In recent years, fossil fuels have been increasingly depleted and environmental pollution has been worsening. The penetration rate of wind and solar photovoltaic power generation has been continuously increasing, occupying some conventional unit space and weakening the power grid regulation capacity. In order to ensure the safe, stable, clean and efficient operation of the power system, it is urgent to have flexible power sources participate in grid frequency regulation and peak shaving to enhance the flexibility of the power system. Solar thermal power generation integrates power generation and heat storage, with advantages such as flexible and controllable output. It can quickly and deeply participate in grid peak shaving and is suitable for complementary operation with new energy generation such as wind power and solar photovoltaic. It is a renewable energy generation technology with great development prospects and has been highly valued by countries around the world. It has been actively researched and promoted for application. As of the end of 2021, the cumulative installed capacity of global photothermal power plants is 6692 MW. Among the first batch of demonstration projects in China, 7 tower type, 1 trough type, and 1 linear Fresnel type photothermal power plants were connected to the grid and put into operation, totaling 550 MW. However, compared to wind power and solar photovoltaic power generation, the large-scale development of solar thermal power generation still has a long way to go. An in-depth analysis of the operational characteristics of solar thermal power generation under multiple uncertainties, and a study of the flexible participation of solar thermal power generation in the operation strategy and benefit evaluation of the power grid, are of great significance in helping solar thermal power generation become an important component of China’s power structure.
Analyze the current situation of solar thermal power generation technology both domestically and internationally, introduce the basic principle, common types, and system composition of solar thermal power generation; Discuss the operational characteristics and advantages and disadvantages of different types of photothermal power generation; Analyze the research progress of photothermal power generation technology; Outlook on the development direction of photothermal power generation technology.
1. Principle and System Composition of Photothermal Power Generation Technology
The basic principle of Concentrated Solar Power (CSP) is to focus the solar radiation around the power plant on the collector area through a large number of mirrors or spotlights. The collector area heats the working fluid to absorb the solar radiation energy and produce high-temperature steam, which drives the turbine generator set to generate electricity, thereby converting solar photovoltaic energy into electricity. A solar thermal power station generally consists of a heat collection system, a heat storage system, a steam generation system, and a power generation device.
1.1 Focusing and heating systems
Focusing and heating systems are the foundation of solar thermal power generation, mainly composed of focusing mirror fields, heat absorbers, etc. The spotlight field is composed of a large number of identical spotlight devices (such as slot shaped parabolic reflectors, planar heliostats, etc.) arranged according to a certain pattern. At present, the investment in the spotlight field accounts for more than 60% of the total system investment in various solar power generation systems. The solar energy absorbed by the focusing mirror field is not only affected by factors such as mirror field layout and reflectivity, but also closely related to the external environment, such as the weather conditions and solar radiation at the location of the mirror field. The collector directly converts the solar radiation energy gathered by the focusing mirror field into thermal energy, heating working fluids such as thermal oil and molten salt. The performance of a heat absorber directly determines the outlet temperature of the heat absorbing medium. Due to factors such as the intermittency of solar photovoltaic heat sources and the corrosiveness of molten salts in the medium, heat absorbers require high technical and process requirements in terms of material selection, optimized design, and reliability.
1.2 Heat storage system
The energy storage system is the key to achieving flexible and adjustable solar thermal output and 24-hour continuous and stable operation of solar thermal power generation. Its heat storage is closely related to the annual power generation of the electric field, the scale of the condenser mirror field, and the total investment of the power station. Therefore, the design of energy storage systems needs to comprehensively consider factors such as heat storage capacity, heat storage cycle, and power generation economy.
1.3 Steam generation system
The main function of the steam generation system is similar to that of conventional thermal power plants, which is to achieve heat exchange between high-temperature fluid media (thermal oil, molten salt) and water working fluid, generate superheated steam to drive the turbine to do work; The difference lies in that the steam heating system of the photovoltaic power plant has a fast heating rate, up to 10 ℃/min, and can achieve rapid start-up of the steam turbine.
1.4 Power generation system
The performance of the power generation system is directly related to the economic efficiency of solar photovoltaic thermal power generation. This system configuration is similar to that of thermal power units, but compared to coal-fired units, the power generation system of photovoltaic power plants has better regulation performance, which requires the turbine to have characteristics such as frequent start stop, fast start, low load operation, and high efficiency.
