WO2014181047A1 - Device for collecting, exchanging, and thermally storing solar energy - Google Patents

Abstract

The invention relates to a device (T) for collecting, exchanging, and thermally storing solar energy, including a solar radiation concentrator (1), a primary tank (3) including an opening (9) that is transparent to the outside and a heat exchanger (7), wherein said device (T) is characterized in that the primary tank (3) contains a solid semi-transparent material (8) selected such as to be capable of being heated to more than 500°C when the concentrated solar radiation (2) that passes through said opening (9) is absorbed, without impairing the physical properties thereof, and capable of maintaining the collected thermal energy and/or redistributing same via the heat exchanger (7).

Description

 DEVICE FOR CAPTURING, EXCHANGING AND THERMALLY STORING SOLAR ENERGY

FIELD OF THE INVENTION

The present invention relates to the field of heat sensors and heat exchangers and more particularly to those used for the storage of solar energy.

STATE OF THE ART.

Thermal storage of solar energy requires converting solar radiation into heat through a thermal sensor that absorbs this light energy and transforms it into calories. The thermal sensor is often a black surface that heats up proportionally to the light intensity it receives, then these calories are transported to a storage medium. The storage medium may for example be a water volume because the water has a high heat absorption capacity. The transport of calories is usually done by a heat transfer fluid which heats up in contact with the thermal sensor and then gives up its calories to the storage material. In order to improve this principle of solar thermal storage, we mainly seek to improve five factors:

 1 - the performance of the thermal sensor, that is to say its ability to transform solar energy into heat energy, trying to reduce as much as possible all losses.

 2 - the quality of heat transfer from the heat sensor to the storage volume, which implies reducing the heat losses due to this transfer, which is essentially a question of isolation, but also a question of heat transfer between the sensor and the heat transfer fluid and between the fluid and the storage volume. These thermal interfaces are what we will call heat exchangers.

 3 - the quantity of calories stored per unit of volume, which requires choosing highly absorbent materials for the storage volume, but also materials that must withstand high temperatures.

4 - the storage temperatures must be as high as possible in order to reduce the volume of the tank and allow applications that require high temperatures.

 5 - the overall cost of the device must be as low as possible in order to make solar energy competitive with other available energies.

 The most successful known devices generally use concentrated solar radiation. This solar concentration is produced by an optical device that uses mirrors and / or lenses. The thermal sensor is generally a metal surface such as steel or copper, possibly protected by a glass to limit heat loss. This metal surface may take the form of a pipe that is in contact with a heat transfer fluid in motion. This fluid is often water vapor at high temperature, possibly under pressure. There are also phase-change fluids, such as molten salt. Finally, the storage volume may be composed of molten salt, concrete, oil, natural stones, or any solid or liquid material having a high heat capacity.

 Heat exchangers work mainly on the principle of thermal conduction between the two faces of an element, often a very conductive plate of heat. Schematically one of the two faces of the plate is the thermal sensor which heats up under the solar radiation and the other face of the plate is in contact with the flow of a fluid which is charged with the passage of the calories transmitted by the plate . The performance of the heat exchanger mainly depends on the exchange temperature, the exchange surface, and the thermal conduction of the materials. Furthermore, to increase the thermal storage capacity of the system and also for applications that require high temperatures, it is necessary to store the calories in solids and not in liquids because the liquids pass rather quickly in the vapor phase, and the steam is not easy to store. But storing heat in a solid requires either a storage material that has both a good storage capacity and a good thermal conductivity, which is the case of metals such as aluminum or steel (but these materials are expensive), or use a storage material that has a very good storage capacity and low thermal conductivity, such as concrete, the latter characteristic being then compensated by a heating circuit consisting of a multitude of pipes acting as heat exchangers and disseminated throughout the storage volume.

This multitude of heat exchangers necessary for heat transfer inside the storage volume, which is therefore solid, then increases necessarily the overall cost of the system.

PURPOSE OF THE INVENTION

The object of the invention is to describe a sensor and heat exchanger of solar energy, with a high performance storage of calories in a solid material that resists and remains in the solid state at high temperatures, and at a lower cost. This will aim at solving the problem of the cost of the multitude of heat exchangers that are necessary inside a storage volume when it is solid.

SUMMARY OF THE INVENTION

The invention relates to a device for capturing, exchanging and thermal storage of solar energy, comprising:

 a primary reservoir comprising a transparent opening towards the outside;

a solar radiation concentrator able to concentrate the solar radiation towards the opening of the primary reservoir and to heat up the contents of said primary reservoir;

 a heat exchanger able to recover the thermal energy generated in the primary reservoir,

- This device being characterized in that said primary reservoir contains a semi-transparent solid material capable of being heated to temperatures of above 500 ° C without changing state and while maintaining its physical properties, and being able to maintaining the collected heat energy and / or redistributing it through the heat exchanger (7).

