US20130025587A1

US20130025587A1 - Vacuum tube solar collector with overheating protectionby means of a rotating reflector

Info

Publication number: US20130025587A1
Application number: US13/261,329
Authority: US
Inventors: Adolfo Luis Lopez Ferrero
Original assignee: Individual
Current assignee: Individual
Priority date: 2008-12-31
Filing date: 2009-12-18
Publication date: 2013-01-31
Also published as: EP2520871A1; WO2010076352A1; ES2352939B1; JP2013514519A; ES2352939A1

Abstract

The aim is to add a device to vacuum-tube solar collectors so as to prevent overheating thereof. FIG. 1 shows an illustration summarizing the invention applied, in this case, to a vacuum-tube collector of the type consisting of a single glass tube (1) with heat pipe (2) centred inside the glass tube (1) and making contact with heat-absorbing fins (6). The invention is characterized in that the part of the inner wall of the glass tube (1) which is hidden from the sun incorporates a curved reflector (3) with a small thickness along the entire length of the glass tube (1), which reflector is secured by means of various rings (4) and is able to rotate while being operated by means of a nitinol or bi-metal torsion spring (5) which is screwed in and makes good thermal contact with the evaporator tube of the heat pipe (2) so that, when the temperature of the evaporator tube of the heat pipe (2) increases beyond a certain value, it causes the nitinol spring to change form, rotate and cause rotation of the reflector (3), protecting the heat-absorbing fins (6) from the solar radiation and preventing overheating of the collector and the solar installation. Rotation is reversed when the temperature of the evaporator tube (2) drops, the collector remaining in the normal heat supply condition. This invention is applicable, with certain modifications, to any type of vacuum-tube solar collector according to the accompanying description. Thus not only is overheating prevented in solar installations, but also the collectors are made more efficient due to the reflection of the radiation heat losses of the fins back towards themselves by means of the reflector.

Description

PRIOR ART

The capacity of control the heat energy supplied by a solar collector is very important in a thermal solar installation. If the collector is always supplying heat without the possibility of stopping this heat supply, the cost of the installation increases and the performance and durability of the installation decreases.
A collector that is supplying heat without the possibility of stopping this heat supply has the following disadvantages:

- Increase design and installation costs: The design of the installation is more complex and expensive due to the measures to prevent overheating, with additional installations like security valves, over dimensioning of expansion tanks, more resistant welding and so on.
- Reduces the performance and lifetime of the installation: In general all thermal solar installations suffer when they are subjected to high temperatures. The collectors decrease performance and deteriorate. The antifreeze that is normally added to the water in the installation degrades with high temperatures endangering the whole installation due to the frost unprotected situation.
- The impossibility of a proper design of the solar thermal installation for central heating, that needs a large solar absorbing area, which leads to an overheating situation in summer.
- Many other problems not mentioned here that are obvious due to the fact that the installations were not designed to stand so high temperatures.

All these problems lead also to prevention costs, so the price of a nowadays thermal solar installation would be more economic if the collector heat supply were under control.
Nowadays there are quite a number of ways to prevent overheating in thermal solar installations.
Some of these ways commonly used are:

- Cover the Collectors Manually
- Disadvantages: the overheating situation still exists when the collectors are not covered. In case of situate the collectors in roofs the access to the collectors is normally difficult. Cost of the covers.
- Incline the Collectors More than Necessary in Order not to have so Much Heat Supply During Summer
- Disadvantages: Does not prevent the overheating situation in all cases. Decrease the performance of the installation because the collectors are not inclined in optimal angle.
- Install Fewer Collectors than Necessary
- Disadvantages: Does not prevent the overheating situation in all cases. Low energy supplied by the installation when is required.
- Install Heat Dissipation Equipments
- Disadvantages: Cost of the equipment. Overheating in case of pump failure.
- Circulating the Water Through a Pool:
- Disadvantages: Does not apply to homes without a pool. Unnecessary use of the pump when not using the pool. Overheating in case of pump failure.
- Waste Hot Water and Substitute with Cold Water:
- Disadvantages: Unnecessary water consumption. Overheating in case of pump failure.
- Empty the working fluid of the collector:
- Disadvantages: Cost of the installation. Overheating with dry collectors
- Other More Sophisticated Systems that Generally Lead to Costs
- Etc.

