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Characterising the discharge properties of a thermochemical storage system via the hydration of chemical salts

Robbie Sutton, Eifion Jewell Orcid Logo, Justin Searle, Jon Elvins

Sustainable Energy Technology (SET2016),

Swansea University Author: Eifion Jewell Orcid Logo

Abstract

A transpired solar collector (TSC) is a renewable technology developed to harness useful thermal energy from the sun. An example of such technology is Colorcoat Renew SC®, TATA Steels own steel cladding solar collector, being the initial project driver. The energy available over a typical year varie...

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Published in: Sustainable Energy Technology (SET2016),
Published: Singapore Sustainable Energy Technology (SET2016) 2016
Online Access: http://set2016.chbe.nus.edu.sg/confprog.html
URI: https://cronfa.swan.ac.uk/Record/cronfa37735
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Abstract: A transpired solar collector (TSC) is a renewable technology developed to harness useful thermal energy from the sun. An example of such technology is Colorcoat Renew SC®, TATA Steels own steel cladding solar collector, being the initial project driver. The energy available over a typical year varies seasonally. Due to seasonal variations, excess solar thermal energy is generated in summer, when the thermal demand of a building is significantly lower, and vice versa in winter. Thermochemical storage methods can be utilised to store the excess thermal energy generated in summer for use in winter when building demand is much greater. The thermochemical storage method under consideration involves the hydration and dehydration of chemical salts, storing and releasing thermal energy via the fully reversible endo/exothermic chemical reaction: salt∙x(H_2 O)+heat ↔salt+x(H_2 O). Chemical salts housed within matrices (SIMs) have been shown to have minimum hysteresis and material degradation over hydration and dehydration cycles. Highly porous vermiculite impregnated with CaCl2, LiNO3 and MgSO4 are shown to be promising candidates. The maximum air temperature attainable via a TSC (typically 80°C) dictates the lowest hydrated state available for each salt and thus the maximum attainable storage capability, based on the dehydration temperature of the hydrated states of the salt and the associated change in enthalpy. The storage capacities attainable are calculated to be 1204.03 kJ.mol-1, 757.43 kJ.mol-1 and 1492.43 kJ.mol-1 for CaCl2, LiNO3 and MgSO4 respectively, being more than 66% of the overall storage capacity.This work looks to investigate the performance of the SIMs under discharge properties representative of a typical thermal storage system. The thermal output of the SIMs relates to the change of enthalpies associated with the change of hydrated state following the addition of H2O molecules. Experiments are developed to simulate typical discharge properties of a thermochemical storage system; monitoring the variation of temperature and humidity through differing volumes of active SIM, to track the moisture absorption of the SIM and the associated thermal release. An input humidity of 75% at 20°C results in a moisture content of 12.9 gm-3, representative of maximum moisture content of air during winter months; when the discharge cycle occurs. A number of studies are completed by varying the type and amount of active salt within the SIM, along with, material volumes, input flow rate, duration and cyclic efficiency. A larger change in temperature (DT) is expected with higher salt content SIM, due to the overall increase in storage capacity. However, a higher salt content within the SIM does not result in an increases DT, due to the reduction in free space within the SIM constricting air flow. Typically a higher flow rate results in a larger DT attained due to the increase moisture content over a period of time, and thus an increased opportunity for water adsorption of the SIMs. Initial experiments confirm the occurrence of the thermochemical reaction, with a DT of 25°C and 12°C for the CaCl2 and LiNO3 SIMs respectively observed within the bulk of the material. A DT of 5°C is observed at the exit of the material for both SIMs. This study will look to optimise the discharge process and increase the exit DT to match the DT of bulk material. No DT is observed with the MgSO4 SIM indicating that the moisture content of 12.9 gm-3 is insufficient to initiate a reaction. A larger amount of energy can be theoretically stored within this SIM under the operational conditions of a TSC, however, there is an apparent challenge relating to the energy recover under the proposed discharge cycle of a thermochemical storage system.The energy densities of the SIMs is determined using a Thermal Activity Monitor, and is linked to the input humidity conditions of the experiments undertaken. The results from this study will feed into the development of a building integrated thermochemical storage system currently under development.
Keywords: Thermal storage, hydrated salts
College: Faculty of Science and Engineering

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