Glass heat pipe evacuated tube solar collector

• US 4,474,170 A
• Filed: 08/06/1981
• Issued: 10/02/1984
• Est. Priority Date: 08/06/1981
• Status: Expired due to Fees

• Current Assignee: United States Department of Energy
• Original Assignee: United States Department of Energy
• Inventors: McConnell, Robert D., Vansant, James H.
• Primary Examiner(s): Scott, Samuel
• Assistant Examiner(s): Kamen, Noah
• Application Number:
  US06/290,282
• Time in Patent Office: 1,153 Days
• Field of Search: 126/433, 126/901, 126/417, 165/104.26, 427/230, 427/234, 427/236, 427/181, 118/317
• US Class Current: 126/636
• CPC Class Codes: F24S 10/45 the enclosure being cylindrical
• CPC Class Codes: F24S 10/95 having evaporator sections and condenser sections, e.g. heat pipes
• CPC Class Codes: F24S 70/225 for spectrally selective absorption
• CPC Class Codes: F24S 80/30 Arrangements for connecting the fluid circuits of solar collectors with each other or with other components, e.g. pipe connections; Fluid distributing means, e.g. headers
• CPC Class Codes: F28D 15/046 characterised by the material or the construction of the capillary structure
• CPC Class Codes: Y02E 10/44 Heat exchange systems
• CPC Class Codes: Y10S 126/907 Absorber coating

Abstract

A glass heat pipe is adapted for use as a solar energy absorber in an evacuated tube solar collector and for transferring the absorbed solar energy to a working fluid medium or heat sink for storage or practical use. A capillary wick is formed of granular glass particles fused together by heat on the inside surface of the heat pipe with a water glass binder solution to enhance capillary drive distribution of the thermal transfer fluid in the heat pipe throughout the entire inside surface of the evaporator portion of the heat pipe. Selective coatings are used on the heat pipe surface to maximize solar absorption and minimize energy radiation, and the glass wick can alternatively be fabricated with granular particles of black glass or obsidian.

Claims

1. Solar apparatus comprising:

a heat pipe absorber for absorbing incident solar radiation and for transferring thermal energy derived therefrom to a working fluid, said heat pipe absorber including an elongated glass tube with a hollow interior sealed from the exterior environment, a first portion of said glass tube being transparent and adapted for exposure to said solar radiation and a second portion of said glass tube being adapted for exposure to the working fluid;

a heat transfer fluid in said transparent tube for transferring said thermal energy from said first portion to said working fluid, said heat transfer fluid being a volatile liquid adapted to evaporate readily upon conducting said thermal energy and to condense to a liquid when in the environment of the working fluid;

selective coating means on the outside surface of said first portion, said selective coating means being adapted to maximize said absorption of said solar radiation on said outside surface of said first portion and to minimize the emission of said absorbed thermal energy;

transparent envelope means surrounding said first portion in space relation thereto, the space between said first portion and said envelope being evacuated and hermetically sealed from the exterior environment; and

porous wick means for uniformly distributing said heat transfer fluid throughout said inside surface of said first portion, said wick means comprising a layer of glass particles and a layer of sodium silicate derived from a water glass solution previously heated to remove said water for bonding said glass particles to said inside surface of said first portion, said layer of glass particles being in intimate and uniform contact with said inside surface of said first portion, said layer of glass particles being fused together and being permanently fused to said inside surface of said first portion through said layer of sodium silicate, said glass particles being previously heated in a manner to cause said glass particles to be said fused together and be fused to said inside surface of said first portion through said layer of sodium silicate.

2. The solar collector apparatus of claim 1, wherein said glass particles are of sizes in the range of about 470 to 270 microns in diameter.

3. The solar collector apparatus of claim 2, wherein said glass particles are transparent, and said selective coating means is a dark colored film on the outside surface of said first portion, said film being adapted to absorb solar radiation and to emit only a minimum of infrared radiation.

