Dynamic demand (electric power)
Encyclopedia
Dynamic Demand is the name of a semi-passive technology for adjusting load demand
s on an electrical power grid. (It is also the name of an independent not-for-profit organization in the UK
supported by a charitable grant from the Esmée Fairbairn Foundation
dedicated to promoting this technology.) The concept is that by monitoring the frequency
of the power grid, as well as their own control parameters, individual, intermittent loads would switch on or off at optimal moments to balance the overall system load with generation, reducing critical power mismatches. As this switching would only advance or delay the appliance operating cycle by a few seconds, it would be unnoticeable to the end user. This is the foundation of dynamic demand control. In the United States, in 1982, a (now-lapsed) patent for this idea was issued to power systems engineer Fred Schweppe.
in electricity
is able to schedule just enough base load
in advance. Any remaining imbalance would then be due either to inaccuracies in the prediction, or unscheduled changes in supply (such as a power station
fault) and/or demand. Such imbalances are removed by requesting generators to operate in so called frequency response mode (also called frequency control mode), altering their output continuously to keep the frequency near the required value.
The grid frequency is a system-wide indicator of overall power imbalance. For example, it will drop if there is too much demand because generators will start to slow down slightly. A generator
in frequency-response mode will, under nominal
conditions, run at reduced output in order to maintain a buffer of spare capacity. It will then continually alter its output on a second-to-second basis to the needs of the grid with droop speed control
.
This spinning reserve
is a significant expense to the power utilities as often fuel
must be burned or potential power sales lost to maintain it. The kind of generation used for fast response is usually fossil fuel
powered which produces emissions of between 0.48 and 1.3 tonnes of CO2 equivalent for every megawatt hour (MWh) generated. Thus a significant environmental burden, in the form of in increased greenhouse gas
emissions, is associated with this imbalance.
that operates to a duty cycle
(such as industrial or domestic air conditioners, water heaters, heat pump
s and refrigeration
) could be used to provide a constant and reliable grid balancing service by timing their duty cycles in response to system load.
Because it is possible to measure grid frequency from any power outlet on the grid, it is possible to design controllers for electrical appliances that detect any frequency imbalance in real time. Dynamic-demand enabled appliances would react to this same signal. When the frequency decreases they would be more likely to switch off, reducing the load on the grid and helping to restore the balance. When the frequency increases past the standard, they would be more likely to switch on, using up the excess power. Obviously, the controller
must also ensure that at no point does the appliance stray out of its acceptable operating range. As line frequency
is directly related to the speed of rotation of generator
s on the system, millions of such devices acting together would act like a huge, fast-reacting peaking power plant
.
, such as aiding blackstart recovery
—the ability of a power grid to be brought back to service after a power outage
– if programmed with that function. Generally blackstarts are made more difficult because of the large number of reactive loads
attempting to draw power simultaneously at start up when voltage
s are low. This causes huge overload
s that trip local breaker
s delaying full system recovery. The dynamic controller could have these loads "wait their turn", as it were, until full power had been restored.
Another vital balancing service is ‘fast reserve’ which is the use of standby plant to replace possible lost generation (e.g. due to a failed power generator or lost power line
). By shedding load quickly while the running generators spin up, then switching back in to bring the frequency back to standard, dynamic controllers could spare the high cost of fast reserve generators
. Also the fast response speed of this method would avoid possible brownouts
occurring.
The technology could also help facilitate greater use of generation from variable sources, like wind power
. Demand-side techniques could be an efficient and cost effective way to help integrate this resource onto the grid. In particular it would allow these sources to work in conjunction with virtual power reserves like municipal water tower
s to provide a reasonably predictable dispatchable
capacity.
by the services they provide. But before dynamic demand control can be widely incorporated regulation must be put in place to mandate installation on at least new appliances or an effective market mechanism must be created to reward installation of the technology fairly. One method contemplated is to enable the electricity meter
that measures the electricity consumption also measure the grid frequency, and switch to a higher tariff
if the frequency drops below a certain level. The monthly electricity bill will then say that so many hours (and so many kilowatt hours) were on the Regular tariff and a few hours on the Short Supply tariff. Those consumers without smart demand management have to pay the extra cost, but those who install smart technologies that adapt to the short supply periods will save money.
