Study of thermal characteristics of a heating module with parabolic trough concentrator and linear wedge-like photoelectric receiver

New solar modules intended for typical solar collectors containing semiparabolic trough concentrators and receivers that convert solar energy into thermal energy are considered. Mathematical modeling is carried out to develop an algorithm for estimating the structure of a heating module with the assigned energy parameters according to the laws of geometrical optics, as well as heat and mass transfer. When using such modules, which are based on a parabolic concentrator and a receiver with a system of coolant flow, cogeneration plants can be designed to produce electricity and heat. The mockups developed using this procedure are studied on the corresponding facilities and are tested under in-situ conditions. A solar module with an asymmetric parabolic trough concentrator and a linear wedge-like photoelectric receiver of concentrated radiation with a system of coolant flow provides the maximum power of 386 W at a temperature of 40°C and an efficiency of 60%, and 319 W at 60°C and 49%, respectively. Such modules are proposed for use to design solar collectors with the required performance.     

Energy performance and numerical optimization of a screw expander–based solar thermal electricity system in a wide range of fluctuating operating conditions

This paper provides fundamental principles to study the thermodynamic performance of a new screw expander–based solar thermal electricity plant. While steam turbines are generally used in direct steam generation solar systems without admitting fluid in two-phase conditions, steam screw expanders, as volumetric machines, can convert thermal to mechanical energy also by expanding liquid-steam mixtures without a decline in efficiency. In effect, steam turbines are not as competitive as screw expanders when the net power is smaller than 2 MW and for low-grade heat sources. The solar electricity generation system proposed in this paper is based on the steam Rankine cycle: Water is used as both working fluid and storage, parabolic trough collectors are used as a thermal source, and screw expanders are used as power machines. Since screw expanders can operate at off-design working conditions in several situations when installed in direct steam generation solar plants, studying expander performance under fluctuating working situations is a crucial issue. The main aim of the present paper is to establish a thermodynamic model to study the energetic benefits of the proposed power system when off-design operating conditions and variable solar radiation occur. This entails, first and foremost, developing overexpansion and underexpansion numerical models to describe the polytropic expansion phase, which considers all the losses affecting performance of the screw expander under real operating conditions. To assess the best operating conditions and maximum efficiency of the whole power system at part-load working conditions under fluctuating solar radiations, parametric optimization is then improved in a wide range of variable working conditions, assuming condensation pressures of water increasing from 0.1 to 1 bar, under an evaporation temperature rising from 170°C to 300°C.     

Liquid-based high-temperature receiver technologies for next-generation concentrating solar power: A review of challenges and potential solutions

To reduce the levelized cost of energy for concentrating solar power (CSP), the outlet temperature of the solar receiver needs to be higher than 700 °C in the next-generation CSP. Because of extensive engineering application experience, the liquid-based receiver is an attractive receiver technology for the next-generation CSP. This review is focused on four of the most promising liquid-based receivers, including chloride salts, sodium, lead-bismuth, and tin receivers. The challenges of these receivers and corresponding solutions are comprehensively reviewed and classified. It is concluded that combining salt purification and anti-corrosion receiver materials is promising to tackle the corrosion problems of chloride salts at high temperatures. In addition, reducing energy losses of the receiver from sources and during propagation is the most effective way to improve the receiver efficiency. Moreover, resolving the sodium fire risk and material compatibility issues could promote the potential application of liquid-metal receivers. Furthermore, using multiple heat transfer fluids in one system is also a promising way for the next-generation CSP. For example, the liquid sodium is used as the heat transfer fluid while the molten chloride salt is used as the storage medium. In the end, suggestions for future studies are proposed to bridge the research gaps for > 700 °C liquid-based receivers.     

