Retrofitted Solar Domestic Hot Water System for Single-Family Electrically-Heated Houses: Development and testing

Sammanfattning: In Sweden, there are roughly half a million single family houses that use electric heating for both domestic hot water production and space heating. Since electricity is the most expensive energy source in Sweden, it is interesting to install a high solar fraction solar thermal system to reduce the energy use in these houses. One of the most significant bottlenecks for deployment of solar heating technology is economic profitability due to high investment costs. During this investigation a system that retrofits existing domestic hot water heaters was developed. This has the potential to significantly reduce the investment costs regarding both material and installation. Three different collectors were also tested. Their suitability to be part of the retrofitted solar domestic hot water system was addressed. These were a one-axis tracking photovoltaic/thermal (PV/T) concentrating hybrid and two stationary compound parabolic concentrating (CPC) collectors with the same geometry. One of the CPC collectors was a thermal collector while the other was a PV/T hybrid collector. The aim for PV/T concentrating hybrid collectors is to produce both heat and electricity at a lower cost than conventional alternatives. The CPC-thermal collector design aims to adapt the solar production to the yearly consumption profile in order to achieve a higher annual solar fraction without increasing the stagnation periods. Outdoor measurements were carried out to characterise the collectors. For the retrofitted system, several system configurations were theoretically analysed by means of simulation models using TRNSYS software. A prototype of the retrofitted system was built and the simulation model was revised and validated against measurements. The validated model was further used to optimize the performance of the system and to carry out a sensitivity analysis. Furthermore, an add-on retrofitting unit that is able to connect solar collectors to existing hot water heaters was also designed and built. Measurement results showed that the efficiency values of the tracking concentrating PV/T hybrid are lower than those of conventional flat plate collectors and PV modules. The usable incident irradiation on the one-axis tracking concentrating surface is also lower than the usable irradiation incident on an optimally tilted flat surface. Even though the studied hybrid has potential for improvement, the combination of low efficiencies with low usable irradiation levels makes it difficult for concentrating PV/T hybrids to compete with conventional alternatives, especially in countries where the annual beam irradiation values are low. The geometry of the tested stationary CPC-PV/T hybrid collector was shown not to be the most appropriate for hybrid applications. The studied CPC-thermal collector system was found to achieve a higher annual solar fraction than a conventional flat plate collector system while making use of less absorber surface, one of the most expensive components in the collector. It is concluded that the CPC-thermal collector is adequate to be part of the retrofitting solar domestic hot water system in Swedish single family houses. The decrease in absorber area together with the increase in performance must compensate for the additional cost of extra materials such as reflectors, glass and frames. The investigation of the retrofit system showed that an annual solar fraction of 58 % can be achieved in Sweden when a 200-litre hot water heater is retrofitted and 6 m2 of flat plate collectors is used. The investment cost of the developed retrofit solution was estimated to be reduced by roughly one third compared with a conventional solar domestic hot water system. This means that the developed retrofit system achieves a comparable performance with conventional solar domestic hot water systems in single-family houses with a significant reduction in investment cost. The developed retrofitting solution is therefore considered to have the potential to become a competitive solution in the solar domestic hot water market.

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