Abstract:
The invention relates to a device for controlling the power consumption of one or more electric resistance heating element in a vessel containing a heatable liquid, at least one inlet for receiving a liquid having a first temperature, at least one outlet for removing liquid having a second higher temperature, and at least one thermostat for directing power to at least one heating element. The device has a detector for detecting one or more predetermined conditions in the vessel relating to the amount of liquid having the second higher temperature remaining in the vessel, and for generating an initiating signal corresponding to the predetermined condition. A controller is responsive to the initiating signal, and is configured for outputting a corresponding switching signal. A power modulator is responsive to the switching signal, and is configured for modulating power to at least one heating element.

Description:
FIELD OF THE INVENTION 
     The present invention relates generally to electrically heated liquid heaters. More particularly, the present invention relates to methods and device for adjusting electrical power to an electric resistance heating element in the liquid heater in a manner to either reduce its average electrical power or to allow the liquid heater to receive a significant quantity of heat from an auxiliary heat source. 
     BACKGROUND OF THE INVENTION 
     A hot water heater typically includes a vertically mounted cylindrical water tank, a cylindrical shell coaxial with and radially spaced apart from the water tank to form an annular space between the outer wall of the water tank and the inner wall of the shell, and insulating at least a portion of the annular space for providing thermal insulation to the water tank. Polymer foam expanded directly within the annular space is an effective insulating material. 
     The typical water tank has various appurtenances such as inlet, outlet and drain fittings. Especially, the water tank is provided with water heating and temperature control means. Typically for electrically heated water heaters, the water heating means comprises one or more electrical resistance heating elements. Each heating element extends through a fitting in the wall of the water tank such that the resistance heating element is inside the tank and means for connecting the resistance heating element to an electrical power source is outside the tank. 
     Electric water heaters with storage tanks between 30 and 120 gallons typically have two electric-resistance elements that heat the stored water. One element is located near the bottom of the storage tank and the second is located at a height approximately one-fourth to one-third down from the top of tank. Both elements commonly have the same heating rate. Although elements for 240 V (the voltage at which most 30 to 120 gallon water heaters operate) electric water heaters are available with heating rates between 750 W and 6000 W, 4500 W elements are most commonly installed at the factory. This gives the water heaters a high reheat capability without requiring wiring changes to the building to handle more than 20 A on the water heater&#39;s circuit. 
     The temperature control means for an electrically heated water heater commonly comprises a mechanical thermostat which operates a switch to apply electrical power to the electrical resistance heating element when water in the tank is sensed to be below a selected set point temperature, and operates the switch to disconnect electrical power from the electrical resistance heating element when the water in the tank is at or above the set point temperature. With such temperature control means, electrical power to the electrical resistance heating element is either full on, passing full electrical current, or completely off. 
     Electric water heaters with storage tanks almost always operate with stratified thermal conditions inside the tank. Hot water is drawn from the top of the tank, while at the same time, cold water enters near the bottom. Since the cold water is more dense than the hot, it tends not to mix with the hotter water above. 
     Typically, when a hot-water draw occurs, the tank&#39;s lower thermostat will be the first to sense the cold water entering the tank. This triggers the lower heating element. If a large volume of hot water is quickly drawn from the tank, the level of cold water within the tank can reach the upper thermostat. This will simultaneously trigger the upper element and turn off the lower element. The upper element reheats the top 25% to 33% of the tank. Once the upper thermostat has been satisfied (i.e., the top of the tank has been reheated), the upper element turns off and the lower element resumes heating the remainder of the tank. 
     Electric resistance water heaters are generally simple and inexpensive devices. However, such heaters are expensive to operate and have a very high instantaneous demand for power in comparison to their average power demand. 
