Efficient cooling system for nutrient solution in hydroponic greenhouse technologies

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The temperature of the nutrient solution plays an important role in the successful cultivation of plants. There are two significant factors that depend on the temperature of the solution – the solubility of oxygen in water and the oxygen demand of plants. The higher the temperature of the solution, the less oxygen there is in the solution. With an increase in temperature from 0 to 30°C, water loses about half of its oxygen. Pure water at 20°C contains about 9.5 mg/l of dissolved oxygen. At 30°C, the oxygen content drops to 7.6 mg/l. But these values are valid only for pure water. An increase in the temperature of the nutrient solution leads to an increase in plant metabolism. As a result, the oxygen demand in the root zone increases. Plants in the ground close their stomata to conserve water when the temperature rises too much (more than 30°C). In hydroponics, plants can grow at higher temperatures. However, this is possible only with good water circulation, which ensures a high oxygen level [1-4].

The normal range is considered to be the temperature of the solution from 18 ° C to 24 ° C. In greenhouses with hydroponic technologies, for example, in India, daytime temperatures reach 35-40 ° C, which slows down plant growth, and fruits do not appear after flowering. In addition, greenhouses use the NFT (Nutrient Film Technique) "nutrient Film" method. This method is suitable for crops with lower water consumption, such as arugula, lettuce, as well as for growing tomatoes with high requirements [5].

For the southern regions of Russia and the republics of Central Asia, when growing vegetable crops in greenhouses, it is necessary to maintain the same temperature and humidity conditions [6]. In [3], the problems of climate control and various cooling systems in greenhouses are considered in detail, which are the same for both greenhouses with soil and hydroponic technologies. The cooling process is divided into two main categories: "passive" and "active" systems. "Passive" greenhouse cooling refers to a constructive approach (shape, shelter materials, openings, overnight cooling of soil or nutrient solution). At the same time, the temperature inside the greenhouse decreases without additional supply of water or electrical energy. "Active" cooling refers to all cooling systems that use electrical equipment such as pumps, fans, refrigerating machines, and heat pumps. The integration of passive cooling methods followed by active cooling can simultaneously provide adequate conditions for crop growth and reduce power consumption [3].

Compressor refrigerating machines and heat pumps are used for cooling and heating in greenhouses. Most studies agree that temperature, humidity, and CO2 emissions can be effectively controlled with such machines. But energy consumption is very intensive and uneconomical.

Absorption refrigerating machines and heat pumps are widely used in industry and construction, but very little research has been conducted on the integration of these greenhouse cooling machines and nutrient solution in hydroponic greenhouses. In [7], a lithium bromide absorption refrigerating machine was used to cool the air in the greenhouse. Significant savings in electrical energy have been achieved. Several experimental and numerical studies of the use of solar energy for the operation of absorption machines have been conducted [8-10]. Lithium bromide and water-ammonia absorption refrigerating machines require a high-potential hot heat source of about 110-130degrees Celsius for their operation. It is difficult and expensive to obtain such a temperature using solar thermal panels.

3R-Technology Company (https://www.3r-t.com, Israel) has been promoting sorption refrigerating machines (absorption – AbHM and adsorption – AdHM) to the markets for a long time. A distinctive feature of these devices is the use of methyl alcohol (methanol) as a refrigerating agent in the proposed sorption machines. The use of methanol makes it possible to reduce the temperature of the heating coolant to 60-65°C. This makes it possible to use conventional solar heaters or any secondary heat sources of low potential (60-75°C) for the operation of sorption refrigerating machines. For agriculture, especially for greenhouses, sorption machines are quite affordable, provide great energy savings, and are simple and cheap to operate.

For greenhouses that require constant operation of the cooling (heating) system, it is advisable to use absorption refrigerating machines. In farms where cooling is required only during the daytime or it is necessary to periodically stop the cooling process, it is most advisable to use adsorption refrigerating machines. Adsorption refrigerating machines have a number of advantages over absorption ones. There is no circulation pump in the AdHM, and there are no mechanisms (self-acting return valves open at a minimum pressure difference and close with a simple spring), they have been operating non-stop for 20-25 years.

