Concentrated Solar Thermal Power Plants Research Articles

Generalized distributed state space model of a CSP plant for simulation and control applications: Single-phase flow validation

Concentrating solar thermal power plants, also known as CSP plants, can be of different configurations depending on type of collectors, temperatures, heat transfer fluid, working fluid, and the thermodynamic cycle used in the plant. This leads to complex behavior with nonlinear dynamics, potential instability and parameters that vary in both space and time. In this work, a distributed state space model is proposed to ensure computational flexibility and facilitate industrial applications, such as optimization, control and automation. The format used allows the model to represent the thermal dynamics at different operation points including phase changes (liquid or gas) along the spatial dimension. To validate the model, some experimental tests have been made on an operating solar thermal plant located at the University of South Florida, in United States, where real input disturbances were applied to compare measurements with model predictions. Preliminary results show good agreement with experimental observations. Literature data of water and steam properties were used in the model, that can be easily extended to direct steam generation (DSG) plants.

Simulation of Triplex Tube Thermal Energy Storage System Using High Temperature Phase Change Material

To increase the capacity factor of a concentrated solar thermal power plant (CSTPP) beyond the hours of sunlight, the use of thermal energy storage systems (TES) can be a promising solution. Phase change materials (PCMs) can store latent thermal energy in the course of the melting process and release it when solar energy is not available. Generally, PCMs have low thermal conductivity. One of the most commercially promising solutions is the application of an extended heat transfer surface inside the PCM container. Moreover, the distance of the heat transfer fluid (HTF) to the core of the PCM in a TES system can affect the storage performance. Accordingly, a triplex tube heat exchanger with eight fins is considered in this paper, to investigate the impact of the different velocity of HTF and different entrance pattern in a vertical PCM container. Notably, the middle enclosure of the triplex tube is filled with PCM. Numerical analysis using an enthalpy porosity technique revealed that increasing HTF velocity reduces the charging time. Also, when the HTF enters from the bottom of the container, the storage time will diminish owing to a natural convection side-effect, but if the HTF flows downward, the amount of sensible thermal energy storage is higher than in the other cases.

Performance of a solar thermal power plant with direct air-cooled supercritical carbon dioxide Brayton cycle under off-design conditions

The use of an efficient and compact supercritical carbon dioxide (sCO2) Brayton cycle in concentrated solar thermal power plants has the potential to reduce costs of electricity generation. Heat rejection in the hot-arid climate is of great concern to the power cycle, especially by natural draft dry cooling technologies. For this purpose, a comprehensive design and rating analyses of the sCO2-air cooling process was conducted based on short natural draft dry cooling towers. This approach is featured with the capture of non-linear characteristics in physical properties of CO2 and geometry of fin-tube air-cooled heat exchangers. It is found that the proposed methodology successfully predicted the experimentally observed outlet temperatures of an existing cooling tower. By utilizing off-design models of heat exchanger and turbomachinery, a direct air-cooled recompression sCO2 cycle was investigated for a parabolic trough solar plant with thermal energy storage (TES). The impacts of pressure ratio, recompression fraction, shaft speed and boundary conditions, i.e., ambient air temperature and solar intensity, were investigated on the power output and key parameters of the power plant under quasi steady state conditions. The results show that the recompression fraction significantly affects the pitch point in the recuperators, the optimum value of which decreases with an increase in compressor inlet pressure and in shaft speed. In addition, the direct air-cooled power system depends strongly on ambient environments, and is able to handle lower solar intensities without deterioration in electricity generation by the buffering of TES. The cooling tower approach decreases non-linearly as the ambient temperature increased, indicating that a fixed approach of typical 15 °C results in a conservative electricity production at hot climatic conditions.

Water Saving in CSP Plants by a Novel Hydrophilic Anti-Soiling Coating for Solar Reflectors

In this work, results of the outdoor exposure campaign of a newly developed hydrophilic anti-soiling coating for concentrated solar thermal power (CSP) mirrors are presented. The material was exposed for nearly two years under realistic outdoor conditions and the influence of two different cleaning techniques was evaluated. Mirror samples were analyzed during exposure and their reflectance and cleanliness were measured. The performance of the anti-soiling coated mirror samples was compared to conventional uncoated silvered-glass mirrors. The coatings showed appropriate anti-soiling and easy-to-clean behavior, with a mean cleanliness gain of 1 pp and maximum values under strong soiling conditions of up to over 7 pp. Cleanliness of the coated samples stayed higher throughout the whole campaign before and after cleaning, resulting in lower soiling rate compared to the reference material. Taking into account these values and supposing a threshold for cleaning of 96%, the number of cleaning cycles could be decreased by up to 11%. Finally, the coated material showed negligible degradation, not exceeding the degradation detected for the reference material.

