FIELD OF THE INVENTION

The present invention relates generally to improving energy efficiency of heating, ventilating, and air-conditioning systems. More particularly, the present invention relates to systems, devices, and methods for improving efficiencies of over-sized heating, ventilating, and air-conditioning systems by controlling and extending cyclical run times of the systems.

BACKGROUND OF THE INVENTION

Utilities need to match generation to load, or supply to demand. Traditionally, this is done on the supply side using Automation Generation Control (AGC). As loads are added to an electricity grid and demand rises, utilities increase output of existing generators to solve increases in demand. To solve the issue of continuing long-term demand, utilities typically invest in additional generators and plants to match rising demand. As load levels fall, generator output to a certain extent may be reduced or taken off line to match falling demand. As the overall demand for electricity grows, the cost to add power plants and generation equipment that serve only to fill peak demand becomes extremely costly.

In response to the to the high cost of peaking plants, electric utility companies have developed solutions and incentives aimed at reducing both commercial and residential demand for electricity. In the case of office buildings, factories and other commercial buildings having relatively large-scale individual loads, utilities incentivize owners with differential electricity rates to install locally-controlled load-management systems that reduce on-site demand. Reduction of any individual large scale loads by such a load-management systems may significantly impact overall demand on its connected grid.

In the case of individual residences having relatively small-scale electrical loads, utilities incentivize some consumers to allow them to install demand-response technology at the residence to control high-usage appliances such as air-conditioning (AC) compressors, water heaters, pool heaters, and so on. Such technology aids the utilities in easing demand during sustained periods of peak usage. Traditional demand-response technology used to manage thermostatically-controlled loads such as AC compressors typically consists of a demand-response thermostat or a load- control switch (LCS) device. Such demand-response devices traditionally receive commands over a long-distance communications network for controlling the electrical load. A demand- response thermostat generally controls operation of a load by manipulating space temperature or other settings to control operation. An LCS device can be wired into the control circuit of the AC compressor or power supply line of another electrical load, and thereby interrupts power to the load when the load is to be controlled.

Such demand-response thermostats, LCS devices, and other known demand-response devices are designed to be used with a wide variety of ducted, thermostatically-controlled heating, ventilating, and air conditioning (HVAC) systems as commonly used in single-family residences in the United States. Typical ducted HVAC systems in the United States utilize distinct and separate thermostat devices, circulation fan controls, electrical contactors, switches, and so on, that are easily accessible for connection to demand-response devices. Further, most control logic relies on analog control voltages for operation. For example, 24V AC is commonly used for thermostatic control. As such, demand-response devices are designed to operate with such systems, and may be installed into most ducted, thermostatically-controlled HVAC systems.

However, while the traditional demand-response schemes described above shed demand during peak times, especially for systems utilizing AC units, that demand is often time-delayed and merely pushed to another time along the utility demand timeline. In other words, the traditional demand-response schemes are suitable for reducing peak loads, but don't affect an actual decrease in energy usage. A key problem lies in the energy consumed by AC units typically used in thermostatically-controlled HVAC systems. A majority of the energy consumed by such a system is spent powering the AC compressor. In a recent Environmental Protection Agency report, it was reported that air conditioning accounts for 13% of total home energy expenses on average, and over 20% in hot, humid regions. This statistic is made more significant by the fact that AC units are typically used between three to five months per year, so their effect on the peak demand during summer periods is very significant.

The accurate sizing of HVAC equipment, and specifically, the AC unit, is often quite challenging. Many factors contribute to the proper sizing of an AC unit, including the angle at which the sun contacts the home, the type of windows installed in the home, the interior window shading of the windows, the insulation installed in the home, the air circulation patterns, the efficiency of the duct system, and the size of the living space, among others. In addition, those factors change over time as the home and landscaping ages. Because those involved with home construction or AC unit selection, like homeowners and homebuilders, do not want to undersize an AC unit and have to replace the unit later, AC units tend to be oversized. Additionally, oversized units typically provide cooling more quickly, thus avoiding any chance of not meeting the cooling demand of the occupants.

However, the oversizing of AC units contributes to the problem of energy overusage, among other issues. A primary problem is the short run times of oversized units where the units run for shorter periods of time than are engineered for optimum operation. The efficiency of air conditioners is low when first starting, and increases gradually, reaching peak efficiency in about 10 minutes for most residential AC units(e.g. long enough for the unit to be running at optimum efficiency). In addition, even a properly sized unit will have short run times on days where cooling demand is low. The problem of AC unit efficiency is illustrated in FIG. 1, a graph of energy efficiency ratio (EER) as a function of AC unit run time.

A number of other problems arise because of short run times. Relatively short operation times followed by relatively long off times do not allow the HVAC system to effectively lower humidity levels. Improperly dehumidified air effects home comfort, reduces AC cooling efficiency, and can also promote the growth of mold and mildew indoors. Likewise, short run times decrease overall air circulation, resulting in repercussions on air quality and home comfort. Short run times also increase wear and tear on HVAC systems. Problems like dirty filters, leaky ducts, and improper refrigerant are often masked by oversized units. These problems can increase the amount and magnitude of maintenance required by AC units and can potentially shorten the operable life of the units. And, perhaps most importantly, short run times cost homeowners and commercial building owners additional money to operate, as the units are not operating at peak efficiency.

One attempt at improving the energy-efficiency characteristics of HVAC systems relies on variable speed AC unit compressors and fans that may be used to increase system turndown. However, such technology remains relatively expensive for new HVAC systems. Further, retrofitting existing, working HVAC systems to replace "single-speed" technology with variable- speed technology does not provide a convenient nor cost-effective solution for improving energy efficiency.

Another attempt at improving AC system efficiency is described in U.S. Patent No. 5,960,639 to Hammer, entitled "Apparatus for Regulating Compressor Cycles to Improve Air Conditioning/Refrigeration Unit Efficiency". Hammer discloses methods and systems for addressing compressor short-cycling. Short-cycling occurs when the time between a compressor stopping and restarting is so short that coolant pressures within the HVAC system do not have time to equalize, and the compressor does not have time to cool. Such conditions may occur in undersized HVAC systems, and result in decreased system efficiency. While the invention disclosed by Hammer addresses inefficiencies for systems experiencing short-cycling, often in undersized units or on peak usage days, Hammer fails to address the energy inefficiencies caused by short run times (as opposed to short off times) occurring in oversized AC systems.

Thus, there remains a need for a solution that reduces energy usage of oversized compressor-based HVAC systems in residential or even commercial buildings.

