Solar panel

There is disclosed a photovoltaic solar panel capable of clearing accumulated ice. The panel includes a plurality of photovoltaic cells arranged in a plane with an overlaying glass sheet. The glass sheet has a first side towards the photovoltaic cells and a second side having a flat planar surface. The panel further includes an electrical heating web on the first side of the glass sheet between the sheet and the photovoltaic cells. The electrical heating web is configured to heat the first glass sheet sufficiently to melt the ice where it contacts the flat planar surface to cause the snow and ice to slide off the photovoltaic solar panel when the photovoltaic solar panel is at an angle from the horizontal. The electrical heating web is thermally separated from the photovoltaic cells by a transparent layer of low thermal conductivity.

FIELD OF THE INVENTION

The invention relates generally to solar panels.

BACKGROUND OF THE INVENTION

Photovoltaic solar panels have been developed to convert sunlight directly into electrical energy. Photovoltaic solar panels are effective whenever they are exposed to direct sunlight, even in cold climates. Unfortunately, photovoltaic solar panels do not generate much electricity when they are covered with snow or ice. Therefore, in cold climates, the overall efficiency of solar panels is reduced due to the fact that snow and ice accumulates on top of the solar panels. To become effective, the layer of snow and ice overburdening the solar panel must be cleared, either by scrapping (or sweeping) the snow away or by melting. Melting the snow and ice overburden can be achieved by heating the solar panels sufficiently to melt away all of the snow and ice; however, given the amount of energy required to do this, the overall gain in efficiency is very low. U.S. Pat. No. 4,063,963 to Bond Jr. discloses the use of electric heating elements placed directly on top of photovoltaic cells to help melt away any accumulated snow and ice. However, such a design has a very low overall efficiency because a large amount of electrical energy is required to heat the photovoltaic cells sufficiently to melt the snow and ice. What is therefore required is an energy efficient way of clearing snow and ice from solar panels.

SUMMARY OF THE INVENTION

In accordance with one aspect of the present invention, there is provided a photovoltaic solar panel capable of clearing ice accumulated on a surface thereof. The photovoltaic solar panel includes a plurality of photovoltaic cells arranged adjacent one another in a plane and a transparent glass sheet overlaying the photovoltaic cells. The glass sheet has a first side positioned towards the photovoltaic cells and an opposite second side having a flat planar surface extending along the entirety of the second side. The panel further includes an electrical heating web extending along the first side of the glass sheet between the glass sheet and the photovoltaic cells, the electrical heating web being thermally coupled to the glass sheet. The electrical heating web is configured to generate enough heat to heat the flat planar surface of the first glass sheet to sufficiently melt the ice where it contacts the flat planar surface so as to cause the snow and ice to slide off the photovoltaic solar panel when the photovoltaic solar panel is held at an angle from the horizontal.

In accordance with another aspect of the present invention, there is provided a photovoltaic solar panel as described above wherein the electrical heating web is thermally separated from the photovoltaic cells by a transparent layer of low thermal conductivity.

With the foregoing in view, and other advantages as will become apparent to those skilled in the art to which this invention relates as this specification proceeds, the invention is herein described by reference to the accompanying drawings forming a part hereof, which includes a description of the preferred typical embodiment of the principles of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring toFIG. 1, the present invention is essentially a flat photovoltaic solar panel10having a flat top surface12which is configured to clear a layer of snow and ice overburden14by a combination of melting and gravity. Solar panel10is preferably set at an angle α from the horizontal to ensure that snow and ice overburden14slides off the solar panel when a layer of ice and/or snow contacting the solar panel is partially melted. Surface12is made of very flat glass which is free of protrusions, cavities, ridges or any other surface feature which would provide additional surface area for ice to cling to and which would obstruct the movement of snow and ice off the surface when the snow and ice are melted. Overburden14remains on top of panel10because a thin layer of ice forms a layer of contact (or adhesion) with surface12. This contact layer is effectively frozen to surface12and causes the rest of the overburden to cling to the panel. As shall be discussed further, solar panel12is provided with a heating element which is configured to generate just enough heat to raise the temperature of surface12sufficiently to melt this contact layer of snow/ice to form a micro layer of water on surface12. Melting this contact layer to form this micro-layer of water permits the rest of the snow and ice piled on top of the panel to simply slide off the panel.