The use of a more resistant adsorbent in AdHM, for example, hardened activated carbon, and methanol as a refrigerating agent, makes it possible to double the productivity of the machine (with the same dimensions of the adsorbers) and obtain a boiling point in the evaporator of the order of -5°C... -2°C. The vacuum in the apparatus of the adsorption refrigerating machine is not lower than 35-40 mm Hg. This makes it possible not to increase the thickness of the walls of the devices, but to dispense with the installation of reinforcing bandages.

3R-Technology has developed and is successfully promoting an efficient nutrient solution cooling system for agricultural greenhouses with hydroponic technologies using sorption refrigerating machines. A patent of the Russian Federation was obtained for the developed nutrient solution cooling system [11].

Fig.1 shows a schematic technological diagram of a hydroponic solution cooling system using an adsorption refrigerating machine (AdRM).

The system consists of several blocks: (A, B, C, D, E). Block (A) is designed to prepare a heating coolant for an adsorption refrigerating machine (7). Block (A) consists of a diesel generator (1) a gas-liquid heat exchanger (2), a tank (3) with a coil (6) for preparing a heating coolant, a circulation pump (4) and a pump (5) designed to supply hot coolant to the adsorbers (Ada) and (Adb) of the refrigerating machine (7).

Block (B) is an adsorption refrigerating machine (7). In addition to the refrigerating machine (7), block (B) includes: a receiver (9) with a pump (10), a primary heat exchanger (11) with a pump (12) and a secondary heat exchanger (13), as well as a fan cooling tower (15). A secondary air-cooled heat exchanger (13) in which the water discharged from the adsorber is cooled at the end of the vapor adsorption process of the refrigerant. The heat exchanger (13) is designed for additional cooling of water, which is periodically supplied to the adsorbers (Ada) and (Adb) during the adsorption of refrigerant vapors.

The unit (C) consists of a hydroponic solution cooler (18), inside which several sections of cooling batteries (19) are located. The coolant cooled in the evaporator (Ev) is fed into the batteries (19), and then returned to the evaporator through the receiver (9). The coolant is circulated by a pump (10).

Block (D) is a hydroponic installation that contains trays (20) for growing plants and a pressure tank (21). Trays (20) are connected to a pressure tank (21) by a pipeline (C18). The dosed supply of the nutrient solution to the trays (20) is carried out, for example, using airlifts (22). The cooled nutrient solution from the pressure tank (21) through the pipeline (C18) enters the hydroponic trays (20). From the trays, the solution is fed through the collectors (23) through the pipeline (C19) by the pump (24) to the filtration unit of the block (E).

Block (E) is a filtration unit consisting, for example, of a multi–hydrocyclone (25), a sump (26) and a sludge collector (27). The sediment separated in the hydrocyclones enters the sump (26). The sludge is discharged into a barrel (27), and the pure solution is drained into a cooler (18).

Fig. 2, 2a, 2b show the basic design diagram of the improved AdRM. The refrigerating machine contains: adsorbers (Ada) and (Adb), a condenser (Cd) and an evaporator (Ev). The condenser is connected to the evaporator by a siphon (8). The refrigerating agent in this machine is methyl alcohol (methanol). To ensure continuous operation of the refrigerating machine, two adsorbers are installed in it. This smooths out the cyclical operation of the adsorbent (the cycle of adsorption of refrigerant vapors by the adsorbent and the cycle of desorption of vapors from the adsorbent). Each adsorber is a sealed box-shaped housing (2.1), which houses a single-pass heat exchanger (2.2). The heat exchanger is placed in a cartridge (2.3) made of a mesh material, for example, a stainless steel mesh. Adsorbent (2.4) is poured into the inter-tube space of the heat exchanger (2.2). Granular or hardened activated carbon or a sorbent based on activated fibrous carbon material (Busofite) is used as an adsorbent. In Fig. 2a and 2b show the cross section of an adsorbent (Adb) for a backfilling adsorbent refrigerating machine.

There is a design option for the adsorbent, which is to apply the adsorbent directly to the surface of the heat exchange tubes. The adsorbers are equipped with nozzles (2.5a, b) and (2.6a, b) for connecting 3-way valves (3T1, 3T2, 3T3 and 3T4). The adsorbers (Ada) and (Adb) are separated by a thermally insulated partition (2.7). The adsorbers are equipped with steam pipes (2.8a, b) and (2.9a, b), to which self-acting check valves (2.10a, b) and (2.11a, b) are connected.

