WO2010015858A2

WO2010015858A2 - Testing photovoltaic transducers

Info

Publication number: WO2010015858A2
Application number: PCT/GB2009/050982
Authority: WO
Inventors: Martin James Bellamy
Original assignee: G24 Innovations Limited
Priority date: 2008-08-06
Filing date: 2009-08-05
Publication date: 2010-02-11
Also published as: GB0814406D0; WO2010015858A3

Abstract

A method is provided for predicting a response characteristic of photovoltaic transducer which is to be cut from an elongate web of flexible dye-sensitised photovoltaic material. The preferred method comprises irradiating (15) a portion of the surface of the photovoltaic material with a regular array of LEDs and sensing (16) the resulting electrical response of the photovoltaic material. It is thereby possible to predict (17) the response characteristic of the resulting photovoltaic transducer module which is to be cut from the web. Furthermore, the electrical response is used to control (24) a manufacturing process of the photovoltaic transducer by way of a feedback loop. The measured electrical response characteristic can be printed (22) in encoded form on to a side edge of the flexible web.

Description

TESTING PHOTOVOLTAIC TRANSDUCERS

The present invention relates to methods of testing photovoltaic transducers, and in particular to methods of testing the response of such transducers to incident light.

The term "light" as used throughout this specification is intended to refer not only to visible radiation but also to infrared and ultraviolet radiation.

A photovoltaic transducer responds to incident light by generating a voltage across its terminals which can give rise to an electric current in an external circuit. In this way, the power of the incident light is converted into useful electric power. The magnitude of the resulting voltage and/or current depend on a number of factors, including the wavelength and intensity of the incident light and the chemical and physical characteristics of the photovoltaic transducer.

Photovoltaic transducers are normally manufactured as an array of photovoltaic cells which are electrically connected together in series, such that the output voltage is the sum of the individual voltages generated across each of the cells. However, since the magnitude of the generated current depends on the area of the light-sensitive surface of the photovoltaic transducer, the photovoltaic cells could alternatively be electrically connected together in parallel so as to increase the effective surface area exposed to the incident light, the effect of which will be that, whereas the voltage remains the same as with an individual cell, the magnitude of the current generated in an external circuit is greater than that generated by a single cell. It is now possible to manufacture photovoltaic cells which are connected together in series and in which the length of the cells in the array, and hence the area of light-sensitive surface, is selected such that a desired level of current is generated.

In practice, such cells can be manufactured in the form of a flexible sheet or web, and the desired length is simply cut from the web at the end of the manufacturing process. The types of photovoltaic transducer which can be manufactured as a web include dye- sensitised, amorphous silicon, cadmium telluride, copper indium diselenide (CIS), copper indium gallium selenide (CIGS) and spheral crystalline silicon and organic. After the desired length is cut, the resulting module can then be encapsulated in a cover to protect the module from ingress of water and oxygen which could cause the performance of the module to deteriorate. The encapsulated module may then be subjected to testing in order to characterise its response to light.

Conventional methods of testing involve applying to the light-sensitive surface of the encapsulated photovoltaic module with light having a spectral content and intensity such as to emulate natural sunlight, and measuring the resulting electrical response, typically in terms of both the resulting voltage across the terminals of the transducer and the magnitude of the current generated in an external circuit connected to the transducer.

A disadvantage of such an arrangement is that, in the event that the measured electrical response is below an acceptable level, the module must necessarily be discarded, with the consequential wastage of valuable component materials, and the waste of the energy which was required to manufacture the module.

It would therefore be desirable to provide a method which reduces the above wastage.

Thus, in accordance with the present invention there is provided a method of predicting a response characteristic of a photovoltaic transducer which is to be cut from an elongate web of photovoltaic material, the method comprising: providing the elongate web of photovoltaic material; irradiating a portion of the photovoltaic material with light; sensing the electrical response of the photovoltaic material to the irradiation; and in response thereto, predicting the response characteristic of the photovoltaic transducer which is to be cut therefrom.

The electrical response can be the output voltage and/or the output electric current, and the predicted response characteristic can be the peak output light power of the transducer, measured in watts.

