AU2010224409A1

AU2010224409A1 - Humidified and cooled greenhouse

Info

Publication number: AU2010224409A1
Application number: AU2010224409A
Authority: AU
Inventors: Alexander Charles Paton
Original assignee: SEAWATER GREENHOUSE AUSTRALIA Pty Ltd
Current assignee: SEAWATER GREENHOUSE (AUSTRALIA) Pty Ltd
Priority date: 2010-03-31
Filing date: 2010-09-24
Publication date: 2011-10-20
Also published as: GB201005445D0

Abstract

An evaporatively cooled and humidified greenhouse has an evaporator in which, in use, air entering the greenhouse is cooled and humidified by evaporation from a flow of salt water, and subsequently passes into a growing area, the evaporator 5 having a first section into which hot salt water flows, such that air from the first section is hot and humid, and a second section into which salt water flows from the first section, such that air from the second section is cooler than air from the second section, and further comprising a condenser in which air from the first section is cooled so as to obtain fresh water therefrom as condensate. 10 [Fig. 5]

Description

AUSTRALIA Patents Act 1990 ORIGINAL COMPLETE SPECIFICATION STANDARD PATENT Invention title: HUMIDIFIED AND COOLED GREENHOUSE The following statement is a full description of this invention, including the best method of performing it known to us: duwm AO115607011v3 120093174 HUMIDIFIED AND COOLED GREENHOUSE Field of the Invention The present invention relates to a method, apparatus and system for humidifying and cooling a greenhouse, or similar facility for growing plants. 5 Background of the Invention Any reference to or discussion of any document, act or item of knowledge in this specification is included solely for the purpose of providing a context for the present invention. It is not suggested or represented that any of these matters or any combination thereof formed at the priority date part of the common 10 general knowledge, or was known to be relevant to an attempt to solve any problem with which this specification is concerned. Agriculture accounts for around 70% of all fresh water consumed worldwide. In the Middle East/North Africa region, this figure is above 85%. Unprotected outdoor cultivation can demand more than 4 times the amount of 15 irrigation water, as compared with shaded or more protected cultivation. Countries in the Middle East, for example, are facing decreasing ground water and increasing salinity. Coastal plain regions, where traditionally most agriculture takes place, suffer from a lowering water table and the consequent saline intrusion. Some farmers are pumping ground water from a depth of 100 20 meters while many farms on the coast have been abandoned due to toxic levels of salinity. Little incentive for water use efficiency combined with low rainfall (average of 100 mm. per year) has the result that farmers extract beyond the natural refill of the aquifers, which has led to falling groundwater tables. This causes salt water to 25 intrude inland. As a consequence, farmers see a gradual decline in marketable produce, both in quality and in quantity, leading to the point where agriculture is no longer viable or possible. In many parts of the world, desalination is the only method of meeting the shortfall. Yet desalination is expensive in terms of energy. Even with the more 30 efficient desalination plant, 1 kg of oil can only yield about 1000 kg of fresh water.

