Harnessing energy from natural forces (wind, sun) has proven difficult in part due to the fact that production and consumption of electricity are rarely synchronized, requiring some kind of energy storage system.
A UK company known as Isentropic has developed a technology known as Pumped Heat Electricity Storage for the storage of electrical energy in the form of a temperature difference. The company and idea appear to have been around for a few years, though the concept is not yet commercialized.
How it appears to operate
The core of the system is two large insulated silos or tanks containing common gravel or stone through which a gas such as air can be moved.
Air (or maybe a more inert gas) is compressed inside of one of the tanks using electricity to drive the compressor.
The hot compressed gas heats the gravel in the tank to a high temperature - 550 degrees Celsius is stated in articles on the concept.
Once the gas has given up most of its heat to the gravel, it is allowed to expand into the other tank. The expansion process removes heat from the gravel in that tank, cooling it to a low temperature.
When the time comes to recover the electricity, a compressor moves the gas (the "working gas" ) into the hot tank where it is heated. The gas then flows through the engine (which, in the case of the Isentropic system, is reciprocating) where it produces mechanical work and subsequently electricity.
The engine exhausts the working gas to the cold tank, which increases the Carnot Efficiency. The compressor then recycles the cold working gas back through the hot tank and engine and maybe past a regenerator/heat exchanger for efficiency.
Because the incoming electricity drives a heat pump to provide the thermal gradient, the efficiency of the system can be much higher than the typical heat engine efficiency such as a power plant where fuel is burned and the heat exhausted at atmospheric temperature. Comments on the popular news articles regarding the system noted the work->heat->work conversion inefficiency but assumed that the electricity would be heating the gravel with resistance (heating elements), resulting in an extremely serious round-trip efficiency hit.
Losses
- As the temperature difference between the hot and cold tanks decreases, the efficiency of the electricity-producing engine decreases.
- As the temperature difference between the hot and cold tanks increases, the efficiency of the electricity-consuming compression-expansion (heat pump) cycle decreases.
- Loss of energy results due to imperfections in machinery and work required to move the working fluid through the gravel bed. We don't get this work back upon reversal of the cycle because both the "charging" and "discharging" of the system requires work to move the gas through the gravel beds, pipes, etc.
- Despite the name (Isentropic), it is not possible to have a 100% reversible process, but it may be possible to have a round-trip efficiency similar to that of battery systems which store electricity directly - 70% and higher. The use of a heat pump in combination with a heat engine makes the process as reversible (efficient) as possible.
- There is also heat loss/gain through the tank walls, piping, etc. which can be minimized with good insulation and timely cycling of the system.
Possible Scenario
An electricity storage system would allow the supply of electricity from less-predictable forces of nature to be matched with the demands of society.
AN IDEAL STORAGE SYSTEM MATCHES SUPPLY WITH DEMAND. THE AMOUNT OF ENERGY PRODUCED IS EQUAL TO THAT CONSUMED (LESS INEFFICIENCIES) BUT THE POWER LEVELS ARE DIFFERENT WITH TIME.