An area of research that our maritime team focuses on is energy; primarily in the form of oil and gas. As we are blown into the season of mists and mellow fruitfulness, our thoughts must also turn to other forms of energy and how we can best store the bounty nature is showering us with. In the northern hemisphere, most of the wind and wave energy that nature can bestow, is delivered in the autumn, winter, and spring months. Unfortunately, the ideal storage method has yet to be invented.
If efficiency is the main criterion, then lithium battery storage would seem optimal if the limitations of recharging, the need to replace difficult-to-recycle batteries, and problems with overheating are ignored. By piggy backing many units (the largest, so far, in Moss Landing, Monterey, California, consists of 122 container sized units with 110,000 batteries), 3,000 MWh of energy can be stored.
If size is the criterion, then dammed hydroelectricity is your go-to if the consumption of large areas of valley are ignored and potential hazards downstream when breaches occur.
If lower capital costs and eco-friendly credentials are a significant factor, then green hydrogen created by the electrolysis of water are definitely a prime choice, if problems of flammability and pressurisation can be accepted.
When we want to store comestibles, then pickling, salting, desiccating (including smoking), canning and inert blanketing (excluding oxygen by displacing air with pure nitrogen) are normal choices.
Do they have any equivalents in the energy storage world?
As far as an analogy to pickling, brines and acids have long been used as thermal storage. These have been mostly confined to domestic, commercial, and small industrial uses such as thermal storage walls in commercial buildings and in breweries. The secret is phase-change which stores the most energy – raising the temperature of 1 kilogram water from 10˚C to 90˚C requires 334.4 kiloJoules, which is the same amount of energy required to melt 1 kilogram of ice at 0˚C ice to 0˚C water. If you choose the right substance at the right pressure, you can store heat or cooling as you wish but, generally, only at low level.
Salts and desiccation are another matter. Sodium hydroxide (which is also called lye or caustic soda) is a very good storage of energy with a specific heat that exceeds water, and it is cheap. The most interesting property for the purposes of energy storage is that hydration is extremely exothermic (adding water releases a lot of energy) which can be used to superheat steam and drive turbines. It is, unfortunately, extremely corrosive.
A Danish start-up, called Seaborg, that specialises in small nuclear reactor modules has developed (as a by-product of its research) a system using sodium hydroxide as an energy storage solution. Whimsically, they claim that if they filled up a container the same size as the Colosseum in Rome (1,320,000 m3 or 13,200 London buses or 132,000 elephants) with the salt and heated it to 700 degrees, they would be able to supply Italy’s entire population with heat and power for ten hours.
The secret is controlling the salt’s corrosive properties which will corrode the steel pipes and tanks used to hold it. Heated to super-high temperatures, the corrosion reactions are increased drastically. The key technology is the chemistry control that limits the corrosion of structural materials in contact with the molten salt. The chemistry control is developed by Seaborg and is the core IP of the company.
A team of engineers and scientists from Seville in Spain, and Pisa in Italy1 have proposed a thermochemical energy storage system for concentrated solar power plants based on the reversible hydration/dehydration process of the calcium hydroxide (slaked lime). It is a single fluidised bed reactor concept with alternating dehydration-hydration processes, charging and discharging alternating reactions, with superheated steam as fluidising agent. So far, the system has only reached the mathematical and engineering model stage, but these show it to be both relatively efficient and cheap.
It should be noted that both the above systems are dependent on changing the salts at regular intervals so the issue of disposal is not well addressed, but given the many industrial uses for these salts, that should not be an overwhelming problem.
Canning, of course, is covered by battery storage and super-capacitors but that is another discussion.
While anything but inert, hydrogen is an ideal energy storage solution. Hydrogen is fairly easy to make from the electrolysis of water and storage. Both are old(ish) technologies. For the amounts needed for grid storage, however, a somewhat different approach might be needed.
The main problems with hydrogen are that it is a little bit explosive and, being a small molecule (H2), quite leaky. The explosive bit merely requires the exclusion of oxygen. The leakiness depends on both differential pressure and the number of fittings (valves and regulators and such) where the gas will leak.
