Renewable Energy Glossary
Advanced Metering System
Also referred to as “AMS”
Please see Smart Grid.
Abbreviation for “Advanced Metering System”
Please see Smart Grid.
Describes the challenge utilities meet to maintain a balance between energy consumption and the amount of power being generated. If not, a system imbalance can occur which causes electrical equipment and industrial processes to malfunction, lights to flicker, and can cause damage to sensitive electrical equipment – all of which are things that society cannot tolerate. If the imbalance is significant enough the entire electric grid can fail causing black outs. More and more utilities are using a combination of conventional generation produced by sources such as natural gas, coal, and nuclear, and renewable sources such as wind, solar, and geo-thermal to achieve this balance.
Base Load Generation
Power generation plants that operate continuously throughout the year, except for periodic maintenance or upgrading, to meet the majority of customer demand. Utilities traditionally use fossil-fuel generation (natural gas, coal, nuclear) to meet the base level of energy demand because of its reliability.
The baseload they produce is “The average amount of electric power that a utility company must supply at a given time.” (PG&E)
This is a category of sources for power generation derived from biomass (crops like corn) or waste feedstocks (think manure). An example is ethanol. Another is biodiesel which is “A fuel made from vegetable oils, animal fats or recycled grease that can be used instead of petroleum-derived fuel.” (PG&E)
“The amount of electricity a generator can produce when it’s running at full blast. This maximum amount of power is typically measured in megawatts (MW) or kilowatts and helps utilities project just how big of an electricity load a generator can handle.” (US Department of Energy, March 2018)
Capacity does not reflect the same level as actual electricity generated, because plants do not run all the time.
Capacity factor is a measure of how much energy is produced by a plant compared with its maximum output. It is measured as a percentage, generally by dividing the total energy produced during some period of time by the amount of energy the plant would have produced if it ran at full output during that time. “(NREL Sept 2013)
“Capacity factors allow energy buffs to examine the reliability of various power plants. It basically measures how often a plant is running at maximum power. A plant with a capacity factor of 100% means it’s producing power all of the time.” (US Department of Energy, March 2018)
Describes power generation that releases net zero carbon dioxide (CO2) emissions into the atmosphere. (Fast Company, 2018)
Power generation considered carbon neutral would include (Nature.org, May 2013):
- Synthetic fuels
- Wave energy
Biofuels are not included in a zero carbon category (Study.com) because producing biofuels contributes more carbon dioxide to the atmosphere than it displaces in energy generation.
Concentrating Solar Power
Definition below an extract from the SEIA.org Site
Concentrating Solar Power (“CSP”) plants use mirrors to concentrate the sun’s energy to drive traditional steam turbines or engines that create electricity. The thermal energy concentrated in a CSP plant can be stored and used to produce electricity when it is needed, day or night. Today, roughly 1,815 megawatts (MWac) of CSP plants are in operation in the United States.
Components of a Concentrating Solar Power System:
- Parabolic Trough Systems
- Compact Linear Fresnel Reflector
- Power Tower
1. Parabolic Trough Systems
Parabolic trough systems use curved mirrors to focus the sun’s energy onto a receiver tube that runs down the center of a trough. In the receiver tube, a high-temperature heat transfer fluid (such as a synthetic oil) absorbs the sun’s energy, reaching temperatures of 750°F or higher, and passes through a heat exchanger to heat water and produce steam. The steam drives a conventional steam turbine power system to generate electricity. A typical solar collector field contains hundreds of parallel rows of troughs connected as a series of loops, which are placed on a north-south axis so the troughs can track the sun from east to west. Individual collector modules are typically 15-20 feet tall and 300-450 feet long.
2. Compact Linear Fresnel Reflector
CLFR uses the principles of curved-mirror trough systems, but with long parallel rows of lower-cost flat mirrors. These modular reflectors focus the sun’s energy onto elevated receivers, which consist of a system of tubes through which water flows. The concentrated sunlight boils the water, generating high-pressure steam for direct use in power generation and industrial steam applications.
3. Power Tower
Power tower systems use a central receiver system, which allows for higher operating temperatures and thus greater efficiencies. Computer-controlled mirrors (called heliostats) track the sun along two axes and focus solar energy on a receiver at the top of a high tower. The focused energy is used to heat a transfer fluid (over 1,000° F) to produce steam and run a central power generator. Energy storage can be easily and efficiently incorporated into these projects, allowing for 24 hour power generation.
Mirrors are distributed over a parabolic dish surface to concentrate sunlight on a receiver fixed at the focal point. In contrast to other CSP technologies that employ steam to create electricity via a turbine, a dish-engine system uses a working fluid such as hydrogen that is heated up to 1,200° F in the receiver to drive an engine. Each dish rotates along two axes to track the sun.
Key Requirements for Concentrating Solar Power Plants
Financing – The primary challenge for any utility-scale energy generating facility, including CSP, is project financing.
Areas of high solar radiation – In order to concentrate the sun’s energy, it must not be too diffuse. This is measured by the direct normal intensity (DNI) of the sun’s energy. Production potential in the U.S. Southwest stands apart from the rest of the U.S., as the map from the National Renewable Energy Laboratory below demonstrates.
An abbreviation for “Concentrating Solar Power.”
Please see Concentrating Solar Power.
The amount of electricity that utility customers are using, or trying to use, at one time. (PG&E) It fluctuates throughout the day and night (see Demand Curve), and utilities must forecast those changes in order to bring more generation and power on the grid just before it is needed or less power when demand dips.
If you use a line graph to track customer demand in a 24-hour period or throughout the year, it produces a curve showing those times of day or seasons of the year when power demand is up and when it is down. Utilities forecast customer demand for electricity so they can produce or purchase the power to deliver through the grid to meet demand. (EnergyMag)
DERs refers to Distributed Energy Resources – please see the definition of Distributed Energy Resources
Distributed Energy Resources
Distributed Energy Resources are often referred to as “DERs”
From the Advanced Energy Economy (AEE) Website: “DERs are physical and virtual assets that are deployed across the distribution grid, typically close to load, and usually behind the meter, which can be used individually or in aggregate to provide value to the grid, individual customers, or both. A particular industry interest seems to be centered on DERs — such as solar, storage, energy efficiency, and demand management — that can be aggregated to provide services to the electric grid.” (Distributed Energy Resources 101: Required Reading for a Modern Grid, on AEE.net)
One other definitionon DERs from ArenaWire in Australia as follows: “…small-scale units of local generation connected to the grid at distribution level….Common examples of DERs include rooftop solar PV units, natural gas turbines, microturbines, wind turbines, biomass generators, fuel cells, tri-generation units, battery storage, electric vehicles (EV) and EV chargers, and demand response applications. These separate elements work together to form distributed generation.” (Arenawire, March 2018)
DERs are advanced technology allowing for two-way power flow on the grid, so these units can generate power that flows back into the grid for use by other customers.
Distributed Generation is sometimes referred to as “DG”
Distributed generation is an approach that employs small-scale technologies to produce electricity close to the end users of power. DG technologies often consist of modular (and sometimes renewable-energy) generators, and they offer a number of potential benefits. In many cases, distributed generators can provide lower-cost electricity and higher power reliability and security with fewer environmental consequences than can traditional power generators. (Virginia Tech)
For example, rooftop solar would be a form of distributed generation. (AECT)
May also be referred to as “Power” or “Power Generation”
Producing electricity (which is not readily available in nature in any significant amount for use) from sources of fuel / primary energy. Electricity is most often generated at a power plant (power station) by electromechanical generators.
These generators are primarily driven by heat engines fueled by combustion or nuclear fission or by other means (kinetic energy of flowing water & wind, photovoltaics, or geothermal power).
