By Scott Simonsen
Sep 21, 2017
When it comes to grid storage our batteries are terrible.
A grid powered entirely from solar and wind wouldn’t work with the current state of energy storage, as solar and wind don’t produce consistently, and they can’t be tweaked to meet demand. That is, solar energy can only be produced during cloudless days and wind energy only when it’s windy. Production also can’t be increased to provide consumers with more electricity during peak demand hours. How, then, will energy companies provide a consistent flow of electricity from renewable sources?
If we’re going to increase our reliance on solar and wind energy, the batteries to store that energy will need to get much better, and fast. Thankfully, many new (some bizarre) types are in the works.
Lithium Ion Batteries
Lithium ion is the most common type of battery, but the bigger it gets, the less useful it becomes. For consumer electronics, lithium ion batteries work well enough because they can be recharged quickly, and they offer high energy density, meaning they provide a lot of power for their size and weight. Even for medium-sized applications, such as powering electric cars, they get the job done.
The problem is that storing enough solar and wind power from commercial farms would require warehouses full of massive batteries, and at this size, two problems become apparent.
Cycling stability (PDF) is defined by “…the number of charging or discharging cycles until its capacity is reduced to a certain amount of its nominal capacity (typically 50 percent to 80 percent).” For lithium ion, it is an average of 1,000 cycles, thus reducing the feasibility of long-term investments.
Such a short life cycle is caused by tiny changes in the physical structure of the electrodes. As the lithium ions are transferred from the anode to the cathode during discharge, the nickel-oxide anode is eroded non-uniformly, and during recharge, the lithium ions crystallize around the cathode. Over time, these processes drastically reduce the performance of the battery, especially at high voltages.
According to a paper in Nature, “Once the rate of heat generation exceeds the rate of heat dissipation into the environment, the temperature of the cell starts to rise; thereafter, a sequence of detrimental events propagates in a process known as thermal runaway.”
In lithium ion batteries, the authors claim, the process can begin between 90 and 120 degrees Celsius, leading to a positive feedback loop of exothermic reactions. The batteries used in small consumer electronics have a number of safety features to prevent this, even though some incidents still occur.
However, the bigger the battery, the higher the temperature, making the likelihood of this happening in large-scale solar and wind energy storage much higher.
Despite these problems, lithium ion batteries are being implemented for large-scale grid storage. The largest system was put online in San Diego in February, providing power to 20,000 people for four hours.
Tesla’s lithium ion Powerwall is designed to be installed in residential housing, but to charge it requires personal solar panels, something out of reach for the average person.
New Types of Batteries
While lithium ion batteries are improving, we need something even better if we’re going to transition to 100 percent renewables.
Redox Flow Batteries (RFBs)
RFBs offer a much longer charge/discharge cycle than lithium ion batteries, and they use an incombustible electrolyte, leading many to believe these might be the solution.
The US Department of Energy suggests (PDF) RFBs offer “a long cycle life (>5,000 deep cycles) due to excellent electrochemical reversibility,” and do not “present a fire hazard and use no highly reactive or toxic substances, minimizing safety and environmental issues.”
RFBs consist of two separate tanks that hold the charged vanadium atoms, which are used due to their unique ability to exist in more than one state. These are pumped past the electrodes, creating the charge, as shown this diagram (Fig. 2).
Other benefits include room temperature operation, high efficiency, and scalability. The downside is the cost, because vanadium is not easy to obtain in large quantities and the solutions need special polymers to contain them, although methods are being developed to make these more cost-effective.
First isolated in 2004, graphene is only one atom of carbon thick, making it the world’s thinnest material. It is also chemically inert, an extremely good conductor, flexible, lightweight, and 200 times stronger than steel. Researchersat the University of Manchester, where graphene was first isolated, think it “could make batteries light, durable, and suitable for high-capacity energy storage from renewable generation.”
Some uses already discovered include enhancing the anode in rechargeable batteries to improve conductivity and using a hybrid of vanadium oxide and graphene to enhance the cathode, which can help charging and discharging speeds and lifespan.
