MX2012009644A - Flat, reversible solar collector comprising protection against damage caused by freezing. - Google Patents

Abstract

The invention relates to a flat, reversible solar collector comprising protection against damage caused by freezing, incrustation and problems associated with the use of hard water and insulation that deteriorates under humid conditions. For damage caused by freezing, the invention comprises flexible plastic tubing sealed at the ends thereof or along the entire length of same and positioned within the two heads and the absorber is built with flexible crossbars, allowing the increase in the volume of water when it freezes to be attenuated by said elements, preventing the fracturing of the tubing. In order to avoid problems of obstruction in the conduits caused by hard water (with high salt contents), the collector is formed by a large quantity of crossbar, using materials that have a high resistance to the adherence of salts compared with metal tubing. In order to prevent the degradation of the plastic absorber in high temperatures and as a result of the materials used to construct the collector, vents have been added to dissipate the heat. Said collector is advantageous in that it is reversible given that the cover and the rear are built from the same material.

Description

Solar Reversible Flat Sensor with Protection against Damage by specifically with the use of solar energy for water heating. One of the most important applications of solar energy is the heating of water, basically to cover the sanitary needs, mainly in the service, domestic and industrial sectors. The invention is related to a common problem in conventional flat solar collectors used throughout the world, related to moisture in insulation, the use of hard water and sudden changes in temperature (both low and high temperatures).

Object of the invention The object of the invention is to prevent the traditional solar collectors used in direct systems from being damaged because: (a) when exposed to temperatures below the freezing point of water, the water freezes and increases in volume, this in many occasions it produces fractures of the pipes; (b) in regions where there is hard water (water with a high salt content), due to. that the pipes in general of these collectors are of small diameters can occur incrustations of salts inside the pipes that could eventually block them; (c) in general, the thermal insulation used in these collectors to avoid heat losses to the environment are mineral wool, or materials such as polyurethane or polyisocyanurate which absorb moisture; since the flat collectors are in no case hermetically sealed, if there is moisture in the thermal insulation as mentioned above. there is an evaporation of the water in it and a condensation in the roof which causes the efficiency of the equipment to be reduced and that the insulation degrades its thermal properties very quickly with time.