Transient performance of direct steam generation solar power plants

Parabolic trough power plants are currently the most commercial systems for electricity generation. In this study, a transient numerical simulation of a solar power plant was developed by using direct steam generation (DSG) technology. In this system, condensate water from a Rankine cycle is pumped directly to solar parabolic trough collectors. The pressurized water is heated and evaporated before being superheated inside the solar collectors and directed back to the steam turbines, where the Rankine cycle is a reheated‐regenerative cycle. The plant performance with saturated steam production is compared with the performance of a superheated plant. A mathematical model of each system component is presented, with the solar power cycle modeled by the TRNSYS‐17 simulation program. Annual transient performance, including plant power and efficiency, is presented for both plants. As expected, the power of the superheated plant outperforms the saturated plant by approximately 45%, whereas the efficiency decreases by approximately 10%. Furthermore, the power of such plants is considerably improved under the weather of Makkah, 22.4°N, and it is approximately 40 MW for both the spring and autumn seasons. The annual generated energy is approximately 8062 MWh. The levelized electricity cost (LEC) was estimated for both the DSG and the corresponding synthetic oil plants. The DSG plant has an approximately 3% higher LEC than a synthetic oil plant with heat storage and an approximately 11.2% lower LEC than an oil plant if the plant has no storage. Copyright © 2016 John Wiley & Sons, Ltd.     

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%.     

Numerical study of conduction and convection heat losses from a half-insulated air-filled annulus of the receiver of a parabolic trough collector

Grid-quality parabolic trough collectors utilize expensive receivers that maintain vacuum in their annuli to reduce convection losses. On the other hand, receivers with air-filled annuli, currently used mainly for process heat applications, are significantly less expensive, but their thermal performance is inferior to evacuated receivers. A promising technique that can bridge the cost and performance gap between the two types of receivers is introduced in this work. A heat-resistant thermal insulation material is fitted into the portion of the receiver annulus that does not receive concentrated sunlight. The presence of this insulation material is expected to reduce not only convection heat losses, but also radiation losses. This study focuses on the calculation of conduction and convection heat losses from the proposed receiver using numerical modeling. The performance of the proposed concept is compared to that of a conventional receiver with an air-filled annulus. The results have shown that the combined conduction and convection heat loss from the proposed receiver can be smaller than that from a receiver with an air-filled annulus by as much as 25% when fiberglass insulation is used. However, the fact that the thermal conductivity of the insulating material increases with temperature reduces the benefit of the proposed concept at high temperatures. As a result, the proposed receiver is expected to be suitable as a replacement for receivers with air-filled annuli or as an economical alternative to evacuated receivers that are used at the lower temperature end of utility-scale solar power plants.     

Thermodynamic performance analysis of the coal‐fired power plant with solar thermal utilizations

A new way of energy saving for existing coal‐fired power plant that uses low‐or medium‐temperature solar energy as assistant heat source was proposed to generate ‘green’ electricity. This paper has built the mathematical models of the solar‐aided power generation system focusing on the NZK600‐16.7/538/538 units. Based on the combination of the first and second law of thermodynamics, the thermodynamic performance of different components of the integrated system was evaluated under the changing operating condition aiming at different substitution options for turbine bleed streams. It has been found that the efficiency of the solar heat to electricity enhances with the increase of the load and the replaced extraction level. Additionally, when the second extraction is replaced, the effect is the best, which makes the power output increase around 6.13% or the coal consumption rate decrease 13.14 g/(kW · h) under 100%THA load and CO₂ emission reduce about 32.76 g/(kW · h), while the energy and exergy efficiencies of the integrated system are 39.35% and 39.12%, respectively. The results provide not only theory basis and scientific support for the design of solar‐aided coal‐fired power plants, but also a new way of energy saving and optimization for the units. Copyright © 2014 John Wiley & Sons, Ltd.     

Improving the optical efficiency of a concentrated solar power field using a concatenated micro-tower configuration

The concatenated micro-tower (CMT) is a new configuration for concentrated solar power plants that consists of multiple mini-fields of heliostats. In each mini-field, the heliostats direct and focus sunlight onto designated points along an insulated tube, where thermal receivers are located. The heat transfer fluid, flowing through a multitude of discrete receivers, is combined and directed towards a single power block. The key advantages of CMT are its dual-axis tracking system and dynamic receiver allocation, i.e., the ability of each heliostat to direct sunrays towards receivers from adjacent mini-fields throughout the day according to their optical efficiency. Here we compare between the annual optical efficiencies of a conventional trough, large tower, and CMT configuration, all located at latitude 36 N. For each configuration, we calculated the annual optical efficiency based on the cosine factor and atmospheric transmittance. CMT’s dynamic receiver allocation provides more uniform electricity production during the day and throughout the year and improves the annual optical efficiency by 12-19% compared to conventional trough and large tower configurations.     

