WO2014017896A1

WO2014017896A1 - Flat, reversible solar collector comprising protection against damage caused by freezing

Info

Publication number: WO2014017896A1
Application number: PCT/MX2013/000089
Authority: WO
Inventors: Octavio GARCÍA VALLADARES
Original assignee: Módulo Solar, S.A De C.V.
Priority date: 2012-07-23
Filing date: 2013-07-17
Publication date: 2014-01-30
Also published as: MX2012009644A

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

REVERSIBLE FLAT SOLAR SENSOR WITH FREEZE DAMAGE PROTECTION

Description of the Invention

Technical field

The field of the invention is related to renewable energy sources, specifically with the use of solar energy for water heating. One of the most important applications of solar energy heating water, basically to meet the health needs, mainly in the ^"Ð¾ ^~ sectors was IEIO, domestic and industrial. The invention relates to a common problem in flat solar collectors Conventional used throughout the world, related to moisture in insulation, hard water use and sudden temperature changes (both low and high temperatures).

Object of the invention

The object of the invention is to prevent traditional solar collectors used in direct systems from being damaged due to: (a) being exposed to temperatures below the freezing point of water, water freezes and increases its volume, this in many cases it produces fractures of the pipes; (b) in regions where hard water is present (water with a high salt content), because the pipes in general of these collectors are of small diameters, salt deposits can occur inside the pipes that could along the length of the time to 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 like those already mentioned, there is an evaporation of water in it and a condensation on the roof which causes the efficiency of the equipment to be reduced and that the insulation is degrading its thermal properties very quickly over 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 they can be expensive to implement. Ambient temperatures above zero degrees can also result in temperatures of the absorber and heat transfer fluid below zero due to heat-loss-r-adia ^ 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 very high radiative losses of the absorber. A fairly common form used for freeze protection can be: (a) drain 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) by heating or replacing the liquid found 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 exchanger is required between the absorber and storage tank. The common antifreeze formed by water solutions with ethylene glycol and propylene glycol are toxic and in brand safety codes, which being so requires the use of at least two interface metals between the toxic fluid and drinking water, which increases the cost of system. Glycols degrade during the life of a solar absorber and therefore require replacement. Another method used for freeze protection in systems where drinking water is used is to pump hot water from the thermo 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 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, normally this equipment could not prevent freezing in the system. Another commonly used method is to drain the absorber liquid or part of it through an antifreeze valve placed at the bottom of the system that opens mechanically once a certain setpoint temperature is reached (usually 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 mainly two disadvantages: (a) it can be opened a large number of times in conditions of extreme temperatures, which results in a significant loss of wasted system water:. (. B) - because- they are mostly made up of moving parts that could be damaged over time, which could result in the device not acting well after some time or that the device could be obstructed by sediments that could accumulate in the lower part of the system, avoiding that when opening the valve the water can drain properly. Inventions related to anti-freeze protection in absorbers and solar systems have been found, below we describe its limitations and differences with the one proposed: US Patent 4044754 is an anti-freeze protection system for solar absorbers with sensors inside the absorber that monitor the temperature of the water and when it approaches the freezing temperature, the sensor operates 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 refill the absorber with water, our model has no electronic actuators or moving parts that can fail over time and works without need of 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 number of times during its useful life. US patent 4557252 is An antifreeze system for solar systems with forced convection (uses a 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 pump injects liquid from the tank at a higher temperature to the absorber, our model has no electronic actuators or moving parts that can fail over time and works without electricity, in addition to the Previously, US 4557252 has the disadvantage that the thermal losses of the system are increased by pumping water already heated to the absorber where it will be cooled. US 4237862 and US 4269167 are for closed and pressurized solar systems where water is drained from the absorber without the need for a sensor or valve (does not contain moving parts), it only requires a circulation system with a pump. The tank where the water is received is filled only up to_cier.to-level-qu

