Patent Publication Number: US-2017358927-A1

Title: Solar synchronized loads for photovoltaic systems

Description:
RELATED APPLICATIONS 
     This application is a continuation of and claims the benefit of U.S. non-provisional application Ser. No. 15/010,369, filed Jan. 29, 2016, which is a continuation of and claims the benefit of U.S. non-provisional application Ser. No. 13/489,412, filed Jun. 5, 2012, which claims the benefit of U.S. Provisional Application No. 61/525,483, filed Aug. 19, 2011. The entire contents of each of said prior-filed applications being incorporated herein by reference. 
    
    
     BACKGROUND OF THE INVENTION 
     Field of the Invention 
     The present invention relates to solar power, and, more particularly, to running DC loads by use of solar power. 
     Description of the Related Art 
     Current solar photovoltaic (PV) systems for residential and commercial buildings typically produce direct current (DC) which is inverted to alternating current (AC) using inverters in and connected to an AC circuit breaker box within or on the building. Such systems suffer from power intermittency caused when the grid must make up any short term reduction in PV output. Current HVAC systems such as heat pumps often utilize AC-to-DC-to-AC, AC motor controls or AC-to-DC power supplies especially when the fans or compressors have variable-speed motors. There are a variety of variable-speed motors which could utilize DC input. Examples are brushless DC motors (BLDC motors) also known as electronically commutated motors (ECM), as well as variable-frequency drives (VFD) for AC motors, which may also be configured to use DC as the input to the VFD. 
     For example, a Bosch brand geothermal heat pump uses AC-to-DC power supply to power a variable speed ECM fan motor. For the purposes of this invention description, the phrase “DC motor” refers to any motor system where DC is the PV input to the motor or motor controls, even in the case of a DC input VFD controlling a variable speed AC motor or similar. 
     It is known to convert all of the DC from PV to AC for use by AC building loads or for export to the utility grid. What is not known in the conventional art is to adjust DC loads such that the DC loads consume a maximum amount of the DC electrical output of the solar panels, and to convert DC power from solar panels to AC only if all of the DC power is not being consumed by a DC load. Moreover, it is not known to purposefully manage loads, in a variable manner, on the demand side of the meter in concert with the variability of PV power for the purpose of reduction in or avoidance of power intermittency on the grid. Further, it is not known in the conventional art that the DC load is in the form of an HVAC system, such as a heat pump with DC input to the DC phase of AC-DC-AC motor controls. 
     SUMMARY OF THE INVENTION 
     The invention is directed to an HVAC power supply arrangement in which the DC output from a solar array is either directly or through DC/DC converters connected to the DC circuits of the HVAC equipment. The variable-speed DC motor may be controlled or adjusted to consume a maximum amount of the electrical output of the solar panels, increasing consumption when output is higher, lowering consumption when output is lower. An inverter that is either within (motor control circuits for instance) or separate from the HVAC equipment may operate in a bidirectional mode to invert the DC output from the solar array to AC for the home circuit if the output energy is not being entirely consumed by the variable-speed DC motor. This technique also enables the integration of a DC power bus within the home or commercial building for direct coupling to DC appliances and other DC devices. 
     In one embodiment, the invention comprises an electrical power supply arrangement including a solar power device that converts sunlight into DC electrical power. An adjustable DC load runs on the DC current electrical power. An electrical output sensing device senses a level of electrical output of the solar power device. A controller is coupled to each of the adjustable DC loads and the electrical output sensing device. The controller receives a signal from the electrical output sensing device, and adjusts the DC load such that the DC loads consumes substantially all of the electrical output of the solar power device. The adjusting of the DC loads is performed dependent upon the received signal, and depending on which other loads need to receive power. For example, it may be determined how much of the available solar- generated electricity should be sent to the HVAC systems and how much should be sent to other loads, such as a charging station for an Electric Vehicle (EV). 
