Can you get “green energy” when the sun’s not shining?

Solar storm (photo by NASA – PD)

It is sometimes claimed that it’s not possible for humans to depend entirely on renewable energy, because green energy 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 turbines. 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 windy and the sky is blue 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. And it has become quite practical to store heat in the form of molten salt, from which electricity can be generated on demand later.

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 quite 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.

An earlier version of this article was posted to Google Knol. The following comments were contributed to that version:

You’ve a done great work, since you are looking for ways to manage electricity with renewable energy. I know it’s hard to research about it and I really apreciate it.

I live in Brazil and here the electricity is, in most cases, generated by hidro-eletric power stations like Itaipu, Furnas, Aimorés and many others. Unfortunately, I have to afirm that hidro-electric power stations have a high cost to our environment, since it has to retain the water in huge reservoirs that reduces the flow the river and also expropriates many communities from their homelands. The reservoirs also affect the soil and the biodiversity of the surroundings.

The hydro-electric power station is a great way to generate energy, but it has a cost that cannot be forgotten.

I’m terrible sorry about my poor english. I’m still studing this language and I hope you can understand my arguments.

Once again, keep up the good work! — Thiago de Mattos

Thiago, Thanks for your comments. I do agree that renewable energy has an environmental cost associated with it (although hydro is lower than many competing energy sources). Ultimately, humans have three choices: reduce the population, consume less, or mess up the planet. Of those, I’d pick reducing the population as the way to go. — eiffel

What method would you use to reduce the population? — Nathan Blew

Nathan, population growth has already stopped in many developed countries. Other countries are likely to follow as levels of female education increase (it is established that children per female is lower for educated mothers). Some estimates predict that human population will peak at around 9 billion before it starts to decline slowly. — eiffel

We have many rivers just going to waste in terms of making power. I’m not talking dams. I mean a different kind of water wheel. If I had the money I would pursue different ways to make the rivers work for us. I can picture floating vanes with water just rolling under them and creating power 24 7. Don’t know if anyone has done this sort of thing. Someone here in Michigan is pushing wind generators in Lake Michigan which object to. — Duane Sugarbaker

Electricity generation from “water turning vanes as it flows past” is certainly possible, and occasionally it is done that way. There’s one such installation not far from me, but it’s very small (just a few kilowatts).

The amount of power that can be generated is much less than from a hydro station at a dam, but of course the visual and environmental impact is much less too. — eiffel

A guy called Zac asked about do-it-yourself wind turbines.

Zac, I don’t have any knowledge of the do-it-yourself wind turbines that you mention, but you need to bear in mind that any wind turbine installation will need some maintenance (lubrication, inspecting and repairing storm damage, etc). That’s one reason why wind power is more practical on a larger scale (although of course plenty of people do install their own turbine successfully).

Photovoltaic is very low-maintenance once it’s installed. — eiffel

The earth’s magma is an inexhaustible source of energy.

The more we get to under the surface of earth, it becomes warmer, the core of earth is molten rock. But even a little deep the heat can be usable. Volatile Fluids can be used to run engines and small turbines. — Anantha Narayan

Biogas is also good source of green power source which can used for generate electricity. We can generate bio gas from many kind of waste. This gives us solution to power crisis and environmental issues due to waste. — Dilantha Subasinghe

Nice work. You’re sounding a bit like T. Boone Pickens (no relation to Slim Pickens, of Dr. Strangelove fame), who’s making quite an alternative energy splash these days on this side of the Atlantic — David Sarokin

Whenever people talk about pumped storage of electrcity, they are generally thinking of projects like Dinorwic in North Wales where an upland reservoir is combined with a pumping/generation plant generally located a long way away from the centres of electricity demand. in UK.

There is an alternative resource which is already part of our national infrastructure which is the municipal water supply network, we have pumps, pipe networks, and storage reservoirs close to communities. It isn’t necessary to generate power from the stored electricity, just turn the pumping off creating less demand on the local electricty network when electricity is scarce and switch the pumps on when generation exceeds demand. This kind of strategy could be combined with a higher nuclear base-load and the peaks and troughs provided by renewables to create a different energy balance which generates less carbon.

I havent done the sums yet in terms of overall efficiency, but the UK water industry uses 3% of the total demand for water pumping, a significant element in the overall energy mix. — Laurie Reynolds

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