On the 1st March 2011, RLtec launched a dynamic demand frequency response service in hot water and HVAC load devices distributed across Sainsbury supermarket sites. This MW scale virtual power plant service provides commercial frequency regulating response to National Grid in the UK.
http://stadium.open.ac.uk/stadia/preview.php?s=1&whichevent=720
Up to 5 GW of such diesel generation is used in France for similar purposes, but these technologies seem to be relatively unknown http://www.claverton-energy.com/conference/programme. There is no reason they should not be massively increased in scope http://www.claverton-energy.com/download/131/ to cope with even the intermittence introduced by wind power, which would in fact be less than the intermittence already inherent due to the unreliability of large power stations. For example, in the UK, Sizewell B can impose an instantaneous cut in generation of 1.2 GW http://david-andrews-wind-energy.wikispaces.com/, and indeed did so in May 2008, which is far more severe than the swings which could currently occur in a 100% wind scenario.
However it was recently announced that domestic fridges are now being sold into the UK incorporating a dynamic load control system http://www.claverton-energy.com/bbc-talks-about-dynamic-demand-smart-fridges-and-smart-metering.html
Demand response
In electricity grids, demand response is similar to dynamic demand mechanisms to manage customer consumption of electricity in response to supply conditions, for example, having electricity customers reduce their consumption at critical times or in response to market prices...
s on an electrical power grid. (It is also the name of an independent not-for-profit organization in the UK
United Kingdom
The United Kingdom of Great Britain and Northern IrelandIn the United Kingdom and Dependencies, other languages have been officially recognised as legitimate autochthonous languages under the European Charter for Regional or Minority Languages...
supported by a charitable grant from the Esmée Fairbairn Foundation
Esmee Fairbairn Charitable Trust
The Esmée Fairbairn Foundation is a registered charity founded in England in 1961. It is one of the largest independent grantmaking foundations in England, making grants to organisations which aim to improve the quality of life for people and communities in the UK, both now and in the future.The...
dedicated to promoting this technology.) The concept is that by monitoring the frequency
Frequency
Frequency is the number of occurrences of a repeating event per unit time. It is also referred to as temporal frequency.The period is the duration of one cycle in a repeating event, so the period is the reciprocal of the frequency...
of the power grid, as well as their own control parameters, individual, intermittent loads would switch on or off at optimal moments to balance the overall system load with generation, reducing critical power mismatches. As this switching would only advance or delay the appliance operating cycle by a few seconds, it would be unnoticeable to the end user. This is the foundation of dynamic demand control. In the United States, in 1982, a (now-lapsed) patent for this idea was issued to power systems engineer Fred Schweppe.
The need for spinning reserve
The power utilities are able to predict to a reasonable accuracy (generally to within one or two percent) the demand pattern throughout any particular day. This means that the free marketFree market
A free market is a competitive market where prices are determined by supply and demand. However, the term is also commonly used for markets in which economic intervention and regulation by the state is limited to tax collection, and enforcement of private ownership and contracts...
in electricity
Electricity
Electricity is a general term encompassing a variety of phenomena resulting from the presence and flow of electric charge. These include many easily recognizable phenomena, such as lightning, static electricity, and the flow of electrical current in an electrical wire...
is able to schedule just enough base load
Base load power plant
Baseload is the minimum amount of power that a utility or distribution company must make available to its customers, or the amount of power required to meet minimum demands based on reasonable expectations of customer requirements...
in advance. Any remaining imbalance would then be due either to inaccuracies in the prediction, or unscheduled changes in supply (such as a power station
Power station
A power station is an industrial facility for the generation of electric energy....
fault) and/or demand. Such imbalances are removed by requesting generators to operate in so called frequency response mode (also called frequency control mode), altering their output continuously to keep the frequency near the required value.
