Energy Storage Systems

A New Era in Energy Storage

The rapid price decline over the last decade of renewable electricity sources has made them increasingly competitive with conventional sources. In particular, residential solar PV is now often cheaper than electricity from the grid, without subsidy.

Making use of abundant low-carbon energy without sacrificing security or affordability of electricity supply will require a combination of electricity storage and flexible generation.

Isentropic PHES, our flagship offering, caters to the first of these requirements, with a 150 kW/600 kWh prototype currently under construction at our facility.

Our technologies can be used to cater to both requirements at a much larger scale through Isentropic GTI-CAES and Isentropic Flexible Gas. We are looking to form strong partnerships with industry players in order to develop and demonstrate these offerings.


Pumped heat electricity storage (PHES) is our flagship energy storage system. It will be the lowest-cost site-independent method of storing electricity.

How Isentropic PHES Works

The cycle uses a proprietary reciprocating heat pump/engine developed in-house by Isentropic Ltd, powered by electricity from the grid, to charge Isentropic Thermal Stores.

The heat pump compresses argon gas up to 12 bar and 500°C. The gas then passes through crushed rock in a pressure vessel, the “hot store”. This crushed rock is an inexpensive, abundant and safe medium of energy storage.

The argon gas, now at near-ambient temperature, exits the hot store and enters a second heat pump cylinder. In that cylinder, it is expanded down to around 1 bar, which cools it to -160°C. This cold gas enters a second thermal store, the “cold store”, cooling its contents.

The argon gas is now at ambient pressure and ambient temperature, and the cycle recommences.

The cycle stores electrical energy as a temperature difference. When the electricity is needed again, the temperature difference is used to drive the heat pump as a heat engine, driving a generator and producing electricity.

Isentropic Ltd has designed a number of key features in the heat pump/engine and thermal stores that will enable a round-trip efficiency of 70-75% (ac to ac).

The use of natural materials allows for a projected ex-factory cost of $140/kWh.

The components are engineered to give each installation a life of 25 years with no limitation on number of cycles or depth of discharge.

PHES combines the three essential features required of an electricity storage technology: high efficiency, low capital cost and long life.

Isentropic PHES is our Flagship Storage System

Opportunities for PHES

Replacing pumped hydro

Traditional pumped hydro electricity storage has been a requirement of most networks since nuclear power plants were first introduced in the 1960s. PHES is cost competitive with pumped hydro, can be sited optimally to minimise grid infrastructure costs, is much less environmentally sensitive and allows phased deployment of capital.

Accommodating disruptive solar PV

Solar PV is a hugely disruptive technology as it displaces traditional generation and creates significant supply volatility. There is an opportunity for PHES to store excess PV generation and to discharge the electricity during peak demand periods.

Replacing diesel generation

Remote locations without access to a strong electricity grid have traditionally needed diesel generators to provide for their electricity needs. This is a very expensive form of electricity generation, costing up to 40¢/kWh. Solar PV can be combined with diesel generation to reduce the diesel bill: however, due to the intermittency of solar PV, the maximum fuel saving that can be realised without storage is around 10%.

Combining solar PV and PHES in sunny, remote locations offers savings by reducing the fuel consumed by up to 70%. This solution does not sacrifice any security of supply, since the diesel generators are kept for non-sunny periods


Isentropic Gas Turbine Integrated - Compressed Air Energy Storage

GTI-CAES (gas turbine integrated compressed air energy storage) integrates an Isentropic Thermal Store and an underground cavern into a gas power plant to store the energy created by the compression stage of the gas turbine. The requirements both for flexible primary electricity generation and for electricity storage are combined in this technology, since the gas turbine can be used as a conventional power source and for storage charge/discharge.

With a capital cost of only $618/kW for 6 hours of storage ($375/kW power machinery and $40/kWh stored energy) and a round-trip efficiency in excess of 90% this is the cheapest and most efficient form of energy storage. These figures are confirmed in a detailed analysis by engineering consultants Parsons Brinckerhoff. For large scale energy storage it has the potential to revolutionise power generation.

The ultra low cost and high efficiency make this a commercially and technically compelling proposition in markets around the world.

The Company is therefore looking to work with power equipment manufacturers and EPCs to commercialise this offering. Contact us to discuss this further.

Flexible Gas

The capital saving and fast response capability of an open-cycle gas turbine (OCGT) is matched by an efficiency penalty: its exhaust gases are emitted to atmosphere at around 580°C. The more efficient combined cycle gas turbine is much more expensive to build and is less flexible.

Isentropic Flexible Gas uses our thermal storage technology to combine the high efficiency of a combined-cycle gas plant (CCGT) with the low capital cost and flexibility of an open-cycle plant. It does this by storing the waste heat from the OCGT for steady release over a longer period, allowing for a lower total investment with a higher utilisation rate.

Alternatively, a thermal store can be used to supply heat to the steam cycle in a CCGT installation prior to gas turbine start-up. This means that the gas turbine can be ramped to full power at its maximum rate for optimum performance at times of peak market prices, without damaging the components in the steam cycle.

Isentropic Ltd is looking for a suitable partner to bring this offering to market. Contact us to discuss this further.