[update March 20: I’ve looked further into how SAP treats CHP and written it up here. So while the method described below is being used elsewhere in the industry, the criticism doesn’t apply to SAP.]
I’ve written on this topic before but maybe I didn’t succeed in making clear just how far off the mark the standard method is when estimating carbon emissions from CHP. Why does it matter? Here are some reasons:
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Right now, big developers and the Housing Corp are assuming CHP can get them to level 4 under the Code for Sustainable Homes and this may not be true.
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These emissions figures can determine whether or not a scheme gets planning permission or passes building regs.
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The nascent micro-CHP industry (expected to be worth £2billion per year across Europe) is using this flawed method to back up its sustainability claims. Changing from a commonsense approach to the much more forgiving “standard” approach explains why the first Carbon Trust interim report on the micro-CHP field trails was so bleak and the second was so rosy.
There’s a good chance that, if I’m right and the standard approach is flawed, when the CLG and BRE realise their mistake, the rules will change, leaving public and private sector developers and the micro-CHP industry with a very costly mess to clean up.
Here’s a quick illustration of how widely estimates of emissions from CHP can vary depending on the methodology you adopt. First let’s take a baseline scheme for comparison with later examples. This is based on a gas boiler for heat and grid electricity for power. The exact energy requirement isn’t important since it stays the same through all the scenarios.
Primary energy | Assumed efficiency | Useful energy | CO2 kg/kWh | kgCO2/yr | |
Space heating | 56.8 | 88% | 50 | 0.19 | 10.8 |
Water heating | 28.4 | 88% | 25 | 0.19 | 5.4 |
Electricity | 50.0 | 100% | 50 | 0.422 | 21.1 |
Total kgCO2 | 37.3 |
Table 1. Baseline: gas heating and grid electricity
Now take the same scheme and assume that CHP is used to meet about 60% of the total heat load. Here’s a list of all the assumptions:
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So now the carbon calculation looks like this:
CHP | Backup systems | ||||||||
Useful energy demand (kWh) | Demand met (kWh) | Primary energy | CO2 kg/kWh | kgCO2/yr | Demand met (kWh) | Primary energy | CO2 kg/kWh | kgCO2/yr | |
Heat | 75 | 45 | 63 | 0.19 | 11.9 | 30 | 37.9 | 0.19 | 7.2 |
Electricity | 50 | 27 | 34 | 0.19 | 6.4 | 23 | 23 | 0.422 | 9.7 |
Subtotal CHP CO2 | 18.3 | Subtotal backup CO2 | 16.9 | ||||||
Total kgCO2 | 35.19 | ||||||||
Saving | 5.6% |
Table 2. Gas CHP: exact counting of emissions from fuel combusted
If we count all the carbon emitted by the CHP, backup boilers, and grid, it gives us a carbon savings of just 5.6%. Pretty modest.
But there are ways of calculating the carbon from CHP other than the one shown above. For example, one of the options put forward by DEFRA for treatment of CHP under the Carbon Reduction Commitment is to set the emissions factor for electricity from CHP to be equal to the grid (0.422 kgCO2/kWh) and to set the emissions factor for heat from CHP to zero. In this case, our scheme looks like this:
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Table 3. Gas CHP: CHP electricity equal to grid and zero carbon heat
So it’s quite a jump in carbon savings, but we’re almost in the same ballpark.
