Technology developed in the UK to reduce the supply burden on electricity systems operators is currently undergoing trials on domestic refrigerators in Europe. The concept, when applied to even a small population of new refrigerators, will reduce CO2 emissions and help to smooth the demand for electricity.
issue: January 2008 APPLIANCE Magazine
Appliance Energy Use
Intelligent Software Control Reduces Global Carbon Emissions
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by David Hirst, technical directory and chief technologist, RLtec
Balancing the energy demands of refrigerators with the availability of supply can reduce CO2 emissions and minimize the risk of power cuts.
The Energy Balancing Controller (EBC) is a device that constantly monitors the availability of the incoming electricity supply and balances the demand automatically. Implemented via software that can be installed by white goods manufacturers as part of the production process, the EBC works by altering the operating time of the coolant compressor.
Balancing the Supply
The frequency of an electricity grid is the result of an extraordinary integration or combination of the behaviors of all the devices on the grid. This arises from original concepts patented by Nikola Tesla around 1887 and the theory of synchronous machines that has been well studied since. All devices on the grid contribute to its frequency, albeit often in a small way, and all are affected to some extent by changes in the frequency. Unless actively controlled, this feedback can make the grid unstable. For example a reduced frequency can reduce the output of generators, which leads to a further reduction in frequency, in a potentially vicious cycle that can lead to collapse and blackout. Active control is generally exercised by governors controlling the output of generators and substantial literature exists on optimizing this control. These stability services are generally known as response, or frequency response, devices.
In the late 1970s, Fred Schweppe was awarded a patent for the Frequency Adaptive Power Energy Rescheduler (FAPER) and, with colleagues, proposed methods for autonomous control of grid devices and thus grids. In April 2000, David Hirst, the author of this article, proposed a similar scheme, called ResponsiveLoad (RL), and was awarded a UK patent for it. The broad concept is that consuming devices that incorporate some sort of energy store—LoadStore devices—will normally operate to some sort of duty cycle. When running, they are replenishing the store, but when not running, the store is providing service to the device user. Such devices can, in principle, modify the timing of their consumption without impact on the end use. The broad concept is to use the grid frequency as an input to a controller that then adjusts the timing of the consumption. While the underlying concept of the RL controller is similar to FAPER, the innovation was to use probabilistic methods within the decision-making as to the timing of the LoadStore devices. This gives reliable and managed behavior across a large population of devices and avoids any need for device settings to be centrally managed and planned. Since then, the concept has been refined and enhanced to provide a reliable, predictable, and continuous service to electricity grids.
Systematic Losses and Smart Energy Use
Designed to monitor the frequency of an incoming supply, the EBC has been developed by engineers at RLtec, a UK-based company established to commercialize new technologies and inventions that will improve efficiencies in electricity generation, supply, and consumption. Fundamentally, the EBC determines the extent of shortage, or excess, of electricity generation at any moment in time by monitoring the frequency of supply. These data are then used to intelligently decide when the appliance should be cycled on or off, while still maintaining the optimum operating temperature. By adjusting the cycle time for the compressor relative to the load on the incoming electricity supply, refrigerators can effectively be used as banks of batteries. Subtle alterations to the operating times of the compressor enable energy to be stored in the form of “coolth” (coolness) when demand is high, and drawn from the supply when demand is reduced.
Although no changes in operating efficiency or overall temperature range are experienced by the end-user, the potential savings, both in terms of generating costs and reduced carbon burden, can be enormous. Working on the basis that a typical domestic refrigerator in Europe consumes an average power of 20 W continuously, in the UK alone domestic refrigeration represents 6.4% of the 345 TWh of electricity consumed annually. During normal operation, with no EBC, a compressor typically consumes 100 W and operates for approximately 5 minutes as the temperature is reduced. Over the next 20 minutes, the temperature rises before the cycle repeats (see Fig. 1). A refrigerator’s energy state is related to its temperature, with maximum energy storage occurring at low temperatures.
Figure 1. Refrigerator compressor duty ratio.
Further features are key to the successful working of the EBC. Smooth and progressive switching of fridges as the frequency changes is vital, as many fridges switching at the same time would make the stability problem even worse. So diversity of timing of on-off cycles across the population of appliances has to be maximized at all times.
Reducing Generator Burden and Hot Standby
By applying EBCs to a population of refrigerators, and adjusting compressor switching behavior to increase or reduce power consumption based on the measured grid frequency, the overall effect is to optimize the actual consumption during times when there is least load on the supply system. Unlike power-saving devices, the EBC does not reduce the overall amount of power consumed by an appliance. Instead, it behaves more intelligently than conventional energy-loss controllers or ac motor optimization systems by actually balancing the draw on an electricity supply with the availability, thus reducing the primary energy consumed at the power station. The population of appliances fitted with EBCs provides an automatic balancing service (ABS) that reduces electricity demand at times of shortage and vice versa. National and regional grid networks in many countries rely on having power stations operating on hot standby, ready to feed power into the network as demand increases. Hot-standby generators, normally operating within highly inefficient coal-fired power stations, are responsible for emitting high levels of CO2. By utilizing the battery capability of a large population of refrigerators fitted with EBCs, the need for hot-standby generators is reduced significantly.
