glass laminate rooftop showing photovoltaic cells
Glass laminate photovoltaic cells at St Pancras Station, London © Historic England Archive
Glass laminate photovoltaic cells at St Pancras Station, London © Historic England Archive

Low and Zero Carbon Technologies

Low and zero carbon (LZC) technologies generate energy from renewable or low carbon sources and emit low or no carbon dioxide emissions.

In 2019 the UK Government announced a target of net zero for UK greenhouse gas (GHG) emissions by 2050. Reaching net zero requires reduction of emissions across the whole of the country including historic properties whether businesses or households. Low or zero carbon technologies that generate electricity or heat or both with low or no carbon dioxide (CO₂) emissions are vital to meeting this target:

In assessing the benefits of these low and zero carbon energy sources for historic buildings you will need to consider:

  • Does it suit the particular building and use?
  • What are the carbon reduction benefits?
  • Will the potential savings exceed the whole-life energy costs?
  • Can the system be fitted safely with no significant adverse impact on the building and its historic fabric
  • What will be the visual impact on the setting of the building or heritage asset?
  • Are there any planning controls that affect your choice and positioning of the installation?

These technologies, particularly those used for generating heat, are more effective within buildings with a highly energy efficient fabric, and where heat demand and loss have been reduced to a minimum.

The output of many of these energy supplies can fluctuate. They will often need to be balanced with electricity supplied from the National Grid, importing or exporting as required.

Apart from the initial set up costs, operation and maintenance costs, and de-commissioning of redundant systems need to be considered. These can be higher than for conventional supplies. Some systems can also have relatively short lifespans which can have implications for life-cycle value.

Photovoltaics (PV)

Photovoltaic cells convert sunshine directly into electricity.

PV cells are modular, which means that they can be combined to create PV panels of different power output and dimension, which can be used to generate electricity in a range applications. PV panels can be mounted atop roofs, but the panel orientation and tilt, visual impact and structural bearing must be carefully considered. Practical installation issues include access for maintenance, availability of internal floor space for ancillary equipment and how and when electricity is needed within the building.

During operation, electricity is generated instantaneously, and must either be used immediately, stored in batteries or exported to the National Grid.

The theoretical energy potential is highly sensitive to environmental factors such as geographical location, time of year and day and PV panels are installed to maximise the duration of time that sunlight strikes the panel. In the UK, PV panels are always installed to be south-facing and inclined. However, partial shading can significantly harm the efficiency and installations should be away from trees, hedges or other obstructions to direct sunshine.

The practical energy potential and net electricity yield will depend upon the size, PV-module efficiency, system type and ancillary equipment, which will all have inherent losses along the energy conversion chain. Internally, sufficient space must be provided for batteries, transformers and meters, which will be part of the installation and located within the building floorplan.

The visual impact of PV panels is particularly important for heritage buildings, their settings and landscapes, and specialist advice must be sought for installation of externally visible equipment. For historic buildings siting is crucial, both in terms of energy generation and heritage preservation. It is important to recognise that PV panel area may need to be large, and it is important to assess how visual damage to the building can be prevented though screens or barriers.

PV panels should be cleaned as necessary where there is a build-up of dust or dirt on the panel surface, as this will harm overall energy output. Electrical installations, including batteries, should be routinely checked to ensure they are operating as they should and may require periodic servicing. Installations will require a support structure for roof-mounting, careful consideration must be given to how this will interface with historic building fabrics and the potential for any detrimental impacts that this may have.

Summary of considerations when planning photovoltaics

Available energy:

  • Location (latitude, longitude)
  • Local environment
  • Blocking of sunshine or shading

Equipment:

  • Type of system (battery-bank, grid exporting, and so on)
  • Specific PV panel power and conversion efficiency
  • Specific ancillary equipment conversion efficiencies

Uses:

  • Electrical demand profile of building
  • Interface with existing electrical supply
  • Appropriate ongoing maintenance

Case studies

Sutton Hoo: The National Trust has installed 172 high-efficiency photovoltaic modules on the new visitor centre roofs at Sutton Hoo, the Anglo-Saxon burial site. These panels will generate around 42,000 kilowatt hours of electricity per year, enough to supply more than 10 average homes.

