Although the idea of a solar collector for both electricity and hot water is not new, the new generation studied here is based on a very innovative concept. During the previous feasibility study we collected comprehensive information about all important aspects (costs, technology, market, contacts with industries, …) to prove that further development is worth being done. The results were encouraging since they showed:
- A potential market does exist for several specific applications (about 10 MW in 2005)
- The photovoltaic (PV) thin film technology is likely to be suited for this application from a technical and a financial point of view, provided that the long term stability of the cells at temperatures above 100°C is confirmed.
- Several photovoltaic industries are ready to collaborate in this development at different levels of participation.
- The several technical concepts proposed are suited for the concerned application.
World-wide (USA, IEA, ...) there are several projects with the same topic, although with a different working plan (absorption, temperature behaviour and emissivity are not addressed in priority).
The feasibility study showed that the competitiveness of a hybrid collector depends on several technical requirements of the integrated PV-module. The most prominent aspects are high solar absorption, compatibility with high temperatures and a low emissivity.
The goal of the present study is to verify whether these technical conditions are met for the available amorphous silicon (a-Si) technology. Commercial unencapsulated samples from 6 different manufacturers based on different substrates (glass, stainless steel, polyimide) were measured.
Absorption values were measured between 78% and 90%. Two mesurement methods were used (spectrometry, microcalorimetry). Both methods gave close results. A link between the figures found and the three substrate categories could not be established.
Thanks to the thermal cycling test, very valuable data (also for other applications) was gained. Some samples that were stored at 210°C for 10 hours kept their original properties. The other samples showed modified characteristics after this treatment. The stability under thermal cycling seems to be connected to absence of aluminium in the back contacts (Al diffusion). An improved stability can be obtained with a layer of oxide between the back contact and the a-Si films.
Further, the emmissivity coefficients of the samples were measured. The results show that conventional top cover materials like glass, EVA or Tefzel are non-selective materials that have a high emissivity coefficient (between 86 and 95%).
Since the a-Si cells seem to be stable under thermal cycling, our recommendation is to develop a new encapsulating material with a low-e surface and an improved durability under high temperatures, which will enable the construction of a hybrid PV/T collector with the thermal performance of a standard flat plate collector.