In normal operation at 50.0 Hz, compressors will typically be running in 20% of refrigerators and 1 million devices will have an average power consumption of 20 mW. If the grid frequency falls to 49.7 Hz, then the algorithms determining the compressor timings cause 75,000 refrigerator compressors to turn off and consumption drops to 12.5 mW, so 7.5 mW of low-side response is provided. As frequency
continues to reduce to 49.2 Hz, all compressors go off and 20 mW of low frequency response is provided. High-side response is also available. All compressors could be turned on when the frequency goes high and up to 100 mW of high-side response can be provided from the 1 million refrigerators. Fig. 2 illustrates the concept simply, and the frequencies shown and levels of response provided can be tailored by modification to the device’s defining constants. An important point is that the level of response megawatts is proportional to the size of deviation from 50 Hz, and this enhances grid stability.
Figure 2. Behavior of 1 million refrigerators as frequency varies (at 100 mW and a 20% duty cycle.)
To ensure the EBC maintains the important feature of diversity, the behavior of any one refrigerator is influenced by a random probability element so that refrigerators do not become synchronized by their response to grid frequency changes. When this is combined with awareness of the temperature state, so that devices become more “willing to switch” as they approach their normal switching times, the behavior of each appliance is slightly different. So there is a progressive and smooth switching of fridges as the frequency diverges from normal.
By combining this energy storage potential of even a small proportion of the domestic refrigerators, estimates show that the adoption of EBCs in new refrigerators will have a valuable smoothing effect on overall electricity demand, and the grid frequency.
Using Populations of Appliances to Best Effect
There are constraints in the compressor operation for real refrigerators, and it is not possible in practice to turn all off or on. Based on EBC-equipped refrigerator trials, the developer of the technology has modeled the expected behavior of a significant population. The model uses grid frequency profiles and predicts the refrigerator consumption for the population. The tool is useful in analyzing response levels provided with typical grid events. A dip in grid frequency from 50.0 Hz, falling within ~8 seconds to reach a low of 49.5 Hz and then recovering to 49.8 Hz, may be the result of a generator failing on the grid.
The predicted consumption of a population of 1 million EBC-equipped refrigerators is shown in Fig. 3. The normal load of 20 mW reduces near instantaneously to ~8 mW, providing 12 mW of response. The grid frequency remains low and the refrigerators continue to provide response; however, as the energy coolth is consumed, the magnitude of response reduces, for about 15 minutes, when the refrigerators return to near-normal operation.
Figure 3. Grid dip simulation.
The instant reaction to grid frequency changes is valuable and useful to system operators and provides improved security of supply, allowing time to dispatch reserve or response from other generating assets, especially in fault conditions. Provided at the point of use, the response is distributed across the grid, and the population provides significant tolerance to single (or multiple) refrigerator failures. Fig. 4 shows another abnormal grid frequency profile and the high-side response provided by the refrigerators. More often the grid operates with smaller frequency excursions. These small excursions are the result of load and supply instantaneous changes on the grid with a proportion of generator sets operating with a governor control loop. Fig. 5 shows response provided in these conditions.
Figure 4. High-side response for 1 million refrigerators. (Rise to 50.5 Hz no recovery. Range equivalent: 2.86 Hz.)
Figure 5. Response under normal grid operation. (Randomly varying frequency. Range 0.5 Hz.)
A population size of 1 million has been used in illustrating the behavior of the ABS service provided to grids. There are more that 25 million refrigerators in the UK alone, with the UK cold-appliance market selling approximately 3.5 million units annually (mostly replacement of broken appliances). These EBC-equipped refrigerators may be used to displace the use of generators under governor control. The energy stored in refrigeration can also be used to displace some spinning reserve by providing time contingency to bring generation on line. The benefits of the ABS service include:
- A more efficient electricity grid both in terms of carbon and cost.
- A more stable electricity grid.
- Removal of some of the barriers to higher proportions of renewable energy.
- Significantly reduced CO2 emissions from power generation.
The developing firm is working with European refrigerator manufacturers and has successfully tested EBC in refrigeration. The key results showed energy consumption has no effect on safe food storage and does not measurably alter compressor lifetime.
Refrigeration is not the only example of appliances connected to the grid which store energy and have discretion over when their electricity is consumed or not. Air-conditioners, interruptible power supplies, ground source heat pumps, and even automotive battery-charging stations can harness this technology. It is also possible to retrofit the EBC to larger compressor units in commercial and retail refrigeration, and gain reward for the service offered to the grid.
Fundamentally, the EBC technology provides a holistic solution to the problem of standby generation of electricity, and the accompanying carbon emissions burden. It provides obvious advantages to electricity supply operators, because their reliance on the use of hot-standby generators will be reduced significantly. Electricity producers themselves will also benefit from reducing their own carbon footprint, while white goods manufacturers will be able to provide consumers with a more environmentally sustainable appliance. The entire refrigerator supply chain, from electricity supplier through to consumer, would be involved in the ultimate reduction of CO2. Future challenges lie in how companies can cooperate to take the initial investment required in the EBC devices in order to achieve the savings in cost and energy.
About the Author
David Hirst is inventor of the various RLtec technologies. His career has centered on IT, telecom, and control, including work in space, oil and gas, electricity, banking, and other industry sectors. In the early 1980s, he worked on the 1000-house trial for Mains-borne Telecontrol for Multiutility Metering. For nearly 20 years he was with Ernst & Young as a management consultant in IT. It was during his work as central design authority for the New Electricity Trading Arrangements (NETA) that he formed the first concepts for ResponsiveLoad. To contact Hirst, e-mail email@example.com.