Performance characteristics of a hybrid solar receiver combustor utilising hydrogen or syngas

The use of hybrid solar thermal devices, which harness the energy from both concentrated solar radiation and combustion, is receiving growing attention due to their potential to provide a firm and dispatchable thermal energy supply while lowering the costs of energy systems and assisting the penetration of renewable energy. The Hybrid Solar Receiver Combustor (HSRC), which directly integrates the function of a solar receiver and a combustor into a single device, is a particularly promising hybrid technology. Its design allows the receiver to operate in three modes: solar-only, combustion-only and a mixed-mode (a combination of both solar and combustion). Compared with the present state-of-the-art in hybrid solar-combustion systems (which collect the thermal energy from the solar and combustion sources in separate devices and then combine them subsequently), the HSRC offers a reduction in total infrastructure (and hence capital costs), heat-exchange surface area, start-up/shut-down losses and pollutant emissions, due to a reduced need to start-up the back-up combustion plant. Also, the use of the HSRC with renewable fuels (e.g. hydrogen, syngas) offers the additional potential for low-cost carbon neutral or carbon-negative energy, although no data on performance for systems fed with alternative fuels are presently available. To this aim, the present work provides the first direct measurement of the performance of a Hybrid Solar Receiver Combustor (HSRC) fed with hydrogen-based fuels. A laboratory-scale HSRC prototype operated in the Moderate or Intense Low oxygen Dilution (MILD) combustion regime was tested at a nominal capacity of 12-kW_th for both the combustion-only and mixed mode, using hydrogen and syngas (H₂/CO=2/1 v/v) as fuels. A 5-kW_el xenon-arc lamp was used to simulate solar radiation into the device. The influence of the mode of operation on the thermal efficiency, heat losses, heat flux distribution within the cavity and pollutant emissions are reported for a range of values of the heat extraction. It was found that the combustion process can be successfully stabilised within the HSRC over a wide range of operating conditions, and in the mixed-mode of operation, providing ultra-low NO_x emissions. The thermal performance was found to be similar for all the modes of operation, despite the different nature of the two energy sources and fuel composition. Overall, this study highlights that, if renewable H₂ or syngas are used as fuels, the device can efficiently operate in all the modes of operation employing 100% renewable energy.