Power generation based on the Ca(OH)2/ CaO thermochemical storage system – Experimental investigation of discharge operation modes in lab scale and corresponding conceptual process design

Abstract

Thermochemical storage systems offer in theory promising advantages for a wide range of applications. In particular the reversible reaction of calcium hydroxide to calcium oxide and water vapour is intensively discussed as an alternative storage solution for concentrated solar power plants. The material is cheap, environmentally friendly and discharge temperatures of the reaction of 600 °C and above fit to the operating range of today´s power plants. However, experimental data on the operation of the system in lab scale and at load conditions comparable to the real application is rarely reported. Therefore the thermal discharge of the reaction system at vapour pressures between 4 and 470 kPa and temperatures between 280-600 °C is experimentally investigated in this study. In particular the influence of the cooling load at various vapour pressures on the achievable discharge temperatures is analysed. The presented data complements the experimental characterisation of the reaction system in the complete temperature and pressure range which is relevant for real process applications. Based on this knowledge the applicability of the storage for various processes can now be assessed more accurate. By means of the experimental results a first integration option of the thermochemical system in a CSP plant is proposed in this work and thermodynamically analysed. The analysis revealed that, when the required steam production during discharge is thermally integrated into the Rankine steam cycle, a high storage efficiency of up to 87 % can be reached compared to only 60 % in the reference case.

Performance of thermochemical energy storage of a packed bed of calcium hydroxide pellets

“…However, a lower hydration pressure tended to increase the difference; the difference in the temperatures in the cases of 38.6, 25.0, and 15.8 kPa were 5.2, 7.2, and 21°C, respectively. This tendency is in agreement with the temperatures reported by Schaube et al and Schmidt et al, who used particle or powder beds, the results of which are shown in Figure . To understand the maximum temperature during hydration in laboratory‐scale reactors, further studies have to be conducted at less than 10 kPa.…”

“…31 Metal oxide/water systems are good candidate for TCES, and it is quite difficult to measure vapor movement in a laboratory scale experimental system because the condensation of water vapor makes flow measurement of water vapor difficult. The unique point of this study is that global conversion of the packed bed was measured through mass-change directly with high precision of 0.1 g with temperature F I G U R E 1 Chemical equilibrium lines of the reaction: [8][9][10] ; conditions of the reaction in laboratory-scale reactors [33][34][35][36] measurements. The experimental data of temperatures and conversion can be used easily for validation of a numerical model for the simplicity of the reactor with high accuracy.…”

“…Chemical equilibrium lines of the reaction: Ca(OH) 2 (s) ⇌ CaO(s) + H 2 O(g); conditions of the reaction in laboratory‐scale reactors …”

Composite material for high‐temperature thermochemical energy storage using calcium hydroxide and ceramic foam

“…Practical storage reactors have two main requirements: a fast response during the heat storage/output process and the durability for repetitive operations. In terms of the CaO/Ca(OH) 2 /H 2 O reaction system, previous studies using laboratory‐scale fixed bed reactors have reported several problems that need to be solved to meet these requirements: a low thermal conductivity of the powder bed, formation of centimeter‐scale agglomerates inside the reactors, and change in the bulk volume during repetitive reactions . Therefore, the need to develop suitable materials for a CaO/Ca(OH) 2 /H 2 O reaction system remains.…”

“…A complementary development of materials coupled with the optimization of the reactor design would facilitate the enhancement of heat transfer through a storage reactor. Linder et al and Schmidt et al studied fixed bed reactors with embedded heat exchanger plates.…”