Patent Application
20130200709
The present invention relates to photovoltaic power systems and more particularly, to concentrator photovoltaic power systems.
Solar photovoltaic power systems electrically connect multiple photovoltaic cells to the power grid. In a typical system this is achieved by serially connecting individual cells to produce high DC voltage and low current. Arrays of these serially connected cells are subsequently connected in serial and parallel topographies to produce strings operating at approximately 600 V peak.
Series connected cells are, however, performance limited by the weakest cell(s) in the string. In a series connected circuit, the current through all cells is the same. Cells that are operating at a current other than the maximum power point will thus be limited to less than optimal performance. This limitation increases in severity with concentration. In the case of parallel connected cells, the current produced is large and the voltage too low to be useful for most applications. Also, producing large current is costly in terms of resistive losses and the amount of conductor necessary to connect the system. Also, regardless of the connection scheme, it is common that the cells are mismatched with regard to current and/or voltage relative to the device(s) to which the power is being supplied.
Therefore, techniques for maximizing performance from a grid connected array of photovoltaic cells, and/or being able to match application-specific current voltage ratio requirements would be desirable.
The present invention provides techniques for electrical power transfer in photovoltaic systems. In one aspect of the invention, a photovoltaic system is provided. The photovoltaic system includes an array of photovoltaic power producing elements; a power receiving unit; and at least one ratiometric DC to DC converter connected to both the array of photovoltaic power producing elements and the power receiving unit. The at least one ratiometric DC to DC converter is configured to alter a voltage output from the array of photovoltaic power producing elements supplied to the power receiving unit. The photovoltaic power producing elements can include concentrator photovoltaic cells. The power receiving unit may include a grid tied commercial power inverter or micro inverter. The power receiving unit can include a maximum power point tracker (MPPT) circuit and a grid tied DC to AC inverter connected to the maximum power point tracker (MPPT) circuit. The power receiving unit may include a DC to DC converter or may simply be a device that consumes DC power.
The array of photovoltaic power producing elements can include a plurality of the photovoltaic power producing elements connected in series. Alternatively, the array of photovoltaic power producing elements can include a plurality of the photovoltaic power producing elements connected in parallel. In the present context a single photovoltaic power producing element may include one or more (e.g., a grouping of) photovoltaic cells connected in serial or parallel.
In another aspect of the invention, a method of transferring electrical power from an array of photovoltaic power producing elements to a power receiving unit is provided. The method includes the following step. At least one ratiometric DC to DC converter is connected to both the array of photovoltaic power producing elements and the power receiving unit. The at least one ratiometric DC to DC converter is configured to alter a voltage output from the array of photovoltaic power producing elements supplied to the power receiving unit.
A more complete understanding of the present invention, as well as further features and advantages of the present invention, will be obtained by reference to the following detailed description and drawings.
Provided herein are techniques for connection of photovoltaic power producing elements, such as photovoltaic cells, to power conditioning and grid tie circuitry. With concentrator photovoltaic cells in particular, these systems are characterized by significantly higher cell currents and increasingly larger distances between cells. In some applications individual cell currents can reach 30 amps per square centimeter of cell area. At these current levels, connectors become expensive and very thick wires are required to connect cells. Thus, maximizing performance from a grid connected array of photovoltaic power producing elements is important, especially in the case of concentrator photovoltaic systems.
The circuits described herein utilize a ratiometric DC to DC converter. In general, a DC to DC converter is a type of power converter circuit that can convert a source of direct current (DC) from one voltage level to another. In particular, a ratiometric DC to DC converter is defined as a circuit that imposes a fixed voltage ratio between its input and output. Power may flow in either a forward or reverse direction through the ratiometric DC to DC converter. Finally, the input and output are electrically isolated. An example of this type of circuit is the VTM48EH040T025A00 made by Vicor Corporation, Andover Mass.
As will be apparent from the following description, some of the following benefits provided through use of the present techniques are as follows: first, the present techniques permit the optimization of individual cell performance. Namely, as will be described in detail below, some embodiments described are configured to allow each photovoltaic power producing element (e.g., photovoltaic cell) or group of elements to operate at its maximum performance level. By comparison, with conventional systems, the overall performance of the system is typically limited by the maximum performance of the weakest cell.
Second, as will be described in detail below, the present techniques permit the voltage produced by an array of photovoltaic cells to be increased (boosted)/decreased (bucked) based on the requirements of the power receiving unit.
