Major industrialised nations are increasingly making pledges to redirect their energy production towards renewable technologies. Many nations, such as the UK and US have pledged to become carbon-neutral by 2050, while Germany hopes 55 to 60 percent of its power will come from renewable sources by 2035. Some suggest such targets could be overambitious, but they are an important part of reducing overall carbon emissions.
Nevertheless, a move away from fossil fuels is considered risky by some. Aboveall, critics complain that renewable energy is subject to fluctuations in availability and efficiency. This is particularly clear with solar energy, which provides high yield in the midday hours of summer, but low yield during the evening hours of winter - precisely when electricity demand is at its highest. In addition to photovoltaics, solar thermal, wind and wave energy are also affected by such fluctuations. However, other renewables such as biomass, geothermal and hydropower are less susceptible and provide the highest overall output.
Despite this, a reliable power supply can be guaranteed with renewable energy sources, under two conditions:
1. Our energy supply system becomes more flexible. If energy production from the sun and wind is insufficient, for example, renewable energy sources with high availability create a balance. Flexible biogas plants can bridge such bottlenecks.
2 Energy storage systems are coming into use. Surplus energy is then stored for later use, namely for when demand exceeds current production.
Energy storage: A Problem with Many Solutions
In fact, there are already many solutions in the field of energy storage. It is easy to lose track of them all. That's why RESET would like to bring more clarity to the conglomeration of possibilities:
1. A basic distinction is made between short-term and long-term storage. The range extends from subseconds to entire years.
2. Further issues include storage capacity (how many kilowatt hours are contained in each kilogram of storage medium) and efficiency (how much of the stored energy is ultimately made available as electricity.
Storage time, capacity and efficiency depend largely on the type of storage system: electrical, chemical or electrochemical, mechanical or thermal.
3. There is no ultimate solution yet, but for a more flexible energy supply based on different sources, different technologies are also needed. Electrical energy storage systems, for example, are capacitors that can only store energy for a very short time window, but are extremely efficient. For the storage of renewable energies, electrochemical storage systems such as batteries (e.g. lithium-ion batteries) can compensate for daily fluctuations. For example, the world's largest solar-powered battery is to be built in Florida by 2021.
However, batteries are usually not exactly sustainable, as both the mining of raw materials for production and the disposal of the end product are ethically questionable or environmentally harmful. In Germany, pumped storage, which along with compressed air storage is one of the mechanical energy storage systems, is a more classic design. They transport water up a slope into a reservoir using surplus energy from renewable sources. When needed, the water is allowed to flow back down by gravity, generating energy via turbines. Another much-celebrated technology - at least in terms of high storage capacity and withdrawal time - is called power-to-gas. Here, water resources are split into gaseous hydrogen and oxygen by energy in an electrolytic process. The hydrogen can then be stored as an energy carrier and converted back later. However, the process is not particularly economical because the conversion process dissipates much of its energy.
With energy storage systems, the focus is usually on economic efficiency. The costs for purchasing and operating the storage unit can be enormous. In addition, when the storage unit is charged or discharged, energy conversion often takes place, which is not without thermal losses. Energy losses can also occur during the actual storage period. Innovations in the field of energy storage, however, are shaking up the industry in a big way. Here are some of the more exciting technologies that have already been examined by RESET:
Back to Basics: Salt and Stones
One method of making renewable energy available for later use is to store it in the form of thermal energy. One exciting approach is to store this thermal energy in molten salt. This concept has existed for decades, but has only recently been developed further. With the help of surplus energy from renewable sources, a temperature difference is created. The heat is stored in molten salt, while the cold element is added to a liquid. When needed, the thermal energy is converted back into electrical energy.
Another new type of storage medium will soon be put into operation in Hamburg: A 1,000-tonne stone storage tank housed in an insulated container will cover the daily energy needs of around 1,500 households. Due to the inexpensive raw material, this technology is said to be cheaper than other storage technologies.
Molecular Potential for Solar Power
What about solar systems on the roof of one's own house? Feeding surplus energy into the power grid currently proves to be less lucrative, but often the producer themself cannot consume all their self-produced solar energy. The start-up Gensoric from Rostock has experimented with storing the energy generated in this way. A biocatalyst in the basement of the house converts the solar energy into methanol by adding water and carbon dioxide molecules. The stored methanol can in turn be used as fuel when needed. The novel, resource-saving process has already won an award and is supported by the EU.
Another approach comes from a research team at the Swedish Chalmers University of Technology, called Molecular Solar Thermal Energy Storage (MOST). Here, a specially designed molecule consisting of carbon, hydrogen and nitrogen becomes an energy-rich isomer as soon as it is irradiated with solar energy. The isomer is thus located in the solar cell itself and can be passed on in a circular process from the roof to the house's heating system. In this form, the energy can theoretically be stored for up to 18 years.
The Grid-Stabilising Effect of Flywheel Masses
There are also new approaches within so-called flywheel mass storage systems. These systems store electrical energy as rotational energy. For example, the company Adaptive Balancing Power has developed the Adaptive Flywheel. It has the advantage that it can be adapted to the respective application due to its design and can still be manufactured in a series. Since this type of storage can only store energy for a few minutes, it is especially important for balancing grid fluctuations.
Existing Structures and their "Second Life”
And what actually happens to the entire fossil energy infrastructure when it soon ceases to be? In fact, there are already approaches to breathe new, more sustainable life into the outdated structures as storage technology. In this way, costs and resources can be saved. For example, a start-up in Scotland is working on making disused mines usable as mechanical energy storage facilities. In the old mine shafts, weights are moved upwards on ropes by means of energy and lowered down again when needed to convert energy. In Germany, it is also being discovered that old coal-fired power plants offer the best conditions for storing renewable energies thermally.
No less interesting is the "second life" of retired e-car batteries: In Amsterdam's football stadium, they serve as storage media to absorb energy load peaks, especially during major events. But even intact batteries of electric cars could be used as temporary energy storage during standstill periods.
When it comes to storage material, researchers are also coming up with unusual ideas for recycling; eggshells that normally end up in organic waste, for example, are perfectly suited for the production of low-cost lithium-ion capacitors.
What Obstacles Remain?
The myth that a complete power supply from renewable energy sources is not possible can therefore be safely buried. There are already a lot of efficient storage technologies that could decisively advance the energy transition. Despite the existing solutions, however, a lot still needs to happen for things to really take off. The biggest problem is likely the lack of incentives. Many industrial nations continue to heavily subsidise fossil fuels, including those with ambitious sustainbaility goals, such as Germany. In addition, those who feed renewable energies into the power grid receive a demand-independent, fixed payment. This means that wind power operators, for example, do not see sufficient benefit in investing directly in storage technologies.
Many innovations are also still in the test phase. For example, the Fraunhofer Institute has tested a pumped storage system that uses huge concrete balls on the seabed. The project, which was initially carried out at Lake Constance, has since expired and would now have to be tested under real conditions, i.e. in the ocean.
More support would be needed to accelerate the transition of such developments into practice. However, as the areas of energy generation, heat supply and transport increasingly grow together, the topic of energy storage is likely to become more important than ever in the future. The keyword here is "sector coupling". Increased cooperation at the international level would also be necessary in the energy sector. Therefore politics, above all, has a key role to play. It can set the right levers and funds in motion so that sensible measures and subsidies advance the energy transition.
Author: Lena Strauß, RESET-Redaktion (Juni 2019)
This article is a translaton of an original from the Germany RESET website.