In the May 12, 2015 “G7 Hamburg Initiative for Sustainable Energy Security”, the energy ministers of Canada, France, Germany, Italy, Japan, the United Kingdom and the United States, plus the European Commissioner for Climate Action and Energy, said this:
An increasing number of countries are following the path of a rapid expansion of renewable energy. There (are) a number of challenges as energy systems change and related greenhouse gas emissions are reduced, one of which is how to integrate growing shares of variable renewable energy into electricity systems.
The G7 energy ministers are correct in their assessment. Integrating growing shares of variable renewable energy into electricity systems is indeed a challenge – and so far one without a good solution.
A few quick facts before proceeding. In 2013 renewables supplied the world with 21.7% of its electricity, according to BP. Take out hydro and they supplied the world with only 5.3% of its electricity. Then take out “other” renewables such as biomass and geothermal and the percentage falls to 3.3%.
Why take out hydro and “others”? Because their growth potential is limited by resource availability – too few good hydro sites, too few high-temperature geothermal fields, not enough wood to make biomass pellets etc. – and for these reasons they may never make a significant contribution to future global energy needs. Their growth performance since 1997, the year the Kyoto Protocol set the renewables bandwagon rolling, has certainly been less than impressive, as illustrated in Figure 1. “Others” have gained market share, but at a painfully slow rate, and hydro has actually lost ground:
Figure 1: Percentage of world electricity generation contributed by different renewable sources, 1997-2014 (data from BP)
Not so, however, for wind and solar, which aren’t resource-limited (the amount of solar energy hitting the earth in a year, for example, vastly exceeds annual global energy consumption). They show rapid growth since 1997, although from small beginnings. Clearly they are the energy sources the world must concentrate on developing if it is ever to “go green”.
And why shouldn’t continued rapid growth in wind and solar allow the world to go green? I’ve discussed the reasons piecemeal before. Here I summarize them all in the same post:
Intermittency, or non-dispatchability, is the Achilles heel of wind and solar. So far it hasn’t caused widespread problems because wind and solar still contribute only a small fraction of total power generation in most countries. Integrating wind power into the UK grid in February 2013, for example, was not difficult because wind only supplied 5% of the UK’s electricity in that month:
Figure 2: UK electricity demand and wind generation, February 2013 (data from Gridwatch)
But if in February 2013 the UK had had enough installed wind capacity to generate 50% of its electricity from wind Figure 2 would have looked like this:
Figure 3: UK electricity demand and wind generation with wind supplying 50% of demand, February 2013
Now it’s a different ball game. How do we match a generation curve like that to demand, or at least smooth it out to the point where it becomes manageable? There is in fact a way of doing it, but we’ll get to it later. First we will discuss the options that won’t work.
This is the obvious solution; store intermittent renewable energy during periods of surplus generation and release it during deficit periods. But the only existing technology that can do this at the scale necessary is pumped hydro, and as discussed at length in previous posts here, here and here the amount of pumped hydro storage needed is enormous. At only moderate levels of solar & wind penetration the UK would need several terawatt-hours of storage, maybe as much as a hundred times the capacity of its existing pumped hydro plants, while Europe and the US would need tens of TWh each and the world proportionately more. There is no realistic prospect of bringing this much new pumped hydro – or even conventional hydro, which can also function in an energy-storage mode – into service in the foreseeable future even if enough suitable hydro sites could be found.
The alternative is battery (or flywheel, or compressed air, or thermal) storage. These technologies are so far from deployment on the multi-terawatt-hour scale that they can be discounted. (According to Wikipedia total world battery + CAES + flywheel + thermal storage capacity still amounts to only about 12GWh, enough to fill global electricity demand for all of fifteen seconds.)
Another option that’s been mooted as a potential solution to the storage problem is electric vehicle batteries, which can be charged from the grid during periods of generation surplus and discharged back into the grid during periods of deficit. But this option also founders on the rock of scale. Assuming a 100% charge/discharge capability and no energy losses during the charge/discharge process we would still need 12 million 85kWh Teslas (or 42 million 24kWh Nissan Leafs) to get a single terawatt-hour of storage.
It’s frequently assumed that a smart grid covering a large enough area, like the proposed European supergrid, will be able to smooth out local spikes and troughs in renewables generation and provide “reliable electricity” to all. Unfortunately it won’t. Figure 4, reproduced from Wind Blowing Nowhere compares 2013 wind generation in Spain, the largest producer, with combined wind generation in Belgium, the Czech Republic, Denmark, Finland, France, Ireland, Germany, Spain and the UK. Combining wind generation from all nine countries doesn’t flatten out the Spanish spikes or fill in the Spanish troughs. It just moves them around:
Figure 4 : 2013 wind generation in Spain versus combined wind generation in Spain and eight other countries (data normalized)
What about solar? Seasonal and diurnal variations in solar generation can be smoothed out by combining output from different areas, but the European supergrid would have to link up with New Zealand to do it.
