WO2008051068A1

WO2008051068A1 - Novel geometry for a tubeless solar collector for water

Info

Publication number: WO2008051068A1
Application number: PCT/MX2007/000128
Authority: WO
Inventors: Gerardo GÓNGORA GALLARDO
Original assignee: Gongora Gallardo Gerardo
Priority date: 2006-10-27
Filing date: 2007-10-24
Publication date: 2008-05-02

Abstract

The invention relates to a novel geometry for a flat-plate solar collector for water, having no grid of fluid delivery tubes or head tubes and comprising a pair of plates forming a sandwich containing the fluid. The upper cover is transparent to solar radiation and the lower opaque absorbent plate converts said radiation into heat which is distributed to the water by means of natural convection. The liquid contained between the plates removes the radiation from between same, dispensing with the need for costly selective metal low-emissivity plates which are replaced by dark polymer plates while the Low-e function is provided by the transparent cover. The volume generated by a second transparent cover is used to reduce heat loss employing different techniques, such as: vacuum glazing; injection of gases with low heat transfer, such as argon and krypton; and filling with porous silica aerogels with low heat transfer. The geometry of the collector eradicates bacteria of the Legionella family due to the bactericidal action of the ultraviolet radiation.

Description

NEW SOLAR COLLECTOR GEOMETRY FOR WATER ₅

WITHOUT TUBES

BACKGROUND OF THE INVENTION

Currently, conventional solar water heating collectors (figures IA without cover and IB with cover) use to convert the energy of the solar spectrum to long-wave (infrared) heat radiation, by using selective metal plates (1) chemically treated that increase the absorption capacity of solar radiation and decrease its heat emissivity or, the other option, is coated with black paint. After the above, infrared radiation tries to escape to the environment (which is generally colder) by two paths; the first is on the front side (where the radiation entered) and, to avoid it, a cover (2) of glass (or polymer) transparent to the solar spectrum but opaque to the far infrared is used, to create the greenhouse effect in the air volume (3) and also, so that the resistance to heat escape to the environment increases. The second heat path is in the direction of the tubes (4) that are welded to the absorbent plate and which contain the liquid that finally takes advantage of the heat. The rear face of this plate is lined with high thermal resistance insulator (6) to prevent heat loss to the environment and ensure the highest concentration of heat in the tubes.

The transfer of heat between the walls of the tubes to the liquid is carried out by means of the conduction phenomenon (7) and on a smaller scale by natural convection. The latter is due to the heating of a container from below and the heated liquid (less dense) rises to be replaced by a denser cold liquid, ensuring a better heat transfer than by the phenomenon of thermal conduction, which is through the elastic shock between molecules and electrons.

Raised the temperature of the fluid, contained in the tube rack, rises due to the thermosiphon phenomenon through the head tube (8) transverse to the grill to an accumulator tank, thus renewing the hot water in the collector through the tria. Another option is to recirculate the cold fluid by means of an external pump.

In the first conventional systems by absorbent plates painted black, the heat loss by wind convection was reduced by placing a transparent cover (2), however, with the above, there was another extra loss of heat towards the environment due to the phenomenon of radiation between the dark plate and the cover but, since its value is lower than the loss due to convection to the wind then, it was useful to add covers.

The previous problem, due to the loss of heat by infrared radiation between the black painted plate and the transparent cover, was improved by conventional systems by chemically coating surfaces of metal absorbent plates to convert them into selective plates, which maintain a high coefficient value of solar absorption and a low value of heat emissivity coefficient. This last feature reduced the extra phenomenon of radiation between plates; but this implied the cost of conventional solar collector systems.

There are various geometries of flat plates with tubes: absorbent plate with tubes on its lower face (as in figure -IA); plates with tubes on its upper face; tubes sectioning the plate in the form of fins; also tubes that meander over the absorbent plate; tubes sectioning plates in the form of narrow flat fins but mounted inside a glass tube evacuated at low pressure. However, in all these geometries the thermal conduction is maximized using metallic materials both in the absorbent plate and in the tubes attached to it.

On the other hand, there are specific health regulations in other countries for the elimination of bacteria from water in solar collectors, such as the requirement to raise temperatures at their exit to values higher than

50 ⁰ C. This is due to the latent bacteria circulating in conventional collectors on dark tracks of copper and low temperature pipes, which at the time of being activated by the rise in water temperature (as expected in a collector solar) makes them a public health problem, such as Legionella pneumophila bacteria. However, raise to these temperatures when the water to be used must be between 36 to

42 ⁰ C, makes them inefficient.

DESCRIPTION OF THE INVENTION

The new tubeless solar collector geometry (figures 2A and 2B) ₅ consists of two plates containing the fluid (8) between a transparent upper cover (9), which allows the entry of visible solar radiation, and another plate (10 ) inferior and opaque to the solar radiation that absorbs it and converts it into infrared heat radiation. However, since there are no fluid conductive tubes on the plate and only one insulation lining on its back (13) and on its sides (not shown in Figure 2A), there will be a only way where the heat will try to escape to the environment, which is the same where the radiation enters (the volume of the liquid). Then, the heat will flow from the bottom up through the phenomenon of natural convection (11), canceling the phenomenon of thermal conduction that is the mainstay in conventional systems. After transferring its heat to the liquid then it goes to the transparent covers (9) and (12) where they play their greenhouse role and then let the residual heat escape into the environment.

It is from the scientific domain that the coefficient of heat transfer by convection in a liquid is greater than the coefficient of thermal conductive transfer (used by conventional collectors); in this way, having a single path for the evacuation of heat (through the liquid) and having a higher coefficient of heat transfer by convection ensures a faster distribution of heat in the liquid than in conventional systems.

