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Home is where the heat is

There are various potential routes to zero carbon housing, but one of the more practical is communal microgeneration, says Dave Parker.

The town of Okotoks in Alberta, Canada, maybe on the same latitude as London, but it lies in
the foothills of the Rockies at more than 1,000m above sea level. During the long winters,
temperatures can plummet to a distinctly unLondonlike –300C or even lower. At first sight,
it is an unlikely location for a pioneering solar community. But at Drake Landing, a new development of 52 houses will rely on solar heated water for 90% of its space heating and 60% of its domestic hot water needs.

Space heating by solar-heated water is virtually unknown in the UK – yet, in principle, this heating alternative is much simpler and more cost-effective than many other microgeneration options.

Drake Landing depends on natural gas to make up the shortfall – nevertheless, CO2 emissions overall are cut by 70%. Similar developments could look to alternative energy sources such as wind or biomass to supply the non-solar fraction, and these would be truly zero carbon communities.

So what lessons does Drake Landing have for UK developers, especially those looking at zero carbon housing options?

Most readily available alternative energy sources suffer from the problem of intermittency. How much solar or wind power is available at any given moment is completely unpredictable. Supply is rarely balanced by demand, so the only thing that makes wind or solar microgeneration practical is a buffering energy store to absorb surplus energy and release it on demand.

In the UK, this is usually a domestic hot water tank that can store enough solar-heated water to meet demand for a day or two only. However, at Drake Landing a communal underground thermal store captures enough solar energy during the summer to heat the estate for almost the entire winter (see box above). Basic physics means the larger the thermal store, the more efficient it is.

A borehole store like Drake Landing is usually the cheapest option where space permits, but pits and concrete tanks have also been used elsewhere in the world. Boreholes that contain phase change materials are another realistic alternative.

But whatever the technology, the same principle applies.

Large wind turbines are also far more efficient than small rooftop turbines. As height above the ground increases, so does wind speed – but energy-sapping, building-generated turbulence decreases.

There are few turbines rated at less than 100kW available off the shelf, except at the very low end, but 225kW turbines are virtually a standard product and well-maintained examples are readily available second-hand. It may be harder to get planning permission for one large communal turbine, but its performance will be more predictable and cost effective than an array of much smaller units.

Storing surplus electrical energy from wind turbines or solar photovoltaics (PVs) is more problematic than storing heat. Conventional batteries are rarely cost-effective.

There are a number of high technology alternatives, such as flow batteries, superconducting magnetic energy storage and flywheel energy storage and regenerative fuel cells, which would only be cost effective on a reasonably large scale and would need skilled maintenance.

In theory, the best way to store surplus electrical energy is by exporting it to the grid, and then taking energy from the grid when there is a local shortfall. However, in practice, and particularly in the UK, this can be beset by bureaucratic and technological constraints.

Sometimes the most practical solution is to convert the surplus power to heat and then store it in the project's long-term thermal store. If a heat pump is used for this purpose, the numbers can look very attractive, although such a system would rely on the grid to provide power when the sun or the wind is uncooperative.

There are several other microgeneration technologies that look a lot more interesting when considered as an option for larger developments. Processing biomass into a more convenient and energy efficient form is better in principle than simply burning it in a boiler.

Small-scale biomass is beginning to appear, although the proven hardware is on a larger scale and is only suitable for larger buildings or for housing developments.

The upside is that useful residues that can be sold as soil improvers or processed into fuel briquettes. The downside is the space needed for storage and processing.

But the most significant problem is locating a local and reliable long-term source of suitable biomass. Horticultural and food-industry wastes can all be utilised. However, trucking in woodchip or similar is unlikely to be sustainable in the long term.

Processed biomasses generally yield a hydrocarbon gas with a greater or lesser tar and resin content. Quality permitting, it can be used to fuel anything from conventional boilers to microturbines, producing electricity and heat as well as cooling, via absorption coolers.

Where space is tight, natural gas-fuelled district heating and combined heat and power (cogeneration) schemes can be the answer – and the projects need not be that large.

There are only 78 homes in the new Bourbon Lane development in West London, but gas-fuelled district heating and cogeneration was chosen in the search for an 'Excellent' Eco Homes rating by building research body BRE. Payback time was calculated at between 26 and 27 years – before the recent steep hike in energy prices, which can only make microgeneration an even better option.

A French-sourced cogeneration plant produces 70kW of heat and 30kW of electricity, which is devoted to the landlord's requirements, such as powering lifts and public area lighting. High levels of insulation and heat recovery ventilation in the individual homes slash heating loads, which are met by an 810kW twin boiler set-up.

The best microgeneration option for a project could very well be a combination of two or more separate yet compatible technologies.

If planning and regulatory authorities are sympathetic, we may see a lot more developments like Drake Landing and Bourbon Lane in the next few years.

 Dave Parker is the author of Microgeneration: Low Energy
Strategies for Larger Buildings, will be published later this year by the Architectural Press.

Click here for Drake Landing thermal energy store

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