Solar collector with microstructured absorber surface

Abstract

Solar collector with liquid channels provided directly in an absorber surface (6), which are densely packed next to each other, run parallel to each other in the absorber surface (6) and completely cover the absorber surface (6), wherein the heat transfer medium is guided directly through the liquid channels in the absorber surface (6) without the need for pipe systems inside the collector, furthermore the liquid channels are integrated directly into the absorber surface (6) as microstructures and have cross-sections in the range of a few micrometers to a few millimeters, and the absorber surface (6) is designed in the form of individual, parallel strips. where the strips are arranged at intervals while retaining light-transmitting areas and a mirror surface (10) is formed below the absorber surface (6) such that the strips are located in the focal axes of mirror curvatures running parallel to each other.

Description

[0001] The invention relates to a solar collector with liquid channels that are integrated directly into the absorber surface according to claim 1.

[0002] The channels have cross-sections ranging from a few micrometers to a few millimeters, thus offering the heat transfer medium flowing through them a relatively large exchange surface. The liquid channels are arranged close together, completely covering the absorber surface, which means the collector does not require a conventional internal piping system.

[0003] The absorber surface can be a single plate or in the form of individual, parallel strips with a square or round cross-section, spaced a certain distance apart. It is located inside a housing, the top of which is covered by a transparent glass or plastic pane and appropriately insulated against heat loss. Reduced or minimal air pressure inside the collector can further improve heat output. The solar collector can be designed as either a solar thermal collector or a combined solar thermal / photovoltaic collector.

[0004] The use of solar energy for everyday purposes has gained renewed relevance, not least because of the increasing scarcity of conventional energy sources. Although photovoltaics has made great strides in recent decades, it has yet to achieve widespread adoption. The cost of solar power generated in this way is currently not competitive.

[0005] Solar thermal systems, on the other hand, are gaining popularity, particularly in southern regions. However, their use as a supplement to existing systems for domestic hot water and heating is also increasing in our latitudes. Compared to photovoltaic systems, solar thermal systems offer several advantages: their technology is relatively simple, thus keeping the necessary investment costs manageable.

[0006] One of the most important components of solar thermal systems are the solar collectors. They convert the sun's heat energy into usable thermal energy. Currently, almost all commercially available collectors operate on the same principle: A heat transfer fluid is heated by the sun inside specially designed and arranged pipes or tubes. The heat is then transported out of the collector by circulating this fluid.

[0007] The collectors currently available commercially for domestic hot water heating can be classified as follows: Flat plate collectors; Vacuum tube collector; Absorber mats / hoses. Flat plate collectors:

[0008] This type of collector is constructed as follows: Beneath a cover plate is a dark absorber surface. Below this are pipes through which a heat transfer fluid flows. The absorber surface is heated by solar radiation, and consequently, so is the heat transfer fluid in the pipes, which then transports the heat out of the collector. To minimize heat loss, the collectors are lined with insulation or evacuated. Vacuum tube collectors:

[0009] These are high-performance collectors and significantly more expensive than flat-plate collectors. They can also achieve higher temperatures. This type of collector consists of concentric tubes within a vacuum. The outer glass tube is transparent, while the inner one has a special coating. Inside the inner tube run the heat transfer pipes, which are themselves embedded in metal profiles for improved heat transfer. The concentric tubes are located at the focal point of special mirrors that concentrate the heat radiation onto the tubes. Absorber mats / hoses:

[0010] This is the cheapest and simplest way to extract thermal energy from the sun. Due to the lack of a cover plate or insulation, heat losses are correspondingly high and efficiency is low. These systems often consist of an arrangement of black plastic tubes or dark plastic mats spread out in the sun, through which the heat transfer fluid (usually water) flows. They are frequently used to heat the water in outdoor swimming pools.

[0011] Numerous variations of these collectors are described in the literature. High-tech solutions use vacuum tubes, while low-cost versions use plastic pipes or hoses for heat transfer. The following are some examples of patents representing the state of the art:

[0012] For example, US patent application US 2008 / 0047548A1 describes a flat-plate collector that uses mineral wool for internal thermal insulation. Its construction otherwise corresponds to the classic design of a flat-plate collector with pipes for transporting the heat transfer medium.

[0013] The collector described in application WO 2008 / 051 068 A1, however, completely dispenses with pipes for transporting the heat transfer fluid within it. The fluid flows between two plates, the lower one being dark-colored and the upper one transparent. To reduce heat loss, this sandwich arrangement is covered by another transparent disc within a housing.