King’s Cross: At Grade I listed Kings Cross Station, the building-integrated PV in the glass roofs produce 175,000 kilowatt hours of electricity each year, saving over 100 tonnes of CO₂ emissions per annum. There are 1,392 custom-made glass laminate PV panels over the 2,300 square metres of the glass roofing. 

Gloucester Cathedral: Solar power reduces energy use.

Solar water heating

Solar water heating, or ‘solar thermal’ systems, convert the sun’s energy to generate hot water. A heat transfer fluid is pumped through a solar heat collector, which absorbs thermal energy to generate hot water and can be stored for use within a building.

Both direct sunlight and indirect (diffuse) sunlight can be used to generate hot water. Unlike photovoltaic panels, solar thermal systems and do not need to be in direct sunshine. Geography and location influence in the overall energy yield, however a range of solar thermal technologies are available, which are designed to operate efficiently in different climates. Evacuated tube collectors operate best in cloudy sky conditions and would suit installation in the UK. Conversely, the flat plate collector would better suit geographies with greater direct sunshine.

Key considerations for these systems are similar to influencing factors outlined for photovoltaic system installations. A key distinction being that greater internal space may be required for the storage of thermal energy via a hot water cylinder. Solar thermal systems may provide useful energy in offsetting heating demand, which would otherwise be served from the main building heating system. However, for most applications, a dispatchable energy source will be needed to boost heating during times of peak demand, such as gas boilers.

Further advice on solar water heating is available from the Energy Savings Trust

Heat pumps

Heat pumps are a low carbon technology that use electricity to generate hot water highly efficiently. Heat pumps are not an energy generation source and the overall carbon footprint depends upon the carbon intensity of electrical supply. Heat pumps can only be net-zero in operation if the electricity supply is delivered from zero carbon sources.

Heat pumps work on the basic principle of a heat engine, which converts ‘work’ into a change from a low temperature to a high temperature. Heat pumps use special compounds (refrigerants) to efficiently absorb and transfer thermal energy. The overall result is as if heat is ‘pumped’ from a low to a high temperature by using an electrical supply to power components. Domestic heat pumps can use the ambient air, ground or a water bodies as sources of thermal energy to provide heating or cooling to an internal space.

Ground source heat pumps use pipes buried in the soil as a loop or a vertical borehole to extract low temperature heat, which is then ‘pumped’ up to higher temperature. Horizontal collector loops are buried up to two metres below ground and large areas of land are required outside the building to accommodate these. These can be installed in gardens or car parks where there is enough area. In instances of limited area, vertical boreholes may need to be drilled down up to 200 metres deep. For example the bore hole for the ground source heat pump at Wimpole was 150 metres deep. The exact depth is dependent upon the site geology and thermal energy requirement for the system, typically these would only be done for larger demands, that can justify the high cost of boreholes. For historical buildings and scheduled monuments, site archaeology must be considered, and archaeological surveys will be required.

Water source pumps work in a similar way to ground source, but extract heat from a large body of water such as a lake or river. For example, the Castle Howard closed loop water source heat pump draws heat from the dairy pond via 56 pipe coils immersed in the water. The water source heat pump system has replaced the oil system which used to provide heat and hot water to the property and reuses the existing wet radiator system.

Systems can be either closed-loop or open-loop where the water is taken from the main body of water, pumped to the evaporator and returned to the water source. A closed-loop system will be isolated from the river and the water within the heating system will not mix with the water in the river or lake. An open-loop system, on the other hand, will be open to the atmosphere and will circulate water from the river or lake. Special consideration should be paid to how the pipework gets from the water source to the building and how the low energy heat can be used within the building.

