The HNZ consultation is part of wider changes and proposals, including the December proposals for heat networks in the Future Homes and Buildings Standards (FHS/FBS) and the upcoming regulation of heat by Ofgem. As part of this, the Department for Energy Security and Net Zero (DESNZ) is creating a Heat Network Technical Assurance Scheme, and networks will have to meet its criteria to be allowed to operate.

The current proposals allow for the continuing development of high-carbon networks for years to come, but without doing enough to support their decarbonisation, said CIBSE, which called for the decarbonisation of existing and planned networks to be addressed. It noted that, based on the current data available through the heat metering regulations, 91% of the UK’s 11,847 networks currently use natural gas or oil as their primary fuel. CIBSE also noted that, based on DESNZ data about the pipeline of heat networks, more than 45% of heat network expansions in the pipeline are proposing gas – for boilers or combined heat and power (CHP) – as their primary fuel. However, neither consultation sets out a clear expectation and reasonable timeframe for decarbonisation of heat networks, said CIBSE.

Zoning
The HNZ consultation is significant: in zones identified by a new ‘central authority’ and local zone coordinator, new buildings, existing communally heated buildings, and some existing non-domestic, non-communally heated buildings would have to connect to the heat network. A national zoning methodology would determine which areas are suited to a heat network in terms of value for money and deliverability. It is not clear at this stage how the methodology would assess networks against alternatives, in terms of carbon impacts and costs to consumers. CIBSE does not agree that the main counterfactual to heat network zoning is ‘do nothing’ (as is the case in the heat zoning consultation’s impact assessment), and said there needs to be a focus on other options for heat decarbonisation: ‘This is the real test of whether heat networks offer a cost-effective decarbonisation option,’ it said.

CIBSE is concerned that carbon performance and costs to heat-network consumers did not appear central to the current zoning proposals: for buildings that have to connect to a network, exemptions on grounds of carbon performance and consumer protection are unclear, while they were more prominent in the 2021 consultation, said CIBSE. ‘It is a serious risk to carbon emissions reductions, and to consumers. This must be revisited,’ it said.

CIBSE agrees that building owners should be able to apply for exemptions to connection on grounds of capital cost and timing, but said there must also be exceptions for low carbon buildings. High operational costs for consumers of a heat network must also be a reason for exemption.

Carbon limits
While CIBSE welcomes the fact that networks within a heat zone would have to meet minimum carbon content of heat criteria, it has concerns about the carbon emissions methodology used to demonstrate these criteria, which risks ‘significantly underestimating and misrepresenting the operational carbon impacts of heat networks’.

First, it said the methodology proposes to use a carbon factor for CHP-generated electricity of 304g/kWhe– this is much higher than the current grid average, and is only exceeded by the grid for a small number of hours in the year (eg, around 30-40 hours in 2030, according to data from Carbon Intensity). This means the methodology would attribute benefits to CHP-generated electricity that are unlikely to be realised in practice, and even less so in the future as the Grid continues to decarbonise, said the response.

Second, the methodology does not include secondary losses in the distribution system, which could mean the underestimation of emissions, added CIBSE. While secondary systems are often under different ownership and responsibility, the secondary losses must be accounted for in one way or another, as losses are often significant, contributing to energy waste and overheating in communal parts, said CIBSE. The record of heat networks under the heat metering regulations showed 29% losses  on average, even in communal rather than district networks.

DESNZ should provide a trajectory of carbon limits applying to heat networks, so that networks are encouraged to take early measures before being required to do so, said CIBSE’s response, which also called on government not to wait until the mid-2030s, as proposed, to develop low carbon standards for networks. It proposes standards are created as soon as possible given the significant time lags between planning and operation of heat networks, and would welcome dialogue with DESNZ, as it believes CP1 Heat networks: Code of Practice for the UK could play a role in this.

Public register
CIBSE’s recommendation for the HNZ and the FHS/FBS is to create and maintain a public register of networks, which would be operated by organisations independent from heat networks’ interest. According to CIBSE, this should be publicly available and independently audited, and be the single source of information for any policy relying on operational performance of networks, including Building Regulations, zoning, funding, and so on.

The register should have information currently included in the heat metering register, and have additional data on individual networks – including energy use per fuel – allowing analysis of primary and secondary losses, and carbon content of heat, said CIBSE. It added that this is not the case currently, with the heat metering register providing information on the heat generated and supplied, and on the type of fuel used, but not its amount, preventing an estimate of energy efficiency and carbon content of heat. 

The upcoming regulation of heat by Ofgem creates the perfect opportunity to address this, added the Institution, and it would be happy to discuss this with DESNZ.

The post Heat network zoning: CIBSE response to the recent government consultation appeared first on CIBSE Journal.

When it won the Building Performance Consultancy of the Year (up to 50 employees) at the CIBSE Building Performance Awards 2023, the judges said the consultancy was a good example of a ‘developing practice’, adding that they were impressed by the entrant’s ‘incredible’ focus on net zero carbon and by its demonstration of diversity, inclusion and equality.

‘Our USP has always been a focus on cutting carbon; the clue’s in our name – we’re all about crossing out [X] CO₂,’ says Tom Kordel, a director of the practice and one of its co-founders. ‘While cutting carbon is not new now, 15 years ago it was novel to have a business whose sole focus was to reduce carbon emissions within the built environment,’

From inception, the practice was set up to provide environmental consultancy and MEP design. That is still the case now, Kordel says, although, over time, it has added more strings to the environmental side of the business, such as daylight consultancy and overheating assessments, to provide ‘a more holistic service’. 

Similarly, its MEP services now include energy audits and post-occupancy evaluations, services that Kordel says help its engineers gain an insight into how buildings operate in reality. ‘Without understanding how a building works in operation, engineering designs will never improve,’ he adds.

It is the business’s focus on cutting emissions and on environmental issues that has made it attractive to engineers and consultants with a passion for environmental building design and low carbon engineering. 

Their skills give the practice the ability to look at buildings holistically, which means influencing the architecture to exploit passive design and making designs intuitive to use and operate. ‘If you’re going to focus on driving down energy use and CO₂ emissions, I think the best designed buildings are often the ones with the least amount of building services and that are simplest for people to use,’ says Kordel.

Many of those attracted to work for the practice are women; almost half of its current workforce is female, making XCO2 unusual among engineering consultancies, where, on average, women make up only 14.5% of the staff. ‘We want the background of our team to reflect the society in which we operate, so that balance is important, as is a balance between technical disciplines,’ says Kordel. 

XCO2’s attraction as an employer is no doubt helped by a progressive approach to enabling its employees to achieve a healthy work/life balance. They have the option of working from home three days a week, but, more radically, staff work a nine-day fortnight, with alternate Fridays off. 

Kordel says the policy of working fewer hours was introduced pre-pandemic, initially as a trial to ascertain its impact on business efficiency – but the scheme proved so popular that it has been retained. ‘It gives people the opportunity to do their life admin without having to use their weekend, which has helped a lot in terms of staff retention,’ says Kordel.

Perhaps less unusual in a sector struggling to attract talented engineering students, the business is also culturally diverse, with many of its overseas employees recruited to the business from university Master’s courses. ‘We have always looked to have a diverse workforce in terms of gender and ethnicity, because it brings a lot of ideas and creates a friendly, open, innovative culture within the business – and it does help to win work, because the businesses we work with tend to be culturally very diverse,’ Kordel explains.

The new solar installation at the back-of-house facilities at the Soneva Jani luxury eco resort in the Maldives

Two of the key business sectors in which XCO2 is particularly strong are social housing and luxury resort hotels. On the face of it, these businesses – catering for the opposite ends of the wealth spectrum – would appear to have little in common, but, according to Kordel, both have a vested interest in low energy operation and low CO₂ emissions. 