2 Using standard methods of irrigation, this amount of water may only yield about 1 or 2 kg of edible crop. The Seawater Greenhouse (TM), developed by the present applicant, is a technology for creating fresh water from salt water in arid regions, using an 5 adapted greenhouse that also provides suitable food-growing conditions. The technology involves pumping salt water (or allowing it to gravitate if below sea level) to an arid location and then subjecting it to two processes: first, it is used to humidify and cool the air inside a greenhouse, and second, it is evaporated by solar heating and distilled to produce fresh water. Finally, the humidified air is 10 expelled from the greenhouse and may be used to improve growing conditions for nearby outdoor plants. Slightly more concentrated, residual salt water then gravitates, or is pumped if below sea level, back to the sea and some of the pumping energy can be recovered in turbines. Following earlier projects in Tenerife and the United Arab Emirates, a 1000 15 m2 greenhouse has been constructed near Muscat, Oman, successfully demonstrating the potential for cultivating crops in one of the hottest and driest regions on earth. In common with many other arid parts of the world, the region has experienced over-abstraction of groundwater, causing saline intrusion and loss of fertile land. The Seawater Greenhouse (TM) technology provides an alternative 20 approach that relies entirely on salt water rather than ground water, and may thus be used in coastal regions where cultivation is otherwise difficult or impossible due to lack of water, high temperatures or both. The approach, combined with efficient use of water, enables high value crops to be grown year round. The cooler and more humid conditions enable crops to grow with very little 25 water, and as the crops are not stressed by excessive transpiration, the yield and quality is higher. For example, in Oman, the winter to summer evapo-transpiration rates range from 4-11 litres / m 2 / day, whereas in the Seawater Greenhouse they are reduced to less than 1 to 1.5 litres /m 2 /day; this represents a four to eight-fold saving. Hence, the Seawater Greenhouse Technology presents a solution to the 30 world's water shortage not by producing more water, but by using less water, yet growing better crops.

