Underground thermal energy storage (UTES) uses permeable, water-bearing rock formations called aquifers - or, where aquifers are unavailable, a network of plastic tubing inserted into boreholes drilled into the earth - as underground storage areas for water. With the aquifer system, two well fields are tapped: one for cold storage and the other for heat. These wells, which are usually around 200 meters deep, are capable of maintaining storage temperatures of between 4 and 90 degrees Celsius.
During the summer months, cool groundwater is extracted from one aquifer and circulated through building systems to lower the air temperature. The water, which is heated during the process, is then returned to the other aquifer for use in heating that winter - a cyclic process that can be repeated indefinitely. A UTES system typically reduces cooling costs by 80 percent and heating costs by 40 percent or more. And, because it is a clean technology, it means a significant reduction in emissions of greenhouse gases and ozone-depleting substances.
Environment Canada in the Atlantic region has developed a variety of tools and procedures for implementing this energy-efficient technology - including tests to determine the thermal properties of boreholes, water treatment technologies for high-temperature applications, and environmental screening techniques - and plays an active role in the transfer of UTES technologies, both nationally and internationally.
Four years ago, the department teamed up with the New Brunswick Department of the Environment, New Brunswick Power, Canadian Electricity Association, Panel of Energy Research and Development, and the local community to launch an aquifer-based UTES demonstration project at the Sussex Hospital in New Brunswick. The first hospital in Canada and the second in North America to adopt such a system, it has saved nearly $50,000 a year in energy consumption costs since the project began, and reduced its carbon dioxide emissions by 720,000 kilograms annually. These savings were accomplished after the hospital had already cut its energy consumption in half by installing an energy management system. The UTES system is also playing a significant role in year 2000 compliance, because during an emergency, only small auxiliary generators are required to operate the water pumps used in heating and cooling.
Underground thermal energy storage is commonly used in China and parts of western Europe - particularly the Netherlands and Sweden, where it is growing by 25 percent a year. Although office buildings remain the primary market for the technology, it is gaining ground in industrial and agricultural applications, and a new market is developing in the de-icing and cooling of roads and bridges.
In Canada, the example set by the Sussex Hospital and other successful UTES projects, such as the Saskatoon Airport and Carleton University in Ottawa, has spurred the development of several new initiatives in Ontario, Nova Scotia, and British Columbia. The many other instances where a system of this kind could be implemented hold the potential for considerable long-term environmental benefits, and a reduction in our heavy usage of fossil fuels for heating and electricity.
Reprinted with permission from Environment Canada from their Science and the Environment Bulletin.