‘Social housing is very focused on eliminating fuel poverty, so having low energy and efficient buildings is really, really important, while luxury hotel developers and operators tend to keep assets for a long time, so they too have a vested interest in keeping energy use low,’ he explains.

The creation of more affordable housing ‘chimes with what we want to do as a business’, adds Kordel. However, it was the company’s hotel work in Southeast Asia that led to it opening an office in Singapore in 2016. ‘We have a strong reputation with hotel operators in the area, and we felt that being closer to architects and developers based in Hong Kong and Singapore was really important,’ he recalls.

The completed Jazz Yard project for Sixty Bricks, which delivers 83 new homes (50% of which are affordable) and a new NHS health centre in Waltham Forest

Many of the hotels and resorts that XCO2 work on in the region tend to be located on remote islands, without access to an electrical grid or mains water, so its net zero design is a practical necessity rather than a nice-to-have solution. ‘To have a holistically sustainable development, circularity around water, waste and energy is important, as is solar energy generation,’ Kordel explains.

XCO2 is also involved in charitable work in the region, including with the Hemis Monastic School in Northern India. Here, it is working pro bono on the design of a new school for Buddhist monks, located next to a 17th-century Buddhist monastery. The scheme is designed to use predominantly passive means to achieve thermal comfort in a cold, high-altitude desert with annual temperature swings of 60^oC.

Site photo from Hemis Monastic School, a new residential school for 500 trainee monks in Northern Ladakh , 13,000ft up in the Himalayas

Key to the success of the school’s challenging design is dynamic thermal modelling. This has been used from the earliest design stages to test and fine-tune a variety of measures – such as Trombe walls – to improve the building’s performance. Computational fluid dynamics have also been used to design and size the building’s solar latrines, which feature dark-painted solar chimneys to passively ventilate the toilets.

The use of digital tools and digital innovation are seen as potential growth areas for the business – an opportunity that has led to the company setting up the XCO2 Lab, to help it identify problems and time-intensive processes ripe for automation. The lab is intended to encourage interdisciplinary collaboration and is led by Aidan Kelly, technical lead for the CIBSE Society of Digital Engineering steering group and contributor to CIBSE Journal.

The Soneva Jani luxury eco resort in the Maldives

In addition to contributing articles for publication, XCO2 shares knowledge by making time for its engineers to get involved with industry initiatives such as LETI and, more recently, the UK Net Zero Carbon Buildings Standard. So what’s next for the developing practice? 

‘Now that we’re post-pandemic, we want to push towards growing the business, our client base, and the scale of projects we work on,’ says Kordel. 

‘Alongside the interesting, much smaller, more bespoke projects that we enjoy doing, we’re now working on schemes with thousands of homes where we can have an even bigger impact.’

The post The X Factor: XCO2’s holistic approach to environmental consultancy appeared first on CIBSE Journal.

The proposals, published in December, are intended to bring heat networks into the mainstream and create a viable route for occupiers to decarbonise their properties.

Identifying the lower-cost solution to decarbonising heat will lift the barriers to sector investment, the government says.

Justin Etherington, energy consultant and UK lead for the London energy team at Buro Happold, agrees that cost is key: ‘Heat networks will only succeed if they are competitive on cost and carbon compared with the customer’s alternative solutions.’

Other government initiatives around heat networks will also be vital, says Etherington. ‘Technical standards, policy support and standardisation of approach – as well as the introduction of Ofgem as the regulator – are all key elements in creating confidence.’

The proposals

The government is advocating the creation of a new Heat Network Zoning Authority (Central Authority), which will oversee all heat networks and coordinate zones in England, and Zoning Coordinators, which will implement zoning policy, and work with local authority planning departments.

To encourage investment, the government is proposing that certain buildings in zones connect to a heat network. These will be: new buildings that receive planning permission after designation of a zone; pre-existing communally heated buildings; and other non-domestic buildings that meet certain criteria.

The proposals state that zoning should encourage local authorities and other public bodies to connect their estates to heat networks because it is a cost-effective way to decarbonise. 

Baxi sales director Stephen Hart says it is notable that some housing developers are already choosing community heat networks for larger estates where it is financially more attractive than installing individual heat pumps and associated hot-water cylinders.

‘It would be even more attractive to connect these developments to larger heat networks, where they exist, as the developer can avoid the need to install and operate a local low carbon energy centre,’ he says.

Pre-existing homes with individual heating systems will not have to connect, but Hart says it can make financial sense. ‘Heat networks could prove popular in low-rise, low-density upgrade projects as a cost-effective alternative to individual heat pumps,’ he adds.

Emissions limits

The government is proposing that heat networks in zones will be required to comply with national emissions limits from 2030, and is seeking views on three proposals for this limit – 44g, 83g or 147g CO2e/kWh.

Etherington would opt for the stricter limit: ‘The 44g figure seems a reasonable number for new networks that have the opportunity to start with low carbon sources.’ 

Baxi would advocate for the highest possible carbon intensity at the outset, but with a reducing intensity threshold over time, says Hart, who notes that the current maximum carbon intensity for schemes to qualify for funding under the government’s Green Heat Network Fund is 100g CO_2e/kWh. This allows for hybrid energy centres with a low carbon primary heat source, such as heat pumps, to be combined with a boiler to satisfy peak demand.

Gas-burning boilers in hybrid systems could migrate to hydrogen or direct electric as technology evolves and the electricity grid is reinforced and expanded, adds Hart. 

The government estimates that heat zones could supply 11% of UK heat under its preferred policy option. This means that 70% of buildings that are not required to connect to a heat network in a zone will have to connect voluntarily. For this to happen, the government will have to win hearts and minds, says Etherington: ‘There will need to be significant education and engagement with customers to ensure they are confident in connection to heat networks in zones.’ 

The post Zoning in: the new Heat Network Zoning consultation appeared first on CIBSE Journal.

Factors such as insufficient space for air source heat pumps (ASHPs), oversizing of equipment, noise, vibration, the importance of controls, and inadequate legislation were highlighted as key issues. The event, sponsored by Baxi and held in London, explored ways of optimising heating systems.

The participants began by describing their decarbonisation projects and challenges. Mathew Stark, a senior mechanical engineer at Hoare Lea, cited a current refurbishment scheme. ‘We wanted to remove the gas boilers and use an all-electric system with ASHPs, but planning restrictions wouldn’t allow anything too tall on the roof,’ he said. ‘The cooling system uses air cool condensers on the roof and chillers in the basement. Instead, we changed it from a constant air volume to a variable air volume system so we could at least make the system more efficient and decarbonise in that way.’

Simon Wyatt, partner at Cundall and chair of the CIBSE Knowledge Generation Panel, said many building owners are struggling with the best way to decarbonise existing buildings. They question whether they should do minor works now or carry out a deep retrofit over a phased period. Wyatt is involved in decarbonising many existing buildings. His dilemma is whether to try to keep a high-temperature heating system or upgrade the building’s fabric.

Richard Brimfield

‘The cost of electrification and heat pumps is expensive, but it’s a drop in the ocean compared with the cost of upgrading the fabric,’ he said. ‘If you want to hit those energy intensity targets, this is a real issue and there’s no coherent solution.’ 

Richard Brimfield, associate at Ridge and Partners, and founding committee member of the CIBSE HVAC Group, argued that some of the obvious challenges are finding external space for heat pumps and boreholes, identifying the heat requirement for existing buildings, and simulating how a heat pump operates. He said a current project had hugely oversized existing boilers.

