System is based on a heat pump—a device like an air-conditioner that transfers heat from one place to another. In his case, though, the device is reversible, and when the heat flows back it works like a heat engine, converting thermal energy to mechanical power like a car engine.
Isentropic, a company Mr Howes co-founded with two colleagues, James Macnaghten and Mark Wagner, has developed several prototypes of what the trio call pumped-heat electricity storage (PHES). The firm is now completing a demonstration unit with an output of 1.5 megawatts and a storage capacity of six megawatt-hour (which means that, at full power, it can provide four hours of electricity). This will be used for further testing at its factory near Fareham, on Britain’s south coast.
The PHES system consists of two silos each filled with crushed rock, such as gravel. The silos are connected top and bottom by a series of pipes filled with argon, an inert gas. When electricity is in surplus, it is used to power a motor that operates a compressor to squeeze the argon. Doing work to compress a gas heats it, and in this case the argon reaches 500°C. As it is pumped through the first silo it transfers its heat to the gravel. The cooler gas that emerges is then expanded, which lowers its temperature to -150°C. When it is pumped through the second silo it cools the gravel therein. The resulting difference between the silos can be used to generate electricity by reversing the process. Gas heated by the hot silo flows to the cold one, driving the cylinders of what had been the pump to turn what had been the motor as a generator.
Via Microgen Concepts