SUMMARY OF THE INVENTION

In an embodiment, the present invention comprises a load control device for improving energy efficiency of a heating, ventilating, and air-conditioning (HVAC) system by controlling shed times of a compressor of the HVAC system. The load control device comprises: a compressor cutoff switch comprising a first terminal connectable to a second terminal, the first terminal adapted to receive a control signal from a temperature control device of an HVAC system, the second terminal adapted to transmit the control signal to a device that controls the on/off signal to a compressor of an HVAC system, compressor cutoff switch adapted to selectively cause an electrically-powered compressor of an HVAC system to be disconnected from a power source by disconnecting the first terminal from the second terminal and interrupting the transmission of the control signal to the device controlling power to the compressor; a sensing circuit in electrical communication with the second terminal of the compressor cutoff switch, the sensing circuit configured to detect the presence of the control signal and transmit a signal representative of the control signal; and a processor in electrical communication with the sensing circuit and the compressor cutoff switch, the processor configured to receive the signal representative of the control signal, to determine a subsequent mandatory shed time for a subsequent operating cycle of the compressor based on the run time and the shed time of the previous operating cycle of the compressor, and to cause the compressor cutoff switch to disconnect the first terminal from the second terminal for a period of time substantially equal to the determined subsequent mandatory shed time of the previous operating cycle, the period of time following the previous operating cycle.

In another embodiment, the present invention comprises a method of improving efficiency of a thermostatically-controlled, compressor-based heating or cooling system by controlling a shed time of the compressor with a load-control device that includes a compressor cutoff switch, a sensing circuit. The method includes the steps of determining a mandatory shed time of a previous operating cycle of the compressor; measuring a run time of the previous operating cycle of the compressor; determining using a processor a mandatory compressor shed time of a subsequent operating cycle of the compressor, the subsequent operating cycle occurring after the previous operating cycle and comprising the subsequent mandatory shed time followed by a subsequent compressor run time, and the mandatory shed time determined based on the mandatory shed time of the previous operating cycle, the compressor run time of the previous operating cycle, and a predetermined minimum run time; opening a compressor cutoff switch for a period of time substantially equal to the mandatory shed time of the subsequent operating cycle; removing power to the compressor for at least the period of time substantially equal to the mandatory shed time of the subsequent operating cycle as a result of the opening of the compressor cutoff switch, thereby causing the compressor run time of the subsequent operating cycle to increase and increasing an energy efficiency of the heating or cooling system.

In another embodiment, the claimed invention comprises a load control device for improving energy efficiency of a heating, ventilating, and air-conditioning (HVAC) system by controlling shed times of a compressor of the HVAC system. The load control device includes: means for determining a mandatory shed time of a previous operating cycle of the compressor; means for measuring a run time of the previous operating cycle of the compressor; means for determining a mandatory compressor shed time of a subsequent operating cycle of the compressor, the subsequent operating cycle occurring after the previous operating cycle and comprising the subsequent mandatory shed time followed by a subsequent compressor run time, and the mandatory shed time determined based on the mandatory shed time of the previous operating cycle, the compressor run time of the previous operating cycle, and a predetermined minimum run time; means for opening a compressor cutoff switch for a period of time substantially equal to the mandatory shed time of the subsequent operating cycle; and means for removing power to the compressor for at least the period of time substantially equal to the mandatory shed time of the subsequent operating cycle as a result of the opening of the compressor cutoff switch, thereby causing the compressor run time of the subsequent operating cycle to increase and increasing an energy efficiency of the heating or cooling system.

In another embodiment, the claimed invention comprises a load control device for improving energy efficiency of a heating, ventilating, and air-conditioning (HVAC) system by controlling shed times of a compressor of the HVAC system. The load control device includes: a compressor cutoff switch in electrical communication with a temperature control device of an HVAC system, compressor cutoff switch adapted to selectively cause an electrically-powered compressor of an HVAC system to be disconnected from a power source; and a processor in electrical communication with the compressor cutoff switch, the processor configured to determine a subsequent mandatory shed time for a subsequent operating cycle of the compressor based on a predetermined minimum run time of the compressor, a run time and a shed time of a previous operating cycle of the compressor, and to cause the compressor cutoff switch to cause power to the compressor to be disconnected for a period of time substantially equal to the determined subsequent mandatory shed time of the previous operating cycle, the subsequent mandatory shed time being a first period of time following the previous operating cycle.

While the disclosure herein is focused generally on AC units and specifically, controlling AC compressors, one skilled in the art will appreciate that the embodiments described are applicable to many other areas as well, such as a heat pump system, and can be used for any compressor-based system. For example, air conditioning compressors for indoor space management, heating compressors for indoor space management, and pool heating compressors, among others.

Embodiments of the present invention as described above provide a number of features and benefits. In embodiments, as described above, the AC unit compressor run time is increased toward or to the optimum run time by introducing a variable mandatory shed time after the completion of every "on" cycle. Because the AC unit compressor run time is increased, efficiency is necessarily increased, as is evident by the efficiency slope of the graph of FIG. 1. Increased efficiency therefore allows for the feature and advantage of improved comfort in the conditioned space. When conditioned air is supplied over a longer period of time, the conditioned air is allowed to gradually mix into the space, thus reducing cold drafts near the supply registers. A less drastic and more consistent level of comfort throughout the conditioned space is therefore provided.

Increased efficiency also allows for the feature and advantage of better humidity control. In order for air conditioners to dehumidify or dry the air, they have to cycle long enough for moisture to condense on the coils and drain away. When AC units have short run times, the amount of condensation that drains off the coils is reduced and can even allow some moisture to evaporate back into the air. In contrast, in embodiments of the invention, run time is increased, thereby allowing on cycles long enough to effectively dehumidify the air. As a result, comfort is increased and the risk of mold growth and indoor mildew growth is reduced.

Additionally, increased efficiency allows for the feature and advantage of fewer maintenance problems for the AC unit and HVAC system. Longer run cycles typically expose problems typically hidden by units running with short run times like dirty filters, leaky ducts, and improper refrigerant charge. Therefore, maintenance costs are reduced by spotting maintenance problems properly.

Perhaps most importantly for the individual homeowner, increased efficiency allows for the feature and advantage of lower utility bills. As longer run cycles necessarily increase AC unit efficiency (again, see FIG. 1, less energy is consumed by the compressor and thus, the entire HVAC system to control the temperature of the homeowner's space. Thus, utility bills are likewise decreased, as the homeowner no longer pays for excess energy consumed by system inefficiencies.

