WO1997032466A1

WO1997032466A1 - Greenhouse

Info

Publication number: WO1997032466A1
Application number: PCT/IL1997/000083
Authority: WO
Inventors: Dov Pasternak; Eli Korin; Uri Drori
Original assignee: Ben-Gurion University Of The Negev
Priority date: 1996-03-07
Filing date: 1997-03-06
Publication date: 1997-09-12
Also published as: IL117401A0; IL117401A

Abstract

A greenhouse for growing ornamental vegetables or any other plant or organism, comprising thermal energy storage and heat release means capable of storing solar energy and to release it as heat, wherein the said thermal energy storage and heat release means are enclosed in a confined, at least partially separated and insulated space (10), the said at least partially separated space (10) being provided with a heat barrier (7) consisting of at least one surface which is in contact with the atmosphere of the greenhouse and which can be at least partially removed and restored to its fully extended position at will.

Description

GREENHOUSE

Field of the Invention

The present invention relates to the heating of greenhouses. In particular, the invention relates to a novel greenhouse which allows an effective use of solar radiation, resulting in an inexpensive heating by the absorption of solar radiation throughout the day, and the discharge thereof as heat in a controlled way.

Background of the Invention

Among the various uses of collectable solar energy, space heating is included. The major problems in utilizing solar energy for such a purpose are the necessity of storing large amounts of low grade thermal energy collected on sunny days (or hours) for use at night and/or on cloudy days, while maintaining a cost sufficiently low in comparison with conventional heating means.

Storing solar energy for domestic use has rehed on two types of techniques, the first such being as sensible heat (i.e., heating of water, rocks, etc.) and use of stored thermal energy when required. Such a method was used, for example, by Esquire et al. (Gartenbautechnische Informationen, C. von Zabelitze, Ed., Hannover, 1989) who used in a greenhouse polyethylene sleeves filled with water as the heat-collecting means.

The second method comprises using low-cost phase-change materials (PCMs) such as salt hydrates, paraffins and fatty acids, and others, covering a range of transition temperatures from 8-30°C. Such a method was described, for example, in U.S. Patent No. 4,498,459 and in Korin et al., Int. J. Solar Energy, 1987, Vol. 5, pp. 201-212.

The use of solar heating for greenhouses seems to be of great potential. If an efficient and economical system can be utilized, price of greenhouse plant propagation and growth could be substantially reduced by heat savings. A further factor boosting potential for such a system is that it could use low-grade thermal energy.

In the second half of the 1980s, water-filled transparent polyethylene tubes were introduced as passive greenhouse solar heaters. Said tubes, having a diameter of 30-38 cm, are laid throughout the greenhouse, and absorb the solar radiation. When irradiation and temperature decrease, heat is released by radiation and convection from the tubes.

Greenhouses which qualify as "solar greenhouses" possess the following characteristics:

a. efficiency in collection of solar energy; b. energy-storing ability; c. minimizing of heat losses throughout day and night; d. ability to control the rate of heat release to match the desirable heating rate; e. ability to meet different light and temperature requirements dictated by a variety of growing plants.

Many such passive solar greenhouses have been built and tested in different parts of the world over the years. However, the art has so far failed to provide a solar greenhouse which is both effective and simple to operate, while involving costs which are affordable in agriculture.

It is an object of the invention to provide such a simple and inexpensive system, which can be effectively used to heat the greenhouse atmosphere by utilizing stored solar energy.

It is another object of the invention to provide a method for heating a greenhouse by using stored solar energy.

Other objectives of the invention will become apparent as the description proceeds.

Summary of the Invention

The greenhouse for growing vegetable growths and other crops, according to the invention, comprises thermal energy storage and release means capable of storing solar energy and to release it as heat, wherein the said thermal energy storage and heat release means are enclosed in a confined, at least partially separated and insulated space, the said at least partially separated space being provided with a heat barrier consisting of at least one surface which is in contact with the atmosphere of the greenhouse and which can be at least partially removed and restored to its fully extended position at will.

As will be understood by the skilled person, various types of storage media can be employed, e.g., mass which accumulates thermal energy, such as water, or reactive components, which undergo reactions which Hberate heat, or phase changing materials. However, for the sake of simphcity, reference is made throughout this specification to solar thermal energy, it being understood that any such obvious modifications are also intended to be covered by the invention.