The participants agreed that oversizing is a big problem. Ryan Kirkwood, engineering solutions manager at Baxi, asked whether there is now better guidance on the correct sizing of equipment within the industry. ‘Personally, I have more confidence in reducing the size of things by the methods we use to do the space heating calculations,’ said Brimfield. ‘However, we’re still oversizing things; we’re never going to get everything perfect.’

Daniel Skidmore

Wyatt said that CIBSE is considering producing guidance on efficient sizing. Although there seems to be more confidence in sizing central plant, he agreed that people are still oversizing significantly, partly because of build quality. He cited schools where there is a lot of leakage because of poor airtightness and air permeability, resulting in oversized radiators. 

Daniel Skidmore, director of building engineering at Aecom, echoed these views. He highlighted the BBC, one of Aecom’s biggest clients, which wants to decarbonise much of its estate. Some difficulties encountered with this project, he said, include how flow temperatures with hot water generation are used, roof-plant acoustics and vibration, and long lead times on plant, and how this can affect the project’s planning and timing.

Jeremy Owen, principal mechanical engineer at SVM Building Services Design, said the company is installing ASHPs for a major supermarket. The noise and vibration produced by the heat pumps has been a significant problem given the proximity of housing, he added, so – at some stores – a two-storey gantry has been built above the plantroom where the ASHP and new air handling unit (AHU) are located. 

Jason Donoghue

Noise and vibration produced by ASHPs is an issue that some manufacturers and noise specialists have said is often overlooked. Hoare Lea has an acoustics team that assesses the operation of the ASHP, and it advises architects on the type of materials and vibration mounts that should be used. ‘A lot of the kit we’re using with ASHPs is a set size, but massive generation packages are needed on site, so there are space constraints,’ said Stark. ‘The acoustics are achievable, but the space needed is challenging. So, before we even involve an acoustics specialist, we allow enough space for these packages for all residential projects, because we know we will need it.’

Owen’s other main complaint relates to the Building Regulations. ‘If we want to decarbonise, the regulations need to be amended to make it work and enable the best technologies to be put forward,’ he said, citing a frustrating experience he had with a residential new build. To achieve the SAP calculation, he had to install gas combination boilers with photovoltaics instead of heat pumps. ‘This is not right, because the SAP calculation looks at affordability,’ said Owen. ‘An ASHP is given a C rating because it’s less affordable, while a gas combi boiler with PVs gets an A rating; it’s ludicrous.’

Louis Kimber

Louis Kimber, mechanical engineering technician at AtkinsRéalis, said embodied carbon is a growing issue. He has been working with the Government Property Agency’s (GPA’s) UK sites and exploring what can be done to decarbonise. 

‘Embodied carbon is not a criterion required by the GPA,’ said Kimber. ‘But when I’m writing the reports and suggesting things such as getting rid of boilers and installing PVs on the roof, not only are the costs increasing, but the embodied carbon is going up too, which really concerns me.’

Kirkwood said it was refreshing to hear his own concerns echoed. ‘We find there is a great deal of confusion and people are often looking for a quick fix,’ he added. ‘Engineers need time to engineer, to survey and to monitor. As an industry we’ve almost engineered that out and we need to engineer it back in.’

Matthew Stark

To optimise heating systems, Skidmore recommended metering be installed in existing buildings, to help understand heating use. An existing system with lower temperatures should be run to see how it behaves. Suggested improvements, he added, have included upgrades to building fabric, the introduction of ventilation, using domestic hot water with AHUs, and providing enough space for airflow in heat pumps, as correcting flows can be a problem. 

Skidmore also recommended using high-efficiency chillers to take the rest of the cooling, instead of implementing a blanket approach. In buildings that have a 24/7 operation where there is a constant cooling demand, he has taken advantage of the waste heat element of ASHPs. However, he argued that ASHPs are the biggest offenders in terms of energy consumption, because they are not being controlled properly.

Ryan Kirkwood

‘We often find that buildings aren’t running as smoothly as they should. They are sometimes running at night when they shouldn’t and not ramping down enough when they should,’ said Skidmore. ‘The controls element has been as much a culprit as the plant. We often find that there has been a lack of engagement with the facilities management [FM] team regarding the controls. There have also been issues with the FM team’s expertise of optimising and running the buildings after we’ve done the work.’ 

To optimise heating systems in a supermarket, Owen said they rely on a packaged plant, which is a refrigeration integrated heating and cooling system. It takes the heat of the cold aisles and puts it back into the store, and there are a couple of heating coils in the AHU. 

It was agreed that education would help in situations where a setting has been changed after it was established to optimise heat rejection. Wyatt added that the design performance assessment will possibly shed light on this issue; however, many of his clients don’t take this assessment seriously enough. ‘We have 30-odd projects that have gone through design performance assessments and got their energy target rating,’ said Wyatt. ‘But not one will get anywhere near those targets. The assessment is a de-risking exercise with multiple future scenarios. It’s not like an energy performance certificate, where once you’ve passed it can be forgotten.’

Jeremy Owen

Brimfield then raised the importance of controls and the role artificial intelligence (AI) may have. Wyatt suggested there are two schools of thought: take the fully automated AI path, which seems to be the direction industry is taking, although its complexity and cost can be a barrier; or keep things as simple as possible.

The event concluded with a discussion on hydrogen being piped through the gas grid and the possibility of having localised gas storage. Skidmore said Aecom is focusing on the big district heating schemes and rolling out large projects with low carbon distribution, but he doesn’t predict hydrogen playing a major part.

Baxi has developed and tested 100% H2 boilers, and the tests prove they work just fine said Kirkwood. He says that when there a wide availability of H2, production will start. ‘We have hydrogen boilers installed in a variety of demonstration projects across the EU and UK proving that the technology is a viable solution,’ he added. ‘We’re doing what we believe is the right thing, but we’re not hanging our hat on that. We see a blended source of solutions, including heat pumps, heat networks, and, if required, hydrogen.’

Simon Wyatt

Jason Donoghue, head of marketing at Baxi, said the company had launched two more residential projects, and Baxi will be supplying hydrogen boilers to a new residential trial in Redcar. Kirkwood added that he would like to see a greater focus on heat networks. ‘One of the challenges we face is the medium-temperature heat networks,’ he said. ‘We have a huge number of heat networks sitting on the Grid that are not low carbon. Systems operating beyond where current heat pump trajectory is in terms of performance will be challenging. There’s a dichotomy between heat pumps and heat networks – they are not well suited, which is why we are investing in thermal storage research and development.’

Skidmore argued that it can be challenging to drive heat network improvements, and Wyatt agreed, adding: ‘District heat networks for existing high-density urban environments have a place, but in low-rise density, heat networks are not always the right solution.’ 

The post Taking the heat out of decarbonisation: insights from CIBSE’s roundtable appeared first on CIBSE Journal.

When considering an existing gas-fired heating system that is operating with relatively high water flow temperatures of
70-80^oC –compared with today’s typical designs of ≤55^oC – application of an electrically driven alternative to match those flow temperatures is fairly straightforward. 

Managing the impact on operational costs and capital expenditure (capex) is much more complex, however, especially given the disproportionate cost per kWh of gas vs electricity currently, and the relatively high capex cost of technologies, such as heat pumps, compared with fossil-fuel alternatives. 

This is where a bivalent approach can become attractive and possibly help bridge some of these gaps. Focusing on using heat pumps as part of this bivalent solution, we can consider two broad types of configuration: bivalent in parallel and bivalent changeover (see Figure 1). 

