How heat-pump technology can meet renewable-energy targets
Heat pumps are a very effective way of using renewable energy in commercial buildings, leading Mitsubishi Electric to develop a new product range.
Buildings with air conditioning, especially VRF air conditioning, are generally heated using reverse-cycle heat pump system. As such they can exploit renewable energy, normally from the air but also from other sources, to help meet planning permission requirements for renewable energy and to achieve a higher rating on the Energy Performance Certificate.
The headquarters of Mitsubishi Electric at Hatfield, for example, now has heating and air conditioning provided by heat pumps — improving its Energy Performance Certificate from 117 to 67. The EPC rating required for a new building used for the same purpose is 62, a minimal shortfall.
Despite their benefits in reducing carbon emissions and exploiting renewable energy, heat pumps are generally not used in commercial buildings just to deliver space heating, according to Donald Daw, divisional director of Living Environmental Systems UK with Mitsubishi electric. One of the main reasons is that the capital cost of gas boilers is significantly lower than heat pumps — and why install heat pumps just to provide heat when cooling is not required.
Nevertheless, a demand does exist for heat pumps, with Donald Daw explaining that 46% of energy consumption in the commercial sector is for space heating and 9% for domestic hot water — with only 5% is for cooling and ventilation.
It is that back- ground which prompts Donald Daw to say, ‘We believe heat pumps will become the mass-market alternative to heating based on combustion technology.’
Customers such as consultants and specifiers indicated their agreement when asked, including a focus group of key specifiers. The response when offered heat-pump technology specifically for space heating and hot water was along the lines, ‘This is what we have been waiting for.’
Heat-pump boilers
That research led to the development of a range of heat-pump boilers comprising an air- or ground-source R410A outdoor unit delivering hot gaseous refrigerant to an indoor unit, where it is condensed and the latent heat transferred to water for space heating. To deliver the higher temperatures required for DHW, there is a small cascade compressor using R134a in the heat-pump boiler to deliver hot water at 70°C, compared with 45°C for space-heating purposes.
The outdoor unit is a standard VRF unit, but with cooling totally disabled.
The concept is based on boiler modules with a heating output of 25 kW. Up to eight modules can be linked on a frame to provide a total of 200 kW. The reason for this maximum output is that the cost of a heat-pump installation is directly proportional to its output. Above this output, biomass boilers become more cost effective. However, Philip Ord, product marketing manager, sees heat-pump technology being used for commercial space heating on a larger scale when issues associated with biomass boilers such as fuel delivery and storage and maintenance are taken into consideration.
Mitsubishi’s cost figures per kilowatt of heat output for heating-only systems are £385 for air-to-water, £324 for air-to-air (using fan-coil units), £417 for ground-to-water and £448 for ground-to-air. The 25 kW per module heating output for air-to-air systems is based on -3°C ambient.
With the help of built-in weather compensation, high seasonal COPs are achieved, ranging from 3.26 for air-to-air up to 5.14 for ground-to water.
Renewable energy
One of the driving forces behind considering heat pumps for commercial applications is their use of renewable energy to meet planning-permission requirements.
An air-source heat pump using 1 kW of electricity to draw in 3 kW of energy from the air could be regarded as having 75% renewable-heat output. You can make your own adjustments for the type of fuel used in generating that electricity. A ground-source heat pump could readily achieve an 80% renewable heat output.
The economic case for heat pumps will be strengthened next year with the introduction of the Renewable Heating Incentive, due in April 2011.
Mitsubishi has made a couple of case studies to show the effect of heat pumps for heating and hot water on the use of renewable energy. One is an office development, and the other is student accommodation. The analysis is shown in the two panels.
Philip Ord is predictably extremely enthusiastic about the use of heat pumps in commercial buildings to harness renewable energy — but there are pragmatic reasons why. ‘Other renewable systems such as solar thermal, wind turbines, photo-voltaic, microCHP and biofuels each face limits to their use for one reason or another. For the majority of installations, install costs or the limit in renewable capacity restrict where they can be used. Biomass boilers are not without limits as many people don’t appreciate how much space is required for secure fuel storage and regular fuel delivery, or how often they need maintenance to keep them working efficiently.’
