WO2007113351A1

WO2007113351A1 - Low-cost solar collector

Info

Publication number: WO2007113351A1
Application number: PCT/ES2007/000165
Authority: WO
Inventors: José María ABRIL HERNÁNDEZ; Arturo Granged Pascual; Valeriano RUIZ HERNÁNDEZ; José Maria CÁMARA ZAPATA; Antonio MARTÍNEZ GABARRÓN
Original assignee: Universidad De Sevilla; Universidad Miguel Hernández
Priority date: 2006-03-30
Filing date: 2007-03-27
Publication date: 2007-10-11
Also published as: ES2304289A1; ES2304289B1

Abstract

The present invention relates to a low-cost solar collector arranged horizontally on the ground, which is composed basically of: a) an insulating material in the lower part, covered with black plastic whose function is that of absorber; on which are placed b) a number of pipes in a circular arrangement along the collector, connected to one another via their ends and connected to a hydraulic circuit by means of an inverted return arrangement, which guarantees suitable hydraulic behaviour on the part of the collector; and c) a cover with a transverse section in the form of a circular arc, made from transparent plastic material, for reducing losses of heat through convection. The solar energy collector that is the subject of the invention has been developed basically to boost the conventional heating of greenhouses and to reduce the consumption of fossil fuels.

Description

Title

Low cost solar collector

Object of the invention

The present invention aims at a low cost solar collector arranged horizontally on the ground, which is basically composed of: a) an insulating material in the lower part, covered with black plastic whose mission is that of absorber; on which b) several pipes are placed in circular arrangement along the collector connected to each other at their ends and connected to a hydraulic circuit by means of an inverted return arrangement, which ensures adequate hydraulic behavior of the collector and c) a cover with Circular arc-shaped cross section of transparent plastic material, to reduce heat losses by convection.

The solar energy collector object of the invention has been developed to support conventional greenhouse heating mainly and reduce the consumption of fossil fuels.

State of the art

Greenhouse heating a. Starting from solar energy has been studied in numerous countries. Thus, in the region of Salta (Argentina), Iriarte eí al. (1981) began the development of a water-air heat exchanger with a thin plastic surface. The first tests were carried out using a hot saline solution from a solar well for drying agricultural products and auxiliary heating of a greenhouse (Iriarte eí al. 1981, Lesino eí al., 1983). Subsequent modifications (Saravia eí al., 1992, Iriarte eí al., 1993) achieved a system that fulfilled the dual function: collector - exchanger during daytime hours and exchanger during nighttime. The solar collector was built with long-lasting polyethylene tube (LDT), transparent and black bags. Horizontal ducts were constructed on the bags by welding, so that when they are placed in an upright position, the water that enters through the upper part falls by gravity and circulates inside it through the zigzag-shaped channel. Another prototype successfully used for the heating of greenhouses with hydroponic cultivation consists in the use of a ^' heat accumulator formed by packages of water bottles arranged inside the greenhouse, on which a flow of air collected in the Ia is circulated upper part of the interior of the greenhouse. The air transmits the heat transported through the wall of the container. The accumulators are mounted on the ground and serve as support for the cultivation tables (Saravia et al., 2000).

On the other hand, . The University of Rutgers (New Yersey, USA) developed a system to support the heating of greenhouses by solar energy based on an inclined collector that stored on average 50% of the available solar energy and had a return time of ten years (NRAES, 1992).

In Spain, a series of solar installations for greenhouses have been built and evaluated. In the province of Almería, an installation of 90 m ² of panels outside the greenhouse was built during 1981. The collector absorbent was made of EPDM rubber (ethylene, propylene and diene monomer) in the form of small tubes through which water circulated. The absorbent was covered by a thermal insulating polyethylene film and insulated on its back by 3 cm of polystyrene. The collectors were facing south. with a slope of 55 °. The hot water stored in a reservoir lagging 20 m ^2. The heating of the greenhouses was done by means of buried pipes with a density of 2 m of tube / m ² of soil.

To simplify solar heating systems to the maximum, collection equipment located within the greenhouse itself has been developed. The simplest of these equipments consists of a series of transparent polyethylene tubes filled with water and arranged between the crop lines. It involves capturing solar radiation during the day, heating the water in the bags and slowly giving that heat to the greenhouse, taking advantage of the thermal inertia of the water. In Greece, tubes 30 cm in diameter of 250 microns thick polyethylene are used. Approximately 35-40% of the greenhouse soil is covered with pipes, so that every 1,000 m ² of greenhouse contains between 80 and 100 m ³ of water. In Spain, more modest results have been measured, partly because 40% of the greenhouse soil was never covered and partly ^" because this solar heating is applied to poorly sealed structures. In Almería, with black bags with water, which they are worse than the transparent ones for this application, the minimum temperatures were increased by 1 ⁰ C. When the crop was developed, the bags they were shaded and the effect on the night temperature was negligible. In Cabrils (Barcelona) transparent bags were arranged covering 20% of the soil of a tunnel used to cultivate on a slope. With this device it was possible to save a frost that occurred on a clear night, since the passive solar system maintained a thermal jump of 2 ⁰ C compared to a similar greenhouse without water bags (Matallana and Montero, 1995).

