Discussion

PHES requires the following elements: two low cost (usually steel) tanks filled with mineral particulate (gravel-sized particles of crushed rock) and a means of efficiently compressing and expanding gas. A closed circuit filled with the working gas connects the two stores, the compressor and the expander. A monatomic gas such as argon is ideal as the working gas as it heat/cools much more than air for the same pressure increase/drop – this in turn significantly reduces the storage cost. 

The process proceeds as follows: the argon, at ambient pressure and temperature (top left limb of the circuit on the diagram), enters the compressor (diagram shows a rotating compressor symbol – all equipment is in fact reciprocating). The compressor is driven by a motor/ generator (top) using the electricity that needs to be stored (yellow arrows at top). The argon is compressed to 12 bar, +500°C. It enters the top of the hot storage vessel and flows slowly (typically less than 0.3m/s) through the particulate, heating the particulate and cooling the gas. As the particulate heats up, a hot front moves down the tank (at approximately 1m/hour). At the bottom of the tank, the argon exits, still at nearly 12 bar but now at ambient temperature. It then enters the expander (bottom) and is expanded back to ambient pressure, cooling to minus -160°C. The argon then enters the bottom of the cold vessel and flows slowly up, cooling the particulate and itself being warmed. It leaves the top of the tank back at ambient pressure and temperature. 

To recover the power (i.e. discharge), the gas flow (and all arrows on the diagram) is simply reversed. Argon at ambient temperature and pressure enters the cold tank and flows slowly down through it, warming the particulate and itself becoming cold. It leaves the bottom of the tank at -160°C and enters the compressor. It is compressed to 12 bar, heating back up to ambient temperature. It then enters the bottom of the hot tank. It flows up, cooling the particulate and itself being warmed to +500°C. The hot pressurized gas then enters the expander where it gives up its energy producing work, which drives the motor/generator. The expected AC to AC round trip efficiency is 75-80%.

Conclusion

PHES can address markets that require response times in the region of minutes upwards. The system uses gravel as the storage medium, so it offers a very low cost storage solution. There are no potential supply constraints on any of the materials used in this system. Plant size is expected to be in the range of 2-5 MW per unit. Grouping of units can provide GW-sized installations. This covers all markets currently addressed by pumped hydro and a number of others that are suitable for local distribution, for example, voltage support. Technology is in development stage and commercial systems are due in 2014.

Executive Summary

Liquid Air Energy Storage (LAES), also referred to as Cryogenic Energy Storage (CES), is a long duration, large scale energy storage technology that can be located at the point of demand. The working fluid is liquefied air or liquid nitrogen (~78% of air). LAES systems share performance characteristics with pumped hydro and can harness industrial low-grade waste heat/waste cold from co-located processes. Size extends from around 5MW to 100s+MWs and, with capacity and energy being de-coupled, the systems are very well suited to long duration applications. 

Discussion

Although novel at a system level, the LAES process uses components and sub-systems that are mature technologies available from major OEMs. The technology draws heavily on established processes from the power generation and industrial gas sectors, with known costs, performance and life cycles all ensuring a low technology risk.

LAES involves three core processes:

• Stage 1. Charging the system
  The charging system is an air liquefier, which uses electrical energy to draw air from the surrounding environment, clean it and then cool the air to subzero temperatures until the air liquefies. 700 litres of ambient air become 1 litre of liquid air.
• Stage 2. Energy Store
  The liquid air is stored in an insulated tank at low pressure, which functions as the energy store. This equipment is already globally deployed for bulk storage of liquid nitrogen, oxygen and LNG. The tanks used within industry have the potential to hold GWh of stored energy.
• Stage 3. Power Recovery
  When power is required, liquid air is drawn from the tank(s) and pumped to high pressure. The air is evaporated and superheated to ambient temperature. This produces a high-pressure gas, which is then used to drive a turbine.

Raising efficiencies:

Cold Recycle – During stage 3, very cold air is exhausted and captured by a proprietary high-grade cold store. This is used at a later time during the liquefaction process to enhance the efficiency. Alternatively, the system can integrate waste cold from industrial processes such as LNG terminals.

Thermal store – The low boiling point of liquefied air means the round trip efficiencyof the system can be improved with the introduction of above ambient heat. Highview Power Storage’s standard LAES system captures and stores heat produced during the liquefaction process (stage 1) and integrates this heat to the power recovery process (stage 3). The system can also integrate waste heat from industrial processes, such as thermal power generation or steel mills, at stage 3, recovering additional energy.

Take a virtual tour of Highview Power Storage’s 350KW/2.5MWh pilot plant

Conclusion

LAES plants can provide large-scale, long-duration energy storage, with 100s of MWs output. LAES systems can use industrial waste heat/cold from applications such as thermal generation plants, steel mills and LNG terminals to improve system efficiency. LAES uses existing and mature components with proven long-life times (30 years +), performance, and O&M costs.