However, the most exciting applications are in graphene-enhanced supercapacitors.
A supercapacitor is similar to a battery, except it stores energy in an electrical field rather than in a chemical form. This allows the supercapacitors to charge and discharge quickly and have a much longer lifespan, although they cannot store as much as a typical rechargeable battery and need to be much larger to store an equivalent charge.
By using graphene to improve supercapacitors, they will be able to increase their storage and decrease their size. Dr. Han Lin, a researcher at the Swinburne Centre for Micro-Photonics, claims, “In this process, no ions are being generated or being killed. They are maintained by charge and discharge, and are just moved around.”
Graphene-info.com states, “Graphene-improved performance thereby blurs the conventional line of distinction between supercapacitors and batteries.”
Advancements in 3D printing have allowed researchers to print graphene electrodes for supercapacitors, as well as graphene aerogels, which “will make for better energy storage, sensors, nanoelectronics, catalysis, and separations.”
Printed Energy, an Australian company, “is printing solid state batteries in a thin, flexible format that can be adapted to almost any shape.” These are printed in a roll-to-roll process much like newspapers, and they have a wide range of applications.
“Potential applications for the printed batteries range from powering disposable healthcare devices, sensors, internet-of-things devices, smart cards, wearable electronics and personal lighting to larger-scale applications such as in combination with flexible solar panels to help manage intermittency and energy storage.”
For large-scale solar and wind storage, the idea is to affix the batteries to solar panels or wind turbines, thus allowing them to be both the generator and battery.
Researchers at the Ulsan National Institute of Science and Technology have begun integrating cellulose, the stuff plants are made of, into batteries. They use this plant matter to create a nanolayer, called a c-mat, between electrodes to prevent short circuits, reduce leakage current, and increase capacity retention at high temperatures.
One of the main researchers states, “The c-mat separator is expected to be used for next-generation high-performance batteries with high temperature stability—for example, in large-sized batteries for electric vehicles and grid-scale electricity storage systems.”
Many other uses for cellulose in batteries are currently being researched.
Thermal Energy Storage (TES)
TES is a system for storing excess solar energy by heating or cooling a medium so that it can be used later. The International Renewable Energy Agency states, “TES is becoming particularly important for electricity storage in combination with concentrating solar power (CSP) plants where solar heat can be stored for electricity production when sunlight is not available.”
There are three types of TES:
- Sensible heat storage, which is the most common, stores thermal energy by heating or cooling a substance like sand, molten salt, rocks, or water.
- Latent heat storage is similar except it uses phase change materials, which absorb a tremendous amount of energy when they go from solid to liquid or liquid to gas. Looking at the this graph (Fig. 1), during phase change, energy storage is able to increase without the temperature going up, making this method highly efficient.
- Thermo-chemical storage uses thermal energy “to drive a reversible endothermic chemical reaction, storing the energy as chemical potential.”
This method uses excess energy to pump water uphill and store it in tanks or reservoirs. When needed, it is released to turn turbines.
The National Hydropower Association claims “With an ability to respond almost instantaneously to changes in the amount of electricity running through the grid, pumped storage is an essential component of the nation’s electricity network.”
This method has already found many applications throughout the world, although it is not very efficient when compared to other methods. “…to get the amount of energy stored in a single AA battery, we would have to lift 100 kg (220 lb) 10 m (33 ft) to match it. To match the energy contained in a gallon of gasoline, we would have to lift 13 tons of water (3500 gallons) one kilometer high (3,280 feet).
100 Percent Renewables
A paper recently published in the journal Joule laid out the path for 139 countries to generate 100 percent of their energy from renewables by 2050. The authors make several important claims about their plan, such as avoiding three to five million deaths from air pollution, reducing the cost of global warming by around $28 trillion per year, and connecting four billion people to an adequate supply of electricity.
One of the central components is the ability to store renewable energy. If we are going to transition to 100 percent renewables, the above examples of game-changing energy storage need to become commercially viable.