Air source pumps extract heat from the outdoor air using a fanned heat exchanger. The heat can be used directly air to air, or air to water for a conventional low temperature hot water system. The key considerations for air source heat pumps is the visual impact of the external unit and internal units, which must be located separately, noise generation from the fan, and how the heat will be used within the building.

An additional means of topping up the heating, such as gas boilers, may still need to be retained for peak heat loads. Heat pumps operate at lower temperatures compared to conventional gas systems; therefore, radiators may need to be increased in size to compensate for this. Conversely, underfloor may not need to be re-sized as it operates at lower temperatures compared to radiators. Internal plant room and external space requirements for accommodating the heat pump should also be thought through carefully. From an environmental viewpoint, key considerations for these systems are the assessment of the site’s geology, hydrogeology, and archaeology.

In addition to heritage consents for heat source installations, you may need to get consent, a permit and a licence from the Environment Agency for an open-loop ground source or surface water source heating and cooling system.

Case study – Shrewsbury Flaxmill Maltings ground-source heat pump (March 2021)

The new ground-source heat pump at Shrewsbury Flaxmill Maltings demonstrates that centuries-old buildings can also adapt to use sustainable energy sources and play their part in efforts to tackle climate change. The low carbon energy source is an important part of Historic England’s regeneration project and our organisation’s commitment to climate change mitigation.

Shrewsbury Flaxmill Maltings comprises eight listed buildings. The Grade I listed Main Mill, built in 1797, was the world's first iron-framed building and paved the way for modern skyscrapers. This and the Grade II listed Kiln, added in 1897 when the site was converted to a maltings, are currently the subjects of a major restoration project led by Historic England and largely funded by the National Lottery Heritage Fund.

The ground-source heat pump system is designed to provide an estimated 69% of energy usage for the Main Mill and Kiln, with the remainder provided by a natural gas boiler.

The new pump will reduce carbon emissions associated with space heating by an estimated 46% from 45 tonnes to 23 tonnes per year.

The pump will extract heat from the ground via 10 vertical bore holes at a depth of 187 metres. These are underneath the line of the former Shrewsbury and Newport Canal towpath which will become a green corridor and a pedestrian and cycle route.

The main heat pump plant will be located on the second floor of the New South Engine House at the Flaxmill Maltings. The Engine House used to house the steam engine which powered the flax spinning machinery, a fitting home for the building’s new energy source. The size of the radiators on the upper floors have been increased to cope with the reduced water flow temperatures from the heat pump.

The ground source heat pump installation is being managed by Historic England’s main contractor Croft Building and Conservation Ltd with JPR Mechanical and Electrical Services Ltd as the subcontractor.

Wind turbines

Wind turbines operate by extracting kinetic energy from the wind. The transfer of energy rotates the turbine, which drives an electric generator. Smooth and efficient wind turbine operation is highly sensitive to the direction, consistency and uniformity of the available wind resource. Turbines can be mounted near buildings but may be visually intrusive, could create noise and shadow flicker issues.

Installations are likely to require planning permission as well as heritage consents.

Summary of considerations when planning wind turbines

Available energy:

  • Location and siting
  • Historical wind data (long-term wind profile)
  • Terrain type (short-term wind profile)

Equipment:

  • Type of turbine (classification, controls, and so on)
  • Duration of time operating at rated output
  • Ancillary equipment power and conversion efficiencies

Uses:

  • Demand type (decentralised, UK grid supplying, and so on)
  • Connection to supply (synchronisation, system balancing, and so on)
  • Appropriate ongoing maintenance

Case Study

There have been several examples of community-owned wind turbines in the UK. Mean Moor wind farm is a community-owned wind farm in Cumbria, comprising of three 2.3 MW (megawatt) wind turbines. The electricity generated from Mean Moor is fed to the grid and expected to power approximately 4,500 homes per annum. Local ownership of wind turbines helps to maximise the regional economic benefits from the harnessing of a renewable resource. The initiative has been successful for income generation and for the preservation of the local economies, despite the potential visual detriment to the natural landscape.

