Hot Water Heat Pump Technology
Accent heat pumps have been used in Australia for all forms of water heating for nearly twenty years. The most common application is swimming pool heating as show-cased here but the total range of applications is much broader, ranging from home hydronic heating up to large commercial building water heating at 60°C.
The units operate much like air conditioners, removing heat from the environment and cooling and drying the air. But heat pumps use the heat to raise water temperature rather than simply rejecting it.
A schematic of how a heat pump works is shown below -
How a HWHP works
Like an air conditioner or refrigerator, a heat pump uses a vapour compression cycle. In air-to-water design, refrigerant collects heat from surrounding air for delivery to water. The refrigerant absorbs heat via the evaporator and delivers it to the water in the heat exchanger (or condensor).
- An expansion device lowers the pressure of the warm liquid refrigerant coming out of the heat exchanger, cooling the refrigerant and vaporising a portion of it.
- As the cold liquid and vapour mixture contacts the warmer air in the evaporator, the liquid absorbs heat and boils.
- Flowing to the compressor, the refrigerant is squeezed into a high pressure, high temperature vapour to drive the cycle and increase the heat delivery potential.
- The refrigerant vapor releases its heat to the water in the heat exchanger and returns to a liquid.
The operating characteristics of a HWHP are somewhat different to what is typically expected in refrigeration plant, particularly in relation to superheat and subcooling temperatures. By way of illustration, the following data relates to HWHP designed for pool heating.
OPERATING CHARACTERISTICS - 50KW NOMINAL AIR TO WATER POOL HEAT PUMP
|Amp per Phase||25A|
|Entering Water Temperature||27°C|
|Leaving Water Temperature||32°C|
|Coefficient of Performance (COP)||4.7|
|Saturated Suction Temperature||7°C|
|Saturated Condensing Temperature||4.5°C|
|Air on to Coil||15°C|
A coefficient of performance (COP) indicates the amount of energy (in heating or cooling) delivered compared to the amount of energy required to operate it. A COP is typically used to describe the efficiency of a heat pump under given operating conditions.
The efficiency of heat pumps has steadily improved with advancements in compressor technology, coil design and heat exchanger efficiency. The data provided above in relation to a typical 50KW air to water pool heat pump showed an average COP of 4.7 at nominal conditions. A maximum COP of around 6.0 and minimum of 4.0 would be expected under most Australian conditions. For the air to water unit, the Chart of COP shows increasing efficiency as the air-on temperature rises above the rating condition of 15°CDB. Likewise, COP (and total heat rejection) falls as the air on temperature falls below this point.
Heat pump efficiency is not as high at higher entering water temperatures. The COP in high temperature water heating (60°C) would be expected to be in the range of 3.5 to 4.5.
Heat Pump Design
The heat pump design that most people are familiar with is called air-to-water and the schematic of this operation is shown above. Accent's standard range is air-to-water type and these units are characterised by their larger physical size and the movement of ambient air onto the unit's evaporator and the discharge of the cooled air off the unit's fans.
However, all Accent units can also be provided as water-to-water and ground-sourced design.
A water-to-water heat pump is characterised by an open loop piping arrangement. The water source may be ground-water or surface water. With ground water units, water is drawn from a well, exchanges heat with a refrigerant, and then is discharged into a rejection well, disposal pond or stream. Water in surface water heat pumps is most often obtained through a supply pipe laid in a nearby river, lake or pond and discharged to the same source by a return pipe.
Ground sourced (or ground-coupled) heat pumps are linked to the ground by a heat exchanger, usually plastic pipe. The heat exchanger is called a ground loop or ground coil. The ground loop has a closed configuration and can be installed either horizontally or vertically. The ground-sourced heat pump circulates a heat transfer liquid through the ground loop, absorbing the earth's natural warmth in winter and rejecting heat to the earth in summer. The fluid is most commonly water and antifreeze mixture.