Concentrated Solar Power Systems Research Articles

Estimation of convective heat losses from conical cavity receiver of solar parabolic dish collector under wind conditions and receiver orientations

The parabolic dish collector is one of the recognized concentrated solar power systems based on point focusing, which provides high-temperature heat, high concentration ratio, and low heat loss. This system consists of a parabolic reflector and a cavity receiver situated in the focus line. In this work, the conical cavity receiver with an aperture diameter of 0.5 m is considered for a 100 m2 parabolic reflector having a focal to diameter ratio of 0.48. Due to the complexity of flow and temperature profile, the estimation of convective heat loss is a difficult task in a cavity receiver. More heat losses are associated with high temperature obtained in the cavity receiver of the parabolic dish collector. Due to diverse wind effect, the convective heat losses ramp up, which significantly influences the thermal performance of the concentrating power system. The present work aims to investigate the heat losses due to convection from the conical cavity receiver. The numerical investigation was performed using ANSYS Fluent 20R1 to calculate the convective heat losses from the conical cavity receiver of varying diameter to height ratio for varying wind speed, receiver orientation in head-on, and back-on wind flow directions. The considered influential parameters are varying from 0.5 to 1.5 for diameter to height ratio (d/h), 0° to 90° for receiver orientation (γ), 0 to 10 m/s for wind speed (V). The heat losses are highest at 60° and 75° receiver orientation for d/h = 0.5 and d/h = 1-1.5, respectively, at high wind speed in head-on condition, whereas in back-on wind condition, 30° receiver orientation has more heat losses among all the d/h values at high wind speed. The heat loss at 90° receiver orientation is low for 4-10 m/s. The trends of heat loss curve at receiver orientations for given wind conditions are similar for velocity more than 2 m/s. The result reveals that the considered influential parameters have a remarkable effect on convective heat losses from the cavity receiver.

Thermal characteristics of sensible heat storage materials applicable for concentrated solar power systems

Concentrated Solar Power (CSP) is rapidly increasing as a lucrative renewable energy source. CSP plants are integrated with Thermal Energy Storage (TES) systems to resolve its intermittent nature and enhance its economic feasibility. TES systems also smoothen out the fluctuations in energy demands throughout the day. The efficient design of the thermal storage system has three major aspect i.e., selecting the suitable heat storage material with high thermal conductivity, high energy storage density, and thermally stability. The paper presents an overview of all currently operational CSP plants and the technologies used by them. The paper also reviews the thermal characteristics of potential Sensible Heat Storage (SHS) materials as energy storage media in these plants and provides a critical assessment of each material. This paper presents crucial data needed for optimized selection of materials used for energy storage systems employing sensible heat. A quantitative study of the available data has been made to provide the results for the same.

Beyond the 2D limit: étendue-squeezing line-focus solar concentrators.

Line-focus solar concentrators are commonly designed by extruding a two-dimensional concentrator in the third dimension. For concentration in air, these concentrators are, by the nature of their design, limited by the two-dimensional solar concentration limit of 212×. This limit is orders of magnitude lower than the 45000× concentration limit for three-dimensional solar concentrators. Through the use of étendue squeezing, we conceptually show that it is possible to design line-focus solar concentrators beyond this 2D limit. This allows a concentrator to benefit from a line focus suitable for heat extraction through a tubular receiver, while reaching concentration ratios and acceptance angles previously unseen for line-focus concentrators. We show two design examples, achieving simulated 75× concentration and 218× concentration ratios, with a ±1∘ acceptance angle. For comparison, the 2D concentration limit is 57× at this acceptance angle. Étendue-squeezing line-focus solar concentrators, combined with recent developments in tracking integration, may enable the development of a new class of concentrated solar power systems.

