Method of reclaiming cadmium and tellurium from CdTe for CdTe photovoltaic modules

Abstract

A method of reclaiming cadmium material from photovoltaic (PV) modules is provided. The method includes submerging one or more portions of a PV module in a solution including non-distilled water, wherein the one or more portions of the PV module are submerged until cadmium material present on the PV module dissolves into the solution, boiling the solution until the dissolved cadmium material precipitates, and collecting the precipitated cadmium material.

Claims

1. A method of reclaiming cadmium material from photovoltaic (PV) modules, comprising: submerging one or more portions of a PV module in a citric acid-based buffer solution, wherein the one or more portions of the PV module are submerged until cadmium material present on the PV module dissolves into the citric acid-based buffer solution; boiling the citric acid-based buffer solution until the dissolved cadmium material precipitates; and collecting the precipitated cadmium material.

2. The method as recited in claim 1, further comprising determining if a glass portion of the PV module has broken into a plurality of pieces.

3. The method as recited in claim 2, wherein, if the glass portion of the PV module has broken into the plurality of pieces, the one or more portions of the PV module submerged in the citric acid-based buffer solution includes the plurality of pieces.

4. The method as recited in claim 2, further comprising: if the glass portion of the PV module has not broken into the plurality of pieces: opening an edge of the PV module; and submerging the edge in the solution.

5. The method as recited in claim 1, wherein the cadmium material includes a cadmium compound.

6. The method as recited in claim 5, wherein the cadmium compound includes cadmium telluride.

7. The method as recited in claim 1, wherein the one or more portions of the PV module are submerged for a period of two or more hours.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 shows a graphical representation of Cd concentration over time.

(2) FIG. 2 shows a section of a PV module into which slots are cut.

(3) FIG. 3 shows a graphical representation of leaching of Cd from CdTe solar cells in a solution at room temperature.

(4) FIG. 4 shows a graphical representation of leaching of Cd from CdTe solar cells in rain water at room temperature.

(5) FIG. 5 shows a graphical representation of leaching of Cd from CdTe solar cells in a solution at room temperature.

(6) FIG. 6 shows a flow chart of a method for removing cadmium/cadmium compounds from PV modules, according to an embodiment of the present invention.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

(7) The preferred embodiments of the present invention will now be described with reference to the drawings. Identical elements in the various figures are identified with the same reference numerals.

(8) Reference will now be made in detail to each embodiment of the present invention. Such embodiments are provided by way of explanation of the present invention, which is not intended to be limited thereto. In fact, those of ordinary skill in the art may appreciate upon reading the present specification and viewing the present drawings that various modifications and variations can be made thereto.

(9) The possibility that modules will end up in a landfill, broken, and exposed to groundwater has been examined. Some past studies have indicated that dangerous leaching of CD ions from the panels would be minimal, but these studies used only pure water (distilled water) as the leaching solution. In the real world, one is likely to encounter acidic leaching agents, such as, e.g., groundwater or acid rain. The leaching protocol adopted during experimentation addressed the basic problem of how to take into account the conditions of the broken panels that would be anticipated in a landfill. The idea was to break the panels as realistically as possible to prepare the samples. Breaking them by impact would closely simulate the real world, but the breakage patterns would be inherently random and therefore not precisely reproducible from piece to piece nor in successive experiments. On the other hand, slots cut through the front glass with a diamond saw are reproducible, but not entirely realistic.

(10) An investigation included two independent studies, both using approximately 2-inch (50 mm) squares taken from a First Solar CdTe module. One set of samples was broken by impact while the other set was slotted with a saw. All samples were kept at normal room temperatures (20 to 25 degrees Celsius) throughout the leaching process. The first study examined five pieces broken by impact. Each square was struck a single blow on the front surface with a 5-ounce cross-peen hammer to induce the kind of breakage that would likely occur when modules are thrown into a landfill. Each sample was then submerged in about 150 ml of leaching solution in a covered 8-ounce Mason jar. One jar contained pure (DI) water. The other four samples were submerged in citric acid-based buffer solutions with pH values of 3, 4, 5, and 6, respectively. These were intended to span the range of acidity one might encounter in actual groundwater as rain seeps into the pile of discarded modules. Initial ICP spectrometry analyses (blanks) of the leaching solutions showed no cadmium with a MDL of about 3 ppb. After 32, 120, and 206 days of submersion, each solution was analyzed for cadmium by ICP emission spectrometry. The pH of each buffer solution at the end of 206 days was also tested and it was found to still be within 0.1 pH unit of its nominal value.

(11) The pure water, lacking any buffer, had acquired a final pH value of 6.4. The sample in pure water appeared to suffer no additional destruction beyond that caused by the hammer blow, but those in the more acidic solutions underwent increasing degrees of delamination with decreasing pH values. In fact, the samples in pH 3, and 4 solutions were completely delaminated with all of the front glass fragments having settled to the bottom of the jar, and all the active layers were removed. Only the laminating polymer remained attached to the back glass. The samples in pH 5 and 6 solutions had at least 75% of their front glass area delaminated and the underlying active material gone. The analysis results are shown in FIG. 1, which plots cadmium concentration of the solutions versus time for each pH value. Concentrations are in parts per million (mg/l). For reference, the maximum level of Cd permitted by the EPA in drinking water is 5 parts per billion (5 micrograms/liter). It is not clear why for pH 4 the final (206 days) reading is less than the previous one. One possibility is that the cadmium may not have distributed itself uniformly throughout the solution. Another possibility is that there may also have been some adsorption on the EVA plastic after the sample delaminated.