HiPIMS and DC Magnetron Sputter-Coated Silver Films for High-Temperature Durable Reflectors

High-temperature durable mirrors based on a protected silver sputter coating are attractive for secondary reflector applications in concentrated solar thermal power plants. In this paper, silver films are deposited by high-power impulse magnetron sputtering (HiPIMS) and standard direct current (DC) magnetron sputtering, either as exposed discretely deposited films or in-sequence-deposited thin film systems, where the silver is protected and embedded between adhesion and barrier layers. The unprotected silver films and equivalent protected silver thin film systems are compared and characterized as deposited and after 400 °C oven temperature exposure. The reflectance is measured and grazing incident X-ray diffraction (GIXRD) and scanning electron microscopy (SEM) pictures were taken. The HiPIMS silver film, sputtered with a peak current of 200 A and an approximately equivalent average power density to the DC magnetron sputtered silver, exhibits higher reflectance (and conductivity). Increasing the power density further, yields silver films with lower reflectance, correlating to a reduced grain size. In the protected silver film system, the reflectance does not improve, due to the presence of a less reflective top adhesion layer. The protected film system, with the 200 A HiPIMS, is, however, more durable at 400 °C than the DC magnetron sputtered equivalent.

Non-recourse project financing for concentrated solar thermal power

Non-recourse project financing can play a significant role in scaling up total investment in concentrated solar thermal power (CSP). Non-recourse project financing is designed to identify, allocate, and mitigate risks through project structuring and contracting. These techniques can be utilised to address risks specific to CSP projects. Complementary de-risking techniques, such as debt guarantees, can facilitate non-recourse project financings at bankable levels of return in higher risk contexts. Technological advances and learning through global industry scale-up have delivered major reductions in CSP plant development costs. Ambitious CSP targets in China and other jurisdictions underpin future CSP market development. Hybrid off-take models in current utility-scale CSP projects are utilising thermal storage technology to sell rapid response dispatchable power into electricity wholesale markets at peak periods, enabling lower long-term PPA prices at other times. These developments underpin CSP as cost-competitive with solar PV with battery storage and other forms of low emissions technology. Based on an analysis of selected CSP non-recourse project financings, an emerging contractual model of non-recourse project financing risk mitigation and complementary de-risking measures is identified. While the key features of this model are applicable to large-scale projects in developing countries, it is adaptable to meet the requirements of a wide range of projects. In this way, non-recourse project financing can play a pivotal role in the feasibility of large-scale projects in a potentially critical technology for low-carbon energy development that may otherwise be difficult to finance.

Thermal performance analysis of a concentrated solar power system (CSP) integrated with natural gas combined cycle (NGCC) power plant

In this study, a concentrated solar thermal power plant (CSP) with parabolic trough collectors integrated into a reference triple-pressure NGCC power plant as an Integrated Solar Combined Cycle (ISCC). The aim is to study and assessment the overall performance of different hybrid schemes of ISCC plants. Thus, A detailed thermodynamic model of the proposed ISCC plants has been developed to investigate the thermodynamic impact of solar heat rate injected in the high and intermediate pressure sections (HP, IP) of the NGCC plant at different seasons of the year. The cycles have been characterized and simulated under southern Egypt climate. An increase in the NGCC output power of 52.315 MWe with peak net thermal efficiency of 64.86% and solar to electric conservation efficiency of 46.95% are obtained from the integration process in the HP section. The increase in power output reaches a limit of 43.38 MWe with a net efficiency of 63.82% and solar to electric efficiency of 38.85% for integration in the IP section. The results of the analysis have been indicated that the higher the solar energy injected the higher is the solar electricity production and the higher is the overall thermal efficiency.

Thermodynamic and economic evaluation of a novel concentrated solar power system integrated with absorption refrigeration and desalination cycles

In this study, an innovative concentrated solar power plant integrated with desalination process and absorption refrigeration cycle aimed at supplying power, fresh water and refrigeration, was developed and exergetically assessed. The system comprised a concentrated solar thermal power plant with parabolic dish collectors and steam turbine, a multi-effect desalination process with parallel feed of seawater, and a single-stage ammonia-water absorption refrigeration system. Generally, the collectors provided 21,030 kW thermal power to the steam power plant and 4632 kW of which was converted to electrical power in steam power plant. The absorption refrigeration cycle produced 820.8 kW refrigeration and the desalination cycle provided fresh water at a rate of 22.79 kg/s. The integrated system was simulated in Aspen Hysys and all the components of the integrated system were individually scrutinized based on the second law of thermodynamics; as well, the exergy destruction rate and exergy efficiency of each component were obtained and discussed thoroughly. According to the results, about 86% of the total exergy destruction rate of the system belonged to the distillation column and heat exchangers. The overall exergy efficiency of the cycle was 66.05%, while, the net overall thermal efficiency of the integrated system was 80.70%. The results of the economic analysis showed that the proposed integrated structure had an investment return period of 5.738 years and a net annual profit of 6.828 million US$ per year. Moreover, the impact of various factors on the performance of the integrated system was investigated using sensitivity analysis.

