heat pumps

Heat pumps are commonly used in temperate countries to provide premises with cooling in summer and heating in winter. It employs the concept of reversing the refrigerating cycle as detailed below.

Working Principles of Heat Pumps

Both heating and cooling modes of heat pumps do exactly the same thing. They "pump" the heat from one location to another. In these examples the heat in the air is moved out of or into the space.

Cooling Mode

A heat pump is essentially an air conditioner with a few additions, namely a reversing valve, two thermal expansion valves and two bypass valves. The reversible selection allows the unit to provide both cooling and heating. Fig.1 shows a heat pump in cooling mode. The unit operates as follows:

• The compressor compresses the refrigerant vapour and pumps it to the reversing valve.
  
  
• The reversing valve directs the compressed vapour to flow to the outside heat exchanger (condenser) where the refrigerant is cooled and condensed to liquid.
  
  
• The air blowing through the condenser coil removes heat from the refrigerant.
  
  
• The liquid refrigerant bypasses the first thermal expansion valve and flows to the second thermal expansion valve at the inside heat exchanger (evaporator) where it expands into the evaporator and become vapour.
  
  
• The refrigerant picks up heat energy from the air blowing across the evaporator coil and cool air comes out at the other side of the coil. The cool air is ducted to the occupied space as air-conditioned air.
  
  
• The refrigerant vapour then goes back to the reversing valve to be directed to the compressor to start the refrigeration cycle all over again.

Heating Mode

Fig.2 above shows the heat pump in heating mode. The difference between the two diagrams is the reversing valve directs the compressed vapour refrigerant to the inside heat exchanger first. This makes the inside heat exchanger to act as the condenser and gives out the heat energy. The heat is transferred to the air that blows across the coil and the heated air is ducted to the occupied space. The outside heat exchanger now becomes the evaporator. The liquid refrigerant bypasses the second thermal expansion valve and flows to the first thermal expansion valve where it expands into the evaporator. It becomes vapour and absorbs heat from the outside air.When the heat from inside coil is used to increase the temperature of water in a storage tank, the heat pump acts as a hot water generator. This can be achieved by using a heat exchanger to absorb heat from the inside coil with water circulating through it or by placing the inside coil in the storage tank.

Application Of Heat Pumps In Singapore

The application of heat pumps in Singapore is mainly for the generation of hot water rather than for heating of occupied spaces. When there is consistent and high demand for hot water such as in hotels, catering, laundry and hospitals, large sized heat pumps (better known as heat machines) are commonly used. The following 3 variants of heat pumps are installed in various projects to generate hot water:

1. Heat pumps that reclaims waste heat from the condenser water of the air-conditioning chiller. (Fig. 3)

2. Heat pumps that supply cool air to ventilate the chiller plant room and has its refrigerant compressor enclosed within its storage tank shell. (fig. 4)

3. Heat pumps without refrigerant compressor that is connected to existing split air-conditioners. (Fig. 5)

Some of the local buildings that have installed heat pumps and heat recovery systems are listed in Table 1 below:

Table 1 Buildings with heat pumps/ heat recovery systems

Incentives

Capital expediture incurred by investing in heat-pumps are eligible for the Accelerated Depreciation Allowance.