It is typical in the UK for peak space heating capacity (100% load) to only be required for a small number of hours in the year and, therefore, this makes up a relatively small amount of the total kWh of heat energy delivered. This opens up the possibility of deliberately undersizing your heat pump, in comparison with peak load, and only operating it for certain parts of the year, allowing the gas boiler to provide the extra peak capacity or peak flow temperatures when needed. This approach can help reduce upfront costs and plant space requirements. 

We know that operating a heat pump at the lowest possible flow temperature and the warmest source temperature will usually deliver the highest efficiency and lowest operating cost. But we must also consider how any heat generated is emitted into the building – this is where the infrastructure of the existing system begins to influence the design approach.

The existing heat emitters (fan coils, air handling unit coils, radiators, and so on) must be assessed to understand their deliverable output capacity at different mean temperatures. This will demonstrate what mean temperature is needed at certain ambient conditions to deliver the required capacity. This is a key piece of information needed to model bivalent parallel and bivalent changeover configurations.

Bivalent changeover configuration:

In a bivalent changeover arrangement, the heat pump is deliberately designed not to deliver the peak flow temperature or capacity of the heating system. It will only operate up to a temperature and capacity chosen to match the heat-emitter capabilities and building load at that changeover point.As a result, the heat pump will operate in isolation from the boiler, providing heat to the existing heat emitters until its maximum flow temperature and capacity are reached. At this point, it will turn off and the boiler will take over, delivering the higher-temperature water and increased capacity required to meet the increasing building load. 

This arrangement will deliver a lower proportion of annual space heating load from the heat pump compared with bivalent parallel. However, as the heat pump will not operate at peak design conditions, or be asked to deliver high flow temperatures, it will benefit from increased efficiency, resulting in a lower operating costs.

To maximise the ratio of kWh contribution from the capacity of heat pump provided, our research shows the optimum will be approximately 50-75% of the building peak load, with the maximum flow temperature being approximately 55^oC. Depending on overall system design and existing heat-emitter capabilities, other combinations can also deliver good results. See Table 1 for an example of a bivalent changeover arrangement. Examples are modelled on the Mitsubishi CAHV-R450 YA-HPB.

Bivalent parallel configuration:

This requires the heat pump to be capable of delivering the peak flow temperature of the system, allowing it to work side by side with the boiler at any time of the year. This means the heat pump can be sized to any capacity and, as Figure 4 shows, deliver heat energy on its own when it has the capacity, or in conjunction with the boiler when the load is greater than the heat pump capacity (the example shows a heat pump sized at 50% of peak load). Flow temperature can be fixed or weather-compensated, but the key design principle is that the heat pump is able to deliver the required flow temperature to meet peak heating demands via the existing heat emitters.

This arrangement will deliver a large proportion of annual space heating load from the heat pump, and probably result in the lowest overall carbon emissions. However, operating the heat pump at potentially high flow temperatures and low ambient conditions will reduce its efficiency, so this configuration will probably result in an increased operational cost compared with the gas boiler-only system. 

Special consideration must also be given to the choice of heat pump, to ensure it delivers the necessary flow temperature and capacity in all operational conditions. A cascade arrangement or high-temperature natural refrigerant product may be needed to achieve the required flow temperature. The capacity of the heat pump can be freely selected to meet any site constraints of power supply or plant space, and consideration can be given to capital costs to achieve the optimum balance. 

To maximise the ratio of kWh contribution from the capacity of heat pump provided, our research shows the optimum capacity in relation to the building peak load is likely to be approximately 25-50%. See Table 2 for an example of a bivalent parallel arrangement. Calculations shown are for comparison purposes only

Conclusion

As the two examples show, introducing even a relatively small heat pump into an existing heating system will lead to carbon reductions, but applying that same heat pump capacity in different ways can achieve different outcomes.

Choosing which configuration and capacity of bivalent system is best suited for a specific project depends on budget, existing infrastructure, desired outcome and, most importantly, how it is controlled.

The post The balance of power: the bivalent approach to heat pumps appeared first on CIBSE Journal.

With the rise of electric and hydrogen-powered aircraft, as well as the emergence of advanced air mobility solutions, there is a vital need to redesign existing airspace infrastructure for this new generation of aircraft. Collaborations between airlines, original equipment manufacturers, infrastructure operators, and other key stakeholders are essential to establishing the necessary infrastructure for new forms of flying, such as the electric vertical take-off and landing aircraft(eVTOL) vehicles that will soon be filling our skies.

Existing airport operations and infrastructure need to undergo vast changes to decarbonise while accommodating new flying technology. Atkins is applying its Decarbonomics tool to airports, in a bid to create a roadmap towards net zero for them, and has established a dedicated multidisciplinary Future Flight team to look at what infrastructure is required for new forms of aviation. 

‘We’re actively engaged in decarbonising existing aviation, whether it’s retrofitting airports, introducing new propulsion systems to traditional aircraft, or spearheading genuinely sustainable electric propulsion flight,’ says Andrew Caughey, head of sustainable aviation at Atkins’ Future Flight team.

Decarbonising airports

Atkins’ Decarbonomics tool has gained significant traction in the aviation industry, particularly in the realm of airport operations. Launched in January 2022, the tool was created as a way of tackling the energy use of large portfolios of existing buildings, using data to help make decisions around investment.

The initiative has demonstrated impressive results; it is estimated to have saved 300,000 tonnes of carbon. Initially devised as a strategy for retrofitting housing, its success has prompted an expansion into the industrial sector, including the airport space.

The concept builds a framework for clients that benchmarks their current carbon footprints, devises strategic roadmaps for intervention, oversees implementation, and ensures long-term measurement and verification of decarbonisation efforts. Isabelle Smith MCIBSE, technical lead for Decarbonomics and a director of engineering net zero at Atkins’ parent company SNC-Lavalin, says the approach to airports presented a new set of challenges, with very different types of interventions.

A holistic approach was taken to meet the distinctive demands of airport infrastructure. As well as optimising building fabric and energy usage, the tool delves into the complexities of airport infrastructure and the ground-fleet operations needed to support the transition to zero carbon flight. 

‘We know we are going to have to make significant changes to electricity generation and distribution in airports, so we have to ask what that looks like from an embodied carbon perspective, as well as an operational carbon perspective,’ says Smith.

Scenario modelling is used to anticipate how new technologies, policy shifts, climate change and costs will affect a site. In this way, the development of the roadmap is fully informed and allows planning for resilience and the mitigation of risk as far as possible. This includes assessing the electrification readiness of ground-fleet vehicles, future-proofing infrastructure for energy transitions, and strategising to mitigate stranded asset risks. 

‘We’ve had some great engagement with clients and incredibly positive feedback,’ Smith says. ‘The initiative actively engages with clients, inviting their input and insights to refine and customise the platform.’ 

This ensures that the developed solutions are aligned with the clients’ unique goals and challenges.

Smith stresses that, throughout the process, it is crucial to leave room for future opportunities for renewables. 

‘A lot of it comes down to technology readiness of the fleet vehicles and determining the point at which electrification is going to be available,’ she says. ‘We want to make sure these airports are future-proofed.’ 

Atkins used the latest in data-driven, digital engineering to help maximise investment, future proof, and decarbonise Dublin airport

Decarbonomics extends its impact beyond emissions reduction to encompass financial and risk considerations. Evaluating the economic viability of interventions ensures that investments today will yield long-term benefits and maintain asset value, says Smith. ‘The key question is what is the cost of intervention, and at what point does it become uneconomical? We map that risk and look at ways to mitigate it.’

For example, 25% of the world’s busiest airports are situated less than 10 metres above sea level, at critical risk of sea-level rise, so the risk of that asset stranding is major, says Smith. Engagement ensures that financial and insurance sectors are equipped to support sustainable endeavours while managing the future risks effectively.

