Hybrid Solar Receiver Combustor Research Articles

Experimental investigation of the influence of solar-to-fuel ratio on performance and stability characteristics of hybrid solar-MILD hydrogen processes

This study presents an investigation of the influence of solar-to-fuel energy input ratio (S/F) on performance and stability characteristics of hybrid processes of solar and MILD combustion of H2. A laboratory-scale MILD Hybrid Solar Receiver Combustor was operated at 8-kWth capacity under MILD combustion and in the mixed-mode (MILD plus solar energy simultaneously). An 18-kWe three-lamp metal-halide solar simulator and the combustion of pure hydrogen were used as energy sources. The global combustion performance and stability limits for each mode of operation are reported for different levels of heat extraction and S/F values in the range 5–25%. It was found that similar thermal performance can be achieved for both modes across a wide range of conditions, together with steady operation in response to transients, indicating for the first time that MILD combustion can be used to efficiently compensate for variability in the solar resource, reduce thermal stresses and guarantees constant output. Steady solar-MILD operations retain similar features of conventional MILD processes (nearly-zero emissions, thermal field uniformity) even at relatively high S/F ratio. The global combustion characteristics, performance and stability limits are found to correlate with S/F in the mixed mode, while the operability region for which steady MILD processes can occur was found to increase significantly by adding high-flux concentrated solar radiation to the combustion process and by increasing S/F.

Hybrid Solar-MILD Combustion for Renewable Energy Generation

The intermittent nature of solar energy limits its further deployment to applications where firm supply and constant output is required. While energy storage is a viable option to increase solar share, in itself is not sufficient without an additional dispatchable energy source. Combustion of both fossil-based and renewable fuels can provide the demand ready energy source required and lends itself to hybridisation with tower based Concentrated Solar Thermal, CST, energy. The Hybrid Solar Receiver Combustor, HSRC, is a novel technology that integrates both sources of energy in one device and offers tangible benefits in increased solar share, thermal efficiency and reduced capital and operation costs. This paper reports a brief review of the different findings from experimental and computational research carried out at the Centre for Energy Technology of the University of Adelaide into optimizing the HSRC design, developing the first-of-a-kind laboratory-scale HSRC unit and evaluating its performance under different fuels, operating conditions and modes of operation. It highlights the benefits and need for utilizing MILD combustion in the HSRC to match the heat transfer characteristics and stability required to achieve similar operational range and efficiency from both sources of energy. A 5-kWel xenon-arc solar simulator and the combustion of a wide variety of fuels are used as the energy sources. This paper reports on the effectiveness of MILD combustion under these conditions and in particular it discusses flame stability envelop and its relation to heat extraction, temperature and pollutant emissions. It also reports on thermal efficiency, heat losses and heat flux distribution within the cavity for all fuels and operating conditions. It is found that the HSRC thermal performance is similar under the three operation modes (solar, combustion and combined) and that operating under MILD combustion mode allowed fuel flexibility, homogeneous heat distribution and very low emission of NOx and CO. Also found that H/C ratio plays a minor role in the radiated energy to the heat exchanger within the cavity. Future research and further technology development need is also discussed in this paper.

The influence of aspect ratio on the iso-thermal flow characteristics of multiple confined jets

We report a systematic study of the interaction between four rotationally symmetric jets within a cylindrical chamber obtained with particle image velocimetry, under conditions relevant to a wide range of practical applications including the hybrid solar receiver combustor. The geometry consists of a cylindrical cavity with four inlet jets (representing four burners), which are configured in an annular arrangement and aligned at an inclination angle (αj) to the axis with a tangential component (azimuthal angle θj) to generate a swirl in the chamber. The configurations of αj = 25° were assessed with two azimuthal angles θj = 5° and 15°, a range of chamber aspect ratios (Lc/Dc), and a fixed nozzle Reynolds number of ReD = 10 500. The experimental results reveal a significant dependence of the mean and turbulent flow-fields on the aspect ratio Lc/Dc for the values of αj and θj considered here. Three different flow regimes and their controlling parameters were identified within the range 1 ≤ Lc/Dc ≤ 3. The dependence of the flow characteristics on the chamber length Lc was weak within 1.5 < Lc/Dc ≤ 3, but significant for 1 ≤ Lc/Dc ≤ 1.5. It was also found that the value of Lc/Dc has a controlling influence on the position and strength of large-scale recirculation regions, together with the extent of flow unsteadiness, although this influence is reduced as θj is increased.

