Overview

All energy storage systems return less energy than they receive – the ratio is called the round-trip efficiency – but it’s still worth storing energy when it’s plentiful in order to be able to provide some at times of scarce supply and/or high demand. The value of electricity is particularly high at the instant when demand suddenly surges, or supply from other sources suddenly drops – the ability to generate power at very short notice in these circumstances is especially valuable.

Pumped Hydro

Currently, one of the best methods of energy storage is pumped hydro. Water is pumped from a lower reservoir to a higher reservoir to store energy, and energy is retrieved by running the water back down from the higher reservoir through turbines to power generators.

While pumped storage is a very good method of energy storage, with very low ongoing costs, moderate capital costs, and a good round-trip efficiency, the availability of suitable sites puts a limit on the amount of energy that can be stored. With increasing use of intermittent renewable energy sources, we need more storage than will be possible with pumped hydro alone. (This is true for both the UK, and the world as a whole. There are a few countries, such as Norway, with more potential pumped hydro capacity than they need for their own purposes. They can trade with neighbours, but they can’t provide enough capacity for everyone!)

There are interesting possibilities for synergies with underwater compressed air, see below.

For more information, see Pumped Hydro.

Underwater Compressed Air

Energy can be stored in compressed air anywhere, but to store large quantities of energy in this way requires large air reservoirs, and large volumes of air at high pressure normally require extremely strong – and hence very thick and expensive – reservoirs.

By placing variable volume reservoirs deep under water, the requirement for extreme strength is removed. This is potentially another good method of energy storage, but again it depends on the availability of suitable sites. Worldwide, there are large numbers of suitable sites capable of storing huge amounts of energy – the UK has only a few, but these few might well be worth exploiting.

For more information, see Underwater Compressed Air.

Inverse Pumped Hydro

You can also pump water up from a “reservoir” at the bottom of a lake, the sea, or the ocean:

www.fst.com/corporate/magazine/renewable-energy/underwater-energy-storage/

At first glance this looks similar to underwater compressed air, but the spherical vessels need to be far stronger (and consequently more expensive) than the vessels for compressed air, to avoid being crushed by the water pressure outside them, and the energy stored is far less:

  pv  rather than  pv(1 + ln(p/a))

(see Underwater Compressed Air Maths). It’s possible that the round trip efficiency would be better, but to me the whole concept seems very much inferior.

Chemical Energy Storage

Batteries are the obvious example, but stored fuel is also stored chemical energy. That fuel might be primary fuel such as coal or natural gas being stored before its initial use in producing electricity, it might be synthetic fuel manufactured using electrical energy, or it might be agricultural or other combustible waste.

Batteries are very expensive in both capital and ongoing costs. Manufacturing synthetic fuel and then burning it in conventional power stations has a poor round-trip efficiency, but it has two major advantages: enormous storage capacity already exists for such fuel, and it can be used not only in existing power stations, but also in existing heating systems.

For more information, see Chemical Energy Storage.

Thermal Energy Storage

Energy can be stored as a temperature difference between two masses. (It’s not stored simply as heat: it’s the temperature difference that matters.) A system can work between a hot mass and the environment, between the environment and a cold mass, or between a hot mass and a cold mass. (The environment is neither very hot nor very cold, but it does have an enormous mass!)

The economics of such systems are currently not as good as pumped hydro, and probably never will be: the round-trip efficiency is lower, and the initial and ongoing costs are higher, but they can be built anywhere and there’s no practical limit to the possible size of the energy stores (other than what we’re willing to spend on them).

For more information, see Thermal Energy Storage.

Liquid Carbon Dioxide

Energy can be stored in liquid carbon dioxide – they claim 75% Round Trip Efficiency and half the price of lithium batteries. The obvious questions are about the size of the dome that stores the carbon dioxide at ambient temperature and pressure when the system is discharged, and the self-discharge rates due to heat leakage from the heat store.

Siting Energy Storage Facilities

Pumped hydro systems can only be sited where suitable reservoirs exist or can be made; underwater compressed air systems can only be sited where there is deep water. One of the costs that has to be considered in these cases is the cost of connecting the systems to the grid. In some cases, despite the remote locations, suitable connections already exist. For example, Loch Ness has considerable potential as a site for an underwater compressed air system. A 305MW connection already exists to the Foyers Falls pumped hydro system; this connection is not in continuous use (the system can only store 21 hours worth of 305MW). The new system could extend the period over which 305MW could be delivered (or stored) without any upgrade to the connection. (Actually, it would probably be worthwhile to upgrade the connection: the fact the connection already exists indicates that the cost was not exorbitant.)

Siting Thermal, Chemical, or Liquid CO2 Energy Storage Systems

These can be sited anywhere. One obvious option for large systems is to site them on former nuclear or fossil fuel power station sites: these sites already have suitable connections to the grid, and are already industrial sites, so no new ground is being broken. There may well also be a technologically savvy workforce in need of suitable employment in the locality.

Another option (probably for somewhat smaller systems in general) is to site them close to large conurbations or large industrial consumers (for load balancing purposes, to reduce the stress on their grid connections by spreading the load peaks out – see Not Melting the Grid), or close to large wind farms or solar farms (for supply balancing). These smaller, local systems would generally be mainly for balancing daily variations in local supply or demand, while the much larger systems on former power station sites would be for balancing longer-term variations in national supply and demand.

The local systems would need energy capacities in the megawatt hours, and power ratings in the megawatts; the larger systems would need energy capacities in the gigawatt days, and power ratings in the gigawatts. Since they’ll be charging and discharging frequently, the local systems should have high round-trip efficiencies; the larger systems, charging and discharging at intervals of weeks rather than hours, could sacrifice some round-trip efficiency in favour of high capacity.

Recommendations

  1. Pumped hydro should be developed wherever it is feasible and environmentally acceptable.
  2. The feasibility and cost of developing underwater compressed air energy storage should be investigated as a matter of urgency.
  3. Obsolete nuclear power stations and some fossil fuel power stations should be replaced with thermal energy storage facilities. This is almost certainly more expensive and less efficient than (1) and possibly than (2), but those are both limited by the availability of suitable sites.
  4. Consideration should be given to developing facilities for the production of synthetic fuel using energy from renewable sources.

Renewable Energy Association 2015 report on Energy Storage

Energy Storage in the UK – An Overview