Within different types of collectors, the present invention is applied to the vacuum tube collectors. The stagnation temperature typical of these collectors is about 250° C. The object of this patent is to reduce this temperature to levels that do not cause overheating in solar installations, below 120° C.
Currently there are two types of solar vacuum tubes:

- A. Vacuum tubes of a single crystal with vacuum inside.
- B. Vacuum tubes of two concentric glasses with vacuum in between the glasses.

For the extraction of heat from the tubes there are several methods as:

- 1—With a heat pipe. The heat pipe is a tube, usually made of copper, that has two sections: one long section inside the glass along almost all its length called evaporator, and another short section outside the glass called condenser. The condenser is in a higher position than the evaporator. The evaporator has a little amount of liquid that evaporates when temperature increases and ascends through the evaporator till it reaches the condenser in its top end. This condenser is in thermal contact with the collector header where circulates the fluid to be heated. In the condenser the ascending vapor condenses and transfer the heat of condensation by conduction to the fluid to be heated. The condensed vapor goes down through the tube by its own weight until the evaporator temperature is enough to evaporate it again, ascends and repeat the cycle.
- 2—With a concentric pipe (on the way out and return in the same concentric tube) or in a “U” shape pipe. This pipe carries inside the fluid to be heated. This fluid is heated when passes inside the tube.
  - In both cases, with heat pipe or with concentric pipe or “U” shape, it is common to use solar radiation absorbing fins in contact with the heat pipe or the pipe to gain better performance.
- 3—In case of concentric glass tubes the heat extraction can also be done directly from the fluid to be heated inside the tube. This fluid is heated and ascends by thermosiphon effect towards the header, where it is replaced by colder fluid that goes down through the tube until it is heated again repeating the cycle. There are also concentric glass tubes in which the inner tube is closed at both ends, performing as if it were a heat pipe, with a small amount of liquid to be evaporated inside it. In this case the higher end of the inner glass tube is more prolonged than the outer glass tube, and this prolongation acts as the condenser of the heat pipe.

