Abstract:
Apparatus for controlling an environment in a greenhouse, the apparatus comprising: first and second heat exchangers, each comprising a radiator and a fan system for driving air through the radiator; a first refrigerant circulation system that circulates a refrigerant fluid between and through the radiators; a heater controllable to heat the refrigerant; a controller that controls the apparatus to operate selectively in a maintenance mode or a flush mode, wherein in the maintenance mode the heater heats the refrigerant and the first and second fan systems drive air from outside to inside the greenhouse and through the radiators to acquire heat from the refrigerant, and in the flush mode the first fan system vents air from inside to outside the greenhouse through its respective radiator to deposit heat in the refrigerant and the second fan system drives air from outside to inside the greenhouse and through its respective radiator to acquire the heat deposited in the refrigerant.

Description:
RELATED APPLICATIONS 
     The present application is a US National Phase of PCT Application No. PCT/IB2011/053188, filed on Jul. 18, 2011, the disclosure of which is incorporated herein by reference. 
     TECHNICAL FIELD 
     Embodiments of the invention relate to controlling temperature and humidity in a greenhouse. 
     BACKGROUND 
     Air temperature and relative humidity of an environment in which plants grow affect plant growth and health by affecting photosynthesis and transpiration. Photosynthesis is a process by which plants convert carbon dioxide and water to organic compounds needed for plant growth and metabolism. Transpiration is a process by which plants draw water and nutrients required for plant growth and metabolism from soil into their roots and transport the water and nutrients to their leaves and other plant organs. 
     Photosynthesis and transpiration are temperature and relative humidity dependent. Relative humidity, is a ratio equal to an amount of water contained in the atmosphere per unit volume of air divided by a maximum amount of water a unit volume of the air can contain before the water begins to condense out of the air. Water condenses out of air when the air&#39;s relative humidity is equal to 100%. Photosynthesis generally increases with increase in temperature. Transpiration is affected by a rate at which water drawn in from the soil and transported to plant leaves and organs evaporates from surfaces of the leaves and organs and increases with increase in rate of evaporation. Evaporation of water from plant surfaces also aids a plant in dissipating heat and regulating plant body temperature. Rate of evaporation and therefore transpiration, and a plants ability to cool itself, generally decreases with increasing relative humidity. 
     Plants adapted to different natural environments, for example, desert plants such as cactuses and tropical plants such as orchids, thrive in different temperature and relative humidity ranges. If they are subjected to temperatures and relative humidities outside of the ranges for which they are adapted, they generally do not do well, and may become diseased. Relative humidity in an environment in which a plant grows that is greater than a maximum for which the plant is adapted can result in a reduction in rate of evaporation to such an extent that concomitant reduction in plant transpiration, and the plant&#39;s ability to dissipate heat and regulate its body temperature, damages plant metabolism and health. High relative humidity also tends to result in condensation of water droplets on surfaces of plants when ambient temperature in the environment decreases during the diurnal cycle. The condensed moisture promotes germination of fungal pathogen spores, such as Botrytis and powdery mildew, on the plant surfaces that can damage or kill the plants. 
     Because of the sensitivity of plants to temperature and RH, artificial environments, such as provided by greenhouses, in which plants are commercially grown, must generally be monitored and controlled to maintain air temperature and humidity within desired ranges. For many greenhouse environments in which leafy plants and vegetables are grown, it is advantageous for temperature to be maintained in a range from about 18° C. to about 22° C. and relative humidity in a range from about 75% to about 82%. 
     In the closed environment of a greenhouse, RH tends to increase as a result of plant transpiration and evaporation of water from the soil and can be difficult to control. Typically, relative humidity in a greenhouse is controlled using a longstanding conventional procedure, in which hot humid air in the greenhouse is periodically vented to the outside environment and replaced with cooler air drawn into the greenhouse from the outside. The indrawn cool air is heated to bring its temperature within a desired range of greenhouse air temperatures. Heating the indrawn cool air also reduces its relative humidity. The capacity of air to hold water increases and its RH decreases with increasing air temperature. Relative humidity of indrawn cool air, even if it is 100% (i.e. at which relative humidity water begins to condense out of the air) may be reduced substantially by increasing that air&#39;s temperature. For example relative humidity of outside air at a temperature of 18° C. and 100% relative humidity is decreased to a relative humidity of 50% by heating to a temperature of 25° C. 
     Whereas the longstanding conventional procedure for controlling relative humidity by periodically venting hot humid greenhouse air and replacing it with cooler air drawn into the greenhouse from the outside and heated is generally effective, it exposes greenhouse plants to relatively large fluctuations in air temperature. The procedure also consumes relatively large amounts of energy and is therefore expensive. 
     By way of example, air temperature in a greenhouse using conventional humidity control systems may fluctuate from a low temperature equal to about an outside air temperature, for example, 10° C., to a maximum temperature of about 22° C. Relative humidity of the inside air may suffer a range from about 70% to about 100%. During a diurnal cycle for which outside relative humidity of outside air fluctuates between about 60% to about 70% and temperature of outside air between about 12° C. and 16° C. a conventional system may consume more than about 2,000 kWh (kilowatt hours) of energy. 
