In applications as diverse as space heating, water heating – and even passive and active cooling – heat pumps are finally reaching commercial maturity after a quiet adolescence in the 1990s.

Heat pumps for domestic heating have been available commercially since the 1970s, but it is only in the past few years that units have become sufficiently affordable, reliable, compact and quiet to impress broad consumer markets.

The theoretical principles and energy efficiency of heat pumps have been known for generations. The units are often described by manufacturers as ‘refrigerators in reverse’. They harness the energy that exists in ambient air, the ground or groundwater, then use it in domestic or commercial appliances such as water heaters.

In temperate zones the most common types of heat pumps extract heat energy from the air, but in cooler climates an array of closed-loop coils can be placed underground to make use of latent subterranean heat energy in soil or water. Underground temperatures do not fluctuate dramatically with the seasons and can settle at 5-10˚C even during severe winters in the Northern Hemisphere.

Most heat pumps operate by feeding energy to a heat exchanger (or evaporator). This evaporator contains a liquid refrigerant, which absorbs the heat from the energy source and evaporates. The compressor draws the expanded gas inside, where it is compressed. The temperature rises as pressure builds and a second heat exchanger (or condenser) then transfers the energy to the heating grid. Finally, an expansion valve allows the pressure to normalize, and the cycle continues.

The efficiency of heat pump systems can be impressive. These days it is not unusual to see units with heat energy outputs that are four times higher than electrical energy inputs – representing profound energy savings compared with conventional electric heating systems.

When units that double as air-conditioners are installed there are obvious energy savings to be gained year-round. These dual-purpose devices provide distinct advantages over ‘heating only’ technologies because they can improve the overall energy rating of a household or office building. Don’t forget that the benefits of an efficient water or space heater are often outweighed by wasteful cooling systems in the same building.

World markets and energy regulatory authorities are placing greater value on appliances that conserve power and reduce environmental effects. So it is not surprising that corporate investment in efficient heat pump technology has risen dramatically in the past few years.

Stiebel Eltron, based in the central German town of Holzminden, is a family-owned business with about 2,400 employees worldwide. It has been at the forefront of heat pump research and development for decades and is a leader in the construction and design of new-generation systems for ever-expanding markets.

In May the company augmented its factory floor area by 6,000m2 to accommodate increased heat pump manufacturing and testing facilities. This US$13.3 million investment is evidence that the company predicts long-term growth in heat pump markets. Certainly, sales trends in recent years across Europe indicate extraordinary consumer acceptance of heat pump systems.

Sweden is Europe’s strongest user, with heat pumps securing a 90% domestic market share of heating units in new homes (63,000 units installed in 2005). In Germany 44,000 units were installed in 2006 (forecasts indicate 55,000 units will be sold by the end of 2007, representing a tenfold increase over 2000 figures). Austria recorded 12,716 units installed in 2006, showing 45% growth for the year.

In China, market growth of heat pumps – particularly those extracting energy from the ground – are forecast to exceed 17% a year for several years.