Having made frequent references to storage in this blog, I thought it would be good to read around the subject, this post is more or less a list of links to Wikipedia articles. The list is neither complete or comprehensive. Whilst randomly clicking around, I was intrigued by the number of references to submarines and electric fork lift trucks. Whilst these appear to be diverse applications, both make use of stored energy and both have well developed infrastructures to support their operations. Maybe the starting point of a sustainable energy economy is a submarine, maybe this was the origin the line in the Beatles song which goes “We all live in a yellow submarine”. Storage is the key technology in a sustainable energy economy, generation is the most visible element and attracts most of the attention, but it is storage that bridges the gap between the regular pattern of daily life and the shifting sun and fickle wind.
Traditionally, system efficiency has been principal method used by engineers to assess performance. Whilst it is not unimportant in storage systems, in the authors opinion it is the unit cost of energy as perceived by the end consumer which is the most important measure.
Batteries are the most familiar form of storage. A gross over simplification would be to divide them into two categories defined by weight, the heavy lead acid form which has been in use for a century of more and the lightweight varieties such as NiMH (Nickel Metal Hydride), Ni Cd (Nickel Cadmium), LI (Lithium Ion). Lead Acid accumulators have a long history of use in domestic energy storage providing the energy for lighting, door bells and valve radios. If operated conservatively they have a long life and can provide a few kwh for domestic use and more than 1,000 kwh for submarines. If used in vehicles, the result is the milk float, the high energy density of LI batteries makes it possible to design sleek and elegant high performance vehicles such as the Tesla Model S which has LI batteries with capacities of 60 – 85 kwh. The life of a battery is a function of the way that it is used, high charge and discharge rates will shorten the life of most types of battery, the depth of discharge is also a factor.
Compressed air motors have long been used to provide power where any form of combustion is undesirable, for example in mines or where atmospheric oxygen is not available. Compressed air powered many torpedoes in both the First and Second World Wars. Storage schemes using compressed air range from small pneumatic accumulators to utility scale projects based on underground caverns. Large marine diesel engines often use compressed air for starting. At the time of writing, it seems that most of the utility scale projects are still at the proposed or planning stage. Compressed air storage based on underground caverns maybe less visible than the major civil engineering works required for pumped water systems.
Pumped Water Storage
Pumped water storage is a utility scale technology, often based on worked out quarries and large dams. The system consists two reservoirs, an upper one and a lower one. The energy to be stored is used to pump water from the lower reservoir to the upper reservoir. That energy is reclaimed by letting the water flow back to the lower reservoir through turbines which power generators. Often the machinery is in the form of units which can work as either motor/pump sets or turbine/generators. Pumped water is currently the most common utility scale storage technology.
Thermal storage includes several diverse range of technologies. At the domestic level it includes night storage heaters, these use off-peak electricity to heat up a mass of bricks or water, as these cool during the day they provide space heating. Domestic hot water systems often incorporate an insulated tank, this can be heated using off-peak electricity or solar thermal devices in a suitable climate so that hot water is available for an early evening bath. At the utility scale, heat from large solar concentrators has been used to create a reservoir of molten salt (or similar substance). The heat stored in this material can then be used to create steam for use in a conventional steam turbine generator. The Wikipedia article has a link to an article describing a solar basede Seasonal Thermal Storage System in Canada.
Whilst hydrogen is not a dedicated storage technology as such, it can be produced by sustainable sources, for example by electrolysis from wind generated electricity. It is a versatile fuel and can be used to generate electricity directly in fuel cells and as fuel for reciprocating engines which are adaptions of those used in automotive applications. The German Type 212 submarine uses a form of hydrogen fuel cell to achieve better performance and endurance than a conventional diesel electric vessel. Earlier this year Toyota announced the launch of fuel cell based car.
Flywheels have long been used for storing energy for very short periods of time, for example smoothing out the torque produced by reciprocating internal combustion engines. It maybe an urban myth, but success of the Citroen 2CV (the famous “tin snail”) has been said to be due to a large flywheel which made it well suited to undulating roads of rural France. Flywheels form the basis for for some recuperative braking systems, these capture a vehicle’s kinetic energy as it brakes and then restore it to the drive train on the next acceleration. This type of system has been used in F1 racing cars. The flywheel is an attractive energy storage device, it may have a longer life expectancy than chemical based systems. Despite its apparent simplicity, large systems are heavy, high rotational speed systems which present some design challenges, however, it seems that these are been overcome.