Energy storage and transportation infrastructure
Questions for energy democracy: How do we get energy from where it is available when it is available to where it is needed when it is needed?
The GB energy system currently includes several energy transport infrastructures: gas and electricity networks, road networks for transporting liquid heating and transport fuels, or biomass, and local heat networks. These systems interact with each other, so it is important to consider them together. The boundaries between them will change as we shift the primary source of energy for transport and heating from fossil fuels to renewable energy, and shift our carriers of energy from gas and liquid fuels to electricity.
The gas and electricity networks are national, with a lot of similarities in their historical development, rules and regulations, market structure, and ownership. These network infrastructures are what some people mean when they refer to the GB energy system.
District heating networks are relatively new to the UK, and are operated at the scale of a neighbourhood, campus or maybe city. Energy losses from heat networks are much bigger than from gas and electricity networks, so it does not usually make sense to transport heat over long distances. The UK government has been supporting the development of heat networks, with the HNDU (heat network development unit) providing feasibility funding, but the role of heat networks in a zero carbon system depends on many detailed factors.
Road and freight rail networks are used for transporting fuels, but these are not in themselves designed as ‘energy infrastructure’, so they are not discussed in detail here.
In addition to energy transportation over land in GB, it is important to remember that energy is transported to reach GB in the first place. This includes international shipping of oil, gas, coal and wood, pipelines for oil and gas in the North Sea, and subsea electricity cables. These international transportation connections are not discussed in detail in this guide, but our energy system does currently rely on them, and is likely to continue to do so.
Electricity is generated in power stations or solar farms etc, and consumed in kettles and ovens, and in between it is transported across the transmission and distribution networks. The transmission network is like the motorway system in roads, and the distribution network is like the A and B roads of our road network.
The geography of the GB electricity transmission network is shown below.
Additionally there are interconnectors (under sea cables) between GB and Ireland, Northern Ireland, Netherlands and France, which import and export electricity.
Whilst almost everywhere in mainland Britain is connected to the electricity grid, 14% of households in GB are not connected to the gas network. Off gas properties are relatively common in rural areas such as Cornwall and Wales and highly urban areas such as parts of central London. The geography of the national high pressure UK gas transmission network is shown below.
Detailed maps of regional gas networks are available from the National Grid website.
Heat networks (aka district heating) transport heat from a central boiler to several buildings or properties. These can be at various scales:
- [bullets all green here]
- Apartments within one building
- Buildings on one campus or site with a single owner
- Separate buildings within a neighbourhood
- A whole city with linked-up district heating
Most district heating systems use water in insulated pipes. Heat losses from the network are relatively high (much higher than electricity losses from cables, or gas leakage from pipes). This means district heating only makes sense over short distances. Additionally, the heat loss in the network is reduced by using lower temperature water, as this reduces the temperature difference between the heat network and the surroundings.
There are several ‘generations’ of district heat network. The latest, ‘4th generation’ focus on using low carbon sources of heat, and 5th generation focuses on using low temperature water in the network to minimise losses, with a heat pump in the building to raise the temperature.
For heat networks to be viable, the density of heat demand is important. The more heat is needed within a small area, the less significant the heat losses. The Centre for Sustainable Energy was commissioned by government to produce a national ‘heat map’ showing where the most heat is needed. These are areas where district heating could be viable.
Roads rail and ship
Fuels (e.g. coal, wood, oil, transport fuels) are transported by ship, road and rail. This is one of the ways that the energy system intersects with the transport system (the other being use of energy for transport).
Transport fuel, mainly petrol and diesel, is transported from depots to petrol stations in tankers that travel on roads. Some transport fuel is LNG or LPG. Aviation fuel is particularly difficult to replace with low carbon alternatives.
Many rural areas are not connected to the gas grid, and are dependent on liquid heating oil delivered by road. Consumption is strongly seasonal.
Time balancing of energy production and use requires storage.
Storage is important over different timescales:
- Interseasonal storage – to keep warm in winter, making use of energy from the summer sun
- Weekly – mainly due to changing weather patterns such as a cold spell or a still spell, and consumption patterns such as weekdays and weekends
- Daily – particularly to get light in the evening from solar power from the daytime.
Storage can be in different energy forms:
- Fuel that can be burned
- Fossil fuels such as natural gas and coal – old sun
- Hydrogen – made from something
- Biomass – newer sun
- Various other ways to store electricity
- Batteries, flywheels, compressed air, pumped storage etc
Storage can be a crossover point between different energy vectors.
Currently, storage of gas is a really big part of balancing the electricity system. Storage of heat in hot water tanks in homes reduces pressures on storage in the gas network. A district heating network could These interconnections are important, and it is helpful to think about the energy system as a whole when designing storage, rather than keeping each energy vector (heat, gas, electricity, hydrogen) in a silo.
Storage can be located in different places within the system:
- A battery storing electricity can be located at a windfarm, at a pinch point in the electricity network, or in homes.
