The production of renewable energy is to a large extent variable and difficult to control. In contrast to fossil energy, renewable energy must therefore be harvested when it is available.

The production of renewable energy is to a large extent variable and difficult to control. In contrast to fossil energy, renewable energy must therefore be harvested when it is available, and to maintain a balance between production and consumption, some sort of energy storage is needed or at least of value. This is true of energy used in the power sector, for transport fuels, and of the various thermal sources used to heat and cool buildings.

At present, the balance between consumption and production is relatively easy to maintain, since most of the energy we consume is stored in fossil fuels which can be readily used when needed. However, as the penetration of renewable energy increases, balancing production and consumption will become more challenging.

The challenges depend largely on the mix of renewable energy sources in the different sectors and the strength of the links between the sectors. For example, bioenergy from renewable sources can be transformed into liquid biofuels and biogas, so its consumption in, say, the transport sector does not pose any challenges different from those associated with fossil fuels. For that reason, we will not consider energy storage in the form of biomass and biofuels any further in this chapter.

Instead, we focus on energy storage technologies and their possible uses in a renewable energy system. The biggest challenges are in the electricity sector, where the real-time balance between production and consumption is closely linked to grid stability. However, energy storage technology can also facilitate higher levels of renewable energy in the heating, cooling and transport sectors.

This chapter contains sections covering thermal energy storage, electrical energy storage for stationary applications and energy storage for transport. The focus is on applications and technologies with the potential to facilitate the transition to a fossil-free energy system.

Thermal energy storage

Heating and cooling account for almost half of the total final energy demand in industrialised countries. Heat for buildings and industrial processes today comes primarily from electricity and the combustion of fossil fuels, either just for that purpose or in combined heat and power plants. Cooling is done by electric heat pumps, the electricity often coming originally from fossil fuels.

In a future renewable energy system, heating can come from a number of sources, including geothermal energy, the combustion of renewable fuels, from combined heat and power systems (including nuclear plants), electric heat pumps, solar thermal collectors and electric heaters. In future, cooling will mainly be provided by electric heat pumps.

Since heat demand does not necessarily follow power demand, the balance of production and consumption from combined heat and power plants is sometimes challenged. Furthermore, direct heat sources such as solar heating are typically less available when the heat is actually needed than when it is not. Cooling and heating demands are also not well correlated with renewable power production, and this challenges the use of fluctuating renewable sources to power the necessary heat pumps.

The challenges can be overcome effectively by adding heat and cold stores. These add valuable flexibility to energy systems, not only for heating and cooling but also in their links to electricity production.

Efficiency gains in heating could save about 3,000 PJ/yr by 2050 in the OECD Europe countries, representing 10% of total heat demand. This would be achieved through better energy management in buildings and industrial processes, but not simply through thicker insulation or improved design of industrial processes; about a third of the energy saving would depend on the appropriate use of heat storage.

Thermal energy storage technologies fall into two categories based on the physical processes involved: phase change materials (PCM) and sensible heat storage.

PCMs take advantage of latent heat – the large amounts of heat released or absorbed when materials change phase between solid and liquid, or liquid and vapour. The fact that PCMs absorb and release heat at a constant temperature is an advantage in heat storage applications.

PCMs can have energy densities to the order of 100 kWh/m3, and are commercially available with operating temperatures from –21°C up to 120°C. 

Heat stored or released by changing the temperature of a storage medium is known as sensible heat because the temperature change can be felt (latent heat, in contrast, is “hidden” because it involves no temperature change). The most common application of sensible heat storage is in district and domestic heating and is based on water, which is cheap and safe. Water also has a high heat capacity: The energy stored by heating a cubic metre of water from 20°C to 95°C is about 90 kWh.

Heat storage in large water tanks is commonly used in combined heat and power plants supplying district heating. An example is the system at the Avedøreværket coal-fired power plant in Copenhagen, which stores about 2.6 GWh. Water-based heat storage is also used with solar heat collectors, collecting heat during the summer and releasing it for domestic heating during the winter.

A characteristic of thermal energy storage is that larger systems are more efficient. This is because doubling the dimensions of the tank increases the heat storage capacity eightfold, but the area from which heat is lost increases only fourfold. Very large systems (like underground caverns or aquifers) are therefore a relatively efficient way of storing large amounts of energy (possibly for heating or cooling in urban areas) in the future energy system. Such large energy storage systems could function as energy buffers on a seasonal basis, allowing higher penetrations of fluctuating power sources like wind and solar power.

Electrical energy storage for stationary applications

Integrating more renewable, intermittent energy, like wind energy and solar power, into the electrical grid brings two challenges which could in principle be addressed through electricity storage. Both relate to the fluctuating and unpredictable nature of renewable power sources, but on different timescales.

Fluctuations in renewable power production require the remaining generating units to be very flexible. The first challenge is therefore what to do if fluctuating renewable power sources are to completely replace fossil-fuelled power plants; without electricity storage, security of supply will be compromised when renewable power is not available, for instance at times with no wind. Large electricity storage systems can help by absorbing excess renewable energy during hours or days of high production and low consumption, and then releasing it when production is low and demand is high.

The second challenge is that the variable nature of renewable energy generation makes it more difficult to plan power production, and this in turn increases the need for short-term regulation and reserves. This issue can be overcome by adding electricity storage systems that can provide both up and down-regulation as well as reserves at short notice, sometimes down to below one second.

