Phase Changing Baubles?

Arriving on site just in time for Christmas, these spheres look a bit like baubles waiting to be attached to a tree but they are a much rarer sight and inspire a greater curiosity. They are thousands of spheres filled with an advanced phase change material.

A few quick interesting observations on Phase Change Material (PCM):

It can be tuned to store energy at a wide range of temperatures, in this case +10oC.  Storing cooling potential in these spheres improves the generation efficiency by at least 25% compared to the use of ice storage.

The use of the higher storage temperature also requires less glycol and is much kinder to the heat pumps which sometimes struggle to generate water at the -6oC that is required for ice generation, while heat is recovered usefully from the process at temperatures over 45oC.

While this wonder material does have many advantages, designing systems to get the best from it should not be confused with designing ice storage systems as the material is much more complex to work with.

While the storage temperature is 10oC the charge temperature must be at least 3oC below the storage temperature and the temperature of the water released in cooling mode is at least 3oC above to achieve a reasonable heat transfer rate.

PCM produces a nonlinear relationship between both input and output duties and time. This dynamic performance requires a novel approach to both design and control. As the cylinders change state and their content solidifies, it becomes an insulator, dramatically lowering the rate of heat transfer when compared with ice storage (Ice stores have larger contact areas and ice is a better conductor).  Similarly the discharge profile is non-linear. Due to this effect there is always a proportion of the store’s capacity that cannot be practically used. An effect that doesn’t occur to the same extent with ice.

The rate of heat transfer from store is a function of the surface area to volume ratio of the storage capsules (Which is in turn a function of the diameter of the cylinders in this case). The casing is typically dimensioned to provide a release of energy over 8 to 10 hours which means that the solution cannot easily be used to address large, short term demands and is more suited to steady loads. The peak output is also a function of the total kWh storage capacity which is a property that would not necessarily be expected intuitively. There is a trade-off to be achieved between meeting peak capacities and avoiding over sizing of the total storage capacity.

The latent heat capacity of PCM, while impressive, is notably lower than that of ice and given the space inefficiency of spheres (and the space for expansion of the material that can clearly be seen in the image above) there is a larger storage volume required.  The material also has a limited (but acceptable) lifespan that is equivalent to 20 years of cycling. Unlike ice, care must be taken not to accidentally introduce partial cycling due to the limited cycle capacity.  The material is easily replaceable when it has expired but given its current cost it is prudent to get the most from it.

The cost is a function of the relative infancy of the technology and like all emerging technologies, the cost is a function of supply and demand rather than any fundamental difficulty in producing the material so we can expect dramatic reductions in price as its popularity grows. It takes pioneering companies (like the ESB in this case) to invest in both the use of the material but also to provide a forum for the research and development of understanding of how it can best be used.