     The US Department of Energy&#39;s rating procedure for water heaters assumes a daily average consumption of hot water in residences of 64.3 gallons per day. Assuming that the hot water is heated from 65° F. to 130° F., this usage corresponds to an average daily power of 423 W. However, since a water heater&#39;s upper and lower elements are both typically 4500 W, its instantaneous electrical demand will typically be ten times its demand averaged over 24 hours. Furthermore, there is a high level of coincidence in the operation of residential water heaters within the same geographical region. For most homes, hot-water use is highest in the morning when people wake up and take showers. It is common for an electric utility to have an average (or diversified) demand from all the water heaters within its service territory of 1,500 W during weekday mornings. If these water heaters could be controlled so that most never operate at a power much higher than the average required to meet the daily use of hot water, the morning weekday peak could be reduced by about 1,000 W per water heater. 
     One approach to reducing the operating cost for a water heater is to supplement its operation with an auxiliary heat source such as a desuperheater, a solar thermal collector, a heat pump, and the like. Typically, such a heat source is attached to the water heater so that it draws water from a location near the bottom of the tank, heats the water, and then returns the water to a location near the bottom of the tank. 
     One of the problems associated with auxiliary heat sources is that their heating rate is typically much lower than the heating rate associated with the resistance elements that come with the water heater. Thus, the heating contribution of such heat sources will be greatly diminished if the resistance elements in the water heater are allowed to simultaneously operate. 
     SUMMARY OF THE INVENTION 
     The present invention is generally directed to a device for controlling the power consumption of one or more electric resistance heating elements in a vessel containing a heatable liquid (such as water), at least one inlet for receiving a liquid having a first temperature, at least one outlet for removing liquid having a second higher temperature, and at least one thermostat for directing power to at least one heating element. The device of the present invention comprises detection means for detecting one or more predetermined conditions in said vessel relating to the amount of liquid having said second higher temperature remaining in said vessel, and for generating an initiating signal corresponding to said predetermined condition, controller means responsive to the initiating signal for outputting a corresponding switching signal and power modulating means responsive to said switching signal for modulating power to at least one heating element. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     The following drawings in which like reference characters indicate like parts are illustrative of embodiments of the invention and are not to be construed as limiting the invention as encompassed by the claims forming part of the application. 
     FIG. 1 is a sectional view of a typical two-element electric resistance water heater, showing the main components of the heater; 
     FIG. 2 is a sectional view of a typical two-element electric resistance water heater, showing the main components of the heater and further including an auxiliary heat source; 
     FIG. 3 is a functional block diagram of a water heating system incorporating a preferred load controller of the present invention; 
     FIG. 4 is a functional block diagram of a water heating system incorporating an alternate embodiment of a load controller of the present invention; 
     FIGS. 5-6 collectively show a detailed circuit schematic diagram of various components of the present invention; and 
     FIG. 7 is flowchart illustrating the operation of the present invention. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Referring to the drawings, particularly to FIG. 1, there is shown a sectional view of a water heater  10  comprising a water tank  11 , a shell  12  surrounding the water tank  11 , and foam insulation  13  filling the annular space between water tank  11  and shell  12 . Water inlet  14  enters the top of water tank  11  for adding cold water near the bottom of the water tank  11  by the dip tube  21 . Water outlet line  15  exits water tank  11  for withdrawing hot water from near the top of water tank  11 . Resistance elements  16  and  17  extend through the wall of the water tank  11 . 
     Thermostats  18  and  19  are mounted to the water tank  11  to sense the temperature of the water in the corresponding region of the water tank  11 . If the water in the water tank  11  is sensed to be below a selected set point temperature, the corresponding thermostat  18  or  19  operates a temperature sensitive switch to allow electrical power through the associated resistance element  16  or  17 . Once the water is at or above the set point temperature, the switch in the thermostat  18  or  19  operates to stop electrical power from passing through the resistance element  16  or  17 . 