There are many patents that try to solve the problem of the overheating of vacuum tube collectors, as for example:
The patent U.S. Pat. No. 5,667,003 shows a method of collecting the liquid that is inside the heat pipe by means of a bimetal or a metal with shape memory (to simplify hereinafter called Nitinol for the rest of the description) located in the condenser of the heat pipe. When the temperature of the heat pipe is too high, the nitinol pushes a part that prevents the liquid to return to the evaporator, so that the evaporator tube is finally without any liquid, being all the liquid in retained in the condenser. This prevents the evaporation-condensation process of the liquid, therefore, theorically, the heat transfer towards the header. When the condenser temperature decreases, the nitinol change again its shape and allows the liquid to return to the evaporator, providing heat again by the evaporation-condensation process.
The problem of this method is that because the material of the heat pipe is normally copper, the transfer of heat cannot be avoided totally, since the condenser is receiving heat by conduction from the evaporator copper tube. For this reason, although this method can reduce the stagnation temperature of the collector below 184° C., it is not enough to avoid overheating. (184° C. overheating temperature has been obtained from the test made to these kind of collectors, in the prestigious Swiss laboratory SPF.)
The use of bimetals or materials with memory shape is well known in the state of art of the solar collectors with different purposes. For example the patent JP59231360 proposes a method to avoid overheating by means of these materials attached to the absorbing fins of an evaporator of a heat pipe, so that these fins, in this case absorbing radiation on one side and reflecting radiation on the other, are bended and show its reflecting side by means of the nitinol when the temperature is too high. This patent is able to reduce the stagnation temperature, however, for the same reason as the U.S. Pat. No. 5,667,003 not enough to avoid overheating.
The patent JP59024143 also makes use of a nitinol to avoid overheating. In this case the nitinol, in a spiral spring shape coiled in the condenser, moves an semicircular external reflector when temperature increases. For the same reason as previous patents, the stagnation temperature of this method decreases only until about 200° C., not being enough to avoid overheating. Besides, this patent locates the reflector outside the tube, with the inconvenient that this entails, not being able to withstand adverse weather conditions over the years.
The patent JP59012253 makes use also of nitinol attached to a reflector located below the heat pipe. When temperature increases the nitinol moves the reflector and the heat pipe becomes off center of its focus. This patent assumes that the sun is always perpendicular to the reflector, and does not clarify what happens when the sun is not perpendicular to the reflector (in the morning or in the afternoon). Anyway, even if the reflector will not reflect any radiation at all towards the evaporator of the heat pipe, the heat pipe will be always exposed to the sun, providing heat to the collector. Like above patents, it fails to reduce the temperature to acceptable levels to avoid overheating.
Other patents use different methods, but they make use of electricity, (movement of a motor, for example, activating certain protection against the sun), to avoid overheating. Es desirable not to use electricity, because overheating occurs with power outage, causing the stop of the primary circuit pump and power off of the solar controller. Even if a power outage can be avoided temporary with an UPS, it is not desirable make use of UPS systems because they increase the price of solar installations. At the same time these UPS systems are not valid to avoid overheating with permanent power outage. The present patent uses well known materials in the state of art of the solar collectors, like nitinol, absorbing radiation fins, magnets to move through walls, etc. And design, combine and fit them together in efficient and reliable way so that the temperature of the water to be heated does not exceed 120° C. At the same time the invention is very economic, and makes it possible its commercialization.
Nowadays there is not commercially available a vacuum tube solar collector that do not shows overheating problems.

EXPLANATION

Models of Reflector Hold by Rings

Model 1: Single Glass Vacuum Tube with Heat Pipe
In this section it is described the application of the invention to a vacuum-tube collector of the type of a single glass tube with heat-absorbing fins attached to a heat pipe, and in next sections several easy modifications in order the invention to be applicable to any other type of vacuum-tube collector.

FIGS. 1 & 2

FIGS. 1 &2 show this type of collector. A single glass tube(1) that has vacuum inside. Within the tube there is the heat pipe(2). This heat pipe has attached some fins(6) that has been treated with radiation absorbing material that transmit heat to the heat pipe(2) by thermal contact.
The present invention introduces a reflector(3), made of aluminum or stainless steel or any other material that can stand temperatures over 200° C., inside of the glass tube(1). The shape of this reflector(3), with small thickness, is approximately semicircular and goes along the glass tube(1) in almost all its length. The main mission of this reflector(3) is to protect the fins(6) from the solar radiation once rotated some degrees within the glass tube(1). The more rotation of the reflector(3) the less absorption of the fins(6), if the rotation angle is 180°, the radiation will not be absorbed at all by the fins(6), therefore the collector will not supply heat to the installation. The secondary mission of this reflector(3), when it is in its lower position without been rotated yet, is to reflect the infrared radiation from the fins or radiation absorbing parts towards themselves, with the aim to prevent thermal losses by radiation, increasing the performance of the collector.

FIG. 3

The turn of the reflector(3) is driven by a torsion spring(5) of bimetal or nitinol (Nickel-Titanium alloy) or any other alloy that changes shape with temperature (here onwards called nitinol spring) that is screwed and with good thermal contact with the heat pipe(2).
Excessive increase in temperature of the heat pipe(2) causes a turn of the nitinol spring(5), due to its shape change with temperature. The nitinol spring(5) can only turn by its long end(8) because the other short end is fixed to the heat pipe(2) by means of a clamp(10) or a pressure ring or welded with the heat pipe(2)
The long end of the nitinol spring(8) is attached to the first ring(4) that, in turn, if fixed to the reflector(3) by means of its three tabs(7), so that the turn of the nitinol spring(5) causes the turn of the reflector(3). The more the reflector(3) turns, the less will be the radiation absorbed by the fins(6), and less will be temperature transmitted to the heat pipe(2). When heat pipe temperature decreases there will be a moment in which the nitinol spring(5) will not turn more and stabilized. This temperature of stabilization will prevent the overheating of the collector. In FIG. 3 the nitinol spring has been situated near the end of the vacuum tube, but it can also be situated in other places along the tube, attaching it to any other ring.