[0014] Patent specification AU 2002301425 B2 describes a further collector in which the heat transfer medium circulates between two plates. The disclosure focuses primarily on the method for manufacturing such a plate. Here, two semi-finished plates are pressed together, joined, and connected in such a way that webs and cavities (channels) are formed between them, through which a liquid can flow.

[0015] European patent application EP 1 916 486 A2 also relates to a collector in which the liquid channels are partially integrated directly into the absorber plate. This collector operates on the heat pipe principle. A low-boiling liquid is contained in some of the channels, which evaporates when exposed to heat and releases heat upon condensation. The heat is transferred from the plate to the transport pipe carrying the heat transfer fluid, which is in close contact with the plate.

[0016] Patent application WO 2008 / 017091A1 relates to a solar collector for demand-adapted recording of solar radiation. Here, a specially designed, mirrored reflector surface exposes the absorber element to more or less solar radiation depending on the angle of incidence.

[0017] Low-cost collectors in particular use flexible materials for the absorber surfaces.

[0018] For example, application WO 2007 / 113 351 A1 describes a system in which the heat transfer fluid flows through an assembly of pipe or hose rings, which in turn are placed on a dark, insulating film. To reduce heat loss, the assembly is covered by another, transparent film.

[0019] The collector described in utility model DE 20 2005 001 488 U1 is made entirely of flexible materials and is designed as an inflatable collector. The heat transfer fluid flows through a coiled tube inside the collector, which is embedded in air chambers. The upper part of these chambers is transparent, while the lower part and the tube are dark-colored.

[0020] Although some of the examples listed here forgo internal pipes and the heat transfer fluid circulates through channels inside the collector, they are essentially of a "classic" design. Furthermore, the surface area to volume ratio largely corresponds to that of collectors that use pipes on the absorber surface.

[0021] Microstructures, i.e., liquid channels with diameters of a few micrometers or a few millimeters, exhibit a very favorable surface area to volume ratio. This leads to improved heat exchange thanks to the larger exchange surface area relative to the liquid volume. Microstructures for liquid transport represent the state of the art.

[0022] The closest solution according to US 2007 / 0074514A1 discloses a solar collector containing two parallel pipes. One pipe serves as the inlet and the other as the outlet for the heat transfer medium. The pipes are connected to plates at regular intervals. These plates have parallel fluid channels inside and include translucent sections.

[0023] Based on the prior art described here, the present invention aims to describe an improved solar collector for solar thermal energy in which the absorber surface is traversed by microchannels through which the heat transfer medium flows.

[0024] This problem is solved by the combination of features according to claim 1.

[0025] The very favorable surface area to fluid volume ratio ensures better heat exchange. Furthermore, the absence of additional piping inside the collector simplifies the design, thus reducing manufacturing costs.

[0026] The basic idea behind the new solar collector is to direct the heat transfer fluid directly through an absorber surface, completely eliminating the need for a conventional piping system. To ensure the largest possible contact area for the heat transfer fluid, it flows in parallel through numerous microchannels with dimensions ranging from micrometers to several millimeters. These microchannels are arranged close together, thus covering the entire absorber surface.

[0027] The absorber surface consists of parallel strips with a square or round cross-section, arranged at a certain distance from each other. Integrating the heat transfer channels into the thinnest possible absorber surface ensures high efficiency. The strips can be made of metals such as aluminum, copper, etc., or of plastics or ceramics. The comparatively low thermal conductivity of plastics or ceramics compared to metallic components is advantageous here, as the efficiency of microstructured heat exchangers is known to benefit from low lateral heat conduction. Only polymers with high temperature resistance are suitable for this application. These include, in addition to special thermoplastics such as PTFE, POM, PA, etc., or elastomers such as SI, SR, NBR, PU, ​​EPM, EPDM, etc., primarily thermosets such as UF, MF, PF, UP, EP, etc.

[0028] Depending on whether the absorber surface consists of a single piece ( Fig. 1), or according to the invention is composed of several sections (strips) ( Fig. 2 and Fig. 3) Such solar collectors have different structures. Examples of different cross-sections of the microstructures or strips are shown in Fig. Figure 4 is shown as an example. Combinations of these cross-sectional geometries, as well as other geometric shapes, are possible.

