CPD 1 2020: Ambient loop / carbon zero

This CPD, sponsored by Daikin, considers how the ambient loop heating concept can help reduce energy consumption to meet new targets on carbon emissions

Introduction

Climate awareness is now embedded across most if not all industries, including the construction sector. New legislation aimed at cutting energy consumption and reducing carbon emissions to zero by 2050 is resulting in new technologies that aim to bring about greater energy efficiency. Fossil fuels are increasingly being displaced by renewables and nuclear generation, while planning policies and building regulations are being revised to account for lower emissions projections. These changes will create a huge opportunity for the use of electric heating systems, which have lower associated emissions, both direct and indirect.

This CPD will look at projections for energy emissions, how planners seek to incorporate ambitions to reduce emissions, and how the ambient loop heating concept can help designers, construction firms and end users reduce energy consumption.

Each apartment has its own heat pump that brings the ambient water pumped from the block’s central pump up to the right temperature for hot water and heating

Changing energy and emissions trends

The carbon emissions of grid electricity are set to fall dramatically, due to the decarbonisation of UK grid electricity. Carbon emissions from power stations have already been reduced by more than 60% since 1990. The Department for Business, Energy and Industrial Strategy (BEIS) projects that the carbon emissions from major power producers will have fallen by 80% in the decade up to 2020. Meanwhile, low-carbon energy’s share of UK electricity generation – from renewables and nuclear generation – has risen from 22% in 2010 to 65% in 2020. 

The emissions intensity for grid electricity by 2035 is now projected to be 41gCO2e/kWh, significantly down from BEIS’ 2017 energy and emissions projections of 55gCO2e/kW.

Why have these projections been revised downwards so markedly?

In 2010, the 288TWh generated from fossil fuels accounted for around three-quarters of the UK total. It was also more than 10 times as much electricity as the 26TWh that came from renewables, based on data from climate news source Carbon Brief in 2019. Additionally, the amount of electricity generated in the UK in 2018 fell to its lowest level in a quarter-century – at just 335TWh, a level last seen in 1994 – while output from renewable sources rose to another record high, generating an estimated 33% of the UK total in 2018.

It is encouraging that UK economic growth no longer requires rising generation of electricity. Overall, the amount of electricity generated per person in the UK has fallen by 24% since 2005, down to its lowest level in 34 years. 

While the UK economy has expanded more than two-fold since 1980, electricity generation has nowhere near matched this. In the years up to 2015 it grew by 40%, but then began to fall and is now back down to less than 20% growth since 1980. 

Why is the UK using less electricity despite this economic growth? Rising electricity prices and economic hardship may have played a part, as well as a decline in UK manufacturing and the rise of the service sector as the main driver of economic growth. There are also positive drivers, however, such as product energy efficiency regulations, energy-efficient lighting, the replacement of older appliances with the latest models and more environmentally conscious consumers.

As consumers and industry look to a future featuring less carbon emissions, renewables and nuclear generation will take an increasing share of electricity generation, displacing fossil fuels. Emissions from electricity production are projected to fall steadily over the next 15 years. Scotland is among those countries leading the way, with 75% of its electricity production coming from renewable sources in 2018 – and a forecast of 100% for 2020 .

As a result, planning policies and building regulations are being, and will continue to be, revised to account for these lower emissions projections. These changes will create a huge opportunity for the use of electric heating systems, because of the lower emissions associated, both direct and indirect.

Future systems?

How will this shift in energy sourcing affect heating systems being designed now and in the future? SAP is the Standard Assessment Procedure for Energy Rating of Dwellings. Currently, SAP under version 2012 assumes that any electricity used produces 2.4 times the carbon emissions of mains gas. This is because it uses an outdated carbon factor looking backwards at historic data that does not reflect the current energy mix of the grid, which is increasingly fed by renewable and clean energy.

Under the newer SAP 10 and using a mix of current and projected data, electricity-related carbon emissions have fallen from 0.519kgCO2/kWh to 0.233kgCO2/kWh – a more than 50% reduction, making electricity’s emissions only slightly higher than those for mains gas. Recently matters have taken a step further with SAP 10.1, which went out for consultation late last year and aims for a further reduction to just 0.136kgCO2/kWh, significantly lower than for gas.

Distribution losses

Gas boilers and combined heat and power (CHP) systems have a further disadvantage: distribution losses, due to the use of high-temperature water to distribute heat around the building. In BRE’s 2016 consultation paper on distribution loss factors in heat networks supplying dwelling, typical distribution losses were found to be much higher than the SAP 2012 default factor of 1.05, which has so far been used to calculate the distribution losses from a building’s plantroom to the point of use.

