The problem of energy storage is of central importance for and improved energy economy. Long term heat storage requires very low construction costs. Many underground storage alternatives can meet these requirements. In the big scale, hot water accumulation in rock chambers and in-ground hot water pools are competitive solutions. Direct storage of heat in soil masses is another alternative. The paper describes some new techniques for underground thermal storage.
The consumption of energy is closely related to external conditions like the daily activity pattern of the society and the weather fluctuations. When thermal energy for heating is produced by burning of fuels, then the production can be readily adjusted to the actual consumption level. The fuel itself is a good energy storage. In countries with cold climate, the energy required for heating of buildings is very large. In Sweden, for example, about half of the total energy consumptions used for heating. 90 % of this thermal production is based on oil. Measures for reducting the dependency on oil are discussed in many countries. Where the oil can be substituted with other fuels, like coal or biomasses, there will be no additional need for thermal storage in the system. However, other production methods for thermal energy also utilized or will be used in the near future. Examples of such methods are solar energy, industrial thermal waste and combined power and thermal production. Here the production and consumption patterns are very different on every time scale (daily, weekly, yearly) and thus thermal energy storage is required. Characteristics of thermal storages for heating purposes are low energy density (temperatures usually below 100° C) and low degree of utilization (once on a few times a year). Consequently the installation costs for these storages must be kept very low in order to make them economically competitive. The addition of a thermal storage to a combined power and heat production plant will enable the power production to be adjusted accord.ing to the consumption pattern of electrical energy. It will then not be necessary to waste any heat during peak power generation periods, just because there is not a concurrent thermal load in the system. In pure thermal, production units (central heating stations) a thermal storage has the benefit of reducing the required peak capacity of the heater (as well as of the redundant heater system). Maybe the most important advantage created by thermal, storages is that they permit the utilization of such thermal excess energy from the industry, that otherwise would have been wasted, and of thermal energy from solar collectors. This energy can only be produced during the warm season of the year, but will be consumed during the cold season.
Figure 1 shows the energy consumption for heating of buildings in a cold climate (Stockholm) during a year. This curve has variations on (at least) a daily, a weekly and a seasonal basis.