SOLAR ENERGY SYSTEM AND GEARED DRIVE SYSTEM

Abstract

A control system for a solar energy system causes motor assemblies to pivot the photovoltaic (PV) arrays of the solar energy system about respective longitudinal axes, e.g., to track the sun across the sky. The solar energy system also has an inverter with a known inverter rating, e.g., for a given output level and ambient temperature. The control system is programmed, inter alia, to determine when a calculated electrical output of the PV arrays, e.g., a future electrical output during an imminent future time period, exceeds the inverter rating. The control system then causes some or all of the PV arrays to pivot out of regular solar tracking mode into a position that introduces higher cosine losses, so as to reduce real-time electrical output from at least the direct normal component of real-time solar irradiance incident on the PV arrays involved in order not to exceed the inverter rating, or to exceed it less.

Claims

1. A method of operating a solar energy system, the solar energy system comprising a plurality of photovoltaic (PV) modules, respective motor assemblies configured to pivot the plurality of PV modules about respective longitudinal axes, and an inverter having an inverter rating, the method comprising: a. determining that a calculated electrical output of the plurality of PV modules exceeds the inverter rating; and b. pivoting one or more PV modules of the plurality of PV modules so as to reduce real-time electrical output from at least a direct normal component of real-time solar irradiance incident on the one or more PV modules.

2. The method of claim 1, wherein the calculated electrical output comprises future electrical output.

3. The method of claim 1, wherein the calculated electrical output is calculated using irradiance data.

4. The method of claim 3, additionally comprising, before the determining: accessing irradiance data.

5. The method of claim 4, wherein the accessed irradiance data includes historical irradiance data.

6. The method of claim 4, wherein the accessed irradiance data includes current irradiance data.

7. The method of claim 4, wherein the accessed irradiance data includes forecasted irradiance data.

8. The method of claim 4, wherein the accessed irradiance data includes at least two types of irradiance data selected from historical irradiance data, current irradiance data, and forecasted irradiance data.

9. The method of claim 1, wherein the pivoting includes pivoting at least one of the one or more PV modules so as to reduce the direct normal component by at least 70%.

10. The method of claim 1, wherein the pivoting includes pivoting at least one of the one or more PV modules so as to reduce the direct normal component by at least 50%.

11. The method of claim 1, wherein the pivoting includes pivoting at least one of the one or more PV modules so as to reduce the direct normal component by at least 10%.

12. The method of claim 1, wherein the pivoting reduces real-time electrical output of the plurality of PV modules to be not more than the inverter rating.

13. The method of claim 1, wherein (i) the pivoting reduces the real-time electrical output of the plurality of PV modules to be more than the inverter rating, and (ii) the inverter additionally performs a clipping function.

14. The method of claim 7, wherein the accessing includes acquiring an irradiance forecast for an imminent future time period characterized by having a length of not more than 15 minutes.

15. A control system in communication with one or more motor assemblies configured to pivot, about respective longitudinal axes, a plurality of PV modules of a solar energy system, the solar energy system additionally comprising an inverter having an inverter rating, the control system comprising program code for determining that a calculated electrical output of the plurality of PV modules exceeds the inverter rating, and for causing one or more PV modules of the plurality of PV modules to pivot so as to reduce real-time electrical output from at least a direct normal component of real-time solar irradiance incident on the one or more PV modules.

16. The control system of claim 15, wherein the program code is additionally for accessing irradiance data, and wherein the calculated electrical output is calculated using the irradiance data.

17. The control system of claim 16, wherein the accessed irradiance data includes forecasted irradiance data of an irradiance forecast for an imminent future time period characterized by having a length of not more than 15 minutes.

18. A solar energy system comprising a plurality of photovoltaic (PV) modules, respective motor assemblies configured to pivot the plurality of PV modules about respective longitudinal axes, an inverter having an inverter rating, and the control system of claim 15.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The invention will now be described further, by way of example, with reference to the accompanying drawings, in which the dimensions of components and features shown in the figures are chosen for convenience and clarity of presentation and not necessarily to scale. In the drawings:

[0024] FIG. 1 shows a block diagram of a solar energy system, according to embodiments of the present invention.

[0025] FIG. 2 shows a schematic layout of selected components of a solar energy system according to embodiments of the present invention.

