Ground Source Heat Pump Association

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Groundworks are Infrastructure Investments

by Bean Beanland, BBH Energy Strategies

All ground source heat pump systems comprise of two core elements: the heat pumps themselves and the infrastructure providing the renewable energy resource.

To date, most of the recognised financial models for the technology have tended to assume that the total system has the same useful life and that the total investment should, therefore, be written down at the same rate. This provides an entirely false perspective for the total costs of ownership for a ground-source heat pump system in relation to the design life of any building. This problem is then exacerbated when comparisons are being made with traditional fossil fuel plant equivalents. This argument extends to both open-loop and closed-loop technologies but as closed-loop installations dominate the UK market, this is where the problem is concentrated.

Durable Materials

The materials used for most closed-loop collector array installations, both horizontal in trenches and vertical in boreholes or energy piles are variants of polyethylene and are generally made from the PE100 polymer. This material is internally abrasion resistant, stress cracking resistant, not notch sensitive, resistant to rapid crack propagation in a wide temperature range (+60°C to -30°C) and resistant to a variety of chemicals. The fact that it does not react negatively (chemically or electrically) with most surrounding soil types ensures that the pipework is not susceptible to rot, rust, corrosion or loss of wall thickness.

Polyethylene also has many mechanical benefits being reasonably flexible and lightweight, relatively easy to install with a high degree of tolerance to ground movement and requiring of minimal to zero maintenance. For these reasons, it has been used for many years for a wide variety of infrastructural purposes; "Blue" for UK potable water supply below ground (EN 12201-2, WIS 4-32-17), "Black" for UK potable water above ground and for industrial and general waste applications above and below ground (EN 12201-2, EN 13244-2) and "Orange" for 7-bar natural gas supply below ground (GIS/PL2-8). In addition, PE materials have the ability to quickly form highly reliable end-load resistant fusion weld joints with a strength and minimum design life equivalent to that of the pipe itself.

Extended Design Life

All of the above properties contribute to a very extended design life and an even longer life expectancy. Hot & cold water pipe systems are designed to have the same lifetime expectancy as a typical domestic building, i.e. more than 50 years. PE pipe systems for hot & cold water have been in service since the 1970s. Laboratory ageing tests indicate that the life expectancy of some types of PE could be in excess of 100 years. There are examples of pipes dug up after 60 years of active use being proven to still be fit for purpose when analysed and likely to have a further life expectancy of another 50 years, minimum. Closed-loop ground-source collectors have been extensively deployed in energy piles in Central London Crossrail stations. Research carried out during the design phase verified that they would meet and exceed the 120 year design life of the Crossrail structures. A report produced by Arup's for Crossrail stated "In terms of the PE itself, products using the same material are used as waterproof membranes, linings for landfill sites and more particularly as ducts in PT construction in highways structures requiring a design life of 120 years. The service life of the PE pipe is expected to exceed 100 years according to both the US Plastic Pipe Institute and the Corrugated Polyethylene Pipe Association, with two academic studies giving service life predictions of 265 and 300 years respectively". In support of this, the Minimum Required Strength (MRS) is usually specified at a nominal temperature of 20°C. Many buried pipelines, particularly in Europe, operate at temperatures significantly below this (typically 7°C to 10°C). Hence when operating the PE pipe at pressures based on the MRS, the actual strength of the pipe is significantly greater due to the lower operating temperature. As a result the service life will theoretically be extended well beyond the nominal 50 year design lifetime. PE pipe manufactured to EN 12201-2/ WIS 4-32-17 can withstand transient surges of twice the rated pressure of the pipe. For example, an SDR17 pipe rated to 10bar can withstand surges of 20bar.

Advances in Polymer Science

The success of PE for pipeline applications has been driven through a history of advances in polymer science over the last 60 years. This has resulted in the current availability of premium grade PE100+ materials with further enhanced strength and stability and the improved crack resistance of RC variants.

It is clear then that the design life of ground-source collector arrays, when installed under the auspices of industry best practise and using sector-leading materials, is comparable to the design life of other infrastructural deployments such as water and gas mains. Typically, this can be assumed to be several multiples of the design life of the heat pump plant room installations which are considered to be a maximum of 20 years or so. The equivalent economic models would suggest that gas mains are run into new developments with a design life comparable to the buildings being served but with an expectation that the gas boiler systems will need to be refreshed on a routine basis of 8-12 years. It should, therefore, be the case that the competitive ground-source model should see the design life of the collector array recognised as at least that of the buildings, with good quality northern European or Scandinavian heat pumps requiring refresh every 12-20 years.

Decarbonisation of the Grid

If the design life of the collector array is recognised properly then the carbon argument in support of ground-source heat pump technology becomes much easier to make. As the national grid is progressively decarbonised, the same high quality collector infrastructure will deliver a progressively lower carbon energy resource over time. Using DEFRA data, the carbon factor of grid electricity was 495 gCO2/kWh generated in 2014. In 2015, the equivalent figure was 462, which represents a fall of over 6.5%. In 2016, the figure was 412, a fall of 10.8%. GridCarbon, which reports the current carbon factor of grid electricity, routinely shows a figure of below 400. Such real time reporting starkly highlights the impact of strong sunlight and high winds through solar PV and wind turbine contributions. A typical 100m deep ground-source borehole with a single probe in reasonably standard UK geology will support a 5kW heat pump. If this operates for a typical 2,200 FLEQ run hours per annum at an efficiency (SPF) of 3.4 the following carbon reductions would result:

Thermal energy output kWh Fuel type System efficiency Primary energy consumed kWh Fuel carbon factor gCO2/kWh CO2e emissions kg Emissions reduction vs natural gas Emissions reduction vs natural gas
11,000 Natural gas 85% 12,941 184 2,381 - -
11,000 Electricity 340% 3,235 494
in 2014
1,599 782 35%
11,000 Electricity 340% 3,235 462
in 2015
1,495 886 39%
11,000 Electricity 340% 3,235 412
in 2016
1,333 1,048 47%
11,000 Electricity 340% 3,235 250
est 2020
809 1,572 70%

The government has committed to no coal being burnt by 2023 and DECC's own projection (November 2015) is that the carbon intensity of the grid will continue to fall to below 250g CO2/kWh by 2020. This has profound implications for gas boilers and gas CHP being specified and installed now with a design life of 8-12 years.

Comparisons which take into account actual gas-fired boiler efficiencies and the displacement of more carbon intensive primary fuels such as LPG, oil or direct electrical heating are even more favourable. A seasonal performance factor for a well-designed heat pump system in excess of 3.4 is perfectly achievable in new build dwellings to current Building Regulations. In commercial and urban environments where the same ground-source borehole infrastructure can deliver cooling in addition to heating, the carbon arguments become even stronger.

Infrastructure of the Future

The case for ground-source heat pump collector arrays to be recognised as a core carbon reducing infrastructure is clear and growing. Some utility companies are beginning to see the potential and are exploring ownership and financial models. The existing law around core infrastructure appears to be extendable to this concept without too much effort. Couple these factors to the climatic conditions in the UK which are very favourable for ground-source technology, with relatively benign winter temperatures and relatively high winter rainfall, and the potential for the ground beneath our feet to be the primary fuel infrastructure of the future is highly promising.

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