The value of a dispatchable concentrating solar power transfer from Middle East and North Africa to Europe via point-to-point high voltage direct current lines

Dispatchable solar power from concentrating solar thermal power plants (CSP) combined with thermal energy storage and co-firing option can provide energy according to demand. A transfer of such electricity from CSP in desert regions to distant consumer centres may therefore complement domestic energies. A detailed energy system modelling showing the benefit and drawback of CSP from Middle East and North Africa for Europe was not yet done. This paper closes the scientific knowledge gap applying an energy system model with a least-cost approach and detailed scenario analysis for the year 2050. Energy system analyses describe the effects of including and excluding a transfer of CSP from MENA to EU via a grid or via point-to-point high voltage direct current (HVDC) transmission lines. A multi-criteria assessment reveals the impact of such CSP-HVDC power plants on energy infrastructure, operational behaviour, cost and emission of the energy system. To evaluate national grid expansion, a new grid methodology is used as composed of transmission and distribution grid. The evaluation shows that power plant capacity, electrical storage and grid expansion as well as electrical curtailment can cause a beneficial impact when CSP-HVDC is used to supplement the energy portfolio in Europe.

Integration of Soiling-Rate Measurements and Cleaning Strategies in Yield Analysis of Parabolic Trough Plants

The issue of reflector soiling becomes more important as concentrating solar thermal power plants (CSP) are being implemented at sites subject to high dust loads. In an operational power plant, a trade-off between reducing cleaning costs and cleaning related collector availability on the one hand and keeping the solar field cleanliness (ξfield) high to minimize soiling induced losses on the other hand must be found. The common yield analysis software packages system advisor model (SAM) and greenius only allow the input of a constant mean ξfield and constant cleaning costs. This oversimplifies real conditions because soiling is a highly time-dependent parameter and operators might adjust cleaning activities depending on factors such as soiling rate and irradiance. In this study, time-dependent soiling and cleaning data are used for modeling the yield of two parabolic trough plant configurations at two sites in Spain and Morocco. We apply a one-year soiling rate dataset in daily resolution measured with the tracking cleanliness sensor (TraCS). We use this as a basis to model the daily evolution of the cleanliness of each collector of a solar field resulting from the application of various cleaning strategies (CS). The thus obtained daily average ξfield is used to modify the inputs to the yield analysis software greenius. The cleaning costs for each CS are subtracted from the project's financial output parameters to accurately predict the yield of a CSP project over its lifetime. The profits obtained with different CSs are compared in a parameter variation analysis for two sites and the economically best CS is identified. The profit can be increased by more than 2.6% by the application of the best strategy relative to a reference strategy that uses a constant cleaning frequency. The error in profit calculated with constant soiling and cleaning parameters compared to the simulation with variable soiling and cleaning can be as high as 9.4%. With the presented method, temporally variable soiling rates and CS can be fully integrated to CSP yield analysis software, significantly increasing its accuracy. It can be used to determine optimum cleaning parameters.

Design and experimental validation of a computational effective dynamic thermal energy storage tank model

Concentrating solar thermal power plants rely in thermal energy storage systems in order to provide a stable power supply. However, they might not been able to meet power plant demands, mainly because of their storage sizes which are restricted due to economic reasons. One way of mitigating this effect is to control in an optimal way the charging and discharging processes. For the design and validation of advanced control strategies, an accurate dynamic model is essential. For this reason, a dynamic thermal energy tank model intended to be used in concentrating solar thermal power plant models is presented in this paper. The developed tank model is validated in charging and discharging processes and also at rest state in order to validate thermal losses dynamics. Simulation results are compared against experimental data from the CIEMAT-PSA molten salt testing facility.