FIG. 4

FIG. 4.1 shows the reflector(3) when still has not started its rotation, and it is positioned in the lower part of the glass tube(1), below the fins(6).
FIG. 4.2 shows the reflector after it has turned 180° and situated in its highest position, completely covering the fins from the solar radiation. Although this figure shows a rotated angle of 180° to better understand the covering of the fins, not necessarily will reach this rotation. The rotation will stop when the heat pipe temperature stabilized.
Because the nitinol spring(5) expands with the rotation, it can minimally lose contact with the heat pipe(2). It is desirable that the nitinol spring is in maximum possible contact with the heat pipe(2), so that the heat pipe transmits its temperature by thermal contact. This maximum contact can be achieved by applying thermal grease between the nitinol spring(5) and the heat pipe(2) in order to increase thermal contact between them. This thermal contact of the spring with the heat pipe is necessary in the single glass vacuum tubes, because the spring is inside the vacuum and there is no heat transfer in vacuum, however it is not necessary this thermal contact in case of double glass vacuum tubes, because inside the inner glass(14) there is air.

FIG. 5

The reflector(3) could drop by its own weight when it is rotated and make contact with the fins(6). This is not desirable because there would be a thermal lose by conduction from the fins(6) towards the glass tube(1) by means of the thermal contact with the reflector(3). That is why some thin wall rings, made of aluminum, steel or any other material that stands temperatures over 200° C., has been added, fixed to the reflector, for example by means of three tabs(7) that are bended once introduced in the grooves of the reflector(3) provided for it, or by any other method. It should be noted that these rings provide an added advantage, that is to avoid thermal contact between the reflector(3) and the glass tube(1), thereby increasing the radiation of the fins to themselves, and increasing the performance of the collector. The reflector is in the vacuum, very close to the inner wall of the glass tube(1), without actually contact it because between them there are the rings(4).
All rings(4) are similar except the one that is attached to the spring (in case of FIG. 5, the first ring, but could be any other), that should have an special anchor, for example, another fourth tab (not drawn) with a hole through which the long end(8) of the nitinol spring(5) enters and become fixed.

FIG. 6

If the nitinol spring(5) is of the single memory type, it will turn with the increase in temperature, but it will not go back to its initial position after the temperature decreases. To go back it has to be helped by another normal steel spring(11) with an spiral or torsion shape. This steel spring(11) could be situated in any position, even over the nitinol spring(5) and screwed on it. In the figure it has been drawn an steel torsion spring(11) located after the nitinol spring(5).
If the spring is made of bimetal or nitinol with two memory shapes, it is not necessary to add any other steel spring, because these types of springs will go back by themselves to its original position with the decrease in temperature.
The turn of the reflector should be such that doing so in one direction provides correct overheating protection, and in the other way, when it comes to provide heat to the installation, does not cover at all the fins from the solar radiation. That is why the bimetal or nitinol spring must be designed properly, both in form and in degree of torsion with the temperature. Also the reflector arc length should be appropriate, not necessarily 180°, since with smaller arc overheating protection can be achieved. These measures of springs and reflectors depends, in part, of the type of tube in question.
Model 2: Single Glass Vacuum Tube with Concentric Pipe

FIG. 7

In this type of vacuum tube there is no heat pipe, instead there is a concentric pipes(12 &13). In one of them the working fluid goes in one direction and in the other in the opposite direction. The nitinol spring should be screwed and fixed to the outer pipe(13). Operation is the same as in model 1.
Model 3: Double Concentric Glass Vacuum Tube with Centered Heat Pipe

