Carbon-effective refurbishment

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The vital importance of refurbishment in reducing carbon emissions — Ant Wilson.

Refurbishment projects present an ideal opportunity to address the energy performance of our existing building stock. Ant Wilson stresses that an holistic view is necessary to help meet Government targets for reducing carbon emissions.

It is very difficult to find a silver lining to the cloud of recession, but if there is one it may be that the move away from new-build to refurbishment will force us to take a closer look at the energy performance of our existing building stock. Because if we don’t take more notice of existing buildings we have little hope of meeting the Government’s carbon-emissions targets of an 80% reduction over 1990 levels by 2050.

Thanks to Part L2B of the Building Regulations, any major refurbishment needs to include ‘consequential improvements’ to the building’s energy performance — and the threshold for what constitutes ‘major refurbishment’ is set to fall in the next version of Part L2B. Currently in the consultation phase, the new Building Regulations will also look beyond central plant to encompass ancillary factors such as pump and fan power, as well as imposing higher efficacies for lighting.

Very often, such consequential improve­ments are in the form of installing more efficient lighting and HVAC plant, as these generally offer the quickest payback. While these are important, I would argue that we need to be looking at the full picture to ensure we address issues such as building fabric and the controllability of services.

If we are going to maintain a ‘lean, clean and green’ philosophy to building-services design — and I think we need to — the ‘lean’ is clearly the top priority. By getting the fabric right we can minimise the need for energy-consuming services and drive them with cleaner, greener energy sources. For example, if we reduce space-heating requirements, it becomes more feasible to use lower-grade heat sources such as heat pumps, while also reducing return-water temperatures for more condensing in condensing boilers.

Building-services engineers have a key role to play in modelling a building’s performance and advising architects of appropriate measures to ensure fabric and services work in harmony.

Clearly, this may be more difficult in a refurbishment situation because of the greater expense of retrofitting a solution. But an open-minded and innovative approach can yield significant improvements.

For example, many existing office buildings with floor-to-ceiling glazing are wasting obscene amounts of energy. As well as increasing solar heat gains, the high light levels at the perimeter of a space make the interior seem gloomy, so people further away from windows increase their light levels accordingly.

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Refurbishing existing buildings provides the opportunity to improve their energy performance to help meet Government targets for reducing carbon emissions.

In fact, only the upper two metres of a 3 m-high window serve any useful purpose for daylighting and view out — because that is the bit people look through. A retrofit solution, therefore, might be to put light shelves 2 m up the window to reflect light deeper into the space, using the ceiling as a secondary reflector for improved daylight uniformity.

At the same time, the lower metre could be well insulated and fitted with photovoltaics (PVs) and internally lined with phase-change boards. Emerging concentrated photovoltaic energy generation (CPV has around 1000 times less embedded energy than conventional PVs, and its price is falling rapidly, which will improve the cost-effectiveness of building-integrated PV in coming years.

Similarly, better control plays a major role in achieving a ‘clean’ building; to deliver this we need to move away from operating at full occupancy to create more flexible systems that can respond as occupancy varies. For example, flexible working in many offices has taken many organisations away from traditional 9 to 5 occupancy, and schools are extending the use of their spaces as they open their facilities to local communities.

Consequently, variable-speed control of pumps and fans is becoming a must to optimise performance. The cube relationship between fan power and air volume means that varying air flow will theoretically reduce fan power dramatically So, simply replacing an existing fan with a more efficient one may save 20% on energy — but introducing demand-controlled variable speed fans instead could achieve a much higher energy saving across the year.

At the same time, we need to understand the implications of such actions. Take a chilled water system, for example. The lower the temperature differential between flow and return temperatures the higher the efficiency of the chiller — but the pumping power increases to move more chilled water and achieve the required cooling. So it may be better to use a higher temperature differential and reduce pumping power in the primary and secondary circuits — as well as fan power in the cooling tower if using a water-cooled chiller.

Clearly the best solution will vary with factors such as the efficiency of the pumps and the chillers at different loads, so there is no single solution but one that has to optimise on intelligent principles. The key is for building-services engineers to take a more holistic view and look at how systems interact with one other and with the building. In that way we can start to retrofit some major improvements to our existing building stock.

Ant Wilson is with ECOM,formerly Faber Maunsell

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