Patent Application
20120301979
The present invention relates to preconditioning photovoltaic modules (or photovoltaic panel) for performance testing.
Photovoltaic (PV) module (or panel) manufacturers, distributors, and end-users all have an interest in being able to determine the performance of their PV modules. Unfortunately, transient or metastable current-voltage (I-V) characteristics inhibit the accurate determination of PV module performance and reliability. The transient characteristics of polycrystalline PV modules, such as cadmium telluride (CdTe) or copper indium gallium diselenide (CIGS) photovoltaics, can cause errors in energy ratings and service-lifetime predictions. While there is disagreement about what causes the metastable behavior of CdTe and CIGS devices, the fact that transient I-V phenomena inhibit the accurate determination of PV module performance is well-known.
In order to minimize or eliminate the errors caused with these transient characteristics, it is recommended that a PV module be preconditioned for 8 hours under natural sunlight. Given the impracticality of such lengthy preconditioning in an industry setting, however, some have proposed reducing the preconditioning time to around two hours under natural sunlight or using an artificial light-soak. While the second option offers a significant improvement in preconditioning time, two hours is still a significant time period for manufacturers, distributors, or consumers to wait before being able to determine PV module performance.
In order to help improve the effectiveness of preconditioning (i.e., more closely approximate maximum PV module performance) and to reduce the time period required for preconditioning, methods such as baking (placing the PV module in an environment at elevated temperatures, such as a thermal anneal oven), artificial light-soak (exposing the PV module to lamps with bulbs designed to approximate real sunlight) and combinations thereof have been proposed. These methods, however, still suffer from significant preconditioning time and also create significant costs. Most distributors and consumers are unlikely to have the expensive equipment, such as thermal anneal ovens, necessary to use baking techniques, and are equally unlikely to own the often costly lamps that are used to simulate real sunlight.
In a recent conference paper, Cueto, et al. proposed another method of preconditioning using a “forward-biased dark exposure at elevated temperature.” (Cueto, et al., “Striving for a Standard Protocol for Preconditioning or Stabilization of Polycrystalline Thin Film Photovoltaic Modules,” Conference Paper, NREL/CP-520-44935, July 2009 (hereinafter “Cueto, et al.”).) In this paper, Cueto, et al. disclose preconditioning using “forward-bias (voltage controlled) exposure under dark conditions (dark-soak) at 60° C., with bias between VOC and the optimum power point voltage (VMAX).” (Cueto, et al. at 3-4.) Cueto, et al.'s disclosure is limited in that it relies on an elevated temperature environment and describes a limited voltage controlled forward-bias range. Once again, the equipment required for the preconditioning in Cueto, et al. is expensive and often beyond the access of many distributors and consumers. It is often not practical for a manufacturer to place panels in an elevated temperature environment in order to precondition before performance testing.
Although present methods and devices are functional, they are not sufficiently accurate or otherwise satisfactory. Accordingly, a system and method are needed to address the shortfalls of present technology and to provide other new and innovative features.
Exemplary embodiments of the present invention that are shown in the drawings are summarized below. These and other embodiments are more fully described in the Detailed Description section. It is to be understood, however, that there is no intention to limit the invention to the forms described in this Summary of the Invention or in the Detailed Description. One skilled in the art can recognize that there are numerous modifications, equivalents and alternative constructions that fall within the spirit and scope of the invention as expressed in the claims.
The present invention can provide a system and method for preconditioning a photovoltaic device. In one exemplary embodiment, the present invention can include a method for preconditioning a photovoltaic device, the method comprising applying a forward-bias to the photovoltaic device, wherein a forward-bias current is equal to or greater than IMP(FB) for the photovoltaic device. Applying the forward-bias may comprise applying a current controlled forward-bias, applying a constant forward-bias current, applying a variable forward-bias current, applying a voltage controlled forward-bias, or applying a variable forward-bias voltage. The effective forward-bias current may be sustained at a value equal to or greater than IMP(FB) for the photovoltaic device, or at a value greater than ISC(FB). The forward-bias may be applied for a conditioning time, wherein the conditioning time is based on a magnitude of the forward-bias current.
