Lower carbon emissions should not be at the expense of energy efficiency

Published:  04 August, 2010

Hoval, biomass boilers
Before specifying boiler plant is it essential to understand the thermal performance of the whole building and how this may impact on heating requirements.

With growing recognition that energy efficiency deserves as much attention as carbon emissions, Kevin Stone explains how this principle can be applied to boilers

In recent months there has been some concern expressed that the attention paid to carbon emissions could take some of the focus away from energy efficiency. This is because it is possible to reduce carbon emissions while still consuming the same, or even greater, amounts of energy simply by switching from fossil fuels to another energy source. Indeed, this was one of the reasons for the Carbon Reduction Commitment (CRC) programme changing its name in the UK to the CRC Energy Efficiency Scheme.

Taking these concerns into account, we return to the basic principles that many engineers and energy specialists already understand ó the first step towards any sustainable strategy is to reduce total energy consumption and then to look at how the energy is generated.

In the case of heating systems, this approach means determining the most appropriate plant for the project and then optimising the system that the plant is connected to. Of course, when retrofitting new plant to an existing system there is some cross-fertilisation as the nature of the distribution system and heat emitters will, or should, influence plant selection.

Even in systems where low-carbon heat sources such as heat pumps and solar thermal are to be used, there will almost certainly need to be back-up from boilers, so it is important to ensure the boiler plant is efficient. To that end, using boilers that comply with international efficiency standards is an obvious consideration. In addition, biomass boilers for use in urban conurbations should be listed as smoke-control exempt appliances to comply with the Clean Air Act. All gas boilers should meet the requirements of the Gas Appliance Directive and, for boilers up to 300 kW output, the Boiler Efficiency Directive.

In terms of fuel choice for the boiler, there are many considerations to take into account. Biomass such as wood pellets or wood chips is proving very popular at the moment, not least because wood fuels are very close to carbon neutral. However, there are situations where a condensing boiler may offer higher efficiencies or where a combination of biomass and gas-fired condensing boilers is the optimum solution.

For instance, where there is a relatively constant base load it will often make sense to use a biomass boiler to meet this, perhaps using a condensing boiler at times of high demand. At the Newark Innovation Centre, for example, two Hoval BioLyt wood-pellet boilers supply most of the heating demand, with an UltraGas condensing boiler being brought on at peak times.

In other situations, it may be more appropriate to use biomass for domestic hot water where the return water temperatures are too high for efficient condensing, with condensing boilers meeting space heating requirements and making full use of the lower return water temperatures. It is also worth noting that condensing boilers will still give higher seasonal efficiencies than non-condensing boilers, even when not condensing.

Where biomass is being considered, it is also important to consider factors such as local availability of suitable fuels and the fuel-storage requirements for the type of fuel chosen (typically pellet or chip). The latter will also have a bearing on the types of delivery vehicle and access required. All these factors can mitigate against the practical application of biomass in some instances and favour it in other situations.

In parallel, it is essential to understand the thermal performance of the whole building and how this may impact on the heating requirements. In a modern, well insulated building there may be considerable variation in space-heating loads, influenced by internal and solar heat gains through the day. It is therefore essential to specify plant that can adapt efficiently to variable loads and minimise wasteful stopping and starting. Most traditional boilers with pressure-jet burners allow a turndown of only 3:1, whereas some condensing boilers enable a 6:1 turndown with a single boiler or a 12:1 turndown with twin boilers ó providing much greater flexibility.

Similarly, the control of the system is important, particularly where heating systems are zoned ó a growing trend as more organisations introduce sub-metering to help them manage their energy. If biomass and fossil fuel boilers are being mixed, perhaps with other renewables such as solar thermal, the controls will be particularly important to get the sequencing correct.

In addition, the choice of boiler plant can impact on the energy consumption of other parts of the system. An obvious example is electricity for pumping. A system with low water content and narrow waterways may require a shunt pump to maintain minimum flows against the higher resistance. Inevitably, this results in higher electrical consump≠tion compared to a system that does not require a shunt pump.

Given all of these considerations it is clear that achieving higher energy efficiency and consuming less energy, whatever its source, is not only sensible but also essential to minimise long-term environmental impact. From a building-services perspective that means taking an holistic view that considers carbon emissions but also keeps an eye on the basics.

Kevin Stones is engineering and service director with Hoval



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