Monday, August 08, 2011

Government should think again about supporting air source heat pumps

Last week, a company that installs air source heat pumps was censured by the Advertising Standards Authority for making misleading claims about this technology. But is the Government itself guilty of the same sin?

In a mailshot, ACS Renewable Solutions had said that its air-source heat pumps could "reduce winter fuel bills by 40%", and that they are "over three times as efficient as a traditional heating system".

Energy supplier Calor Gas challenged these and three other assertions, and three of its challenges, relating to winter fuel bills and efficiency, were upheld by the ASA.

Given that off-gas-grid householders are currently able to apply for £12 million-worth of Government £850 vouchers under the Renewable Heat Premium Payment (RHPP) scheme to buy air-source heat pumps, suppliers are falling over themselves to offer the devices.

The RHPP will be replaced from autumn 2012 by the Renewable Heat Incentive, which will pay a fixed tariff for heat pumps, depending on how much heat and hot water they produce.

Ground and water source heat pumps will be eligible for the RHI provided they have a coefficient of performance (COP) of 2.9 or above; this will usually be part of the equipment documentation supplied by the manufacturer.

Some feel that sellers like ACS should not be able to claim that heat pumps are a renewable source of energy. After all, they require electricity for the pump to work, which may not itself be renewable.

The argument is that the heat they pump - from the ground, water or air - is renewable however, as it is ultimately heated by the sun. They are classed as renewable in the EU's Renewable Energy Directive for this reason.

It is right that air source heat pumps will not be supported by the RHI at least to start with, pending further research by the government to better understand the costs, real world performance and benefits. The government will announce the result of the research and how much support will be given in a year's time.

But they need wait no longer for some vital clues. Installers and potential customers now have the benefit of peer-reviewed analysis conducted for the Association of Energy Conscious Builders by the veteran of the industry, John Cantor.

John has been working with the technology for 30 years and no one knows how heat pumps work better than he does.

The first point he makes is that the real value of a heat pump is given not by its COP – which is measured in laboratory conditions - but by its SPF (seasonal performance factor). This is the total useful annual heat produced divided by the total electrical input.

For the SPF, 'real world' measurements need to be done, and these are much harder to standardise.

Crucially, this should include any direct electric 'top-up' heaters which might be necessary to cover any short-fall in the heat output, or to elevate the hot water cylinder temperature.

The new report says that an SPF of 4 is certainly achievable for space heating with a good GSHP (ground source heat pump), connected to a well designed underfloor heating system in an insulated building.

But for ASHPs (air source), results of trials conducted last year by the Energy Saving Trust showed that of 22 evaluated, half fell within the range of 1.6 to 2.2, with one as low as 1.2, and only one achieving a measured efficiency of 3.

This report has been widely interpreted to mean that ASHPs are best avoided. But Cantor's analysis says the EST's results are not the end of the story.

Firstly, he criticises the poor quality of many system installations of heat pumps in the UK (they're better at it in Germany). Installation requirements for heat pumps are "exacting", and the industry is on a learning curve.

This means that if ASHPs ever do get support from the RHI, then installers who want to be accredited under the MCS should have to pass strict tests to make sure they do install them properly.

Secondly, Cantor says that EST should have measured the seasonal performance factor, SPF, not the COP.

Any heat pump, he says, is very sensitive to the way that it is utilised. Its real-world performance will only be as good as the system it is part of.

"A heat pump only has a potential efficiency. The actual performance is that of the entire system, of which the heat pump is only one element," Cantor says.

His best guess for the efficiency of an ASHP installed in the UK is an average SPF of around 2.6. In a new-build dwelling, with a lower heat load, and more freedom to specify a heat-pump friendly system, it might be reasonable to expect SPFs of around 3.0.

The key difference between a heat pump and any other heating system is that a heat pump's energy efficiency is affected greatly by the temperature difference between the source and heat sink. It is at its most efficient when heat is supplied at a relatively low temperature.

If the pipes from the heat pump are hot, the COP is likely to be low, while a heat pump operating at a very high COP will have relatively cool pipes.

An ASPH is not a replacement boiler. You can't expect it to supply heat at 70°C for central heating or 35-40°C for the taps. It works best with ambient, or underfloor heating.

It will also operate at its lowest efficiency - reduced by a third - when we need the most heat – when the air is coldest.

Since the COP of most ASHPs will approach 1.0 when integral direct-acting electric heaters (if fitted - they help defrost the pipes) are engaged, at times of extreme cold and peak demand on the grid, they may pose a real problem if they are large numbers installed in the future.

Ground source heat pumps are around 20% more efficient at these times.

It seems that the Government might be anticipating 150,000 new ASHP installations by 2030, and 425,000 by 2050 (cited by National Energy Action in their paper 'ASHPs – assessing the impact for the electrical distribution system').

And here is the killer argument against air source heat pumps.

Many of these installations might be expected to be replacing gas boilers.

Gas boilers in the home are up to three times more efficient in their use of carbon than electricity supplied to a home from a gas-burning power station, due to inefficiencies in the combustion process and transmission network.

Therefore, there will be no carbon saving, and even a possible increased carbon cost, from installing an air source heat pump to replace a gas boiler – with a COP of 2.6 it will be using the same amount, roughly, of carbon. Add to this the carbon cost of the embodied energy of the installation and there is clearly no carbon benefit.

If the COP is any lower, especially if it is as low as 1.0, then the ASHP will be using more carbon than the gas boiler it is replacing!

The RHPP is a chance for the Government to obtain real-world measurements on the performance of ASHPs. This is why it is specifically targeted at off-gas-grid households.

But given all of the above, the Government needs to take a long hard look at whether there is real benefit for the overall carbon-intensity of the energy mix of encouraging this marginal technology, or whether it's better putting its money into subsidising more carbon-efficient modes of heating.

No comments: