By Claus Krog Ekman, Allan Schr�der Pedersen, Tom Cronin, Ris� DTU; Brian Elmegaard, DTU Mechanical Engineering.
Energy storage
Energy can be stored in six different forms: chemical, thermal, kinetic, potential, electromagnetic and nuclear. All the energy we consume has, until the time of use, occupied one or another of these forms. One example is electromagnetic energy radiated from the sun which is converted and stored as chemical energy in biomass and later burned to yield thermal energy and possibly converted into electricity which is electromagnetic.
This chapter considers a narrower notion of energy storage: artificial energy storage mechanisms that can facilitate a higher proportion of renewable energy in future energy systems. For our purposes, biomass and biofuels are considered to be harvested energy stores rather than deliberate storage mechanisms, so they are not included here.
Since both energy consumption and renewable energy supplies fluctuate over time, energy management is needed to ensure that energy supply balances demand at all times – this is true for heat, electricity, and energy for transport. Energy storage can facilitate this management and has the technical potential to be a key system component as the proportion of renewable energy increases. Transport applications also require high-density mobile energy storage independently of the balancing issue.
Here we describe various storage technologies for thermal energy, grid power and transport.
Thermal energy storage technologies
The possible uses of stored thermal energy depend a lot on the temperature level of the storage. The lower the temperature, the lower the quality of the heat and the more restricted its applications. However, low-temperature heat is abundantly available as waste heat from many processes. In addition, solar radiation may be collected as heat much more efficiently than by converting it to electricity. As a result, heat storage is of great interest.
Compared to energy storage technologies such as liquid fuels and electric batteries, heat storage materials have low energy densities and long response times (typically minutes), mainly because their rate of energy transfer is limited by slow heat transport mechanisms. In addition, the economics of insulation dictate that the round-trip efficiency of heat storage depends not only on the quality of the thermal insulation but also on the length of time for which the heat needs to be stored. Despite these rather poor properties of heat storage technologies, the topic is important because heat is necessary for comfort in a large part of the world.
Sensible heat storage relies on the capacity of matter to absorb heat when its temperature is raised. This principle has a long history of use in domestic hot water tanks and district heating systems, but has caught new interest thanks to research into new materials and other technological developments.
An example from the US is the use of concentrated solar energy to heat liquids to temperatures high enough to generate steam to drive turbines, which then generate electricity. A number of commercial plants are operating with multi-megawatt capacities and capacity factors up to 20% [1]. On the downside, the solar concentrators take up significant areas of land and are only economic in sunbelt regions. There are many types of designs and research is focusing on the development of the heat transfer to energy-storing fluids.
Smaller-scale sensible heat storage is also attractive to households as a means to increase the efficiency and effectiveness of small solar power installations. These systems are in full commercial production, and so benefit from the associated cost reduction.
Phase change materials (PCMs) store and release energy in the form of latent heat as they change from one phase to another. Typical systems store heat by melting a solid material (latent heat of fusion), and recovering the energy by allowing the liquid to solidify.
Compared to sensible heat storage, PCMs have the advantage that – in principle –heat is stored and released at a constant temperature. Conventionally, PCMs have been used to absorb unwanted heat in order to prevent a temperature rise, but attention is increasingly turning to storing heat for later use.
At present, PCM systems have relatively small capacities and are tailored for specific applications, but many new materials and combinations have been developed, and new applications considered. For households and larger entities in the future, PCMs may provide an attractive way to store heat. New ways of integrating PCMs in building materials, for instance, may extend the applications of solar heating [2].
Electrical energy storage technologies
Balancing electricity supply and demand becomes increasingly difficult with the introduction of more renewable (fluctuating) electricity sources. There are challenges on both long time scales (hours, days or longer), where both electricity demand and renewable energy supply fluctuate independently (energy management), and on short time scales (minutes to hours) where uncertainty in the prediction of renewable energy supplies leads to imbalance (power regulation and reserve).
Large-scale electricity storage would be able to shift demand and supply, helping to provide balance over all time scales, and may therefore play an important role in the future power system.
Pumped hydro
Pumped hydro has been in commercial service as an energy storage technique for a long time in many countries where the topography is suitable. However, new techniques such as underground water storage [3] are opening up the possibility of pumped hydro in areas without mountains, like Denmark. Pumped storage can have high efficiencies – up to 75% round-trip on average [4] – with start-up times of a few seconds. Pumped hydro power stations may have power capacities of up to several hundred MW, with the amount of energy storage naturally depending on the volume of water and the height difference. Installations can be designed together with conventional hydro plants, and thus provide a relatively cost-efficient bulk storage mechanism. The technology is mature and the costs are well-known.
Compressed air
Compressed air energy storage (CAES) [5] is not yet used to a large extent, but a few CAES plants have operated on a test basis for decades. The best-known is operated by German energy supplier E.ON in Huntdorf, northern Germany.
A CAES plant is, in principle, a gas turbine plant split into separate compressor and expander sections, and extended by a large underground chamber capable of storing air at pressures from around 50 bar (empty) to 100 bar (full).
Despite the name, a conventional CAES plant is not simply a way to store electricity; it also shares many characteristics with a conventional gas turbine generating plant. When the compressed air is re-expanded, it must be heated in order to keep the turbine exhaust temperature at a practical level (the original heat of compression generally having been lost). The simplest way to do this is to burn natural gas in the compressed air stream, recovering the energy with a high-temperature gas turbine rather than a simple turboexpander.
As a result, the amount of energy supplied by the natural gas is usually greater than that stored in the compressed air. This makes the economics of CAES sensitive to fuel prices, as well as to the leakage rate from the storage chamber and the efficiencies of compression and expansion.
For Danish conditions recent studies show that conventional CAES is not economically viable [6]. Development of CAES with lower fuel consumption by better use of the heat generated during compression is ongoing and may produce significant improvements. At present, however, there is too little experience, and suitable sites are too varied to establish reasonable costs for energy storage using this technology.
Batteries and flow batteries
Several MW-scale battery systems have been developed, and there are a number of demonstration and commercial installations around the world. Most common are lead-acid, nickel-cadmium and sodium-sulphur batteries [7].
Halfway between conventional batteries and fuel cells are devices known as flow batteries. These convert electrical energy to chemical energy in a liquid electrolyte which can be stored in tanks that are independent of the electrical part of the battery. To produce power, charged electrolyte is pumped back through the electrochemical cells. The advantage over conventional batteries is that flow batteries decouple power (MW) from energy storage capacity (MWh). This should reduce costs, and also allow flow batteries to provide power for many hours or even days – something that is difficult with conventional batteries on a scale suitable for power systems.
Flow battery chemistries include vanadium, polysulfide-bromine and zinc-bromine. These do not suffer from memory effects or self-discharge, and can achieve high round-trip efficiencies above 80% (not including AC-DC-AC conversion).
The drawbacks of all large-scale batteries are their large footprints and relatively high prices in terms of both power and storage capacity. However, their very fast response times make them suitable for system services such as frequency support (primary reserves). Unlike pumped hydro and CAES, batteries can be located almost anywhere and can thus be placed in a system where losses are lower and the need for storage greater.
For these reasons there is a growing interest in the potential for large batteries to aid the integration of renewable energy [8]. Parallel research is taking place on the various chemistries to improve efficiency and increase suitability for bulk manufacture, which would lead to cost reductions.
Hydrogen and fuel cells
Using hydrogen as a storage of electrical energy requires firstly the conversion from electrical to chemical energy (electrolysis, i.e. electrochemical splitting of water), secondly the storage of hydrogen, and thirdly conversion back to electricity (by fuel cells or combustion generators). There are several technologies for electrolysis as well as for fuel cells (a detailed description can be found in the Risø Energy Report 3 [9]). Due to its low volumetric energy density (in gaseous form at atmospheric pressure) hydrogen is typically stored as a high-pressure gas, a low-temperature liquid, or in chemically bound form. None of these storage options, however, is easily applied in practice, and partly for this reason there is considerable skepticism about hydrogen as a future energy carrier. The round-trip efficiency for electricity storage via hydrogen is typically well below 50% and the costs of electrolysers and fuel cells are still high. However, research efforts are concentrating on improving the reliability of fuel cells and reducing their costs.
Other storage methods
Other technologies for storing electrical energy include flywheels, superconducting magnets and supercapacitors. These are mainly for special applications, however, and are not likely to play a substantial role in the future energy system.
The concept of using the batteries in electric vehicles (EVs) to store power from the grid (vehicle-to-grid or V2G) is also widely discussed. A large number of grid-connected electric (or hybrid) vehicles could provide energy storage if a certain fraction of their total battery capacity were reserved for grid services. The availability and reserved fraction of the battery capacity could depend on the hour of the day and be restricted by the vehicle owner. Such a system will promote a large battery-powered vehicle fleet and encourage large investments in advanced communication and control.
Transport
Energy storage for transport requires relatively high volumetric and gravimetric energy densities as well as rapid energy flows. Lower efficiency in the overall conversion process (from chemical to kinetic energy) can be tolerated providing the volume and weight of the fuel and power train are acceptable.
Batteries
There are high expectations for the battery technologies aimed at EVs and hybrids. Several projects are being carried out, for instance in Denmark, to assess the technology and develop the market for electric vehicles.
Supplying energy for transport via the power grid has several advantages, including increased flexibility through closer links between the power and transport sectors, increased energy efficiency, and the chance to include transport-related greenhouse gas emissions in carbon trading schemes. However, a serious drawback to the use of batteries in transport is their low energy density.
Even advanced batteries show energy densities (kWh/kg) one or two orders of magnitude below those of gasoline and diesel. This is the reason why battery vehicles have relatively short operating ranges between charges – typically up to 150 km according to the manufacturers. Longer ranges require large, heavy and expensive battery packs. Despite this, most major car manufacturers have electric vehicle R&D programmes and much attention is being paid to battery technology. Car manufacturers are under pressure to deliver electric cars with similar characteristics to today’s conventional vehicles.
The current front runner in electric vehicle applications is the lithium ion battery, mainly due to its superior energy density. Problems associated with traditional lithium ion batteries include a short cycle life and performance degradation with age. However, research has yielded adaptations that promise to avoid these restrictions.
Cost is of course a major issue, as indicated by the fact that the few available performing electric cars are very expensive. The car industry has a history of reducing costs through mass production, however, and if the right battery technology is found, costs can be greatly reduced.
Hydrogen and synthetic fuels
The challenge for hydrogen as a transport fuel is in finding a method to store enough of it safely on board a vehicle. So far, designing a hydrogen vehicle with a range similar to that of a conventional vehicle has proved difficult, whether the hydrogen is stored in gaseous, liquid or solid form.
As well as providing a source of hydrogen, high-temperature electrolysis has also shown the potential to split CO2, allowing synthesis gas (mixtures of carbon monoxide (CO) and hydrogen) to be created directly from electricity [10]. This is of interest because it could be used to create synthetic liquid fuels similar to those we are already used to and for which we already have an infrastructure. Chapter 10 covers this in more detail.
Outlook and recommendations
The need for energy storage in a future energy system dominated by renewable energy depends on many factors, including the mix of energy sources, the ability to shift demand, links between the different energy carriers (see Chapter 10), and the end-use of the energy. Since energy storage always brings extra costs and energy losses, it should be used only when it increases the value of the energy sufficiently from the time of storage to the time of use.
In the personal transport sector, the necessary increase in value comes from fulfilling the need for mobility and high energy output. It is likely that a large fraction of energy for transport in the near future will come from biofuels (not considered a form of artificial energy storage in this context), but batteries and perhaps hydrogen and synthetic fuels will also play an increasing role.
In the heat and power sectors value is represented by changes in the prices of heat and power, so the feasibility of energy storage depends on fluctuations in these prices. District heating systems, as used in Denmark, already incorporate substantial thermal energy storage. This is feasible due to the low cost of excess heat associated with power production, and the relatively low cost of thermal energy storage in the form of hot water. As wind power penetration increases, there will be more hours of low or even negative power prices. Conversion of electricity to heat (by heat pumps or direct electric heaters) will therefore become more common, leading to an increase in heat storage capacity.
In a power system which generates electricity only from thermal and fluctuating sources, system stability and security of supply are only possible either if the thermal plants can meet the entire peak demand, or if the system incorporates large-scale energy storage. In a future power system that is independent of fossil fuels, it may not be realistic to have enough production capacity in thermal biomass power plants to meet peak demand. Large-scale electricity storage will therefore be needed to ensure that power from fluctuating sources will always be available during the hours of peak demand.
In electric power systems, the role of storage can be divided into two tasks:
1. Energy shifting: the movement of bulk electricity in time, either as consumption (charging) or supply (discharging). The amount of energy stored needs to be large and the storage method relatively cheap. Suitable technologies are pumped hydro and compressed air storage.
2. Power balancing and quality issues: rapid response covering smaller amounts of energy. This is suited to batteries and fuel cells, whose comparatively high costs can be justified by the higher value of the electricity stored.
If energy storage is to become widespread then its direct environmental impact also must be considered. The most obvious concern is the safe and cost-effective recycling or disposal of batteries and their electrolytes. This must be accounted for in the life-cycle costs. However, the indirect and positive environmental impact of storage systems should also be included if they allow the use of a higher proportion of renewable energy, with consequent environmental benefits. Storage can also indirectly increase security of supply by increasing the use of renewable energy and reducing reliance on imported fossil fuels.
In a brief survey of energy storage it is not possible to pick out a clear technology leader, not least because much depends on the application. However, recent advances in storage for transport, based on both batteries and fuel cells, are exciting. It appears that lithium ion technology may allow battery-powered cars at last to throw off the slow, short-range and dowdy image that has plagued electric vehicles to date. Society’s need for transport, and the car industry’s desire to fulfill this need profitably, should not be underestimated as drivers for the development of mobile energy storage. Much more work is needed, of course, and technology research and development looks set to increase. We should not forget, however, that alongside research into the technologies themselves, work is needed on finding out how best to use storage devices of all kinds. Given the inevitable losses from energy conversion and storage, it is important to try to use primary energy as it is produced and not store it.
If we have to store energy, we should store excess renewable energy rather than energy produced by conventional means. The losses involved in storage are not insignificant, but since the marginal production cost from wind and solar sources is insignificant, we may need to change our traditional view, formed from the use of fossil fuels, that energy efficiency is the ultimate goal. Until recently, energy experts have agreed overwhelmingly that energy storage is costly, inefficient and something to avoid if at all possible. As we have seen, this may be set to change: the prospects for large energy storage devices have never been better.
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