Solar power system with communication network utilizing magnetic fields

A solar power system with a communications network for reporting the performance status of each solar module in the array. The network includes short range peer-to-peer wireless links between adjacent solar modules along the rows and columns of the array, resulting in a matrix network topology. The wireless links are implemented with modulated magnetic fields.

BACKGROUND

The invention relates generally to the field of solar power, and more specifically to communication networks used to monitor and control photovoltaic solar power arrays.

In a conventional photovoltaic (PV) solar power array, the overall system efficiency is often significantly degraded by partial shading of the array, damaged cells, or mismatches in the characteristics of individual cells. To explain how this occurs,FIG. 1shows a conventional system1consisting of four strings2wired in parallel. Each string consists of a number of solar modules3in series, and a blocking diode4. The array feeds power to an inverter5via a bus with positive6and negative7rails. Ideally, the characteristics of each solar module3are identical to all the others; given equal sunlight, each solar module3outputs the same voltage, and each string2carries the same current. In theory, this allows all the PV cells in the array to simultaneously operate at their maximum power points. But in reality this is never the case. For example, suppose one of the solar modules3in the first string is shaded. This lowers its output voltage, and hence the voltage of the entire string2. The other three strings are forced to the same voltage because the strings are wired in parallel. The maximum power point tracking system in the inverter5is then forced to a point that is suboptimal for all PV cells. So a single shaded or damaged PV cell can prevent every PV cell in the array from operating at its maximum power point.

One solution well known in the art is to monitor the performance of each module3in the array so problems can be identified and corrected. The performance of a module3is measured by various parametric data, such as the modules output voltage. This requires some form of communication network to convey parametric data from each solar module3to a central computer.FIGS. 2A-2Cshow three well known methods to implement such a communication network. In all three examples, a plurality of Solar Modules20(SM0-SMN) are connected to an inverter21via a power bus22, and a computer23receives the parametric data. InFIG. 2Athe communication network is implemented with dedicated wires24. InFIG. 2Bthe network is implemented with radio links25. And inFIG. 2Cthe network is implemented via Power Line Communications (PLC) using a current transformer26such as a Rogowski coil.

All the examples inFIGS. 2A-2Care similar in that they use a bus topology for the network, where each solar module20communicates directly with the computer23via a shared medium, or bus, and the modules20must transmit data one at a time. For the system to work, each solar module20must be assigned a unique network address so the computer23can keep track of which module20each piece of data comes from.

When the parametric data indicates that a failure has occurred, the computer23identifies the failed solar module20by its network address. But a network address is often just a random number programmed into each module20at the factory, which is not very helpful to the repairman who must replace the failed module; he needs to know the physical location of the module20on the rooftop. So someone has to make a map that relates the network address of each module20to its physical location. This task is usually done by the people who install the solar array, and the extra labor adds to the installation cost.

Some prior art references have attempted to solve this problem. For example, U.S. Publication No. 2009-0140719 discloses a system where the solar modules automatically assign their own network addresses based on their location within a chain of series-connected network cables. For instance, the computer might tell the repairman that the bad module is “third from the end of the chain”. If the repairman can see the network cables, then he can trace the chain back to the failed module, but unfortunately the cables are typically hidden underneath the modules.

Accordingly, there is a continuing need in the field of solar power for a communication network that simplifies the installation process by avoiding the task of creating a map that relates the network address of each solar module to its physical location. The present invention fulfills this need and provides other related advantages.

SUMMARY

A solar power system including a plurality of solar power modules arranged in an array. Each solar power module utilizes a modulated magnetic field to communicate with adjacent solar power modules within the array. The solar power system also includes a computer coupled to one of the solar power modules via a gateway device. Each solar power module acquires parametric data related to the conversion of light into electricity (e.g. measurements of its output voltage) and the parametric data is passed module-to-module along the rows and columns of the solar power array, until reaching the gateway device, and thence to the computer.

Each solar power module includes a plurality of PV cells, a transceiver coupled to at least one magnetic loop for communicating with adjacent solar power modules via modulated magnetic fields, a digital processing unit, and a power supply circuit that receives power from the PV cells and provides power to the transceiver and digital processing unit. The digital processing unit supplies data to the transceiver circuit, and processes data received by the transceiver circuit. The communication circuitry (transceiver, processor, and power supply) may be housed in a junction box that is part of the solar power module, or may be housed in a separate communication module that attaches to the solar power module with connectors.