     In another embodiment, the invention comprises an electrical power supply arrangement including a solar power device that converts sunlight into DC electrical power. A DC load runs on the DC current electrical power. A DC-to-AC converter converts the DC electrical power into AC electrical power. A controller enables the DC-to-AC converter to receive a portion of the DC current electrical power from the solar power device only if all of the DC current electrical power cannot be consumed by the DC load. 
     In yet another embodiment, the invention comprises an electrical power supply method including converting sunlight into DC electrical power by use of a solar power device. The DC current electrical power is provided to a plurality of adjustable DC loads. A level of electrical output of the solar power device is sensed. Each of the DC loads is adjusted such that the DC loads conjointly consume a maximum amount of the electrical output of the solar power device. The adjusting of the DC loads is dependent upon the level of electrical output of the solar power device. 
     In yet another embodiment, the invention comprises an electrical power supply method including converting sunlight into DC electrical power by use of a solar power device. The DC current electrical power is provided to an inverter to provide power to an AC load. A level of electrical output of the solar power device is sensed. The AC load or series of AC loads are operated in a variable fashion that optimizes the consumption of the variable PV DC electrical power in a manner which intends to eliminate or reduce AC power demands from the utility grid. The adjusting of the AC loads is dependent upon the level of electrical output of the solar power device. 
     The invention may eliminate the need to have an inverter between the photovoltaic (PV) system and the circuit breaker box. Instead, the DC output from the solar array may be either directly, or through DC/DC converters, connected to the DC circuits of the HVAC equipment. The inverter within the HVAC equipment then may act in a bidirectional mode to invert the DC from the solar array to AC for the home circuit or utility grid, if the output energy is not being entirely consumed by the variable-speed DC motor or other DC loads in the building. This technique also enables the integration of a DC power bus within the home or commercial building for direct coupling to DC appliances and other DC devices. The elimination of the traditional inverter conventionally used for solar systems may greatly reduce system cost, reduce electrical losses due to the inversion, and may result in increased system efficiency at lower cost. The invention may achieve the lower cost, higher efficiency, and higher reliability through simplification of the system. 
     In another embodiment, electrical DC loads (e.g., HVAC systems, appliances, EV chargers, water pumps, etc.) are adjusted in a variable manner such that the power consumption of the loads corresponds as closely as possible to the power available from a PV system. This adjustment of the DC loads is intended to result in less variation in the demand on other power sources in the system (e.g., the utility grid, battery, etc.) by the DC loads. In such applications, the PV output is designed to provide adequate power to the load without call for additional power from the utility or storage device. Utility or storage power is intended to only provide power during evenings or emergencies. This adjustment of the DC loads may also reduce system losses, minimize utility transformer size, lower system costs, and increase the ability of the utility grid to support higher penetration levels of renewable energy. Such application allows the utilities to avoid the added costs of spinning reserves for the purpose of managing the power intermittency caused by traditional PV system designs. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The above mentioned and other features and objects of this invention, and the manner of attaining them, will become more apparent and the invention itself will be better understood by reference to the following description of embodiments of the invention taken in conjunction with the accompanying drawings, wherein: 
         FIG. 1  is a block diagram of one embodiment of an electrical power supply arrangement of the present invention. 
         FIG. 2  is a block diagram of another embodiment of an electrical power supply arrangement of the present invention. 
         FIG. 3  is a flow chart of one embodiment of an electrical power supply method of the present invention. 
         FIG. 4  is a block diagram of yet another embodiment of an electrical power supply arrangement of the present invention. 
         FIG. 5  is a diagram illustrating communication between DC loads according to another embodiment of an electrical power supply method of the present invention. 
         FIG. 6  is a block diagram of still another embodiment of an electrical power supply arrangement of the present invention. 
     
    
    
     Corresponding reference characters indicate corresponding parts throughout the several views. Although the exemplification set out herein illustrates embodiments of the invention, in several forms, the embodiments disclosed below are not intended to be exhaustive or to be construed as limiting the scope of the invention to the precise forms disclosed. 
