The heat is on for BREEAM

Hoval, BREEAM, boiler, CHP, biomass, space heating
Mixing energy sources such as low- or zero-carbon to meet base load and gas-fired condensing boilers to support peak loads is an effective carbon-reduction strategy.

The emphasis on reducing carbon emissions is greater in the latest version of BREEAM than previous versions. Martin Murrish examines how the design of the heating can contribute to a high rating.

Since the BREEAM (BRE Environmental Assessment Method) was first introduced in 1990 it has undergone a number of updates, the most recent of which was in 2011. At 406 pages BREEAM 2011 is a hefty tome with a lot of information to absorb — but one of the key differences is the weighting given to reduction of CO2 emissions, compared to the 2008 version.

This is clear from the number of credits allocated to this ‘BREEAM issue’. While issues such as sustainable procurement or water consumption require one credit for a BREEAM Excellent rating and two credits for the new Outstanding rating, reduction of CO2 requires six and 10 credits, respectively, for these ratings. Interestingly, the Pass, Good and Very Good ratings do not require credits for any of the energy-related issues, including reducing CO2 emissions.

Nevertheless, many end clients are very keen to achieve BREEAM Excellent or Outstanding for their buildings. Given this increased emphasis on reduced emissions, much of the onus is likely to fall on the building-services engineer.

At the same time, it is important to note some of the criticism that has been levelled at BREEAM in the past — notably that many designs include elements that are just there to get BREEAM credits required by the client rather than for sound environmental and engineering reasons. Hopefully, this greater emphasis on reducing emissions will help to address this issue.

To that end I would suggest that the expertise of the building-services engineer in designing the most efficient system, making use of low- or zero-carbon (LZC) technologies when appropriate, rather than just ticking boxes, will come into its own. This can be illustrated by considering the various options for delivering heat for space heating and hot water to the building.

Indeed, BREEAM 2011 does award credits for LZC technologies, with one credit being required for both Excellent and Outstanding ratings. However, up to a further four credits are then available when an LZC system/technology results in reduced CO2 emissions. The question, then, is which LZC technologies will provide the best solution. And a supplementary question may be: ‘Will including non-LZC technologies also help to reduce emissions under certain circumstances?’

Biomass boilers fed, for example, by wood pellets are at their most efficient when meeting constant loads.

Addressing this issue is where sound building-services engineering principles come into their own.

In the case of the space heating, for instance, it will make sense to meet base heating loads using technologies that offer maximum efficiency when operating at full capacity, such as CHP and/or biomass boilers. It may then be most efficient to use a gas-fired condensing boiler to meet peak loads, as they offer high efficiency and a constant flow temperature at variable heat loads.

Thus, a typical scenario might use a CHP unit sized on its thermal output to meet a constant base load throughout the year, with the power generated being exported to the grid or used on site. Ideally the power will be used on site to offset mains electricity, as the latter has a higher carbon footprint than power from CHP, as well as costing more.

As the demand for heating increases during colder weather, biomass boilers may be used to meet this higher base load, again exploiting the higher efficiencies of biomass boilers when meeting constant loads. An added advantage of using biomass in the project is near carbon-neutrality of biomass fuel, as well as the ability to generate revenue for the client through the Renewable Heat Incentive. Also when using biomass, it is important to ensure a local and sustainable source of fuel is used to contribute to the sustainable procurement aspects of the project.

Over and above the all-year and Winter base heating loads there will be a requirement to meet peak loads in most projects. Here, as noted above, the most efficient option may be a non-LZC solution, such as fully modulating gas-fired boilers to achieve optimum achieve optimum efficiency at low firing. Where return-water temperatures are sufficiently low, condensing boilers may maximise these efficiencies.

In the case of domestic hot water (DHW) — where, again, demand will vary through the day — stainless-steel buffers fed by minimum storage, high output calorifiers or plate heat exchangers will provide a flexible and efficient solution. The cold mains water may be pre-heated using solar thermal or heat pump systems and then ‘topped up’ using high efficiency boilers.

The suggestions above are simply one way of dealing with a building’s hot-water requirements; clearly the optimum solution will vary from one project to another. However, the underlying principle of focusing on a design that will minimise carbon emissions is one that will not only give the end client the best solution, it will also give them the best chances of achieving a high BREEAM rating.

Martin Murrish is technical manager with Hoval.

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