Heat pumps start to show their potential for DHW
Chris Davis looks at the role of heat pumps, including air source, to produce domestic hot water and help meet Government targets for renewable energy.
The UK has a challenging and legally binding target of generating 15% of its energy from renewables by 2020. The Government’s renewable-energy strategy has recognised the potential of commercial buildings to contribute to this shift, and the potential for air-source heat pumps is starting to be recognised.
First things first: the debate over whether or not air-source heat pumps are a ‘renewable’ technology is over. In December 2008 the European parliament adopted an EU Directive on the Promotion of Renewable Energy Sources that expanded its definition of renewable energy sources to include air- and water-source heat pumps, in addition to ground-source heat pumps. All three technologies are now being promoted as part of EU policy to get member states to increase their use of renewable energies. This ruling will make its way into UK legislation in 2010.
In the meantime, the Government’s renewable energy strategy sets out a road-map outlining how it intends to achieve its 15% renewables target. It ascribes key roles to be played by both provision of heat within the renewables mix and also states that the non-domestic sector may be able to deliver a significant proportion of the renewable heat required, generally offering lower-cost opportunities than the domestic sector, In addition, the Renewable Heat Incentive (which comes into effect in 2011) will offer financial support at all levels for locally-harvested renewable heat.
Getting into hot water
Another widely held misconception about heat pumps is that they are only suitable for space heating — and even then, only at low water temperatures for use with underfloor heating. While it is certainly true that heat pumps will always optimise their efficiency at low water temperatures, they also have the ability to provide plentiful hot water, either as the sole water-heating appliance or, if preferred, with another heat source such as solar or a fossil-fuelled boiler.
As heating temperatures increase, heat pump efficiency (coefficient of performance) does drop, but even a relatively low CoP of 2 is still equivalent in terms of CO2 emissions to a gas system, and a good heat pump will normally be operating above such a CoP, even for hot-water production.
The new generation of air-source technology offers performance that is comparable to ground source, with high CoPs at low ambient temperatures. Dimplex’s new range, for example, offers typical CoPs of around 3 in water-heating mode (air at 7°C and water at 50°C).
In addition, the incorporation of renewables brings other benefits. For example, a recent installation at a Travelodge used three 28 kW air-source heat pumps to pre-heat domestic hot water, helping it to help achieve the percentage of renewable energy required to meet the Merton Rule as well as complying with Building Regulations.
Specifier’s checklist
There are a number of factors that specifiers need to consider carefully when designing a heat pump system to meet a building’s hot-water requirement.
• Accurate selection of the air-source heat pump to meet the demand at a range of outdoor air temperatures. ASHP output is a function of heat-source temperature, so both efficiency and heat output will decrease at colder times of the year. The capacity of the heat pump needs to be matched to the energy demand during cold periods, and this is true even of heat pumps located indoors, as the source air is still drawn from outside via ducting. Conversely, an air-source heat pump will operate at a much higher output and efficiency during the summer months.
• The ability to provide sufficiently high water temperatures for stored hot water is also important, both from an efficiency point of view and also to meet health-and-safety legislation aimed at preventing legionella. For heat pumps specified primarily for hot-water production, high-temperature models are able to achieve stored hot water temperatures of 65°C without the need for supplementary heating. Lower-temperature heat pumps will require support from a supplementary heat source to reach the maximum temperature – for example, another heat source such as a fossil fuel boiler or a boost electric immersion heater.
• Correct selection of a cylinder with an appropriately sized heat exchanger to ensure maximum heat transfer — normally heat pumps require a larger heat exchanger surface area. Dimplex’s range of commercial cylinders is compatible with heat pumps and is available in sizes up to 4000 litres, with coil size bespoke designed to exact specification.
Hybrid systems of solar thermal and heat pumps are also possible and becoming increasingly popular. Clever heat-pump controls optimise use of the solar energy before bringing on the heat pump compressor.
The use of waste heat is also an interesting application for heat pumps. Dimplex heat pumps have been installed at the first supermarket in Ireland built to PassivHaus standards. In this interesting and exciting project, the heat pumps use the waste heat from the chiller cabinets at around 30°, which is circulated directly through the system and used to provide hot water at 60°C for the staff canteen and washrooms, and customer toilets.
Heat pumps are a viable option in commercial buildings, for hot water as well as space heating — offering great flexibility over installation and system design. It is a question of specifying the right heat pump for the specific task in mind, and siting it to gain the optimum performance.
Chris Davis, head of renewables at Dimplex.
