Wednesday, 22 December 2010

10:10 performance - deeply technical stuff!

In my first post of 2010, I set the target for my home's 10:10 challenge - to cut our domestic emissions to 4.5 Tonnes CO2-e by Christmas eve.  This required our annual energy use to drop by 10%, to 4,770kWh electricity and 12,655kWh gas.

And the results are: SUCCESS with electricity but FAIL with gas.  I am disappointed with this because, despite the cold and snowy weather we had in February, our moving annual total gas consumption was 13,000kWh from February to May and it hadn't risen much over the summer.  It had crept up to 13,600 by mid-November and I was determined to keep the thermostat turned down but this made no difference when one week of snow was followed by our current, much harsher freeze.  In the last four weeks of Arctic weather, our annual total gas usage has jumped to 14,850kWh.

My friend and neighbour, Vincent Jansen, has just introduced me to the concept of degree-days.  The Environmental Change Institute at Oxford University run the i-measure website, where you can benchmark your home's energy use and emissions each week.  They also publish tables of heating demand for buildings, measured at a number of sites around the country - available here.  These record the effective temperature difference that has to be maintained in buildings, in units called degree-days.  You can plot these weekly or monthly data sets against your home's heating energy usage to see how much benefit your insulation improvements or boiler upgrades have given you.  As you can see, the correlation between degree-days and gas usage in my home is remarkably strong.

Each record has a columns of degree-days recorded against a range of reference temperatures and it is important to choose the appropriate reference temperature for your building.  15.5°C is traditionally used for homes and offices (and 18.5°C for hospitals) but my records fit closest to the 14°C curve.  I have split the difference and used a base temperature of 15°C.

If you plot a building's energy use against degree-days, the line is called the performance line and the slope of this line is the building's energy loss rate.  Here are three separate performance lines for my family's home in 2008 (red), 2009 (blue) and 2010 (green).  The slope of the green line is noticeably less than the blue and red, suggesting that the extra-thick loft lagging I added last autumn has been of some benefit.

So the bottom line is that I missed my 10:10 target for home heating energy but, given the extreme weather we are experiencing this month, I could quite well have used 10% more gas than I have done, if it weren't for the extra insulation that I put in to meet my 10:10 commitments.

What about the 'leccy?  You may recall that I fitted LED lights in the kitchen and in the bedrooms.    I also switched my tower PC off last Christmas and replaced it with the amazing Fit-PC 2 net-top computer, running on just 8 watts.  The result has been remarkable.  Our moving annual total usage has dropped smoothly throughout the year and is almost on target, at 4798kWh.

One last picture shows my home's cumulative gas and electricity use through 2009 and 2010.  The effect of our prolonged cold spell is strikingly clear when this year's gas usage jumps ahead of  last year's at the end of November.  Nationally, this must have led to a huge surge in energy demand, emissions and customers' bills.

So far, I have neglected the carbon-intensity of my electricity provider.  In calculating my home's carbon emissions, I use DECC's national average carbon-intensity of 430g CO2 per kWh electricity.  However, as a British Gas customer, my electricity has the lowest carbon-intensity of any of the big 6, at 371g CO2 per kWh supplied.  If I recalculate using the British Gas carbon intensity, my home emitted 4.6 Tonnes CO2 for the year to Christmas Eve 2010.  So I can console myself that, despite missing my target for gas usage, our overall carbon emissions are almost where I had hoped.

For the broader aspects of carbon footprint, such as diet, lifestyle, shopping and travel, I don't have any hard and fast measures.  If we all eat less meat and dairy, buy more local and seasonal food, make our clothes and electronic gadgets last longer and keep our feet firmly on land (or water) when we take our holidays then I suspect we'd make a big dent in Britain's climate-changing carbon emissions.  The transition to a sustainable economy is going to be a major theme of the coming decade and I hope many people here and around the world will jump at the chance to make life better for all the generations to come.

1 comment:

Jeremy said...

Thanks for the detailed analysis. I will have to look up the degree days - I've been using imeasure, but perhaps I should try SMEmeasure instead.