GB2087537A

GB2087537A - An energy transport device for collecting solar energy

Info

Publication number: GB2087537A
Application number: GB8127328A
Authority: GB
Original assignee: Owens Illinois Inc
Current assignee: OI Glass Inc
Priority date: 1980-09-11
Filing date: 1981-09-10
Publication date: 1982-05-26
Also published as: AU7442081A; JPS5780150A; IT8149245A0; IL63727A0; IT1142744B; FR2489943A1; DE3135080A1

Abstract

The device is energized by solar radiation and operated by differential emissivity to transfer its accumulated heat energy. The device comprises an enclosed housing 10 transparent to solar radiation and a thermally conductive member 11 supported in the housing 10. One longitudinal section 15 of the housing 10 is arranged to be exposed to solar radiation, while another longitudinal section 16 is adapted to be exposed to a heat-absorbing medium. A solar selective material 21 of a relatively low emissivity but relatively high absorptivity coats the thermally conductive material 11 that lies within the longitudinal section 15 of the housing 10 that is arranged to be exposed to solar radiation. A material 22 of relatively high emissivity coats the thermally conductive member 11 that lies within the longitudinal section 16 of the housing that is adapted to be exposed to a heat-absorbing medium. Various alternative coating materials are referred to. <IMAGE>

Description

SPECIFICATION An energy transport device and a process for producing same The present invention relates to an energy transport device and a process for manufacturing said device.

More particularly the present invention relates to an energy transport device that collects energy from solar radiation and, by means of differential emissivities within the same device, transports the accumulated heat energy to a heat-absorbing medium that is preferably also heat-transferring.

Realization that the fossil fuel supply of the world is finite and may be rapidly depleted at the present rate of national energy consumption has led to a search for substitute energy resources.

Use of solar radiation is one possibility for providing clean and reliable energy.

Solar energy is an extensive, constant energy source whose economic feasibility depends on efficient collection, retention, and utilization. The efficiency of some solar coliecting systems has been low due to excessive heat losses. One area in which improvement has been sought is in solar selective absorber coatings, that is, coatings which absorb energy particularly well in the solar spectrum. For example, such coatings are designed to collect thermal energy from exposure to solar radiation and then transmit the collected energy through other media either to heat or cool homes and buildings through heat exchangers.

In general, when radiant energy from the sun impinges on a cooler object, part of the energy is reflected and lost and the balance either absorbed or transmitted away. The absorbed energy may be re-radiated at a longer wavelength. Accordingly, a coating which absorbs in the range of solar radiation becomes heated, provided the surface does not re-radiate or emit most or all of the energy collected.

Solar radiation reaching the surface of the earth is aimost entirely confined to the range of 0.3 to 2.5 microns. It is estimated that about 90% of solar radiation is at wavelengths of about 0.4 micron to about 1.5 microns. The amount of radiation above 2.5 microns is negligible. Solar energy selective coatings, therefore, are designed to differentiate in their absorption, refiection or transmission characteristics between wavelengths above about 2.5 microns and wavelengths below about 2.5 microns. Thus, solar energy can be collected at wavelengths below about 2.5 microns and the collected energy then transferred to useful application at wavelengths above about 2.5 microns.

This also means that for effective collection and retention, a solar collector should absorb strongly at wavelengths below about 2.5 microns and not radiate at wavelengths greater than 2.5 microns. A coating which has a high absorptivity, usually termed alpha, in the solar spectrum but a low emissivity, epsilon, at the temperature at which the collector operates may be called a solar selective coating. Even though a high alpha to epsilon ratio is desirable, it is essential that the alpha value be near one to collect as much of the available energy as possible. Solar selective coatings are one important way to increase the efficiency of solar energy collectors, primarily by maximizing the absorption of solar energy and minimizing the energy lost by radiation.

An aim of the present invention is to provide an energy transport device suitable for use as a solar energy collector, using solar collective and emissive coatings and their differential emissivites for its operation.

According to the present invention there is provided an energy transport device comprising a thermally conductive member having sections of differential emissivity, one section having a relatively low emissivity but high absorptivity and adapted to be exposed to solar radiation, and another section having a higher emissivity than said one section and adapted to be exposed to a heat-absorbing medium.

Preferably the present invention is constructed as an integral energy transport device, such as an evacuated tubular solar collector, having one longitudinal section that is solar selective and absorptive and adapted to be exposed to solar energy, and a companion longitudinal section of the same device that is relatively more emissive and adapted to be exposed to a heat-absorbing medium. The two sections of the transport device are coated with materials effective to obtain the results desired.

