WO2013098850A2

WO2013098850A2 - Aluminum reflectors for solar collectors

Info

Publication number: WO2013098850A2
Application number: PCT/IN2012/000793
Authority: WO
Inventors: Vivek Srivastava; Saikat ADHIKARI
Original assignee: Aditya Birla Science And Technology Company Limited
Priority date: 2011-12-07
Filing date: 2012-12-05
Publication date: 2013-07-04
Also published as: WO2013098850A3

Abstract

A method for enhancing the reflectivity of an aluminum element having an initial reflectivity below 75 percent includes cleaning the element with a first cleaning agent, drying the cleaned element,' reducing roughness of at least one surface of the dried element by electro-polishing or chemical polishing, cleaning the at least one surface of the element with a second cleaning agent and drying the element to obtain an element having at least one surface having final reflectivity greater than 80 percent.

Description

ALUMINUM REFLECTORS FOR SOLAR COLLECTORS

FIELD OF THE DISCLOSURE

The present disclosure generally relates to aluminum reflectors for solar collectors.

BACKGROUND

Solar power is the power obtained by the conversion of sunlight into electricity or any other form of energy. Solar radiations are directly converted into electricity by means of photovoltaic panels. Alternatively, solar radiations are indirectly converted into electricity by means of various concentrated solar power generation systems. Amongst all concentrated solar power generation systems, parabolic trough systems are comparatively more developed and idely used systems. In parabolic trough systems, the incoming solar radiation is collected and concentrated by using trough collectors of large aperture onto a single line focus element called receiver. The solar energy is captured by the receiver is utilized by heating a high temperature fluid (HTF) up to 450° C. The heated HTF is passed through a heat exchanger to generate steam which in turn drives a turbine to generate power.

Trough collector having mirrors are used as reflectors with high total and specular reflectivity for concentrating the solar radiation onto a single point. Reflectivity is defined as the ratio of the energy of a light wave reflected from a reflective surface to the energy possessed by the light wave striking the reflective surface. Apart from high total and specular reflectivity, the desired properties required in a reflector material are high durability (low reflectivity degradation in outdoor conditions), high abrasion resistance, light weight and low cost.

Traditionally, silver coated glass mirrors have been used as reflectors due to their relatively high reflectivity and resistance to corrosion in atmospheric conditions. However, these glass mirrors are comparatively expensive to produce and are heavy. Nowadays, emerging trend is towards using lightweight reflectors based on either laminated or front surface aluminum mirrors. However, the existing aluminum reflectors typically suffer from high degradation rate due to corrosion, Ultraviolet (UV) light effect or delamination. In order to achieve high reflectivity and corrosion resistance for the existing aluminum reflectors, complex surface modification techniques are used. However, because of the complex and expensive surface treatments, the existing aluminum reflectors have high manufacturing and maintenance cost.

Further, with continuous use of the conventional solar reflectors and with exposure of the conventional reflectors to the harsh and adverse environmental conditions, the surface roughness of the conventional solar reflectors increases and as such the reflectivity reduces. Accordingly, the efficiency of the solar reflectors drastically reduces with use. The solar reflectors are expensive components and as such it is uneconomical to dump the solar reflectors. There is a need to efficient and economically restore the reflectivity of the solar reflectors by reducing the surface roughness thereof, thereby enabling reuse of the solar reflectors once surface reflectivity thereof reduces due to continuous use and exposure to adverse environmental conditions. In order to enhance the reflectivity of the solar reflectors the solar reflectors are subjected to various treatments, however, most of the reflectivity enhancing surface treatment methods known in the prior art adversely affect the solar reflector as well and may also cause damage to the solar reflector. Further, these reflectivity enhancing treatments can be used only for limited number of cycles and hence the number of times the solar reflector can be reused is limited.

Accordingly, there is need for a simple method for enhancing the reflectivity of the solar reflectors that effectively enhances the reflectivity of an aluminum element having an initial reflectivity below 75 percent. Further, there is a need for method for enhancing reflectivity of corroded aluminum reflectors that can be repetitively used on the same reflector, thereby enhancing the service life of the aluminum reflector. Furthermore, there is a need for a method for enhancing reflectivity of the aluminum reflectors that can be used for any type of reflector and even for coated aluminum reflectors. Furthermore,, there is a need for a method for enhancing reflecti vity of the aluminum reflectors that is economical, convenient, and does not affect the durability of the aluminum reflectors.

