Pumped Heat Electrical Storage (PHES)
In Pumped Heat Electrical Storage (PHES), electricity is used to drive a storage engine connected to two large thermal stores. To store electricity, the electrical energy drives a heat pump, which pumps heat from the “cold store” to the “hot store” (similar to the operation of a refrigerator). To recover the energy, the heat pump is reversed to become a heat engine. The engine takes heat from the hot store, delivers waste heat to the cold store, and produces mechanical work. When recovering electricity the heat engine drives a generator.
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%.
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.