Combined solar energy and combustion of hydrogen-based fuels under MILD conditions

This work presents the stability and performance characteristics of a Hybrid Solar Receiver Combustor operating in the Moderate or Intense Low oxygen Dilution (MILD) combustion regime. The device was operated at 12-kWth in two different modes of operation, i.e. combustion-only (MILD) and mixed (combustion and solar introduced into the device simultaneously), using natural gas (NG), liquefied petroleum gas (LPG), hydrogen (H2), NG/H2 and LPG/H2 blends. A 5-kWel xenon-arc lamp was used to simulate the concentrated solar radiation introduced into the device. The influence of the mode of operation and fuel composition on the combustion stability, thermal efficiency, energy balance, pollutant emissions, heat losses and distribution of heat flux within the receiver are presented for a range of values of the heat extraction. It was found that MILD combustion can be successfully stabilised within the HSRC over a broad range of operating conditions and fuel type, and in mixed operations, with low CO (for carbon-based fuels) and NOx emissions. The addition of H2 and/or concentrated solar radiation to the MILD process was found to increase its stability limits. Mixed and combustion-only operations showed similar performance, regardless of the fuel type, providing further evidence that the fuel flow rate can be used dynamically to compensate for variability in the solar resource. Also, the heat extracted from the heat exchanger and the specific fuel consumption were found to increase and decrease, respectively, by adding H2 to the system for both modes of operation, showing that hydrogen addition is beneficial. The numerical analysis revealed that the higher performance with H2 is attributable to a higher radiative heat transfer rate than for NG and LPG under MILD conditions.

An experimental study of the stability and performance characteristics of a Hybrid Solar Receiver Combustor operated in the MILD combustion regime

This study describes the performance and stability characteristics of a Hybrid Solar Receiver Combustor operated in the Moderate or Intense Low oxygen Dilution (MILD) combustion regime, in which the functions of a solar receiver and a combustor are integrated into a single device. The device was built and tested at a nominal capacity of 20 kWth for both the combustion-only (MILD and conventional combustion) and mixed-mode (a combination of both solar and combustion). Here, a 5 kWel xenon-arc solar simulator and natural gas were used as the energy sources, while the combustion mode was operated in the MILD combustion regime. The thermal efficiency, wall cavity temperature, heat flux distribution within the cavity and pollutant emissions are reported for the two modes of operation for a range of energy input, equivalence ratio, heat extraction, air preheat and solar-to-fuel energy input ratio. The stability limits for stable operations are also identified for each mode of operation. It was found that MILD combustion can be successfully stabilised within the HSRC in a wide range of operating conditions with and without air preheating, and in the mixed-mode of operation, providing ultra-low NOx and CO emissions. Also, the stability limits were found to increase by adding concentrated solar radiation to the combustion process. The thermal performance was found to be similar in both combustion-only (conventional combustion and MILD) and mixed-mode (up to ≈ 88% assuming reasonable heat recovery from the exhaust gas), confirming that an overall benefit can be derived from the device.

