BE1028418A1 - METHOD AND DEVICE FOR GAINING A DEMANDED POWER FROM AN ENERGY STORAGE - Google Patents

Abstract

According to one embodiment, the invention includes a method of recovering a demanded power (127) from an energy storage means (121) when tracked through a DC-DC converter (122) on the solar panel side of a maximum power point inverter (140) (141), MPPT, is connected; and wherein the method comprises the steps of: determining a current-voltage curve having a solar panel characteristic and having a power demand as maximum power point (127); and driving the DC-DC converter (122) so that the output follows the current-voltage curve whereby the converter evolves to the maximum power point and recovers the demanded power (127) from the energy storage means.

Description

-1- BE2020/5457

METHOD AND DEVICE FOR WINNING A QUESTION

POWER FROM AN ENERGY STORAGE Technical Field The present invention relates, among other things, to the recovery of a demanded power from an energy storage medium when it is supplied through a DC-DC converter on the solar panel side of an inverter with maximum power point tracking, MPPT. connected.

State of the art

[01] A solar panel installation comprises one or more solar panels that are coupled to an inverter. In operation, solar panels typically behave like a power source with the power delivered depending on the amount of light converted. The purpose of the inverter is to convert the power supplied by the solar panels and to supply a stable voltage at the output, i.e. a stable AC or DC voltage.

[02] In addition to the inverter circuit, the inverter usually also includes a maximum power point tracking or maximum power point tracking module, in short an MPPT module. The MPPT module must ensure that the solar panels operate in their optimal operating point. The MPPT module then ensures that the electrical load is such that the solar panels operate in their ideal operating point, i.e. that the solar panels deliver their maximum power. This ideal operating point is called the maximum power point or maximum power point, the MPP for short.

[03] An energy storage system, eg a battery, can also be coupled to solar panels that are coupled to an electricity grid by means of an inverter with MPPT. The energy storage system can then store energy from the solar panels, whether or not in parallel with the inverter. When there

-2- BE2020/5457 is not a yield, as for example at night the energy storage system can be discharged by supplying energy to the electricity grid.

Since most energy storage systems supply a DC voltage, it is most efficient to supply the energy storage system on the DC side of the inverter.

For example, one solution is to provide the energy storage system between the MPPT module and the inverter circuit so that the energy storage system does not interfere with the MPPT. A problem with this solution is that an inverter is usually provided as a whole and it is therefore not possible to provide an energy storage system afterwards without modifications to the inverter.

[06] Another solution is to place the energy storage system in front of the inverter, i.e. between the inverter and the solar panels. In this way, the energy storage system can be supplied afterwards by simply switching it between the solar panels and inverter. However, a problem with this solution is that the battery will affect the operation of the MPPT module and vice versa. More specifically, it then becomes difficult to have the battery deliver a certain preset power.

Summary of the Invention

The object of the present invention is to solve the above problem by providing a simple solution for providing an energy storage system between the solar panels and the inverter with a adjustable output power.

In a first aspect, the invention relates to a method for recovering a demanded power from an energy storage means when it is fed to the power supply via a DC-DC converter.

-3- BE2020/5457 solar panel side of an inverter with maximum power point tracking, MPPT, is connected; and wherein the method comprises the following steps: - determining a current-voltage curve with a solar panel characteristic and with a maximum power point, the MPP, the required power; and - driving the DC-DC converter so that the output follows the current-voltage curve whereby the converter evolves to the maximum power point and recovers the demanded power from the energy storage means.

In other words, upon discharging, the DC-DC converter of the energy storage means is controlled such that the MPPT module sees the current-voltage curve of a solar panel at its input poles. The MPPT module will therefore adapt its load to evolve towards the MPP of this current-voltage curve as quickly as possible. Since the current-voltage curve is selected to have the requested power as MPP, the energy storage means will thus start to supply the requested power.

[10] It is an advantage that such an energy storage means can be placed on the solar panel side of the inverter while having an adjustable power output. This further allows the energy storage means to be discharged in a controlled manner, eg in a demand-driven manner.

[11] A possible parameter for the current-voltage curve to be determined is the open-circuit voltage, i.e. voltage at which no current flows according to the current-voltage curve. According to one embodiment, determining the current-voltage curve comprises determining this open-circuit voltage of the solar panel characteristic. Controlling the DC-DC converter can then further comprise setting a bias voltage of the DC-DC converter with this open-circuit voltage. in other words the DC-DC converter is asked to supply a voltage, ie set, equal to the open-circuit voltage of the current-voltage curve. This results in a

-4- BE2020/5457 simple relationship between a parameter of the DC-DC converter, i.e. the bias voltage, and a parameter of the current-voltage curve, i.e. the open-circuit voltage.

