Distributed energy, also district or decentralized energy is generated or stored by a variety of small, grid-connected devices referred to as distributed energy resources (DER) or distributed energy resource systems.
Conventional power stations, such as coal-fired, gas and nuclear powered plants, as well as hydroelectric dams and large-scale solar power stations are centralized and often require electricity to be transmitted over long distances. By contrast, DER systems are decentralized, modular and more flexible technologies that are located close to the load they serve, albeit having capacities of only 10 megawatts (MW) or less.
DER systems typically use renewable energy sources, including, but not limited to, small hydro, biomass, biogas, solar power, wind power, geothermal power. These systems increasingly play an important role for the electric power distribution system. A grid-connected device for electricity storage can also be classified as a DER system, and is often called a distributed energy storage system (DESS). By means of an interface, DER systems can be managed and coordinated within a smart grid. Distributed generation and storage enable collection of energy from many sources, decreasing environmental impacts and improving security of electricity supply.
Microgrids are small, local electric grids that can disconnect from the traditional grid and generate electricity using distributed energy resources such as natural gas turbines, solar PVs, and storage. These systems have been shown to provide power to critical community services during extreme weather events such as Hurricane Sandy and the June 2012 derecho storm. Microgrids provide benefits to building owners and the utility electric grid by offering grid-hardening services (fortifying the grid against outages) that are critical to the resilience of the utility electric system. Microgrid technologies are becoming increasingly cost-effective investments, especially in situations where uninterruptable quality power is required 365 days per year such as hospitals.
Microgrids are being developed throughout the United States, especially in the Northeast. Governments are partnering with businesses to strategically deploy microgrids where they can best benefit communities during an emergency. Many of these systems are privately financed with revenues from selling energy services into the regional energy market. As energy project developers continue to create new business models and value streams—such as energy efficiency or renewable energy generation projects, or energy pricing based on reliability factors—microgrids will increase in popularity as an option for local electricity distribution.
Storing and distributing electricity as effectively as possible is very important because electricity supply (generation) and demand (consumption) vary daily and yearly, and suppliers must meet demand instantaneously. If excess electricity is produced during periods of low demand, it will be lost if it can’t be stored for use at a later time. Similarly, electricity produced in one location might go unused because of low demand, while another location is underserved by supply because too little electricity is produced. Without efficient storage technologies, renewable generation cannot address this issue, due to the intermittent nature of many renewable technologies.
A 100% efficient energy storage device would not lose any energy during the energy storage process—all energy input to the device would be available for output. This level of efficiency remains elusive, but some technologies exist at lower efficiency levels that are able to capture excess energy to be used when needed:
An example of an advanced battery technology are lithium-ion batteries, which are available in varying sizes from utility scale to consumer electronics. Batteries composed of other chemistries offer different advantages. In general, advanced batteries offer several advantages:
- More efficient than lead-acid batteries
- Provide more energy with a smaller unit than traditional batteries
- Last twice as long as conventional batteries
- Some varieties, such as sodium sulfur batteries, can operate under much higher temperatures
- High-power and high-capacity uses, such as in a power grid
- Charge is circulated through the battery from a rechargeable and portable external unit that can be moved to where it is needed on demand
Tesla Motors announced on April 30, 2015 they would begin selling an advanced battery pack for home energy storage called Powerwall, and deliveries would begin summer 2015. Powerwall will come in 10 kWh weekly cycle and 7 kWh daily cycle models. “Multiple batteries may be installed together for homes with greater energy need, up to 90 kWh total for the 10 kWh battery and 63 kWh total for the 7 kWh battery”, said Elon Musk, Tesla CEO, “and the suggested retail price for each unit will be $3,500.”
The chatter in the solar industry is that this new storage product is exactly what the PV solar industry needs: a way for residential- and utility- scale solar generators to store power for use when the sun isn’t shining, or for storing power for use during peak grid energy use when the cost of electricity and demand is higher . Early adopters of new battery technologies may pay a premium, but as with other technologies, the greater the scale and volume the lower the cost of adoption.
Fuel cells are not a recent technology. They were first invented in the 1800’s, and they were also used in NASA space flights for a source of backup power and drinking water a by-product of the conversion of hydrogen to electricity.
Hydrogen is the most abundant substance in the universe, but on Earth it is combined with other elements and therefore must be extracted from other sources to be used. This extraction process requires an energy input. For utility-scale electric production, hydrogen is typically extracted using natural gas, which can be costly and releases carbon dioxide, albeit at significantly lower amounts than combustion.
In the U.S., where there is an abundance of natural gas and an existing infrastructure of pipelines for delivery to homes and businesses, fuel cells can utilize natural gas more efficiently and with significantly lower emissions than by burning it. Emissions are so low that some areas in the U.S. have exempted natural gas-fueled fuel cells from air permitting requirements. There are natural gas fuel cells that have been operating in the U.S. for more than a decade.
- Fuel cells are capable of running off hydrogen generated via renewable pathways such as solar and wind, or from methane produced during wastewater treatment, or by anaerobic digestion of waste products from the food, beverage and farming industries.
- Fuel cells are versatile, able to provide primary, backup or auxiliary power, work in tandem with batteries as a range extender, or partner with renewable energy storage systems.
- Fuel cells provide electricity similar to batteries, but unlike batteries, they require a constant input of a fuel source such as hydrogen.
The hydrogen extraction process can be costly and energy-intensive, which currently limits mass adoption. However, when used as part of a backup power system, fuel cells are a component that will add resiliency to a distributed power system. Most of the work toward the advancement of fuel cells is in states where policies support fuel cell technologies, and Maryland is one of top 5 states.
- California’s state government announced it will provide $46.6 million to open 28 new hydrogen fueling stations in the state.
- More than 43 MW of stationary fuel cells have been installed or ordered in California, Connecticut, Maryland, Nebraska, New Jersey and New York.
- The U.S. Department of Energy awarded more than $75 million to various fuel cell and hydrogen projects across the country.
For more on fuel cells, see the U.S. DOE report, State of the States: Fuel Cells in America 2014.
Important research in electrochemical energy conversion and storage is being done here in Maryland at UMERC, University of Maryland Energy Research Center.
Compressed Air Energy Storage
Compressed air on a near-utility scale or a utility scale functions similarly to air compressors used in construction applications. Air is compressed into a reservoir, such as a rock cavern or an abandoned mine, relatively slowly during low energy demand periods. When needed as an energy source during high demand times, the air is released rapidly. The released air can turn a turbine to generate electricity.
Superconducting Magnetic Energy Systems (SMES)
SMES provide peak power for short durations, which can be helpful to manage quick fluctuations in power quality. SMES store energy in the magnetic field created by electricity flow through a superconducting coil.
- Used to bridge periods of power instability or short-term interruptions, such as what may occur while switching from grid electricity to a backup power supply
- Store energy in the magnetic field created when current travels through supercooled conducting material
- SMES produce immediate high power, but for a very short time. New R&D is underway to improve the cooling process, using either liquid helium or liquid nitrogen
Flywheel Energy Storage (FES)
Flywheel energy storage involves bringing a large-sized (large mass) wheel up to a high rotational speed using available power during low demand times. The flywheel turns on a very low friction shaft, so very little energy is needed to keep it spinning once it has reached speed. When energy demands increase to peak, the kinetic energy of the flywheel is used to provide power back to the grid very quickly. Because flywheels typically remain running and available at a moment’s notice, they can be used to meet quick energy demands while grid operators wait for other peaking power systems to spin up to full operation.