Energy Storage and Scale: The Key to 100% Renewable Energy

16th July 2019

The world of energy storage is ever changing as different technologies jockey for position. One of the largest hurdles for emerging energy storage technologies is scale, i.e. being able to build and operate cost effectively at a size which can match existing energy infrastructure. This is with the ultimate aim to enable a 100% renewable energy grid by being able to release solar and wind energy when the sun is not shining and the wind is not blowing.

Tesla Inc, the electric car and battery manufacturer, recently purchased a company called Maxwell Inc who produce a range of electrical energy storage components known as supercapacitors [1]. Supercapacitors are energy storage devices which are constructed in a very similar way to batteries and share a lot of the same underlying technology. The internet is awash with speculation over the reasons why Tesla purchased this company, but most hypotheses centre on the fact that Tesla wants to get its hands on some of Maxwell Inc’s technology, relating to the way in which batteries/supercapacitors are made.

It is well known that Tesla doesn’t currently produce its own batteries from start to finish. Currently, Tesla gets its batteries from Panasonic who produce them either in Japan or alongside Tesla’s own packaging facilities in factory in Nevada. However, at a recent shareholder meeting the Tesla executives explained that recent bottlenecks in production were being caused by battery supply issues [2], suggesting that this is an area that Tesla are looking to develop. Crucially, much of the technology that has been developed by Maxwell Inc for their supercapacitors could potentially be used by Tesla to produce their own batteries [3].

Much of the internet speculation around this acquisition is focused on Maxwell’s dry electrode technology. An electrode is one half of a battery into which the lithium ions enter or leave during either charging or discharging. Maxwell’s technology has the potential to reduce the cost of manufacture of lithium ion batteries, by switching from a wet process (which uses more equipment and toxic solvents) to a dry process [4-8]. This one change has the potential to reduce the area required to manufacture battery electrodes by a factor of 16. Thereby allowing Tesla to fit x16 the number of electrode manufacturing lines in the same space where one would exit now. This will enable Tesla to keep marching to its ultimate manufacturing goal, which is scale.

Energy and Scale

When it comes to industrial energy projects, scale is always key. Whether that’s large offshore oil and gas rigs, or the ever increasing size of offshore wind turbines. Usually the larger the project/infrastructure, the more efficient and more cost effective to use. Take offshore wind turbines for example. The larger the turbine, the more electricity produced at a single location. Therefore fewer turbines are required, therefore lower overall construction cost and fewer turbines to maintain.

The same is and will be true for energy storage projects. Enabling the energy produced by ever-increasing wind turbines and solar farms to be stored in ever increasing energy storage facilities.

The trials of increasing the scale and production capacity of lithium ion batteries is well-documented by many news articles. For example, debates rage over the sourcing of the materials for these batteries such as cobalt and nickel as well as the building of new factories to make large numbers of batteries [9,10]

One of the reasons that lithium-ion receives such attention is that batteries are intricately linked with the production of electric cars, which is by far the dominant technology to fuel zero emission vehicles. But there are other forms of electricity storage which will be needed to store energy from renewable energy sources, such as intermittent wind and solar.

Below is a look at five different scalable non-lithium ion energy storage technologies used for grid energy storage:

Liquid air energy storage

Liquid air energy storage (LAES) stores energy by cooling air to below its boiling point, thereby turning it from a gas to a liquid. The process of liquifying air requires energy. The liquid air can then be stored in insulated tanks at -170℃. To release the energy the air is reheated, expands, and as it expands it flows through a turbine generating electricity. This technology has the benefit that it can be placed anywere, regardless of geography, and it uses a lot of existing technology for cooling and storing the liquid air. The technology is scalable, as to have a larger storage capacity you need a larger tank and more of the same equipment. One drawback is that is has slightly lower efficiencies (60-75%) compared with other technologies e.g. pumped hydro is 80% [11]. A large scale demonstration plant is operating in the UK and the company that built it – Highview Power – is looking to build grid scale plants in the near future].

Pumped hydro storage

Pumped hydro storage uses the weight of water held high up at mountainside to store energy. It usually consists of two reservoirs one at the bottom of a mountain and the other at the top. To store energy, water is pumped from the lower reservoir to the higher reservoir. To release the energy the water is allowed to flow from the higher reservoir to the lower reservoir in the process turning a turbine and generating electricity. The main advantage of pumped hydro storage is that it is easily scalable since the storage capacity depends on the size of the dam / reservoirs. One drawback is that it is heavily dependent on geography, requiring large elevation changes and suitable geology. Another is the upfront investment cost, as this can be quite high.

As of 2019, there are only 4 pumped hydro schemes in the UK, the last of which finished construction in 1984 [12]. But there is scope for an expanded role for pumped hydro in the UK [13], with the potential to double the UK’s pumped storage capacity through the addition of just one or two new projects.

Non-hydro gravity storage

There are also alternative methods of energy storage using gravity use weights instead of water. Primarily these work by lifting a weight using an electric motor which takes energy and charges the system. When the weight is lowered the motor is used as a generator and generates electricity.

The main issue with this technology is that gravity storage is not very energy-dense. That is, a large number of weights are required, which takes up a lot of room. The advantage is that it is very simple to implement and can have very high efficiency (90% [14]).

Therefore, the challenge with this technology is one of engineering and cost, i.e. in order to make the technology cost-effective the design has to be optimised for the scenario in which it will be used. There are several companies trying to non-hydro gravity based storage, using all different configurations of weights, pulleys, and geographical settings, for example, Energy Vault and Gravitricity.

Sodium sulphur batteries

Sodium sulphur batteries have been known about for a longtime and were a contender for use in early electric cars [15]. They use two very commonly available materials as the cathode and anode: sodium and sulphur. However, these materials must be kept molten between 300 and 350℃ [16]. Therefore additional heating is required. Otherwise a small amount of energy will be required to heat the battery.

If the batteries are used frequently (every 4 to 8 hours) then the heat generated by the batteries in operation will keep the sodium and sulphur molten. But that restricts their use to only a very specific charging and discharging cycles. Luckily, however, that charging and discharging cycle is very similar to that required by solar energy. In fact, one of the world’s largest non-hydro energy storage facilities is a huge network of sodium sulphur batteries in Abu Dhabi [17], which stores excess solar power. The rugged nature of the batteries suits the Middle Eastern climate.

Despite the sodium sulphur battery having been known about for over 50 years, there are few manufacturers of this technology, the largest being NGK Insulators in Japan.

Mix of All of the Above

The storage of renewable energy is going to require a mixture of all the technology above, as well as others which have not been mentioned. But there are two key take home messages:

Energy storage must work at scale. Simply in order to store the energy required to power humanity from 100% renewable energy, the infrastructure will have to be big but at the same time it’s also got to be cost effective.
Energy storage will not be a one size fits all solutions. Different countries with different geographies will be suited to one storage solution over another. The mix of renewable sources will influence the energy storage technology required and the different technological capabilities of each storage technology will need to analysed to match the scenario.

References

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