Field of Invention

The invention relates to a greenhouse comprising an environmental control system. In particular, a greenhouse for use in dry environments using a passive buffering system to control temperature, humidity and C0 ₂ concentration.

Background to the Invention

It is well established that there are significant food shortages around the world and that this problem is likely to increase as the climate becomes less hospitable. The land available for conventional agriculture is also becoming smaller as the climate changes.

Accordingly, there is a demand for innovative approaches to improving food yields and making better use of the earth's surface for agricultural purposes. One particular region of interest are deserts. A desert is typically characterised as a barren area with little precipitation, it does not necessarily need to be hot. Most non-polar deserts receive large amounts of sunlight throughout the year. Although this sunlight is very good for promoting photosynthesis, it typically removes any trace of water from the environment. Moreover, the world's deserts are expanding every year as the Earth's climate becomes warmer. These non-polar deserts also tend to have a substantial variation in temperature with many fluctuating as much as 15°C to 20°C in a single day. These factors pose a significant challenge to plants.

It is possible to prevent evaporative water loss in such environments by shielding crops within an enclosed atmosphere and regulated atmosphere usually in a greenhouse. Crops can also be grown in contained spaces so that moisture is not permitted to escape through the ground. However, this can cause problems regulating temperature as it is not possible to both vent hot/cold air and simultaneously retain water vapour. If the temperature is not controlled carefully then plants will overheat and die. Moreover, the humidity can increase until the cultivating atmosphere is saturated with water vapour, preventing plants from performing nutrient absorption. Excessive levels of humidity also make the growth of fungus, mold and other diseases more favourable.

Accordingly, it is necessary to control the temperature and humidity to within tolerable limits. Conventionally, this is done using dehumidifiers, actively cooling the atmosphere (e.g. using powered air conditioning systems) and by venting to atmosphere. However, it is very difficult to reliably provide power to devices such as dehumidifiers in many desert environments. Further, even temporary disruptions in power supply can result significant crop degradation or failure. Therefore, maximising the time a system can sustain its crops before power is restored is very valuable. Moreover, the energy consumption required is substantial and almost always not practical in remote desert locations.

As such, there is required a system for controlling the temperature, humidity and carbon dioxide conditions in an enclosed growing environment situated in a desertlike environment which has low to negligible power requirements.

The invention is intended to overcome or at least ameliorate some of the above problems.

Summary of the Invention

A greenhouse comprising, a growing chamber for the cultivation of crops and a fluid treatment system; the fluid treatment system comprising at least one passive buffering system selected from: a thermal buffer, a desiccant and a C0 ₂ buffer; wherein the growing chamber is in fluid communication with the fluid treatment system; and wherein the greenhouse is a closed system.

The term "passive buffering" is intended to mean that it does not require power, typically electrical power, in order to moderate the parameter in question. Buffering occurs automatically. Such systems have a maximum buffering capacity and in order to provide the desired level of control, i.e. without being overwhelmed, they must be provided in sufficient quantities (or at least have sufficient capacity) and be able buffer at an appropriate rate. For instance, the rate of absorption into and out of a carbonate solution should ideally be sufficient to meet the demand of plants growing within growing chamber.

The term "greenhouse" is not intended to be construed as exclusively encompassing glass structures but to cover any structure for growing crops. There is no minimum size nor specific dimensions associated with said greenhouse. There is no requirement for the greenhouse to be made substantially, or even partially, of a transparent material. However, it is typically the case that the greenhouse will comprise transparent portions in order to enhance the temperature within the greenhouse. The greenhouse will typically comprise a roof, walls, and a base which communicate with one another so as to form an enclosed environment. Said roof, walls and base may include thermal insulation to avoid heat loss, for instance in the form of paddled material or integral air pockets within the roof, wall and base material. In addition, the greenhouse may also have regions adapted to radiate or absorb heat in situations where the internal temperature of the greenhouse exceeds or approaches the upper limit of acceptable desired temperature conditions. These could be portions of thermally conductive material in contact with the external environment. Such portions could be portions of the walls, roof, base of the greenhouse or combination thereof which may have geometries to maximise surface area to enhance the rate of heat transfer. The entire roof and/or walls of the greenhouse may be made from a transparent material or may comprise a plurality of windows made from transparent material. The choice of transparent material used is not limited to glass. Any transparent material would be sufficient provided it can be manufactured into a suitable shape. Often, transparent polymer materials are used as the transparent material. The skilled person would be familiar with a variety of typical polymers suitable for such purposes such as polyethylene or polypropylene. It is typically the case that a flexible polymer will be used as such materials can be easily transported and often have better performance in desert conditions. For instance, glass can be more prone to scratching in high winds which can compromise optical properties. Moreover, shattered glass can end up in the growing environment and present a hazard. The transparent material may be reinforced or modified to control the reflectance of the material or augment the thermal properties of the material. Shading could also be employed to control the amount of sunlight incident on the external surfaces of the greenhouse at certain times of the day.

The term "closed system" as used herein is intended to mean an enclosed environment such that the growing atmosphere, and preferably the entire growing environment is unable to communicate (i.e. come into physical contact) with the external environment. This prevents the air within the greenhouse from escaping and hence, prevents the loss of heat via convection. It also prevents the loss of valuable growth materials present in the air such as water vapour, oxygen, C0 ₂ and the like. This also prevents water and nutrients escaping through the base of the greenhouse. Whilst it is preferable that the greenhouse recirculates the atmosphere or fluid within the greenhouse between the growing chamber and the air treatment system in a continuous loop, this is not essential.

Utilising passive buffering systems allows for low power buffering of conditions within the closed environment of the greenhouse. As such, the conditions remaining essentially constant and fluctuations in temperature, CO2 and humidity are accommodated by the passive systems and regenerated over 24 hours. The greenhouse typically comprises at least two passive buffering systems selected from : a thermal buffer, a desiccant and a C0 ₂ buffer and more often, the greenhouse comprises each of a thermal buffer, a desiccant and a CO2 buffer. Moreover, it is often the case that the greenhouse includes at least two of the said passive buffering systems.

This allows the system to synergistically regulate two interrelated parameters. The greenhouse when positioned in warm non-polar desert environments is typically arranged such that, during the day when it is hot, excess heat is stored within the buffers and then releases said heat during the night when the temperature drops. This may be facilitated with a plurality of valve arrangements to improve the level of control with which fluids can be moved around the system. By providing two passive buffering systems described above, this increases the overall capacity of the system to buffer changes in temperature, humidity and carbon dioxide.

Further, it is often the case that at least two of said passive buffering systems are in thermal communication with one another. This is advantageous as it effectively allows different passive buffering systems to work together to better resist changes in the system environment, especially as CO2 concentration, humidity and temperature influence one another in a closed system.

The inventors have found that by thermally coupling at least two passive buffering systems together selected from the desiccant, thermal buffer and CO2 buffer described above, both systems together provide improved environmental buffering.

The greenhouse usually has a volume greater than 10m ³ . More typically, greater than 20m ³ and more typically still greater than 100m ³ .

The term "growing chamber" is intended to refer to that region of the greenhouse in which crops are grown. For instance, it may be the case that the greenhouse comprises a plurality of planters comprising soil or may simply comprise soil covering the base of the greenhouse, in which plants can grow.

The term "fluid treatment system" is intended to refer to a device which modifies the properties of the atmosphere or fluid within the greenhouse. In particular, temperature and humidity. It is usually the case that the fluid treatment system is an air treatment system. Accordingly, the atmosphere within the growing environment is treated to moderate the described variables. However, the atmosphere, usually air, may be treated by passing it through a liquid. In such embodiments, the air may be bubbled through or passed over the liquid in order to promote the exchange of heat, CO2 concentration and water content. In such embodiments, the fluid treatment systems moderates the properties of the air by treating the liquid to which the air is exposed. In some embodiments, the fluid treatment system combines both a liquid treatment system and an air treatment system combined. This can be advantageous as, when the liquid medium is water, it can be difficult to regulate the water content of the air using purely a water treatment system. The liquid through which the air is passed in a liquid treatment system is typically water.

