Future of Energy Storage

 

Source- MIT Energy Initiative (Future of Energy Storage Study )

A decarbonized electric sector will require replacement of existing fossil fuel resources with intermittent renewable energy resources. Energy storage technologies are a key constrain for this transition. The energy storage technologies and products available now are not in the scale that is needed for the decarbonation. A study estimates than upto 12 TWh of grid storage will be needed by 2050, this is more than 500 times the existing 24 GWh of storage that exists in the US.

Battery Storage Technologies

The characteristics of energy storage can be classified into two types- power (kW) vs. energy (kWh).  Based on the chemistry battery storage technologies used, each of them can be better suited for providing large power or providing large volume of energy storage. For example, of the three types promising energy storage technologies, lithium-ion batteries are more for providing high power, but not suited for storing large volume of energy as the materials is expensive, Redox-Flow batteries are neutral, while Metal-to-Air batteries can provide cheaper volume but not large power. Looking at the grid need, to replace the existing gas-powered plants, we need long duration battery storage that can provide multiday energy storage.

Li-ion is not the sole solution for grid need- Based on the costs, the energy storage technology can only compete with natural gas system if priced at $20/kWh. This is more than 10 times more expensive than the existing lithium-ion battery storage price of $250/kWh. To get to this lower price will require finding cheaper battery chemistry than lithium-ion, which inherently is a more expensive element. Even while the costs of lithium ion have decreased astronomically in the past years, to meet the grid need just with lithium-ion will require more than 20% annual growth for the next 20 years. This growth will be challenging to sustain. While li-ion will continue to play an pivotal use in EV and for high power applications, li-ion is not the solution for long duration battery energy storage applications.

Redox flow batteries are better suited for long duration storage- Redox flow batteries are essentially a electrochemical reactor. The flow batteries where different chemicals that have different electric potentials, and these chemicals are passed through a electrochemical reactor that allows the passage of electric charge creating a energy source. The energy produced is proportional to the size of the tanks of electrolyzes, and the power is depended on the size of the reactor. The challenge with the redox is that they have lower energy density than compared to li-ion. The state of art is the vanadium redox flow battery. There are companies such as Largo Clean Energy and Cell Cube are developing this technology. However, the there is only Vanadium as it is a rare element.  A solution should rely on working on using the elements that are abundant like sulphur and iron, or engineering new organic compounds.

Metal-air batteries hold promise for long duration storage- Metal air batteries – Aluminum Air, Iron Air, Zinc Air- chemistry works where the metal or the metalloid is oxidized (essentially the chemistry works as rusting-discharging and reducing the rust-charging) and can used to produce electricity. Air electrodes are currently inefficient; however the lower cost of electrode metals hold promise that this technology could lead to development of cost-effective large duration battery storage. Form Energy is developing Iron-Air battery storage, Noon Energy is working on Carbon-Air batteries. This technology needs more research and development.

Mechanical (Non-battery related) energy storage technologies

There are several promising technologies that are being developed that do not use battery chemistry for energy storage. These technologies include use of hydrogen storage, thermal storage, compressed air, and gravity storage technologies.  

Pumped hydro- The most mature long duration energy storage is the pumped storage hydro that has more than ten hours of energy storage capabilities. This pumped hydro storage essentially requires a selective geographical terrain where water can pumped up (stored energy) that can be later discharged when needed. In US, the areas geographical areas where the pumped hydro facilities have been build has already been build out.

Compressed Air- Compressed air technology compresses atmospheric air to high pressure, which creates considerable heat and compressed air. The heat is stored at a thermal energy storage. The compressed air is pumped underground caverns for storage. To discharge, the compressed air is pulled from underground and send through a turbine (and uses the previous thermal energy for are expansion) to generate electricity when needed. If the thermal energy is not stored, then later in the discharging process, the system will require external fuel to create heat to decompress the compressed air.

Thermal Energy storage- Thermal energy uses heat as storage and use the heat later for generating electricity. The thermal energy is stored upwards of 1200 degree Celius. There are designs that retrofit  the existing coal or natural gas plants where these steam turbines are run through thermal storage rather than burning coal or natural gas. This technology has a potential for  long duration energy storage.

Hydrogen- The hydrogen storage can use as either electrolyzer or used as combustion. The challenges with the using hydrogen is two fold- ability to cost effectively store hydrogen, and second the challenge of using hydrogen as hydrogen is orderless, colorless, and highly combustible. However, the commercial