     When hot water  8  is drawn, the lower thermostat  19  will be the first to sense the cold water  9  entering the water tank  11 . This triggers the lower resistance element  17 . This is called the “baseload heating” state in which only the lower 67% to 75% of the water tank  11  requires heating. If a large volume of hot water  8  is quickly drawn from the water tank  11 , the level of cold water  9  within the water tank  11  can reach the upper thermostat  18 . This will simultaneously trigger the upper resistance element  16  and turn off the lower resistance element  17 . This is the “incipient runout” state because there is a low amount of hot water  8  in the water heater  11  and the danger of a runout or delivery of low temperature water from the water heater  11  is present. This state can also be referred to as the “quick recovery” state because all the power is focused at the upper resistance element  16  which quickly reheats the top 25% to 33% of the water tank  11 . 
     Once the upper thermostat  18  has been satisfied (i.e., the top of the water tank has been reheated), the upper resistance element  16  turns off. The water heater  10  returns to the “baseload heating” state. The lower resistance element  17  turns on and resumes heating the remainder of the water tank  11  until the set point temperature at the lower thermostat  19  is reached. Once that point is reached the water heater  10  is in the “stand-by” state, and no heating occurs. 
     Electric resistance water heaters are generally provided with safety devices. A high temperature safety shut off switch (not shown) is installed in the electric power line (not shown) which cuts off power to the electrical resistance element when the temperature in the water tank  11  rises above a safe level. Also, the water tank  11  is provided with a high temperature and pressure relief valve  22  which opens when it detects either excessively high temperature or pressure in the water tank  11 . A drain  23  is also provided for regular maintenance purposes. 
     FIG. 2 shows the same water heater  10  as the one depicted in FIG. 1 except for the addition of an auxiliary heat source  26 . The auxiliary heat source  26  typically includes a water pump (not shown) and a heat exchange means (not shown) for heating water drawn in from the water tank  11  through inlet line  24  and returned back to the water tank  11  through outlet line  25 . Auxiliary heat sources come in different forms such as desuperheaters, solar thermal collectors, heat pumps and the like. They are typically connected near the bottom of the water tank  11  to assist the heating of the water. As discussed before, the problem associated with auxiliary heat sources  26  is that their heating rate is typically slower than the heating rate associated with resistance elements  16  and  17 . As will be described hereinafter, one embodiment of the invention provides a means to coordinate the resistance elements  16  and  17  and the auxiliary heat source  26  to provide significant heating contribution by the auxiliary heat source  26 . 
     Referring to FIG. 3, one embodiment of a load control device  30  in accordance with the principles of the present invention is shown. The load control device  30 , referred to as a “device” hereinafter, is especially useful in modulating the power consumption of the resistance elements  16  and/or  17  in the water heater  10 . In this embodiment, the device also detects the operating state of the water heater  10  (“baseload”, “incipient runout” or “stand-by”) and uses this information to modulate the average power consumed by the resistance elements  16  and/or  17  to a low value, while still meeting the demand for hot water as will be described hereinafter. 
     The device  30  can be implemented with water heaters having more than two resistance elements and functions in the same manner as with two resistance element water heaters. One group of resistance elements would be designated as meeting the “baseload demand” (i.e. lower resistance element  17 ) and the other group of elements would be designated as “quick recovery demand” (i.e. upper resistance element  16 ). As will be further described hereinafter, the operation of the quick recovery demand group could be used to determine when the system is having trouble delivering adequate hot water. The baseload demand group would have its power modulated to the lowest value that avoids runouts or delivery of relatively cold water. 
     FIG. 3 depicts the placement and connection of the device  30  in relation to the water heater  10 . The device  30  is coupled to the power leads  48  to the water heater  10 . This allows the device  30  to be connected to the water heater  10  without having to modify the existing tank circuit. The device  30  includes a signal generator  36 , a reflection detector  38 , a controller  40 , a relay switch or contactor  34  and a low pass filter  32 . The low pass filter  32  is coupled to the power supply line  42  at the outset to prevent the high-frequency electrical signals produce by the signal generator  36  from being transmitted onto the power supply line  42 . The signal generator  36  and the reflection detector  38  are each coupled to the power supply line between the low pass filter  32  and the water heater  10 . The controller  40  is coupled to the output of the reflection detector  38 . The relay switch  34  carries and interrupts power to the water heater  10  under the control of the controller  40  through output line  41 . 