4. Heat pipe apparatus comprising:

a transparent envelope means;

an elongated glass tube within said envelope means, said glass tube having a hollow interior sealed from the exterior environment, said glass tube including inner and outer opposed surface portions, said outer portion of said glass tube being adapted for exposure to a source of thermal energy and for absorbing said thermal energy from said source, and said inner portion of said glass tube being adapted for exposure to a heat sink cooler than said source of thermal enery and for transferring said thermal energy to said heat sink through thermal conduction;

a heat transfer fluid in said glass tube which is volatile and adapted to vaporize upon conducting said thermal energy and to condense to the heat sink;

a porous wick means on the inside surface of said glass tube for enhancing capillary drive distribution of said heat transfer fluid over the inside surface of said glass tube, said wick means including a layer of granulated glass particles, said layer of glass particles being in intimate and uniform contact with said inside surface of said glass tube, said glass particles being previously heated in a manner to bond said glass particles together and to cause said bonded glass particles to be permanently adhered to the inside surface of said glass tube; and

a film means for uniformly and permanently said adhering the bonded glass particles to said inside surface of said glass tube, said film means comprising a sodium silicate residue derived from a previously heated water glass solution, said film means enabling said thermal energy absorbed at said outer portion to be uniformly conducted therethrough to said wick means.

5. The heat pipe apparatus of claim 4, wherein said glass particles are of sizes in the range of about 470 to 270 microns in diameter.

7. The heat pipe apparatus of claim 5, wherein said solution includes approximately 25% sodium silicate and 75% water by weight.

6. In an evacuated tube insulated solar collector apparatus including solar energy absorber means surrounded by an evacuated, hermetically sealed transparent envelope in spaced-apart distance thereto, the improvement comprising;

a solar energy absorber in the form of an elongated glass heat pipe, a first portion of which is positioned in said envelope and adapted to absorb solar radiation and a second portion of which extends outside said envelope and is adapted for exposure to a heat sink working fluid;

a heat transfer fluid in said glass heat pipe for transferring said thermal energy said absorbed by said first portion to said heat sinking fluid, said heat transfer fluid being a volatile liquid adapted to evaporate readily upon conduction of said thermal energy and to condense to a liquid when in the environment of said heat sinking fluid;

selective coating means outside said first portion of said heat pipe, said selective coating means being adapted for maximizing said absorption of solar radiation by said first portion of said heat pipe and minimizing emission of said absorbed thermal energy;

a porous wick means on the inside surface of said first portion for enhancing capillary drive distribution of said heat transfer fluid over the inside surface of said first portion, said wick means including a layer of granulated glass particles, said layer of glass particles being in intimate and uniform contact with said inside surface of said first portion, said glass particles being previously heated in a manner to bond said glass particles together and to cause said bonded glass particles to be permanently adhered to the inside surface of said glass tube; and

a film means for enabling said wick means to be said intimately and uniformly adhered to said inside surface of said first portion, said film means comprising a sodium silicate residue derived from a previously heated water glass solution, said film means further enabling said thermal energy to be uniformly conducted from said selective coating of said heat transfer fluid.

8. A method of fabricating a capillary wick on an inside surface of a glass heat pipe, comprising the steps of:

preparing a thermally conducting bonding medium consisting of a water glass solution of a sufficient viscosity to maintain a film on said inside surface of said glass heat pipe and to allow said capillary wick to uniformly and permanently adhere to said inside surface after depositing said wick thereon;

depositing said glass solution on said inside surface of said glass heat pipe while simultaneously rotating said glass heat pipe so as to form said film and to enable said solution to be evenly distributed on said inside surface;

rotating a conveying means in a first direction while simultaneously conveying a glass powder on said film formed on said inside surface of said glass pipe to effect forming said wick;

rotating said glass pipe in a second opposite direction while said glass powder is being conveyed on said inside surface while forming said wick;

adapting said film and said wick to conduct thermal energy from an outside surface of said glass heat pipe by fusing said glass powder together and to said inside surface with said film such that said fused glass powder has sufficient porosity and capillary characteristics for glass heat pipe applications;

firing said heat pipe to accomplish said fusing of said glass powder together and to said inside surface through said film;

cooling said heat pipe to normal temperature; and

deposing an absorption-type selective coating on an exterior surface of said glass heat pipe for maximizing an absorption of incident sun radiation.

