Peak Load Limiting

The present invention relates in general to peak load limiting and more particularly concerns novel apparatus and techniques for economically and reliably limiting peak loads in residential installations by automatically interrupting deferrable loads, such as water heaters, dryers and cooking ranges, according to a predetermined priority when the total load drawn by a higher priority appliance exceeds a predetermined value.

A serious problem for utilities providing electrical energy is meeting instantaneous peak capacity. Typical costs for adding peaking capacity are approximately $200 per kilowatt of capability for a combustion turbine installation of the 100 megawatt range and about $1,500 per kilowatt for a large base load coal plant. The energy costs on the busbar for these types of plants are approximately 200 mills per kilowatt-hour and 55 mills per kilowatt-hour for the combustion turbine and coal plant, respectively. All costs stated are 1981 dollars.

Accordingly, a number of utilities are using peak load management techniques to reduce peak load demands. Numerous techniques are available for managing loads. A search of subclasses 35, 38 and 492 of class 307 uncovered U.S. Pat. Nos. 2,199,639, 2,521,171, 2,565,631, 2,637,822, 2,645,726, 3,423,598, 3,858,110, British Pat. No. 553,183, Japanese Pat. No. 53-4030 and United Kingdom Pat. No. 2,019,138. U.S. Pat. Nos. 4,324,987 and 4,336,462 were also found.

It is an important object of the invention to provide an improved yet simple electrical load controller.

According to the invention, there is means for sensing the power or energy drawn by a first applicance and interrupting the flow of power or energy to a second appliance when the power or energy drawn by the first appliance exceeds a predetermined value. In a specific form of the invention, there is current sensing relay means responsive to the current being drawn by the first appliance exceeding a predetermined value for operating the current sensing relay means and opening associated normally closed contacts in series with the second appliance to interrupt power flow to the second appliance until the current drawn by the first appliance again becomes less than the predetermined value to release the current sensing relay means and again close the normally closed contacts.

Numerous other features, objects and advantages of the invention will become apparent from the following specification when read in connection with the accompanying drawing, the single FIGURE of which is a combined block-schematic diagram of an exemplary embodiment of the invention having an electric range, dryer and water heater controlled in accordance with the principles of the invention.

With reference now to the drawing, there is shown a combined block-schematic diagram of an exemplary embodiment of the invention for a residence having an electric range, an electric dryer and an electric water heater controlled in accordance with the invention. An electric range 11, electric dryer 12 and water heater 13 are connected in parallel between lines 14 and 15 providing typically 230 volts a.c. therebetween and 115 volts with respect to neutral line 16. Range 11 is connected in series with two current sensing relay means S1 and S2. Current sensing relay means S1 is associated with normally closed contacts S1' in series with dryer 12. Current sensing relay means S2 is associated with normally closed contacts S2' in series with water heater 13 and line 15 and normally open contacts S2" in series with water heater 13 and line 16.

Operation is as follows: Range operation has priority and is not interrupted. Current sensing relay S1 detects current flow in the range branch circuit when the range is in use. When the current rises to a predetermined value, typically 10.9 amps, corresponding to 2.5 kilowatts of power, the relay operates to open normally closed contacts S1' and interrupt the flow of energy to dryer 12. Reducing range load below 2.5 kilowatts releases relay S1 to close contacts S1' and allow dryer 12 to be restarted. 2.5 kilowatts is sufficient to allow operation of one cooktop burner element at maximum heat and another at partial heat. This mode of operation continues up to a range load of typically 7.5 kilowatts corresponding to 32.6 amps. This load is typically drawn by two oven elements at 3 kilowatts each and one cooktop burner at maximum heat drawing typically 1.5 kilowatts. At this load of 32.6 amps or 7.5 kilowatts, current sensing relay S2 operates to open normally closed contacts S2' and close normally open contacts S2". This action causes the voltage applied to water heater 13 to drop from 230 volts to 115 volts a.c. and thereby reduce the water heating load by substantially 3.4 kilowatts to leave a residual heating capacity of substantially 1.1 kilowatts.

The connected loads of most household ranges, dryers and electric hot water heaters are typically 12, 5 and 4.5 kilowatts, respectively, for a total of 21.5 kilowatts. For each residence having these three electric appliances, the serving electric utility supply system is exposed to that load. With the invention installed, the maximum load imposed by the three appliances is reduced to no more than 13.1 kilowatts, a saving of 8.4 peak kilowatts of load. For 10,000 homes in a service area each equipped with all three appliances, the invention could reduce the utility peak load by 84 megawatts. The actual reduction would be less than the theoretical 84 megawatt upper limit due to normal diversity in the simultaneous use of all three appliances in the hypothetical 10,000 homes. However, the actual reduction would still be substantial. In a city the size of Seattle, Washington, for example, 84 megawatts would represent a reduction in peak load of around 5% for only 10,000 homes equipped with the invention (1981 load data).

These advantages are achieved with relatively minor change in lifestyle that may be classified as a minor inconvenience on occasion. Operation of the dryer cannot occur if the range is operating with one oven element or two burners on. This means that significant cooking activity stops clothes drying, and the dryer requires restarting after the range load is reduced below the threshold of 2.5 kilowatts. Furthermore, use of the oven and two burners curtails 3/4 of water heating capacity, but only when the oven element is on. Typically the oven element is cycling. Thus, water heating would take longer during range use, but families could readily adjust to minor inconveniences in bath and shower schedules for a significant rate reduction on their electrical power consumption. And people would readily adjust to using their electric clothes dryers when not cooking. Some cooking might also be deferred to microwave ovens or other efficient means.

The invention would have the overall effect of shifting the time of day usage of electrical energy. The net change in total annual kilowatt-hour consumption would most likely be none or a slight reduction.

The invention is believed to be capable of being manufactured, sold and installed for less than $400.00 for 8.4 kilowatts in peak generation capacity savings, or less than $50 per kilowatt compared with $200 per kilowatt to increase capacity with a combustion turbine and $1,500 per kilowatt to increase peak capacity with a coal plant. Furthermore the invention has economic advantages from the standpoint of operating and maintenance costs which is equivalent to the lifecycle cost to the utility in mills per kilowatt-hour of the rate reduction offered to the homeowner for installing the device. The utility could readily justify a rate reduction of 1 to 2 cents per kilowatt-hour. In Seattle, Washington, annual consumption is about 12,000 kilowatt-hours for the average home with non-electric heat, and 24,000 kilowatt-hours average for those electrically heated. Using this consumption as an example, homeowners could save from $120 to $480 annually, easily justifying the cost of installation.

There has been described novel apparatus and techniques for significantly reducing peak load demands with reliable apparatus relatively inexpensive to manufacture and install in a safe manner. It is evident that those skilled in the art may now make numerous uses and modifications of and departures from the specific embodiments described herein without departing from the inventive concepts. Consequently, the invention is to be construed as embracing each and every novel feature and novel combination of features present in or possessed by the apparatus and techniques herein disclosed and limited solely by the spirit and scope of the appended claims.