High-efficiency, air-stable manganese–iron oxide nanoparticle-pigmented solar selective absorber coatings toward concentrating solar power systems operating at 750 °C

Solar selective absorber coating with long-term thermal stability at high temperatures ≥750 °C in air is an important component to reduce the levelized cost of energy (LCOE) of concentrating solar power (CSP) systems toward 50% power efficiency and dispatchable solar electricity. Conventionally, solar spectral selectivity requires multilayer-interference coatings implemented by stringently controlled vacuum deposition, and these coatings degrade significantly at >700 °C in air. Herein, we established a quantitative design approach and demonstrated a proof-of-concept air-stable, manganese–iron oxide nanoparticle (NP)-pigmented solar selective coatings with a high solar absorptance of ~93%, a relatively low thermal emittance of ~52%, and an optical-to-thermal energy conversion efficiency >89% under 1,000× solar concentration at 750 °C toward Generation 3 CSP systems. These coatings demonstrate spectral selectivity using cost-effective spray coating approach, a notable improvement over conventional vacuum-deposited, multilayer solar selective coatings for low-cost, high-efficiency solar thermal receivers. In contrast to the thermal degradation of spectrally non-selective benchmark Pyromark 2500 coatings at 750 °C in air, the solar absorptance of the MnFe2O4-pigmented coatings on stainless steel 310 (SS 310) substrates is increased to ~92.9% and the optical-to-thermal energy conversion efficiency is improved to 89.7% after serving at 750 °C in air for 700 h. X-ray diffraction results reveal that this improvement is due to the transformation of MnFe2O4 NPs into more thermodynamically stable, non-stoichiometric manganese-rich manganese ferrite and iron-rich manganese–iron oxide phases after 500 h aging at 750 °C. For >1,000 h-endurance testing at 750 °C in air and the subsequent 19 day-night thermal cycling between 750 °C (12 h/cycle) and 25 °C (12 h/cycle) on SS 310 substrates, the thermal degradation is mainly due to the CrOx microflake formation from SS 310 substrates rather than the coatings, which can be suppressed by preoxidizing the surface of SS 310. With lower emittance matrix material and further optimization of pigment NP stoichiometry, concentration, and coating thickness, it is promising to achieve an optimized thermal efficiency ≥92.5% with long-term thermal stability at 750 °C for Generation 3 CSP systems.

A GIS-based multi-criteria decision making method for the potential assessment and suitable sites selection of PV and CSP plants

By combining geographic information systems (GIS) and multi-criteria decision making (MCDM) method, this paper presents a framework to evaluate the suitable location and technical potentials for large-scale solar photovoltaic (PV) and concentrated solar power (CSP) plants. With the assist of spatial tools, the potential locations are identified by extracting restrictive criteria (e.g. protected area, land cover, transport infrastructure, and orography). Using MCDM, the land suitability for PV and CSP plants is evaluated with the comprehensive consideration of different factors, such as climate, orography, water availability, and location, where factor weights are calculated by the analytic hierarchy process (AHP) method. The study framework is illustrated by Ningxia, China, a key energy project area along the Belt and Road with abundant solar radiation. The obtained results suggested that the middle area of Ningxia is recommended for the solar power plant deployment. Besides, due to the limitation of water resource availability, PV technology is more appropriate for the study area than CSP technology. The estimated total technical potentials of PV and CSP systems could be 443 TWh/year and 308 TWh/year, indicating great potential for future solar energy development to enhance the renewable energy supply and participate in energy cooperation with the other regions. Moreover, sensitivity analysis is conducted to investigate the impacts of system efficiency variation on energy output. The results can be used for the identification of the potential areas for solar power plant installation, as well as support for the synergic expansion of generation plants and the power grid.

Finite-size effects on thermal property predictions of molten salts

It is of great importance to obtain the accurate thermal properties of molten salts over the entire operating temperature for system design in concentrated solar power or other thermal storage systems. In this work, we explore size effects on density, viscosity and thermal conductivity predictions of molten nitrate and chloride salts by molecular dynamics. Equilibrium molecular dynamics and reverse nonequilibrium molecular dynamics methods are applied to calculate thermal conductivities of NaNO3 and NaCl regarding different simulation cell sizes at the specific temperatures to study size effects on thermal conductivity predictions of melts. The results show that the size effects are neglected on density and viscosity predictions while a linear positive relationship between 1/λ and 1/Lz is found in the reverse nonequilibrium molecular dynamics method. Through analyzing the phenomenon from a view of microstructure, phonon-phonon-scattering rates are found to be the key factors. The influences of temperature and particle species on size effects are further discussed. Subsequently, appropriate simulation cell sizes are selected for NaNO3, KNO3 and Hitec salt, and the calculated thermal properties are in good agreement with the experimental values. The mean relative errors and fitting formula for density, thermal conductivity, and viscosity for the three salts are also obtained.

