Patent Application
20120176076
This invention relates to photovoltaic (PV) systems and, more particularly, to powering auxiliary devices in a PV system.
PV systems typically include an array of PV panels that capture sunlight and convert it into direct current (DC) power, and an inverter that converts the DC power into alternating current (AC) power that is supplied to a power grid. PV systems also routinely include auxiliary devices at the PV system installation site. Such auxiliary devices may include, for example, PV system performance monitoring devices, environmental monitoring devices, data storage devices, wireless communication devices and communication infrastructure devices.
Auxiliary devices in a PV system are often powered by batteries that must be replaced. In a PV system having a large number of battery powered auxiliary devices, significant field maintenance issues arise. For example, each auxiliary device's battery pack must be individually swapped in the field and the device must be carefully resealed to protect against often harsh environmental conditions at the installation site. Moreover, if an auxiliary device's batteries drain before replacement, operability of the auxiliary device is temporarily lost.
These field maintenance issues can be alleviated by allowing auxiliary devices to draw power from the PV panels, rather than relying on batteries. However, using PV panels as the power source can render the auxiliary devices inoperative during periods of darkness, low light and/or snow coverage when PV panels generate little or no power. Moreover, having auxiliary devices draw power from PV panels can bias PV system performance data. For example, the parasitic load of an auxiliary device can result in performance data showing that a PV panel is producing less power than it is in fact, and reliance on biased performance data can lead to suboptimal business and technical decisions.
The present invention, in a basic feature, addresses shortcomings of conventional PV systems by harvesting unused electricity generated by a PV system and using it to power the PV system's auxiliary devices. The invention takes advantage of the fact that inverters in PV systems have a harvesting threshold below which they do not harvest power generated by PV panels. Rather than discarding PV panel power that is below an inverter's harvesting threshold (e.g., power generated during periods of low light), the invention applies this below threshold PV panel power to charge rechargeable batteries in the PV system's auxiliary devices. The invention offers significant advantages over conventional PV systems, including: (1) full-time operability of auxiliary devices by virtue of rechargeable batteries that are charged using below threshold PV panel power, (2) reduced field maintenance requirements for auxiliary devices (e.g., battery pack replacement), and (3) elimination of bias in PV system performance data caused by parasitic load of auxiliary devices.
In one aspect of the invention, a PV system comprises a PV panel array having one or more PV panels adapted to generate PV panel power, an inverter operatively coupled with the PV panel array and adapted to harvest PV panel power, and one or more auxiliary devices operatively coupled with the PV panel array and having one or more rechargeable batteries, wherein the auxiliary devices continually determine whether the inverter is harvesting PV panel power, wherein when the auxiliary devices determine that the inverter is harvesting PV panel power the auxiliary devices prevent harvesting of PV panel power by the auxiliary devices, and wherein when the auxiliary devices determine that the inverter is not harvesting PV panel power the auxiliary devices allow harvesting of PV panel power whereby the rechargeable batteries are charged.
In some embodiments, the auxiliary devices determine whether the inverter is harvesting PV panel power at least in part by analyzing a DC voltage supplied by the PV panel array.
In some embodiments, the auxiliary devices determine whether the inverter is harvesting PV panel power at least in part by analyzing a DC current flowing to the inverter.
In some embodiments, the auxiliary devices determine whether the inverter is harvesting PV panel power at least in part by analyzing AC power supplied by the inverter.
In some embodiments, the auxiliary devices determine whether the inverter is harvesting PV panel power at least in part by analyzing an operating mode reading taken from the inverter.
In some embodiments, the auxiliary devices determine whether the inverter is harvesting PV panel power at least in part by analyzing a feedback signal indicative of whether the inverter is harvesting PV panel power.
In some embodiments, the auxiliary devices comprise one or more PV system performance monitoring devices.
In some embodiments, the inverter harvests PV panel power at least in part by converting DC power supplied by the PV panel array into AC power and supplying the AC power to a power grid.
In some embodiments, the auxiliary devices harvest PV panel power at least in part by using DC power supplied by the PV panel array to charge the rechargeable batteries.
In another aspect of the invention, an auxiliary device for a PV system comprises a processor, and a rechargeable battery operatively coupled with the processor, wherein the rechargeable battery is charged using PV panel power, and wherein the processor continually determines whether an inverter of the PV system is harvesting PV panel power and regulates use of PV panel power to charge the rechargeable battery based at least in part on whether the inverter is harvesting PV panel power.
In some embodiments, the processor prevents use of PV panel power to charge the rechargeable battery upon determining that the inverter is harvesting PV panel power.
In some embodiments, the processor allows use of PV panel power to charge the rechargeable battery upon determining that the inverter is not harvesting PV panel power.
In some embodiments, the determination comprises analyzing a DC voltage supplied by a PV panel.
In some embodiments, the determination comprises analyzing a DC current flowing to the inverter.
In some embodiments, the determination comprises analyzing AC power supplied by the inverter.
In some embodiments, the determination comprises analyzing an operating mode reading taken from the inverter.
In some embodiments, the determination comprises analyzing a feedback signal indicative of whether the inverter is harvesting PV panel power.