2. Classification and characteristic analysis of photothermal power generation technology
According to the energy gathering method and structure, solar photovoltaic thermal power generation technology can be divided into four categories: tower type, trough type, disc type, and Fresnel type.
2.1 Tower solar thermal power generation
Tower power generation is a centralized solar photovoltaic thermal power generation technology: a heat absorbing tower with a height of several hundred meters stands at the center of a circular mirror field composed of thousands of independently controlled heliostats. The heliostat field that tracks the sun independently gathers sunlight onto the receiver at the top of the tower to generate high temperature, heat the working fluid, and generate superheated steam to drive the turbine to generate electricity. Tower solar thermal power generation is shown in Figure 1.
2.2 Slot solar thermal power generation
The slot type photothermal power generation utilizes the optical focusing principle of a parabolic surface to focus solar radiation parallel to the main axis of the slot shaped parabolic surface onto the heat collecting tube. In practical applications, multiple trough shaped parabolic concentrators are combined in series and parallel to form a concentrator system, which is used to absorb solar radiation energy and generate superheated steam to drive the generator set for power generation. The structure of the slot type photothermal power generation device is shown in Figure 2.
2.3 Disk solar thermal power generation
The disc-shaped solar photovoltaic thermal power generation system adopts a disc-shaped focusing system, with the solar radiation reflection surface arranged in a disc-shaped (disc-shaped) shape. The solar light is reflected and focused on the receiver through a disc-shaped parabolic reflector, and the generated heat energy is driven by a Stirling machine installed at the focal point to drive the thermal power generation unit for power generation. The workflow is shown in Figure 3. Disc power generation adopts point focusing, which has the characteristics of high focusing ratio, high collection temperature, and small heat loss of the collector. Currently, the peak photoelectric conversion efficiency can reach about 30%. However, its single unit capacity is limited by price factors, and the scale of individual power generation capacity is relatively small, making it suitable for distributed power generation.
2.4 Fresnel solar thermal power generation
The Fresnel solar photovoltaic thermal power generation system adopts a linear Fresnel solar concentrator. The linear Fresnel concentrator system evolved from the parabolic concentrator system, and its working principle is similar to slot type photothermal power generation. The difference is that the linear Fresnel mirror arrangement does not need to maintain the parabolic shape. The direct solar radiation is focused on the top of the tower through a primary plane mirror and then transmitted to the linear collector through a secondary mirror, heating the working fluid, generating steam, and driving the turbine to generate electricity. The linear Fresnel solar thermal power generation device is shown in Figure 4.
2.5 Comparison of Characteristics of Various Solar Thermal Power Generation Technologies
The above four types of solar thermal power generation technology routes have different operational performance and promotion and application levels due to differences in the structure of the concentrating system, the collection method of solar thermal power generation, and the working parameters of each link of the solar thermal power generation system.
In summary, disc and tower power generation technologies have higher optical efficiency in the focusing mirror field, with more concentrated energy during the focusing and photothermal conversion processes, resulting in higher system operating temperature and efficiency. The disc-shaped system is limited by the scale and cost of individual units, and currently the capacity of a single unit in a demonstration project under construction is in the kW level; The solar island control system of a tower system is complex and has high maintenance costs, but it has characteristics such as high concentration factor, high solar thermal conversion efficiency, and short heat transfer path, making it very suitable for large-scale and high-capacity commercial applications. Therefore, tower solar thermal power generation systems are considered a highly promising technological route, currently accounting for about 20% of the total installed capacity of global solar photovoltaic thermal power generation.
Slot solar thermal power generation is currently the most mature and widely used solar thermal power generation technology, accounting for approximately 76% of the total installed capacity. The slot type solar thermal power generation system has the characteristics of simple structural components of the spotlight and collector system, and easy energy collection and tracking control. However, compared to the tower type, the trough type has lower spotlight, larger heat dissipation area, and lower efficiency and operating temperature.
Although Fresnel style photothermal power generation uses Fresnel structured condenser mirrors instead of parabolic surfaces, which reduces the technical difficulty and cost of condenser mirror production, the overall efficiency of the system needs to be improved. At present, the only linear Fresnel solar thermal power generation project that has been built in China is the Lanzhou Dacheng Dunhuang 50 MW solar thermal power generation project.
