Micro-boilers (illustration)

Combined heat and power boilers boost carbon savings for the home

Massed ranks of gas central heating boilers, serving as mini CHP plants, are being recruited in the battle against global warming as E&T discovers.

Gas central heating enjoyed by some 20 million UK homes is one of the great conveniences of modern life. Like most conveniences, however, it's something we tend to take for granted.

Timers and thermostats turn it on and off as required, and an annual maintenance visit, hopefully, keeps it going. All the home owner has to do is pay the bills and enjoy the benefits. We can be obsessive about the make of car we drive, the features of our new HDTV, but it's a rare individual who has the remotest interest in the gas boiler heating their home. All this could be about to change.

During 2010, increasing numbers of homes across the UK, and in parts of mainland Europe, will be switching to an entirely new type of gas boiler. Like the old model it will heat the water and keep the house warm, but alongside this traditional role the boiler will also be functioning as a mini thermal power plant, pumping out around 1kW of electricity. Home owners, potentially in their millions, could be going into the combined heat and power (CHP) business.

Free heat

CHP is a great idea at least in principle. All thermal power plants produce copious quantities of waste heat, and finding a use for this 'free' resource offers real financial and environmental benefits. The trick is to find an application where there's a well-matched simultaneous application for both electricity and heat, with the likes of leisure centres, hotels and hospitals dominating the traditional CHP market.

Figures from the CHP Association indicate that these three applications represent over 85 per cent of total UK CHP sites, with an average site electrical and thermal capacities of around 217kW and 369kW respectively.

The domestic CHP market that this new generation of boilers will create represents a radical departure from traditional CHP. There's a much greater emphasis on heat generation; also, at 1kW the electricity output is minuscule. For obvious reasons the whole business is labelled microCHP.

Despite these differences, the case for micoCHP is identical with that applied to conventional installations it saves money and it saves carbon. Think about generating 1kW of electricity using the power plant incorporated into your new boiler. In conventional CHP plants the electricity to heat ratio is around 1 to 2.

In microCHP, the ratio is much more like 1 to 6; so along with the 1kW of electricity you get around 6kW of heat. The gas flow required to generate these outputs will obviously depend on the overall efficiency of the boiler, but a bit over 8kW is a good realistic figure. Domestic electricity is much more expensive than domestic gas around 13.5p/kWh as opposed to 3.5p/kWh and, assuming the 6kW of heat is used in the home, the net effect of these various inputs and outputs is an overall saving on the price of a kWh of electricity that, Elaine Ball, new technology director for boiler manufacturer Baxi, puts at between 8 and 10p.

The carbon savings come in two forms. First, generating electricity at the point of use saves on losses in transmission and distribution. Secondly, because the waste heat associated with generation is used rather then dumped, as with centralised generation, there's a net reduction in CO2. A lot of assumptions have to go into any calculation, but Graham Meeks, director of the CHP Association, believes microCHP can reduce household carbon emissions by around 10 per cent.

We have the technology

The attractions of microCHP are undoubtedly strong, what has held it back until now the idea's been around for well over a decade is the lack of a suitable power plant to go in the boiler. It's quite a challenge. The power plant has to be small (so you can still hang the boiler on on your kitchen wall), quiet (it's going in the kitchen), reliable (boilers are only serviced once a year), and, of course, reasonably cheap (home owners have to be willing to pay for it).

There are two contending technologies: the Free-Piston Stirling Engine (FPSE), used in Baxi's Ecogen, and the Organic Rankine Cycle (ORC), used by Genlec.

The Ecogen boiler comprises two basic units: a compact FPSE with an electrical output of 1kW and a heat output of 6kW, and a supplementary 18kW burner, giving a maximum combined heat output of 24kW. The unit's Stirling Engine is supplied by the Microgen Engine Corporation, partly owned by the four boiler manufacturers Baxi, Remaha, Viessman and Vaillant. Stirling Engines have a reputation for being slow to start, and requiring high levels of maintenance. The Ecogen's FPSE reaches full power in 5 to 10 minutes , and the helium-filled unit is sealed for life.

Field trial of the Ecogen began in 2006, with under 10 units, and by the end of 2009 there were nearly 500 units installed in UK homes. The commercial launch of the Ecogen is planned for the first quarter of 2010, with UK marketing organised through a two-year deal with British Gas. Each year around one and half million gas boilers are replaced in UK homes split roughly 2 to 1 between combi boilers and condensing boilers. Baxi's Ecogen is a condensing boiler. A combi boiler variant, using the same FPSE, is being developed by Remaha. Given the availability of combi and condensing designs, Baxi's Ball believes that, by 2015, microCHP units could cover 30 per cent of the 1.5 million gas boilers replaced in the UK each year.