Experimental demonstration of the hybrid solar receiver combustor

We report the first-of-a-kind experimental demonstration of a direct hybrid between a solar cavity receiver and combustor, in which the functions of a solar receiver and a combustor are integrated into a single device. The device was built and tested at a nominal capacity of 20-kWth for all the three modes of operation, namely solar-only, combustion-only and mixed-mode (a combination of both solar and combustion). Here, a 5-kWel xenon-arc solar simulator and natural gas were used as the energy sources, while the combustion mode was operated in the Moderate or Intense Low oxygen Dilution (MILD) combustion regime to offer low NOx emissions and good heat transfer. The thermal efficiency, heat losses, heat flux distribution within the cavity and pollutant emissions are reported for the solar-only, combustion-only and mixed-modes of operation. The thermal performance was found to be similar in all modes of operation, assuming reasonable heat recovery from the exhaust gas. Since system losses are reduced by integration (e.g. by avoiding start-up and shut-down losses), this confirms that an overall benefit can be derived from the device. Nevertheless, there is a need to manage the significantly different heat flux distribution for the three modes of operation.

Iso-thermal flow characteristics of rotationally symmetric jets generating a swirl within a cylindrical chamber

We present a systematic experimental study of the interaction between four rotationally symmetric jets within a cylindrical chamber, under conditions relevant to a wide range of engineering applications, including the technology of a Hybrid Solar Receiver Combustor (HSRC). The HSRC geometry is simplified here to a cylindrical cavity with four inlet jets (representing four burners) which are configured in an annular arrangement and aligned at an inclination angle to the axis with a tangential component (azimuthal angle) to generate a swirl in the chamber. In this study, the jet inclination angle (αj) was varied over the range of 25°–45°, while the jet azimuthal angle (θj) was varied from 5° to 15°. The inlet Reynolds number for each injected jet and the number of jets were fixed at ReD = 10 500 and 4, respectively. Measurements obtained with Particle Image Velocimetry were used to characterise the large-scale flow field within selected configurations. The results reveal a significant dependence of the mean and root-mean-square flow-fields on the jet azimuthal angle (θj) and the jet inclination angle (αj). Three different flow regimes with distinctive flow characteristics were identified within the configurations investigated here. It was also found that θj can significantly influence (a) the position and strength of an external recirculation zone and a central recirculation zone, (b) the extent of turbulence fluctuation, and (c) the flow unsteadiness. Importantly, the effect of αj on the flow characteristics was found to depend strongly on the value of θj.

Thermal performance analysis of a syngas-fuelled hybrid solar receiver combustor operated in the MILD combustion regime

ABSTRACTThis article examines the overall performance of a hybrid solar receiver combustor when operated under the moderate or intense low oxygen dilution (MILD) combustion mode and contrast it with performance under the solar-only mode. A three-dimensional computational fluid dynamics model, coupled with analytical model data, is utilised to investigate the influence of fuel type (syngas, natural gas and tail gas) on the thermal efficiency, heat transfer mechanisms and heat flux distribution within the cavity. It was found that irrespective of the fuel type, similar thermal performance characteristics can be achieved under the MILD combustion mode and the solar only mode of operation (given a cavity of sufficient length and an appropriate arrangement of the heat transfer fluid (HTF) pipes). Nonetheless, it was found that the type of fuel influences significantly the rate of radiative heat transfer and the ratio of radiative to convective heat transfer rates, and so the configuration must be optimised for each type of fuel. The total heat absorbed by the HTF pipes and the thermal efficiency are predicted to be higher for the fuelled-syngas MILD hybrid solar receiver combustor case than for the CH4-fuelled and a simulated tail gas-fuelled MILD cases (where the tail gas is the by-product from a Fischer–Tropsch process) owing to a greater rate of radiative heat transfer (the latter increases with the H2 concentration in the syngas). Also, the level of dilution of the fuel stream was found to significantly influence the thermal performance of the device. In addition, the calculated distribution of heat flux to the receiver pipes was found to be significantly different under the solar and combustion modes of operation, which needs to be considered in the selection of design strategies and materials.

