12. Plant Growing Structures

The term of plant growing structures involves all the technical solutions by which both the value of climatic parameters of ornamental and vegetable growing (as temperature, illumination and air humidity) and the components of air (especially the concentration of carbon-dioxide) can be regulated. Both regulation and keeping on the same level of every climatic parameter cannot be accomplished precisely by a single device. Therefore, plant growing structures involve many technical solutions all over the world. The importance of the climatic parameters for regulation is determined by the geographical features of the site of plant growing structures (note it that air humidity is not a state function). For example while ensuring the so-called t _i temperature in Hungary, then in the Mediterranean region the (limitation) of lighting is the most important task in order to realize economic yield. Other, non-technical devices also can be found in plant growing structures allowing the regulation of the climatic features mentioned above, which determine profitability of cultivation in certain cases (e.g. energy conserving screens in Fig. 12.1). Before reviewing of the operation of plant growing structures it is necessary and useful to define the geometry dimensions (width, average interior height and average length) of the units.

Fig. 12.1. Energy screen in open position

12.1. Geometry of Plant Growing Structures

The large-scale (industrial) ornamental and vegetable plant cultivation requires both the biggest floor space and coating matters with high-level heat insulation and light transmitting ability. Based on this the possible covering materials:

• glass,

• light-transmitting polymer foils [polyethylene (PE), polyvinyl chloride (PVC), ethylene vinyl acetate (also known as EVA), polyvinyl fluoride (PVF)] and hard or semi-hard disks [polymethyl methacrylate (PMMA), polycarbonate (PC), or glass fiber reinforced polyester].

At the early stage of establishing plant growing structures (which happened in the 1960’s) gutter-connected greenhouses with glass covering were used widely. Two or more open plan bays were attached by their lateral sides with eliminating the common glass walls and carrying greenhouse blocks out this way. Before this period window frames of seedling nursery greenhouses of 1.2 x 0.8 m with a special glass type – the 0.4m wide and 3.6 mm thick “gardener –glass” – were used and later this 0.4 m width or rather its integer multiply became decisive referring to the module size. Therefore, 3.2 m and later 6.4 m wide multispan structures were adapted in general (shown in Fig. 12.2.). Considering the gradient of the roof at 30 degrees with the horizontal, the height of the greenhouse ridgeline used to not exceed the value of either 3.2 or 4.5 m which was determined by the 2.2 - 2.7 m height of the gutter providing rainwater diversion. This is the reason why the value of the average height of the gutter-connected (also known as multispan) structures is 2.7 - 3.6 m.

Fig. 12.2. Gutter-connected plant growing structure

We can state that the value of thermal inertia of the facilities established in this way is too small. The small value of specific heat capacity of air also has a negative impact on plants influenced by the parameters of the air temperature regulation (as the regulation intervals or the amplitude of the temperature fluctuation). This negative effect was verified in the course of cucumber growing and also justified the necessity of increasing the value of thermal inertia by raising the average ceiling height. Due to the same intention the value of the module width of multispan structures has been increased since the 1970’s, too. Plant growing bays with 12 m module width m bays have been established mainly in Hungary, while 16, 18, 24 or 32 meter wide units may be also found in other countries. Considering both in soil (e.g. at carnation cultivation) and on table cultivation (as potted plants) the technology process of ornamental cultivation set up a claim to wider greenhouse bays in the past few decades. Although it is a fact that the area of plant growing structures used for ornamental cultivation is one order of magnitude smaller than the one used for vegetable production in Hungary. Considering cropping technology aspects and based on the feedbacks of the growers, plant growing structures with either 3.2 m or 6.4 m wide and up to 50 m long bays are the most appropriate ones for plant and vegetable cultivation.

Considering vegetable cultivation, a significant change in plant variety occurred in the past decade. The demand increased for all the varieties of plants with longer crop cycle than 11 months since greenhouse vegetables in the southern (Mediterranean) regions can be cropped with smaller heat energy input throughout most of the year. Therefore, cropping in greenhouses in Hungary is marketable in that period of time, when fruiting of the plants because of the summer heat either ends or starts not yet in these countries. This situation explains why the use of varieties with 5 – 6 month long crop cycle is preferred to the earlier adapted 11-month-cycled ones. In consequence of double time cultivation both biomass production and growth in size of plants make double. The wire system used for directing the growth inside plant growing structures is usually attached to the ridgeline. In regards to the general value of the gutter height, there was not enough room for the growth of plants in the existing plant growing facilities in Hungary. Due to the changes above 4.5 or even more then 5 meter high greenhouses used for vegetable growing can be regarded as the most advanced ones.

