Accent Water & Energy

GENERAL

HOT WATER HEAT PUMP TECHNOLOGY
OPERATING CHARACTERISTICS
OPERATING EFFICIENCY
DESIGN CONSIDERATIONS
HEAT PUMP DESIGN
REFRIGERANT HEAT RECOVERY
CURRENT AND POTENTIAL MARKET APPLICATIONS

HOT WATER HEAT PUMP TECHNOLOGY

Accent hot water heat pumps (HWHPs) are typically air-to-water design, providing water up to 65°C. They operate much like air conditioners, removing heat from the environment and cooling and drying the air. But HWHPs use the heat to raise water temperature rather than simply rejecting it.

A schematic of how a HWHP works is shown below -

How a HWHP works

Like an air conditioner or refrigerator, a HWHP 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).

(1) 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.
(2) As the cold liquid and vapour mixture contacts the warmer air in the evaporator, the liquid absorbs heat and boils.
(3) 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.
(4) The refrigerant vapor releases its heat to the water in the heat exchanger and returns to a liquid.
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OPERATING CHARACTERISTICS

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

Nominal Output

50KW

Nominal Input

10.5KW

Amp per Phase

24A

Entering Water Temperature

27°C

Leaving Water Temperature

5°C

Coefficient of Performance (COP)

4.7

Superheat Temperature

6°K

Sub-cooling Temperature

6°K

Saturated Suction Temperature

7°C

Saturated Condensing Temperature

4.5°C

Suction Pressure

520KPa

Discharge Pressure

1,550KPa

Air on to Coil

15°C

Refrigerant

R22

Compressor

Scroll

OPERATING EFFICIENCY

A coefficient of performance (COP) indicates the amount of energy (in heating or cooling) delivered by an appliance compared to the amount of energy required to operate it. A COP is typically used to describe the efficiency of a HWHP under given operating conditions.

The efficiency of HPWHs 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 Sydney 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 THR) falls as the air on temperature falls below this point.

HPWH efficiency is not as high at higher EWTs. The COP in high temperature water heating (65°C) would be expected to be in the range of 3.5 to 4.5.
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3. DESIGN CONSIDERATIONS

The technical and economic success of a HPWH system is dependent on site specific planning and system design. Key areas for design consideration are -

Capability
Where water temperatures are up to 65°C, HPWHs can meet the entire water heating need or alternatively, backup electric element heating may be incorporated in hot water tank design. HPWHs can also be specifically designed to provide pre-heating only, where they operate at highest efficiency due to the lower water temperature.
Load
Correct HPWH system design relies on a detailed understanding of actual hot water usage.
Sizing
The higher capital cost of HPWHs means that there is a closer relationship between installed capacity and the peak demand. For hot water applications, HPWH sizing is most likely made to typical demand with peak loads met by a supplemental heating system, whether element or gas. Alternatively, system design for HPHW systems will utilise increased storage capacity to meet peak loads in favour of increased HPWH capacity.
Integral/Remote
HPWH units are manufactured as either integral or remote units. In an integral unit, the HPWH includes the hot water tank. These units are low capacity and are available for domestic hot water market along with gas and element storage heaters. The units generally take up the same floor space as conventional water heaters. A remote HPHW unit is stand-alone from the tank. These units are available in much larger capacity than the integral HPHW.
Space Cooling
Air to water HPWHs provide cooling in response to hot water demand. This air can be fed into the fresh air inlet of the air conditioning system or ducted directly for space cooling.

HEAT PUMP DESIGN

As mentioned, typical HPWH design is air to water type. The Australian market also shows significant examples of 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 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 GCHP 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.
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REFRIGERANT HEAT RECOVERY

In general terms, "heat recovery" involves the use of waste heat from one process for a useful purpose (in the same or different process) to reduce the total energy required by the plant. The refrigerant heat recovery unit (HRU) delivers substantial energy savings by heating potable water by recovering waste heat from refrigeration, air conditioning and heat pump systems. The HRU can effectively heat water up to 60°C, and can be connected to systems from 5HP to 200HP using reciprocating, scroll or screw compressors.

The HRU is piped in series between the compressor gas discharge and the condenser. In this position, the HRU captures the heat rejection of the compression cycle. The heat is comprised of both superheat and condensing heat energy. The HRU uses the superheat (which accounts for around 15% of the total heat rejection) to heat domestic hot water. By removing the superheat, the addition of the HRU effectively adds 15% more condenser capacity to the host plant, resulting in a 5% to 10% reduction in its operating cost. The compressor pressure of the host plant is also lowered with the decrease in the discharge gas pressure.
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CURRENT AND POTENTIAL MARKET APPLICATIONS

The high efficiency of hot water heat pump technology presents a strong economic case for its use across many market sectors across Australia. For example, applications have been made by Accent for hot water up to 65°C for uses such as hospitals, restaurants, carwashes, nursing homes, commercial laundries, hospital laundries, supermarkets, piggeries, abattoirs, gymnasiums, defence force bases, dormitories, hostels, clubs, mine sites and private residences.

The base market for heat pump technology is the Australian aquatic and recreation industry, with heat pumps dominating this sector in NSW and Queensland. Hundreds of pool heating systems are installed across Australia, with recent exports of Australian technology to South East Asia.

Substantial energy efficiencies can be gained by the integration of heat pump technology with refrigerant heat recovery or desuperheating, as well as integration with solar and gas.
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