FIG. 8

The reflector(3) is in vacuum and located between the two concentric glass tubes(1 &4), therefore the long end of the nitinol spring(8) cannot be hooked directly to the reflector(3), because there is a glass in between. To be able to move the reflector(3), the long end of the nitinol spring(8) has fixed a north magnet(16) that attracts another south magnet(17), through the inner concentric glass(14) fixed at one of the rings(4) that holds the reflector(3). The turn of the north magnet(16) makes the south magnet(17) to turn through the glass, thus making the reflector(3) to turn. The south magnet(17) can be substituted by an iron part or high iron content part, so that it can be attracted by the north magnet(16).
Since these magnets are exposed to high temperatures and need quite a high magnetic attraction power, they must be neodymium magnets treated to stand high temperatures, or samarium or other kind of magnets.
Model 4: Double Concentric Glass Vacuum Tube with an U Shape Pipe

FIG. 9

This type of tube does not have a heat pipe, instead the working fluid flows through an U shape pipe located in the inner glass tube(14) that goes through all its length, making thermal contact with it by means of an aluminum molded sheet (not drawn for this model). A copper or aluminum part(19) is interposed between the two pipes with good thermal contact, welded or fixed by pressure. The nitinol spring(5) in this case is screwed over this part(19). Although the turn of the reflector(3) is limited to something less than 180 degrees, due to the collision of the long end of the nitinol spring(8) with the U shape pipe(18), overheating can be effectively prevented, because it is not necessary to reach a turn of 180 degrees.
Because the tubes are concentric it shall proceed to place magnets as in model 3.
Model 5: Double Concentric Glass Vacuum Tube with an Off-Center Heat Pipe

FIG. 10

This type of tube is the same as in model 3 except that the heat pipe(2) is off-center. The invention is adapted to this type of tube in the same way as in model 4, but the copper or aluminum part is now shorter(20) and is fixed or welded to the heat pipe(2).
Model 6: Double Concentric Glass Vacuum Tube without Heat Pipe Neither Pipes Inside, with Working Fluid Inside.
Although this type of tubes are commonly used for the well known “thermosiphons”, that incorporate the tank horizontally in its top part, with the tubes directly inserted in the tank, can also be used to produce collectors, which are often of low cost because cannot stand much pressure.
The inner part of this kind of tubes is filled with working fluid, transmitting heat by thermosiphon effect. The invention is applicable to this kind of tubes in the same way as in model 3, but with the nitinol spring immersed in the working fluid, so that the excessive temperature increase of the working fluid will make the nitinol spring turning. In this case, since there is no heat pipe to place the nitinol spring over, a part with a central axis (not drawn) will be provided for the invention, made of metal, simple, that can be fixed by pressure to the walls of the inner tube and allows free turn of the nitinol spring.
Model 7: Double Concentric Glass Vacuum Tube without Heat Pipe Neither Pipes Inside, Provided with Liquid that Actuates as Heat Transmission when Evaporates.
In these tubes there is a small amount of liquid inside the inner tube that evaporates when the tube temperature increases, making the whole tube working as if it were the shaft of a heat pipe. The inner tube is closed at both ends, protruding from the outer tube in its top end, and this protruding part working as if it were the condenser of a heat pipe. Usually there is vacuum inside the inner tube to control the temperature at which the inside liquid starts vaporizing.
The invention is adapted to this type of tube in the same way as in model 6 but with the spring immersed inside the vapor that rises through the tube(14), turning or not depending on the vapor temperature.
Models with Reflector Fixed without Rings for Concentric Double Glass Vacuum Tubes