According to a further aspect of the present invention there is provided a solar energy collector comprising an enclosed housing transparent to solar radiation, a thermally conductive member supported within the housing, one longitudinal section of said housing being adapted to being exposed to solar radiation and another longitudinal section being adapted to be exposed to a heat-absorbing medium, a solar selective coating of relatively low emissivity but relatively high aborptivity being provided on said thermally conductive member within that longitudinal section of the housing that is adapted to be exposed to solar radiation, and a coating of relatively high emissivity being provided on said thermally conductive member within that longitudinal section of the housing that is adapted to be exposed to a heat-absorbing medium.

A preferred embodiment of the further aspect of the present invention comprises an enclosed envelope, the interior of which is maintained at a subatmospheric pressure. One longitudinal section of the envelope is adapted to be exposed to solar radiation, and another longitudinal section is adapted to be exposed to a heat-absorbing medium. A thermally conductive member is resiliently supported within the envelope and has a coating of relatively low emissivity and high solar absorptivity within that section of the envelope adapted to be exposed to solar radiation, and a coating of a relatively high emissivity that is within that section of the envelope adapted to be exposed to a heat-absorbing medium that is preferably also heat-transferring. The coatings may be formed on the thermally conductive member by various means such as vacuum deposition.

In operation, upon exposure to solar radiation, that section of the thermally conductive member having the solar selective and absorbing coating becomes hotter due to its high absorptivity and low emittance. The collected heat energy is passed by conductance through the thermally conductive member to its companion section having a much higher emissivity from which the collected heat energy is radiated.

According to a further feature of the present invention there is provided a process for forming an energy transport device having differential emissivities, comprising coating one section of a thermally conductive member with a material having a relatively low emissivity and a relatively high solar absorptivity, coating another section of said thermally conductive member with a material having a relatively high emissivity, and placing said thermally conductive member within an enclosed housing that is transparent to solar radiation, said one section of the thermally conductive member and corresponding section of said housing defining a zone adapted to be exposed to solar radiation, and said another section of the thermally conductive member and corresponding section of the said housing defining another zone adapted to be exposed to a heatabsorbing medium.

The present invention will now be further described, by way of example, with reference to the accompanying drawings, in which: Figure 1 is a longitudinal, cross-section of a preferred embodiment of a solar energy collector constructed according to the present invention; and Figures 2 and 3 are cross-section of Figure 1 on the lines 2-2 and 3-3, respectively, An energy transport device according to the present invention, in the form of a solar energy collector is shown in the accompanying drawings, the construction of the solar energy collector being described herein followed by a description of the operation of the collector upon exposure of solar radiation. One example of a specific solar energy collector is given.

The illustrated solar collector relies on differential emissivity of two parts or sections of a single energy transport unit which perform opposite roles. One section is designed to absorb heat energy upon exposure to solar radiation while possessing relatively low emissivity. This section conducts its heat energy to another section of the collector which has a relatively high emissivity.

Because of the differences in emissivities, the second section is capable of readily emitting or radiating the heat energy it receives, to a desired receptor.

The illustrated preferred embodiment of solar energy collector of the present invention is tubular, however, it will be understood that other geometric shapes can alternatively be used within the scope of the present invention.

Referring to the drawings, the illustrated embodiment comprises an outer envelope or housing 10 that is transparent or transmissive to solar radiation. Ideally, housing 10 transmits all of the solar radiation to which it is exposed.

Transmissivity as used here a-nd in the claims may be considered to be a numeric that is the ratio of the energy which is transmitted by a particular object to the total energy available for transmission. However, it is normally impossible to achieve a transmissivity value of 100 since there are always some losses, such as those due to reflection. Glass is a good transmitter of solar energy, and therefore housing 10 is preferably fabricated from glass. The composition of the glass is not critical and may comprise a lime-silica or borosilicate glass.

A thermally conductive member 11 is suitably supported within housing 10. Any material that is a good conductor of heat may be used in constructing this member. Preferably the thermally conductive member 11 is made from a metal, metals being good heat conductors. Any common metal may be used, such as copper, iron, zinc, molybdenum, compatible alloys of metals, and the like. Gold and silver are also excellent thermal conductors, but their high costs make their use prohibitive.

The thermally conductive member 11 may also be of any shape as long as it extends into both the absorptivity section and the emissivity section of the solar collection as hereinafter described.