From the aforementioned problem in the background, by the cost of conventional solar collectors to have selective plates; With the new tubeless collector, the emissivity problem of the absorbent plate ceases to be important, because the phenomenon of radiation transfer between plates is nullified by having liquid in between, leaving therefore only the phenomenon of heat transfer by natural convection over the liquid; Then, any other type of cheaper absorbent opaque plates such as polymeric materials, among other materials, can be used. The low heat emissivity function for the collector is still necessary, but in this new geometry it is now performed by the transparent low cover (9) emissivity as noted below.

With the elimination of the rack of temperature risers and the pair of head tubes in the new manifold, the frictional loss of the flow of the liquid that passes through is reduced and, its flow velocity is increased causing two benefits: Less is maintained " heat "to the collector and also, its connection can be extended to the heat accumulator tank without detriment to the thermosiphon effect (or for effects of loss of power in the systems with pumping). Therefore, the new tubeless collector: increases efficiency; It cuts production costs against conventional solar collectors, by saving both tubes and labor by welding, as well as expensive metal selective plates. Now the manufacture of these new collectors can be carried out with polymeric materials that facilitate molding on an industrial scale.

Even with the above, better performances are presented by sealing the air enclosure that forms between the transparent covers (9) and (12), and then using various techniques, elements and compounds that reduce heat losses to the environment, such as: a) The application of low, medium or high vacuum on the air enclosure to eliminate its convection and decrease or eliminate its thermal conductance but used supporting pillars between the covers (9) and (12) against the atmospheric pressure generated "Vacuum Glazing". b) The application of watertight gases to eliminate the phenomenon of convection and reduce thermal conductance. These are: Argon, Krypton. Xenon, CO ₂ , among others. c) Use aerogels of porous and ultralight silica structures that eliminate thermal emissivity between plates, as well as convection, achieving low thermal conductances. d) Use thin film on the faces of the transparent covers (9) and (12), to reduce the solar reflection "AR" and therefore, increase its solar transmission. e) Use thin film on the inner face of the recess in the transparent cover (9), to reduce the emissivity of heat "low-e", by means of transparent conductive oxides "TCO" with Tin, Zinc, Cadmium, Silver, Titanium , among others.

The objective of the new collector are applications that use different liquids, such as: Natural water; fluids used by heat exchange systems (antifreeze, silicone, petroleum derivative, opaque liquids to solar radiation, etc.). This new collector is useful and adaptable in single-family, multi-family homes, in offices and for the preheating of liquids in industrial heat transfer processes, as well as in the heating of swimming pools or sinks. This new collector covers the ways to transfer heat, whether they are the thermosiphon system or using pumping equipment. This new collector is suitable for use as a receiver for solar radiation concentrating reflector attachments.

The geometry of the new collector allows through its two transparent covers (9) and (12) (or more) to expose the bacteria, contained in the water, to the bactericidal action of ultraviolet radiation from the transmitted solar spectrum. The Legionella pneumophila bacteria and other Legionella species are eliminated with low levels of solar UV radiation and their exposure time that depends on the flow rate of the water. In this way the new collector does not require temperatures above 50 ⁰ C to eliminate bacteria as in the collectors conventional, and therefore, better performance is achieved by lower temperatures.

Claims

1, - New flat solar collector geometry that does not use heating tubes welded to the opaque absorbent plate, as in conventional collectors; and is characterized by a transparent upper cover (9) (glass or polymer) to solar radiation and an opaque lower plate (10) to solar radiation containing the liquid as in a sandwich and, two lateral openings to allow its flow. Conventionally lined with thermal insulating material on the back and sides. The incident solar radiation is converted into heat by the opaque absorbent bottom plate (10), which is then efficiently distributed in the liquid by means of natural convection. The water can be distributed by thermosiphon to the storage system or by a pump for distribution. Residual heat escapes into the environment through the enclosure between transparent covers (9) and (12) (or other covers).

2.- Derived from claim one, the air enclosure formed by the two transparent covers (9) and (12), is characterized by reducing heat loss to the environment and is also characterized by using elements, compounds and various techniques of individual or combined way to reduce thermal loss; same that are described: a) Filling of low conductance gases that reduce thermal convection, such as: Argon, Krypton, Xenon, CO ₂ , among others; b) as well as, by applying low, medium or high gas vacuum to reduce convection and thermal conduction, and using support pillars against atmospheric pressure "Vacuum Glazing" between the covers (9) and (12 ); c) likewise, by filling porous silica structures called aerogels, of low thermal conductance that reduce emissivity and convection thermal, d) Also by the application of thin film with transparent conductive oxides "TCO" on the inner face (to the enclosure) of the roof (9) reducing the emissivity of heat "low-e". e) Also with the application of thin film on the faces of the roofs (9) and (12), which produce the "AR" solar anti-reflection effect to increase its solar transmission.

3- Derived from claim one, that by containing liquid between the transparent cover (9) and the absorbent plate (10), the phenomenon of heat transfer by radiation between them ceases to have effect, then its function is characterized by using plates dark absorbents regardless of their emissivity, selecting polymeric materials (or other type of material) that are easily constructed using molds and therefore, cheaper.

4- Derived from claim one, by eliminating the tubes in the new manifold, its function is characterized by decreasing the frictional loss of the liquid within the new manifold. Having consequences such as: the increase of the speed of the fluid (decrease of the temperature of the collector) and the increase of the distance between collector and accumulator without affecting the phenomenon of thermosiphon.

5, - Derived from claim one, the legionella pneumophila bacterium is exposed to the ultraviolet radiation that crosses the covers (9) and (12) of the new collector, and which characterizes it as a bactericidal agent depending on the time imposed by the velocity flow regulated by thermosiphon or, where appropriate, by an external pump.