Recently Dr Kai Wang presented one of our daytime seminars, a very interesting overview of state-of-the-art, hybrid solar energy systems for provisions of cooling, heating and/or power. He has written up this blog post as a compementary overview of the topic. You can also download his slides from the talk as a PDF.

Solar energy is the most abundant renewable energy source in the world and is expected to feature prominently in low-carbon, sustainable future energy systems. Solar energy can be harnessed either as heat by solar thermal technologies (flat-plate or evacuated-tube collectors, etc.) or electricity by photovoltaic (PV) or concentrated solar power (CSP) technologies. However, heat and electricity are inherently different energy forms, and the associated collectors would have to occupy additional space if both heat and electricity are required which is, in fact, often the case in many practical applications.

Hybrid photovoltaic-thermal (PVT) systems are highly suitable for meeting the multi-vector energy needs of end-users (e.g. cooling, heating, and power), as they combine PV and solar thermal elements in one component, allowing for electrical and thermal outputs to be generated simultaneously from the same collector area with a much higher overall (electrical+thermal) efficiency than standalone systems. Great flexibility and many options for integration with other technologies (e.g. for cooling, heating, clean water, storage) are available in such PVT-based “all-in-one” energy systems.

Motivation for a hybrid PV-thermal (PVT) collector

A PV module converts incident sunlight into electrical energy, but unlike a solar thermal collector, it cannot utilise the full range of solar wavelengths. The remaining solar energy, which is not converted into electricity, and which amounts to about 80% of the total incident solar energy, is lost as heat, thereby increasing the temperature of the module, and leading to a loss in efficiency, depending on the material used in the solar panels.

To overcome this problem, PV cells can be cooled by an appropriate fluid flow, reducing their temperature and increasing their electrical efficiency, while providing an additional useful thermal output in the form the heated coolant for purposes such as space or water heating, or cooling through thermally-driven refrigerators. Hybrid PVT collectors feature this dual-generation of electricity and thermal energy by combining PV cells and a heat exchanger for collecting heat. The total useful energy output of PVT collectors is significantly higher than that of PV modules since the additional thermal output can be more than double the electrical output (e.g. >300 W/m² thermal vs. 150 W/m² electrical), and total efficiencies of around 40-80% (electrical+thermal) are attainable.

In the Clean Energy Processes (CEP) Laboratory, Professor Christos Markides and I, together with a group of researchers, are working on these “all-in-one” solar energy technologies. Our research spans component innovation, design and characterisation, system integration and thermoeconomic assessments, technology commercialisation, with a particular interest in distributed and off-grid applications.

Hybrid PV-thermal (PVT) collector design and characterisation

A PVT collector involves both thermal and electrical energy conversion, with interactions between optical, electrical and thermal processes, which synergistically affect the operation of the collector, thereby requiring sophisticated characterisation efforts to uncover the coupled, underlying processes that determine overall performance.

In the CEP Laboratory, we have established both experimental and computational characterisation methodologies for hybrid PVT collectors, but also a range of other solar collector types. This includes an indoor solar simulator and a dedicated outdoor testing facility which is situated on the roof of the Department of Chemical Engineering. The design of this facility is based on standard configurations employed for solar-thermal system testing following international standards, which allows the thermal and electrical outputs, efficiencies and dynamics to be determined accurately over a range of outdoor solar conditions. Apart from the outdoor facilities, an indoor controllable thermal testing chamber is also available for characterising solar cells, solar thermal collectors, and innovative conceptional PVT designs.

We have also developed a range of validated modelling tools for the design and prediction of the performance of solar collectors, including a three-dimensional, dynamic full-physical model of PVT collectors. These models have allowed an understanding of the role of key design parameters on the operation and performance of these collectors, and how these can be used to optimise performance in different conditions.

The experimental and modelling capabilities have provided us with powerful capabilities that extend from a much-improved fundamental understanding of the key underlying processes, to an ability to design and engineer next-generation PVT technology, and to enable system-level integration, operation, control, and application assessments.

