Study on design of molten salt solar receivers for beam-down solar concentrator

Systems using molten salt as thermal media have been proposed for solar thermal power generation and for synthetic fuel production. We have been developing molten salt solar receivers, in which molten salt is heated by concentrated solar radiation, in the Solar Hybrid Fuel Project of Japan. A cavity shaped receiver, which is suitable for a beam-down type solar concentration system, was considered. In order to design molten salt solar receivers, a numerical simulation program for the prediction of characteristics of receivers was developed. The simulation program presents temperature distributions of a receiver and molten salt with the use of heat flux distribution of solar radiation and properties of composing materials as input data. Radiation to heat conversion efficiency is calculated from input solar power and heat transferred to molten salt. The thermal resistance of molten salt and the maximum discharge pressure of molten salt pumps were taken into account as restrictions for the design of receivers. These restrictions require control of maximum receiver temperature and pressure drop in the molten salt channel. Based on the incident heat flux distribution formed with a 100 MWth class beam-down type solar concentration system, we proposed a shape of solar receiver that satisfies the requirements. The radiation to heat conversion efficiency of the designed receiver was calculated to be about 90%.     

塔式太阳能电站熔盐换热器设计发展概述

熔盐换热器作为塔式太阳能熔盐储热发电系统中的一个重要设备,随着太阳能储热发电技术的发展,产生了各种不同形式的设计。从熔盐物性特点和太阳能储热发电技术特点的角度出发,对熔盐换热器设计要点进行了阐述。并通过归纳总结新型换热器技术在熔盐换热器上的应用,来展望熔盐换热器技术的发展趋势。     

A novel PVT/PTC/ORC solar power system with PV totally immersed in transparent organic fluid

A novel hybrid PVT/parabolic trough concentrator (PTC)/organic Rankine cycle (ORC) solar power system integrated with underground heat exchanger has been proposed. The evaporator unit consists of a transparent flat PVT solar collector and a PTC connected in series. The first transparent solar collector has transparent covers and consists of solar cells totally immersed within a pressurized transparent organic fluid that allows the solar radiation to reach the solar cells, cools them effectively, and captures all thermal losses from the solar cells. The second concentrator is a conventional one with opaque black receiver used to reheat the transparent organic fluid to higher temperatures. Both solar collectors (the PVT and PTC) perform as the boiler and superheater for the ORC. The performance of the proposed system is investigated by a steady‐state mathematical model. The results show that, at design conditions, the efficiency of the PV modules stabilizes around 12%, absorber efficiency varies within 64% to 75%, and the ORC efficiency varies within 7% to 17%.     

Experimental and theoretical analysis of a dynamic test method for molten salt cavity receiver

Test methods for estimating the thermal performance of the molten salt receiver are a matter of ongoing concern. To date, test methods in the literature require receiver to be operated in steady state or quasi-steady state. However, the receiver is always operating in the unsteady state with ongoing changes in power absorption and flow rate. Therefore, research into dynamic test method for the molten salt cavity receiver is required. The Transfer Function Method (TFM) is a successful dynamic test method for solar collectors. In this paper, a theoretical analysis of the TFM was applied to the molten salt cavity receiver and then verified by indoor transient experiments. The TFM predicted outlet temperature of the receiver was compared with experimental data. The results showed that the TFM accurately predicted the outlet temperature trends despite some errors between predicted and measured outlet temperature. The errors may have originated from the changing flow rate. The TFM is a good candidate as a dynamic test method for the concentrated solar receiver.     

Theoretical investigation of the performance of a novel loop heat pipe solar water heating system for use in Beijing,China

A novel loop heat pipe (LHP) solar water heating system for typical apartment buildings in Beijing was designed to enable effective collection of solar heat, distance transport, and efficient conversion of solar heat into hot water. Taking consideration of the heat balances occurring in various parts of the loop, such as the solar absorber, heat pipe loop, heat exchanger and storage tank, a computer model was developed to investigate the thermal performance of the system. With the specified system structure, the efficiency of the solar system was found to be a function of its operational characteristics - working temperature of the loop heat pipe, water flow rate across the heat exchanger, and external parameters, including ambient temperature, temperature of water across the exchanger and solar radiation. The relationship between the efficiency of the system and these parameters was established, analysed and discussed in detail. The study suggested that the loop heat pipe should be operated at around 72 °C and the water across the heat exchanger should be maintained at 5.1 l/min. Any variation in system structure, i.e., glazing cover and height difference between the absorber and heat exchanger, would lead to different system performance. The glazing covers could be made using either borosilicate or polycarbonate, but borosilicate is to be preferred as it performs better and achieves higher efficiency at higher temperature operation. The height difference between the absorber and heat exchanger in the design was 1.9 m which is an adequate distance causing no constraint to heat pipe heat transfer. These simulation results were validated with the primary testing results.     