In addition to this, more questions have been raised about the length of plant growing structures. More technical solutions exist for leading the rainfall off from the gutter. In spite of this potential, more expensive technical solutions have to be adapted for the structures longer than 100 m. Due to the growth of expenses, the installation of several multispan structures connected into one is cheaper than the establishment of one than 100 m, which fact explains the absence of 100m or more long structures.

There are differences in geometric features of greenhouses concerning those functions as well.

12.2. Outgoing Heat Flux on The Scope of Different Designs

Having regard to greenhouses adapting conventional transfer medium (e.g. water) for heating, the outgoing thermal energy in one second (also known as outgoing heat flux) can be divided into two parts: heat conduction through the boundary surfaces and heat convection by air exchange through the gaps of the cover (also concerning on the value of basic air exchange rate).

12.2.1. Outgoing Heat Flux by Heat Conduction

W

where: A_hat.  boundary surface area of the plant growing structure m²,

k_vez. total heat transfer coefficient W·m^-2·K^-1.

The difference between internal and external temperature (in other words the value of (t_i – t_e)) is usually indicated by t. This temperature difference is provided by the adapted heating system. In the light of this, plant growing structures can be classified by their function as:

• plant growing-,

• propagating-,

• wintering plant growing structures.

In the case of plant growing structures, the value of t must be 25 C at least which is also considered in calculations. Since the boundary surface of the plant growing structures is given by the geometric parameters, thus the concrete value of heat transfer coefficient is needed only, which can be determined by the next formula:

W·m^-2·K^-1

where: _i – heat transfer coefficient concerning with the internal air mass and the wall

_e – heat transfer coefficient concerning with the external air mass and the wall W·m^-2·K^-1

n – number of light transmittance layers ,

_j – thickness of j^th layerm

_j – heat conductivity coefficienct of the j^th layerW·m^-1·K^-1

In so far as gaseous stated layer (e.g. air) with _lev. thickness is located between n number solid phase layers, then the following regression equation should be used instead of , since not just heat conduction takes place there:

0,023ln() + 0,242.

The value of total heat transfer coefficient based on this:

W·m^-2·K^-1

Adapting the datas of the most commonly used 3-layered (foil-air-foil) cover, the value of the outgoing heat flux calculated per m² is: W, while this value is 200 W in case of adapting a conventional, one-layered cover (4 mm thick glass).

12.2.2. Outgoing Heat Flux by Air Exchange

[W]

where: volume of air inside the structure of the greenhouse [m³]

z  air exchange rate, shows that how many times exchanges the volume of plant growing structure with the external one in one hour [h^-1]

specific heat capacity of the air at constant pressure [Wh·kg^-1·K^-1]

r  value of the latent heat of evaporation of water [Wh·kg^-1]

(x_i – x_e)  difference in the value of absolute mositure content of internal and external air mass

The value of air exchange rate is always bigger than 0 even in case of closing every door and window, though it would be better from the aspect of energetics. This occurs because the internal growing area is not separated utterly from the external one. However, this purpose would not be preferred, since the plants in the greenhouse transform some carbon dioxide by absorbing light energy in order to convert it to chemical bonds. Carbon dioxide supply can be provided through the gaps of the cover only. Due to the higher partial pressure of the carbon dioxide content of the external air and density difference affected by the diversity of the external and internal temperature, air flows into the growing area from outside. There are also differences in the so-called basic air exchange rate considering the material of the cover. By the use of glass cover this rate is 1.2 h^-1, while in case of polyethylene foil covers it equals 0.8 h^-1. In accordance with above, the value of outgoing heat flux by air exchange is: W/m² assuming a well-designed and maintained plant growing structure.

Other heat fluxes (as heat flux to the ground water evolving in the deeper layers of the soil) are usually not taken into consideration at practical calculations or planning, having regard the order of their magnitude and temporal alteration. Thus, the sum of the two heat fluxes gives the maximum value of the total outgoing heat flux, which is essential for planning, and the range of its value:

per square meters depending on the design.

Considering the above and in accordance with the need of the farmers, 150 – 250 W per square meter in case of vegetable cultivation, and 250 – 320 W per square meter in case of ornamental cultivation can be expected for the value of outgoing heat flux in reference to a well designed, modern plant growing structure.

Knowing the maximum value of the heat flux leaving the plant growing structure is necessary for planning the heating system. But this energy loss is not occurring longer than 10 – 15 days a year or per production cycle. Hence, it is more important to know the total energy demand for heating besides the value of a given t_i (e.g. daily average) temperature.