FIGS. 11, 12 & 13

These models are valid for any kind of concentric double glass vacuum tubes. The reflector(3) can be hold without using any ring at all in the following way: The reflector(3) has attached four iron cylinders with tip(22) (iron or any metal with high iron content so that it can be attracted by magnets), two of them at the same height located near the top end of the tube(14) near the header, and two, also at the same height, but near the bottom part of the tube(14). These cylinders(22) are fixed to the reflector(3) so that they can freely rotate by its tips, either cutting, bending and punching the reflector, so that the tips enters freely in the bended parts (like FIGS. 11 & 12), or using metal clamps, not drawn, that accommodates the cylinders(22) and allow them to turn freely. These parts will be fixed by pressure to the reflector(3).
The cylinders are moved by two magnets(21) with a “V” shape, preferably of neodymium, with a hole where the heat pipe(2) is introduced, so that they can freely rotate over it. These magnets are moved by two nitinol springs(5) and fixed to one of its end. In turn, these springs are fixed, at the other end, at the heat pipe(2) by means of pressure rings(10) or welded. The rotation of the nitinol springs causes the rotation of the magnets, that attract the cylinders through the inner tube glass wall and make them rotate. When the cylinders rotate, like if they were a wheel, do so by resting on the outside face of the inner glass tube. The cylinders can be manufactured a little more thin in the central part(22B), with the aim of making the least possible damage to the surface of the inner glass when the cylinders rotate.
The reflector can turn in this way over the inner glass tube without falling down when turning due to the attraction of the magnets with the cylinders.
To center the heat pipe on the inner glass tube, three elongated aluminum plates(23), molded according to FIG. 13, will be added. These plates, also claimed, not only allow to center the heat pipe, but also provides more effective collector performance, due to the simultaneously heat transferring of the heat inside the tube(14) and the heat of the inner tube walls(14) through their six curved radius, improving radiation losses of the inner glass tube because the temperature difference between the inner glass tube and the heat pipe is less.
These plates should be cut longitudinally to such an extent which allows to place between them the nitinol springs and the magnets.

Claims

1. Solar collector of the type of vacuum tubes consisting of two concentric glass tubes(1 and 14) between which vacuum has been made, that transmits the sun's radiant energy to the working fluid by means of a heat transfer tube(2) in thermal contact with the inner glass tube(14) by means of aluminum molded fins(23), characterized by it incorporates a curved reflector(3) of small thickness, located between the two concentric glasses along almost all their length, hidden from the sun, and being of arc less or equal to 180 degrees, and diameter something less that the outer glass(1), and surrounded by rings(4) fixed to the reflector by several tabs(7) or by pressure parts, so that the reflector can rotate between the glasses pushed by a bimetal or nitinol (or any other alloy that changes shape with temperature) torsion spring, coiled around the heat transfer tube(2). The sprig, in turn, is connected with s north magnet(16) located in the inner wall of the inner glass(14), that attracts another south magnet(17), located in the outer wall of the inner glass(14). A temperature increase inside the inner glass(14) rotates the nitinol spring(5) and this, in turn, rotates the reflector(3) due to the magnets. The rotation of the reflector(3) covers the inner glass(14) from the solar radiation, preventing the overheating of the working fluid and it is reversible in function of the temperature of the inner glass(14) so that in a normal situation of the collector supplying heat, the reflector(3) returns to its original position below the inner glass hidden from the solar radiation, by itself in case of using a bimetal of nitinol spring with two memory shapes, or by means of another steel spring(11) in case of a nitinol spring(5) (or any other alloy that changes shape with temperature) of a single shape memory. In addition to the overheating protection, this invention is intended to increase the collector performance when the reflector(3) is located in its normal lower position of supplying heat, below the inner glass(14) reflecting the radiation heat loss of the inner glass(14) towards itself, being optimum the increasing in performance due to the fact that the reflector(3) is not in contact with the glass tube(1), existing a separation, in vacuum, created by the rings(4) interposed between the inner wall of the glass tube(1) and the reflector.

2. Solar collector according to claim 1 characterized in that the heat transfer tube is a heat pipe, centered in the inner glass tube(14).