The grid frequency is a system-wide indicator of overall power imbalance. For example, it will drop if there is too much demand because generators will start to slow down slightly. A generator
Electrical generator
In electricity generation, an electric generator is a device that converts mechanical energy to electrical energy. A generator forces electric charge to flow through an external electrical circuit. It is analogous to a water pump, which causes water to flow...
in frequency-response mode will, under nominal
Real versus nominal value
In economics, nominal value refers to a value expressed in money terms in a given year or series of years. By contrast, real value adjusts nominal value to remove effects of price changes over time...
conditions, run at reduced output in order to maintain a buffer of spare capacity. It will then continually alter its output on a second-to-second basis to the needs of the grid with droop speed control
Droop speed control
In electricity generation, droop speed control is the primary instantaneous system using net frequency deviations to distribute with stability load changes over generating plants....
.
This spinning reserve
Peaking power plant
Peaking power plants, also known as peaker plants, and occasionally just "peakers," are power plants that generally run only when there is a high demand, known as peak demand, for electricity.-Peak hours:...
is a significant expense to the power utilities as often fuel
Fuel
Fuel is any material that stores energy that can later be extracted to perform mechanical work in a controlled manner. Most fuels used by humans undergo combustion, a redox reaction in which a combustible substance releases energy after it ignites and reacts with the oxygen in the air...
must be burned or potential power sales lost to maintain it. The kind of generation used for fast response is usually fossil fuel
Fossil fuel
Fossil fuels are fuels formed by natural processes such as anaerobic decomposition of buried dead organisms. The age of the organisms and their resulting fossil fuels is typically millions of years, and sometimes exceeds 650 million years...
powered which produces emissions of between 0.48 and 1.3 tonnes of CO2 equivalent for every megawatt hour (MWh) generated. Thus a significant environmental burden, in the form of in increased greenhouse gas
Greenhouse gas
A greenhouse gas is a gas in an atmosphere that absorbs and emits radiation within the thermal infrared range. This process is the fundamental cause of the greenhouse effect. The primary greenhouse gases in the Earth's atmosphere are water vapor, carbon dioxide, methane, nitrous oxide, and ozone...
emissions, is associated with this imbalance.
Local load control
In principle, any applianceHome appliance
Home appliances are electrical/mechanical machines which accomplish some household functions, such as cooking or cleaning. Home appliances can be classified into:*Major appliances, or White goods*Small appliances, or Brown goods...
that operates to a duty cycle
Duty cycle
In engineering, the duty cycle of a machine or system is the time that it spends in an active state as a fraction of the total time under consideration....
(such as industrial or domestic air conditioners, water heaters, heat pump
Heat pump
A heat pump is a machine or device that effectively "moves" thermal energy from one location called the "source," which is at a lower temperature, to another location called the "sink" or "heat sink", which is at a higher temperature. An air conditioner is a particular type of heat pump, but the...
s and refrigeration
Refrigeration
Refrigeration is a process in which work is done to move heat from one location to another. This work is traditionally done by mechanical work, but can also be done by magnetism, laser or other means...
) could be used to provide a constant and reliable grid balancing service by timing their duty cycles in response to system load.
Because it is possible to measure grid frequency from any power outlet on the grid, it is possible to design controllers for electrical appliances that detect any frequency imbalance in real time. Dynamic-demand enabled appliances would react to this same signal. When the frequency decreases they would be more likely to switch off, reducing the load on the grid and helping to restore the balance. When the frequency increases past the standard, they would be more likely to switch on, using up the excess power. Obviously, the controller
Controller (control theory)
In control theory, a controller is a device which monitors and affects the operational conditions of a given dynamical system. The operational conditions are typically referred to as output variables of the system which can be affected by adjusting certain input variables...
must also ensure that at no point does the appliance stray out of its acceptable operating range. As line frequency
Utility frequency
The utility frequency, line frequency or mains frequency is the frequency at which alternating current is transmitted from a power plant to the end-user. In most parts of the world this is 50 Hz, although in the Americas it is typically 60 Hz...
is directly related to the speed of rotation of generator
Electrical generator
In electricity generation, an electric generator is a device that converts mechanical energy to electrical energy. A generator forces electric charge to flow through an external electrical circuit. It is analogous to a water pump, which causes water to flow...
s on the system, millions of such devices acting together would act like a huge, fast-reacting peaking power plant
Peaking power plant
Peaking power plants, also known as peaker plants, and occasionally just "peakers," are power plants that generally run only when there is a high demand, known as peak demand, for electricity.-Peak hours:...
