Heat pumps, hydrogen, re-used waste heat; we have the technology to cut down CO2 emissions in homes and offices, but what do the experts advise?
The road to net zero 2050 is fraught with challenges and the need to decarbonise heating in buildings – the single biggest source of greenhouse gas emissions comprising around 37% of national CO2 output – is the most pressing of all. This has been recognised in the long delayed government's Heat and Building Strategy.
But the move to greener fuel sources for heating remains in its infancy. One scenario, from UK Power Networks, forecasts that 7.6 million electric heat pumps would be needed across Britain by 2050 – from around just 10,000 in operation in 2019.
Swapping out methane for hydrogen in the gas network offers an opportunity to reuse existing infrastructure, but building services experts claim the fledgling state of development means market-ready solutions may not be practical until the 2030s.
‘We need to make an impact on decarbonising heat immediately, and hydrogen is not ready to be rolled out at a large scale,’ explains Jon Gregg, associate for sustainability at Buro Happold. ‘It’s definitely part of the mix but we’re not in a position where homes can be easily transferred to the technology. Heat pumps, whether air or ground source, are really the only technology that can be delivered at scale today.’
Regulation can give businesses and consumers a head start on the changes needed to decarbonise heating and encourage investment, but clear and concise guidance has yet to emerge from Whitehall.
A national Heat and Buildings Strategy, due to be published this autumn, has been pushed back to 2021. Meanwhile a 10 point plan for a green industrial revolution, revealed last month, includes positive initiatives, such as a target to install 600,000 heat pump systems per year by 2028, an extension to the Green Homes Grant and more funding for the Public Sector Decarbonisation Scheme. However, it ditches an earlier plan to implement a Future Homes Standard by 2023, which would have required all new-build homes to include low-carbon heating technologies. Compare that to Norway where oil-fired heating systems are already banned in all buildings.
Considering that any heating systems installed today might still be in use in 30 years’ time, when net zero kicks in, designers need to think hard about sustainable options for newbuilds and retrofits and for buildings of different size, use class, heating and cooling loads.
Transferring the 85% of UK homes currently connected to the gas grid to low carbon alternatives is a Herculean task, not least because burning gas is cheap, delivers high temperatures quickly and the technology is familiar to installers and users.
Heat pumps, both air and ground source, tend to have higher capital costs, are not plug and play and require various modifications to building services. They also run at lower temperatures, which in houses is likely to mean installing larger radiators and/or upgrading the performance of the building fabric to reduce heating demand.
Air-source heat pump units are not compact and must be installed outside, typically at ground level, and can also be noisy. Each unit draws several kW of electrical power so widespread deployments within a community will place greater demands on network capacity.
‘Mass uptake of heat pumps will have a big impact on the electrical infrastructure of the grid, so retrofitting houses to reduce peak demand on the coldest days will be really important,’ says Gregg. Self-generation via rooftop photovoltaics and solar thermal would help take the strain off the grid, he adds, while providing occupants with hot water at a decent temperature. They could also generate an income through feed-in-tariffs.
An alternative option for hot water is to introduce a storage tank, similar in size to a regular hot water tank.
The transition to net zero might see an initial preference for hybrid heat pumps that combine an air-source heat pump with a traditional gas condensing boiler. The Daikin Altherma hybrid heat pump automatically determines the most economical and energy efficient combination based on energy prices, outdoor temperatures and indoor heat capacity, cutting energy consumption by up to 35% compared to a regular boiler. Eon offers a system that transforms an oil or liquid petroleum gas heating system into a hybrid.
Incremental retrofit strategies for existing homes could provide a ‘quick win’ reduction in carbon intensity using heat pumps with gradual upgrades to the building fabric, says Gwilym Still, Passivhaus leader at Max Fordham.
‘We’ve worked through strategies on some projects that involve the initial installation of heat pumps for low carbon electric space heating and/or domestic hot water which can then move towards lower system temperatures and more efficient operation as the fabric is improved over time,’ he says. ‘The end goal looks the same, but the pathway to get there is slightly different, depending on the building.’
Increasingly stringent insulation and air-tightness standards, combined with heat loads from IT and occupancy, make offices increasingly cooling-led, with a limited demand for heating.
Polyvalent, or hybrid, heat pumps that simultaneously deliver heating and cooling and recover and store waste heat from spaces that need to be cooled, are therefore an attractive solution in low carbon office schemes.