The fluid treatment system need not be contained within the growing chamber of the greenhouse (usually that space within the region bounded by the base, roof and walls of the greenhouse) but need only be in fluid communication with the greenhouse. The fluid treatment system typically comprises a "thermal buffer" which is capable of absorbing heat from the greenhouse fluid and releasing energy to the fluid as heat, depending on the relative temperatures of the system components. There is no particular restriction on the type of thermal buffer used and the term "thermal buffer" is intended to take its typical meaning, i.e. a volume of material, typically having a high specific heat capacity, capable of readily absorbing and radiating heat. Alternatively (or in addition to an in-built thermal buffer) the greenhouse may also comprise a heat sink. Typically a means to permit the evolution of heat into the external atmosphere or into the ground. The term "heat sink" is intended to take its conventional meaning i.e. a body that absorbs heat. There is no particular restriction as to how this is achieved. For instance, radiators may be provided through which the fluid from within the greenhouse may circulate. Said radiators may be positioned external of the greenhouse, exposed to the external environment, for embedded within the ground. This permits heat to be transmitted to the external environment or the ground depending on the relative temperatures. The preferred heat sink is typically the ground because the temperature of the ground (certainly a few meters below ground) often remains at a relatively constant level compared to surface temperatures and does not change substantially. Accordingly, large amounts of heat energy can be deposited there by the system. It is particularly advantageous to have both a heat sink and a thermal buffer as the thermal buffer is capable of fine tuning the temperature of the system whereas the heat sink can remove excess heat from the system entirely.

Often, the thermal buffer is comprised of a phase change material. The term "phase change material" (PCM) is intended to refer to material which undergoes a change of state at a temperature close to the optimum crop growing temperature. This ensures that as the temperature of the system increases above the optimum temperature, excess heat energy is used to change the state to the PCM. Accordingly, the PCM acts as a passive temperature buffering system. Typically, this is a change from solid to liquid or liquid to gas. Conversely, if the temperature drops below the optimum temperature then the reversion of the PCM to a more ordered state relinquishes heat and, in doing so, resists the change of temperature. The PCM need not be selected to have a critical phase transition at the optimal temperature of greenhouse atmosphere. Materials that undergo partial phase changes, e.g. the evolution of water vapour from liquid water, are also envisaged. Typical examples of PCMs includes, but are not limited to: waxes, water, volatile organic solvents such as alcohols or combinations thereof. It is typically the case that water is the phase change material.

The term "desiccant" is intended to take its traditional meaning, i.e. a material that is capable of absorbing water and releasing water. This water absorption is typically associated with a release of heat energy as water vapour becomes associated with the desiccant in a more ordered form. The desiccant is typically a solid desiccant, though liquid desiccants are also envisaged. There is no particular restriction on the type of desiccant that is used, though it is usually a chemically inert material. However, it is typically the case that the desiccant is selected from : silica, activated alumina, activated charcoal, calcium sulfate, calcium carbonate, molecular sieves, magnesium sulfate or combinations thereof. The desiccant may be coupled to a desiccant recharging system. The term "recharging system" as used herein is intended to refer to a portion of the system where desiccant can be passed in order to either pick up more water or release water. This may be in a separate portion of the system not in communication with the fluid of the fluid treatment system e.g. the air from the growing chamber.

