Keeping our feet on the ground
Micro generation, zero carbon and passive houses are all great ideas for showing where we should be heading to meet the targets that will make the energy future, especially in housing, one that will meet the problems of the next ten years and beyond.
How though do we approach the project on the drawing board this morning? Clearly we have to stay within the bounds of today’s budgets, and technical skill we have right now.
Bench marks or handcuffs
Providing a replacement energy for fossil fuel might be seen as an end in its self. What cost for saving the planet? However faced with real budgets and real buildings we surely need to exercise some judgement of what will give the best return? Let’s look first at the economics, since the discipline of seeing what saves most gives us a benchmark – how many kW saved for your buck.
The ideal start for this should be to look at the approach that beats most of the others hands down- insulation. In most cases stopping the need for a kWh is cheaper and simpler than investing in a machine to generate it. So insulate the building to beyond Building Regulation levels if possible. It may not be as sexy as a turbine on the roof but it makes much better money sense.
Now we have reduced the base load this is the point to consider which investment in renewables makes sense.
Clearly the best return is a condensing boiler. If the building has a conventional boiler and a boiler will still be needed, change it for a condensing type – fast. While you are at it ensure the control includes a weather sensitive option, that way the system will achieve savings in the 35 to 40% range rather than say 25%
Sunny days are here again
Solar water heating is usually the renewable that gives the best return. But that is only where water use is reasonably high and consistent. The other key point is designing and siting the installation correctly. There are some vital steps, like orientation or the use of a sufficient cylinder size that, if not considered carefully, can end up halving the output from the system.
Even in northern latitudes the sun can provide up to 60% of annual domestic hot water requirements. However the trick is ensuring that this free energy is maximised and the 40% balance of the heating load is still delivered with the highest possible efficiency. To provide this, Viessmann have developed the Vitosol system. This matches all the components needed for integrating solar heating into a heating system.
Collectors come in either flatbed or vacuum tube versions. The collected heat is then sent by a Divicon pump/diverter control unit to the bottom coil of a Vitocell dual coil cylinder. Should the cylinder need further heat input, this is sensed and the upper coil is heated from the boiler. To complete the system a Solartrol control unit continuously monitors available heat and ensures the system maximises the use of free solar energy.
Wood you like to save more?
Wood burning (biomass) provides an ideal solution to substituting existing boilers, especially where an oil boiler is being replaced. Wood burning boilers, drawing their fuel from managed and therefore sustainable forests, is both carbon neutral and makes use of wood that is usually wasted.
There are some issues with biomass, and generally that means boilers using pellets. Firstly distance from the source of fuel adds both to cost and to the CO2 element of delivery – 25 miles is regarded as the ideal maximum distance. Next that fuel must be of consistent quality and lastly the fuel takes a lot of space. So buildings with storage space around them make the best candidates. There are other options for biomass and I will come to that later.
Heat in the air or below your feet
Next in our pecking order are heat pumps; either ground source or air sourced.
Ground source to water heat pumps, using bore holes or coils buried about 2m deep; rely on space around the building. This tends to rule them out of high density, urban building. Capital costs, especially for the ground extraction is high and directly related to the heat demand of the building. This, combined with the limitation of single phase operation of 12kW, points to newly built and very well insulated projects.
Air to water heat pumps, on the other hand, do not have the space drawbacks of their ground source counterparts. The heat pump can be sited outside or inside the building, passes large quantities of outside air through an evaporator which carries refrigerant. The heat picked up by this evaporator passes through a scroll compressor and condenser and then to the water in the heating system.
How effective are they? Surprisingly even in Britain’s cold climate there is more heat available than you might appreciate. The coefficient of performance (that is the amount of heat delivered against the power needed to run it) is around 3•0 in colder weather rising to 4•0 or more in the warmer weather of autumn and spring.
Air to water heat pumps therefore suit high density dwellings, especially if they are low energy construction, but care must be taken to ensure that they are sited where noise problems will not arise.
Using this analysis the best buys are; • In a family house with a gas boiler- change it for condensing and add solar panels.
• A newly built, well insulated home, with a plot size that offers space – a ground source heat pump
• Replacing an oil fired boiler – wood pellet if storage is available.
• Existing buildings in high density – air to water heat pumps.
What does zero carbon mean? Carbon is easy; it’s the zero bit that’s difficult. There have been several projects in the UK to achieve a zero carbon rating. I am told that there are about 20 buildings in existence. In Germany, the same issue has been addressed with the Passiv Haus programme with some 4,000 Passiv Hauses.
What these test installations have shown is that to actually reach year round zero carbon means, for an individual building, that it will have to be ‘negative carbon’. It will have to be a nett exporter of energy, not merely reach equilibrium at the magic zero. And guess what, it’s not easy and it isn’t cheap.
…. in clusters? Another approach, for both new and existing projects, is to consider the zero carbon rule as applying to groups or clusters of buildings. This then brings in the economies of scale, so that for example, combined heat and power, could be used for the buildings in the group with the facility to export surplus electricity or hot water. This would become a rather different form of energy trading but just as valid in achieving carbon free communities.
This approach is already used widely in Northern Europe, mainly based on biomass fuels which are already carbon neutral. With a grouping of buildings the biomass heat station can also include power generation at economical cost. These types of projects have been carried out widely by Mawera, a European leader in biomass, which this year has become part of the Viessmann Group.