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(Updated 8/4/2023 to include inter-seasonal storage requirements for green hydrogen heating.)
Acentral issue in the low carbon future is large-scale energy storage. Due to the variability ofrenewable electricity (wind, solar) and its lack of synchronicity with the peaks of electricity demand, there is an essential need to store electricity at times of excess supply, for use at times of high demand. This article reviews some of the key issues concerning electricitystorage. In particular, it compares ''Green Hydrogen'' storage with the available alternatives.
In the UK''s nuclear and fossil-fuelled electricity system of 30 or more years ago, large scale nuclear and coal-fired thermal power stations provided a constant ''base load'' of power. The peaks in demand were supplied by switching-on ''open cycle gas turbine'' generators, that can run up to full load in minutes, plus some hydro-electricity, and oil-fired steam turbinesthat can also be switched on quickly from a warm start.
The peak power consumption was 39.3 GW at 17:40 on October 16. The minimum was 22.9 GW, at around midnight. The average value across the whole 24 hour period was 33 GW, as shown bythe horizontal blue dashed line on Fig. 1a. The total electricity consumption during the day was 790 GWh, which is the area under the curve ( also equal to the area under the average line). The peaks and troughs in demand were managed mainly by varying the amount of electricity supplied by CCGT and pumped hydroelectricity.
These numbers: 65GWh and 8GW are probably underestimates because of 3 factors:
Conversely, electrification of transport provides opportunities for storing electricity through charging demand management and''vehicle to grid'' (V2G)systems. Storage of heat also provides similar opportunities (see later). Sothe overall effect of their electrification on storage requirements is unclear.
The alternative to building generating capacity for the average electric power requirement and storing the peak to trough variation would be to install sufficient generating capacity to meet the peak demand and then ''curtail'' (switch-off) generators when they aren''t needed at other times of the day.
For our example on October 16th, this would require renewable electricity generation of 39.3GW at 17:40. Of this, only 22.9GW would be needed at midnight, meaning that up to 16.4GW (42% of the installed capacity) would be switched-off for some of the day.
The average ''power factor'' of wind energy in the North Sea is 38.9% (seethis article). So if all of the peak power was to be provided by off-shore wind turbines, the 39.3 GW of electricity would require 39.3/0.389 =101 GW of installed wind turbine capacity, corresponding to 8,400 of the largest (12MW) offshore wind turbines.
If the average power of 33 GW was to be provided by the installed base and the peak to trough variation managed through intra-day storage, as described above, then the installed base of turbines wouldneed to be 33/0.389 = 85 GW, corresponding to 7100 turbines. The cost of providing the energy storage must be compared with the cost of installing and maintaining the additional 1300 wind turbines to decide which is more financially attractive.
Of course, the economic calculation is more complex than that, because the ''levelized cost of electricity'' depends on the utilization of the generating assets. So the economics of electricity generation changes ifsome of the wind turbines are to be curtailed for up to half of the day.
The remainder of this article discusses electricity storage. Load shedding won''t be discussed further.
There are many applications for electricity storage: from rechargeable batteries in small appliances to large hydroelectric dams, used for grid-scale electricity storage. They differ in the amount of energy that has to be stored and the rate (power) at which it has to be transferred in and out of the storage system. This article is concerned with large-scale intra-day and inter-seasonal storage needed to balance-out fluctuations in energy supply and demand at national scale.
Proponents of a ''Green Hydrogen'' economypropose to solve the electricity storage problem by using excess electricity to electrolyse water and make Hydrogen; storing the Hydrogen in ''geological storage'', (underground salt caverns); and converting it back to electricity using fuel cells at peak times.This process is shown in theleft branch of Fig. 3. It has the advantage of potentially high storage capacity, depending on the internal volume of the salt caverns. However, the locations for hydrogen storage are limitedby the available geology, which limits the flexibility.
The US-based Energy Storage Association says that up to 100 GWh of Hydrogen could be stored in a salt cavern with volume of 500,000 cubic metres, at a pressure of 200 bar. This would be sufficient to cover the estimated intra-day requirement of 65 GWh. To provide the 16.3 TWh estimated inter-seasonal storage for the electricity system, about 160 such salt cavern facilities would be needed. If inter-seasonal energy storage was needed for heating the country’s homes with green hydrogen, it would take 2,080 such salt caverns, as calculated in https://
Table 9 (p151) of the 2019 International Energy Association report ''The Future of Hydrogen'' [2] states (without any supporting references) that there are 3 suitable salt cavern sites for hydrogen storage in the USA and another 3 sites in the UK. It also says there is "little experience with depleted oil and gas fields or water aquifers for hydrogen storage (e.g. contamination issues)". This raises questions about whether the quantity of storage available is anywhere near sufficient to meet demand.
Hydrogen storage at this scale is completely undeveloped. There are no examples or prototype systems in existence. Its technology readiness is at the lowest level on the TRL scale. Given this starting point, it is difficult to imagine how sufficient Green Hydrogen storage could be built in time to significantly affect the UK''s electricity or heat decarbonisation trajectories by 2050.
Compressed Airstorage follows a similar pattern. Electricity is used to compress air and force it into similar underground salt caverns.When electricity is needed,air is released back to the atmosphere through air turbines that drive electricity generators. Although small-scale compressed air storage systems have been built, the salt cavern technology is, as yet, untested in this application.