Solar Thermal Power Towers

The solar thermal central receiver technology, known as solar power towers, is rapidly evolving to a state of near-term energy availability for electrical power generation and industrial process heat applications. The systems consist of field arrays of heliostat reflectors, a central receiver boiler, short term thermal storage devices, and either turbine-generators or heat exchangers. Fluid temperatures up to 550°C are currently achievable, and technology developments are underway to reach 1100°C. Six solar power towers are now under construction or in test operation in five countries around the world.     

An experimental study: Thermal performance of molten salt cavity receivers

In solar power plants, a molten salt receiver always works in unsteady state conditions. Therefore, it is necessary to research the thermal performance of a receiver in an unsteady state condition. For this purpose, an indoor testing system with a molten salt cavity receiver was developed. Experimental research was conducted to determine the thermal performance of a 100 kWt molten salt receiver. The effect of the input power and flow rate on the thermal performance of the receiver was investigated. In addition, a simple unsteady model was established to research the characteristics of the variation of the internal energy of the receiver and the characteristics of the heat loss. The results indicated that the efficiency of the receiver was in direct proportion to the flow rate. However, the influence was small. In the initial stage of the transient process, the increments of the internal energy of the receiver and the fluid were large (approximately 20% in the energy which is not removed by the mass flow of the fluid). Over time, the thermal inertia of the receiver decreased with the transient process. As a result, any energy not taken away by the fluid was transformed into heat loss.     

Energy and exergy analyses of a solar based hydrogen production and compression system

In this study, a solar thermal based integrated system with a supercritical-CO₂ (sCO₂) gas turbine (GT) cycle, a four-step Mg–Cl cycle and a five-stage hydrogen compression plant is developed, proposed for applications and analyzed thermodynamically. The solar data for the considered solar plant are taken for Greater Toronto Area (GTA) by considering both daily and yearly data. A molten salt storage is considered for the system in order to work without interruption when the sun is out. The power and heat from the solar and sCO₂-GT subsystems are introduced to the Mg–Cl cycle to produce hydrogen at four consecutive steps. After the internal heat recovery is accomplished, the heating process at required temperature level is supplied by the heat exchanger of the solar plant. The hydrogen produced from the Mg–Cl cycle is compressed up to 700 bar by using a five-stage compression with intercooling and required compression power is compensated by the sCO₂-GT cycle. The total energy and exergy inputs to the integrated system are found to be 1535 MW and 1454 MW, respectively, for a 1 kmol/s hydrogen producing plant. Both energy and exergy efficiencies of the overall system are calculated as 16.31% and 17.6%, respectively. When the energy and exergy loads of the receiver are taken into account as the main inputs, energy and exergy efficiencies become 25.1%, and 39.8%, respectively. The total exergy destruction within the system is found to be 1265 MW where the solar field contains almost 64% of the total irreversibility with a value of ～811 MW.     

Dynamic model of a solar thermochemical water-splitting reactor with integrated energy collection and storage

Water-splitting solar thermochemical cycles are important in meeting the challenges of global climate change and limited fossil fuels. However, solar radiation varies in availability, leading to unsteady state operation. We propose a solar receiver-reactor with integrated energy collection and storage. The reactor consists of a double-pipe heat exchanger placed at the focal line of a parabolic trough solar concentrator. Molten salt passes through the jacket, absorbing energy from the irradiated outer surface while driving the endothermic oxygen production step of the copper-chlorine water-splitting cycle in the reactor bore. Excess energy is stored in a thermal storage tank to buffer the reactor from changes in insolation. Dynamic simulation indicates that the reactor can sustain steady 100% conversion during 24/7 operation with a reasonable plant layout. The technology employed is extant and mature. This is important in view of the urgency to reduce dependency upon fossil fuels as primary energy sources.     

