﻿Reducing carbon with efficiency
With the current focus on reducing carbon emissions, it is just as important, if not more so, not to lose sight of the need for efficiency. Kevin Stones explains how to tackle both issues.
Changing the name of the Carbon Reduction Commitment to the CRC Energy Efficiency Scheme (CRC EES) was more than just a whim; it was in recognition that reducing carbon emissions does not always improve efficiency. And it is important to address both issues because efficiency has a direct impact on fuel consumption and, therefore, costs.
For example, electricity generated by a nuclear power station may have a low carbon footprint but if the systems using the electricity are inefficient the building operator will still end up with a big energy bill. So a low-carbon installation that is expensive to run will clearly fail to meet all of the needs of the end client.
Another, more common, example is biomass fuels. A biomass boiler reduces carbon emissions because wood fuels are close to carbon neutral, on the grounds that the CO2 released during combustion was fixed by the plants used as fuel just a few years earlier. But it is still important that the biomass boiler is used efficiently.
Modern biomass boilers are very efficient (up to 92% for the better designs), but as with all building services plant their efficiency will vary with the nature of the system they serve. For instance, they cope effectively with variable heat loads but not, perhaps, as efficiently as a gas-fired condensing boiler. So biomass boilers operate at maximum efficiency with a steady base load.
Consequently there will be occasions when a mix of biomass and conventional boilers achieves the highest overall efficiency, in relation to the way the building is used and the demands it makes on its heating systems. An obvious example would be the biomass boiler meeting a constant load and ‘topped up’ by a condensing boiler as and when required.
There are other considerations the building-services engineer has to take into account when designing such systems. These include local planning consents requiring a percentage of renewable energy sources and/or the desire of the end client to be seen to be green.
As a result, there will certainly be a need to consider the use of other renewable heat sources, such as solar thermal and heat pumps, as part of the overall mix. Because the heating capacity of solar-thermal and heat-pump systems shows considerable seasonal variation, there will be a need to design a suitable level of back-up into the system. This, in turn, may impact on capital costs and resulting payback periods.
For that reason, solar thermal systems and heat pumps are often used to pre-heat cold water for hot water systems, with other heat sources (such gas, oil or biomass boilers) being used to bring the water up to the required temperature and for pasteurisation cycles. This approach tends to work well and increases efficiency by minimising the use of the boiler plant. However, it is essential to include efficient controls and suitably designed thermal-storage vessels to ensure that each heat source is used to optimum effect.
Other situations may point to a different solution. Swimming pools, for example, act as a large heat sink with relatively low operating temperatures (typically 26 to 30°C) that can easily be achieved with solar thermal, with no need to buy thermal-storage vessels. These low temperatures result in high collector efficiencies and solar fractions. Additionally most indoor pools have a large roof area, which is ideal for the solar collectors.
New-build projects may offer more scope to make wider use of renewable heat sources, as early collaboration at design stage can help ensure the fabric design reduces heating loads. This, in turn, could open the door for using lower-grade heat sources such as heat pumps, with underfloor heating or suitably sized heat emitters to compensate for the lower water temperatures. In such cases, the lower return-water temperatures may make condensing boilers the best choice as a back-up heat source because of the extra condensing that will be achieved.
Refurbishment projects, where the requirement for consequential improvements under Part L of the Building Regulations is likely to encourage the use of low-carbon technologies, can be more challenging. For instance, if the central plant is being replaced but the distribution system and heat emitters remain, the original design water temperatures may need to be retained. Here, biomass boilers offer a good solution as they will generally deliver hot water at around 82°C, equivalent to conventional boilers.
Ultimately, the key to arriving at the best solution is to decide which renewable and conventional heat sources will work together in a controllable fashion. Further, these choices need to be made in the context of the nature of the building and how it is used on a day-to-day basis, local planning requirements, the end client’s aspirations and the budgetary realities.
Kevin Stones is technical service director with Hoval.