According to a preferred embodiment of the invention, the confined, at least partially separated space, consists of the walls and floor of the greenhouse, coupled with a removable heat barrier. In one particularly preferred embodiment of the invention, the removable heat barrier comprises an adjustable curtain.

As will be appreciated by the skilled person, it is not always necessary or desirable to allow heating of the greenhouse, through discharge of stored heat, at its full power. According to a preferred embodiment of the invention, therefore, the removable heat barrier, when actuated, bares varying areas the size of which is proportional to the amount of heat to be released, thereby acting as a "continuous thermal switch".

It should be understood that the terms "heat barrier" and "separated", as used herein, are not intended to indicate absolute prevention of heat passage, but rather are intended to indicate a marked heat separation effect, which prevents - although not always entirely - heat from being delivered to the greenhouse atmosphere, unless the "thermal switch" is operated.

As will be appreciated by a person skilled in the art, the positioning of the thermal energy storage means may vary according to the position of the greenhouse relative to the sun, in order to take advantage of improved solar energy collection. According to one embodiment of the invention, this is taken into account such that the thermal energy storage and heat release means are located on the southern side of the greenhouse, if the longitudinal axis thereof is directed in an east -west direction, or on both east and west sides of the greenhouse, if the longitudinal axis thereof is directed in a north-south direction.

The heat storage and release means can be of any suitable type, but they typically comprise opaque and dark elements filled with a liquid. According to one preferred embodiment of the invention the transparent elements are polyethylene sleeves. According to another preferred embodiment of the invention the storage liquid is water. Water, among its many advantages, is cheap, is not dangerous to the crops and to the greenhouse, and has sufficient storage capacity at the temperature levels involved.

As will be appreciated by the skilled person, an auxiliary heating system can also be provided, to further heat the liquid within the transparent members. This is useful, for instance, when it is necessary to heat the greenhouse but the thermal energy collected through sun radiation is not sufficient to provide the required heating.

According to one embodiment of the invention, heating is achieved by natural convection, whereby hot air flows from the enclosed space, which is at a higher temperature, towards the colder parts of the greenhouse. However, in some instances it is necessary to apply forced convection or water circulation, in order to obtain a satisfactory homogenization of the temperature, for instance when high plants and substantial foliage are involved, which may block the flow of air by natural convection. 97/32466 PCI7TL97/00O83

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Therefore, according to another alternative - but more expensive - embodiment of the invention, the greenhouse further comprises air and/or water circulation means to homogenize the temperature within the greenhouse.

Preferably, but non-limitatively, the heat barrier, as well as any other reflecting surfaces, is made of reflecting material.

According to another preferred embodiment of the invention, the thermal energy storage and releasing means are positioned at an elevated position with respect to the ground, e.g., are positioned on support means which keep them in a separated positioned relationship from one another. Suitable support means will be apparent to the skilled person and may comprise, e.g., a rack with one or more layers. In one preferred embodiment of the invention the thermal energy storage and heat releasing means comprise plastic sleeves filled with a liquid, which sleeves are positioned on a plurality of superimposed layers.

The invention also encompasses a method of heating a greenhouse during nighttime in cold weather, comprising:

a) providing thermal energy storage and heat release means capable of storing solar energy and to release it as heat, the said thermal energy storage and heat release means being enclosed in a confined, at least partially isolated space, the said at least partially isolated space being provided with a heat barrier consisting of at least one surface which is in contact with the atmosphere of the greenhouse and which can be at least partially removed and restored to its fully extended position at will; b) causing sun radiation to reach the said thermal energy storage and heat release means during daytime, while the heat barrier is closed, thereby causing an increase in the heat stored therein;

c) at least partially opening the said thermal energy storage and heat release means to release heat to the colder atmosphere in the greenhouse.

According to the invention, thermal energy may be stored as sensible heat, or as latent heat of a phase-change material. According to a preferred embodiment of the invention, thermal energy is stored in a sensible form, and it is preferred that the collectors used be transparent tubes or sleeves filled with liquid, more preferably, water.