A solar-driven combined cycle power plant

The main results of a feasibility study of a combined cycle electricity generation plant, driven by highly concentrated solar energy and high-temperature central receiver technology, are presented. New developments in solar tower optics, high-performance air receivers and solar-to-gas turbine interface, were incorporated into a new solar power plant concept. The new design features 100% solar operation at design point, and hybrid (solar and fuel) operation for maximum dispatchability. Software tools were developed to simulate the new system configuration, evaluate its performance and cost, and optimize its design. System evaluation and optimization were carried out for two power levels. The results show that the new system design has cost and performance advantages over other solar thermal concepts, and can be competitive against conventional fuel power plants in certain markets even without government subsidies.     

Evaluation of solar aided thermal power generation with various power plants

Solar aided power generation (SAPG) is an efficient way to make use of low or medium temperature solar heat for power generation purposes. The so‐called SAPG is actually ‘piggy back’ solar energy on the conventional fuel fired power plant. Therefore, its solar‐to‐electricity efficiency depends on the power plant it is associated with. In the paper, the developed SAPG model has been used to study the energy and economic benefits of the SAPG with 200 and 300 MW typical, 600 MW subcritical, 600 MW supercritical, and 600 and 1000 MW ultra‐supercritical fuel power units separately. The solar heat in the temperature range from 260 to 90°C is integrated with above‐mentioned power units to replace the extraction steam (to preheat the feedwater) in power boosting and fuel‐saving operating modes. The results indicate that the benefits of SAPG are different for different steam extracted positions and different power plants. Generally, the larger the power plant, the higher the solar benefit if the same level solar is integrated. Copyright © 2010 John Wiley & Sons, Ltd.     

槽式太阳能电站集热管热性能测试

采用硅碳棒加热技术和热平衡法测试了桑普生产的具有自主知识产权的槽式太阳能电站集热管的热性能。在40～300℃温度范围内,共测试8个工况下集热管热性能。实验结果显示,集热管中低温性能与肖特公司的PTR70相差不大,完全满足中低温槽式太阳能电站和其他太阳能中低温利用领域的应用。红外图像结果表明,玻璃-金属封接温度明显高于玻璃外管温度,对集热管进行理论分析时不能忽略此部分漏热量。实验数据的获得为国内太阳能槽式电站的设计、建设提供了实验参数,为集热管漏热测试相关标准的制定提供了基础。     

Hydrogen production by coupling pressurized high temperature electrolyser with solar tower technology

Solar hydrogen production by coupling of pressurized high temperature electrolyser with concentrated solar tower technology is studied. As the high temperature electrolyser requires constant temperature conditions, the focus is made on a molten salt solar tower due to its high storage capacity. A flowsheet was developed and simulations were carried out with Aspen Plus 8.4 software for MW-scale hydrogen production plants. The solar part was laid out with HFLCAL software. Two different scenarios were considered: the first concerns the production of 400 kg/d hydrogen corresponding to mobility use (fuel station). The second scenario deals with the production of 4000 kg/d hydrogen for industrial use. The process was analyzed from a thermodynamic point of view by calculating the overall process efficiency and determining the annual production. It was assumed that a fixed hydrogen demand exists in the two cases and it was assessed to which extent this can be supplied by the solar high temperature electrolysis process including thermal storage as well as hydrogen storage. For time periods with a potential over supply of hydrogen, it was considered that the excess energy is sold as electricity to the grid. For time periods where the hydrogen demand cannot be fully supplied, electricity consumption from the grid was considered. It was assessed which solar multiple is appropriate to achieve low consumption of grid electricity and low excess energy. It is shown that the consumption of grid electricity is reduced for increasing solar multiple but the efficiency is also reduced. At a solar multiple of 3.0 an annual solar-to-H₂ efficiency greater than 14% is achieved at grid electricity production below 5% for the industrial case (4000 kg/d). In a sensitivity study the paramount importance of electrolyser performance, i.e. efficiency and conversion, is shown.     