     SUMMARY 
     An embodiment of the invention relates to providing a greenhouse environment control (GECO) system for controlling temperature and relative humidity in a greenhouse by periodically venting warm humid air in the greenhouse and replacing it with air drawn in from the outside that is heated by heat extracted from the vented warm humid air. Between periods when warm humid air is vented, the GECO system generates and heats a moderate flow of outside air into the greenhouse. The process is relatively energy efficient and characterized by relatively moderate fluctuations in greenhouse air temperature that results from exchanging greenhouse inside air with air from the outside. 
     In accordance with an embodiment of the invention, the GECO system comprises an air circulation and heat exchange system and a controller that controls the circulation and heat exchange system selectively to operate in a “flush” mode or in a “maintenance” mode. The circulation and heat exchange system comprises a first “vent” heat exchanger that is coupled by a refrigerant fluid and a refrigerant flow system to a second, “intake” heat exchanger. The vent heat exchanger comprises a vent fan system selectively controllable to drive warm moist air from inside the greenhouse to outside the greenhouse or to drive air from outside to inside the greenhouse, through a relatively long air flow path in a large efficient “vent” radiator. The intake heat exchanger comprises an intake fan system controllable to draw relatively cold air from outside the greenhouse to inside the greenhouse through a relatively long air flow path in a large and efficient “intake” radiator. 
     In the flush mode, the GECO controller controls the vent fan system to drive hot humid air from the greenhouse through the vent radiator to the outside, and the intake fan system to draw air from the outside into the greenhouse through the intake radiator to replace the vented air. The vent radiator extracts heat from the vented air to heat the refrigerant fluid and cool the vented air. The refrigerant flow system transports the refrigerant heated by heat extracted by the vent radiator from the vented air to the intake radiator. The intake radiator heats air drawn into the greenhouse by the intake fan system and cools the refrigerant. After heat is removed from the refrigerant to heat the intake air, the cooled refrigerant is recycled by the refrigerant flow system to the vent radiator where it is heated again and recycled back to the intake radiator. Optionally, the vent heat exchanger cools venting air to a temperature substantially equal to an ambient temperature of the outside air and the intake radiator heats drawn in air to a desired greenhouse temperature. 
     In the maintenance mode the GECO system operates to maintain temperature and RH in the greenhouse within desired ranges by generating a relatively slow and steady influx of heated outside air into the greenhouse. To generate the influx, the GECO controller controls both the intake and venting fan systems to draw outside air into the greenhouse and heat the drawn in air to a desired greenhouse temperature. The rate of influx is determined to create an air pressure inside the greenhouse that is slightly greater than atmospheric pressure, and a resultant leakage of air out from the greenhouse equal to the rate of influx. Optionally, air leakage out of a greenhouse having a floor area of about 1,000 m 2  (square meter) and height of about 3 m is greater than or equal to about 2,500 m 3 /hr (cubic meters per hour). Optionally, the air leakage is less than about 3,500 m 3 /hr. In an embodiment of the invention the air leakage may be equal to about 3,000 m 3 /hr (cubic meters per hour). Optionally, the desired greenhouse temperature is equal to about 22° C. To provide heat for heating the drawn in outside air, the controller couples the refrigerant flow system to a heat source. 
     By controlling durations and frequency of switching between flushing and temperature maintenance modes of operation in accordance with an embodiment of the invention, the GECO system provides substantial savings in amounts of energy required to control temperature and RH in a greenhouse and reduces amplitude of fluctuations in temperature and RH of air in the greenhouse. 
     An embodiment of the invention relates to providing a system, hereinafter a water agitator (WAGIT), that operates to clean surfaces of leaves and plant parts of moisture that may have accumulated on the surfaces. The system comprises a source of acoustic energy controllable to transmit sound waves which generate vibrations in the leaves and plant parts that agitate and shake water droplets from their surfaces. In an embodiment of the invention, the acoustic source is tunable to transmit acoustic waves at resonant vibration frequencies of plant leaves. 
     There is therefore provided in accordance with an embodiment of the invention, apparatus for controlling an environment in a greenhouse, the apparatus comprising: first and second heat exchangers, each comprising a radiator and a fan system for driving air through the radiator; a first refrigerant circulation system that circulates a refrigerant fluid between and through the radiators; a heater controllable to heat the refrigerant; a controller that controls the apparatus to operate selectively in a maintenance mode or a flush mode, wherein in the maintenance mode the heater heats the refrigerant and the first and second fan systems drive air from outside to inside the greenhouse and through the radiators to acquire heat from the refrigerant, and in the flush mode the first fan system vents air from inside to outside the greenhouse through its respective radiator to deposit heat in the refrigerant and the second fan system drives air from outside to inside the greenhouse and through its respective radiator to acquire the heat deposited in the refrigerant. Optionally the apparatus comprises a third heat exchanger controllable to heat air inside the greenhouse. Optionally, the third heat exchanger comprises a radiator, a second refrigerant flow system that streams a refrigerant through the radiator, a heater that heats the refrigerant in the second refrigerant flow system and a fan system that drives air inside the greenhouse through the radiator to acquire heat from the refrigerant and remain in the greenhouse. 