Analysis on the long-term relationship between DNI and CSP yield production for different technologies

Abstract Ground measured hourly data from the Solar Radiation Monitoring Laboratory of the University of Oregon over 34 years have enabled a detailed analysis of the relations between direct normal solar irradiance (DNI) and energy yield from concentrating solar thermal power (CSP) generation systems. Six different plant configurations have been selected for simulating the energy yields with System Advisor Model (SAM), based on different parabolic through and central receiver CSP and linear Fresnel commercial plants. The long-term yield energy of each CSP reference plant has been estimated by two different approaches: by modeling a typical DNI year estimated by the Sandia procedure for computing typical meteorological years, or by applying the Sandia methodology directly on the 34 years of hourly gross energy output resulting from multi-year modeling with SAM of the 34 years of meteorological input. Important differences are observed on the typical yield year resulting from each methodology. This work presents thus a complete analysis on how the statistical long-term characteristics of solar resource input might represent the statistical characteristics of long-term yield energy.

Minimum system entropy production as the FOM of high temperature heat transfer fluids for CSP systems

Abstract High temperature heat transfer fluids are used in concentrated solar thermal power (CSP) plants to receive heat from a solar concentrator and then transfer it to a working fluid (such as water, air, or super critical CO 2 ) in a thermal power system. For various high temperature heat transfer fluids, such as, synthetic oils, various molten salts, and liquid metals, a general criterion is proposed in this work to evaluate the merit of fluids regarding their transport properties. For the goal of transferring a desired amount of heat, a fluid that causes less entropy production is believed to have better figure of merit (FOM). This is due to the fact that entropy production is associated with the destruction of exergy or useful energy. The entropy production in a heat transfer system in a solar thermal power plant includes the part due to the processes of heat addition and removal and the other part due to pressure losses in the flow in heat exchangers and pipes. Theoretical analysis and relevant equations for total entropy production are derived. As an example, the FOM for several heat transfer fluids used in CSP industry are compared for the goal of heat transport in the range of 50 MW th to 600 MW th . The modeling provides a fundamental approach for the comparison of various heat transport systems which may have different designs and using different heat transfer fluids/media (gas, liquid, or solid particles) in CSP systems.

Development and experimental validation of a CFD model for PCM in a vertical triplex tube heat exchanger

Thermal energy storage in concentrated solar thermal power plants improves the dispatchability and eliminates the miss-match between the energy supply and demand. Recently, considerable attention has been made to latent heat thermal energy storage due to its high energy density per unit mass and volume at nearly constant temperature. This paper presents a computational fluid dynamics (CFD) model using ANSYS FLUENT 15.0 for phase change material (PCM) in a vertical triplex tube thermal energy storage system and its validation through experimental results. To enhance the heat transfer inside the PCM, eight fins have been incorporated between the internal and external tubes. Experiments were conducted for both freezing and melting processes. The CFD model endeavoured to simulate both the freezing and melting processes of the PCM. The inlet and outlet temperatures of the heat transfer fluid (HTF) as well as six temperature locations in the PCM were compared with the CFD results. The average effectiveness as well as the duration of the phase change process at each experimental point were compared with results from the CFD model and found to be in good agreement. The variation between the experimental and CFD for the phase change duration are within an average of 5.8% for freezing and 1.6% for melting.

Advanced Modeling of CSP Plants with Sensible Heat Storage: Instantaneous Effects of Solar Irradiance

A flexible model for concentrated solar thermal power plants with two tank active direct storage system is developed on gPROMS platform. Eutectic mixture of potassium and sodium nitrates salt melt is used as heat transfer fluid and storage medium. The solar radiation data obtained from a weather station, during 10 years in the city of Isfahan are used. It is showed that the Bird’s clear sky model well represents the normalized weather data. The storage system is designed to feed the boiler for 15 hours in the absence of sun. The model predicts the instantaneous effects of solar irradiance change and calculates variable with time parameters of the CSP plant; namely, the salt melt flow rates through receiver and boiler, and tank levels during 24 hr. The results indicate that a collector surface area of 35 m 2 and 250Kg mixture of nitrate salts, per 1 KW electricity generation, is necessary. With respect to the “variable with time” nature of solar irradiance, the role of storage tanks to damp fluctuations in process parameters of a CSP plant is further quantified. A 2500 Kg of heat storage material eliminates variable collector surface area need of 100 m 2 to 1500 m 2 or equivalently, eliminated increasing boiler temperature greater than 1000 o C.

Maximising revenue via optimal control of a concentrating solar thermal power plant with limited storage capacity

Concentrating solar thermal power plants with thermal energy storage is a potential source of clean electrical power. This work has the same four optimal control modes established in a previous paper where Pontryagin's principle demonstrated that to maximise revenue from a plant with unlimited storage the optimal modes were to: (i) store all collected power, without generating; (ii) generate using collected power only; (iii) generate at maximum capacity using both collected and stored power; (iv) generate a maximum capacity, storing any surplus power. This paper presents an expanded analysis to cover a more realistic case where there is limited storage capacity, introducing 2 differences. First, it is sometimes necessary to discard power if the store is full. Second, the critical prices used to determine the optimal control can change whenever the store becomes full or empty. Again we use Pontragin's principle to derive necessary conditions for an optimal control and present an algorithm to find the sequence of critical prices required to construct it. The analysis gives more insight into the nature of the optimal control than mathematical programming or probabilistic search techniques, and the calculation is fast-finding the optimal control for a yearlong operation in about 30 seconds.