Third, the ability to vary (increase/decrease) the voltage output from the array of photovoltaic cells (e.g., by way of a ratiometric DC to DC converter, see below) as according to the present techniques preserves the ability to operate maximum power point tracking (MPPT) systems. Maximum power point tracking (MPPT) systems operate by varying the input impedance of the power receiving unit and observing the current variation until the optimum power point is identified (and thereafter operating at that point) and thereby allowing the photovoltaic cells to operate at the most efficient voltage for optimum power transfer. With conventional (non ratiometric) DC to DC converters which output a fixed voltage an MPPT cannot be used because the output voltage is constrained to a constant value. MPPT systems will be described in detail below.
Fourth, the present techniques may be used to reduce the total amount of copper used for interconnects (higher currents require more copper to avoid resistive losses). By enabling the array of power producing elements to produce power at a higher voltage, the current that would otherwise be produced is reduced. In some concentrator elements current output can reach 30 A. Higher currents naturally require more copper in the various connections to avoid resistive losses. Thus, by reducing the current output, the amount of copper employed in the present systems can overall be reduced. This can mean a significant savings in terms of cost of materials, weight, etc.
It is noted that all of the connections shown in the figures and described herein may be formed using conventional electrical wiring. By way of example only, the term electrical wiring includes any type of insulated conductor that can be used to carry electricity. Examples of suitable electrical wiring include, but are not limited to insulated copper wires.
In conventional systems, arrays of individual photovoltaic cells are generally connected either in series or in parallel to a device(s) to which power is being supplied. Connecting the photovoltaic cells in series has the advantage that the photovoltaic cells produce a large voltage that is proportional to the number of cells in the series. This allows for less series resistance power losses. However, the photovoltaic cells are current limited by the weakest cells in the series. In small arrays, optical misalignment of a cell or a poor quality cell can strongly impact the performance of the series. In larger systems, shadowing (i.e., shading caused by neighboring structures, clouds . . . ) will have a degrading effect on the series. By contrast, connecting the photovoltaic cells in parallel has the advantage that the photovoltaic cells are not current limited by the weakest cell (since the cells are connected in parallel). A parallel connection has the further advantage that current can be monitored for individual cells using hall effect sensors and used as a direct measure of cell performance including optical alignment. However, conventional parallel-connected photovoltaic cell systems produce high current (e.g., 2 to 20 amps from a single cell) and thus are challenging to implement in practice.
As highlighted above, the present techniques relate generally to the coupling of power from photovoltaic cells to MPPT and inverter systems. The following discussion will focus on the use of a micro inverter (i.e., the low power (e.g., from about 100 watts to about 500 watts) version of a combined MPPT/DC to AC inverter) with the understanding that the techniques presented are equally applicable to higher power (e.g., 50,000 watts or more) commercial inverter units.
Further, concentrator photovoltaic systems operating at very high concentration may require optical alignment of individual receivers to achieve optimal power performance. Cells are most easily aligned by monitoring short circuit current or individual cell current at the maximum power point. This is challenging for serially connected cells. Thus, connecting the power producing elements in parallel (as shown in
It is important to the operation of a grid connected array of photovoltaic cells that the operating current and voltage be optimized to the maximum power point. This is achieved by the maximum power point tracker (MPPT). By way of example only, some maximum power point tracker circuits function (i.e., “track” the maximum power point which may be constantly changing) by sampling the output of the photovoltaic power producing elements and applying an amount of impedance sufficient to obtain maximum power output from the photovoltaic power producing elements. See also description of
In one exemplary embodiment, the maximum power point tracker (MPPT) is a single unit (computer-driven circuit configured to, e.g., based on the output of the photovoltaic power producing elements apply an amount of impedance sufficient to obtain maximum power output from the photovoltaic power producing elements) that outputs a fixed DC voltage. The output(s) of the MPPT unit(s) is/are connected to the DC to AC inverter. The inverter in a photovoltaic system, as is known in the art, is used to convert the DC output of the photovoltaic power producing elements into alternating current that can be fed into the grid. Photovoltaic inverters are commercially available, for example, from SMA Solar Technology, Rocklin, Calif. and Enphase Energy Corporation, Petaluma, Calif.
In another exemplary embodiment the function of the MPPT is combined with the grid tied DC to AC inverter in a single unit. Low power versions of this system are sometimes referred to as micro inverters (e.g., micro inverters are typically rated at 200 W to 400 W). Micro inverters are commercially available, for example, from Direct Grid® Technologies, Edgewood, N.J.