Experimental investigation of a molten salt thermocline storage tank

Thermal energy storage is considered as an important subsystem for solar thermal power stations. Investigations into thermocline storage tanks have mainly focused on numerical simulations because conducting high-temperature experiments is difficult. In this paper, an experimental study of the heat transfer characteristics of a molten salt thermocline storage tank was conducted by using high-temperature molten salt as the heat transfer fluid and ceramic particle as the filler material. This experimental study can verify the effectiveness of numerical simulation results and provide reference for engineering design. Temperature distribution and thermal storage capacity during the charging process were obtained. A temperature gradient was observed during the charging process. The temperature change tendency showed that thermocline thickness increased continuously with charging time. The slope of the thermal storage capacity decreased gradually with the increase in time. The low-cost filler material can replace the expensive molten salt to achieve thermal storage purposes and help to maintain the ideal gravity flow or piston flow of molten salt fluid.     

Opportunities and challenges in using particle circulation loops for concentrated solar power applications

Concentrated Solar Power (CSP) is an electricity generation technology that concentrates solar irradiance through heliostats onto a small area, the receiver, where a heat transfer medium, currently a fluid (HTF), is used as heat carrier towards the heat storage and power block. It has been under the spotlight for a decade as one of the potential or promising renewable and sustainable energy technologies.Using gas/solid suspensions as heat transfer medium in CSP has been advocated for the first time in the 1980′s and this novel concept relies on its possible application throughout the full CSP plant, i.e., in heat harvesting, conveying, storage and re-use, where it offers major advantages in comparison with the common heat transfer fluids such as water/steam, thermal fluids or molten salt. Although the particle suspension has a lower heat capacity than molten salts, the particle-driven system can operate without temperature limitation (except for the maximum allowable wall temperature of the receiver tubes), and it can also operate with higher hot-cold temperature gradients. Suspension temperatures of over 800 °C can be tolerated and achieved, with additional high efficiency thermodynamic systems being applicable. The application of high temperature particulate heat carriers moreover expands the possible thermodynamic cycles from Rankine steam cycles to Brayton gas cycles and even to combined electricity generating cycles.This review paper deals with the development of the particle-driven CSP and assesses both its background fundamentals and its energy efficiency. Among the cited systems, batch and continuous operations with particle conveying loops are discussed. A short summary of relevant particle-related properties, and their use as heat transfer medium is included. Recent pilot plant experiments have demonstrated that a novel bubbling fluidized bed concept, the upflow bubbling fluidized bed (UBFB), recently adapted to use bubble rupture promoters and called dense upflow fluidized bed (DUFB), offers a considerable potential for use in a solar power tower plant for its excellent heat transfer at moderate to high receiver capacities.For all CSP applications with particle circulation, a major challenge remains the transfer of hot and colder particles among the different constituents of the CSP system (receiver to storage, power block and return loop to the top of the solar tower). Potential conveying modes are discussed and compared. Whereas in solar heat capture, bubbling fluidized beds, particle falling films, vortex and rotary furnaces, among others, seem appropriate, both moving beds and bubbling fluidized beds are recommended in the heat storage and re-use, and examined in the review.Common to all CSP applications are the thermodynamic cycles in the power block, where different secondary working fluids can be used to feed the turbines. These thermodynamic cycles are discussed in detail and the current or future most likely selections are presented.Since the use of a back up fuel is recommended for all CSP systems, the hybrid operation with the use of alternative fuel back-up is also included in the review.The review research is concluded by scale-up data and challenges, and provides a preliminary view into the prospects and the overall economy of the system. Market prospects for both novel concentrated solar power are expected to be excellent. Although the research provided lab- and pilot-scale based design methods and equations for the key unit operations of the novel solar power tower CSP concept, there is ample scope for future development of several topics, as finally recommended.     

