While ground-source heat pumps continue to make most of the headlines, Simon Keel of Daikin Airconditioning UK Ltd puts the case for its air-source relative.
THE reduction of greenhouse gas emissions demanded by the Kyoto Protocol ten years ago made it obvious that energy utilisation patterns in buildings would need to undergo significant changes.
The UK government went further in its Sustainable Energy and Climate Change bill by aspiring to cut overall UK carbon emissions by as much as 60% below their 1990 level by 2050. Furthermore, the Part L 1a building regulations Conservation of Fuel and Power directive seeks to achieve a zero carbon dwelling by 2016.
As a result, the many cost and efficiency benefits inherent in the use of heat pumps, widely known and successfully applied in residential property in Scandinavia, Germany and France for many years, have also gained belated but growing recognition in the UK. Now, driven by the need to reduce energy usage and costs, heat pump installations are making extensive inroads into the traditional fossil fuel boiler market.
This sector however, is still responsible for around 27% (or 152 million tonnes) of all UK carbon emissions, with some 80% of it, according to the BRE, caused by central heating and dhw production. It comes as no surprise therefore, that manufacturers, suppliers and installers have been quick to recognise and promote electrically driven air-to-water and ground-to-water heat pumps as environmentally conscious and energy efficient alternatives to the fossil fuel boiler.
Greater outlay
Whilst ground-source heat pumps are perceived to provide efficiencies some 10 -15% higher than those of air-to-water units, careful consideration must be given to the type and size of the bore hole(s) or trench and the ground sub strata and it is debatable whether (in domestic installations) the extra efficiency can ever repay the greater outlay in capital expenditure that they require.
Air-to-water heat pumps however, using low temperature free heat from the atmosphere in the form of renewable solar energy, have substantially lower capital cost, are clean, easy and quick to install and service and can return COPs of between 3 and 5. Also, their low carbon emission characteristics, readily accessible heat source, good low ambient performance and the potential to reduce valuable energy input by up to 80% present a compelling argument for their use in residential applications.
That said, of all these benefits, perhaps the most telling is the low carbon emission factor. For example, a 6kW output air to water heat pump with a COP of say, 3.1 and a grid electrical input of 2kW emits 2.0 x 0.422kg/kW (or 0.844kg carbon per hour). A 90% efficient condensing gas boiler at 6kW output however, produces 0.194 ÷ 0.9 x 6 (or 1.293kg carbon per hour) ie 53% more per hour.
Inverter control
The heart of modern sophisticated air to water heat pumps is the compressor, which takes heat from the outside air and transfers it to the building water circuit, transporting the precise quantity of heat to meet the required duty. In spring and early autumn the heat needed to bring a building up to temperature is obviously less than in deep mid winter.
Traditional methods therefore, whereby a burner cycles on and off, overheats the water, turns off the flame and then allows the water to cool to below the set point before re-igniting the gas, uses excess energy and represent a far from satisfactory method of operation.
However, an inverter-controlled compressor, like that used in Daikin’s Altherma, is able to vary its speed relative to the heating. This ensures against wide fluctuations in temperature within the heated space, keeps running costs to a minimum and provides the all important environmental benefit of significantly reduced levels of emitted CO2. Also, the refrigerant charge in domestic format Altherma units (up to 8.4kW capacity) is just 1.7kg and is therefore outside the scope of the main F-gas regulation applying to larger systems.
Weather compensation is also important. Thus, if the ambient gets warmer, the system can sense the need for less heat and throttles back the compressor accordingly to reduce the water temperature, saving energy, reducing CO2 and ensuring the maximum level of comfort.
Domestic hot water
Altherma also handles the domestic hot water requirements throughout the year maintaining a high heat output relative to power input. Even in the winter however, the system can return effective COPs at temperatures down to the -15ºC to -20ºC levels experienced in countries such as Norway.
The combined heating and comfort cooling version of the system provides cooling by reducing the water temperature to 4ºC and supplying it to indoor fan coil units without compromise to the domestic hot water supply.
System design and capacity selection is naturally, critical, particularly if the heat pump is required to cover 100% of the heating load and, in the UK, capacity must be determined on the basis that the ambient temperature rarely drops far below zero for more than relatively short periods. On the rare occasions when it does fall below a pre-determined set level, a small electrical booster heater will provide the supplementary heat needed.
As the market expands, further improvements in heat pump COPs can be expected and during the next few years, international and national government legislation will no doubt, drive the technology onwards and upwards.
In this regard, initiatives such as the so called ‘Merton’, effect, encapsulated in the government’s Policy Planning Statement number 22, which requires all new commercial buildings of more than 1,000m2 to generate a minimum of 10% of their energy requirements from renewable sources, merely reflects acceptance that the heat pump has well and truly, come of age in the UK.
ALTHERMA is Daikin’s residential air-to-water heat pump system. Its main components are an outdoor unit and an indoor hydro-box. In heating mode, Altherma provides low temperature water to underfloor heating and/or radiator circuits.
The domestic hot water option can heat to 65ºC or above via a purpose built stainless steel tank, the lower part of which is heated by water from the heat pump to 55ºC. The electric booster heater in the upper part of the tank boosts the water temperature when required
The domestic hot water facility can also be extended for use in conjunction with solar heating panels, a special kit enabling the water tank to be connected to a solar collector and hot water production via the solar collector prioritised by built in control.
If this is unable to generate sufficient energy, the system switches automatically to heat pump operation or the electric booster heater in the water tank. This adds between 30 and 70% of the solar heated water to the heat pump system, which also uses solar heated air to provide the rest.