From an infrastructure perspective, it’s crucial to decarbonise because people will still be flying in the future. ‘Airports will look and feel totally different from what we are used to today, but they will exist,’ says Smith. ‘From a financial and social perspective, the sector is very aware that it needs to do this to remain viable; it’s at the top of everyone’s agenda.’

Evolving air mobility

The growing interest in fully electric and hydrogen aircraft, including eVTOLand  drone taxis, is also reshaping the aviation landscape. Atkins’ Future Flight team has been created to expedite the decarbonisation of aviation and brings together professionals from building services, infrastructure, architecture, and structural and civil engineering.

‘Whether you are talking about sustainable aviation in its broadest sense or narrowing it down to emerging eVTOL technology, for example, the change to established transportation networks will be massive,’ says Caughey. 

‘We’re looking beyond traditional air travel. Urban air mobility hubs and vertiports are emerging to facilitate short-range travel. They require scalable eVTOL reception facilities, energy efficiency, and advanced technology, such as biometrics for passenger journeys,’ Caughey says. 

eVTOLs will operate like air taxis, and their potential promises greener, safer and faster travel. ‘Air mobility is a really exciting area,’ says Daniel Jones, director at Atkins, who oversees the company’s work in the airport sector. ‘We’ve investigated the application of eVTOL drones in various case studies, including passenger transport, cargo, and last-mile logistics.’

An artist’s impression of what landing pods for small drones might look like

The team’s first project, the Advanced Mobility Ecosystem Consortium, comes under the UKIR Future Flight Challenge, and aims to demonstrate real-life mobility scenarios in the southwest of the UK. It involves demonstrating the eVTOL ecosystem in the region to showcase key use cases and address challenges such as integration with existing traffic and ground support. ‘We’re enthusiastic about our role in this project, collaborating with partners such as Vertical Aerospace, Virgin Atlantic Sky Port, and various airports, including Heathrow and London City. We’re continuously learning, progressing, and moving towards the outcomes funded by Innovate UK’, Caughey says.

Andrew Querée, associate at Atkins, has been collaborating with Atkins’ eVTOL partners on the innovative projects, and with county councils, helping them to establish transport hubs in park and ride facilities. ‘The focus is on developing air mobility solutions with a range of up to 100 miles, connecting various places, such as airports, eVTOL hubs and venues,’ he says.

Taking healthcare to new heights

Scottish First Minister Humza Yousaf (centre) visits the Project Caelus drone project

Atkins’ Future Flight team is supporting the infrastructure component on a project led by AGS Airports to deliver vital medical supplies via drones. Project Caelus (Care & Equity Logistics Unmanned Air Systems) is a partnership with NHS airports in Aberdeen, Glasgow and Southampton. Already operational, it looks at how drones can be used to transport blood tests from GPs to hospitals and deliver cancer therapies to patients in remote locations.

Infrastructure development is a key challenge, particularly the creation of specialised housing for the drone fleet operating within the network. This involves the testing and development of next-generation systems to support the network effectively. A variety of drones are already operational across different distances in Scotland. Climate-controlled units are also being designed to securely house these systems.

The goal is to use drones to eliminate the need for individuals to take lengthy ferry rides or taxi journeys to access essential medical treatment. ‘If we can safely deliver medications while adhering to pharmaceutical parameters, it’s an avenue worth exploring,’ says Andrew Caughey.

The infrastructure needed to support these new transport systems is also something Querée’s team has explored. Rapid charging facilities are essential to ensure commercial viability and efficient passenger movement between hubs. The emphasis is on enhancing the passenger experience, using modern construction methods, and optimising energy efficiency in hub design. 

‘Multimodal connectivity is crucial too; there’s no sense in everyone driving diesel cars to an airport parking lot that negates the progress of decarbonising,’ Caughey said. 

Caughey is keen to stress, however, that the zero carbon vision for aviation isn’t just about hydrogen jet planes and drone taxis, but about the improvement to existing buildings and infrastructure. 

‘It’s important to inject a dose of reality when working with clients to create a roadmap for their developments,’ he says. ‘Our shared goal is clear: achieving truly sustainable, emission-free aviation while preserving the essential connectivity that aviation provides.’ 

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The retention and reuse of a building can limit additional carbon production and policy makers across the UK and Europe are increasingly requiring developers to justify their decision to demolish existing buildings.  

Dublin City Council’s 2022-2028 development plan, for example, requires ‘robust justification’ for demolition and reconstruction works and that re-use should always be considered as a first option.

The policy appears to be having an impact, with property agent Savills saying that ‘emerging evidence points to [the retention and reuse of buildings] as best practice,’ for many developments in its 2023 review of the Dublin office market.

The following contrasting schemes show how consultant engineers in Ireland are helping to trailblaze the reuse of two very different types of office building.

 Tropical Fruit Warehouse

A warehouse building, formerly used for tropical fruit imports (and more recently as a studio by U2), on the quays of the River Liffey in Dublin, has been redeveloped as an office building.

Owned and developed by IPUT Real Estate, designed by architect Henry J Lyons, built by Dutch design and build contractor Octatube, with a façade designed by Arup and building services by O’Connor Sutton Cronin, the scheme comprises three interconnecting elements: a restored 1890s warehouse with a new lightweight cantilevered glass box extension above and a new seven-storey office building behind.

Reuse of the fabric of the double-bay existing warehouse reduced the carbon embodied in the project. A two-storey glass-box extension appearing to float above these is the most eye-catching part of the project.

The early involvement of Arup’s façades team ensured that the lightweight, transparent box could be delivered in keeping with the architectural aspirations.

The double-skinned façade features an outer skin of 8.5m high, 2.5m wide giant laminated glass panes, that reach full height of the two-storey box. The inner double-glazed skin, forming the building’s thermal envelope, is a bespoke steel and glass module. It is recessed top and bottom to give it the appearance of being frameless; all the occupants see is a thin black silicone 24mm-wide vertical joint between units. The double-skin façade has a U-value of 1.1 W·m^-2·K^-1.

A series of glass fins, positioned in line with the joints on the outer leaf, separate the outer leaf from the inner unit. The fins are hung from the building’s roof. The weight of the outer glazed skin is supported on a bespoke stainless steel frame fixed back to the bottom floor slab. 

‘We worked diligently with the contractor to develop bespoke toggled connections that allowed accommodation of the differential vertical movements with the fins providing horizontal restraint to both skins,’ explains Lee Corcoran, senior façade engineer at Arup.

An additional movement challenge was that the cantilevered structure onto which the façade was attached was predicted to deflect under imposed loads. 

Corcoran says: ‘We worked closely with Octatube and structural engineers Torque Consulting Engineers to fully understand the movements associated with the structure at façade connection points so that we could design the system to accommodate the anticipated racking movements and minimise the joint sizes with minimal visual impact.’

The glazed corners of the outer skin were carefully considered through extensive structural calculations and detailing. The final solution to ‘lock’ the corner units incorporates a closed loop structural solution; the large glass panels transfer the lateral load into bespoke stainless steel connections and concealed stainless steel tension rods hidden within the silicone joints at the head and base of the units. 

‘It is a fantastic, discrete solution that no-one will ever see, which is aligned with the essence of the project,’ says Corcoran.

Arup used a low-iron glass for enhanced clarity in both the inner and outer units. 

In the drive to maintain transparency, the architect was keen that the office floors were kept free of window blinds and that the double-skinned façade remained uncluttered by interstitial maintenance walkways. 

A high-performance solar control coating on the outer skin of the double-glazed unit combined with a solar control PVB interlayer on the laminated single-layer outer skin and ventilation to the interstitial cavity removes excess heat.

