Low-carbon grid renews the case for heat pumps
Published: 06 October, 2016
The technical and economic arguments in favour of heat pumps are becoming increasingly persuasive, according to David Pepper of Lochinvar.
The Government’s latest annual energy statistics show that renewable energy is making up ground on fossil fuels and rapidly decarbonising the UK’s electricity supply.
Last year 25% of our electricity was generated from renewables — up by 29% on 2014. By comparison, coal generated 22% of the country’s electricity — down from 30% in 2014. In some parts of the country more than 50% of electricity supplies are now being generated by renewables, and on one day this summer 100% of Scotland’s electricity demand was met by renewables alone.
The more we strip carbon out of our electricity, the stronger the case for heat pumps that use electricity as a primary fuel source. The fact that they can multiply input energy by a factor of three (or sometimes four) was already a strong energy-efficiency argument for retrofitting heat pumps in place of ageing and inefficient heating systems. If that primary energy is decarbonised then they start to look even more impressive in all round environmental performance.
Replacing some traditional heating and cooling equipment with energy-efficient modern equivalents like heat pumps can deliver energy savings of between 25% to 40% and payback times of five to 10 years, depending on the type of building. Heating generated by heat pumps also continues to attract subsidies under the Renewable Heat Incentive (RHI), although this has been under-exploited by users.
However, experience shows that in a growing number of UK commercial buildings, the most appropriate and cost-effective solution is to integrate heat pumps with boilers, water heaters and thermal stores. An integrated approach will deliver the best return on investment for the end user, as well as the necessary flexibility to meet heating and hot water demand.
|Smaller air-source heat pumps increasingly offer the refinements of larger units.|
The refinement of heat pump technology to bring features previously only available in large multi-stage compressors to smaller models is also making a huge difference and improving performance in tandem with other heat sources. The availability of ‘economised vapour injection’ (EVI) is an example of how the technology has evolved and improved.
Systems that use EVI have a much more efficient refrigeration cycle because the compression process is separated into two stages within a single compressor. As a result, most of the refrigerant is cooled during compression, reducing the electrical energy consumed. This is particularly beneficial at lower evaporating temperatures — i.e when the outdoor air temperature is around 0°C or below. It also increases the evaporating effect — the useful amount of heat that the refrigerant can extract from the outside air.
Gas absorption heat pumps (GAHP) have the potential to gain traction in our market and are capable of delivering even more impressive coefficients of performance (CoPs) than ‘standard’ vapour-compression air-source heat pumps. Their efficiency is also less affected by changes in outdoor temperature. A gas absorption system will only lose around 10% of its operating capacity when the outdoor temperature falls from 5°C to -5°C.
So, by correctly sizing a GAHP system, a designer can provide a reliable source of hot water to feed heating and domestic hot water systems at a CoP of around 1.4 (seasonally adjusted across a whole year).
Heat pumps work particularly well if they are integrated with a condensing boiler to provide low-temperature hot water for underfloor heating or low-temperature radiators; the use of weather compensation control in the heating mode will also significantly enhance system efficiency. GAHPs can also be used to supply pre-heated hot water to a gas-fired condensing water heater.
Another growth area for heat-pump technology is as part of multi-valent systems where energy is gathered from a number of separate sources — including condensing gas-fired boilers and/or water heaters, as well as other renewables.
|Gas absorption heat pumps have the potential to gain traction in our market.|
In such a system, the use of a suitably flexible and properly sized thermal store becomes the critical element. The integrated-system set up ensures that the heat pump will supply the main heat load, and the non-renewable plant (such as a gas condensing boiler) only runs at times of particularly high demand or during very cold periods — so minimising fossil-fuel use.
The use of a thermal store/buffer vessel capable of accepting multiple energy sources, in tandem with the latest heat-pump technology, enables the design engineer to provide an extremely flexible solution with a high output. The thermal store effectively acts as a large, low-resistance header that can accept heat from up to three sources. This smooths out the system capacity to maximise efficiency. It also prevents legionella risk because it does not store domestic hot water.
Integrating technologies in this way requires good control strategies and a thorough commissioning process to ensure the various parts of the system work in correct sequence. The system should be set up to ensure the renewable technologies are the first to respond to any call for heating and hot water, with gas-fired boilers acting as back-up.
Manufacturers can support the work of design engineers and contractors by providing expertise on how the different technologies can best be integrated. If the industry’s supply chain can get these calculations right and take advantage of the rapidly changing carbon picture, the march of heat pumps looks unstoppable.
David Pepper is managing director of Lochinvar.