Renewables such as wind and solar generate power intermittently. Once they become a significant part of the total generation mix, storage will be required to manage their impact of the grid. Today storage is used to balance supply and demand and reduce the need to use inefficient power plants to meet peak demand.
Pumped hydro accounts for 99% of all large-scale electricity storage. Pumping water from a lower to a higher elevation is to date the most practical way to store large amounts of energy that can then be released during periods of high, or peak, demand. However, because pumped hydro needs to be built in very large sizes and at specific geographical locations, it is severely limited by a lack of suitable sites.
Japan has about 10% of its generating capacity in pumped hydro storage because it has an abundance of suitable sites close to population centres but North America and Europe only about 2.5%. To quote a recent market research report on storage: “The theoretical potential in energy management applications for a pumped-hydro look alike that does not requie mountains or deep caverns, seems to be of the order of 10-15% of the installed generating capacity. This suggests a ltent opportunity of about 550GW at present (additional to the separate requirement for intermittent renewables), compared to a little over 100GW installed today.”
Isentropic is developing such a system.
Pumped heat electricity storage
Isentropic has designed a system that used the Isentropic heat pump to store electricity in thermal form (“Pumped heat”). The storage comprises two large container of gravel, one hot (500C) and one cold (-150C). Electrical power is input to the machine, which compresses/expands air to (+500C) on the hot side and (-150C) on the cold side. The air is passed through the two piles of gravel where it gives up its heat/cold to the gravel. In order to regenerate the electricity, the cycle is simply reversed. The temperature difference is used to run the Isentropic machine as a heat engine.
The round trip efficiency is over 72%-80%. Because gravel is such a cheap and readily available material, the cost per kWh can be kept very low-$55/kWh-and $10/kWh at scale.
Compared to pumped hydro
Bath Count Pumped Storage, Virginia, USA. Two reservoirs covering 820 surface acres (3.32 sq.km). 30 GWh storage capacity (largest in the world). Pumped Heat Storage Plant of the same capacity would occupy 1/300th of the area.
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