(12) The leaching solutions, especially the more acidic ones, show cadmium levels 4 orders of magnitude greater than the EPA limit. Thus, even if this experiment does not mimic the conditions in a landfill perfectly, it is hard to argue that the cadmium in CdTe modules is safe against polluting the environment. The other set of tests also used squares from a First Solar module approximately 50 mm on a side, but these were carefully cut through the front glass with a diamond saw to make six slots each approximately 25 mm long. All the samples had the same pattern as shown in FIG. 2. Two such samples were immersed in buffer solutions with pH values of 4 and 10, respectively. The pH 4 simulated acid rain, as in the broken-sample tests, while the alkaline solution simulated the properties of some groundwater that passes through calcium salts. The third sample was put into rainwater collected during a storm on 3 Jun. 2010 in Washington, D.C., which had a measured pH of 6.9 (essentially neutral). All three samples were kept in plastic containers with airtight lids and maintained at room temperature. After 7, 10, 14, 21, 30, 45, and 64 days, we withdrew approximately 10 ml specimens of the leaching solution for analysis by DCP-AES to determine the Cd concentrations. To maintain constant solution volume, each sample withdrawn was replaced with 10 ml of fresh solution.

(13) The results of the analyses are summarized in Table 1, and graphed in FIGS. 3-5. Comparing results, at 64 days the broken sample in pH=4 solution had about 85 mg/l Cd, while the scribed one had 54 mg/l. The difference might be explained by noting that the broken sample had a much closer pattern of cracks so as to expose more of the active layer to the leaching solution. Both, however, had Cd concentrations four orders of magnitude above the EPA limit for drinking water. What is even more striking is that the sample in the alkaline solution with a pH of 10 reached a concentration of 110 mg/l Cd after 64 days. According to NREL's Cadmium Telluride Fact Sheet [ref 5], CdTe PV modules contain between 3 and 10 grams of Cd/m2. The nominal value is 7. So a 55 cm (25 cm2) sample would contain about 17.5 mg of Cd. If all of this dissolved in 0.15 liter of solution the concentration would be 117 mg/l.

(14) TABLE-US-00001 TABLE 1 Cadmium concentration in solutions for scribed samples in Rain Water, Buffer pH 4, and Buffer pH 10. Days RW (ppm) B4 (ppm) B10 (ppb) 7 7.10 30.00 10 1.30 14 14.79 80.00 21 20.80 90.00 30 2.89 31.11 120.00 45 45.02 170.00 64 53.86 140.00

(15) The final concentrations of cadmium given in Table 2 show that at least the majority of this element did leach into solution for the jars with pH values of 3, 4, and 5, and that a significant fraction leached out at pH 6. We were not able to measure the original amounts of cadmium in the samples, but based on the nominal value it should be clear that substantial amounts of the element leach out effectively in all but the pure water. One observation that supports the conclusion that most of the cadmium is being leached is that the concentration-versus-time curves tend to level off after 60 to 100 days. Therefore, it is very probable that in the real world virtually all the cadmium will be leached out within the first year after the modules are dumped.

(16) TABLE-US-00002 TABLE 2 Cadmium concentration in ppm (mg/l) of broken leaching samples. Days pH = 3 pH = 4 pH = 5 pH = 6 DI 0 0 0 0 0 0 32 35.4 27.8 11.1 1.11 41 55.8 120 94.6 174 123 82.7 2.84 206 162 145 122 107 7.28

(17) Referring now to FIG. 6, a flow chart of a method 600 for removing cadmium/cadmium compounds from PV modules is illustratively depicted, in accordance with an embodiment of the present invention.

(18) The conclusion of all the scientific research conducted is that CdTe is very soluble in all forms of water, except distilled water. This property is being used in the present invention to reclaim the carcinogenic CdTe and Cd from PV modules.

(19) At step 605, it is determined whether a PV module (or, e.g., the glass components of the PV module) is broken up into pieces. If the PV module is broken up into pieces, the pieces, at step 610, are placed into water having a fairly high acidic pH or a basic pH.

(20) At step 615, after period of time of two hours or more, the components of the PV module, even encapsulated pieces of the module, fall apart, enabling the CdTe to dissolve in the acidic or base water

(21) At step 620, once the Cd or the Cd compound is dissolved, the Cd or Cd compound is precipitated out by the boiling off the water. It is noted, however, that other methods of precipitating the Cd or Cd compound may also be used, while maintaining the spirit of the present invention. According to an embodiment, once precipitated, the precipitated Cd or Cd compound is collected.

(22) If we want to save the glass substrate that the CdTe is deposited on, and the cover plate, we can still reclaim the Cd and CdTe and save the glass pieces in one. This is due to the laminar action of the penetration of the water between the two pieces of glass. This action is also called capillary action. If the PV module is determined, at step 605, to not be broken up into pieces, at step 625, one edge of a sealed CdTe PV module is opened and then, at step 630, this edge is immersed in water having a fairly high acidic pH or a basic pH.

(23) At step 635, the water dissolves the material nearest to it and, by capillary action, moves on to penetrate between the two glass plates. Since the edge of the module is in a volume of water, the water dissolving the CdTe will continuously be diluted and displaced with fresh unsaturated water and, while the capillary action the water traveled throughout the module until the two pieces of glass are separated. The Cd or Cd compound, such as CdTe, is then, at step 620, precipitated out as before.

(24) When introducing elements of the present disclosure or the embodiment(s) thereof the articles a, an, and the are intended to mean that there are one or more of the elements. Similarly, the adjective another, when used to introduce an element, is intended to mean one or more elements. The terms including and having are intended to be inclusive such that there may be additional elements other than the listed elements.

(25) Although this invention has been described with a certain degree of particularity, it is to be understood that the present disclosure has been made only by way of illustration and that numerous changes in the details of construction and arrangement of parts may be resorted to without departing from the spirit and the scope of the invention.