By Jason Deign
The Norwegian firm EnergyNest is expected to announce the outcome of a pilot of its concrete-based thermal energy storage (TES) product soon, potentially within days.
In September the company’s CEO, Christian Thiel, told SmartGridToday that the company was due to complete tests of the product, Heatcrete, “by next month.”
The company’s website, meanwhile, says a “first-of-its-kind” solid-state TES pilot at Masdar Institute’s Solar Platform in Abu Dhabi will deliver full validation of EnergyNest’s “potentially game-changing technology” at some point in 2015.
The Abu Dhabi pilot is part of a joint research project that has been underway since 2013. The 500kWh pilot itself began in May.
“The project will effectively demonstrate the operational and economic feasibility of concrete-based energy storage vis-à-vis other TES systems currently in the market,” says EnergyNest.
“EnergyNest and Masdar Institute aim to prove that incorporating this new technology into commercial solar thermal projects allows project developers to derive significant benefits and savings.”
A special concrete formula
For all EnergyNest’s marketroid-friendly product talk, Heatcrete essentially appears to be based on a special concrete formula developed in association with HeidelbergCement Group, a German building materials giant.
So far, so humdrum. But the fact that concrete is a ubiquitous material might be the ace up the sleeve for EnergyNest and other companies pursuing similar forms of TES.
In terms of supply chain, handling and construction, few materials are likely to be as cost effective, easy to obtain and simple to use.
Concrete has decent thermal properties and is less likely than loose TES substrates, such as gravel, to suffer from thermal ratcheting.
This is where the substrate packs itself into the bottom of the storage tank after repeated heating and cooling cycles cause the material to expand and contract.
TES costs with concrete
Research published in 2013 concluded TES costs with concrete could be in the range of USD$0.88 to $1/kWhth, compared to $30/kWhth quoted for other media.
The fact that it does not rely on complex manufacturing also means concrete might be expected to have a higher energy return on investment than most other forms of storage, although it still has a significant carbon footprint.
And unlike the molten salt currently used for large-scale concentrated solar power TES, because concrete is needed universally for construction its capital cost is unlikely to vary widely over time.
In essence, there only appear to be a couple of reasons why concrete should not be used more widely for TES… and none of them are very convincing. One is that it loses water when heated, which can lead to cracking.
However, work by the German Aerospace Centre (Deutsches Zentrum für Luft- und Raumfahrt) suggests selecting a basalt concrete mix and adding steel needles and reinforcement to the TES can overcome this problem.
The same temperature as molten salt
Another possible argument against concrete TES is that it might not be able to reach the same operational temperature as molten salt.
As it stands, though, special mixes of concrete have been shown to work at more than 600ºC, while research to develop molten salt systems that operate in excess of 565ºC is only just getting underway.
Concrete could potentially have plenty of TES applications even without reaching these super-hot temperatures, however.
“Those 550ºC solar farms like Ivanpah, Nevada, can’t use the heat they produce,” he said. “They are so large in array area that they must be out in the desert, far from a heat user.
“Meanwhile our microgrid module arrays can be sited next to heat users. Since we are within 100m of heat users, the heat we collect is useful. And that heat is valuable for space heating, process heat, desalination, plus chilling.”
Better transfer of heat
Buckley said gravel beds can be used in the same way but “we prefer concrete because once installed it can be its own support and there is better transfer of heat to and from the pipes embedded in it.
“Either way, the costs of storage are very low. We estimate a day’s storage for each collector block costs $50 of concrete. Our storage is $5/kWh compared to Elon Musk batteries at $200/kWh.”
In principle, concrete is already being used widely for low-cost, low-temperature TES as a common component of storage heaters.
To prove whether it can be usefully employed at much larger scale and higher temperatures, said the authors of the 2013 study: “Long-term testing of concrete is required to validate its use.”
It is possible EnergyNest’s pilot will provide some answers. The results will be awaited with interest.