English

For concentrated solar thermal (CST) power plants located in dry and arid areas, dry cooling is the only cost effective alternative. If using existing dry cooling technology, the CST power generation will be reduced significantly at high ambient temperatures. This paper presents design options of a novel cooling tower design concept with the potential to maintain the design-point performance even on very hot days. In this new concept, a natural draft dry cooling tower is modified by adding a solar collector around its base. The function of the solar collector is to make the air inside the cooling tower hotter so the buoyancy force increases. In a natural draft dry cooling tower, increased buoyancy means increased air flow rate through the heat exchangers and, therefore, increased heat transfer capacity. The end result is power plant efficiencies significantly higher than what could be achieved with the unmodified design.

Comparative study on the performance of natural draft dry, pre-cooled and wet cooling towers

Most geothermal and concentrated solar thermal (CST) power plants are dry cooling due to the lack of water sources. Unfortunately, the performance of dry cooling is particularly reduced when the ambient air is hot, which is because dry cooling relies mainly on convective heat transfer to reject heat from the working fluid rather than water evaporation as is in wet cooling systems. This is even worse for natural draft cooling towers as their air flow is created by means of buoyancy effects due to the difference in air density between the inside and outside of the towers. The heat rejection capacity of natural draft dry cooling towers is significantly reduced at high ambient temperatures because the driving buoyancy force is small, resulting in small air flow through the cooling towers. To offset the low performance of dry cooling, pre-cooled cooling systems which combine dry and wet cooling are proposed. The wet cooling system only operates at high ambient temperatures to assist dry cooling. This paper is to investigate the variations in annual performance of three proposed natural draft cooling towers, i.e., a Natural Draft Dry Cooling Tower (NDDCT), a pre-cooled NDDCT and a Natural Draft Wet Cooling Tower (NDWCT). The water consumption of the pre-cooled NDDCT is compared with that of the NDWCT. The present study finds that: the performances of the NDDCT, the pre-cooled NDDCT and the NDWCT are significantly affected by the ambient conditions in operation; the NDWCT provides the highest heat rejection rate, followed by the pre-cooled NDDCT and the NDDCT; the pre-cooling enhancement can go up to 46% by increasing the NDDCT heat rejection rate from 93 MW to 136 MW in the hottest month. The use of the pre-cooling system is season-dependent (usually for hot summers); the pre-cooled NDDCT consumes 70% less water for every MW heat rejection than the NDWCT.

Effects of the selection of heat transfer fluid and condenser type on the performance of a solar thermal power plant with technoeconomic approach

Renewable electricity generation systems have an increasing trend in terms of usage due to aiming to decrease greenhouse gas emissions and energy source diversification strategies of countries. Parabolic trough, Fresnel, and solar tower systems have been used to generate solar thermal electricity around the world. In this study, the effects of the selection of collector heat transfer fluid (HTF) and condenser type on a concentrated solar thermal power plant were analyzed. Net power, net electrical efficiency, and economic analysis were carried out for the selected HTFs for different collector outlet temperature cases. In the case of condenser type selection four different systems were considered; water cooled, air cooled (dry air) and air cooled with water spraying (spraying before fan and spraying before and after fan). Levelized cost of energy (LCOE) and specific investment cost were calculated. According to the results, specific investment cost and LCOE were found to be 4000 USD/kWel and 0.207 USD/kWh, respectively. Carbon tax/credit was also included to the calculations of LCOE and a comparison study was carried out for gas turbine, combined cycle and solar thermal power plant with thermal storage. Including carbon tax/credit to the LCOE shows that solar thermal power plant with heat storage can be competitive when compared to gas turbines.

Dual Mode MPC for a Concentrated Solar Thermal Power Plant

A model predictive control strategy for a concentrated solar thermal power plant is proposed. Design of the proposed controller is based on an estimated linear time-invariant state space model around a nominal operating point. The model is estimated directly from input-output data using the subspace identification method and taking into account the frequency response of the plant. Input-output data are obtained from a nonlinear distributed parameter model of a plant rather than the plant itself. Effectiveness of the proposed control strategy in terms of tracking and disturbance rejection is evaluated through two different scenarios created in a nonlinear simulation environment.