For electric power industries (utilities) – this is the first stage in delivering electricity to the end user.
This term may be used to describe two things.
- First, Energy Shifting may be used to describe the process of using of energy storage systems (ESS) to store excess solar energy to be distributed at night or when there is a peak in demand on a very hot day. A more in-depth discussion about the types of ESS and how they improve the reliability of energy can be found at this link: Implementing Energy Storage at the Caterpillar Industries Website.
- Second, Energy Shifting may also be used to describe (from a social-political-cultural point of view) the move from traditional fossil-fuel generation to renewables for electricity generation. Check out this link NY Times, December 2018 to see how each state in the U.S. is implementing Energy Shifting in this regard.
Energy Storage Systems (ESS)
Energy Storage Systems (often referred to as “ESS“) are the set of methods and technologies used to store electricity.
From the StudentEnergy.org site, as follows:
There are many different forms of energy storage.
- Solid State Batteries: a range of electrochemical storage solutions, including advanced chemistry batteries and capacitors
- Flow Batteries: batteries where the energy is stored directly in the electrolyte solution enabling longer charge/discharge cycles, usually four hours each.
- Flywheels: mechanical devices that harness rotational energy to deliver instantaneous electricity
- Compressed Air: utilize compressed air to create energy reserves
- Pumped hydro-power: creates energy reserves by using gravity and the manipulation of water elevation
- Thermal: capturing heat or cold to create energy
Flexibility in this case refers to power system operation, specifically as follows from the National Renewable Energy Laboratory website:
“Flexibility of operation—the ability of a power system to respond to change in demand and supply—is a characteristic of all power systems. Flexibility is especially prized in twenty-first century power systems, with higher levels of grid-connected variable renewable energy (primarily, wind and solar).” (National Renewable Energy Laboratory: 21st Century Power Partnership, May 2014)
Per EnergyPedia.info: As the world shift to more renewable energy, especially variable ones such as wind and solar, a paradigm shift in the power sector has gradually taken place to meet the transition. In particular is the growing trend of more focus on the so-called “Power System Flexibility” in the academic and industrial sectors in this field.
According to the International Energy Agency, the flexibility of a power system refers to “the extent to which a power system can modify electricity production or consumption in response to variability, expected or otherwise”. Another source described it as “the modification of generation injection and/or consumption patterns in reaction to an external signal (price signal or activation) in order to provide a service within the energy system” .
Flexibility can therefore also refer to the capability to change power supply/demand of the system as a whole or a particular unit (eg. a power plant or a factory). (EnergyPedia)
Without sufficient flexibility, system operators may need to frequently curtail (decrease the output of) wind and solar generation. Although low levels of curtailment (e.g., less than 3%) may be a cost-effective source of flexibility, significant amounts of curtailment can degrade project revenues and contract values, impact investor confidence in renewable energy revenues, and make it more difficult to meet emissions targets. (From NREL)
Describes the rate per second at which electrical current changes direction or alternates between positive and negative voltage. It is measured in hertz (Hz), an international unit of measure where 1 hertz is equal to 1 cycle per second.
The US standard power frequency is 60 Hertz. In other parts of the world, 50 Hertz is used. The frequency for all types of power generation needs to be at the standard to keep on our lights.
If the different generators don’t spin at the same speed, the system becomes unstable. If there is more demand for electricity than there is supply — frequency will fall. If there is too much supply the frequency will rise. Increases or decreases in power frequency as little as one percent will put equipment and power infrastructure at risk of damage.
“The maximum demand that a given generator or group of generators can meet at a given time. For example, a 1,000 megawatt power plant could meet the demand of 1,000 homes using 1 kW of power simultaneously.” (AECT)
Please refer to Geo-Thermal Energy.
“This is energy available as heat emitted from within the earth’s crust, usually in the form of hot water or steam.” (IEA) In the U.S. geothermal energy is often used at the household or campus level.