3. Current research status of photothermal power generation technology
3.1 Modeling and operating characteristics of photothermal power generation
The solar thermal power generation system is a dynamic system that integrates and coordinates the control of multiple systems such as solar photovoltaic heat collection, heat transfer and storage, heat exchange, and power generation. Therefore, analyzing the accumulation and flow of system energy and the operating characteristics of solar thermal power generation at different time scales is the basis for achieving optimal operation of solar thermal power and improving the efficiency of solar thermal power generation. The current models of solar thermal power generation characteristics mostly consider the conversion relationship of internal heat in solar thermal power plants, or display the relationship between heat and electricity as a functional relationship. Establish a static model suitable for scheduling operation and economic analysis when studying the simplified internal energy flow process of solar thermal power plants. Sioshansi et al. established a static model of hourly energy flow by simplifying the dynamic process of energy exchange within a photothermal power plant. However, the model did not consider the factors of heat rejection and unit climbing. The model that does not consider factors such as climbing and backup cannot be directly applied to the power grid scheduling problem with CSP power stations. Chen Runze et al. improved the static energy flow model and proposed a scheduling model with the participation of solar thermal power plants. Zhang Zhongdan et al. reasonably simplified the energy flow of each subsystem of photothermal power generation and established a static energy flow mathematical model for power generation optimization. In terms of establishing a thermodynamic dynamic model considering heat exchange, Li Huanbing studied the real-time dynamic simulation model of the thermal system of a conventional trough solar photovoltaic thermal power plant using the SEGS VI unit as the object, and analyzed the dynamic characteristics of this type of power plant based on the simulation model. Geng Zhi et al. built a thermodynamic simulation model for a medium and low temperature trough solar thermal power generation system with thermal storage devices, and conducted hourly simulations on four typical days: the spring equinox, summer solstice, autumn equinox, and winter solstice. Li Guoying established thermodynamic dynamic equations for each link of the tower solar photovoltaic power generation system, and based on the model and actual operating data, further studied the overall efficiency of the 1 MW tower solar thermal power plant in Yanqing, Beijing. Alfeidi et al. developed a probability model for CSP to determine the impact of solar radiation and temperature changes on the reliability of power systems. The study also used this model to evaluate the impact of a series of factors such as system load, installed capacity, and installation site parameters on the reliability and effective carrying capacity of solar thermal power generation.
3.2 Optimization operation methods for power systems containing photothermal power generation
In recent years, incorporating solar thermal power plants as flexible regulation resources into the power system to support the flexible operation needs of the power grid has become a research hotspot. Starting from the characteristics of solar thermal power plants, Xiao Bai et al. established a coordinated optimization scheduling model for solar thermal wind solar photovoltaic power generation with the goals of maximizing system revenue, maximizing load following capacity, and smoothing wind power output fluctuations. Utilizing the flexible output characteristics of solar thermal power plants, they carried out peak shaving and valley filling for wind power and solar photovoltaic power generation grid connection, and smoothed the output curve. This type of research has achieved optimized operation of solar thermal, wind power, and solar photovoltaic power generation systems under different operating conditions and control modes. In terms of source load coordination and scheduling, Zhao Yuxuan studied the coordination and scheduling problem between solar thermal power plants and multiple source load resources, as well as the optimization problem of pricing curves. He proposed a multiple coordination and scheduling strategy between solar thermal power plants and multiple source load resources, which improved the scheduling economy and flexibility when aggregating solar thermal power plants and multiple source load resources. Liu Xinyuan et al. constructed a multi time scale source load coordination and scheduling model for power systems containing solar thermal wind power to address the problem of flexible source load coordination and scheduling in multiple time scales. This model not only improves the flexibility of power system scheduling but also solves the peak shaving problem caused by large-scale wind power integration. Cui Yang et al. proposed a method for coordinated scheduling of source network load, using solar thermal power plants as regulating resources, effectively improving the local capacity for new energy consumption. In terms of considering the participation of solar thermal power plants in peak shaving auxiliary services in power system scheduling, Cui Yang et al. established a pricing model for solar thermal power plants to participate in peak shaving services. Based on this, they proposed a scheduling method for thermal power plants and solar thermal power plants to jointly participate in peak shaving auxiliary services, improving the level of wind and solar energy consumption in the power system while reducing operating costs.
