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

Abstract

Solar energy can play a leading role in reducing the current reliance on fossil fuels and in increasingrenewable energy integration in the built environment, and its affordable deployment is widely recog-nised as an important global engineering grand challenge. Of particular interest are solar energy systemsbased on hybrid photovoltaic-thermal (PV-T) collectors, which can reach overall efficiencies of 70% orhigher, with electrical efficiencies up to 15–20% and thermal efficiencies in excess of 50%, dependingon the conditions. In most applications, the electrical output of a hybrid PV-T system is the priority, hencethe contacting fluid is used to cool the PV cells and to maximise their electrical performance, whichimposes a limit on the fluid’s downstream use. When optimising the overall output of PV-T systemsfor combined heating and/or cooling provision, this solution can cover more than 60% of the heatingand about 50% of the cooling demands of households in the urban environment. To achieve this, PV-Tsystems can be coupled to heat pumps, or absorption refrigeration systems as viable alternatives tovapour-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 housingsector. First, the technical viability and affordability of the proposed systems are studied in ten Europeanlocations, 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 m2(4–5 persons) and a rooftop area of50 m2. Based on annual simulations, Seville, Rome, Madrid and Bucharest emerge as the most promisinglocations from those examined, and the most efficient system configuration involves coupling PV-T pan-els to water-to-water heat pumps that use the PV-T thermal output to maximise the system’s COP. Hourlyresolved transient models are then defined in TRNSYS, including thermal energy storage, in order to pro-vide 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 indicatethat PV-T systems have the potential to cover 60% of the combined (space and hot water) heating andalmost 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 ofenergy of these systems is found to be in the range of 0.06–0.12€/kW h, which is 30–40% lower than thatof equivalent PV-only systems