When the Winds Don't Blow

by Dave Elliott

New Ground 62
Winter 2001

Renewable energy sources like wind, wave and solar power are very nice, but not reliable. What happens when the winds don’t blow and the sun doesn’t shine? We have all heard this argument. Do we have an answer? Yes, we do. Dave Elliott casts some light on the debate.

We tend to think the existing UK power system is very reliable. It has been so far, despite occasional blackouts that are usually caused by extreme weather. But this level of security has cost us a lot; we have to have a significant excess generating capacity to meet unexpected peaks in demand. This excess capacity has been about 20% for some while. Relying on large centralised power stations linked via, the national grid, to loads scattered around the UK can lead to significant losses from the heating of long-distance power cables. Maybe as much as 10% of power is wasted in this way. A system with smaller, local power stations, meeting local needs directly, can be more efficient. In this system, only excess power is exported on the grid and top-up power is imported when needed. The buzz word is that such plants are embedded in the local area power system. Many renewables sources are best tapped using small, local power plants, and are available in areas where the national grid is weak, at the far end of the power lines. So they provide well matched power – which ought to be seen as of high value.

But isn’t this power intermittent? Well, yes. On average, winds only blow at reasonable speeds around 30% of the time in any one place – more in some, less in others. But they blow strongest and most reliably in the winter, so there is good general seasonal matching to demand. Wind speed prediction techniques have recently improved, making it easier to plan ahead. Wind is even better matched than you might think. As we feel colder when it is windy (the wind chill effect), the conventional power system has to have capacity to meet extra requirements which can be equivalent to the output of a nuclear power plant. Wind farms can meet this demand automatically as their wind-driven output just about matches the extra wind chill load.

Then there is wave power. Waves are really just stored wind, available, predictably, for hours or even days after the wind stops blowing over the seas. They are even better matched to the general pattern of seasonal demand. Tidal power of course operates on a lunar cycle, which means it is often available at times when there is no wind and no waves.

Solar energy is the worst case – it is only available in the day and for a few months of each year is only weakly available, if at all. Even so, the UK gets about 50% as much solar energy as tropical or desert countries, and you can harvest useful heat using solar collectors, even in winter. Indeed, the clear winter days make solar popular in northern latitudes, where the heating season is longer and the heat more valuable. In some northern countries, it has proved worthwhile to store summer solar heat until winter in insulated, rock-filled heat stores underground.

So is energy storage the answer to the variability of renewables? Yes and no. It turns out that if we have lots of windfarms and lots of wave and tidal projects, all scattered around the country and linked via the grid, then the variation in their output due to weather cycles does not matter much as long as their total contribution to UK power generation is less than about 20% of the power generated from conventional plants. In effect, the grid averages out local variations, which at that 20% level are small compared with normal variations in supply and demand. The grid system already deals with large variations in demand, so it is not difficult for it to deal with variations in supply, as long as the two do not overlap. With wind, wave and tidal energy all working in different ways, the chance of this happening is low. And if you also take into account the efficiency gains and better matching of demand and supply from a decentralised system, coupled with lowered demand as consequence of conservation measures and more efficient energy-using devices, then you can push the variable renewables figure up to maybe 30%-40% of conventional generation – without any need for energy storage.

We have been talking here about variable, intermittent, renewables: wind wave, solar and tidal. There are also renewable sources which are not variable. for example energy crops. If necessary, they can be used to fuel power plants to balance variations from other renewables sources, as well as contributing to base load, or continuous, electricity supply. Geothermal energy, although out of favour in the UK, might also be a non-variable option. So could hydropower. When we want to go beyond the 30%-40% limit on intermittent renewables, in a couple of decades time, then we will have to have energy storage facilities. There are short-term mechanical, pneumatic and hydraulic storage options such as compressed air, flywheels and so on, which can help buffer variations, and medium-term systems like pumped storage reservoirs feeding hydro plants, but for the longer term we will look to hydrogen as the key storage option. Intermittent outputs from wind, wave, tidal and solar devices could power the electrolytic splitting of water, generating hydrogen gas, which could be stored or transported, much as with conventional gas. We could be moving towards a hydrogen economy.

The message is that there are technical solutions to intermittency. The new green power system will be different; it will be more decentralised and more efficient. But there is no reason why it should be less reliable. Studies suggest that, as you might expect, a system based on a large number of small units using a diverse range of sources is likely to be more reliable than one based on a few large units using a few sources. As long as we plan and develop the system properly, we will not be freezing in the dark.