Author: Robert Wilson

Lulls in Californian wind farm output can be severe

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Last week I wrote a post on wind farm capacity factors in America. Below is the key graph showing monthly variations in capacity factor:


This tells a simple story. The average capacity factor of America’s wind farms is very higher, much higher than most other countries. However, to imagine this is the same throughout America would be a mistake. The climate in California is rather different to that in North Dakota or Texas. And wind farm output follows a very different pattern.

We can find out how much wind farm output varies in California by looking at the hourly data provided by CAISO. Let’s look at it from a number of angles. Read the rest of this entry »

Don’t press publish by accident

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My hand is clearly looser than I imagine. I was just chucking a couple of plots into WordPress for a blog post I’ll throw out in the next few days, and I seem to have pressed publish.

So if anyone arrived here via Twitter or email expecting a piece on the variability of German solar output you will just have to wait, til Thursday or Friday.

Germany will never run on solar power. Here is why.

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If you rely too heavily on social media for your information, you are liable to believe that Germany now gets half of its energy from solar panels. The reality of course is that Germany gets nowhere near to half of its energy or electricity needs from solar panels. Far from it.

Last year, 5.7% of Germany’s electricity generation and 2.5% of primary energy consumption came from solar panels (BP Statistical Review of World Energy). The contribution solar panels make towards Germany’s renewable energy target (which uses the final energy consumption metric) is even lower. Less than 1.5% of German final energy consumption now comes from solar panels according to Eurostat.

Headline writers of course are still awed by renewables in Germany. Statistics be damned. A windy and sunny day on a German weekend will result in stories about record high production of renewable energy. Normally there is no need to tell readers that these record highs are very unrepresentative.

There is also no need to explain to readers that these record highs are a problem, and not necessarily something we should shout too loudly about. Consider that Germany only gets 5.7% of its electricity needs from solar panels, but on some afternoons it can meet just over 50% of instantaneous demand from solar panels.

Now, run a simple thought experiment. What if Germany increased solar panels so that they produced ten times more electricity? They will then be capable of producing half of Germany’s electricity.

But wait. Think of those afternoons when Germany now gets half of its electricity needs from solar panels. In this imaginary future it will be getting 500% of its electricity needs from solar panels on those record breaking afternoons. What can it possibly do with this much electricity? Can it store it? Of course not. No technology exists that can credibly store that much electricity. Can it export it? Absolutely not. The excess electricity produced by Germany’s solar panels will far exceed the electricity demand of Germany’s neighbours. They will not want it. So what Germany will have to do is dump the electricity. It will not be consumed. It will be wasted.

That is the plain reality of these headline grabbing record highs. Germany’s renewables occasionally produce far more power than they do on average. The simple consequence is that in the future they will have to curtail massive amounts of electricity on these occasions.

These are the simple consequence of attempting a massive expansion of solar power in a high latitude and cloudy country. Germany, of course, despite the hype has not expanded solar massively. But let me show you why it cannot.

Solar panels operate on a simple principle, they convert solar radiation to electricity. The main factor determining the output of a solar panel is the solar radiation striking it and the efficiency of the panel. Other factors such as humidity come into play, but are not major determinants of the geographic variation in solar potential.

All things being equal, solar radiation striking a solar panel would essentially be determined by latitude. However, because of large scale differences in climate, cloud cover results in pronounced differences in solar radiation in different latitudes. Florida, the so called “Sunshine State”, has lower solar potential than a large number of American states at higher latitudes. This is even more pronounced in China, where the incredibly cloudy Sichuan Province sits at the same latitudes as western regions which are essentially desert.

Germany, of course, manages to combine a high latitude location with high cloud cover. This is clearly a perverse location for the world’s leader in installed solar capacity.

To see the problems with solar in Germany, let’s start with something nice and comprehensive.

The figure below shows the output from Germany’s solar panels in every hour of every day in 2014.


Record high solar output may garner headlines, but they are clearly not representative. Yes, German solar output on incredibly rare occasions hits around 25 GW. But in December it basically never gets above 5 GW. Where are the headlines about German solar output then?

The graph above makes clear that the record highs aren’t even representative of summer months. May, June and July may have the potential for days when output gets up to around 25 GW, but it also has days when output never gets above 7 or 8 GW.

But if record highs are worth discussing, why not record lows? Obviously solar panels do not produce electricity at night, a fact that continues to be glossed over by environmentalists and advocates of solar energy. So, there is little point talking about the record low hourly output. This is zero at midnight every night, and it always will be.

What is more interesting is daily output. Last year, the best day was 6th June, when mean output was 8,850 MW. The worst day, unsurprisingly was around the time of the winter solstice. On 30th December solar output averaged 141 MW. This is 63 times lower than the production on the 6th June. Any Germans delusional enough to be considering disconnecting from the grid and “running on the sun” must ask how they will get through Christmas and New Years.