     The operation of the preferred embodiment of the device  30  of the present invention will now be explained with references to FIGS. 1 and 3. In this embodiment, information about the operational state of the thermostats  18  and  19  is used to limit the amount of time that the water heater  10  operates in the “incipient runout” state, thereby reducing the probability that the water heater  10  will run out of hot water. 
     During the operation of the device  30 , the signal generator  36  is continuously transmitting a periodic signal (such as a square wave) along the power leads  48  to the water heater  10 . As the thermostats  18  and  19  open and close, the impedance across the water heater  10  circuit changes and in turn changes the reflection of the periodic input signal. The reflection detector  38  analyzes the signal reflected from the water heater  10  to determine which operational state the water heater is in. 
     A preferred method called time-domain reflectometry requires the application of a low-current short rise-time pulse across the two electrical leads to the water heater. This pulse travels down the leads and is reflected when it encounters changes in electrical impedance. Both the shape and time of the reflected signal or waveform will change according to the impedance encountered by the input pulse signal. The reflected signal or waveform that returns to the detector  38  when either the upper thermostat  18  is on or just the lower thermostat  19  is on are sufficiently different to reliably distinguish the state of the water heater  10  (i.e. baseload heating, quick recovery, stand-by) using low cost circuits. Since the operating states for a water heater will almost always occur in a predictable sequence (i.e., “stand-by” to “baseload” to “quick recovery” to “baseload” to “stand-by”; or “stand-by” to “baseload” to “standby”) and the “stand-by” state is easily distinguished when the water heater  10  draws no current, each operating state can be associated with reflected waveform shape. 
     Another detection method is called spectral detection where a low level signal at one or more frequencies is applied and changes in the frequency dependent input impedance is detected. As an example, the signal generator  36  alternately applies low-power sinusoidal current signals at 5 Mhz, 10 Mhz, 20 Mhz and 40 Mz onto the power leads  48  to the water heater  10 . The reflection detector  38  using synchronous detection means measures the amplitude and phase angle of the voltage signal that is reflected by the water heater  10  for each frequency of input current signals. These amplitudes and phase angles will be depend on whether the water heater  10  is in the “stand-by”, “baseload” or“quick recovery” state. Changes in the measured amplitudes and phase angles indicates that the water heater  10  has changed operating states. Since the operating states for a water heater will almost always occur in a predictable sequence (i.e., “stand-by” to “baseload” to “quick recovery” to “baseload” to “stand-by”; or “stand-by” to “baseload” to “stand-by”), each operating state can be associated with a set of measured amplitudes and phase angles. Although the preceding measurement can be made at one frequency, the use of four frequencies insures that differences in frequency-dependent input impedances for different water heaters will always be detected. 
     When the reflection detector  38  detects that the water heater  10  is in the stand-by state, the controller  40  reduces the set point for the average power of the lower resistance element  17 . For example, this set point could be reduced as an exponential function in time that decreases to half its value every 12 hours. 
     When the reflection detector  38  detects that the water heater  10  is in baseload heating, the controller  40  again reduces the set point for the average power of the lower resistance element  17 . When the water heater  10  is in either the “baseload” or “stand-by” states, the rate at which the set point is reduced could be the same. 
     However, if the water heater  10  remains in the “baseload” state for a very long time, the water heater  10  may be having trouble meeting the demand for hot water. To reduce the probability that the water heater  10  will run out of hot water, it is desirable to increase the set point for the average power of the lower element  17  when the reflection detector  38  detects extended operation in the “baseload” state. For example, if the water heater is in the “baseload” state for more than 12 hours, the set point for the average power of the lower resistance element  17  is exponentially increased so that it doubles every four hours. 