Stirling Engines might be simple in principle, but the FPSE used in the Ecogen represents the cutting edge of small heat engine design. Genlec, part of Energetix Group, has taken a deliberately low-tech approach in the development of its microCHP boiler, employing a variant of heat engine already found in most UK homes.

An ORC power plant, as used in the Genlec boiler, is essentially a fridge running backwards. It uses the Rankine cycle, just like a conventional thermal power plant, but the working fluid is an organic (as in organic chemistry) compound, instead of superheated steam, to allow operation at much lower temperatures. The maximum internal temperature of the Genlec unit is 150C. Lower operating temperatures, relative to a conventional power plant, means the electricity to heat ratio will be much lower, at around 1 to 10.

Adrian Hutchings, chief executive for the Energetix Group, cites three basic advantages for the ORC approach. First, low cost attributable to design simplicity and standard components. 'All we're effectively doing is adding an ORC unit on to a boiler,' he explains. 'The ORC module takes some of the heat from the boiler and uses it within the module. The heat that is rejected by the ORC module is then used back to heat the hydraulic system in the home. It's a simple, single-burner system.' For a 'turbine', the ORC module uses a scroll expander made by the million for automotive car air-conditioning systems.

Hutchings's other plus points for the Genlec design are a fast start-up of around 20 to 30 seconds which could be significant in combi boiler applications and a very flat efficiency curve. The efficiency of the ORC unit at half power, 0.5kW electrical output, is virtually identical with that at full power, so that it's easy to moderate the power up and down.

The Genlec boiler will be available in combi and condenser variants. Holland will be a key 'early-adopter' market, with Daalderop BV committed to selling 30,000 combi units, under the brand name CombiVolt, over the next three years. The Dutch government has committed 10m to its target of installing 10,000 microCHP units. UK marketing will be organised through a collaboration with E.ON and a major European appliance manufacturer.

Feed-in tariffs

The technology behind micoCHP is undeniably smart, but it will only be adopted by significant numbers of home owners if they can be sure of getting a reasonable return on their money. In principle, the UK government's feed-in tariffs scheme should ensure that this is exactly what happens.

Feed-in tariffs are designed to reduced carbon emissions by 'paying' producers to use approved, low-carbon forms of generation. In Germany, where they have been widely adopted, feed-in tariffs are credited with putting the country comfortably on target towards deriving 20 per cent of its energy from renewable sources by 2020. There are two components to feed-in tariffs: a technology-specific payment for generation, and a technology-independent export price, for electricity fed-out to the distribution network. The idea is that the generation payment is set at a level to give the home owner an adequate rate of return (defined by the UK government as being in the range of 5 to 8 per cent) while the export price is kept low, to encourage home consumption.

The UK feed-in tariff scheme is due to start operating on the 1 April. Somewhat belatedly, tariff rates for the various eligible technologies, which include hydro, solar, wind and microCHP, were finally issued by the UK government on the 1 February. Solar does best, with a generation tariff of up to 41.3p/kWh, while the generation rate for microCHP is 10p/kWh. The export tariff is the same for all technologies, and is set at 3p/kWh.

Almost inevitably, the microCHP industry would have preferred a higher figure, In a letter sent to to Ed Miliband, Secretary of State for Energy and Climate Change, in October last year, industry representatives pushed for a minimum figure of 15p/kWh. Both Baxi and Genlec believe their microCHP boilers will produce savings without the support of feed-in tariffs, with Hutchings estimating the additional cost of a the Genlec boiler over a conventional condensing boiler at around £650. Nevertheless, there's a clear acknowledgement of the marketing boost that that feed-in tariffs will generate.

'They're a Godsend,' say Ball. 'The feed-in tariff is just the sort of just the sort of support mechanism that will encourage early adopters, when product costs are high. We can then work on volumes, and economies of scale, which will mean that, eventually, you won't need the subsidy.'

Hutchings believes feed-in tariffs have made the UK market much more attractive for micoCHP, and Genlec has now moved its plans for a UK launch forward a year, with the first evaluation products going into UK, and Dutch, homes this winter.

Strength in numbers

A generating capacity of 1kW per home doesn't sound like much, but run forward a few years, assume a fairly modest national take up of 10 per cent (two million households), and you've added 2GW nationally say around two Sizewell Bs. When it really matters, all these individual microCHP plants are likely to be working in concert.

The peak winter demand for electricity occurs between 4pm and 7pm when people come in from work and start preparing dinner in their centrally heated homes. At such times, the most marginal plants will be brought onto the grid, and generating costs will be at their maximum. Given a modicum of government encouragement, functioning technology and gas to burn, microCHP offers the prospect of shaving gigawatts of demand off this peak. It's hard to think of a form of home-based electricity generation likely to exercise a more profound influence on the nation's generating system.

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