EP4169141A1

EP4169141A1 - Method and apparatus for extracting a requested power from an energy storage medium

Info

Publication number: EP4169141A1
Application number: EP21739442.8A
Authority: EP
Inventors: Kristof GAUKEMA; Ismaël BEN-AL-LAL
Original assignee: Futech
Current assignee: Futech
Priority date: 2020-06-22
Filing date: 2021-06-21
Publication date: 2023-04-26
Also published as: BE1028418A1; WO2021260522A1; BE1028418B1; CN115885444A

Abstract

A method for extracting a requested power (127) from an energy storage medium (121) when it is connected via a DC-to-DC converter (122) to the solar-panel side of an inverter (140) with maximum power point tracking (141), MPPT; and wherein the method comprises the following steps: determining a current-voltage curve with a solar panel characteristic and with the requested power (127) as maximum power point; and controlling the DC-to-DC converter (122) so that the output follows the current-voltage curve, resulting in the inverter moving toward the maximum power point and extracting the requested power (127) from the energy storage medium.

Description

METHOD AND APPARATUS FOR EXTRACTING A REQUESTED POWER

FROM AN ENERGY STORAGE MEDIUM Technical Field

The present invention relates, inter alia, to extracting a requested power from an energy storage medium when it is connected via a DC-to-DC converter to the solar-panel side of an inverter with maximum power point tracking, MPPT.

Prior Art

[01] A solar panel installation comprises one or more solar panels which are coupled to an inverter. In operation, solar panels typically act as a current source in which the delivered current is dependent on the amount of converted light. The purpose of the inverter is to convert the power delivered by the solar panels and to deliver a stable voltage, i.e. a stable AC or DC voltage, at the output.

[02] In addition to the inverter circuit, the inverter typically also comprises a maximum power point tracking module, or MPPT module for short. The MPPT module must ensure that the solar panels are operating at their optimal operating point. The MPPT module then ensures that the electrical load is such that the solar panels are operating at their ideal operating point, i.e. that the solar panels are delivering their maximum power. This ideal operating point is called the maximum power point, or MPP for short.

[03] An energy storage system, e.g. 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, potentially in parallel with the inverter. When there is no yield, for example at night, the energy storage system can be discharged by delivering energy to the electricity grid. [04] Patent application GB 2541431 A describes a battery storage system for use with a microgeneration installation on the electricity grid (e.g. photovoltaic panels) in order to limit the import and export of power to and from the distribution network or electricity grid. The battery storage system is connected to the input of a solar panel emulator that presents one or more settable voltage- current curves on its output to an MPPT (maximum power point tracking) circuit within a PV inverter. The battery storage and emulator arrangement allows variable selection of the battery discharge rate and connection to a standard MPPT solar inverter. The battery storage system further comprises a controller for activating charging when energy export is above a set threshold, and for adjusting the maximum power point presented for discharge rate control when energy import is above a set threshold. A battery control module can be provided for each battery or battery bank, the control module having at least one from among an overcharge-, overdischarge-, overcurrent- or temperature-protection function or a balancing function.

[05] Since most energy storage systems deliver a DC voltage, it is most efficient to fit the energy storage system on the DC side of the inverter. [06] One solution is e.g. to fit the energy storage system between the MPPT module and the inverter circuit, so that the energy storage system does not interfere with the MPPT. One problem with this solution is that an inverter is usually supplied as one unit and it is therefore not possible to retrofit an energy storage system without adapting the inverter.

[07] 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 retrofitted just by connecting it between the solar panels and the inverter. However, one problem with this solution is that the battery will affect the operation of the MPPT module and vice versa. More specifically, it is then difficult to have the battery deliver a given preset power.

Summary of the Invention [08] The aim of the present invention is to address the above-mentioned problem by providing a straightforward solution for providing an energy storage system between the solar panels and the inverter with an adjustable deliverable power.

[09] According to a first aspect, the invention relates to a method for extracting a requested power from an energy storage medium when it is connected via a DC-to-DC converter to the solar-panel side of an inverter with maximum power point tracking, MPPT; and wherein the method comprises the following steps:

- determining a current-voltage curve with a solar panel characteristic and with the requested power as maximum power point, the MPP;

- controlling the DC-to-DC converter so that the output follows the current- voltage curve, resulting in the inverter moving toward the maximum power point and extracting the requested power from the energy storage medium;

- wherein the determining of the current-voltage curve further comprises determining an open-circuit voltage of the solar panel characteristic; and

