Anyone observing the media recently will be aware of the widening coverage of renewable energy. As public interest has grown, so has the range of technologies and devices available. The question inevitably arises, 'Which is the best for my project?' The honest answer is that in many cases, no one technology offers the complete solution. Different technologies offer different strengths, and the best solution is usually the combination which provides complementary advantages. Solar thermal and biomass are a good example of this. With oil, gas or electric heating added as a back-up, the resulting system will contain at least three sources of heat.
In order to integrate these with each other, and with the systems they supply, we have to consider how the heat is used. For the heating engineer, mostly concerned with space heating and domestic hot water, this brings the number of elements in the design to at least five. Designing the whole system to combine energy efficiency with user comfort and value for money is the challenge, and demands some understanding of the principles involved.
While accompanying sections covers the individual technologies, the principles of combining them are outlined below. They have been analysed in two ways:
While conventional heat sources produce heat 'on-demand', some renewable sources produce heat intermittently as the energy becomes available. Since the peaks in production rarely coincide with peaks in demand, short-term storage of heat is required. The larger this is, the greater the capacity to capture surplus energy and to bridge any gaps in supply. At it's simplest, this is a store of hot (usually primary) water, fed by the various heat sources, and supplying the space heating and DHW. Priority of supply is given to the renewable sources, with fossil fuels only used if none is available and the store is depleted.
While finding space for the store can be a problem, it does provide other advantages. Crucially, it separates the requirements of supply and demand, allowing much more freedom in boiler sizing. Properly controlled, it allows non-modulating boilers (oil and some biomass) to run for extended periods at full output, followed by long rests. This reduces fuel consumption by maximising combustion efficiency and reducing cycling, especially important when demand is low. If a biomass boiler is used, it reduces the frequency of manual re-lighting. It also allows simpler systems, with fewer complicated controls. It's buffering ability is increasingly useful in modern houses, which have small space heating requirements with large DHW loads.
The various sources of renewable heat also have different preferred flow temperatures. For example, solar thermal and ground-source heat-pumps (GSHP) lose efficiency as the required flow temperature rises. By contrast, most biomass boilers require a return temperature of at least 60°C to prevent tarring of the heat exchanger, hence giving a flow temperature of 80°C or more. On the demand side, a radiator system may require a flow temperature of 70°C, a DHW supply 50°C, and an underfloor heating circuit as low as 35°C. This gives the store conflicting demands: the hotter the store, the greater the quantity of useable heat it contains. However, the cooler the store, the greater the ability to capture 'low-grade' heat, from solar, GSHP or a condensing boiler. The solution is to promote stratification within the store, by maintaining a large temperature difference between top and bottom. This is more difficult than it sounds, and there are various attempted solutions on the market. One common version is to provide two smaller stores: one high temperature buffer for boiler and space heating, the other a lower temperature store for DHW, solar and GSHP.
Although more complicated, the separation of functions allows greater efficiencies (and gives stores small enough to fit through doorways !).
In summary, the suggested design principles for successful renewable heat integration are: