Long-term energy storage
Continuously changing the power infrastructure from primary energy to secondary energy comes with some downfalls. The biggest challenge to overcome for these rather momentary power sources is the imminent need to consume the generated electricity or heat. As in contrast to fossil fuels, control is another characteristic renewable energy sites usually don’t provide. Due to this, some argue that the application of battery farms to renewable generation sites is the best solution. However, in this blog post, we will look at other, often underrated, long-term storage options.
The problem – Thermodynamics
One of the main problems with renewable energy is that peak generation times are not in synchronisation with peak consumption times. Solar plants, for example, generate most of their power around midday. In contrast, most electricity is consumed in the evening and/or when it is dark outside. Positively correlated market prices towards this heterogeneous generation/consumption relation call for action. As more demand for electricity increases power prices, these ‘out-of-peak-power’ times represent a business niche.
The first law of thermodynamics is a version of conservation of energy, adapted for thermodynamic processes. In general, the conservation law states that the total energy of an isolated system is constant; energy can be transformed from one form to another but can be neither created nor destroyed.
Therefore, the choice of the correct storage solution is of utmost importance and should be of far more interest than it currently is. There are several ways to classify or categorise energy storage (for example, based on duration of response time) but the most commonly used classification is by the form of energy.
This new technology proposal might sound odd in the beginning, but it is brilliant. By stacking colossal cement blocks on top of each other to form a circle around the lifting tower, energy can be used when available and released when needed.
This technology received an investment of $110 million in 2019 and connected its first commercial demonstration unit to the Swiss grid in July 2020. This tower can deliver between 20 MWh to 80 MWh of storage capacity. Naturally, this is depending on the height and usage of mechanical lifts. Usually, one such building operates comfortably at a 35 MWh capacity.
Pumped hydro storage
This well-known gravity-based technology finds use worldwide since 1907 when it first was demonstrated in Switzerland. Pumped-storage hydroelectricity (PSH) stores energy in the form of potential energy of water pumped from a lower elevation reservoir to a higher elevation. To work towards a business plan and for load balancing reasons, water is usually pumped uphill at times of low electricity price and demand. In contrast, it usually is released at times of high electricity prices and need.
PSH is still widely used and can hold vast amounts of energy (1,300 MWh facility in California) . According to the U.S. Department of Energy, 95 per cent of the U.S. grid storage comprises pumped hydro storage solutions. The only problem with these facilities is the highly complex permitting processes to build new plants.
These batteries have been popular in discussions about new storage solutions over the last decade. However, a successful utility-scale installation is yet to be built.
Flow batteries can work like a fuel cell, whereby the spent fuel is extracted and the new fuel is added to the system. They can also work like a a rechargeable battery where an electric power source drives the regeneration of the fuel. A typical flow battery consists of two tanks of liquids that are pumped past a membrane held between two electrodes.
Advanced compressed air energy storage (A-CAES)
This storage model is pretty low tech. Excess electricity is used to pump compressed air into an underground tank. This is released again when the electricity demand increases. Old mines or caves are suitable for such storage plants to host high-pressure containers.
Frankly, this technology is most likely a non-residential solution but could be a success for utility-scale plants as seen in in Ontario at the Goderich A-CAES Facility. Many companies tried to roll-out A-CAES storage plants to a broader audience but so far have failed. The most promising player on the market seems to be Hydrostor.
Cryogenic energy storage
To create liquid air or liquid nitrogen (cryogenic liquids), gaseous air/nitrogen needs to be cooled down to cryogenic temperatures (-195°C) so that the gas condensates into its liquid aggregate state. Furthermore, it needs to be thermally isolated from temperature to remain in the liquid form. Hence, specialised vacuum containers are the preferred way of storing, as they would turn back into the gaseous form instantly when in contact with ambient air.
Cryogenic liquids in the power sector are usually used when a power surplus needs to be bypassed. Modern nuclear plants, for example, could use this technology to balance demand and supply at off-peak hours and at times when the baseload requirements are exceeded. This way a part-load operation of the power plant could be avoided, resulting in cheaper electricity prices and risk avoidance.
Likewise, this technology is also applicable to renewable energy power plants. This avoids downtimes due to the electricity surplus of intermittent energy sources.
In contrast to CAES, cryogenic energy strorage does not depend on topographic circumstances. The storage units are built above ground on any even surface available.
With the selection of long-term storage solutions above, a variety of options are available to help balancing the demand and generation issues associated with intermittent energy resources. Instead of shutting down power plants, the additional implementation of such a storage faciility could help massively towards implementing more renewable energy sources into the electricity mix.
The author of this discussion neglects the nature of the variability of weather-dependent energy. Storage capability for this problem has to anticipate infrequent extended shortages of energy – energy droughts – of uncertain magnitude. The basic problem with weather as an energy source is associated with the statistic of that variability. Weather events do not occur randomly. They are highly correlated, both in time and space. The consequence is the fractile nature of the statistics, and an exaggeration of extreme events – extended periods with high or low energy outputs.
Thank you, Robert, for emphasising the importance of storage solutions due to an increasing share of volatile energy sources in our energy mix. We do not neglect anything, but, same as you, we have taken the volatile energy generation of most renewable energy sources as a call for long-term storage solutions in this blog post.