All the above and other characteristics and advantages of the invention will be better understood through the following illustrative and non- limitative description of preferred embodiments.

Brief Description of the Drawings

In the drawings:

- Fig. 1 schematically illustrates the operation of a greenhouse, according to one embodiment of the invention; - Fig. 2 shows a greenhouse, according to another embodiment of the invention, wherein two thermal energy storage and delivery sections are provided;

- Fig. 3 schematically shows a support for the curtain of the heat barrier of Fig. 1; The width of the support can vary from 0 to 50% of the width of the curtain and it is possible to have a few of these barriers along the curtain;

- Fig. 4 illustrates the operation of the heat barrier of Fig. 1;

- Fig. 5 schematically illustrates a greenhouse according to another embodiment of the invention;

- Fig. 6 illustrates a greenhouse similar to that of Fig. 5, but with a single energy center. In the embodiment described in Figs. 5 and 6 the sleeves facing the inner side of the greenhouse serve as the heat barrie; and

- Figs. 7-10 show results of experiments detailed in the examples.

Detailed Description of the Invention

In Fig. 1, a greenhouse, generally indicated by numeral 1, is used to grow a plurality of plants, 2. The outer shell 3 of the greenhouse is conventional, and can be of any suitable shape and material. The greenhouse can further be provided with any suitable accessory, such as a thermal screen 4, which reduces heat losses and dead air volumes, and which may further facilitate hot air circulation in the greenhouse. The addition of a thermal switch is particularly important when dealing with relatively low temperatures in the energy storage material. The arrangement according to the invention functions as an "energy center", and provides three functions: solar energy collection, solar energy storage, and heat dissipation by natural or forced convection to the greenhouse space.

In the embodiment shown in this figure, a plurality of polyethylene sleeves 5 he on the floor 6 of the greenhouse. Of course, an additional layer may be provided on the floor, which isolates, partially or fully, the sleeves from the soil, which may be made of insulating material and/or of reflecting material.

The sleeves are filled with water, and during daytime (Fig. IA) the water is heated by solar radiation, S, with diffused or direct radiation and/or with convected heat, which passes through one or more layers of a transparent material, e.g., a polyethylene window, W, in the outer shell of the greenhouse (not shown). This is a relatively efficient system for collection and release of solar energy, since it works at relatively low temperatures. During night or in clouded days, the said window may be closed, e.g., with a curtain, as will be more fully illustrated with reference to Fig. 5, to minimize heat losses to the outside. The curtain is preferably made of a reflecting and insulating material, the reflecting surface facing inwards. The external angle of the window can be adjusted relative to the sun, to obtain maximal solar energy collection.

The corner of the greenhouse which houses the sleeves 5 is closed to the volume of the greenhouse by a barrier 7, which consists, in this particular embodiment of the invention, of a curtain 8, which is at least partially insulating, with an additional element 9, which will be discussed in greater detail below. The enclosed space defined by them is generally indicated by numeral 10. At nighttime (Fig. IB), curtain 8 is folded so at to permit heat transfer from the energy center to the greenhouse space. When the air in the main part of the greenhouse reaches a temperature T which is lower than the temperature Ti of the enclosed space 10, hot air (indicated by H) will begin to flow into the greenhouse, while cold air therefrom (indicated by C) will flow into the thermal center. In this way, heat is constantly removed from sleeves 5 by convection, thus achieving heating of the greenhouse.

Of course, as explained above, an auxiliary heating system (not shown) may be installed for use as a back-up system. Furthermore, a fan 11 can also be provided, to create forced convection and to improve hot air circulation and delivery of heat to the greenhouse space.

In Fig. 2 another embodiment of the invention is shown, in which two thermal energy storage and delivery sections, 10 and 10', are provided at two ends of the greenhouse. This may be desirable, e.g., when the greenhouse is large, to facilitate temperature homogenization therein, or depending on the orientation of the greenhouse relative to the sun. The resulting air flow by natural convection is schematically shown in the figure.

Looking now at Fig. 3, a section of a support 9 for the curtain 8 is shown, which is suitable for use with the embodiment of the invention shown in Fig. 1. The support consists of a flat, convex surface 12, on which the curtain rests, which is supported by legs 13 secured into the ground, and by supports 14 attached to the greenhouse structure. The operation of the -I I-

barrier 7 is illustrated in Fig. 4. In Fig. 4A a curtain 8 is stretched along the whole opening, as seen in Fig. IA, and its central position is supported on support 9. The extremities of the curtain are held in then- extended position by appropriate strings which are conventional and, therefore, are not shown herein in detail. The same strings are used to gather the whole curtain together on support 9, as seen in Fig. 4B, when it is desired to open the heat barrier, by pulling it in the directions of the arrow. In this way, openings 15 and 16 are created, which permit air circulation and contact between the enclosed space and the atmosphere of the greenhouse.

Fig. 5 illustrates a greenhouse, according to another preferred embodiment of the invention, in which the polyethylene sleeves are not in contact with the ground. The sleeves 17 are arranged on a rack, 18, having layered supporting means 19 which may be, e.g., metallic tubes or shelves. In this embodiment of the invention the sleeves are more separated from one another, and maximal heat transfer area is exploited, with minimal losses to the ground. As is shown in the figure, a space 20 is left at the bottom, between the ground and the lowermost row of sleeves. Cold air C enters at the bottom, and flows out at the top, as indicated by the arrows. The greenhouse is also provided with window 21, which can be opened to let fresh air into the greenhouse space.

The greenhouse of Fig. 5 is adapted to be positioned along a North-South axis, so that its two energy centers are in the West and East positions. This greenhouse, therefore, is provided with two energy centers (ECl and EC2). It can be seen that, for a greenhouse having a width of 8.6 m, the width of the energy center is of about 1.5 m, and its height of about 1.3 m (dimensions shown in the figure).

Looking at EC2, the inner solar screen 22 is seen, which functions as a separator between the energy center and the greenhouse volume. The external solar curtain 23, which is rolled up in EC1, is in its open position in EC2, where the energy center is entirely covered, and is clearly seen in the figure.

Fig. 6 shows a greenhouse of the type of that of Fig. 5, but which is adapted to be positioned along the East- West axis, with the energy center directed towards South. In this embodiment only one energy center is used, with its various parts being numbered as in Fig. 5, which are not described again, for the sake of brevity. It can be seen that, with a single energy center, the dimensions . of the greenhouse change. For a greenhouse width of 9 m, a width of 2.2 m and height of 1 m are used for the energy center since larger amounts of heat must be supplied by it.

The greenhouses of Figs. 5 and 6 have been used in a number of experiments, as more fully detailed below.

Example 1

The air temperature of the greenhouse was measured at a height of 1 meter from the soil, and at its center. Furthermore, the air temperature outside the greenhouse was also recorded. Recordings were taken for both the greenhouses of Figs. 5 and 6, identified as GH 10 and GH 11, respectively, on January 10, 1995, in which tomatoes were grown. The results are shown in Fig. 7, for a period starting at 8:00 AM and ending at 6:00 AM of the following day.

As it can be seen, during the critical hours (nighttime), the addition to the greenhouse air temperature obtained by the invention was about 7°C, which is a significant addition for most plants and yield. In this experiment the inner screen was opened at night when the inside temperature dropped to 11°C, and remained open until 8:00 AM of the next day.

Example 2

The experiment of Example 1 was repeated on January 23, 1995, with the following changes: I GH 11 the thermal screen was spread over the plants. The results are shown in Fig. 8. As it can be seen, the difference between the two greenhouse is of about 3°C, which is substantial in the relevant temperature range. Furthermore, since heat was discharged from the energy center, a difference of about 4°C exists between GH 11 and the outdoors temperature.

It should further be noted that during the hot hours (10:00 AM - 2:00 PM) the temperature in the greenhouse may raise to well above 30°C which, for some growth is excessive and undesired. An additional advantage of the invention is that it is possible to ventilate the greenhouse with the heat curtain in the closed position, thus continiiing to accumulate energy for the cold hours. Example 3

Operating as in Example 1 and additional test was carried out on January 25, 1995, in which greenhouse 11 operated with closed inner solar screen and without thermal screen thermal curtain and solar heating. Fig. 9 shows the results of this experiment, and it can be seen that the temperature in GH 11 during the cold hours approached that measured outdoors. Since there is heat transfer from the energy center to the growing section through the closed solar screen, a difference of 2°C exists between GH11 and the outdoor temperature.