[0026] FIG. 3 shows a block diagram of a control system for a solar energy system, according to embodiments of the present invention.

[0027] FIG. 4 shows a block diagram relating to a forecasting process, according to embodiments of the present invention.

[0028] FIGS. 5A and 5B show flowcharts of methods and method steps for operating a solar energy system, according to embodiments of the present invention.

[0029] FIG. 6 is a schematic illustration of a PV module tracking the position of the sun, according to embodiments of the present invention.

[0030] FIGS. 7, 8A, 8B, 9, and 10 are schematic illustrations of the PV module of FIG. 6 pivoted to an orientation in which real-time electrical output is reduced, according to embodiments of the present invention.

[0031] FIG. 11 shows a graph, based on computer modeling, of irradiance values equivalent to delivered quantities of energy, illustrating the effect of reducing real-time electrical output, e.g., to the level of an inverter rating, according to embodiments of the present invention.

[0032] FIG. 12 shows a graph, based on computer modeling, of irradiance values equivalent to delivered quantities of energy, schematically illustrating the effect of reducing real-time electrical output, e.g., to a level above an inverter rating, according to embodiments of the present invention.

DETAILED DESCRIPTION OF THE ILLUSTRATED EMBODIMENTS

[0033] The invention is herein described, by way of example only, with reference to the accompanying drawings. With specific reference now to the drawings in detail, it is stressed that the particulars shown are by way of example and for purposes of illustrative discussion of the preferred embodiments of the present invention only, and are presented in the cause of providing what is believed to be the most useful and readily understood description of the principles and conceptual aspects of the invention. In this regard, no attempt is made to show structural details of the invention in more detail than is necessary for a fundamental understanding of the invention, the description taken with the drawings making apparent to those skilled in the art how the several forms of the invention may be embodied in practice. Throughout the drawings, like-referenced characters are generally used to designate like elements.

[0034] Embodiments disclosed herein relate to mechanically clipping the electrical output of an array of photovoltaic (PV) modules of a solar energy system when the electrical output exceeds or is projected to exceed the rating of an inverter. ‘Mechanical clipping’ as the term is used herein means mechanically adjusting the orientation of PV modules to receive less solar radiation, thereby reducing the DC electrical output of the PV array.

[0035] A ‘solar energy system’ as used herein means a system for generating electricity using an array of (PV) modules. The system can include an inverter for converting the direct-current (DC) electricity generated by the PV modules to alternating current (AC) electricity, e.g., for delivery to an electricity grid.

[0036] ‘Clipping’ (as opposed to ‘mechanical clipping’) means limiting or reducing the amount of DC electricity converted to AC electricity by the inverter so as not to exceed an AC output rating of the inverter. An inverter rating can be expressed, e.g., in terms of its maximum AC output, or in terms of the amount of DC input equivalent to a maximum level. The rating can be a variable dependent upon factors such as (and not exhaustively) the amount of DC input, or environmental factors such as temperature and humidity. Throughout this specification, an inverter rating is referred to as a constant rather than a variable for purposes of brevity and clarity, but within the scope of any of the disclosed embodiments of the present invention, the rating can be alternatively and more precisely regarded as a variable, and the resulting calculations and determinations can be adjusted accordingly, including in real time. Solar energy systems are frequently deployed with inverters having ratings up to 10%, up to 20%, up to 30% or even a higher percentage below the equivalent DC output of the PV array. Typically, the PV array operates at or close to its maximum output capacity a limited number of hours per year, making the incremental investment in raising the inverter rating less worthwhile. It can be desirable, according to the embodiments disclosed herein, to shift a least a part of the clipping burden to the PV array itself. An illustrative benefit of employing such a strategy is that by implementing the ‘mechanical clipping’ disclosed herein, one may extend the operating life of the inverters of a solar energy system, which already can have shorter lifespans than the PV arrays and which may need to be replaced one or more times during the life of the solar energy system.

[0037] . Referring now to the figures, and in particular to FIG. 1, a solar energy system 100 according to embodiments includes a PV array 95 comprising a plurality of PV modules 57 (shown in FIG. 2). In embodiments, the PV array 95 includes a tracking component, i.e., a solar tracker, for increasing cumulative electricity generated over the course of a period of time.