3.3 Integration Strategy for Multi energy Systems Including Photothermal Power Generation
Equipped with an energy storage system, the solar thermal power generation unit has energy time shift characteristics and fast adjustment capabilities. At present, scholars have reasonably matched solar thermal power generation with wind power, solar photovoltaic, energy storage, heat storage and other systems, and constructed a multi energy complementary joint power generation system. Dai Jianfeng et al. proposed a coordinated control strategy for wind and solar thermal power plants suitable for grid scheduling in the wind solar thermal power plant joint system, and established a multi-objective optimization model for wind solar complementary control with thermal storage devices. Zheng Lianhua et al. comprehensively considered the impact of the collaborative operation of CSP power plants and hydrogen energy storage on the scheduling of the integrated energy system, and proposed a low-carbon optimization operation strategy for the integrated energy system, which includes solar thermal power plants and hydrogen energy storage. This optimized the operational flexibility of the system and improved energy utilization efficiency. Zang Haixiang et al. found through studying the energy coupling relationship of wind power solar thermal biomass hybrid power plants that the thermal storage system and biomass boiler of the solar thermal power plant not only improve the operational flexibility of the hybrid power plant, but also optimize its operating strategy to increase the bidding volume of the hybrid power plant in the electricity market, in order to obtain higher electricity market returns. Sakellaridis et al. constructed a wind power, pumped storage power station, and solar thermal combined power generation system model based on the energy storage and regulation characteristics of photothermal and pumped storage power stations, and evaluated the operational reliability of the system. Peng Yuanyuan and others utilized the controllable output of photovoltaic power plants to aggregate them into wind thermal virtual power plants, and proposed a two-stage optimization scheduling model for photovoltaic wind thermal virtual power plants containing solar heat, in order to fully tap into the regulatory potential of CSP and increase power plant revenue through internal collaborative optimization. Zhao Lingxia studied the construction of a multi energy virtual power plant consisting of wind power, solar photovoltaic, solar thermal, and energy storage batteries, in response to the shortcomings of the integrated energy system. Zeng Xianqiang et al. constructed a simplified regional integrated energy system with the participation of solar thermal power plants for a multi energy coupled regional integrated energy system. Through simulation, it was found that the participation of solar thermal power plants improved the utilization efficiency of renewable energy and the coordination and optimization ability of the regional integrated energy system.
3.4 Optimization configuration of photovoltaic power plants
The energy storage system is the key to ensuring the continuous and stable operation of solar thermal power generation with adjustable performance. Therefore, the impact of optimizing the capacity parameter configuration of thermal storage systems on the operational characteristics of photovoltaic power plants and the economic feasibility of flexible participation of solar and thermal power in wind photovoltaic complementary operation are currently hot research topics. Kueh et al. studied the impact of heat storage systems on the operation of photovoltaic power plants and provided important factors affecting heat storage capacity. Boukelia et al. compared and analyzed eight different configurations of slot solar photovoltaic power plants based on two thermal media, molten salt and thermal oil, from four dimensions: energy efficiency, thermoelectric efficiency, environmental friendliness, and economy. The results showed that solar power plants equipped with molten salt heat storage and fuel backup systems had the highest overall efficiency. Levelized Power Cost (LCOE) is the most commonly used parameter in economic research and analysis of solar photovoltaic thermal power plants. Praveenr et al. aim to optimize the performance of the proposed CSP power plant by changing the solar photovoltaic multiple and full load hours of the Thermal Energy Storage System (TES), with the lowest LCOE value at the highest annual power generation rate. Boukelia et al. studied the optimization of solar multiple and heat storage capacity parameters in the 50 MW photovoltaic thermal combined power generation model in Algeria, in order to minimize the LCOE of the combined power plant. In the study of heat storage capacity configuration that takes into account scheduling economy in solar thermal power generation scheduling, Cui Yang et al. comprehensively considered the impact of peak shaving cost and heat storage cost of thermal power units on the capacity configuration of heat storage systems, and proposed a heat storage capacity configuration method for solar thermal power plants to reduce peak shaving cost of thermal power units. Yao Yuanxi comprehensively considers factors such as the cost of generating thermal power units, the environmental benefits and operation and maintenance costs of grid connected solar thermal power generation, and the cost of system rotation and backup for scheduling economy. He explores the balance point between the configuration cost of thermal storage devices and scheduling economy, and determines the configuration of thermal storage capacity for solar thermal power plants. Kost et al. found that different subsidy mechanisms and operating strategies have an impact on the optimal photo capacitance ratio and optimal heat storage capacity of photothermal power generation.