In terms of a country or region that uses this to a large extent: the unique geology of Iceland allows for 25% of its total electricity production to come from geothermal power facilities. (National Energy Authority of Iceland)
Green Certificate may also be known as a Renewable Energy Certificate (REC)
“A Green Certificate is a tradeable asset which proves that electricity has been generated by a renewable (green) energy source.” (Kyos.com)
Owners of renewable generation have one REC for each megawatt of generation. They can keep or sell their certificates. REC purchasers can claim that the energy they used came from a renewable source. Such purchases support renewable power generation.
The EPA has a video on RECs at this link: RECs: Making Green Power Possible.
Sometimes referred to as “GHG” or “GHGs” – a term for gases that trap heat in the atmosphere; essentially, “gases that contribute to the greenhouse effect by absorbing infrared radiation (heat).” (IEA)
Primary examples of these gases as follows (courtesy of the EPA.gov site):
- : Carbon dioxide enters the atmosphere through burning fossil fuels (coal, natural gas, and oil), solid waste, trees and other biological materials, and also as a result of certain chemical reactions (e.g., manufacture of cement). Carbon dioxide is removed from the atmosphere (or “sequestered”) when it is absorbed by plants as part of the biological carbon cycle.
- : Methane is emitted during the production and transport of coal, natural gas, and oil. Methane emissions also result from livestock and other agricultural practices and by the decay of organic waste in municipal solid waste landfills.
- : Nitrous oxide is emitted during agricultural and industrial activities, combustion of fossil fuels and solid waste, as well as during treatment of wastewater.
- : Hydrofluorocarbons, perfluorocarbons, sulfur hexafluoride, and nitrogen trifluoride are synthetic, powerful greenhouse gases that are emitted from a variety of industrial processes. Fluorinated gases are sometimes used as substitutes for stratospheric ozone-depleting substances (e.g., chlorofluorocarbons, hydrochlorofluorocarbons, and halons). These gases are typically emitted in smaller quantities, but because they are potent greenhouse gases, they are sometimes referred to as High Global Warming Potential gases (“High GWP gases”).
“The electrical energy derived from turbines being spun by fresh flowing water. This can be from rivers or from man-made installations, where water flows from a high-level reservoir down through a tunnel and away from a dam.” (IEA)
“A snapshot of the amount of electric power required to meet customers’ demand at a given time, expressed in kilowatts (kW) or Megawatts (MW).” (AECT)
Additionally, from Memphis Light, Gas, and Water (MLGW): At any given moment, the actual amount of power a customer is using is that customer’s load. Peak load is the highest amount of electricity drawn from the utility during a given increment of time—for example, per month, per day, or per hour. Each utility must plan to have the capacity needed to satisfy the anticipated peak load on its system at any time.
“Microgrids are localized grids that can disconnect from the traditional grid to operate autonomously. …Microgrids support a flexible and efficient electric grid by enabling the integration of growing deployments of distributed energy resources such as renewables like solar.” (US Dept of Energy)
A more technical definition comes from the Lawrence Berkeley National Laboratory (Berkeley Lab):
A microgrid is a localized group of electricity sources and sinks (loads) that typically operates connected to and synchronous with the traditional centralized grid (macrogrid), but can disconnect and maintain operation autonomously as physical and/or economic conditions dictate.
There is also no specific guidance on the size of microgrids. Instead, microgrid definitions focus primarily on two features:
- A microgrid is a locally controlled system
- A microgrid can function both connected to the traditional grid (megagrid) or as an electrical island.
There are two major types of microgrids, as discussed in building-microgrid.lbl.gov/types-microgrids. These include microgrids wholly on one site, akin to a traditional utility customer, which are usually called customer microgridsor true microgrids (µgrids), and microgrids that involve a segment of the legacy regulated grid, often called milligrids (mgrids).