9. The method of claim 8, including the step of firing the heat pipe by slowly raising the temperature to approximately 300°

C. and maintaining that temperature for about three hours.

10. The method of claim 8, wherein said water glass solution comprises approximately 25% sodium silicate and 75% water by weight.

11. The method of claim 8, wherein said glass powder comprises granulated glass particles sized in a range of from about 470 to about 270 microns in diameter.

12. A method of fabricating a glass heat pipe solar collector, comprising the steps of:

placing an elongated glass heat pipe evaporator tube having one closed end and an absorption-type solar selective coating on its exterior surface concentrically inside a large diameter glass enclosure tube leaving a space around said evaporator tube inside said enclosure tube and leaving the open end of said evaporator tube extending outwardly from said enclosed tube;

swaging the one end of said enclosure tube adjacent to said open end of said evaporator tube and fusing said one end of said enclosure to an exterior of said evaporator tube to form a vacuum-type seal;

evacuating the space inside the enclosure tube and sealing its outer end in a vacuum tight seal to maintain a vacuum around said evaporator tube;

preparing a thermally conducting binding medium consisting of a water glass solution of a sufficient viscosity to maintain a film on said inside surface of said glass heat pipe and to allow a capillary wick defined by the glass heat pipe to uniformly and permanently adhere to said inside surface upon depositing said film and aid wick thereon;

depositing said said solution on said inside surface of said glass heat pipe while simultaneously rotating said glass heat pipe so as to enable said solution to be evenly distributed in a uniform film throughout said inside surface;

rotating a conveying means in a first direction while conveying glass particles on said surface of said glass pipe with said conveying means, while simultaneously rotating said glass tube in a second opposite direction so as to effect uniformly forming said wick on said inside surface;

adapting said film and said wick to thermally conduct heat energy from said selective coating by fusing said glass particles together and to said inside surface with said film such that said fused glass powder has sufficient porosity and capillary characteristics for glass heat pipe solar collector applications;

firing the evaporator and vacuum envelope assembly to accomplish fusing said glass powder together and to said inside surface of said heat pipe through said film;

fusing a cap member having an opening in an end thereof over the open end of the evaporator tube to form a condenser portion and to complete the structure of the heat pipe comprised of the evaporator portion inside the evacuated envelope of the condenser portion outside the evacuated envelope;

placing a thermal transfer fluid into the evaporator portion through the opening in the cap member; and

evacuating the inside of the heat pipe and sealing the opening in the cap member to maintain said transfer fluid in the heat pipe.

13. The method of claim 12, wherein said water glass solution consists of approximately 25% sodium silicate and 75% water by weight.

14. The method of claim 12, wherein said glass particles are sized in a range of from about 470 to about 270 microns in diameter.

15. The method of claim 12, wherein said binding medium is a viscous slurry of fine powdered glass mixed and suspended in an inert carrier medium.

16. Solar collector apparatus comprising:

a heat pipe absorber for absorbing incident solar radiation and for transferring thermal energy derived therefrom to a working fluid, said heat pipe absorber including an elongated glass tube with a hollow interior sealed from the exterior environment, a first portion of said glass tube being transparent and adapted for exposure to said solar radiation and a second portion of said glass tube being adapted for exposure to the working fluid;

a heat transfer fluid in said transparent tube for transferring said thermal energy from said first portion to said working fluid upon conducting said thermal energy;

selective coating means on the outside surface of said first portion, said selective coating means being adapted to maximize said absorption of said solar radiation on said outside surface of said first portion; and

a porous wick means for uniformly distributing said heat transfer fluid throughout said inside surface of said first portion, said wick means comprising a layer of glass particles and a layer of sodium silicate derived from a water glass solution previously heated to remove said water for bonding said glass particles to said inside surface of said first portion, said layer of glass particles being in intimate and uniform contact with said inside surface of said first portion, said layer of glass particles being fused together and being permanently fused to said inside surface of said first portion through said layer of sodium silicate, said glass particles being previously heated in a manner to cause said glass particles to be said fused together and be fused to said inside surface of said first portion through said layer of sodium silicate.