Pumped Heat Electrical Storage (PHES)
Pumped Heat Electrical Storage (PHES) is another scheme for energy storage useful to bridge the gap between availability patterns of renewable energies (like solar, wind, etc), and the human demands for power. In fact, it may be more appropriate to term it as Pumped Heat Energy Storage for two reasons. Firstly, the storage is in the form of heat, and secondly, the scheme need not use electrical input energy only. Even mechanical form of wind or other energy could be used to drive the storage mechanism directly.
In Pumped Heat Electrical Storage (PHES), electric power, from whatever sources, drives a storage engine which is connected between two large heat stores. When we want to store, the input electric energy is supplied to a heat pump. The heat pump works like a water pump. It draws heat energy from a system or a ‘store’ at a lower temperature and feed the energy to another heat storage at a much higher temperature.
When we need to recover the energy, the function of the heat pump is reversed to make it a heat engine. This engine now develops electrical power when it allows heat from the hot store to flow through it to the cold store. Can you think of a water mill working off a water fall? In fact, Pumped Heat Electrical Storage is similar in basic concept to the pumped hydro-electric storage where spare electric power is used to pump water up into a raised water reservoir thus storing energy as the potential energy of water. When needed the raised water is allowed to flow down allowing energy to be extracted.
Typically, argon gas at ambient temperature and pressure is compressed by a pump using the electrical energy which is required to be stored. The gas reaches a pressure of 12 bar and temperature of above 500 C during this compression. It enters the “hot” reservoir and flows through it at reduced speed losing heat to it, cooling itself, but still at about the same pressure. Outside the hot reservoir, the gas is allowed to expand, cooling itself to about 165 C below zero. This cold gas enters the ‘cold’ reservoir, draws energy from it, and travels out to be compressed again.
During energy extraction a reverse process occurs. Gas at ambient temperature and pressure enters the cold reservoir and exits at about 160 C below zero. It is compressed in the same compressor to 12 bar and gets heated to near ambient temperature. It travels through the hot reservoir at 12 bar, and exits at a temperature above 500 C. Its expansion drives a generator to produce electrical power.
Pumped Heat Electrical Storage 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 heats/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, enters the compressor. The compressor is driven by a motor/ generator (top) using the electricity that needs to be stored . 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 is simply reversed. Argon at ambient temperature and pressure enters the cold tank and flows slowly down through it. It warms the particulate and itself becoming cold. Then, it leaves the bottom of the tank at -160°C and enters the compressor.After that, 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%.
Pumped Heat Electrical Storage 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.