And a full year’s data clearly highlights that the lower the heat load per apartment, the higher the distribution losses as a percentage of the total load. As a result, the default value of 1.05 has been raised to 1.5, which equates to 33% losses, an almost 700% increase. Otherwise, if the system is designed in accordance with CIBSE/ADE Heat Networks: Code of Practice for the UK, the default value is 2, which equates to 50% losses.

The distribution loss factor not only is important from an efficiency point of view but also affects comfort levels; heat losses from the distribution pipework in a thermally efficient building can lead to increased air temperatures, contributing to overheating of risers, corridors and apartments. It can also lead to increased ventilation to mitigate overheating problems and mechanical cooling if increased ventilation proves insufficient. This can result in higher costs and a less efficient building.

This completely changes the preferred choice of heat plant for any communal heating system. While connection to a district heating system remains the goal, the most achievable – and beneficial – low-carbon solution has shifted from CHP or communal boiler systems to a site-wide communal heat pump system. Such a system is future-proof in that it can be connected to a district heating system should one become available at some point in the future. This shift in strategy gives an ideal opportunity for heat pump technologies. 

Another key driver for heat pumps over gas-fired CHP systems and conventional boilers is that heat pumps run on electricity, which is clean at the point of use. Gas combustion emits nitrous oxide and particulates that are a major contributor to poor air quality, so avoiding the need for flues is a great advantage. Tightened regulations are calling for new developments to be air quality neutral. If gas CHP or boiler systems are proposed, local authorities will require applicants to provide sufficient information to justify their use and explain how the carbon and air quality impact will be minimised.

Planning policies

In its draft London Plan published in December 2017 and expected to come into effect later this year, the Greater London Authority (GLA) adopted a new heating hierarchy. The first goal is to use less energy from the outset, along with better insulation of homes. Subsequently in October 2018 the GLA published its Energy Assessment Guidance document, which encourages energy assessors to use the SAP 10 carbon emission figures for any new development. And thanks to the efficiency of heat pumps, these fare better than gas boilers for the heating that is required.

As well as requiring a clean and efficient supply of energy, the London Plan insists on the use of renewable energy and, for all new housing developments, an on-site carbon emissions reduction of 35% greater than the Building Regulations baseline.

Any difference between the on-site reduction and net zero carbon must be paid at a rate of £60 per tonne of carbon emissions per year for 30 years, according the 2018 guidelines. And the new draft London Plan expected this year recommends this is increased to £95 per tonne. While this is currently only a London guideline, regional authorities are likely to adopt the same or something similar.

Heat pump solution

Heat pumps work by moving heat from inside the building to the outside in cooling and vice versa for heating. They are able to do this thanks to the vapour compression cycle, a cycle that uses a refrigerant as the working fluid. 

During the cycle, pressure changes occur that are used to govern the properties of the refrigerant to absorb or reject heat energy.

The refrigerant is passed through an expansion valve, which allows the refrigerant to reduce in pressure; this also reduces the refrigerant’s temperature. The cold refrigerant then passes through a heat exchanger, also known as an evaporator. In an air-source heat pump, the evaporator would be a coil in the outdoor unit. As air passes across the coil, heat is transferred from the air to the refrigerant, which is at a very low temperature. This has the effect of boiling off or evaporating the refrigerant. The warmed refrigerant vapour passes to the compressor where its pressure – and therefore its temperature – is increased, ready to reject its heat across the second heat exchanger, known as the condenser. As the heat in the refrigerant is rejected it condenses into a cooled liquid and is ready to start the cycle once again.

The efficiency of a heat pump is described as the energy efficiency ratio (EER), which is the ratio of the heat absorbed by the heat pump divided by the energy consumed by the heat pump. When talking about heating efficiency, the term “coefficient of performance” (COP) is used, which is the ratio of rejected heat divided by power input. The rejected heat is always the sum of the absorbed heat plus the power input. And with the aid of some simple numbers, we can see that heating outputs and efficiencies are higher than cooling.

EER and COP are important when considering how much energy the heat pump consumes and how this equates to carbon emissions and primary energy consumption. With the advent of the Energy Related Products Regulations, the efficiency in cooling and heating of a heat pump system is now measured using seasonal efficiency, which takes into account the performance of the system throughout the year – as represented in SEER and SCOP. The details of how these values are derived will not be explained here, but the main principle of the change is to more reflect accurately how a system is used through the year as well as the changes in the external temperature. However, COP and EER illustrate clearly the key principles involved when comparing different systems. 

To get from the electricity consumed by the heat pump to the associated carbon emissions of that electricity, one must multiply the energy consumption by a carbon emission factor (CEF). A primary energy factor (PEF) connects primary to final energy; in other words, it indicates how much primary energy is used to generate a unit of electricity or a unit of usable thermal energy. Importantly, primary energy is set to form the main basis of compliance under new Part L guidance.