3.5 Comprehensive Benefit Evaluation of Photothermal Power Generation
Photothermal power generation has regulatory performance comparable to conventional power sources, providing peak shaving and backup services for the power grid and promoting the consumption of new energy such as wind and solar power. Therefore, in addition to considering its own operational benefits, the evaluation of the benefits of photothermal power generation also includes objective benefits such as the capacity benefits of replacing conventional units, the electricity benefits of multi energy complementary power generation with other power sources, and the benefits of providing flexible services such as peak shaving. Fu Xu et al. proposed an equivalent annual fee method for comprehensively evaluating the capacity and electricity benefits of CSP, taking into account unit start stop and cross day regulation of energy storage power sources. Shayun et al. conducted a quantitative analysis of the flexible operation benefits of photovoltaic power plants from three perspectives: operational economy, peak shaving effect, and green electricity benefits, in order to optimize the operation of interconnected systems with a high proportion of new energy transmission ends connected by ultra-high voltage direct current. Chen Runze et al. analyzed the objective benefits of grid connection of solar thermal power plants in terms of power generation costs, renewable energy acceptance, and improving the utilization rate of concentrated transmission lines, under the premise of fully accepting solar thermal power generation.
Further combining relevant literature with the electricity market, this study analyzed the operational strategies of solar thermal power plants participating in the electricity market, in order to achieve maximum market returns for solar thermal power plants. He et al. established an optimal pricing strategy for CSP power plants in the joint day ahead energy, backup, and regulation markets to address the uncertainty of solar photovoltaic and market prices, in order to improve the operational efficiency of solar thermal power plants. Liang Zheng and others considered solar thermal power generation companies as market bearers and constructed a market trading decision model based on the multi oligopoly Cournot model for electricity spot market trading. Zhao Yuxuan et al. considered the non stochastic uncertainty of thermal production in CSP power plants, the stochastic uncertainty of market prices, and the risks associated with CSP power plants, and proposed the optimal operating strategy for solar thermal power plants in the day ahead and real-time electricity market, which improved the economic benefits of solar thermal power generation.
Based on the above literature, it can be seen that at present, in terms of optimizing the operation of power systems containing solar thermal power generation, scholars focus on the research of solar thermal power stations participating in wind photovoltaic joint generation, addressing the uncertainty of wind and solar power, and promoting flexible scheduling strategies for high proportion renewable energy power systems; However, there is relatively little research on the reliability of the power generation capacity of photovoltaic power plants and the methods to support safe and flexible operation of the power grid from the perspective of the overall power system. In the integration of multi energy systems containing solar heat, existing research exploring the integration strategy of wind solar heat storage multi energy complementary energy systems mostly considers the formation of a “solar heat+” multi energy complementary power generation form on the power supply side between solar heat, photovoltaic, wind power, and energy storage; However, further exploration and research are needed to explore the operational strategies of photovoltaic power plants in the integrated system of electricity/heat/cold/gas multiple energy sources, as well as to provide certain capacity support and regulation capabilities for the power system. In terms of optimizing the configuration of solar thermal power plants, most of the current optimization focuses on optimizing the collection and storage efficiency of a single solar thermal power plant. Some literature has studied the configuration strategy of thermal storage parameters that takes into account scheduling economics, but there are few reports on the optimal capacity matching method for solar thermal power plants to flexibly participate in grid regulation under the maximization of power system benefits. In terms of comprehensive benefit evaluation of solar thermal power generation, some literature has studied and analyzed the operating strategies of solar thermal power plants participating in the electricity market in specific scenarios from the perspective of operating benefits of solar thermal power plants. However, it has not fully considered the uncertain influencing factors such as CSP operation mode, heat storage duration, new energy penetration rate, peak shaving demand, and grid constraints, as well as the benefit evaluation methods of solar thermal power plants under multiple optimization objectives.