Combining Generation from Different Renewable Sources
It’s also been claimed that because the wind and the sun blow and shine at different times we will get smoother power output when we combine them. That doesn’t work either. Figure 5 re-plots the Figure 2 case with the UK getting 40% of its electricity from wind and 10% from solar instead of 50% from wind. Adding the midday solar spikes, which lead evening peak demand by about five hours in the winter, if anything makes things worse:
Figure 5: UK electricity demand and wind generation with wind supplying 40% and solar 10% of demand, February 2013
A lot of faith is pinned on the potential of DSM, which instead of matching generation to demand seeks to match demand to generation, or at least to match it as closely as possible. But there’s no way demand could be matched to the generation curves shown in Figures 3 or 5. The best that could be hoped for is an incremental improvement, maybe a flattening of the daily demand curve and/or a reduction in total demand, but the larger problem of how to smooth out bursts of intermittent power into a manageable form would remain unresolved.
And then there’s the great unexploited renewable resource:
It’s predictable, infinitely renewable and has near-unlimited potential. What’s not to like about it? As discussed in the Swansea Bay post (link above), quite a lot. Arguably the best indicator of tide power’s lack of potential, however, is that almost fifty years after the world’s first tide power plant went in at La Rance in France it still supplies less than 0.005% of the world’s electricity.
So if energy storage, supergrids, combining output from different sources, demand-side management and tide power won’t work, what will? Only one thing:
Fossil Fuel Backup
The concept is simple: use load-following fossil fuel capacity – I’m going to assume gas turbines – to generate the electricity needed to meet demand whenever renewable energy can’t generate enough. The approach requires no storage and imposes no theoretical limits on the level of wind & solar penetration, as discussed in How much windpower can the UK grid handle and Wind power and the island of Denmark. Figure 6 illustrates how it would apply to the 50% wind penetration case shown in Figure 2:
Figure 6: Combined wind and backup gas generation matched to UK demand, February 2013
Inevitably, however, there are problems. One is that there are times when wind generation exceeds demand and has to be curtailed, and as a result the UK gets only about 47% of its electricity from wind instead of 50% in the above case. Another is the generation curve the gas turbines would have to follow to fill demand when wind generation can’t, which looks like this:
Figure 7: Generation curve gas turbine generation must achieve to balance UK wind generation fluctuations, February 2013
Tracking this erratic generation curve would severely stress the gas turbines (and probably the grid operators too). Wear, tear, downtime and generation costs would all increase, as would fuel consumption because of the constant start-up and shutdown, thereby offsetting some of the CO2 emissions reductions generated by the wind energy.
And that’s with 47% wind penetration. At higher levels the system becomes progressively more inefficient until at 80-90% penetration it’s running at load factors as low as 10% and well over half of the wind generation has to be curtailed (more details in the tables in the How much windpower post linked to above). We can therefore anticipate that this approach will also eventually run up against the hard wall of reality, if only because sooner or later it will occur to someone that it would be a lot easier to keep the dispatchable gas generation and do away with the non-dispatchable wind generation altogether.
But the way things are going there’s a good chance that this point will never be reached. Why? Because of a problem that’s rarely taken into consideration:
Lack of Investment
Every year UNEP publishes a chart of annual global investment in renewable energy, the lion’s share of which (92% in 2014) goes to wind and solar. Here’s the latest version:
Figure 8: Global investment in renewable energy, $ billion, reproduced from UNEP
Total investment in renewables since 2004 now exceeds $2 trillion – a lot of money, but it’s still far short of what’s needed to stimulate growth to the point where renewable energy, assuming it can be made to work, eventually powers the world. The $232 billion invested in renewables in 2013 was dwarfed by the $1.6 trillion total global energy investment in that year reported by IEA, and of the 235GW of new generation capacity installed globally in 2012 only 76GW was wind or solar, according to EIA and BP. If investments in conventional generation continue to dominate to this extent then wind and solar are doomed to remain also-rans. A very substantial transfer of investment from conventional generation to wind and solar will be needed if they are ever to become the dominant players, but the investment climate needed to achieve this just isn’t there.
Another question is whether global renewables investment might not already have peaked (as shown in Figure 8, it’s certainly flattened out). Renewables investment is still increasing in the developing countries – notably China – but it’s been essentially flat in the US since 2008 and in Europe it’s been declining since 2011. Europe in particular bears watching because if the decline continues at the rate shown in the Bloomberg New Energy Finance chart below it won’t be long before Europe will have had all the clean energy it’s going to get:
Figure 9: Investment in clean energy in Europe (graphic from Bloomberg, first quarter 2015 data from Reuters added by RA)
And finally the big problem. Even if the world succeeds in developing wind and solar to the point where they supply 100% of its electricity the job is still less than half-done because electricity supplies the world with only about 40% of its energy. The remaining ~60% comes from the oil, gas and coal consumed in transportation, heating etc. How to decarbonize that? Again no solution is presently in sight.