     DESCRIPTION OF THE PRESENT INVENTION 
     Referring now to the drawings, and particularly to  FIG. 1 , there is illustrated one embodiment of an electrical power supply arrangement  10  of the present invention including a solar PV array  12 , and a DC power bus  14  electrically connecting array  12  to a bidirectional inverter  16  of an HVAC system  18  (or DC input to the DC phase in an AC-DC-AC motor controller circuit). Inverter  16  may convert DC voltage from array  12  to a DC voltage level that is appropriate for use by HVAC system  18 . Array  12  may include a DC-to-DC converter which may convert the DC voltage output by the solar array to a voltage level suitable for transmission on bus  14 . Inverter  16  may also convert DC voltage from array  12  to an AC voltage that may be used through the remainder of a house  20  with which arrangement  10  is associated. Excess AC power that cannot be used within house  20  may be provided to a grid  22  for use outside of house  20 . In one embodiment, DC voltage from array  12  may be converted to AC voltage by inverter  16  only if HVAC system  18  (or other variable AC or DC loads) cannot consume all of the DC power from array  12 . 
     By transferring the DC voltage from array  12  directly to HVAC system  18 , arrangement  10  may eliminate the need for a separate inverter to convert the DC voltage from array  12  to AC voltage for transmission and use by both AC loads and DC loads (after conversion of the AC voltage back to DC voltage). The omission of the separate inverter may reduce electrical losses, reduce cost, and provide improved reliability of the system. Moreover, the invention may provide a high voltage DC power bus  14  which may be used by other appliances within the building or EV charging systems. 
     In another embodiment, the bidirectional inverter may simply be a DC drive or DC motor, and an optional, often smaller, inverter may be added to the DC bus. The need for an inverter and the size of the inverter may be determined by the amount of PV energy, if any, that could not be consumed by the DC loads under certain building circumstances. 
     In an alternative embodiment (not shown), the DC/DC conversion at PV array  12  is omitted. That is, the DC voltage may be transmitted on bus  14  in the same voltage as produced by the solar cells of array  12 . 
     In another embodiment ( FIG. 2 ), an electrical power supply arrangement  100  of the present invention includes a DC load  118  which can be adjusted by a controller  124  to maximize the use of a varying PV output of a solar array  112 . That is, a power meter  126  or a similar device may sense the output power of solar array  112  and communicate the sensed reading to controller  124 , as indicated at  128 . In turn, controller  124  can adjust DC load  118 , as indicated at  130 , such that load  118  consumes substantially all of the power that solar array  112  can produce. Thus, no inverter, or a reduced size inverter, may be needed to feed excess PV output into the utility grid (not shown). A practical example would be to run air conditioning motors at lower speed for a longer time under the reduced solar power output of lower light (e.g., cloudy) conditions. This could achieve the same building cooling, while reducing the variation (intermittency) of power on the grid, which is highly desirable to the utility companies. Also, by the load(s) using the entire amount of electrical power produced by the solar array, the invention avoids the energy losses associated with converting the DC power from the solar array to AC power, transferring the AC power to the grid, receiving AC power from the grid, and converting the AC power from the grid to DC power. 
     The same principle may apply to systems utilizing battery storage in place of, or in addition to, solar arrays. In this case, the battery controller and/or charge controller could be reduced or eliminated by adjusting the load to consume all of the power output of the battery. In one embodiment, the battery ensures that a minimum required level of electrical power may be provided to the load when the solar array cannot provide the minimum required level of electrical power. For example, the battery may power the electronic controller as well as any electronics operating in association with the load. For example, a battery may keep lights operating on the load at night so that a user can interact with the load, even though the solar array cannot provide power to keep the load fully operating until daylight in the morning. The battery may be recharged by the solar array in the event that the solar array produces more power than is required by the load. In addition, synchronizing the loads to the available solar energy could reduce the amount and frequency of charging and discharging batteries in the system which may extend the battery life. Solar Synchronized Loads can also improve battery life by reducing the intermittency in battery charging/discharging in the same way as intermittency in utility grid demand is reduced. 
