PERFORMANCE ASSESSMENT AND COMPARISON OF SOLAR ORC AND HYBRID PVT SYSTEMS FOR THE COMBINED DISTRIBUTED GENERATION OF DOMESTIC HEAT AND POWER

Solar-thermal collectors and photovoltaic panels are effective solutions for the decarbonisation of electricity and hot water provision in dwellings. In this work, we provide the first insightful comparison of these two competing solar-energy technologies for the provision of combined heating and power (CHP) in domestic applications. The first such system is based on an array of hybrid PV-Thermal (PVT) modules, while the second is based on a solarthermal collector array of the same area (based on a constrained roof-space) that provides a thermal-energy input to an organic Rankine cycle (ORC) engine for electricity generation. Simulation results of the annual operation of these two systems are presented in two geographical regions: Larnaca, Cyprus (as an example of a hot, high-irradiance southern-European climate) and London, UK (as an example of a cooler, lower-irradiance northern-European climate). Both systems have a total collector array area of 15 m, equivalent to the roof area of a single residence, with the solarORC system being associated with a lower initial investment cost (capex) that is expected to play a role in the economic comparison between the two systems. The electrical and thermal outputs of the two systems are found to be highly dependent on location. The PVT system is found to provide an annual electricity output of 2090 kWhe yr in the UK, which increases to 3620 kWhe yr in Cyprus. This is equivalent to annual averages of 240 and 410 We, respectively, or between 60% and 110% of household demand. The corresponding additional thermal (hot water) output also increases, from 860 kWhth yr in the UK, to 1870 kWhth yr in Cyprus. It is found that the solar-ORC system performance is highly sensitive to the system configuration chosen; the particular configuration studied here is found to be limited by the amount of rejected thermal energy that can be reclaimed for water heating. The maximum electrical output from the solar-ORC configuration explored in this study is 450 kWhe yr (50 We average, 14% of demand) for the UK and 850 kWhe yr (100 We average, 26% of demand) for Cyprus, however, the study helps to identify aspects that can lead to significant improvement relative to this estimate, and which will be at the focus of future work. An economic analysis is also undertaken to investigate the installed costs and lifecycle costs of the two systems. Without financial incentives both systems show long payback periods (14 years in Cyprus and 18 years in the UK for the PVT, and >20 years for the solar-ORC). NOMENCLATURE A [m] Area a1 [m K/W] Collector efficiency coefficient a2 [m K/W] Collector efficiency coefficient C [J/m/K] Effective heat capacity d [-] Discount rate D [-] Annual demand provision, diameter G [W/m] Irradiance i [-] Inflation rate, simulation time-step J [€] Cost Kθ [-] Incident angle modifier L [m] Length M [kg] Mass P [W] Electrical power Q [J] Energy T [K] Temperature t [s] Time U [W/Km] Heat loss coefficient V [m] Volume W [J] Work Special characters β [K] PV cell temperature coefficient η, η0 [-] Efficiency, zero-loss efficiency Subscripts a Ambient, annual b Beam irradiance aux Auxiliary c Collector (array), capital expenditure comp Components cond Condensation db Deadband d Demand, diffuse irradiance e Electricity exp Expander fan Fan gen Generator hx Heat exchanger hw Hot Water i Inlet main Mains water mod Module no Normal operating o Outlet pp Pump sup Supply t Tank th Thermal 12th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics