“Renewable” includes wind, solar, hydroelectric, geothermal, tidal and biomass. Low carbon includes all these plus nuclear, which is not technically renewable because it burns fuel. “Intermittent” sources are wind and solar, because we cannot control when the sun shines or when the wind blows. One of the biggest communication errors I encounter is confusing “renewable” with “intermittent”, so I want to keep these terms clear.There is no controversy over the fact that above 30% or so of wind and solar penetration the cost of new capacity goes up. This is because intermittent sources require overcapacity and grid storage to work at higher percentages. This is a geometric progression, with the inflection point being somewhere around 30%. This is not a limit or ceiling, it is an inflection point. The curve really starts to turn up at around 60%.The other big variable is grid storage. With sufficient grid storage, well distributed, we can have 100% wind and solar. We don’t currently have such grid storage, so again we are betting on a future capacity we don’t currently have. Grid storage also needs to be able to shift energy production to demand over hours, days, and even seasons (for solar).Again, the question is, what is the probability that a 100% renewable scheme (which would need to include about 80% wind and solar) will succeed, meaning that we have sufficient grid updates and grid storage? And if it does not succeed, what is our plan B? The default plan B is to burn fossil fuel, and that will cost us our climate goals. This is why I have found compelling the arguments of experts who say we need to maintain, at least, our nuclear capacity, and perhaps even expand it a bit, to reduce the probability that we will need to burn fossil fuels to make up for any shortfall in a 100% renewable scheme. This further means we need to develop nuclear capacity now, because it will be too late in 20 years.Pumped hydro has always been one of the best grid storage options, and today is responsible for 99% of grid storage power. The idea is to pump water from a lower reservoir to an upper reservoir when the grid has excess energy, and then to flow the water from the upper to the lower through turbines to generate electricity when needed. This has a round trip energy efficiency of about 80%, which is pretty good as grid storage goes. Only lithium-ion batteries are better, at around 92%.Attention, however, has shifted to closed-loop pumped hydro. These installations are not connected to a river, and cannot be used as a source of hydroelectric power. They solely serve the function of grid storage. They can be built wherever there are two reservoirs of water that are close enough and have a sufficient difference in altitude (called the head). If, for example, the reservoirs have a head of 300 meters and are 1km apart, that is a good location for a closed loop system. There are other geological details of importance. The two reservoirs are connected by tunnels or pipes, so it matters how much rock will need to be drilled.
A recent analysis of the global potential for closed-loop pumped hydro finds 616,000 potential sites around the world. They also conclude that we only need 0.1% of these sites to develop sufficient pumped hydro grid storage to support wind and solar energy. This means we can pick the best 0.1% of sites to develop. They also argue that prior analyses of the potential of pumped hydro did not include the potential for closed loop, non-river systems.
An analysis of potential sites in the US includes an interesting discussion. First, this also finds great untapped potential, 35 terawatt-hours of capacity. But they also point out that such calculations are extremely sensitive to technical assumptions about the suitability and cost of developing specific sites.
A closed-loop pumped hydropower
energy storage diagram
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