Renewable Energy Global Innovations features: Hybrid photovoltaic-thermal solar systems for combined heating, cooling and power provision in urban environment

Significance Statement

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.

Schematic diagram of the proposed PV-T system for solar heating and cooling provision.

Selected locations and global horizontal irradiation in Europe. (b) Space heating, DHW and cooling demands and global irradiance profiles for Seville and Vienna.

About The Author

Dr Alba Ramos Cabal is an Aeronautical Engineer who holds a Masters and a Doctorate degree in Photovoltaic Solar Energy from the Technical University of Madrid (Spain). Currently, she is a Postdoctoral Research Associate in the Clean Energy Processes (CEP) Laboratory at Imperial College London.

Her research involves the modelling, design, fabrication and testing of novel hybrid solar PV-thermal (PV-T) collector technology, as part of wider solar-energy heat, power and cooling (tri-generation) systems. Her previous experience includes the theoretical modelling of physical and chemical vapour deposition (CVD) processes, as well as experimental research with CVD reactor prototypes in the laboratory and at industrial scales. She was also involved in the modelling, design, fabrication and testing of a novel thermal energy storage (TES) system that utilizes high-temperature phase change materials (PCMs) and thermo-photovoltaic (TPV) cells. In addition, during her PhD she was involved with the Solar Energy Institute, a Spanish R&D centre, worked at the Institute for Energy Technology (IFE) in Norway, and undertook postdoctoral research stays at the Cyprus University of Technology (CUT) for the purpose of outdoor solar PV-T collector testing and characterization.

Contact: a.ramos-cabal@imperial.ac.uk 

About The Author

Ms Maria Anna Chatzopoulou is a PhD student in the Clean Energy Processes (CEP) Laboratory at the Department of Chemical Engineering of Imperial College London, working under the supervision of Dr Christos Markides. Her research focuses on the design of low-carbon cogeneration technologies with applications in buildings. She is interested in the design and optimization of novel technologies, such as the organic Rankine cycle (ORC) and absorption refrigeration systems, which are not currently widely adopted in the built environment.

Her research is co-funded by the Imperial College President’s PhD Scholarship scheme, and the Climate-KIC and European Institute of Innovation and Technology. She holds an MEng degree in Mechanical Engineering (Distinction) and an MSc in Environmental Engineering and Business Management (Distinction). Prior to her PhD, she worked as a design engineer in an international engineering consultancy in London. Her main responsibilities involved the design of HVAC systems for critical facilities, such as data centres, aiming to reduce their energy consumption and carbon emissions by considering novel cooling technologies.

Contact: maria-anna.chatzopoulou@imperial.ac.uk

About The Author

Dr Ilaria Guarracino holds a PhD in Chemical Engineering from Imperial College London. She completed her PhD at the Clean Energy Processes (CEP) Laboratory under the supervision of Dr Christos Markides and Dr Ned Ekins-Daukes who led the Quantum Photovoltaics group in the Blackett Laboratory at Imperial College. Her research focused on the design of hybrid solar PV-thermal (PV-T) systems for electricity, hot water and cooling, for which she developed a flexible computer tool for the evaluation of the performance of solar thermal and PV-T collectors and wider systems. The tool was validated against outdoor experimental data generated during a research visit to the Cyprus University of Technology (CUT) where she worked under the supervision of Professor Soteris Kalogirou.

Prior to the PhD, she completed a double Masters degree in Mechanical Engineering at the Royal Institute of Technology (KTH, Stockholm) and at the Universitat Politecnica de Catalunya (UPC, Barcelona) focusing on solar energy systems. She also worked as a summer intern at the Fraunhofer Institute for Solar Energy (ISE) working in a project conducting research into the thermodynamics of molten salts in steam generators for concentrated solar power systems.

Contact: ilaria.guarracino12@imperial.ac.uk

About The Author

Dr James Freeman is a Postdoctoral Research Associate in the Clean Energy Processes (CEP) Laboratory at the Department of Chemical Engineering, Imperial College London. He completed his PhD in solar thermal combined energy systems for distributed applications in 2017. His previous work has included the design, testing and modelling of a small-scale organic Rankine cycle (ORC) engine for use with non-concentrating solar collectors. He has also worked within the CEP Laboratory to develop methods for the testing and modelling of solar thermal and hybrid solar PV-thermal (PV-T) collectors and systems.

His current research focuses on a small-scale modular solar-cooling system for rural cold-chain applications based on a diffusion-absorption refrigeration cycle. He recently participated in an ongoing project to field-test the system in India, which runs until 2018.

Contact: j.freeman12@imperial.ac.uk

About The Author

Dr Christos N. Markides is a Reader in Clean Energy Processes at the Department of Chemical Engineering, Imperial College London, where he heads the Clean Energy Processes (CEP) Laboratory, and leads the Department of Chemical Engineering Energy Research Theme and the cross-faculty Energy Efficiency Network. After graduating with a PhD in Energy Technologies from the University of Cambridge, he co-founded a Cambridge University spin-out company to develop and commercialize a novel thermally-powered device without moving parts capable of converting low-temperature waste heat or solar energy into fluid pumping. He acted as its Technical Director until his appointment at Imperial in 2008.

His research expertise lies in the application of fundamental aspects of thermodynamics, fluid flow, heat and mass transfer, both computationally and experimentally, to novel processes, components, technologies and systems for the recovery, utilization, conversion and storage of thermal energy. He has a particular interest in high-efficiency energy systems, renewable energy technologies, and the efficient utilization of solar and waste heat for cooling, heating and power. He has written over 200 scientific articles in these areas. He is an Executive Editor of Applied Thermal Engineering and is a Subject Editor of Renewable Energy, is on the Editorial Board of Energy (Elsevier) and Frontiers in Solar Energy (EPFL), and is also a member of the UK National Heat Transfer Committee and Scientific Board of the UK Energy Storage SUPERGEN Hub.

Contact: c.markides@imperial.ac.uk

The Clean Energy Processes (CEP) Laboratory, headed by Dr Christos Markides, is based in the Department of Chemical Engineering at Imperial College London. Currently, approximately 35 members (research fellows, postdoctoral and doctoral researchers, and postgraduate students) are at the core of this research laboratory. Its mission is to conduct high-impact research in applied thermodynamics, fluid flow, heat and mass transfer, as applied to technologies for high-performance power generation, heat and/or cooling provision. The CEP Laboratory fosters close interactions with industry and actively collaborates with international research centres and universities. The CEP solar-energy team focuses on innovation, research and development aimed at high-performance solar thermal and hybrid PV-thermal technologies for heating, cooling and power generation.

Reference

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)

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