4. Outlook on Photothermal Power Generation Technology
Solar photovoltaic thermal power generation, as a stable and reliable new energy generation technology, is an indispensable and important technological means to achieve China’s energy transformation goals. This technology involves the integration and coordination of multiple systems such as solar photovoltaic heat collection, heat transfer and storage, and power generation. In future development, attention should be paid to the following aspects.
(1) Research on key technologies and equipment for photothermal high-temperature collection/storage. The solar thermal power generation system has the advantages of high power generation efficiency, high economy, and suitability for large capacity development under high temperature operating parameters. Therefore, conducting research on high-temperature heat collection/storage technology and promoting the development of photothermal power generation towards high operating parameters and large capacity is crucial for this technology.
(2) In terms of predicting photothermal power. Photothermal power generation mainly utilizes the normal direct radiation of the sun. Normal direct radiation is greatly influenced by clouds. Environmental factors such as irradiance under different natural resource environments will have an impact on solar thermal power generation, heat collection, and energy storage systems. We should study how to improve the prediction accuracy of solar thermal power, in order to better provide decision-making basis for the efficient and economical operation of solar thermal storage systems and power plant output.
(3) Research on the characteristics of photothermal operation. The development of new energy sources such as wind and light has been accelerated by the establishment of the “dual carbon” target, which puts forward higher requirements for the acceptance and regulation capabilities of the power system. In the future, comprehensive consideration should be given to the high uncertainty of both the source and load sides, and the dynamic characteristics analysis of various types of photothermal power plants under different operating conditions should be carried out based on actual operating conditions. Mathematical models of photothermal power generation equipment should be constructed at different time scales to analyze the reliability of different types of photothermal power generation capacity, support for grid safety and other operating characteristics, and improve the stability and flexibility of the power grid; Conduct research on coordinated and optimized configuration and scheduling strategies for different types of solar thermal power generation to participate in the high proportion of new energy transmission and acceptance of new energy in the power grid, in order to improve the operational efficiency and economy of the power grid.
(4) Standards related to solar thermal power generation. At present, solar thermal power generation technology is in its early stage in China, with each technology acting independently and lacking clear standards. It has not fully corresponded to the actual operation of solar thermal power generation, making it difficult to form strong support for the industry. In the future, based on the actual operation of photothermal systems, we should conduct in-depth research on the selection methods of different types of photothermal capacity configuration parameters from the perspectives of system stability, dynamic performance, and economy. We should also propose corresponding control strategies for photothermal participation in power grid peak shaving and frequency regulation with different capacity ratios, as well as the “solar thermal+wind solar power” multi energy complementary integrated operation standard.
(5) Cost benefit evaluation of photothermal power generation. At present, the high investment cost of solar thermal power is the main factor affecting the development of solar photovoltaic thermal power plants. Therefore, analyzing and evaluating the economic and comprehensive benefits of solar thermal power generation is the key to enhancing its competitiveness. In the future, further exploration is needed in terms of optimizing the cost of solar thermal power plants, participating in grid peak shaving, and achieving multi energy complementary power generation benefits with other power sources. One is to improve the economic efficiency of photovoltaic power plants and reduce LCOE by optimizing the materials of photovoltaic power generation technology, planning and optimizing the solar island heat collection system, and configuring heat storage capacity for each subsystem of photovoltaic power plants. The second is to combine the electricity market and consider the overall demand for auxiliary services such as peak shaving in the power grid. Analyze the operational mode and decision-making analysis model of flexible participation of photovoltaic power plants in grid regulation under the maximization of power system benefits, explore the benefits of photovoltaic power generation providing flexible services such as peak shaving, and explore the multi energy complementary power generation benefits with other power sources.
5. Conclusion
In order to improve the flexibility of high proportion renewable energy power systems, a comparative study was conducted on four solar thermal power generation systems. Solar thermal power generation integrates power generation and energy storage, with advantages such as flexible and controllable output, and is a highly promising renewable energy generation technology. This article introduces the principle and system structure of photothermal power generation technology; Summarized the current research status of photothermal power generation technology; Explored the issues and research directions that need to be addressed in future photothermal power generation, hoping to provide reference for future research and development of high-performance photothermal power generation.