Patent Publication Number: US-2013233304-A1

Title: Design of Integrated Heat Exchanger into Solar Absorber for Affordable Small-scale Concentrated Solar Power Generation (SCU)

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
     This application claims priority from U.S. Provisional Patent Application 61/607,747 filed Mar. 7, 2012, which is incorporated herein by reference. 
    
    
     STATEMENT OF GOVERNMENT SPONSORED SUPPORT 
     This invention was made with Government support under contract SU-83603201 awarded by U.S. Environmental Protection Agency. The Government has certain rights in this invention. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates generally to solar energy devices. More particularly, the invention relates to solar absorbers for concentrated solar power (CSP) dish systems. 
     BACKGROUND OF THE INVENTION 
     Concentrated Solar Power (CSP) technology seeks to replace fossil fuels in mechanical compression cycles for power generation. Existing dish systems require large areas to achieve sufficient conversion efficiency for the cost of the system. In addition, the conversion efficiencies which have been achieved are limited by the materials and manufacturing processes used. Moreover, a solar absorber for small scale CSP has not been economically viable due to high cost for small systems. 
     The current technologies in the art use metal components in one form or another, which limits the overall performance that can be achieved. Common high temperature metals will begin to fail around 1300 K and these metals have a low thermal conductivity. Since metal has high reflectivity, the receivers are often large to increase their internal surface area, and their surface is often coated with a radiation absorbent layer. 
     SUMMARY OF THE INVENTION 
     According to one aspect, the invention provides a solar absorber for a concentrated solar power (CSP) dish system. The solar absorber includes a cylindrically shaped blackbody cavity receiver and a cylindrically shaped heat exchanger shell covering the receiver. An exterior surface of the cavity receiver has grooves embedded to create a duct spiraling about the exterior of the cavity receiver so that the duct forms tubes of a tube-style heat exchanger when covered by the heat exchanger shell. An interior diameter of the cavity receiver is greater than or equal to a diameter of an aperture of the cavity receiver, and a longitudinal interior depth of the cavity is greater than or equal to twice the interior diameter of the cavity receiver. The cavity receiver is composed of a first monolithic ceramic material having emissivity greater than 0.9 and the heat exchanger shell is composed of a second monolithic ceramic material having emissivity greater than 0.9. The first and second ceramic materials are not necessarily distinct, but may be. The cavity receiver and heat exchanger shell are fused to form a monolithic cavity receiver and heat exchanger. 
     In a preferred embodiment, the first monolithic ceramic material is silicon carbide and the interior diameter of the cavity receiver is equal to the diameter of the aperture of the cavity receiver. Alternatively, the first monolithic ceramic material may be silicon nitride. Preferably, the duct has a rectangular cross-section. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  shows a solar receiver with a blackbody cavity absorbing solar radiation and heating air as it circulates around the cavity, according to one embodiment of the invention. 
         FIG. 2  shows a schematic drawing of the air passage integrated onto the surface of the receiver (dimensions are in millimeters), according to one embodiment of the invention. 
         FIG. 3  shows experimental data gathered on the silicon carbide cavity receiver with a theoretical prediction also shown for comparison. 
     
    
    
     DETAILED DESCRIPTION 
     In one aspect, a scalable and modular concentrated solar thermal dish-Brayton system is provided in response to growing demand for renewable energy and distributed power generation. One embodiment of the current invention reduces production costs and creates a viable small-scale solar power system for home or neighborhood use by achieving better conversion efficiency. 
     One embodiment of the invention includes a low cost, high efficiency solar receiver with the capability to achieve much higher operating temperatures than current receivers. The current embodiment of the invention uses a cylindrical shaped blackbody cavity to absorb solar radiation at a temperature of about 1700 K, much higher than existing receivers at 1250 K. This is made possible by using silicon carbide to manufacture the cavity. The fabricated cylindrical part boasts high absorption and thermal conductivity at a low fabrication cost. The advantage of the high absorption allows for the cylindrical design and a more compact absorber than current designs, which require much larger surface areas.  FIG. 1  shows a schematic drawing of one embodiment of the invention having an air passage entrance and exit as well as its path around the solar cavity receiver. 
     The advantage of the high thermal conductivity is for an integrated heat exchanger to heat a working fluid to a design temperature of 1500 K. The increased working fluid temperature will increase the conversion efficiency by 20% over comparable systems. The increased working temperature also enables better waste heat recovery by increasing the temperature of the Brayton Cycle exhaust. The integrated heat exchanger also achieves the desired fluid temperature with very little loss in pressure from entrance to exit. 
       FIG. 2  shows a detailed drawing of the integrated heat exchanger, according to one embodiment of the invention. The grooves embedded in the surface of the receiver create a square shaped duct for the air to travel through once the shell is covering the receiver. This duct design heats the air effectively without large losses in air pressure. 
     According to another embodiment of the invention, achieving fewer parts and a more compact package is done by integration with a power turbine. The final section of the heat exchanger is designed as a nozzle in the same way that the first stage stator would be for a gas turbine. This will allow the omission of the first stage stator making the overall design lighter, cheaper and less expensive. To design with fewer parts is always advantageous for manufacture, and is an ongoing challenge for designers in the gas turbine industry as well as Stirling engine manufacturers. 
     In one embodiment, the invention includes heating air to a high temperature. The application of this high temperature air can be made much broader than a Brayton power cycle. For instance, the air can be used to heat water for process applications, or to superheat steam to supplement fossil fuels in a coal fired power plant. 
     The heat exchanger is not limited to air by any means. The chemical stability of silicon carbide makes this absorber a candidate for thermochemical reactions for the solar production of synthetic fuels, or reformation of light hydrocarbons into heavier ones. Absorption chillers or water heaters may also take advantage of the waste heat from any of the aforementioned methods of heat utilization. 
     In a further embodiment, the invention can be scalable to significantly larger sizes, such as 2.5 kW to sizes as large as 50 kW for utility scale power production. 
     According to one embodiment of the invention, the high emissivity of silicon carbide allows this invention to be highly absorptive in a compact size and without any absorptive coatings. Silicon carbide is also operable to temperatures in excess of 2000 K, which is beyond the requirements of this system. The 1500 K fluid target temperature of this system will boast a 20% increase in efficiency over similar systems. 
     The completed research by the inventors, verifies that the absorption efficiency of the absorber is concurrent with prediction. The experimental results presented in  FIG. 3  show that the prediction for the maximum achievable temperature and the time response prediction of a scaled model are accurate. The temperature recorded is taken from a thermocouple attached to the outer surface at the back end of the cavity. 
     The invention provides commercialization of small-scale solar energy production for the end user. The solar receiver is designed to be reproducible for mass manufacture. The prominence of ceramic components in the system makes this invention of particular interest to established ceramics manufacturers. 
     Further details, variations and embodiments are described in the attached APPENDICES which are hereby incorporated to this application.