     Optionally the apparatus comprises a fluid flow control valve controllable to connect the first and second refrigerant flow systems so that heated refrigerant from the second refrigerant flow system can flow into the first refrigerant flow system. Optionally, in the maintenance mode, the controller controls the fluid control valve to connect the first and second refrigerant flow systems. 
     In an embodiment of the invention, in the maintenance mode, the controller controls the third heat exchanger to substantially refrain from heating air inside the greenhouse. 
     In an embodiment of the invention, the controller controls the third heat exchanger to heat air inside the greenhouse when temperature of the inside air drops below a predetermined minimum air temperature. 
     In an embodiment of the invention, in the maintenance mode the controller controls the fan systems of the first and second heat exchangers to draw air from outside to inside the green house at an average flow rate that is substantially proportional to a volume of the greenhouse. Optionally, the flow rate is greater than about 2,500 m 3 /hr (cubic meters per hour) per 3,000 m 3  of greenhouse volume. Additionally or alternatively, the flow rate is less than about 3,500 m 3 /hr per 3,000 m 3  of greenhouse volume. Optionally, the flow rate is equal to about 3,000 m 3 /hr per 3,000 m 3  of greenhouse volume. 
     In an embodiment of the invention, the controller controls the apparatus to operate in a flush mode if relative humidity in the greenhouse is greater than a predetermined minimum relative humidity. 
     In an embodiment of the invention, the controller switches operation of the apparatus between flush and maintenance modes at regular intervals. Optionally, duration of a period of operation in the flush mode is the same for a plurality of consecutive periods of operation in the flush mode. Optionally, the flush mode periods are repeated at a repetition frequency greater than about 0.8 per hour. Additionally or alternatively, the repetition frequency is less than about 1.2 per hour. In an embodiment of the invention, the repetition frequency is equal to about 1 per hour. 
     In an embodiment of the invention, periods of operation in the flush mode have duration less than or equal to about 10 minutes. In an embodiment of the invention, periods of operation in the flush mode duration greater than or equal to about 5 minutes. In an embodiment of the invention, periods of operation in the flush mode have duration equal to about 6 minutes. In an embodiment of the invention, the controller initiates periods of operation in the maintenance mode substantially at times at which periods of operation in the flush mode end. 
     There is further provided in accordance with an embodiment of the invention, a method of controlling an environment in a greenhouse, the method comprising: periodically, during first periods, venting air from inside to outside the greenhouse while drawing air from outside to inside the greenhouse and heating drawn in air with heat extracted from the vented air; and during second periods between the first periods, drawing in air from outside to inside the greenhouse and heating the air as it is drawn in. 
     Optionally the method comprises initiating first periods when the relative humidity becomes greater than a predetermined relative humidity. Alternatively or additionally the method comprises switching between first and second periods at regular intervals. Optionally the method comprises determining a same duration for a plurality of consecutive first periods. 
     In an embodiment of the invention the method comprises initiating second periods substantially at times when first periods end. 
     In an embodiment of the invention, an average flow rate at which air is drawn in from outside to inside the green house during the second periods is substantially proportional to the greenhouse volume. Optionally, the flow rate is greater than about 2,500 m 3 /hr per 3,000 m 3  of greenhouse volume. Additionally or alternatively, the flow rate is less than about 3,500 m 3 /hr per 3,000 m 3  of greenhouse volume. Optionally, the flow rate is equal to about 3,000 m 3 /hr per 3,000 m 3  of greenhouse volume. 
     In an embodiment of the invention, first periods have duration less than or equal to about 10 minutes. In an embodiment of the invention, first periods have duration greater than or equal to about 5 minutes. In an embodiment of the invention, first periods have duration equal to about 6 minutes. 
     There is further provided in accordance with an embodiment of the invention, a method of removing water droplets from surfaces of plants growing in a greenhouse, the method comprising: providing an acoustic generator configured to generate acoustic waves in the greenhouse; and operating the acoustic generator to transmit sound waves that are incident on, and generate vibrations in, surfaces of the plants that cause water droplets on the surfaces to roll or be shaken off the surfaces. Optionally, the sound waves are characterized by a frequency that is substantially equal to a resonant frequency of vibration of the plant surfaces. Additionally or alternatively, the sound waves are characterized by a frequency that is substantially equal to a resonant frequency of vibration of the water droplets. 
     In the discussion unless otherwise stated, adjectives such as “substantially” and “about” modifying a condition or relationship characteristic of a feature or features of an embodiment of the invention, are understood to mean that the condition or characteristic is defined to within tolerances that are acceptable for operation of the embodiment for an application for which it is intended. 
     This Summary is provided to introduce a selection of concepts in a simplified form that are further described below in the Detailed Description. This Summary is not intended to identify key features or essential features of the claimed subject matter, nor is it intended to be used to limit the scope of the claimed subject matter. 
