Renewable energy is on the rise while utility providers are required to either reduce carbon dioxide emissions or to increase their percentage of renewable energy sources. One of the major issues hindering this rise is to overcome the intermittent nature of renewable energy such as solar and wind, as the sun is not shinning at night and wind tends to be strongest during night. Mechanisms to store surplus energy to meet the continuous demand throughout the day and to meet the peak load demands are critical for a robust and vibrant renewable energy development. Popular methods to achieve this are Pumped-Storage Hydropower (PSH) system and Compressed-Air Energy Storage (CAES). A new method, Pumped-Heat Electricity Storage (PHES) system, is catching on to further the energy storage technologies in helping mitigate the global warming.
Pumped-Storage Hydropower (PSH) utilizes water and gravity and its center piece is two reservoirs at different height. Water from the lower reservoir is pumped to the higher one using off-peak electricity. It is released to the lower reservoir to turn gravitational energy back to electricity by spinning the turbine of generator. This kind of traditional PSH is not easy to find a suitable location for and it is hard to increase capacity. New types of PSH are emerging. A Denmark company, Gottlieb Paludan, devised Green Power Island concept—an artificial island with wind turbines and a deep central reservoir: wind power pumps water out of the reservoir and when power is needed, seawater flows into the reservoir via turbines to generate power. A California based company, Gravity Power, devised a subterranean hydropower concept—a U shaped tube built underground with a piston inside: the piston is pushed up with surplus power and when power is needed, the piston sinks down to force water through a generator. Another California company, Advanced Rail Energy Storage, utilizes modified railway cars and specially built track: off-peak electricity is used to pull the car uphill and when energy is needed, the car is released and its motion powers a generator. This requires certain topography but it is more efficient than the traditional PSH.
According to the Electric Power Research Institute (EPRI) which is the research arm of US power utilities, 99% of bulk storage capacity worldwide uses PSH. The distant second is Compressed-Air Energy Storage (CAES). This process compresses air and stores it in large repositories, such as underground salt caverns. The air is released to drive a turbine during peak hours for electricity generation. This method is ineffective because the heat generated during compression is lost during its expansion, therefore the air must be reheated before expansion. There are a few improvement ideas out there—focusing on capturing the heat during compression for later reheating—with small scale demonstration factories already. The third kind is converting surplus energy to heat for storage.
Now a brand new method of energy storage is gaining momentum. The innovation in the energy storage technologies shows this industry is blooming and promising, which is essential to help mitigate global warming.
The Pumped-Heat Electricity Storage (PHES) system is based on a heat pump, transferring heat from one place to another. But it is reversible and when the heat flows back, it converts thermal energy to mechanical power like a car engine. It has an efficiency of 72 to 80 percent, similar to the 74 percent of PHS. The advantage of PHES is it does not require the geographical condition pumped-hydro needed. The system is designed by Isentropic, a Britain-based company which is now completing a demonstration unit, with an output of 1.5 megawatts and a storage capacity of six megawatt-hours.
The PHES system is consist of two silos each filled with gravel. The silos are connected top and bottom by a series of pipes filled with argon, an inert gas. Surplus electricity is used to compress the argon, heating it to 500 degree Celsius. The hot gas is then pumped through the first silo, heating the gravel and becoming cooler. The cool gas out of the first soli is then expanded and its temperature is lowered to -150 degree Celsius. The cool gas is then pumped through the second silo, cooling the gravel there. The resulting difference between the silos can be used to generate electricity if the process is reversed.
With the lowering cost and increasing efficiency, solar energy is already the cheapest form of electricity is most sunny climates. Solar will lead the renewable sectors to thrive in the battle against global warming. Department of Energy sets a goal for solar energy to provide 27 percent of the electricity by 2050, from less than one percent now. Wind energy has seen significant development in several states in recent years. Efficient grid-scale storage is a critical for the continuous investment and advancement in the development of these forms of renewable energy. The performance of PHES in its demonstration plant and further tests is exciting to watch as this new energy storage technology can be the best method so far in helping mitigate the global warming.
By Tina Zhang