Obtaining higher intensity using solar simulator

Figure 5.8 shows a photograph of the experimental setup established for this study and a schematic diagram of the system is shown in Figure 5.9. The experimental setup in section 4.5.1 was adopted and improved by including: a flow meter, a DC Pump, a heat exchanger, a DC power supply and type K thermocouples.

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Figure 5. 8 A photograph of the experimental setup

- Flow meter

The water flow meter was purchased from RS (UK-RS/198-3029. http://uk.rs-online.com/web/p/flow-sensors-switches-indicators) and was used to measure the mass flow rate of the cooling fluid. This would allow the heat removed from the cold side of the TEG to be calculated. The specifications and details of the flowmeter are given in Appendix.11.

-DC Pump

A DC pump (model DC30A-1230) was used to circulate the cooling fluid to the heat exchanger. The voltage applied was constant at 3 V, and a photograph of the DC pump is shown in Figure 5.10.

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Figure 5. 9 A schematic diagram of the experimental setup

Figure 5. 10 Photograph of the DC pump

98 - Heat exchanger

A heat exchanger (9 cm X 9 cm X 2.2 cm) was used to transfer the heat effectively from the cold side of the TEG. It consists of a copper plate (9 cm x 9 cm x 0.2 cm) with seven fins (0.7 cm x 0.6 cm x 6 cm), which can remove the heat efficiently from the cold side of the TEG. Figure 5.11 shows a photograph of the heat exchanger.

Figure 5. 11 Photograph of the heat exchanger - DC power supply

A DC power supply (Type UK-FARNEKK) was used to supply the voltage to the DC pump in order to circulate the fluid and take the heat away from the cold side of the TEG.

- Type K thermocouples

Four type K thermocouples were used to measure the temperature in four places. They were positioned to measure: TH, TC, the inlet fluid temperature (Tin) and the outlet fluid temperature (Tout).

5.2.2.2 Results and Discussions

By changing the distance between the solar lamp and the PV surface, the maximum light intensity of 2 suns can be obtained in this system. An increase of light intensity from one sun to two suns was achieved by reducing the distance between the PV and the solar simulator from 18 cm to 10.5 cm. The optimised PV/TEG hybrid system in Chapter four

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was selected to apply 2 suns and investigate the Ptot and the thermal power (Pther). The PV cell was characterised initially without the TEG by attaching it directly to the cold side of the cooling system. One sun and two suns of light illumination were applied respectively.

Figure 5.12 shows the I-V curves of the PV cell under one sun and two suns. It can be seen that an increase in the light intensity from one sun to two suns resulted in an increase in the ISC from 50 mA to 103 mA (i.e., more than doubled), while the VOC remained almost constant. It can also be seen that the temperature of the PV cell only increased from 25 ⁰C to 26.5 ⁰C when the light intensity was increased from one sun to two suns because all the heat was transferred quickly from the PV cell to the heat exchanger.

Subsequently, the TEG was inserted between the PV cell and the cold side of the heat exchanger, which introduce an additional thermal resistance between the PV cells and the cold side of the heat exchanger, depending on the geometry of the TEG. Figure 5.12 also shows the I-V curves of a photovoltaic cell on top of a thermoelectric generator in the hybrid system under 1 sun and 2 suns.

Figures 5.13 shows the P-V curves of the PV cell under one sun and two suns while attached directly to the cold side of the heat exchanger, without the TEG. It can be seen that the Pmax was also improved by increasing the light intensity.

Figure 5.14 shows the P-V curves for the PV cell on top of a TEG in a hybrid PV/TEG system. An increase in the temperature of the PV cell was observed due to an increase in the thermal resistance between the PV cells and the cold side of the heat exchanger. The temperature of the PV under one sun was 42 ⁰C in this case and increased to 51 ⁰C under two suns.

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Figure 5. 12 The I-V curves of photovoltaic cell alone under 1 sun and 2 suns, and a photovoltaic cell on top of a thermoelectric generator in the hybrid system under 1 sun and 2 suns

Figure 5. 13 The P-V curves of the photovoltaic cell alone under 1 sun and 2 suns 0

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Figure 5. 14 The P-V curves of a photovoltaic cell on top of a thermoelectric generator in the hybrid system under 1 sun and 2 suns

The increase in the temperature of the PV cell caused a drop in Pmax from 74 (mW) to 71.4 (mW) under the one sun illumination. This represents a decrease of the power output by 3.5 %. A decrease by 4.5 % was observed for the case of two suns illumination.

The power output of the TEG in the PV/TEG hybrid system was measured under the same test conditions. The I-V and P-V curves for the TEG are presented in Figures 5.15 and 5.16 respectively. Unlike in the PV cell, both ISC and VOC of the TEG were increased with increasing ΔT as shown in Figure 5.15. The increase in the VOC of the TEG was significant, from 0.21 V to 0.38 V, representing an increase by 81 %.

There was a power gain from TEG when moving from one sun and two suns as shown in Figure 5.16. The power generated from the TEG in the hybrid PV/TEG system was 5.6 mW under one sun, and 18 mW under two suns because the T across the TEG was increased from 17 ⁰C under one sun to 26 ⁰C for two suns.

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Figure 5. 15 The I-V curves of a thermoelectric generator in the hybrid system under 1 sun and 2 suns

Figure 5. 16 The P-V curves of a thermoelectric generator in the hybrid system under 1 sun and 2 suns

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As a result, an increase in the overall power output of the hybrid system (Ptot) due to integrating the TEG is achieved, which is of about 4 % under one sun and 8.9 % under two suns, respectively as shown in Table 5.5.

The ηPV, ηTE and ηtot arepresented in Table 5.5. It can be seen that there is an increase of 6.5 % under one sun and 8.9 % under two suns due to the top-up power from the TEG. It is interesting to mention here, the ηTE was calculated using the total power input to the system.

Table 5. 5 The maximum power output and conversion efficiency of TEG, PV and PV/TEG under one sun and two suns

Clearly, integrating TEG with PV can improve the power output from the system and has little effect on removing heat from the PV cell to the heat exchanger, which will help to increase the lifetime of PV, in particular if the operating temperature of the PV is close to room temperature.

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