Referring now toFIG. 2, a photovoltaic solar panel made in accordance with the present invention is shown generally as item10and consists of a flat array of photovoltaic cells18formed as a flat plane. Solar cells18are aligned in a side by side flat parallel plane. Overlaying photovoltaic cells18is first glass sheet15. Glass sheet15is dimensioned to overlay the entire photovoltaic array18and to extend as much as a centimeter or so beyond the edges of the photovoltaic array. Extending the glass sheet beyond the edges of the photovoltaic array may be necessary to physically contain the solar panel in a suitable frame. Glass sheet15is a very flat highly transparent glass sheet of low Fe glass. Glass sheet15has a first side24which is directed towards photovoltaic cells18and an opposite second side22which forms a very flat surface. An anti-reflective coating26can be formed on side/surface22to prevent the reflection of sunlight and thereby increase the photovoltaic performance of the solar panel. The dimensions of anti-reflective coating26are exaggerated inFIG. 2; in practice, an anti-reflective coating would be a fraction of a mm in thickness. A second glass sheet16is positioned between glass sheet15and solar cells18. Glass sheet16has opposite surfaces28and30, with surface30positioned adjacent solar cells18. Glass sheet16is also made from highly transparent low Fe glass.

Electrical heating web20is positioned between glass sheets16and15. Electrical heating web20consists of a flat electric heating element which is configured to turn an electric current into heat. Heating web20extends along first surface24of glass sheet1overlaying the entire photovoltaic array18. Electrical heating web20is thermally coupled to glass sheet15such that heat generated by heating web20is passed to glass sheet15and thereby to surface22. Heating web20must be sufficiently rated to generate enough heat to form a micro layer of water by melting a contact layer of ice which may form on top of sheet15. Of course, this melting must occur when the temperature of the environment is freezing. It has been discovered that for northern climates, heating web20should preferably be rated at one or two watts per decimeter; however, the exact watt rating will be determined by the desired performance of the heating web, the likely operating temperatures and the amount of expected snow and ice overburden. In particularly cold climates with large amounts of snow, it is likely that increased heating would be required. Successful tests in Sudbury, Canada have been run using solar panels having heating webs rated at 6 watts per decimeter. This ensures that the heating element is sufficiently powerful. Preferably, the heating element is selected such that power is introduced into the panel sufficient to satisfy a gain in panel temperature over ambient above freezing to create a micro layer of water

Electrical heating web may consist of any flat heating element which has high transparency such as an electrically conductive silver film. It has been discovered that electrical conducting tungsten microfilaments having a diameter of between about 50 to 10 microns is particularly useful. Preferably, heating web20consists of a plurality of spaced apart parallel tungsten micro-filaments31having opposite ends which are coupled to an electric buss bar32and electrical connector34. Buss bar32and electrical connector34are positioned to make the panel easy to construct with the electrical terminals close together. Micro-filaments31are positioned as close to surface24as possible. Preferably, micro-filaments31are embedded in a transparent polymer layer36which acts to secure the micro-filaments and keep them in the appropriate orientation. Transparent polymer layer36is preferably between about 0.4 mm to 4 mm in thickness. Depending on the transparent polymer used, layer36can act to decrease the flow of heat from glass sheet15to glass sheet18. Several suitable polymers are commercially available which are highly transparent.