Another feature and advantage of the various embodiments of the present invention is that demand is reduced during peak load times. In an embodiment, the present invention comprises a load-control device (LCD) configured to receive commands over a long-distance communications network from a controlling utility and subsequently act on these commands to interrupt power to the load when the load is to be controlled. As individual LCSs can be installed at numerous locations having both large-scale individual loads and small-scale individual loads, utility load can be very precisely controlled.

Another feature and advantage of the various embodiments of the present invention is the ability to adapt to changing weather conditions. Because of the iterative algorithm implemented by embodiments of the invention, the run times of the compressor are automatically adjusted for day-to-day temperature differences, and even day-to-night differences. Further, no additional shed time set points need to be defined, and nothing further needs to be programmed by the utility, customer, or HVAC technician after the initial installation. The iterative algorithm is defined to account for the differences in temperature experienced by an interior space to be cooled, and is defined so that further maintenance or involvement is limited.

Another feature and advantage of the various embodiments of the present invention is that the methods implemented by embodiments are suitable for use not only in systems having LCSs for receiving commands from a remote utility, but are also appropriate for local use by individual homeowners. For example, a homeowner can utilize embodiments of the invention to simply increase the efficiency of his existing HVAC system, thereby receiving all of the benefits described above with respect to embodiments implemented to control peak load.

The above summary of the invention is not intended to describe each illustrated embodiment or every implementation of the present invention. The figures and the detailed description that follow more particularly exemplify these embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

The invention may be more completely understood in consideration of the following detailed description of various embodiments of the invention in connection with the accompanying drawings, in which:

FIG. 1 is a graph of efficiency versus run time for an exemplary, theoretical air- conditioning system;

FIG. 2 is a diagram of a facility receiving electricity through an electrical-distribution network and having a heating, ventilating, and air conditioning (HVAC) system with a load control switch (LCS) device, according to an embodiment;

FIG. 3 is a block diagram of a portion of the HVAC system with the LCD device of FIG.

2 in a commanded-off mode, according to an embodiment;

FIG. 4 is a flowchart of operation of a demand-response load control system according to an embodiment;

FIG. 5 is a flowchart of an algorithm implemented by an LCD according to an embodiment; and

FIG. 6 is a flowchart of an algorithm for determining a system efficiency.

While the invention is amenable to various modifications and alternative forms, specifics thereof have been shown by way of example in the drawings and will be described in detail. It should be understood, however, that the intention is not to limit the invention to the particular embodiments described. On the contrary, the intention is to cover all modifications, equivalents, and alternatives falling within the spirit and scope of the invention as defined by the appended claims.

DETAILED DESCRIPTION

Referring to FIG. 2, a demand-response load control system 100 is depicted according to an embodiment. Demand-response load control system 100 generally includes master station 102, electrical power generator 104, electrical distribution network 106, long-distance/long-haul communications network 108, and one or more facilities 110. Demand-response load control system 100 generally operates in tandem with improved heating, ventilating, and air conditioning (HVAC) system 112, which can be located a facility 110 in an embodiment. Though the term "HVAC" is generally understood to mean "heating, ventilating, and air conditioning", it will be understood that improved-efficiency HVAC system 112 may comprise heating and cooling capability, just cooling capability, or just heating capability. As such, when specific reference is made to a cooling configuration and operation, it will be understood that the same configuration and operation may exist and operate as a heating configuration and operation.

Master station 102 can comprise the utility or power company headquarters, in an embodiment. Master station 102 can originate signals or commands relating to energy load in order to control the energy load demanded by the aggregation of the demand of individual facilities 110. In an embodiment, master station 102 contains electrical power generator 104.

Electrical power generator 104 is contained within master station 102 in an embodiment, or, in another embodiment, is not contained within master station 102 but is under the direction of master station 102. Electrical power generator 104 comprises the source of electrical energy for system 100. Electrical power generator 104 therefore includes electricity generation equipment such that electricity can be generated by electricity generation equipment.

Electrical distribution network 106 is configured to carry electricity from electrical power generator 104 at master station 102 to appropriate facilities 110. In an embodiment, electrical distribution network 106 generally comprises power lines. Electrical distribution network 106 can further comprise substations, pole-mounted transformers, and distribution wiring.

Long-distance/long-haul communications network 108 is configured to carry the signals or commands originated by master station 102 to the appropriate component or components within individual facilities 110 to effect one-way communication. In another embodiment, communications network 108 is configured to carry signals from the appropriate component or components of facilities 110 back to master station 102 to effect two-way communication.

The invention can be implemented with such long-haul one-way and two-way communication interfaces or protocols that include, but are not limited to, 900 MHz FLEX oneway paging, Sensus Flexnet, Cellnet, IEEE 802.15.4, AERIS/TELEMETRIC Analog Cellular Control Channel two-way communication, SMS Digital two-way communication, or DNP Serial compliant communications for integration with SCADA/EMS communications currently in use by electric generation utilities. Other short-haul wired or wireless communications protocols may be employed, including, but not limited to, ZigBee ^® , Bluetooth ^® , WiFi ^® , and others.

Facility 110, as depicted in FIG. 2, is a residential home having an interior space requiring heating or cooling. Facility 110 can also be a commercial building, industrial building, or any such building or structure having an interior space requiring heating or cooling. Facility 110 generally houses the components of improved-efficiency HVAC system 112.

In general operation of the electricity generation and transmission components, electricity is generated by electrical power generator 104 at master station 102 and transferred to facilities 110 via electrical distribution network 106. Actual electricity consumption at any individual facility 110 may be measured by electricity meter 114.

Electricity meter 114 may be a standard non- communicative device, or may be a "smart meter" tied into an Advanced Meter Infrastructure (AMI) or an electricity "smart grid", capable of communicating with master station 102 over long-haul communication network 108 and in some cases capable of communicating with local devices a short-haul communication network (not depicted) at or near facility 110. Electricity meter 114, as depicted in FIG. 2, is connected to electrical distribution network 106 and one or more components of improved-efficiency HVAC system 102.

Improved-efficiency HVAC system 102 includes temperature control device 116, load control device (LCD) 118, outdoor unit 120, and forced air unit (FAU) 122. In an embodiment, as depicted, temperature control device 116 is in electrical communication with LCD 118 and FAU 122. LCD 118 is further in electrical communication with outdoor unit 120.

Temperature control device 116 may be any of a number of known temperature control devices or thermostats used to regulate a temperature of a space within facility 110. As such, temperature control device 116 may be programmable, non-programmable, digital, mechanical, communicative, and so on. Thermostat 104 may operate on 24V AC, line voltage, or another voltage as needed.