3 When compared with conventional greenhouse cooling and desalination, the Seawater Greenhouse (TM) technology uses very little electrical power, as the thermodynamic work of cooling and distillation is performed by energy from the sun and the wind. The modest electrical demand enhances the potential for 5 driving the entire process using photovoltaic power, yet without the need for batteries, inverters or the like. Such a development would enable food and water self-sufficiency, especially in remote, arid regions. It has also been observed that the use of pesticides was not required with the Seawater Greenhouse (TM) technology, while crops in the comparison 10 greenhouses had to be sprayed for whitefly 7 times during the growing seasons. One possible reason for this is that the salt-water evaporators had an air scrubbing effect, such that any airborne insects or contaminants were washed out of the incoming ventilation air. Details of the Seawater Greenhouse (TM) technology have been disclosed 15 in the following: e The Seawater Greenhouse Brochure', available on 26 March 2010 at www.seawatergreenhouse.com/downloads/Seawater%20Greenhouse%2OBr ochure.pdf * 'The Seawater Greenhouse: Cooling, Fresh Water And Fresh Produce From 20 Seawater', Paton C & Davies P, 2nd International Conference on Water Resources in Arid Environments, King Saud University, Riyadh 2006 e 'Potential of the Seawater Greenhouse in Middle Eastern Climates', Davies P, Turner K & Paton C, International Engineering Conference (IEC) "Mutah 2004" Mutah University, JORDAN, April 26-28. Pages 523-540 25 e 'The Seawater Greenhouse in the United Arab Emirates: thermal modelling and evaluation of design options', Davies P & Paton C, Desalination Volume 173, Issue 2, 10 March 2005, Pages 103-111 * 'Greenhouse plans to reap the sea wind', Coghlan A, New Scientist issue 1875, 29 May 1993 30 The opportunity has now arisen for the first commercial scale operation in South Australia, an area which faces similar problems of water salinity, water 4 scarcity, rising input costs, as well as lack of year-round supply of locally sourced fruits and vegetables. It would therefore be desirable to simplify and reduce the cost of greenhouse technology of the type described above. It would also be desirable to 5 reduce the amount of cooling that is required to produce fresh water, particularly in winter when little if any cooling is required for growing plants. WO-A-2004/049783 discloses another example of a greenhouse in which air is humidified by an evaporator and subsequently cooled by a condenser after passing through a growing area. 10 Summary of the Invention According to a first aspect of the present invention, there is provided an evaporatively cooled and humidified greenhouse having an evaporator in which, in use, air entering the greenhouse is cooled and humidified by evaporation from a flow of salt water, and the cooled and humidified air subsequently passes into a 15 growing area, the evaporator having at least a first section into which hot salt water flows, such that air from the first section is hot and humid and the hot salt water is cooled, and a second section into which the cooled salt water flows from the first section, such that air from the second section is cooler than air from the first section, and further comprising a condenser in which air from the first section 20 is cooled so as to obtain fresh water therefrom as condensate. According to a second aspect of the present invention, there is provided a method for evaporatively cooling and humidifying a greenhouse, comprising: (a) passing a flow of salt water through an evaporator through which air entering the greenhouse is cooled and humidified by evaporation, 25 the evaporator having at least a first section into which hot salt water flows, such that air from the first section is hot and humid and the hot salt water is cooled, and a second section into which the cooled salt water flows from the first section, such that air from the second section is cooler than air from the first section, 30 (b) passing the air from the first section to a condenser for cooling so as to obtain fresh water as condensate; and 5 (c) passing the cooled and humidified air subsequently into a growing area. Advantageously, this arrangement provides an evaporator that combines two different functions: producing hot humid air for the condenser, and cool, 5 humid air for the growing area. The condenser may then be brought close to the evaporator, upstream of the growing area, so that the salt water plumbing can be made more compact. The first and second parts of the evaporator need not be discrete, but may be continuous parts of a unitary evaporator. Alternatively, the first and second 10 parts may be discrete but connected to one another, so that salt water flows from the first part to the second part. The air cooled by the condenser may then enter the growing area, to assist in providing a cool environment. The flow of air through the first part of the evaporator and/or into the growing area may be regulated to achieve the desired 15 temperature and/or humidity in the greenhouse. The flow of salt water through the evaporator may be regulated to achieve the desired temperature profile across the evaporator, and preferably so that salt water leaves the evaporator substantially at wet bulb temperature. Preferably, the hot salt water passes downwards through the evaporator 20 under gravity, so that the first portion of the evaporator is higher than the second portion. An upper portion of the condenser may be positioned higher than the top of the evaporator, so as to capture the hot, humid air rising from the first, higher part of the evaporator. The condenser is preferably cooled by the cold salt water leaving the 25 evaporator. This further simplifies the salt water plumbing, as the condenser is close to the evaporator and there is no need to obtain a separate supply of cold salt water. Preferably, the hot salt water is heated by solar radiation entering the greenhouse; this reduces the heating effect of solar radiation on the growing area. 30 However, alternative or additional heating may be provided by solar radiation outside the greenhouse. A heat exchanger may be used to heat the hot salt water; 6 for example, a fresh water circuit may be heated directly by solar radiation, and the heat exchanger may pass heat from the fresh water circuit to the salt water. This avoids the need to pass salt water directly through a solar heating area, which reduces the risk of corrosion and/or deposits in that area. 5 Brief Description of the Drawings There now follows, by way of example only, a detailed description of preferred embodiments of the present invention, with reference to the figures identified below. Figure 1 is a schematic diagram of a system using the known Seawater 10 Greenhouse (TM) technology; Figures 2a and 2b are schematic diagrams of the evaporator and condenser arrangements in a known arrangement, and in an embodiment of the present invention; Figure 3 is a diagram of a model of the evaporator; 15 Figures 4a and 4b are charts of air temperature across the model for different salt water flow rates; Figure 5 is a diagram of airflow through the evaporator and condenser in the embodiment; Figure 6 is a diagram of the temperature of salt water as it flows through the 20 evaporator 1 and the condenser 4; and Figure 7 is a schematic diagram of the flow of hot and cold salt water and fresh water through the embodiment. Detailed Description of the Embodiments In the following description, functionally similar parts between the 25 described prior art and the embodiments carry the same reference numerals, but this does not imply that any specific part of the embodiments is wholly or partially conventional. The drawings are intended to be schematic, and dimensions and angles may not be determined accurately from them. Note that the term 'greenhouse' is intended herein to cover broadly a 30 structure which admits solar radiation and thereby provides an advantageous 7 environment for growing plants. The term is not intended to be limited to greenhouses, such as hothouses, designed to provide a warm environment in colder climates, by trapping air warmed by solar radiation. Instead, the term is specifically intended to include greenhouses designed to provide a cooler, more 5 humid environment in hot, arid climates. In the description of the invention and in the claims, except where the context requires otherwise due to express language or necessary implication, the word "comprise" or variations such as "comprises" or "comprising" is used in an inclusive sense, i.e. to specify the presence of the stated features but not to 10 preclude the presence or addition of further features in various embodiments of the invention. Prior Art Seawater Greenhouse (TM) Technology Figure 1 shows a greenhouse using the known Seawater Greenhouse (TM) technology, comprising generally a structure 10 having a transparent or 15 translucent roof that admits solar radiation SR. An air flow AF enters the structure and passes through a first evaporator 1, through which cold salt water CS is trickled, so as to cool and humidify the air. The cool, humid air flow AF then passes over a growing area GA forming the main part of the structure. In a hot salt water circuit HS, salt water is heated by solar radiation in a 20 solar heater 3, comprising for example a network of black pipes above the growing area GA, which removes heat from the greenhouse roof. The heated salt water passes over a second evaporator 2, through which the air flow AF passes on leaving the growing area GA, making the air hot and humid. The air flow AF then passes through a condenser 4 where the air is cooled 25 below its dewpoint temperature by the cold salt water CS that has been cooled by, and circulated from the first evaporator 1. The condensation is collected from the condenser 4 and stored as fresh water FW and/or supplied to the growing area GA. The air flow AF is driven by one or more fans 11 which draw air into the structure, but may be aided by sitting the structure so that the prevailing wind is in 30 the desired direction of the air flow AF. The cold salt water CS is drawn from and returned to a supply of salt water, such as a well or directly from the sea, and is 8 pumped to the condenser 4 and thence to the first evaporator 1 by a first pump 12. The hot salt water HS is pumped to the solar heater 3 and second evaporator 2 by a second pump 13. Embodiments of the Invention 5 In the prior art arrangement described above, the present inventors have observed a strong vertical temperature gradient in the second evaporator 2, which was hot and humid at the top, but cool and humid at the bottom. The effect is such that, almost irrespective of the temperature of the hot salt water HS flowing onto the pad, the salt water will almost invariably come off the second evaporator 10 2 at the wet bulb temperature, which is the coolest temperature that can be achieved by evaporation alone. The present inventors have therefore devised an improved design of the Seawater Greenhouse' technology. As shown schematically in Figure 2a and described in more detail above, the conventional Seawater Greenhouse T" 15 technology has a first evaporator 1 upstream of the growing area GA, a second evaporator 2 downstream of the growing area GA, and a condenser 4 downstream of the second evaporator 2, with reference to the direction of the air flow AF. In an embodiment of the present invention, as shown schematically in Figure 2b, the second evaporator 2 is eliminated and the condenser 4 is moved 20 upstream of the growing area GA. The condenser 4 is reduced in size, for example to approximately 25% of the area of the first evaporator 1, and is positioned downstream of an upper portion of the evaporator 1, through which the salt water flows first. Apart from any performance benefit, this arrangement reduces the number 25 of elements required, simplifies the plumbing, and concentrates both the salt water and fresh water plumbing into one smaller area. It may at first seem counter-intuitive that one evaporator can be used to: * cool the greenhouse * humidify the greenhouse 30 e heat and humidify air onto the condenser; and 9 cool the condenser all at the same time. However, a combination of observations, trials, modelling and testing have verified that this is the case. The air and water flow rates, together with the ability to adjust these 5 throughout the day and seasons, may be regulated. The following modelled illustrations describe the process and illustrate the effect of different flow rates. A model of the evaporator 1 is shown in Figure 3. The model divides the pad of the evaporator 1 into 50 x 50 compartments. For each compartment, the properties of air and water are calculated. The model is then run with nominal 10 climate conditions. In the case illustrated below in Table 1, the meteorological data for the midday conditions at the height of summer are used. The effect of two different flow rates of water are illustrated in Figures 4a and 4b, respectively for 61/s and 1 1/s. 15 Table 1 Year Week Greenhouse Temp Relative Temp Airflow Air Time dry humidity wet (m 3 /s) speed bulb bulb (m/s) 2008 1 12.3 13.3 35.8 20.7 19.9 5 0.5 As shown in more detail in Figure 5, in one embodiment of the invention the condenser 4 is raised slightly higher than the evaporator 1, to make best use of the hotter and more humid air from the upper portion of the evaporator 1. Air 20 passes through the evaporator 1 and hot salt water HS flows over it. Thus the hottest and most humid air HA comes off the top section of the evaporator 1 and the salt water cools progressively as it flows down the evaporator 1. The lower section of the evaporator has a greater airflow than the upper section, cooling the salt water down to the wet bulb temperature and resulting in colder air CA which 25 flows into the growing area GA.

