How low can you go?

Chris Davis, GLA, Low Carbon Heat: Heat Pumps in London, gas-fired CHP, heat networks, Evinox Energy
Chris Davis

Chris Davis considers changes to the carbon emission factors in the updated GLA planning guidance will have for the specification of heat networks and selection of heat interface units.

London has ambitions plans to become a zero carbon city by 2050, with the Mayor’s London Environmental Strategy stressing the need to depend less on natural gas and increase the use of low carbon electricity and heat.

To achieve this, new planning applications for residential developments must already meet a zero-carbon target – with at least 35% improvement in onsite carbon emissions beyond building regulations Part L; and must also be accompanied with a commitment to connect to a communal or district heating network.

This has been good news for the heat network sector in recent years, with gas and CHP central boiler plants serving Heat Interface Units via distribution pipework being the norm. Consulting engineers and M&E contractors have become much more familiar with the design, installation and commissioning aspects of such systems, whilst advancement of prevailing standards and system design guidance aimed at improving the overall efficiency of heat networks in operation has all helped the sector move forwards.

From January 1st 2019 however, the GLA has introduced a new requirement for all energy assessments accompanying planning applications to be based on the proposed SAP10 carbon emission factors. This is set to have a fundamental impact on the design of heat networks (including evolving requirements for HIU’s) for one simple reason – the presumed carbon content of electricity (kgCO/kWh) has been reduced by a whopping 55% over SAP2012. This in turn is likely to mean two things.

Firstly, the benefits of the “free” electricity produced by CHP plant becomes less significant to the overall carbon saving target. In other words, will have to produce twice as much carbon-free electrical power to achieve the same level of carbon saving as before; and secondly, lower carbon factors will encourage the use of electrically powered heat generation technologies – specifically heat pumps – which typically operate at lower temperatures than boilers or CHP. Couple this with the allied targets around air quality and NOx emissions, and it is clear to see that electricity is about to replace gas as the primary fuel for new build heating schemes, especially for small-to-medium residential developments that do not have a year-round base load for optimum operation of CHP.

Grid electricity has decarbonised enormously since the last update to Building Regulations Part L in 2014, specifically as a result of an increased net contribution from renewable sources (in particular wind and solar PV) and a reduction in the amount of coal used to produce electricity for the grid.

So, reflecting these changes in our electricity generation mix makes a lot of sense. And while we won’t see SAP 10 come into effect nationally for some time yet, the GLA has taken the lead in future proofing all new planning applications right away. The table below provides a comparison of SAP2012 and SAP10 carbon factors.

Almost certainly then, heat pumps will play a growing role in the delivery of low carbon heat in London, both as part of low carbon heat networks (for example using waste heat as a heat source) and in building-level “communal” systems.

The graph shown here, from a recently published report entitled “Low Carbon Heat: Heat Pumps In London” (September 2018) clearly illustrates that when applying more up to date carbon factors for electricity – such as those proposed in SAP10 - heat pumps offer a substantially lower carbon alternative than gas boilers or gas-fired CHP. Coupled with the fact that the carbon factor of electricity is expected to fall further in the future, increased uptake of electrified heat is an obvious conclusion to draw.

A key consideration however for the wider uptake of heat pumps in heat networks is the mechanical system design – particularly relating to supply temperatures and domestic hot water provision.

Chris Davis, GLA, Low Carbon Heat: Heat Pumps in London, gas-fired CHP, heat networks, Evinox Energy

There are myriad of ways in which heat pumps might be deployed within the scope of communal and district heating systems, including as the sole heat source, or in combination with other (higher temperature) heat generation equipment. However, an inherent feature of the way heat pumps work is that their efficiency reduces as the temperature they are required to supply increases, meaning the requirement for heat networks to operate at lower primary flow temperatures – in the region of 60°C or lower – will become much more commonplace.

Of course, running communal or district heating systems at lower primary temperatures does bring obvious benefits – lower system heat losses for one. But the careful consideration for all aspects of system design for consulting engineers and specifiers will be crucial in ensuring systems are able to offer the correct balance of low carbon heat and system performance within the apartment – particularly with regards to the supply of domestic hot water.

Communal and district heating systems typically feature heat interface units to provide a hydraulic break between primary and secondary systems. Most commonplace are twin plate HIU’s which decouple the space heating system from the primary network by means of a plate heat exchanger; similarly, domestic hot water is produced instantaneously by heating incoming mains water from the heat in the primary network via a second heat exchanger. With internal space at a premium, the ability to provide domestic hot water without the need for a storage cylinder is a popular choice among consultants and developers.

However, with lower primary temperatures, specifiers will need to look carefully at the performance characteristics of their selected HIU, particularly in terms of the ability to deliver adequate hot water temperatures and capacities, as few HIU’s available on the market are capable of operating effectively at primary flow temperatures below 70°C.

The latest Evinox ModuSat XR-ECO models, for example, have been developed specifically with lower temperature heat networks in mind, providing the ability to deliver useful peak hot water capacities (i.e. around 40kW depending on model) at 50-52ºC with a primary water temperature as low as 55ºC – testament to the superb performance of the units highly efficient plate heat exchangers, making them the ideal choice for systems where heat pumps are deployed as the primary heat generator.

Emissions kgCO2/kWh 
SAP2012  Draft SAP10 
Mains Gas  0.216  0.210 
Electricity  0.519  0.233 

It is conceivable also that with the need for lower primary temperatures, domestic hot water storage with local temperature top up may also become more popular. So HIU’s such as Evinox’s FS range allow volumes of up to 400L of stored water to be heated by the primary network via a conventional cylinder coil. The cylinder temperature can then be boosted locally via an electrical immersion – either to maintain ~60ºC in the tank on a continual basis; or to allow water to be stored at whatever temperature can be achieved by the primary network and boosting this periodically using the HIU’s legionella boost function. With much lower electricity carbon emissions factors in play, the use of boost immersions carries a far lower carbon compliance penalty than in the past, so can now be reasonably considered a viable option.

In summary then, the new GLA planning requirements are certainly going to have an impact on the choice of heat generation plant for heat networks, which in turn will have a knock-on effect on choice of HIU’s. For sure, the selection of HIU’s with characteristics that lend themselves well to lower temperature systems will be a key consideration for consulting engineers and system specifiers as the driver for next generation of heat networks now appears to be upon us.

Chris Davis is head of sales and marketing at Evinox Energy

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