[Update March 20 – while it’s true that SAP gives misleadingly high emissions savings for CHP, I got the methodology wrong. See an updated post here. Points 3 and 4 below are still valid.]
The SAP results for dwellings using CHP are badly skewed. This may cause large developers to formulate strategies for meeting the Code for Sustainable Homes which fall well short of the targets.
Doing some research this week, I read the Housing Corporation’s report on the estimated costs of meeting various levels of the Code for Sustainable Homes. It’s an interesting document, but at a certain point I was confused by their claim that you can meet the carbon reductions required by code level 4 (i.e. a 44% reduction in DER relative to TER) just by using gas CHP. In fact, when I looked closer I found that in some cases, they were claiming an emissions reduction of over 50% – an extremely high figure. Something closer to 10 and 15% is much more reasonable, unless you want to get Orchardesque.
The source of these wild claims is the Government’s Standard Assessment Procedure for dwellings – SAP 2005. As hinted at in the CHP debate running in the BSJ over the summer, SAP does some funny things when it comes to CHP. Have a closer look at the SAP worksheets and you find that SAP:
- Assumes that for each kWh of electricity generated by CHP, you displace a kWh of grid electricity and so are awarded an emissions reduction of 0.568 kgCO2/kWh. So for a kWh of electricity from a typical engine, rather than emitting 0.65 kgCO2, SAP says you only emit 0.08 kgCO2.
- Assumes that 100% of the electricity demand is met by CHP regardless of the proportion of heat demand met
- Doesn’t allow for pumping energy
- Omits the secondary heating system from dwellings served by CHP
So taking each of these points in turn:
Point 1. I’ve been trying to spot the logic and I can’t see it. As a comparison, if you look at the way SAP treats renewables for a dwelling supplied by the grid, you’ll see that SAP first totals up the emissions as if all electricity were met by the grid (at 0.422 kgCO2/kWh) and only then subtracts the 0.568 figure. In other words, the bonus for on-site renewables is only 0.146kg CO2/kWh (0.422 – 0.568). This is a nice perk for renewables but very little compared to the whopping 0.568 that’s awarded to CHP.
Assuming the 0.568 figure is justified on the grounds that you’re displacing future generation capacity, shouldn’t CHP get the same bonus as renewables, i.e. 0.146 and not 0.568?
Point 2. This doesn’t make sense either. For example in the Housing Corp report they assumed the CHP engine would run 4500 hours a year (see appendix B) but meet 100% of the electrical load and 50% of the heating load. Running an average of 12 hours a day, they’re going to fall well short of meeting the total annual power requirement.
This likelihood of the shortfall is further increased when you consider that the engine size will be limited by the base summer heat load (dump too much heat in the interests of generating power in summer and you’ll lose your “good quality CHP” status – and your LECs). Even running 6500 hours a year, they might only expect to meet say two-thirds of site power demand. SAP doesn’t appear to allow for any of this and simply assumes that any time there is electricity demand on the site, the CHP engines will be running and have sufficient capacity to meet demand.
Point 3. For a dwelling with its own boiler, SAP does a great job of including the power requirement for pumps and fans. But move the boiler into a communal energy centre and the energy demand magically disappears.
Point 4. SAP forces you to assume that, for dwellings with individual heating systems, 10% of the space heating demand is met by a secondary system. This is usually an electric bar fire, which drives up the DER owing to the higher carbon intensity of grid electricity. If you move to a communal system, the secondary heating disappears, which straight away reduces your emissions from space and water heating by more than 10%. But why should dwellings served by communal schemes be treated differently from dwellings with their own boilers?
There may be justifications for SAP’s assumptions, or I may have misinterpreted the calculations. If you have an answer, please leave a comment. But if I’m right, then SAP is giving engineers unfair and unrealistic results when evaluating schemes that use CHP.
Most importantly, right now the big house builders are considering how they’re going to meet future requirements under the Code for Sustainable Homes. And when they do the numbers, they may decide to put most of their eggs in the CHP basket assuming it will exceed requirements for code level 4. But I strongly suspect that the BRE is going to have to reconsider the way it treats CHP and bring it’s assumptions more in line with reality. And when they do, it’s going to pull the rug out from under the developers who trusted SAP 2005.
carbon limited 
Nice analysis. Unfortunately I’m miles from software which could help verify your results, but from my previous experience, I suspect you’re right. I once got a bee in my bonnet about the previous Building Regs and SAP, especially regarding electric heating. I even had an article published in BSJ, but I doubt it was even noticed. Sigh.
The libertarian in me is beginning to think the government should start to leave the calculations to those who can do them rather than imposing random targets which have been picked from thin air. But then the cynic in me thinks that targets are necessary to rein in the greater excesses of money spinning developments. Yikes, I’ve gone all philospohical.
This all points to something which I’ve been mulling over about the role of BRE, government departments and CIBSE within the industry. How much independence they have, where they get funded from, etc…
Thanks Mel. Nick did a quick check in NHER and confirmed that this is indeed what SAP is doing.
I sympathise with your conflict of libertarian vs. cynic. If you want to get a clear idea of how CHP is going to perform on a given site you need a lot more a few cells in a SAP spreadsheet can provide. So either you publish a standard assessment procedure just for CHP (which includes things like hourly demand profiles) or you let engineers get on with it themselves. Maybe CHP Sizer could be adapted for this? It’s quick and simple but gives a much better idea of actual performance, though it doesn’t currently do residential.
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Your article came as no surprise to me. I’ve been sizing CHPs for different projects in UK and it seems that the sustainable code 4 and 6 is a fictional code set by government to encourage the use of renewable energy technologies. Another confusion that we normaly came across is the wrong preception of Gas CHP being renewable energy technology. CHPs can only be renewable, if they are running on biomass; which again raises questions about the commerical viability and availability of this technology. Albeit CHPs are being widely used as an embedded heat and power generation source; it is not proven as yet that they can reduce 44% carbon emissions. In my experience with small CHPs less than 500 kWe a figure between 10-15% and for large CHP greater than 1 MWe 25-30% is quite a promising figure.