Experimental and numerical investigation of the iso-thermal flow characteristics within a cylindrical chamber with multiple planar-symmetric impinging jets

We present a joint experimental and numerical study of the flow structure within a cylindrical chamber generated by planar-symmetric isothermal jets, under conditions of relevance to a wide range of practical applications, including the Hybrid Solar Receiver Combustor (HSRC) technology. The HSRC features a cavity with a coverable aperture to allow it to be operated as either a combustion chamber or a solar receiver, with multiple burners to direct a flame into the chamber and a heat exchanger that absorbs the heat from both energy sources. In this study, we assess the cases of two or four inlet jets (simulating the burners), configured in a planar-symmetric arrangement and aligned at an angle to the axis (αj) over the range of 0°–90°, at a constant inlet Reynolds number of ReD = 10 500. The jets were positioned in the same axial plane near the throat and interact with each other and the cavity walls. Measurements obtained with particle image velocimetry were used together with numerical modeling employing Reynolds-averaged Navier-Stokes methods to characterize the large-scale flow field within selected configurations of the device. The results reveal a significant dependence of the mean flow-field on αj and the number of inlet jets (Nj). Four different flow regimes with key distinctive features were identified within the range of αj and Nj considered here. It was also found that αj has a controlling influence on the extent of back-flow through the throat, the turbulence intensity, the flow stability, and the dominant recirculation zone, while Nj has a secondary influence on the turbulence intensity, the flow stability, and the transition between each flow regime.

Techno-economic evaluation of modular hybrid concentrating solar power systems

This paper assesses the influence on techno-economic performance of modularising hybrid Concentrating Solar Power (CSP) systems with fossil fuel backup for both a Hybrid Solar Receiver Combustor (HSRC), which integrates a combustor into a solar cavity receiver, and a Solar Gas Hybrid (SGH) system with a similar cavity receiver and a back-up boiler. It was found that the energy losses in a system of small-sized modules, which employs molten salt as its Heat Transfer Fluid (HTF), are dominated by trace heating owing to the increased piping over their larger receiver counterpart. However, this can be reduced significantly by using alternative HTFs with a lower melting point such as sodium. In addition, for modularisation to be cost effective requires it to also enable access to alternative, lower-cost manufacturing methods. That is, the benefit of standard learning rates is insufficient to lower the Levelized Cost of Electricity (LCOE) on its own. For a plant with 30 units of 1 MWth modules the LCOE is competitive, relative to a single unit of 30 MWth, after ∼10 plants are installed if the modularised components (i.e. heliostats, receivers and towers) can be decreased by >80% and >40% for molten salt and sodium as the HTF, respectively.

Comparison of system performance in a hybrid solar receiver combustor operating with MILD and conventional combustion. Part I: Solar-only and combustion-only employing conventional combustion

The paper reports a detailed heat transfer study of a hybrid solar receiver combustor operating in either the solar-only or combustion-only modes. It aims to investigate the differences between performance in these two modes of operation and to reduce the uncertainties of an analytical model of the device developed previously for the techno-economic assessments. The influence of the dominant geometrical parameters, burner configuration and flux distribution of the incident solar radiation on the thermal efficiency, heat losses, heat transfer mechanisms and heat flux distribution within the cavity was investigated with a three-dimensional computational fluid dynamics (CFD) model of the device. For the solar-only mode of operation, the model employs a Monte-Carlo ray tracing approach to calculate the heat flux distribution incident on the pipes containing the heat transfer fluid (HTF) and the overall thermal efficiency. For the combustion-only mode, two different burner designs were considered as representative cases for non-premixed and partially premixed flames, one with and the other without swirl. It was found that the device can achieve a similar thermal performance in each mode of operation given the use of either a cavity of sufficient size (length-to-diameter cavity ratio, L/D>5) or an appropriate arrangement of the HTF pipes within the cavity. In addition, that the thermal efficiency of the device was found to increase with the length-to-diameter cavity ratio and/or solar peak flux at the focal plane. A comparison of the heat flux distribution on the HTF pipes in the two modes of operation showed that the solar-only and the combustion-only mode, employing swirl-stabilised flames, can provide similar heat flux distributions in comparison with non-swirl flames, suggesting that the materials selection would be controlled by the combustion mode. The investigation also highlights that the analytical model developed previously can be effectively employed to evaluate the thermal efficiency of the device although its use should be limited to long cavities (L/D>5). Recommendations are also made on how to further refine the analytical model for the solar-only mode of operation.