     When the reflection detector  38  detects that the water heater  10  is in “quick recovery” heating (i.e., the “incipient runout” state), the water heater  10  is close to running out of hot water (or it has already run out of hot water). When this state is detected, the controller  40  increases the set point for the average power of the lower element  17 . For example, the power set point to the lower element  17  is increased according to a function that has one term that is proportional to time and a second term that is proportional to time squared (e.g., C 1 *time+C 2 *(time) 2 ). The inclusion of the term that is proportional to time squared makes the adjustment more rapid during longer runs of the upper element  16 , for example, 40 minutes of upper element  16  operation will cause the set point for the lower element  17  to be set at full operating power during baseload heating. 
     When the water heater  11  is in “baseload” heating (i.e., the lower thermostat  19  is on) the controller  40  will adjust the average power supplied by the lower resistance element  17  so that it equals the set point. This adjustment is made by opening and closing the contactor  34  so that power is periodically applied to the water heater  10 . For example, the contactor  34  remains closed for a fraction of every fifteen minute interval. If the set point for the power to the lower element  17  is 50% of the element&#39;s maximum power, the contactor  34  would remain closed for 7.5 minutes. Cycling the contactor open and closed over an interval no longer than 15 minutes is particularly useful because electric utilities will frequently bill commercial customers a monthly charge that depends on the customer&#39;s maximum power use averaged over fifteen minutes. 
     The device  30  may be modified to utilize other power adjustment methods such as modulating the applied voltage or current to the water heater  10 . 
     it is understood that the controller  40  may be modified to selectively the power to either the upper element  16  only, the lower element  17  only or both elements  16  and  17 . The adjustment of the power set point for the upper element  16  is not preferred, though the controller  40  can be easily modified to do so, if the user&#39;s needs require. It is not generally preferred because during impending runouts, it becomes imperative to heat the water that is leaving the tank  11  and any decrease in power to the upper element  16  would be detrimental to the ability of the water heater to deliver hot water rapidly during such conditions. 
     FIG. 4 illustrates an alternate embodiment of the device  30 . The basic circuit is similar to the embodiment shown in FIG.  3 . In this embodiment, the controller  40  is modified to include an additional output terminal to a second relay switch  46  that can interrupt or supply power to the auxiliary heat source  26 . The device  30  effectively controls the heating activity of the auxiliary heat source  26 . In this manner, the device  30  can coordinate the respective heating contributions of the auxiliary heat source  26  and the resistance elements  16  and  17 . 
     The operation of the embodiment shown in FIG. 4 is essentially the same as the embodiment depicted in FIG.  3 . The device  30  of FIG. 4 can switch the auxiliary heat source  26  on during baseload heating (i.e. lower thermostat  19  is on). During baseload heating the controller  40  can reduce or turn off the power consumption of the lower element  17  to permit the auxiliary heat source  26  to contribute the bulk or all of the heating needs. If the heating by the source  26  is inadequate to prevent run outs, the power set point of the lower element  17  is raised accordingly, as the feedback control steps described above. If the heat source  26  provides heat at a cost that is lower than that of the water heater  10 , the device  30  will reduce the cost of heating water by increasing the fraction of heat required for water heating that is provided by heat source  26 . 
     In this embodiment, if the auxiliary heat source  26  is a heat-pump water heater, the device  30  can be used to run the heat-pump water heater and disconnect power to the lower element  17  when the reflection detector  38  detects that the lower thermostat  19  is closed, and disconnect the heat-pump water heater and apply power to the upper element  16  when the upper thermostat  18  is closed. This will allow a water heater  10  to be heated by a heat-pump water heater during the base-load state while solely using the upper resistance element  16  during the quick recovery state. 
     In an alternate embodiment of the invention, a temperature sensor is either attached to the wall of the water tank  11  or inserted into the water tank  11  through either the hot water outlet  15 , cold water inlet  14 , or other fitting on the tank. The temperature sensor is located so that it measures the temperature of the water in the upper region of the water tank  11 . Since the water temperature in the upper region of the tank will decrease when the water heater  10  is approaching a run-out condition, the temperature measured by this sensor can be used by the controller  40  to modulate the power to the resistance elements  16  and/or  17  to levels that are low but sufficient to prevent a run-out condition. 