It is typically the case that the fluid treatment system comprises a carbon dioxide (C0 ₂ ) buffer. In addition to regulating the temperature and humidity of the growth environment, it is also important to control the concentration of CO2. Controlling the concentration of CO2 within optimal limits is important for plant productivity. If the concentration of CO2 is permitted to increase too much, crops (and microbes in soil) can suffocate resulting in crop failure. If the concentration of CO2 is permitted to decrease too much, crops will be restricted in their ability to photosynthesise. There is an optimal CO2 concentration to maintain in order to ensure optimal plant growth and maximise crop yield. It is often the case that the CO2 buffer is a solution containing Dissolved Inorganic Carbon (DIC). This DIC comprises a mixture of dissolved CO2, carbonic acid, bicarbonate ions and carbonate ions. The 15 DIC solution is in equilibrium with the C0 ₂ in the air of the greenhouse growth environment, known as the headspace. At a given temperature and pressure, the DIC solution will reach equilibrium with CO2 in the headspace above it at a particular partial pressure of CO2. As plants photosynthesise, CO2 will be withdrawn from the headspace, resulting in a drop in the partial pressure of CO2 in the headspace. The CO2 buffer will respond to the decrease in partial pressure of CO2 in the headspace by degassing CO2 from the DIC solution thereby maintaining the partial pressure of CO2 in the headspace. In this way the partial pressure of CO2 in the headspace can be maintained within optimal bounds. When the plants respire at night, resulting in the release of CO2 into the headspace, the DIC solution will again buffer the system, drawing down CO2 into the solution so as to maintain the equilibrium partial pressure of CO2. Overall, as the amount of biomass increases there will be a net removal of CO2 from the DIC solution. If left unmaintained, the DIC solution will gradually lose its ability to buffer the system. The DIC solution can be refreshed by addition of carbonate ions. Further, there is an optimal CO2 concentration to maintain in order to ensure optimal plant growth and maximise crop yield. It is often the case that the CO2 buffer is a carbonate solution. The carbonate ions in the carbonate solution are in equilibrium with the CO2 in the air of the greenhouse growth environment. The carbonate solution may be overly saturated with carbonate in order to ensure a sufficient supply of carbonate ions.

It is typically the case that each of the greenhouse comprises all three passive buffering systems. Moreover, said three passive buffering systems are typically in thermal communication with one another. It has been found by the inventors that, thermally coupling the thermal buffer, desiccant and CO2 buffer results in a synergistic improvement in the overall passive buffering of the system. For instance, the excess thermal energy received by the greenhouse during the day results in heat transfer to the thermal buffer. By thermally coupling the thermal buffer to the CO2 buffer, more CO2 is released which can be used to meet the high CO2 demand during the day.

It is usually the case that the greenhouse comprises a fluid recirculation means. This ensures that the fluid, typically air, in the greenhouse is able to effectively communicate with each of the passive buffering systems and the growing chamber and maintain a substantially homogeneous atmosphere. Typically, the fluid circulation means comprises a pump or fan together with a network of conduits connecting the growing chamber atmosphere or other liquid source to the fluid treatment system in a loop arrangement. The fluid recirculation system is typically electrically powered. Moreover, the pump or fan arrangement may be used as a means of introducing ingredients into the atmosphere and/or the growing chamber. For instance, fertilisers, antibiotics, hormones, seeds or the like may be introduced using the fluid recirculation means. The location of the pump or fan within the greenhouse is typically such that any additives end up in the appropriate portion of the greenhouse. In particular, the recirculation means may be positioned so as to prevent large particulates, such as seeds, from blocking conduits, radiators and the like. This may be achieved with the use of a suitable filtration system.

The greenhouse may utilise the ground as an alternative or additional thermal buffer and/or heat sink to the heat sink described above. Typically, this is achieved by embedding a radiator within the ground through which air from the growing chamber, or liquid with which said air contacts, can be passed in order to relinquish any excess heat. The term "radiator" as used herein is simply intended to encompass a conduit for fluid from which heat can be dissipated. The radiator is therefore typically fabricated from a thermally conductive material and usually has a large surface area in order to ensure maximum heat dissipation (or absorption) as fluid passes through it. The radiator may be integral to the conduits through which air from the greenhouse atmosphere travels. For instance, portions of the conduits may be equipped with a plurality of blades or ribs about the external surface of the conduit to maximise the surface area. The advantage of using the ground as a heat sink is that, whilst surface temperatures fluctuation substantially, the temperature below the surface of a desert is much more consistent. Accordingly, the ground has essentially unlimited capacity to receive and donate heat (provided the region about the radiator is able to conduct heat sufficiently well).

