Automation of greenhouse electric heating using contact temperature sensors

Automation of heating for protected soil allows maintaining optimal temperature conditions without constant human presence. Using a simple electrical circuit and sensors, heating elements turn on as soon as the soil or air cools below the norm and turn off upon reaching the target temperature. This prevents the overcooling of seedlings and optimizes electricity consumption.

• Sensor embedding depth in the soil — 5–7 cm
• Relay contact current — at least 5 A
• Spark-suppression capacitor capacity — 1000 pF
• Breaking capacity of mercury thermometers — 2–4 W

Electro-contact sensors and automation circuits

To assemble an automatic system, an AC relay (e.g., MKU-48) or an alternative circuit with D226 diodes and spark-suppression capacitors C1 and C2 is used. The system operates in a closed cycle, reacting to temperature changes in the root zone.

1. A temperature sensor (electro-contact thermometer KT) is installed in the soil. When the temperature drops to a critical point, its contacts close.
2. Closing the contacts activates relay K1, which uses its K1.1 contacts to supply power to the heater (heating element).
3. The heater raises the temperature in the greenhouse or hotbed to the required value.
4. Upon reaching the target temperature, the thermostat sends a signal to break the K1 relay power circuit, the contacts open, and the heating element switches off.

Mercury contact thermometers are used to monitor temperature conditions. They consist of a capillary tube with mercury and embedded electrodes: the bottom one is in constant contact with the mercury, and the upper ones are located at a height corresponding to the target temperature. When the temperature rises, the expanding mercury closes the circuit between the contacts, and during cooling, it opens it.

Manometric electro-contact thermometers of TPR-SK, TPP-SK, TPG-SK, and TIP-SK types are installed on electrode heaters. The pressure of the gas or liquid (nitrogen, argon, freon, methyl chloride, or acetone) in the hermetic thermal bulb changes during heating, bending the manometric spring and rotating the needle with the movable contact until it closes with the fixed contact on the scale.

Resistance thermometers — copper or platinum wire on an insulating frame in a protective casing — as well as semiconductor thermistors, are also suitable for automatic regulation. They work in conjunction with a device that measures the electrical resistance of the sensor. Semiconductor variants are compact and highly sensitive, although their characteristics are less stable compared to metallic ones.

Biofuel as an alternative to electric heating

If providing electricity to the greenhouse is impossible, heating is organized using biofuel. As organic materials decompose, they release heat, which warms the soil and protects the root system of plants. Livestock animal manure, household waste, sawdust, tree bark, straw, and plant residues are used as heat sources.

Cow manure and fallen forest tree leaves provide the highest heat output. Leaf harvesting begins in autumn: they are collected and placed under the canopy of garden trees. In winter, this layer protects the garden roots from freezing, and in spring, the compacted and partially decomposed leaves are transferred to the greenhouse as a heating cushion. Surplus raw material is sent to compost.

This work must be carried out as early as possible: as soon as the snow melts in the garden and the ground in the greenhouse has thawed enough to be dug. To speed up the ground thawing process, the following actions are performed:

• frames are placed on the greenhouse;
• the soil on the beds is irrigated with heated water;
• after irrigation, the soil is covered with paper (packaging paper or old newspapers) and plastic film covers.
• the thawed soil located on one side of the aisle is tossed to the other side, and the resulting trench is filled with leaves.

The thickness of the leaf layer is brought to 30–40 cm. This layer is irrigated with warm water, or even better, with a manure solution, and left covered only with plastic covers for several days. When the decay process with heat release resumes in the leaves (determined by a thermometer), the tossed soil is returned to its place and laid in an even layer over the leaves. Biofuel is incorporated into the bed on the other side of the aisle in the same way.

Household waste and straw are heated for a week before being placed into the greenhouse, while loosening it with a pitchfork. The biofuel is laid

Thermosiphon: 1 — biofuel; 2 — soil; 6

3 — air environment; 4 — siphon body; 5 — rib; 6 — rubber stopper 3 and into the prepared greenhouse and watered with hot water or manure slurry. 1 77 4 It is also possible to use a 0.6% urea solution. After 3—4 days, the material warms up, and a layer of soil can be spread over it.

Biofuel not only heats the soil but also improves the temperature and gas conditions of the air in the greenhouse.

If there is soil heating, it is possible to protect plants from light frosts by using thermosiphon pipes, which carry out intensive heat exchange between the soil and the air in the greenhouse. One of the designs is shown in the figure.

A thermosiphon is a metal pipe with a length of 600 mm and an internal diameter of 18 mm. For intensive heat dissipation, six ribs measuring 150x20 mm are welded to the upper part of the pipe. A thermosiphon of this design is installed at an angle of 60° to the horizontal. When installing thermosiphons (at a rate of one device per 0.4 m² of area), the air temperature in the greenhouse can be increased by 2—3°C.

A special place in the design of greenhouses is occupied by the ventilation system. Ventilation of individual greenhouses is carried out mainly by opening transoms, and the area of ventilation openings should be 10-15% of the greenhouse surface.

Ventilation openings are arranged in the end walls (using curtain doors), in the side walls, or on the roof.

In film-covered greenhouses, the side enclosure can be rolled up using special reels.

Structurally, vents can be designed with a suspension on one of the sides to the ridge or the top rail of the side wall. The vents are attached with hinges. To prevent the leakage of warm air when closed, special sealing gaskets are used. The force required to open or close the vent can be reduced by placing the axis of rotation of the latter not on one of the sides, but with some offset towards the center.

