€/kWh could be achieved. Large-scale thermal storage with water. Large scale STES water storage tanks can be built above ground, insulated, and then covered with soil. Horizontal heat exchangers. For small installations, a heat exchanger of corrugated plastic pipe can be shallow-buried in a trench to create a STES. Earth-bermed buildings. Stores heat passively in surrounding soil. Salt hydrate technology. This technology achieves significantly higher storage densities than water-based heat storage. See Thermal energy storage: Salt hydrate technology Conferences and organizations The International Energy Agency's Energy Conservation through Energy Storage (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 were InnoStock 2012 (the 12th International Conference on Thermal Energy Storage) in Lleida, Spain and GreenStock 2015 in Beijing. EnerStock 2018 will be held in Adana, Turkey in April 2018. 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 initially 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 buildings 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 in the soil, the temperature is naturally stable within a year-round range, if the drawdown does not exceed the natural capacity for solar restoration of heat. Such storage systems 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