- wherein the controlling of the DC-to-DC converter further comprises setting a bias voltage for the DC-to-DC converter with the open-circuit voltage of the current-voltage curve and setting a maximum output current for the DC-to-DC converter as obtained according to the current- voltage curve with the voltage imposed by the inverter as voltage. [10] In other words, during discharging, the DC-to-DC converter of the energy storage medium is controlled such that the MPPT module sees the current- voltage curve of a solar panel on its input poles. The MPPT module will therefore adjust its load so as to move as quickly as possible toward the MPP of this current-voltage curve. Since the current-voltage curve is selected so that it has the requested power as MPP, the energy storage medium will thus deliver the requested power. [11] It is advantageous that such an energy storage medium can be placed on the solar-panel side of the inverter because its power delivery is adjustable. This further allows the energy storage medium to be discharged in a controlled manner, e.g. in a demand-driven manner.

[12] One possible parameter for the current-voltage curve to be determined is the open-circuit voltage, i.e. the voltage according to the current-voltage curve when no current is flowing. The determining of the current-voltage curve further comprises determining this open-circuit voltage of the solar panel characteristic. The controlling of the DC-to-DC converter can then further comprise setting a bias voltage for the DC-to-DC converter with this open-circuit voltage. In other words, the DC-to-DC converter is requested to deliver a voltage, thus set, that is equal to the open-circuit voltage of the current-voltage curve. This results in a straightforward relationship between a parameter of the DC-to-DC converter, namely the bias voltage, and a parameter of the current-voltage curve, namely the open-circuit voltage.

[13] The controlling of the DC-to-DC converter further comprises setting a maximum output current for the DC-to-DC converter as obtained according to the current-voltage curve with the voltage imposed by the inverter as voltage. In other words, the current-voltage curve is obtained by continuously adjusting the maximum output current of the DC-to-DC converter according to the current- voltage curve. [14] Since the MPPT module determines the voltage, this can simply be performed by measuring the voltage at the output of the DC-to-DC converter and then determining the corresponding current value by means of the current- voltage curve. It is additionally advantageous that DC-to-DC converters with an adjustable output voltage and adjustable maximum output current are readily available. The method can therefore be carried out using a DC-to-DC converter that is already available on the market. [15] According to one embodiment, the determining of 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. in the case of a short circuit. The determining of the current-voltage curve can then further comprise determining the short-circuit current on the basis of the requested power and the open-circuit voltage.

[16] According to one embodiment, the determining of an open-circuit voltage of the solar panel characteristic is performed on the basis of a voltage delivered by the energy storage medium. Preferably, an open-circuit voltage is selected that is lower than the voltage delivered by the energy storage medium.

[17] Some DC-to-DC converters can only deliver an output voltage that is lower than the input voltage, i.e. the voltage delivered by the energy storage medium. By choosing the open-circuit voltage to be lower than this voltage, it is ensured that the entire current-voltage curve can be followed.

[18] According to one embodiment, the method further comprises the following steps:

- detecting when the voltage delivered by the energy storage medium falls below a given limit voltage;

- determining a new open-circuit voltage on the basis of the detected voltage;

- determining a new current-voltage curve on the basis of a newly determined open-circuit voltage;

- controlling the DC-to-DC converter so that the output follows the current- voltage curve, resulting in the inverter moving toward the maximum power point and extracting the requested power from the energy storage medium.

[19] The output voltage of the energy storage medium can slowly drop during discharging, resulting in the maximum output voltage of the converter falling too. By performing the above-mentioned steps, it is ensured that the DC-to-DC converter can follow the current-voltage curve at any given moment, more specifically the zones with the highest voltage. [20] According to a second aspect, a setup for storing and delivering energy in a solar panel installation is provided, comprising:

- an energy storage medium;

- a DC-to-DC converter for converting voltage from the energy storage medium to an output voltage and that is connectable to the solar-panel side of an inverter with maximum power point tracking, MPPT; and

- a control unit configured to carry out the method according to the first aspect.

[21] According to a third aspect, a solar panel installation is provided, comprising:

- at least one solar panel;

- an inverter; and

- the setup according to the second aspect. [22] According to a fourth aspect, a computer program product is provided, comprising instructions that can be run on a computer in order to carry out the method according to the first aspect when said program is run on a computer.

[23] According to a fifth aspect, a computer-readable storage medium is provided, comprising the computer program product according to the fourth aspect.

Brief Description of the Figures

[24] Figure 1 shows a solar panel installation with an energy storage system according to one embodiment; [25] figure 2 shows a current-voltage curve according to one embodiment;

[26] figure 3 shows steps performed to control a DC-to-DC converter according to one embodiment;

[27] figure 4 shows further steps performed to control a DC-to-DC converter according to one embodiment; and

[28] figure 5 shows a control unit suitable for carrying out various methods according to the described embodiments.