    
    
     
       BRIEF DESCRIPTION OF FIGURES 
       Non-limiting examples of embodiments of the invention are described below with reference to figures attached hereto that are listed following this paragraph. Identical structures, elements or parts that appear in more than one figure are generally labeled with a same numeral in all the figures in which they appear. Dimensions of components and features shown in the figures are chosen for convenience and clarity of presentation and are not necessarily shown to scale. 
         FIG. 1A  schematically shows a conventional environment control system operating to maintain temperature and relative humidity in a greenhouse; 
         FIG. 1B  shows a flow chart descriptive of operation of the conventional environment control system shown in  FIG. 1A ; 
         FIGS. 1C and 1D  show graphs of relative humidity and temperature respectively of air in a greenhouse environment controlled by the conventional environment control system shown in  FIG. 1A ; and 
         FIG. 2A  schematically shows a GECO greenhouse environment control system operating to maintain temperature and relative humidity in a greenhouse, in accordance with an embodiment of the invention; 
         FIG. 2B  shows a flow chart descriptive of operation of the GECO system shown in  FIG. 2A , in accordance with an embodiment of the invention; 
         FIGS. 2C and 2D  show graphs of relative humidity and temperature respectively of air in a greenhouse environment controlled by the GECO system shown in  FIG. 2A , in accordance with an embodiment of the invention; and 
         FIG. 3  schematically shows operation of a WAGIT moisture removal system operating to remove moisture from a leaf in accordance with an embodiment of the invention. 
     
    
    
     DETAILED DESCRIPTION 
     In the following detailed description, components and operation of a conventional greenhouse environment control system are described and discussed with reference to  FIGS. 1A and 1B .  FIGS. 1C and 1D  show graphs of relative humidity and temperature of air in a greenhouse environment controlled by conventional environment control system such as that shown in  FIG. 1A . Components and operation of a GECO environment control system in accordance with an embodiment of the invention are described and discussed with reference to  FIGS. 2A and 2B .  FIGS. 2C and 2D  show graphs of relative humidity and temperature of air in a greenhouse environment controlled by a GECO system in accordance with an embodiment of the invention such as the GECO system shown in  FIG. 1A . Operation of a WAGIT system for accelerating removal of water from plant surfaces is discussed with reference to  FIG. 3 . 
       FIG. 1A  schematically shows a greenhouse  20  having windows  22  and exhaust fans  24  mounted respectively in opposite walls  26  and  28  of the green house. Greenhouse  20  comprises a conventional environment control system  30  for controlling temperature and RH in the greenhouse. The environment control system comprises an “inside” heat exchanger  40  for heating air inside greenhouse  20  to a desired temperature, manifold flow sleeves  60  for distributing the heated air to different regions in the greenhouse and sensors  31  and  32  for monitoring temperature and RH respectively of air inside the greenhouse. A controller  33  controls the environment control system, windows  22  and exhaust fans  24  of greenhouse  20  responsive to measurements of temperature and RH provided by sensors  31  and  32 . 
     Heat exchanger  40  comprises a radiator  42  and a refrigerant flow system comprising a refrigerant heater  41  and a refrigerant pump (not shown) that streams heated refrigerant, generally water, into and out of the radiator. The refrigerant flow system is connected to radiator  42  by inlet and outlet pipes  43  and  44  respectively. Heat exchanger  40  optionally comprises two blowers  46  controllable to drive air in the greenhouse through radiator  42 , where the air is heated to a desired temperature by the refrigerant streaming through the radiator. Heated refrigerant enters radiator  42  via inlet pipe  43  and after heating greenhouse air blown through the radiator by blowers  46 , the refrigerant is cooled and leaves the radiator via outlet pipe  44  to return to the refrigerant flow system and heater  41  where it is reheated and returned to again flow through the radiator. It is noted that in  FIG. 1A  and figures that follow, heater  41  is schematically shown located in greenhouse  20  and close to heat exchanger  40 . Heater  41  does not of course have to be located inside greenhouse  20 , and in practice the heater is generally located outside the greenhouse, and often far from the green house. 
     Air blown through and heated in radiator  42  flows out of heat exchanger  40  and, optionally, into manifold flow sleeves  60  through coupling sleeves  62 . The manifold sleeves are typically made of plastic sheeting and/or fabric, and are inflated by the heated air that enters them from the heat exchanger. Sleeves  60  are formed having holes (not shown) through which heated air from heat exchanger  40  flowing in the sleeves flows out of the sleeves to mix with air in the greenhouse to maintain a desired greenhouse air temperature and relative humidity. Arrows  64  schematically represent air flowing out of sleeves  62 . Whereas in  FIG. 1A  heated air exiting heat exchanger  40  is directed into manifold sleeves  60  for dispersion into the greenhouse volume, in some greenhouses heated air is dispersed differently. For example, in some greenhouses heated air from a heat exchanger flows directly from the heat exchanger into the greenhouse volume. By way of another example, in some greenhouses heated water is streamed through a network of pipes on the greenhouse floor to heat the air inside the greenhouse. 
     Controller  33  optionally controls environment control system  30  to control temperature and relative humidity in greenhouse  20  by periodically replacing hot humid air inside the greenhouse with air drawn in from the outside and heated, in accordance with a conventional exemplary algorithm schematically represented by a flow diagram  100  shown in  FIG. 1B . The numeral  100  is used to refer to the flow diagram and to the algorithm which it represents. 
