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The sun doesn't shine brightly enough everywhere and is sometimes clouded. Winds don't blow always and are expected to become, overall, even weaker (that's what the IPCC report says anyway). Can energy be both sustainable and reliable? I heard good stories on tidal power, but this technology is unlikely to ever be scaled up and be used across the globe (solar panels are at least relatively cheap).

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    Hydropower? Geothermal? Biomass? Wind and solar may be getting all the publicity right now, but there are far more sources of renewable energy than just those two.
    – Mark
    Oct 13 at 1:08
  • Guess why some entrepeneurs thought about building a gigantic solar array in the Sahara to power half of Europe: Because there the sun shines almost every day.
    – Erik
    Oct 18 at 7:48
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If you want to include a means of energy storage within the system then yes.

Batteries can be used to store electrical energy for when the sun doesn't shine and when the wind doesn't blow, when the tide is not moving in tidal systems or when waves are flat in wave energy system.

Solar or wind powered pumps can be used to return water to hydroelectric dams.

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There are very many ways energy can be sustainable and reliable at the same time.

Firstly, you can observe the fact that there is no renewable energy. Most so-called "renewables" (a misnomer) actually use fusion energy from the sun, although some less common "renewables" like geothermal power or tidal power use other finite sources of energy like geothermal heat or gravitational energy in Earth-Moon system. The sun has a finite life, the Earth will cool if actively cooled and the orbit on the Moon will change if interfered with. You also can't find any other truly renewable power source because it's forbidden by the laws of thermodynamics. If we could find a terrestrial non-intermittent power source that lasts for 5 billion years then that would be enough. There are three such terrestrial power sources that could create enough energy for 5 billion years in the future: (1) fusion energy but that has always been at least 50 years away in the future, (2) uranium-238 breeder reactors, (3) thorium breeder reactors. Both (2) and (3) are realistic but even non-breeder fission reactors are quite expensive nowadays and presumably breeder reactors would be even more expensive than current nuclear reactors. Current nuclear reactors can't produce power 5 billion years into the future because they run on uranium-235 that makes only 0.7% of all uranium. Thus, due to very inefficient fuel use, they require lots of cheap fuel. Breeder reactors would require very little fuel that can be very expensive, and thus for example uranium extraction from seawater would become financially feasible.

Secondly, you can utilize energy sources that obtain their energy from the sun and come with included storage. The most important of these is hydropower. The main problem is that hydropower is limited by the size of hydropower reservoirs (mainly natural reservoirs in mountainous areas can be used because constructing artificial height differences would cost way too much) and the rain fall amount unless you use pumped hydro in which case only reservoir size limits you. Because mountainous areas are non-evenly distributed, it is unlikely this would be a truly global solution valid for even areas far away from hydropower reservoirs, because transferring electricity for large distances is expensive and has high lossees.

Thirdly, you can construct transfer of energy from orbit into earth. This is today too expensive but space launch costs are exponentially decreasing so might become feasible someday. An open question is how to beam this energy to earth in a non-dangerous manner. The energy density of the beams would be very high. This solution is probably very far away in the future, possibly even further away than fusion power.

Fourthly, you can transfer energy from sunny/windy areas to areas where there is no sun and no wind. Unfortunately, transferring electricity is expensive and has high losses. Perhaps superconductors might change the situation some day, but it's uncertain if they ever become working in common temperatures and cheap enough for bulk energy transfer. There is always sunshine and wind somewhere on the planet.

Fifthly, you can use thermal storage with concentrating solar-thermal power which is cheap. Unfortunately, it is feasible only in areas where solar power is not seasonal and is being outcompeted by batteries and solar photovoltaics. The thermal storage isn't good enough for seasonal energy storage. Because of competition from photovoltaics and batteries, this solution is doomed.

Sixthly, you can use batteries. Areas that receive lots of sunshine every day and have no real winter can even use lithium-ion, but new forms of batteries such as iron-air batteries developed by Form Energy and funded by Bill Gates are rapidly appearing. Still, it is unlikely they would become cheap enough for seasonal solar energy storage for areas that have real winter. The new battery chemistries could perhaps be used with wind power where calm periods usually last for weeks at the most. Wind power and iron-air batteries could allow a global energy solution.

Seventhly, you can produce green hydrogen. Producing it with electrolysis has 75% efficiency and converting it back to electricity in fuel cells or combined cycle power plants has 50% efficiency. So 37.5% round-trip efficiency. Yet, if solar and/or wind power become very cheap the 37.5% round-trip efficiency might still be enough to outcompete for example natural gas.

Eighthly, biomass can be burned in limited amounts to energy. One possibility is old-fashioned power plants that use it like coal. Unfortunately, varying the output of these steam power plants is very inflexible and thus can't be realistically used for balancing intermittent renewables. However, there are new technologies such as integrated gasification combined cycle (IGCC) that could allow these biomass power plants to work like natural gas peaker plants, allowing both a high efficiency and also having very good adjustment properties.

Also, don't underestimate demand-side management. Electric cars are a flexible demand that can shift charging in the future. A typical home doesn't need to be continuously heated or air-conditioned, as a typical home has about a day worth of thermal energy storage. You can build larger water-based thermal energy reservoirs in district heating networks allowing for weeks or even months of thermal energy storage. If some processes such as steelmaking or fertilizer production shift to using green hydrogen, deciding when you create it allows demand-side management. Putting a time-dependent variable price on electricity solves most of the supply/demand mismatch issue.

Besides, for the next 100 years we won't need sustainable energy. We only need net-zero emission energy. For example, capturing all carbon dioxide emissions from natural gas power plants and pumping them back under the ground would solve the CO2 emission problem. One cubic meter of natural gas burned produces one cubic meter of carbon dioxide. So if the natural gas was stored under the ground, there will be enough space to store the produced carbon dioxide. This reduces environmental risks, as natural gas (methane) is a far more potent greenhouse gas than carbon dioxide. So if the gas is released, it's far better if the gas is in carbon dioxide form than in the methane form.

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    Not all renewables use solar fusion. Tidal power uses the orbital potential energy of the Moon instead, while geothermal uses the Earth's internal radioactivity.
    – Mark
    Oct 14 at 20:07
  • That's true. I was mainly thinking about the "renewables" that are globally outcompeting fossil fuels today. Tidal power isn't cost-effective today and geothermal is limited to some rare sites where the internal heat of Earth is near the surface. Now I fixed the answer and mention alternative power sources.
    – juhist
    Oct 16 at 7:55
  • What exactly hydropower uses is a crazy mix of all of them :) Just to confuse the issue.
    – Móż
    Oct 17 at 23:54

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