Introduction

Power Generation, Transmission, and Use

Markets, Regulation, and Oversight

Impacts of Power Generation and Transmission

Looking Ahead

Appendices

CEIR Report Map

PPRP Home

Maryland Power Plants and the Environment (CEIR-18)

5.5.2 Innovations in Transmission Technologies

New emerging transmission technologies are being developed to endure higher electrical and mechanical stresses and provide greater power transfer capacity and flexibility. Currently available technologies are already able to provide twice the capacity of similar traditional equipment with half the energy losses. Minimizing transmission losses effectively reduces energy demand and increases system efficiency.

High-Voltage Transmission Line Technologies

Electricity can be transmitted several ways and at various voltages. The majority of current bulk power transmission systems in the U.S. consists of overhead AC transmission lines that are generally rated at 230 kV or higher. High-voltage direct current lines (HVDC) comprise only about 2 percent of the total installed high-voltage transmission line mileage (see Section 2.5.2). These direct current systems have been used mainly for large scale one-way bulk power transfers, such as undersea cables, or to transmit power over long distances. HVDC systems are capable of carrying significantly more power over longer distances with fewer losses than traditional AC systems. Ultra-HVDC systems are being installed outside the U.S. in overhead configurations that operate at 800 kV and can carry 6,000 MW of electricity. 

HVDC transmission lines are especially effective for transmitting power from remote and renewable generation facilities like offshore wind, solar, and hydro. Several HVDC projects for renewable power transmission are currently planned or under construction in the US. In January 2016, Vermont’s Public Service Board approved the New England Clean Power Link Transmission Line. This HVDC line will carry Canadian-generated hydro and wind power to the Northeastern US. The Clean Power Link has a 1,000 MW capacity and will run 150 miles from the US-Canadian border to Ludlow, Vermont. Another HVDC project of note is the TransWest Express Transmission Project. This project will carry renewable energy from Wyoming to the Southwestern US, and has been under development since 2005.

The technology with perhaps the greatest potential for future transmission grid improvements is high-temperature superconductors (HTS), which will typically be designed for underground installations. Advances in materials sciences are steadily increasing the temperature requirements for superconductivity, which function only in extreme cold. These HTS can potentially carry up to 100 times more power with few, if any, line losses as there is no electrical resistance in superconducting wires.

A nearly half-mile 138 kV HTS cable was energized in 2008 as part of the Long Island Power Authority grid. The current in the Long Island cable is carried through HTS wires, which exhibit zero resistance when cooled to about -321°F with liquid nitrogen. Several smaller scale demonstration projects are in progress worldwide, including the Hydra project in New York City, which is funded in part by the U.S. Department of Homeland Security.

Electricity Storage Technologies

Electricity storage technologies might serve to support intermittent renewable resources such as wind and solar. Electricity storage devices currently in use include pumped hydroelectric power, compressed air facilities, batteries, and flywheels.

Pumped hydro is the most widespread energy storage system in use today. With an efficiency rate of more than 80 percent, pumped storage provides for approximately 20 GW of energy storage in the United States. Water is pumped into an upper reservoir when electricity prices are low, generally during night-time off-peak periods, then used to generate electricity for sale to the grid during peak hours. The Muddy Run pumped storage facility on the Susquehanna River in Pennsylvania has been in operation since 1966 and has a capacity of 1,070 MW.

Compressed air energy storage (CAES) makes use of natural and manmade (abandoned gas and oil wells) caverns to store compressed air and recover it for use in a turbine. Excess and inexpensive electricity is used to compress and pump high pressure air into an underground cavern. When electricity is needed, the air is released, mixed with natural gas, and combusted via a turbine to generate electricity.

Lithium-ion batteries and sodium sulfur batteries are already being used to provide 15 to 60 minutes of energy storage as regulation service. In 2011, AES began operation of its Laurel Mountain facility, which provides 32 MW of lithium-ion battery energy storage for a 98 MW windpower facility in West Virginia. AES plans to assemble a similar, much larger 400 MW facility for the Long Island Power Authority (LIPA) in New York. Some energy companies are also testing the use of batteries for grid management and energy storage.

A flow battery is a type of battery that uses liquid chemicals to store energy. Total energy storage is limited only by the size of tank used to hold the liquid. These systems are being targeted for peak shaving and utility-scale storage of solar and wind power. Prototype flow battery demonstration systems have been deployed throughout the world. The U.S. Department of Energy announced in April 2013 a breakthrough in flow batteries that utilizes a less expensive design with increased performance. UniEnergy Technologies has developed the largest capacity flow battery in North America and Europe; it entered service in June 2015. The 1 MW and 4 MWh vanadium-redox battery is located near Pullman, Washington, and is owned and operated by Avista Utilities. 

Flywheel systems utilize large rotating masses and are a good fit for providing regulation services. This technology can be used as a short-term buffer to smooth local output fluctuations from a wind facility or PV array. Flywheels are commercially available for development as “regulation power plants” providing up to 20 MW of regulation capacity. A flywheel storage regulation power plant has been shown to be capable of providing full power within four seconds of receiving a control signal.

Rail cars are also becoming a viable alternative for energy storage. In 2014, the Southeastern Pennsylvania Transportation Authority (SEPTA) piloted a battery storage network program that captures and stores energy from braking subway cars. Constellation Energy (a subsidiary of Exelon) will partner with Viridian Energy to expand this pilot program to a 10 MW battery storage network at seven SEPTA stations in 2016. Similarly, a company called ARES recently developed a railcar test-system as an alternative to hydro-pumped storage in Southern California. The storage system moves weighted rail cars uphill when receiving excess energy from wind and solar generation, and releases the cars back down the hill to generate additional power during lulls in solar and wind production. ARES plans to build a 50 MW commercial-scale rail car storage system in Nevada with operations targeted for 2019.

In addition to traditional storage devices, the electricity grid itself can be considered a mechanism for storing electricity. For example, a home powered by a solar PV installation may ship (sell) excess electricity generated to the grid during daylight hours and utilize (buy) electricity from the grid during evening hours and overnight.