In a comment on the recent power-to-methane post I made the following observation:
It would be an interesting exercise to take a high-renewables-penetration DECC scenario that meets UK emissions targets, convert it to hourly generation by factoring actual Gridwatch generation and compare it to demand for, say, 2013 or 2014. I’d be willing to bet the UK would be freezing in the dark for much of the time during the winter.
Well, the interesting exercise is now complete and this post documents the results. They are based on a DECC Pathways Calculator scenario that meets the UK’s 80%-by-2050 emissions reduction target and uses the Centre for Alternative Technology’s (CAT) 100% renewables generation mix listed in the power-to-methane post. And to eliminate any suspense as to the outcome Figure 1 previews what we get when we compare February 2050 generation for this scenario with February 2050 demand, with both projected using factored February 2013 data from Gridwatch and other sources:
Figure 1: Generation deficits using DECC 80%-emissions-reduction-by-2050 and CAT 100% renewables generation mix scenario, February 2050 simulated hourly data.
I would have won my bet.
To construct the scenario I began with the DECC Pathways Calculator, which allows one to juggle 42 different input variables until the desired goal of an 80% reduction in UK emissions by 2050 is achieved. It took me a little while to get to this point, but in the process of getting there I noted a few interesting features that I will mention before proceeding. According to the DECC Calculator:
• Major reductions in emissions from transportation, heating etc. are needed to meet the 80% target. Simply decarbonizing electricity generation doesn’t do it.
• The biggest emissions-reduction bang-for-the-buck comes from expanding nuclear.
• Biomass generation significantly increases CO2 emissions.
The scenario I eventually developed is summarized in the Figure 2 screenshot and in this link which shows the scenario with the appropriate boxes clicked. The columns on the left show how I had to redline many of the demand side options such as transportation and heating to get to the 80% target, the column in the middle replicates the CAT energy mix reasonably closely and the redlined variable at the upper right is, unsurprisingly, energy storage:
Figure 2: The DECC 80%-emissions-reduction-by-2050 scenario
The CAT scenario contemplates that annual UK electricity generation will double between 2013 and 2050 from 359TWh to 738TWh. The added generation goes mostly towards the electrification of transport, including 100% zero-emissions vehicles (presumably EVs), 100% electrified railways and 50% electrified buses.
Having developed the scenario the next step was to convert it into 2050 electricity generation relative to the 2030 CAT generation mix shown in Figure 3:
Figure 3: The CAT 100% renewables generation mix
To do this I again used February 2013 as my “average winter month” and assumed that the wind, tide, solar etc. conditions in that month would be duplicated in February 2050 and that the shape of the demand curve, although not its amplitude, would be the same. The scaling factors and other assumptions used to derive the generation numbers are detailed below:
Demand: February 2013 Gridwatch demand was scaled up by a factor of 2.06 (738TWh annual generation in 2050/359TWh annual generation in 2013).
Wind: February 2013 Gridwatch wind generation (offshore + onshore combined) averaged 2,053MW. With the CAT energy mix offshore+onshore wind averages 68,055MW. February 2103 wind generation was therefore scaled up by a factor of 68055/2053 = 33.14.
Hydro: February 2013 Gridwatch hydro generation averaged 412MW. With the CAT energy mix it averages 937MW. February 2103 hydro generation was therefore scaled up by a factor of 937/412 = 2.28.
Geothermal : The CAT scenario calls for ~24TWh/year of geothermal, which works out to an average of ~2,800MW. This was input as constant baseload generation.
Solar PV: Gridwatch gives no solar generation data, so I used the February 2013 data for France taken from the PF Bach data base (astronomical noon in France occurs on average only about 15 minutes beforeastronomical noon in UK). Solar generation in France in February 2013 averaged 381 MW while the CAT scenario calls for an average of 6,794MW, so the February 2013 data were scaled up by a factor of 6,794/381= 17.83.