.
Ancillary services
The dynamic controller could also provide other ancillary servicesAncillary services (electric power)
The United States Federal Energy Regulatory Commission defines the ancillary services as:"those services necessary to support the transmission of electric power from seller to purchaser given the obligations of control areas and transmitting utilities within those control areas to maintain...
, such as aiding blackstart recovery
Black start
A black start is the process of restoring a power station to operation without relying on the external electric power transmission network.Normally, the electric power used within the plant is provided from the station's own generators...
—the ability of a power grid to be brought back to service after a power outage
Power outage
A power outage is a short- or long-term loss of the electric power to an area.There are many causes of power failures in an electricity network...
– if programmed with that function. Generally blackstarts are made more difficult because of the large number of reactive loads
Power factor
The power factor of an AC electric power system is defined as the ratio of the real power flowing to the load over the apparent power in the circuit, and is a dimensionless number between 0 and 1 . Real power is the capacity of the circuit for performing work in a particular time...
attempting to draw power simultaneously at start up when voltage
Voltage
Voltage, otherwise known as electrical potential difference or electric tension is the difference in electric potential between two points — or the difference in electric potential energy per unit charge between two points...
s are low. This causes huge overload
Overcurrent
In electricity supply, overcurrent or excess current is a situation where a larger than intended electric current exists through a conductor, leading to excessive generation of heat, and the risk of fire or damage to equipment. Possible causes for overcurrent include short circuits, excessive load,...
s that trip local breaker
Circuit breaker
A circuit breaker is an automatically operated electrical switch designed to protect an electrical circuit from damage caused by overload or short circuit. Its basic function is to detect a fault condition and, by interrupting continuity, to immediately discontinue electrical flow...
s delaying full system recovery. The dynamic controller could have these loads "wait their turn", as it were, until full power had been restored.
Another vital balancing service is ‘fast reserve’ which is the use of standby plant to replace possible lost generation (e.g. due to a failed power generator or lost power line
Electric power transmission
Electric-power transmission is the bulk transfer of electrical energy, from generating power plants to Electrical substations located near demand centers...
). By shedding load quickly while the running generators spin up, then switching back in to bring the frequency back to standard, dynamic controllers could spare the high cost of fast reserve generators
Peaking power plant
Peaking power plants, also known as peaker plants, and occasionally just "peakers," are power plants that generally run only when there is a high demand, known as peak demand, for electricity.-Peak hours:...
. Also the fast response speed of this method would avoid possible brownouts
Power outage
A power outage is a short- or long-term loss of the electric power to an area.There are many causes of power failures in an electricity network...
occurring.
The technology could also help facilitate greater use of generation from variable sources, like wind power
Wind power
Wind power is the conversion of wind energy into a useful form of energy, such as using wind turbines to make electricity, windmills for mechanical power, windpumps for water pumping or drainage, or sails to propel ships....
. Demand-side techniques could be an efficient and cost effective way to help integrate this resource onto the grid. In particular it would allow these sources to work in conjunction with virtual power reserves like municipal water tower
Water tower
A water tower or elevated water tower is a large elevated drinking water storage container constructed to hold a water supply at a height sufficient to pressurize a water distribution system....
s to provide a reasonably predictable dispatchable
Dispatchable generation
Dispatchable generation refers to sources of electricity that can be dispatched at the request of power grid operators; that is, generating plants that can be turned on or off, or can adjust their power output on demand...
capacity.