     An electrical power supply method  300  ( FIG. 3 ) of the invention is described below with reference to  FIGS. 4 and 5 . In a first step  302 , sunlight is converted into DC electrical power by use of a solar power device. For example, electrical power supply arrangement  400  ( FIG. 4 ) includes a solar array  412  which converts sunlight into DC electrical power. 
     In a next step  304 , the DC current electrical power is provided to a plurality of adjustable DC loads. As shown in the specific embodiment of  FIG. 4 , a plurality of DC loads  418   a  through  418   n  each receives and runs on the DC current electrical power generated by solar array  412 . Alternatively, DC loads  418  may be AC loads. 
     Next, in step  306 , a level of electrical output of the solar power device is sensed. For example, a power meter  426  or a similar device may sense the output power of solar array  412  and communicate the sensed reading to controller  424 , as indicated at  428 . 
     In step  308 , each of the DC loads is adjusted such that the DC loads conjointly consume a maximum amount of the electrical output of the solar power device. The adjusting of the DC loads is dependent upon the level of electrical output of the solar power device. In the example embodiment of  FIG. 4 , controller  424  can adjust each of DC loads  418   a - n,  as indicated at  430   a - n , such that loads  418   a - n  in combination consume substantially all of the power that solar array  412  can produce. Thus, no inverter, or a reduced size inverter, may be needed to feed excess PV output into the utility grid (not shown). For example, loads  418   a - n  may represent n number of air conditioning motors each of which is primarily responsible for cooling a respective section of a building. In an alternative embodiment, loads  41   8   a - n  may represent n number of refrigeration and/or ice-making motors. Although the various sections of the building may be at or below the desired set temperature (e.g., 72 degrees F.), AC motors  418   a - n  may continue to run during and around the noon hour when sunlight is most direct and thus the output of solar array  412  is greatest. Thereby, the various sections of the building may be “overcooled” to a temperature of approximately between 68 and 71 degrees F., for example. By virtue of such overcooling, AC motors  41   8   a - n  may require less electrical power output from solar array  412  in the late afternoon when sunlight is less direct and the electrical power output capability of solar array  412  is lower. Thus, the building may be efficiently cooled with only small variations in temperature which may not be noticeable to the inhabitants of the building. Also, the variation in the amount of power require from the grid may be reduced, which is highly desirable to the utility companies. Further, by the loads using the full amount of electrical power produced by the solar array, the invention avoids the energy losses associated with converting the DC power from the solar array to AC power, transferring the AC power to the grid, receiving AC power from the grid, and converting the AC power from the grid to DC power. 
     In a next step  310 , a respective portion of the electrical output of the solar power device to be consumed by each of the loads is determined, possibly by use of an algorithm or a lookup table. For example, if DC loads  418   a - n  represent n number of air conditioning motors cooling respective sections of a building, then each of the sections of the building may be at different actual temperatures, and possibly may have different set temperatures. Thus, controller  424  may adjust air conditioning (or cooling equipment) motors  418   a - n  such that each motor consumes an amount or portion of electrical power that varies with the difference between the actual temperature and the set temperature of the motor&#39;s respective building section. The algorithm or lookup table may take into account expected thermal conditions in each of the building sections in the immediate future (e.g., in the next three hours). For example, a section on the west side of the building that is more exposed to sunlight may be expected to heat up more in the next few afternoon hours, and thus the respective air conditioning motor may be adjusted to consume a greater portion of the electrical power output of solar array  412 . 
     Generally, the HVAC equipment may no longer operate in a digital fashion as in all on or all off. Instead, the compressor and blower unit may operate at levels consistent with the output of the solar array. When the sun shines strongest, heat loads tend to be highest. The same principle applies to ice-making and refrigeration. 