[0038] A solar tracker, or simply ‘tracker’, is an arrangement that changes the orientation of the PV panels so as to capture, i.e., convert, a higher proportion of the direct irradiance falling on the panels over the course of nearly any given period of time. Capture and conversion of the diffuse radiation component is largely unaffected by the tracking. A single-axis tracker is one that rotates PV panels around a single axis, usually from east to west over the course of a day around a north-south axis. A double-axis tracker is one that is designed to generally have the PV panels ‘face’ the sun directly at all times so as to capture and convert the entire amount of available direct irradiance. Some double-axis trackers operate using Euler angles and are not, strictly speaking, rotating the PV panels about two Cartesian axes, but the results are substantially the same. The embodiments disclosed herein are described in terms of single-axis tracking, but their application, mutatis mutandis, to double-axis tracking, is within the scope of the present invention.

[0039] The solar system 100 of FIG. 1 additionally includes an inverter 190 for conversion of DC electricity to AC. An inverter can include electronic circuitry, for example for synchronizing the phase, and for matching the voltage and frequency of the power output to those of the grid. The PV array typically has an output rating in kilowatts peak (kWp) which is the maximum DC power output rating for a given set of standard of environmental and operating conditions such as, e.g., temperature.

[0040] As is known in the art, an inverter 190 can have a rating that is lower than the output rating of the array of PV modules. This is usually because the PV array 95 may have a sharp output peak in midday, and configuring the inverter 190 to convert and deliver all of the peak energy would mean that the inverter 190 is not fully utilized during most hours of the day—and of the year. Thus, the inverter 190 can be configured to ‘clip’ the peak output of the PV array so as to achieve better utilization of the inverter. An inverter may perform the clipping functionally electronically and/or electrically, for example by changing the electrical working point (current and voltage) of the PV array to make the PV modules less efficient.

[0041] FIG. 1 further illustrates a non-limiting example of a power flow scheme for a solar energy system 100: power generated by the PV array 95 flows to a charge controller 40 as indicated by arrow 901. Two-way power flow takes place between the charge controller 40 and an energy storage device 165, as indicated by two-way arrow 902. Power from the PV array 95 and the energy storage device 165 flows through the charge controller 40 to the inverter 190, as indicated by arrow 903. The inverter 190 can deliver energy to the electric grid 15, as indicated by arrow 904.

[0042] Referring now to FIG. 2, a solar energy system 100 according to embodiments includes one or more PV modules 57. The PV module 57 includes an array of n PV panels 55.sub.1 through 55.sub.n, joined to a support subassembly 58. The support subassembly 58 includes an array of frames 56 for mounting the PV panels 55, and a central elongated member 59 to which the frames 56 are joined. The central elongated member 59 serves to transfer a torque to rotate the frames 56 as a unit together with the central elongated member 59 and the PV panels 55. The PV module 57 is rotated about a central longitudinal axis indicated in FIG. 2 by dashed line 900, and the rotation is schematically represented by arrows 1100. The central elongated member 59 is pivotably supported by ground supports 12. As shown by axes 1000, the panels are facing generally east, indicating that FIG. 2 shows a morning orientation. The tracking of the PV module 57 is shown as being east-west tracking as is the case in the vast majority of current installations of PV modules, but the principles disclosed here are equally applicable to north-south tracking systems, mutatis mutandis.

[0043] A control system 150 for a solar energy system 100, according to embodiments, is illustrated schematically in FIG. 3 to show selected components. The exemplary control system 150 of FIG. 2 includes one or more computer processors 155, a computer-readable storage medium 158, a communications module 157, and a power source 159. The computer-readable storage medium 158 can include transient and/or transient storage, and can include one or more storage units, all in accordance with desired functionality and design choices. The storage 158 can be used for any one or more of: storing program instructions, in firmware and/or software, for execution by the one or more processors 155 of the control system 150. In embodiments, the stored program instructions include program instructions for operating a solar energy system 100. Data storage 154, if separate from storage 158, can be provided for historical data, e.g., actual irradiance and/or forecast values, e.g., forecasted irradiance values, and other data related to the operation of the solar energy system 100. In some embodiments, the two storage modules 154, 158 form a single module. The communications module 159 is configured to establish communications links, e.g., via communication arrangements 70 with a forecasting system 200 (described below and illustrated schematically in FIG. 3), and with the charge controller 40 via communications arrangements 75. In some embodiments, a control system 150 does not necessarily include all of the components shown in FIG. 2. The terms “communications arrangements” or similar terms such as “communications links” as used herein mean any wired connection or wireless connection via which data communications can take place. Non-limiting and non-exhaustive examples of suitable technologies for providing communications arrangements include any short-range point-to-point communication system such as IrDA, RFID (Radio Frequency Identification), TransferJet, Wireless USB, DSRC (Dedicated Short Range Communications), or Near Field Communication; wireless networks (including sensor networks) such as: ZigBee, EnOcean; Wi-fi, Bluetooth, TransferJet, or Ultra-wideband; and wired communications bus technologies such as. CAN bus (Controller Area Network, Fieldbus, FireWire, HyperTransport and InfiniBand.

