As the drive towards the elimination of carbon emissions accelerates, heat pumps are being viewed as an increasingly important weapon in the armoury of building services engineers and clients. However, says Maurice Diamond, it is crucial to choose the heat pump carefully.
Until now the main interest in these heat pumps has been from the domestic sector with many companies either developing or acquiring access to products, for what is perceived to be a potentially huge market. However, with lower than expected fuel costs and the removal of grant support, this market has stalled somewhat with the result that alternative outlets for these products are being sought. The commercial market was an obvious target. However, this segment requires a different product and approach. It also requires the correct selection both in type and size of unit if the system is to achieve its full potential.

Because of the intense promotion, most people are familiar with the two main types of heat pump, air source and ground source. For various reasons, there has been a perception that the ground source type are the best, offering a high COP that does not vary with changing weather and never requiring energy to defrost the source exchanger. Furthermore, due to limitations of some air source units, there has developed a perception that they will only operate at low ambient conditions if augmented with other heat sources.

Have all the facts available

Like all things in life, nothing is absolutely right or absolutely wrong. What is more important is to approach the subject with a clear mind and have available all of the facts in order to determine which unit is the best selection for the system of which it will be a part.

To make any decision regarding selection, it is vital to have the performance information of the unit at the conditions under which it is to operate. Simple, one condition snapshots are useful but a simulation that examines the whole year's operation and takes account of both defrosting and the generation of hot water at higher temperatures is essential if correct decisions are to be made.

Hidros has developed such software (for both ground and air source units) that enables the performance at different locations and conditions to be compared. It is an energy-based approach that utilises degree day data for the location to calculate the annual energy use in kWh for any selected unit for the proposed building into which it is to be applied. It can also calculate the energy use for domestic hot water and for defrosting if an air source unit is to be chosen.

A further refinement is the calculation of not only the unit COP but, more importantly, the system COP. The software also includes a correction factor that enables the annual energy use calculated to be adjusted to match the figure produced by sophisticated dynamic building modelling programmes that take account of both internal and solar gain. Finally, with the cost of a unit of electricity input to the programme, the annual projected running costs can be produced for the client.

Without such modelling it is not readily seen which product offers the best solution for the particular project.

If we analyse a typical commercial building with the software we can demonstrate which is best under various conditions.

If we take a building located in London that has a heat loss at -5 deg C of 54kW for an indoor temperature of 22 deg C, has 20 occupants that use 50 l/24hr of DHW that is heated from 10 to 60 deg C, and is heated by an underfloor system that operates at 35/30 deg C, then we can use the simulator to examine the performance of various units.

First, using an air source unit, the most economic selection is to use a unit that meets the load for 90/95 per cent of the time and augments this with direct acting energy to cater for the extreme conditions. The unit selection is a LZT52 with a system COP of 3.8 (the heat pump alone is in the region of 4.2) and an estimated annual running cost of £5,362.

The ground source unit that matches the load with negligible direct acting energy use is the WDH039 with a system COP of 4.6 (the unit COP is 4.7) and an estimated running cost of £4,400. This is with realistic source temperatures of 2/-1 deg C although it is found that if heat is constantly being drawn from the ground source that this can drop to 0/-3 deg C as a typical figure. This lower figure will drop the COP to 4.3 and raise the running cost to £4,730.

On the face of the above, the ground source appears to be the best option however, this is not the whole story. Capital costs must also be taken into account.

For the two heat pumps described above, the capital cost would be in the region of £16,000 for the air source and £8,000 for the ground source. Again it would appear that the ground source is the best option however, the ground source unit also requires around eight 100m boreholes or collectors that will cost something in the region of £30,000. The payback then changes dramatically in favour of the air source unit. The air source is cheaper by £22,000 while the running cost savings are, at best £900 in favour of the ground source. A pay back of >18 years would not normally be considered acceptable to most accountants.

The above scenario demonstrates that first impressions are not always correct, however, the colder the location, the better the argument for ground source units. Conversely, the milder the climate, the better air source units perform.

Relatively unaffected

In fact, if the above example were in Dublin, it would be possible to drop a unit size and reduce further the air source capital costs for around the same running costs, whereas if the project were in Stuttgart in Germany, the air source unit would have to dramatically increase in size and the amount of direct acting energy use would also increase, with a reduction in COP to 2.8. The ground source unit remains relatively unaffected.

The most important thing is to have the tools and information to change such parameters thereby enabling the correct selections to be made. Incorrect application because of equipment limitations or incorrect information will result in heat pumps in general getting a bad name when in fact they offer the most practical way forward to low or zero carbon buildings. It is vitally important that equipment that has been designed specifically for commercial application is used for this segment as the attempt to force small domestic type product into the sector will undoubtedly lead to poor results.

• Maurice Diamond is general manager UK & Ireland of Hidros