To finalise the glazing and PVB selection, Octatube built a series of small-scale mock-ups at its Delft HQ. Once the selection was confirmed, a full-scale mock-up was built and subjected to the CWCT sequence B weather performance test. 

Access for cleaning within the cavity is provided by a single walkway concealed at the base of the façade. To enable personnel to move between the two skins, Arup worked with Octatube to shorten the glazed fins so that they stop 1.6m above this walkway. A bespoke abseiling solution was developed to allow for cleaning of individual bays in between the glass fins. 

The location of the glass box above the existing warehouses meant that when it came to constructing it, sequencing was key. As part of the restoration of the warehouses, all of their roof trusses were removed and taken off-site for restoration. This allowed construction of the core and structure to support the glazed box. The box was pre-assembled as far as possible to minimise work at height, ensure build quality and to complete the installation before the warehouse roof trusses were reinstalled.

‘What made the project so successful was our involvement at such an early stage, which meant the design was well considered quite early in the process and that allowed us to engage with a specialist contractor very quickly,’ Corcoran says.

The 85,000ft² office is now entirely leased to TikTok.

Tom Johnson House

Tom Johnson House is a five-storey 1970s office that will become home to Ireland’s Department of the Environment, Climate and Communications

The retrofit of Tom Johnson House, Dublin, is set to turn a five-storey over-basement, 1970s office building into one of the most sustainable buildings in Ireland, ready to become the new headquarters for the Department of the Environment, Climate and Communications (DECC).

The Irish Government has made it a top priority to decarbonise public sector projects and drive the green transition to help mitigate climate change. This project, funded by the EU under Ireland’s National Recovery and Resilience Plan 2021 as part of the European Union’s response to the global pandemic, is intended to be an exemplar. 

It will demonstrate that the project’s client, the Office of Public Works (OPW), is helping lead that transition. As such, Tom Johnson House has been designated a Public Sector Retrofit Pathfinder Project by the OPW.

Designed in-house by the commissioners of public works in Ireland and engineered by Lawler Consulting, the refurbishment is designed to take the building from a C3 Building Energy Rating to an A2, which the OPW predicts will reduce primary energy use by 75%, greatly extending the building’s useful life. ‘The OPW brief was for the building to be A2 energy rated; that set strict criteria for what we had to achieve in terms of both fabric and MEP systems’ energy performance,’ says James Long, associate director at Lawler Consulting.

Application of the OPW’s Green Procurement Policy will further mitigate the project’s carbon impact as will the requirement for compliance with EU rules for material input and waste management, re-use and recycling.

The retrofit retains the existing 1970s concrete structure and external brick façade to minimise the carbon embodied in the refurbishment. ‘We’ve reused the existing window openings and fitted new glazing to enable the office to be naturally ventilated,’ explains Long.

A BIM model of Tom Johnson House

Internally, the existing cellular office layout has gone, to be replaced by a predominantly open plan office arrangement. 

The biggest modification by far, however, is the introduction of a new atrium punched through the centre of the building, from roof to ground floor, to allow daylight to enter deep into the building’s core and to further facilitate natural ventilation.

The new atrium divided the existing rooftop plantroom into two halves. As a result, Lawler Consulting’s scheme now serves each half of the building from its own dedicated rooftop plant. ‘We’ve lost a third of the plant space and yet we’re going to deliver enhanced levels of comfort,’ says Long.

Lawler Consulting’s all-electric design includes removal of the existing boilers. These are being replaced by two, 600kW multifunction chiller heat pumps to provide both heating and cooling to the offices. 

A smaller high-temperature heat pump boosts the heating water temperature to heat the domestic hot water. There is no fossil fuel on site.

Office floors are heated by radiators on a low-temperature hot-water system. High occupancy areas, such as meeting rooms, all incorporate active cooling, primarily provided by fan coil units. 

Additional carbon reductions are provided by a 50kW roof-mounted solar PV array.

The scheme also incorporates a large number of EV charging stations. These are controlled to ensure the building’s electrical demand remains within the capacity of the site’s existing 600kVA transformer.

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However, there can be a significant increase in spatial requirements and capital expenditure when using heat pumps as the lead heat-generation source for heat networks, compared with gas CHP and gas boiler solutions.

To tackle this issue, I propose a hybrid approach that combines heat pump and thermal storage with electric boilers. Thermal storage increases operational flexibility of heat pumps and maximises the annual target heat fraction that can be provided by the heat pumps.

My research, How hourly load modelling is revolutionising heat pump and thermal store sizing in hybrid energy centres, was presented at the CIBSE ASHRAE Technical Symposium 2023, at the University of Strathclyde, Glasgow, in April (www.cibse.org/symposium).

It provides a ‘rule of thumb’ for hybrid energy centres to support designers and developers early in the design stages, ensuring that heat pump size and thermal storage capacity are optimised to achieve the required target heat fraction contribution, which is the proportion of the total annual network consumption that is provided by the heat pumps.

The heat pumps are sized to deliver the majority of the site-wide annual heat demand, with large thermal storage used to reduce the required heat pump size by storing heat during times of lower demand and using it during periods of higher demand. Electric boilers are sized to act as ‘top up’ during periods of peak load demand.

It is important to consider equipment sizing early on in the design stages, as such decisions could be constrained later by architectural layouts, floor plans and building elevations. This is where my research comes in.

Modelling heating and hot water use

In my research, an hourly load model was built, which aims to model the domestic hot water (DHW) and space heating usage that can be assumed for each hour across an entire year for any given development.

The model takes into account several inputs to investigate the impact of equipment sizing on the heat pump’s annual heat fraction contribution. These include: heat network heat losses; heat interface unit heat losses; DHW hourly profile; space heating hourly profile; and annual DHW and space heating loads.

To be as flexible as possible, the model allows for easy variation of building size, DHW, space heating loads, expected heat losses, and geographical location. By doing this, the impacts of these factors on heat pump and thermal store sizing can be easily understood and assessed. The final model was then validated against load profiles taken from operational heat networks to provide confidence in the findings of the research.

While there are many benefits to oversizing thermal storage, a key constraint will be the spatial requirement within the energy centre. This is especially key when comparing this with traditional CHP thermal storage, which operates at higher temperatures and, therefore, requires a smaller volume.

The hourly load model allowed the thermal storage size to be modelled against the heat pump to determine the point at which any increase to thermal storage will have minimal impact on reducing the heat pump size. The study found that, as a rule of thumb, thermal storage should be sized at 50-75L·kW^-1, as shown in Figure 1. The percentages under the graph represent the annual target heat fraction contributions (for the heat pumps).

Because of the impact of the higher electricity tariffs compared with gas, it is critical that heat pump sizing is considered carefully. If the heat pump is undersized and unable to meet the required target heat fraction, end users will see a significant increase in their cost of heat because of the reliance on electric boiler top-up.

Equally, oversizing a heat pump presents a number of challenges, particularly when considering the spatial requirements against a gas CHP or gas boiler solution.

As such, the hourly load model was used to provide a rule of thumb for a given target heat fraction contribution as a percentage of the site-wide peak load. The findings demonstrate that a target heat fraction that is as high as 99% of the site-wide annual demand can be achieved by installing heat pump capacity of less than 50% of the peak load (see Figure 2).

A summary of the ‘rule of thumb’ findings are presented in Table 1. These findings provide a benchmark to support developers and designers early on in the design stages, to ensure energy centre sizing is fully considered until more detailed design calculations can be carried out at a later stage.

In conclusion, the research demonstrates that sizing a hybrid system for a heat network requires careful consideration of various factors, including the capacity of the heat pump and thermal store. Designers must take spatial limitations into account when estimating the required heat pump and thermal store size, to ensure these will not be constrained later in the design development, when the risk of undersizing will be critical.