Modeling analysis on solar steam generator employed in multi-effect distillation (MED) system

Recently the porous bilayer wood solar collectors have drawn increasing attention because of their potential application in solar desalination. In this paper, a thermodynamic model has been developed to analyze the performance of the wood solar collector. A modeling analysis has also been conducted to assess the performance and operating conditions of the multiple effect desalination (MED) system integrated with the porous wood solar collector. Specifically, the effects of operating parameters, such as the motive steam temperature, seawater flow rate, input solar energy and number of effects on the energy consumption for each ton of distilled water produced have been investigated in the MED desalination system combined with the bilayer wood solar steam generator. It is found that, under a given operating condition, there exists an optimum steam generation temperature of around 145°C in the wood solar collector, so that the specific power consumption in the MED system reaches a minimum value of 24.88 kWh/t. The average temperature difference is significantly affected by the solar heating capacity. With the solar capacity increasing from 50 kW to 230 kW, the average temperature difference increases from 1.88°C to 6.27°C. This parametric simulation study will help the design of efficient bilayer wood solar steam generator as well as the MED desalination system.     

Optimization of nanofluid volumetric receivers for solar thermal energy conversion

Improvements in solar-to-thermal energy conversion will accelerate the development of efficient concentrated solar power systems. Nanofluid volumetric receivers, where nanoparticles in a liquid medium directly absorb solar radiation, promise increased performance over surface receivers by minimizing temperature differences between the absorber and the fluid, which consequently reduces emissive losses. We present a combined modeling and experimental study to optimize the efficiency of liquid-based solar receivers seeded with carbon-coated absorbing nanoparticles. A one-dimensional transient heat transfer model was developed to investigate the effect of solar concentration, nanofluid height, and optical thickness on receiver performance. Simultaneously, we experimentally investigated a cylindrical nanofluid volumetric receiver, and showed good agreement with the model for varying optical thicknesses of the nanofluid. Based on the model, the efficiency of nanofluid volumetric receivers increases with increasing solar concentration and nanofluid height. Receiver-side efficiencies are predicted to exceed 35% when nanofluid volumetric receivers are coupled to a power cycle and optimized with respect to the optical thickness and solar exposure time. This work provides insights as to how nanofluids can be best utilized as volumetric receivers in solar applications, such as receivers with integrated storage for beam-down CSP and future high concentration solar thermal energy conversion systems.     

Review of supercritical CO2 power cycles integrated with CSP

Increasing demand of electricity and severer concerns to environment call for green energy sources as well as efficient energy conversion systems. SCO₂ power cycles integrated with concentrating solar power (CSP) are capable of enhancing the competitiveness of thermal solar electricity. This article makes a comprehensive review of supercritical CO₂ power cycles integrated with CSP. A detailed comparison of four typical CSP technologies is conducted, and the cost challenge of currently CSP technologies is pointed out. The thermophysical properties of sCO₂ and the corresponding two real gas effects are analyzed elaborately to express the features of sCO₂ power cycles. An extensive review of sCO₂ layouts relevant for CSP including 12 single layouts and 1 combined layout is implemented logically. Strengths and weaknesses of sCO₂ power cycles over traditional steam-Rankine cycle generally adopted in current CSP plants are concluded, followed by metal material degration summary in CSP relevant temperature sCO₂ environment, which shows that the nickel-based alloy is a proper structural material candidate for sCO₂-CSP integration. Thermodynamic analyses of sCO₂ power cycles when integrated with CSP are divided into three level of which design-point analysis and off-design modeling are conducted and compared, more researches into the off-design point analysis, dynamic modeling, especially the transient behavior are suggested. Economic analysis of the integrated system is concluded and presents a considerable levelized cost of electricity reduction of 15.6% to 67.7% compared to that of state of art CSP. Taking the thermodynamic and economic analysis into consideration, target designs of sCO₂ power cycles for CSP are summarized in three aspects. Finally, current theoretical and experimental researches of sCO₂ power cycles integrated with CSP for market penetration are introduced. The strengths, weaknesses, and potential solutions to the gaps of three potential pathways (molten salt pathway, particle pathway, and gas phase pathway) to realize the integration of sCO₂ power cycles in the next CSP generation plants up to 700°C are reviewed. In general, the integration of sCO₂ power cycles with CSP technologies exhibits promising expectations for facilitating the competitiveness of thermal solar electricity.     