Background Without a form of protection, conventional flat solar collectors are subject to damage when exposed to temperatures low enough to cause the heat transfer fluid (water) to freeze, resulting in an expansion of its volume by approximately 9%. Most of the methods adopted to avoid the problem are not completely reliable or can be expensive to implement. Ambient temperatures above zero degrees can also result in absorber and heat transfer fluid temperatures below zero due to radiative heat losses with a clear cycle. This effect is very common in areas where there is low humidity in the air giving rise to a high clarity of the sky and with this to quite high losses of the absorber. A fairly common form used for freeze protection may be: (a) draining the liquid or part of it from the absorbers; (b) the use of water solutions with glycols (antifreeze mixtures that reduce the freezing point of the mixture used); (c) heating or replacing the liquid that is in the absorber. These well-known methods work with the principle of avoiding freezing but have some disadvantages. Example, when an antifreeze solution is used in the absorber circuit, a heat intcrearbia is required between the absorber and the storage tank. Common anticongclantes formed by solutions of water with cyclcnglicol and propi-lenglicol are toxic and in safety codes mark, that being so requires the use of at least two metals interface between the toxic fluid and drinking water, which makes the system. The glycols degrade during the life of a solar absorber and therefore require replacement. Another method used for freeze protection in systems where potable water is used is to pump hot water from the thermos tank to the absorber when necessary to prevent it from freeze This method has the disadvantage that the thermal losses of the system are significantly increased and requires an additional electronic control to perform this function when a certain temperature (generally close to the freezing point) is reached in the absorber. If there is an electrical failure, this equipment could not prevent freezing in the system. Another commonly used method is to drain the liquid from the absorber or part of it through an anti-convulsion valve placed in the lower part of the system that opens mechanically once a certain setpoint temperature is reached (normally between 4 and 7 ° C), when draining part of the water it is occupied by water coming from the tank at a higher temperature which prevents it from freezing. This antifreeze valve has two main disadvantages: (a) it can be opened a large number of times under conditions of extreme temperatures, which results in a significant loss of water from the system that is wasted; (b) because they are formed mostly by moving parts, these could be damaged over time, which could produce that the device will not act well after some time or that the device could be obstructed by sediments that could accumulate in the lower part of the system, preventing the water from draining properly when opening the valve. We have found inventions related to antifreeze protection in absorbers and solar systems, then we describe their limitations and differences with the proposed one: US patent 4044754 is an anticongclastic protection system for solar absorbers with sensors inside the absorber that monitor the temperature of the water and when approaching the freezing temperature, the sensor activates a valve to drain the water from the absorber. When the temperature of the absorber is raised to a predetermined safe value, the sensor again acts to close the valve and again fill the absorber with water, our model has no electronic actuators or mobile pads that can fail over time and work without need for electricity, in addition to preventing water from the system having to drain with the consequent waste of it, which could be considerable if the system opens and closes, a good many times during its useful life. The patent US 4557252 is An antifreeze system for solar systems with forced convection (uses pump), consists of a sensor that records the temperature of the water inside the absorber and a valve to drain the water from the absorber when its temperature decreases from a predetermined value. While it is drained, the liquid is injected from the tank at a higher temperature into the absorber through the pump, our model has no electronic actuators or moving parts that can fail over time and operate without the need for electricity, in addition to the previous patent US 4557252 has the disadvantage that the thermal losses of the system are increased when pumping water already heated to the absorber where it will be cooled. Patent US 4237862 and US 4269167 is for closed and pressurized solar systems where water is drained from the absorber without the need of a sensor or valve (it does not contain moving parts), it only requires a circulation system with a pump. The tank where the water is received is filled only to a certain level that allows all the fluid of the circuit including the absorber and the pipes to be collected in the tank once the pumping system stops, which produces the water in the system is returned by gravity to this deposit, leaving the absorber full of the air that contained the tank thus preventing its freezing, our system can be used in systems with natural convection (thermosiphon systems widely used in Mexico and in other countries) or forced convection like these patents and it is not necessary in our case that the absorber is necessarily at a higher level than the tank so that it can be operated or that the tank is oversized in order to capture the water coming from the whole system once it is drained towards it. Patent US 4308856 is an antifreeze device consisting of an eight-shaped tube welded to the fin of the absorber having on its two sides a steel bar and both bars are joined by a U-shaped steel profile. so that increasing the volume of the inner fluid of the tube in the form of an eight will produce an elastic deformation of the same and the profile of steel in U, this deformation will return to its original shape once the ice has been undone thanks to the U-shape with which the tube is shaped in the form of eight, our system does not require special geometries for the pipes or of additional profiles (which increases its cost) to counteract the effect of water dilation when it freezes, our system can be placed in virtually any normal configuration of absorbers that exist in the market without appreciably increasing the cost of the system. GB 2117110 uses a system of different energy dissipation means associated with each conduit of the absorber; these means are tabs or fins what the author does according to the freezing occurs progressively from one point of the tube (stringer) to the other end and the expansion occurs outside these stringers which can cause the expansion to be absorbed in the heads of the absorber where there is a greater area designed to absorb this dilation; In addition to the above because it is more difficult for the heads to freeze because they have a much larger volume than the stringers (risers), our system can be placed in practically any configuration of the absorbers that exist in the market without increasing the cost of the system sensibly and without changes as in this case in which fins must be added to the pipes of the absorber to produce the effect mentioned. The patent US 5413091 uses the risers in a conical shape and the heads are completely insulated. The conical shape of the stringers is effective to prevent the rupture of the pipeline since it initiates the freezing and therefore the expansion of the water in a region of the stringer located in its central part where there is the minimum cross section of the conical section and successively it is extended towards the ends of the stringers, where because the head is isolated causes that in this area the freezing is not reached or is much slower, causing that it can absorb the expansion produced by the water in the stringers, our system does not require that the heads are insulated (which decreases the solar collection area) or that special conical pipes are manufactured with their minimum cross section in the center of the beam so that this effect can occur (these special beams make the absorber expensive since they are not commercially available in the market). On the other hand, there is no knowledge on our part, in a review of the patents of flat solar collectors, that a reversible sensor has been registered, that is, that it can be used indistinctly by facing the sun to the top or turn and put the bottom; this can extend the life of the system because it can be turned if for some reason the cover suffers some degradation over time and the collector will continue to work normally. Additionally, it was not found in any patent of a flat solar collector that will be used as insulation a cellular polycarbonate with air in steady state trapped in its small interior ducts, this air has a thermal conductivity similar to polyurethane close to 0.024 W / mK which allows it to act as an insulator, with the additional advantage that it does not degrade like the insulation materials commonly used in flat solar collectors such as mineral wool or polyurethanes that absorb a large amount of moisture and degrade easily over time. contact with water.

Detailed description of the invention The reversible flat solar collector, object of this invention, is encapsulated in a rectangular box with the face exposed to the sun covered by some transparent material for example: cellular polycarbonate, while the four sides are made of metal or plastic sheet, for example sheet pintro or aluminum and the backrest is also a cover of some cellular polycarbonate material equal to that used as a cover. Inside the box, exposed to the sun, an absorber plate is placed (for example of plastic materials such as polypropylene). The absorber plate is composed of a large number of flexible conduits (for example plastic) attached to the inlet and outlet heads through which a heat transfer fluid flows (usually water, although glycol can be used, or a mixture of both). The absorber plate is made of a material that does not degrade with ultraviolet rays and that acts as a selective surface (finished in dark color) thus increasing the absorption of heat from the sun's rays. The object of the invention is to provide a reversible flat solar collector with protection against freeze damage, incrustations and problems related to insulations that deteriorate with moisture, which can be coupled to a direct system (thermosyphonic systems or systems of forced convection, where the water that will be used later passes through its ducts). In particular, solve the problem that occurs in this type of direct systems when they are exposed to temperatures low enough to cause the internal fluid to freeze, which causes it to expand and fracture the pipes of the absorber leaving it unusable , as well as the possible obstruction of the ducts generally used of small diameters due to the use of hard waters (with high contents of salts), and avoid using traditional insulation (mineral wool, polyurethane, etc.) that degrade with moisture . The materials and manufacturing processes used in this collector can be of low cost to encourage its massive use and not have mobile parts that can be damaged during its useful life that must be at least 10 years.

The parts that compose the invention are the following: Figure 1 shows the components of the reversible flat solar collector consisting of a plastic absorber (1) protected by a rectangular box with the face exposed to the sun formed by a cellular polycarbonate cover (2) ), while the four sides are made of pintro, aluminum or plastic sheet (3) with some grooves or vents (4) to avoid high temperatures and the back (5) also made of cellular polycarbonate equal to the one used as a cover. Inside the head of the absorber is the device to prevent the damage of the absorber in the case of freezing of water (6), which is detailed in Figure 2. Inside the box, exposed to the sun, is placed an absorber plate (of plastic materials such as polypropylene). The absorber plate is made of a material that does not degrade with ultraviolet rays and that can act as a selective surface (finished in black or dark) thus increasing the absorption of heat from the sun's rays.

Figure 2 shows one of the important devices of the model to prevent the absorber from suffering fractures during the process of freezing the water inside its pipes (Fig. 1 6). This damping device consists of a flexible plastic pipe (eg polyethylene), which has been sealed by means of heat in one of its two versions: (a) at the ends of this flexible pipe; (b) or a series of additional intermediate seals are made to the ends. In both cases, air remains trapped in its interior that absorbs the pressure exerted by the water to solidify and present volumetric expansion. Figure 3 shows the plastic absorber (polypropylene) and a detail of a cross section formed by a large number of flexible beams (7) of black or dark plastic (to increase the absorption of solar radiation) attached to the heads (8) The fluid to be heated circulates in these pipes. The plastic damping device (9) shown in Fig. 3 is inserted along the entire length of both heads. This device acts in the following way: the absorber used (Fig. 3) either in direct systems of forced convection (where a pump is used to circulate the water through the collector) or of natural circulation (as is the case of systems) thermosiphonic widely used in Mexico and other regions), is completely filled with water, once the environmental conditions either by room temperature or by radiative losses with clear skies cause the water to fall to temperatures below 0 ° C, in this way the water freezing process begins with its consequent increase in volume by approximately 9%. This increase in the volume of freezing water that can cause fractures in the pipes is avoided on the one hand because the damping device has been designed (Fig. 3) in such a way that its interior formed by air has a volume of at least 11% of the total volume that the water occupies in the absorber, whereby the increase in the volume of the water when freezing is absorbed by this damping device (9) that is compressed as the water freezes, because it is flexible and contains air inside it. On the other hand, in case the freezing is sudden and to avoid putting these damping devices (9) also on all the stringers (7) of the system, which is problematic due to the small diameter of the same and their large number , flexible tubing is used in the construction of the stringers (7), which means that they can slightly modify its shape and also dampen freezing if it is abrupt. Some advantages of the freeze protection device is that it does not have moving parts therefore it does not require maintenance, nor does it use electricity to operate (it is a passive system) and unlike some that exist in the market it is not necessary to drain the water from the absorber (with its consequent waste in most cases) or that the water of the absorber is replaced with water from the tank, which causes the thermal losses of the system to increase significantly, as water is already being pumped to the absorber where it will be cooled. Another very important advantage of this system is its low cost compared to the cost of implementing antifreeze valves or electronic control systems, or in other cases to make special pipe geometries or add additional fins to dissipate energy, which obviously It has to be ordered to make special pipes that would greatly increase the cost of the absorber. This device additionally produces that the absorber and the system in which this installed can also dampen sudden increases in pressure such as those that can be produced by water hammer or in the initial filling of the system. Another serious problem is the use of flat solar collectors in direct systems of hard waters (with high contents of salts) that generally due to the fact that the flat solar collectors are formed by few metallic stringers (risers, from 6 to 14 typically) of diameters less than an inch in diameter where it circulates, in the case of thermosyphonic water systems at low speeds and sometimes at high temperatures, it can eventually produce deposits of these salts that can sometimes clog the conduit, making it unusable. system. In the design of the collector this problem is avoided by two fundamental aspects: (a) the collector is formed by a large number of conduits (7) so that the flow of water is divided into a large number of branches which makes it more difficult that all of these can be sealed; (b) the plastic material used (polypropylene or other similar) has a greater resistance to the adhesion of salts compared with metal pipes. Another important part of the invention is that the sensor is isolated in its front and back (where the largest area of losses is compared to the lateral sides) with an area of air trapped in it. the cellular polycarbonate; which, being in a stationary state, has a thermal conductivity similar to polyurethane close to 0.024 W / mK, this advantage allows this insulation does not degrade as the insulation materials commonly used in flat solar collectors such as mineral wool or polyurethanes They absorb a large amount of moisture and degrade easily over time in contact with water. In addition, on both sides of the sensor (cover and backrest); a cellular polycarbonate plate (2 and 5) has been used that has air trapped inside (low conductivity) acting as insulation; In addition to the above, it gives rigidity to the sensor. This system allows as an additional advantage that if the cover (2) for some reason should deteriorate over time, the collector can turn over and continue to operate in the same way as when it was initially installed. Another additional advantage of the sensor given the materials with which it is built: in general plastics, pintro or aluminum sheet is that all its materials do not degrade to be in direct contact with environmental conditions of temperature, humidity, ultraviolet rays, etc. what lengthens the system life. Additionally, it allows the collector not to be totally hermetic and can have vents (4) to prevent the absorber from reaching high temperatures, such as those produced under stagnation conditions in flat collectors, which can cause the plastic of the absorber to degrade over time and does not reach the planned useful life or avoid using more expensive plastics to prevent their degradation due to exposure to high temperatures.