[0029] At the in Fig. In the collector shown in Figure 1, an outer shell or frame [1] surrounds the mounting [4] for the individual plates. A thermal insulator [3] made of plastic or another insulating material is located between the frame [1] and the mounting [4]. A transparent cover plate [2], the absorber surface [6], the insulation layer [9], and the lower cover plate [5] are connected to the mounting [4]. The transparent cover plate [2] can be made of either glass or a transparent plastic and is designed to minimize heat loss. Similar to the thermal insulator [3], the insulation material for the insulation layer [9] can be either plastic or another material with good insulating properties. Microchannels are incorporated into the absorber surface [6] along the entire length of the plates. They are densely packed and run parallel to each other.Their ends are open and open into the collecting channels [8] and [7] in the bracket, which extend across the entire width of the plate. Both the collecting channel [7] and [8] can be closed at one end, if necessary, by means of appropriately sealed screw connections. This allows the collectors to be operated either individually or combined into arrays.

[0030] If a heat transfer medium (e.g., water or glycol / water) is pumped through the collection channel [7] and this channel is closed at one end, the fluid is forced through the channels inside the absorber surface [6], exits at the opposite end, and is collected in the collection channel [8]. As it flows through the absorber surface [6], the fluid absorbs the heat energy radiated through the cover plate [2]. At the open end of the collection channel [8], the now warm heat transfer fluid is discharged from the solar collector and is available for the external circuit. To minimize heat losses, the collector may be evacuated, or the air pressure inside it may have been reduced to a minimum.

[0031] If the absorber surface [6] is divided into individual segments (strips) and these are arranged parallel to each other at certain intervals, the following is obtained: Fig. 2 shown in the inventive collector. These strips can either be individually connected to the support [4] or themselves be attached in a sandwich construction between two transparent support plates. Inside this sandwich, the air pressure can be reduced to a minimum. The fact that the in Fig. 2 the absorber area shown [6] not as in Fig. The fact that the collector element (strip) is not closed but has several translucent sections has the advantage that, according to the invention, a mirror surface

[10] can be installed below the collector surface (strip). This mirror surface

[10] is structured and positioned such that the collector elements (strips) are located in the focal axes of the parallel mirror curvatures. This exposes the collector elements (strips) to thermal radiation from both sides, which further improves heat transfer. As already described in [reference to relevant section], there is a [reference to relevant section] between the mirror surface

[10] and the lower cover plate [5]. Fig. Figure 1 shows an insulating layer [9]. The further structure, including the other designations, as well as the operation of the collector, corresponds to that of Figure 1. Fig. 1.

[0032] A constructive variant of the in Fig. The collector shown in section 2 is in Fig. 3 is shown. Here, each strip with the microchannels for the heat transfer medium was installed inside its own tube

[11] , following the model of vacuum tube collectors. The cross-sectional geometry of this tube

[11] can vary (e.g., square or round). The air pressure inside these tubes

[11] can be reduced to a minimum. The upper transparent cover plate and the additional thermal insulation under the mirror surface were omitted. The function and all other designations correspond to those from Fig. 2 or Fig. 1.

[0033] Based on this design, a similar construction is conceivable in which the mirror surface is also divided into individual strips. The individual tubes with the underlying mirror half-shells are mounted at a certain distance from each other. This has the advantage that, for example, snow can fall through the individual strips in winter, thus reducing the snow load.

[0034] The in Fig. The cross-sectional geometries of the channels inside the absorber surface shown in Figure 4 [6], as well as the geometries of the strips themselves, are to be understood as exemplary embodiments. Combinations of these cross-sectional geometries and other geometric shapes are possible.

[0035] If photovoltaic modules are attached to the absorber surface(s) of the structures shown above, a combined solar thermal / photovoltaic collector is obtained. Alternatively, the microstructures shown here can also be integrated directly into the photovoltaic modules. The advantage is that this prevents overheating of the photovoltaic elements and simultaneously allows heat energy to be recovered for other applications.

Claims

[1] Solar collector with liquid channels provided directly in an absorber surface (6), which are packed closely together, run parallel to each other in the absorber surface (6) and completely cover the absorber surface (6), wherein the heat transfer medium is guided directly through the liquid channels in the absorber surface (6) without the need for pipe systems inside the collector, furthermore the liquid channels are integrated directly into the absorber surface (6) as microstructures and have cross-sections in the range of a few micrometers to a few millimeters, and the absorber surface (6) is designed in the form of individual, parallel strips, where the strips are arranged at intervals while retaining light-transmitting areas and a mirror surface (10) is formed below the absorber surface (6) such that the strips are located in the focal axes of mirror curvatures running parallel to each other. [2] Solar collector according to claim 1, characterized by , that each strip is arranged inside its own vacuum tube (11).

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