The US Department of Energy has done some work on hydrogen storage, but the focus is mostly on portable storage for cars, buses, and trains. Hydrogen has the highest energy per mass of any fuel, but its low density at ambient temperatures means low energy per unit volume. On a mass basis, hydrogen has nearly three times the energy content of petrol—120 MJ/kg for hydrogen versus 44 MJ/kg for petrol. On a volume basis, however, the situation is reversed; liquid hydrogen has a density of 8 MJ/L, whereas gasoline has a density of 32 MJ/L. They are therefore promoting the development of advanced storage methods that might allow higher energy density. They have set specific system targets which include the following:
- 1.5 kWh/kg system (4.5 wt.% hydrogen)
- 1.0 kWh/L system (0.030 kg hydrogen/L)
- $10/kWh ($333/kg stored hydrogen capacity).
Hydrogen storage is normally as a gas or a liquid. As a gas, it is generally stored in high-pressure tanks or cylinders at 350–700 bar (5,000–10,000 psi). As a liquid, hydrogen must be lowered to −252.8°C at normal atmospheric pressure. Hydrogen can also be stored on the surfaces of solids (by adsorption) or within solids (by absorption) and in lattices of ice (Hydrogen Clathrates).
Clathrates occur naturally on continental shelves and in cold regions of the world (tundra and steppes) and normally contain either methane or carbon dioxide, so could possibly be used for carbon capture, although global warming might cause mass release and a runaway warming.
Chart Illustrates How Hydrogen Storage Works
Is large scale hydrogen possible for grid storage (and where tens of thousands of tons are meant)?
The answer is that it is already being done…in salt caves. Where these are not available, abandoned mines, rock caverns and depleted underground seams that held natural gas must also be considered. Metal pipes, spheres, bottles, and gas holders are used, and it has been suggested that placing these in ocean trenches where differential pressures between the compressed hydrogen and water pressure are much less might be useful, although few countries have access to deep water trenches within their jurisdiction. There is also a problem with hydrogen embrittlement requiring frequent inspection of the vessels. Carbon fibre vessels might be a possible route, but some further research is needed.
Liquid hydrogen storage is costly. Alternatives such as adsorption in metal and chemical hydrides can also be expensive. Hydrogen storage as ammonia, methanol, or liquid organic hydrogen carrier (LOHC) is an option, as these compressed gases and cryogenic liquids are cheaper to store and recover hydrogen from.
There is no system for hydrogen storage that does not have some problems associated with it. There is still active research on all the options described for hydrogen storage and there needs to be substantial improvement in technology and cost before any of them are adopted universally. Methanol, ammonia, and LOHCS would seem to be the better options so far as they require less electrical energy input for releasing the hydrogen. Although there are some problems with the thermal demand, which might easily be met by burning hydrogen carrier as an external fuel, or by burning some of the released hydrogen.
If you are interested in delving deeper into this topic, please contact a member of our team here. The maritime team at Valour Consultancy writes a number of different reports across the energy spectrum, and will soon be publishing ‘Deep Dive: IoT and Crew Communications with Offshore Energy Producers’. Here, we will review the evolving business models of maritime oil, gas and energy suppliers, and explore the future development of IoT-related market sectors.
1 – Analysis of an energy storage system using reversible calcium hydroxide in fluidised-bed reactors
O. Bartoli a,b, R. Chacartegui a,d,*, A. Carro a, C. Ortiz c, U. Desideri b, J.A. Becerra a,d
a Dpto. Ingeniería Energ´etica, Universidad de Sevilla, Camino de los Descubrimientos s/n, 41092 Sevilla, Spain
b Department of Energy, Systems, Territory and Construction Engineering, University of Pisa, Largo Lucio Lazzarino 1, Pisa 56122, Italy
c Materials and Sustainability Group, Department of Engineering, Universidad Loyola Andalucía, Avda. De las Universidades s/n, 41704 Dos Hermanas, Seville, Spain
d Universidad de Sevilla, Laboratory of Engineering for Energy and Envorinmental Sustainability, 41092 Seville, Spain