Author: Sándor Nagy
Spring frosts – when temperature drops below 0 °C – are the most dangerous ones in Hungary because the water content level is increased within the plants making the intracellular solution dilute and resulting higher (closer to 0 °C) freezing point as well. Several factors must be considered during the selection of the appropriate frost protection method or process. Frost protection methods can be divided into two separate groups: active and passive ones. It is necessary to note that none of the so far known methods can be considered as the best available method against frosts.
Using passive protection methods we do not intend to interfere with the temporal temperature change processes but we protect the plants from the harmful effects of frost by different methods. The number and the effect of these – prior to plantation – methods are limited and used under large-scale conditions principally. The profitability of their use is always determined by the total value of the protected plants located on a certain area. Passive protection methods are:
We alter the physical progresses of the air close to the ground level with the purpose of moderating the temperature decrease gradient. To achieve this result we have to:
In order to select the proper frost protection method it is practical to make an overview of reasons for frost. Two groups could be formed:
Considering the temperature field in vertical direction (Figure 7.1) of the radiation (Figure 7.1. case ‘C’) and advection frost (Figure 7.1. case ‘B’) are opposite of the ordinary conditions (Figure 7.1. case ‘A’). Regularly, under ordinary conditions air temperature decreases by nearly 1 °C per 150 meters vertically. The opposite state of the air temperature field, when the ground close layers are cooler than overhead ones is called inversion.
Figure 7.1. The temperature field over the ground
The profitability of specific frost protection methods is influenced by different ‘demand values’ such as weight of energy resources, volume flow, etc., which necessitates the creation of a balance equation related to the radiation frost. Considering the fact that each body of which absolute temperature (T) differs from 0 K – in accordance with an ET(l) distribution – emits electromagnetic waves, thus transmits energy of which value can be determined by Stefan-Boltzmann law:
[W·m-2]
where:
cr - radiation factor, which can be calculated by (c) the speed of light, kB - Boltzmann and (h) Planck constant, and its value: 5,6687·10 -8 W·m-2·K-4,
c0 - perfect blackbody radiation factor: 5,6687 Wm-2K-4
Since radiant and advective energy flux land on a particular surface thus the energy balance equation transforms in the following way:
where:
ε - relative emissivity of the surface of an object (of a plant in our case), its value is between 0-1,
Φ - mutual irradiation factor; its value is approximately 0.5,
Téga - absolute temperature of surface of an object in the direction of the radiation or rather the temperature of the surface which is colder than the surface of the plant. (In our case it is the lower region of the sky). Téga = -40 °C should be used during the calculations for achieving the same results of the measurements.
ttal - temperature of the soil with λtal heat conductivity in δtal depth. It is practical to use the value of the soil depth where the temperature equals to the value of the local annual average temperature.
q - value of the specific heat flow in watts per square meter of the plant providing a certain tnöv. temperature (min. 0 °C in this case). In absence of this, the average temperature of the plant decreases.
After substituting the actual values into the equation above it turns out that the heat flow from the ground to the plant via heat conduction is a small percentage of the heat loss resulted via heat radiation flow difference of the plant and its environment. This is the reason for the decrease in average temperature of plants in unclouded, clear nights (ideal weather for heat radiation).
In this case the purpose of frost protection is the change of the value of the balance equation in a favorable way. This can be done either by altering the mutual irradiation factor (Φ) and/or the temperature of the surface which is irradiated by the plant (Téga.). It can be accomplished by:
- covering, which can include covering both the ground and/or the plants. The dissipation of heat energy stored during the day can be slowed down by the usage of this method. Ground covering (or mulching) is accomplished by using some material such as a blanket on the surface of the ground. Both organic matters (which can be decomposed during its use) and non decomposable ones can be used for this purpose. Wooden residues (bark, sawdust, woodchip), vegetable residues (straw, hay, corn stalk, corn cob, cane, grass, fallen leaves) and other resources (paper, compost, marc, peat, manure) are involved in the first group and sand, pebble, stone and brick debris in the second one. The root system of peony may be protected safely by creating a depth of 7-8 cm of winter mulching. Another option is the utilization of ‘hasura’ (‘blanket’) which is suitable not only for the protection of the ground but the crop. Hasura is a wrapping woven from rye straw, cane, bulrush, weed and used for covering seed beds or glass houses enhancing the heat insulation and/or shading features of the facilities. Plant covering can be accomplished by both a single plant (as the bag isolation is shown in Figure 7.2.) and on a whole row as well (Figure 7.3.). The material of the cover is seldom paper, mostly foil or a mesh made of polyethylene (PE) is used. The adventages of these procedures without any doubt are the low labour demand and particularly at vegetable cultivation could be profitable due to the extra income of the early maturing (but it can also be adapted at ornamentals). The disadvantage of this method is the difficult implementation at fruit trees or plants with higher growth habit. During the implementation the contact of the plant with the cover material has to be avoided because a powerful fall in temperature can occur by heat conduction between the plant and the cover.
