Construction, Operation and Measurement of TEPVIS in Africa

In September 1995 a TEPVIS-tank was ordered at a locksmith’s shop in Harare (Zimbabwe). The material used for the construction was galvanized steel [21] (see Fig. 10.20).

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Fig. 10.20. Tank for reduction of cell temperature and serving as module mount/ foundation, manufactured in Harare (Zimbabwe) by galvanized sheet steel (without internal convection aid).

Two PQ 10/40 multi-crystalline PV modules (by Telefunken, now ASE – Schott/RWE) have been selected out of a series of twelve, according to similar short-circuit currents and open circuit voltages (see Fig. 10.21). To eliminate even small possible measurement errors, the modules were interchanged and retested after each day of measurement. The figures below are showing the average values of the measurements. The Maximum-Power – Point (MPP) was tracked manually by power metering and a variable ohmic load. Irradiance was measured by a BM 5 pyranometer.

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Fig. 10.21. Comparative measurement of TEPVIS (in the back) with a standard reference PV module (in the front), both based on PQ 10/40 together with a BM 5 pyranometer during the measurements in 1995 in Zimbabwe.

 

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Fig. 10.22. Comparison of module temperatures for a conventional multi-crystalline PV module with TEPVIS (both based on PQ 10/40) during a clear day in Bulawayo, Zimbabwe (20°10′ S, 38°37′ E).

 

The results for the tests carried out in Bulawayo, Zimbabwe are presented in the Figures 10.23, 10.24, and 10.25. Despite quite favorable weather conditions –

 

relatively cold nights and cloudless days (however breezy) – the gains attained at the experiments in Berlin, Germany could not be achieved: The yields increase of TOEPVIS (without optical improvements) over the conventional installation (averaged over two tests with exchanged solar modules) amounted only 6.4%. The reasons for the lower gain could be found in the flatter inclination of construction (20° instead of 33° module elevation angle), and the absence of a “convection accelerator” (a plate of 2/3 of module length, installed inside the tank parallel to the module surface at a distance of 15 cm to it). Both facts reduced thermal convection inside the tank and led to a high thermal stratification (ca. 10 K temperature difference the top and at the bottom of the tank), which caused higher operation temperatures for the module. To avoid the negative effects of stratification, a new design with a tank having the main part of its reservoir above the module is under construction. On the other hand it has to be examined whether the cost reducing effect by substitution of the concrete foundation with the tank is still possible with that new design.

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Fig. 10.23. Comparison of PV conversion efficiencies for a conventional multi­-crystalline PV module (PQ 10/40) with TEPVIS during a clear day in Bulawayo, Zimbabwe.

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Fig. 10.24. Electrical power output at MPP of TEPVIS during a clear day in comparison to a conventional PV module (Location: Bulawayo, Zimbabwe, 20°10′ S, 38°37′ E).

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