Description of Embodiments [29] Drawing 1 illustrates a solar panel installation 100 with a setup 120 according to one embodiment. The solar panel installation comprises one or more solar panels 101 which are coupled to a power network 143 via an inverter 140. Such an installation 100 can, for example, be provided on the side of a private user of the power network 143, an industrial user or as an industrial- scale power solution. Inverter 140 comprises a DC-to-AC converter 142 which converts the DC voltage level from the solar panels to the AC voltage level of the network 143. Alternatively, this can also be a DC-to-DC converter when it is e.g. a local DC voltage network 143 being supplied. Inverter 140 further comprises a module 141 for maximum power point tracking, or MPPT. MPPT module 141 then ensures that the solar panels 101 are operating at their ideal operating point, i.e. the maximum power point, or MPP for short. Under certain circumstances, solar panels will operate as a non-ideal current source as illustrated by current-voltage curve 210 in figure 2. When the terminals of a solar panel are shorted, the panels deliver what is termed a short-circuit current 213. With increasing load, this short-circuit current will slowly drop with rising voltage until, at a given moment, it reaches zero at the open-circuit voltage 212. MPPT module 141 will then adjust the load of the solar panels and thus the output voltage 105 such that the solar panels deliver their maximum power, i.e. are situated in the MPP 211.

[30] Setup 120 is an energy storage system that can be coupled between the solar panels 101 and the inverter 140. Setup 120 comprises an energy storage medium 121, e.g. a battery 121, that delivers a DC voltage VBAT (126). Setup 120 can also comprise a connection in order to be connected to such an energy storage medium externally. This output voltage 126 can vary depending on the level of charge of battery 121. Typically, it will decrease with increasing level of charge. Energy storage medium 121 is coupled to a DC-to-DC converter 122 which is in turn coupled to the inverter 140. The DC-to-DC converter can be configured to deliver a determined open-circuit voltage VDC (124) and to limit the delivered current to a maximum upper limit IMAX (125). The setup 120 further comprises a control unit 128 configured to carry out various steps of a method according to one embodiment as explained in more detail hereinbelow with reference to figure 3 and figure 4. This control unit 128 is configured to control the DC-to-DC converter 122 toward maximum current 125 and bias voltage 124 based on the measured voltage 105 and a requested power 127 so that the energy storage medium 121 delivers this requested power 127 to the network 143 via the inverter 140. Control unit 128 can further perform the control on the basis of the voltage 126 delivered by the energy storage medium 121.

[31] Setup 120 can further comprise switches 129 that allow the solar panels 101 to be decoupled from the inverter 140 and DC-to-DC converter 122 during the discharging of the energy storage medium 121. Control unit can further be configured to charge the energy storage medium 121 with energy generated by the solar panels, potentially in parallel with the inverter 140.

[32] Figure 3 shows steps of a method 300 that are able to be carried out by control unit 128 in order to allow the energy storage medium 121 to be discharged into the inverter 140 with a requested power 127. This preferably occurs when the solar panels 101 are not in operation, e.g. by decoupling the solar panels 101 by means of switches 129, by carrying out discharging at night or when it is dark, or through a combination of both. Method 300 can therefore be carried out during a discharge cycle of the energy storage medium 121. The method 300 ensures that the DC-to-DC converter 122 behaves in accordance with a current-voltage curve 210 of a solar panel with respect to the inverter 140. This current curve 210 is selected such that the MPP 211 corresponds to the requested power 127. Since the inverter 140 operates with MPPT, the DC-to- DC converter 122 will move toward the MPP 211 over time and thus deliver the requested power 127 to the inverter. [33] 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, or FF for short. These then determine the MPP 211 and, therefore, the requested power 127. The method 300 illustrates the steps for selecting a determined current- voltage curve 210 for use by the control unit 128.

[34] In a first step 301 , the open-circuit voltage Voc (323, 212) is determined. This is selected within the operating range of the inverter 140 and is preferably chosen so as to be lower than the maximum open-circuit voltage of the solar panels 101. Typically, a plurality of solar panels are placed in series in a "string" and the maximum open-circuit voltage is that of the entire string. Preferably, the open-circuit voltage Voc (212) is taken to be as high as possible because then discharging will proceed as efficiently as possible. Thus, the chosen open-circuit voltage 323 can be determined as a predetermined percentage of the maximum voltage of the inverter, e.g. 80% or 90%.

