Exploiting the operational flexibility of a concentrated solar power plant with hydrogen production

Global warming has profoundly changed the energy landscape. Consequently, the China government aims to reach the peak of its carbon dioxide emissions before 2030 and reach carbon neutrality by 2060. The goal of carbon neutrality has further accelerated the development and utilization of renewable energy in China. With the increased proportion of wind and solar energy (Kroposki et al., 2017); research on the efficient and low-carbon operation of energy systems is urgently needed.

Concentrated solar power (CSP) is a novel technology for harnessing solar energy to generate power. CSP is different from photovoltaics (PV) generation in the aspect of its thermal energy storage and generating mode, which can shift the utilization and generation of solar energy. As a result, CSP makes volatile renewable energy schedulable and has an important role in smoothing volatile wind power and PV power (Du et al., 2016). Currently, CSP plants, such as the Delingha Power Plant in China and the Ivanpah Power Plant in the U.S., have been in grid-connected operation for several years. At the end of 2020, the gross installed capacities of CSP plants had reached 420 MW in China and approximately 7000 MW worldwide (Tschopp et al., 2020). The International Energy Agency predicted that the global installed capacities of CSP plants would reach 11.3 % of gross power generation by 2050 (Usaola, 2012), which represents enormous market development potential. Although CSP has a high cost per kWh, a win–win situation could be achieved by coordinating fluctuating energy sources, such as wind and solar energy. However, the flexible regulation abilities of CSPs remain underutilized with the current policy of peak regulation compensation, it is necessary to explore novel applications for CSP plants and comprehensive energy utilization to realize the flexible regulation ability of CSP.

Hydrogen production does not generate pollution and emissions, and thus, has an extremely important role in modern industry, human life, and production (National Development and Reform Commission, National Energy Administration). It is estimated that renewable energy will provide 70 % of China’s hydrogen after 2050 (Chin, xxxx). Currently, available technologies for hydrogen production from renewable energy electrolysis include alkaline electrolysis, proton exchange membrane electrolysis, and high-temperature solid oxide electrolysis (International Renewable Energy Agency). The first two techniques are extensively utilized as low-temperature technologies. However, their hydrogen production costs are high due to expensive electrode materials, and their hydrogen production efficiencies are lower than those of high-temperature solid oxide electrolysis. Ref. (Mu et al., 2017) indicates that the hydrogen production efficiencies of the three methods are among 54–66 %, 70–85 % and 90–100 %, respectively. Over the past few years, solid oxides have been employed as electrolytes at 600–1000 °C for high-temperature electrolytic hydrogen production (HEHP), which has improved the hydrogen production efficiency and obviated the use of expensive metal electrode materials (Mu et al., 2017). However, the high temperature needed for this technology is usually provided by burning fossil fuels, which may cause environmental pollution. This can be resolved by combining CSP, which is a zero-carbon heat source, with HEHP technology. The high-temperature and high-pressure steam produced by CSP can be utilized for HEHP. The use of CSP instead of fossil fuel enables hydrogen production to be obtained in a cleaner, safer, and renewable way. The energy utilization of CSP increases, and if necessary, hydrogen energy can be subsequently converted into electricity. In addition, the fluctuating output of renewable energy utilized for water electrolysis hydrogen production can be smoothed by CSP, which is conducive to the steady operation and loss reduction of an electrolytic cell. Therefore, CSP with HEHP technology is complementary and promising.

In recent years, some research institutions have devoted themselves to CSP research with HEHP technology and have made some progress. In 2017, a German R&D team completed a 750 kW demonstration project of solar-driven hydrogen production in Spain, which involved a complete process, including power generation, extraction of high-purity hydrogen and hydrogen storage (SaCk et al., 2016). It has been reported that the project heated steam to 700 °C in a collector and that the maximum thermal conversion efficiency was 40 % (Houaijia et al., 2014). The calculation results demonstrate that the maximum efficiency for a high-temperature electrolytic cell is 93 % (Buttler et al., 2015); which indicates the feasibility and merits of HEHP heated by solar energy. In addition; HEHP coupled with solar heating is listed in a 2019 report issued by the U.S. Department of Energy as a future development orientation for large-scale hydrogen production (Stetso, 2019). Thus; creating an energy carrier with electricity and hydrogen at the core can stimulate the growth of the electricity market; carbon trading market, and hydrogen market.

While CSP with HEHP technology contributes to a lower-carbon, flexible, integrated energy system (IES) and serves as a flexibility asset, several challenges remain to be addressed. First, it is crucial to propose a novel mode of operation for CSPs with HEHP technology in electric-hydrogen energy systems to exploit the operational flexibility of CSP with HEHP. Second, facing the complex coupling system of electricity and hydrogen energy, it is vital to establish a simplified and optimal operation model of an IES containing CSP with HEHP technology. Third, it would be desirable to analyse and quantify the flexible operation benefits of CSP with HEHP in the IES, especially in terms of carbon reduction benefits. This paper aims to address these three challenges.

