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Storage of energy is required, not only at junctions in transportation networks, as basic stores for large energy consumers or for strategic and military purposes, but also as a means of smoothing deviations in production and consumption in most energy supply systems. Energy consumption on almost any time scale (hourly, daily or monthly) is highly variable. Pipelines and electrical power transmission systems having finite capacities are usually not capable of supplying all users at high demand periods. Construction of storage facilities at producing points and in close proximity to the users is thus necessitated. Novel energy sources like wind and solar energy, when introduced to an appreciable extent in the energy systems, will require increased storage capacity due to their intermittent production characteristics. For many reasons, including costs, land savings, safety, environmental benefits and heat insulating properties of rock, underground energy storage can be advantageous as compared to surface storage. This paper will deal specifically with problems in connection with "unconventional" underground energy storage: high pressure gages, cryogens and hot water. Thus, the comparatively well-known techniques of storing crude oil, oil products and LPG will not be discussed.
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Traditionally, facilities for natural gas storage have been constructed in depleted oil or gas reservoirs, or in aquifers which can provide safe and economical methods of containing large volumes of gas. However, favorable geological conditions for this type of storage do not exist in all parts of the country. One example of this is the Atlantic coast region of the United States which has almost one third of the population, but is sorely lacking in gas storage facilities. However, most areas in this region are favorable for storage in mined rock caverns. Pipeline natural gas can for example be stored as compressed gas in deep caverns for use when needed along the pipe system. In the near future, large amounts of liquefied natural gas (LNG) will be shipped to the United States, most of it on the East coast. It is possible, with very large cost benefits, to store this LNG underground in mined caverns. Figure 1 describes a possible layout for a compressed natural gas chamber. Depending on the operation pressures of the pipeline and the storage, compression of the gas may or may not be needed. The access tunnel (or shaft) is excavated with all precautions taken to minimize drainage of groundwater from surrounding rock masses. Before blasting the storage cavern, a water injection system with tunnels and drillholes is constructed and pressurized with water above the cavern roof. By storing the gas on water, hydraulically connected to a reservoir at the surface, the gas pressure can be kept almost constant; however, there is also the possibility of choosing a completely closed chamber system with varying gas pressure. In the hydraulic type storage, compression requirements on the gas may be very small and very limited volume of cushion gas is required. Under atmospheric pressure, LNG is at a temperature of -259°F( -162°C) and requires heavily insulated tanks for storage at the surface.