塔式太阳能热发电系统性能模型及其仿真研究

结合塔式太阳能热发电系统中主要设备的机理模型和变工况下的运行方式,建立一套能完整反映电站工作过程的性能模型.发电系统在太阳法向直射辐照度扰动下的动态过程研究结果表明:吸热器出口熔盐温度、表面最高温度、散热功率的过渡过程较平缓,效率在瞬时突变后会逐渐恢复到接近扰动发生前的水平;吸热器出口熔盐温度、表面最高温度、散热功率与...     

Exergetic optimization for solar heat receiver with heat loss and viscous dissipation

The exergetic efficiency of heat receiver in solar thermal power system is optimized by considering the heat loss outside the receiver and fluid viscous dissipation inside the receiver. The physical models of heat loss and pumping power consumption for solar heat receiver are first proposed, and associated exergetic efficiency is further induced. As the flow velocity rises, the pumping power consumption and heat absorption efficiency significantly rises, and the maximum absorption efficiency and optimal incident energy flux also increase. Along the flow direction of solar receiver, the exergy flux increment and the flow exergy loss almost linearly increase, while the exergetic efficiency varies very slowly at high flow velocity. According to the exergetic efficiency loss from flow viscou’s dissipation, the exergetic efficiency of solar heat receiver will first increase and then decrease with the flow velocity. Because of the coupling effects of heat absorption efficiency and exergetic efficiency from fluid internal energy, the exergetic efficiency of solar heat receiver will approach to the maximum at proper inlet temperature. As a result, the exergetic efficiency of solar heat receiver will reach the maximum at optimal inlet temperature, incident energy flux and flow velocity.     

Liquid sodium versus Hitec as a heat transfer fluid in solar thermal central receiver systems

Selection of an appropriate HTF is important for minimising the cost of the solar receiver, thermal storage and heat exchangers, and for achieving high receiver and cycle efficiencies. Current molten salt HTFs have high melting points (142–240 °C) and degrade above 600 °C. Sodium’s low melting point (97.7 °C) and high boiling point (873 °C) allow for a much larger range of operational temperatures. Most importantly, the high temperatures of sodium allow the use of advanced cycles (e.g. combined Brayton/Rankine cycles). In this study, a comparison between the thermophysical properties of two heat transfer fluids (HTFs), Hitec (a ternary molten salt 53% KNO₃ + 40% NaNO₂ + 7% NaNO₃) and liquid sodium (Na), has been carried out to determine their suitability for use in high-temperature concentrated solar thermal central-receiver systems for power generation. To do this, a simple receiver model was developed to determine the influences of the fluids’ characteristics on receiver design and efficiency. While liquid sodium shows potential for solar thermal power systems due to its wide range of operation temperatures, it also has two other important differences – a high heat transfer coefficient (～an order of magnitude greater than Hitec) and a low heat capacity (30–50% lower than Hitec salt). These issues are studied in depth in this model. Overall, we found that liquid sodium is potentially a very attractive alternative to molten salts in next generation solar thermal power generation if its limitations can be overcome.     

Heat transfer enhancement and performance of the molten salt receiver of a solar power tower

This paper investigates the interaction between the heat transfer performance and the thermal efficiency of a molten salt receiver used in the solar power tower plant. A test-bed is built, and a series of experiments of heat transfer enhancement for two types of molten salt receiver tubes, including smooth and spiral tubes, have been carried out under the high temperature and the high heat flux conditions. The experimental results show that the Nusselt numbers of spiral tube with heat transfer enhancement are in the range of 400–1200, which is about 3 times than that of the smooth one on average. The wall temperature of the spiral tube is decreased by about 30 °C comparing with that of the smooth tube under the identical heat transfer conditions. The results of the experiment show that, by using the spiral tube as the heat transfer tube, the heat transfer performance of the molten salt receiver is obviously improved, and the radiation and convection losses are significantly reduced. The results will be helpful for the design of the molten salt receiver.     