[0044] In embodiments, it can be desirable to access forecasted irradiance data, e.g., for calculating electrical output of a PV array for an imminent future time period, e.g., a future time period beginning immediately following the time of the forecasting. This is sometimes called ‘now-casting’, or simply ‘short-term forecasting’. FIG. 4 shows examples of components, according to embodiments, provided for working with a short-term forecasting system 200. The non-exhaustive list of components includes one or more irradiance sensors 81, local meteorological sensors 82, and a source of satellite imagery 83. A future time period having a short-term forecast available from the forecasting system 200 can be as short as 5, 10 or 15 minutes, or as long as 30, 45 or 60 minutes.

[0045] Referring now to FIG. 5A, a method is disclosed for operating a solar energy system 100, e.g., the solar energy system 100 of FIG. 1. According to the method, the solar energy system 100 comprises a plurality of PV modules 57 and an inverter 190 having an inverter rating. As illustrated by the flow chart in FIG. 5A, the method comprises at least the two method steps S01 and S02.

[0046] Step S01 includes: determining that a calculated electrical output of the plurality of PV modules 57 exceeds the inverter rating. In some embodiments, the calculated electrical output comprises future electrical output, e.g., electrical output of a future period or an imminent future period. In some embodiments, the calculated electrical output is calculated using irradiance data. The irradiance data can include historical irradiance data and/or current irradiance data and/or forecasted irradiance data. In some embodiments, the irradiance data includes at least two types of data selected from historical, current and forecasted data. In embodiments, the forecasted irradiance data is acquired from a forecasting system 200 such as the forecasting system 200 of FIG. 4.

[0047] Step S02 includes: pivoting one or more PV modules 57 of the plurality of PV modules 57 so as to reduce real-time electrical output from at least a direct normal component of real-time solar irradiance incident on the one or more PV modules 57. As will be explained in greater detail in the discussion of FIGS. 6-10, pivoting the PV modules 57 from tracking the sun to be in an ‘on-sun’ position (e.g., as illustrated in FIG. 6) to being ‘off-sun’ (e.g., as illustrated in FIGS. 7-10), reduces the electrical output of the PV modules 57 by presenting a reduced surface to the direct radiation that is incident on the faces of the PV modules 57. In some embodiments, Step S02 includes pivoting at least one of the one or more PV modules 57 so as to reduce the direct normal component by at least 70%, or at least 50%, or at least 10%. In some embodiments, a smaller number of PV modules 57 (e.g., at least one PV module 57 and at most half) is included in the group of pivoted PV modules 57 and a higher direct normal component reduction per module is necessary, e.g., at least 50%, or at least 60%, or at least 70%, or at least 80%, or at least 90%. In some embodiments, a larger number of PV modules 57 (e.g., at least half and up to all of the PV modules 57 of the solar energy system 100) is included in the group of pivoted modules 57 and a lower direct normal component reduction per module is necessary, e.g., at least 10%, or at least 20%, or at least 30%, or at least 40%. In other words, the number or percentage of PV modules 57 selected for pivoting to an off-sun position for reducing electrical output is selectable by the user, or by the control system 150, as long as a total desired reduction in electrical output can be achieved for the solar energy system 100 by the selected PV modules. Further, not all PV modules 57 need be pivoted to the same degree. In an illustrative example of performing Step S02, a first group of PV modules 57 can be pivoted to reduce the direct normal component by at least 70%, and a second group of PV modules 57 is pivoted to reduce the direct normal component by about 10%.

[0048] In some embodiments, the method additionally comprises method step S03, which is illustrated by the flow chart in FIG. 5B:

[0049] Step S03 includes: accessing irradiance data. According to the embodiments, Step S03 is performed before Step S01. As described hereinabove, irradiance data can include any one, two or three of the three types of irradiance data: historical (including recent), current, and future (forecasted).

[0050] In embodiments, some or all of the steps of the method can be carried out by a control system 150 of the solar energy system 100, e.g., the control system 150 of FIG. 3.

[0051] We now refer to FIGS. 6, 7, 8, 9, and 10, which illustrate, inter alia, the concept of reducing electrical output by pivoting PV panels 55 in order to deliberately create or increase ‘cosine losses’. Each of FIGS. 6-10 shows a schematic end view, i.e., parallel to axis 900) of selected components of a PV module 57, including a PV panel 55 supported by a ground support 12. The amount of direct normal irradiance actually incident on the active face of a PV panel 55 is proportional to the cosine between the angle of incidence of direct normal irradiance, indicated in each of FIGS. 6-10 by an arrow 600, and a normal vector, i.e., normal to the active face of the PV panel 55, indicated by arrow 500 in each of the figures.

[0052] The difference between the electricity output available by facing the sun directly and the electricity output of the PV panel 55 when it is oriented such that there is a non-zero angle between incidence and normal vectors 600, 500, is referred to as cosine losses. This simple formula ignores, and only for purposes of simplifying the discussion herein, the fact that there can also be a small increase in optical losses in a PV panel not facing the sun directly resulting from the direct radiation having a longer path through the covering, non-active layer of the panel, e.g., a glass layer. The additional optical losses can generally be assumed to be much smaller than the cosine losses, e.g., an order of magnitude smaller, or even smaller.

[0053] The formula also ignores, and also only for the purpose of simplifying the discussion, additional cosine losses from the PV panels 55 being slightly off-sun in the north-south axis (i.e., in an east-west tracking solar energy system 100 such as is illustrated in FIG. 2). The vectors 500, 600 drawn in FIGS. 6-10 are really the east-west component vectors of the actual respective vectors, and the actual respective vectors can additionally have a north-south component vector. In this way, the teachings of FIGS. 6-10 are applicable to two-axis trackers as well as the single-axis trackers shown.

[0054] When, for example, FIG. 6 shows in an end-view illustration a zero angle between incidence and normal vectors 600, 500, a corresponding side view might show a non-zero angle. Just as the actual angle between incidence and normal vectors 600, 500 can be broken down into east-west and north-south vectors, the cosine of the actual angle equals the cosine of the east-west vector multiplied by the cosine of the north-south vector. Pivoting the PV modules 57 east-to-west (or vice versa) on a single axis as per arrows 1100 of FIG. 2 does not change the north-south vector of the angle determining cosine losses (the vector parallel to the longitudinal axis 900 of the PV module 57). Therefore the north-south cosine loss need not be taken into account—the north-south cosine loss was already accounted for in the calculation of expected electrical output before the reduction caused by the east-west pivoting.

[0055] In some implementations, the PV module 57 can be installed with a north-south tilt, e.g., having higher ground supports 12 at a northern end than the ground supports at the southern end (in the northern hemisphere and the reverse in the southern hemisphere), in order to generally reduce average cosine losses over the course of the year. When the north-south tilt is fixed, this type of implementation can increase cosine losses at some times and reduce cosine losses at other times. Other types of implementation can include a variable north-south tilt. Nonetheless, for purposes of this simplified discussion, only changes in the east-west vector of the angle between incidence and normal vectors 600, 500 are addressed.

[0056] In FIG. 6, the panel 55 faces the sun 1 directly. The normal vector 500 is therefore parallel to the direct normal radiation 600. In this orientation, the full surface area of the panel 55 faces the sun 1. In contrast, FIG. 7 shows the PV module 57 of FIG. 6 with the panel 55 having been pivoted 90° counter-clockwise, as indicated by pivot-direction arrow 550, resulting in an angle of 90° between the direct normal radiation vector 600 and the normal vector 500. It can be clearly seen that the panel 55 of FIG. 7 receives no direct solar radiation, and the pivoting of the panel 55 has resulted in a 100% reduction in the direct normal component of the solar irradiance that would be incident on the panel 55 had it remained in the orientation of FIG. 6. A 90° rotation of the PV panel 55 is equivalent to a 100% reduction in the portion of the electrical output from photovoltaic conversion of direct normal irradiance, as the zero-angle cosine of 1.0 between incidence and normal vectors 600, 500 (in FIG. 6) has been reduced all the way to zero (in FIG. 7).