OBJECTS

Some of the objects of the present disclosure, aimed to ameliorate one or more problems of the prior art or to at least provide a useful alternative, are described herein below:

An object of the present disclosure is to provide a method for enhancing the reflectivity of an aluminum element having an initial reflectivity below 75 percent.

Another object of the present disclosure is to provide a simple method for refurbishing corroded aluminum reflectors.

Yet another object of the present disclosure is to provide a method for enhancing reflectivity of corroded aluminum reflectors that can be repetitively used on the same reflector, thereby enhancing the service life of the aluminum reflector.

Still another object of the present disclosure is to provide a method for enhancing reflectivity of the aluminum reflectors that can even be used for coated aluminum reflectors.

Further, an object of the present disclosure is to provide a method to for enhancing reflectivity of the aluminum reflectors that does not affect the durability of the aluminum reflectors.

Also, an object of the present disclosure is to provide an economical method for enhancing reflectivity of aluminum reflectors.

Additionally, an object of the present disclosure is to provide a method for enhancing reflectivity of the aluminum reflectors without increasing the weight of the aluminum reflectors.

Furthermore, an object of the present disclosure is to provide a method for enhancing reflectivity of the aluminum reflectors that is efficient and reliable.

These and other objects of the present disclosure are dealt in great extent by the accompanying drawing and the descriptive matter, in which there are illustrated exemplary embodiments of the disclosure.

In accordance with an embodiment of the present disclosure, there is provided a method for enhancing the reflectivity of an aluminum element, typically a solar reflector, having an initial reflectivity below 75 percent. The method for enhancing the reflectivity of the aluminum element includes cleaning the element with a first cleaning agent, drying the cleaned element, reducing roughness of at least one surface of the dried element by at least one process step selected from the group consisting of electro-polishing and chemical polishing, cleaning the at least one surface of the element with a second cleaning agent and drying the element to obtain an element having at least one surface having final reflectivity greater than 80 percent.

Typically, the step of reducing roughness of at least one surface of the dried element is performed by electro polishing that includes the steps of immersing the element in an electro-chemical cell containing cool electro-polishing electrolyte, connecting the element to the positive terminal of a DC voltage source, connecting an immersed graphite rod to the negative terminal of the voltage source, stirring the cooled electro polishing electrolyte for maintaining uniform temperature and composition thereof, maintaining a voltage in the range of 20-40 Volts in the cell for a predetermined time period and removing the electro-polished element from the electro-chemical cell.

Typically, the electro-polishing electrolyte is at least one selected from the group consisting of methanol, ethanol, 2-butoxy ethanol and perchloric acid.

In accordance with one embodiment of the invention, the electro-polishing electrolyte comprises ethanol and perchloric acid in the proportion of 80:20 v/v.

Alternatively, the electro-polishing electrolyte comprises methanol, 2-butoxy ethanol and perchloric acid in the proportion of 70:20: 10 v/v/. Typically, the cool electro-polishing electrolyte is cooled to a temperature between 15 to -5 °C, preferably 10 °C.

Generally, the electro-polishing electrolyte is cooled with the help of an ice bath.

In accordance with another embodiment of the present disclosure, the step of reducing roughness of at least one surface of the dried element is performed by a chemical polishing process^'using a chemical polishing solution selected from the group consisting of sulphuric acid, nitric acid and phosphoric acid to which copper nitrate is added.

In accordance with one embodiment of the disclosure, the composition of the chemical polishing solution is 70:20:10 v/v/v of sulphuric acid, nitric acid and phosphoric acid to which 5 gram per liter of a noble metal salt such as copper nitrate, copper sulphate and silver nitrate is added.

Further, the step of reducing roughness of at least one surface of the element reduces the thickness of the element by 20 microns for each iteration.

In accordance with an embodiment of the present disclosure, the method can be carried out for at least forty cycles, _f

Typically, the first cleaning agent is a mixture of acetone and De-ionized (DI) water.

Furthermore, the second cleaning agent is De-ionized (DI) water. In accordance with one practical embodiment of the disclosure, the element selected for the method has an initial reflectivity in the range of 60-70 percent.

