The Heat Is On

Heat pumps have a valuable role to play in cutting carbon emissions and home energy costs, says the Hot Water Association. 

 

Heat Pumps make use of low-grade heat either directly from the air or indirectly from ground sources - which may include lakes or rivers. The heat pump converts this low-grade heat into a more useful higher flow temperature.

The temperature achieved depends on the design of the heat pump system: virtually all heat pumps can achieve temperatures of 50 degrees celcius and some go to 65 degrees celcius or above, albeit at lower efficiency. These flow temperatures are high enough to be used in central heating systems and can also make a useful contribution to heating domestic hot water.

Hot water storage systems are perfect for harvesting renewable and low-carbon energy sources such as solar heat, as the energy can be banked and used throughout the day when required. The same principle applies with heat pumps. Energy from the heat pump is used to indirectly heat a store of hot water via a heat exchanger, usually a coil, although externally mounted plate heat exchangers are sometimes the preferred choice as these can be designed specifically to the application requirements. This energy is banked in the usual way and used when required.

Because heat pumps operate at lower flow temperatures than gas boilers, the heat exchanger performance is usually enhanced to provide better heat transfer and ensures the heat pump doesn't cycle too often during the re-heat period. This enhancement usually involves significantly increasing the coil surface area to achieve a closer match to the requirements of the heat pump.

It should be noted that the characteristics of different heat pumps should be taken into consideration when specifying hot water storage systems, especially with regard to heat exchanger performance.

Plate heat exchangers can be specified to close tolerances to match the requirements of specific heat pumps. The hot water storage tank can be open-vented or unvented.

One improvement that can help to further reduce carbon emissions and is ideally suited to hot water storage and heat pump installations is an additional coil or other heat exchanger for solar water heating. This coil is situated low down in the water tank to maximize solar gain. Effectively the heat pump isn't used during the summer, when most domestic hot water demand can be met by the sun from April to September.

Efficient system

This 'best of both worlds' solution would improve SAP ratings and offer householders an excellent energy efficient system with all the benefits of hot water storage such as quick bath filling and multi-outlet use without significant falls in flow rate across the system.

It is also possible to use primary storage systems with heat pumps. These can help to buffer the demand on the heat pump, particularly for space heating applications where the instantaneous load once a property has warmed up may be very small. These primary stores can also be used with heat pumps to preheat the 'heat bank' for domestic hot water. Inevitably, further top-up will be required from a back-up heat source and a seperate hot water cylinder is usually required.

This naturally leads to consideration of group heating schemes with commercial duty heat pumps. These heat pumps can achieve outputs for a number of dwellings with the aid of buffer stores and individual heat exchangers or buffer vessels for space heating requirements.

Whatever system is chosen, attention must be paid to low temperature systems to ensure that any legionalla risk is minimized. In practice, this means that an electric immersion heater or back-up boiler is needed to provide additional heat to the water at given periods.

 

Source - Gas Safe Register

 

Posted Date: 01st Jul 2010