DISTRICT HEATING AND COOLING

Thermal energy delivered to a building from an outside source is known as district heating and cooling, which can range in size from small systems serving two or three buildings to networks serving entire cities. District heating and cooling is widely used in developed countries throughout the world and offers numerous advantages over individual building apparatus, including greater safety and reliability, reduced emissions, and greater fuel flexibility, particularly in using alternative fuels such as biomass or waste.

The earliest examples of district heating were Roman hypocausts, a type of hot-air furnace often adapted to warm several buildings in close proximity, such as the three temples at Carnutum (Vienna). The hypocaust and other Roman technologies were reintroduced during the Renaissance, serving primarily as starting points for improvements. Meanwhile, city fathers in Chaudes Aigues, a small town in the volcanic Cantal region of southern France, had by 1322 levied a tax on several houses heated by a natural hot spring channeled through open trenches dug in the rock. The history of this system, which still operates warming 150 residences, includes the introduction of wooden pipe, later replaced by plastic conduits. Accounts of this system appeared in numerous architectural works and may have been the inspiration for the proposed introduction of district heating in London in 1622 by Dutch polymath Cornelius Drebbel. This scheme was primarily intended to distribute heat for cooking and warming and thus reduce air pollution, caused by individual coal stoves. Unfortunately, Drebbel's patron, Prince Charles, was more concerned with wooing the Spanish Infanta, and Drebbel used his talents for other purposes, including building the first working submarine.

Although Roman engineers almost exclusively used hot air for heating, they extensively employed hot water in public baths. This technology was also reborn in the late sixteenth and early seventeenth centuries by Sir Hugh Plat, Solomon DeCaus, and others during what some have called the Rosicrucian Enlightenment.

Steam and hot water were also used to heat extensive horticultural nurseries in England and Sweden. A French engineer, Bonnemain, used hot water to heat several large residences, and in 1785 a Bonnemain-type system was used to heat the three large buildings of Count Potemkin's Taurida Palace in St. Petersburg. A competitor of James Watt's, Matthew Murray, heated his house in 1804 by piping steam from his nearby factory, and in 1808 a Scottish engineer, Robertson Buchanan, wrote that "a number of neighboring buildings might be served with one boiler." In 1826, the religious utopian community of Old Economy, Pennsylvania, used the waste heat from its steam engine to heat several other buildings by means of buried steam pipes.

Despite a strong awareness of the potential advantages of district heating in the early nineteenth century, widespread adoption did not occur until technology was developed to handle the numerous problems associated with heat distribution. The Great Exhibition of 1851 marked one of the positive turning points, with a separate boiler plant providing steam to power exhibits in the enormous Crystal Palace, a 600-meter-long glass structure. A similar exhibition, complete with separate boiler house, was held in New York two years later.

District heating began to appear more frequently in institutional complexes, which were becoming more numerous in both Europe and America. A new state capital complex in Springfield, Illinois in 1867 was served by a steam system, and the 1876 Croydon Asylum in the outskirts of London used hot water. Another large steam system was built for the American Centennial Exposition of 1876 in Philadelphia.

In March 1877, New York inventor Birdshill Holly introduced the Holly Steam Combination System to provide heat and power using steam distributed through an underground piping network. The first Holly Steam system was installed in Lockport, and within five years was installed in nearly fifty cities in North America and Europe. Although not all of the early systems were successful, and many were later abandoned for various reasons, the Holly System serving the downtown district of Denver, Colorado began service in November 1880 and has been operating ever since. A variant of Holly's design has operated in New York City since March 1882, providing reliable steam service to a large portion of Manhattan.