Outdoor unit 120 in an embodiment is a condensing unit of an air-conditioning system or HVAC system 112. Outdoor unit 120 includes compressor 124, and as understood by those skilled in the art, generally includes a heat exchanger with condensing coils, a fan, valving, electrical components including a compressor contactor, and so on. Although generally referred to an "outdoor" unit, it will be understood that although condensing units and other such units of an HVAC system are typically located at an exterior of a building, such as facility 110, unit 120 could in some embodiments be located inside facility 110. Further, it will also be understood that while outdoor unit 120 may comprise a condensing unit of an air-conditioning system designed for cooling, outdoor unit 120 may also be a unit of a heat-pump or other such system, providing heating, rather than cooling.

FAU 122 includes circulation fan 126, and may also include electrical control circuitry having several electrical terminals, as discussed further below. FAU 122 may be any of several known types of forced air units used to condition and circulate air. FAU 122 may also include heating and cooling elements, filters, dampers, and other related HVAC equipment not depicted. FAU 122 and circulation fan 126 may be connected to ductwork for distributing conditioned air to all or portions of facility 110. Circulation fan 126 in an embodiment may be a single-speed electric fan located within FAU 122, and turned on and off to move air through facility 110. In other embodiments, circulation fan 126 may be a variable-speed or adjustable-speed fan controlled to vary the rotation speed of the fan, and hence the air volume output of the fan.

Load-control device (LCD) 118, as described further below, in an embodiment may comprise a load-control switch (LCS) which receives signals or commands from master station 102 by way of long-distance/long-haul communications network 108 to interrupt compressor 124 of outdoor unit 120 in order to reduce energy demand, even when temperature control device 116 calls for run operation. In another embodiment, LCD 118 may receive signals or commands over a short-haul network. In yet another embodiment LCD 118 operates locally without receiving external communications.

Further, it will be understood that when compressor 124 is interrupted, circulation fan 126 can and will often still run.

In general operation without interruption from LCD 118 and master station 102, air is heated or cooled by HVAC system 112, and forced through a network of air ducts by circulation fan 126. Based upon a temperature set point at temperature control device 116, temperature control device 116 calls for heating or cooling based on feedback from a temperature sensor within the conditioned space of facility 110. In the case of cooling, temperature control device 116 signals compressor 124 to turn on, and for circulation fan 126 to circulate cooled air through the ductwork to various points about facility 110. The duration of time that compressor 124 is powered and runs may generally be considered the compressor "run time". When a temperature set point is reached, temperature control device 116 ceases signaling compressor 124 and fan 126 to run, and power is removed from compressor 124. The duration of time that compressor 124 is not powered, or is off, before restarting may generally be considered the compressor "off time", or "shed time". As explained further below, the "shed time" may be determined solely on the basis of the on-off control of compressor 124 by temperature control device 116 as temperature control device 116 seeks to hold a constant space temperature ("natural" shed time), or may be determined wholly or in part by control of compressor 124 by LCD 118 ("mandatory" shed time). A single compressor 122 cycle comprises an off/shed time followed by a run time. When the space temperature rises, temperature control device 116 again calls for cool, and the process repeats.

Referring again to FIG. 1, an efficiency versus run time chart for an exemplary HVAC system is depicted. The vertical axis of the chart represents a range of system energy efficiency ratings (EER) ranging from "Min" for minimum efficiency to "Max" for maximum efficiency. The horizontal axis of the chart represents system run time in minutes. In this depicted example chart, energy efficiency ranges from 0 to 7 EER, while time ranges from 0 to 10 minutes.

Three points, Point A, Point B, and Point C are also depicted on the EER vs. Run Time chart of FIG. 1. At Point A, after 1 minute, the system efficiency rating is 3 EER; at Point B, after running 5 minutes, the system efficiency has improved to 6 EER; and at Point C, after running 9 minutes, which may be considered an optimal amount of time, or TO _PT , system efficiency is essentially maximized at 7 EER.

Although the depicted EER v. Run Time chart is only an example of performance of a particular theoretical HVAC system, the chart illustrates the general concept that when a compressor-based HVAC system begins to operate, system efficiency may be rather low, then, after some time has passed, energy efficiency increases non-linearly to its maximum after a period of time.

In the graph depicted in FIG. 1, at time t = 9 minutes, system energy efficiency is maximized. Such a time is referred to as TO _PT . For the portion of time that HVAC system runs beyond T _0PT , 9 minutes for the example depicted in the chart of FIG. 1, the system will generally operate at maximum system efficiency.

Consequently, in an HVAC system where a compressor is regularly cycled on and off, such as improved-efficiency HVAC system 112, it is generally desirable to size and operate the system such that the system runs for at least a minimum run time T _MIN which may be equal to, or greater than TO _PT , SO as to maximize energy efficiency. Alternatively, T _MIN may be less than TO _PT , resulting in an efficiency below a maximum efficiency. As explained further below, in such a case T _MIN will result in an improved efficiency, though the efficiency will not be Optimum.

However, in an oversized system, one with excess cooling or heating capacity, the system can run for significantly less time than TO _PT - LCD 118 provides a solution for improving the efficiency of such an oversized HVAC system by extending compressor 124 off time to thereby subsequently increase compressor 124 run time such that run time T _MIN approaches or exceeds TOPT-

Referring to FIG. 3, a block diagram of a portion of HVAC system 112 is depicted according to an embodiment. The portion depicted includes temperature control device 116, LCD 118, and compressor 124, as well as cooling contactor 144.

Temperature control device 116 comprises call-for-fan output signal 128 and call-for- cool output signal 130. Call-for-fan output signal 128 is electrically connected to FAU 122. Call-for-cool signal is electrically connected to LCD 118, and specifically, compressor cutoff relay 140, described further below.

LCD 118 generally includes, according to the embodiment of FIG. 3, processor 132, memory 134, optional radio transceiver 136, power supply 138, compressor cutoff switch 140, monitoring line 142 and sensing circuit 145.

Processor 132 can comprise a microprocessor, microcontroller, microcomputer, and any other such processing device. Processor 132 can comprise a central processing unit, microprocessor, microcontroller, microcomputer, or other such known computer processor. Processor 132 is in communication with memory 134, radio transceiver 136, power supply 138, and compressor cutoff switch 140. Further, processor 132 is connected with the line controlled by compressor cutoff switch 140 via monitoring line 142.

Memory 134, which may be a separate memory device or memory device integrated into processor 132, may comprise various types of volatile memory, including RAM, DRAM, SRAM, and so on, as well as non-volatile memory, including ROM, PROM, EPROM, EEPROM, Flash, and so on. Memory 134 may store programs, software, and instructions relating to the operation of LCD 118.

