Dynamic Demand is the name of a semi-passive technology to support demand response by adjusting the load demand 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 controls, intermittent domestic and industrial loads switch themselves on/off at optimal moments to balance the overall grid 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. Other patents have been issued based on this idea.
Dynamic demand is similar to demand response mechanisms to manage domestic and industrial consumption of electricity in response to supply conditions, for example, having electricity customers reduce their consumption at critical times or in response to prices. The difference is that dynamic demand devices passively shut off when stress in the grid is sensed, whereas demand response mechanisms respond to transmitted requests to shut off,
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 market 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 increased greenhouse gas emissions, is associated with this imbalance.
Local load control
In principle, any appliance that operates to a duty cycle (such as industrial or domestic air conditioners, water heaters, heat pumps 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 generators on the system, millions of such devices acting together would act like a huge, fast-reacting peaking power plant.
Ancillary services
The dynamic controller could also provide other ancillary services, 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 voltages are low. This causes huge overloads that trip local breakers delaying full system recovery. The dynamic controller could have these loads "wait their turn", as it were, until full power had been restored.