Compressed air is highly energetic and complex to manage from a safety point of view. Industrial pressure vessels require regular safety inspections and proof testing to ensure that microscopic cracks do not grow to cause rupture, which can be explosive. Subterranean salt caverns would need to be monitored continuously, regularly pressure tested and inspected so that their safety is ensured. Further information is available from the Health and Safety Executive.
Cryogenic (Liquid Air Energy Storage – LAES)is an emerging star performer among grid-scale energy storage technologies. From Fig. 2, it can be seen that cryogenic storage compares reasonably well in power and discharge time with hydrogen and compressed air.
The Liquid Air Energy Storage processis shown in the right branch of figure 3. It has 3 main steps: ''charging'' which involves liquefaction of air using a Claude cycle; ''Storage'' of the liquid air in standard double skin, low pressure tanks (large-scale ''thermos flasks''); and ''Recover'', which involves expanding the liquid air through a turbine to generate electricity. Despite not being very well known, this storage system has high technology readiness level (TRL), is safe and benign, uses standard off-the-shelf components and can be located anywhere in the country, using a small land area no need to be near salt caverns. Liquid air storagehas the advantage that the energy conversion and storage systems are uncoupled. Need more storage?addsome more low coststoragetanks.
In 2018, start-up companyHighviewPoweropened a 15MWh''grid scale'' demonstrator plantin Bury near Manchester. It can nominally provide5MW of power for 3 hours. Highview recently started construction of the first commercial facility ''CRYObattery'' with 250MWh storage capacity that can nominally provide50MW of electricity for 5 hours. This technology is in its infancy and can be scaled-up substantially.
Batteriesof various chemistries are possible for small and medium scale electricity storage, but the technologies do not scale as well as the other high capacity systems because the energy conversion and storage systems are coupled. Consequently there is no significant reduction in cost per kWh with increasing storage capacity each additional kWh of battery storage costsabout the same as the first.. Batteries also have finite lives. They can only be charged and discharged a limited number of times. They use expensive and scarce materials such as Lithium and Cobalt and so have significant environmental resource impact.
Conclusion: The technologies thatcan be used to supplement existingPumped Hydro forgrid-scale electricity storage are:Green Hydrogen, Compressed Air and Cryogenic (Liquid Air) storage. Of these, Liquid Air looks most promising for intra-day storage. It has a high technology readiness level, unlike the competing technologies which are untested at scale. It is not limited to locations near underground salt caverns.
The cost of energy storage is strongly dependent on the ''round-trip'' energy efficiency of the storage process (the amount of energy that comes out of the storage device divided by the amount that went in).
The left branchof Fig. 3 shows that if you start with 100kWh of renewable electricity, produce and store Green Hydrogen; then pass the Hydrogen through a fuel cell to generateelectricity, only 32kWhis returned to the electricity gridafter the storage process. This is because 68% of the input energyis wasted as low grade heatduring the process.The major losses occur in the electrolysis step, which is at best 75% efficient and the fuel cell, which is about 50% efficient [3]. The ''round-trip'' efficiency (electricity-Hydrogen-electricity) is therefore approximately 32%.
As an example of a more efficient storage technology, cryogenic (Liquid air) storage has around-trip efficiency of up to 70%as shown in the right branch of Fig. 3. This means that 30% of the energy is lost as low grade heat in the round trip [4].The process steps involve some losses through the change of state of nitrogen (gas-liquid-gas), but these steps are much more efficient than the chemical reactions needed to make and split hydrogen (water-Hydrogen-water). Other large-scale storage technologies, including compressed air and pumped hydro have similar round-trip efficiencies – in the region of 70%.
Conclusion:A number of storage technologies such as liquid air, compressed air and pumped hydro are significantly more efficient than Green Hydrogen storage. Consequently much less energy is wasted in the energy storage round-trip.
Suppose a company was set up to purchase electricity from the grid off-peak, when the wholesale market price is low and sell it back to the grid when the price is high. The company could make money on the difference between the sale and purchase prices and use that to fund its capital and operating costs and generate a profit. This business model is known as ''arbitrage''. Suppose, for example, that the purchase price was 10p per electricity ''unit'' (kWh). This is theequivalent to £10 per 100 kWh, as shown in the top block of figure 3.
If the company used Green Hydrogen to store the electricity, 68 kWh of the original 100 kWh would be wasted by the inefficient process and only 32 kWh would be available to sell back to the grid. The value of that 32 kWh would be £3.20 at the initial purchase price, so inefficiency would have thrown away£6.80 of the original electricity purchase. This means that the storage company would have to sell the stored electricity for 10.00/0.32 = 31.25 p/kWh (ie £31.25per 100kWh) in order to break even on their£10purchase. Add to this, the interest payments on capital for their plant, operating cost and profit and they would need a sale price nearer to 40 p/kWh in order to maintain their business.
The right-hand side path shows that if you use liquid air storage, pumped hydro or compressed air storage with an efficiency of 70%, then 70 kWh of stored electricity would be available to sell back to the electricity grid. Consequently, the break-even sale price would be 10/0.7 = 14.29 p/kWh or £14.29 per 100kWh. This simple example shows how important energy efficiency is to the arbitrage business model.No consumer would purchase electricity for31 p/kWhfrom the Green Hydrogen storage company when they could purchase itat 14 p/kWh from the Liquid Air storage company.
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