Further advice is available on our Wind Energy web page. 

Hydroelectric power

Hydroelectric power uses the energy stored in bodies of water to generate electricity through a water turbine. The power output is dependent on the volume flow rate and height through which the water falls (the ‘head’). Hydroelectric power generation can vary in scale of power output, but the fundamental principles for energy generation remain the same, which involve diverting water to drive a turbine. Due to the high level of environmental intervention required, capital costs for hydroelectric projects are often substantial, making them more suitable for large-scale generation and long-term operation.

A high degree of maintenance is required, particularly parts exposed to the water. This may involve clearing screens of debris, cleaning filters and more generally, ensuring that the body of water is healthy. In addition, the mechanical and electrical equipment must be routinely serviced.

Systems usually require Environment Agency approval and conditions can be onerous. An environmental impact assessment will usually be a condition of any planning consent.

Micro-hydro schemes have a typical arrangement involving flow diversion from a water body and channelling this towards the pump house at a lower elevation and discharge of water back to the water body. The key considerations are rainfall catchment area, flow regimes, visual impacts of the dam or weir, the environmental impact of diverting a water flow, impact of the pipeline to transport the water to the turbine and then the outflow where the water returns to the water course.

Hydroelectric generation is often well-suited to sites with historic significance, particularly sites with historic watermills. More broadly, these sites may contain:

  • buildings such as watermills with existing mill wheels
  • buildings which were once mills but have since been converted to other uses, and so only remnants of the original use are visible
  • sites with existing infrastructure suitable for a hydroelectric scheme, or
  • other sites where potential for hydroelectric generation has been identified.

Such sites can often be in isolated areas, where on-site generation can cost less than installing a connection to the National Grid. Location can bear strong significance on the project scope and overall economics. An example is the National Trust’s Gibson Mill, a 17th century cotton mill.

Micro-Hydroelectric Power and the Historic Environment provides further advice on micro-hydroelectricity schemes and historic sites.

Summary of considerations when planning hydroelectric power

Available energy:

  • Location and scheme layout
  • Flow duration curve
  • Hydraulic head available

Equipment:

  • Type of turbine (classification, controls, and so on)
  • Duration of time operating at rated output
  • Ancillary equipment power and conversion efficiencies

Uses:

  • Demand type (decentralised, UK grid supplying, and so on)
  • Connection to supply (synchronisation, system balancing, and so on)
  • Appropriate ongoing maintenance

Case study:

The National Trust’s video Power from the past lights up Cragside describes the Trust’s green energy project at this Grade I listed Northumberland home. In 1883-7 Lord Armstrong built a hydroelectric turbine system to generate electricity for his house. The powerhouse is listed Grade II* and includes a Thompson double vortex turbine and a R E Crompton double magnet "Trade"-type Gramme ring compound dynamo which is the earliest known surviving example. The new Archimedes screw turbine produces enough energy to light all 350 light bulbs in the house.

Combined heat and power (CHP)

Combined Heat and Power (CHP) systems provide both electricity and heating. The principle of operation is based on capturing heat produced during electricity generation from combustion processes that would otherwise be wasted and using this as a heat supply. CHP systems can offer significant efficiency savings in comparison to combustion-based electricity production alone and are often applied in district heating schemes where there is a predictable electricity demand.

CHP systems perform best operating under constant demand profiles; therefore, they are often installed where there is a predictable and steady large aggregate demand. Where a demand for both heat and electricity exist in the same location, CHP can reduce energy costs whilst reducing carbon emissions and air pollution. They are used widely for university campuses, housing developments and manufacturing facilities.

CHP schemes can encompass a range of different generation technologies and can be fuelled by fossil fuels or renewables. Traditional CHP systems use fossil-fuel generated electricity, which means that they are not a net zero carbon technology. Natural gas CHP is recognised as a ‘bridging fuel’ which could bring efficiency benefits whilst transitioning to lower carbon fuels. The future for CHP, would be to use zero carbon fuels, such as hydrogen for the combustion process.