Net Zero Carbon
Please see “Carbon Neutral“
Net-Zero Emissions means that carbon dioxide and other greenhouse gas emissions are reduced 100%, to zero, although some can be allowed if compensatory carbon negative processes are introduced, for example, air capture of carbon dioxide.” (PhysicsWorld, Oct 2018)
Fuels which were not formed by the process whereby prehistoric plants & animals died and were gradually buried by layers of rock.
This category of fuels includes renewables like solar, wind, biomass, wave energy, and nuclear energy. (US Energy Information Administration)
Please see “Peak Demand“
Also referred to as “Peak Load” or “On-Peak”
“The maximum expected load for a given period of time….” usually expressed in a 24 hour period. (AECT) In winter months, peak demand is often in the morning when people are preparing to go to work and school and in the evening when they return home from work and school. In summer months, peak demand is often from mid-afternoon to early evening, when outside temperatures are at their hottest and the air conditioning is always running
Peak demand is typically characterized as annual, daily or seasonal and has the unit of power. Peak demand, peak load or on-peak are terms used in energy demand management describing a period in which electrical power is expected to be provided for a sustained period at a significantly higher than average supply level. Peak demand fluctuations may occur on daily, monthly, seasonal and yearly cycles. (Wikipedia)
Please see “Peak Demand“
Generation that only operates at times of high demand, and is not needed at times of low demand. (AECT)
This is a semiconductor device that directly converts “solar energy into electricity.” (IEA) It is one of the main processes solar panels use to produce electricity.
The term “photovoltaic” comes from the Greek φῶς (phōs) meaning “light”, and from “volt”, the unit of electromotive force, the volt, which in turn comes from the last name of the Italian physicist Alessandro Volta, inventor of the battery. (From Wikipedia).
Renewable Energy Certificate
Please see Green Certificate.
May be referenced as “REC” or “RECs”
Renewable Portfolio Standard (RPS)
“A legislative or regulatory mandate or goal for power generated from renewable sources.” (AECT)
Below is an expanded definition and explanation from the The National Renewable Energy Laboratory (NREL) Website on the the RPS Topic
A Renewable Portfolio Standard (RPS) is a regulatory mandate to increase production of energy from renewable sources such as wind, solar, biomass and other alternatives to fossil and nuclear electric generation. It’s also known as a renewable electricity standard.
An RPS is most successful in driving renewable energy projects when combined with the federal production tax credit. States often design them to drive a particular technology by providing “carve out” provisions that mandate a certain percentage of electricity generated comes from a particular technology (e.g. solar or biomass). States can choose to apply the RPS requirement to all its utilities or only the investor owned utilities. States can also define what technologies are eligible to count towards the RPS requirements.
Having adequate transmission capacity to accommodate generation from renewable resources is important for the success of an RPS. States with successful RPSs either have adequate transmission available or plans to build it.
Ratepayer impacts of an RPS can also derail its adoption politically. A counterbalance to the impacts on ratepayers is that RPS mandates usually drive local economic growth. Under a well designed RPS, costs are shared fairly by all ratepayers. Another way to address ratepayer impacts is to include provisions in the RPS to prevent costs from escalating excessively.
Design Best Practices
When designing an RPS, incorporate the following best practices:
- RPS targets should be stable, ramp up steadily over time and not be subject to sudden or uncertain shifts
- An RPS program should be of sufficient duration to allow for long-term contracting and financing
- An RPS program should apply to all load-serving entities: investor owned, municipal, and electric cooperatives, including suppliers of last resort
- The eligibility of specific renewable energy technologies and generators should be well defined
- Use of tradable renewable energy credits for RPS compliance should be considered and adhered to with a robust tracking system
- The cost of RPS compliance should be allocated fairly across all utility customers
- An RPS program should be mandatory and impose non compliance penalties on those entities that fail to meet requirements.
