In article ,
Tony Bryer writes:
On Sun, 16 Nov 2008 09:51:17 +0000 (GMT) Dave Plowman (News) wrote :
Given the overall efficiency is only 92% which is poor for a modern
boiler it would be fair to assume that it's the generating part
which is pulling it down in the absence of any actual figures.

But with your record on quoting efficiency it's not surprising you
think it is over 90% efficient when producing electricity.

Forgetting Drivel's understanding of things, where MicroCHP units
deliver is in reducing CO2, given that grid-sourced electricity is
assumed to have a CO2/kW figure of just over twice that for mains gas
(0.422 v. 0.194). The test cases we were sent when I was writing SAP
software assumed a heatower ratio of 2:1, which may or may not
realistic, but assuming this and a heat load of 20kW

Gas boiler 92%: 20 x 0.194 x 100/92 = 4.22kg CO2

CHP 90% 2:1 20 x 100/60 = 33.33kW input
33.33 x 0.194 x = 6.47kg CO2 for heating
electricity generated = 10kW

These units are 1.1kW max, just using waste heat.

CO2 saving from displayed = 10 x 0.422 = 4.22
Net CO2 = 6.47 - 4.22 = 2.25 - 47% reduction

The docs I have read (a while back now) did emphasise that unlike
modulating gas boiler systems, Micro CHP is very sensitive to correct
sizing as you need the load to keep it running for long periods.

That's what I imagine to be a big failing. Considering a house
I've done detailed heatloss calculations for, 3 bedroom, semi or
end-terrace, built 1909 (9" brick walls), only insulation added
since built was double glazing (before April 2002) and 4" loft
insulation, i.e. probably below average insulation for UK
housing stock. Calculated heat loss at -3C is 11kW. (Based on
subsequent experience running the system, I suspect this was an
over-estimate. At a more typical temperature of 5C, heat loss is
barely 7kW, which is what the boiler will modulate down to.
There's nowhere near enough wasted energy to run a waste energy
generator at this level. The only time the boiler runs at 25kW
(where it could run a stirling enging) is for about a minute
when it's first switched if the radiators are stone cold.

--
Andrew Gabriel
[email address is not usable -- followup in the newsgroup]