Seasonal Thermal Store - STES Technologies

STES Technologies

There are several types of STES technology:

• UTES (underground thermal energy storage), in which the storage medium may be geological strata ranging from earth or sand to solid bedrock, or aquifers. UTES technologies include:
  • ATES (aquifer thermal energy storage). An ATES store is composed of a doublet, totaling two or more wells into a deep aquifer that is contained between impermeable geological layers above and below.. One half of the doublet is for water extraction and the other half for reinjection, so the aquifer is kept in hydrological balance, with no net extraction. The heat (or cold) storage medium is the water and the substrate it occupies. Germany’s Reichstag (parliament building) has been both heated and cooled since 1999 with ATES stores, in two aquifers at different depths. In the Netherlands there are now well over 1,000 ATES systems, which are now a standard construction option. A significant system has been operating at Richard Stockton College (New Jersey) for several years. ATES has a lower installation cost than BTES because usually fewer hole are drilled, but ATES has a higher operating cost. Also, ATES requires particular underground conditions to be feasible.
  • BTES (borehole thermal energy storage). BTES stores can be constructed nearly anywhere, and are composed of one to hundreds (potentially a few thousand) vertical boreholes, typically 155 mm in diameter. Systems of all sizes have been built, including many quite large. The strata can be anything from sand to crystalline hardrock, and depending on engineering factors the depth can be from 50 to 300 metres. Spacings have ranged from 3 to 8 metres. After 3–5 years of operation a stable thermal gradient is established horizontally in the rock, and heat loss is through the top. Storage efficiency increases with the volume of rock accessed by the borehole heat exchangers (BHEs), and decreases with storage temperature. A BTES operating at Richard Stockton College since 1995 at a peak of about 85 oF consists of 400 boreholes 425 feet deep under a 3.5 acre parking lot. It has a heat loss of 2% over six months. The upper temperature limit for a BTES store is 80-90oC due to characteristics of the PEX pipe used for BHEs, but most do not approach that limit. Boreholes can be either grout- or water-filled depending on geological conditions, and usually have a life expectancy in excess of 100 years. Both a BTES and its associated district heating system can be expanded incrementally after operation begins, as at Neckarsulm, Germany. BTES stores generally do not impair use of the land, and exist under buildings, agricultural fields and parking lots for example.
  • CTES (cavern or mine thermal energy storage). STES stores are possible in flooded mines, purpose-built chambers, or abandoned underground oil stores (e.g. those mined into crystalline hardrock in Norway), if they are close enough to a heat (or cold) source and market.
  • Energy Pilings. During construction of large buildings, BHE heat exchangers much like those used for BTES stores have been spiraled inside the cages of reinforcement bars for pilings, with concrete then poured in place. The pilings and surrounding strata then become the storage medium.
  • Horizontal heat exchangers. For small installations, a “slinky” heat exchanger of plastic pipe can be shallow-buried in a trench to create an STES.

• Pit storages. Lined, shallow dug pits that are filled with gravel and water as the storage medium are used for STES in many Danish district heating systems. Pit storages are covered with a layer of insulation and then soil, and are used for agriculture or other purposes. Marstal, Denmark’s system is a case study, initially providing 20% of the village’s year-round heat but now being expanded to provide twice that.

• Large-scale water storages. Large scale STES water storage tanks can be built above ground, insulated, and then covered with soil.