Most sustainability strategies include the use of low carbon energy sources, so it's important to ensure they deliver maximum benefits. Graham Rodd considers some of the key factors that affect heat pump performance
As organisations seek to improve their sustainability performance there is a growing requirement for low carbon heating - either as the sole heat source or as part of an integrated system that incorporates both traditional and low carbon heat sources. A typical scenario might be a condensing gas or oil fired boiler that supplements 'renewable' heat sources such as heat pumps or solar thermal.

Clearly, when different heat sources are brought together in this way it is important to establish the optimum control strategy and hydraulic design in support of what needs to be achieved. This will vary from one project to another so the purpose of this article is to explore the options that can be applied to suit the specific requirements of the project. To that end the article will consider boiler/heat pump hybrid units and how they can be configured, as well as opportunities for using air source heat pumps (ASHPs) to dramatically reduce the installation costs of ground source heat pumps (GSHPs).

In our experience there is growing interest in the use of combined boiler/ASHP units but the overall sustainability benefits will vary with how these heat sources are configured in relation to the bivalence point.

For example, the bivalence point for an ASHP might be at an ambient temperature of 5 deg C. Below this temperature, the Coefficient of Performance (CoP) of the heat pump will be at an unacceptable level if the design water flow temperature is to be maintained. In a typical scenario, therefore, the ASHP will be turned off at this point and the boiler will be used to meet all heating loads.

However, with a good control strategy the ASHP can be configured to work with a variable bivalence point, whereby an acceptable COP can be maintained. Here the ASHP effectively meets part of the load leaving the boiler to provide the remainder so that the use of fossil fuels is minimised.

Help with variable heating loads
This means that if a building requires 30kW of heating it is possible to meet the heating loads with a 10kW ASHP and a 20kW boiler. For optimum control and efficiency a modulating boiler with a good turn-down will help to meet variable heating loads.

However, it is traditional for specifiers to seek some peace of mind by 'over-sizing', so it's far more common to have at least half of the load being covered by the ASHP combined with a boiler sized to meet the entire load. Again a modulating boiler with a good turn-down will help to meet variable heating loads with maximum efficiency.

A potential complexity that arises from using heat pumps and condensing boilers together is that if you pre-heat the water with the heat pump it will raise the return temperature of the water and reduce the amount of condensing that can be achieved. The solution is to use a thermal store as a buffer with very good control of stratification. In this way, the different temperature layers are maintained and each heat source is used to maximum benefit.

Good thermal stores incorporate devices such as perforated plates to prevent turbulence and have multiple temperature sensors for efficient monitoring of the system.

The relatively high installation cost of ground source heat pumps (a ballpark figure would be around £1,000/kW), compared to ASHPs, has proved to be a strong deterrent to their use in the UK. However, if they are combined with ASHPs the cost of groundworks can be reduced considerably.

The extent of the groundworks is proportional to the heating loads, because when heat is extracted from the ground it requires some time to recuperate. Thus a GSHP on its own may require considerable groundworks to allow for recuperation time.

However, if an ASHP is also brought into the mix it will reduce the running time of the GSHP, so that less recuperation time is required and the extent of the groundworks can be reduced accordingly. Our experience shows that running the ASHP only when ambient temperatures are above the bivalence point will reduce groundworks by around 25 per cent. And if the ASHP is used below the bivalence point to supplement the GSHP, groundworks can be reduced by as much as 50 per cent.

A single unit that combines both ASHP and GSHP and incorporates the required high level of controllability is already available in the UK.

In all such cases, it is the skills of the building services engineer that make the difference between exceptional and disappointing performance for the end user. The important thing is to be aware of the possibilities and design the system accordingly - calling on specialist support if required.

// The author is owner of MHG Heating //