By measuring the response characteristic of the photovoltaic transducer before cutting off the desired length for a given application, it is possible to reduce wastage in a number of ways. For example, should the response be slightly below (or above) what is required for a given application, it would be possible to select a correspondingly greater (or lesser) length of the photovoltaic transducer to be cut. Thus, in one preferred embodiment, the size of the photovoltaic transducer to be cut from the elongate web is determined in dependence on the sensed electrical response.

Furthermore, it is not necessary for the final photovoltaic transducer to be cut from the web at the time or place of manufacture of the web. Thus, a customer, such as a retailer or manufacturer of a product which includes a cut length of the photovoltaic transducer stock, could purchase the entire web and cut the desired lengths at a different location and indeed at some time in the future. In this case, the customer would need to know the electrical response of the stock.

For this reason, the method preferably further comprises applying an indicium to a surface of the photovoltaic material which indicates the sensed electrical response.

This provides a convenient way of indicating the electrical response of the stock without the need for the indicating to be provided separately, e.g. on an information sheet.

Since the electrical response may well vary to some extent along the length of the web, it would be desirable to indicate the "local" electrical response at the corresponding positions along the entire length of the elongate web.

Alternatively, a log could be created of the various measured responses at the different positions along the length, in which case the method preferably further comprises determining the position of the portion of the photovoltaic material within the elongate web at which the electrical response has been sensed, in conjunction with the actual value of the electrical response.

Thus, the method preferably further comprises sequentially irradiating a plurality of portions of the photovoltaic material and sensing the electrical response to the irradiation of each portion, the method further comprising determining the position of each of the portions within the elongate web, thereby to generate a profile of the varying electrical response with position along the elongate web.

Since the electrical response is dependent to some extent on the manufacturing parameters of the photovoltaic transducer, the measured response could itself be used to control the manufacturing process by way of a feedback loop, such that the response is maintained within desirable limits. Thus, the method of the present invention preferably further comprises adjusting a process in the manufacture of the elongate web of photovoltaic material in dependence on the sensed electrical response.

For some manufacturing processes, the electrical response characteristics may be substantially constant throughout the length of the web of photovoltaic transducer material, in which case a single measurement of the electrical response will be sufficient to characterise the entire web. However, it is expected that the upstream and downstream ends of the web will still exhibit at least a slight reduction in electrical response resulting from various production factors, and it is therefore desirable to be able to characterise the electrical response of each end of a length of web as well as that of the bulk of the web.

The method of the present invention permits an "in-line" measurement of the electrical response of a newly manufactured web of photovoltaic transducer stock, and this measurement can form a part of the overall manufacturing process, such that there is no additional time required for the measurement itself. A suitable measuring station can be placed at the end of the manufacturing assembly.

Thus, the elongate web of photovoltaic material is preferably irradiated and the electrical response sensed at a testing station as the photovoltaic material moves past.

The irradiation used for testing the photovoltaic material is preferably pulsed but may alternatively be continuous. The advantage of pulsing the light is not only a saving in energy, but a reduction in the likelihood of damaging the photovoltaic material by excessive heating which can result from subjecting the light-sensitive surface to a continuous beam of high-energy light radiation.

In order to provide a uniform illumination across the entire width of the photovoltaic material, the light source is preferably in the form of an array of light-emitting elements. Light-emitting diodes (LEDs) are preferred, since the spectral content and intensity can be controlled either by controlling the electrical power supplied to the LEDs or simply by selecting the type of LEDs accordingly. LEDs provide the following substantial advantages over other artificial sources of light:

(a) LEDs are available which generate light at a large number of different wavelengths without the need for optical filters which would necessarily absorb some of the light; (b) the intensity of the output light of LEDs can be controlled by adjusting the applied voltage without significantly changing the wavelength of the light - this contrasts with incandescent lamps which change colour in accordance with the well-known Wien's displacement law of black bodies;

(c) the higher efficiency of LEDs, i.e. the light output power divided by the electrical input power, when compared with other types of light source;

