AIR SOURCE HEAT PUMPS IN THE UK
Air Source Heat Pumps (ASHPs) are a mature technology, and widely used in Europe. Due to their efficiency they can greatly reduce the energy needed for heating homes.
However, they are relatively new to the UK home heating market, and their status a low carbon technology is controversial here. This is partly because an ASHP relies on electricity rather than gas. The primary fuel mix of the UK grid means that CO2 emissions associated with this technology can be higher than for conventional heating.
Furthermore, because of the ASHP’s reliance on electricity, many observers have disputed their status as a renewable technology. It has been an open question as to whether ASHPs can contribute to Local Planning Requirements for on-site renewables.
A European Directive provided guidance on this in December 2008, and it went in favour of the Air Source Heat Pump by recognising Aerothermal energy as a renewable energy source.
An ASHP may require electricity to run a compressor, but that enables the device to extract heat from external air. ASHP are typically able to recover 1. kW of aero thermal energy for every 1kW electricity (see below).
As a result, UK planning authorities are more receptive to the specification of the ASHP. This has been good news for developers, because it offers a relatively low cost way to deliver both on-site renewable planning conditions as well as the requirements of the Code for Sustainable Homes.
Recent advice from the Greater London Authority has confirmed that the Air Source Heat Pump is permissible for consideration under the 20% on-site CO2 reduction target in the Mayor’s London Plan.
So, what kind of contribution can the Air Source Heat Pump make to carbon reductions on a housing development?
Heat Pumps – A Background
Heat Pumps are devices that cycle a refrigerant through a compressor to move heat from one location to another. The same technology used in refrigerators can be put into reverse to heat the interior of a house rather than cool it.
Ground Source Heat Pumps (GSHPs), rely on the consistent year round temperatures below ground for their heat, have typically been considered as are renewable technology. The efficiencies of these systems are such that for 1 kWh of electricity put into the GSHP, 2 kWh of heat energy is extracted from the environment. This is an efficiency of 300%. This is also described as a ‘ecoefficient of performance’ (COP) of 3.
Air Source Heat Pumps (ASHPs), rely on the fluctuating ambient air temperature for their efficiency, so performance varies much more than for GSHPs. At present, the government’s standard assessment procedure (SAP) assumes that all such systems operate with a COP of 2.5.
There are a number of UK planning policies that set minimum targets for onsite renewables.
- 10% Total onsite energy demand to come from onsite renewable technology [eg the original Merton Rule, & many of the first regional SPDs to adopt a target]
- 10% reduction in the CO2 emissions associated with total onsite energy through the use of onsite renewable technology. [Many of the later Regional SPDs]
- 20% reduction in the CO2 emissions associated with total onsite energy through the use of onsite renewable technology. [The London Plan, updated 2008]
- 25% reduction in the CO2 emissions associated with regulated (excluding appliances) onsite energy through the use of onsite renewable technology AND fabric improvements [The Code for Sustainable Homes, adopted by the HA and some local authorities]
Renewable Energy Strategies
In light of the EU directive on aerothermal energy, the ASHP can be recommended as a renewable energy technology. However, its applicability to different schemes will depend on the Planning Conditions to be met. It must be remembered that:
- Apparent savings in ENERGY are substantial in comparison to gas boilers, due to ASHP ‘efficiencies’ of 150% or greater.
- When it comes to CARBON reduction however, comparing gas boilers with ASHPs is not so straight forward. Only systems with high COPs will show a carbon saving.
This is illustrated in the two examples given below:
Meeting a 10% Energy target:
A simple comparison of the energy required by a Gas Boiler and an ASHP with a COP of 2.5 shows that a 36% saving can be demonstrated.
Meeting a 10% or 20% CO2 reduction target:
A comparison between the CO2 emitted from a Gas Boiler and Air Source Heat Pumps of different efficiencies is shown on the right.
A COP of 2.5 results in a negative CO2 balance in this example; it takes a COP of 3.5 to achieve a 10% CO2 reduction.
These results will differ depending on the property. The Air Source Heat Pump makes the greatest percentage contribution where space heating demands make up a large part of the total load.
It should be noted that these calculations include a reduction in coefficient of performance for the immersion heating of Hot Water, which is typical for ASHPs.