Developers in the late nineteenth century also recognized the need for artificial cooling in their customers' buildings. The New York Steam Company offered steam-driven absorption chillers as early as 1886, and four years later a district cooling company began operating in Denver by distributing chilled brine through underground pipes. Several other cities had such service by the 1920s, with systems distributing either brine or ammonia, often serving meat-packing and similar industries. Many electric companies also made ice in their central plants and distributed it to their customers for domestic refrigeration. During the 1930s, large, chilled water refrigeration plants were installed at Rockefeller Center in New York city and at the U.S. Capitol in Washington. In 1940 Southern Methodist University installed a central chiller plant and district cooling system at its campus in Dallas. Many institutional and industrial users installed large district cooling systems, and in 1962 commercial district cooling service was started in downtown Hartford, Connecticut, the first of many such systems.

Despite the early synergies between electric generation and district heating in America, the tremendous growth in generator size made it impractical to locate plants close to urban areas. Coal shortages during World War I (primarily caused by government takeover of railroads) sparked a movement to locate power plants close to coal mines, since it was thought to be more economical and even more environmentally desirable to transport electricity via wires. Small district heating systems that distributed heat from a power plant measured in kilowatts could not compete against plants making tens or even hundreds of megawatts of power. Many systems, however, benefited from managers who recognized the advantages of district heating and located plants where they could still serve heating customers.

At about the time that many American systems were being dismantled, district heating began to be incorporated into European cities such as Paris and Hamburg. The new Soviet Union studied various heating technologies throughout the world and adopted district heating to warm its new cities. In Iceland, the pipe for a planned geothermal system for the capital of Reykjavik was still in Copenhagen harbor when the Nazi invasion stopped the shipment. The United States eventually assumed responsibility for supplying Iceland, and calculated that it was more efficient to provide and transport twenty-four miles of steel pipe rather than deliver oil for Icelandic furnaces.

Many European cities, such as Rotterdam, incorporated district heating while rebuilding after World War II. Investigators from the American Strategic Bombing Survey were surprised to discover that aerial bombs had little effect on high-temperature hot-water piping in German factories. Within a short time such systems appeared on American military bases and, later, on the many colleges and universities opened in the post-war period. A Danish engineer working at the American air base in Thule, Greenland, invented a better way to prefabricate heating pipe. He returned home to start a company that was able to take advantage of the oil embargo that hit Denmark especially hard in the early 1970s.

Since Denmark imported nearly all of its energy in the form of petroleum, the Danish government adopted an ambitious plan to become energy self-sufficient. This was accomplished over the next twenty years through a combined effort to increase energy efficiency and to make use of alternative fuels, especially renewable resources such as wind and biomass. District heating played a large role in this effort, allowing previously wasted heat from factories and power plants to be used for useful heating. By 2000, more than 55 percent of Danish residences were connected to district heating networks. The environmental advantages of district heating have also become of importance due to global warming concerns. Utilizing the energy wasted in industrial and electric generation plants avoids burning additional fuel in individual buildings.

District energy systems primarily serve commercial and institutional buildings in the United States, although some systems serve multi-unit residential buildings. According to U.S. Department of Energy survey data, about 10 percent of commercial building floor space used district heating and about 4 percent used district cooling as of 1995. Steam was the predominant type of energy produced and distributed, accounting for about three-quarters of distributed energy. The median size system serves about two million square feet of floor space.

Overall, there were about six thousand district energy systems operating in the United states in 2000. They collectively provide over one quad of energy annually—about 1.3 percent of all energy used in the United States. Most systems serve institutions such as colleges, hospital complexes, and military installations, that is, they serve a number of buildings owned by a single organization. Utility systems that sell heating and/or cooling to separately owned buildings account for only around 16 percent of the district energy provided in the United States.

About 10 percent of the district energy systems are part of combined heat and power systems where both electric energy and useful thermal energy are produced. The electric generation capacity of these systems totals about 3,500 MW, about 0.5 percent of total installed electric generation capacity in the United States. However, the use of combined heat and power along with district energy distribution is growing, with new systems installed in recent years in Philadelphia, Penn., Trenton, N.J., St. Paul, Minn., and elsewhere.

Morris A. Pierce