(d) the quantity of heat generated by LEDs is substantially less than that generated by incandescent light sources, and it is much easier to dissipate the heat away from LEDs, which is necessary in order to prevent any significant variation in the response characteristics of the photovoltaic transducer and also to prevent permanent damage to the transducer;

(e) the quick response of LEDs (typically of the order of microseconds), as compared with that of other light sources;

(f) LEDs are structured such that the output light can be focussed without the need for separate lenses and/or reflectors; (g) LEDs have greater longevity (typically tens of thousands of hours), as compared with other types of light source, and the longevity is not adversely affected when the light is pulsed - this contrasts with incandescent lamps and fluorescent lamps which have a significantly reduced lifetime (typically thousands of hours) when used as a source of pulsed light, and with high-intensity discharge lamps which require a long "refresh" time between pulses;

(h) LEDs retain their calibration significantly longer than most, if not all, other known light sources;

(i) LEDs are significantly more robust and less fragile than either fluorescent or incandescent lamps; (j) LEDs can be fabricated in extremely small sizes - and this is of particular benefit in applications such as the preferred embodiments of the present invention, in which arrays of a large number of LEDs are required to be arranged within a relatively small area; a further advantage of this feature is that the size of the LED array can be selected with high precision; and (k) a light source comprising an array of LEDs can be made arbitrarily large without loss of uniformity of the emitted light.

The spectral content and/or intensity of the irradiation preferably emulates that of sunlight, since for many applications the resulting photovoltaic transducer will be subject to natural outdoor light. However, for applications where the photovoltaic transducer will be located in an environment which is lit artificially, the spectral content and/or intensity is selected such as to emulate the expected artificial light source. For example, the spectral content and intensity of a first source of artificial light, such as a strip fluorescent light source, could be emulated using a second source of artificial light, such as an array of LEDs. Photovoltaic cell arrays of the type which are based on photosensitive dyes are of particular use in artificially lit environments, since the electrical response to such artificial light is significantly greater than that exhibited with earlier types of photovoltaic transducers, such as those manufactured from amorphous silicon.

Such dye-sensitised photovoltaic arrays are now available in the form of flexible sheets, in which case the photovoltaic material can be supplied to a customer as a roll.

The photovoltaic material is preferably in the form of an array of photovoltaic cells connected together in series, so that the output voltage of the array is equal to the sum of the output voltages of the individual cells. Furthermore, the number of cells can be selected so as to "tune" the resulting photovoltaic transducer to a desired output voltage.

The method preferably further comprises the step of cutting the said photovoltaic transducer from the elongate web, although as mentioned above, the step of cutting could alternatively be carried out by the customer.

The present invention extends to apparatus for predicting a response characteristic of photovoltaic transducer which is to be cut from an elongate web of photovoltaic material, the apparatus comprising: means for irradiating a portion of the photovoltaic material; means for sensing the electrical response of the photovoltaic material to the irradiation; and means, responsive to the sensing means, for predicting the response characteristic of the photovoltaic transducer which is to be cut therefrom. Preferred embodiments of the present invention will now be described with reference to the accompanying drawings, in which:

Figure 1 illustrates apparatus for use in a preferred method of the present invention;

Figure 2 illustrates a light source and associated power supply and control system for use with the method of Figure 1 ; and

Figure 3 is a flowchart illustrating the method steps of the preferred embodiment of the present invention.

Referring to Figure 1 , a flexible web of photovoltaic transducer material 1 , in the form of an array of dye-sensitised photovoltaic cells, is transported toward a testing station 2 at a speed of approximately 10 metres per minute (0.1 m s^"1) in the direction indicated by the arrow 3, where part of the surface of the web, indicated by the dashed outline 1 ', is irradiated with radiation from a light source 4 which comprises an array of 480 high- power LEDs arranged in a regular 16 x 30 matrix. Electrical power is supplied to the light source 4 from a power source 5 via a control module 6 which controls the power supplied to the LEDs. This light source is described in greater detail below.

