ATES & BTES: Geothermal Energy Storage

There are some very convincing sales people currently in the geothermal market and their message sounds almost too good to be true.

There are some foreign firms who are joining forces with US companies that specialize in ground energy storage. One of the methods they are discussing is ATES which stands for “aquifer thermal energy storage”. This is what is commonly called a withdraw-recharge geothermal system where well water is pulled from some wells and returned to others – then reversed seasonally. This is a great idea except for the fact that you need 400+ feet of free draining sand and gravel under your site. In the Netherlands and coastal plains of New Jersey and possibly other coastal regions this might be feasible but as a general rule these geologic conditions are fairly rare in the US.

They also are discussing BTES, which is borehole thermal energy storage. This type of system uses geothermal closed loops to store energy at high temperature. These systems are designed for high temperature applications where waste heat from some process such as combined heat and power, industrial waste heat, or solar thermal is directed to the center of the geothermal well field to deliberately heat the ground up and then mined later for winter heating with heat pumps. For BTES to work cost effectively it is necessary to have a very hot water source that is essential free. When this heat source is later mined with a heat pump the efficiencies can be very high, in the COP of 6 range compared to regular ground source of 3.5 or so. These sorts of elevated efficiencies are only seen when you have access to very high temperature water – not normal building heating/cooling discharge temperatures from standard geothermal equipment. The high temperature system COP’s cannot be attributed to the lower temperature, normal system designs.

There is some confusion between these two technologies and the traditional ground source heating and cooling applications due to the fact that one relies on very rare ground conditions and the other relies on high temperature ‘heat mining”, which is not normally available to standard ground source geothermal applications on college campuses.

Some of the proponents of these technologies assert that with the same number of wells as a standard ground source project the efficiencies can be doubled by simply seasonally reversing the flows. This is simply not true.