In this way, thanks to the semitransparency of the material disposed in the primary tank, the light energy passes through the tank in all its depth very quickly and heats all the solid material, much faster than in the case of a heating by thermal conduction.

The concentrated solar radiation is for example a solar radiation which is concentrated by Fresnel lens type optics, parabolic mirrors, Fresnel mirrors, or a multitude of heliostats. The primary reservoir may take any suitable shape, but preferably it will be elongate so that the distance between any point of its volume and its surface is as short as possible taking into account other design constraints. This distance, between any point of the primary reservoir and its surface, in fact determines the capacity of the system to exchange its calories with the outside.

 The wall of the primary reservoir is preferably made of a highly thermally conductive material, such as for example copper, steel or aluminum. The tank is filled with a solid and semi-transparent material. This material is preferably glass or crushed glass, which has a high heat capacity, that is to say which is capable of storing a large amount of calories, while withstanding high temperatures, that is to say ie to remain in the solid state and not to lose its thermal absorption properties even at temperatures well above 500 ° C. An interesting option is to include in the glass metal or carbon particles or other types of particles that absorb some of the solar radiation. These particles can also be of nanometric size.

 Solar radiation focuses on the opening of the primary reservoir and penetrates inside the semi-transparent material. As sunlight penetrates into the primary reservoir, this radiation will be gradually absorbed by the semi-transparent material. This semi-transparent material will then warm up quickly and in a substantially homogeneous manner throughout its volume.

 The energy of the solar radiation will thus be transferred to the totality of the semitransparent matter with a high speed, since this speed of propagation is that of the light, whereas if the material were opaque it would be the speed of thermal conduction to inside the material that would be taken into account. This high thermal transfer rate inside the primary tank will allow said tank to have such shapes that they promote large areas of heat exchange with the outside while maintaining a lower overall volume. For example, this will be the case of a primary tank in the form of a rectangular parallelepiped, a cylinder or a cone, when one of their dimensions is much larger than the others, in any case when one of the dimensions is at least three. times higher than the others, which will allow for example to create two parallel plates slightly spaced in the case of a rectangular parallelepiped.

Thus, the primary reservoir according to the invention allows rapid and efficient heat exchange thanks to important heat exchange surfaces with outside, while allowing a diffusion and a fast and homogeneous storage of the received solar energy. The same result could have been obtained with a primary tank containing a non-transparent material such as aluminum or steel because these materials also have a high thermal storage capacity and a relatively fast heat transfer rate, although less than that of light, but the advantage of the device according to the invention is that it is much less expensive since glass, for example, is five times more economical than these metals.

 In a particular embodiment of the device, the inner wall of the primary reservoir is made reflective in order to facilitate the penetration of solar radiation into said reservoir. An interesting option for optimally distributing the intensity of the radiation inside the primary tank is to make the inner surface of said tank less and less reflective as one moves away from the opening of the tank primary. This being made possible for example by positioning a multitude of reflective zones on the inner wall of the primary reservoir so that the surface density of said reflective zones is smaller and smaller as one moves away from the opening. In return, it will be observed that a multitude of absorbing zones of the inner wall of the primary reservoir will have an increasing surface density as one moves away from the opening. Thus, the lower the intensity of the solar radiation inside the primary reservoir and the greater the absorption of the wall of said reservoir, which will substantially equalize the distribution of calories along the reservoir.

 In order to increase the thermal storage volume of the device, the primary reservoir, except for its opening, is surrounded by a secondary thermal reservoir of high thermal capacity, such as for example plaster, concrete or clay. This secondary thermal reservoir possibly being in contact with the surface of the primary reservoir, it will gradually heat up itself. To reduce heat losses, both tanks can be surrounded by a highly insulating material such as air film, vacuum or glass wool.

 In order to recover the calories stored in the two reservoirs, a fluid circulation heat exchanger is positioned in one or the other or both reservoirs, and preferably positioned near the surface of the primary reservoir. This fluid-circulating heat exchanger is for example a coil that surrounds the primary tank and is traversed by air or water or steam.

In a particular embodiment, the primary reservoir contains a material semi-transparent whose volume density is gradually increasing as one moves away from the opening. For example, if the semi-transparent material is composed of a multitude of pieces of glass or glass beads, the average density of the glass pieces or beads is progressively increasing as one moves away from the opening. This gradient of semi-transparent material density has the effect of equalizing throughout the volume of the primary container the absorption percentage of solar radiation. This is because as solar radiation penetrates the semitransparent material, the radiation loses intensity due to its partial absorption, and this loss of intensity is compensated by a density of more important matter that locally increases the percentage of absorption, so the amount of calories absorbed. The product of the luminous intensity by the percentage of its absorption then remains substantially constant along the path of solar radiation in the material of the reservoir.