10 Preferably, adjustable flaps 6 regulate the air flow from the upper portion of the evaporator 1 through the condenser 4, preferably so that salt water flowing off the evaporator 1 achieves the lowest possible (wet bulb) temperature. The cooled air coming off the condenser 4 may be allowed to pass into the growing area. 5 Figure 6 is a salt water flow diagram of the embodiment, showing the change of temperature as salt water flows through the evaporator 1 and the condenser 4. The cool salt water flowing off the evaporator 1 passes through the condenser 4 where it is warmed slightly by the latent and sensible heat of condensation from the hot and humid air HA. From the condenser 4, the salt 10 water is heated further by heat exchange with the solar heater 3 before circulating back to the evaporator 1. Note that the salt water becomes more concentrated as it circulates through the evaporator 1 and condenser 4. The more concentrated salt water may be passed to a salt making process, or to a well outlet, and replaced by salt water 15 from the source. Figure 7 is a schematic diagram of the water flow in Summer in the embodiment, showing: The flow of cold salt water CS from a well inlet WI, driven by pump 12, through the condenser 4 and thence as hot salt water HS through a heat 20 exchanger 5, over the evaporator 1 and out to a well outlet WO. * The flow of cold fresh water FW as condensate from the hot humid air HA off the condenser 4. * The circulation of fresh water through the solar heater 3, which may comprise panels 3a in the roof of the structure and/or panels 31 outside the 25 structure and the heat exchanger 5, to heat the hot salt water HS before it flows onto the evaporator 1. * The flow of hot fresh water from evacuated tubes 16 to a salt making process 14, from which cold fresh water is returned. Evacuated tubes provide hot water around 60-80 0 C which is circulated through a salt water 30 evaporation pan. The water vapour that is driven off may be re-condensed by a condenser.