     In another alternate embodiment of the invention, a meter measures the water flow either into or out of the water heater  10 . This measurement of water flow is then used by the controller  40  to calculate whether the water heater  10  is approaching a run-out condition. Once again, the controller  40  would modulate the power to the resistance elements  16  and/or  17  to levels that are low but sufficient to prevent a run-out condition. This embodiment would be more effective if sensors were added to measure the cold inlet water  9  and hot outlet water  8  temperatures and their measurements sent to the controller  40 . The controller  40  would then calculate the energy required to heat the water and compare it to the energy provided by the resistance elements  16  and  17 . 
     Referring now to FIGS. 5 and 6, the preferred embodiment of FIG. 3 utilizing time-domain reflectometry for detecting the states of the water heater  10 , is shown in specific detail with parts numbers identified as appropriate. The device  30  consists of five main components: a power supply, a power relay assembly  34 , a signal generator  36 , a signal detector  38 , and a microprocessor controller  40 . 
     The power supply is a conventional transformer isolated regulated 5 volt power supply. It includes components F 1 , F 2 , T 1 , DB 1 , C 1 , U 3 , and C 6  through C 13 . The power relay assembly includes a low pass filter  32  consisting of L 1  through L 4  and C 2 . This filter prevents the signal superimposed on the water heater power leads  48  from being applied to the power supply line  42  and causing electromagnetic interference. It also isolates the signal generator  36  at high frequency from low line impedance. Relay K 1  carries and interrupts power to the water heater  10 . Triac Q 1  is turned on momentarily each time K 1  is opened or closed to prevent contact arcing, thereby prolonging the life of the relay K 1 . Q 1  is triggered by optoisolator U 2 . Transistor Q 2  provides the drive current for the relay coil of relay K 1 . 
     The signal generator consists of a flip-flop U 5 A being operated as an inverter to drive ceramic resonator X 1 . The 4 MHZ output drives the clock input of the microprocessor and flip-flop U 5 B which ensures 50% duty cycle and splits the 2 MHZ resultant into opposite phases for the detector circuit. The 2 MHZ signal is also applied to the output terminal of the controller U 1  through current limiting resistor R 1  (providing about 1 ma of signal injection) and low frequency AC and DC blocking capacitor C 4 . 
     The signal detector  38  is a gated synchronous detector which will measure the voltage generated across the water heater  10  by the injected signal current at a number of different delays from the transition of a 2 MHZ square wave. Four different time delays can be generated by R 14 , R 15 , R 17  and C 16  depending on which of R 14  and R 15  are shorted by analog switches U 9 A and U 9 B. Component U 7 D buffers and squares the delayed signal, and U 8 C, U 7 A, U 8 D, R 16  and C 17  form a 15 nanosecond pulse generator. These delayed pulses alternately connect C 14  and C 15  to the water heater  10  through high pass filter C 3 -L 5 . This acts to sample the voltage caused by the applied signal while rejecting noise at other frequencies. The difference in voltage caused by the applied voltage between C 14  and C 15  is amplified by U 6 A. Although the signals are sampled in the 10 to 200 nanoseconds time scale, the output of the detector is smooth and responds in about 5 milliseconds. 
     The microprocessor U 1  digitizes the output of the detector with an internal analog to digital converter. It also controls the triac, the relay, and the delay selectors of the signal detector. All the software for decoding changes in the detector signatures resides in the microprocessor. EEPROM U 4  provides non-volatile memory for carrying information about operation through power failures. 
     The foregoing discussion discloses and describes merely exemplary embodiments of the present invention. One skilled in the art will readily recognize from such discussion, and from the accompanying drawings and claims, that various changes, modifications and variations can be made therein without departing from the spirit and scope of the invention as defined in the following claims.