For more information about renewable portfolio standards, see the following NREL publications:
- International Best Practices for Implementing and Designing Renewable Portfolio Standard Policies
- So you have questions about…Renewable Portfolio Standards: Resources & Technical Assistance
- Survey of State-Level Cost and Benefit Estimates of Renewable Portfolio Standards
- Including Alternative Resources in State Renewable Portfolio Standards Current Design and Implementation Experience
- State of the States 2010: The Role of Policy in Clean Energy Market Transformation
- State Clean Energy Practices: Renewable Portfolio Standards
The following resources may also prove helpful:
- Database of State Incentives for Renewable and Efficiency (DSIRE). A comprehensive source of information on incentives and policies that support renewables and energy efficiency in the United States. Also includes “Solar Carve-outs in Renewable Portfolio Standards,” which provides a high level summary of solar carve-outs in RPS policies and a summary of best practices.
- National Conference of State Legislatures’ Energy and Environment Legislation Tracking Database Searchable database for information on renewable energy bills that have been introduced in the 50 states and the District of Columbia.
- Recommended Principles and Best Practices for State Renewable Portfolio Standards. State-Federal RPS Collaborative (2009). Identifies principles and best practices emerging from state experiences to assist policymakers.
- Renewables Portfolio Standards in the United States: A Status Update. Lawrence Berkeley National Laboratory (2013). The presentation provides a review of current RPS policies and highlights carve-outs and recent political challenges; shows positive impacts on renewable energy development.
- Renewable Portfolio Standards Resources. Lawrence Berkeley National Laboratory. A resource for data, presentations and LBNL reports on RPS.
- The State of State Renewable Portfolio Standards PDF. Clean Energy States Alliance. Provides an overview of RPS track records, highlights their strengths, and outlines ways of addressing weaknesses.
Grid instability that occurs as a result of renewable generation because of its dependence on ideal weather condition for generation. Advances in technology (specifically with energy storage and grid management) are part of reducing such volatility.
A Smart Grid is also referred to as an “Advanced Metering System” or simply as “AMS”
“An enhancement to the electric grid that allows customers to use more technologically advanced electric meters that provide greater detail and increased control over their electric usage. The system also creates new efficiencies and capabilities for transmission and distribution utilities, such as remote meter-reading and improved storm response.” (AECT)
The benefits associated with a Smart Grid include (from the SmartGrid.gov site):
- More efficient transmission of electricity
- Quicker restoration of electricity after power disturbances
- Reduced operations and management costs for utilities, and ultimately lower power costs for consumers
- Reduced peak demand, which will also help lower electricity rates
- Increased integration of large-scale renewable energy systems
- Better integration of customer-owner power generation systems, including renewable energy systems
- Improved security
Solar Power Generation
Describes processes that use energy from the sun to create electricity.
From the US Department of Energy:
There are two main types of solar energy technologies:
Photovoltaic (often referred to as “PV”) is utilized in panels. When the sun shines onto a solar panel, photons from the sunlight are absorbed by the cells in the panel, which creates an electric field across the layers and causes electricity to flow.
Concentrating Solar Power (often referred to as “CSP”) – It is used primarily in very large power plants and is not appropriate for residential use. This technology uses mirrors to reflect and concentrate sunlight onto receivers that collect solar energy and convert it to heat, which can then be used to produce electricity. (US Department of Energy)
Solar water heating
“This term comprises various technologies that convert sunlight into renewable energy that heats water using a solar thermal collector.” (PG&E) It is one of the processes solar panels use to produce electricity.
This describes the amount of extra generation capacity not being used at a generating unit or units that can be ramped up within 10 minutes to meet unexpected peaking demand.
Zero Net Carbon
Please see “Carbon Neutral“
Zero-emission technologies do not emit greenhouse gas into the atmosphere. “In the energy sector that means using non-fossil energy sources, i.e. nuclear or renewables. However, neither is entirely carbon-free – at present we use fossil fuel to make the materials for the energy conversion technologies involved and, in the case of nuclear, to extract and process nuclear fuel. Nevertheless, they are both low-carbon options.” (PhysicsWorld, Oct 2018)