Radio transceiver 136 receives the signals or commands originated by master station 102. In an embodiment, radio transceiver 136 thus allows for one-way communication from the outside world to LCD 1 18. In such an embodiment, radio transceiver 136 may be considered a radio receiver. In another embodiment, radio transceiver 136 can originate signals for receipt by master station 102 or any other component along long-distance/long-haul communications network 108. In such an embodiment, radio transceiver 136 thus allows for two-way communication between the outside world and LCD 118.

Power supply 138, receives power from an external power source, such as from FAU 122, and as understood by those skilled in the art, conditions the power to provide an appropriate power to processor 132, radio transceiver 136, and other components of LCD 118 as needed. In an embodiment, power supply 138 receives a 24 VAC power from FAU 122. In other embodiments, power supply 138 may receive a 120V AC or other such power as is locally available.

Compressor cutoff switch 140 comprises an electrically operated switch, which in an embodiment comprises a relay, such as a normally-closed single-pole, double throw relay switch. Compressor cutoff switch 140 may also comprise other types of switching devices, in addition to any of various types of known relays. Compressor cutoff switch 140 as depicted includes first terminal 141a and second terminal 141b. When compressor cutoff switch 140 is closed, first terminal 141a and second terminal 141b are electrically connected, such that control line COOL is electrically connected to cooling contactor 144 via control line 143. Compressor cutoff switch 140 is driven by a control signal received from processor 132. In an embodiment, LCD 118 also includes a relay driver (not shown) intermediate processor 132 and compressor cutoff switch 140 such that the relay driver receives the control signal from processor 132 and drives switch 140 to open or close.

Monitoring line 142 is connected to control line 143. Monitoring line 142 connects control line 143 to processor 132, such that processor 132 can monitor the control line 143 voltage to determine whether call-for-cool output 130 has been commanded and is operative. In an embodiment, sensing circuit 145 may be located between processor 132 and control line 143. In an embodiment, monitoring line 142 is positioned subsequent to compressor cutoff switch 140. In such an embodiment, the logic of processor 132 is reduced in determining whether call- for-cool output 130 is commanded and operative with respect to compressor cutoff switch 140, as the line can merely be monitored after compressor cutoff switch 140, instead of a case where the line is monitored prior to compressor cutoff switch 140, whereby both the state of compressor cutoff switch 140 and call-for-cool output 130 would need to be monitored and then acted on. In an embodiment, processor 132 samples the voltage of control line 143 between call- for-cool output 130 and cooling contactor 144 every 5 seconds. Other monitoring algorithms utilizing monitoring line 142 can also be implemented.

In an embodiment, LCD 118 may include sensing circuit 145 in communication with control line 142, monitoring line 143, and processor 132. Such a sensing circuit may sense the absence or presence of a voltage or current signal by sampling control line 143 at a predetermined sampling frequency f _s . In an embodiment, a sensing circuit may comprise a Schmitt trigger that senses voltage, or a current sensor that senses current flow in control line 143. In other simplified embodiments, sensing circuit 145 may not be present, or may merely comprise an electrical connection between processor 132 and control line 143, i.e., monitoring line 142.

Cooling contactor 144, in an embodiment, is a contactor relay or other similar switch that switches line voltage to compressor 124 on and off based on a received control signal, such as COOL. Contactor 144 may be one of many known contactors or other known controlling devices for switching the power of compressor 124, wherein compressor 124 may be an air- conditioning compressor, heat pump, or other such compressor of a heating or cooling circuit. Contactor 144 may operate on alternating current (AC) or direct current (DC), and at a control circuit voltage appropriate for the particular control circuit, such as 24V AC. In operation generally where compressor cutoff switch 140 is in a closed position such that first terminal 141a and second terminal 141b are connected, and the line between call-for- cool output 130 and cooling contactor 144 is uninterrupted, temperature control device 116 is allowed to directly control the operation of compressor 124. In the case where cooling is desired, temperature control device 116 places an appropriate voltage on call-for-cool output 130. Cooling contactor 144, upon receiving the call-for-cool signal 130 from temperature control device 116 switches line voltage on to compressor 124. Thus, cooling is commanded and implemented by cooling contactor 144 through compressor 124. Note that the aforementioned operation is how a typical HVAC system would operate without an LCD 118.

In operation wherein a remote, commanding master station 102 implements load controls, compressor cutoff switch 140 can be, most basically, commanded open or closed. Referring to FIG. 4, master station 102 originates a load control signal at 146. Master station 102 transfers the signal to long-distance/long-haul communications network 108 at 148. Long-distance/long- haul communications network 108 subsequently conveys the signal to individual facilities 110, and specifically, to radio transceiver 136 of LCD 118 at 150. After receipt by radio transceiver 136 of the signal of master station 102, the signal is communicated to processor 136 so that processor 136 can interpret the signal and subsequently act on compressor cutoff switch 140 at 152. As described above, most basically, LCD 118 can command compressor cutoff switch 140 to be closed or opened. In the case where compressor cutoff switch 140 remains in its normal closed position, HVAC system 112 operates as described above wherein the line between call- for-cool output 130 and cooling contactor 144 is uninterrupted. If, however, master station 102 signal to facilities 110 is to lessen demand on the utility, compressor cutoff switch 140 can be commanded open. The line between call-for-cool output 130 and cooling contactor 144 is then broken such that temperature control device 1 16 signals to cooling contactor 144 are not received. Thus, compressor 124 does not run when it normally would have and energy demand is lessened.

As such, LCD 118 may implement a variety of load-shedding and load-control algorithms, including known algorithms, such as those described in US Patents 7,355,301, 7,242,114, 7,702,424, and 7,528,503, 7,869,904, assigned to the assignees of the present invention, and herein incorporated by reference in their entireties.

However, while such operation reduces overall energy load, such operation does not necessarily address or improve, energy efficiency. To improve energy efficiency, LCD 118 can implement various compressor run time algorithms that result in longer run times of compressors. Such algorithms can be commanded by master station 102, transmitted via long- distance/long-haul communications network 108, received by radio transceiver 136 and subsequently stored in memory 134 by processor 132 and ultimately implemented by processor 132. Alternatively, such algorithms may be preprogrammed and stored in LCD 118 on site, or prior to installation.