The research provides targeted guidance for sizing heat pumps and thermal storage at concept design, in advance of detailed design, helping to pave the way towards more efficient and sustainable heat networks in the UK. To read the research paper, visit the ‘Research & writing’ page on FairHeat’s website: fairheat.com/research-writing

The post Formula for success: Sizing heat pumps in hybrid energy centres appeared first on CIBSE Journal.

The current reality is that a paltry 60,000 were installed in the UK last year – which, at two heat pumps per household, put the country at the bottom of the European heat pump league.

To increase the number of installations in new homes, the government is taking a ‘big stick’ approach and proposing to ban gas and oil boilers in new homes from 2026.

It’s a huge challenge, especially for small to medium-sized housebuilders that are familiar with gas boilers, and may not have the resources to design and develop homes based on alternative heating systems.

Building merchant Travis Perkins has recognised this predicament and launched a digital platform called WholeHouse, aimed at streamlining the design and delivery of low carbon homes.

Lee Jackson, WholeHouse director at Travis Perkins, says the industry lacks the skilled workers required to meet government heat pump targets, and a new digital approach is required that simplifies design and installation, and encourages the prefabrication of building systems.

‘I’ve opened too many airing cupboards where I’ve been met by a wall of pipework that’s 15 layers deep,’ says Jackson. ‘The average plumber has spent years installing combi boilers, and now they are expected to put back in a cylinder and install an air source heat pump [ASHP].’

Without help, Jackson believes plumbers will have to learn from job to job, and will deliver inefficient heat pump systems until they have gained experience.

Baxi’s is the only appropriate heating system featured in the software at the moment

WholeHouse is a 3D online design tool that allows the housebuilder to design any size home using a variety of materials and systems, including heat pumps. The software has strict parameters to ensure whatever is selected leads to the most energy efficient and cost-effective design possible. Once a design is settled on by the user, the program produces a set of drawings and the bill of materials in just 45 minutes.

In developing the software, Travis Perkins assembled a team of 20 suppliers and consultants to come up with design parameters that guarantee the software’s outputs are optimised for cost and energy efficiency.

‘Upfront design is undervalued in housebuilding,’ says Jackson, who is a trained architect. ‘It’s critical, with new technologies, to make sure the design is correct – then we have a fighting chance of delivering what we intend to.’

Some designs, such as the heating system, can be prefabricated off site, which cuts down on waste and reduces the need for people on site. Jackson calls this process ‘the industrialisation of design’.

The designs are incorporated in the software, alongside the latest housing standards and calculation methods, including SAP 10 and the CIBSE TM59 Overheating risk calculation, which is included in the latest Part L of the Building Regulations. This means that if users change window sizes, for instance, the software will automatically adjust other elements – such as insulation – to ensure compliance with Building Regulations.

Radiator sizes will be adjusted according to whether a boiler or heat pump is used as the heat source, and the system automatically inserts the most efficient pipework runs for plumbing, and ductwork for ventilation, positioning the necessary voids and holes in the floors and joists. It can include photovoltaics and decentralised mechanical extract fans, while more sophisticated mechanical ventilation with heat recovery units are also included.

One of the design partners is Baxi Heating, which supplied details of 160 understairs configurations featuring its range of ASHPs, water cylinders and pumps. (Gas boilers are also included.)

Andrew Miele, senior application engineer, offsite, at Baxi, was responsible for ensuring the design outputs did not feature a tangle of pipes in the understairs cupboard. By working with other partners on the design, he was able to ensure the plantroom was as streamlined as possible and could be prefabricated in the Baxi factory.

A prototype helped Baxi position its heating system under the staircase

It was decided by WholeHouse team to locate the cylinder under the stairs with the rest of the heating system, because it only took up 0.9m² of floor space compared with 1.5m² of floor space if it was in an upstairs utility cupboard.

Baxi worked closely with staircase manufacturer Staircraft to devise an efficient heating design that worked in the spaces created by the risers and treads. Staircraft provided Baxi with a staircase to create a prototype that validated software designs and calculations. The mock-up will soon be installed at a Baxi training facility.

To improve the efficiency of the design, Baxi also requested that a wall be moved 100mm, something that would have been impossible in a regular build, where Baxi would have come onto the project at a much later design stage.

Miele says he was given much more time to optimise the design: ‘In other projects, I will try to feed in design suggestions, but you can only make limited changes. Whereas here, everything’s up for discussion.

‘Going into this amount of design detail was of great benefit to us, because we can then repeat the design in the factory.’

Baxi used its experience with prefabricated utility cupboards and commercial heating and cooling systems to feed into the design discussions.

With more standardisation of design and repetition, offsite manufacturers will be able to achieve better economies of scale, says Jackson. They will also be able to plan for production because WholeHouse knows when working designs have been downloaded and when materials and systems need to be manufactured. ‘The visibility means that Baxi can make sure its supply chain is always sized to suit the demand coming through,’ says Jackson.

Any manufacturer can potentially have its products featured in WholeHouse – there are a number of suppliers’ door kits featured, for instance – but Jackson says Baxi’s is the only appropriate heating system featured at the moment.

A digital twin of the house is created, which can be used in the future if, for example, the occupier wants to add an extension (the project teams purposely created designs that could be extended in the most efficient and economical way possible).

It has not yet been decided who takes ownership of the digital twin  – it could be WholeHouse or it could be the regional housebuilder. ‘It will be whoever adds the most value,’ says Jackson.

The first two houses designed using the system are nearing completion and Jackson has ambitious plans for the system. As well as targeting the SME housebuilder market of up to 2,000 companies, he thinks it could also be used by selfbuilders and major housebuilders, with whom he is already in discussion about using elements of WholeHouse.

Jackson has also had talks with the Health and Safety Executive about incorporating manuals into the software and highlighting known safety issues. ‘We can bring more people to WholeHouse to refine the software further and iron out other issues,’ he says.

While Travis Perkins is advocating a standardised approach, it doesn’t mean that homes will look the same everywhere. This is part of the appeal to regional housebuilders, as they can reference the local vernacular with their choice of materials, says Jackson.

There is ‘beauty in standardisation’, he adds, with some of Britain’s best, most historical housing being based on only two or three house types, such as the Royal Crescent in Bath and Islington’s Georgian townhouses.

‘Pattern books are a fundamental part of our history of architecture and the landscape of towns and villages,’ says Jackson, who is talking to The Prince’s Foundation about using WholeHouse in heritage developments.

‘We tend to associate the term “house types” with some of the homes that major housebuilders churn out, but we shouldn’t think of standardisation in that way,’
says Jackson.

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In its first year of operation, the £3.1m scheme is expected to signifcantly reduce the college’s heating bill and cut carbon emissions by 70%, saving 248 tonnes of CO₂. By 2050, with the further decarbonisation of the electrical Grid, that figure is predicted to increase to more than 90%.

Comberton Village College is a large secondary school located close to the city of Cambridge. It is formed from a diverse collection of buildings of various ages, most of which have been constructed as self-contained units, complete with their own plantrooms. The campus has no gas supply, so the majority of buildings were heated by fuel oil, with a few heated using liquefied petroleum gas (LPG).

Bouygues Energies & Services (E&S) is a contractor under the Greater London Authority and Partnership’s energy performance contracting initiative Re:fit. As part of this, in 2018 it was approached by Cambridgeshire County Council and the Cam Academy Trust to develop proposals to decarbonise Comberton Village College.

Bouygues E&S’ senior project manager Ornella Sangermano, and Colin Marchant, site manager – clerk of works

Bouygues E&S energy design manager Tom McGrath describes the process: ‘We start each project by developing an energy baseline that provides us with an overview of the current energy consumption and carbon emissions of the site.