In some embodiments, the processor further regulates use of the PV panel power to charge the rechargeable battery based at least in part on a comparison of a charge on the rechargeable battery with a critical low charge threshold.
In some embodiments, the auxiliary device further comprises a wireless modem operatively coupled with the processor, and the processor receives via the wireless modem a feedback signal indicative of whether the inverter is harvesting PV panel power.
In some embodiments, the auxiliary device further comprises sensor logic and a power switch operatively coupled with the processor, and the processor determines a connection state for the power switch based at least in part on a reading of PV panel power taken by the sensor logic.
In some embodiments, the auxiliary device further comprises a wireless modem operatively coupled with the processor, and the processor reports via the wireless modem readings taken by the sensor logic.
In another aspect of the invention, a method for powering an auxiliary device in a PV system comprises the steps of continually determining by the auxiliary device whether an inverter of the PV system is harvesting PV panel power, and regulating use by the auxiliary device of the PV panel power to charge a rechargeable battery based at least in part on whether the inverter is harvesting PV panel power.
These and other aspects of the invention will be better understood by reference to the following detailed description taken in conjunction with the drawings that are briefly described below. Of course, the invention is defined by the appended claims.
PV panels 111, 112, 113 capture incident sunlight, convert it to DC power and supply DC power to inverter 130 via inverter input terminals 129. The voltage of the DC power supplied by PV panels 111, 112, 113 is measured by monitoring devices 121, 122, 123, respectively. The DC power supplied to inverter 130 varies with incident sunlight. When incident sunlight is strong, such as at midday on a cloudless day, a relatively large DC power with a relatively high voltage level is supplied to inverter 130. When incident sunlight is weak, such as at dawn, dusk or a period of heavy cloud cover, a positive but relatively small DC power with a relatively low voltage level is supplied to inverter 130. When incident sunlight is absent, such as after dark or when PV array 110 is covered with snow, the DC power and voltage supplied to inverter 130 is near or at zero.
Inverter 130 converts DC power received from PV panel array 110 on inverter input terminals 129 into AC power and supplies the AC power on inverter output terminals 131 to a power grid. However, inverter 130 has an operating range and does not perform power conversions when the DC voltage on inverter input terminals 129 is below an inverter harvesting threshold. For example, inverter 130 may have an operating range of 240 to 550 volts and require a minimum 235 volt DC input before attempting conversion to a 240 volt AC output to the power grid. Accordingly, a DC voltage on inverter input terminals 129 that does not reach the inverter harvesting threshold does not enable inverter 130 to contribute to grid power.
Monitoring devices 121, 122, 123 are auxiliary devices of PV system 100 that run on rechargeable batteries. In illustrated embodiments, monitoring devices 121, 122, 123 are assigned to monitor PV panels 111, 112, 113, respectively. Monitoring devices 121, 122, 123 measure DC voltage supplied by the PV panel to which they are assigned and may also measure DC current flowing to inverter 130. In other embodiments, a monitoring device may be assigned to monitor multiple PV panels or even an entire array of PV panels. Monitoring devices 121, 122, 123 continually monitor and report to data collection and feedback system 140 on PV panel performance and environmental conditions at the installation site.
Monitoring devices 121, 122, 123 continually assess whether inverter 130 is harvesting PV panel power to determine when to charge their rechargeable batteries. When monitoring devices 121, 122, 123 determine that inverter 130 is harvesting PV panel power, monitoring devices 121, 122, 123 refrain from charging their batteries so as not to impose a parasitic load on PV system 100 that reduces grid power and biases PV panel performance metrics. On the other hand, when monitoring devices 121, 122, 123 determine that inverter 130 is not harvesting PV panel power, monitoring devices 121, 122, 123, except in special circumstances, charge their batteries so as to extend the operating life of monitoring devices 121, 122, 123 without the need for field maintenance.
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Sensor logic 210 continually measures the DC voltage supplied by the PV panel to which monitoring device 200 has been assigned on PV panel output terminals 211. Sensor logic 210 may also continually measure the DC current flowing to inverter 130, environmental conditions (e.g., air temperature), and the charge on battery pack 250. Readings taken by sensor logic 210 are fed to processor 220 for local analysis and storage on monitoring device 200 and are periodically transmitted to data collection and feedback system 140 via a wireless modem 260 for remote analysis and storage.
Power switch 230, under direction of processor 220, conditionally isolates charge circuit 240 and battery pack 250 from the DC voltage supplied on PV panel output terminals 211. When processor 220 determines that inverter 130 is harvesting PV panel power, processor 220 disconnects power switch 230 which inhibits the supply of DC power to charge circuit 240 and battery pack 250. On the other hand, when processor 220 determines that inverter 130 is not harvesting PV panel power, processor 220 connects power switch 230 which supplies DC power to charge circuit 240 and battery pack 250. An exception arises when the DC voltage supplied on PV panel output terminals 211 is below a monitoring device harvesting voltage threshold, in which case power switch 230 generally remains disconnected even though inverter 130 is not harvesting PV panel power. Power switch 230 may be implemented in transistor circuits using MOSFETs or in reed switches with bi-stable hysteresis.
