To improve the energy efficiency of an industrial process thermochemical energy storage (TCES) can be used to store excess or typically wasted thermal energy for utilisation later. Magnesium carbonate (MgCO₃) has a turning temperature of 396 °C, a theoretical potential to store 1387 J/g and is low cost (~GBP 400/1000 kg). Research studies that assess MgCO₃ for use as a medium temperature TCES material are lacking, and, given its theoretical potential, research to address this is required. Decomposition (charging) tests and carbonation (discharging) tests at a range of different temperatures and pressures, with selected different gases used during the decomposition tests, were conducted to gain a better understanding of the real potential of MgCO₃ for medium temperature TCES. The thermal decomposition (charging) of MgCO₃ has been investigated using thermal analysis techniques including simultaneous thermogravimetric analysis and differential scanning calorimetry (TGA/DSC), TGA with attached residual gas analyser (RGA) and diffuse reflectance infrared Fourier transform spectroscopy (DRIFTS) (up to 650 °C). TGA, DSC and RGA data have been used to quantify the thermal decomposition enthalpy from each MgCO₃.xH₂O thermal decomposition step and separate the enthalpy from CO₂ decomposition and H₂O decomposition. Thermal analysis experiments were conducted at different temperatures and pressures (up to 40 bar) in a CO₂ atmosphere to investigate the carbonation (discharging) and reversibility of the decarbonation–carbonation reactions for MgCO₃. Experimental results have shown that MgCO₃.xH₂O has a three-step thermal decomposition, with a total decomposition enthalpy of ~1050 J/g under a nitrogen atmosphere. After normalisation the decomposition enthalpy due to CO₂ loss equates to 1030–1054 J/g. A CO₂ atmosphere is shown to change the thermal decomposition (charging) of MgCO₃.xH₂O, requiring a higher final temperature of ~630 °C to complete the decarbonation. The charging input power of MgCO₃.xH₂O was shown to vary from 4 to 8136 W/kg with different isothermal temperatures. The carbonation (discharging) of MgO was found to be problematic at pressures up to 40 bar in a pure CO₂ atmosphere. The experimental results presented show MgCO₃ has some characteristics that make it a candidate for thermochemical energy storage (high energy storage potential) and other characteristics that are problematic for its use (slow discharge) under the experimental test conditions. This study provides a comprehensive foundation for future research assessing the feasibility of using MgCO₃ as a medium temperature TCES material. Future research to determine conditions that improve the carbonation (discharging) process of MgO is required.

A primary drawback of solar thermal technologies, especially in a domestic setting, is that collection of thermal energy occurs when solar irradiance is abundant and there is generally little requirement for heating. Thermochemical Energy Storage (TCES) offers a means of storing thermal energy interseasonally with very little heat loss. A combination of Solar Thermal Collectors (STC) and TCES systems will allow a variety of different heating applications, such as domestic space and hot water heating as well as low temperature industrial process heat applications to be met in a low carbon way. This paper describes and assesses the feasibility of two novel technologies currently under development at Loughborough University; i) an evacuated flat plate STC and ii) composite TCES materials, coupled together into a system designed to store and supply thermal energy on demand throughout the year. The predicted performance of an evacuated flat plate STC is described. The objective of this paper is to evaluate the economic, energy and carbon saving potential of conceptual STC + TCES systems suitable for domestic use. This research uses experimental results from Differential Scanning Calorimeter tests to evaluate the total enthalpy, dehydration enthalpy and sensible component enthalpy of composite TCES materials. The experimental results along with predicted performance of STC are used within a developed model to assess key metrics of conceptual STC + TCES systems feasibility, including; charging time, payback time, cost/kWh, energy savings and CO₂ savings. Preliminary results suggest the combination of these two technologies has significant potential for domestic applications.

One problem with utilising solar renewable energy sources, for domestic space heating and domestic hot water in the UK is the mismatch between energy supply and energy demand due to the seasonal variation. One way to mitigate this phenomenon is to utilise seasonal thermochemical heat storage. 13X molecular sieves have received recent attention as a thermochemical energy storage material for domestic use due to the heat stored when dehydrated and released when rehydrated. Minimal attention has been paid to the dehydration conditions and the impact this has on the heat release from dehydrated 13X. The reported study experimentally characterised the charge and discharge enthalpy of 13X molecular sieves using a custom designed novel Differential-Scanning-Calorimeter apparatus and Thermogravimetric-Analyser for mass loss investigations. This study shows how employing different thermal analysis testing methodologies can produce different results. The impact that the grade of nitrogen employed has on the mass change of the 13X samples, and the length of time exposed to the nitrogen purge as the charged sample cools, can have a dramatic impact on the mass of the 13X sample. When using a nitrogen generator or technical grade nitrogen as the purge gas in the cooling phase after a 500 ℃ dehydration, (charging), the mass of the 13X sample increased by 13% and 17%, respectively, this could potentially impact the later adsorption potential leading to reduced hydration and energy output on discharge. The hydration enthalpy (energy output) of 13X for varying charge (60–150 ℃) and discharge (25–60 ℃) temperatures, with a discharging partial vapour pressure (pH₂O) of 0.64 kPa are also presented. This work provides reference values for the expected energy output from 13X for a range of charge and discharge temperatures, for example, showing that the energy output varies from 660–500 J/g with different discharge temperatures if the sensible heat is not utilised.