The condenser (Cd) of the refrigerating machine has a rectangular shape and is a heat exchanger with a floating tube grid. A drain pan (2.12) is installed under the pipe bundle, through which refrigerant condensate enters the throat (2.13) of the siphon (8).

The evaporator (Ev) is designed for cooling a coolant, for example, a 30% ethylene glycol solution. Also, aqueous solutions of sodium chloride or calcium chloride can be used as a coolant. The evaporator is of the flooded type, since the tube bundle is located in the liquid refrigerant of the lower part of the rectangular body (2.14). The upper row of the tube bundle is located in the steam space of the apparatus. The coolant is cooled in the tubes when the refrigerant boils on their surface. The heat exchange tubes of the tube bundle have knurled fins, which provide an increase in the heat transfer coefficient during boiling of the refrigerant. A throttling valve (EV) is fixed in the end wall of the housing (2.14), through which the refrigerant flows from the condenser (Cd) into the evaporator. The bottom (2.15) of the evaporator is slightly inclined towards the drain nozzle (2.16). Since this refrigerating machine is designed to produce negative temperatures, the evaporator body and adsorbers are provided with thermal insulation (2.17). A support frame is attached to the body (2.14) (not shown in the design diagram), which is mounted on supports (2.18).

Various types of water coolers, chillers, are used in hydroponic technologies to cool the nutrient solution. The chillers use freon refrigerating machines with an installed capacity of 0.25 kW electric motors for a cooler of 100 liters per hour and 3.5 kW for a cooler of 2000 liters per hour. These coolers are equipped with a compressor refrigerating machine and a bath for cooling the hydroponic solution. A coil is placed in the bath, through which either a refrigerating agent or a coolant is pumped [12].

In hydroponic greenhouses based in hot and dry climatic conditions, coolers from the following companies are used: Carrier (USA), E2PS-GmbH (Germany), DEGA SPA (Italia) [13]. These companies supply compressor heat pumps that cool the nutrient solution in tanks or baths.

TAICH (China) manufactures industrial water and solution coolers with screw compressors. When cooling clean water, when the speeds of the cooled liquid and coolant are commensurate and within the limits of energy efficiency, these coolers are quite efficient. When cooling hydroponic solutions, which have a low flow rate and a very low flow rate, all used coolers have low efficiency. In addition, the reduction in cooling intensity is influenced by the deposition of organic and inorganic particles on the heat exchange surface of the cooling batteries. The developed solution cooling system provides for filtration of the solution before feeding it to the cooler.

Our project has developed nutrient solution coolers for greenhouses with different capacities. The main attention was paid to the intensification of heat exchange during cooling. Figure 3 shows the design of the cooler, which uses recirculation of the solution using a pump and a jet collector located in the central part of the device. This design is used in the technological scheme shown in Fig.1. For another greenhouse, a solution cooler with a screw-type mixing device with a diffuser nozzle was used (Fig. 4). In this cooler design, the solution is mixed and moved from the inlet to the separation wall. In both designs, the heat transfer coefficient was 2.5-3.0 times higher than in traditionally used coolers. The designs of the proposed coolers include chutes (3.5) with covers (3.6), which are designed to trap dirt particles from the cooled solution. The chutes (3.5) are installed in the bottom (3.4) of the cooler body (3.1). A pipeline (C21) for the supply of flushing water and a pipeline (C22) for the discharge of mud mixture are connected to the gutters (3.5).