In one such energy-saving embodiment, run time of compressor 122 is extended by manipulating the off time, or shed time of compressor 122. As explained further below, LCD 118 implements a mandatory shed time based on a preceding mandatory shed time and a preceding measured run time such that a subsequent run time will generally be longer than the preceding measured run time. In one embodiment, a mandatory compressor shed time duration, S(x), is determined as follows:

EQN. 1 : S(x) = S(x-l) + T - R(x-l),

Where x represents a particular cycle in a sequence of cycles, e.g., x=l represents a first cycle, x=2 represents a second, subsequent cycle, and so on, and wherein each cycle comprises a mandatory shed time duration followed by a run time duration; S(x-l) is the mandatory shed time duration of the previous cycle; T is the duration of a minimum preferred compressor run time, and R(x-l) is the measured duration of the run time of the (χ-1) ^Λ cycle. For the sake of understanding, S(x) may also be referred to as the "present" mandatory shed time duration to distinguish from a previous mandatory shed time duration S(x-l).

As such, Equation 1 provides that a mandatory shed time duration is determined to be substantially equal to the previous mandatory shed time plus the difference between the minimum preferred compressor run time and the measured previous run time. In an embodiment, run time durations R will be measured or estimated run time durations, while mandatory shed times S will be predetermined durations (based on the above algorithm), rather than measured durations.

In a theoretical example comprising five compressor cycles, an operating sequence of the five compressor cycles may be described as (SI, Rl), (S2, R2), (S3, R3), (S4, R4), and (S5, R5). The first cycle, x=l, comprises mandatory shed time SI followed by run time Rl, the second cycle, x=2, comprises mandatory shed time S2 followed by run time R2, and so on. In such a sequence, mandatory shed time S2 is determined by the previous shed and run time durations, mandatory shed time S 1 and run time Rl .

As evident by Equation 1, mandatory shed time duration S(x) will be longer than the previous mandatory shed time duration S(x-l) when the previous run time R(x-l) is less than the minimum run time T. The result of the increase in mandatory shed time duration S(x) is to cause subsequent run time R(x), R(x+1), and so on, to generally increase in duration, even though compressor run times R(x) are not directly controlled by LCD 118. Subsequent run time durations R(x) tend to increase due to an increased incremental load on compressor 124. The incremental load on compressor 124 is caused by a space temperature falling further below a temperature set point than would normally have been allowed by temperature control device 116. For example, temperature control device 116 might normally call for cool after a temperature rises 0.5 degrees above a temperature set point, and after a 6 minute off time or shed time duration. When the shed time duration is extended from the "natural" shed time of 6 minutes, to a mandatory shed time S(x) of, for example, 9 minutes, as implemented by LCD 118, a space temperature might rise to 0.8 degrees above the desired temperature set point thereby causing compressor 124 to run for a longer subsequent period of time, R(x).

Referring to FIG. 5, a flowchart of the above energy-efficiency improving algorithm implemented by LCD 118 is depicted. The algorithm illustrated is implemented upon installation of LCD 118 into a facility 110, or upon the transmission of the specific algorithm to LCD 118 by master station 102, or as is appropriate. The algorithm may be stored in a non- transitory memory device, such as memory 134, or another such memory device (see also FIG. 3).

The utility, homeowner, HVAC technician, processor 132, or other entity determines minimum run time T at 154, which may be stored in memory 134. In one embodiment, minimum run time T may be derived from a system efficiency chart similar to the one depicted in FIG. 1. In an embodiment, minimum run time T may be set to maximize system efficiency. For example, T = 9 minutes (referring to the exemplary system efficiency depicted in FIG. 1). In other embodiments, T may be determined and set based upon a minimum run time that does not necessarily maximize system efficiency, but merely improves compressor and system efficiency, for example, T = 6 minutes (again referring to the exemplary system efficiency depicted in FIG. 1).

In another embodiment, T may be set to exceed the time required to maximize system efficiency, for example, T=12 minutes (again referring to the exemplary system efficiency depicted in FIG. 1). The operating characteristics of the system of FIG. 1 are based on an assumed set of climatic conditions or other influencing factors. Weather changes, solar radiation, elevation, humidity, and so on affect the actual characteristics. Having a minimum run time T above a run time that theoretically corresponds to a maximum run time makes it more likely that a maximum system efficiency will be met, especially under changing climatic conditions.

Other methods and criteria may be used or considered to determine an appropriate minimum run time T. Such criteria may include maximum humidity levels (implies longer minimum run times), measured or perceived temperature variation at facility 110, compressor manufacturer recommended minimum run times, and so on.

The previous mandatory shed time duration S(x-l) is retrieved at 156. In an embodiment, S(x-l) is a calculated value stored in memory 134, such that it is not necessary to measure and/or store actual measurements of shed time durations of compressor 124. In the case where the algorithm is being executed in the first instance and thus there is no previous shed time duration, the value of S(x-l) may be set to a default of zero. In other embodiments, an initial default value of S(x-l) may be non-zero, such as S(x-l) being set equal to a previous known mandatory shed time, or an estimated previous known mandatory shed time. However, upon all subsequent iterations, a previous mandatory shed time is available and thus factors in to the adaptability of the algorithm to account for changes in temperature from day-to-day and within days, as well.

At 158, the previous run time R(x-l) is determined. In an embodiment, according to FIG. 3, processor 132 determines the previous run time duration based on data sampled at monitoring line 142 or control line 143, which may be via sensing circuit 145.

It will be understood that steps 156 and 158 may be interchanged, such that a determination of a previous run time R(x-l) is made prior to a determination or retrieval of previous mandatory shed time S(x-l), as both are required for determining a new or present mandatory shed time S(x).

A new mandatory shed time, S(x) can then be calculated at 160 by processor 132 according to the above Equation 1.

Referring also to FIG. 3, at step 162, processor 132 causes compressor cutoff switch 140 to open, thereby starting the new mandatory shed time period. If temperature control device 116 was calling for cool, power to compressor 124 would be removed via cooling contactor 140. If temperature control device 116 was not calling for cool, cooling contactor 150 would remain open, such that power remained off at compressor 124.

At step 164, if the new mandatory shed time S(x) is not expired, at step 166, compressor cutoff switch 140 remains open, and compressor 124 is not powered, regardless of whether temperature control device 116 calls for cool. At step 164, when mandatory shed time S(x) expires, at step 168, processor 132 causes compressor cutoff switch 140 to close. If temperature control device 1 16 is calling for cool, upon the closure of compressor cutoff switch 140, cooling contactor 144 will cause power to be applied to compressor 124, causing compressor 124 to begin to run, starting a new run time period R(x).

Following step 168, the algorithm returns to the step of retrieving the previous mandatory shed time S(x-l) at 156 in order to iteratively operate following any run cycle. Other algorithms can also be implemented.