It is notable that while the power receiving unit in the embodiments shown and described herein includes a grid tied DC to AC inverter (and potentially a maximum power point tracker (MPPT)) this is merely one exemplary configuration. Namely, the power receiving unit can be any device that either passes the power on or uses the power itself. By way of example only, the power receiving unit might include a device, such as a battery, or electric motor that can use the DC power directly. In that case, no inverter is necessary.
Concentrator photovoltaic cells utilize lenses and/or mirrors to concentrate incident solar power onto a photovoltaic cell(s). Concentrator photovoltaic cells are described, for example, in Luque, Antonio; Hegedus, eds (2003). “Handbook of Photovoltaic Science and Engineering.” John Wiley and Sons. ISBN 0471491969, the contents of which are incorporated by reference herein.
By employing a ratiometric DC to DC converter (such as in the circuit of
The power receiving unit 103 is further connected to an AC power grid via connection 104. By way of example only, connection 104 is a wired connection to the power grid. Alternatively, the power receiving unit may be connected (via connection 104) to a device that runs on AC power (such as a machine or appliance) or to a connector (e.g., outlet) to which a device that runs on AC power can be connected. Further, as shown in
It is notable, however, that the ratiometric DC to DC converters described herein may also be employed to lower the voltage output (also referred to as bucking which is the opposite of boosting) from an array of photovoltaic power producing elements. The ratiometric DC to DC converters add the additional capability to regulate the input/output voltage ratio. Therefore, the ratiometric DC to DC converters described herein are configured to alter, i.e., increase (boost) and/or decrease (buck), the voltage output from the array depending, e.g., on the input voltage requirements of the power receiving element. Further, as provided above, and as is apparent from
The input and output stages of the ratiometric DC to DC converter 105 are electrically isolated. In some embodiments this allows the possibility to physically localize high voltage elements of a solar panel to reduce costs.
In one exemplary embodiment, such as that shown in
According to an exemplary embodiment, the maximum power point tracker (MPPT) circuit is a computer driven circuit that varies the input impedance to the ratiometric DC to DC converter with a fixed voltage output. The output of the MPPT is connected to the DC to AC inverter that applies the power to the grid. This may be done in a one to one ratio or multiple systems may be connected in parallel to a large inverter to achieve higher efficiency or lower cost.
The circuit shown in
Vcells+Vcells*R,
wherein R is the voltage ratio of the ratiometric DC to DC converter. Vcells is the voltage produced by the array of photovoltaic cells. By way of example only, a ratiometric DC to DC converter with a (input/output) voltage ratio of 1:2 will have an R value of 2. The current produced by the array of photovoltaic cells will be inversely reduced. The benefit of the circuit shown in
Second, a further benefit of the circuit of
It is notable that a significant amount of useful power point tracking and power inversion technology is becoming available for the flat panel photovoltaic modules market. It is desirable to be able to utilize this technology in the concentrator photovoltaic sector. Concentrator systems may have different current to voltage ratios than similar size flat panel embodiments. In some concentrator embodiments, use of flat panel technology is facilitated by increasing the voltage to current ratio to more closely match values typical of flat panel photovoltaic modules.
As shown in
The power receiving unit 204 is further connected to an AC power grid via connection 205. By way of example only, connection 205 is a wired connection to the power grid. Alternatively, the power receiving unit may be connected (via connection 205) to a device that runs on AC power (such as a machine or appliance) or to a connector (e.g., outlet) to which a device that runs on AC power can be connected. Further, as shown in
The circuit shown in
As shown in
The power receiving unit 304 is further connected to an AC power grid via connection 303. By way of example only, connection 303 is a wired connection to the power grid. Alternatively, the power receiving unit may be connected (via connection 303) to a device that runs on AC power (such as a machine or appliance) or to a connector (e.g., outlet) to which a device that runs on AC power can be connected. Further, as shown in
Thus, the circuit shown in
The circuit shown in
Each of the circuits shown above in which a ratiometric DC to DC converter is placed in the power path consumes a portion of the power produced by the cells due to the efficiency of conversion. This is typically 95%. In other words, the aforementioned advantages carry the cost of 5% of the power that flows through them.