In another embodiment, the present invention may comprise a method for preconditioning a photovoltaic device, the method comprising applying a forward-bias current to the photovoltaic device, wherein a forward-bias current is substantially equal to or greater than IMP(FB) for the photovoltaic device. In yet another embodiment, applying a forward-bias for preconditioning may comprise applying a first forward-bias for a first time period; and applying a second forward-bias for a second time period. The magnitude of the first forward-bias may be the same or different than the second forward-bias, and the length of the first time period and the second time period may be the same or different. In one embodiment, the magnitude of the first forward-bias is the same as the magnitude of the second forward-bias, and the length of the first time period and the second time period are different. In one embodiment, the magnitude of the first forward-bias is different than the magnitude of the second forward-bias, and the length of the first time period and the second time period are the same. In one embodiment, the magnitude of the first forward-bias is different than the magnitude of the second forward-bias, and the length of the first time period and the second time period are different.
As previously stated, the above-described embodiments and implementations are for illustration purposes only. Numerous other embodiments, implementations, and details of the invention are easily recognized by those of skill in the art from the following descriptions and claims.
Various objects and advantages and a more complete understanding of the present invention are apparent and more readily appreciated by reference to the following Detailed Description and to the appended claims when taken in conjunction with the accompanying Drawings wherein:
Referring now to the drawings, where like or similar elements are designated with identical reference numerals throughout the several views, and referring in particular to
In general, an in-sun I-V curve shows the possible combinations of current and voltage output for a PV module at standard conditions. At one end of the in-sun I-V curve, the maximum current output of the module, the short circuit current (ISC), occurs when the resistance drops to zero. At the other end, the maximum voltage of the device, the open circuit voltage (VOC), occurs when the circuit is broken or when the resistance grows infinitely large. The maximum power point (PMAX) occurs somewhere in-between these two points, on the knee of the I-V curve. The voltage and current at this maximum power point are labeled VMP and IMP, respectively.
As is understood by those of skill in the art, an in-sun I-V curve is determined using standard conditions of sunlight (i.e., one sun or 1000 W/m2) and standard device temperature. It is noted that when these conditions change, the in-sun I-V curve may also change.
For the forward-bias condition, the I-V curve is representative of the PV module acting as a power consumer, rather than a power producer. Although the shape of the forward-bias I-V curve is similar to the in-sun I-V curve, the shapes of the curves are not the same. This is due to multiple factors. For example, the resistance characteristics of the PV module can vary based on being in an illuminated state (e.g., an in-sun condition) or a dark state (e.g., a forward-bias condition). Moreover, for the forward-bias condition there is no current flow until the barrier potential of the forward diode is overcome. Just as the in-sun IV curve will vary for different PV modules, and different types of PV modules, the forward-bias I-V curve for different PV modules, and different types of PV modules, will also vary. It is further notable that the forward-bias I-V curve is not a fixed curve. Just as the in-sun I-V curve is affected by changes in sunlight and temperature, a forward-bias I-V curve can also fluctuate due to certain operational factors such as temperature and loading conditions. Accordingly,
The present invention provides a method for preconditioning PV modules using an applied forward-bias, wherein the forward-bias current is substantially equal to, or greater than, IMP(FB). By applying a forward-bias that is substantially equal to IMP(FB) the preconditioning time can be reduced to less than ten (10) minutes without the need for light-soak or an elevated temperature environment. Moreover, as the forward-bias is increased, the preconditioning time can be further reduced. For example, in one embodiment, a forward-bias that is substantially equal to ISC(FB) may be used in order to reduce the preconditioning time to less than one (1) minute. In yet another embodiment, the magnitude of the forward-bias current may be a multiple of the magnitude of IMP for the PV module (e.g., a 1.5·IMP(FB), a 2·IMP(FB), 3·IMP(FB), 4·IMP(FB), etc.) in order to reduce preconditioning time to less than forty-five (45) seconds, or even less than thirty (30) seconds.