Figure 7.2. Covering separately
Figure 7.3. Row covering
- smoking , which means an intended increase in the rate of the solid aerosol contaminent in the air (e.g. by perfect burning). Using this pollution, a part of the direct eradiation transforms into scattered one resulting in a decrease in both portion of space and cooling velocity. Wet hay or other organic raw materials can be used for smoking but polluting feedstocks (as well as wet chaff with gasoline, car tyres) must be avoided. The fuming pile is made of a pole surrounded by some combustible matter (vine cane, straw) and covered by smoke providing feedstocks (wet leaves, manure or wet straw). Finally the whole pile is wrapped by soil. Before setting it on fire – when the air temperature is 1-2 °C – the center pole is pulled out and after that the lower section is inflamed. The area of 40-50 m2 can be protected from the frost by one smulding pile. The smoke or smog candle also creates aersol, but partly its price partly its short-term warranty limit the use of it. The smoke generated in the way above (Figure 7.4.) contains a large amount of carbon-dioxide which is a greenhouse gas and this is the reason why the radiation loss decreases. Smoking is effective in wind lulls and just if it is started before the temperature drops below 0 °C and the smoke or smog cover the whole protected area thickly.
Figure 7.4. Smoking in the fruit garden
Artificial smog can be produced from fluids (water). Considering the absorption of electromagnetic waves of 8-12 μm wavelength, which are decisive in the temperature radiation, water is more effective than solid aerosols. Therefore it warms up more dynamically and if the vapour in the air is not saturated, it vaporizes quickly, it can have a marked influence. The adaptation of this method can be effective only if the generated (cold or warm) smog is continuous. Methods reviewed before this one are more often used due to the time-consuming feature of the use of artificial smogs.
Heating
For the heating of the cold air mass causing frost damages energy is needed. This energy can be taken from the decrease of the internal energy of other resources. The energy transfer can be accomplished by
Other options are the methods of generating heat energy from:
Air mixing
The possible cases of vertical temperature gradient in Hungary are shown in Figure 7.1. In case of inverse temperature distribution (case ’ B’ and ’C’) the air layer located at a particular altitude is warmer than the one close to the ground. Therefore mixing the ground close air mass and the higher, warmer one can be a secure protection method against ground close chills and frosts. There are two ways for bringing the mixing to effect: either by stable wind machines or mobile helicopters. The alignment of the wind machines can be done by the position of its axis:
Vertical axis machines :
Figure 7.5. Helicopters used for frost protection
Figure 7.6. Vertical axis wind machine
Horizontal axis wind machines can be grouped into the following clusters:
advantage: shorter cycle time (if the wind blows in the same direction),
disadvantage: if the wind shifts round, its effect may cease.
Wind machines draw warm air from above down and mixes with the colder one on the surface. The height of the wind machines is 10-12 meters. The wind machine rotor made up of 2-3 rotor blades, each blade is 2-2,5 m long and spins at 400-600 rpm in use. The most common form is the simple set-up rotating wind machine with constant angular velocity (Figure 7.7.).
Figure 7.7. Horizontal axis wind machine
Altering angular velocity rotors are also used, which spins faster upwind. The wind machine is powered by either electric or internal combustion engine, although it can also be run by the PTO shaft of a tractor. Thick-walled poles with large diameter are usually made of tubes and the horizontal rotor takes a whole turn in every 4-5 minutes by the bevel gears on the upper edge making the air mass drifting around the tower (Figure 7.8.). It is effective up to minus 5-7 °C and replenishing with thermo-ventilators it heats the air with an additional 4-5 °C. 5-5,5 hectare of plantation can be protected against the frost by this kind of wind machines, though it costs more than 10 million HUF now. Alternatively, it can also be used as a wind engine at the sites where the wind velocity reaches 5-7 m/s. It must be considered that the start up of the unit has to be launched significantly sooner than the frost appears. The larger the temperature inversion, the more effective it is. Using wind machines can be very harmful if there is no inversion!
