Green Electricity When The Sun's Not Shining

It is sometimes claimed that it's not possible for humans to depend entirely on renewable energy, because it is inherently unreliable and therefore it will always be necessary to have conventional power stations as backup. But this turns out not to be the case.

Photovoltaic cells ("solar cells") generate electricity from sunshine. Naturally, they work best when the sun is shining, but they also work reasonably well under a bright but cloudy sky. So what about night-time, or winter days with dull grey skies?

Conveniently, winds tend to be stronger during winter and we can make use of wind generators. They're a bit more fickle than solar cells because the wind needs to be not too strong, not too weak, and not too gusty. Windy countries like the United Kingdom and the Netherlands have a high capacity to generate electricity from the wind. But what about when the sun's not shining and the wind's not blowing?


It's quite practical to generate electricity from the tidal movement of the oceans, although it can't be done everywhere because it works best where there is an estuary or bay of the right shape to concentrate the tidal currents. Tidal power is not continuously available, but has the advantage of being reliably predictable. The maximum output is generated at the times midway between high tide and low tide, when the water level is changing at the fastest rate. But what about when the sun's not shining and the wind's not blowing, and the tide's not flowing?

Hydro-electricity generates power from the flow of water. A river flows into a mountain lake or reservoir, then a pipe takes the water down the hill to a power station at the bottom where it turns the turbines of the generator with great force. Hydro power is readily controllable - it can be turned on or off at any time just by adjusting the flow of the water. It's ideal for generating power when the sun's not shining and the wind's not blowing and the tide is not flowing.

But it gets better than that. The generators at the hydro-electric power station can be run in reverse. Water can be pumped uphill, back into the reservoir, using surplus power generated when it's sunny and windy and the tides are flowing. The more water that's pumped back uphill, the more energy that is available for generation later when it's needed.

There are other ways to store energy, of course. Batteries may seem an obvious answer, yet they are not practical to store energy on an industrial scale due to their cost and environmental considerations. They may have some contribution to make though. For example, future electric cars could keep their charge topped up whenever there's plentiful power. When power is in shorter supply, the cars would be charged up as and when required by their owners, rather than being kept fully-charged at all times.

Other storage technologies are becoming practical. Compressed air can be pumped into underground caverns, then used later to drive generators. Flywheels with superconducting bearings can be used to store energy on a domestic scale.

It can be difficult to match power generation to power consumption because of peaks in demand. There is a daily peak (typically in the early evening) and an annual peak (usually during the winter heating season, but in hot climates it's during the summer air-conditioning season).

The daily peak can be managed through smart pricing - charging consumers less for energy that is used when it is plentiful. When the price difference is great enough, consumers will find ways to heat their water or run their washing machines away from the time of peak demand. Industry, too, will align its power use with the cheaper rates offered by smart pricing.

The seasonal peak can be managed through ground storage of heat. A heat pump is a very efficient way to heat or cool a house, particularly in conjunction with underfloor piping. The heat pump can be run in reverse during the off-season to increase the ground storage of heat (or coolness in hot climates). Due to the high insulating properties of the earth, most of this heat (or coolness) will be available for extraction six months later when it is needed. Ground storage heat pumps are relatively expensive to install, but have very low running costs and work very well in conjunction with renewable energy generation.

If it seems like a lot of trouble to mix-and-match the many different sources of renewable energy in order to achieve a reliable supply, it's worth remembering that it's also difficult to achieve a reliable supply with other technologies. If a country depends on gas-fired power stations, an interruption to the gas supply can have major consequences. If a country depends on nuclear power, the discovery of a design fault can result in the shutdown of all reactors sharing the same design. In each case, large quantities of backup generation capacity must still be available.

Therefore, the broad diversity of renewable energy sources makes a positive contribution to the robustness of the overall energy supply. This will only increase in the future as additional sources of green energy become practical. For example, current work on solar thermal towers is likely to yield practical benefits in the future.

It's certainly not the case that non-renewable sources of energy are inevitably required to "back up" sources of renewable energy.