Accordingly, the thermally conductive member 11 may be a flat strip, a curved strip, several strips joined edgewise together to form a hollow member of polygonal cross-section, and the like.

The thermally conductive member 11 desirably has as large a surface area as possible, and therefore a tube or other hollow member of circular cross-section is preferred. In the illustrated embodiment, a tube made from metal forms the thermally conductive member 11 and extends from one end of housing 10 to the other, stopping short of the very ends of housing 10.

Any suitable means may be used to mount tube 11 within housing 1 0. Preferably, the mounting means permits expansion and contraction of tube 11 relative to the housing, since the metallic tube 11 will expand and contract at different rates as compared to glass housing 10 during heating and cooling of the collector. In Figure 1, a resilient mounting generally represented at 12 fits within each end of housing 10 and resiliently grips an adjacent end of tube 11. Each mounting 12 includes a circular end plate 13 having a plurality of fingers 14 extending from end plate 1 2 longitudinally of the collector between housing 10 and tube 11 to define a spider-like construction which embraces an end of tube 11. Fingers 14 are corrugated as shown to impart resiliency and to grip tube 11 loosely to accommodate its contraction and expansion.

Although not critical, it is advantageous to coat the resilient mountings 1 2 with what is termed in the art as a "getter", such as a barium alloy. Upon evaporation within housing 10 which is normally under a subatmospheric pressure, the getter coating vaporizes and scavenges residual gases contained within the housing.

Housing 10 is divided into an absorbtivity section represented in Figure 1 by the bracketed section 1 5 and an emissivity section represented by the bracketed section 1 6. The relative lengths of sections 1 5 and 1 6 are not critical and may be varied to accommodate anticipated on-site working conditions. As a rule, a larger surface area is desired for absorbing solar radiation than for and therefore absorptivity section 1 5 is normally longer than emissivity section 1 6 when the outside diameters of these section are uniform throughout their lengths.As indicated, either section 1 5 or 1 6 may be radially enlarged, if desired, with respect to the other, depending upon the total absorptivity or emissivity desired in those sections.

Section 1 5 is adapted to be exposed to solar radiation and section 1 6 is adapted to be exposed to a heat-absorbing medium. In practice, section 1 6 may be immersed in a heat-absorbing enclosure partially illustrated at 1 7 which contains a heat-absorbing medium 18, such as a gas or a liquid. He.at-absorbing medium 1 8 transports the heat energy received from section 1 6 as in a direction indicated by arrows 20 through enclosure 1 7 to a useful site, as is known in the art, such as to a heat exchanger.For example, heat-absorbing enclosure 1 7 may define a manifold within which a number of solar energy collectors like the collector of Figure 1 are immersed. The use of a manifold per se with a different type of tubular solar collector is disclosed in U.S. Patent Specification No. 4 016 860 to Moan.

In order for sections 1 5 and 1 6 to perform their respective functions, tube 11 has a coating 21 of high absorptivity and relatively low emission in section 1 5 of the solar collector and a coating 22 of relatively high emissivity in section 1 6. These coatings may be applied onto tube 11 by any known means such as electro-plating, vacuum deposition, sputtering, the use of an electron beam, thermal evaporation, and the like. For purposes of illustration, coatings 21 and 22 have been illustrated in Figure 1 as being slightly spaced apart. However, the coatings can abut one another edgewise or slightly overlap if desired.

Coating 21 is preferably a semiconductive material that absorbs strongly radiation in the solar spectrum, and that is essentially transparent in the infrared spectrum. As used here and in the claims, the terms "semiconductor" and "semiconductive" are taken to means a material as defined by the "American Institute of Physics Handbook", second edition, 1963, page 9-31, namely, a material in which the highest occupied energy bond (valence bond) is completely filled at absolute zero; and in which the energy gap between the valence bond and the next higher bond (conduction bond) is of the order of 0.4 to 5 electron volts.

In general, incompletely oxidized oxides of the transitional metal elements provide the best results and are therefore preferred as the semiconductor materials. Specific semiconductor materials useful for coating 21 include black chrome, black nickel, black platinum, black molybdenum, black copper, black iron, black cobalt, black manganese, and compatible alloys thereof. Black chrome is a mixture of the oxides of chromium and is designated in the art as Crow.

Similarly, black nickel is a mixture of the oxides of nickel. Black platinum, black molybdenum, black copper, black iron, black cobalt and black manganese are oxides of these metals.