Review on Performance Analysis of the Power Block in Concentrated Solar Power Plants

Concentrated solar power (CSP) is a promising option to generate flexible and green power for the power grid. Power block is important to enhance energy efficiency and flexibility of CSP plants, which mostly operate in off‐design conditions and dynamic processes due to the intermittency of solar energy. Many studies have been conducted on the performance analyses of power block in CSP plants, and they are reviewed herein. Steady‐state models of power block are summarized, and influences of power block on the entire CSP system are discussed. Through improving the dual‐pressure‐level steam generator and declining back‐pressure of condensers, power block efficiency increases by 0.5% and 1.78%, respectively. Then, the dynamic characteristics of CSP plants, particularly on the power block, are reviewed for start‐up/shut‐down processes, sudden factor change situations, and control system design. Recent advances on the thermodynamic and economic optimizations of the power block in CSP plants that aim to achieve fast start‐up/shut‐down and continuous economic stable power are discussed. Moreover, the performance of global in‐service CSP plants is discussed. Turbine efficiency increases with operation temperature (349–395 °C), ranging from 30.6% to 38.7%. Several research gaps are proposed by investigating previous studies, and suggestions on future studies are provided accordingly.

Study of a metal hydride based thermal energy storage system using multi-phase heat exchange for the application of concentrated solar power system

Thermal energy storage system is of great significance for the concentrated solar power system to keep the balance between power generation and demand. Metal hydride based thermal energy storage system is regarded as a promising method due to its good reversibility, low cost, and no by-product. Multi-phase heat exchange has much higher heat transfer coefficient than single-phase fluid heat exchange, thus facilitating the steam generation. In this study, a two-dimensional model of the metal hydride reactor using multi-phase heat exchange is proposed to estimate the performance and its feasibility of application in the concentrated solar power system. The results show that the velocity of the heat transfer fluid should match well with the thermal conductivity of the metal hydride bed to maintain the heat flux at a relatively constant value. The match of thermal conductivity of 3 or 5 W/(m·K) and fluid velocity of 0.0050 m/s results in the heat flux up to about 19 kW/m2, which is increased by 3 orders of magnitude than single-phase heat exchange. In the thermal energy storage system, the reheating cycle is recommended to improve the utilization of the thermal energy. The efficiency of the system could be improved from 18.4% to 30.8% using the reheating cycle. The increased efficiency is comparable to the previously reported efficiency of 39.2%. Besides, the operation strategy of raising the steam temperature by increasing the hydrogen pressure or the superheater temperature is suggested for the system to obtain higher efficiency.

Hydrogen Production from Two Commercial Dish/Stirling Systems Compared to the Photovoltaic System-Case Study: Eastern Morocco

In the present work, a comparative analysis of electricity and hydrogen production using concentrated solar power (CSP) and photovoltaic (PV) is carried out. For this purpose, an identical capacity is taken equal to 25 and 10 kWe for both CSP and PV systems have been considered. One-year of metrological data was taken via a high precision weather station consisting of a Kipp and Zonen (K&Z) SOLYS 2 installed in Oujda. The System Advisor Model (SAM) software, version 2018 from NREL laboratory is used for electricity production, and then the hydrogen production has been calculated. The results show that both commercial dish/stirling systems produce more electricity than PV with an annual production of 42.9 MW h instead of 38.4 MW h for PV system with a capacity 25 kWe. Furthermore, the annual hydrogen production from the SES system is 13.6% higher than the PV. Considering the capacity equal to 10 kWe, the WGA dish/stirling system gives an annual electricity production equal to 15.89 MW h instead of 14.07 MW h for the PV system. The annual hydrogen production is around 302.2 kg, while about 267.8 kg is for PV system (10 kWe).

Technoeconomic Impacts of Storage System Design on the Viability of Concentrated Solar Power Plants

For future concentrating solar power (CSP) plants, a levelised cost of electricity (LCOE) of$0.06/kWhe is targeted, with their greatest value being their low-cost storage. However, low-cost storage is not the only factor that should be considered when selecting storage systems as this choice can significantly impact other parts of the CSP plant, potentially leading to a higher LCOE. Therefore, the current study aims to economically investigate the integration of various storage system designs into a sodium/sCO2 CSP plant. Using component cost targets coupled with storage cost estimates, the impact of storage design conditions could be evaluated. From this analysis it was found that while a molten chloride two-tank system had the lowest storage cost, integration led to a higher or comparable LCOE to other studied designs. Therefore, it is important that future storage designs and studies account for the complete CSP system when system configurations and technologies are proposed.