[12] According to one embodiment, driving the DC-DC converter further comprises setting a maximum output current of the DC-DC converter obtained according to the current-voltage curve with the voltage being the voltage imposed by the converter. In other words, the current-voltage curve is obtained by continuously adjusting the maximum output current of the DC-DC converter according to the current-voltage curve.

[13] Since the MPPT module determines the voltage, this can easily be done by taking a voltage measurement at the output of the DC-DC converter and then determining the corresponding current value from the current-voltage curve. It is also an advantage that DC-DC converters with adjustable output voltage and adjustable maximum output current are readily available. The method can therefore be performed with a DC-DC converter that is already available on the market.

[4] According to one embodiment, determining the current-voltage curve further comprises determining a short circuit current of the solar panel characteristic. The short-circuit current is another possible parameter that determines the current-voltage curve, i.e. the current when the voltage is zero, i.e. at short circuit. Determining the current-voltage curve may then further comprise determining the short-circuit current based on the power demand and the open-circuit voltage.

[15] According to one embodiment, the determination of an open-circuit voltage of the solar panel characteristic is based on a voltage supplied by the energy storage means. Preferably, an open-circuit voltage is selected that is lower than the voltage supplied by the energy storage means.

-5- BE2020/5457

[16] Some DC-DC converters can only supply an output voltage lower than the input voltage, i.e. the voltage supplied by the energy storage means. By choosing the open-circuit voltage lower than this voltage, it is certain that the entire current-voltage curve can be followed.

[17] According to an embodiment, the method further comprises the following steps: - detecting when the voltage supplied by the energy storage means falls below a determined limit voltage; - determining a new open-circuit voltage based on the detected voltage; - determining a new current-voltage curve based on a newly determined open-circuit voltage; - controlling the DC-DC converter so that the output follows the current-voltage curve so that the converter evolves to the maximum power point and obtains the required power from the energy storage means.

[8] The output voltage of the energy storage means may slowly decrease during discharge, causing the maximum output voltage of the converter to also decrease. The above steps prevent that at a given moment the current-voltage curve can no longer be followed by the DC-DC converter, in particular the zones with the highest voltage.

[19] According to a second aspect, there is provided a device for storing and releasing energy in a solar panel installation, comprising: - an energy storage means: - a DC/DC converter for converting voltage from the energy storage means to an output voltage and connectable to the solar panel side of a converter with maximum power point tracking, MPPT; and

-6- BE2020/5457 - a control unit configured to perform the method according to the first aspect.

[20] According to a third aspect, there is provided a solar panel installation comprising: - at least one solar panel; - an inverter; and - the device according to the second aspect.

[21] According to a fourth aspect, there is provided a computer program product comprising computer executable instructions for performing the method according to the first aspect when this program is executed on a computer.

[22] According to a fifth aspect, there is provided a computer readable storage medium containing the computer program product according to the fourth aspect.

Brief Description of the Figures

[23] Figure 1 shows a solar panel installation with an energy storage system according to an embodiment;

[24] Figure 2 shows a current-voltage curve according to an embodiment;

[25] Fig. 3 shows steps performed for driving a DC-DC converter according to an embodiment;

[26] Figure 4 shows further steps performed for driving a DC-DC converter according to an embodiment; and

[27] Figure 5 shows a control unit suitable for carrying out various methods according to the described embodiments. Description of Embodiments

[28] Drawing 1 illustrates a solar panel installation 100 with a device 120 according to one embodiment. The solar panel installation comprises one or more solar panels 101 which are coupled to a power grid 143 via an inverter 140. Such an installation 100 can be provided, for example, at a private user of the power grid 143, at an industrial user or as an industrial-scale power solution. Converter 140 comprises a DC-AC converter 142 which converts the DC voltage level of the solar panels to the AC voltage level of the grid 143. Alternatively, this may also be a DC-DC converter if, for example, a local DC voltage grid 143 is supplied. Transducer 140 further includes a maximum power point tracking module 141, also referred to as maximum power point tracking, hereinafter referred to as MPPT. MPPT module 141 then ensures that the solar panels 101 operate in their ideal operating point, i.e. the maximum power point, also known as maximum power point, or MPP for short. Under given circumstances, solar panels will operate as a non-ideal current source as illustrated by current-voltage curve 210 in FIG. 2. If the terminals of a solar panel are short-circuited, the panels deliver a so-called short-circuit current 213. With increasing load, this short-circuit current will gradually decrease with an increasing voltage until it at a given moment evolves to zero at the open-circuit voltage 212. MPPT module 141 will then regulate the load on the solar panels and thus the output voltage 105 such that the solar panels deliver their maximum power, ie is located in the MPP 211.