of the circuit included the absorber and the pipes can be collected in the tank once the pumping system stops, which causes that the water of the system is returned by gravity to this tank, leaving the absorber ueno of the air that contained the tank preventing In this way its freezing, our system can be used in systems with natural convection (teimosiphonic systems widely used in Mexico and other countries) or forced convection such as these patents and it is not necessary in our case that the absorber is necessarily in a level higher than the tank so that it can be operated or the tank is oversized to be able to capture the water coming from the entire system once it drains into it. US 4308856 is an antifreeze device consisting of an eight-shaped tube welded to the absorber fin that has a steel bar on its two sides and both bars are joined by a U-shaped steel profile. so that when the volume of the inner fluid of the tube is increased in the form of eight, an elastic deformation of the tube and the U-shaped steel profile will occur, said deformation will return to its original shape once the ice has been undone thanks to the U-profile with which the tube is attached in the form of eight, our system does not require special geometries to be made for pipes or of additional profiles (which increases its cost) to be able to counteract the effect of water expansion when it is frozen, our system can be placed in virtually any normal configuration of the absorbers that exist in the market without appreciably increasing the cost of system. GB 2117110 uses a system of different energy dissipation means associated with each absorber duct; These means are eyelashes or fins, which the author says is that 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 into the absorber heads 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 greater volume than the stringers (risers), our system can be placed in virtually any configuration of the _. a.bspr.bedores.que-existe-en-el-mer = cado without significantly increasing the cost of the system and without changes as in this case in which fins must be added to the absorber pipes to produce the aforementioned effect. US 5413091 uses conical stringers and heads are completely insulated. The conical shape of the stringers is effective to avoid the rupture of the pipe since it starts freezing and hence 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 extends towards the ends of the stringers, where, because the head is insulated, the freezing is not reached in this area or is much slower, causing the expansion produced by the water in the stringers, our system does not require that the heads be isolated (which decreases the area of solar collection) or that special conical pipes are manufactured with their minimum cross-section in the center of the crossbar so that this effect can occur (these special stringers make the absorber more 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 collector is registered, that is, that it can be used indistinctly by facing the sun its upper part or turn it and put the lower part; This can extend the life of the system because it can be rotated if for some reason the cover undergoes some degradation over time and the collector will continue working normally. Additionally, it was not found in any patent of a flat solar collector that will be used as insulation a cellular ρο-licarbonate with stationary air 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 easily degrade over time by 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 metallic or plastic foil, for example foil 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, there is an absorbent plate (for example of plastic materials such as polypropylene). The absorber plate is composed of a large number of flexible ducts (for example plastic) attached to the inlet and outlet heads through which a heat transfer fluid flows (generally water, although glycol, or a mixture of both, can be used). The absorber plate is made of a material that does not degrade with ultraviolet rays and acts as a selective surface (dark colored finish) thereby increasing the heat absorption of the sun's rays. The object of the invention is to provide a reversible flat solar collector with protection against freeze damage, scale and problems related to insulation that deteriorates with moisture, which can be coupled to a direct system (thermosiphon systems or forced convection systems, where the agiia that will be used later passes through its ducts). In particular, solving the problem that arises in this type of direct systems when they are exposed to temperatures low enough to cause the internal fluid to freeze, causing it to expand and fracture the absorber pipes leaving it unusable. , as well as the possible obstruction of the ducts in general used of small diameters due to the use of hard waters (with high salt contents), and avoid using traditional insulations (mineral wool, polyurethane, etc.) that degrade with moisture . The materials and manufacturing processes used in this sensor can be inexpensive to promote its massive use and not have moving parts that can be damaged during its useful life that must be at least 10 years.

The-parts-that-compose the invention are the ^ igüieñtes'. 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 that very high temperatures are reached inside and the backrest (5) also of cellular polycarbonate equal to that used as a cover. Inside the absorber heads is the device to prevent damage of the absorber in case of freezing of water (6), which is detailed in Figure 2. Inside the box, exposed to the sun, an absorber plate is placed (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 color) thereby increasing the heat absorption of 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 those of the ends. In both cases, air is trapped inside that absorbs the pressure exerted by the water when it solidifies and presents volumetric expansion. Figure 3 shows the plastic absorber (polypropylene) and a detail of a cross section formed by a large number of flexible stringers (7) of black or dark plastic (to increase the absorption of solar radiation) attached to the heads (8) In these pipes circulates the fluid to be heated. The plastic damping device (9) shown in Fig. 3 is inserted along the length of both heads. This device acts as follows: the absorber used (Fig. 3) either in direct systems of forced convection (where a water pump is used to circulate the water through the collector) or of natural circulation (as is the case of systems terrn £ siphonLC.os_ampliamen ^