Studies on CSP over the past few years have focused on the planning and operation of CSP plants. Analyses of the cost-effectiveness of CSP plants in (Petrollese et al., 2017, Liu et al., 2019, Brand et al., 2012, Xu and Ning, 2016) show that CSP plants with a thermal energy storage system connected to a grid offer considerable benefits in terms of power generation, reserve service, and improving the utilization rate of aggregated transmission lines. In (Guo et al., 2020, Du et al., 2018, Botterud et al., 2013, Santos-Alamillos et al., 2015), the rational scheduling of CSP plants was shown to effectively alleviate the fluctuation and intermittence caused by off-grid wind and PV power, thereby reducing renewable energy curtailment. These studies highlight the role and benefits of CSP as a flexible regulating electric source in the field of power systems. However, few studies have described the heat-power co-generation capabilities of CSP plants. Although (Wang et al., 2019, Qi et al., 2017) directly uses the heat collected by CSP plants to support the heat load of a micro-grid, the application of CSP thermal energy with high temperature characteristics at the planning and operation levels has not been fully explored.

Studies on CSP with HEHP focus on the realization process and thermodynamic principle analysis of the technology. (Schiller et al., 2019) shows that the enthalpy of water ΔH0 and the required minimum energy for water splitting are greatly reduced during the phase transition from liquid water to steam. The electric power required for the electrolytic reaction decreases with an increase in temperature, while the demand for thermal energy increases. Therefore, electricity can be replaced to some extent by heat, which is much cheaper. Several existing CSP technologies are generally grouped into line focusing systems and point focusing systems. (Monnerie et al, 2016) shows that only the normal operation of the point focusing system can meet the high temperature demand above 600 °C, so solar tower technology is chosen for coupling with HEHP. In (Cheng et al., 2019, Cheng et al., 2019); a new parabolic trough solar receiver-reactor system was proposed, and a comprehensive kinetic model was established for efficient solar thermal hydrogen production through a methanol-steam reforming technology reaction. However, this hydrogen production technology produces carbon dioxide during the reforming process. In (Alzahrani and Dincer, 2016, Almahdi et al., 2016, Mehdi and Marzie, 2020, Mohammadi and Mehrpooya, 2019); the authors mainly investigated the hydrogen production process with the coupling of a high-temperature electrolytic cell and CSP from the perspective of thermodynamics. However, hydrogen loading was given priority, and the flexible regulation role of CSP plants in the power system was not considered in the coordination in these studies. In addition, they focused on the realization process and control methods within the specific model, and there was no system-level optimization and benefit analysis research. Therefore, to date, the potential of hydrogen production from CSP and its flexibility benefit have not been thoroughly investigated at the system level.

Previous studies have primarily focused on the feasibility of CSP plants coupling with the HEHP process from the perspective of thermodynamics, or the contribution of CSP plants as peak shaving power sources in the power system. To the best of our knowledge, few studies have been conducted from the point of view of system operation to model CSP with HEHP and evaluate the benefits of the coupling, e.g., the benefit of CSP with the HEHP process during the midday alleviating the regulation pressure of conventional units, and reducing the carbon dioxide emission of the system. To close this gap, we exploit the operational flexibility of CSP with HEHP in an island IES. Our contributions are threefold:

(1)

A novel application mode of CSP with HEHP is proposed, and its operating principle is briefly described. Based on participating in peak shaving of the power grid, CSP plants can provide the required electric power and heat energy for the HEHP process through the storage of heat or its power-heat co-generation characteristics, so that hydrogen energy can be obtained more cleanly, efficiently, and renewably.

(2)

An optimal operation model of an IES containing CSP with HEHP is formulated to schedule the power generation facilities, HEHP device, and energy storage devices in the system, including the CSP energy model of both power generation and hydrogen production, and the linearized HEHP operation model. The optimal capacity configuration of energy storage devices in the system is explored to ensure the optimal operation of the electric-hydrogen integrated system.

(3)

The flexible operation benefits of CSP with HEHP in the IES are analysed and quantified. The effects of CSP with HEHP on the operating benefit of the system are evaluated from the aspects of system economy, energy utilization, and carbon emission reduction. The carbon emission trajectory of the IES is also depicted.

The remainder of this paper is organized as follows. Section 2 presents a detailed description of an IES containing CSP with HEHP. Section 3 provides the optimal operation model of the IES. Section 4 presents the results and the discussion of the case studies. Section 5 outlines the conclusions.