In accordance with an embodiment of the present disclosure, the final reflectivity of the element is greater than 85 percent.

In accordance with another embodiment of the present disclosure the element is a coated element.

In accordance with an embodiment of the present disclosure, there is provided an aluminum element whose reflectivity is enhanced by a method including the steps of cleaning the element with a first cleaning agent, drying the cleaned element, reducing roughness of at least one surface of the dried element by a method step selected from electro-polishing and chemical polishing, cleaning the at least one surface of the element with a second cleaning agent and drying the element to obtain an element having at least one surface having reflectivity greater than 80 percent.

Typically, in accordance with an embodiment of the present disclosure, the element is a solar radiation reflector.

In accordance with another embodiment of the present disclosure, the element is an arcuate element.

DETAILED DESCRIPTION

A preferred embodiment will now be described in detail. The preferred embodiment does not limit the scope and ambit of the disclosure. The description provided is purely by way of example and illustration. The embodiments herein and the various features and advantageous details thereof are explained with reference to the non-limiting embodiments in the following description. Descriptions of well-known components and processing techniques are omitted so as to not unnecessarily obscure the embodiments herein. The examples used herein are intended merely to facilitate an understanding of ways in which the embodiments herein may be practiced and to further enable those of skill in the art to practice the embodiments herein. Accordingly, the examples should not be construed as limiting the scope of the embodiments herein.

However, conventional aluminum solar reflectors having reflecting surfaces have various limitations such that in the course of use and as result of environmental corrosion the reflectivity of the aluminum element of the solar reflector drops to below 75%. Generally, the reflectivity of conventional aluminum reflectors drops to between 60% and 70% and may go as low as 25% which results in them becoming inefficient and they are either dumped or require complex surface treatments to be reused which are expensive.

With continuous use of the conventional solar reflectors and with exposure of the conventional reflectors to the harsh and adverse environmental conditions to which the conventional solar reflectors are subjected to, the surface roughness of the conventional solar reflectors increases and as such the reflectivity reduces. Accordingly, the efficiency of the solar reflectors drastically reduces with use and the solar reflectors fail to provide the desired output or desired effect. The solar reflectors are expensive components and as such it is uneconomical to dump the solar reflectors after use for some time. There was therefore felt a need to efficiently and economically restore the reflectivity of the solar reflectors by reducing the surface roughness thereof, thereby enabling reuse of the solar reflectors once the surface reflectivity thereof reduces due to continuous use and exposure to adverse environmental conditions. In order to enhance the reflectivity of virgin aluminum elements or solar reflectors of aluminum elements, the aluminum elements or the solar reflectors are subjected to various treatments, however, most of the reflectivity enhancing surface treatment methods known in the prior art adversely affect the solar reflector as well and may also cause damage to the solar reflector. Further, these reflectivity enhancing treatments can be used only for limited number of cycles and hence the number of times the solar reflector can be reused is limited.

The present disclosure envisages a method for enhancing the reflectivity of virgin aluminum elements or the solar reflectors made of aluminum elements that effectively enhance the reflectivity of the aluminum element of ths reflector. The method for enhancing the reflectivity in accordance with the present disclosure efficiently and economically restores the reflectivity of the solar reflectors by reducing the surface roughness thereof, thereby enables reuse of the solar reflectors once surface reflectivity thereof reduces due to continuous use and exposure to adverse environmental conditions. The method for enhancing the reflectivity in accordance with the present disclosure can be used for a number of cycles and hence the number of times the solar reflector can be reused is enhanced and as such the service life and reusability of the solar reflector is enhanced.

In accordance with the present disclosure, the method for enhancing the reflectivity of the solar reflector includes the following steps: cleaning the aluminum element with a cleaning agent which could include a mixture of acetone and De-ionized (DI) water;

> drying the cleaned element; > reducing roughness of at least one surface of the dried aluminum element;

> cleaning the at least one surface of the aluminum element with a cleaning agent which can include De-ionized (DI) water; and

> drying the element to obtain an aluminum element having at least one surface having high reflectivity.

