Energy Storage Enables Just-in-Time Generation

Pumped Heat Electricity Storage

A potential electricity storage game-changer is United Kingdom startup Isentropic Energy’s proposed Pumped Heat Electricity Storage (PHES) system. Unlike conventional pumped hydro energy storage plants, the PHES system uses a heat pump to store electricity in thermal form or as “pumped heat.”

Isentropic Energy’s chief technology officer, Jonathan Howes, describes how the system operates: “The Isentropic PHES system utilizes a highly reversible heat engine/heat pump to pump heat between two insulated storage vessels containing gravel. A gas is employed in this process from which work is extracted. At first, the compressed gas is made to pass through one of the stores. This heat[s] up the gravel up to 500 degree[s] Celsius and during heat exchange, the temperature of the gas almost drops to ambient condition. In the second step, this gas is made to expand to its original pressure and as a result the temperature drops to –160 degree[s] Celsius. It is then passed through the other store and it exchanges heat with the gravel by direct contact. In this process, the gas is warmed back to its original temperature. The energy supply is mainly by employing a motor as it not only supplies electrical power to it but also acts as an energy storage element. The cycle is reversed to release the energy. As the energy passes from hot to cold, it powers a generator from which electricity is produced.” Figure 5 illustrates the concepts.

5. Cold and hot storage. Isentropic Energy has proposed the simplest of energy storage systems. Two large containers of gravel, one hot (500C) and one cold (–160C) provide the temperature difference to operate a heat pump, based on the Ericsson Cycle. Surplus electricity is used to produce and maintain the temperature in the high-temperature tank by running the heat pump in reverse during times when surplus wind power is available. Source: Isentropic Energy

The simplicity of the design is extremely attractive. In fact, Howes claims the installed cost of energy storage using the PHES system is currently $55/kWh, dropping to perhaps $10/kWh with utility-scale systems. The overall operating efficiency of the energy storage process is claimed to be 72% to 80%.

The advantages of the system are high power density, being geographically unconstrained, and the fact that the heat pump operates on air, using no refrigerants, chemicals, or water.

The first pilot PHES is targeted at 16 MWh, or 2 MW produced for eight hours.The system can be scaled as required. This pilot system would require a footprint of about 8 m x 16 m x 7 m high. Design of the utility storage prototype is under way and is expected to require about two to two and a half years, according to Howes, but it will require an investment of about $16 million.

Distributed Compressed Air Storage

Professor Seamus Garvey of Nottingham University has proposed combining floating offshore wind turbines and compressed air storage as a cost-effective way to store excess electricity produced by the turbines. A new spin-off company, NIMROD Energy Ltd., has been formed to commercialize the Integrated Compressed Air Renewable Energy System (ICARES) that has been under development since 2006.

The process is simple in concept but requires a new, much larger, and less-expensive fleet of offshore wind turbines to become economic. Wind-produced electricity is used to compress air that is stored in balloons or “Energy Bags” located on the seabed or in geologic formations when deep water is not available. A set of bags is connected to a common compressed air pipe that runs along the seabed. When peaking electricity is required, the compressed air is released from the bags and expanded through a conventional air turbine to produce electricity. During off-peak hours, the energy bags would again be inflated and made ready to repeat the cycle.

The energy bags are to be constructed of a thin polyethylene material that can handle only a 0.4 atmosphere pressure difference. However, in this application the pressure difference is nil because the compressed air in the bag and the external water pressure are in equilibrium. Garvey says that the overall efficiency of the system is about 90%. The design, supported by E.ON, has been successfully tested in laboratory water test tanks. Further prototype testing in seawater is scheduled to begin in May 2011 and is expected to lead into development of a commercial product in the near future.

Thinking big, Garvey says that storing the equivalent of 2 GW for four days will require 7 million cubic meters of air storage. “The optimal dimensions for energy bags are around 20 meters in diameter and each has a volume (when full) of about 4,000 cubic meters,” he says. “For 7 million cubic meters, we would need 1,750 of these bags. The seabed area covered by these would be less than one square kilometer and the total surface area of bag material would be 2.2 million square meters.”

Left unsaid was the how much energy is required to fill the bags in relation to the number of floating wind turbines. Using Garvey’s numbers, it would take about 230 10-MW wind turbines four days to fill the bags, and actually more than twice that time when a reasonable capacity factor for the wind turbines is applied. Garvey is obviously thinking of offshore wind-produced electricity on a much larger scale than is now under development.

Garvey believes that it’s possible to deploy this utility-scale system at a cost well below $16/kWh; that, he says, is less than 20% of the cost for pumped hydro energy. That number also assumes that offshore wind turbines can be scaled up to even larger sizes and that more radical designs can be used to reduce installed costs, by up to a factor of four, by reducing the amount of structural material required per kilowatt of rated power, according to Garvey.

“I foresee that at least 25 per cent of offshore wind power in the UK will use this integrated compressed air approach by 2025,” he says. “Although I expect that the direct-generating wind turbines will catch up with us on cost per unit power output, the role for systems which put energy directly into store is clear. If you have 1 MW of integrated compressed air system (including large energy stores) for every 3 MW of conventional generation, then the whole set of offshore wind equipment starts to look like a very versatile power generating system which can adjust its output to match demand—notwithstanding what the wind is doing.”

Final Thoughts

Each of these technologies is championed primarily by independent entrepreneurs, venture capital firms, many private investors, and several utilities. In contrast, the latest DOE budget request asks for only $34 million for a new Energy Innovation Hub that would focus on “battery and energy storage.” Electricity storage may not be a DOE priority, but commercial utility-scale electricity storage has significant potential, in the short term, to completely reinvent the power generation industry.