Changing times

Strebel Heating Technology, Adrian Walker, energy, SAP, BREEAM, SBEM, NOx, energy, heating, carbon
Adrian Walker

Adrian Walker discusses how the UK’s shifting energy sources and decarbonisation of the grid are changing the choices for heating in sectors such as hotels, schools and leisure centres.

As a nation we can all now see that we are gradually reducing our reliance on coal as our main source of power. It isn’t necessary to be a building services engineer to be aware of this. Today this is mainstream news. The Times, The Guardian and also the BBC reported after the first May Bank Holiday of 2019 that the country had already passed 1000 coal-free hours in 2019 with wind turbines and natural gas keeping the lights on.

The transformation of the reducing carbon content of electricity generation is set to continue with the Government target to shut the door completely on coal by 2025. However, whilst it’s true that wind and solar power means that the need for burning fossil fuels is sometimes very low it is still the fact that natural gas regularly provides more than half of the UK’s electricity.

Also factor in that converting fully to electric transport will require a doubling of the UK’s current electricity capacity and that during the winter months heating requires over three times the capacity that electricity generators can provide today.

To address this it can only be assumed that a significant investment in generation capacity alongside a grid extension will be required – and over a period significantly longer than some of the carbon zero date aspirations often heard. Having said that, one thing is clear: Our energy generation mix has become more diverse and that is now beginning to have significant impacts downstream in the heating sector.

So how does all this affect the commercial heating world when it comes to selecting plant to heat our schools, health centres, offices and other multi-populated buildings? What does this all mean for plant selection criteria when it comes to compliance with the Building Regulations?

The quick answer is that technology selection stems from carbon emission factors that are included in the Standard Assessment Procedure of Energy Rating of Dwellings (SAP). On behalf of BEIS the Building Research Establishment (BRE) publishes the carbon intensity of generated electricity, gas, oil and solid fuels and includes these in the appendices within SAP. This document is currently under review (Draft Version 10 24/07/2018) with significant proposed reductions in both the Primary Energy Factor (1.738) and Emissions for electricity (2.33g CO2/kWh) over previous SAP figures – whilst there are no changes proposed for Natural Gas (2.10g CO2/kWh).

On the face of it electrically generated heat would look to be the winner if these figures are confirmed (Table 12, page 169 of SAP 10.0 July 2018). Translated to product terms this would appear to favour heat pumps and electrical water heaters over gas fired appliances as primary heating appliances. However, balance in all things, as modern condensing gas boilers equipped with ultra high efficiencies and ultra low NOx emissions will continue to provide an excellent method of either meeting whole system needs or, in conjunction with heat pumps, achieving winter peak loads as well as whole load back-up.

Included in SAP Table 12 is an estimate of the unit price of Natural Gas and Electricity. With Natural Gas commercially available at 3.94p/kWh and electricity at 16.55p/kWh the cost of this low carbon lean towards heat pumps could be considered to be putting a premium rate of operating cost onto the building operator or owner.

Strebel Heating Technology, Adrian Walker, energy, SAP, BREEAM, SBEM, NOx, energy, heating, carbon

It’s widely accepted that heat pumps are more efficient with low grade heat circuits operating with flow temperatures of around 35oC. As a result, the widespread adoption of heat pumps will necessitate the adoption of low-grade heat circuits and appropriate emitters such as high surface area radiators and under floor heating. Some modern heat pumps can generate flow temperatures up to around 60oC but such high temperatures come at the expense of efficiency, especially so when ambient air temperatures are sub-zero.

It is generally accepted that with return temperatures around 35oC modern condensing gas boilers will operate with Gross Seasonal Efficiency around 95% (Part L method), so for the operational costs of running a heat pump to be similar to this the Coefficient of Performance (COP) of a heat pump needs to be four times as efficient as a gas boiler, or the COP needs to be greater than 4 when heat is needed. Heat Pump technology has evolved today to where such COPs are available although great care needs to be taken when looking at COP reductions with falls in ambient air temperature. Generally split design air source heat pumps with stepless pumps will provide the best available Seasonal Coefficient of Performances (SCOP), some models in excess of 5, and will still offer COPs of over 4 at ambient air temperatures close to freezing if operating with low system flow temperatures.

Consequently, care needs to be taken when selecting a heat pump to avoid criticism after the install, as it could be considered commercially insensitive to design a heat pump as primary heat source with a ‘winter air temperature’ COP of less than 4, in the knowledge that the operational costs of running may prove significantly higher than that of a modern condensing gas boiler during the critical months when temperatures fall to below freezing.

Given the variation in seasonal heat loads and wide range of ambient air temperatures in the UK adopting the use of heat pumps as the primary heat source alongside modern gas condensing boilers in cascade arrangement is a carbon friendly and pragmatic way to ensure lowest carbon footprint as well as full back-up and peak load matching. Critically installing both technologies not only offers the building operator heat security but also choice on operating costs if electricity prices spike. Notwithstanding this as there could be significant strains placed on the electrical network over the coming decades having gas boilers installed may prove useful in the event of unforeseen future power outages.

As with heat pumps there have also been recent advancements in boiler performance with emphasis placed on the lowering of NOx emissions in response to growing concerns over air quality. In December 2018 BRE published an update to the BREEAM UK New Construction Technical Manual in which there were significant reductions published to NOx emission limits of gas fired heating boilers. Following on from BREEAM 2014 where 3 credits were available for achieving 40mg/kWh the revised limits were reduced to 27mg/kWh for 1 credit and a lower level of 24mg/kWh for 2 credits.

As the name of this publication suggests these NOx limits are only pertinent for new construction projects and do not apply to the largest proportion by far of commercial heating installations that is the retrofit of existing gas fired plant. However, as with previous BREEAM updates it is anticipated that the new limits will become the norm within time and the air quality benefits will flow through then.

Additionally, the nature of the gas which is fired in boilers is likely to follow the decarbonisation trend over the years ahead with the adoption of bio-methane and hydrogen mixes coming to the fore.

What’s clear is that decarbonisation of the power grid is shaping our energy future when it comes to selecting commercial heating plant. Led by recent and potential changes in SAP, SBEM, BREEAM and ultimately Part L of the Building Regs, combinations of ultra-high SCOP air source heat pumps and ultra-low NOx gas fired boilers working together are bringing the best available technology to the market. It is up to manufacturers, consultants and contractors to bring these products together to work in harmony for the buildings of today and the future.

Adrian Walker is national sales manager for Strebel Heating Technology

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