regions), it is completely filled with water, once the environmental conditions either by ambient temperature or by radiative losses with the clear sky cause the water to descend to temperatures below 0 ° C, in this way the water freezing process with its consequent increase in volume by approximately 9%. This increase in the "volume of water on freezing that can cause fractures in the pipes is avoided on the one hand because the damping device (Fig. 3) has been designed so 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 water when frozen is absorbed by this damping device (9) that is compressed as the water freezes, because it is flexible and contains air inside. 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 these and their large number, flexible pipe is used in the construction of the stringers (7), which causes them to slightly modify its shape and also dampen the freezing in case This is abruptly. Some advantages of the freeze protection device is that it has no 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 absorber's water is replaced with water from the tank produce which causes the thermal losses of the system to be significantly increased, by pumping already heated water to the absorber where it will cool. 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 that of making special pipe geometries or adding additional fins to dissipate energy, which obviously It makes them have to be sent to make special pipes that would greatly increase the cost of the absorber. This device additionally produces that the absorber _y_eLsist.ema_in_the_that-is-installed- can also dampen sudden increases in pressure such as those that can be caused by water hammers or in the initial filling of the system. Another serious problem is the use of flat solar collectors in direct hard water systems (with high salt contents) which in general because flat solar collectors are formed by few metal stringers (risers, typically 6 to 14) in diameters smaller than an inch in diameter where it circulates, in the case of thermosyphonic systems water at low speeds and sometimes at high temperatures, it can eventually produce deposits of these salts that can sometimes block the duct leaving the duct 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 ducts (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 similar) has a greater resistance to the adhesion of salts compared to metal pipes. Another important part of the invention is that the collector is insulated at its front and rear (where there is the largest area of losses compared to the lateral sides) with an area of air trapped in cellular polycarbonate; which, being in a steady state, has a thermal conductivity similar to polyurethane close to 0.024 W / mK, this advantage allows this insulator not to degrade as the insulation materials commonly used in flat solar collectors such as mineral wool or polyurethanes which absorb a large amount of moisture and easily degrade over time upon 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 the collector stiffness. This system allows as an additional advantage that if the cover (2) for some reason deteriorates over time, the collector can turn 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, pintrq or aluminum foil_is_that_all-materials-are not-degraded by being in direct contact with environmental conditions of temperature, humidity, ultraviolet rays, etc. which extends the system life. Additionally, it allows the collector to not be completely airtight and can have vents (4) to prevent the absorber from reaching high temperatures, such as those produced in conditions of stagnation in flat collectors, which can cause the absorber plastic to degrade over time. and do not reach the planned useful life or avoid using more expensive plastics to prevent degradation due to exposure to high temperatures.

This invention therefore extends the life of the collector because it cannot reach excessively high temperatures that degrade the materials from which the collector is made; the inside of the collector cannot reach excessive temperatures due to the vents (4) that have been designed in the frame of the collector so that by means of natural convection, hot air is removed from them inside and replaced by temperature air environment considerably reducing the maximum temperatures that can be reached inside it. Description of figures

Figure 1. Reversible flat solar collector components.

Figure 2 Damping device formed by a flexible plastic pipe sealed at its ends (a) or all along it (b).

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

Claims

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

The flat solar collector with protection against freeze damage according to claim 1 characterized by having a construction of the absorber by means of flexible stringers allows the increase in the volume of water when frozen to be damped by said elements preventing the fracture of the pipes of the absorber

The reversible flat solar collector, which can be used interchangeably by facing the sun its upper part or turn it and put the lower part.

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

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

The device for protection in case of freezing in absorbers according to claim 1 characterized in that it has a damping device formed by flexible plastic tubing sealed at its ends or along the entire length thereof used to absorb the increase in volume suffered by the water when frozen. The diameter of the flexible pipe of this device will depend on the total volume that the water occupies in the absorber, in such a way that the air or vacuum space inside it is at least 11% of the volume that the water occupies in the system, this to be able to absorb the increase in volume of water when frozen without affecting the absorber.

The device for protection in case of freezing in absorbers according to claim 1 characterized by stringers of the flexible pipe absorber to also dampen in case of abrupt freezing by its flexibility to move and subsequently return to its original form.

The device for 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 hammers 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 by being in direct contact with moisture, ultraviolet rays, etc. (in general materials such as plastics, pintro or aluminum foil) extend the life of the system.