Seasonal thermal energy storage (STES)
Seasonal thermal energy storage (or STES) is the common umbrella term for several technologies for storing heat or cold for periods of up to several months. The uncommon term inter-seasonal thermal storage is a better descriptor, because the thermal energy can be collected whenever it is available and be used whenever needed, even in the opposing season. For example, heat from solar collectors or waste heat or air conditioning equipment can be gathered in hot months for space heating use anytime needed, including during winter months. Waste heat from industrial process can similarly be stored and be used much later. Or the natural cold of winter air can be stored for summertime air conditioning. STES stores can serve district heating systems, as well as single buildings or complexes. Among seasonal storages used for heating, the design peak annual tempertures generally are in the range of 27 to 80 oC (80 to 175 oF), and the temperature difference occurring in the storage over the course of a year can be several tens of degrees. Some systems use a heat pump to help charge and discharge the storage during part or all of the cycle. For cooling applications, often only circulation pumps are used.
An example of one of the several kinds of STES storages illustrates well the capability of interseasonal heat storage. At Alberta, Canada’s Drake Landing Solar Community (in operation since 2007), the homes get 97% of their year-round heat from a district heat system that is supplied by solar heat from solar-thermal panels on the garage roofs. This feat – a world record – is enabled by interseasonal heat storage in a large mass of native rock that is under a central park. The thermal exchange occurs via a cluster of 144 boreholes, drilled 37 metres into the earth. Each borehole is 155 mm in diameter and contains a simple heat exchanger made of small diameter plastic pipe, through which water is circulated. No heat pumps are involved.
STES technologies 
There are several types of STES technology, covering a range of applications from single small buildings to community district heating networks. Generally, efficiency increases and the specific construction cost decreases with size.
- 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.
- 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.
- Horizontal heat exchangers. For small installations, a “slinky” heat exchanger of plastic pipe can be shallow-buried in a trench to create an STES.
- Earth-bermed buildings, with passive heat storage in surrounding soil (further described below).
Conferences and organizations 
The International Energy Agency's Energy Conservation through Energy Conervation (ECES) Programme has held triennial global energy conferences since 1981. The conferences originally focused exclusively on STES, but now that those technologies are mature other topics such as phase change materials (PCM) and electrical energy storage are also being covered. Since 1985 each conference has had "stock" (for storage) at the end of its name; e.g. Ecostock, Thermastock. They are held at various locations around the world. Most recent was Innostock 2012 (the 12th International Conference on Thermal Energy Storage) in Lleida, Spain. Greenstock 2015 will be held in Beijing.
The IEA-ECES programme continues the work of the earlier International Council for Thermal Energy Storage which from 1978 to 1990 had a quarterly newsletter and was initially sponsored by the U.S. Department of Energy. The newsletter was initiially called ATES Newsletter, and after BTES became a feasible technology it was changed to STES Newsletter.
Use of STES for small, passively heated buildings 
Small passively heated building typically use the soil adjoining the building as a low-temperature seasonal heat store that in the annual cycle reaches a maximum temperature similar to average annual air temperature, with the temperature drawn down for heating in colder months. Such systems are a feature of building design, as some simple but significant differences from 'traditional' buildings are necessary. At a depth of about 20 feet (6m) in the soil, the temperature is naturally stable within a year-round range, if the draw down does not exceed the natural capacity for solar restoration of heat. Such storages operate within a narrow range of storage temperatures over the course of a year, as opposed to the other STES systems described above for which large annual temperature differences are intended.
Two basic passive solar building technologies were developed in the US during the 1970s and 1980s. They utilize direct heat conduction to and from thermally isolated, moisture-protected soil as a seasonal storage medium for space heating, with direct conduction as the heat return method. In one method, “passive annual heat storage” (PAHS), the building’s windows and other exterior surfaces capture solar heat which is transferred by conduction through the floors, walls (and sometimes) the roof into adjoining thermally buffered soil. When the interior spaces are cooler than the storage medium, heat is conducted back to the living space. The other method, “annualized geothermal solar” (AGS) uses a separate solar collector to capture heat. The collected heat is delivered to a storage device (soil, gravel bed or water tank) either passively by the convection of the heat transfer medium (e.g. air or water) or actively by pumping it. This method is usually implemented with a capacity designed for six months of heating.
Small buildings with internal STES water tanks 
A number of homes and small apartment buildings have demonstrated combining a large internal water tank for heat storage with roof-mounted solar-thermal collectors. Storage temperatures (up to 90 oC) are sufficient to supply both domestic hot water and space heating. The first such house was MIT Solar House #1, in 1939. An eight-unit apartment building in [Oberburg]], Switzerland was built in 1989, with three tanks storing a total of 118 m3 (4,200 cubic feet) that store more heat than the building requires. Since 2011, that design is now being replicated new buildings.
In Berlin, the “Zero Heating Energy House”, was built in 1997 in as part of the IEA Task 13 low energy housing demonstration project. It stores water at temperatures up to 90 °C (194 °F) inside a 20 m3 (710 cubic feet) tank in the basement,.
A similar example was built in Ireland in 2009, as a prototype. The solar seasonal store consists of a 23 m3 tank, filled with water, which was installed in the ground, heavily insulated all around, to store heat from evacuated solar tubes during the year. The system was installed as an experiment to heat the world's first standardized pre-fabricated passive house in Galway, Ireland. The aim was to find out if this heat would be sufficient to eliminate the need for any electricity in the already highly efficient home during the winter months. The system is monitored and documented by a research team from The University of Ulster and the results will be included in part of a PhD thesis.
Use of STES in Greenhouses 
STES is also used extensively for applications as the heating of greenhouses. ATES is the kind of storage commonly in use for this application. In summer, the greenhouse is cooled with ground water, pumped from the “cold well” in the aquifer. The is heated in the process, and is returned to the “warm well” in the aquifer. When the greenhouse needs heat, such as to extend the growing season, water is withdrawn from the warm well, becomes chilled while serving its heating function, and is returned to the cold well. This is a very efficient system of free cooling, which uses only circulation pumps and no heat pumps.
See also 
- Central solar heating
- District heating
- Ice pond
- List of energy storage projects
- Solar thermal collector
- Solar pond
- Thermal energy storage
- Zero energy building
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- See slide 15 of Snijders (2008), above.
- December 2005, Seasonal thermal store being fitted in an ENERGETIKhaus100
- October 1998, Fujita Research report
- Earth Notes: Milk Tanker Thermal Store with Heat Pump
- Heliostats used for concentrating solar power (photos)
- Wofati Eco building with annualized thermal inertia