Glass sheet18helps to protect electric heating web20and also acts to add additional strength. Furthermore, glass sheet16acts as a thermal barrier between heating web20and photovoltaic cells18. As shall be better explained below, the heat energy generated by heating web20is preferably to be used to melt a layer of ice clinging to the surface22of glass sheet15. While heating the entire panel to a high temperature would be effective in melting accumulated snow, the electrical energy required to do so would greatly decrease the overall efficiency of the solar panel. Silicon (the principle ingredient in photovoltaic cells) is metallic and is an efficient thermal conductor with a conductivity of about 150 W/m-K. By contrast, aluminum has a conductivity of about 250 W/m-K and carbon steel has a conductivity of about 54 W/m-K. Since silicon is a good conductor of heat, placing the heating element directly on the photovoltaic cells would require large amounts of electrical energy to be consumed since the silicon would radiate away most of the heat. In such an arrangement, most of the electrical energy consumed by the heating element would be used to heat the silicon solar cell and then radiated away. Very little of the electrical energy would be used to melt ice. Placing a transparent material having a low thermal conductivity between heating web20and photovoltaic cells18would greatly decrease the heat lost to the photovoltaic cells. Glass has a thermal conductivity of about 1 W/m-K, therefore, placing a glass sheet between the heating web and the silicon photovoltaic cells, results in a vastly less heat being radiated away by the photovoltaic cells. As a result, a far greater percentage of the electrical energy consumed by the heating element will be used to melt the snow and ice covering the solar panel. Glass sheet16could be made thicker than glass sheet15to maximize the transfer of heat towards glass sheet15.

Referring now toFIG. 3it is possible to construct a solar panel in accordance with the present invention, shown generally as item50, with only a single layer of glass. In panel50, a single glass sheet52is used. Glass sheet52has flat upper surface62having an anti-reflective coating64formed thereon. Electrical heating web56is positioned between glass sheet52and photovoltaic cells54. Again, heating web56preferably consists of a plurality of tungsten micro-filaments60coupled to an electrical buss bar66. Again, micro-filaments60are positioned as close to glass sheet52as possible in order to thermally couple glass sheet52to electrical heating micro-filaments60. Again, the micro-filaments are contained within a transparent polymer layer58. In order to decrease the amount of heat transferred from the heating micro-filaments to the silicon photovoltaic cells, the thickness of polymer layer58should be increased, depending on the thermal conductivity of the polymer used. Several transparent polymers have thermal conductivities as low as 0.1 W/m-K to 0.2 W/m-K. The thicker the layer, the less thermal energy is wasted in heating photovoltaic cells. Of course, the transparency of the layer may be compromised if the layer is made too thick. It is important to ensure that there is some relatively non-heat conductive material separating heating web56and photovoltaic cells54. Since the embodiment shown inFIG. 3does not include a layer of glass separating heating web56and photovoltaic cells54, polymer layer58must be sufficiently thick to decrease the flow of heat from the heating web to the photovoltaic cells.

Referring now toFIGS. 4 and 5, snow overburden14forms a contact layer70which is frozen onto surface75of panel50. This contact layer prevents overburden14from falling off panel50. By engaging heating web56, heat is transferred to surface75which causes contact layer70to melt to form a water micro layer72separating surface75from overburden14. This very thin layer of water acts to greatly decrease the coefficient of friction between the overburden and surface75, which causes overburden14to slide off the solar panel due to the action of gravity. Since panel50is at an angle from the vertical, the force of gravity acts on overburden14to cause it to slide off the panel when the coefficient of friction between the overburden and surface75is low enough. Generally, only a few minutes of heating is required to form water micro-layer72. The vast majority of overburden14is not heated or melted, but simply slides off. Since heating web56is thermally insulated (isolated) from photovoltaic cells54, very little heat is wasted and more of it is used to form water micro-layer72. In this way, a very small amount of energy can be used to clear solar panel50.

Referring now toFIG. 6, it is possible to decrease the flow of heat from the heating element to the underlying photovoltaic cells by setting off the heating element from the photovoltaic cells to form an “air gap” between the heating element and the photovoltaic cells. A solar panel made with this in mind is depicted inFIG. 6and is shown generally as item80. Panel80has a glass upper sheet82with heating element84thermally coupled thereto as in the previous embodiment. However, unlike the previous embodiment, gap88is positioned between heating element84and photovoltaic cells86. Preferably, gap88could be filled with a dry low conductive gas such as argon. Gap88greatly reduces the flow of heat from heating element84and photovoltaic cells86.

A specific embodiment of the present invention has been disclosed; however, several variations of the disclosed embodiment could be envisioned as within the scope of this invention. It is to be understood that the present invention is not limited to the embodiments described above, but encompasses any and all embodiments within the scope of the following claims