However, the semiconductor material can be other than metal oxides. For example, carbides of the same and other metals having semiconductor properties may be used, such as copper carbide, hafnium carbide, nickel carbide, molybdenum carbide, and the like. Also, sulfides of the same and other metals having semiconductor properties may be similarly used, such as silver sulfide, iron suifide, manganese sulfide, copper sulfide and the like. Still further, elemental metals like silicon and gemanium can be used as the semi-conductor material.

Absorptivity can be expressed by the equation: A = 1 -R, in which A represents absorptivity and R represents reflectivity. Thus, absorptivity is a numeric although it is sometimes also expressed as a percentage such as 70%, meaning that 70% of the energy to which a material is exposed is absorbed. Coating 21 normally has an absorptivity within the range of about 70% to about 95%.

Emissive coating 22 may comprise any material providing relatively high emissivity in the infrared spectrum. Among useful materials for this purpose are powdered glass, silica, lampblack, and graphite. The emissivity of a material is a measure of energy radiated from it. Coating 22 may have an emissivity within the range of about 80% to about 96%.

The thicknesses of solar collective coating 21 and emissive coating 22 are not critical. As a rule, the thickness of each coating may range from about 0.05 to about 5 mils.

To prepare a solar collector of the type illustrated by the figures, solar selective coating 21 and emissive coating 22 are deposited, in turn, on tube 11 by standard known techniques, such as vacuum evaporation, electric resistance heating of coating material, the use of an electron beam, sputtering, and the like, followed by condensation on tube 11 to produce the desired coating. For example, a small reservoir of the coating material may be heated in vacuum by electrical resistance circuits to evaporate the material and deposit it on tube 11.The resilient mountings 12, which are preferably metallic, are then fitted about tube 11 and the assembly placed within housing 1 0. The housing is evacuated by standard means to a subatmospheric pressure, such as to 10-4 to 10-6 Torr, and glass housing 10 then sealed off in a common weld 21 of the sides of the housing in a known manner.

The collector harnesses solar energy in the following manner. Solar radiation, represented by arrows 23 in Figure 1, passes through housing 10 and the interstitial space between housing 10 and tube 1 The solar flux, diminished by the amount absorbed and reflected by housing 10, strikes coating 21 in section 1 5 and heats it due to the high absorptivity and low emissivity of the coating.

The resulting heat energy is conducted through metal tube 11 to section 1 6 that lies within enclosure 1 7 where it passes to emissive coating 22. Being highly emissive, coating 22 radiates the heat energy it receives outwardly to that portion of housing 10 in the immediate vicinity. Heatabsorbing medium 1 8 which surrounds housing 10 is therefore heated both by conduction and convection and may be utilized in any known desired manner, such as by passing through heat exchangers to heat or cool the interior of a home.

The following example only illustrates the invention and should not be construed as imposing limitations on the claims.

WORKING EXAMPLE A metal tube measuring about 12 inches in length and about two inches in outside diameter was coated with black chrome by reactive evaporation of chromium. By this technique, pure chromium is evaporated but reacts in transit to the tube to form the oxide. This coating extended for about eight inches along the tube at one end. The rest of the tube was spray coated with an ultra flat black enamel comprising 50% carbon black and 50% alkali silicates by weight as a binder. Each coating was about 0.05 to about 0.25 mil in thickness.

An end support clip of the type illustrated in Figure 1 was placed around each end of the metal tube and the resulting assembly placed in a tubular glass housing. The housing was closed at one end and has an internal diameter of a size resiliently to receive and hold the tube and its end clips. The glass of the housing has a transmissivity of at least 90% or more. The tube was then sealed in the glass housing except for an evacuating port, vacuum backed for 1 6 hours at 7500F while the housing was being evacuated, and finally tipped off at the evacuating port to form a solar collector.

The collector was then ready for installation in a manifold.

The solar energy collector of the present invention is of relatively low cost and requires no central, longitudinal feeder tube to pass a heatabsorbing medium actually through the collector.

Since the collector contains no fluid at all, it need never be drained like other tubular solar collectors in use. Either a gas or liquid may be used as the heat-transfer medium.

Although the foregoing describes several embodiments of the present invention, it is understood that the invention may be practiced in still other forms within the scope of the following

Claims

claims. CLAIMS

1. An energy transport device comprising a thermally conductive member having sections of differential emissivity, one section having a relatively low emissivity but high absorptivity and adapted to be exposed to solar radiation, and another section having a higher emissivity than said one section and adapted to be exposed to a heat-absorbing medium.

