There’s a new GSHP technology on the block and rumour has it, it’s set to make big changes

An emerging form of ground source heat pump system (GSHP) known as ‘shared ground loop arrays’ is making a big impact on climate change and air pollution at the same time as providing low-cost heat for households and a return for investors in the technology. Here Dr Matthew Trewhella, Contracts Director at the Kensa Group, explores the GSHP industry’s latest newcomer.

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onnecting multiple homes and businesses together with individual heat pumps in each property connected to a low-temperature shared ground loop array provides a technically robust district heating system, yet allows simplified billing and administration as each heat pump is installed inside each dwelling and connected to the individual property’s own electricity supply. Each heat user, therefore, pays for the heat they have consumed naturally through their own independent electricity bills.

A further advantage of this design in comparison to traditional district heating systems is that it prevents overheating; a further climate change focus area for our major cities. In a typical apartment block, conventional central plant systems rely on circulating hot water, typically at around 65 to 85°C through a network of pipes around the building; because the system doesn’t know when any one flat might want heating or hot water, this has to be maintained hot 24/7.

The pipes are insulated, but they still leak a small amount of heat into the building; in summer, this can cause the building to overheat. Over time, the pipe insulation degrades and often parts of it get damaged, making the problem worse and, of course, these heat losses also have to be paid for in energy bills.

The ambient temperature shared ground loop design avoids this problem entirely. The ‘heat’ circulating the building is low temperature, and the heat for each dwelling is produced at the point of use, i.e. inside the heat pump within the flat, so heat losses are minimised, and pipes are only insulated to stop the possibility of condensation, not to retain heat.

The design also opens the opportunity for passive cooling to provide some free cooling into the building, at the same time energy taken from the building is used to increase temperatures in the ground array further increasing the efficiency of the system.

The efficiency of ground source heat pump systems is high – with the heat pumps producing three to four units of heat for each unit of electricity they consume. Running costs and CO2 emissions are therefore extremely low – running costs compete well with mains gas boilers whilst CO2 emissions are half that of a gas system (and reducing further as grid electricity decarbonises).

Air quality

A ground source heat pump extracts naturally occurring heat from the ground and uses it for space heating and hot water production in buildings (domestic and commercial). The heat pump itself is an electrically driven device with no combustion. There is therefore no point of use emissions of any pollution – particulates, NOx or SOx (and no point of use CO2 emissions either).

Air quality in the UK’s cities is a major issue which has been linked to 40,000 premature deaths per year. The air pollution comes in three main forms: particulates; nitrogen oxides (NOx) and sulphur oxides (SOx). Much of this air pollution is caused by vehicle transport, but there is also a contribution from combustion heating systems that burn gas, oil, LPG or biomass to produce heat. In London, the NOx pollution contribution directly from domestic and commercial gas boilers is estimated to be 16%, but that percentage is set to grow as transport becomes cleaner.

A particular benefit is the use of ground source heat pump systems in cities. As an example, if a modern condensing gas combi boiler heated a typical three-bedroom property that consumes 12,000kWh per year of heat, it would produce approximately 480kg of NOx emissions. For older, non-condensing boilers this could be over two tonnes of NOx per annum.

Whilst ensuring that all gas boilers in London are modern condensing boilers would produce significant air pollution savings, it is worth stating again that ground source heat pumps produce no point of use emissions. There are estimated to be 900,000 domestic gas boilers in London, and if these were all replaced with ground source heat pumps, we could reduce the NOx emissions in our city alone by over 400,000 tonnes per annum compared to even the best gas condensing boilers.

It is important to note that there are some NOx emissions associated with electricity generation; however, power stations are typically located outside of cities where air pollution is greatly reduced, and they are much simpler to monitor, measure and control local pollution effects. The air pollution effects of UK grid electricity is also reducing due to the decommissioning of coal-fired power and the increasing role of renewables in the UK energy mix.

Simon Lomax, Managing Director of Kensa Heat Pumps, comments: “Thankfully, Government is starting to show interest in air quality issues linked to heating systems; the recent Clear Air Quality consultation poses the question as to whether biomass installations in urban locations should even be eligible for the Renewable Heat Incentive. As an alternative, ground source heat pumps are the perfect solution for cities.”

Climate change

As well as promoting initiatives to improve local air quality, the Greater London Authority is set to introduce strong CO2 targets. Policy SI2 proposes that all new developments achieve an on-site reduction of at least 35% beyond the baseline of Part L of the current Building Regulations. Any CO2 emissions remaining after are allowed to be ‘offset’ at a suggested rate of £95/tonne (actual rates are to be determined by individual boroughs). Current Building Regulations express CO2 emissions as the Dwelling Emission Rate (DER), and it is already mandatory that these are below the Target Emission Rate (TER) which is the emissions of a notional building of the same size and shape as the development. Using the carbon factors in SAP 2012, a shared ground loop array ground source heat pump system would produce a saving of 27.9% compared to a gas combi boiler. Using the proposed carbon factors in SAP 2016, the saving would be 42.4%. As the UK grid becomes increasingly decarbonised, the savings become ever greater – using 2017 average grid carbon factors predicts a saving of 49.4%. If ground source heat pumps are combined with local thermal storage and time of use tariffs, even further savings will be realised as the heat pumps will typically operate at times when the grid has lowest emission rates.

What this means in practice is that properties are becoming much more expensive to meet DER targets using gas boilers, which significantly reduces the historic capital cost gap between ground source heat pumps and gas boiler systems. For example, an 82m² representative property that just passes the 35% rule would be required to pay a carbon offset price of £2626. If you take the same building and change the heating to ground source heat pumps with shared ground arrays then the carbon offset price would be £1893 – a saving of £733 per property or £219,900 on a 300-unit development.

Non-Domestic RHI

The passing of the long-awaited reforms to the Renewable Heat Incentive (RHI) regulations in May 2018 have cemented the financial appeal of shared ground loops. The most significant change in the reforms is the decision to base Non-Domestic RHI payments for residential properties linked to shared ground loops on the deemed heat consumption taken from the property’s Energy Performance Certificate. This arrangement mirrors previous policy for standalone ground source heat pump installations supported by the Domestic RHI and gives certainty to system owners. The returns from the Non-Domestic RHI combined with the carbon offset savings make a compelling case for investors in ground arrays.

According to Simon: “This refinement is long overdue. Thankfully, we can finally stimulate increased deployment in line with the ambition detailed in successive carbon budgets and multiple Government publications. In many cases, shared ground loop systems will benefit tenants who are living in fuel poverty as landlords can now invest with confidence and deliver systems which offer the lowest running costs. And the falling carbon intensity of electricity generation means the carbon emissions linked to ground source heat pump installations has never been more appealing with further savings forecast.”

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