Optimization of the heat exchanger in a flat plate indirect heating integrated collector storage solar water heating system

Due to the environmental impact of energy usage, consumers need to be encouraged to use renewable energy sources such as solar energy. The indirect heating flat plate integrated collector storage solar water heating system is one of the compact systems for domestic water heating. It incorporates the collection of a solar energy component and a hot water storage component in one unit. The objectives of this study were to investigate the effect of different parameters on the thermal performance of this system with the aim of reducing both the initial and the running costs. The outlet service water temperature was used as a measure of performance, because it is an indicator of the energy acquired from the solar radiation. The continuity, momentum and energy equations of the fluids involved in the system were numerically solved in a steady state condition, using FLUENT software. Three-D CFD models were developed and validated using previous experimental results. A standard k–ω turbulent model was used in the optimization of the heat exchanger because it produced good agreement with the experimental results. The surface-to-surface radiation model was included. The effect of single and double row heat exchangers with different lengths was investigated. Circular and elliptic cross-section pipes were also examined. Mass flow rates of 500 and 650 L/h were chosen. The results showed that the single row HX of 10.8 m length for both the elliptical and type B tube gave high service water outlet temperature (acceptable for heat exchanger design) and with low pumping power. This resulted in an increase in the thermal efficiency and a significant reduction in both the initial and the operating costs of the system.     

Numerical and Experimental Investigation on Heat Transfer Performance of a Solar Single Storage Tank

The single-tank latent heat thermal energy storage(LHTES) of solar energy mainly consists of two modules: the first one is the phase change material(PCM) module heated by solar energy; the second is a module of heat transfer between melted PCM and the user's low-temperature water. This paper mainly focuses on the former one. To investigate the heat transfer performance of the paraffin-based solar single storage tank and find a more suitable experimental configuration, as basic research work, we established a single-tank thermal storage platform and then conducted a numerical simulation on the heat transfer process with Fluent. The result of numerical simulation shows that the test situation was basically reflected and the data agreed well with the experiment results. The numerical simulation analysis is accurate and the method is reliable. To obtain the heat transfer performance of paraffin in a single tank and strengthen heat transfer, the aspect ratio, the melting temperature of paraffin, and the heating power of the electric heater were analyzed based on simulation. The results show that the heat transfer gets more uniform when the aspect ratio is lower. This results in an increase in the liquid fraction of 61.83% to 76.47% one hour after heating when the aspect ratio of the tank reduced from 2.8 to 1.1. The higher the melting temperature of paraffin, the longer it takes for PCM to reach a stable state. And the curvature of liquid heating is greater than that of solid heating at the bottom layer. Under the constant total work, the heating power has little effect on the heat transfer performance of the paraffin. This study will provide some reference value for the optimization design of single-tank LHTES systems in the future.     

High-temperature Solar Thermal Energy Storage

The mislocation of solar energy production facilities and points of demand and the temporal mismatch of solar energy availability and energy demand make transport and storage of solar energy essential. Research at the Solar Energy Research Institute has focused on high-temperature, diurnal storage because of the frequency of use and the potential for conservation of premium fossil fuels. Also, high-temperature thermal energy storage can reduce the cost of hydrogen production, electricity and heat produced by cogeneration, and methane reforming. SERI research is concentrating on containment techniques (including materials corrosion, internal insulation, and storage medium for high-temperature molten salts) and direct-contact heat exchange (including cost-effective heat exchanger design and heat transfer of various materials). After initial screening tests we selected carbonates for further study. We are now constructing test equipment that will allow heat transfer experiments with molten carbonate to 700°C     

槽式太阳能光热发电油盐换热装置设计优化

简要介绍油盐换热装置的工作原理,熔盐、导热油介质的特性。分析在设计槽式太阳能光热发电的关键设备油盐换热装置时需要考虑的问题,并详细描述油盐换热设备的设计方案。     

Solar thermochemical hydrogen: The heat source-process interface

A discussion of issues and considerations related to the interface between a solar heat source and a thermochemical hydrogen process and some details of a tubular heat exchanger operating as such an interface in a cavity-type receiver are presented. The issues include the temperature and heat input requirements for the endothermic reaction, type of receiver, heat storage, transient operations, and control. A thermal performance analysis of a tubular reactor/heat exchanger operating in a cavity-type solar receiver is applied to SO₃ decomposition. The analysis produces axial distributions of temperature tube wall and process fluid, reaction rate, conversion, velocity, density, pressure and residence time. Process fluid conditions at the inlet, tube characteristics, reaction kinetics and cavity operating temperature are inputs. The cavity temperature affects average heat flux and, therefore, heat-exchanger cost and receiver efficiency and, therefore, mirror field cost. A design which minimizes the combined cost may be found and examples are shown.