The magnetic loops may include a trace on a printed circuit board, a conductor in a frame around the periphery of the solar power module, a conductor embedded within the laminate of the solar power module, or the magnetic loop may be implemented utilizing at least some of the PV solar cells.

DETAILED DESCRIPTION

FIG. 3Ashows a high level block diagram of the solar power system30disclosed herein. The system30includes a plurality of solar modules31(SM00-SMNM) arranged in a two dimensional array32of N rows by M columns. To keepFIG. 3Asimple, the inverter21and power cables are not shown. Each solar power module31communicates with adjacent solar power modules via short range peer-to-peer communication links33. Thus, the communication network has a matrix topology, with a structure directly corresponding to the physical layout of the modules31in the array32, as shown inFIG. 3B.

In the context of this document, the term “adjacent” hereinafter refers to the spatial relationship between two solar power modules31; two modules are considered to be adjacent if they share the same row or column in the solar power array32, and with no other solar power module31between them.

At least one module31is connected to a gateway device34via a first interface35. The gateway device34is also coupled to a computer23via a second interface36. The main function of the gateway device34is to provide electrical isolation between the first interface35and second interface36, because the solar power module31will typically be at voltage potential much higher than the computer23. The second interface36may be a cable, a radio link, or a link via Power Line Communications (PLC). The computer23may be a part of the inverter21, or may be a stand-alone computer such as a laptop. The second interface36may also comprise a network connection; for example, the gateway device34may communicate with the computer23via a local area network, or the internet.

Each solar power module31in the array32converts light into electricity, and acquires parametric data related to the conversion of light into electricity. For example, the parametric data typically includes measurements of the output voltage produced by the solar power module31. The parametric data may also include measurements of the current flowing through the solar power module31. The parametric data is passed from module to module along the rows and columns of the array32via the communication links33, and thence to the computer23via the gateway device34. Additionally, each module31uses its unique array coordinates as its network address when communicating to the computer23.

The system30differs from the prior art in significant ways. First, the communications medium is different. Magnetic fields are very different from radio waves. For example, magnetic fields do not propagate like radio waves, and magnetic fields are able to penetrate nonferrous conductors that block radio waves. Second, the network topology is fundamentally different. The matrix topology disclosed herein offers the unique advantage of being able to self-organize and automatically assign network addresses to the modules31that correlate with their physical location in the array32.

FIG. 4Ashows a high level block diagram of a solar module31having a plurality of photovoltaic solar cells39, bypass rectifiers40, a transceiver41coupled to at least one magnetic loop42, a digital processing unit43, and a power supply44. The power supply44receives power from PV cells39and provides at least one supply voltage47to power the processor43, and at least one supply voltage48to power the transceiver41. The processor43has an interface49to the transceiver41for sending and receiving data. In some embodiments the transceiver41operates in a plurality of discrete frequency channels, and the interface49also selects the channel. The processor43also has an interface35for communicating with a gateway device34.

FIG. 4Ashows a preferred embodiment, wherein the communication circuitry41,43, and44is built into the solar module31. A junction box70, typically affixed to the back of the solar module31, houses the communication circuitry41,43,44and the bypass rectifiers40. The junction box70has a power interface51for receiving power from the PV cells39. The power interface51typically includes connections to the conductive tape commonly used to connect the PV cells39in series. The junction box70also typically includes a positive cable45and a negative cable46that connect to other solar modules31in the array32.

FIG. 4Bshows a high level block diagram of another embodiment, wherein the communication circuitry41,43, and44is housed in a communications module50. The communications module50has a power interface, typically consisting of connectors52, for receiving power from a conventional solar module3. This arrangement allows conventional solar systems, such as the conventional system1and the like, to be retrofitted with the network communication hardware, as disclosed herein, by attaching communications modules50via the connectors52.

