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Abstract:One of the biggest obstacles to economic profitability of solar water heating systems is the investment cost. Retrofitting existing domestic hot water heaters when a new solar hot water system is installed can reduce both the installation and material costs. In this study, retrofitting existing water heaters for solar water heating systems in Swedish single-family houses was theoretically investigated using the TRNSYS software. Four simulation models using forced circulation flow with different system configurations and control strategies were simulated and analysed in the study. A comparison with a standard solar thermal system was also presented based on the annual solar fraction. The simulation results indicate that the retrofitting configuration achieving the highest annual performance consists of a system where the existing tank is used as storage for the solar heat and a smaller tank with a heater is added in series to make sure that the required outlet temperature can be met. An external heat exchanger is used between the collector circuit and the existing tank. For this retrofitted system an annual solar fraction of 50.5% was achieved. A conventional solar thermal system using a standard solar tank achieves a comparable performance for the same total storage volume, collector area and reference conditions.

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One of the most important goals on solar collector development is to increase the system's annual performance without increasing overproduction. The studied collector is formed by a compound parabolic reflector which decreases the collector optical efficiency during the summer period. Hence, it is possible to increase the collector area and thus, the annual solar fraction, without increasing the overproduction. Collector measurements were fed into a validated TRNSYS collector model which estimates the solar fraction of the concentrating system and also that of a traditional flat plate collector, both for domestic hot water production. The system design approach aims to maximise the collector area until an annual overproduction limit is reached. This is defined by a new deterioration factor that takes into account the hours and the collector temperature during stagnation periods. Then, the highest solar fraction achieved by both systems was determined. The results show that, at 50° tilt in Lund, Sweden, the concentrating system achieves 71% solar fraction using 17 m 2 of collector area compared to 66% solar fraction and 7 m 2 of a flat plate collector system. Thus, it is possible to install 2.4 times more collector area and achieve a higher solar fraction using the load adapted collector. However, the summer optical efficiency reduction was proven to be too abrupt. If the reflector geometry is properly design, the load adapted collector can be a competitive solution in the market if produced in an economical way.