Hybrid photovoltaic-thermal solar systems for combined heating, cooling and power provision in the urban environment

Solar energy can play a leading role in reducing the current reliance on fossil fuels and in increasing renewable energy integration in the built environment, and its affordable deployment is widely recognised as an important global engineering grand challenge. Of particular interest are solar energy systems based on hybrid photovoltaic-thermal (PV-T) collectors, which can reach overall efficiencies of 70% or higher, with electrical efficiencies up to 15-20% and thermal efficiencies in excess of 50%, depending on the conditions. In most applications, the electrical output of a hybrid PV-T system is the priority, hence the contacting fluid is used to cool the PV cells and to maximise their electrical performance, which imposes a limit on the fluid's downstream use. When optimising the overall output of PV-T systems for combined heating and/or cooling provision, this solution can cover more than 60% of the heating and about 50% of the cooling demands of households in the urban environment. To achieve this, PV-T systems can be coupled to heat pumps, or absorption refrigeration systems as viable alternatives to vapour-compression systems. This work considers the techno-economic challenges of such systems, when aiming at a low cost per kW h of combined energy generation (co-or tri-generation) in the housing sector. First, the technical viability and affordability of the proposed systems are studied in ten European locations, with local weather profiles, using annually and monthly averaged solar-irradiance and energy demand data relating to homes with a total floor area of 100 m(2) (4-5 persons) and a rooftop area of 50 m(2). Based on annual simulations, Seville, Rome, Madrid and Bucharest emerge as the most promising locations from those examined, and the most efficient system configuration involves coupling PV-T panels to water-to-water heat pumps that use the PV-T thermal output to maximise the system's COP. Hourly resolved transient models are then defined in TRNSYS, including thermal energy storage, in order to provide detailed estimates of system performance, since it is found that the temporal resolution (e.g. hourly, daily, yearly) of the simulations strongly affects their predicted performance. The TRNSYS results indicate that PV-T systems have the potential to cover 60% of the combined (space and hot water) heating and almost 100% of the cooling demands of homes (annually integrated) at all four aforementioned locations. Finally, when accounting for all useful energy outputs from the PV-T systems, the overall levelised cost of energy of these systems is found to be in the range of 0.06-0.12 epsilon/kW h, which is 30-40% lower than that of equivalent PV-only systems. (C) 2017 The Authors. Published by Elsevier Ltd.