One interesting technology for short-term regulation is mechanical flywheels. These have made considerable technical progress in recent years thanks to companies such as Beacon Power in the USA. Working at timescales of seconds to minutes (between inertial reserve and spinning reserve), they combine high regulating effectiveness with almost instantaneous response.

Many other stationary electricity storage technologies have been developed, and several systems are operating around the world, although not in widespread use. In addition to flywheels, other well-known technologies are stationary batteries (typically used for fast response), compressed air energy storage (CAES) and pumped hydro, both of which are suitable for spot market arbitrage.

It is the opinion of the authors that storage systems are likely to increase their market in the near future, while new technologies currently being developed will reach commercial maturity in the next few decades. An example of a system which might be seen by 2050 is the “energy island”. The underlying technology is simply pumped hydro, but unlike pumped hydropower, the energy island does not require mountains – though there are other problems to be overcome. It illustrates the kind of vision we need to overcome the challenges posed by the widespread use of sustainable energy.

Energy storage for transport

The transport sector accounts for approximately 20% of the world’s total energy demand (30% in many developed countries like Denmark) and is powered almost exclusively by fossil fuels. There is little doubt that this picture will change as we move towards 2050: Renewable sources will have to take over for environmental reasons and due to a decline in fossil fuel resources.

The energy currently used for transport is mostly stored on board vehicles in tanks containing liquid fossil fuels, exceptions being electric trains and buses supplied directly from the grid. A shift to sustainable energy in the form of electricity (i.e. bioenergy disregarded) will require mobile technologies which can store electricity from wind power and solar sources in concentrated form, guaranteeing driving ranges similar to those of gasoline and diesel vehicles. Range is especially important for drivers of private cars, who appreciate the freedom their vehicles provide and are likely to demand the same capability from future electric vehicles.

Batteries may at a first glance seem the obvious way of storing electrical energy for transport. Unfortunately, the driving ranges guaranteed by even advanced batteries fall far below those possible with fossil fuels. The fundamental problem is that the energy density of batteries is almost two orders of magnitude lower than that of fossil fuels.

This means that about 1.5-2 tonnes of batteries are required to provide the same driving range as a tank holding 50 kg of gasoline, even taking the different conversion efficiencies into account.

Since such a great weight of batteries is clearly not viable, the idea of combining batteries with other fuel systems (sometimes called range extenders) is attracting interest among car manufacturers. The parallel fuel system could be hydrogen or another synthetic fuel made using wind or solar power. At some point in the future, it may even become economical to use synthetic hydrocarbons made from hydrogen and carbon dioxide extracted from the atmosphere. The technologies required to do this are complicated but well-known.

It is worth noting that consumers may value the convenience of liquid fuels over future energy storage methods that are more direct and more efficient, even if they end up paying more as a result. Today’s liquid-fuelled vehicles have driving ranges of around 1,000 kilometres. The distribution system for liquid fuels is already in place, so new synthetic liquid fuels would not need huge investments in infrastructure.

Engines for liquid fuels are highly developed and affordable, so there may be no need to rush new traction systems such as fuel cells and batteries to market before they are fully developed. Finally, synthetic liquid fuels are easily blended with biofuels.

Nevertheless, batteries will undoubtedly see dramatic changes in the coming decades. The trend towards new battery types with higher power outputs and greater energy densities is already noticeable. By 2050, rechargeable lithium-air batteries with properties much better than those of current batteries are likely to be on the market, since intense development is going on in many countries.

We anticipate that electricity storage for transport applications will become embedded in future electricity grids and markets. Depending on the market, producing synthetic transport fuels and charging transport batteries may become the preferred use for surplus electricity. In this way, transport-related energy storage may come to play the same balancing role in the power system as we discussed above under electrical energy storage for stationary applications.

The logical conclusion is to devolve control for exchanging energy with vehicle batteries or generating synthetic fuels to the companies which control the transmission grid, and are therefore responsible for balancing electricity supply against demand. This fits in with the concept of a future intelligent energy system, where the supply and demand of electricity are both controlled and optimised according to needs as well as prices.


The vision of fossil-free energy by 2050 is not unrealistic, provided we are determined to make it happen and perhaps willing to pay a little extra, at least for a while. Fossil fuel resources are certain to run out eventually, and before this happens we can expect fossil energy prices to increase dramatically. As fossil energy becomes more expensive, sustainable energy will become competitive.

A fossil-free future will require energy storage, but to what extent is difficult to judge. So far, electricity storage has received much R&D attention, probably because it has the most obvious, straightforward and urgent role to play in the energy market.

However, heat storage has considerable technical and economical potential. Unfortunately, heat and cold storage is currently not very efficient, especially over long storage periods, because it depends on thermal insulation. In future, huge underground thermal storage reservoirs may be used for seasonal storage of heat and cold wherever there is an appropriate balance of local climate and geology. If these reservoirs are large enough, their energy losses could be relatively small.

The technical potential for energy storage is enormous, but unfortunately the costs are often considerable and some times prohibitive. New ideas and technologies which could lower energy storage costs should therefore be encouraged.

Some emerging technologies could also reduce the need for storage. Examples are smart management of electricity demand and transmission of electricity over very long distances, perhaps using superconductors, so that we could move electricity economically from a region with surplus wind power, for instance, to one where the wind is not blowing. However, this would not solve the problem of mobile energy storage for transport.

We have no doubt that many types of electricity storage will be important in the coming decades as components of the future integrated and sustainable energy system.

Allan Schrøder Pedersen, Risø DTU, Denmark; Robert Remick, NREL, USA; Claus Krog Ekman, Danish Energy Agency, Denmark.