Referring again to
It should be understood that if a fixed forward-bias voltage is used, the effective forward-bias current of the PV module may vary over time. As discussed above, the forward-bias I-V curve for the device can fluctuate due to certain operational factors such as temperature, loading conditions, and the incoming state of the module (e.g, from dark storage versus a partially lighted environment). As the temperature of the PV module increases due to the applied forward-bias, the shape of the forward-bias I-V curve can change. Thus, for the same forward-bias voltage, the effective forward-bias current will increase over time as the forward-bias is sustained.
For purposes of the present invention, a voltage controlled forward-bias or a current controlled forward-bias method may be used. Here, the terms voltage controlled and current controlled reflect the fact that during forward-biasing, the forward-bias I-V curve may change. Thus, while the forward-bias voltage is controlled (this includes holding the voltage at a constant value or varying the voltage in a controlled manner) the effective forward-bias current is not.
Alternatively, the forward-bias current may be controlled while the effective applied voltage is not controlled. Once again, changes in operational characteristics of the PV module can affect the forward-bias I-V curve resulting in changes to the relationship between current and voltage.
In one embodiment, a current controlled forward-bias is used, wherein the forward-bias current is sustained at a value substantially equal to, or greater than, IMP(FB). In another embodiment, a current controlled forward-bias is used, wherein the forward-bias current is sustained at a value equal to, or greater than, ISC(FB). In yet another embodiment, a voltage controlled forward-bias is used, wherein the effective forward-bias current is sustained at a value substantially equal to, or greater than, IMP(FB). In accordance with these and other embodiments a forward-bias of sufficient magnitude can be used to significantly reduce preconditioning and eliminate the need for light-soak or an elevated temperature environment. Various embodiments will be readily apparent to those of skill in the art based on the present description.
Referring now to
Due to variances between different types of PV modules (CdTe, CIGS, amorphous silicon (a-Si), etc.), between different PV module manufacturers and even between individual modules, a relationship between forward-bias current and conditioning time for all PV modules cannot be defined. However, a person of skill in the art can easily determine the conditioning time for a manufacturer's PV modules. This can be achieved by comparing (a) the optimal performance test results for a PV module using previously adopted preconditioning, with (b) the performance test results received using various conditioning times consistent with the present invention.
Although a precise relationship between forward-bias current and conditioning time cannot be established for all embodiments, initial testing provides the following guideposts for the conditioning time required based on the magnitude of the forward-bias current (measured as compared to a multiple of IMP(FB)):
Referring again to
In
In one embodiment, the controller 2300 may contain computer-readable instructions for preconditioning PV panels using a forward-bias. For example, the flow-chart in
Referring now to
If, however, the controller 2300 is unable to acquire preset precondition values the controller 2300 may still determine precondition values using the operational parameters of the panel. For example, the controller 2300 may request and receive operational parameters for the panel (e.g., IMP, VOC, ISC, maximum recommended voltage, maximum recommended current, etc.). These operational parameters may then be used to determine the proper forward-bias and conditioning time for the panel. For example, if a user enters an IMP for the panel, the controller 2300 could select to apply a forward-bias of IMP(FB) for a predetermined conditioning time of 4-6 minutes. In this way, the device 2300 can be programmed to operate within the operational parameters of the panel. The conditioning time may be set based on a length of time generally known to be sufficient for the selected forward-bias. Referring back, by way of example, to Table 1, a conditioning time of 150 seconds could be selected when the forward-bias is 3·IMP(FB) (a value 25% greater than the high end of the conditioning time range for 3·IMP(FB)).
In another embodiment, there may be an option for the user to input a forward-bias and conditioning time for the panel. This may be a separate option, or it may be incorporated with the flow chart in
It is further notable that the device 2000 in
Referring now to
Referring to
In conclusion, the present invention provides, among other things, a system and method for preconditioning photovoltaic devices. Those skilled in the art can readily recognize that numerous variations and substitutions may be made in the invention, its use and its configuration to achieve substantially the same results as achieved by the embodiments described herein. Accordingly, there is no intention to limit the invention to the disclosed exemplary forms. Many variations, modifications and alternative constructions fall within the scope and spirit of the disclosed invention as expressed in the claims.
