Patent Application
20190280605
The invention relates to bidirectional DC DC converters between sources, loads and energy storage devices.
Solar power in the US is primarily grid tied to avail the owner of net metering provisions offered by the local utility. In many instances, net metering provisions are under going change which decreases the net metering credit to the owner by changing the on-peak hours, when net metering credits are of highest value, to later times when power feed from the owner to the grid is more valuable to the utility. The power to be fed back is dependent on the sun's profile, not time, and so maximum solar energy harvest is now out of sync with utility demand. To accommodate this change, the solar system can be equipped with an energy storage component, such as a battery, that stores the energy harvested at peak solar production and then releases it to the grid, or the homeowner, during the new on-peak hours, thereby benefiting both the utility grid and the owner of the solar system.
Solar systems deployed over the past several years were provisioned primarily for net metering and storage was not contemplated. As such, retrofitting the solar system with storage or additional power inputs is a difficult and engineering intensive, costly task. The invention reduces the cost and effort of retrofitting such solar arrays, as well as significantly reducing the engineering required to effect such a change.
Current state of the art for adding energy storage to an existing solar PV installation includes replacing the current inverter with an inverter configured to connect to a battery system, or to include an inverter within each battery pack. In addition, either dedicated load circuits or transfer switches must be used to be able to provide self consumption on-site.
Batteries present a fixed (or limited range) voltage which makes adding their DC voltage to a PV DC voltage buss impractical because of the very wide variability of the PV system voltage. And, since each system has different PV configurations, a one-size fits all solution has not been realizable.
The market growth of energy storage systems for renewable energy applications continues to grow exponentially, although for new installations primarily. The invention provides a solution for both new installations and for retrofitting existing with energy storage.
The invention provides an electrical adapter between renewable energy sources and loads such that energy storage elements can be added to the system with minimal effort and disruption. In one embodiment, the invention is inserted into an existing solar energy system that consists of a photovoltaic array connected to a grid tied inverter and where it is desired to add an energy storage element to said system. Such energy storage systems have a duality in that they are both loads (charging) and sources (discharging) and must match the system's electrical characteristics in order to function properly. Typically, such existing systems have high DC voltage magnitudes that are incompatible with low DC voltage magnitude energy storage elements and therefore a voltage converter is required to adapt the storage element to the array. Photovoltaic arrays need maximum power point tracking (MPPT) elements to ensure maximum power harvesting from the array and this function is typically provided by the load, in most cases a grid tied inverter. During the charge phase of the energy storage system, the existing load which typically provides the MPPT function remains operational as designed and the invention routes a configurable amount of the power to the battery to be charged. During the discharge phase, the converter must match the input characteristics of the load so that to the load it simply appears as a PV source, thereby allowing optimum blending of battery energy and maximum power PV energy to be presented to the load.
The invention combines the MPPT-following function, the voltage conversion function, the intelligent transfer function, with intelligent electronics for control of operation, either preprogrammed or user adjustable. In an embodiment, the invention also provides the charge control function for energy storage systems such as batteries and can interface to the battery's management system for optimal control and operation. In an embodiment, detailed operational data is collected by the invention and transmitted to a collection system that provides such data to the system owner or operator. In another embodiment, additional energy can be provided through the apparatus from an AC power source such as the utility grid, microgrids or AC generating equipment. In another embodiment, a secondary off-grid inverter can be connected to the system providing power to dedicated, critical loads when the primary, grid tied inverter is off-line.
During daylight, when the PV array [101] is energized and harvesting energy, its maximum power point (MPP) will vary considerably depending on irradiance, temperature, PV-module mismatch and soiling, shading and other external factors. The inverter [102] contains a maximum power point tracking (MPPT) algorithm to adjust the input voltage to match that of the PV-modules. The variance in MPP points can range from tens to hundreds of volts, preventing the insertion of fixed voltage batteries. In an embodiment of the invention, the converter automatically adjusts its N1 and N2 operating point to match the MPPT point established by the inverter [102] so that maximum energy harvesting from the PV array [101] is undisturbed and the converter [103] is transparent to the PV array [101] and the inverter [102] while they are operating.
As the PV array [101] harvests energy, the user can set business rules via the controller's [201] data lines [206] to specify when energy will be stored, how much of that energy harvest will be stored, and how much will be assigned to the inverter. Similar business rules are assigned to when the stored energy will be discharged and in what ratio. This operation provides the user with the ability to time shift the use or sale of energy harvested by the system to its most advantageous time.
The controller [201] also receives data from the battery's Battery Management System (BMS) [205] to determine the State of Charge (SOC), State of Health (SOH), the charge rate (C) and the discharge rate (C) defined by the user or by the battery manufacturer. In this respect, the converter performs as a charge controller and by measuring voltage and current at node 3, N3 [204] and using data input by the user, virtually any battery chemistry charge/discharge profile can be used, making the system agnostic to battery technologies and chemistries.
When operating in discharge mode, the controller [201] monitors voltages and currents at node N1 [202] and N2 [203] and executes its discharge operation to match the operational characteristics and MPPT of the PV array [101] and inverter [102]. The timing, the amount of power and its rate of discharge are governed by the controller [201]. A key feature of the controller [201] is the monitoring of the node voltages and currents, so the converter [103] continuously adjusts its output voltage and current to prevent current backflow from the energy storage [104] to the PV array [101], while still maintaining an optimal voltage at node 2, N2 [203] for the inverter's [102] operation.
In an embodiment of the invention, the operating parameters and their execution are governed by a combination of electronic control circuitry and business rules established by a user. An example of these business rules and operation is shown in
While many of the operational functions are handled by the controller [201] and its operational algorithm, the electrical properties of the converter [103], which make the system functional, reside in an innovative bidirectional DC DC converter shown in
An embodiment of the invention is shown in
In discharge mode of this embodiment of the invention, the power flow is reversed using the energy stored in the energy storage [413]. The secondary side [302] is now operated as a DC DC boost converter through charging and discharging the inductor [411] with switches [409] and [410] which produce an amplitude approximately half the value desired on the high voltage side. The windings [407] of the transformer [408] magnetically couple the AC signal to the primary side [406] where switches [404] and [405] are now turned off and act as diodes (through the body diode of the switch). The combination of these diodes [404] and [405], and the capacitors in the network [403] act as a voltage doubler to produce the desired high voltage, which is filtered and balanced by the networks of [403] and [402].
The switching operation of switches [409] and [410] is critical for rapid power flow reversal and for continuously variable voltages. Instead of operating the switches 180 degrees out of phase with each other, the switches' drive signals are overlapped and the degree of overlap is determined by the required voltages and power flow characteristics.
As will be understood by those familiar with the art, the invention may be embodied in other specific forms without departing from the spirit or essential characteristics thereof. Likewise, the particular naming and division of the members, features, attributes, and other aspects are not mandatory or significant, and the mechanisms that implement the invention or its features may have different names, divisions and/or formats. Accordingly, the disclosure of the invention is intended to be illustrative, but not limiting, of the scope of the invention, which is set forth in the following Claims:
This application claims the benefit of U.S. Provisional Application No. 62/641,197 “DC DC Converter with MPPT and Transfer Switch” filed on Mar. 9, 2018, the entirety of which is incorporated herein by this reference thereto.
| Number | Date | Country | |
|---|---|---|---|
| 62641197 | Mar 2018 | US |
