Developing domestic technologies

Over the last few years companies such as BG Group have been investigating Stirling engines as combined-heat-and-power plants for domestic and commercial uses. Two products based on very different applications of the cycle were investigated.

The New Zealand company Whispertech began work on Stirling engines in 1989 and released its first commercial DC units in 1998. Whispertech says its version combines four piston-cylinder sets in an axial arrangement, with the hot end of one cylinder attached to the cold end of the adjacent cylinder.

The company says that, if the power from the pistons was transferred to a rotary motion by a traditional crank type of mechanism, it would put considerable side loading on the pistons and cause rapid guide and seal wear – traditionally a life-limiting factor in Stirling engines. Instead, it has developed a ‘wobble-yoke’ system to convert the linear motion of the engine’s four pistons into the rotary motion necessary to drive a generator, while putting very little side load on the piston seals and guides. The wobble-yoke mechanism connects the pistons to a single rotating shaft and alternator, which are sealed into the compact housing.

The Microgen microCHP was based on a design by US-based Sunpower and based on a linear-free motor. The CHP unit is started up in synchronization with the grid and a planar spring acts with the control system to maintain its frequency at 50Hz.

8.5 Development issues

Development of both microCHP units has been problematic. The target is a tough one: it is hoped the technology will replace conventional boilers, but that means reducing its size to fit a standard kitchen spacing. It is unlikely that capital cost and installation charges will ever be as low as standard boilers, so customers will have to be convinced that the benefit of lower electricity bills over time will outweigh the upfront cost.

The opportunity to export excess power to the grid could be a major selling point for such products. But the grid structure in England and Wales is notoriously unprepared for such small-scale export.

Combined heat and power 81 As late as November 2001, for example, a framework document for design and planning of low-voltage networks in greenfield housing estates referred to PV gen- eration as a possibility but said that domestic generation was unlikely in greenfield estates – by then, BG Group already had nine microCHP prototype units in operation in UK houses.

As distributed generators of all sizes have found, connection requirements are not uniform across the UK. New generation coming onto the system usually has to apply to the local DNO, which operates as a regulated monopoly, for permission to connect, and the DNO sets the conditions. One significant victory has already been won: the requirement to pre-notify the DNO and obtain agreement for a microCHP installation would be replaced by a ‘fit and notify’ arrangement. The original requirement would have made it almost impossible to install microCHP ‘on the spot’ – for example to replace a broken-down boiler – and removed a major opportunity.

A second issue is metering output from the microgenerator. Companies involved in domestic generation have lobbied for bidirectional meters that would simply record the net import of generation, but the industry regulator Ofgem has consistently opposed this approach, and says that, since in every case a new meter would be required, ‘there is merit in being able to measure import and export’.

In the past, Ofgem’s distributed-generation coordinator has pointed out, ‘If you don’t export much, it doesn’t matter too much. But say you have a development of three or four hundred houses and a distributed CHP plant. You have a considerable amount of generation on the network and at certain times not much of it is being used. Then it is helpful to know the import/export profile.’

However one important development has been made in metering: microgener- ation has been exempted from the requirement for half-hourly metering normally mandatory for potential exporters.

Microgen pointed out that there are some users who may have significant exports, highlighting the old, who have constant heat demand throughout the winter and little power consumption. But in most cases the power available for export will be limited. The microCHPs would be generating power at times when they are producing hot water and that also coincides with peak electricity consumption, when most domestic users are consuming more than the kilowatt or so that they can generate.

If microCHP proved popular, its most significant effect is likely to be on the country’s load profile, rather than in providing large quantities of power for export. Up to 13 million homes in the UK could use gas-fuelled microCHP and, with all of them generating at peak times, the country’s winter peak demand would be significantly reduced. That would mean lower peak prices and reduced requirements for reserve capacity.

But once again Ofgem has counselled caution, noting that when heating demand disappears in the summer all those customers will once again need to meet their demand from the grid.

The units also offer greatly improved efficiency. Using gas to generate electricity in a power station and then transmitting the power overall has a fairly low efficiency. The best gas-fired generating stations convert around 60 per cent of the energy in the gas to electric power, and many are far less efficient. More of the power is dissipated

82 Local energy

as it is transmitted through the power network. But because all the heat produced by a microgenerator is used in the house, and electricity is a useful by-product, these small units can claim efficiency percentages in the high nineties. Even without allowing for the energy required to transmit the gas to the point of use, that is a huge improvement.