The cost of producing electricity from wind and solar is falling fast. It’s become the cheapest way to generate power, even in northern industrialized nations, as reported by Bloomberg. The reason we’re not immediately moving to a zero-carbon economy is not about our ability to produce electricity without carbon. The challenge is producing it close to the point of use and when it is needed. At certain times of day, a solar plant in the desert or a wind farm on an isolated coast can produce electricity at a fraction of the cost of fossil fuels. But a coal or gas power station can be built just outside a city and fire up just as people are turning their lights on. When the cost of long-distance transmission and energy storage is added, the cheap renewable energy can start to look very expense.
The only economical way of storing really large amounts of energy is pumped-storage hydroelectricity (PSH). Surplus electricity is stored as potential energy by pumping water from a low-lying reservoir up to another reservoir located much higher up, often in mountains. When electricity is needed, the water is allowed to flow back down to the lower reservoir, passing through a turbine that generates electricity. Round-trip energy efficiency is typically 70 to 80 percent. There is currently about 127GW of PSH, over 99 percent of the world’s bulk-storage capacity.
Although new battery technologies may one day provide economical grid energy storage, this technology remains speculative. Furthermore, there may be additional environmental issues battery manufacture and disposal on that scale required.
How Much Energy Can PSH Store?
Potential energy is simply the force due to gravity multiplied by the vertical distance through which the mass is moved:
E = m a h
Where E is the energy stored, m is the mass of water pumped to height h and a is the acceleration due to gravity. If the mass is given in kg, the acceleration as 9.81m/s2 and the height is in m, then the energy is given in j.
It’s interesting to note that the energy stored by pumping water to a height doesn’t always add up in a way that seems intuitive. For example, it might seem that this could be applied to store electricity locally, perhaps storing energy from a solar panel within a house. Let’s say the water is pumped to a height of 5m within the roof. A typical household uses on average 1kW of power. To power the house for 12 hours would require 12kWh, approximately 43,000,000 j. Dividing this by the distance of 5m and the acceleration due to gravity (9.81m/s2) gives a required mass of water of 880 tons. This is equivalent to a pretty big swimming pool. Clearly this is far too much weight to be supported by the structure of a house, let alone what would fit in an average roof.
Small PSH reservoirs have been built into the base of wind turbines that are able to store 17.5MWh in the base of each 3.4MW turbine, providing five hours of storage. When considered on an even larger scale, the numbers start to add up quickly. Taking things to the extreme, the energy required to supply the entire United States with electricity for five days is approximately 560TWh, or 2 x 1017 j. This energy could be stored by a 25m deep lake 30km by 30km, elevated by 1,000m. This doesn’t seem so big in relation to the entire U.S.
The Problem with Pumped
PSH power plants require very special sites. Elevation differences of 500m or more between the upper and lower reservoirs means they are usually located in mountains. The reservoirs themselves take up a considerable amount of land and usually require geological features to contain them. Filling these reservoirs requires an abundance of water. The high-pressure pipes may also require the support of local geology. Sites that combine all of these requirements are often in beautiful natural areas such as national parks. This can mean there are significant social and environmental costs to constructing large dams and reservoirs in these locations. Together, this makes siting new PSH plants a considerable challenge.
The Stored Energy at Sea Solution
The Stored Energy at Sea (StEnSea) system eliminates the need to locate PSH plants in mountains by placing concrete reservoirs on the seabed. No upper reservoir or pipe is required. A concrete cylinder with an integrated turbine acts as the lower reservoir. Energy is stored by pumping water out of this cylinder. The pressure of the sea water above creates the hydrostatic head to drive the turbine when water is allowed back into the cylinder. This approach removes the need for land and minimizes the required structures. It eliminates the danger of a dam collapse and improves storage efficiency since evaporation from the upper reservoir does not reduce the energy stored.
A 10m diameter demonstration unit has completed a four-week test at a depth of 100m. In its commercial form, it is planned to have an inner diameter of 30m and be anchored at a depth of 700m. This configuration would yield an 18.3MWh storage capacity and would have a power output of 5MW. Economic analysis has been carried out showing that, when located within an energy park containing over 100 units, the capital cost per unit would be approximately $7 million. The storage cost would be a few cents per kWh, making this a commercially viable system.
The units would need a sufficient depth of water close enough to population centers to avoid high costs and losses in transmission. It is envisaged that the storage units would be located close to offshore wind turbines, further reducing the need for transmission infrastructure. Suitable locations have been identified around Norway, Spain, U.S. and Japan. Considering these limited sites, a worldwide storage capacity of approximately 900GWh appears feasible. This represents approximately 20 minutes of the worlds average electricity use.
Pumped-storage on the seabed, particularly in combination with offshore wind, looks set to play a significant role in the move to a zero-carbon economy.