Micro-CHP units are the size of conventional domestic boilers and offer the potential to substantially reduce carbon emissions by generating electricity along with useful heat in homes. Advocates of microgeneration argue it could help to displace large-scale, inefficient generation in power stations and reduce electricity transmission losses.
But that dream appeared to be punctured in November when the Carbon Trust released early results of its trials of commercial micro-CHP units.
The performance of micro-CHP was "not as encouraging as had been hoped," the Trust concluded. It suggested that, unless new data changes the picture, "there is unlikely to be a significant carbon emissions reduction from wide deployment of the technology at this stage in its evolution."
The Trust found that a third of installations appeared to reduce emissions by up to 18%, but that another third increased emissions by the same amount as a domestic condensing boiler and electricity from the grid. The remaining third showed no difference in carbon intensity.
In contrast, industry trials and modelling suggest the technology may achieve emission savings of 15-40%. The Trust suggests the discrepancy is because the real-world efficiency of the units is lower than assumed by modelling exercises and less electricity is generated than forecast.
The results appear to be bad news for the micro-CHP industry, which benefits from Government support in the form of a reduced rate of VAT and credits under the energy efficiency commitment (EEC) on energy supply companies.
The industry is angered by the report. David Sowden, chief executive of the Micropower Council, told ENDS: "We are a bit peeved that the Carbon Trust has taken it upon itself to put this out with such an inconclusive data set."
The trials were very slow to get underway, largely because of delays in developing and installing the micro-CHP units (ENDS Report 357, pp 10-11 
). The Trust originally planned to have statistically valid data on over one hundred units operating for 12 months by the end of 2005. This now looks unlikely until spring 2007 at the earliest.
By the third quarter of 2005, the Trust had only 40 units in the trial, of which 31 were micro-CHP units and nine were larger CHP units intended for small businesses such as nursing homes and hotels.
Mr Sowden told ENDS that the Trust only had full-year data for just six units. He argued that most of the Trust's data came from units operating in the summer months, when significant savings would not be expected because units would only be used for short periods to heat water. In the winter, the heat load would be greater and the units will run for longer, reducing heat losses through on-off cycles.
"The data set is so incomplete we do not believe they can draw conclusions," Mr Sowden concluded. "We also have reservations about the analysis because even the same consultants used by the Carbon Trust using the same data have been unable to reach the same answers."
However, the Carbon Trust's director of strategy and marketing Michael Rea insisted the data were "robust" and "the vast majority" of units tested had been through both winter and summer operation.
"When we look at the differences between winter and summer they are not very great," Mr Rea said. "We need to understand why some units are performing better than others. One of the big reasons may be the variation in heat load in different applications and the sensitivity to installation."
Although Mr Rea said units from several manufacturers were used in the trial, many are believed to have been manufactured by Whispertech - a New Zealand company that is supplying Powergen in the UK. British manufacturer MicroGen has postponed participation in trials because of delays in developing its product. Other firms in the market include Baxi and Disenco.
Russell Marsh of the Green Alliance commented: "The report clearly shows the need for the trials to continue and that further development may be needed to improve efficiency. It may be that micro-CHP is better suited to some applications than others."
However, the trials were more positive on the benefits of the larger small-CHP units, although the sample size was smaller than for micro-CHP. The Trust did not give a headline figure for carbon savings, but suggested that the units' larger scale - up to 25kW of electrical output compared to around 1kW for micro-CHP - and longer running times are likely to produce more efficient performance.