Charge circuit 240, depending on the connection state of power switch 230, conditionally charges battery pack 250 using DC power supplied on PV panel output terminals 211. When power switch 230 is connected, charge circuit 240 charges battery pack 250 unless battery pack 250 is fully charged. On the other hand, when power switch 230 is disconnected, charge circuit 240 does not charge battery pack 250. Charge circuit 240 is a passive circuit that does not depend on power from battery pack 250 in order to perform its battery charging function.
Battery pack 250 is a factory sealed pack and has one or more batteries. By way of example, the batteries in battery pack 250 may be nickel metal hydride (NiMH), nickel cadmium (NiCd), nickel zinc (NiZn), lead acid or lithium ion (Li-ion) batteries and may be size AA, AAA, C, D or 9 Volt.
Wireless modem 260 is a bidirectional wireless communication interface that transmits and receives data to and from data collection and feedback system 140. Wireless modem 260 may implement one or more standard wireless communication protocols, such as wireless Ethernet (WiFi), ZigBee wireless mesh networking, Worldwide Interoperability for Microwave Access (WiMAX), Code Division Multiple Access (CDMA), Global System for Mobile Communication (GSM) and/or Universal Mobile Telecommunications System (UMTS).
Monitoring devices 121, 122, 123 may determine whether inverter 130 is harvesting PV panel power in several ways. In some embodiments, monitoring devices 121, 122, 123 determine whether inverter 130 is harvesting PV panel power by analyzing a feedback signal indicative of whether inverter 130 is harvesting PV panel power. In some of these embodiments, data collection and feedback system 140 continually measures AC power supplied by inverter 130 on inverter output terminals 131 and determines when the power flow is towards the power grid. When the AC power is flowing into the power grid, system 140 presumes that inverter 130 is harvesting PV panel power and transmits to monitoring devices 121, 122, 123 a feedback signal indicating that inverter 130 is harvesting PV panel power. On the other hand, when the AC power is not flowing towards the power grid, system 140 presumes that inverter 130 is not harvesting PV panel power and transmits to monitoring devices 121, 122, 123 a feedback signal indicating that inverter 130 is not harvesting PV panel power. In other of these embodiments, data collection and feedback system 140 communicates directly with inverter 130 and continually reads the operating mode of inverter 130, then transmits to monitoring devices 121, 122, 123 a feedback signal indicating whether inverter 130 is harvesting PV panel power.
In some embodiments, monitoring devices 121, 122, 123 determine whether inverter 130 is harvesting PV panel power by analyzing DC voltage supplied by PV panels 111, 112, 113. In these embodiments, monitoring devices 121, 122, 123 continually and individually measure DC voltage supplied by PV panels 111, 112, 113 that monitoring devices 121, 122, 123 have been assigned to monitor on PV panel output terminals (e.g., 211) and compare the DC voltage with individual thresholds configured on monitoring devices 121, 122, 123. These thresholds are set based on system design and configuration and/or empirical studies to levels above which harvesting of PV panel power by inverter 130 can be presumed. Different ones of monitoring devices 121, 122, 123 may utilize the same or different thresholds. When a given one of monitoring devices 121, 122, 123 determines that the DC voltage supplied by its assigned PV panel is above its threshold, that monitoring device presumes inverter 130 is harvesting PV panel power. On the other hand, when a given one of monitoring devices 121, 122, 123 determines that the DC voltage supplied by its assigned PV panel is below its threshold, that monitoring device presumes inverter 130 is not harvesting PV panel power.
In some embodiments, monitoring devices 121, 122, 123 determine whether inverter 130 is harvesting PV panel power by analyzing DC current flowing to inverter 130. In these embodiments, monitoring devices 121, 122, 123 continually and individually measure DC current flowing to inverter 130 and compare the DC current with a threshold. The threshold is set based on system design and configuration and/or empirical studies to a level above which harvesting of PV panel power by inverter 130 can be presumed. When a given one of monitoring devices 121, 122, 123 determines that DC current flowing to inverter 130 is above the threshold, that monitoring device presumes that inverter 130 is harvesting PV panel power. On the other hand, when a given one of monitoring devices 121, 122, 123 determines that DC current flowing to inverter 130 is below threshold, that monitoring device presumes that inverter 130 is not harvesting PV panel power.
In some embodiments, monitoring devices 121, 122, 123 invoke two or more of the above methods to determine whether inverter 130 is harvesting PV panel power. For example, inverter 130 may be presumed to be harvesting PV panel power if two or more of the invoked methods indicate that inverter 130 is harvesting PV panel power.
It will be appreciated by those of ordinary skill in the art that the invention can be embodied in other specific forms without departing from the spirit or essential character hereof. For example, while embodiments have been described in which the auxiliary devices that harvest and run on unused PV panel power are PV performance monitoring devices, other types of auxiliary devices, such as environmental monitoring devices, data storage devices, wireless communication devices and communication infrastructure devices may harvest and run on unused PV panel power. The present description is therefore considered in all respects to be illustrative and not restrictive. The scope of the invention is indicated by the appended claims, and all changes that come with in the meaning and range of equivalents thereof are intended to be embraced therein.