‘We then review potential decarbonisation options and quantify the impact of these for carbon and cost. The most effective options are then presented in a business case that demonstrates that the intervention will save X tonnes of carbon and energy annually, and will offer a financial payback of Y years.’

The metrics form the basis of an energy performance contract (EPC), in which Bouygues E&S guarantees that it will be achieved, says McGrath. ‘We undertake responsibility for full design, planning and procurement, and include this in our investment-grade proposal, and this provides a significant amount of security to the client,’ he adds.

Bouygues E&S was given a blank slate by Cambridgeshire County Council and the Trust, and its proposals included an option to upgrade the college’s existing lighting installation to an LED-based solution, which would go some way to reducing its electricity consumption. It also included: the installation of photovoltaic (PV) panels on the roofs of some of the buildings, to help meet a proportion of the college’s electrical demand from renewable sources; the addition of insulation to some of the heating circuits; and improvements to the building management system (BMS), to enable it to operate more efficiently.

The proposal also addressed the legacy of the college’s ad hoc expansion over the years, which meant virtually every building on the campus had its own plantroom, many of which housed oil-fired boilers that were at the end of life and in need of replacing.

As part of its initial assessment, Bouygues looked at like-for-like replacement of the various oil-fired and LPG-fired boilers with individual air source heat pumps (ASHPs). However, McGrath says it soon became apparent that this was unworkable for most buildings because of a lack of suitable external space and noise constraints for the heat
pump condensers.

Heat is pumped from the thermal store to 11 plantrooms

Instead, Bouygues E&S came up with a more radical proposal of combining all the satellite plant into a single, centralised system. This also had the benefit of removing the college’s reliance on oil heating. ‘The big-ticket item was that we saw an opportunity to decarbonise the college’s heating significantly by installing a large-scale heat network using centralised plant,’ says McGrath.

The decarbonisation programme started in 2019. Initially, what McGrath calls ‘the more straightforward elements’ were undertaken – replacing the lighting and the installation of 140kW_p of PVs. ‘As soon as the college saw the impact of these interventions on its carbon footprint, its ambitions grew and we were tasked with further developing the heat network proposal,’ says McGrath.

The starting point for developing the detailed design for the heat network was to make an assessment of the college’s peak heat demand. ‘All we had to go on was the number of boilers, some of which had known operating efficiencies, and incomplete sets of fuel oil and LPG delivery notes,’ says McGrath.

To try to quantify the peak heat demand, external consultants were employed to undertake a heat loss assessment for each of the college’s buildings. By combining this information and the fuel consumption data, a notional peak demand of 705kW was estimated. However, when it came to sizing the system, because of the uncertainty about the accuracy of the peak heating, and to increase the system resilience, the system was sized based on two 500kW Carrier ground source heat pumps (GSHPs), providing a total peak output of 1,000kW.

The two 500kW ground source heat pumps

These deliver heat at 65^oC to a 15m³ buffer tank that smooths out the fluctuation in demand and allows the heat pumps to operate at their optimum setpoint. Hot water is then pumped from the thermal store around the campus to 11 individual plantrooms through insulated pipes, which are either buried or attached to buildings.

As an additional contingency, an electrode boiler has been installed, which has the capacity to meet the peak demand and can provide top-up heat as and when required, such as if a heat pump is out of service.

The heat pumps draw heat from ground loops contained in 60 boreholes, each 200m deep, located beneath three of the college’s car parks: 30 boreholes in one, 18 in another, and 12 in another. The boreholes are spaced at one per 9m².

‘One of the biggest challenges was installing boreholes on a live site where parking is at a premium,’ says McGrath. ‘To mitigate this, we spread them over the various car parks and phased the installation to minimise disruption to parking. This was a real success for the project and wouldn’t have been possible without the close relationships developed between the client, design team and project managers.’

Bouygues E&S did consider air source instead of ground source, but the same issues of a lack of space and concern about noise from the condenser fans made this impractical. It also considered other heating options, including biomass and direct electric, which would decarbonise the college over time as the Grid decarbonises. ‘If you want heating costs comparable with those of gas, then you need to use heat pumps,’ says McGrath.

To maximise the operating efficiency of the GSHPs, the heat network is designed to operate at a system temperature of 65^oC flow, 55^oC return, which is lower than the existing heating. At these temperatures the heat pumps are expected to operate with a coefficient of performance (COP) of approximately 2.7.

New pipework installed for the heat network

Bouygues E&S had to assess whether the reduced system temperature would render the college’s existing radiators and fan convector units undersized for the space. McGrath says heat loss assessment data showed that it was primarily the fan convector units that would have to be replaced because, at the lower circuit temperature, their output dropped to the point where it was insufficient to meet the heat demand. ‘Generally speaking, 70% of the emitters are able to cope with the reduced temperature, so only about 30% of emitters needed to be upgraded,’ he says.

Improvements to the thermal performance of the building fabric was outside the scope of the project. Instead, fabric improvements will be carried out ‘as and when each building is refurbished’, says McGrath, which will reduce the amount of energy needed by each building over time.

‘What we have said to the school is: “You’ve currently got a boiler supplying, say, 100kW of heat to a building, so – under the decarbonisation scheme – we’re going to install a plate heat exchanger that will deliver an equivalent heat output,’ he explains.

‘As the buildings are refurbished, this will provide the opportunity to improve the thermal performance of the fabric, which, in turn, should allow the supply temperature to drop and further improve the efficiency of the heat pumps’.

An incidental benefit of replacing the large, ageing fossil fuel-fired boilers with much smaller heat exchangers is that the project has improved maintenance access significantly.

Heat from the GSHPs is delivered at 65°C to a 15m3 thermal store

Currently, the system is designed to operate year-round at 65°C, while there is no space heating demand in summer, there is a swimming pool, and the system supplies hot water to calorifiers throughout the site.

As a result the domestic hot water temperature needs to remain above 60°C to manage the domestic hot water quality

‘In theory, we could lower the temperature on the heat network significantly during summer, to give us additional heat pump efficiency improvements, but we’re restricted because of the hot water demand,’ McGrath says. ‘The intention in the future, as the calorifiers are replaced, is that we will be able to drop the system flow temperature to 45°C in summer, and use this to preheat the hot water, and then use electric immersion heaters in the hot water cylinders to top up the water temperatures.’

Altering the system flow temperature in summer is the only major change planned. Although the capacity of the heating system can, in principle, be expanded incrementally later to supply other heat demands, constraints on plantroom space and space for new boreholes mean there is no capacity to extend the scheme beyond the college boundaries – to serve local residents, for example.

However, Cambridgeshire County Council, again working with Bouygues E&S, has built a low carbon heat network at nearby Swaffham Prior (see CIBSE Journal February 2022), which it may look to reproduce elsewhere in the county.

The project involved installing 140kWp of PVs on the roofs

The heat decarbonisation project at Comberton Village College started in October 2021 and the heat network has been running since January – although the scheme has not yet reached practical completion.

‘One of the things we have had to make clear to the college is the importance of the operational strategy,’ says McGrath. ‘At the end of the year, when we do the energy reconciliation, if the scheme is using more energy than we anticipated we need to be able to understand whether this is related to design or an intervention from the college’s facilities management team.’

The pioneering project is expected to become  a showcase for retrofitting renewable energy technologies and heat network installations to deliver significant long-term carbon emissions reductions and future life-cycle costs.

While projects such as Comberton are novel now, they are set to become the new norm as colleges and local authorities continue to move away from fossil-fuelled combustion to heat their buildings

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