Thursday, 15 September 2016

Does the UK need solar electricity?

I am not against solar PV panels in other countries where the pattern of demand is different, but in the UK our peak heating demand is in the winter time when solar power is low. Wind on the other hand peaks in the winter too so is a natural fit for our demand pattern. But does wind generation really match our demand well, or do we need mix of other types of generation? The answer was not as clear cut as I expected.

As a thought experiment, I decided to see what would happen if all our electricity was delivered by wind power. Here I have charted our current demand pattern through the year versus wind generation scaled up to match the most demanding quarter which is Q1.
Current demand pattern (left hand bars) matched by wind power only. The red hatch area shows where supply does not match demand.

I was surprised to find that there was not enough wind in the summer time to meet demand - so maybe we could use a bit of solar in the mix too. This next chart shows demand matched optimally by a mix of wind and solar. We have more energy than we need in Q4 but only just enough in Q1 and Q3.

Current demand pattern matched by a mix of wind and solar power

But then I thought, what about the future? To decarbonise our energy use substantially we need to convert a lot of homes that are currently heated by gas to electricity. Gas heating for buildings is never going to be zero carbon - carbon capture and storage is completely impractical on such small scales. The National Grid recently published this year's Future Energy Scenarios in which they suggest how our energy supply and demand might change in the future. As in previous years they describe four scenarios but only one of them meets our carbon targets - the 'Gone Green' scenario. This one has a lot of electric heating, which increases our demand in the winter time and so will match solar even worse than now.

The following chart is based on our current demand pattern with some changes, based on the Gone Green scenario in 2040.

  • Added 40 TWh/year of domestic heating demand, divided into the quarters by typical requirements. (This represents about 12 million homes with electric heat pumps but also a great deal of improved insulation reducing heat loss)
  • Removed 30 TWh/year due to increasing appliance efficiency, evenly spread through the year.
  • Added 20 TWh/year for transport, representing nearly 10 million electric cars, evenly spread through the year - actually I think this is likely to be seasonal with greater use in winter but I do not have the data for that.

I have ignored changes in commercial and industrial energy use as the Gone Green scenario shows little overall change - improvements in efficiency more or less match growth in demand due to economic growth and switching from gas to electricity. The next chart shows the new pattern of demand and supply from wind and solar optimised to match.

Future demand based on changes in the 2040 Gone Green scenario (see text above), matched by wind and solar.

As you can see there is still a smidgeon of solar in there - though not much.

Of course a purely renewables based scenario is extremely unlikely. In the National Grid scenarios even Gone Green has only about 50% renewables. There is also some nuclear power and some fossil fuels with carbon capture and storage (CCS). These types of generation are available throughout the year. Adding just 10 GW of other base load generation into the mix means we do not need any solar at all.

Future demand based on changes in the 2040 Gone Green scenario matched by wind and solar and 10 GW of other generation spread evenly throughout the year.
So the answer to my question turned out to be complicated. With our current demand pattern a mix of solar power as well as wind probably does make sense. However if we are to seriously address our carbon emissions then we need to convert a lot of heating to electricity, as well as reducing heat loss. This increases the heating demand in the winter which means wind becomes more useful and solar power less. Solar generation installed now should last 25 years - after that we may not need it any more.

Sources and assumptions:
The current demand pattern was derived from electricity demand data 2013 to 2015 from the National Grid.

The data on renewables generation was based on load factors and installed capacity from Energy Trends: Renewables 6.1.

The split of heating demand through the year is taken from Energy Trends: Weather section 7.1.

National Grid Future Energy Scenarios is downloadable here.

My charts show matching of demand to supply over quarters in the year. I expect electricity and heat storage will help to match supply and demand over shorter timescales. However the prospects for inter-seasonal power storage are still poor as far as I can see.

My optimal matching of demand and supply minimises wasted power - not cost. Judging by the subsidies wind is still cheaper than solar power, though perhaps not by much. However both are capital intensive and wasting the power generated increases payback time and reduces the return on investment.

There are other types of renewables that I have not mentioned. Tidal power is fairly steady through the year so would add to 'other' generation. Biomass power stations tend to operate all year as well. Waves are generated by wind so wave power is likely to have a similar pattern to offshore wind.

I have assumed that the efficiency savings from residential appliances are spread throughout the year. In practice some appliances (such as freezers) use more in summer while others (such as kettles) use more in winter. This is probably fair overall.

I am not sure if the National Grid Future Energy Scenarios has taken into account changes in climate causing milder winters. This would decrease heating demand and so favour solar. Also hotter summers will favour more air conditioning load in summer. The Consumer Power scenario has the most residential air conditioning - up to 2 TWh/year in 2040. If this were spread over 3 months it would add another 1 GW to demand in summer - this favours solar but only a little.

In general I have ignored the seasonality of changes to industrial and commercial electricity demand. The overall total electricity use is largely unchanged in Gone Green for 2040 but an increasing proportion is due to space heating switched from gas to electricity - this favours wind. On the other hand hotter summers will increase demand for air conditioning which favours solar.

The 'other' supply will be a mix of nuclear power and fossil fuels with CCS. Technically this could be flexible and could be used just for topping up when renewables fail. However, nuclear power is fairly inflexible and is far more likely to be used continuously (at least when available). Since it is very expensive to build and relatively cheap to run it has to be used a great deal to pay back its investment. Also there are technical difficulties with running nuclear power up and down (see Nuclear power plants aren't just for base load). Gas power stations can be very flexible but if they are used in combination with CCS there are, again, difficulties with ramping power up and down.

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