The method as described herein above enhances the reflectivity of a virgin aluminum element or the reflectivity of an element which is a solar reflector whose reflectivity has reduced due to continuous use thereof, particularly, the method as described herein above enhances the reflectivity of an aluminum element or of an aluminum solar reflector that has dropped down to below 75 percent to more than 80%, thereby enabling reuse of the solar reflector.

Typically, in accordance with the present disclosure, the reflectivity of the solar reflector is increased by reducing roughness of the surface of aluminum element. More specifically, the reflectivity of the solar reflector is increased by either electro polishing or chemical polishing or both, wherein both are economical ways of enhancing the reflectivity of the solar reflector. Such methods of enhancing the reflectivity enable reuse of the solar reflector.

In accordance with one embodiment of the present disclosure, the reflectivity of the reflective surface of an aluminum solar reflector is restored by electro polishing. The electro polishing/chemical polishing method of removing surface roughness in the process of enhancing the reflectivity and restoring the reflectivity of the reflective surface of the aluminum solar reflector is eco- friendly and cost effective as compared to other methods and accordingly is preferred over other methods. In accordance with the present disclosure, electro polishing involves the following steps:

immersing the element in an electro-chemical cell containing cool electro-polishing electrolyte (typically cooled to 8-12 °C in an ice bath) which could be a judicious mix of all or some of the following : methanol, 2-butoxy ethanol, perchloric acid and ethanol, ;

connecting the element to the positive terminal of a DC voltage source;

connecting an immersed graphite rod to the negative terminal of the voltage source;

stirring the cooled electro polishin electrolyte for maintaining uniform temperature and composition thereof;

> maintaining a voltage in the range of 20-40 Volts in the cell for a predetermined time period; and . .

> removing the electro-polished element from the electro-chemical cell.

The electro-polishing electrolyte can comprise ethanol and perchloric acid in the proportion of 80:20 v/v. .

Alternatively, the electro-polishing electrolyte can comprise methanol, 2-butoxy ethanol and perchloric acid in the proportion of 70:20: 10 v/v.

The electrolyte is maintained at a uniform temperature. A magnetic stirrer can be used to maintain uniform temperature and composition of the electrolyte. The electro polishing is achieved by applying a potential of 40V for duration of about 2 min while maintaining the temperature of the solution constant. The electrochemical cell needs to be cooled to a temperature between -5 to 15 degrees C because when a high voltage (or current) is applied through a conductive solution (perchloric acid in the case of electropolishing), it tends to further heat up. Too much heating of the electrolytic solution can lead to sparking hence this becomes a potential fire hazard. The optimum temperature is found to be around 10 deg C although anything between -5 deg to 15 deg C is found to work. Too low a temperature can also be a problem because it reduces the kinetics of electropolishing and hence it will take much longer time to get the same surface finish (reflectivity) at a lower temperature as compared to a higher temperature. »

The initial reflectivity of a mill-finished new aluminum sheet is also around 70 %. The present invention is to protect both new as well as degraded aluminum solar reflectors. In both cases, the sheets could to electropolished to enhance the reflectivity beyond 80%.

Using these methods it has been possible to achieve reflectivity of even greater than 88% repeatedly without any damage to the body of the element.

The method for enhancing the reflectivity of the aluminum reflector in accordance with the present disclosure can be used for restoring the reflectivity of outdoor solar reflectors whose reflectivity has reduced due to continuous exposure to detrimental environmental conditions and whose efficiency has reduced. The energy reflected by the aluminum reflector is collected and concentrated at a receiver where solar energy is captured and can be used to generate steam which in turn drives a turbine to generate power. The durability of the highly reflective aluminum solar reflector is generally enhanced by appropriate coatings. The method for enhancing the reflectivity in accordance with the present disclosure is also applicable to coated aluminum reflectors. Accordingly, the coated aluminum reflectors can also be subjected to the reflectivity enhancing treatment of the present disclosure to facilitate reuse of the coated aluminum reflectors, thereby further enhancing the durability thereof. The electro-polishing process in accordance with the present disclosure can be repeatedly performed on the surface of an aluminum solar reflector and as such the aluminum reflectors can be repeatedly used after enhancing the reflectivity of the aluminum reflectors, thereby providing economical reusability of the aluminum solar reflector. The method for enhancing reflectivity of the solar reflectors using electro polishing and periodic re-polishing of the corroded/spent aluminum reflectors after exposure thereof to detrimental environmental conditions holds a significant economic advantage over existing methods for restoring reflectivity of reflectors and as such provides considerably long life to the aluminum solar reflectors.