Usually, the greenhouse is for cultivating macrophytes. A macrophyte is a plant which grows on water, typically on the surface of water so as to be able to photosynthesise efficiently. Macrophytes are distinct from microphytes, the latter being small, unicellular plants such as algae. A typical construction adopted to grow macrophytes involves a fluid bed, typically involving a plurality of channels about which a fluid (typically water) is continually circulated. The fluid may be salt water or fresh water and is typically in communication with a thermal syphon and often a filter. Reference to a "fluid bed" as used herein is intended to describe a container for holding water typically at the base of the greenhouse. The container is usually shaped to ensure a large surface area of water is available on which crops can be grown. One or more channels are typically provided to provide a circulating path of the water and macrophytes growing on the surface thereof. In such arrangements, it may be desirable for the fluid bed to function as thermal buffer and in a typical embodiment the atmosphere may be bubbled or passed through the fluid on entry to or exit from the growing chamber so as to promote rapid heat exchange between the two fluids and provide aeration to the water.

The thermal syphon typically comprises: an inlet, an outlet and a heat exchanger. In use, fluid is drawn into the thermal syphon, potentially with the aid of a pump, though this is not essential. Often, the inlet is positioned near the surface of the fluid and the outlet is positioned near the base of the greenhouse (i.e. near to the bottom of the fluid bed). Warm water will rise to the top of the fluid bed and then enter the heat exchanger. Heat can be released in the heat exchanger and then cooler water is expelled at the bottom of the fluid bed via the outlet. The heat exchanger is not particularly restricted. It is typically a radiator having a plurality of conduits which create a large surface area so as to maximise the amount of transfer of heat. Accordingly, it is desirable that the radiator be made from a thermally conductive material. The thermal syphon is typically positioned outside the greenhouse but is in fluid communication with the fluid bed. The thermal syphon may include an active means for cycling fluid there thorough (such as a pump) but it is typically the case that the syphon operates without such systems, such as through the change in temperature of the fluid passing through the system.

Typically, the thermal syphon includes one or more valves. The valves can be used to prevent the flow of fluid into the thermal syphon, typically at night when the external temperature drops below the desired temperature of the greenhouse in order to prevent undue heat loss. However, operation of such control means will change depending on the conditions both inside and outside the greenhouse and the desired optimal growing conditions.

Where the greenhouse is for cultivating macrophytes, it may be the case that the thermal syphon acts as the heat sink or the thermal buffer. Additionally, the thermal syphon may be used as an added thermal management system to those already described above.

The greenhouse is typically equipped with one or more sensors to monitor the greenhouse environment. In particular, temperature and humidity sensors are desirable as well as sensors to monitor the concentration of various gases in the environment such as C0 ₂ and oxygen. Sensors may also be used to monitor the conditions of the fluid bed where the greenhouse is for cultivating macrophytes. Said sensors, may communicate with the control systems within the greenhouse, such as the thermal syphon and valves to control the operation thereof and ensure a consistent growing environment.

One or more filters may be provided, in order to remove unwanted particulate matter from the greenhouse atmosphere or from the fluid bed. The skilled person would be familiar with such systems. Further, sterilisation systems made be included to ensure that undesirable pathogen growth is minimised or eradicated. Passive and active sterilisation systems may be used. Passive sterilisations systems include, for example, biocides and pesticides whereas active systems of sterilisation system include UV irradiation or heating. A typical filtration system suitable for use with the fluid bed would be a slow-sand filter.

The thermal syphon may be provided with one or more tanks for temporary storage of fluid from the fluid bed. For instance, a hot fluid tank and a cold fluid tank may be provided in communication with the heat exchanger. A nutrient storage tank may also be provided to maintain the levels of nutrients in the fluid bed. The nutrient storage tank may also communicate with one or more sensors in order to respond to changes in nutrient levels.

There may also be provided means for catching and storing rain water such that, when rain does fall, it is collected and can be used to augment any inadvertent water loss from the greenhouse, such as when harvesting takes place.