     The step  310  of determining a respective portion of the electrical output of the solar power device to be consumed by each of the loads may include controlling a rate of change of an aggregate consumption of the electrical output of the solar power device by the loads such that the rate of change does not exceed a threshold rate of change. Thus, the amount of power drawn from solar array  412  may change with a gradual ramp up in order to avoid spikes of power being sent to and/or from the grid. Such spikes may result in inefficiencies and wasted energy. For example, controller  424  may adjust loads  418   a - n  such that a rate of change of power consumption by loads  418   a - n  does not exceed a threshold or maximum level of Watts per second of time. If the load requires more energy than can be delivered from the array, a signal may be sent to the utility advising that the load is planning a gradual ramp and an increased power call from the utility. The timing of the ramp may be adjustable based upon a feedback loop (power availability) from the utility. 
     Each of the loads may include a respective processor, and the step of determining a respective portion of the electrical output of the solar power device to be consumed by each of the loads may include communication between the processors. For example, as shown in  FIG. 5 , loads  518   a - e  each includes a respective one of processors  532   a - e . Each of processors  532   a - e  may communicate directly with each of the other ones of processors  532   a - e , as indicated by the dashed double-sided arrows in  FIG. 5 . The communication between processors  532   a - e  may occur wirelessly, or may be carried by same the electrical conductors that carry power to loads  518   a - e . In one embodiment, processors  532   a - e  communicate with each other in order to conjunctively determine how much power will be drawn by each of loads  518   a - e . For example, processors  532   a - e  may conjunctively determine how much power will be drawn by each of loads  518   a - e  such that all of the power produced by the solar array is consumed. Alternatively, or in addition, processors  532   a - e  may conjunctively determine how much power will be drawn by each of loads  518   a - e  such that a rate of change of power consumption by loads  51   8   a - e  does not exceed a threshold or maximum level of Watts per second of time. 
     In a final step  312  ( FIG. 3 ), if the DC loads cannot consume all of the electrical output of the solar power device, then a remaining portion of the electrical output of the solar power device is converted into AC electrical power. There may also be situations in which the load  418   a - n  are capable, strictly speaking, of consuming all of the electrical output of the solar power device  412 , but processor  424  does not allow one or more the loads  418   a - n  to consume all the power they are capable of consuming in order to avoid damaging or overheating loads  418   a - n . In the above-described embodiment including loads in the form of air conditioning motors, processor  424  may not allow one or more the motor to consume all the power they are capable of consuming in order to avoid burning out the motors. Regardless of whether the loads are physically incapable of consuming all of the electrical output of the solar power device or the processor prevents the loads from consuming all of the electrical output of the solar power device strictly to avoid damaging the loads, the remaining portion of the electrical output of solar array  412  may be converted in AC electrical power. This AC electrical power may then be consumed by AC appliances within the building, consumed by full home, commercial building loads, or sent to the grid for use by other consumers. 
     Other features of electrical power supply arrangement  400  may be substantially similar to those of electrical power supply arrangement  100  as described above, and thus are not further described herein in order to avoid needless repetition. 
     In  FIG. 6  there is illustrated still another embodiment of an electrical power supply arrangement  610  of the present invention including a solar PV array  612 , and a DC power bus  614  electrically connecting array  612  to a bidirectional inverter  616  associated with a DC load  618 . Inverter  616  may convert DC voltage from array  612  to a DC voltage level that is appropriate for use by DC load  618 . A uni-directional (e.g., one-way) power connector  634  may couple DC load  618  to inverter  616 . Connector  634  may allow current to flow in only one direction (e.g., from inverter  616  to load  618 ), and may prevent current from flowing from load  618  to inverter  616  if load  618  happens to function as a generator for a brief period. In one embodiment, connector  634  may be in the form of one or more diodes (not shown). Thus, uni-directional connector  634  may prevent power from load  618  from damaging inverter  616 . 