In response to the light emitted by the light source 4, the photovoltaic transducer material 1 generates an output voltage across its conductive terminals (not shown) which are located along the sides of the transducer material 1. A corresponding pair of electrical sliding contacts 7 are maintained in contact with the conductive terminals by suitable biasing means (not shown), and an external circuit is formed by the brush contacts within a microprocessor 8. The microprocessor 8 is arranged to sense the electrical response of the photovoltaic transducer material in terms of both the generated voltage and the resulting current in response to the light from light source 4, and to calculate from the response one or more desired characteristics of the photovoltaic material. The calculated characteristics are then displayed on a monitor 9 and/or printed on a printer (not shown).

After passing the testing station 2, the flexible photovoltaic material 1 is supplied to a wind-up reel 10, where it is stored for subsequent processing or supply to a consumer. Since the response characteristics of the photovoltaic material may vary along the length of the web, it may be desirable either to sense the position along the web at which a particular response characteristic has been measured and to log the response characteristic in conjunction with the sensed position. Alternatively, the response characteristic could be marked on to the photovoltaic material in encoded form as an indicium at the longitudinal position at which the characteristic was measured, e.g. at one side thereof.

The subsequent processing of the photovoltaic transducer web material typically involves cutting one or more desired lengths to form photovoltaic modules for use in specific applications. However, the length of web required for a given transducer module will depend on the response characteristics of the transducer material.

The determined response characteristic is therefore used to determine the length of the photovoltaic material which will be required for a given application, and it can be seen that such an arrangement provides a major technical advantage over conventional testing arrangements in which a photovoltaic module is tested for its electrical response characteristics only after having been manufactured in its final form.

The light source 4 is illustrated in greater detail in Figure 2. It is constructed from two different types of high-power LED. The test source comprises a total of 576 LEDs arranged in an 18 x 32 array, of which a first group of 96 LEDs, constituting one sixth of the total number of LEDs, are of a first type 1 1 indicated by an "X" in Figure 2, and of which a second group of 480 LEDs, constituting five sixths of the total number of LEDs, are of a second type 12 indicated by an "0". The array is arranged to illuminate an area extending over approximately 170 mm by 300 mm. Each LED is arranged to emit light in the form of a cone with overlaps the cones of light emitted by the adjacent LEDs within the array. This arrangement enhances the uniformity of the resulting light which is incident on the photovoltaic transducer.

As can be seen from the arrangement of the individual LEDs in Figure 2, the LEDs of each group are arranged at approximately regular intervals within the array such that the illumination from each group is substantially uniform over the total illuminated area. A control system 13 supplies electrical power from a power source 14 independently to each group of LEDs within the test light source, such that the intensity of the light emitted by the LEDs of each group can be set independently at one or more desired levels. It will be appreciated that both the spectral content and the overall intensity of the light can be controlled in this way. In a preferred embodiment, a constant level of power is supplied to the larger group of 480 LEDs, while the power level supplied to the smaller group of 96 LEDs is varied so as to provide a fine tuning of the spectral content and overall intensity. In this way, the majority of the spectral content and overall intensity of the light emitted by the array is defined by the characteristics of the 480 LEDs of the larger group.

The light emitted by the array can be constant or pulsed, depending on the nature of the electrical response characteristics to be determined.

The method steps according to the preferred embodiment of the present invention are illustrated in the flowchart of Figure 3.

A length of flexible photovoltaic transducer web material is irradiated at the testing station (step 15), and the resulting electrical response of the transducer material is sensed (step 16). From the sensed electrical response, a desired electrical response characteristic is determined (step 17). This could be, for example, the open-circuit voltage, the closed- circuit current or the efficiency of the photovoltaic material. At this stage, the method diverges (step 18) according to whether or not it is desired to cut a suitable length of the photovoltaic material from the elongate web at this stage. If so, then the length required is calculated in accordance with the determined electrical response characteristic (step 19), and this length is cut (step 20) and subsequently encapsulated (step 21 ) in a protective covering to prevent ingress of moisture and/or oxygen in use. If it is not required to cut a length of the photovoltaic material from the web at this stage, then the determined electrical response characteristic is printed in encoded form on one side edge of the photovoltaic material (step 22) and the web rolled up into a form suitable for supply to a customer (step 23). Since the electrical response characteristics may vary along the length of the web of photovoltaic material, it is desirable to print the encoded characteristic at substantially the same longitudinal position at which the response measurement took place. Alternatively, or in addition, the web can be marked with longitudinal position-indicating indicia during the manufacturing process, and the longitudinal position of the web of photovoltaic material can be determined by sensing these indicia and the determined electrical response characteristics logged separately in conjunction with the sensed longitudinal position data. The roll of the photovoltaic material would then be supplied to the customer together with the log of a roll of the photovoltaic material.