     Generally, a greenhouse environment control system, such as environment control system  30 , is off during the day in climates for which there is sufficient solar energy incident on the greenhouse to maintain greenhouse air temperature above a desired minimum. In flow diagram  100  it is assumed that initially, as shown in a block  102  of the flow diagram, that controller  33  controls heat exchanger  40  to be off and therefore environment control system  30  to refrain from heating air in greenhouse  20 . 
     In a block  104 , controller  33  optionally acquires a measurement “T” of air temperature in greenhouse  20  from temperature sensor  31 . In a decision block  106  the controller determines if the measured temperature T is less than a predetermined desirable minimum temperature “T Min ”. Whereas T Min  is dependent upon a type of plants grown in greenhouse  20 , for many plants T Min  is advantageously equal to about 20° C. If, in decision block  106 , controller  33  determines that T is less than T Min , as generally might occur towards nightfall, the controller optionally proceeds to a block  108  and turns on heat exchanger  40  to heat air in greenhouse  20  to a temperature above T Min . Turning on the heat exchanger generally involves turning on blowers  46  and the refrigerant flow system to stream hot refrigerant through radiator  42  ( FIG. 1A ). Thereafter, controller  33  optionally proceeds to a block  110 . 
     If instead of finding in decision block  106  that T is less than T Min  as assumed in the preceding paragraph the controller  33  finds that T is greater than or equal to T Min  the controller skips block  108  and proceeds to block  110 . 
     In a block  110 , whether or not controller  33  skips block  108 , the controller acquires a measurement “RH” of relative humidity of the air in greenhouse  20  from humidity sensor  32  and in a block  112 , the controller compares RH to a given desired maximum, “RH Max ”. In a decision block  112  the controller also, optionally, determines whether at a time at which RH is acquired in block  110 , an elapsed time since air in the greenhouse was last replaced by heated air from the outside is greater than an optionally predetermined time interval “τ”. If in decision block  112  RH is less than RH Max , or the elapsed time is less than τ, controller  33  skips a block  114  discussed below, and advances to a block  116 . 
     In block  116  the controller acquires a temperature measurement T, and in a decision block  118  determines whether T&gt;T Min . If T is greater than T Min  the controller returns to block  102  and turns off heat exchanger  40 . On the other hand, if T≦T Min , controller  33  returns to block  110 , acquires measurement new RH, and in block determines if the new RH is greater than RH Max . 
     If in decision block  112  controller  33  determines that RH is greater than RH Max  and the elapsed time is greater than τ, controller  33  proceeds to block  114  to replace overly humid air in greenhouse  20  with outside air to reduce humidity in the greenhouse. To accomplish the replacement, the controller opens windows  22  and controls fans  24  to vent air from inside greenhouse  20  and draw air in from the outside through open windows  22  to replace the vented air. 
     In block  116 , after replacement of air in greenhouse  20 , controller  33  acquires a temperature measurement T, and in decision block  118 , if T&gt;T Min  the controller returns to block  102  and turns off heat exchanger  40 . On the other hand, if T&lt;T Min , controller  33  continues to heat air (block  108 ) in greenhouse  20  and returns to block  110 . 
     Generally air drawn in from outside greenhouse  20  to replace air inside the greenhouse is relatively cold, and typically has a temperature that is substantially less than T Min . As a result, immediately after replacing air inside greenhouse  20  with outside air, temperature of air in greenhouse  20  is less than T Min . For a period after air replacement therefore, from decision block  118  controller  33  generally repeatedly returns to block  110  to cycle through blocks  110 - 118 , heating air in greenhouse  20  until the controller determines in decision block  118  that temperature of air in the greenhouse is greater than the desired minimum T Min . 
     For many greenhouse environments RH Max  is advantageously equal to about 85%. Time interval τ is determined to prevent cold air from outside greenhouse  20  being drawn in to replace greenhouse air so frequently that a rate at which cold air drawn into greenhouse  20  must be heated to maintain a desired greenhouse temperature exceeds a capacity of the heat exchanger to heat the drawn in air. 
       FIGS. 1C and 1D  show graphs  201  and  202  of relative humidity and temperature respectively of air inside and outside of greenhouse  20  having an environment control system  30  operating in accordance with an algorithm similar to algorithm  100 . In graphs  201  and  202  solid curves  211  and  212  show relative humidity and temperature respectively for air inside greenhouse  20  as a function of time for a period of two days. Time in hours is shown along the graphs&#39; abscissas. Dotted curves  214  and  215  show relative humidity and temperature respectively for air outside greenhouse  20  as a function of time for the same two day period. The curves in graphs  201  and  202  were experimentally determined for a greenhouse, hereinafter also referred to as a 3 m×1,000 m 2  greenhouse, having height equal to about 3 m and floor space equal to about 1,000 m 2 . Heat exchanger  40  when turned on provided 290 kW of energy to heat air streaming at 14,000 m 3 /hr (cubic meters/hr) through radiator  42 . On the average, for each diurnal cycle the heat exchanger operated for about seven hours. In consequence, conventional environment control system  30  consumed about 2,030 kWh (kilowatt hours) of energy during each diurnal cycle. 
