Taking gas to new levels of efficiency
Boilers can’t get much more efficient, so how can you reduce carbon emissions much further? Mike Wheeler of Shorts Environmental took Ken Sharpe round an installation of gas-fired air-source heat pumps.
How does a gas-fired heating system that is potentially over 60% more energy efficient than even the best condensing boiler appeal to you? That is what gas-fired air-source heat pumps using an absorption cycle based on ammonia and water can achieve, says Mike Wheeler, national sales manager with Shorts Environmental.
Shorts specialises in low-carbon building services, including the Robur range of gas-fired absorption heat pumps (GAHP) and another manufacturer’s biomass boilers.
One of the largest recent projects for Shorts is a primary school in Milton Keynes with six GAHPs, each having a natural-gas input of 22 kW and a heat output of 38 kW at 7°C ambient. Those figures indicate that 40% of the overall heat output comes from renewable energy. According to Shorts’ literature, these units achieve an energy efficiency (GAHP people talk efficiency, not COP) of 152% at 7°C ambient and a flow temperature of 50°C. The efficiency is still 100% at minus 20°C ambient.
The implication is that heating costs can be reduced by up to 40% compared to the best condensing boiler, giving a payback of two to four years.
GAHPs are, however, much larger and heavier than straightforward gas-fired boilers with an output of 40 kW, corresponding to one GAHP, or 230 kW, corresponding to the six GAHPs at this school — but renewable energy has never been neat and compact.
However, air-source heat pumps are installed outdoors, so the square-metre footprint of a single unit is less of a problem than a similar-sized piece of kit installed in a plant room. They are pretty heavy, too, with a low-noise version weighing 400 kg. You can shave 10 kg off the weight with a standard version, but with a 9 dB(A) increase in noise level to 54 dB(A).
The installation at the Primary School in Milton Keynes has two modules of three heat pumps each which are connected to a low loss header. Onward connections to underfloor heating circuits and radiators were installed by the M&E contractor for the school, which opened in September 2010.
There is also a gas-fired boiler to support the heat pumps in very cold weather and to provide domestic hot water.
Space heating is almost entirely underfloor. There are just four radiators, generously sized to operate with the low flow temperatures associated with the underfloor heating. One is next to the main entrance by the reception office. Another is by the staff door. The other two are at the bottom of the staircase to the upper floor, where there is no underfloor heating.
Mike Wheeler’s advice to consultants and end users considering the use of GAHPs is to work with Shorts throughout the project to avoid the risks that may be associated with contractors who may not understand the intricacies of gas-fired absorption heat pumps. Such involvement means that Shorts can guarantee the flow rates and temperature to primary circuits.
Shorts can also provide a bespoke maintenance service.
Robur GAHPs have been installed in many countries around the world for many years, and many installations are much larger than anything in the UK. A Holiday Inn in northern Italy, for example, has about 20 units as the primary source of heating and hot water.
How do gas-fired heat pumps compare with electric heat pumps? Mike Wheeler’s response is that the capital cost of GAHPs is 30% less and that they are also cheaper to install — especially if supplied with a primary connection module.
And when it comes to assessing the renewable-energy benefit, gas is a primary fuel, unlike electricity, so the benefit is immediately evident — without the complexity of the carbon-conversion factor required for electricity.
One setback to the economics of a project is that GAHPs do not currently qualify for the Renewable Heat Incentive. Mike Wheeler explains, however, that the intention is that they will be included in the second phase of the RHI coming into effect in summer 2013.
Another importance factor to the economics of a project is an accurate calculation of the heat requirement because of the high incremental capital cost of heat pumps compared with boilers. ‘You certainly do not want to install six heat pumps when four will do,’ says Mike Wheeler.
This school installation comprises six heat pumps, but a project in the near future will have eight units. At the other end of the spectrum, the 38 kW of a single unit is well suited to small commercial projects