[29] Device 120 is an energy storage system that can be coupled between the solar panels 101 and inverter 140 . Device 120 comprises an energy storage means 121, e.g. a battery 121, which carries a DC voltage (126)

-8- BE2020/5457 delivers. Device 120 may also include a terminal for externally connecting such an afternoon energy storage. Depending on the charge level of battery 121, this output voltage 126 may vary. Typically it will decrease with decreasing charge level. Energy storage means 121 is coupled to a DC-DC converter 122 which in turn is coupled to the inverter 140 . The DC-DC converter can be configured to supply a certain open-circuit voltage Vpc (124) and to limit the supplied current to a maximum upper limit Imax (125). The apparatus 120 further includes a control unit 128 configured to perform various steps of a method according to an embodiment as further explained below with reference to FIG. 3 and FIG. 4. This control unit 128 is configured to drive the DC-DC converter 122 to maximum current 125 and bias voltage 124 based on the measured voltage 105 and a power demand 127 so that the energy storage means 121 delivers this power demand 127 to the grid 143 through the converter 140. Control unit 128 can further do the driving based on the voltage 126 supplied by the energy storage means 121.

Device 120 may further include switches 129 that allow to disconnect the solar panels 101 from the inverter 140 and DC-DC converter 122 during the discharging of the energy storage means 121. Control unit can be further configured to store the energy storage means 121. charging with energy generated by the solar panels, whether or not in parallel with the inverter 140.

[31] Figure 3 shows steps of a method 300 that can be performed by control unit 128 to discharge the energy storage means 121 at a demanded power 127 to the inverter 140. This is preferably done when the solar panels 101 are inoperative, e.g. by disconnecting the solar panels 101 by means of switches 129, by performing the discharging at night or in darkness, or by a combination of both. Method 300 is therefore executable during a discharge cycle of the energy storage means 121. The method 300 causes the DC-DC converter 122 to behave according to a current-voltage curve 210 of a

-9- BE2020/5457 solar panel to inverter 140. This current curve 210 is selected such that the MPP 211 corresponds to the demanded power 127. Since the inverter 140 works with MPPT, the DC-DC converter 122 will evolve over time to the MPP 211 and thus supply the demanded power 127 to the converter.

[32] A current-voltage curve of a solar panel can be characterized by short-circuit current Isc (213), open-circuit voltage Voc (212) and a fill factor, also called fill factor, FF for short. These then determine the MPP 211 and, thus, the power demand 127. The method 300 illustrates the steps to select a particular current-voltage curve 210 for use by the control unit 128.

[33] In a first step 301, the open-circuit voltage Voc (323, 212) is determined. It is selected within the operating range of the inverter 140 and is preferably chosen to be lower than the maximum open circuit voltage of the solar panels 101. Typically, multiple solar panels are arranged in series in a 'string' and the maximum open circuit voltage is that of the full string. Preferably, the open-circuit voltage Voc (212) is taken as large as possible because then the discharging will proceed most efficiently. Thus, the selected open circuit voltage 323 can be determined as a predetermined percentage of the maximum voltage of the converter, eg 80% or 90%.

[34] Some DC-DC converters can only supply a voltage that is lower than the input voltage, the so-called buck converters. In this case, the selected open-circuit voltage 323 will thus have to be chosen lower than the voltage Vear (126) of the energy storage means. Preferably, the open-circuit voltage 323 is chosen to be somewhat lower than the voltage Vear (126) because the voltage 126 may drop further upon discharge. Thus, the selected terminal voltage 323 in this type of DC-DC converters can be determined as a predetermined percentage of the voltage V 126, eg 80% or 90%.