- Hydrogen could replace natural gas in the networks, or could simply be used as storage at an offshore windfarm and used to generate electricity there when needed, or could be produced from electricity at an industrial site to power industrial processes.
Storage involves losses. The efficiency of conversion, and rate of loss of stored energy over time are therefore important.
The holy grail of energy storage is interseasonal heat storage – meaning we store up heat in the summer, from the sun, and use it in the winter when it is cold. This is an issue faced by all temperate climates – everywhere where we get a lot of sun in summer, and need to keep ourselves warm in winter.
Daily energy storage is also important. In a solar PV-based energy system, we can easily produce electricity in the daytime, and need light at night. Having a mixed energy system with wind power and other sources of energy reduces the need for both daily and seasonal storage, but there will still be periods (in the region of 5 days a year or so) when it’s not windy for days, it’s really cold, and we need to heat our homes. We need big storage to deal with this.
The size and location of electricity and gas storage affects the design of the electricity and gas networks of the future. If we don’t have storage, large amounts of energy need to flow back and forth. If we store more locally, the flow between places is supplementary rather than fundamental, so it needs less big infrastructure. Just as when people walk and cycle locally rather than commuting for 3 hours by car, congestion on motorways decreases. However, electricity network operators aren’t currently allowed to own and operate storage, or to invest in their networks ‘ahead of demand’ – they have to be responsive. This policy may change in the future, but would be quite a fundamental change to regulation. This is discussed more in the section on governance and decision-making.
Electricity itself cannot be stored. Traditionally, fluctuations in electricity demand have been met by increasing or decreasing generation to match demand at any time. As we come to rely more on intermittent renewable energy, storage becomes an important part of the electricity system. Electricity is converted to another form of energy to be stored, and then converted back again. This leads to some losses of energy in conversion.
The most common form of storage for electricity is pumped hydro facilities where water is moved to high reservoirs at times of low demand before being allowed to run downhill through hydro turbines at times of high demand.
There is increasing emphasis on other forms of storage for electricity including:
- [also green bullet points]
- compressed air
Different forms of storage operate effectively over different timescales.
Gas consumption has strong daily and seasonal variation. Short and long term storage in the gas network is used to buffer these variations. Natural gas is a fossil fuel that is ‘produced’ from the ground by drilling, often in the same place as oil or coal. More gas is produced in winter, when it is needed for heating, than in summer, which effectively means that the gas fields under the seabed are a form of storage.
In addition to ‘natural’ storage, the gas network includes both long and medium term storage facilities. Natural gas is actively stored in a disused oil field called “Rough” and in onshore tanks. There is also storage within the pipes themselves, commonly known as ‘linepack’ storage, in the form of pressure changes. As a fossil fuel, natural gas has no or limited place in a zero carbon energy future, but other gasses e.g. hydrogen or biogas could potentially be stored in existing gas storage facilities.
Hydrogen gas could be transported through our gas network instead of using fossil gas. This is a form of energy storage, as to be zero carbon the gas would need to be produced from surplus renewable energy (unless we use carbon capture and storage – which is currently only envisioned to be commercially viable when used to increase oil and gas production from oil fields… but that’s another story).
Storing hydrogen in existing gas storage facilities could prove to be a challenge, as hydrogen is a smaller molecule than methane, and so more prone to leakage.
Heat can be stored in many ways, some of which are probably already familiar to you:
Hot water tank in houses
Most houses in GB used to have insulated hot water tanks, but the shift to ‘combi-boilers’, gas boilers which provide instantaneous hot water rather than storing it, led to many tanks being removed. This has some immediate energy efficiency advantages, but removes important storage from the system. Reinstating hot water tanks would be challenging as people are now using the space freed up in their homes. Hot water tanks can be used to:
- reduce stress on the gas network at peak hours (e.g. morning showers)
- store excess solar electricity in a useful place using immersion heaters
- enable the installation of solar thermal as part of a hot water system
- enable other forms of alternative heating
The fabric of the house/building itself
If you have a house with big stone or brick walls, the walls of the house itself will store heat. This is called thermal mass. It means that the temperature inside the house changes more slowly. Thermal mass in buildings is a useful form of heat storage if there is external insulation on the walls and if drafts are managed effectively.
‘Heat store’ big water tanks
Interseasonal heat storage can be achieved by building very highly insulated water tanks – typically underground – that can store large amounts of heat for a long time. Imagine a basement swimming pool surrounded by 1m of insulation on all sides, and sealed at the top. This is the form of storage used in Drake Landing in Canada and in places in Denmark.
Phase change materials
A high tech form of energy storage is to use ‘phase change materials’ that turn from liquid to solid and back again. The process of changing ‘phase’ from liquid to solid releases a lot of heat, without changing temperature too much, and changing from solid to liquid absorbs a lot of heat (i.e. stores it). This is more expensive than hot water, but uses less space, and has less heat loss during storage so it can store heat for a long time.
Molten salt is one example of a phase change material, and is used in solar furnaces to enable them to generate electricity when the sun isn’t shining.