Comments on: emissions from CHP: the standard method is wrong

By: Carbon Trust micro CHP interim report finally out « carbon limited

Carbon Trust micro CHP interim report finally out « carbon limited — Tue, 07 Oct 2008 21:06:05 +0000

[...] is. In summary, micro-CHP saves 5% to 10% of carbon in large inefficient houses but only if you use a flawed methodology and give yourself extra-extra credit for displacing grid electricity. A few of the key [...]

By: CHP and SAP - part II « carbon limited

CHP and SAP - part II « carbon limited — Fri, 21 Mar 2008 08:50:42 +0000

[...] CHP. It’s not as simple as I thought here, although the result is similar. The incorrect method I wrote about a few weeks back is still in use, but it’s different from what’s [...]

By: Casey

Casey — Thu, 20 Mar 2008 21:37:04 +0000

Gordon, in the case of SAP, I’ve taken a more thorough look at the method and written it up in a new post here: http://carbonlimited.org/2008/03/20/chp-and-sap-part-ii/

By: Casey

Casey — Tue, 18 Mar 2008 20:59:10 +0000

Hi Gordon. Thanks for the comment.

To reply to your queries:

1. It doesn’t follow that a CHP engine sized to meet 60% of the heat load would meet 60% of the electricity load. It will depend on a) the proportions of energy consumption by end use, b) the “peakiness” of the consumption curves for each day of the year, and c) on the electrical/thermal split for the CHP engine. I used proportions of space / water / and electricity that I think you might find on a mixed use scheme. The 60% figure for demand met came from experience on other projects. In this case, the CHP is meeting 54% of electricity demand.

2. The distribution losses are for heat only. For example, the primary energy for CHP in table 1 is equal to the demand met divided by one minus the distribution losses and then divided by CHP efficiency. So 45 / (1-0.1) / 0.8 = 62.5. The difference between demand met and primary energy for electricity from CHP is based on CHP efficiency only, with no distribution loss (i.e. 27 / 0.8).

3. See 2.

4. The proportions for energy demand are based on what you might see in a mixed use scheme where there’s higher density of electricity demand. For a purely residential scheme built to 2006 regs I think you might see a breakdown in demand of 40%/30%/30% for space/DHW/electricity. The proportions do affect the savings (for example using 40/30/30, the savings in table 1 would go up to 7.1%), but my aim in writing the post was to highlight the differences in results of the different methods rather than the the exact savings figures.

I hope that helps. If you can’t get your spreadsheet to work out, get in touch via the contact page and I’ll send you my original calcs.

Also, let me know if you get good info from your experts.

By: Gordon Callaway

Gordon Callaway — Mon, 17 Mar 2008 13:15:11 +0000

Hi, your thesis makes very interesting reading and I am in agreement with the principles. However, I have a couple of queries as I am also trying to get to the bottom of this issue in order to gain some clarity. Firstly, does it necessarily follow that a CHP engine that is sized to provide 60% of heat demand will also provide 60% of the electicity demand? Secondly, your calculations (unless I have done something wrong!) seem to apply distribution losses in the CHP assumptions to electricity but not to heat – is there a reason for this? Thirdly, your figure of 63 in the CHP primary energy cell – how is this calculated as I cant get my spreadsheet to replicate the answer? Finally, are the figures for heating, HW and electricity proportionally correct? I was under the impression that the figures for space htg and water should be about the same and togtether account for 60% of the total demand leaving 40% for electricity demand (based on a typical 3bed house built to 2005 bregs). Does this impact on the overall savings significantly?

I look forward to your response. I’ve also sent your link to a large ‘expert’ firm to gain their views on the ‘correct’ way to calculate savings.

Regards
Gordon Callaway
The Hyde Group
023 8083 6890