The short range communication links33between solar modules31utilize modulated magnetic fields to convey information. Magnetic loops42produce and detect the magnetic fields. A magnetic loop42is simply a loop of conductor with one or more turns, but it is different from a loop antenna. A loop antenna produces radio waves because its circumference is roughly equal to the carrier wavelength, allowing the formation of standing waves. In contrast, a magnetic loop is far smaller than the carrier wavelength, and its impedance is predominantly inductive, with a small radiation resistance. So, while a magnetic loop can produce electromagnetic waves in the far field (with low efficiency) in the near field, it can produce and detect only magnetic fields. The term “near field” generally refers to the region within about one wavelength radius around the magnetic loop. For example, at 800 kHz, the wavelength is 375 m, which is much larger than the dimensions of most solar power arrays. Thus, magnetic loops offer the benefit of being relatively insensitive to electric and electromagnetic fields generated by near field sources, such as switching mode power supplies.

To describe the principle of communication via magnetic fields,FIG. 5shows an equivalent circuit where a signal source53produces an ac current flow in a first magnetic loop54, thus creating an ac magnetic field55. Some of the magnetic flux lines55couple to a second magnetic loop56, inducing an ac current which the resistor57converts to an output voltage. Thus the two circuits are essentially transformer-coupled; the two magnetic loops54and56form the windings of an air-core transformer with very low coupling coefficient. So magnetic loop communication is wireless, but it is very different from radio, and it has very short range because magnetic fields cannot propagate like radio waves.

The transceivers41are similar in architecture to radio transceivers, so a person of ordinary skill in the art will recognize that there may be many different variations.FIG. 6Ashows a high level block diagram of a first exemplary embodiment of the transceiver41utilizing two separate magnetic loops42aand42b. A frequency synthesizer60generates a carrier signal, typically in the range of 100 kHz to 800 kHz. The digital processor43sends a serial data stream to a modulator61which modulates the carrier signal. The modulated carrier signal then goes through a first amplifier62which drives a first magnetic loop42ato transmit the data. Data is received by a second magnetic loop42b. The received signal goes through a second amplifier63, a tuner64, and then a demodulator65which sends the recovered digital data stream to the digital processor43.

The processor43comprises the subunits typically found in a microcontroller chip: a Central Processing Unit (CPU), Random Access Memory (RAM), Non-Volatile Memory (NVM), and Analog-to-Digital Converter (ADC). The ADC acquires the parametric data related to the conversion of light into electricity, such as the output voltage of the solar power module31. Typically, the ADC also quantifies the output of the demodulator65, as a measure of the received signal strength to facilitate the self-organization process described below.

FIG. 6Bshows another exemplary embodiment of the transceiver41where a single magnetic loop42is used in conjunction with a transmit/receive switch66for half-duplex operation.

The magnetic loops have several embodiments.FIG. 7Ashows a first embodiment of the magnetic loop42that comprises a trace on a printed circuit board. The circuit board is typically housed within the junction box70on the back of the solar module31, and contains the rest of the circuitry: the processor43, the transceiver41, and the power supply44.

FIG. 7Bshows another embodiment wherein the magnetic loop42is external to the junction box70. Running the loop42near the periphery of the module31greatly increases the coupling with the magnetic loops in adjacent modules, which simplifies the design of the transceiver41, for example by reducing the required gain in the receiving amplifier63, or reducing the required current in the transmitting amplifier62. Solar power modules are typically constructed by laminating the PV cells between a stiff glass layer that acts as substrate, and a plastic layer that seals out moisture; the magnetic loop42inFIG. 7Bmay be constructed with a piece of wire, or conductive tape embedded within the laminate of the solar module31. Alternatively, the magnetic loop42may be constructed as a wire inside a frame around the edges of the module31.

FIG. 7Cdiscloses another embodiment of the magnetic loop. The PV cells39in a solar power module31are typically arranged in a meandering path, with bypass diodes40connected in parallel with segments of PV cells. By adding a capacitor73in parallel with one of the bypass diodes40a magnetic loop42is formed, as indicated by the shaded area. When light shines on the PV cells39they conduct electricity like a wire. The capacitor essentially acts as a short circuit at the frequencies utilized by the transceiver41, closing the segment of PV cells into a conductive loop42. A transformer74couples the transceiver41to the magnetic loop42to avoid high dc currents in the transceiver41. The transformer74typically has a high turns ratio, for example 100:1, to reduce the current it the transceiver41.