 In another particular embodiment, the increase of the density gradient inside the primary reservoir is achieved by balls whose diameter is gradually decreasing.

 In another particular embodiment, the increase of the density gradient inside the primary reservoir is achieved by glass plates that are closer and closer together as one moves away from the opening of the primary tank.

 In another particular embodiment, the primary reservoir is in a vertical position, its opening is located in its upper part, and said primary reservoir is driven into the ground which then serves as a secondary reservoir. Preferably the soil will be made of materials with high heat capacity such as clay or sand.

 In another particular embodiment, the opening is temporarily covered, for example at night or when the sun is hidden, by a cover on which thermoelectric cells are disposed, one of the faces of which is in contact with the interior of the primary reservoir. and the other side is in contact with the ambient air, so that said thermoelectric cells are able to produce electricity even in the absence of sun, thanks to the temperature difference between the primary reservoir and the outside air.

DETAILED DESCRIPTION OF THE INVENTION

The invention is now described in more detail with the aid of the description of the Figures 1 to 3 indexed.

 Figure 1 is a block diagram in section of the device.

 Figure 2 is a sectional diagram of a particular embodiment in which the primary reservoir has a density gradient.

 Figure 3 is a sectional diagram of a particular embodiment of the device according to the invention, which uses the soil as additional thermal storage tank for agricultural application.

 With reference to FIG. 1, the device (T) for capturing, exchanging and storing solar energy according to the invention comprises a primary reservoir (3) consisting of a semi-transparent solid material (8) such as for example pieces or glass beads. It receives through an opening (9) formed in the primary reservoir (3), the concentrated solar radiation (2). The solar radiation is concentrated by known optical methods (1), such as for example Fresnel lenses, concave mirrors or a multitude of heliostats.

 The concentrated solar radiation (2) introduced into the primary reservoir (3) disperses (6) in all the semi-transparent solid material (8) by being progressively absorbed as it is propagated in said solid (8). The walls (4) of the primary reservoir (3) are partially reflective to facilitate the propagation of solar radiation (6) throughout the volume of the tank (3).

 The reservoir (3) then heats up by absorbing and transforming the solar radiation into calories (6). The temperature of the solid (8) can be several hundred degrees Celsius and the solid (8) can remain solid, or become soft or liquid while maintaining its partial transparency. In order to increase the calorie storage volume, the primary reservoir (3) surrounded by a secondary reservoir (5) which heats in contact with the wall (4) of the primary reservoir (3). The secondary tank (5) is preferably made of a solid material with a high heat capacity such as concrete, plaster or clay.

 To limit heat loss, the two tanks (3,5) can also be thermally insulated from the outside by an insulating envelope (not shown) such as glass wool, an air film or a vacuum space. To recover the stored calories, a heat exchanger (7) traversed by a fluid (not shown) is placed in the primary reservoir (3) and / or in the secondary reservoir (5) or preferably between the two reservoirs (3, 5).

FIG. 2 illustrates a particular case in which the primary reservoir (3) is composed of a semi-transparent material (10) whose volume density is gradually increasing as one moves away from the opening (9) of the primary reservoir. For example, the primary reservoir (3) is filled with a multitude of glass plates (10) arranged parallel to each other and whose thicknesses are gradually increasing and / or whose distances between them are gradually decreasing towards the bottom of the primary tank (9). Thus, the volume density of the glass inside the primary reservoir (3) is progressively increasing as one moves away from the opening (9), which makes it possible to obtain a quantity of absorption heat, and therefore a temperature, substantially equal throughout the volume of the tank (3).

EXEMPLARY EMBODIMENT

FIG. 3 illustrates an exemplary embodiment for heating an agricultural greenhouse: a device (T) for capturing, exchanging and storing solar energy is composed of a cylindrical primary reservoir (3) of 40 cm diameter and 200 cm deep, filled with glass beads whose diameter is gradually decreasing as one moves away from the window (9) for receiving concentrated solar radiation (2). The diameter of the beads varies from 3 cm to 1 cm.

 The primary tank (3) is made of steel 1 mm thick and its inner wall is polished to be reflective. The primary reservoir (3) is encircled by a copper tubular coil (7) 20 mm in diameter which is welded to the wall of said primary reservoir (3). The primary reservoir (3) and said coil (7) are surrounded and in contact with a cylindrical secondary reservoir (5) 2m high and 80 cm in diameter which is composed of plaster.