Effects of CaCl2 on cyclic carbonation-calcination kinetics of CaO-based composite for potential application to solar thermochemical energy storage

The calcium looping (CaL) process is a promising thermochemical energy storage option for concentrated solar power systems due to its great advantages of high energy density and low material cost. However, CaO-based sorbents often suffer from continuous degradation in their reversibility due to the formation of a passivation layer and evolution of a sintering effect. In this work, we focused on the effects of CaCl2 as an additive to CaO sorbents with the aim to enhance reversibility by addressing the mass-transfer issue. A variety of CaO-based sorbents with CaCl2 were synthesized, characterized, and examined their reaction patterns during cyclic carbonation-calcination. The experimental results show that the addition of CaCl2 enhances the rate of the carbonation reaction, based on enhanced reactant diffusion. The CaCl2 causes reduction of surface area; however, the addition of MgO effectively delays sintering. Therefore, CaO-based sorbents with CaCl2 and MgO achieved high reversibility with enhanced durability.

Nuclear Air-Brayton Power Cycles with Thermodynamic Topping Cycles, Assured Peaking Capacity, and Heat Storage for Variable Electricity and Heat

Electricity markets are changing because of (1) the addition of wind and solar generating capacity and (2) the goal of a low-carbon electricity grid. The large-scale addition of wind and solar photovoltaics results in low wholesale electricity prices at times of high wind and solar output and high prices at times of low wind and solar input. Today, gas turbine combined cycle (GTCC) plants burning natural gas or oil provide dispatchable electricity and provide the most economic method to match electricity production with demand. Nuclear Air-Brayton Combined Cycles (NACCs) with heat storage and a thermodynamic topping cycle enable base-load nuclear plants with sodium or salt coolants to provide dispatchable electricity to the grid and heat to industry. This capability maximizes nuclear plant revenue and enables a base-load nuclear reactor with NACCs to be a low-carbon replacement for a GTCC. The NACC power cycle, alternative heat storage technologies, and development status of the different technologies are described. The technology applies to other heat generating technologies including high-temperature concentrated solar power and future fusion systems.

Dynamic corrosion behavior of 316L stainless steel in quaternary nitrate-nitrite salts under different flow rates

Nitrate salts have been applied as heat transfer and heat storage medium in the Concentrating Solar Power (CSP) plants. Corrosion is critical to the long-time safe operation of the CSP system. This experimental study was conducted to research the dynamic corrosion behaviors of 316 L stainless steel in high-temperature quaternary nitrate nitrite mixture. The effects of corrosion on 316 L stainless steel were investigated under the varied flow rates of 0.6 m/s, 1.3 m/s and 2 m/s with 565 °C constant temperature for 1000 h. The results showed that oxidation corrosion occurred in 316 L stainless steel at all working conditions. The dynamic corrosion curves were parabolic, and the value corrosion weight loss (mmass) increased with the increase of the flow rate. Meantime, when the products flow rates were 0.6 m/s, 1.3 m/s and 2 m/s, the values of mmass were 3.6 mg cm−2, 3.77 mg cm−2 and 4.36 mg cm−2. The values of corrosion depth (Rdepths) were 0.039 mm/a, 0.041 mm/a and 0.048 mm/a, respectively. Besides, the corrosion products were consisted of hematite (Fe2O3), magnetite (Fe3O4), trace amounts of MgFe2O4 and NaFeO2 with the structure of lamellar. Furthermore, the corrosion products gradually transformed into FeCr2O4 with structures of dense and massive, and the increase of the flow rate deepened the degree of corrosion, but it had little effect on the types of corrosion products.

Accelerated charging of PCM in a horizontal shell and multi-finned tube energy storage system

An experimental study is carried out to analyze the melting behaviour of erythritol in an energy storage unit with multi-finned tubes. Temperature variation of erythritol in different locations of the storage unit and rate of heat storage during the charging process were obtained. Melting of PCM was a faster process owing to the effect of additional natural convection. During melting, molten PCM primarily occupies in the upper region of energy storage unit (ESU) and then extends towards the bottom region. The lower part of the storage unit is mostly dominated by conduction while the upper part of the ESU is controlled by natural convection. Compared to the heat transfer fluid (HTF) inlet temperature, volume flow rate shows slight influence on the melting process. Furthermore, storage unit with longitudinal multiple tubes gave the best thermal performance and significant increase in the rate of PCM melting were achieved during the ESU charging process. The result of this analysis is expected to be helpful for concentrated solar power systems and other thermal systems design.

