(257d) A Comparison of Solid-Gas Thermochemical Energy Storage Reactions and Process Configurations for CSP Applications

Concentrating solar power (CSP) integrated with thermal energy storage is considered to be a promising option to deliver cost effective and dispatchable renewable power. Thermochemical energy storage (TCES), which is the reversible conversion of solar-thermal energy to chemical energy, allows high operating temperatures, high storage density and low heat loss over long periods. Many fluid-phase and solid-gas reactions have been studied as candidates for TCES.(Pardo et al., 2014) Recently, solid-gas TCES reactions have been receiving more attention because they require less gas storage, have higher reaction temperature and easier product separation.

Today, TCES is still at an early stage of development. While considerable work has been done in reaction selection and reactor design, there are limited system-level studies for the design of integrated processes with TCES systems embedded in CSP plants. As a result, it remains unclear what is the best process configuration, reactor design, and chemistry for CSP systems with TCES. Accordingly, the goal of this work is to (1) explore process configurations for solid-gas TCES (2) evaluate the performance of different TCES strategies via optimization models, and (3) determine how the characteristics of a reversible reaction affect the selection of a TCES process configuration.

The key units of a TCES process are two reactors: R1, where the solar heat drives an endothermic reaction ( ) and R2, where the reverse exothermic reaction takes place and the reaction heat is used for power generation. To systematically develop TCES processes, we identify four key process characteristics with two options available for each: (1) R1 heat receiving (directly from solar irradiance/ indirectly from heat transfer fluid (HTF)), (2) heat transfer between R2 and power cycle (working fluid direct/ indirect contact with reactants), (3) reactor operation mode (continuous/ batch), and (4) presence of gas storage (closed-loop/ open-loop). The selection of an option for each characteristic leads to a configuration. After a preliminary screening on the possible combinations, we select the six most promising configurations for this analysis.

A TCES strategy is defined by the combination of a reversible reaction and a process configuration. We select three reactions, each representing one reaction group: (Carbonate group), (hydroxide group), and (redox group). Carbonate and hydroxide reactions can only be paired with closed-loop configurations, while redox reactions can be combined with all six configurations. Therefore, the three reactions and six configurations selected in this work lead to 12 TCES strategies in total.

We develop optimization-based process synthesis models to investigate the energetic and economic performance of these 12 strategies (Peng et al., 2017). We build models for different TCES units. Special emphasis is placed on the modeling of fixed-bed reactors, which operate in a cyclic batch mode and their first-principal model involves partial differential equations (PDEs) in time and space. Solving this first-principal model, especially for optimization, is computationally expensive, so we build surrogate models with simple algebraic equations to replace the complex first-principal models. Specifically, we first identify key input and output (e.g. heat storage and reaction conversion) variables; then generate data points using expensive numerical simulations implemented in COMSOL (which are carried out offline); generate surrogate models via data fitting; and, finally, integrate the surrogate models into the CSP-TCES process synthesis optimization model.

To design the plant under seasonal and daily variability in solar irradiance, we formulate the optimization problem as a two-stage stochastic programming model with day and night modes nested inside each scenario (Peng et al., 2018). By solving the model, we obtain the first stage design decisions (e.g. solar field area and all equipment sizes) and second stage operational decisions (e.g. stream flow rates, unit operating conditions) that yield the minimum plant levelized cost of electricity (LCOE). Other performance metrics such as energy efficiency and storage density of the optimal design are also calculated.

The analyses allow us to identify cost-efficient TCES systems. Among the 12 strategies, the hydroxide reaction + configuration with direct irradiated R1 and continuous reactor shows the best economic performance, achieving a 20% LCOE reduction over the commercial two-tank molten salt CSP plants. From this analysis, several general design rules and insights are also generated to serve as guidelines for the future development of TCES processes.

Reference

Pardo, P., Deydier, A., Anxionnaz-Minvielle, Z., Rougé, S., Cabassud, M., Cognet, P., 2014. A review on high temperature thermochemical heat energy storage. Renew. Sustain. Energy Rev. 32, 591–610. https://doi.org/10.1016/j.rser.2013.12.014

Peng, X., Root, T.W., Maravelias, C.T., 2018. Optimization-based Process Synthesis under Seasonal and Daily Variability: Application to Concentrating Solar Power. AIChE J. 0, 1–16. https://doi.org/10.1002/aic.16458

Peng, X., Root, T.W., Maravelias, C.T., 2017. Storing Solar Energy with Chemistry: The Role of Thermochemical Storage in Concentrating Solar Power. Green Chem. 2427–2438. https://doi.org/10.1039/C7GC00023E