Passive stagnation control for solar collectors

Abstract

A method for controlling stagnation in a solar collector, comprises: providing an solar energy absorbing substrate and a first layer; providing a second layer disposed between the first layer and the solar energy absorbing substrate; coupling an actuator to the solar energy absorbing substrate; and expanding the actuator when the solar collector is exposed to a stagnation temperature to form a gap between the first layer and the second layer.

1 . A solar collector, comprising:
 a solar energy absorbing substrate;   a first layer and a second layer above the solar energy absorbing substrate, each of the first layer and the second layer including respective surface features facing one another and defining an interface, the surface features configured to transmit a selected portion of solar energy across the interface when in alignment with one another, the transmission being greater than or equal to 80% measured according to ISO 9060:1990; and   an actuator coupled to the solar energy absorbing substrate in thermal communication with the solar energy absorbing substrate, the actuator mechanically coupled with at least one of the first layer and the second layer;   wherein, the actuator is configured to receive thermal energy from the solar energy absorbing substrate and move the first layer and the second layer to move the respective surface features out of alignment with light transmission through the first layer being less than or equal to 10% measured according to ISO 9060:1990.   
     
     
         2 . The solar collector of  claim 1 , wherein the respective surface features comprise complementary geometric configurations at the interface between the first layer and second layer. 
     
     
         3 . The solar collector of  claim 1 , wherein each geometric configuration includes at least one of trapezoidal, saw-tooth, sinusoidal, lamellar, triangular, abs(sin), cycloid, and pyramidal geometric configurations. 
     
     
         4 . The solar collector of  claim 1 , further comprising an intermediate layer located between the first layer and second layer. 
     
     
         5 . The solar collector of  claim 4 , wherein the intermediate layer comprises a material having a light transmission equal to the light transmission of the first layer. 
     
     
         6 . The solar collector of  claim 1 , wherein the actuator and the solar energy absorbing substrate have identical coefficients of thermal expansion. 
     
     
         7 . The solar collector of  claim 1 , wherein the actuator has a coefficient of thermal expansion greater than the coefficient of thermal expansion of the solar energy absorbing substrate. 
     
     
         8 . The solar collector of  claim 1 , wherein the light transmission through the first layer and the second layer is less than or equal to 5% measured according to ISO 9060:1990 at the stagnation temperature. 
     
     
         9 . The solar collector of  claim 1 , wherein the first layer and the second layer comprise a polymeric material selected from polyester, polycarbonate, polystyrene, poly(methyl methacrylate), poly(styrene-co-methyl methacrylate), poly(styrene-co-acrylonitrile), poly(methyl methacrylate-co-styrene-co-acrylonitrile), and combinations comprising at least one of the foregoing. 
     
     
         10 . The solar collector of  claim 1 , further comprising an air gap between the second layer and the solar energy absorbing substrate. 
     
     
         11 . The solar collector of  claim 10 , wherein the actuator is located in the air gap. 
     
     
         12 . A method for controlling stagnation in a solar collector, comprising:
 providing an solar energy absorbing substrate and a first layer;   providing a second layer disposed between the first layer and the solar energy absorbing substrate;   coupling an actuator to the solar energy absorbing substrate; and   expanding the actuator when the solar collector is exposed to a stagnation temperature to form a gap between the first layer and the second layer.   
     
     
         13 . The method of  claim 12 , wherein each of the first layer and the second layer include respective surface features facing one another and defining an interface, the surface features configured to transmit a selected portion of solar energy across the interface when in alignment with one another, the transmission is greater than or equal to 80% measured according to ISO 9060:1990 and wherein when the surface features of the first layer and the second layer are moved out of alignment, light transmission through the first layer is less than or equal to 10% measured according to ISO 9060:1990. 
     
     
         14 . The method of  claim 12 , wherein the respective surface features of the first layer and second layer comprise complementary geometric configurations at the interface between the first layer and second layer. 
     
     
         15 . The method of  claim 12 , wherein the actuator and the solar energy absorbing substrate comprise identical coefficients of thermal expansion. 
     
     
         16 . The method of  claim 12 , wherein the light transmission through the first layer and the second layer, when the surface features are in direct contact with one another, is greater than or equal to 80% transmission measured according to ISO 9060:1990. 
     
     
         17 . The method of  claim 12 , wherein the light transmission through the first layer and the second layer at the stagnation temperature is less than or equal to 5% transparency measured according to ISO 9060:1990. 
     
     
         18 . The method of  claim 12 , wherein the first layer and the second layer comprise a polymeric material selected from polyester, polycarbonate, polystyrene, poly(methyl methacrylate), poly(styrene-co-methyl methacrylate), poly(styrene-co-acrylonitrile), poly(methyl methacrylate-co-styrene-co-acrylonitrile), and combinations comprising at least one of the foregoing. 
     
     
         19 . The method of  claim 12 , further comprising providing an air gap between the second layer and the solar energy absorbing substrate. 
     
     
         20 . The method of  claim 19 , further comprising locating the actuator in the air gap.