[0057] In the example of FIG. 7, pivot-direction arrow 550 shows that the PV panel 55 is rotated counter-clockwise from its orientation before the mechanical-clipping rotation. The direction of rotation is not substantially relevant to the reduction in electrical output, and other operational factors may be taken into account when deciding in which direction to pivot: for example, it may be desirable for the off-sun position to be closer in terms of rotation to the orientation to which the PV panel 55 will be rotating. In another example, a PV module 57 may be geared to rotate faster in one direction than the other.

[0058] FIGS. 8 and 9 show the PV panel 55 of FIG. 6 as having been rotated by 72°—clockwise in FIG. 8A and counter-clockwise in FIG. 8B. In both cases, the rotation reduces electrical output generated from the direct normal component of the solar radiation by about 70%, because the cosine of 72° is about 0.3, and the direction of rotation is not substantially relevant to the cosine loss induced by the pivoting. FIG. 9 shows the PV panel 55 as having been rotated by 60°—the cosine of 0.5 being effective to reduce the quantity of direct normal radiation received by 50%. FIG. 10 shows the PV panel 55 rotated by 26°—the cosine of 0.9 being effective to reduce the quantity of direct normal radiation received by 10%.

[0059] In some embodiments, the mechanical clipping procedure, i.e., the pivoting of selected PV modules 57 to respective off-sun orientations can be targeted to fully address the difference between an inverter rating (e.g., a maximum rating for a given set of environmental conditions) and the electrical output, e.g., for an imminent future forecasting period. In some embodiments, the mechanical clipping procedure can be targeted to partially address the excess electrical output in excess of the inverter rating, e.g., half or most or most of the electrical output in excess of the inverter rating, and then leaving the ‘remaining’ excess electrical output, e.g., current or forecasted electrical output to the inverter to clip, e.g., by changing an electrical set point of the PV modules 57.

[0060] We now refer to FIGS. 11 and 12, which show graphs of computer-modeled output of a solar energy system according to embodiments. For both of the figures, the x-axis shows time (in minutes) from sunrise over the course of a 12-hour solar day, and the y-axis shows the solar radiance in W/m.sup.2 (watts per square meter) that is equivalent to the electrical output with and without reduction from mechanical clipping. The figures are simplified and ignore, for example the effect of temperature on solar panel conversion efficiency and other factors unrelated to the disclosed embodiments. Thus, FIG. 11 shows a solar radiation curve with clear-sky irradiance peaking at 900 W/m.sup.2 during the middle of the day. This produces the electrical output without mechanical-clipping reduction represented by the solid line. The dashed line shows the solar radiation equivalent of electrical output that reduced to the level of an inverter rating equivalent to a maximum irradiance of 800 W/m.sup.2. As described above, some or all of the plurality of PV modules of the solar energy system are pivoted away from their on-sun orientation to increase cosine losses (including pivoting to 90° from the direct normal vector) for the modules involved. FIG. 12 illustrates an embodiment in which the inverter rating is the electrical-output equivalent of a maximum irradiance of about 750 W/m.sup.2 (dotted line). The PV modules are again controlled to be pivoted away from their on-sun orientation to increase cosine losses for the modules involved. This ‘mechanical clipping’ is sufficient in the example of FIG. 12 to reduce electrical output to an electrical output level equivalent to a maximum irradiance of 800 W/m.sup.2. The inverter additionally performs an inverter clipping function to reduce electrical output further, equivalent to a maximum irradiance of 750 W/m.sup.2, i.e., the inverter rating.

[0061] The present invention has been described using detailed descriptions of embodiments thereof that are provided by way of example and are not intended to limit the scope of the invention. The described embodiments comprise different features, not all of which are required in all embodiments of the invention. Some embodiments of the present invention utilize only some of the features or possible combinations of the features. Variations of embodiments of the present invention that are described and embodiments of the present invention comprising different combinations of features noted in the described embodiments will occur to persons skilled in the art to which the invention pertains.