With all eyes focused on carbon emissions it’s important not to lose sight of the need to combine carbon reduction with energy efficiency. Kevin Stones, Engineering and Service Director with Hoval, explains how the two can be brought together, with particular reference to heat sources.
In recent years, when referring to the energy performance of buildings, there has been an apparent switch in focus from efficiency to carbon emissions. And while many people treat a reduction in carbon emissions as being synonymous with an increase in energy efficiency, this isn’t always the case. This is because it’s possible to reduce carbon emissions while still using energy inefficiently. So it’s important to combine the two, to generate the energy with minimum emissions and use it as efficiently as possible.
For example, a combined heat and power (CHP) system may deliver low carbon/low cost electricity to a building but if the systems using that electricity are inefficient, some of that electricity will be wasted. That means potentially having to use back-up power sources more often, or having less surplus power to export to the grid. Both of these scenarios clearly have both financial and environmental implications.
Similarly, low carbon heat sources such as solar thermal and heat pumps are invariably backed up by other heat sources. So the heat exchange mechanisms, distribution systems and heat emitters all need to be as efficient as possible to make maximum use of the low carbon heat and minimise the use of back up heat sources.
In many cases the low carbon element of such a project can be increased by using biomass boilers as the backup heat source, as biomass is classified as a near carbon neutral fuel source. This is because the CO2 released during combustion was fixed by plants a few years earlier, in contrast to fossil fuels where the carbon fixed over millions of years is being released over just a few hundred years. Furthermore, wood is a sustainable fuel that is easily replaced.
So biomass effectively reduces the carbon emissions of a building, but the building operator still has to pay for the wood chips or wood pellets. So the more efficient the biomass boiler, the lower the running costs.
Fortunately, 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 are able to cope effectively with variable heat loads but not, perhaps, as efficiently as a gas-fired condensing boiler. So biomass boilers will operate at maximum efficiency when combined with a steady base load.
Consequently there will be occasions when a mix of biomass and conventional boilers gives 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 where the biomass boiler meets a constant load and is then topped up by a condensing boiler as and when required. If the system served by the condensing boiler uses relatively low return water temperatures, condensing will be maximised.
However, there are many projects that are influenced by more than engineering considerations and local planning requirements to use a percentage of renewable energy sources are now impacting on many specifications. In parallel, end clients often want to be seen to be green.
Consequently, there may be good reasons for considering the use of other renewable heat sources, such as solar thermal and heat pumps, as part of the overall mix. And, as noted earlier, because the heating capacity of both solar thermal and heat pump systems shows considerable seasonal variation, it will be necessary to design a suitable level of back-up into the system. This, in turn, may impact on capital costs and the resulting payback periods.
It is also important to ensure that each heat source is used to optimum effect, so the control aspects are also very important. For example, solar thermal systems and heat pumps are often used to pre-heat cold water for hot water systems, with other heat sources (e.g. 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 type of 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 26-30°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.
Work in harmony
With new build projects there is more scope to make the building fabric work in harmony with the services, so in the context of heating the design of the fabric can (and should) reduce heating loads. Consequently, it becomes more practical to use lower grade heat sources such as heat pumps, in conjunction with underfloor heating or suitably sized heat emitters to compensate for the lower water temperatures. In such cases, the lower return water temperatures will facilitate a higher level of condensing in a condensing boiler (compared to standard return water temperatures) so that condensing boilers may be the best choice as a back-up heat source.
Refurbishment projects can be more challenging, especially if the central heating plant is being upgraded but still has to work with the existing distribution system and heat emitters – as is often the case with consequential improvements under Part L of the Building Regulations. In these situations the original design water temperatures may need to be retained, so that scope for lower grade heat sources is less. Here, biomass boilers offer a good solution as they will generally deliver hot water at around 82°C, equivalent to conventional boilers.
Ongoing system efficiency is another important consideration and this will only be achieved if the plant is maintained and optimised regularly following installation. In these cases, simply ensuring the plant operates reliably is only part of the picture; the maintenance regime should also maintain the efficiency of each individual item of plant and also the control of the system as a whole – especially if building usage has changed since the plant was commissioned. This is best achieved by using maintenance engineers who have specialist expertise in this area.
The best solution
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. Furthermore, 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 building owner’s aspirations and the budgetary realities.