As described generally above, if H VAC system 1 12 traditionally had short run times due to oversizing or weather conditions, the aforementioned algorithm causes the mandatory shed time to increase after each run, until individual run times are gradually extended to meet or even exceed the minimum run time. For example, the following theoretical data illustrates an example set of run times generated according to a typical interior space cooling scenario and the related calculated mandatory shed times, summarized in Table 1. In this example, the minimum run time T is 10.

Table 1

As illustrated by the values of Table 1, the calculated mandatory shed time (S(x)) continues to increase as the run times approach the set minimum run time T. As the calculated mandatory shed time increases, the space is warmed by heating environmental forces, and therefore, subsequent future run times are increased in length as the compressor needs to run longer to cool the increasingly warmed space. As the run time approaches the minimum run time T, the calculated mandatory shed time levels off and reaches an equilibrium point. Traditionally, HVAC system 112 has natural on (run) and off (shed) times based on the space and the desired temperature, whereby the compressor runs for a period of time to cool the interior space, then remains off for a period of time while the space gradually heats up. The aforementioned algorithm forces deviations from the natural times, both shed times and run times. Specifically, run time is extended via an increase in commanded off time. Note that there will necessarily be larger swings in temperature in a space when such an algorithm is implemented, due to the underlying assumptions of the algorithm that it will take longer to subsequently cool the space (and therefore the compressor will be required to run longer and hence, more efficiently) when HVAC system 1 12 is commanded off when naturally it would have been commanded on by temperature control device 116, had LCD 118 not been able to interrupt.

As understood by the above description, LCDs 118 of the present invention may be configured to operate as load-shedding devices, receiving commands from master station 102 via transceiver 136, and implementing predetermined or received load shedding algorithms so as interrupt power to compressors 124 and decrease overall energy demand on the utility. LCDs 118 may also, or instead, be configured to operate as an energy-efficiency improving device by indirectly extending run times of compressor 124 by controlling shed times of compressor 124 using the methods and algorithms described herein.

In an embodiment, LCD 118 is configured to continually operate as an energy-efficiency improving device, and upon receiving a load-shedding command from master station 102, override the energy-efficiency algorithm in favor of a load shedding event. Such a loadshedding event may comprise LCD 118 opening compressor cutoff switch 140 for a predetermined period of time based upon a predetermined duty cycle, for example, 15 minutes out of every hour. Such a load-shedding event may interrupt the energy-efficiency algorithm, causing it to restart after the load-shedding event. In such an instance, a previous mandatory run time duration S(x-l) may be reset to a default value, or return to a pre-load-shedding-event value, without having to measure an actual shed time. It will be understood that other combinations and interactions of the at least two operational aspects of LCD 118, namely traditional load shedding, and energy-efficiency improvement, are intended to be within the scope of the present invention.

In an embodiment, the methods and algorithms for indirectly extending run times to improve efficiency may comprise an efficiency feature that may be turned on or off in a particular system 112 and its LCD 118. In other words, a system 100 or HVAC system 112 may be allowed to run without being controlled so as to indirectly extend the run times, but have the feature built in, ready to be enabled as needed. In one such embodiment, the criteria for turning the efficiency feature on or off may be based on local or remote factors, including whether a measured system efficiency falls below a threshold or optimal efficiency. A system 112 that is already operating at or near an optimal efficiency may not have the feature enabled, while a system 112 that regularly operates at a low efficiency, may be directed to enable the efficiency feature of the claimed invention. As will be discussed further below, the decision to enable, or turn the feature on or off, may be made by a utility, an end user/utility customer, or both.

In an embodiment, LCD 118 determines a system efficiency, EFF system. As will be described further below, the system efficiency may be based on one or more determined cycle efficiencies, EFFcycle, such as on an average of a number of determined cycle efficiencies. A cycle efficiency EFFcycle may be based on a determined system efficiency of a single operating cycle of system 112.

In an embodiment, cycle efficiency EFFcycle for a cycle is determined based on a measured compressor run time R(i) of an operating cycle according to Equation 2:

EQN 2: EFFcycle(x) = R(i) / TOPT where EFFcycle(x) is the cycle efficiency, R(i) is the compressor run time for the ith cycle, and TO _PT is an optimum run time. For instance, if compressor 124 runs for 4.5 minutes during a particular cycle, the ith cycle, and an optimum run time is previously determined to be 9 minutes, the cycle efficiency is 0.5, or 50%.

While cycle efficiency represents system efficiency for a single cycle, an improved method of determining system efficiency EFFsystem determines system efficiency based on multiple data points, or multiple cycle efficiencies, EFFcycle. In one such embodiment, system efficiency EFFsystem is simply an average cycle efficiency, such as determined according to Equation 3 :

1 ^N

EQN. 3: EFFsystem(X _N )= (—∑EFFcycle(Xi) )

N i=\

Where EFFsystem(X _N ) is the system efficiency of the xth cycle, EFFcycle(Xi) is the cycle efficiency of an ith cycle, and N is the number of sampled cycles.

In the above embodiment, LCD 118 captures N run time values for N cycles, determines a cycle efficiency EFFcycle for each cycle, then determines an average system efficiency EFFsystem to be an average of the cycle efficiencies over the N cycles. In an embodiment, the N cycles are consecutive cycles, and in another embodiment, the N cycles are not consecutive. It will be understood that the system efficiency may be calculated in similar ways, such as determining a number of run times, averaging the run times, then dividing the average run time by an optimal run time TO _PT , to determine a system efficiency.

Referring to the flow diagram of FIG. 6, and to FIGS. 2 and 3, a method for determining system efficiency is depicted and described.

At step 180, if a command or request to reset system efficiency EFFsystem is received by LCD 1 18, or LCD 1 18 otherwise determines to reset system efficiency EFFsystem, at step 182, system efficiency EFFsystem is reset to an initial system efficiency EFF system-initial. The initial system efficiency, EFFsystem-initial, corresponds to a baseline efficiency determined for a particular HVAC system 120, a particular region, a particular climate, and so on. In some embodiments, the initial system efficiency may be 100%.

It will be understood that LCD 1 18 may store in memory 134 data corresponding to system efficiency, including EFFsystem-initial and EFFsystem. A value corresponding to an initial system efficiency EFFsystem-initial may be stored in memory 134 of LCD 1 18. In an embodiment, this initial system efficiency value may be entered into memory prior to deployment of LCD 1 18. Further, EFFsystem-initial may remain constant, or may be subject to change via processor 132. Processor 132 may change EFFsystem-initial based on local data or conditions, or may change or update EFFsystem-initial based on commands received over network 1 18.