More recently, it was tested with a collector-accumulator, in the form of a covered raft, located inside a greenhouse subjected to cold climates and concluded that the installation can reduce heating costs for a long period of time (Al-Hussaini and Suen, 1998). In Morocco, flat plate metal collectors have also been tested and compared with a system that uses methylene blue to perform a selective absorption of solar radiation (Tadili and Dahman, 1997; Bargach et al., 2004). In Turkey, flat plate collectors with water and phase change material have been used as a working fluid for heating greenhouses with satisfactory results (Kürklü and Bilgin, 2004). In these and other works, the possibility of using solar energy for greenhouse heating is positively concluded. One of the main problems of the use of solar thermal energy to reduce the consumption of petroleum-derived fuels is that of accumulation (Sen, 2004).

References:

Al-Hussaini, H. and Suen, KO 1998. Using shallow solar ponds as a heating source for greenhouses in cold climates. Energy Convers. Mgmt VoI 39, N ⁰ 13. p. 1369-1376. Bargach, MN, Tadili, R. Dahman, AS and Boukallouch, M. 2004. Comparison of the performance of two solar heating systems used to improve the microclimate of agricultural greenhouses in Morocco. Renewable Energy 29. p. 1073-1083.

Iriarte, A .; Luna, D. and Saravia, L. 1981. Development of water-air exchangers for use in solar dryers. Minutes 7 ^a . ASADES Rosario meeting, p. 30-34.

Iriarte, A .; Biagi, S. and Saravia, L. 1993. Characterization of a heat exchanger for greenhouse heating. "Minutes 16 ^a . ASADES La Plata Meeting, p. 461 - 466, Volume I. Kürklü, A. and Bilgin, S. 2004. Cooling of a polyethylene tunnel type greenhouse by means of a rock bed. Renewable Energy 29. p. 2077-2086.

Lesino, G .; Saravia, L .; Castro, PL and Blasco, D. 1983. Construction and monitoring of a greenhouse and adjacent premises with auxiliary heating by solar well. Acta 8 meeting ASADES, La Pampa, p. 9-16.

Matallana, A. and Montero, J. I. Greenhouses: design, construction and setting. 1995. Mundiprensa.

Natural Resource, Agriculture, and Engineering Service (NRAES) 1992. Greenhouse engineering. Cooperative extension, pp: 212. Saravia, L .; Echazú, R .; Cadena, C. and Cabanillas, CL. 1992. Solar heating of greenhouses in the Province of Salta. Acts 15th. ASADES National Meeting, Volume I, p. 371-375.

Saravia, L., Echazú, R., Quiroga, M. and Robredo, P. 2000. Water accumulator for greenhouse air conditioning armed with PET bottles. Averma Magazine, VoI. Four.

Sen, Z. 2004. Solar energy in progress and future research trends. .Progress in Energy and Combustion Science 30. p. 367-416.

Tadili, R. and Dahman, AS 1997. Effects of a solar heating and climatization system on agricultural greenhouse microclimate. Renewable Energy, VoI. 10, N ⁰ 4. p. 569-576.

Description of the figures

Figure 1.-. Scheme of a tunnel tunnel sensor, with small elevation of the ground for water evacuation, thermal insulation layer, and cover. The arrangement of the windings of the PE lines (connected in parallel to the input and output heads) is indicated below.

Figure 2.- General view of the collector field next to the greenhouse whose heating needs to be contributed. The collectors are oriented from East to West to maximize the use of solar radiation. The outer feed pipe of the collectors with inverted return is appreciated to improve the behavior of the hydraulic circuit. Figure 3.- Detail of the assembly process of the collector field.

Figure 4.- Detail of the collector field

Figure 5.- Performance curve of a tunnel sensor.

Description of the invention

The invention that concerns us is a low-cost solar energy collector to save energy in greenhouse heating characterized in that the working fluid (water) circulates inside several plastic pipes with small diameter (12-16 mm) and large length (up to 10-15 m of line per linear meter of collector) located in curls on top of a black plastic absorber and an insulator on the ground and has a plastic cover of high transparency and low thermalness, in configuration of tunnel The insulator is made of polystyrene plates lined with black plastic to avoid possible heat losses as a result of the wetting of the insulator. It is evident that the floor surface must be slightly smoothed before placing the insulator, although it is not necessary that the leveling is very precise.