In accordance with another embodiment of the present disclosure, the reflectivity of the reflective surface of an aluminum solar reflector is restored by using chemical polishing using a chemical polishing solution selected from the group consisting of sulphuric acid, nitric acid and phosphoric acid to which the salt of a noble metal is added such as silver nitrate or copper sulphate or nitrate is added.

The composition of the chemical polishing solution can be 70:20:10 v/v/v of sulphuric acid, nitric acid and phosphoric acid to which 5 grani per liter of copper nitrate / silver nitrate / copper sulphate is added. For the chemical polishing process, the polishing is achieved using a strong oxidizing acid (like phosphoric, nitric, sulphuric acids or their combinations) solution. However addition of a metal less active than aluminum (like copper or silver) in the form of their salts can produce a much better surface finish (or reflectivity). The noble metal is introduced in the solution in the form of ionic salts like copper nitrate, copper sulphate, silver nitrate. The copper or silver is present in the form of a fine precipitate on the polished aluminum surface and can be wiped off or rinsed off with washing in water.

Further, the step of reducing roughness of at least one surface of the element either by electro polishing or chemical polishing reduces the thickness of the element by about 20 microns for each iteration.

In accordance with an embodiment of the present disclosure, the method can be carried out for at least forty cycles without causing any damage to the aluminum element of the reflector.

Although, the method for enhancing reflectivity of the present embodiment is described in accordance with trough collectors, the method for enhancing reflectivity in accordance with the present disclosure is applicable to any type of solar reflectors such as solar dish collectors having arcuate surfaces.

TEST DATA:

In accordance with the present disclosure, experiments were performed to determine the best possible proportions and compositions of solutions used by the various methods for enhancing reflectivity of an aluminum solar reflector for a solar collector. Various non limiting experiments are performed by varying the various parameters to determine the possible proportions and compositions of solutions used and conditions for achieving enhanced reflectivity of the aluminum reflector but they do not in any way limit the scope of the disclosure or the ambit of the claims. These experiments are explained herein below:

Experiment 1:

A sheet of aluminum alloy AA1050 is cleaned with acetone and De-ionized (DI) water and then air dried. The sheet is then immersed in the electro polishing electrolyte and is connected to the positive terminal of a DC voltage source to act as anode. Further, a graphite rod is immersed into the solution and is connected to the negative terminal of the DC voltage source to act as cathode. The electro polishing electrolyte is a 70:20:10 v/v/v mixture of methanol, 2- butoxy ethanol and perchloric acid. The electro polishing electrolyte is cooled to about 10 °C using an ice bath. A magnetic stirrer is used to maintain uniform temperature and composition of the electrolyte. The electro polishing is achieved by applying a potential of 40V for duration of about 2 min while maintaining the temperature of the solution constant. After the electro polishing cycle, the aluminum sheet is removed from the electrolyte and is cleaned in DI water and then air dried. The average thickness loss for AA1050 during one electro polishing cycle at 40V for about 2 min is found to be 20 microns. Reflectance of the sample is then measured using a Shimadzu UV-2550 spectrophotometer in the range of 200-800nm. Average reflectivity in the measured spectrum is found to be at least 88%. The electro polished aluminum sheet can be used as a solar reflector either in the bare condition or with coating for improving durability.

Experiment 2:

An electro polished AA1050 sheet is prepared as described herein above in experiment 1. One sample is intentionally corroded using a combination of exposure conditions including outdoor exposure, immersion in salt solution (NaCl), acid solution (H2S04) and alkaline solution (NaOH) to simulate decrease in reflectivity of the aluminum under working conditions. More aggressive solutions are used for accelerating the corrosion process. After the reflectivity value for the sheet has dropped to below 75%, the sample is electro polished again using the same procedure described in experiment 1. The average reflectivity value of re-electro polished sheet is then measured and found to be greater than 88%. The steps of intentionally corroding the aluminum sheet and electro polishing the aluminum sheet after intentionally corroding the aluminum sheet define one complete cycle of re-polishing. The entire cycle is repeated 3 more times with accelerated corrosion and electro polishing. After each polishing cycle, the initial average reflectivity of 88% is achieved. Each polishing cycle caused a loss of 20 microns from electro polishing and 1-5 microns due to the corrosion process. Thus a 2 mm thick sheet of aluminum can be re-polished for at least 40 cycles before it loses half of its thickness.