     Array  612  may include a DC-to-DC converter which may convert the DC voltage output by the solar array to a voltage level suitable for transmission on bus  614 . Inverter  616  may also convert DC voltage from array  612  to an AC voltage that may be used through the remainder of a house  620  with which arrangement  610  is associated. Excess AC power that cannot be used within house  620  may be provided to a grid  622  for use outside of house  620 . In one embodiment, DC voltage from array  612  may be converted to AC voltage by inverter  616  only if HVAC system  618  (or other variable AC or DC loads) cannot consume all of the DC power from array  612 . A uni-directional (e.g., one-way) power connector  636  may couple array  612  to inverter  616 . Connector  636  may allow current to flow in only one direction (e.g., from array  612  to inverter  616 ), and may prevent current from flowing from inverter  616  to array  612 . In one embodiment, connector  636  may be in the form of one or more diodes (not shown). Thus, uni-directional connector  636  may prevent power from inverter  616  from damaging electronics associated with array  612 . 
     As the invention may be applied to an HVAC system as the load, if heating or cooling is not needed while the PV electrical output is available, the circulation fan could still run to provide air filtering functionality. Similarly, the compressor could run in a dehumidification mode. Buildings can also be “overcooled” or “overheated” by a slight amount, reducing future energy demand. Thus, by use of these techniques, all of the electrical power produced by the PV may be consumed by the HVAC system. 
     As the invention may be applied to a freezer or refrigerator as the load, the motor speed of the freezer&#39;s compressor could be varied to match or completely consume the PV electrical output. Any excess electrical output could be used to “overcool”, or further lower, the freezer temperature beyond the temperature that would ordinarily be called for. This allows the thermal energy stored in the freezer to be recovered at night (when there may be no PV electrical output), thereby enabling the freezer to draw a lower level of electrical power from other sources. 
     As the invention may be applied to a heat pump or conventional electric hot water heater as the load, the load can be adjusted to match or completely consume the PV electrical output. Any excess PV electrical output may be used to “overheat” the water above the temperature that would ordinarily be called for. Thus, the need to draw power from other non-PV sources during the day or at night is reduced. 
     As the invention may be applied to a water pump as the load, the pump could be run at variable speed to match or completely consume the PV output. Any extra water that is pumped at a higher motor speed may be stored or deployed during the daytime, thereby reducing the nighttime needs for pumping water. 
     As the invention may be applied to electric vehicle charging as the load, the battery charge rate can vary with PV electrical output. A DC power bus between the PV system and the DC motor loads may provide an efficient way to vary the load. PV modules output DC voltage, so utilizing the DC voltage directly via the DC power bus without conversion to AC voltage may reduce conversion losses. 
     In one embodiment, a DC input Variable Frequency Drive (VFD) for an AC motor or DC brushless motor provides an efficient way to vary the speed of a motor and thereby vary the load to match the PV electrical output. The DC input Variable Frequency Drive may be employed to vary the amount of work done instead of the conventionally-used on/off cycle or duty cycle. A DC input Variable Frequency Drive may be employed in an HVAC system, an irrigation water pump, or a refrigerator, for example. 
     In one embodiment, the application is applied to HVAC systems on commercial rooftops. Wiring costs and electrical losses may be reduced by directly feeding the PV power into the HVAC rooftop compressor or air handling unit. Losses may be further reduced by utilizing a DC bus to a DC motor. 
     The principle of solar synchronized DC loads may be applied to standardized DC building bus systems (e.g., the emerging 380 VDC standard). In one embodiment, PV modules may contain DC/DC converters, with the known inherent benefits of ease of application, flexibility to modify the system to accommodate rooftop changes, tolerance to shadowing, etc. Various building electrical loads may be developed to operate off a standardized voltage (e.g. 380 VDC), and all loads may be synchronized to PV output via central control to minimize variation in grid demand. 
     The invention may beneficially influence electricity consumption via a smart grid. For example, the peak power load and the peak power generation by the utility company may be reduced by the invention. 