Another exemplary embodiment that addresses this short coming is shown in
As shown in
The power receiving unit 403 is further connected to an AC power grid via connection 404. By way of example only, connection 404 is a wired connection to the power grid. Alternatively, the power receiving unit may be connected (via connection 404) to a device that runs on AC power (such as a machine or appliance) or to a connector (e.g., outlet) to which a device that runs on AC power can be connected. Further, as shown in
The outputs of the ratiometric DC to DC converters 405 are connected in parallel and thus are constrained to a common floating voltage. The terms “float” and “floating voltage” as used herein refer to the concept that the photovoltaic power producing elements in the array are permitted to equilibrate to an average output value (i.e., power will be shunted from over performing photovoltaic power producing elements in the array to under-performing photovoltaic power producing elements in the array to achieve an average output value. By constraining the outputs of the ratiometric DC to DC converters 405 to a common voltage, the individual photovoltaic power producing element voltages are also constrained to a common (average) voltage. Accordingly, an input voltage to the ratiometric DC to DC converters 405 is thus constrained to a common value. In the case where an individual photovoltaic power producing element is underperforming, current is applied from the other photovoltaic power producing elements through the ratiometric DC to DC converters 405 to that underperforming individual photovoltaic power producing element's output. In the case where a photovoltaic power producing element is producing more than the average, its power is shunted to adjacent underperforming photovoltaic power producing elements. Thus power is balanced. Unlike the aforementioned embodiments, the amount of power flowing through the ratiometric DC to DC converters is limited to the difference of a photovoltaic power producing element's power from the average value. This is much smaller than in the aforementioned embodiments where all the photovoltaic power producing elements' power flowed through the ratiometric DC to DC converters (e.g., in this embodiment the amount of power flowing through each of the ratiometric DC to DC converters is only from about 2 percent to about 20 percent that of the power flowing through the ratiometric DC to DC converters in the previously described examples). Thus, the circuit of
As shown in
As shown in
In the circuit configuration shown in
In this exemplary embodiment shown in
Based on the exemplary embodiments described above,
In step 602, at least one ratiometric DC to DC converter is connected to both the array of photovoltaic power producing elements and the power receiving unit. The ratiometric DC to DC converter(s) is/are configured to alter (increase/decrease) a voltage output from the array of photovoltaic power producing elements. See description above. As shown in
By way of example only, step 602 can be performed by connecting the array of photovoltaic power producing elements to an input of the at least one ratiometric DC to DC converter (step 602a); and connecting an output of the at least one ratiometric DC to DC converter to the power receiving unit (step 602b). This is in accordance, for example, with the exemplary system configuration shown in
Alternatively, step 602 can be performed by connecting the array of photovoltaic power producing elements to both the at least one ratiometric DC to DC converter and the power receiving unit, such that the array of photovoltaic power producing elements is connected to an input of the at least one ratiometric DC to DC converter, and an output of the at least one ratiometric DC to DC converter is connected to the power receiving unit in series with the array of photovoltaic power producing elements (step 602c). This is in accordance, for example, with the exemplary system configuration shown in
Alternatively, as described above, multiple ratiometric DC to DC converters may be employed. In this case, step 602 can be performed by connecting each of the photovoltaic power producing elements in the array to one of the ratiometric DC to DC converters in a one to one relationship (step 602d); and connecting outputs of the ratiometric DC to DC converters in parallel to the power receiving unit (step 602e). This is in accordance, for example, with the exemplary system configurations shown in
Alternatively, as described above, multiple ratiometric DC to DC converters may be employed. In this case, step 602 can be performed by connecting each of the photovoltaic power producing elements in the array to one of the ratiometric DC to DC converters in a one to one relationship (step 602f); and connecting outputs of the ratiometric DC to DC converters in parallel or otherwise constraining the outputs to a common voltage and/or current (step 602g). This is in accordance, for example, with the exemplary system configurations shown in
As provided above, the maximum power point tracker (MPPT) (as used herein) adjusts its input impedance to allow series/parallel connected photovoltaic cells to operate at the voltage and current that produces the maximum power transfer. This can be better understood by way of reference to
With the present techniques a very significant improvement in system efficiency can be realized. By way of example only, employing any one of the above-described configurations can result in at least a 10% improvement in system efficiency with approximately 1% loss due to the DC to DC converter power consumption. The resulting net benefit is thus 9%, which represents a significant savings.
Although illustrative embodiments of the present invention have been described herein, it is to be understood that the invention is not limited to those precise embodiments, and that various other changes and modifications may be made by one skilled in the art without departing from the scope of the invention.