11 The evaporator 1, or humidifier, may comprise a porous or perforated wall, comprising one or more panels or pads over which the sea water trickles down under gravity, and through which the incoming air passes. The panels or pads may be constructed from corrugated cardboard. Salts from the seawater may be 5 deposited onto the cardboard, strengthening and prolonging the life of the evaporator. The condenser 4 may be made of tubes through which cold seawater passes, and fins for increasing the surface area that comes into contact with the air and thereby enhancing the cooling effect. The tubes may be made from polythene 10 or other plastic that resists corrosion. The tube walls may be sufficiently thin to compensate for the low thermal conductivity of the plastic. The roof of the greenhouse is preferably double-walled, with the solar heater 3 arranged between the walls. At least the inner wall of the roof may be arranged to filter out those wavelengths that are less important for photosynthesis. 15 The inner wall of the roof may absorb or reflect infrared radiation, so as to enhance the effect of the solar heater 3 while reducing the solar heating of the growing area GA. Instead of sea water, embodiments of the invention may use salt water from a source other than the sea, such as saline ground water, water from tidal rivers or 20 estuaries, or hard or brackish water. Alternative Embodiments The embodiments described above are illustrative of rather than limiting to the present invention. Alternative embodiments apparent on reading the above description may nevertheless fall within the scope of the invention. 25

Claims

1. An evaporatively cooled and humidified greenhouse having an evaporator in which, in use, air entering the greenhouse is cooled and humidified by evaporation from a flow of salt water, and the cooled and humidified air 5 subsequently passes into a growing area, the evaporator having at least a first section into which hot salt water flows, such that air from the first section is hot and humid and the hot salt water is cooled, and a second section into which the cooled salt water flows from the first section, such that air from the second section is cooler than air from the first section, and 10 further comprising a condenser in which air from the first section is cooled so as to obtain fresh water therefrom as condensate.