3. Solar collector according to claim 1 characterized in that the heat transfer tube is an off center heat pipe and because the bimetal or nitinol spring(5) is coiled and hooked at an axis of a copper or aluminum part(20) with good thermal contact, hold by pressure or welded with the heat pipe(2).

4. Solar collector according to claim 1 characterized in that the heat transfer tube is a concentric pipe where the water to be heated flows through.

5. Solar collector according to claim 1 characterized in that the heat transfer tube is an “U” shape pipe(18) that runs inside the inner glass(14) in all its length, and which water to be heated flows through, and because the bimetal or nitinol spring(5) is coiled and hooked at an axis of a copper or aluminum part(19) with good thermal contact, hold by pressure or welded with the “U” shape pipe(2).

6. Solar collector according to claim 1 characterized in that the heat transfer tube is the inner glass tube(14) itself, that acts as a heat pipe, with a small amount of liquid inside the inner glass tube(14), that evaporates when the temperature increases and ascends through the tube up to its top end, and that protrudes from the outer glass as a heat pipe condenser in contact with the water to be heated, and because nitinol springs and the magnets are secured and rotate freely over the axis of a part hooked by pressure inside the inner tube, immersed in the vapor that ascends along the tube.

7. Solar collector according to claim 2 characterized in that the aluminum molded fins(23) are multiple, three or more, with a radial section. The radius of the aluminum fins(23) may be curved or straight, although it is preferable curved, especially when their number is low, six or eight, with the aim of allow a separation between them for a better heat absorption inside the inner tube(14).

8. Solar collector according to claims 1 to 7 Characterized in that the reflector incorporates four cylinders(22) with tip, made of iron or a material with high content in iron, that substitutes the surrounding rings and that can rotate freely hold by its tips to the reflector, two of them located at the same height near the end of the outer tube, and the other two also at the same height, but near the opposite end, and that are rotated by the magnetic field through the inner glass(14) of the two magnets(21) with a “V” shape located at the same height as the cylinders, that in turn are rotated by two bimetal or nitinol springs.

9. Solar collector of the type of a single glass vacuum tube(1) that transmits the sun's radiant energy to the working fluid by means of a heat-absorbing fins(6) in contact with a heat transfer tube(2), characterized by it incorporates a curved reflector(3) of small thickness along the glass tube(1), of arc something less or equal to 180 degrees and diameter something less than the inner wall of the inner glass tube(1), hidden from the sun and surrounded by rings(4), and fixed to it by several tabs(7) or by pressure parts, so that it can be rotated inside the glass tube(1) pushed by a bimetal or nitinol (or any other alloy that changes shape with temperature) torsion spring(5) coiled around the heat pipe(2) with good thermal contact and hooked to the reflector(3) by its long end(8) and to the heat pipe(2) by its short end(9) by a clamp(10) or a pressure ring or welded. A temperature rise of the heat pipe(2) rotates the nitinol spring(5) and this, in turn, rotates the reflector(3). This rotation of the reflector(3) covers the absorbing fins(6) from the solar radiation preventing the overheating of the working fluid, and it is reversible in function of the temperature of the heat pipe(2), so that in a normal situation of the collector supplying heat, the reflector(3) returns to its original position below the absorbing fins hidden from the solar radiation, by itself in case of using a bimetal of nitinol spring with two memory shapes, or by means of another steel spring(11) in case of a nitinol spring(5) (or any other alloy that changes shape with temperature) of a single shape memory. In addition to the overheating protection, this invention is intended to increase the collector performance when the reflector(3) is located in its normal lower position of supplying heat, below the fins(6), reflecting the radiation heat loss of the fins(6) towards itself, being optimum the increasing in performance due to the fact that the reflector(3) is not in contact with the glass tube(1), existing a separation, in vacuum, created by the rings(4) interposed between the inner wall of the glass tube(1) and the reflector.

10. Solar collector according to claim 9 Characterized in that the heat transfer tube is a heat pipe.

11. Solar collector according to claim 9 Characterized in that the heat transfer tube is a concentric pipe where the water to be heated flows through.