A Look into Emerging Long-Duration Energy Storage Technologies 

As discussed in earlier articles, the world is speeding up its shift to renewable energy sources like solar and wind. Hence, there has never been a greater demand for effective energy storage solutions. 

Particularly, the need to store energy for many hours or days, which is known as Long Duration Energy Storage or LDES.  

Although conventional lithium-ion batteries have dominated the energy storage market, new and developing battery technologies are set to become essential for long-term energy storage. Let's take a look at some of these:

1. Flow Batteries 

One of the most promising technologies for LDES is flow batteries, especially vanadium redox flow batteries (VRFBs). Flow batteries are perfect for situations where storage times exceed a few hours since they store energy in liquid electrolytes housed in external tanks that may be scaled to give a significant amount of energy over extended periods.

Source: Flow battery systems with solid electroactive materials  

In contrast to conventional batteries, which store energy in solid electrodes, flow batteries store energy in liquid electrolytes that move through the system when the battery charges and discharges. Both power and energy capacity can be scaled freely with this design by varying the size of the electrochemical cells and the electrolyte tanks, respectively.  

Key advantages of flow batteries: 

1. Flow batteries can discharge energy over several hours or even days, offering a much longer duration of storage compared to lithium-ion batteries. 

2. The energy capacity (amount of electricity a battery can generate through electrochemical reactions) can be scaled independently of power output by increasing the size of the electrolyte tanks, making it possible to store large quantities of energy for longer periods. 

3. Flow batteries use water-based electrolytes, which are safer compared to the flammable electrolytes used in traditional lithium-ion batteries. They also have a longer lifespan, with fewer degradation issues during charge and discharge cycles. 

However, flow batteries are currently less energy-dense than lithium-ion batteries, which limits their adoption in certain applications, such as electric vehicles (EVs).  

2. Compressed Air Energy Storage (CAES) 

Despite not being a battery in the traditional sense, compressed air energy storage, or CAES, is another technology that could be used in conjunction with batteries for LDES applications.  

CAES system compresses ambient air during the charging phase using inexpensive, off-peak electricity. After that, the compressed air is kept in a pressurized reservoir, which could be an aboveground tank or a subterranean cavern.  

Now, when it is time to discharge, the energy stored by compressing the air is released. A turbine system draws air from the reservoir, heats it, and then expands it at high temperatures and pressures. Electricity produced by this expansion process can then be sent to the grid.  

Source: - European Association for Storage of Energy 

Advantages of CAES: 

1. CAES systems can store energy for much longer durations, ranging from several hours to days or even weeks. 

2. CAES systems can be built on a large scale, making them ideal for grid-level energy storage. 

3. The technology can use existing infrastructure like underground caverns, making it a relatively low-cost storage solution. 

However, CAES is not as energy-dense as batteries, and the system efficiency can be lower. Nonetheless, it remains an important complementary solution for large-scale, long-duration storage.  

3. Gravity Energy Storage 

Gravitational potential energy is the energy that is stored by gravity energy storage (GES). The fundamental idea underlying GES is rather straightforward: a heavy mass is raised to a specific height and then released when energy is required. When the mass is allowed to fall, the energy that was utilized to raise it is transformed back into electrical power using gravitational potential energy.  

During the charging phase, a heavyweight or mass is lifted using extra grid electricity, usually during off-peak hours or when demand for renewable energy exceeds supply. This mass could consist of a number of lesser weights or it could be a solid object, such as a big block of concrete. 

When energy is needed (typically during periods of high demand or when renewable generation is low), the stored gravitational potential energy is released. The heavy mass is allowed to fall, either through a shaft or along a track. As the mass descends, it drives a generator or a turbine, converting the gravitational potential energy back into electrical energy. 

Source: Gravitricity 

Advantages 

1. Gravity energy storage systems do not rely on harmful chemicals or rare-earth metals like lithium-ion batteries, making them environmentally friendly. The materials used in many gravity energy systems, such as concrete and steel, are abundant and widely available. 

2. One of the major advantages of gravity energy storage is its relatively low cost compared to other storage technologies like pumped hydro. 

3. Gravity-based systems can also be integrated with existing infrastructure, such as abandoned mines, shafts, or towers, reducing the need for new construction and minimizing environmental impact. 

4. Unlike traditional batteries, which typically discharge energy over a period of hours, gravity energy storage systems are designed for long-duration energy storage. 

However, while gravity storage is well-suited for long-duration storage, it generally has lower energy density than chemical storage systems like lithium-ion batteries. The efficiency of gravity-based systems can be affected by factors such as friction, mechanical losses, and the need for precise control over the lifting and lowering processes.

4. Liquid Air Energy Storage (LAES) 

Liquid Air Energy Storage is a type of cryogenic energy storage, which involves storing energy in a material at extremely low temperatures. In this technology, energy is stored using liquid air as the storage medium. 

The charging system is an industrial air liquefaction plant where electrical energy is used to release heat from ambient air drawn from the environment, generating liquid air (cryogen).  

The liquid air is stored in an insulated tank at low pressure, which functions as the energy store. When power is required, liquid air is drawn from the tank, pumped to high pressure, and evaporated. This produces gaseous air that can be used to drive a piston engine or turbine to do useful work that can be used to generate electricity.

Source: - European Association for Storage of Energy 

Advantages 

1. LAES systems can be scaled to meet different energy storage requirements. The size of the storage tanks, the number of compressors, and the turbine capacity can all be adjusted based on the specific needs of a given application. 

2. One of the major advantages of LAES is its ability to provide long-duration storage, capable of discharging energy for hours or even days. 

3. LAES is considered a relatively environmentally friendly energy storage solution. 

While LAES offers the advantage of using low-cost, abundant materials, the initial capital costs for setting up a cryogenic storage facility can be high. The energy density is lower compared to some other storage technologies, such as lithium-ion batteries. 

5. Iron-Air Batteries 

Iron-air batteries are another type of metal-air battery that uses iron as the anode and oxygen from the air as the cathode. During discharge, iron reacts with oxygen in the air to form rust (iron oxide), releasing energy in the process. When recharged, electricity from an external source reverses this reaction, converting the rust back into iron and releasing oxygen into the atmosphere. 

This technology has garnered interest because it has the potential to provide inexpensive, long-duration storage. Because iron is readily accessible, and plentiful. 

Advantages: 

1. Iron is one of the most abundant and inexpensive metals, which makes iron-air batteries potentially very affordable. 

2. Iron is environmentally friendly, and iron-air batteries could be easily recycled, making them a sustainable option for energy storage. 

3. Iron-air batteries are well-suited for long-duration energy storage, with the potential to store energy for days or even weeks. 

The sluggish charging and discharging rates of Iron-Air batteries are a drawback that may restrict their applicability in some situations. Compared to lithium-ion batteries, iron-air batteries have a poorer round-trip efficiency (the ratio of energy input to energy extracted).