Design of high-temperature atmospheric and pressurised gas-phase solar receivers: A comprehensive review on numerical modelling and performance parameters

Abstract

Gas-phase solar receivers can operate over a wider temperature range than receivers which use conventional liquid heat transfer media. However, due to their relatively poor heat transfer performance, gas-phase receivers require substantially more heat transfer area and/or higher flow rates to extract the same amount of heat which leads to more complicated designs (e.g. a porous-media absorber with small feature/pore size) and decreased reliability (e.g. internal temperature gradients and higher thermal stresses). Detailed numerical modelling techniques can help elucidate the fundamental heat and mass transfer mechanisms/interactions in such to help mitigate these risks and to create innovative, high-performance, designs. To collate the research efforts towards gas-phase receiver modelling, this paper systematically reviews—and critically compares—the latest numerical studies of various high-temperature gas-phase receiver designs. It was found that, from a numerical point of view, gas-phase receivers can be categorised by whether they contain a porous medium or not. This categorisation is crucial, because it dictates the numerical techniques needed to capture the underlying phenomena. It was also found that no standardised performance metrics are reported for gas-phase receivers. This study suggests that a holistic figure of merit, such as the one developed by Lenert et al. (2012), be adopted in this field to merge all performance criteria for comparative evaluation. Overall, the present review is expected to serve as a guide for the development/enhancement of receiver designs based on linking best practices in simulation/modelling with the key features and limitations of gas-phase receivers.

Introduction

The low power density of solar resources (e.g. ~1 kW/m²) limits its usefulness in our modern energy-intensive economy relative to conventional energy systems (e.g. a gas turbine is ~1000 kW/m²) (Zhou et al., 2015). Concentrated solar thermal technologies effectively ‘densify’ this distributed resource to achieve similar specific power levels (Bijarniya et al., 2016). In a spot-focus solar concentrator, numerous heliostat mirrors track and concentrate sunlight, often by more than 1000×, to a central receiver, where the solar flux is converted into heat (Polo et al., 2015, Emes et al., 2019). Over the course of a sunny day, this thermal energy can be subsequently used to offset gas purchases for industrial processes. This represents a low-carbon mechanism to drive energy-intensive thermochemical processes (Chintala, 2018, Bhatta et al., 2019, Bush et al., 2019, Wang et al., 2019, Zhang and Smith, 2019), or to generate electricity (Giostri and Macchi, 2016, Wang et al., 2017a, Abutayeh et al., 2019, Awan et al., 2019), or to simply store energy for later use during times of peak demand (Nithyanandam and Pitchumani, 2013, Nithyanandam and Pitchumani, 2014, Mostafavi Tehrani et al., 2017, Nithyanandam and Stekli, 2017, Mostafavi Tehrani et al., 2018a, Mostafavi Tehrani et al., 2018b, Seddegh et al., 2018, Mostafavi Tehrani et al., 2019). In all of these applications, pushing towards higher temperatures yields a denser, more utilisable form of energy.

A solar thermal receiver consists of an absorbing material, a working fluid, and a surrounding structure (which usually includes insulation). The absorbing material can be either a solid, a porous material, or particles embedded in the flow (Tan and Chen, 2010, Ho, 2016, Morris et al., 2016). The working fluid, or heat transfer medium (HTM), is typically either a liquid such as molten salt or liquid metal (Raade and Padowitz, 2011, Deangelis et al., 2018, Turchi et al., 2018, Fritsch et al., 2019, Li et al., 2019) or a gas including air, steam, or helium (Lenert et al., 2012, Sullivan et al., 2016, Stadler et al., 2019).

Due to temperature limitations on liquid-phase receivers, they are unlikely to serve the current research trend which is pushing towards higher temperatures. Solid-phase receivers (e.g. particle receivers) are able to provide high temperatures, but face several technical/practical barriers, including: attrition/erosion, particle loss, clogging of conveyance mechanism, low thermal efficiency for the receiver, and working fluid-to-particle heat exchanger efficiency concerns, all of which will require extensive further study (Ho, 2016, Mehos et al., 2017).

Thus, gas-phase receivers represent the most promising, near-term high-temperature solution since they remain inert at high-temperature and can be considered more dependable than liquid or solid HTMs (Mehos et al., 2017). Gas-phase receivers also have minimal environmental hazards and can potentially be cost-effective. However, gaseous heat transfer fluids (HTF) suffer from poor thermal and optical properties (Mehos et al., 2016, Ho, 2017). To deal with these issues, different types of gas-phase receivers have been developed which essentially seek to increase the heat transfer rate between solar radiation absorber and HTF (Poživil et al., 2015, Wang et al., 2016b, Sedighi and Padilla, 2018, Wang and Laumert, 2018). At present, none of the proposed gas-phase receivers has become commercially successful (Sedighi et al., 2019). To develop and optimise innovative designs, the most efficient preliminarily approach is to employ numerical methods before conducting any experimental test (Capuano et al., 2016, Capuano et al., 2017). Numerical analysis can allow for rapid parametric studies and is much more cost-effective than experimental methods. With modern, high-fidelity models, it is now possible to capture all salient phenomena, even in complex geometries, accurately via simulation (Kumaresan et al., 2017).

In the design procedure of any gas-phase receiver, choices must be made about its material properties, geometry, operating pressure and temperature, mass flow rate, and solar radiation input. These options should be parametrically analysed according to some key criteria in order to enhance/optimise a design. Performance criteria, therefore, must be selected to help evaluate the impact of these choices on the optical, thermal, and fluid flow performance. Briefly, the optical efficiency indicates how much of the incident radiation flux is captured by the receiver. Thermal efficiency indicates how much thermal power can be delivered by the receiver relative to the incident power. The fluid flow performance must also be considered on a system level to determine parasitic losses, pressure drop, and to predict failure modes.

In the authors’ previous paper (Sedighi et al., 2019), different types of high-temperature gas-phase receivers (e.g. directly-irradiated HTF, indirectly-irradiated HTF, and hybrid) and their applications were reviewed. This review will cover the essential steps towards the development and enhancement of the high-temperature gas-phase solar receivers from a numerical modelling perspective. As an integral part of the commentary in this field, a critical review of the performance criteria and design parameters used in this context will also be presented. As Fig. 1 illustrates, the task of simulating gas-phase receivers has several interlocking components (not just running simulations) including; design work, determining input parameters, numerical modelling, performance assessment, and design selection/optimisation (i.e. a feedback loop to modify and enhance a design).