The use of renewable energy resources offers a secure and sustainable solution to the energy problem, which encompasses complex and multifaceted aspects such as an increasing global energy demand notwithstanding stagnating oil production capacities and price volatility, concerns relating to energy independence and security, and the detrimental effects to human health and the environment that arise from the continued consumption of fossil fuels. Renewable energy resources currently supply only up to 14% of the world’s total energy, so there is considerable scope to develop and implement new renewable energy technologies.
Solar energy is a particularly promising renewable and sustainable source of energy, which can be tapped to address the energy problem simultaneously from environmental, health and economic perspectives. Nevertheless, the widespread delivery of affordable solar energy through suitable technologies remains an engineering grand challenge that continues to attract a strong interest from academia, industry, government, and beyond. In particular, solar energy systems based on hybrid photovoltaic-thermal (or, PV-T) collectors have been recently receiving an increased interest due to their higher overall efficiencies compared to conventional PV panels. These hybrid panels can reach overall efficiencies (electrical plus thermal) of over 70%, with electrical efficiencies reaching 15-20% and thermal efficiencies of over 50%.
In a PV-T panel, PV cells convert sunlight directly to electricity and thermal energy is removed from the cells via a contacting coolant fluid (liquid or gas); the heated fluid flow is then used downstream for hot water provision or space heating (and/or cooling if necessary), with the added benefit of actively cooling the PV cells and increasing their electrical efficiency. In most applications, the electrical output of a hybrid PV-T system is the main priority. Hence, the contacting fluid is kept at low temperature in order to maximize the system’s electrical performance. However, this imposes a limit on its posterior use and a design conflict arises between the electrical and thermal performance of hybrid PV-T systems.
When optimising the overall output of PV-T systems for combined electricity and heating/cooling provision, this solution can cover about 60% of the heating and between 50-100% (depending on the location) of the cooling demands of households in the urban environment, based on typical household demands and available roof space. To achieve this, PV-T systems can be coupled to vapour-compression heat pumps or absorption refrigeration units.
Researchers in the Clean Energy Processes (CEP) Laboratory at Imperial College London considered the techno-economic advantages and shortcomings of a range of such systems, while aiming at a low cost per kWh of combined energy generation (co- or tri-generation) in the housing sector. They studied the technical feasibility, performance and affordability of proposed systems in ten different geographical locations covering all European climates, with local weather profiles using monthly and annually averaged solar-irradiance and energy-demand data relating to houses with a 100-m2 total floor area and 50-m2-rooftop area. The results of this research have been published in Energy Conversion and Management.
Amongst the locations studied, Seville, Rome, Bucharest and Madrid were the most promising for the proposed hybrid PV-T systems. The authors found that the most effective system arrangement entailed the coupling of PV-T panels to water-to-water heat pumps. The electrical output of the panels was used to run the heat pump or an air conditioning system, while the thermal output was used to maintain the source-side temperature of the heat pump at around 15 °C year round. This practice maximizes the heat pump and/or air-conditioning coefficient of performance (COP) and enables a reduction in their electricity consumption. The authors found that the temporal resolution of the simulations affects strongly the predicted system performance. Detailed high-resolution hourly simulations indicated that such PV-T systems are capable of covering 60% of the combined heating demands and almost 100% of the cooling demands of the examined households in middle and low European-latitude regions.
Moreover, the authors estimated the cost of solar thermal, PV and PV-T systems. For PV-T systems, the levelized cost of energy (LCOE), i.e., the total net present value of the system per unit total energy over the system’s lifetime, was found to be mainly influenced by the system size, which will be larger at higher latitudes (lower irradiance). Nevertheless, the calculated LCOE for the PV-T systems varied between 0.06 and 0.12 €/kW h, which is 30-40% lower than the LCOE of small-scale PV-only installations in Europe.
Finally, the authors identify important barriers for the market adoption of PV-T technology, which at present act to limit the PV-T market size, including high initial costs, and uncertainties caused by poor knowledge of the technology due to its limited penetration. They suggest that demonstration projects exploring the potential of PV-T co-generation or tri-generation systems should be encouraged, supported and advertised to the public, to increase the awareness of this technology and to accelerate the adoption rates of these much-promising systems.
Alba Ramos, Maria Anna Chatzopoulou, Ilaria Guarracino, James Freeman, Christos N. Markides. Hybrid photovoltaic-thermal solar systems for combined heating, cooling and power provision in the urban environment. Energy Conversion and Management, Vol. 150, 838-850 (2017)Go To Energy Conversion and Management