Resetting system efficiency EFFsystem to be equal to an initial system efficiency EFFsystem-initial may be desireable when previous data relating to system efficiency is not required for determining a current system efficiency or to measure system efficiency after enabling/disabling the efficiency feature.

At step 184, an optional step, system 100 determines whether the current time is within a predetermined time window. In an embodiment, cycle and system efficiencies are only calculated within a permissible time window. The time window generally includes a start time and an end time. The start time is generally the time of day that cycle run-times and related data are collected for inclusion in the system efficiency calculation. The window end time is generally the time of day to end collection of data for determining system efficiency. Data used for determining cycle and system efficiencies may be collected continuously during the predetermined time window, or may be collected periodically during the duration of the time window. In an embodiment, the start time and end time remain the same for each day. In other embodiments, the start and end time may change. In one such embodiment, the start and end times change on a seasonal basis, for example, the time window for summer may be later in the day as compared to fall. The start and end times may also be modified or optimized based on the processing needs of LCD 1 18. In one such embodiment, the time window may be shifted if LCD 1 18 is collecting data for other purposes, or transmitting data over network 108.

If at step 184, a current time is not within the time window, cycle and system efficiencies are not updated.

If the current time is within the time window, data is collected, and cycle efficiency and/or system efficiency is calculated.

At step 186, cycle efficiency, EFFcycle is calculated for a particular cycle. The cycle efficiency may be calculated according to EQN. 2 as described above. In an embodiment, if a measured run time R(x) exceeds an optimal run time, TO _PT , the cycle efficiency EFFcycle(x) for that cycle is set to 1 or 100%.

In an embodiment, the optimal run time TO _PT is saved in memory 134 of LCD 1 18, as is the minimum run time T _MIN - AS discussed above with respect to FIG. 1 , the optimal run time is generally determined by the operating characteristics of HVAC system 1 12, under a predetermined set of climatic conditions such as air temperature, air humidity, and so on. The optimal run time, T _0PT , may be set to a predetermined value, such as 9 minutes in the example of FIG. 1 , and saved in LCD 1 18 prior to deployment at a facility 1 10. However, the optimal run time may be modified on site by an installer based on local conditions, by a master controller communicating to LCD 1 18 over network 108, or otherwise modified. The modifications, an increase or decrease, to the optimal run time may be due to regional or local conditions such as expected air temperature, air humidity, solar radiation (cloud cover), and elevation. In an embodiment, a current state of the optimal run time, as well as other stored or saved parameters including minimum run time, window start and end times, and initial system efficiency, may be read at LCD 1 18 or communicated over a network, such as network 108.

At step 188, system efficiency EFFsystem is determined. In an embodiment, system efficiency is determined by EQN. 3, or by similarly averaging cycle efficiencies EFFcycle. In another embodiment, system efficiency EFFsystem is iteratively determined by adjusting the currently-stored system efficiency, which could be the system efficiency calculated at a previous time, or the initial system efficiency, EFFsystem-initial, following a reset, plus a weighted average of cycle efficiencies EFFcycle. In such an embodiment, system efficiency may be determined by EQN. 4 as follows: EQN. 4: EFFsystem(X _N ) = EFFsystem (X _N-1 ) + EFFcycle(Xi) / Weighting Factor where EFFsystem(X _N ) represents a current system efficiency, EFFsystem(X _N -i) represents a previously determined system efficiency value, EFFcycle(Xi) represents a current determined cycle efficiency, and Weighting Factor is a weighting factor. The weighting factor determines the effect that an individual sample may have on the overall, determined system efficiency, EFFsys(X _N ). In an embodiment, Weighting Factor is 64, such that a current efficiency is the sum of a previous system efficiency plus l/64 ^th of a current cycle efficiency. In other embodiments, the weighting factor may be greater or lesser, resulting in a current cycle efficiency have more or less influence on determined system efficiency.

At step 190, the system efficiency, EFFsystem(X _N ), may be stored in memory 134 of LCD 118, may be transmitted over network 108, or LCD 118 may continue to monitor and collect run time data for efficiency determination as needed.

Any of the system efficiency or cycle efficiency data may be stored locally or transmitted to a remote location, such as transmitted over network 108 to a utility 102. Such efficiency data may be used in any number of ways, including to determine whether to turn on the extended runtime feature of the claimed invention.

In an embodiment, an authorized user, such as a utility, an installer, or in some cases, an authorized end-user, may activate or deactivate the extended run-time feature. In the case of the authorized user being a utility, the utility may authorize use of the feature as part of an energy- efficiency program. If the authorized user is an end-user consumer, the feature may be activated by the end-user as part of a rate-based program. If an end-user consumer has purchased or otherwise owns LCD 118 (as opposed to a utility), the end-user will generally be able to modify the parameters or authorize a third party to make modifications.

The activation may be implemented as a binary state stored in memory 134 of LCD 118. When activated system 100 and LCD 118 will perform the efficiency control functions described above; when deactivated, LCD 118 will not perform the extended run time control, but in some embodiments, may continue to measure, calculate, and store or transmit, cycle and system efficiencies.

In an embodiment, extended run time control is implemented if a system efficiency is below a predetermined threshold. In one such embodiment, the threshold may be 80%; in another such embodiment, the threshold may be 70%. It will be understood that such a threshold may be determined, and may comprise any desired value. The embodiments above are intended to be illustrative and not limiting. Additional embodiments are within the claims. In addition, although aspects of the present invention have been described with reference to particular embodiments, those skilled in the art will recognize that changes can be made in form and detail without departing from the spirit and scope of the invention, as defined by the claims.

Persons of ordinary skill in the relevant arts will recognize that the invention may comprise fewer features than illustrated in any individual embodiment described above. The embodiments described herein are not meant to be an exhaustive presentation of the ways in which the various features of the invention may be combined. Accordingly, the embodiments are not mutually exclusive combinations of features; rather, the invention may comprise a combination of different individual features selected from different individual embodiments, as understood by persons of ordinary skill in the art.

Any incorporation by reference of documents above is limited such that no subject matter is incorporated that is contrary to the explicit disclosure herein. Any incorporation by reference of documents above is further limited such that no claims included in the documents are incorporated by reference herein. Any incorporation by reference of documents above is yet further limited such that any definitions provided in the documents are not incorporated by reference herein unless expressly included herein.

For purposes of interpreting the claims for the present invention, it is expressly intended that the provisions of Section 112, sixth paragraph of 35 U.S.C. are not to be invoked unless the specific terms "means for" or "step for" are recited in a claim.