Development of a hardware-in-the-loop simulator for small-scale concentrated solar combined heat and power system

• A HiL framework of a novel integrated solar mCHP plant is developed. • The HiL simulator is designed in Matlab/Simulink and coupled with the real controller of the plant. • The HiL simulator is used for control algorithms optimization and control strategy definition. • Optimal proportional and integral gains of the PI-based control are assessed for all the diverters. • The best control strategy to assure constant inlet temperature at the ORC consists in acting on the aperture of a diverter. In this work a hardware-in-the-loop (HiL) simulator of a novel micro combined heat and power system is presented and its use for control algorithm optimization is demonstrated and discussed. The plant under investigation consists of a concentrated Linear Fresnel Reflectors solar field, a 2 kWe/18 kWt Organic Rankine Cycle unit and an advanced latent heat thermal energy storage tank equipped with reversible heat pipes as developed by a consortium of universities and companies within the EU funded project Innova Microsolar. A smart control unit manages their integration and monitors the operation of each subsystem. In order to support the optimization of the control algorithms and the definition of the best control strategy of the micro-CHP plant at different working conditions, a simulation framework based on Matlab/Simulink has been developed by the authors and connected to the real control unit according to a HiL approach. Ad-hoc models of the different subsystems together with those of the components (i.e. valves and variable speed pumps) regulating the plant operation have been included. The use of the HiL simulator has permitted to optimize the control logic of the integrated plant prior to its future commissioning, thus helping to overcome some of the technical and reliability issues occurring during the setup of the real system. In particular, the HiL has allowed: (i) to define the proportional and integral gains of the diverters in order to assure a robust and fast response of the plant during the switch among the different operation modes; (ii) to prove the limits of acting on the oil pump flow rate in assuring the nominal oil temperature at the inlet of the ORC unit, due to the inherent fluctuations caused by this control strategy; and (iii) to assess the best control strategy which is obtained by acting on the aperture of the diverter which controls the oil mass flow rate to the ORC unit. Hence, the scientific approach here proposed can be extended also to many other complex energy conversion systems in order to significantly reduce the potential critical issues during their commissioning.

Electrical power generation through concentrated solar technology for the southern cities of Iraq

With a continuing mismatch between generating capacity and demand requirements, Iraqi cities are still enduring scheduled power outages. In this work, concentrated solar power (CSP) technology is proposed and designed for Iraqi cities to inject power into distribution networks with the objective of boosting the generating power capacity. Since CSP systems require a preliminary study of the direct normal irradiance (DNI), analyses of monthly data is carried out for each of the candidate cities. This is followed by determination of the amount of solar irradiance that falls on a titled collector per month considering the effects of reflection and diffusion. Finally, a thermal power plant is proposed and simulated using the system advisory model (SAM) per city. Results presented show an encouraging number of metrics and confirm the feasibility of such a plant in southern Iraq. The levelised cost of electricity and capacity factor shows a considerable decrease and increase respectively, when the plant is backed up by a fossil fuel steam cycle under circumstances when a plant loses over 80% of the MW capacity due to drop in solar irradiance. To provide a comparision platform, for each city, a photovolitaic (PV) plant is designed with an indentical electric capacity to that of the CSP plant. Findings from this work confirm that CSP plants can provide a suitstanable and enviroemntl friendly solution to electrical power shortages in the country compared to the current PV trends.