Implementation issues
Dynamic demand devices have the potential to save considerable amounts of energyEnergy
In physics, energy is an indirectly observed quantity. It is often understood as the ability a physical system has to do work on other physical systems...
by the services they provide. But before dynamic demand control can be widely incorporated regulation must be put in place to mandate installation on at least new appliances or an effective market mechanism must be created to reward installation of the technology fairly. One method contemplated is to enable the electricity meter
Electricity meter
An electricity meter or energy meter is a device that measures the amount of electric energy consumed by a residence, business, or an electrically powered device....
that measures the electricity consumption also measure the grid frequency, and switch to a higher tariff
Tariff
A tariff may be either tax on imports or exports , or a list or schedule of prices for such things as rail service, bus routes, and electrical usage ....
if the frequency drops below a certain level. The monthly electricity bill will then say that so many hours (and so many kilowatt hours) were on the Regular tariff and a few hours on the Short Supply tariff. Those consumers without smart demand management have to pay the extra cost, but those who install smart technologies that adapt to the short supply periods will save money.
On the 1st March 2011, RLtec launched a dynamic demand frequency response service in hot water and HVAC load devices distributed across Sainsbury supermarket sites. This MW scale virtual power plant service provides commercial frequency regulating response to National Grid in the UK.
Frequency service and reserve service
The national grid in the UK already is a massive user of this technology at an industrial scale - up to 2 GW of load can be lost instantaneously by frequency sensitive relays switching off steelworks etc., which is matched over a 20 minute cycle by up to 2 GW of quite small emergency diesel generators. For a complete description of this complex system see for example "Emergency Diesel Standby Generator’s Potential Contribution to Dealing With Renewable Energy Sources Intermittency And Variability" - a talk by David Andrews of Wessex Water who works closely with the UK National Grid to provide this service, given at the Open University Seminar "Coping with Variability - Integrating Renewables into the Electricity System" 24 January 2006.http://stadium.open.ac.uk/stadia/preview.php?s=1&whichevent=720
Up to 5 GW of such diesel generation is used in France for similar purposes, but these technologies seem to be relatively unknown http://www.claverton-energy.com/conference/programme. There is no reason they should not be massively increased in scope http://www.claverton-energy.com/download/131/ to cope with even the intermittence introduced by wind power, which would in fact be less than the intermittence already inherent due to the unreliability of large power stations. For example, in the UK, Sizewell B can impose an instantaneous cut in generation of 1.2 GW http://david-andrews-wind-energy.wikispaces.com/, and indeed did so in May 2008, which is far more severe than the swings which could currently occur in a 100% wind scenario.
UK government investigation
In August 2007, the UK government published a report outlining what potential it sees for dynamic demand technology.http://www.berr.gov.uk/files/file41011.pdf The report stops short of recommending the government encourage its introduction. It lists a number of technical and economic barriers to its introduction and recommends these be investigated before the government encourage the use of dynamic demand. Dynamic demand is one element of a wider government investigation into technologies that can cut greenhouse gas emissions.However it was recently announced that domestic fridges are now being sold into the UK incorporating a dynamic load control system http://www.claverton-energy.com/bbc-talks-about-dynamic-demand-smart-fridges-and-smart-metering.html
See also
- Advanced Distribution AutomationAdvanced Distribution AutomationAdvanced Distribution Automation is a term coined by the IntelliGrid project in North America to describe the extension of intelligent control over electrical power grid functions to the distribution level and beyond. It is related to distribution automation that can be enabled via the smart grid...
- Distribution management system
- Electricity marketElectricity marketIn economic terms, electricity is a commodity capable of being bought, sold and traded. An electricity market is a system for effecting purchases, through bids to buy; sales, through offers to sell; and short-term trades, generally in the form of financial or obligation swaps. Bids and offers use...
- Electricity retailingElectricity retailingElectricity retailing is the final process in the delivery of electricity from generation to the consumer. The other main processes are transmission and distribution.- Beginnings :...
- Energy demand managementEnergy demand managementEnergy demand management, also known as demand side management , is the modification of consumer demand for energy through various methods such as financial incentives and education...
- Flexible AC transmission systemFlexible AC transmission systemA flexible alternating current transmission system is a system composed of static equipment used for the AC transmission of electrical energy. It is meant to enhance controllability and increase power transfer capability of the network...
- Grid friendlyGrid friendlyElectrical devices are considered grid friendly if they operate in a manner that supports electrical power grid reliability. Basic grid-friendly devices may incorporate features that work to offset short-term undesirable changes in line frequency or voltage; more sophisticated devices may alter...