The invention will now be described with respect to the accompanying figures and drawings.

Brief Description of the Drawings

Figure 1 show a schematic system view of the greenhouse of the invention.

Figures 2a and 2b show a schematic cross section view of different thermal syphon arrangements.

Figure 3 shows a representation of the fluid bed and fan of the air treatment system.

Figure 4a and 4b show two alternative schematic arrangements of the greenhouse.

Figure 5 shows a representation of the fluid bed and fluid treatment system.

Figure 6 shows a schematic representation of the fluid bed and fluid treatment system.

Figure 7 shows possible configurations for the heat exchanger. Specific Description

Figure 1 shows a greenhouse 101 comprising a growing chamber 103 comprising a water bed 105 on the surface 107 of which macrophytes are grown. Heat from the sun 102 is incident upon the green house which increases the temperature of air within the growing chamber 103. A thermal syphon 109 is provided in fluid communication with the water bed 105 adapter to moderate the temperature of the water bed.

A fan (not shown) is provided which draws air from the growing chamber 103 into a radiator 113 via conduit 111. Depending upon the temperature of the incoming air, heat is able to transfer between the air and the heat sink 115. Air is subsequently drawn into a desiccation chamber 117 comprising a desiccant. Moisture from the air may be absorbed by the desiccant depending on the saturation of the desiccant and temperature of the air. The desiccant may be moved between the desiccation chamber 117 and a desiccant recharging chamber 118. In said chamber 118, desiccant is exposed to an air cycle 120 which passes through a precipitation chamber 122. The air is heated, often using solar energy or by using latent heat of the passive conditioning systems within the system, in order to remove to provide water to the desiccant. The desiccant's water can thus be controlled and a reintroduced into the desiccation chamber 117 at optimum saturation and temperature.

Air passing through the greenhouse system is, after desiccation chamber 117, then drawn into a C0 ₂ buffer chamber 119 which is in communication with a DIC solution which acts as a CO2 buffer. The CO2 buffer is a sodium carbonate solution which is connected to a CO2 reservoir 125 which can be used to top up the CO2 content of the C0 ₂ buffer 129.

A further radiator 121 is provided between the growing chamber 103 and the CO2 buffer chamber 119 as a means of venting additional head to the external atmosphere, though this could easily be a second heat sink or phase change medium. Flow into the further radiator is controlled by a valve and sensor arrangement (not shown). Air is then conveyed back to the growing chamber 103 via conduit 123. The system can be operated in either a clockwise or anticlockwise direction.

Figure 2a shows a representation of one possible thermal syphon 201, comprising an inlet 202 for incoming hot water into the water tank 203. The water tank includes an inlet conduit 207 for transporting hot water to the heat exchanger 209. A valve is positioned within the inlet conduit 207 to control operation of the heat exchanger 209. Heat is radiated from (or received by) the exchanger from the external environment and the subsequently passed into the outlet conduit 213 and into the water tank 203 at a lower temperature. The water at the bottom of the tank is of a lower temperature than the water at the top of the tank and it may be the case that the tank is partitioned into top and bottom portions in order to clearly separate the warm and cold water and improve stratification. Water is able to exit the water tank 203 via the outlet 221 either back into the fluid bed or into subsequent water treatment systems.

In an alternative arrangement, the water tank 203 can be separated into two separate tanks (a hot water tank 204 and a cold water tank 206 either side of the heat exchanger 209) each adapted for storing and conveying hot and cold water respectively.

The arrangement of the fan 311, within the greenhouse 301 is shown in figure 3. The greenhouse 301 has an elongate structure which includes two channels 305, defined by the walls 309 of the greenhouse and a central barrier 307, around which the water 303 of the fluid bed is circulated . The fan 311 is located at one end of the greenhouse 301 in order to draw air over the full length of the channels 305.

Figure 4 shows two arrangements 401, 402 both of which can be operated in a clockwise or anticlockwise direction.

