Evacuated glass tube solar thermal collector

FIELD OF THE INVENTION

This invention relates to an evacuated glass tube solar thermal collector.

BACKGROUND OF THE INVENTION

Solar thermal collectors are generally concentrating or non-concentrating types. Concentrating solar thermal collectors include a tracking mechanism for tracking the position of the sun, whereas non- concentrating solar thermal collectors are generally fixed in position. Concentrating solar thermal collectors are generally used for medium and high temperature applications, while non-concentrating ones are used for low temperature applications. Concentrating solar thermal collectors are complicated and complex in construction and are expensive. Besides, they also require heavy maintenance. On the other hand, non-concentrating solar thermal collectors are simple in construction and are cost effective. Maintenance of non-concentrating solar thermal collectors is also low. Flat plate collectors (FPCs) and evacuated glass tube solar thermal collectors (EGTSCs) are typical examples of non- concentrating solar thermal collectors.

An evacuated glass tube solar thermal collector comprises a tubular manifold describing a flow passage therein for a heat receiving fluid to flow therethrough and get heated up by the heat from the solar radiation. The solar thermal collector also comprises a plurality of evacuated glass tubes with heat pipes arranged along the length of the manifold perpendicular thereto. Each of the evacuated glass tubes comprises a single walled or double walled glass tube (tube-in-tube glass tube). In the case of an evacuated single walled glass tube, the heat pipe is preferably located at the center of the tube in spaced apart relationship with the inner wall of the tube. A double walled evacuated glass tube comprises an inner absorbent tube and an outer protective tube disposed concentrically with each other with the space between the inner and outer tubes being evacuated. In the case of the double walled evacuated glass tube, the heat pipe is preferably located within the absorber tube at the center thereof in spaced apart relationship with the inner wall of the inner tube. The outer surface of the inner tube is optionally coated with a selective absorbent layer.

A typical heat pipe comprises a good thermal conductor material pipe containing a working fluid and defining an evaporator section and a condenser section. The condenser section protrudes out of the glass tube and communicates with the flow passage in the manifold. The heat pipe may comprise a wick located therein. Conductive metal connector blocks like copper blocks or conductive metal female adapters are used to support and connect the condenser sections of the heat pipes to the manifold. The working fluid in the evaporator sections of the heat pipes absorbs the heat of the solar radiation falling on the evacuated glass tubes and gets heated up and evaporates. The working fluid vapours travel to the condenser sections of the heat pipes and deliver the heat to the hear receiving fluid flowing through the manifold and get condensed. The condensate returns to the evaporator sections of the heat pipes under gravity or due to wick action, as the case may be and the cycle of evaporation and condensation continues. (http://www.carbonsmart.us/pdf/Apricus components.pdf, last accessed on 9/12/2010; http://www.freefuelforever.com/Scripts/gravity system.php, last accessed on 13/12/2010; http:///www.solarcollectorinc.c0m admin main upload 2010092109560149005.pdf. last accessed on 12/12/2010; and http://www.sundasolar.com/product seido 10 series collector, html, last accessed on 9/12/2010; US 4653471 ; US 7412976 B2 and EP 1203915 A2). . In order to increase the amount of solar radiation falling on the evacuated glass tubes, the solar collectors are provided with reflecting surfaces like flat diffuse reflectors or parabolic reflectors.

In an evacuated glass tube solar thermal collector, each of the heat pipes is separately evacuated and charged with the working fluid and sealed. Cost of manufacture of the solar collector is increased because of the manufacture of the heat pipes individually and the techniques used for the manufacture thereof. As the condenser sections of the heat pipes are contacted with the manifold with the help of conductive metal connector blocks or female adapters, additional thermal resistance is introduced at the contact between the condenser sections of the heat pipes and the manifold. Because of this, the heat transfer efficiency of the solar collectors is reduced. Further, the size, weight and cost of the manifold increases as the condenser sections of heat pipes are accommodated in the manifold. The response time of the solar collector is also increased due to increased thermal mass and additional thermal resistance at the contact between the condenser sections of the heat pipes and the manifold. Because of the connector blocks or female adapters, the flow passage within the manifold is also non- uniform and has stagnation zones. Therefore, the flow of the heat receiving fluid within the manifold is not smooth and uniform. Since the flow passage within the manifold is not uniform and has stagnation zones, it is also difficult to clean and service the manifold. As the manifold size increases, a large area of the manifold is exposed to atmosphere. In order to minimize heat loss the manifold requires heavy insulation.

