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Is a Heat Pump and Gravel the Answer to Energy Storage for the Grid?

isentropic2Some energy storage technologies for the power grid are expensive but can be deployed anywhere, like advanced batteries, while others are cheap but can only be built in certain locations, like pumping water up and down hilly terrain (known as “pumped hydro”). But a group of English engineers have built an advanced heat pump and connected it to an energy storage system using two silos full of plain old gravel that they say is as cheap as pumped hydro, as location-agnostic as a battery — and is super efficient. The startup they founded two years ago called Isentropic, named after a reversible process in thermodynamics, is now looking for a Series B round of $5 million and will be showing off its technology at the Energy Storage Association conference next week in Washington, D.C.


Founders and engineers Jon Howes and James Macnaghten developed the design of the heat pump a decade ago, and a couple years ago brought on Mark Wagner as chairman to help with business direction. Heat pumps are basically engines that can work in reverse; Isentropic’s device is indeed reversible, both storing and releasing energy when needed. Wagner told us in a phone interview that the key to the company’s heat pump is that it can be reversed extremely efficiently, and has an isentropic efficiency (reversible efficiency) of 99 percent.

Using the heat pump as the key, the team built an energy storage system that compresses argon gas to produce a temperature differential and deposits heat and cold into two separate large silos of gravel. Wagner says any inert substance with particles around the same size as gravel can also be used — the filling just needs to hold the heat and cold well. Energy is stored in the gravel and when the process is reversed, it can be released.

Wagner says the low cost of all the basic ingredients — the heat pump, gravel and gas — make the system among the cheapest energy storage options out there, undercutting options like batteries, which often require costly materials. Wagner says the system costs between a half and a third of that of implementing flow batteries on the grid. The system is relatively quick to set up, too, says Wagner, compared to a technology like compressed air, which has a years-long regulatory process to approve the underground caverns where compressed air needs to be stored. It’s almost plug-and-play, says Wagner — as a result he says that wind turbine companies have approached the team to see if their storage device could be installed for each wind farm, enabling wind developers to help combat the variability of wind (the wind doesn’t always blow when energy is needed).

Another benefit, says Wagner, is that the system could be used underground and underwater, which could be useful when it comes to providing storage for offshore wind deployments. Danish utility DONG Energy, German energy company E.ON and Abu Dhabi’s clean energy initiative MasdarWagner are building the world’s largest offshore wind project called the London Array. Wagner says the company will soon be starting a study using its technology for the offshore wind plants that are being developed in the UK.

For now, Isentropic’s energy storage benefits are still just company intentions. It’s been building out two demonstrations of its technology and is looking for $5 million to help construct a larger, commercial-scale demo project. Last year Isentropic raised a Series A round from Credit Suisse Securities Europe and won a £250,000 ($380,112) research grant from The Carbon Trust; Wagner says so far the company has raised about $1 million. Next week at the Energy Storage Association conference in Washington, D.C. Isentropic will be discussing its technology and showing off its designs. It’s a good time to jump into the U.S. energy storage market, which, as we pointed out this week, is just starting to get attention from Congress, investors and entrepreneurs.

7 Responses to “Is a Heat Pump and Gravel the Answer to Energy Storage for the Grid?”

  1. From Isentropic’s website:

    “The round trip efficiency is over 70%.”

    I am assuming they are now talking about thermal efficiency. The website also states:

    “the machine works both as an engine and a heat pump (an engine turns a temperature difference into work, a heat pump turns work into a temperature difference – every fridge has one). High reversibility means that if it first turns electricity into a temperature difference, it can then regenerate most of the electricity from the temperature difference it has created.”

    They further refer to the “Ericsson cycle of 1833” which implies that a Stirling-type external engine is used to power a generator.

    This article would have benefitted from greater technical scrutiny of the claims of Isentropic, which are somewhat misleading. If the technology is being used for storage of electrical energy, then only the overall power out vs. power in is relevant, regardless of how efficient internal components may operate.

  2. The claim of 99% energy efficiency is unheard of, particularly where thermodynamic heat transfer is involved. Think of all the possible losses from all the components that would have to be insulated with vacuum-like isolation If it can be proven, it is truly amazing. My gut feeling is that this number reflects only some core aspect, rather than effective power out vs. power in.

    For comparison, an isolation transformer is typically only 98% efficient, due to capacative losses, but no moving parts.

  3. Jeanne

    Could this heat pump and gravel technology be used here? This is an abandon power plant in Morro Bay. The smoke stakes seem like it would work well and it is right on the ocean which seems like it would also work well with the hydro idea.

    Thank you.