This invention therefore lengthens the life of the collector because it can not reach excessively high temperatures that degrade the materials from which the collector is made; the interior of the sensor can not reach ex-cessive temperatures due to the vents (4) that have been designed in the frame of the sensor so that, by natural convection, hot air is removed from them inside and replaced by air at a temperature ambient considerably reducing the maximum temperatures that can be reached inside it.

Description of figures Figure 1. Components of the reversible flat solar collector.

Figure 2 Damping device formed by a flexible plastic pipe sealed at its ends (a) or its entire length (b).

Figure 3 Plastic absorber with cross section of the head where the device for antifreeze protection is displayed. The numbers are repeated because they are the same parts, but viewed from different perspectives.

Claims (6)

1. The flat solar collector with protection against freeze damage that is characterized by being formed by flexible plastic tubing sealed at its ends or the entire length thereof and placed inside both absorber heads,

2. The flat solar collector with protection against freeze damage according to claim 1 characterized by having an absorber construction by means of flexible beams allow that the increase of the volume of water upon freezing is cushioned by said elements preventing the fracture of the pipes of the absorber

3. The reversible flat solar collector, which can be used indistinctly by facing the sun to the top or turn it and put the bottom.

4. The flat solar collector with protection against freeze damage characterized by having a cellular polycarbonate insulation with air in a stationary state trapped in its small interior ducts.

5. The flat solar collector characterized by being formed by a large number of conduits (sometimes more than 100) that have a much greater resistance to the adhesion of salts compared to metallic pipes.

6. The device for protection in case of freezing in absorbers according to claim 1 characterized by having a damping device formed by flexible plastic tubing sealed at its ends or along its length used to absorb the volume increase suffered by the water when freezing. The diameter of the flexible pipe of this device will depend on the total volume occupied by the water in the absorber, in such a way that the air space or vacuum inside it is at least 11% of the volume occupied by the water in the system, this to be able absorb the increase in water volume when freezing without affecting the absorber. The device for the protection in case of freezing in absorbers according to claim 1 characterized by stringers of the flexible pipe absorber to cushion also in case of sudden freezing by its flexibility to move and return later to its original shape. The device for the protection in case of freezing in absorbers according to claim 1 characterized by a passive system that also allows to absorb high peaks of pressure in the absorber that can be produced by water hammer or during the initial filling of the system. The flat solar collector with cellular polycarbonate insulation according to claim 3 since the materials do not degrade when in direct contact with humidity, ultraviolet rays, etc. (in general, materials such as plastics, pintro or aluminum foil) extends the life of the system.