In order to improve, the heat transfer from the evacuated glass tube to the heat pipe a heat transfer element of good thermal conductivity like copper or aluminum is known to be located in the glass tube over the evaporator section of the heat pipe. The heat transfer element generally comprises one or two fins which provide a heat path between the evacuated glass tube and the heat pipe. As the number of fins is limited, the fin length increases and the efficiency of the heat transfer element is reduced. As a result, either the collector efficiency is reduced or the weight of thermal mass and cost of the solar collector is increased.

SUMMARY OF THE INVENTION

According to the invention there is provided an evacuated glass tube solar thermal collector comprising a tubular manifold having at least one good thermal conductor material heat receiving fluid flow pipe located therein in spaced apart relationship with the inner wall thereof and extending along the length thereof and defining a common condensation zone therein along the length thereof in the space surrounding the heat receiving fluid flow pipe, the tubular manifold further having a resealable port, a plurality of evacuated glass tubes arranged along the length of the tubular manifold perpendicular thereto and mounted to the tubular manifold at one ends thereof packed with breathable thermal insulation, the evacuated glass tubes being mounted to a protective support at the other ends thereof, each of the evacuated glass tubes having a thermal conductor material evaporator pipe located therein preferably at the center thereof along the length thereof in spaced apart relationship with the inner wall of the evacuated glass tube, the evaporator pipes containing a working fluid and communicating with the common condensation zone in the tubular manifold and a cover for the tubular manifold. BRIEF DESCRIPTION OF THE ACCOMPANYING DRAWINGS

In the accompanying schematic drawings:

Fig 1 is an isometric view of the evacuated glass tube solar thermal collector according to an embodiment of the invention; Fig 2 is an enlarged view at A in Fig 1 ;

Fig 3 is a sectional view of the evacuated glass tube solar thermal collector of Fig 1 without the spacer;

Fig 4 is a sectional view of the tubular manifold of the solar thermal collector of Fig 1 with the spacer;

Fig 5 is a sectional view of a double walled evacuated glass tube of the solar thermal collector of Fig 1 ;

Fig 6 is a sectional view of a single walled evacuated glass tube of the. solar thermal collector according to another embodiment of the invention including the evaporator pipe; Figs 7 and 8 are different embodiments of the tubular manifold of the solar thermal collector of the invention;

Figs 9 and 10 are different fluted versions of the heat receiving fluid flow pipe of the solar thermal collector of the invention;

Fig 1 1 is a twisted helical construction of the heat receiving fluid flow pipe of the solar thermal collector of the invention;

Fig 12 is a view of two interconnected solar thermal collectors according to another embodiment of the invention; Fig 13 is a sectional view of a double walled evacuated glass tube of the solar thermal collector of Fig 1 according to another embodiment of the invention; Fig 14 is an elevation of the heat transfer element of the solar thermal collector of Fig 13;

Fig 15 is a sectional view of the heat transfer element of Fig 14;

Fig 16 is a view of the solar thermal collector of the invention disposed in a reflector according to another embodiment of the invention;

Fig 17 is a view of an array of the solar thermal collectors of Fig 17; and

Figs 18 and 19 are graphical representations of experimental results obtained by typical solar thermal collectors of the invention for steam generation and water heating, respectively.

DETAILED DESCRIPTION OF THE ACCOMPANYING DRAWINGS

The evacuated glass tube solar thermal collector 1 as illustrated in Figs 1 to 5 of the accompanying drawings comprises a square shaped tubular manifold 2 having three circular heat receiving fluid flow pipes 3 located therein in spaced apart relationship with the inner wall thereof and extending along the length thereof. Heat receiving fluid flow passages in the pipes 3 are marked 4. Heat receiving fluid flow pipes are connected to a common inlet header 5 at one ends thereof and to a common outlet header (not shown but can be visualized from Figs 1 and 2) at the other ends thereof. Both the common inlet header and common outlet header are similar in construction. The heat receiving fluid flow pipes are spaced apart from one another and are held at intervals with the help of spacers 6. The tubular manifold defines a common condensation zone 7 provided therein along the length thereof in the space surrounding the heat receiving fluid flow pipes. The heat receiving fluid flow pipes are made of a good thermal conductor material selected from copper or aluminium or alloys thereof or corrosion resistant conducting material like stainless steel. The tubular manifold also has a resealable port 8.