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Layout schemes for vents in individual greenhouses:

a — sliding end vents; b — in side walls; c — on the roof; d — curtain style in side walls

Individual greenhouses can be equipped with forced supply and exhaust ventilation. To do this, an electric fan is installed in one of the greenhouse ends with a calculated air flow of 1-1.5 m³/min per 1 m² of greenhouse area. For the ventilation of small greenhouses, household fans with a capacity of 20-30 m³/min are quite suitable. The fans are turned on for exhaust, and louvers are installed at the opposite end of the greenhouse to ensure air intake. The louvers should be permanently closed, and when the fan is switched on, they open due to the air pressure drop in the greenhouse.

If the greenhouse is equipped with vents, they should be provided with an electric drive. An electromagnet or an actuator with an electric motor can be used as an opening device. Commercially available PR-1M industrial drives with a power of 50 W or drives for rotating Christmas trees, which are available for sale, are often used. Since

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Scheme of automatic ventilation for an individual greenhouse: a — block diagram of greenhouse ventilation with electric drive of end vents; b — schematic diagram of vent electric drive control; 1 — TL-3 thermostat; 2, 3 — intermediate control relays for the electric drive; 4 — temperature sensor; 5, 6 — limit switches for opening and closing the vents; 7 — PR-1M electric drive; 8 — shaft; 9 — cable; 10 — vent electric drive is based on a reversible electric motor, it is necessary to include it through an intermediate relay, providing two control signals. It should be noted that such an electric drive operates more reliably than an electromagnetic one. One of the possible kinematic schemes for opening the vents and the drive control scheme are presented in the figure. Greenhouse ventilation is solved much more simply when using direct-acting thermostats. In them, the thermostat itself and the actuator are combined into one device.

Automatic ventilation: direct-acting thermostats

For the ventilation of greenhouses without the use of electricity, direct-acting regulators are used. Their operation is based on the physical properties of materials — the expansion of liquids and gases when heated or the difference in the linear expansion of metals. These devices are autonomous, simple to manufacture, and inexpensive.

• Tulpan cylinder diameter — 60 mm
• Tulpan cylinder length — 450 mm
• Technical oil volume — 1 l
• Piston rod working stroke — 170 mm
• Opening temperature range — 20–25 °C
• Actuator bimetallic strip size — 1100x170 mm

In addition to liquid-based devices like the Tulpan thermostat, actuators based on the expansion difference between metal and rigid PVC are used. A bimetallic strip is attached with hinges to the greenhouse base and the vent rod. When the air heats up, the strip bends, opening the vent, and returns to its original position upon cooling.

There are also air-based designs. In these, the vent is lifted due to the expansion of air in a sealed vessel under a dome, which is connected by a hose to a car inner tube. The expanding air inflates the tube inside a barrel of water, causing it to float and pull the vent rod with it. Another option is the redistribution of water between two containers under the pressure of expanding air.

Air-based actuators are simple but bulky due to the low heat capacity of air. Replacing air with low-boiling liquids (e.g., Freon) allows for reducing the system dimensions; however, in practice, such actuators often fail due to leakage of the working substance through the seals.

Organizing an automated irrigation system and water preparation

Regular water supply to plants is a mandatory condition for high yield, especially if the greenhouse is located on a remote plot. For the uninterrupted operation of automatic irrigation, a storage tank is required, which solves three tasks at once: it accumulates a water supply, creates the necessary pressure in the network, and allows the water to warm up in the sun.

1. Install a storage barrel with a volume of 200–250 l at a height of 1.5–2.5 m from the ground to create gravity-fed pressure. If a water supply is available, mount a float valve from a toilet flush tank inside the barrel to automatically maintain the water level.
2. Lay the main distribution pipeline along the center of the bed — a polyethylene hose or pipe with a diameter of 15–20 mm.
3. Prepare microtubes for targeted water supply. Use 50–60 cm long segments of PVC insulation from electrical wires with an internal diameter of 0.9–1.0 mm.
4. Puncture holes in the main pipe with an awl and tightly insert the ends of the microtubes into them. Secure the second end of each tube to a support near the plant: the water outlet hole should be raised 2–3 cm above the soil and positioned 5–6 cm from the stem.

To automate irrigation, you can use ready-made factory kits (for example, the "Vodomer" system) or assemble the line yourself. If irrigation is carried out manually from a hose, install a faucet-type valve 10–15 cm from its free end. This will allow you to quickly shut off the water when moving between beds and adjust the pressure, while a shower spray head at the end of the hose will protect the soil from erosion.

The hose is also convenient to use when watering from a watering can. A piece of hose about 1 m long is placed on the spout of the watering can, the frame is removed from the greenhouse wall, the watering can is placed on the lower frame beam, and the end of the hose is brought to the line of planted crops. The watering can is tilted and the hose is guided along this line; the water flow rate is regulated by the tilt of the watering can. The force should be such that the soil does not erode and the root system is not exposed.

If the greenhouse design allows for the frames to be easily removed, it is more convenient to water from a watering can while standing outside the greenhouse.

Managing plant irrigation is easy to implement using soil moisture sensors. Several principles of moisture measurement can be used. One of them is based on the change in the bulk density of the soil during moistening. The moisture regulator contains a moisture sensor in the form of a water-filled chamber 1, and a siphon 3 connected to it, suspended on a spring-loaded lever 5. One end of the lever is equipped with a valve 19, which blocks the drain pipes 17 and 18 of the hydraulic cylinder 10. An increase in soil moisture leads to an increase in its mass, deflection of the membrane 2 of chamber 1, and the overflow of part of the water into siphon 3. An increase in the mass of the siphon leads to re-

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