One of the most rapidly accelerating trends in the commercial-buildings industry — driven by rising energy costs and the growing influence of the U.S. Green Building Council's Leadership in Energy and Environmental Design (LEED) Green Building Rating System — is the use of water-source-heat-pump (WSHP), or thermal-energy-transfer (TET), systems.

The latest generation of WSHPs can extract as much as 5 kwh of heat from a water loop for every 1 kwh of electricity used to power a compressor and fan, delivering 6 kwh of heat to the air. This 6-to-1 ratio, or coefficient of performance, can be equated to 600-percent efficiency.

THERMAL-ENERGY TRANSFER

A TET system is comprised of highly efficient heat pumps interconnected by way of a closed water loop. Each pump, using a simple vapor-compression refrigerant circuit, satisfies the air-comfort requirements of the zone in which it is installed.

When heat is needed, the pumps extract thermal energy from the water loop and reject it into the air. When cooling is needed, they extract thermal energy from the air and reject it into the water loop, where it is available for use by other heat pumps in the building, rather than wasted to the outdoors.

All buildings contain year-round sources of energy that can be recovered and recycled by a TET system. These include:

• Lighting. The electrical energy used for lighting in most structures varies from 1 to 4 w per square foot.

• People. Humans emit thermal energy ranging from 300 to 500 Btuh (88 to 147 w per hour), depending on their activity.

• Equipment. The energy consumed by computers, printers, copiers, pumps, motors, and the like is emitted as heat.

• Solar gain. Perimeter zones with large glazed windows may require daytime cooling, even during cold weather.

The thermal energy recovered by a WSHP system has many uses, including for space heating, water heating (domestic hot water, swimming pools and spas, radiant-heat systems, snow-melt systems), and ventilation.

WHY WATER?

Water is the most efficient way to move thermal energy. A 2-in. water pipe can carry the same amount of cooling as a 24-in. air duct, while requiring up to 90-percent less transport energy and taking up far less space. Another advantage is thermal storage, provided by the mass of a water loop. Thermal storage allows a substantial amount of heat to be carried from occupied periods into morning warm-up.

WSHP heat exchangers are both compact and efficient. This is because of the high mass and thermal conductivity of water. WSHPs operate at lower condensing temperatures than traditional HVAC systems because, when used with a cooling tower, they are linked to outdoor wet-bulb temperature, and, when used with a ground heat exchanger, they are linked to deep-earth temperature. This leads to higher efficiencies and longer service life.

WSHPs are combined with a heat rejector and a boiler, which are used to maintain the temperature of circulating water within a controlled range — typically, 60°F to 95°F. The most common heat rejectors are open cooling towers with isolating heat exchangers, closed-circuit evaporative coolers, or dry coolers.

Each heat pump uses the water loop to provide heating or cooling at any time, regardless of the operating mode of the other heat pumps. This is accomplished with neither the duplicate operation of heating and cooling systems nor the double waste inherent in reheat modes.

Water-to-water heat-pump systems are highly efficient under part-load conditions, such as when a small portion of a building remains occupied after hours. In such cases, only the required zone heat pumps are used.

Acceptable Use Policy blog comments powered by Disqus