The absorber is a black plastic on which the plastic pipes are placed through which it circulates in water. As for its thickness, it is convenient that it has a certain consistency so that if the plastic used is thin, it can be arranged in two or four layers. The width of the absorber must coincide with that of the insulator, so that to ensure its support the absorber is buried slightly on the sides and the front of the sensor. The pipes through which water circulates inside the collector have a circular arrangement along the absorber, which allows the pipes to cover the entire base of the collector and the shade between them is minimal. In this way the solar radiation collection is maximum and in the pieces of pipe where the solar radiation does not affect, the heat is transferred by conduction. The configuration of diameters, lengths and numbers of lines in parallel is adjustable depending on the specific applications, being able to design tunnel collectors from 3-4m ² to more than 5Om ² , The recommended width is 1m, to facilitate the tasks of assembly Limitation to size is given for the tolerable load loss (according to applications, but 7 mca - water column meters - in the primary for conventional solar thermal installations). The selection of larger diameters and the increase in the number of lines in parallel allows the configuration to be adapted to the tolerable load loss in each case (tables can be provided to assist in the selection of the configuration, with option between 3 and 8 lines in parallel) . It also highlights the ease of assembly, without requiring specific knowledge or skills. The preferred orientation should be EW to reduce reflection losses. For a 27.5m ² sensor with 6 lines of 16mm polyethylene pipes, the yield curve is characterized by an optical factor of 0.53 and a loss coefficient of 4.82 Wm ^-20 C, with average daily yields close to 30% .

The collector cover is made of transparent plastic with a circular arc shape and arranged along the sensor. To facilitate its fastening, they are placed along the sensor on metal joists in the form of circumference arches joined together with metal wire. The sides and the front of the roof are buried carefully to prevent the wind from causing damage to the solar collector.

The water that circulates through the different plastic pipes comes and goes to thermally insulated tanks where solar thermal energy is stored in the form of hot water to be used in the greenhouse heating system, consisting of a water distribution network through plastic pipes

The object of this invention can also be used in other activities with heating requirements and they can not cope with the cost of the sensors flat plate domestic hot water, such ^as' the pig farms, among others.

Embodiment of the invention

The collectors object of the invention can be constructed with cork plates of 2 mx 1m, that is, with a width of 1 m and a length of 24 m using 12 insulating plates. The insulator is previously protected with black plastic material to enhance the effect of the absorber and ^' prevent the cork from It can get wet and heat losses occur. The absorber can be made with thin black plastic although it is necessary to use several layers of it. To conduct the water inside the collector, six pipes of 14 mm internal diameter and 125 m long each conveniently arranged so as to cover the entire base of the collector can be used. In this case, each pipe must extend over the absorber occupying 4 m in length. The six pipes can be joined at the ends by a copper head, so that at one end a temperature probe can be included in the head. The transparent cover is installed after checking the tightness of the hydraulic circuit. To facilitate clamping, they are placed along the sensor on metal joists in the form of circumferential arcs separated 2 m from each other and connected to each other with metallic wire. The maximum height of the roof over the pipes must not exceed 50 cm. The sides and the front of the roof are buried carefully to prevent the wind from causing damage to the solar collector. The separation between two sensors can be approximately 50 cm.

Claims

1. Low cost solar collector characterized in that it is arranged horizontally on the ground and comprises polyethylene pipes with a diameter that ranges between 12-16 mm and one. 10-15 m length per linear meter of collector, located in curls on top of a black plastic absorber and an insulator on the ground, and has a plastic cover with high transparency and low thermality, in tunnel configuration.

2. Low cost solar collector according to claim 1 characterized in that the size ranges from 3 to 50 m ² with a width of 1 m preferably, to facilitate assembly tasks.

3. Low cost solar collector according to previous claims, characterized in that the pipes through which the working fluid circulates, preferably water, has a circular arrangement between 3 and 8 lines parallel to the length of the absorber covering the entire base of the collector

4. Low cost solar collector according to previous claims, characterized by average daily yields close to 30%, for a 27.5 m ² collector with 6 lines of 16 mm diameter polyethylene pipes.

5. Low cost solar collector according to previous claims, characterized in that the insulator is made of expanded polystyrene plates of dimensions 2 x 1 m ² preferably, lined with black plastic to avoid possible heat losses as a result of the insulator get wet.

6. Low cost solar collector according to previous claims for use in the greenhouse heating system, essentially.