[35] Some DC-to-DC converters can only deliver a voltage that is lower than the input voltage, i.e. buck converters. In this case, the chosen open-circuit voltage 323 will thus have to be chosen so as to be lower than the voltage VBAT (126) of the energy storage medium. Preferably, the open-circuit voltage 323 is chosen so as to be somewhat lower than the voltage VBAT (126) because the voltage 126 can drop further during discharging. Thus, with this type of DC-to- DC converter, the chosen terminal voltage 323 can be determined as a predetermined percentage of the voltage VBAT 126, e.g. 80% or 90%. [36] In the next step 302, the short-circuit current 213, 327 of the current- voltage curve 210 is determined. When 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 thus the requested power 127 and on the basis of the open-circuit voltage 323. The short-circuit current 213, 327 can be determined by means of a table 310 in which, for every possible set of the open-circuit voltage 323 and the requested power 127, a corresponding short-circuit current 327 is given. For this, the open-circuit voltage 323 and the requested power 127 can be rounded to a set of values in the table 310. The execution of step 302 thus determines the necessary parameters needed to define the current-voltage curve. In this way, a relationship 304 is obtained in which the current can be derived from a given input voltage, i.e. the voltage VMPPT as imposed by the inverter 140. For example, this can be achieved by means of an analytical function on the basis of the derived values. One example of such an analytical function is: where c is a constant that determines the fill factor FF of the curve. An exemplary value for c is c = 1. 1Cr¹⁰.

[37] The method then continues with step 303 for continuously controlling the DC-to-DC converter. At the start of step 303, the open-circuit voltage VDC of the DC-to-DC converter is adjusted so as to correspond to the derived open-circuit voltage Voc 323. The maximum current IMAX 125 is then set according to the derived current-voltage characteristic 210 on the basis of the measured voltage VMPPT. Typically, an MPPT module 141 will first measure the open-circuit voltage of a solar panel and thus start at point 212. IMAX=0 is then set accordingly in step 303. Then, the MPPT module 141 will iteratively move toward the MPP 211 and thus the requested power PD 127 by adjusting the voltage 105. The current IMAX has to be controlled continuously so that the MPPT algorithm in the MPPT module 141 is not disrupted. Consequently, the current IMAX is preferably adjusted within the same time interval as the MPPT algorithm. Typically, this time interval will be shorter than a second, even of the order of milliseconds, e.g. between one and a hundred milliseconds.

[38] Some types of DC-to-DC converters 122 can only deliver an output voltage 105 that is lower than the battery voltage 126. Additionally, the battery voltage 126 can drop during discharging. This can result in the battery voltage falling below the set voltage 124 and thus below the open-circuit voltage 212. When that happens, the set current-voltage characteristic 210 can no longer be adjusted in step 303. Figure 4 shows extra steps 401-403 in order to avoid this situation. During step 303, the battery voltage 126 is constantly controlled in a step 401 , for example 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 in step 302 (see arrow 402). If the measured battery voltage 126 falls below the limit voltage VT, the method returns, via step 403, to the first step 301 in order to determine a new open-circuit voltage 301 and thus determine a new characteristic 304 again.

[39] Figure 5 shows a suitable computer system 500 for carrying out the steps according to the methods of the above embodiments. Computer system 500 can generally take the form of a computer suitable for general purposes with 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 communication interface 512, a storage element interface 506 and one or more storage elements 508. Bus 510 can comprise one or more conductors, which enable communication between the components of the computer system 500. Processor 502 can comprise any type of conventional processor or microprocessor which interprets and executes program instructions. Local memory 504 can comprise a random access memory (RAM) or another type of dynamic storage device which stores information and instructions for execution by processor 502, and/or a read-only memory (ROM) or another type of static storage device which stores static information and instructions for use by processor 502. Input interface 514 can comprise one or more conventional mechanisms which allow an operator to input information into the computer device 500, such as a keyboard 520, a mouse 530, a stylus, voice recognition and/or biometric mechanisms, etc. Output interface 516 can comprise one or more conventional mechanisms which provide information to the operator, such as a display 540, a printer 550, a speaker, etc. Communication interface 512 can comprise a transceiver-type mechanism, such as for example one or more Ethernet interfaces, which allow the computer system 500 to communicate with other devices and/or systems. The communication interface 512 of computer system 500 can be connected to such another 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 can comprise 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 with one or more storage elements 508, such as one or more local disks, for example SATA disk drives, and for reading and writing data to and/or from these storage elements 508. Although the storage elements 508 above are described as a local disk, in general any other suitable medium that can be read by computer, such as a removable magnetic disk, optical storage media, such as a CD or DVD, CD- ROM, solid-state drives, or flash memory cards, can be used. [40] Although the present invention has been illustrated by means of specific embodiments, it will be clear to a person skilled in the art that the invention is not limited to the details of the above illustrative embodiments, and that the present invention can be carried out with various changes and modifications without thereby departing from the area of application of the invention. Therefore, the present embodiments have to be seen in all areas as being illustrative and non-restrictive, and the area of application of the invention is described by the attached claims and not by the above description, and any changes which fall within the meaning and scope of the claims are therefore incorporated herein. In other words, it is assumed that this covers all changes, variations or the like which fall within the area of application of the underlying basic principles and the essential attributes of which are claimed in this patent application. In addition, the reader of this patent application will understand that the terms "comprising" or "comprise" do not exclude other elements or steps, that the term "a(n)/one" does not exclude the plural and that a single element, such as a computer system, a processor or another integrated unit, can perform the functions of various auxiliary means which are mentioned in the claims. Any references in the claims cannot be interpreted as a limitation of the respective claims. The terms "first", "second", "third", "a", "b", "c" and the like, when used in the description or in the claims, are used to distinguish between similar elements or steps and do not necessarily indicate a sequential or chronological order. In the same way, the terms "top side", "bottom side", "above", "below" and the like are used for the sake of the description and do not necessarily refer to relative positions. It should be understood that these terms are interchangeable under the appropriate circumstances and that embodiments of the invention can function according to the present invention in different sequences or orientations than those described or illustrated above.