-10- BE2020/5457

[35] In the next step 302, the short circuit current 213, 327 of the current voltage curve 210 is determined. If the fill factor of curve 210 is fixed, the short-circuit current 213, 327 can be determined on the basis of the MPP 211 and therefore the required power 127 and on the basis of the open-circuit voltage 323. The short-circuit current 213, 327 can be determined on the basis of of a table 310 in which a corresponding short-circuit current 327 is given for a possible set of the open-circuit voltage 323 and the requested power 127. The open-circuit voltage 323 and the power demand 127 can hereby be rounded to a set of values in the table 310. Executing step 302 thus determines the necessary parameters needed to define the current-voltage curve. In this way a relation 304 is obtained in which the current can be derived from a certain input voltage, i.e. the voltage VmerT as imposed by the converter 140. This can be done, for example, on the basis of an analytical function based on the derived values. An example of such an analytic function is: Vuppr xn) Imax = Ilse — € Voc Where c is a constant that determines the filling factor FF of the curve. An example value for c is c = 1.10 70.

[36] The method then continues with step 303 for continuously driving the DC-DC converter. At the start of step 303, the open-circuit voltage Vpc of the DC-DC converter is set in accordance with the derived open-circuit voltage Voc 323. The maximum current Imax 125 is then set according to the derived current voltage characteristic 210 based on the measured voltage Vmerr. Typically, an MPPT module 141 will first measure the open circuit voltage of a solar panel and thus start at point 212. Accordingly, in step 303, Imax=0 is set. Thereafter, the MPPT module 141 will iteratively evolve to the MPP 211 and thus the requested power Pp 127 by adjusting the voltage 105 . The driving of the current Imax should be done continuously so that the MPPT algorithm in the MPPT module 141 is not disturbed. Therefore, the current Imax is preferably set

-11- BE2020/5457 within the same time interval as the MPPT algorithm. Typically this time interval will be below a second, even on the order of milliseconds, eg between one and one hundred milliseconds.

[37] Some types of DC-DC converters 122 can only provide an output voltage 105 that is lower than the battery voltage 126. Furthermore, the battery voltage 126 may drop during discharge. As a result, it is possible that the battery voltage falls below the set voltage 124 and thus below the open-circuit voltage 212. At that moment, in step 303, the set current-voltage characteristic 210 can no longer be set. fig. 4 shows additional steps 401-403 to avoid this situation. At step 303, the battery voltage 126 is constantly monitored at step 401, e.g. every second. As long as the measured battery voltage 126 is above a limit voltage VT, step 303 continues to use the characteristic 304 determined under step 302 (see arrow 402). When the measured battery voltage 126 falls below the threshold voltage VT, the method returns via step 403 to the first step 301 to determine a new open-circuit voltage 301 and thus determine a new characteristic 304 again.

[38] FIG. 5 shows a suitable computer system 500 for performing the steps according to the methods of the above embodiments. Computer system 500 may be configured generally as a suitable general purpose computer having a data bus 510, a processor 502, a local memory 504, one or more optional input interfaces 514, one or more output interfaces 516, a communications interface 512, a storage element interface 506 and one or more storage elements 508. Bus 510 may include one or more conductors that enable communication between the components of the computer system 500 . Processor 502 may include any type of conventional processor or microprocessor that interprets and executes program instructions. Local memory 504 may include a Random Access Memory (RAM) or other type of dynamic storage device that stores information and instructions for execution by processor 502, and/or a Read-Only Memory (ROM) or other type of static

-12- BE2020/5457 storage device, which stores static information and instructions for use by processor 502 . Input interface 514 may include one or more conventional mechanisms that allow an operator to input information into the computing device 500, such as a keyboard 520, a mouse 530, a pen, voice recognition and/or biometric mechanisms, etc. Output interface 516 may include one or more conventional mechanisms that deliver information to the operator, such as a display 540, a printer 550, a speaker, etc. Communications interface 512 may include a transceiver-like mechanism, such as, for example, one or more Ethernet interfaces, that computer system 500 to communicate with other devices and/or systems. The communication interface 512 of computer system 500 may be connected to such other computer system by means of a Local Area Network (LAN) or a Wide Area Network (WAN), such as, for example, the Internet. Storage element interface 506 may include a storage interface, such as, for example, a Serial Advanced Technology Attachment (SATA) interface or a Small Computer System Interface (SCSI), for connecting bus 510 to one or more storage elements 508, such as one or more local disks, e.g., SATA - disk drives, and control the reading and writing of data to and/or from these storage elements 508. Although the storage elements 508 have been described above as a local disk, in general any other suitable computer-readable medium, such as a removable magnetic disk, optical storage media, such as a CD or DVD, ROM disk, solid state drives, flash memory cards, can be used.