Tidal Power: Tidal power causes difficulties because there are no large-scale tide power generation records to factor up. Eventually I settled on a spring-neap tide approximation based on a peak spring tide on February 12th. I had insufficient information to estimate semidiurnal variations, so the approximation implicitly assumes that these are canceled out by mixing output from tidal plants where tide times are three hours apart, as discussed in the Swansea Bay post. Also as discussed in the same post, however, they almost certainly won’t be, so this gives tidal power a break in that it makes it look a lot easier to match to demand than it really is.
Figure 4 shows generation from the five energy sources listed above during February 2050. Generation is dominated by wind with relatively minor contributions from solar and tidal. Geothermal and hydro barely lift off the X-axis:
Figure 4: Generation by source, February 2050 simulated hourly data
Figure 5 sums generation from all five sources and compares it with demand. Note that generation and demand are the same at 58TWh for the month:
Figure 5: Total generation versus demand, February 2050 simulated hourly data
And Figure 6 plots generation surpluses and deficits relative to demand:
Figure 6: Surpluses and deficits, calculated as generation minus demand, February 2050 simulated hourly data
FIgure 6 doesn’t look good from the standpoint of matching generation to demand, but we haven’t yet allowed for storage. My scenario uses DECC Level 4, the largest storage option, in which DECC assumes:
that by 2050 the UK has 20GW of storage, with a storage capacity of 400GWh, and 30GW of interconnectors. This level also assumes that around 75% of electric cars allow flexible charging for co-ordinated demand shifting.
I gave DECC’s storage option two breaks. First I assumed that output from storage isn’t limited to 20GW – it can be whatever it needs to be to follow demand. Second I assumed that all of the electric cars used for co-ordinated demand shifting will be available at the time they are needed and not marooned somewhere with dead batteries. Using the 400GWh of storage capacity to balance the surpluses and deficits shown on Figure 6 now gives the energy-in-storage plot shown in Figure 7. There is no energy left in storage for half the time:
Figure 7: Gigawatt-hours in storage, February 2050 simulated hourly data
After allowing for the load-following contributions that storage is able to make we are left with the generation deficits shown earlier in Figure 1, which is reproduced below for reference as Figure 8:
Figure 8: Generation deficits, February 2050 simulated hourly data
What happens to the generation surpluses? Those that can’t be fed into storage have to be curtailed, resulting in curtailments aggregating 9.8 TWh, or 17% of the total power generated in the month (Figure 9):
Figure 9: Surplus power curtailed, February 2050 simulated hourly data
But DECC still has one last string to its bow – its 30GW of assumed interconnector capacity. With a cold winter anticyclone covering a renewables-heavy Europe no one will of course have any power to spare, but we will nevertheless assume that the UK’s helpful neighbors somehow scrape together enough to export up to 30GW to UK whenever the UK needs it. Adding this imported power gives us Figure 10:
Figure 10: Generation deficits after allowing for 30GW imports, February 2050 simulated hourly data
And the lights still go out; just for not as long.
Time to sum up. Here is an example of how the DECC Pathways Calculator manufactures a pathway to a green, sustainable future that looks good on paper but won’t work in practice, and it’s far from being the only one. In fact no pathway that combines high levels of intermittent renewables generation with inadequate storage is going to work in practice, and because the DECC Calculator conveniently ignores this flaw in its logic it gives results that can only charitably be described as misleading.
Is there any way the DECC Calculator could be modified to give more objective results? Indeed there is. It could perform an analysis like the one I perform above for each scenario submitted, which is not beyond the capabilities of an Excel spreadsheet. It is, however, unlikely that it ever will, partly because the vast majority of renewables-heavy scenarios would flunk this test and partly because the 80%-emissions-reduction-by-2050 target enshrined in the Climate Change Act implicitly assumes that the UK can solve the problem of decarbonizing the electricity sector simply by throwing renewables at it, which of course it can’t.