Papers by Willem Le Roux
Heat loss analysis for an open-cavity tubular solar receiver
16th International Heat Transfer Conference (IHTC-16), Beijing, China, 2018
A small-scale open solar-thermal Brayton cycle using a parabolic dish as the concentrated solar p... more A small-scale open solar-thermal Brayton cycle using a parabolic dish as the concentrated solar power system is a solution to the problem Southern Africa faces, which is that communities stay too far from the national grid and therefore do not have access to electricity and hot water. This small-scale system will be mobile and offer the ability to produce off-grid electricity and hot water by using concentrated solar power. An open-cavity tubular solar receiver, and the heat losses that occur due to conduction, convection and radiation is investigated. The solar receiver receives concentrated solar energy from a 4.8 m parabolic dish, which then heats compressed air to turn the turbine of a turbocharger, which would power a compressor and an electric generator. The insulated receiver was tested in a solar dish set-up by sending hot compressed air through it. By measuring surface temperatures at specific intervals from inlet to outlet, as well as the inlet and outlet air temperatures, as well as the temperatures on the exterior of the insulation, a thorough heat loss analysis can be done. Testing done without solar exposure in stow position (or an angle of 0°), taking weather conditions into account proved that most of the heat lost at this angle is due to radiation, and on very windy days the convection losses are greater than normal. The receiver analyzed in this paper had a total heat loss rate of 6.3 kW at an average surface temperature of about 950K, with the conduction, radiation and convection heat losses accounting for 17%, 45% and 38% respectively. Assuming the receiver receives about 13 kW of solar energy from the collector, the receiver efficiency is calculated to be approximately 50%.
Optimum tilt and azimuth angles for fixed solar collectors in South Africa using measured data
Renewable Energy, 2016
Performance study of a solar-assisted organic Rankine cycle using a dish-mounted rectangular-cavity tubular solar receiver
Applied Thermal Engineering, 2016
Experimental investigation and parametric analysis of a solar thermal dish collector with spiral absorbe
Applied Thermal Engineering, 2017
Numerical comparison of a solar dish concentrator with different cavity receivers and working fluids
Journal of Cleaner Production, 2018
Analysis of a novel rotating disk cylinder engine concept for power generation
International Journal of Energy Research, 2019
Applied Sciences (Switzerland), 2019
A parabolic solar dish concentrator, as the heat source of an organic Rankine cycle (ORC), can be... more A parabolic solar dish concentrator, as the heat source of an organic Rankine cycle (ORC), can be used for power generation. Different types of tubular cavity receivers with different nanofluids can be considered for use in the solar dish collector to improve its efficiency. In the current research, an ORC with three different cavity receivers including hemispherical, cubical, and cylindrical are investigated using three nanofluids: Al 2 O 3 /oil, CuO/oil, and SiO 2 /oil. A numerical model is validated using experimental data. The ORC analysis is done for a constant evaporator pressure of 2.5 MPa, and condenser temperature of 38 • C. Methanol is employed as the ORC's working fluid and a non-regenerative, ideal ORC system with different turbine inlet temperatures is considered. Furthermore, a fixed solar heat transfer fluid flow rate of 60 mL/s and dish diameter of 1.9 m is investigated. Results show that, compared to pure oil, the thermal efficiency of the cavity receivers increases slightly, and the pressure drop increases with the application of nanofluids. Furthermore, results show that the cubical cavity receiver, using oil/Al 2 O 3 nanofluid, is the most efficient choice for application as the investigated solar ORC's heat source.
Recuperated solar-dish Brayton cycle using turbocharger and short-term thermal storage
Solar Energy, 2019
Many studies have been published on the performance and optimisation of the Brayton cycle and sol... more Many studies have been published on the performance and optimisation of the Brayton cycle and solar thermal Brayton cycle showing the potential, merits and challenges of this technology. Solar thermal Brayton systems have potential to be used as power plants in many sun-drenched countries. It can be very competitive in terms of efficiency, cost and environmental impact. When designing a system such as a recuperative Brayton cycle there is always a compromise between allowing effective heat transfer and keeping pressure losses in components small. The high temperatures required in especially the receiver of the system present a challenge in terms of irreversibilities due to heat loss. In this paper, the authors recommend the use of the total entropy generation minimisation method. This method can be applied for the modelling of a system and can serve as validation when compared with first-law modelling. The authors review various modelling perspectives required to develop an objective function for solar thermal power optimisation, including modelling of the sun as an exergy source, the Gouy–Stodola theorem and turbine modelling. With recommendations, the authors of this paper wish to clarify and simplify the optimisation and modelling of the solar thermal Brayton cycle for future work. The work is applicable to solar thermal studies in general but focuses on the small-scale recuperated solar thermal Brayton cycle.
