Energy Storage

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General Description

Energy storage involves methods and apparatus used to store heating, cooling, or power. In district energy applications these encompass a wide variety of applications, but all are designed to be recharged on a cyclical basis (usually daily, occasionally seasonally) and fulfill one or more of the following purposes:

Increase system generation capacity
Demand for heating, cooling, or power is seldom constant over time, and the excess generation available during low demand periods can be used to charge the energy storage apparatus in order to increase capacity during high demand periods. This allows a smaller production unit to be installed (or to add capacity without purchasing additional units) and results in a higher load factor on the units.
Enable dispatch of cogeneration plants
Combined heat and power, or cogeneration, plants are generally operated to meet the demands of the connected thermal load, which often results in excess electric generation during periods of low electric use. By incorporating thermal energy storage, the plant need not be operated continuously and can be dispatched within some limits.
Shift energy purchases to low cost periods
This is the demand-side application of the first purpose listed, and allows an energy consumer subject to time-of-day pricing to shift energy purchases from high cost to low cost periods.
Increase system reliability
Any form of energy storage, from a small personal computer uninteruptable power supply (UPS) to a large pumped storage project, will almost certainly increase system reliability.
Integration with other functions
In applications where on-site water storage is needed for fire protection, it may be feasible to incorporate thermal storage into a common storage tank. Likewise, apparatus designed to solve power quality problems may be adaptable to energy storage purposes as well.

A Short History of Energy Storage

The oldest form of energy storage involves harvesting ice from lakes and rivers, which was stored in well insulated warehouses and sold or used throughout the year for almost everything we use mechanical refrigeration for today, including preserving food, cooling drinks, and air conditioning. The Hungarian Parliament Building in Budapest is still air conditioned with ice harvested from Lake Balaton in the winter. Quite an impressive system, which I was able to visit last summer.

Chemically-charged batteries became quite common in the mid-nineteenth century to provide power for telegraphs, signal lighting, and other electrical apparatus. By the 1890s central stations were providing both heating and lighting, and many did both. Electric systems were almost all direct current (DC), so incorporating batteries was relatively easy. In 1896, Toledo inventor Homer T. Yaryan installed a thermal storage tank at one of his low temperature hot water district heating plants in that city to permit capturing excess heat when electric demand was high. Other plants used steam storage tanks, which were not as successful for some reason.

Other forms of energy storage were used to power street cars in the 1890s, including compressed air and high temperature hot water that was flashed into steam to run a steam engine. Electric cars and trucks were quite common prior to World War I until gasoline-powered internal combustion engines ran them off the road.

Energy storage has always been closely associated with solar installations, including both solar heating and photovoltaic (PV) applications. Today you can find compressed air storage, batteries, chilled and hot water storage, ice storage, and the occasional flywell in use, all designed to meet one or more of the purposes listed above. Many utilities provide incentives for energy storage applications, while time-of-day rates and stiff demand charges also entice customers to consider these opportunities.

Applications

This material will take me some time to sort out and get on line, but in the meantime here is some introductory material. The fundamental basic never-to-be-forgotten rule of energy storage is to remember that it is part of a system, and whatever storage mechanism is used has be properly engineered into that system.

Electric Power Storage

There are some sites on the web with information on pumped storage facilities, which I will track down and add here. I also know some people who have done compressed air storage, and have some information on flywheels and other devices that might have application in district energy applications. Lots of research in this area going on.

Ice Thermal Storage

Water Thermal Storage

Energy Storage Publications

District Energy Library

Last updated 2 June 2000