Wednesday, 12 November 2014

Solar electricity is like ice cream

In the UK our peak electricity demand is in the winter. Solar PV panels deliver 6 times more power in the summer than in the winter. Solar PV delivers carbon-free electricity with little environmental impact and helps mitigate climate change but it doesn't help 'keep the lights on' in the winter unless we have inter-seasonal energy storage as well. Last year the government gave out £2 million to 16 projects for innovative energy storage solutions - of which £40,000 went towards one project with the potential to deliver inter-seasonal storage [1][2]. This is a paltry sum compared to the many £millions/year paid out in subsidy for PV panels. So why aren't we doing more?

The chart below shows the pattern of electricity demand and PV (solar electricity) yield through the year. Demand is highest in January but PV yield is less than 40% of average at that point. In December it is down to a quarter of the the average. If PV delivered 20% of our electricity in July it would give only 2.4% of our needs in December. Wind power can fill in part of this gap because it is generally windier in the winter than in the summer - but the difference is not nearly so marked as with PV.

Trends in electricity demand and PV yield through the year. Electricity demand is based on the last 5 years from [3], PV is from the PVWatts calculator [4]

The other 15 energy storage projects that won funding last year were for battery storage, compressed air, liquid air, flywheels, even elevated heavy weights. Since then, the liquid air option has won another £8 million [5]. But none of these can practically be used for storing energy for more than a few weeks at best.

The one project that can could save summertime solar electricity for use in the winter was a power to gas project called P2G Biomet. This uses electricity to separate hydrogen from water by electrolysis and then combines this with CO2 from a bio energy plant (such as biological sewage waste treatment) to make methane - the same as natural gas. There is a demonstration plant working on similar principles in Germany [6]. We already have a lot of infrastructure to handle methane, though in terms of storage we only have a couple of weeks of winter demand.

Solar electricity is the most popular of the renewable technologies in the UK. In the Public Attitudes Tracker survey published in April this year, 85% of us supported solar electricity compared to 70% for onshore wind and 77% for offshore wind [7]. If we want this technology so much we need to invest more in the storage technology needed to use it sensibly. It seems solar PV is like ice cream - everyone likes it and we consume lots of it in summer, but by itself it doesn't give you a balanced diet.

[1] £21 million for carbon cutting technologies ( May 2013
[2] Power-to-Gas Project Awarded DECC Grant Funding
Storing Electricity as Renewable Methane in the UK Gas Grid (CNG Services) May 2013
[3] National Grid Data Explorer
[4] PVWatts Calculator
[5] Liquid air technology gets full demonstrator funding (Eureka magazine) Feb 2014
[6] Audi opens renewable energy E-gas plant in Germany ( July 2013
7] Public Attitudes Tracking Survey wave 9 ( April 2014


  1. Though if average temperatures rise by even half a degree, in UK, peak temperatures leading to extensive use of air conditioning are likely to become much more common.

    However my own view is that we should be looking at greatly increasing the amount of wind generation.

  2. Good blog, I think saving heating energy in winter gives us energy cheaper at the right time. But I also noticed ITM power do energy to gas conversion and have a plant in Germany. The director talks about hydrogen or methane storage in the gas grid and says at 51min in that there is interseasonal storage capacity in the gas grid because it is so big, and for no extra cost.

    1. Germany has huge amounts of storage capacity in their grid - 5 times as much as we do. See This says they have 20 billion m3. We have 3.8 m3 in depleted oil fields (from DUKES

  3. There is interseasonal storage of GAS, just leave it in the ground for longer!