A plurality of evacuated glass tubes 9 are arranged along the length of the tubular manifold perpendicular thereto and mounted to the tubular manifold at one ends thereof thermally insulated (thermal insulation marked 10) from the tubular manifold. The evacuated glass tubes are mounted to a protective support 1 1 at the other ends thereof. Each of the evacuated glass tubes is a double walled evacuated glass tube comprising an inner absorber tube 12 and an outer protective tube 13 disposed concentrically in spaced apart relationship with each other. Preferably, the outer surface of the inner absorber tube is coated with an absorber layer (not shown) like aluminium or copper nitride or molyblednum silicon oxide. 14 is an evaporator pipe located in the inner absorber tube at the center thereof along the length thereof in spaced apart relationship with the inner wall of the evacuated glass tube. The evaporator pipe is made of a thermal conductor material like copper or aluminium or alloys thereof or a corrosion resistant conducting material like stainless steel. The evaporator pipes contain a working fluid (not shown) like water, methanol or ethanol and communicate with the common condensation zone in the tubular manifold. 15 is a cover for the tubular manifold.

When the solar thermal collector is exposed to solar radiation, the working fluid in the evaporator pipes absorbs the heat of the solar radiation falling on the evacuated glass tubes and gets heated up and evaporate. The working fluid vapours in the evaporator pipes travel to the common condensation zone and exchange the heat with the heat receiving fluid flowing through the fluid flow passages 4 in the fluid flow pipes 3 and get condensed. The heat receiving fluid includes water, liquid desiccant, thermic fluid or air. The condensate returns to the evaporator pipes in the evacuated glass tubes under gravity to get heated up and to evaporate and the cycle of evaporation and condensation continues.

In another embodiment of the invention, the evaporator pipes may comprise wicks (not shown) located therein. In case the evaporator pipes include wicks, the condensate will also return to the evaporator pipes due to wick action.

According to another embodiment of the invention the solar thermal collector comprises a plurality of single walled evacuated glass tubes in the place of the double walled evacuated glass tubes. A single walled evacuated glass tube 16 as illustrated in Fig 6 of the accompanying drawings comprises an evaporator pipe 17 located in thermal communication with an absorber plate 17a which is disposed in the evacuated glass tube such that the evaporator pipe is at the centre of the glass tube along the length of the glass tube in spaced apart relationship with the inner wall of the glass tube. The evaporator pipe protrudes out of the glass tube and is connected to the tubular manifold. The glass tube is vacuum sealed to the evaporator pipe at 18. A solar collector comprising a single walled evacuated glass tube has distinct advantages like lower mass including thermal mass and lower cost.

According to the embodiments of the invention as illustrated in Figs 7 and 8 of the accompanying drawings, the evacuated glass tube solar thermal collector comprises a tubular manifold having six and seven heat receiving fluid flow pipes held in the manifold spaced apart from one another with spacers 19 and 20, respectively. The tubular manifold of Fig 8 is circular and is marked 2a. The common condensation zone of the tubular manifold of Fig 8 is marked 7a. Spacer 19 of Fig 7 is a disc with spaced apart holes (not marked but can be seen in Fig 7) for the heat receiving fluid flow pipes to pass therethrough. The spacers at intervals along the length of the heat receiving fluid flow pipes help to prevent the heat receiving fluid flow pipes from touching one another and to avoid bridging between neighbouring pipes with condensate. This will ensure that the entire outer peripheral surface of the heat receiving fluid flow pipes is available for condensation of the working fluid. As a result, heat exchange between the working fluid vapours and the heat receiving fluid in the heat receiving fluid flow pipes is increased.

The heat receiving fluid flow pipes 3a and 3b as illustrated in Figs 9 and 10 of the accompanying drawings respectively are fluted construction. The heat receiving fluid flow pipe 3c as illustrated in Fig 1 1 of the accompanying drawings comprises multiple tubes 21 twisted together in a spiral configuration. In such a construction of the heat

receiving fluid flow pipe as shown in Figs 9 to 1 1 the surface area for heat transfer between the heat receiving fluid and working fluid per unit length of the tubular manifold is increased. Further stratification of two phase mixture is avoided so as to increase heat transfer rate by effectively wetting the inside surface of the heat receiving fluid flow pipe in a two phase heat transfer situation. Twisted or fluted pipes force the liquid from two phase mixture towards the wall of the pipes so as to provide better heat transfer. In a single phase heat transfer situation, twisted or fluted construction of the heat receiving fluid flow pipes helps to achieve periodic breaking up of boundary layer and to increase heat transfer between the working fluid vapours and the heat receiving fluid.

In the embodiment as illustrated in Fig 12 of the accompanying drawings, the outlet header of one solar thermal collector is connected to the inlet header of another solar thermal collector through an orifice or constriction 22 provided in one of the headers. The constriction creates a spray of two phase mixture entering a solar thermal collector and helps to evenly redistribute two phases of the fluid in the downstream of the heat receiving fluid flow pipes of the following solar thermal collector. According to the embodiment of the invention as illustrated in Figs 13 to 15 of the accompanying drawings, the solar thermal collector comprises a transfer element 23 disposed in the evacuated glass tube 9 of Fig 5 over the evaporator pipe 14 in contact with the evaporator pipe and the absorber tube 12 of the evacuated glass tube. The heat transfer element comprises a segmented cylindrical body of foil disposed in the evacuated glass tube over the evaporator pipe along the length thereof in contact with the evaporator pipe and the absorber tube of evacuated glass tube. Each of the segments 23a comprises six bends marked a, b, c, d, e and f. The heat transfer element is made of a good thermal conductor material foil selected from copper or alluminium or alloys thereof. The heat transfer element as illustrated in Figs 13 to 15 covers the entire inner circumference of the evacuated glass tube. Therefore, the heat path between the evacuated glass tube and the evaporator pipe is substantially reduced. As a result, the temperature drop across the evacuated glass tube is reduced and heat recovery efficiency of the solar thermal collector is improved. The solar thermal collector 24 as illustrated in Fig 16 of the accompanying drawings comprises a parabolic reflector 25 surrounding any one of the solar collectors described above and commonly marked as 26. The reflector is mounted on a rigid surface (not shown) with a pair of mounting brackets 27 which are fixed to the rigid surface with bolts (not shown) tightened in bolt holes 28 in the brackets and matching bolt holes (not shown) in the rigid surface. Each of the mounting brackets comprises a head 29 with a pair of engaging grooves 30 at opposite sides thereof. Ends of the reflector are simply press or push fitted into the engaging grooves of the heads of the mounting brackets and glued if necessary. Preferably the parabolic reflector comprises a flexible sheet of reflective material such as alluminium sheet or a flexible sheet such as plastics sheet coated with a reflective material at the inner surface thereof. u

Fig 17 of the accompanying drawings illustrates an array 31 of the solar thermal collector of Fig 16. In the case of the solar thermal collector of Fig 16 or solar thermal collector array of Fig 17, the parabolic reflector intercepts and concentrates a large amount of solar energy onto the evacuated glass tubes so as to increase heat collection of the solar thermal collectors and to increase the temperature rise of the heat receiving fluid. Using multiple solar thermal collectors as shown in Fig 17, the capacity of the solar thermal collector and temperature of the heat receiving fluid can be substantially increased.

It is to be understood that depending upon the heat receiving fluid used and temperature rise to be achieved, the geometry and design parameters of the heat receiving fluid flow pipes and tubular manifold and common condensation zone within the tubular manifold are selected and designed. It is also to be understood that different types of heat receiving fluids may be allowed to flow through the heat receiving fluid flow pipes.