It shows two greenhouse systems 401, 402 each comprising a growing chamber 403 comprising a water bed 405. A water storage tank 407 is provided connected to a thermal syphon 409 to moderate the temperature of the water bed.

A fan 413 is provided which draws air from the growing chamber 403 into and through the heat sink 417. Depending upon the temperature of the incoming air, heat is able to transfer between the air and the heat sink 117. Air is subsequently drawn into a desiccation chamber 418 comprising a desiccant. The desiccant may be moved between the desiccation chamber 418 and a desiccant recharging chamber 420. In said chamber 420, desiccant is exposed to an air cycle which passes through a precipitation chamber 422. In the first arrangement 401, the desiccant chamber 418 is thermally coupled to the C0 ₂ buffer chamber 424. The CO2 buffer chamber 424 typically comprises a sodium carbonate solution.

The air is then passed back into the growing chamber via return conduit 425 after passing through filter 427. In the second arrangement 402, air existing the heat sink 417 enters a thermal buffer chamber 419 which typically comprises an aqueous solution which can evaporate based on the relative temperature and humidity of the air. The thermal buffer chamber is thermally coupled to a desiccant chamber 423. Air leaving the desiccant chamber subsequently enters the C02 buffer chamber 426 before being returned to the growing chamber via return conduit 425 after passing through filter 427.

Figure 5 shows a schematic top down view of the greenhouse 501 together with the water treatment system 502. The walls of the greenhouse 503 together with the central barrier 507 form two channels 505 about which the water 507 is able to flow. Water is drawn into inlet 511, through conduit 513 into a slow sand filter 515 as would be familiar to a person skilled in the art. The resulting purified water is passed through a UV filtration system 516 to ensure safe levels of pathogens and into a storage tank 517 which may, together with heat exchanger 519 function as a thermal syphon. Water exiting the thermal syphon is passed into a nutrient balance chamber 521, comprising a plurality of nutrient reservoirs and control means 523 for introducing nutrients into the water depending on the optimum growth requirements.

Once nutrient levels have been adjusted, water is passed back into the greenhouse channels 505 via outlet conduit 525 and outlet 527.

Figure 6 is a top down view of a typical greenhouse 601 of the invention. It shows a growing chamber 603 comprising a fluid bed 602 separated by a central barrier 606 into two channels 605. A harvesting machine 607 is provided comprising a collection apparatus 609 which periodically removes macrophytes from the surface of the water and is deposited in a storage chamber 609. The temperature of the fluid bed 602 is controlled using a thermal syphon 613, feed by a storage tank 611 from an inlet conduit 610. Fluid is returned to the fluid bed from the thermal syphon 613 via outlet conduit 612.

Air from the growing chamber 603 passed through air conduit 615 into a radiator embedded below ground which functions as a heat sink 617. Air exiting the heat sink 617 encounters desiccant chamber 619 which is provided in communication with a desiccant recharging chamber 621 which is communication with a radiator 623 containing fluid for heating and modifying the saturation level of the desiccant. Air passing through the desiccant chamber 619 subsequently passes into the C02 buffer chamber 625 before being returned to the growing chamber 603 via return conduit 627.

Figures 7a to 7c show three alternative heat exchanger arrangements 701, 702, 703 suitable for dissipating or absorbing heat. In the first arrangement 701, the air is passed through a plurality of pipes 707 surrounded by a chamber 709 comprising a heat transfer medium, such as water. The outer wall of said chamber 705 abuts the ground in which the system 701 is buried and heat transfer is possible therewith. Typically the outer wall of the chamber 705 is made of a steel or plastic material.

In a second embodiment 702 the air to be cooled (or heated) is passed through a first channel 713 and a heat transfer medium, typically water, is passed through an adjacent channel 715. The two channels are separated by an intervening wall 717, typically with good thermal conductivity, through which heat transfer is possible. In a third embodiment 703, the intervening wall 717 referred to above is removed and air to be cooled (or heated) is passed directly over the surface of a heat transfer fluid 721, again typically water, in a channel 719.