One advantage of the present invention is the ability of the system to self-organize Self-organization means that each module31automatically assigns itself a unique network address, and communication pathways along the rows and columns of the array32are automatically formed. These pathways allow parametric data from every module31to be conveyed back to the computer23via the gateway34, and also allow commands from the computer23to reach all the modules31.

By way of example, one simplified method of self-organizing, using four steps, is outlined.FIG. 8shows a 2×2 subsection of an array32where each solar module31(SM00-SM11) includes a junction box70containing the transceiver41and magnetic loop42, similar toFIG. 4A. The modules31have a length (L) to width (W) ratio of 5:3. The gaps between modules31are typically very small compared to W and L, so the distances between junction boxes70are essentially the module dimensions W and L.

In the first step, each module31inhibits transmissions until it has determined its own coordinates within the array32. Once the coordinates are determined, the module31begins broadcasting the coordinates to all nearby modules. So initially, when the array32is first installed, all modules31are only receiving, not transmitting. The coordinates are represented herein using the format <row,column>.

In the second step, a module31connected to a gateway34is automatically assigned coordinates <0,0>. In this example, the gateway34is always plugged into the module31in the upper left corner of the array32. So according to the first step, module SM00is the first to begin transmitting, and it broadcasts a message containing the coordinates <0,0> to all the nearby modules.

In the third step, when a module receives a signal level above a predetermined threshold, it automatically assigns itself coordinates in the same column as the transmitting module, and a row number that is one greater. SM10receives the strongest signal since it is closest to SM00. Signal strength decreases with the square of the distance between junction boxes70, and given the 5:3 ratio, SM01receives a signal about 4.4 dB weaker than SM10. The predetermined threshold is set between these two signal levels. SM10received coordinates <0,0> at a signal strength above the threshold, so it assigns itself coordinates of <1,0>. The other modules SM01and SM11continue to listen because they received signals below the predetermined threshold. So now, both SM00and SM01are broadcasting their coordinates, and the other two modules, SM01and SM11, receive both signals.

In the fourth step, when a module receives two adjacent coordinates in the same column, and both signals have strengths below the predetermined threshold, the module automatically assigns itself coordinates based on the stronger of the two signals, by using the same row number and incrementing the column by one. SM01and SM11both receive coordinates <0,0> and <1,0>, which satisfy the conditions for step four because the column numbers are identical and the row numbers differ by only one. SM01sees <0,0> as the stronger signal because it is closer to SM00, so SM01assigns itself coordinates <0,1>. Also, SM11sees <1,0> as the stronger signal and assigns itself coordinates <1,1>. Now all four modules broadcast their coordinates, and the process continues until all modules31in the array32have their coordinates assigned.

The next step in the self-organization process is to establish data flows from each module31to the gateway34so that the computer23can gather and analyze the parametric data for the entire array32. Continuing the simplified example from above, where the gateway34is connected to SM00,FIG. 9shows a 6×6 matrix where the modules31are assigned to four groups A, B, C, and D. The phases90,91,92, and93represent four time intervals in a continuously repeating cycle. The circles represent modules31that are transmitting, and the squares represent modules31that are receiving. In some embodiments the four groups A, B, C, and D are discrete frequency channels; during each phase, each transmitting module is adjacent to four receiving modules, but only one of those receiving modules is tuned to the same frequency as the transmitting module. In other embodiments the four groups A, B, C, and D are different time slices or sub-intervals of each phase; all modules use the same frequency, but at any given instant only one of the modules adjacent to a transmitting module is receiving.

The four phases shown inFIG. 9allow data to be passed across the array32in two directions: up and left. Alternating between phases90and91passes data along each row from right to left. Similarly, alternating between phases92and93passes data along each column from bottom to top. This organized behavior allows each module31to send its parametric data to the gateway34connected to SM00via intermediate modules31that act as simple routers. Additionally, it will be obvious to one with ordinary skill in the art that four more phases could be added to the cycle allowing data propagation from left-to-right and top-to-bottom so that commands from the computer23can enter the array32via the gateway34and reach all the modules31.

Although several embodiments have been described in detail for purposes of illustration, various modifications may be made to each without departing from the scope and spirit of the invention. Accordingly, the invention is not to be limited, except as by the appended claims.