“Candidates For Long Duration Bulk Energy Storage”

The energy transition will require new approaches to energy supply and demand, while vetting newer technologies for energy generation. As the globe works to assimilate intermittent solar and variable wind generation into a demand profile that demands energy on a 24/7 basis, the future “Holy Grail” of the transition will be energy storage.

Background:

According to Precedence Research, the global stationary energy storage market size is expected to hit over US$ 224.3 billion by 2030 and is expanding growth at a compound annual growth rate (CAGR) of 24.9% from 2022 to 2030.

The key will be utility-scale, long-duration storage that will bridge renewable energy generation variability with residential, commercial and industrial 24/7 energy demand.

The International Energy Association (IEA) estimates that, in order to keep global warming below 2 degrees Celsius, the world needs 266 GW of storage by 2030, up from 176.5 GW in 2017. Under current trends, Bloomberg New Energy Finance predicts that the global energy storage market will hit that target, and grow quickly to a cumulative 942 GW by 2040 (representing $620 billion in investment over the next two decades).

According to the U.S. Department of Energy (DOE), pumped-storage hydropower (PSH) has increased by 2 gigawatts (GW) in the past 10 years. In 2015, the United States had 22 GW of PSH storage incorporated into the grid. Yet, despite the widespread use of PSH, in the past decade the focus of technological advancement has been on battery storage.

There is a delicate balance between the timing of energy generation and consumer demand. Bridging this gap is energy storage during periods of relatively high production and low demand, then release it back to the electric power grid during periods of lower production or higher demand.

According to the U.S. Department of Energy, the United States had more than 25 gigawatts of electrical energy storage capacity as of March 2018. Of that total, 94 percent was in the form of pumped hydroelectric storage, and most of that pumped hydroelectric capacity was installed in the 1970s. The six percent of other storage capacity is in the form of battery, thermal storage, compressed air, and flywheel, as shown in the Graph 1:

Graph 1 – Electricity Storage Capacity In The U.S. By Technology

Source: U.S. DOE Global Energy Storage Database

Types of energy storage:

Hydro Pumped Storage

The most common form of energy storage on the grid and accounts for over 95% of the storage in use today. Electricity is used to pump water up to an upper reservoir when electricity is easily-available and cheap. When water is released to the lower reservoir, it flows down through a turbine to generate electricity to satisfy higher demand at higher prices. Siting these systems can be difficult because of the terrain needed (an upper and lower pool of water) and large footprint as shown in Figure 1.

Figure 1 – Hydro Pumped Storage

Advanced Compressed Air

Electricity is used to compress air at up to 1,000 pounds per square inch and store it, often in underground caverns. The standpipe (vertical shaft) and cavern is filled with water from a surface reservoir/lake in a discharged state when energy supply is tight and prices are high. The system is recharged with energy supply is abundant and prices are low by compressing air to move the water from cavern to the surface reservoir as shown in Figure 2.

Figure 2 –Advanced Compressed Air

Liquid Air Energy Storage (LAES)

Liquid Air Energy Storage (LAES) uses electricity to cool air until it liquefies, stores the liquid air in a tank, brings the liquid air back to a gaseous state (by exposure to ambient air or with waste heat from an industrial process) and uses that gas to turn a turbine and generate electricity. LAES systems use off the shelf components with long lifetimes (30 years +), resulting in low technology risk.

LAES is a long duration, large scale energy storage technology that can be located at the point of demand. The working fluid is liquefied air or liquid nitrogen (~78% of air). Size extends from around 5MW to 100MW+ and, with capacity and energy being de-coupled, the systems are very well suited to long duration applications as shown in Figure 3. 

Figure 3 – Liquid Air Energy Storage (LAES)

Gravity

Energy is stored when power demand is low by raising 35-ton blocks, to the top of the crane where they remain until energy is needed. At that time, the bricks are lowered, releasing kinetic energy back to the grid, as shown in Figure 4.

Figure 4 – Gravity-based Energy Storage

Flywheel

Electricity is used to accelerate a flywheel (a type of rotor) through which the energy is conserved as kinetic rotational energy. When the energy is needed, the spinning force of the flywheel is used to turn a generator.

Flywheels store energy in a rapidly spinning mechanical rotor and are capable of absorbing and releasing high power for typically 15 minutes or less, although longer duration systems are being developed. These systems can balance fluctuations in electricity supply and demand where they respond to a control signal adjusted every few seconds. They also recapture braking energy from electric trains in some installations or provide short-term power until backup generation comes online during a grid outage, such as in a critical manufacturing process where product would be lost by a momentary electric interruption.

Thermal energy storage

Electricity can be used to produce thermal energy, which can be stored until it is needed, such as chilled water or ice during times of low demand, in addition to capturing heat for a later energy use.

Thermal systems use heating and cooling methods to store and release energy. For example, molten salt stores solar-generated heat for use when there is no sunlight. Ice storage in buildings reduces the need to run compressors while still providing air conditioning over a period of several hours. Other systems use chilled water and dispatchable hot water heaters. In all cases, excess energy charges the storage system (heat the molten salts, freeze the water, etc.) and is later released as needed.

Pumped Heat Electrical Storage (PHES)

In Pumped Heat Electrical Storage (PHES), electricity is used to drive a storage engine connected to two large thermal stores. To store electricity, the electrical energy drives a heat pump, which pumps heat from the “cold store” to the “hot store” (similar to the operation of a refrigerator). To recover the energy, the heat pump is reversed to become a heat engine. The engine takes heat from the hot store, delivers waste heat to the cold store, and produces mechanical work. When recovering electricity the heat engine drives a generator.

Batteries

There are various forms of batteries, including: lithium-ion, flow, lead acid, sodium, and others designed to meet specific power and duration requirements.

Initially used for consumer products, lithium-ion batteries now have a range of applications including smaller residential systems and larger systems that can store multiple megawatt hours (MWh) and can support the entire electric grid. These systems typically house a large number of batteries together on a rack, combined with monitoring and management units.

Hydrogen

Excess electricity generation can be converted into hydrogen via electrolysis and stored for later generation.

Table 1 compares the different types of energy storage technologies.

Table 1 – Energy Storage Technology Comparison

Since the discovery of electricity, we have sought effective methods to store that energy for use on demand. Over the last century, the energy storage industry has continued to evolve, adapt, and innovate in response to changing energy requirements and advances in technology.

Importance of energy storage:

Many also expect there to be significant synergies with the emergence of electric vehicles (EVs) powered by Li-ion batteries. The flexibility of Li-ion technology in EV applications, from small high-power batteries for power buffering in hybrids, to medium-power batteries providing both electric-only range and power buffering in plug-in hybrids, to high-energy batteries in electric-only vehicles, has similar value in stationary energy storage.

Summary:

Energy storage is truly the “Holy Grail” of the energy transition by bridging the intermittency of solar and variability of wind to match 24/7 energy demand by all types of consumers, residential, commercial, and industrial. As society is able to attack this need with reliable and cost-effective solutions, the transition will progress.

Copyright © May 2023 Ronald L. Miller All Rights Reserved