How Energy Storage?

Capacitor

How Energy Storage?

Renewable energy storage — "One of the greatest challenges facing the electric power industry worldwide is how to harness the immense renewable energy resources and deliver them in a useable form as a higher-value product. By storing the power produced from renewable sources off-peak and releasing it during on-peak periods, energy storage can transform this low-value, unscheduled power into schedulable, high-value 'green' products. Developing these resources will not only lessen environmental impacts but also increase each country's domestic energy security (lowering payments for imported energy)."

"Two challenges in particular are well known to renewable energy proponents. First, many of the potential power generation sites are located far from transmission facilities. Although the costs of connecting these sites to the transmission system will delay these resources from being tied into the local grid for a long time, it leaves the possibility for off-grid applications. The second challenge is the timing of resource itself. Generally, renewable energy sources are intermittent or vary in intensity throughout the day, with much of the potential for generated power not coincident with the peak demand. Renewables - especially wind - suffer from lower prices in the wholesale market due to the inability to guarantee delivery levels."

"Electric power has a tremendous weakness: it must always be used precisely when it is produced. Thus, one of the greatest challenges facing the electric power industry is how to harness the immense renewable energy resources and deliver them when they are needed..."

Electricity Value Chain

The continuing evolution of the U.S. electric power industry has been stymied because the electric power market is incomplete. Unlike other successful markets, it has no growing storage component. Energy markets need storage in order to successfully emerge from deregulation—witness the successful evolution of the gas industry. Without a storage component, the market will not function properly and price spikes, instability and volatility will remain chronic problems. Many of the market-based services (Like tailored end-use products) that make the gas industry as flexible as it is today only became available by leveraging storage capacity.

What is Energy Storage?
At its heart, energy storage is an economic decision. Without storage, an industry must develop and maintain an entire delivery network capable of meeting the highest peak of the year at any given moment. Without storage, the industry must operate within a "justin-time" framework that is dependent not only on variable end-use demands, but is also completely at the mercy of one of the most uncontrollable variables known: weather. With storage, the owners only must build out what is necessary to carry a heavy, but normal load—resulting in a much higher utilization of the existing equipment, and hence a higher return on their investments (ROI).

Although electricity cannot be directly stored (cheaply), it can be easily stored in other forms and converted back to electricity when necessary. The additional value of the electricity during peak demand can cover the cost of storing power produced at night. As demand continues to expand, storage can play a crucial, multi-functional role since storage facilities are designed to excel in a dynamic environment.

How Storage Helps
Without a means to store electricity, a number of conditions will remain endemic to the industry: 1) raised volatility, 2) reduced reliability and stability, and 3) threatened security. By utilizing energy storage technologies, each of these challenges facing the industry can be greatly diminished thereby leading to a more efficient market that costs less to operate, that is more responsive to market changes, and that is more reliable in the event of a disruption.

Energy storage technologies provide a wide spectrum of capabilities that will perform a number of applications throughout the market. These capabilities can be grouped into three market roles: 1) energy management, 2) bridging power, and 3) power quality & reliability. Since the purpose is to act as a ‘shock absorber’ to the system, the incremental and beneficial impacts will accumulate as they are implemented.

Market Role
By supplying power when and where needed, energy storage will create a far more responsive market. It will:

1) Reduce the need for additional transmission assets,
2) Be the preferred supplier of ancillary services,
3) Provide better integration of renewables into the system,
4) Support more efficient use of existing assets,
5) Improve the reliability of electricity supply,
6) Increase the efficiency of existing power plant and transmission facilities, and
7) Reduce the investment required for new facilities.

The current power market suffers from uncertainty. The regulatory framework is incomplete, obvious market leaders have fallen, and the winning business models are still not defined. Energy storage can help by improving the economic efficiency and utilization of the existing system, not by replacing it. By optimizing the existing assets in the market and creating more opportunities, increased private industry investment will move into the market prompting greater competition and lower prices.

"Energy storage can improve the efficiency and reliability of the electric utility system by reducing the requirements for spinning reserves to meet peak power demands, making better use of efficient baseload generation, and allowing greater use of intermittent renewable energy technologies. Energy storage technologies include utility battery storage, flywheel storage, superconducting magnetic energy storage, compressed air energy storage, pumped hydropower, and supercapacitors."
- U.S. DOE Energy Storage Technologies


Energy storage technologies do not generate electricity but can deliver stored electricity to the electric grid or an end-user. They are used to improve power quality by correcting voltage sags, flicker, and surges, or correct for frequency imbalances. Storage devices are also used as uninterruptible power supplies (UPS). by supplying electricity during short utility outages. Because these energy devices are often located at or near the point of use, they are included in the distributed energy resources category.

The following technologies are discussed in this section, with more details below:

Battery Storage
Flow Batteries
Flywheel
Superconducting Magnetic Energy Storage (SMES)
Supercapacitor
Compressed Air Energy Storage (CAES)
Photo Source: UP Networks


Battery Storage

Utilities typically use batteries to provide an uninterruptible supply of electricity to power substation switchgear and to start backup power systems. However, there is an interest to go beyond these applications by performing load leveling and peak shaving with battery systems that can store and dispatch power over a period of many hours. Batteries also increase power quality and reliability for residential, commercial, and industrial customers by providing backup and ride-through during power outages.

The standard battery used in energy storage applications is the lead-acid battery. A lead-acid battery reaction is reversible, allowing the battery to be reused. There are also some advanced sodium/sulfur, zinc/bromine, and lithium/air batteries that are nearing commercial readiness and offer promise for future utility application.

Flow Batteries

Flow batteries differ from conventional rechargeable batteries in one significant way: the power and energy ratings of a flow battery are independent of each other. This is made possible by the separation of the electrolyte and the battery stack (or fuel cell stack). A flow battery, on the other hand, stores and releases energy by means of a reversible electrochemical reaction between two electrolyte solutions.

There are four leading flow battery technologies: Polysulfide Bromide (PSB), Vanadium Redox (VRB), Zinc Bromine (ZnBr), and Hydrogen Bromine (H-Br) batteries.

Flywheel

A flywheel is an electromechanical device that couples a motor generator with a rotating mass to store energy for short durations. Conventional flywheels are "charged" and "discharged" via an integral motor/generator. The motor/generator draws power provided by the grid to spin the rotor of the flywheel. During a power outage, voltage sag, or other disturbance the motor/generator provides power. The kinetic energy stored in the rotor is transformed to DC electric energy by the generator, and the energy is delivered at a constant frequency and voltage through an inverter and a control system.

Traditional flywheel rotors are usually constructed of steel and are limited to a spin rate of a few thousand revolutions per minute (RPM). Advanced flywheels constructed from carbon fiber materials and magnetic bearings can spin in vacuum at speeds up to 40,000 to 60,000 RPM. The flywheel provides power during period between the loss of utility supplied power and either the return of utility power or the start of a sufficient back-up power system (i.e., diesel generator). Flywheels provide 1-30 seconds of ride-through time, and back-up generators are typically online within 5-20 seconds.

Superconducting Magnetic Energy Storage (SMES)

Superconducting magnetic energy storage systems store energy in the field of a large magnetic coil with direct current flowing. It can be converted back to AC electric current as needed. Low temperature SMES cooled by liquid helium is commercially available. High temperature SMES cooled by liquid nitrogen is still in the development stage and may become a viable commercial energy storage source in the future.

A magnetic field is created by circulating a DC current in a closed coil of superconducting wire. The path of the coil circulating current can be opened with a solid state switch which is modulated on and off. Due to the high inductance of the coil, when the switch is off (open), the magnetic coil behaves as a current source and will force current into the capacitor which will charge to some voltage level. Proper modulation of the solid-state switch can hold the voltage across the capacitor within the proper operating range of the inverter. An inverter converts the DC voltage into AC power. SMES systems are large and generally used for short durations, such as utility switching events.

Supercapacitor

Supercapacitors (also known as ultracapacitors) are DC energy sources and must be interfaced to the electric grid with a static power conditioner, providing 60-Hz output. A supercapacitor provides power during short duration interruptions and voltage sags. By combining a supercapacitor with a battery-based uninterruptible power supply system, the life of the batteries can be extended. The batteries provide power only during the longer interruptions, reducing the cycling duty on the battery. Small supercapacitors are commercially available to extend battery life in electronic equipment, but large supercapacitors are still in development, but may soon become a viable component of the energy storage field.

Compressed Air Energy Storage (CAES)

Compressed air energy storage uses pressurized air as the energy storage medium. An electric motor-driven compressor is used to pressurize the storage reservoir using off-peak energy and air is released from the reservoir through a turbine during on-peak hours to produce energy. The turbine is essentially a modified turbine that can also be fired with natural gas or distillate fuel.

Ideal locations for large compressed air energy storage reservoirs are aquifers, conventional mines in hard rock, and hydraulically mined salt caverns. Air can be stored in pressurized tanks for small systems.