Thursday, 21 November 2013

Carbon accountancy

In Energy cost, energy use and carbon I wrote that the new house emits about half the carbon of our old house. At first sight this doesn't seem so impressive, but it's important to note that the whole of the new house stays warm throughout the winter and relatively cool through the summer, while the old house was literally freezing in many places for some of the winter, and although cooler than a lot of other modern buildings, was not coping with the record temperatures of the decade we lived there.

Having noted this, I should still re-emphasise that we were only using twice the energy in the old house, so while the efficiency of our new house may be many times better, the actual carbon emissions have only halved. This is an example of the Jevons paradox, and the old adage: a little energy efficiency is a dangerous thing. We would probably emit a lot less carbon if we lived in a tent, but I'm not sure whether I'd still have a job and a family. 

Also I should note that the calculation of our carbon emissions did not consider the electricity we produce, which may substantially change the equation.

The electricity we use from the grid was probably turned from burnt fossil fuel, with all the inefficiency involved in the burning process and the turning of turbines and generators. Then it's been stepped up, sent hundreds of kilometres and stepped down again, wasting little energy on the way, but more in the stepping processes, and then some more in the wires on the way to your house. So by the time you get one kWh to your electrical appliances, you've used something like 2.7 kWh of fossil fuel. Each kilowatt hour of domesticated electricity is going to release around half a kg of carbon dioxide, and its equivalents, into the atmosphere. 

But what about the electricity from our solar panels. That's green isn't it? Not completely green, but perhaps a lighter shade of grey. 

It's relatively straightforward to get a figure for the electricity we buy over the grid, making assumptions about the overall energy picture in Japan. Of course the exact carbon cost of electricity varies around the country, from region to region and even from house to house depending on whether you're next door to the stepping-down station, or up a valley at the end of a few kilometres of cable. Nagano prefecture has a lot of hydroelectric power, but most is owned by Tokyo Electric rather than Chubu Electric, which supplies our electricity.

The performance of each region's power company can be found here and here here at the department of environment's site. Chubu electric (which supplies power to Nagano) produced 518 grammes per kWh sold in 2011, and 473 g/kWh in 2009, when the nuclear power stations were running. Tokyo electric, with the help of the hydroelectric power stations they run in Nagano, produced 464 g/kWh in 2011 and 375 in 2009. They  had more nuclear power stations to switch off, as you may have heard. There improved carbon performance is possibly due to more urban consumers living within shorter cable lengths.

We also have to estimate the carbon emitted from the electricity we use from our own solar panels, which amounts to something like 10% of what we generate and 30% of what we use.

Our solar panels were made in China. They may be leading the green revolution on many fronts, most of which are invisible to a western media usually not even trying to understand what's happening behind the Great Wall, but they still use a fair bit of coal, so most of the electricity used to make the panels is from dirty fossil fuels. They make shoes for everyone, but walk barefoot.

The estimate here at EDF Energy, based on a 25-year life time of the panels, is that each kWh of solar electricity produces 72 grammes of carbon. This is around one seventh of the grid electricity, so as the site says, it's low carbon, not zero carbon. Nuclear power is similarly encumbered with carbon costs in the extraction, purification and transportation of radioactive materials, and windmills also need to be made out of something other than air. There's no such thing as a carbon free lunch.

Of course the estimate of kg carbon per kilowatt hour of solar power depends very much on the insolation, in other words how much sunshine you get. We perhaps get twice the sunlight to somewhere like Glasgow, so our 25 years are going to produce twice the electricity for the same embedded carbon costs, and our carbon per kWh would be half of theirs. We're also closer to China, so transportation costs are a little less, but they're still using those dirty coal power stations. Conservative estimate: 50 grammes of carbon per kWh.

So our consumption of electricity, both from the grid and from our own panels emits something like 2.7 tonnes of carbon per year.

Next, how do we account for the electricity we supply to the grid in calculating our carbon emissions?

In the simplest terms, we sell a little under 12,000 kWh to the grid per year. So instead of 12,000 kWh of fossil fuels being generated, at 500 g CO2/kWh, we're generating solar electricity at 50 g CO2/kWh. This is a saving of about 5.4 tonnes of CO2 per year. So we're in carbon credit.

This is simple, but almost certainly wrong. This assumes that the moment the electricity hits our meter it's going to be used by hungry consumers. In fact there is over ten metres of cable before it even gets to the next house, and the chances are that they'll be away at work while we're generously generating their power. Ten metres may not sound far, but my ballpark estimate inside the house was that we lose 1% of electricity every 5 metres.

In the worst-case estimate, the electricity from our panels is not going to make any difference at all. Chubu electric employees are not going to be sitting at the controls of their gas-fired power stations, looking at the weather forecast and turning down the volume because my house is putting out a couple of extra kilowatts.

We can perhaps split the difference and say that half of our solar electricity is going to be of use to someone, so the amount of dirty power is reduced by 6,000 kWh per annum, and we're saving 2.7 tonnes. This balances out the 2.7 tonnes we produce, so we're carbon neutral.

If you're interested in carbon,'s may be useful.