Hybrid PVT-based solar energy systems and applications

Hybrid PVT collectors have both electrical and thermal outputs, which makes them highly suitable for meeting the complete energy needs of end-users, also with excellent flexibility for integrating with other technologies (refrigerators, heat pumps, power generation units, energy storage units, etc.), as shown in below.

PVT-based “all-in-one” solar energy systems. Depending on the specific application and requirements, the system layout and integrated technologies may vary.The tools and knowledge gained from component-level characterisation and system-level integration in the CEP Laboratory are being used to assess and understand the technoeconomic potential of such “all-in-one” solar technologies, and to identify appropriate designs and optimal operation strategies for specific applications of interest, such as for meeting the energy demands of things like domestic buildings, sport centres, university campuses, dairy farms, and greenhouses.

A case study of PVT-based combined cooling, heating and power systems aimed at the domestic sector has indicated that, with appropriate component and system design and operation, there is strong evidence that such systems can cover more than 60% of the combined heating demand, i.e., including for space and hot-water heating, more than 50% of the cooling demand, and 30-100% of the electrical demand of households given access to reasonable installation areas in ten representative European locations.

Another case study compared the technoeconomic performance of various hybrid vs. conventional solar energy systems for a university sport centre, and showed that PVT systems outperform conventional solar-energy systems (PV, evacuated-tube collector, and their combinations) in terms of total energy output, with annual electrical and thermal energy yields accounting for >80% and >50% of the sport center’s demands, respectively. The CO₂ emission reduction potential of  PVT systems were also shown to be considerably higher than those of the other solar systems (438 tCO₂/year vs. 253-310 tCO₂/year). Economic analyses showed that the payback time of PVT systems is typical around 8-15 years, depending on the application, local weather, energy costs, and subsidies for renewables in specific locations, and can be further reduced if more cost-effective collector technologies can be developed.

Next-generation PVT collectors

Previous studies have suggested that “all-in-one” solar energy systems based on PVT collectors have an attractive energy-supply capability, an excellent decarbonisation potential and promising economic performance. Further efforts should be directed towards more effective designs that are suitable for the delivery of heat at higher temperatures and with lower costs, as this is expected to make this technology more competitive over existing energy-supply solutions. Applying advanced loss suppression techniques and spectral splitting concepts into hybrid PVT collector designs have emerged as key routes towards next-generation PVT collectors.

The CEP Laboratory has successfully demonstrated that high electrical and thermal performance at high temperatures are attainable by PVT collectors featuring advanced heat-suppression designs, including vacuum insulation, low-emissivity coatings, and low temperature-coefficient PV cells. The most advanced collector design is projected to have double the thermal efficiency compared to present commercially available PV-T collectors, and to provide 1.5 and 2 times the revenue or carbon savings of separate PV modules and solar-thermal collectors, respectively.

One of the most promising solutions for lowering the cell temperatures and overcoming the temperature limitations of conventional PVT collectors, and therefore for unlocking applications requiring high-temperature heat, involves splitting the incident sunlight into separate bands, one that is well-suited to conversion into electricity, which is directed to the PV cells, and a second that is absorbed as thermal energy.

Innovative PVT collectors employing advanced heat-loss suppression techniques and innovative spectral splitting concepts are now being developed in the CEP Laboratory. A recent Imperial College London spin-out company, Solar Flow, has been founded to develop and commercialise such next-generation hybrid solar technologies. Early predictions have shown that these designs deliver +20% more electricity and +80% more hot water at 60°C relative to the existing PVT market leader.

Kai Wang

Kai Wang is a Postdoctoral Research Associate in the Clean Energy Processes (CEP) Laboratory at Imperial College London. He is also the Managing Editor of journal Applied Thermal Engineering (Elsevier). He specialises in energy conversion systems for harvesting waste heat and solar energy for the provision of cooling, heating and power.

His research interests include but are not limited to, solar energy systems, co-/trigeneration energy systems, waste heat recovery and other clean and renewable energy technologies. In 2019, he was awarded with the Sadi Carnot Award from the International Institute of Refrigeration (IIR), one of the IIR Scientific Awards for young researchers in the field of thermodynamics.