emissions from CHP: the standard method is wrong

[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:

• 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.
• These emissions figures can determine whether or not a scheme gets planning permission or passes building regs.
• 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.

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:

So now the carbon calculation looks like this:

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:

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:

* 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.

* 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:

* 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.