Power Demand Limiting Circuit

Consumers of electrical power today are often charged by electrical utilities for both total power consumption during a billing period as well as for peak demand. This charge for peak demand is made irrespective of actual consumption made by the consumer during any specific period. The justification for this charge rests in the necessity of the utility having to have the resources available to provide power during peak demand periods. In other words, it compensates the utility company for having to have the capacity available to service their customers during brief peak demand periods which capacity may remain idle during the remaining periods of each day and month.

Though billing periods of power companies normally cover a substantially long period of time such as a month, peak demand is calculated on a relatively short period of time such as a quarter or half hour basis. Accordingly, utility companies meter both the total power supplied to their customers during each billing period as well as the maximum power delivered during any brief period in a succession of periods that cumulatively form the billing period. The maximum demand recorded during any of these brief periods in any one month is then imputed into a billing formula, exemplary of which is: ##EQU1## WHERE T.sub.2 is the aforementioned relative brief period of time such as a quarter hour or half hour and T.sub.1 is the relative long billing period such as a month.

Typically, the electrical load that imposes the greatest demand for electricity for the majority of utility company customers is their temperature control systems, be it an air conditioning or electrical heating unit. Thus, on hot summer days, air conditioners usually account for peak demand of customers whereas in winter their heaters account for peak demand. These temperature control units are themselves ordinarily controlled by thermostats which cyclically couple the system with line voltage dependent upon sensed temperatures at which they are set to respond. Their control function therefore is eventually divorced from the just described method or formula by which their consumption cost is calculated.

Accordingly, it is the general object of the present invention to provide means for limiting peak power demand.

More specifically, it is an object of the present invention to provide an electrical circuit for limiting peak power demand of air temperature control systems such as air conditioning and electrical heating systems.

Another object of the invention is to provide a power demand limiting circuit of the type described which is easily incorporated into preconstructed circuits supplying power to air temperature control systems.

Yet another object of the invention is to provide a power demand limiting circuit of the type described which may be selectively activated and deactivated with facility.

In one form of the invention a power demand limiting circuit is provided comprising air temperature control apparatus coupled with a municipal source of electric current through a thermo-responsive switch, through a meter having means for measuring maximum power demand for any period in a succession of periods of preselected duration, and through a timer programmed to cycle a timer switch at least once during each of the periods whereby the air temperature control apparatus is rendered incapable of demanding power continuously throughout any of the metered periods.

In another form of the invention a power demand limiting circuit is provided for limiting peak power demand of an air temperature control system adapted to be coupled with a municipal power source through metering means that meters the maximum power demand during any period in a succession of periods of the same duration. The power demand limiting circuit comprises circuit means for coupling the air temperature control system with the municipal power supply serially through a pair of switches. One of the switches comprises a thermostat responsive to the temperature of air controlled by the air temperature control system while the other switch comprises a timer programmed to cycle a timer switch on and off at least once during each period in the succession of periods.

The drawing is a block diagram of a power demand limiting circuit embodying principles of the present invention in a preferred form.

Referring now in more detail to the drawing, a power demand limiting circuit is shown in block diagram form for limiting power to an air temperature control system of a building having two spacial areas independently serviced by two air conditioning units and two electrical resistance heating units. Power is supplied to these units from a municipal power source through a meter. The meter measures both total power consumption in kilowatt-hours for billing periods of duration T.sub.1 as well as maximum power demand measured during the succession of time periods T.sub.2 that form T.sub.1.

With continued reference to the drawing the air conditioner compressors and the electrical resistance heating elements are both coupled to a municipal power supply by power circuits through time delay relays that prevent all elements in the compressors and heaters from being simultaneously energized and thereby creating substantial surge currents. A selector switch is provided for each spacial unit A and B whereby air conditioning may be selected in summer by the customer and heating in the winter. Thermostats are also provided independently for both units A and B. A programmed timer is also provided in the power circuit in series with the thermostats which timer is programmed to make a complete switching cycle in each period T.sub.2 for which peak power demand is metered. An override switch is further provided in parallel with the programmed timer to deenergize the timer by short-circuiting it upon switch closure. From the timer the power circuit extends to the municipal power supply through the previously described meter that measures total power consumption during the billing period T.sub.1 plus peak demand during the succession of T.sub.2 periods.

In operation, the timer is preferably programmed to make but one complete cycle during each successive period for which the meter measures maximum power consumption. For example, if the local utility measures peak power demand every thirty minutes the timer may be programmed to cycle the timer switch on or closed for, say, twenty minutes followed by an open or deenergized period of ten minutes. In this manner the customer is assured that his air temperature control systems will never be energized for more than two-thirds of any demand period T.sub.2 and thereby be assured that peak demand never surpasses two-thirds maximum. Therefore, even on a hot summer day when the thermostats may energize the air conditioning compressors almost continuously, the programmed timer prevents the compressors from being energized more than two-thirds of any one demand period time T.sub.2. Upon the arrival of autumn and cooler days the customer may close the override switch and thereby deactivate the power demand limiting feature. Thus, typically the override switch is cycled seasonally, or every three months for a T.sub.3 cycle. In the override mode the air conditioning compressors or electrical heating units are controlled just by the thermostats, selector switches and time delay relays. During this time moderate ambient temperatures themselves normally prevent any continuous demand for air temperature control from arising and thus maximum peak demand from also arising.

It should be understood that the just described embodiment merely illustrates the principle of the invention in a preferred form. Many modifications, additions and deletions may, of course, be made thereto without departing from the spirit and scope of the invention as set forth in the following claims.