An innovative design of the nutrient solution cooler has been developed. The design of this cooler is described in detail in [14]. Figures 5, 5a show the design diagram of the new nutrient solution cooler. The proposed design of the solution cooler uses a two–body arrangement of the device, and heat pipes (HP – heat pipe) or closed two-phase thermosiphons (CTFT - closed two-phase thermosiphon) with an intermediate coolant are used as heat transfer elements. The internal unit has a square shape, which makes it possible to place the maximum possible number of heat transfer elements in the walls. The external device has the shape of a hexagon in plan (Fig. 5a). The heat pipes are located in the walls of the internal apparatus. A warm nutrient solution is fed into the cavity of the internal apparatus through the holes in the upper lid. Since the flow rate of the solution into the internal apparatus is very low and does not exceed 0.125 m/ sec, the intensification of heat exchange on the heat-receiving surface of the heat pipes is increased by rotating the internal apparatus. This makes it possible to increase the heat transfer coefficient by 2.5-3.0 times. The heat-dissipating sections of the heat pipes are located in an annular space, which is located between the walls of the internal and external apparatus. The heat-removing areas are washed by a coolant stream with a temperature of +2 °C or -2 °C. The temperature of the coolant at the entrance to the annular space depends on the required cooling interval. To prevent the coolant from entering the nutrient solution, labyrinth seals are installed on the covers of the internal apparatus and on the mating surface of the annular flanges of the external apparatus. A hollow shaft is fixed in the lower flange of the internal apparatus, through which the cooled solution is drained into the sump. The use of elements with ultra-high thermal conductivity made it possible to reduce the dimensions and reduce the weight of the cooler several times, compared with known cooler designs. Considering that the operating pressures of the circulating cooled solution and coolant are close to atmospheric pressure, the devices can be made of plastic materials. This option additionally reduces the weight of the cooler structure and their manufacture is much cheaper.

Conclusions:

1. A highly efficient solution cooling system has been developed in hydroponic technologies based on the use of an adsorption-type refrigerating machine in which methyl alcohol is the refrigerating agent;
2. The design of the solution cooler provides for recirculation of the solution or mixing it with a screw agitator, which significantly increases the heat transfer coefficient when cooling the solution;
3. The use of heat pipes in a hydroponic solution cooler makes it possible to increase the amount of heat transferred tenfold;
4. The reliability and operability of such a device on heat pipes is at least 2 times higher than any known heat exchanger.

Captions to drawings:

Fig. 1. Basic technological scheme of nutrient solution cooling. Specification of pipelines in the diagram: Hot water C1 – C4; Coolant C5 – C8; Cold water C9 – C14; C15 – C16; Nutrient solution C17 – C18. Water pumps: 4. 5, 12, 16. Nutrient solution: 24, 28. Coolant: 9

Fig. 2, 2a, 2b. New generation adsorption refrigerating machine Notation – in the text

Fig. 3. Nutrient solution cooler with recirculation of the cooled solution. 19 – Sections of cooling batteries; 21– Pressure tank; 29 – Circulation pump; 29 – Tubular partition; 3.1 – Cooler body; 3.2 – Cooler cover; 3.3 – Supports; 3.4 – Bottom; 3.5 – Gutter; 3.6 – Gutter cover; 3.7 – Coolant supply pipe; 3.8 – Coolant outlet pipe; 3.9 – Circulation pump connection pipe; 3.10 – Nozzle comb; 3.11, Solution supply pipe into the nozzle comb; 3.12, 3.13 – Cooling solution supply pipes; 3.14 – Self-acting dampers; 3.15 – Overflow pipe; 3.16 – Control valve.

Fig. 4. Solution cooler with a screw-type mixing device with a diffuser nozzle. 4.1 – Cooler body; 4.2 – Cooler cover; 4.3 – Supports; 4.4 – Bottom; 4.5 – Chute; 4.6 – Chute cover; 4.7 – Mixing device; 4.8, 4.9 – Cooling solution supply pipes; 4.10 – Drive of the mixing device.

Fig. 5, 5a. Highly efficient solution cooler on heat pipes. 1 – 6-faceted outer apparatus; 2 – Square-shaped inner apparatus; 3 – Base; 4 – Bearing support; 5 – Bearing housing; 6 – Base plate; 7 – Labyrinth seals; 8 – Drive for rotating the internal apparatus; 9 – Heat pipes; 9 – Evaporative sections of heat pipes; 9 – Condenser sections of heat pipes.

About the authors

Ilya N. Mirmov

Author for correspondence.
Email: miily@yandex.ru
Russian Federation

Naum I. Mirmov

Email: naumir@yandex.ru

Sergey A. Schiptsov

Email: pd6@bk.ru

References

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