     From the graphs it is seen that both relative humidity and temperature of air in greenhouse  20  cyclically fluctuate with relatively large amplitudes in cadence with the repeated replacement of hot humid greenhouse inside air with cold, relatively low humidity outside air. Temperature fluctuates with amplitude of about 7° C. between about 14° C. and about 21° C. and relative humidity fluctuates with an amplitude of about 20% between about 75% and 95%. 
       FIG. 2A  schematically shows a greenhouse  320  comprising a greenhouse environment control system  330 , that is a GECO system  330 , also referred to as GECO  330 , used to control the environment in the greenhouse, in accordance with an embodiment of the invention. 
     GECO system  330  optionally comprises components, such as an inside heat exchanger  40  and vent fans  24  comprised in environment control system  30 , and in addition comprises an air circulation and heat exchange system  340 , hereinafter also referred to as a climate control system (CCS)  340 , in accordance with an embodiment of the invention. 
     CCS  340  optionally comprises a controller  342  and a vent heat exchanger  350  coupled by a refrigerant fluid flow system  360  to an intake heat exchanger  370 . Vent heat exchanger  350  comprises a vent radiator  352  and vent fan system  354 . The vent fan system is selectively controllable to drive warm moist air from inside the greenhouse to outside the greenhouse or to drive air from outside to inside the greenhouse, through a relatively long air flow path in a large efficient “vent” radiator  352 . Airflow arrows  355  pointing from vent heat exchanger  350  towards the outside of greenhouse  320  and airflow arrows  356  pointing from the vent heat exchanger towards the inside of the greenhouse, schematically represent the selectable directions in which vent fan system  354  can drive air. Intake heat exchanger  370  comprises an intake fan system  374  controllable to draw relatively cold air from outside the greenhouse in a direction indicated by airflow arrows  371  to inside the greenhouse through a relatively long air flow path in a large and efficient “intake” radiator  372 . 
     Fluid flow control system  360  comprises refrigerant circulation pipes  362  that connect intake radiator  372  with vent radiator  352  and a refrigerant pump  364  controllable to pump refrigerant in the circulation pipes between the vent and intake radiators. Circulation pipes  362  are connected by a fluid flow control valve  366  to inlet pipe  43  through which hot refrigerant from refrigerant heater  41  is introduced into radiator  42 . The circulation pipes are optionally connected by a T joint  367  to outlet pipe  44  through which relatively cold refrigerant leaves radiator  42 . Controller  342  controls heat exchanger  40 , and controls flow valve  366 , pump  364 , vent and intake heat exchangers  350  and  370  to selectively operate CCS in a flush mode or a maintenance mode. 
     In the flush mode, controller  342  controls vent fan system  354  to drive air from inside greenhouse  320  in a direction indicated by airflow arrows  350  to outside of the greenhouse and intake fan system  374  to drive air from outside the greenhouse to inside the greenhouse in a direction indicated by airflow arrows  371 . In the flush mode the controller closes flow valve  366  and operates refrigerant pump  364  to circulate refrigerant from vent radiator  352  to intake radiator  372 . 
     Hot humid air driven by vent fan system  354  through vent radiator  352  in the direction of airflow arrows  355  is cooled in passing through the vent radiator and heats refrigerant fluid in the radiator. Pump  364  pumps heated refrigerant from the vent radiator to intake radiator  372  where it is cooled in heating air driven by intake fan system  374  through the intake radiator. In the flush mode CCS  340  replaces hot humid air vented by vent heat exchanger  350  from inside greenhouse  320  with cold air drawn into the greenhouse by intake heat exchanger  370  and heats the indrawn air with heat that the vent heat exchanger extracts from the vented air. In an embodiment of the invention, heat extracted from the vented air is sufficient to heat indrawn air to a temperature substantially equal to a desired greenhouse air temperature. 
     In the maintenance mode, controller  342  controls vent fan system  354  to drive air from outside greenhouse  320  to inside the greenhouse in a direction of airflow arrows  356  and intake fan system to drive air from outside to inside in a direction of airflow arrows  371 . The controller also opens flow valve  366  to connect circulation pipes  362  to inlet pipe  43  so that refrigerant fluid in the inlet pipe heated by heater  41  that heats refrigerant fluid for heat exchanger  40  can enter circulation pipes  362 . Controller  342  operates pump  364  to circulate the heated refrigerant fluid entering the pipes from inlet pipe  43  through radiators  352  and  372  to heat air drawn in from the outside by vent and intake fan systems  354  and  374 . The controller controls a flow rate at which the indrawn and heated air enters greenhouse  320  so that air pressure in the greenhouse is slightly greater than atmospheric pressure and heated air from outside flows into the greenhouse at a moderate rate and replaces air inside the greenhouse. 
