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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.
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