SUMMARY The Brayton cycle's heat source does not need to be from combustion but can be extracted ... more SUMMARY The Brayton cycle's heat source does not need to be from combustion but can be extracted from solar energy. When a black cavity receiver is mounted at the focus of a parabolic dish concentrator, the reflected light is absorbed and converted into a heat source. The second law of thermodynamics and entropy generation minimisation are applied to optimise the geometries of the recuperator and receiver. The irreversibilities in the recuperative solar thermal Brayton cycle are mainly due to heat transfer across a finite temperature difference and fluid friction. In a small‐scale open and direct solar thermal Brayton cycle with a micro‐turbine operating at its highest compressor efficiency, the geometries of a cavity receiver and counterflow‐plated recuperator can be optimised in such a way that the system produces maximum net power output. A modified cavity receiver is used in the analysis, and parabolic dish concentrator diameters of 6 to 18 m are considered. Two cavity construction methods are compared. Results show that the maximum thermal efficiency of the system is a function of the solar concentrator diameter and choice of micro‐turbine. The optimum receiver tube diameter is relatively large when compared with the receiver size. The optimum recuperator channel aspect ratio for the highest maximum net power output of a micro‐turbine is a linear function of the system mass flow rate for a constant recuperator height. For a system operating at a relatively small mass flow rate, with a specific concentrator size, the optimum recuperator length is small. For the systems with the highest maximum net power output, the irreversibilities are spread throughout the system in such a way that the internal irreversibility rate is almost three times the external irreversibility rate.
This article appeared in a journal published by Elsevier. The attached copy is furnished to the a... more This article appeared in a journal published by Elsevier. The attached copy is furnished to the author for internal non-commercial research and education use, including for instruction at the authors institution and sharing with colleagues. Other uses, including reproduction and distribution, or selling or licensing copies, or posting to personal, institutional or third party websites are prohibited. In most cases authors are permitted to post their version of the article (e.g. in Word or Tex form) to their personal website or institutional repository. Authors requiring further information regarding Elsevier's archiving and manuscript policies are encouraged to visit: http://www.elsevier.com/copyright a b s t r a c t The small-scale open and direct solar thermal Brayton cycle with recuperator has several advantages, including low cost, low operation and maintenance costs and it is highly recommended. The main disadvantages of this cycle are the pressure losses in the recuperator and receiver, turbomachine effi-ciencies and recuperator effectiveness, which limit the net power output of such a system. The irre-versibilities of the solar thermal Brayton cycle are mainly due to heat transfer across a finite temperature difference and fluid friction. In this paper, thermodynamic optimisation is applied to concentrate on these disadvantages in order to optimise the receiver and recuperator and to maximise the net power output of the system at various steady-state conditions, limited to various constraints. The effects of wind, receiver inclination, rim angle, atmospheric temperature and pressure, recuperator height, solar irradiance and concentration ratio on the optimum geometries and performance were investigated. The dynamic trajectory optimisation method was applied. Operating points of a standard micro-turbine operating at its highest compressor efficiency and a parabolic dish concentrator diameter of 16 m were considered. The optimum geometries, minimum irreversibility rates and maximum receiver surface temperatures of the optimised systems are shown. For an environment with specific conditions and constraints, there exists an optimum receiver and recuperator geometry so that the system produces maximum net power output.
International Journal of Energy Research, 2012
The Brayton cycle's heat source does not need to be from combustion but can be extracted from sol... more The Brayton cycle's heat source does not need to be from combustion but can be extracted from solar energy. When a black cavity receiver is mounted at the focus of a parabolic dish concentrator, the reflected light is absorbed and converted into a heat source. The second law of thermodynamics and entropy generation minimisation are applied to optimise the geometries of the recuperator and receiver. The irreversibilities in the recuperative solar thermal Brayton cycle are mainly due to heat transfer across a finite temperature difference and fluid friction. In a small-scale open and direct solar thermal Brayton cycle with a micro-turbine operating at its highest compressor efficiency, the geometries of a cavity receiver and counterflow-plated recuperator can be optimised in such a way that the system produces maximum net power output. A modified cavity receiver is used in the analysis, and parabolic dish concentrator diameters of 6 to 18 m are considered. Two cavity construction methods are compared. Results show that the maximum thermal efficiency of the system is a function of the solar concentrator diameter and choice of micro-turbine. The optimum receiver tube diameter is relatively large when compared with the receiver size. The optimum recuperator channel aspect ratio for the highest maximum net power output of a micro-turbine is a linear function of the system mass flow rate for a constant recuperator height. For a system operating at a relatively small mass flow rate, with a specific concentrator size, the optimum recuperator length is small. For the systems with the highest maximum net power output, the irreversibilities are spread throughout the system in such a way that the internal irreversibility rate is almost three times the external irreversibility rate.