2. The greenhouse of claim 1, wherein the air cooled by the condenser enters the growing area.

3. The greenhouse of claim 1 or claim 2, including means for regulating the 15 flow of air through the first section.

4. The greenhouse of any one of the preceding claims, including means for regulating the flow of salt water through the evaporator.

5. The greenhouse of claim 3 or claim 4, wherein the means for regulating are arranged such that salt water leaves the evaporator substantially at wet bulb 20 temperature.

6. The greenhouse of any one of the preceding claims, wherein an upper portion of the condenser is positioned higher than the top of the evaporator.

7. The greenhouse of any one of the preceding claims, wherein the condenser 25 is cooled by a supply of cold salt water.

8. The greenhouse of claim 7, wherein the cold salt water supply comprises salt water from the evaporator.

9. The greenhouse of any one of the preceding claims, wherein the hot salt water is heated by solar radiation. 13

10. The greenhouse of claim 9, wherein the hot salt water is heated by solar radiation entering the greenhouse.

11. The greenhouse of claim 9 or claim 10, wherein the hot salt water is heated by solar radiation outside the greenhouse. 5

12. The greenhouse of any one of claims 9 to 11, wherein the hot salt water is heated by heat exchange with fluid heated by solar radiation.

13. The greenhouse of any one of the preceding claims, wherein the first and second sections are continuous sections of the evaporator.

14. A method for evaporatively cooling and humidifying a greenhouse, 10 comprising: (a) passing a flow of salt water through an evaporator through which air entering the greenhouse is cooled and humidified by evaporation, the evaporator having at least a first section into which hot salt water flows, such that air from the first section is hot and humid and the 15 hot salt water is cooled, and a second section into which the cooled salt water flows from the first section, such that air from the second section is cooler than air from the first section, (b) passing the air from the first section to a condenser for cooling so as to obtain fresh water as condensate; and 20 (c) passing the cooled and humidified air subsequently into a growing area

15. The method of claim 14, wherein the air cooled by the condenser enters the growing area.

16. The method of claim 14 or claim 15, including regulating the flow of air 25 through the first section.

17. The method of any one of claims 14 to 16, including regulating the flow of salt water through the evaporator.

18. The method of any one of claims 14 to 17, wherein the salt water leaves the evaporator substantially at wet bulb temperature. 14

19. The method of any one of claims 14 to 18, wherein an upper portion of the condenser is positioned higher than the top of the evaporator.

20. The method of any one of claims 14 to 19, wherein the condenser is cooled by a supply of cold salt water. 5

21. The method of claim 20, wherein the cold salt water supply comprises salt water from the evaporator.

22. The method of any one of claims 14 to 21, wherein the hot salt water is heated by solar radiation.

23. The method of claim 22, wherein the hot salt water is heated by solar 10 radiation entering the greenhouse.

24. The method of claim 22 or claim 23, wherein the hot salt water is heated by solar radiation outside the greenhouse.

25. The method of any one of claims 22 to 24, wherein the hot salt water is heated by heat exchange with fluid heated by solar radiation. 15

26. The method of any one of claims 14 to 25, wherein the first and second sections are continuous sections of the evaporator.

27. An evaporatively cooled and humidified greenhouse substantially as herein described with reference to any one of Figures 2b to 7 of the accompanying drawings. 20

28. A method of evaporatively cooling and humidifying a greenhouse substantially as herein described with reference to any one of Figures 2b to 7 of the accompanying drawings.