Entropy generation and Carnot efficiency comparisons of high temperature heat transfer fluid candidates for CSP plants

Heat transfer fluid is a critical component in a concentrating solar power plant. A large quantity of heat transfer fluid is required to transfer heat between the solar collector and the power block, thus it is crucial to select the most appropriate heat transfer fluid in order to maximize the system performance. The present study compared the performances of five molten-salt eutectic mixtures in regarding with the entropy generation rate and the Carnot efficiency of using them as heat transfer fluids. All the five molten-salt eutectic mixtures have thermal stability temperatures above 600 °C. Effects of the tube lengths in the steam generation heat exchanger and the receiver heat exchanger as well as the heat transfer fluid flow rate on both the entropy generation rate and the Carnot cycle efficiency were investigated. The results indicate that the carbonate salts has the worst performances compared to the other eutectic mixtures. The three chloride salts have slightly higher entropy generation rate and 5% higher Carnot efficiency than the Solar Salt. Therefore the three chloride salts are suggested to be used in advanced concentrating solar power tower plants as potential high temperature heat transfer fluids.     

Experimental Study on Melting Process in an Industrial Level Molten Salt Tank

Thermal energy storage(TES) is an important part of concentrating solar power(CSP) plants. The primary advantage of TES in CSP plants is the ability to dispatch electrical output to match peak demand periods and reduce the levelized cost of electricity. The major challenge of the molten salt is its high freezing point, leading to additional complicating freeze protection. This paper presents the experimental results of melting process of a mixed nitrate salt with a melting temperature of 115℃ in a 20 m^3 industrial level tank. Twenty electrical heaters inside the tank are used to heat the salt with a total maximum input power of 240 kW. In order to ensure a safe and fast melting process, the whole process adopted an operating strategy of combining automatic control with manual control. The whole melting process lasted for 314 hours. The salt temperature showed the greatest increase in the first 38 hours. Finally, an economic operation mode of molten salt heat storage tank was obtained.     

Numerical investigation of flow and heat transfer in a volumetric solar receiver

Volumetric solar receivers are used in solar power plants to convert concentrated solar radiation into high temperature heat to operate a thermal engine. In general, porous high temperature materials are used for this purpose. Since the pore geometry is important for the efficiency performance of the receiver, current R&D activities focus on the optimization of this quantity. In this study, the influence of slight geometry changes of this component on its temperature distribution and efficiency has been investigated with the objective of an overall improvement. A numerical analysis of the mass and heat transfer through the receiver has been performed. The investigated receiver was an extruded honeycomb structure made out of Silicon Carbide. Additionally, experimental tests have been performed. In these tests, selected receiver samples have been exposed to concentrated radiation. From these tests solar-to-thermal efficiency data have been derived, which could be compared with the calculated data. Two numerical models have been developed. One makes use of the real geometry of the channel (single channel model), the other one considers the receiver to be “porous continuum”, which is described by homogenized properties such as permeability and effective heat conductivity. The experimental parameters such as the average solar heat flux and the mass flow were taken into account in the models as boundary conditions. Various quantities such as the average air outlet temperatures, the temperature distributions and the solar-to-thermal efficiency were used for the comparison. The correspondence between the experimental and numerical results of both numerical models confirms the capability of the approaches for further studies.     

Technical and economical system comparison of photovoltaic and concentrating solar thermal power systems depending on annual global irradiation

Concentrating solar thermal power and photovoltaics are two major technologies for converting sunlight to electricity. Variations of the annual solar irradiation depending on the site influence their annual efficiency, specific output and electricity generation cost. Detailed technical and economical analyses performed with computer simulations point out differences of solar thermal parabolic trough power plants, non-tracked and two-axis-tracked PV systems. Therefore, 61 sites in Europe and North Africa covering a global annual irradiation range from 923 to 2438 kW h/m² a have been examined. Simulation results are usable irradiation by the systems, specific annual system output and levelled electricity cost. Cost assumptions are made for today's cost and expected cost in 10 years considering different progress ratios. This will lead to a cost reduction by 50% for PV systems and by 40% for solar thermal power plants. The simulation results show where are optimal regions for installing solar thermal trough and tracked PV systems in comparison to non-tracked PV. For low irradiation values the annual output of solar thermal systems is much lower than of PV systems. On the other hand, for high irradiations solar thermal systems provide the best-cost solution even when considering higher cost reduction factors for PV in the next decade. Electricity generation cost much below 10 Eurocents per kW h for solar thermal systems and about 12 Eurocents/kW h for PV can be expected in 10 years in North Africa.     

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.