Solving the stability issue of heat-storage materials to reduce fuel poverty
09 Dec 2022




Domestic thermostat

​​​​​​​​​​​​​​​​​​Photo of domestic thermostat​​​

Credit: Pixabay

​This case study is part of a set reflecting the impact of ISIS science reported as part of the UK Research Excellence Framework (REF 2021).​​​​​

REF Case Study: New phase-change materials enable commercialisation of energy-efficient, sustainable and cost-effective domestic heat storage, resulting in robust company growth and a reduction in fuel poverty
Principal Investigator: Prof Colin Pulham
Institution: University of Edinburgh
Funding: EPSRC, Innovate UK, Advanced Propulsion Centre, Energy Technology Partnership (ETP)
Company involved: Sunamp Ltd​

The Challenge

Heat-storage technology has a key role to play in increasing both energy efficiency and the use of renewables in the heating and cooling sector. Heating and cooling contribute almost 40% of global energy-related carbon emissions. The core component of heat-battery technology is a phase change material (PCM) that absorbs heat on melting and releases it on freezing.

Salt hydrates are leading candidates for use as PCMs due to their high energy density, plentiful supply, low cost, safe use and sustainability. In particular, sodium acetate trihydrate (SAT) provides heat release at an ideal temperature for domestic use. However, SAT suffered from two key issues; non-reproducible performance and poor long-term stability, preventing its practical use as a heat store.

Evidence of Impact

UK-based company Sunamp Ltd began developing heat battery technology in 2006, but found that the failure of SAT prevented progress. The company made contact with Professor Colin Pulham at the University of Edinburgh, who brought significant expertise in the science of crystallisation, built up from years of experience using ISIS Neutron and Muon Source and the neighbouring Diamond Light Source, to understand why these failures occurred.

Collaborative research led by Prof Pulham for Sunamp developed polymer-based additives that at low concentrations act as crystal-habit modifiers, suppressing the formation of the solid anhydrous sodium acetate. As a result, the new SAT-based material withstands repeated melting and freezing cycles without degradation, while maintaining both its energy density and the desired melting point of 58°C. In situ X-ray powder diffraction experiments at the Diamond Light Source were used to interrogate, in real time, the structural and chemical behaviour of these new SAT formulations during repeated temperature cycling, and confirmed complete inhibition of the precipitation of anhydrous sodium acetate over a wide range of conditions.

The role of ISIS

Experiments carried out on the high pressure instrument Pearl provided Professor Pulham and his team with fundamental insights into the behaviour of polymorphs and solvates at high pressures, and particularly how salt hydrates crystallise. Professor Pulham says, “My longstanding collaborations with staff at ISIS, combined with the use of world-leading instrumentation, have been of enormous benefit to many of our research programmes. These studies have enhanced our understanding of crystallisation processes for a range of technologically important materials, especially under extreme conditions of pressure and temperature.”       

ISIS Instrument(s): Pearl ​             
ISIS Technique(s): High-pressure neutron diffraction


I.D.H. Oswald, A. Hamilton, C. Hall, W.G. Marshall, T.J. Prior and C.R. Pulham, “In-situ characterization of elusive salt hydrates - the crystal structures of the heptahydrate and octahydrate of sodium sulfate", J. Am. Chem. Soc., 2008, 130, 17795-17800. DOI: 10.1021/ja805429m.

I.D.H. Oswald, I. Chataigner, S. Elphick, F.P.A. Fabbiani, A.R. Lennie, J. Maddaluno, W.G. Marshall, T.J. Prior, C.R. Pulham and R.I. Smith, “Putting pressure on elusive polymorphs and solvates", CrystEngComm, 2009, 11, 359-366. DOI: 10.1039/B814471K.

Contact: Fletcher, Sara (STFC,RAL,BID)