Experiment 3:

A sheet of AA1050 is electro polished using the same procedure as described in experiment 1. However, the electrolyte used is a 80:20 v/v mixture of ethanol and perchloric acid. The mean reflectivity of the sheet after the electro polishing is again in excess of 88%. In another experiment the polishing is achieved using a chemical polishing process instead of electro polishing. The chemical polishing solution is a 70:20: 10 v/v/v mixture of phosphoric acid, sulphuric acid and nitric acid respectively with 5 g/lit of copper nitrate added to it. In this case the chemical polishing process yielded an average reflectivity value of 85% for the AA1050 aluminium sheet.

Experiment 4:

A sheet of AA3003 is electro polished using the same procedure as described in experiment 1. The maximum reflectivity achieved for AA3003 aluminum sheet is found to be 87% in the wavelength range of 200-800 nm. In another experiment an AA3105 sheet is polished using the chemical polishing solution described in experiment 3. The maximum reflectivity in the wavelength range 200-800 nm is found to be 87%.

Experiment 5:

A sheet of AA1050 is electro polished using 20V for 3 min using the same procedure as in experiment 1. The mean reflectivity obtained for the sample was 87.9%. In another experiment, a sample of AA1050 is electro polished using 20V for 4.5 min and a reflectivity of 87.9% is obtained. A list of electro polishing voltage and time during the experiments are given below in Table 1 along with the corresponding reflectivity values obtained.

Tab!e 1

Experiment 6:

A sheet of AA1050 is electro polished using the procedure described in experiment 1 above. The polished sample after electro polishing is coated with a sol-gel silica coating. In another experiment, an electro polished AA1050 sheet is coated with a polymer based primer by a dip coating method. In yet another experiment, an electro polished AA1050 is coated with a silsesquioxane based super-hydrophobic coating. In all cases, the electro polished and coated surface could be used as a durable solar reflector. The coated and corroded samples were again electro polished using the procedure described in experiment 1 and the initial reflectivity before coating was recovered. Thus, the repolishing procedure was found to work with coated samples also.

TECHNICAL ADVANCEMENTS AND ECONOMIC SIGNIFICANCE:

The method for enhancing the reflectivity of an aluminum element having an initial reflectivity below 75 percent described herein above have several technical advantages including but not limited to the realization of:

• a method for enhancing the reflectivity of an aluminum element that is simple and convenient to use;

• a method for enhancing reflectivity of the aluminum reflectors that can be repetitively used on the same reflector, thereby enhancing the service life of the aluminum reflector;

• a method for enhancing reflectivity of the aluminum reflectors that can even be used for coated aluminum reflectors;

• a method to for enhancing reflectivity of the aluminum reflectors that does not hamper the durability of the aluminum reflectors;

• a method for enhancing reflectivity of the aluminum reflectors that is economical;

• a method for enhancing reflectivity of the aluminum reflectors that enhances reflectivity of the reflector without increasing the weight of the aluminum reflectors; and • a method for enhancing reflectivity of the aluminum reflectors that is efficient and reliable.

The numerical values given of various physical parameters, dimensions and quantities are only approximate values and it is envisaged that the values higher or lower than the numerical value assigned to the physical parameters, dimensions and quantities fall within the scope of the disclosure and the claims unless there is a statement in the specification to the contrary.

Throughout this specification the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated element, integer or step, or group of elements, integers or steps, but hot the exclusion of any other element, integer or step, or group of elements, integers or steps.

Any discussion of documents, acts, materials, devices, articles or the like that has been included in this specification is solely for the purpose of providing a context for the disclosure. It is not to be taken as an admission that any or all of these matters form part of the prior art base or were common general knowledge in the field relevant to the disclosure as it existed anywhere before the priority date of this application.