Figure 7.8. Rotary wind machine
Sprinkler Frost Protection
The base of this procedure is that situation when 1 litre 0 °C water freezes cca. 700 Wh energy releases and there is available fluid H2O e.g on the surface of the plant its temperature does not drop below 0 °C. The intracellular fluid of the plant is some kind of solution, of which freezing point is always lower than the water’s one (it is usually minus 1-2 °C).
Nowadays modern plantations are designed together with an irrigation system, which is suitable for frost protection beside its watering and climate regulating function.
One of the indirect versions of frost protection is the so called bloom delay irrigation, which means cooling the plantation by watering via sprinklers causing the heat sum for the blossoming delay for the buds. One of the methods is watering the plantation when the daily air temperature reaches 7-9 °C with the intensity of 2-3 mm in every 10-20 min interval for the period of 2-3 min for at least 2-3 weeks. According to the experiment results, the blossoming of peach, apple can be delayed even by 2-3 weeks. Since the irrigation must be started already in February and harsh night frosts are also prognosticated for this period, the irrigation system must be protected from frost damages as well. It may happen by sprinkling as mush water that makes the area totally impassable for the period of soil cultivation works. In contrast with this method there is a direct procedure called watering before frost, which means irrigating the surface of the soil. Being a heat container, the heat conduct ability of wet soil is higher than the dry one, thus the heat eradiating from the surface of the soil can replace a part of the heat loss of the plant. It is effective only in the height of 1-1,5 meters hence it can be adapted in berry plantations.
On frosty days still before the chill comes, watering has to be started by accomplishing the method of direct frost protection irrigation (Figure 7.9.) with fine spray sprinklers. The amount of water needed depends on the amount of water (which is calculated from the balance equation) that has to freeze on the surface of plants. Water is continuously sprinkled to the surface of the frozen water (ice) causing the temperature of water-ice mixture not to drop below 0 °C. The ice pulp forming in this way slips the ice crust (Figure 7.10.) on the surface of the plant off. According to experimental observations, 1,5-3 mm of water intensity (15-30 m3 water on a hectare) is needed for fending off the frost in vegetable cultivations. Frost protection irrigation has to be executed without intermission until the frost layer melts (a couple of hours after the sunrise). Either ignoring this or watering at lower intensity than required cause the branches and the twigs of the trees breaking off by the load of the ice. Even the effect of minus 12 °C- frost can be avoided by a properly designed irrigation system. Due to the principle of its operation, it can be applied in case of both advective and radiation frost. This is the reason why frost protection irrigation is the most commonly used method in Hungary, too.
Figure 7.9. Frost protection irrigation at dawn
Figure 7.10. Ice crust formation by frost protection irrigation
Flooding is a direct protection method when the whole area is overflown by water. Having high specific heat capacity, the water either does not freeze or freezes just on the surface (but below the ice layer its temperature remains above 0 °C). This procedure can be applied in specific cases, only when the plant tolerates flooding and water in- and outlet is available and economic. Low height berry plantations (mostly blueberry) are protected in this way.
Warm air blowers
A hydro-engine powered axial blower - located at the front of a mobile cylindrical furnace - delivers and mixes the air with heat energy liberated from stack gases as it is shown in Figure 7.11.
Figure 7.11. Mobile warm air blower
Similarly to the markings of Figure 7.12. the air needed for combustion flows bottom-up (7) to the burner (1). The drawn and warmed air flows due to the suction of baffles (3) and the ventilator between the section of internal and external cylinder (4) of the blower. Meanwhile, it mixes the warmed air mass with the external cold air in a share which helps to avoid the outlet air temperature rising over 40 °C. The lower density of warmer air would make it drift upwards, resulting in a decrease in the frost protection effect.
Figure 7.12. The set up of a mobile warm air blower
It is a fact stated by users that hauling the installation the warm and cold air is not mixed properly and 5-8 m/s initial velocity of air flow is effective in a 15-20 m distance only. In favor of enhancing efficiency a deflector setup has been designed, which is suitable for blowing the air in one (using 1 fan) or both (using 2 fans) directions perpendicular to the heading. By the latter installation almost 15 hectare of fruit plantation was maintained and frost protected at -3 °C.
For the effective protection of larger plantations a maximum 65-m-long foil hose with 1000- mm diameter is attached and tapped by maximum 125-m-long side hoses with 30-mm diameter according to the row spacing. The number of the side hoses cannot exceed 50 pieces on each side. According to the installation in figure 7.13 large areas can be meshed at low cost level and warm air flow can be distributed relatively steadily. The heat oil demand of the burner may be more than 100 liter per hectare.