     In an embodiment of the invention, controller  342  controls switching between flushing and maintenance modes of CCS  340 , and durations of the modes, to maintain a relatively steady response to changes in temperature and relatively humidity of air in greenhouse  320 . Cycling of CCS  340  between flushing and maintenance modes obviates the periodic greenhouse air replacements that characterize operation of conventional greenhouse environment control systems and provides relatively efficient control of greenhouse temperature and relative humidity.  FIG. 2B  shows a flow diagram  400  of an exemplary algorithm, also referenced by numeral  400 , that describes operation of GECO  330  in controlling temperature and humidity in greenhouse  320 , in accordance with an embodiment of the invention. 
     In flow diagram  400  it is assumed that, as in flow diagram  100  ( FIG. 1B ), initially, GECO  330  is in a quiescent state, in which radiators  42 ,  352  or  372  are not operating to heat air in or being drawn into greenhouse  320 . Accordingly, a block  402  of the flow diagram shows that/greenhouse heating is off. In a block  404  controller  342  receives a measurement “T” of temperature in greenhouse  320  from temperature sensor  31  and a measurement “RH” of relative humidity of air in the greenhouse from humidity sensor  32 . In a decision block  406 , if T is greater than a desired minimum temperature T Min  for example, 20° C., controller  342  returns to block  402 . If however, T is less than or equal to T Min , in a block  408  the controller turns inside heat exchanger  40  on, and in a block  410  turns CCS  340  ( FIG. 2A ) on in the flush mode. In the flush mode as noted above, heat exchanger  350  is turned on to vent air from inside greenhouse  320  and extract heat from the vented air and heat exchanger  370  is turned on to draw air into the greenhouse from the outside and heat the drawn in air with the heat extracted from the vented air. In a block  412  controller  342  acquires another measurement T of temperature and another measurement RH of relative humidity. 
     In a decision block  414  controller  342  determines whether T is less than or equal to T Min . If T≦T Min , the controller leaves inside heat exchanger  40  on and CCS  340  in the flush mode, and returns to block  412 , to acquire further measurements of T and RH and in decision block  414  to compare T to T Min . If on the other hand, in decision block  414  the controller determines that T&gt;T Min , the controller continues to a decision block  416  and determines whether RH&lt;RH Max . If RH is greater than or equal to RH Max , the controller optionally turns off inside heat exchanger  40  in a block  418  and returns to block  412  to again cycle through to block  418  leaving inside heat exchanger  40  off, until in decision block  416  controller  342  determines that a measurement RH is less than RH Max . Upon determining that RH is less than RH Max  controller  342  proceeds to a block  420  and switches CCS  340  to the maintenance mode. 
     In a block  422  controller  342  acquires measurements of T and RH and in a block  424  determines whether T≦T Min . If T less than or equal to T Min , the controller returns to block  408  to turn on inside heat exchanger  40 , turn on CCS  340  in the flush mode, and cycle through blocks in flow diagram  400  to block  424 . If in decision block  424  T&gt;T Min , in a block  426  controller  342  determines whether temperature T is greater than a maximum desirable temperature T Max . If T is greater than T Max  the controller returns to block  402  and shuts down heating of air inside greenhouse  320 . Optionally, T Max  is a temperature equal to about 22° C. If on the other hand, T is less than or equal to T Max , the controller proceeds to a decision block  428  to determine whether RH&lt;RH Max . If RH is less than RH Max , the controller leaves CCS  340  in the maintenance mode and returns to block  422 . If on the other hand RH is greater than or equal to RH Max , the controller returns to block  410  and switches CCS  340  to operation in the flush mode. 
     Operation of GECO system  330  in accordance with an algorithm, such as algorithm  400  reduces magnitude of fluctuations in greenhouse temperature and relative humidity, and results in substantial savings in costs and amounts of energy required to control temperature and relative humidity in a greenhouse.  FIGS. 2C and 2D  show graphs  501  and  502  of relative humidity and temperature respectively of air inside and outside of greenhouse  320  controlled by a GECO system similar to GECO system  330  operating in accordance with an algorithm similar to algorithm  400 . 
     In graphs  501  and  502  solid curves  511  and  512  respectively show relative humidity and temperature respectively for air inside greenhouse  320  as a function of time for a period of two days. Time in hours is shown along the graphs&#39; abscissas. Dotted curves  514  and  515  show relative humidity and temperature respectively for air outside greenhouse  20  as a function of time for the same two day period. 
     The curves in graphs  501  and  502 , as were the curves in graphs  201  and  202  ( FIGS. 1C and 1D ), were experimentally determined for a 3 m×1,000 m 2  greenhouse. Vent and intake radiators  352  and  372  had a length in a direction of air flow through the radiators equal to about 100 cm and a cross section perpendicular to the air flow equal to about 60 cm×60 cm. Each radiator comprised in its 100 cm×60 cm×60 cm volume, an array of 16 sets of 16 rows each of ⅝ inch copper pipe. Fan systems  354  and  374  were capable of streaming 1,500 m 3 /h (cubic meters of air per hour) through their respective associated radiators. Heat exchangers  350  and  370  were capable of extracting heat from heated water flowing through their copper pipes, or introducing heat into cooled water flowing in the pipes at rate of about 10 kW. Heat exchangers  350  and  370  were turned on for about 7 hours during each diurnal cycle. Whereas, when turned on, heat exchanger  40  in GECO system  330 , operated at an energy consumption of about 290 kW, during each diurnal cycle it was turned on for about three and a third hours. An overall average energy consumption of GECO system  330  per diurnal cycle was about 1030 kWh. 