     In another aspect, the invention may realize system synergies. For example, the invention may enable the coordination of run times, prevent overloading, and enable the scheduling of events based on the current weather or forecast (e.g., the amount of cloud cover). 
     In another aspect, the communication between the solar array and the loads, and the potential communication between the various loads enabled by the invention may enable the homeowner/user/operator to locally and remotely control the user interface. For example, the user may control the user interface from his couch within the building, from his work place remotely, or from his vacation home remotely. This local and/or remote user control may include reviewing performance data, scheduling operation of appliances based on power cost rates, reviewing billing, and optimizing operation. 
     In another aspect, the invention may include features utilized by a professional appliance installer. For example, the installer may be provided with an interface for remotely servicing, diagnosing, and/or troubleshooting. This interface may eliminate the need for the installer to make a trip to the job site for troubleshooting. The interface may also enable the installer to offer system optimization as a service. 
     In still another aspect, the invention may enable an appliance manufacturer to optimize the appliance or system remotely. The appliance manufacturer may also remotely diagnose the appliance or system to thereby assist the installer. The invention may also enable the appliance manufacturer to monitor the performance of the appliance or system, as well as monitor the appliance or system on a long-term basis. 
     In a further aspect, the invention may provide a central user interface in the form of an existing device that is already familiar to the user, such as an iPhone, television or laptop computer. By using a familiar interface, the number of questions from the user is reduced, the number of errors made by the user is reduced, and the user is enabled to have an intuitive interaction with the appliance. 
     In a still further aspect, the invention may operate with a universal open source protocol, such as Zigbee, WLAN, etc. Thus, interaction with other devices may be enabled. 
     In another aspect of the invention, there may be a gateway with firewall in each appliance or device. Thus, a high level of operational security may be provided. 
     In yet another aspect, the invention may be integrated in a home network. Such home networks may include the Google power meter or Apple Apps, and/or may be marketed by the Microsoft HomeStore. 
     In a further aspect, the invention may include a parallel DC network to eliminate DC-to-AC conversion or AC-to-DC conversion. Thus significant losses caused by such conversions may be eliminated. 
     The invention has been described in some embodiments above as applying to an HVAC system. However, in other embodiments, the invention is applied to domestic hot water (DHW), building access control/alarm systems/security systems/fire alarms, automobile charging (electric vehicle/plug-in hybrid electric vehicle), kitchen appliances, media, internet, etc. 
     The invention has been described above as applying to a DC power source in the form of a solar array. However, in other embodiments, the invention is applied to other sources of DC power, such as the rectified output of a wind turbine, for example. The invention may be applied to any DC power source, but may be particularly applicable to a DC power source whose level of DC power output fluctuates with time. 
     In another embodiment, a bidirectional inverter, similar to bidirectional inverter  16  in  FIG. 1 , includes a DC solar variable frequency drive (VFD) optimizer. This embodiment may be advantageous with an electric vehicle charger or DC LED lighting being powered on the bus between the solar PV array and the DC VFD optimizer. The solar VFD optimizer may remove the need for a solar inverter and may minimize AC cycling on the utility supply (e.g., demand smoothing). 
     With a variable frequency drive used in conjunction with an HVAC, the speed of rotation may be driven by solar radiation. Also, there may be longer cycles if necessary to offset slower compressor rotation. Further, this variable frequency drive to HVAC embodiment may seek constant draw from the utility as opposed to ON/OFF cycling. 
     The solar variable frequency drive (VFD) optimizer may enable post-installation changes to the solar configuration, and may enable the use of a high voltage DC power bus, such as could be used with LED lighting and/or EV changing as mentioned above. Also, the solar variable frequency drive (VFD) optimizer embodiment may apply to refrigeration, pumping, etc. 
     While this invention has been described as having an exemplary design, the present invention may be further modified within the spirit and scope of this disclosure. This application is therefore intended to cover any variations, uses, or adaptations of the invention using its general principles.