The government provides support to improve the commercial case for investing in CHP because of its relatively long payback period, the environmental benefits of cogeneration, and technical complexity. The CHP Quality Assurance Scheme (CHPQA) is an annual assessment process, that ensures that all CHP plants that benefit from government support meet a required level of energy efficiency.

Advantages of CHP systems are best realised when applied at scale. Efficiency gains can offer a lower cost option to grid electricity. Distribution and transmission losses are reduced through a district energy network and operators can save on energy bills. Additional benefits include the ability to modulate output during peak demand, security of supply during network outages and heat storage which should be incentivised to encourage flex within the grid.

For further advice see the government’s Department for Business, Energy and Industrial Strategy guidance on CHP support.

Biomass boilers

Biomass boilers are like conventional boilers but use wood chips or pellets as fuel to generate heat. In doing so, they are carbon neutral as emissions from the combustion process are balanced by the planting and regrowth trees, which sequester carbon dioxide during their lifetime.

Biomass boilers tend to be larger and fully automated systems, they have an auger to feed the boiler, a hopper and a fuel and ash storage area. Due to combustion of a solid fuel, biomass installations require more frequent and complex servicing than conventional oil or gas boilers. Boilers need to be cleaned regularly in order to keep them free of ash, and the feed systems need to be checked weekly.

Biomass fuel can be bought commercially or harvested and processed locally but supplies need to be from Forest Stewardship Council (FSC) certified sustainable sources. Biomass must be dispatchable therefore, the fuel store needs to be dry and big enough to keep enough stock of chips or pellets to minimise the number of deliveries. Incorporating the indirect carbon emissions from fuel deliveries and logistical operations, a poorly operated biomass installation may not be carbon neutral. Regular delivery of biomass pellets may preclude certain sites with limited access and must be taken into careful consideration in heritage sites, which may be in remote locations. Guaranteeing long-term supply chains are essential, the operation of a biomass system will be dependent on the regular availability of biofuel.

Biomass boilers are covered by several regulations including the Clean Air Act. Local authorities have the powers under this act to request the measurement of dust emissions from biomass boiler exhaust stacks and require arrestment plant to be installed to control dust emissions. Where the local authority has designated a ‘Smoke Control Area’ biomass boilers must be approved as an 'exempt appliance'. Biomass energy generation is unlikely to be acceptable in urban areas due to the emissions and issues surrounding entrapment of air within built up regions.

Further advice on biomass fuel is available from the Energy Savings Trust.

Case study:

The National Trust has installed a biomass boiler at Dyrham Park as part of a new conservation heating system to help preserve the English tapestries and Dutch paintings. The wood chip is sourced from woodlands just north of Dyrham Park. The chips are a woodland management by-product and the supply chain can be overseen effectively.

The National Trust has also installed biomass boilers at other historic properties: Croft Castle, Barrington, Dunham Massey and Killerton.

Fit for the Future network

Fit for the Future led by the National Trust, is a cross-sector network supporting hundreds of practitioners to make their organisations more climate-friendly, adaptive and resilient and achieve carbon net zero targets. Historic England and other heritage organisations are members.

The network helps people share ideas and practical experience through events, workshops, showcases, newsletters and online resources.

Learn more

View the recording of the 2020 webinar on Climate change adaptation: low carbon heat sources.

Heating and hot water for buildings make up 40% of energy use and 20% of greenhouse gas emissions in UK buildings. To reach net zero emissions by 2050 the question is what will the future of heat in our buildings look like? As we move away from the familiar fossil fuels like natural gas and oil, what does that mean for older buildings? This webinar looks at what the future could look like for heating and discusses the options available.

For the best webinar experience, please use Google Chrome browser or download Adobe Connect.

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