A polyvalent heating system plays a vital role in the building services strategy for 80 Charlotte Street in London, an exemplar sustainable office designed by Make for client Derwent. The BREEAM Excellent-rated building has all-electric operation completely independent of fossil fuels.
‘In simultaneous operation the COP (coefficient of performance) of polyvalent heat pumps can reach 50% more than that of cooling mode alone,’ says Rahul Patel, a mechanical engineer and associate director at Arup, which devised the building services strategy for 80 Charlotte Street. ‘Rather than chucking waste heat energy into the atmosphere, you’re putting it back into the building where it’s needed.’ Air source heat pumps are typically larger than office chillers and ‘really need to see sky’ to optimise the extraction of heat from the air outside. Visual impacts need careful consideration because covered roofs or ventilated louvres on the sides can create issues with air flow and over-pressurize the refrigerant circuit, increasing the risk of a complete shut down.
The electric plant is at the ‘top end of the spectrum’ in terms of noise emissions from the main plant , says Patel – a particularly important consideration if there are mixed uses at the top of the building, if there are homes nearby, or if the building has a thin roof slab or timber deck that might let noise penetrate to the inside.
‘Acoustic packs can be fitted but they erode energy performance and strategically positioned acoustic louvres or plant screens can mask sound, but distance is the best attenuator for sound if possible,’ he adds.
As buildings become larger, heat distribution losses can become more significant, which in residential buildings can lead to overheating in corridors, communal areas and risers.
Ambient energy loop systems offer an innovative solution by pumping water at low temperatures around a building, with a flow/return temperature of 25º/20º versus 70º/40º for a regular boiler system.
The two-stage approach sees tepid water, generated by central plant using air or ground source heat pumps, circulated through pipework to individual water heat pumps inside each apartment, which take heat out of the loop for domestic hot water and space heating. Architects need to factor in space in each apartment for a small heat pump and a hot water tank.
The results can be impressive: the Zeroth Energy System by GenDimplex is claimed to cut peak heat losses by up to 90% and halve the plant room footprint.
Ambient loops can be applied to other building types or mixed use developments that require a mix of heating and cooling, explains Max Fordham’s Still: ‘A light industry building that wants year round cooling, or a one with refrigerated display cases or intensely occupied retail spaces, can get useful cooling from a loop of water. It’s an effective mechanism to shunt heat around a building.’
The systems have also been posited as an alternative to district heat networks, which typically operate at higher temperatures, increasing heat loss from pipework.
But there are potential drawbacks. The requirement for individual heat pumps in every dwelling makes things complicated when parts need replacing, and they could increase the embodied energy of the heating system, reigning in operational CO2 savings.
Incremental retrofits for existing homes could offer a ‘quick win’ using heat pumps and gradual fabric upgrades
Neighbourhoods / developments
Heat networks that distribute energy from a central source to communal buildings, like newbuild apartments or clusters of homes, are an attractive option in urban areas, but currently serve only 500,000 customers across the UK.
The energy saving potential is much greater if they can tap into waste heat from industrial sources, waste incinerators or the London Underground.
Gregg says: ‘Reusing waste heat and sharing energy between buildings and sources is going to be key. We don’t want to be adding more gas into these systems, this requires strategic thinking at a government and local authority level to prioritise connections into low carbon sources of heat.’
It’s an area that needs closer scrutiny: incinerators continue to gain planning permission without any obligation to integrate district heating, potentially locking in CO2 waste for decades to come.
Fabric plus efficiency
Working with low-energy design specialist Enhabit, Cousins & Cousins has used internal wall insulation, high levels of roof and floor insulation applied with thermal breaks in the structure, and an integrated airtight layer. A high-efficiency ventilation system with heat recovery manages moisture and reduces indoor pollutants. The high-efficiency gas heating system, used in conjunction with solar thermal panels on the roof to heat water, and with internet-enabled smart controls, means no energy is wasted.
So, at Canonbury Park South, the PHPP modelling (calculating the effectiveness of a Passivhaus approach) predicts a 75% reduction in heating demand, a 65% cut in energy demand and a 60% drop in carbon emissions. The space heating demand should be only 50kWh/m2 – very low for a building of this age and type. The result is a home that not only has a very low energy demand but is also pleasant and healthy to live in.