Figure 7.13. Distribution of warm air flow by a hose system
Stack heaters
Stack heaters are effective machineries in ground close frost defence. The bottom of stack heater (stoven) is slightly conical shaped, perforated, and a cca. 20-liter fuel oil container is mounted with filling and air gap (Figure 7.14.). The surface of the cylindrical stove tunnel can warm up to 600 °C, which heats the air effectively by heat radiation. Low quality fuel oils can also be used by tracing the stack gases partly back to the combustion chamber from the upper cylindrical part of the device causing lower soot production by preheating the oil fumes. Because of the radiation heat flow, stack heaters must be placed at a half meter distance from the plants (fruit, grape, etc.). The protection is effective if the number of heaters (the density of heaters) in case of triangle binding is minimum 200 or even 500 on a hectare. Warm stack gases admix to the ascendant warm air mass resulted from the radiation heat flow of heaters. This warm and less dense air starts floating on the whole area of the plantation. This leaving airstream is replaced by colder one from the edges of the territory irrespectively of winds, too. Compensating this freezing effect appearing at each edge, the density of heaters must be doubled compared to the density inside the crop field. Therefore each tree can have even an own separate heater. The larger the inversion is, the slower the air elevation is. Warm air drifting upwards cools rapidly down in this case and it becomes colder than the layer above the “ceiling” causing temperature growth only in the lower sections of frost dangered air mass.
Figure 7.14. Structure of a stack heater
Even a 4 °C air temperature decrease can be balanced by the use of this method. The oil consumption of stack heaters is 1,2 – 3,5 litre per hour, heating power can be about 11-32 kW. This frost protection method has not been adapted widely in Hungary so far and the present day prices (investment, operation costs + heat oil demand) do not favour to its adaptation at all. On the other hand its use is widespread in the United States. (Figure 7.15.). A simpler way of heating is the use of paraffine candles, which are more expensive than the run of sack heaters but effective even at minus 6-7 °C. A single candle is enough for the protection of each tree. One candle costs 1500 HUF (cca. 5€ in 2012) but it can be used for 2 years.
Figure 7.15. Stack heater at a fruit plantation
Capacitive frost protection
A metal mesh (of which surface is signed by “A”) insulated from the ground is placed above the plantation as it is shown in Figure 7.16. This mesh (the one plate) located almost parallel with the ground (the other plate) at “d” average distance effectuate a capacitor by connecting to an electric power supply.
Figure 7.16. Adjustment of a capacitive frost protection system
In one plate of a charged capacitor more charge carrier can be accumulated than in the other one, which can be quantified by capacity [C]:
As it can be seen, not just geometrical parameters but the dielectric constant of the medium between the two plates [εr] define the capacity while ε0 the dielectric constant of vacuum is described with the value of 1/(4·π·9·109 F). In view of dipole structure water molecules in an electric field between the two plates of a capacitor is shown by Figure 7.17.
Figure 7.17. The structure of water
Figure 7.18. Water molecules in alternative electric field
In accordance with Figure 7.16. ’G’ generator (high-voltage power supply) establishes an alternative electric field, forcing water molecules moving either to the left or to the right direction as it is shown in the figure 7.18. During the realignment the molecules friction to one another (because of viscosity feature) causing continuous heat generation with steady distribution. The value of liberated heat energy can be modulated by either the value of effective voltage between the two plates and/or changing the frequency of polarity switching. The adjustment of the wanted heat flow value can be automated easily. The field current of the generator can be regulated by using a thermo sensor located at the typical place of the plant which brings an effect on the value of effective voltage measured between the plates. According to experiments at a -7 °C external temperature and a 3 °C of plant temperature, the leaving heat flux is about 100 W/m2. For the replacement of the heat loss, 1 kW power of generator at 11kV effective voltage is required. Unfortunately, the investment cost of this kind of instruments is rather high and its operation is highly expensive. Therefore, it is used at some precious crops (e.g. at plant collections, plant breeding sites or for experimental purposes). However, it also works as a net reducing bird and hail damages.
Az "Angol és magyar nyelvű, digitális tananyagok fejlesztése a BCE kertészettudományi kar kertészmérnök és multiple degree hallgatói számára" pályázat a TÁMOP-4.1.2.A/1-11/1-2011-0028 pályázati projektek támogatásával készült.