Energy, 2011
The small-scale open and direct solar thermal Brayton cycle with recuperator has several advantag... more The small-scale open and direct solar thermal Brayton cycle with recuperator has several advantages, including low cost, low operation and maintenance costs and it is highly recommended. The main disadvantages of this cycle are the pressure losses in the recuperator and receiver, turbomachine efficiencies and recuperator effectiveness, which limit the net power output of such a system. The irreversibilities of the solar thermal Brayton cycle are mainly due to heat transfer across a finite temperature difference and fluid friction. In this paper, thermodynamic optimisation is applied to concentrate on these disadvantages in order to optimise the receiver and recuperator and to maximise the net power output of the system at various steady-state conditions, limited to various constraints. The effects of wind, receiver inclination, rim angle, atmospheric temperature and pressure, recuperator height, solar irradiance and concentration ratio on the optimum geometries and performance were investigated. The dynamic trajectory optimisation method was applied. Operating points of a standard micro-turbine operating at its highest compressor efficiency and a parabolic dish concentrator diameter of 16 metres were considered. The optimum geometries, minimum irreversibility rates and maximum receiver surface temperatures of the optimised systems are shown. For an environment with specific conditions and constraints, there exists an optimum receiver and recuperator geometry so that the system produces maximum net power output.
Energy, 2012
The Brayton cycle's heat source can be obtained from solar energy instead of the combustion of fu... more The Brayton cycle's heat source can be obtained from solar energy instead of the combustion of fuel. The irreversibilities of the open and direct solar thermal Brayton cycle with recuperator are mainly due to heat transfer across a finite temperature difference and fluid friction, which limit the net power output of such a system. In this work, the method of total entropy generation minimisation is applied to optimise the geometries of the receiver and recuperator at various steady-state weather conditions. For each steady-state weather condition, the optimum turbine operating point is also found. The authors specifically investigate the effect of wind and solar irradiance on the maximum net power output of the system. The effects of other conditions and constraints, on the maximum net power output, are also investigated. These include concentrator error, concentrator reflectivity and maximum allowable surface temperature of the receiver. Results show how changed solar beam irradiance and wind speed affect the system net power output and optimum operating point of the micro-turbine. A dish concentrator with fixed focal length, an off-the-shelf micro-turbine and a modified cavity receiver is considered.
Renewable and Sustainable Energy Reviews, 2013
Many studies have been published on the performance and optimisation of the Brayton cycle and sol... more Many studies have been published on the performance and optimisation of the Brayton cycle and solar thermal Brayton cycle showing the potential, merits and challenges of this technology. Solar thermal Brayton systems have potential to be used as power plants in many sun-drenched countries. It can be very competitive in terms of efficiency, cost and environmental impact. When designing a system such as a recuperative Brayton cycle there is always a compromise between allowing effective heat transfer and keeping pressure losses in components small. The high temperatures required in especially the receiver of the system presents a challenge in terms of irreversibilities due to heat loss. In this paper, the authors recommend the use of the total entropy generation minimisation method. This method can be applied for the modelling of a system and can serve as validation when compared with first-law modelling. The authors review various modelling perspectives required to develop an objective function for solar thermal power optimisation, including modelling of the sun as an exergy source, the Gouy-Stodola theorem and turbine modelling.
Energy Conversion and Management, 2014
The first law and second law efficiencies are determined for a stainless steel closed-tube open r... more The first law and second law efficiencies are determined for a stainless steel closed-tube open rectangular cavity solar receiver. It is to be used in a small-scale solar thermal Brayton cycle using a micro-turbine with low compressor pressure ratios. There are many different variables at play to model the air temperature increase of the air running through such a receiver. These variables include concentrator shape, concentrator diameter, concentrator rim angle, concentrator reflectivity, concentrator optical error, solar tracking error, receiver aperture area, receiver material, effect of wind, receiver tube diameter, inlet temperature and mass flow rate through the receiver. All these variables are considered in this paper. The Brayton cycle requires very high receiver surface temperatures in order to be successful. These high temperatures, however, have many disadvantages in terms of heat loss from the receiver, especially radiation heat loss. With the help of ray-tracing software, SolTrace, and receiver modelling techniques, an optimum receiver-to-concentrator-area ratio of A 0 % 0.0035 was found for a concentrator with 45°rim angle, 10 mrad optical error and 1°tracking error. A method to determine the temperature profile and net heat transfer rate along the length of the receiver tube is presented. Receiver efficiencies are shown in terms of mass flow rate, receiver tube diameter, pressure drop, maximum receiver surface temperature and inlet temperature of the working fluid. For a 4.8 m diameter parabolic dish, the larger the receiver tube diameter and the smaller the mass flow rate through the receiver, the higher the receiver surface temperature and the less efficient the collector becomes. However, the smaller the receiver tube diameter, the higher the pressure drop through the tube and the smaller the second law efficiency. It was found that the receiver with larger tube diameter would perform better in a solar thermal Brayton cycle. An overall solar-toheat efficiency of between 45% and 70% is attainable for the solar collector using the open-cavity receiver.