     From graphs  501  and  502  it is seen that neither the relative humidity, curve  511 , and temperature of air, curve  512 , in greenhouse  320  exhibit the large cyclical changes exhibited by relative humidity and temperature controlled by conventional environment control system  30  in greenhouse  20  ( FIG. 1A ). Temperature in greenhouse  320  fluctuates with amplitude of about 2° C. between about 20° C. and about 22° C., and relative humidity in the greenhouse fluctuates with an amplitude of about 8% between about 80% and about 87%. Not only does GECO system  330  provide substantially improved control of temperature and relative humidity in a greenhouse but it does it with substantially reduced energy consumption compared to a conventional greenhouse environment control system. 
     For example, as noted above, for external conditions of temperature and relative humidity of outside air indicated by curve  215  in graph  202  and curve  214  in graph  201  respectively, conventional greenhouse environment control system  30  may consume about 2,030 kWh of energy per diurnal cycle to control air in greenhouse  20  with proficiency represented by curves  212  and  211  in the graphs. A GECO system in accordance with an embodiment of the invention similar to GECO system  330  on the other hand, for conditions of relative humidity and temperature of outside air indicated by curve  514  in graph  501  and curve  515  in graph  502  respectively, may control humidity and temperature for greenhouse  320  with substantially improved proficiency exhibited by curves  511  and  512  in the graphs at an energy cost of 1,030 kWh per diurnal cycle. Whereas the conditions of temperature and relatively humidity of outside air under which GECO system  330  operates to control temperature and relative humidity of air in greenhouse  330  are substantially more demanding than the conditions of temperature and relative humidity of outside air under which conventional environment control system  30  operates, the GECO system operates at an average power consumption that is about half that at which the conventional system operates. 
     It is noted that the energy consumption and flow rates referred to above for GECO system  330  that controls an environment for a 3 m×1,000 m 2  greenhouse and provides performance substantially as shown in graphs  501  and  502 , scale substantially linearly with greenhouse size. For example, a GECO system in accordance with an embodiment of the used to control the environment in a 3 m×2,000 m 2  greenhouse may be configured to consume twice the energy and provide twice the flow rates provided by a GECO system that controls the environment in a 3 m×1,000 m 2  greenhouse. 
     In some embodiments of the invention, controller  342  controls GECO  330  to switch between flush and maintenance modes at optionally predetermined regular intervals. For example, a GECO system similar to GECO  330  in accordance with an embodiment of the invention may operate in flush and maintenance modes for about six and about fifty four minutes respectively every hour can maintain a greenhouse temperature between about 20° C. and about 22° C., and relative humidity between about 80% and about 87%, for outside air and relative humidities for which graphs  501  and  502  were obtained. 
     To provide added protection for plants against disease encouraged or promoted by water condensation on plant leaves and body parts, a greenhouse may comprise a WAGIT in accordance with an embodiment of the invention that operates to sonically clean surfaces of leaves and plant parts of moisture that may have accumulated on the surfaces. 
       FIG. 3  schematically shows a WAGIT  600  operating to remove water droplets  650  condensed on a plant leaf  652 , in accordance with an embodiment of the invention. WAGIT  600  optionally comprises an acoustic transducer  602 , such as a piezoelectric crystal, driven by a power source  604  to generate acoustic waves, schematically represented by dashed arcs  610  that propagate to leaf  652 . When sonic waves  610  are incident on leaf  652  they generate large amplitude vibrations, represented by dashed silhouettes  654 , in the leaf that shake water droplets  650  off the leaf. The removal of the water droplets is schematically indicated by arrows  656 . 
     In an embodiment of the invention, power source  604  drives transducer  602  to generate waves  610  at a frequency substantially coincident with a resonant frequency of leaf  652 . As a result, acoustic waves  610  generate relatively large vibrations in leaf  652  that are relatively efficient in shaking droplets  650  off the leaf. Optionally, power source  604  drives acoustic transducer  602  to generate acoustic waves at a resonant frequency of water droplets  650 , which generate relatively large vibrations in the bodies of the droplets. The vibrations cause the droplet to “roll” off leaf  652 . 
     In the description and claims of the present application, each of the verbs, “comprise” “include” and “have”, and conjugates thereof, are used to indicate that the object or objects of the verb are not necessarily a complete listing of components, elements or parts of the subject or subjects of the verb. 
     Descriptions of embodiments of the invention in the present application are provided by way of example and are not intended to limit the scope of the invention. The described embodiments comprise different features, not all of which are required in all embodiments of the invention. Some embodiments utilize only some of the features or possible combinations of the features. Variations of embodiments of the invention that are described, and embodiments of the invention comprising different combinations of features noted in the described embodiments, will occur to persons of the art. The scope of the invention is limited only by the claims.