems was investigated by Ogbonnaya et al. (2013). Van Sark (2011) developed a model to analyse the feasibility of a PV-thermoelectric module in outdoor conditions and provided very optimistic results by considering ideal conditions of operation. Advances in the related fields such as: (1) the development of high-performance spectrally-selective solar absorber based on a yttria-stabilized zirconia cermet with high- temperature stability by Wang et al. (2011), (2) thin-film TEG model by Weinstein et al. (2013) which can be used in place of conventional TEGs with minimal losses, and (3) the multi-hybrid cell by Yang et al. (2013), which can harvest mechanical, solar and thermal energy at the same time, provided strength to the optimistic feasibility predictions of van Sark and Zhang et al. (2013) to come true. One such promising field is the solar spectrum splitting for energy co-generation. Within all these works, the splitting of the solar spectrum was discussed theoretically but not investigated in an extensive practical manner, except for Mizoshiri et al. (2012) who generated an open voltage of 79 mV.
This study will investigate the performance of a thermoelectric generator by changing its material constitution and design features. The TEG is anticipated to be integrated with PV modules to form a hybrid photovoltaic–thermoelectric generator and increase the overall conversion efficiency from solar irradiance to electricity.
The first system setup
The test rig was designed to allow independent movements of the system components. It offers enough space to test different types of PV cells, absorbers and beam splitters. In the test rig both the PV cell and the TEG can be cooled, the input and output temperature of both coolers can be monitored. Both coolers use a liquid cooling media provided by a radiator cooling tower with an estimated cooling power of 1000 W. The lowest possible temperature depends on the surrounding temperature during measurements.
It has to be evaluated within the project if a tailored mirror with 800 nm or another cut-off wavelength will achieve better performance or not. As the project aims to use commercially available parts to minimize system costs for the final hybrid module, the mirror from OpticBalzers was considered to initiate the tests.
In the next simulation step, a TEG and absorber were included in the model with a parameterized footprint area and height. The cold side of the TEG was attached to a 45 °C surface with a thermal conductivity of 1000 W/mK. The thermal conductivity between TEG and absorber plate was set to infinite.
For the next steps, the TEG model will be enhanced using the Comsol models developed by Jägle et al. (2008). Using the material data of the real TEG modules, the real performance of the system can be evaluated with a good accuracy.
The second system setup
A second set of tests was conducted to compare the performance of the Hybrid PV system with a standard system. The Hybrid system consisted of a small size (15 cm × 15 cm) monocrystalline, custom made, low power PV Panel and a comparable sized TEG, Model HiZ-2, (2.9 × 2.9 cm). The setup makes use of the Bismuth Telluride based ‘HZ-2’ TEG Model from Hi Z (Product Page: Hi-Z 2015) which accommodates 97 thermocouples in 2.9 cm × 2.9 cm × 0.508 cm and has a conversion efficiency of 4.5 %. The TEG typically produces 2.5 Watts at 3.3 volts at Matched Load with a 200 °C temperature gradient between the surfaces at 30 °C ambient temperature. The standard system had a similar PV only setup. The testing was conducted in a solar simulator chamber (Model: SEC 1100, Manufacturer: Atlas) [Product Page: Atlas SEC 1100 (2015)].
Results and discussion
The first setup results
As revealed in Fig. 4, the absorbance of “Metal Velvet™” keeps at 100 % corresponding to any values of wavelength from 0 to 10,000 nm. The maximum temperature difference was around 18 K under the solar radiation level at 1.1 suns as shown in Fig. 7a. The corresponding power output at 1.1 suns was about 32 mW. However, due to the selective absorbance characteristic of “Tinox® energy Al”, the absorber greatly reduced the radiation emission heat loss and sustained a higher temperature between the hot and cold surface. As shown in Fig. 7a, b, the power output and the temperature difference between the two sides of “Tinox® energy Al” absorber-TEG assembly were always higher than the “Metal Velvet™” absorber-TEG assembly under different solar irradiation conditions of 500, 700 and 1000 W/m2.
The two absorbers being compared in this part were “KG-1” and “Tinox”. The thickness of the heat absorber glass KG-1 was 3 mm. The thermal mass of “KG-1” is 40 times higher than the non-transparent absorber “Tinox” which was deposited on a 0.2 mm aluminum foil. Owing to the high thermal mass, the response time to a radiation change of “KG-1” absorber was much higher.
The second setup results
This study investigated the performance of a photovoltaic (PV) and thermoelectric generator (TEG) assembly by changing its material constitution and design features. The TEG is anticipated to be integrated with PV modules to form a hybrid photovoltaic along with a sunbeam splitter to increase the overall conversion efficiency from solar irradiance to electricity.
The thermoelectric conversion efficiency is proportional to the temperature difference between the absorber’s hot and cold surfaces; however, the PV efficiency reduces with the increase of its temperature. The methods used to enhance the hybrid system performance were proposed. Their corresponding experiments were performed and the initial results were presented. Conclusively, proper selections of selective absorbance materials of the absorber are contributive to the thermoelectric generation. Alleviation of the convective heat loss from the surface of the absorber results in substantial positive impact to a TEG. The PV showed a better overall performance with the beam splitter. The proposed concepts and the positive experimental results provide useful information and reference for the further development of a hybrid PV-TE system for field testings.
The authors would like to acknowledge the Qatar National Research Fund for funding the presented work in the NPRP: 5-363-069.
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