As an illustrative experimental example of the invention, three sets of arrays of evacuated glass tube solar thermal collectors of Figs 1 to 5 without parabolic reflectors were used for heating up water and generation of steam for 9 h measurement period and the results were as shown in Figs 18 and 19 of the accompanying drawings. Fig 18 and 19 represent test results for steam and water, respectively. Wet steam generated and water were separated with the help of steam separators. Water was recirculated in the collectors through a recirculation tank with a pump in case of steam generation. In case of water heating tap water was heated in the collectors in once through mode. The results show variation of global radiation, I _g , heat output of the solar collectors per square meter of absorber area, Q sc.m2> inlet temperature of water t _{. w} .j, outlet temperature of water or steam, t _{w o} , steam generation rate per square meter absorber area, mf.st.m2 and efficiency of the collectors based on absorber area of the collectors h _.sc . Data points are averaged over half hour periods, starting from 830 hrs and ending at 1730 hrs, 9 h period. Absorber area of the collector array was 10.5 m ² and the collector array was nearly horizontal (b~0°) with individual evacuated glass tubes and evaporator pipes in them, inclined at 2° to horizontal.

As shown in Fig 18, in case of steam generation, the flow rate of water at inlet of the collectors, volf.w.sc.i was 31.5 lph. Total heat incident on the collector array over 9 h period Q. _S c.i.9h was 59.1 kWh and the collector array heat output during the period Q _sc. was 33.8 kWh, giving an average efficiency h.sc.9h of 58.5% over the specified period. The fluctuation of mass flow rate of steam over its mean value curve was due to fluctuation of water level in the steam separator. It may be noted that the fluctuations in individual measurement cancel each other and do not affect accuracy of 9 h average values of parameters.

As shown in Fig 19, in case of water heating, the average flow rate of water at inlet of the collectors volf.w.av was 2.38 1pm. Total heat incident on collector array over the 9 h period Q. _sc .i.9h was 59.8 kWh and the collector array heat output during the period Q. _sc .9h was 45.5 kWh, giving an average efficiency h _SC 9h of 77.9% over the specified period. Average outlet temperature of water t _{w o} .av.9h was 60° C during this period.

The experimental results clearly establish the high temperature rise gain obtained according to the invention.

According to the invention the tubular manifold includes a common condensation zone surrounding the heat receiving fluid flow pipe(s) and the evaporator pipes in the evacuated glass tubes are directly connected to the common condensation zone. All the evaporator pipes and common condensation zone can be together evacuated and then charged with the working fluid at a time after opening the resealable port 8. On completion of evacuation and charging, the port 8 is sealed vacuum tight. As a result, the manufacture of the solar thermal collector is simplified. Cost manufacture of the solar thermal collector is also reduced. The heat receiving fluid flow passages in the heat receiving fluid flow pipes are without stagnation zones. As a result, pressure drop of the heat receiving fluid within the heat receiving fluid flow passages is reduced. There is low fluid hold up in the heat receiving fluid flow pipes and this helps to achieve fast response time to reach high temperatures. Also the heat receiving fluid flow pipes and manifold can be easily accessed for cleaning and servicing.

As the evaporator pipes are directly connected to the common condensation zone, the overall size of the tubular manifold is reduced correspondingly reducing the weight, thermal mass and cost of the manifold. The manifold is compact and the thermal insulation required for the manifold is reduced. Insulating paints like those used with vacuum ceramic beads may sufficiently insulate the manifold thereby further reducing cost and size of the manifold. Because of the increased heat recovery, the solar thermal collectors of the invention can be advantageously used for medium and high temperature applications. The invention also provides the option and flexibility to heat different or multiple heat receiving fluids in the heat receiving fluid flow pipes of the solar thermal collector.

The above embodiments of the invention are by way of examples and should not be construed and understood to be limiting the scope of the invention. Several variations of the invention are possible without deviating from the scope of the invention. For instance, the number of heat receiving fluid flow pipes within the tubular manifold can vary. There can be one heat receiving fluid flow pipe or there can be two or more than two or more than six or seven heat receiving fluid flow pipes. The evacuated glass tube construction and configuration can vary. The evaporator pipe need not necessarily be located at the centre of the evacuated glass tubes. The solar thermal collector can be without the heat transfer element. The heat transfer element configuration can be different. The number of bends of the heat transfer element can be different. The headers, wicks and spacers are all optional. Instead of using spacers, the heat receiving fluid flow pipes can be mounted in the tubular manifold spaced apart from one another using different holding means. The spacer geometry and construction can be different. The reflector geometry can be different. The geometry of the heat receiving fluid flow pipes and tubular manifold can be different. The protective support for the evacuated glass tubes can be different. Such variations of the invention are obvious to those skilled in the art and are to be construed and understood to be within the scope of the invention.