Alternatively, or in addition, if the determined electrical response characteristic of the photovoltaic material is found to be below standard, the manufacturing process can be adjusted to correct for this (step 24) using, in effect, a feedback control loop.

It will be appreciated that the preferred embodiments described above can be modified without departing from the scope of the present invention, which is defined solely by the accompanying claims. For example, the specific configuration of the light source and the speed of movement of the web are intended to be merely exemplary, and alternative configurations will be apparent to those skilled in the art.

Claims

CLAIMS:

1. A method of predicting a response characteristic of a photovoltaic transducer which is to be cut from an elongate web of photovoltaic material, the method comprising: providing the elongate web of photovoltaic material; irradiating a portion of the photovoltaic material; sensing the electrical response of the photovoltaic material to the irradiation; and in response thereto, predicting the response characteristic of the photovoltaic transducer which is to be cut therefrom.

2. A method as claimed in claim 1 , further comprising determining the size of the photovoltaic transducer to be cut from the elongate web in dependence on the sensed electrical response.

3. A method as claimed in claim 1 or claim 2, further comprising applying an indicium to a surface of the photovoltaic material which indicates the sensed electrical response.

4. A method as claimed in any one of claims 1 to 3, further comprising determining the position of the portion of the photovoltaic material within the elongate web.

5. A method as claimed in any one of claims 1 to 4, comprising sequentially irradiating a plurality of portions of the photovoltaic material and sensing the electrical response to the irradiation of each portion, the method further comprising determining the position of each of the portions within the elongate web, thereby to generate a profile of the varying electrical response with position along the elongate web.

6. A method as claimed in any one of claims 1 to 4, further comprising adjusting a process in the manufacture of the elongate web of photovoltaic material in dependence on the sensed electrical response.

7. A method as claimed in any one of claims 1 to 6, wherein the elongate web of photovoltaic material is irradiated and the electrical response is sensed at a testing station as the photovoltaic material moves therepast.

8. A method as clamed in any one of claims 1 to 7, wherein the irradiation is pulsed.

9. A method as claimed in any one of claims 1 to 7, wherein the irradiation is substantially continuous.

10. A method as claimed in any one of claims 1 to 9, wherein the photovoltaic material is irradiated using an array of light-emitting elements.

1 1. A method as claimed in claim 10, wherein the light-emitting elements are light- emitting diodes.

12. A method as claimed in any one of claims 1 to 11 , wherein the irradiation has a spectral content which emulates that of sunlight.

13. A method as claimed in any one of claims 1 to 11 , wherein the step of irradiating a portion of the photovoltaic material comprises irradiating the portion with a first type of light source which is selected to emulates a second, different type of artificial light source.

14. A method as claimed in any one of claims 1 to 13, wherein the photovoltaic material is flexible and the elongate web is supplied on a roll.

15. A method as claimed in any one of claims 1 to 14, wherein the photovoltaic material comprises an array of electrically connected photovoltaic cells.

16. A method as claimed in any one of claims 1 to 15, further comprising the step of cutting the said photovoltaic transducer from the elongate web.

17. Apparatus for predicting a response characteristic of photovoltaic transducer which is to be cut from an elongate web of photovoltaic material, the apparatus comprising: means for irradiating a portion of the photovoltaic material; means for sensing the electrical response of the photovoltaic material to the irradiation; and means, responsive to the sensing means, for predicting the response characteristic of the photovoltaic transducer which is to be cut therefrom.