Comparison of system performance in a hybrid solar receiver combustor operating with MILD and conventional combustion. Part II: Effect of the combustion mode

This paper reports on the effects of the influence of the MILD combustion mode relative to conventional combustion on the overall performance of a hybrid solar receiver combustor (HSRC) operating under the combustion-only mode of operation. The influence of the dominant reactor geometrical parameters and level of dilution of the combustion air stream on the thermal efficiency, heat transfer mechanisms and heat flux distribution within the cavity were investigated using a three-dimensional computational fluid dynamics (CFD) model of the device. It was found that the use of MILD combustion leads to an increase of the thermal performance of the device in comparison with a conventional flame, owing to a larger rate of convective heat transfer. Also, the level of dilution of the combustion air influences significantly the rate of radiative heat transfer and the ratio of radiative to convective heat transfer rate. The thermal efficiency of the device was also found to increase with the length-to-diameter cavity ratio (L/D) and/or a reduction of the jet inclination angle. In addition, operation in the MILD regime can achieve a similar thermal performance to the solar mode of operation given suitable configurations, such as the use of either a cavity of sufficiently length (L/D>5) or an appropriate arrangement of the HTF pipes within the cavity. However, the calculated heat flux distributions on the receiver pipes were found to be significantly different under the two modes of operation, implying that the difference in heat flux distribution is a key parameter that needs to be considered in the design strategies and materials selection. The present investigation also identified the configurations for which the analytical model of the device (operating in the MILD regime), developed previously, yields reasonable accuracy for the thermal efficiency of the device, notably for the case of long cavities (L/D>5).

Impact of start-up and shut-down losses on the economic benefit of an integrated hybrid solar cavity receiver and combustor

The impact of avoiding the start-up and shut-down losses of a solar thermal power plant by directly integrating the back-up boiler into a tubular solar-only cavity receiver is studied using a multiple time-step, piecewise-continuous model. A steady-state analytical model of the mass and energy flows through both this device and a solar-only cavity receiver reported previously are incorporated within a model of the solar power generating plant with storage. The performance of the Hybrid Solar Receiver Combustor (HSRC) is compared with an equivalent reference conventional hybrid solar thermal system employing a solar-only cavity receiver and a back-up boiler. The model accounts for start-up and shut-down losses of the boiler, threshold losses of the solar-only cavity receiver and the amount of trace heating required to avoid cooling of the heat transfer fluid. The model is implemented for a 12month/five year time-series of historical Direct Normal Irradiation (DNI) at 1h time-steps to account for the variability in the solar resource at four sites spanning Australia and the USA. A method to optimize the size of the heliostat field is also reported, based on the dumped fraction of solar power from the heliostat field. The Levelized Cost of Electricity (LCOE) for the HSRC configuration was estimated to be reduced by up to 17% relative to the equivalent conventional hybrid solar thermal system depending on the cost of the fuel, the storage capacity and the solar resource, while the fuel consumption was estimated to be reduced by some 12–31%.

Analytical assessment of a novel hybrid solar tubular receiver and combustor

This paper presents a novel hybrid between a tubular solar receiver and a combustor, called the Hybrid Solar Receiver Combustor (HSRC), and reports on the development and application of an analytical model to describe its performance. The analytical model accounts for the variability of the solar resource with a pseudo steady state balance of the mass and energy flow, which calculates the heat transfer throughout the device. A systematic investigation of the influence of variation in all controlling parameters on performance and weight is undertaken. The results provide new understanding of the performance of the device relative to the solar-only and combustion-only counterparts, and further justification for its ongoing development.