Hybrid photovoltaic and solar thermal (PVT) systems for solar combined heat and power
Creators: name: Guarracino, Ilaria
Publisher: name: Imperial College London
Year: 2017

description: Solar is a particularly promising sustainable energy source in terms of its potential to displace the burning of fossil fuels for heat and power, heating and even cooling, albeit at a cost. The sun load-factor profile has a close and predictable match to the daily varying energy demand for heat and electricity, both thermal and electrical, and thermal storage for periods of low irradiance can be made readily available. In addition, solar thermal technologies can provide a significant fraction of the hot water demand in households, as well as space heating and cooling in residential buildings and for industrial facilities. In fact, solar heating has been proposed as one of the leading solutions in terms of its potential for greenhouse gas abatement [1]. At the small scale, photovoltaic systems presently dominate the domestic solar market with solar to electrical conversion efficiencies of around 15% and at a competitive cost for the building owner. Solar photovoltaic installations were encouraged in Europe at the local level with financial support and now constitute a large and mature market with continuously falling prices. Solar thermal systems are able to make use of a larger proportion of the solar resource as they convert solar energy into heat with a higher efficiency than the PV conversion efficiency. Moreover, the low temperature heat may be used to satisfying the largest portion of the demand for thermal energy that is currently met by fossil fuels. The development of the solar thermal market is strongly dependent on the availability of the local irradiance level and on the cost of the alternative sources of thermal energy. In some countries in Europe the solar thermal market is quite mature (e.g. Austria), whilst in others, such as in the UK, solar thermal energy still contributes marginally to the energy mix and solar thermal systems are not yet cost competitive. Due to the high costs of solar thermal energy systems, these constitute a relatively small market at present with the potential to grow substantially in the near future. A competitive solution for energy (heat and power) provision in buildings is the development of combined solar photovoltaic/thermal (PVT) systems which produce both electricity and heat simultaneously from the same aperture area. This solution is particularly suited to residential applications in urban areas, where the demand for electricity is accompanied by a demand for low temperature heat, and space for solar installations is scarce. Many alternative technologies for PVT integration exist and PVT units can be coupled with various systems for domestic hot water generation and/or space heating. At the design stage of a PVT system, decisions have to be made on the absorber characteristics (consisting of thermal collector and PV laminate), on the thermal to electrical yield ratio and on the application (industrial or residential application, stand alone or grid connected). These design parameters influence the requirements on the fluid temperature and electricity output, and the overall efficiency. In addition, system control can significantly impact the potential of such systems in terms of their performance characteristics in different applications. The aim of this present research effort was to demonstrate the technical and practical feasibility of a novel, high-efficiency hybrid PVT water system, by considering an affordable, small-scale, modular unit that can be scaled easily to cater to varying demand levels. The research investigated the technical issues related to PVT panel technology, by looking in particular at the optical efficiency of the PV cells, at the heat transfer from the PV cells to the fluid, and at the integration of such a unit in a heat and power provision system that attempts to match generation and local demand. A detailed numerical model was developed that constitutes a tool for testing various collector and system designs. The model was validated against experimental data. An experimental apparatus was designed and constructed for the purpose of evaluating the collector model and for collecting a database of performance data on PVT collectors. Collector performance data are scarce at the moment due to the relatively small market size, thus the work constitutes a reference for further development and analysis of this type of collectors. Steady-state tests and dynamic tests were performed on PVT collectors and the results were used to develop a reliable model of collector performance over a wide range of time-varying operating conditions. The model allowed for assessments of various solar PVT system designs under different operating conditions and control strategies. Result showed that such systems may underperform if their operation and design is not designed specifically for the local weather conditions and user-demand specific requirements. It is envisaged that emissivity control applied to the solar cells should be adopted for PVT system application, especially if higher operating temperatures are required (e.g. in combination with thermally driven/heat powered cooling systems). The numerical model confirms that solar cells a with low emissivity coating can maximise the thermal energy output of a PVT system. The potential of improved PVT systems is finally assessed from an economic perspective, in an analysis that considers the potential cost reduction of PVT systems in relation to alternative technologies used as a benchmark.