12.3. The Evolution of Annual Heat Energy Demand

The energy demand of a given term (year, heating cycle) can be quantified by the definite integral of the outgoing heat flux in the interval of the examined term. With the substitution of the equations above we receive the connection as follows:

Wh

where: k_lég. = heat demand coefficient quantified from the outgoing heat flux by air exchange

[Wm^-2K^-1]

₁ = start of heating date,

₂ = end of heating date,

_{1 - 2} = heating degree days during the period of (₁ - ₂), which can be quantified by the upcoming definite integral:

 d Ch

The t_e() function represents the temporal change of the external temperature. This function quantifies the actual outdoor temperature at a certain moment of a given day of the year by average of several years’ data. Having regard to the difficulties of determining this function, and the fact that is not necessary to endeavor to make an error less than 2 – 3 % hence using the function of daily external average temperature () is adequate. The general form of this function:

t_e()  C

After integrating, Γ_{τ1 – τ2} function can be determined easily for any period between the days of ₁ and ₂:

The constant “24” in this connection represents the term of a day as the unit of time for simplifying the calculation. Due to this simplification, the value of internal daily average temperature () presumed constant is adapted in the integrand instead of the function of temporal change of the internal temperature demand t_i(). This does not exclude the possibility of changing the value of average daily temperature during the cultivation process. In this case, the mean of average daily internal temperature calculated for the heating period has to be adapted.

In regard to above both the parameters mentioned before (geometric measures, internal and external temperature and heat fluxes, thermal parameters) and the coefficients of external daily average temperature („A”, „B” and „C” in the equation above) have to be considered for the definite calculations.

To the quantification of these parameters the function above has to be fit to the meteorological data series of a certain geographical place. After the fitting has been carried out with applying meteorological datasets of some places, the values of coefficients can be determined this way:

Table 12.1.

	A [ ° C]	B [ ° C]	C [ ° C]
Budapest	10,8	11,1	9,2
Kaposvár	10,3	10,5	9,8
Kecskemét	10,2	11,5	9,2
Miskolc	9,5	11,7	9,3
Nagykanizsa	10,2	10,7	9,3
Szeged	11,2	11,5	10,1

In order to calculate the energy demand, there is nothing more to define just the lower and upper limit of the definite integral, in other words the start and the end of heating. As the result of this, both the starting and the ending date of heating is represented by that calendar day, when the outdoor daily average temperature equals the desired internal one. These limiting days of the heating interval can be also determined by the regression equation. There is some error of this approximation of course: it does not take into account those nightly hours when the external temperature is lower than the average daily one, hence the heating must be operated longer by 2-3 days or more. This extra time may result in a 5 – 6 % surplus of the annual heating energy. The correction in calculation as this one can increase the time consumption of accomplishing the regression analysis, meanwhile the magnitude of error of the differences between real outdoor temperature and data series does not decrease. The fluctuation of the relative error can be more than 25 – 30% between the real external temperature and the average of meteorological data series of the past 100 years.

For example: in accordance with the regression equation above, in a plant growing structure located at the border of Szeged with the outgoing heat flux of 210 W, 230 kWh heat energy is needed per one square meter of the surface area in a year in order to keep 20 °C internal temperature.

12.4. Heat Rejecting (Heating) Systems and Their Temperature Level

 

The heat flux leaving the growing area has to be replaced by heating. The heating can be performed indirectly (e.g. using electric heaters) or via transfer medium (e.g. air, water, oil). The indirect method can be adapted in plant growing structures with small power demand (maximum 10 kW_th). At the other variety of heating the medium can be either gaseous or liquid. The medium can transfer the heat energy from the production site (e.g. from the furnace) to the location of use by means of open or closed thermodynamic cycle, while the medium temperature decreases maximum 20 °C or changes phase. In case of direct air heating, a thermodynamically opened cycle is realized since the internal air mass of the plant growing structure is heated by either flame-to-air heaters or heat exchangers. In regard to the low value of the specific heat capacity of air, this heating method results in intensive temperature increase, therefore it demands a non-neglible level of (usually electric) energy for the circulation of the medium. In order to provide the same level of thermal comfort for the plant with the static air, a higher air temperature is needed which increases the energy consumption by 10%. The inhomogeneous temperature field evolved by the air flow along the longitudinal axis of the bays of the structure does not help the plant cultivation. This is the reason why this method is seldom adapted nowadays.