Now look at SAP and the typical method of calculating emissions from CHP. This methodology awards an extra carbon reduction for electricity generated on site that would otherwise have come from the grid. Specifically, SAP and similar methodologies award a carbon reduction of 0.568 kgCO2 for each kWh of electricity generated by CHP on site. All of a sudden our scheme looks like this:
CHP | Backup systems | ||||||||
Useful energy demand (kWh) | Demand met (kWh) | Primary energy | CO2 kg/kWh | kgCO2/yr | Demand met (kWh) | Primary energy | CO2 kg/kWh | kgCO2/yr | |
Heat | 75 | 45 | 62.5 | 0.19 | 11.9 | 30 | 37.9 | 0.19 | 7.2 |
Electricity | 50 | 27 | 34 | 0.19 | 6.4 | 23 | 23.0 | 0.422 | 9.7 |
Subtotal CHP CO2 | 18.3 | Subtotal backup CO2 | 16.9 | ||||||
SAP carbon reduction | -15.3* | ||||||||
Total kgCO2 | 19.9 | ||||||||
Saving | 46.8% |
* A reduction of 0.568kgCO2 is awarded for each kWh of electricity generated by CHP, so in this case 27 x 0.568 = 15.3kgCO2
Table 4. Gas CHP – SAP (carbon reduction of 0.568kgCO2/kWh of CHP elec)
Holy cow, suddenly we’ve met the requirement for Level 4 under Code for Sustainable Homes! But unfortunately it’s not true. We shouldn’t be applying the 0.568 figure to CHP in this way.
Hang on a minute though. The 0.568 figure applies to renewables too, right? As a form of on-site generation, CHP should get the same break as renewable energy. Yes absolutely, except that under SAP, CHP gets a much much bigger break than renewables.
That’s because for renewables, the 0.568 figure is applied only after tallying up all the carbon resulting from meeting the electricity demand from the grid. In other words, they first assume you’ve met all the demand from the grid (at 0.422kgCO2/kWh) and then they take away 0.568kgCO2 for each kWh from renewables. The net carbon benefit for renewables is therefore (0.568 – 0.422) or 0.146kgCO2/kWh. Here’s an example based on our baseline scheme to show how the carbon benefit is applied for renewables.
Primary energy | Assumed efficiency | Useful energy | CO2 kg/kWh | kgCO2/yr | |
Space heating | 56.8 | 88% | 50 | 0.19 | 10.8 |
Water heating | 28.4 | 88% | 25 | 0.19 | 5.4 |
Electricity | 50.0 | 100% | 50 | 0.422 | 21.1 |
Total kgCO2 | 37.3 | ||||
Electricity from renewables (kWh) | 10 | ||||
Carbon saved (kg) | -5.68* | ||||
Adjusted total carbon kgCO2 | 31.6 | ||||
Saving | 15.2% |
* A reduction of 0.568kgCO2 is awarded for each kWh of electricity generated by renewables, but only after it’s assumed that all the electricity requirement is met by the grid.
Table 5. Example of awarding a carbon reduction to renewables
In fact, if you apply this 0.146 figure to CHP, suddenly we’re back in the ballpark:
CHP | Backup systems | ||||||||
Useful energy demand (kWh) | Demand met (kWh) | Primary energy | CO2 kg/kWh | kgCO2/yr | Demand met (kWh) | Primary energy | CO2 kg/kWh | kgCO2/yr | |
Heat | 75 | 45 | 62.5 | 0.19 | 11.875 | 30 | 37.87879 | 0.19 | 7.19697 |
Electricity | 50 | 27 | 34 | 0.19 | 6.4 | 23 | 23.0 | 0.422 | 9.7 |
Subtotal CHP CO2 | 18.3 | Subtotal backup CO2 | 16.9 | ||||||
Carbon subsidy | -3.9* | ||||||||
Total kgCO2 | 31.25 | ||||||||
Saving | 16.2% |
* Here the reduction is awarded to CHP in the same way as for renewables, i.e. as if it is assumed that all electricity comes from the grid before the 0.568 figure is applied. So in this case 27 x (0.568 – 0.422) = 3.9
Table 6. Gas CHP – carbon reduction of 0.146kgCO2/kWh of CHP elec
So it looks like SAP and similar methodologies have got it badly wrong. The Housing Corp is unintentially misleading all the housing associations, micro-CHP manufacturers need to head back to the drawing board, and big developers have to consider their options.
Or it could be me who’s wrong. I’m just one guy right? I’m sitting in a cattle stall trying to work this out and I might have made a mistake. If you think that’s the case, then leave me a comment or get in touch via the contact page and let me know. I need to get to the bottom of this.
Because if I’m right, then all the companies and public sector bodies who are putting their faith in the standard calcs need to think again. And fast.
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
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.
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/
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