The intermittent nature of some renewable energies, such as solar and wind power, presents significant challenges for transitioning to an entirely renewable energy grid. Simply put, wind and solar energy do not generate a consistent amount of power over even short time spans.
At night solar power lies dormant—ending power generation—while on calm days wind turbines become 800-foot-tall, motionless lawn ornaments. In other circumstances, solar and wind might produce too much power for immediate demand.
Faced with this situation, the common solution is to sell that energy at negative wholesale electricity prices—essentially paying consumers to use up excess power. Of course, this is not a road to sustainable or prosperous business success, and it is one of the reasons states and energy providers still rely on fossil fuels. Despite their dirty nature, fossil fuel-fired plants can be quickly and easily modified to match energy demand. Need more power? Simply burn more coal (or natural gas).
But what if there was another way to put that excess renewable power to use? That is where renewable energy storage solutions come in. If the excess power can be stored, it can be released later when demand is higher. In this way, the peaks and troughs of renewable energy production can be smoothed over, making it more competitive economically.
The Simple—But Expensive—Solution
Fundamentally, the goal is to convert the original renewable energy into another form, which can be contained, before being passed back into the grid, often via a turbine or generator. Known as "Power-to-X," this can be achieved in several ways, with the best solutions losing the least amount of power between stages.
The most straightforward method is to use surplus power to charge large lithium-ion batteries. However, these are expensive, unwieldy, and potentially hazardous. Moreover, for lithium-ion batteries to be economical, their estimated cost should be around $20 per kilowatt hour (kWh). Currently, they average around $132 per kWh.
Efforts are underway to produce the next generation of batteries. One solution is instead of spreading lithium-ion batteries across a grid—increasing costs—they are pooled into utility-scale or grid-scale facilities. These larger utility scale batteries can maintain more power at a cheaper comparative rate, and potentially replace older fossil fuel plants. By placing them at critical junctions in a grid, they can also serve multiple renewable power generation sites.
By placing larger utility scale lithium-ion batteries at critical junctions in an electric grid, they can also serve multiple renewable power generation sites.
Unfortunately, this approach may exacerbate one of the biggest drawbacks with lithium-ion batteries: They contain elements such as lithium and cobalt. These metals are often extracted in open-pit mining, placing pressure on local environments. Cobalt, in particular, is largely imported from the Democratic Republic of the Congo, a nation with a poor track record in human rights.
Faced with this, researchers are also developing new breeds of batteries that do away with lithium and cobalt by using sodium or organic polymer-based batteries. Many of these, however, cannot match the power density of lithium-ion batteries.
Luckily, there are also other ways to store renewable energy.
Storing Power with Water, Weights, and Heat
One common solution is pumped storage. When lots of energy is being produced, and electricity prices are low, water is pumped uphill into a reservoir. The electricity is therefore being converted into potential kinetic energy. When energy is in demand and prices are higher, that water can be released downhill through turbines like a traditional hydroelectric dam. Although fairly efficient, this system is expensive, large, and dependent on local geographic features. However, attempts are underway to refine it. Switzerland is inaugurating its state-of-the-art pumped storage power plant Nant de Drance on September 10-11, with a storage capacity of 400,000 EV batteries.
One Dutch project aims to replicate the process on the seafloor using inflatable bladders, while a German start-up has developed shipping container-sized batteries that recreate the process using air and gas. A similar effect can also be achieved using weights and pulleys. One Scottish project is using excess power to lift weights in disused mine shafts, before lowering them again through generators.
Another method is "power-to-heat." This involves using surplus renewable energy to create so-called "Carnot batteries." Within a Carnot battery, materials are superheated and stored. When power is needed, this thermal energy is converted back into electricity, often via heat engines or steam turbines. The best materials for Carnot batteries are those that retain heat over long periods, such as sand, stones and, more recently, molten salt. On the plus side, Carnot batteries can be built anywhere and use cheap, readily available materials. The downside to this approach is that it is comparatively inefficient. Many Carnot batteries aim for around 40%-70% conversion efficiency, while pumped storage averages around 80%.
A more experimental approach, dubbed molecular solar thermal energy storage (MOST) is also under development. With MOST, a specially designed molecule consisting of carbon, hydrogen, and nitrogen is irradiated with solar energy via a special dish. The isomer can then be stored at room temperature until solar energy is required—for example at night or on overcast days. When passed through a catalyst, the molecule releases the energy and reverts to its former state, ready to be irradiated again. In this way, solar energy could theoretically be stored for up to eighteen years with degradation.
Hydrogen as ‘Energy Carrier’
One other promising storage solution comes from the universe's most abundant element: hydrogen. Although hydrogen is not a common fuel itself (yet), it can be used as an “energy carrier.” Surplus renewable energy can be used to power the process of pyrolysis—making hydrogen from gas—or electrolysis—making hydrogen from water. Once in the form of hydrogen, the renewable energy can be stored indefinitely. When it is needed again, the hydrogen goes through reverse electrolysis and is combined with oxygen to create water and electricity. Attempts are being made to develop hydrogen “batteries” that overcome some of the issues and dangers of storing hydrogen under high pressure.
Surplus renewable energy can be used to power the process of making hydrogen from gas or from water—and once in the form of hydrogen, the renewable energy can be stored indefinitely.
As the above suggests, there are numerous potential ways to store renewable energy, but none do it perfectly. Almost every solution comes with additional challenges in cost, efficiency, or construction. In addition, many systems are too expensive for small-scale, local renewable energy projects, but not efficient or cost-effective for large-scale producers. Pumped storage has quickly become the most popular method—accounting for 90% of all renewable energy storage—but logistics and construction requirements also make pumped storage difficult to scale and expand.
In the short term, lithium-ion batteries are likely to become the preferred method for both small- and large-scale producers. The technology behind them is well understood and can be easily adjusted to match the size of their associated renewable power plant. Need more storage? Connect more batteries. As mentioned above, such batteries are still prohibitively expensive and come with ethical concerns, but their overall cost is dropping. It is worth noting that although a lithium-ion battery averages $132 per kWh today, in 2010 that figure was over $1,200 per kWh.
*Mark Newton is a Berlin-based freelance journalist and researcher originally from the UK. After specializing in conflict and security studies, he has recently shifted his focus towards sustainability and environmental concerns.