A development that uses a central air source heat pump in each block to warm an ambient temperature loop; the water is then heated higher within each apartment

Performance and compliance

While future building regulations will be based upon primary energy, currently the measure of compliance is still based upon carbon. Looking at independent analysis that considers a range of technology types and applications, the impact of the change in carbon emission factor is clear. Using the old carbon emissions factor of 0.519kgCO2/kWh, CHP systems just about clear the carbon reduction target when used in residential blocks of 70 units or more, or in 100-bedroom hotels. Conversely, heat pump systems don’t make the grade.

But looking at how heat pumps perform and taking into account the SAP 10 carbon emissions factor of 0.233kgCO2/kWh,heat pump systems can be shown to decisively deliver the carbon reductions required. In comparison, CHP systems, communal boilers and direct electric systems would require additional carbon reduction measures to achieve the 35% carbon reduction target.

If the newer SAP 10.1 figure of 0.136kgCO2/kWh is adopted, the difference will be even greater. So while historically planning policy has prioritised combined heat and power systems in medium- and large-scale residential developments, it is expected that future policy will virtually wipe out the advantages of CHP. If such a 35% carbon reduction target in residential settings is being sought, the solution is now a heat pump, rather than CHP.

Ambient loop concept

An ambient loop system circulates low-grade heat to all the apartments in a development. Within each apartment, a water-to-water heat pump draws water from the ambient loop and upgrades this to a usable temperature for heating and/or domestic hot water.

The apartment heat pumps can operate as long as the loop temperature is maintained above -10°C and below +30°C.

Therefore, a wide range of heat sources can be considered:

  • Air source heat pumps
  • Boreholes and thermal piles
  • Surface water such as a river, sea or canal
  • A fourth or fifth generation district heating
  • Waste heat recovery. 

By moving energy around the building at these temperatures the heat losses are close to zero, thus solving one of the main issues in high-temperature distribution with overheating of corridors and other similar spaces. The apartment heat pump can also produce chilled water for cooling if required.

Importantly if cooling is required no additional central plant or pipework is needed as the apartment heat pump works in reverse cycle and instead of absorbing energy from the loop rejects energy to the loop. This can deliver cost savings compared with having a central plant heating system and a central plant cooling system installed in parallel.

The efficiency delivered by the apartment heat pump varies depending on the source temperature. For example:

  • A shared borehole array with an average temperature of 10°C to the heat pump will deliver a COP of 6 in heating (at 35°C for underfloor heating) or 3.2 for domestic hot water (at 55°C).
  • Similar efficiencies for surface water could be expected, although there is more of a seasonal variation as river and sea water temperatures vary.
  • Where the loop is preheated by a central air source heat pump to 25°C, a COP of 11.2 in heating (at 35°C for underfloor heating) or 5.7 for domestic hot water (at 55°C). When considering both the power consumption of the in-apartment heat pumps and the central air source heat pump, this can result in a seasonal efficiency of the system of approximately 3 to 3.2. The efficiency sweet spot for the primary loop is around 25°C.

The illustration on the first pageshows what a development might look like using central air source heat pumps to warm the ambient loop. In this block-by-block approach, each block has its own heat pump. Water is circulated around the building at near ambient temperature of 25°C, when the system is operating at its highest efficiency.

Each in-apartment heat pump is connected to the loop. When the heat pump needs to heat the apartment or heat the domestic hot water storage cylinder, it draws the near-ambient water from the loop to use as a source of low-grade energy. The heat pump then upgrades the water temperature, to anywhere from 35°C to 60°C, depending on user requirements. For larger developments with multiple blocks, the system can be interconnected or served by a common energy centre where the heat pumps are centrally located.

Calculating carbon reductions

To fully understand the benefit of the ambient loop system one can look at an example SAP calculation, based on a building where an ambient loop system was compared with a combined central boiler/CHP system connected to heat interface units. The results were assessed based on current SAP 2012 and then adjusted to consider the changes in carbon factor and distribution losses highlighted in SAP 10.

Under SAP 2012 there is little difference between the two systems, so it was not driving change. But once the results under SAP 10 were considered there was not only a sizeable reduction in the dwelling emission ratio for the ambient loop system but also a significant increase in the dwelling emission ratio for the boiler/CHP system.

Looking at that scenario across the entire block, comparing the current CHP/boiler system with the future ambient loop heat pump system results in savings of 143 tonnes of CO2 emissions. 

If one assesses the carbon reduction in terms of what that means for the difference in carbon offset payments the argument to use ambient loop heat system technology can become compelling.

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