Thermal power forecasting of solar power tower system by combining mechanism modeling and deep learning method

Concentrating solar power systems are proliferating around the world. However, the non-schedulable characteristic of thermal output presents considerable challenges to safe utilization of solar heat. So, seasonable forecasting of thermal output is going to be essential to both power grid and plant. In this paper, a hybrid forecasting method based on mechanism modeling and deep learning method is proposed. By mechanism modeling, the major meteorological factors used for forecasting are identified accurately, avoiding the subjectivity of chosen of model input features. By convolutional neural network and long short-term memory network, the input features are reconstructed, and the spatial-temporal coupling characteristics between meteorological factors are fully explored. Finally, the thermal power is output by fully connected layers. As a case study, the thermal power of a Solar Two-like concentrating solar power plant based on meteorological conditions of Zhangbei is forecasted, and the mean values of root mean square error and mean absolute error are 5.061 MWt and 2.871 MWt on the whole year, respectively. The results show that the methods proposed in paper can be effectively used for thermal power forecasting of concentrating solar power system.

Decomposition kinetics of Al- and Fe-doped calcium carbonate particles with improved solar absorbance and cycle stability

Calcium-based materials are considered to be promising heat storage methods for the upcoming 3rd generation concentrated solar power systems (CSP) due to their high operation temperatures and energy storage densities. However, pure calcium carbonate (CaCO3) particles suffer from poor solar absorptance and stability. In this work, we successfully enhance solar absorptance, cycle stability, and decrease decomposition temperature, simultaneously, based on proposed doped CaCO3 particles. A fabrication method, which is cheap and suitable for large scale applications, is proposed based on doping Al and Fe elements into CaCO3 powders via sol–gel processes. The average solar absorptance is enhanced by about 560%, and the energy storage density decay rate after 50 cycles is prominently reduced to be as low as 4.5% from 35.5%. The decomposition temperature is reduced by 15 to 24 K depending on the atmospheres, and the decomposition kinetics of both doped and pure CaCO3 particles is found to follow the equation of phase boundary controlled reaction. The activation energy increases only slightly after doping, but will have a sharp increase when switching the atmosphere from N2 to pure CO2. This work paves the way to the design of high-performance calcium-based materials for next-generation high temperature thermal energy storage system.

Investigation on the Ca(OH)2/CaO thermochemical energy storage system with potassium nitrate addition

Thermochemical energy storage system (TCES) is a novel generation of concentrated solar power (CSP) heat storage system, which has the characteristics of higher heat storage density and long-term heat storage. Ca(OH)2 is a low-cost and widely available material with great application prospects, especially in CSP system because of the suitable heat storage temperature. This research focuses on improving the heat storage efficiency of Ca(OH)2/CaO TCES system by KNO3 addition. In order to further study the effect of KNO3 addition on Ca(OH)2/CaO heat storage process, the kinetic analysis of the thermal storage process of pure and KNO3 added Ca(OH)2 was carried out and the kinetic control equations were obtained, which can provide guidance for the design of the Ca(OH)2/CaO TCES system. Results show that 10 wt% KNO3 addition can reduce the heat storage temperature to 428.49 °C and greatly accelerate the reaction with nearly no loss of heat storage density. The cycling stability of the pure as well as KNO3 added Ca(OH)2/CaO system was investigated in air and nitrogen atmospheres. The failure process of the KNO3 added Ca(OH)2/CaO in air atmosphere is revealed and it is verified that the composite material has a good cyclic stability in nitrogen atmosphere.

Improvement of thermal stability of ZrSiON based solar selective absorbing coating

Solar selective absorbing coatings (SSACs) are required to have not only excellent optical property but also outstanding thermal stability for high temperature applications. The optical properties of Mo/ZrSiN/ZrSiON/SiO2 SSAC had been optimized successfully before. Herein, we are focusing on the evaluation and mechanism of thermal stability of this multilayer coating for its potential applications in concentrated solar power (CSP) systems. Fortunately, the coating exhibits excellent thermal stability after aging at 400 °C for 1500 h in vacuum. At aging temperature of 500 °C for 1000 h or 600 °C for 300 h in vacuum, the slight inter-diffusion between Mo layer and stainless steel (SS) substrate occurs. At higher aging temperature of 700 °C for 100 h in vacuum, the serious inter-diffusion between Mo layer and SS substrate leads to invalidation of the coating, which has been evidenced by Rutherford backscattering spectrometry (RBS) and X-ray diffraction (XRD) technologies. Additionally, this coating also has an outstanding thermal stability after aging at 400 °C for 300 h in air. A heating-cooling cycling (HCC) treatment evidences the good thermal stability of this coating working in cold environment (−60 °C). The results reveal that this coating can be a promising candidate not only for CSP system in high temperatures but also for usage in cold environment.