The case for using phase-change materials (PCMs) for thermal energy storage is being bolstered through research using a modelling technique out this year.
The technique, developed by a team in Catalonia, Spain, has uncovered a PCM tank approach that could theoretically allow up to 74% of stored thermal energy to be returned from storage.
It might also reduce the amount of storage material needed in solar thermal plants by 32% compared to traditional two-tank molten salt systems, CSP Today reports.
The approach, called multi-layered solid PCM (MLSPCM), works by using two PCM layers separated vertically by a layer of filler materials such as cheap, readily available granite or quartzite.
The filler keeps the top and bottom PCM layers close to their optimum discharging and charging temperatures, respectively. Overall, the amount of PCM material needed would be much less than in a PCM-only setup, which should help reduce costs.
And since each PCM layer is kept close to its optimum charge or discharge temperature, the efficiency of the system is increased.
Latent thermal energy storage
The research, unveiled last year at the Solar Power And Chemical Energy Systems annual conference but only now brought to mainstream attention, was carried out using a latent thermal energy storage modelling system due for launch this year.
The system is being commercialised by TermoFluids, a spinoff of the Heat and Mass Transfer Technological Centre of the Polytechnic University of Catalonia (Universitat Politècnica de Catalunya or UPC in Catalan).
It is one of a number of solar thermal planning tools developed by Tesconsol, a project consortium made up of UPC with Sweden’s KTH Royal Institute of Technology, Total, Gas Natural Fenosa and Tecnalia, with backing from KIC InnoEnergy.
“The idea is to combine PCMs with thermoclines, which traditionally use filler materials such as rocks or sand,” says UPC’s Prof Carlos-David Pérez-Segarra.
“There are already proposals for using cascade PCM with encapsulated material throughout the tank. What we’ve done is to study using a conventional filler material for most of the tank with encapsulated PCM layers at the top and bottom.”
Although the researchers are confident the MLSPCM technique will work in practice, it is important to note that its predicted performance is based on mathematical models rather than field trials.
Prompting developers to embrace PCM
Furthermore, it remains to be seen whether the discovery will be enough to prompt plant developers to embrace PCM.
As previously reported, there has so far been little appetite for the technology among solar thermal companies, even though PCM storage developers are convinced the technology could help cut costs by up to 40% compared to current molten salt systems.
There is much at stake here.
The current generation of solar thermal plants, including Crescent Dunes in the US and Khi Solar One in South Africa, can claim an advantage over other renewable energy sources because they come with storage and can provide dispatchable power.
However, there is growing concern in the solar thermal sector over the prospect of lower-cost storage helping cheaper renewable energy sources, such as PV or wind, overcome the challenge of intermittency to offer dispatchable power too.
The main threat is from PV, which is already being widely sold along with battery power in the residential and commercial market.
Wind developers focusing on storage
Interestingly, Siemens is working on both batteries, in conjunction with Aquion Energy, and thermal energy storage. For the time being, these developments do not appear to pose a serious competitive threat to the solar thermal industry.
However, that situation will not last forever.
Plus a potential problem for solar thermal developers is that there is little they can do to control the cost of their current favoured storage medium, molten salt, which is largely priced according to the vagaries of industrial raw materials markets.
A move to PCM might help solar thermal and along the way demonstrate more fully the value of energy storage in general. But it probably needs to happen soon.
And with demand for new solar thermal plants already drying up in key markets such as Spain and the US, there are fewer and fewer opportunities for developers to experiment with such a new and untested storage technology.
Written by Jason Deign
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