Below is a graph showing average daily output in each day of the year, split out by month.

Solar2014_daily_monthAgain, look at December. Solar panels have risible output in December in all high latitude countries. In fact, they often might as well not exist in the middle of winter. This is simple physics.

Now read the opening of a story from the Guardian website last year

It’s the shortest day of the winter, but the bright blue morning means the UK’s biggest solar farm is powering away. The 120,000 matt-black panels laid out in long, neat rows above sheep-shorn grass are running at about three-quarters of their peak capacity.

Solar panel output in Germany and Britain are not particularly different, so to call the sentences above inaccurate would be putting it mildly. But this kind of innumerate dross pervades energy debates. Do the math on climate change. Absolutely, this is what we must do. The math on actual solutions to climate change. That is something else.

It is no great sin to imagine that Germany gets half of its energy needs from solar panels. Believing this will not call into question how seriously you take climate change, or how well you understand the issues. Far from it. But it should.

Here is the above data replotted so that average daily output is on a single graph.


Solar2014_dailyNow. Let me do a simple trick. Right now Germany get around 5% of its electricity needs from solar. Let’s multiply current output by 10 and compare it with the average daily electricity load.


Not so good. Clearly you will need to curtail a lot of electricity in summer. But, it’s actually much worse than this. Solar panels only really produce electricity between around 8 am and 8 pm, and within a narrower time frame in winter. What we really need to look at is hourly solar output versus hourly demand.

Here it is. Each point represents an hour of the year. The red points show when solar output will exceed electricity load at that time.



Clearly there are a lot of hours when solar output will exceed demand. Furthermore, a lot of solar energy will need to be curtailed in this fantasy scenario.

How much? Most grid operators believe that wind and solar cannot exceed something like 60% of instantaneous demand. However, future levels of curtailment will be significantly lower if this level can be raised, as shown by a recent study of wind farms in Ireland.

To estimate it for Germany, I will start by assuming that all electricity in excess of electricity load will be curtailed. This will give a lower limit to levels of curtailment. Under this assumption 45% of solar energy in Germany will have to be curtailed if Germany was to increase the number of solar panels by a factor of 10.

This, of course, is a significant underestimate. Actual curtailment will be much higher. If we assume that anything above 80% of load will be curtailed, we find that 55% of solar energy is curtailed. And I haven’t even looked at what happens if Germany simultaneously expands wind farms by a factor of ten. If this happens curtailment will be even higher.

So there you have it. If Germany wanted to get the majority of its electricity needs from solar panels, the majority of energy those panels produce will be wasted.

Note on data

Hourly data was taken from the website of PF Bach. Analysis was carried out in R, and plotted using ggplot2 and a custom theme. If anyone wants the R code I will share it with them.



How reliable are Britain’s wind farms?

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According to the Telegraph website, Drax (the owner of Britain’s largest biomass power plant) has attacked wind and solar power for their lack of reliability.

The Telegraph reports:

Dorothy Thompson, Drax chief executive, said the UK’s wind and solar farms were “increasing the instability of the grid”, with “mounting” cost implications, citing forecasts suggesting the bill for balancing supply and demand would rise from £1bn this year to £2bn by 2020.

Yet “aggregate wind farm and solar output in the UK can be expected to fall below 1pc of total electricity production with reasonable regularity”, she argued.

Read the rest of this entry »

Hillary Clinton’s climate change goals aren’t going to save the planet

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Hillary Clinton announced her climate change policies last night. What should we make of them?

Let’s look at her two main policies.

The first:

Have more than half a billion solar panels installed across the country by the end of Hillary’s first term.

To say that this policy is worded in a deliberately misleading way is accurate. The average voter knows nothing about energy or climate change. Open season for a politician like Clinton. Read the rest of this entry »

Do they check facts and logic at The Conversation?

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One of the easiest ways to get away with dodgy analysis of energy matters is to assume that your readers are completely ignorant about what is going on in other countries. If you live in Britain, you can point to Germany, Denmark, or any other country, and claim they are doing much better than you on renewables, climate change, or whatever you choose. If you are German, you can point to Britain. Read the rest of this entry »

What are the capacity factors of America’s wind farms?

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In energy it is important know what to count.

Consider this simple question. Which country is number one in terms of wind energy? In terms of total installed wind capacity it is China. At the end of last year it had 114 GW of installed capacity, in contrast to 66.2 GW in the United States (BP, 2015).

China is number one, then, by a long way. Not so fast. Last year America’s wind farms produced a total of 183.6 TWh of electricity, whereas China’s produced 158.4 TWh (BP, 2015).
Read the rest of this entry »