The foregoing description of the specific embodiments will so fully reveal the general nature of the embodiments herein that others can, by applying current knowledge, readily modify and/or adapt for various applications such specific embodiments without departing from the generic concept, and, therefore, such adaptations and modifications should and are intended to be comprehended within the meaning and range of equivalents of the disclosed embodiments. It is to be understood that the phraseology or terminology employed herein is for the purpose of description and not of limitation. Therefore, while the embodiments herein have been described in terms of preferred embodiments, those skilled in the art will recognize that the embodiments herein can be practiced with modification within the spirit and scope of the embodiments as described herein.

Claims

A method for enhancing the reflectivity of an aluminum element having an initial reflectivity below 75 percent said method comprising the following steps:

• cleaning said element with a first cleaning agent;

• drying said cleaned element;

• reducing roughness of at least one surface of said dried element by a method step selected from the group consisting of electro- polishing and chemical polishing;

• cleaning said at least one surface of said element with a second cleaning agent; and

• drying said element to obtain an element having at . least one surface having reflectivity greater than 80 percent.

The method as claimed in claim 1, in which the aluminum element is selected from a group consisting of a new aluminum element, a used aluminum element and a corroded aluminum element.

The method as claimed in Claim 1, wherein said step of reducing roughness of at least one surface of said dried element is performed by electro polishing comprising the following steps:

• immersing said element in an electro-chemical cell containing cool electro-polishing electrolyte;

• connecting said element to the positive terminal of a DC voltage source;

• connecting an immersed graphite rod to the negative terminal of said voltage source;

• stirring said cooled electro polishing electrolyte for maintaining uniform temperature and composition thereof; • maintaining a voltage in the range of 20-40 Volts in said cell for a predetermined time period; and

• removing said electro-polished element from said electrochemical cell.

4. The method as claimed in Claim 2, wherein said electro-polishing electrolyte is at least one selected from the group consisting of methanol, 2-butoxy ethanol, ethanol and perchloric acid.

5. The method as claimed in Claim 2, wherein said electro-polishing electrolyte comprises ethanol and perchloric acid in the proportion of 80:20 v/v.

6. The method as claimed in Claim 2, wherein said electro-polishing electrolyte comprises methanol, 2-butoxy ethanol and perchloric acid in the proportion of 70:20:10 v/v/v.

7. The method as claimed in Claim 2, wherein said cool electro-polishing electrolyte is cooled to between -5 deg C and 15 deg C, preferably 10 °C.

8. The method as claimed in Claim 2, wherein said cool electro-polishing electrolyte is cooled with the help of an ice bath.

9. The method as claimed in claim 1, wherein said step of reducing roughness of at least one surface of said dried element is performed by a chemical polishing process using a chemical polishing solution selected from the group consisting of sulphuric acid, nitric acid and phosphoric acid to which copper nitrate is added.

10. The method as claimed in claim 9, wherein the composition of said- chemical polishing solution is 70:20:10 v/v/v of sulphuric acid, nitric acid and phosphoric acid to which 5 gram per liter of a noble metal salt such as copper nitrate, copper sulphate and silver nitrate is added.

11. The method as claimed in claim 1, wherein said step of reducing roughness of at least one surface of said element reduces the thickness of said element by 20 microns.

12. The method as claimed in claim 1 carried out for at least forty cycles.

13. The method as claimed in claim 1, wherein said first cleaning agent is, a mixture of acetone and De-ionized (DI) water.

14. The method as claimed in claim 1, wherein said second cleaning agent is De-ionized (DI) water.

15. The method as claimed in claim 1 , wherein said initial reflectivity of said element is in the range of 60-70 percent.

16. The method as claimed in claim 1, wherein said final reflectivity of said element is greater than 85 percent.

17. The method as claimed in claim 1, wherein said element is a solar reflector.

18. The method as claimed in claim 1, wherein said element is an arcuate element.

19. The method as claimed in claim 1, wherein said element is a coated element.

20. An aluminum element whose reflectivity is enhanced by a method comprising the following steps:

• cleaning said element with a first cleaning agent;

• drying said cleaned element;

• reducing roughness of at least one surface of said dried element by a method step selected from electro-polishing and chemical polishing;

• drying said element to obtain an element having at least one surface having reflectivity greater than 80 percent.

21. The aluminum element as claimed in claim 20 is a solar reflector.

22. The aluminum element as claimed in claim 20, which is new and is used in the manufacture of a solar reflector.