Claims

1. A computer-implemented method (300) for extracting a requested power (127) from an energy storage medium (121) when it is connected via a DC-to- DC converter (122) to the solar-panel side of an inverter (140) with maximum power point tracking (141), MPPT; and wherein the method comprises the following steps:

- determining (301, 302) a current-voltage curve (211, 304) with a solar panel characteristic and with the requested power (127) as maximum power point (211);

- controlling (303) the DC-to-DC converter (122) so that the output follows the current-voltage curve, resulting in the inverter moving toward the maximum power point (211) and extracting the requested power (127) from the energy storage medium; - wherein the determining (301, 302) of the current-voltage curve further comprises determining (301) an open-circuit voltage (212, 323) of the solar panel characteristic; and

- wherein the controlling (303) of the DC-to-DC converter (122) further comprises setting a bias voltage (124) for the DC-to-DC converter with the open-circuit voltage (212, 323) of the current-voltage curve and setting a maximum output current (125) for the DC-to-DC converter (122) as obtained according to the current-voltage curve with the voltage (105) imposed by the inverter (140) as voltage.

2. The method as claimed in claim 1 , wherein the determining (301 , 302) of the current-voltage curve further comprises determining a short-circuit current (213, 327) of the solar panel characteristic.

3. The method as claimed in one of the preceding claims, wherein the determining (301, 302) of the current-voltage curve further comprises determining the short-circuit current (213, 327) on the basis of the requested power (127) and the open-circuit voltage (212, 323).

4. The method as claimed in one of the preceding claims, wherein the determining of an open-circuit voltage (212, 323) of the solar panel characteristic is performed on the basis of a voltage (121) delivered by the energy storage medium.

5. The method as claimed in claim 4, wherein the open-circuit voltage (212, 323) is lower than the voltage (121 ) delivered by the energy storage medium.

6. The method as claimed in claim 4 or 5, wherein the method further comprises the following steps:

- detecting (401 ) when the voltage (121) delivered by the energy storage medium falls below a given limit voltage (VT);

- determining (301) a new open-circuit voltage (212, 323) on the basis of the detected voltage; - determining (302) a new current-voltage curve (211, 304) on the basis of a newly determined open-circuit voltage (212, 323);

- controlling (303) the DC-to-DC converter (122) so that the output (134) follows the current-voltage curve, resulting in the inverter moving toward the maximum power point (211) and extracting the requested power (324) from the energy storage medium.

7. A setup for storing and delivering energy in a solar panel installation, comprising:

- an energy storage medium (121); - a DC-to-DC converter (122) for converting voltage from the energy storage medium to an output voltage and that is connectable to the solar- panel side (132) of an inverter (102) with maximum power point tracking (103), MPPT; and

- a control unit configured to carry out the steps as claimed in one of claims 1 to 6.

8. A solar panel installation, comprising:

- at least one solar panel; - an inverter; and

- the setup as claimed in claim 7.

9. A computer program product comprising instructions that can be run on a computer in order to carry out the method as claimed in one of claims 1 to 6 when said program is run on a computer.

10. A computer-readable storage medium comprising the computer program product as claimed in claim 9.