 The two tanks (3,5) are positioned vertically and buried under the ground surface (12) so that only the window (9) is visible on the surface. The solar radiation (2) is concentrated on the window (9) by coupling a heliostat (16) whose rectangular mirror is 2 x 3 m and a parabolic mirror (1) circular 2 m in diameter. The window (9) receives a solar irradiation of a power of 4 kW in strong sunlight for an average of 4 hours per day, a heat accumulation in the two tanks equivalent to about 16 KWh per day.

The coil (7) is traversed by a coolant which is a mixture of water and glycol and which acts as a heat exchanger between the storage device (T) and a greenhouse (14) 10 meters away. The floor of the agricultural greenhouse (14) is traversed by a pipe circuit (17) which is buried at a depth of 50 cm and which is connected to the coil (7) of the thermal storage device (T) in order to heat the soil of the agricultural greenhouse (14). An automatic system (11) equipped with a circulation pump calculates the optimum flow rate of the coolant in the pipes (17) in order to regulate the soil temperature of the greenhouse (14), generally at a temperature above 6 ° C. to prevent freezing of roots and plants (13). The regulation is done either during the day when the intensity of solar radiation (15) is not sufficient to maintain this temperature in the greenhouse (14), or at night, so in the absence of sun, through the only device (T) for solar thermal storage.

ADVANTAGES OF THE INVENTION

Ultimately, the invention responds well to the goal set by allowing the efficient storage of solar energy in a solid material that is resistant to high temperatures and at a lower cost.

Claims

1 - Device (T) for capturing, exchanging and thermal storage of solar energy, comprising:  - a primary reservoir (3) comprising a transparent opening (9) to the outside;  a solar radiation concentrator (1) able to concentrate the solar radiation towards the opening (9) of the primary reservoir and to heat the contents of said primary reservoir;  a heat exchanger (7) capable of recovering the thermal energy generated in the primary reservoir (3), - This device (T) being characterized in that said primary reservoir (3) contains a semi-transparent solid material (8) capable of being heated to temperatures of above 500 ° C while remaining in the state solid, and adapted to maintain the heat energy collected and / or redistribute through the heat exchanger (7). 2 - Device according to claim 1, characterized in that the average volume density of said semi-transparent material (8) has a density density gradient and is gradually increasing when moving away from the opening (9) of the primary reservoir (3), so as to equalize heating of said material along the primary reservoir. 3 - Device according to claim 2, characterized in that said density density gradient of the transparent material (8) is formed by balls whose diameter is gradually decreasing when moving away from the opening (9), or by glass plates (10) parallel to each other and closer and closer to each other as one moves away from the opening (9). 4 - Device according to any one of the preceding claims, characterized in that said concentrator is constituted by an optical (1) Fresnel lens type, parabolic mirror, Fresnel mirrors, or a multitude of heliostats. 5 - Device according to any one of the preceding claims, characterized in that the wall of the primary reservoir (3) is steel, aluminum or copper. 6 - Device according to any one of the preceding claims, characterized in that said semi-transparent solid material (8) is taken from glass, a multitude of pieces of glass, a multitude of glass plates, glass beads, crushed glass, or glass containing particles, possibly of nanometric size and able to absorb a part of the solar radiation such as metal particles or carbon. 7 - Device according to any one of the preceding claims, characterized in that the heat exchanger (7) is fluid flow, said fluid being air or water or water vapor. 8 - Device according to any one of the preceding claims, characterized in that the heat exchanger (7) is positioned:  - either in the primary tank (3);  - either in the secondary tank (5);  - is positioned near the surface of the primary reservoir (3);  - is made in the form of a coil and positioned so as to surround the primary reservoir (3). 9 - Device according to any one of the preceding claims, characterized in that the primary reservoir (3) has the shape of a rectangular parallelepiped, a cylinder or a cone of which one of the dimensions is at least three times greater to the other dimensions of said primary reservoir (3). 10 - Device according to any one of the preceding claims, characterized in that the inner wall of the primary reservoir (3) is wholly or partly reflecting to facilitate the penetration of solar radiation (6) into said primary reservoir (3) . 11 - Device according to any one of the preceding claims, characterized in that said primary reservoir (3) is surrounded, and is optionally in contact with a secondary thermal reservoir (5), with the exception of its opening (9) who remains free. 12 - Device according to any one of the preceding claims, characterized in that the primary reservoir (3) is in a vertical position, its opening (9) is located in its upper part and is pressed into the ground. 13 - Device according to any one of the preceding claims, characterized in that reopening (9) of the primary reservoir (3) is covered by a cover on which are disposed thermoelectric cells, one of the faces is in contact with the interior of the primary tank (3) and the other side is in contact with the ambient air, so that said thermoelectric cells are able to produce electricity.