2. A solar energy collector comprising an enclosed housing transparent to solar radiation, a thermally conductive member supported within the housing, one longitudinal section of said housing being adapted to being exposed to solar radiation, and another longitudinal section being adapted to be exposed to a heat-absorbing medium, a solar selective coating of relatively low emissivity but relatively high absorptivity being provided on said thermally conductive member within that longitudinal section of the housing that is adapted to be exposed to solar radiation, and a coating of relatively high emissivity being provided on said thermally conductive member within that longitudinal section of the housing that is adapted to be exposed to a heat-absorbing medium.

3. A solar energy collector as claimed in claim 2, in which the interior of said enclosed housing is at subatmospheric pressure.

4. A solar energy collector as claimed in claim 2 or 3, in which said enclosed housing is made of glass.

5. A solar energy collector as claimed in any one of claims 2 to 4, in which said thermally conductive member is metallic.

6. A solar energy collector as claimed in any one of claims 2 to 5, in which said thermally conductive member is tubular.

7. A solar energy collector as claimed in any one of claims 2 to 6, in which said coating of relatively low emissivity but relatively high absorptivity, is a semiconductive material.

8. A solar energy collector as claimed in any one of claims 2 to 6, in which said coating of relatively low emissivity but relatively high absorptivity, is an incompletely oxidized oxide of a transitional metal element.

9. A solar energy collector as claimed in claim 2, in which said coating of relatively low emissivity but relatively high absorptivity is selected from the group consisting of black chrome, black nickel, black platinum, black molybdenum, black copper, black iron, black cobalt, black manganese, molybdenum carbide, copper sulfide, copper carbide, hafnium carbide, nickel carbide, and compatible alloys thereof.

10. A solar energy collector as claimed in any one of claims 2 to 9, in which said coating of relatively high emissivity is selected from the group consisting of powdered glass, silica, lampblack, and graphite.

1 A solar energy collector as claimed in any one of claims 2 to 10, including resilient means to support said thermally conductive member with respect to said enclosed housing.

12. A solar energy collector as claimed in any one of claims 2 to 11, in which said heatabsorbing medium is part of a heat transport system.

13. A solar energy collector as claimed in any one of claims 2 to 12, in which each of said coatings of relatively low and relatively high emissivity, has a thickness of from about 0.05 to about 5 mils.

14. A solar energy collector of differential emissivities comprising: (a) an enclosed, solar transparent glass envelope having a subatmospheric pressure, one longitudinal section of said envelope being adapted to be exposed to solar radiation and another longitudinal section being adapted to be exposed to a heat-absorbing and heat-transferring medium, (b) a metallic tube supported adjacent its end within said envelope, (c) a coating of relatively low emissivity and high solar absorptivity on said metallic tube that is within said section of the envelope adapted to be exposed to solar radiation, and (d) a coating of a relatively high emissivity said metallic tube that is within said section of the envelope adapted to be exposed to a heatabsorbing and heat-transferring medium.

1 5. A process for forming an energy transport device having differential emissivities, comprising coating one section of a thermally conductive member with a material having a relatively low emissivity and a relatively high solar absorptivity, coating another section of said thermally conductive member with a material having a relatively high emissivity, and placing said thermally conductive member within an enclosed housing that is transparent to solar radiation, said one section of the thermally conductive member and corresponding section of said housing defining a zone adapted to be exposed to solar radiation, and said another section of the thermally conductive member and corresponding section of the said housing defining another zone adapted to be exposed to a heat-absorbing medium.

1 6. A process as claimed in claim 15, including evacuating said housing to a subatmospheric pressure.

17. A process as claimed in claim 15, in which said housing is glass.

18. A process as claimed in claim 15, in which said thermally conductive member is metallic.

1 9. A process as claimed in claim 15, in which said material having a relatively low emissivity and relatively high absorptivity, is selected from the group consisting of black chrome, black nickel, black platinum, black molybdenum, black copper, black iron, black cobalt, black manganese, molybdenum carbide, copper sulfide, copper carbide, hafnium carbide, nickel carbide, and compatible alloys thereof.

20. A process as claimed in claim 15, in which said material having a relatively high emissivity is selected from the group consisting of powdered glass, silica, lampblack, and graphite.

21. A solar energy collector constructed and arranged substantially as hereinbefore described with reference to and as illustrated in the accompanying drawings.

22. A process for producing an energy transport device having differential emissivities, substantially as hereinbefore described with reference to the accompanying drawings.