[39] While the present invention has been illustrated with reference to specific embodiments, it will be apparent to those skilled in the art that the invention is not limited to the details of the foregoing illustrative embodiments, and that the present invention may be embodied with various modifications and modifications without departing from the scope of the invention. The present embodiments are therefore to be regarded as illustrative and not restrictive in all respects, the scope of the invention being

-13- BE2020/5457 is described by the appended claims and not by the foregoing description, and all modifications which come within the meaning and scope of the claims are therefore incorporated herein.

In other words, it is understood that any changes, variations or equivalents that fall within the scope of the underlying principles and whose essential attributes are claimed in this patent application are understood to be included.

In addition, the reader of this patent application will understand that the words "comprising" or "comprising" do not exclude other elements or steps, that the word "a" does not exclude the plural, and that a singular element, such as a computer system, a processor or other integrated unit can perform the functions of various tools stated in the claims.

Any references in the claims should not be construed as limiting the claims in question.

The terms "first", "second", "third", "a", "bp", "c" and the like,

when used in the specification or in the claims, are used to distinguish between similar elements or steps and do not necessarily describe a sequential or chronological order.

Likewise, the terms "top", "bottom", "over", below" and the like are used for purposes of description and do not necessarily refer to relative positions.

It is to be understood that those terms are interchangeable under appropriate circumstances and that embodiments of the invention are capable of functioning according to the present invention in other orders or orientations than those described or illustrated above.

Claims (13)

-14- BE2020/5457 CONCLUSIONS A computer-implemented method (300) for recovering a demanded power (127) from an energy storage means (121) when fed through a DC-DC converter (122) on the solar panel side of an inverter (140) with maximum- power point tracking (141), MPPT, is connected; and wherein the method comprises the following steps: - determining (301, 302) a current-voltage curve (211, 304) with a solar panel characteristic and with a maximum power point (211) and the requested power (127); and - driving (303) the DC-DC converter (122) so that the output follows the current-voltage curve whereby the converter evolves to the maximum power point (211) and recovers the demanded power (127) from the energy storage means. The method of claim 1, wherein determining (301, 302) the current-voltage curve comprises determining (301) an open-circuit voltage (212, 323) of the solar panel characteristic. The method of claim 2 wherein driving (303) the DC-DC converter (122) further comprises setting a bias voltage (124) of the DC-DC converter with the open-circuit voltage (212, 323) of the current voltage curve. The method of any preceding claim wherein driving (303) the DC-DC converter (122) further comprises setting a maximum output current (125) of the DC-DC converter (122) as obtained according to the current voltage curve with the voltage (105) imposed by the inverter (140) as voltage. The method of any preceding claim wherein determining (301, 302) the current-voltage curve further comprises determining a short circuit current (213, 327) of the solar panel characteristic. -15- BE2020/5457 The method of claims 2 and 5 wherein determining (301, 302) the current-voltage curve further comprises determining the short circuit current (213, 327) based on the power demand (127) and the open circuit voltage (212, 323). ). The method of claim 3, wherein determining a terminal open voltage (212, 323) of the solar panel characteristic is based on a voltage (121) supplied from the energy storage means. The method of claim 7 wherein the open-circuit voltage (212, 323) is less than the voltage (121) supplied by the energy storage means. The method according to claim 7 or 8, wherein the method further comprises the steps of: - detecting (401) when the voltage (121) supplied by the energy storage means falls below a certain limit voltage (VT); - determining (301) a new open-circuit voltage (212, 323) based on the detected voltage; - determining (302) a new current-voltage curve (211, 304) based on a newly determined open-circuit voltage (212, 323) - driving (303) the DC-DC converter (122) so that the output (134) is the current voltage curve follows through which the inverter moves to the maximum power point (211) and gains the demanded power (324) from the energy storage medium. A device for storing and releasing energy in a solar panel installation, comprising: - an energy storage means (121); - a DC/DC converter (122) for converting voltage from the energy storage means to an output voltage and connectable to the solar panel side (132) of an inverter (102) with maximum power point tracking (103), MPPT; and BE2020/5457 - a control unit configured to perform the steps of any one of claims 1 to 9. 11. A solar panel installation comprising: - at least one solar panel; - an inverter; and - the device according to claim 10. A computer program product comprising computer executable instructions to perform the method according to any one of claims 1 to 9 when this program is executed on a computer. A computer readable storage medium containing the computer program product of claim 11.