Example 4

An experiment was carried out to show the controllable nature of the heat-release system of the invention. The heat curtain (8 in Figs. 1 - 4) was connected to an electrical motor which opens and closes it according to a command received from a control circuit. The control circuit was operated so that it received temperature readings from the greenhouse, and was set so as to command the opening of the curtain when the temperature in the greenhouse reached 10°C, and its closure when it raised to 11°C. The results are shown in Fig. 10, from which the substantially homogeneous temperature profile obtained can be appreciated.

The above examples and description have been provided for the sake of illustration only, and are not intended to limit the invention in any way. Many modifications can be effected in the various devices, curtains, heat storage and release means and materials, greenhouse shape, size and orientation, all without exceeding the scope of the invention.

Claims

CLAIMS:

1. A greenhouse for growing ornamental vegetables or any other plant or organism, comprising thermal energy storage and heat release means capable of storing solar energy and to release it as heat, wherein the said thermal energy storage and heat release means are enclosed in a confined, at least partially separated and insulated space, the said at least partially separated space being provided with a heat barrier consisting of at least one surface which is in contact with the atmosphere of the greenhouse and which can be at least partially removed and restored to its fully extended position at will.

2. A greenhouse according to claim 1, wherein the confined, at least partially separated space, consists of the walls and floor of the greenhouse, coupled with a removable heat barrier.

3. A greenhouse according to claim 1, wherein the removable heat barrier comprises an adjustable curtain.

4. A greenhouse according to claims 1 to 3, wherein the removable heat barrier, when actuated, bares varying areas the size of which is proportional to the amount of thermal energy to be released, thereby acting as a "continuous thermal switch", and wherein the said barrier can optionally be controlled by thermostates and automatically adjusted to provide preset inside night air temperature at will.

5. A greenhouse according to claim 1, wherein the thermal energy storage and heat release means are located on the southern side of the greenhouse, if the longitudinal axis thereof is directed in an east-west direction, or on both east and west sides of the greenhouse, if the longitudinal axis thereof is directed in a north-south direction.

6. A greenhouse according to claim 1, wherein the thermal energy storage and release means comprise opaque and dark elements filled with a liquid.

7. A greenhouse according to claim 6, wherein the opaque elements are polyethylene sleeves.

8. A greenhouse according to claim 6 or 7, wherein the liquid is water.

9. A greenhouse according to any one of claims 1 to 8, wherein an auxiliary heating system is provided to further heat the liquid within the transparent members.

10. A greenhouse according to any one of claims 1 to 9, further comprising air circulation means to homogenize the temperature within the greenhouse.

11. A greenhouse according to claim 1, wherein the heat barrier is made of reflecting material.

12. A greenhouse according to any one of claims 1 to 11, wherein the thermal energy storage and heat releasing means are positioned at an elevated position with respect to the ground.

13. A greenhouse according to claim 12, wherein the thermal energy storage and heat releasing means are positioned on support means which keep them in a separated positioned relationship from one another.

14. A greenhouse according to claim 13, wherein the support means comprise a rack with one or more layers.

15. A greenhouse according to any one of claims 12 to 14, wherein the thermal energy storage and heat releasing means comprise plastic sleeves filled with a liquid, which sleeves are positioned on a plurality of superimposed layers.

16. A method of heating a greenhouse during nighttime in cold weather, comprising:

a) providing thermal energy storage and heat release means capable of storing solar energy and to release it as heat, the said thermal energy storage and heat release means being enclosed in a confined, at least partially isolating space, the said at least partially isolating space being provided with a heat barrier consisting of at least one surface which is in contact with the atmosphere of the greenhouse and which can be at least partially removed and restored to its fully extended position at will;

b) causing sun radiation to reach the said thermal energy storage and heat release means during daytime, while the heat barrier is closed, thereby causing an increase in the heat stored therein; c) at least partially opening the said thermal energy storage and heat release means to release heat to the colder atmosphere in the greenhouse.

17. A greenhouse, substantially as described and illustrated.

18. A method of heating a greenhouse, substantially as described and illustrated.