(449d) On the Role of Thermochemical Energy Storage in Concentrating Solar Power

Concentrating solar power (CSP) with thermal energy storage has the potential for grid-scale dispatchable power generation. Thermochemical energy storage (TCES), that is, the reversible conversion of solar-thermal energy to chemical energy, has high energy density and low heat loss over long periods. Previously, several reaction types have been studied as candidates, including dissociation of ammonia, hydrides, carbonates, oxides, hydroxides and methane reforming [1]. Although considerable work has been done, past research has mostly focused on experimental studies and reactor design. The limited system-level studies that are available in the literature are for specific reactions [2] and, furthermore, consider TCES in isolation, neglecting its interactions with the solar field and power generation system [3].

Accordingly, in this work, we establish a framework to systematically analyze and compare candidate reactions for TCES systems [4]. First, we develop a general process for CSP plants employing TCES systems. A general TCES system includes five tasks: reaction, storage, separation, material handling and heat recovery. We then formulate an optimization-based model for the process, which optimizes the plant design towards the minimum levelized cost of electricity (LCOE). We use this model to study ammonia and methane systems.

We identify that, for fluid phase TCES systems, the main cost driver is gas storage using pressure vessels, accounting for 77% of the TCES cost. This is because, even if reaction products are compressed and stored at 550 bar, both ammonia and methane systems require 65,000 m³ of gas storage at a vessel price of 8900 $/m³. The main energy driver is the electricity consumption for storage compression, which consumes 18% of the generated power. Adopting underground gas storage, where products are stored at lower pressure in much cheaper underground caverns, could significantly reduce the compression power consumption (from 18% to 3%) and improve the overall energy efficiency and thus cost. The methane system combined with underground storage is identified as the most cost-efficient alternative, achieving a 13% LCOE reduction over the current two-tank molten salt CSP plants.

Finally, we discuss the impact of the reaction temperature and operating pressure. The endothermic and exothermic reaction temperature determines not only the reaction conversion, but also the receiver and turbine efficiency. We find that both reaction temperature should be in the vicinity of the reaction turning temperature (at which the equilibrium constant K=1) and separation can increase the reaction conversion, allowing lower endothermic reaction temperature. The operating pressure, including process pressure, highest and lowest storage pressure, influences both the storage volume and compression work. For pressure vessel cases, the storage tank pressure varies in a wide range (150-550 bar), reducing the storage volume at the expense of more compression power consumption. When cheap underground storage is employed, a narrow storage pressure range (135-165 bar) leads to smaller electricity consumption and larger storage volume.

The proposed framework can be used to study a wide range of TCES systems, including solid-gas phase reactions, which will be the topic of future research.

References:

[1] P. Pardo, A. Deydier, Z. Anxionnaz-Minvielle, S. Rougé, M. Cabassud and P. Cognet, Renew. Sustain. Energy Rev., 2014, 32, 591–610.

[2] A. Luzzi, K. Lovegrove, E. Filippi, H. Fricker, M. Schmitz-goeb, M. Chandapillai and S. Kaneff, Sol. Energy, 1999, 66, 91–101.

[3] C. Corgnale, B. Hardy, T. Motyka, R. Zidan, J. Teprovich and B. Peters, Renew. Sustain. Energy Rev., 2014, 38, 821–833.

[4] X. Peng, T. W. Root and C.T. Maravelias, Green. Chem, 2017, DOI: 10.1039/c7gc00023e.