Investigation into the potential of MgSO4 for interseasonal domestic thermochemical energy storage

Approximately 26% of the UK s primary energy consumption is used specifically for Domestic Space Heating (DSH) and Domestic Hot Water (DHW) production [1]. The majority of this, 88%, comes directly from gas and oil with only 2% coming from renewable energy sources [1]. Decarbonising DSH and DHW represents a huge challenge for the UK s government which is targeting a reduction of CO2 emissions of 80% by 2050 [2]. The amount of energy utilised from renewable sources can be increased by effective Thermal Energy Storage (TES). In a domestic environment thermal energy is typically required when the energy supplied from renewable sources is low (i.e. thermal energy demand is high in the winter and low in the summer), interseasonal Thermochemical Energy Storage (TCES) offers a solution to this problem. TCES has the ability to store thermal energy from the summer months within chemical bonds and release the stored heat when required with heat losses of only around 15%. Magnesium sulphate heptahydrate (MgSO4.7H2O) has the potential to store 2.8GJ/m3 of energy, is a low cost, non-toxic, safe material that can be dehydrated to MgSO4.0.1H2O (fully charged) at 150˚C making it suitable for domestic integration. Research has shown that when MgSO4 is used for TCES it suffers from problematic issues such as agglomeration. However, more research is needed to understand the characteristics of MgSO4 further, develop high energy density TCES materials containing MgSO4 and to understand if MgSO4 should be used within a domestic interseasonal TCES system on a large scale, which is the aim of this research. Throughout this research several thermal analysis devices were used to characterise TCES materials. The devices used include a Differential Scanning Calorimeter (DSC), Thermogravimetric Analyser (TGA), Residual Gas Analyser (RGA), Scanning Electron Microscope (SEM), Energy Dispersive X-ray Spectrometer (EDX) and several custom built laboratory experimental rigs. MgSO4 and novel composite materials containing MgSO4 were characterised at a small (10mg) scale to investigate their energy density and what impact the dehydration heating rate and particle size had on the charging of MgSO4. The results have shown that MgSO4 has a dehydration enthalpy of 1118J/g that is not impacted by the heating rate used. MgSO4 was dehydrated and hydrated at a larger (100g+) scale and it was shown to have significant agglomeration and permeability problems. To solve the problems caused when using pure MgSO4 a novel sample preparation method to create TCES pellets from powdered materials was developed. The novel pellet preparation methodology was optimised and the results showed that the initial dehydration heating rate and preparation methodology used (mix or impregnation) did have an impact on the TCES potential of the synthesised pellets. A high energy density novel TCES material (ZMK) was synthesised and experimentally tested. The ZMK had a dehydration enthalpy of 715J/g and a performance of 85%. The dehydration enthalpy and TCES performance of 13X absorbent pellets were improved through an ion exchange process. A large scale (40kg) modular TCES experimental test rig was designed and built to test TCES materials at a larger scale for experimental investigations into optimisation of efficient charge and discharge cycles. This research shows the future potential of interseasonal domestic TCES through experimental results. Novel composite energy dense TCES materials containing MgSO4 have been shown to have potential for larger scale testing. Future work is required to optimise the novel ZMK material developed and also test TCES materials at a larger scale to understand the associated scaling losses of TCES materials and understand better their role in future domestic TES systems.