Conference Presentations by Willem Le Roux
Power generation for African rural communities: initial assessment of high temperature thermal energy storage for small-scale solar Brayton system
International Conference on Applied Energy (ICAE2019), Västerås, Sweden, 2019
Experimental testing of a small-scale solar thermal Brayton cycle recuperator
16th International Heat Transfer Conference (IHTC-16), 2018
The open Brayton cycle with open cavity receiver utilises a parabolic dish to concentrate solar i... more The open Brayton cycle with open cavity receiver utilises a parabolic dish to concentrate solar irradiance so that it may be captured by the working fluid (air). The cycle has been analysed and optimised to work with a simple receiver so that complexity and cost may be reduced. To maintain sufficient cycle effectiveness, a large efficient recuperator has to be implemented to allow for the high temperatures in order of 1000 K needed by the Brayton cycle. The proposed micro-turbine, an automotive turbocharger, cannot operate at high pressure ratios. The recuperator allows for lower pressure ratios to be considered. The purpose of this research is to test a design for a low-pressure and high-temperature recuperator that can be implemented within the solar Brayton cycle, and can be locally manufactured for a relatively low cost, as no current solution for such a cycle exists. Current solutions involve complex designs, expensive manufacturing processes and permanent joining methods that would eliminate the possibility for inspection and maintenance. The key element in the proposed design would see a high temperature sealant being used along with a clamped plate heat exchanger layout, in place of welding, allowing for ease of assembly as well as the ability to dismantle the unit for inspection. To ascertain the possibility of implementing the design, the recuperator was first modelled and shown to adhere to the necessary criteria. Results from the theoretical model show that at the proposed cycle conditions the recuperator plate bank would consist of 350 channels with an effectiveness of 90% and a total pressure drop of 3.49 kPa. A small scale model of the recuperator was constructed and tested, using both in-stream and surface thermocouples. Due to combustion issues with the LPG, the data was slightly skewed, however enough results were attained to show that the design could prove effective with a few modifications and further testing, and that the high temperature sealant works well with the clamped plate design.
Feasibility study of a hybrid small-scale dish-mounted solar thermal Brayton cycle with cogeneration
16th International Heat Transfer Conference (IHTC-16), 2018
A small modular solar power plant which uses the sun, air and a turbocharger (micro-turbine) to g... more A small modular solar power plant which uses the sun, air and a turbocharger (micro-turbine) to generate electricity and hot water for a household or small community in South Africa is investigated. Solar rays are focused onto a solar receiver which heats compressed air to turn the turbine of a turbocharger, which powers a compressor and an electric generator. A recuperator is used to collect waste heat from the turbine, to pre-heat air before it enters the solar receiver and any other waste heat is used to heat water (cogeneration). It has been proposed that the setup can also be powered with gas during the night or when solar radiation levels are low, making it a hybrid, however, a cost analysis is required. The feasibility of such a small-scale dish-mounted hybrid solar thermal Brayton cycle, in terms of running costs, is therefore investigated. For a constant power load and water heating load, an expected energy utilization factor (EUF) is found by considering previous work and its effect is investigated. The annual running costs of the system are calculated and compared with similar costs when using the municipal grid. Two locations in South Africa are considered by using measured hourly solar beam irradiance data. The effects of EUF, receiver heat loss rate and solar dish area are also investigated. Results show that the hybrid solar thermal Brayton cycle with cogeneration, using an off-the-shelf turbocharger as micro-turbine, can be competitive with the national grid when the EUF is higher than 52%.
5th Southern African Solar Energy Conference (SASEC2018), 2018
Concentrated Solar Power (CSP) is quickly becoming a necessity as a result of it being a renewabl... more Concentrated Solar Power (CSP) is quickly becoming a necessity as a result of it being a renewable energy power source, as well as showing potential for high efficiencies. A parabolic dish is an attractive form of CSP, since it can be scaled down and used for off-grid power supply to rural areas. The accuracy of a parabolic dish depends mainly on the parabolic function, specularity and slope error, shadowing and reflectivity. These factors affect the heat flux distribution at the receiver. By decreasing these errors, the collector will experience less energy spillage. A lunar test was done in order to analyse the spillage and flux map of a parabolic dish in its construction phase, using the full moon. The lunar flux map was analysed and compared to a flux map modelled with SolTrace, so that the cause of spillage could be determined. The lunar flux mapping method is cost effective and a simple technique for pinpointing areas in a collector that need improvement.
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Papers by Willem Le Roux
Conference Presentations by Willem Le Roux