In case of using H₂O as transfer medium warm-, hot water and steam operated heating systems can be distinguished by the status indicators of the medium. The convective and radiant heat flux have different ratio in the total heat flux emitted by these systems. However, heat conduction can be neglected by each method. Warm water heating can be operated at the lowest over-temperature compared to the t_i temperature of the growing space, hence this method provides the lowest heat radiation ratio. Warm water heating is the most commonly used procedure because its use means the smallest risk of scorching for the cultivated plants. If warm water gets to the heat emitters from the furnace, the medium has to provide heat energy to the heat emitters in closed process. Closed system has to be meant in the sense of thermodynamics namely zero amount of water flows in and out the system. This is the reason why no need to reckon with scaling inside the furnace. However, the system is also opened in the mechanical sense: higher (over)pressure than at the growing area exists cannot be formed inside the system. If it happens then it is considered as a pressurized water heating method. An opened system is realized by the utilization of thermal water as heating medium, because after a fall in the temperature the water leaves the system to the environment. This opened cycle method of heating generates a significant dissolved salinity which precipitates to the walls of the system because of temperature decreasing. For this reason the heating water can become saturated or even oversaturated easily, furthermore its salinity content can precipitate continuously resulting in less heat transfer and flow section within the heat rejecting units. To avoid these harmful effects, heat exchangers have to be inserted into the heating system, which primer circuit is run by open-surfaced thermal water while the secondary circuit is operated in closed way.

Inside the growing areas, four independent kind of warm water operated heating circuits can be estabilished, namely:

1. air heating :, warm water streams in the heat dissipating units at maximum 90C and flows back into the furnace by losing no more than 20 C of its temperature. Heat rejecting surfaces can be built from plain (Fig. 12.3.) or cross finned pipes (Fig. 12.4.). Pipe bundles mean another technical solution, which can be placed either in the plain of gutter or on the side of the gutter-holding columns at the height of 1 m.

Fig. 12.3. Air-heating Fig. 12.4. Air heating

by plain surfaced pipes by cross finned pipes

2. “Plant shoot heating”: this method is used at vegetable cultivation only. The heating pipes with 2 or 3 fins (Fig. 12.5.) made from metal or polyethylene are suspended. The vertical position of the pipes can be set by the suspension which enables to follow the height of the stem. Both the temperature of 30 C in the pipes and the temperature difference of 3 - 4 C between the inflow and outflow water should not be exceeded. For this reason this heating method can provide just 1 – 2% of the total heat energy demand. However the temperature close to the heat rejection surfaces is higher by 2 – 3 °C, which results in smaller partial vapor pressure in the air around the shoot. The air humidity decrease hinders the growth of fungi. This is the reason why this “heating” also gains ground as a plant protection method. Due to radiation heat flux the average temperature of stem increases by 1 – 2 °C which has a beneficial effect on the growth as well.

Fig. 12.5. “Plant shoot” heating

3. Vegetation heating : it can be installed both on the ground and above the surface level (usually on stands) and it also can be used at the cultivation in both soil and hydroponics. The diameter of the pipes is usually 2.5” or 3” (Fig. 12.6.). The inlet temperature of the heating water is 40-42 C, while the outlet is colder than this by 5-6 C. Considering vegetable cultivation, heating pipes are located in the row spacings, and the pipelines also function as the rails of harvester/collector trolleys. In case of ornamental cultivation, benches hold the polyethylene pipes wherein the water is led at the temperature of 30 – 35 °C and it flows back to the furnace via collector pipes after cooling down by 2-3 °C (Fig. 12.8).

Fig.12.6. Vegetation heating

Fig. 12.7. Vegetation pipline as the rail of the harvester trolley

Fig. 12.8. Vegetation heatin on benches

4. Ground heating : In order to avoid corrosion, polyethylene pipes are laid into the ground at the depth of 60 cm from the surface level at least. The value of row spacing should be no more than the depth is. The inlet temperature of heating water cannot exceed 30-32 °C permanently, having regard to the optimal temperature of nutrient uptake at the root zone. Considering vegetable cultivation in hydroponics this method has lost its importance by now. Its portion of the required heat flux is no more than 4-5% either. Metal pipe bundles are usually placed under the planting tables in case of ornamental cultivation (Fig. 12.9). However, the inlet temperature of heating water may exceed 55 °C because planting tables (or benches) are heated from below indirectly by pipes emitting heat flux via heat convection and radiation.

Fig. 12.9. Using ground heating at ornamental cultivating benches

Spatially and temporally optimal temperature field can be provided for the plants by operating heating circuits described above and also relative air humidity may be reduced slightly. Heating circuits can be connected in serial due to the inlet and outlet temperature of each heating circuits, which procedure effects the greatest fall in the temperature of heating water by either thermal water or waste heat utilization. Based on a scientific statement, the most effective allocation of heating pipelines can be considered if the plant gets into the way of heating flux. In accordance with the above the method of air heating with top layout can be considered the most favorable heating process for the plants. Heat flux emitted by convection and radiation cannot be achieved without the presence of air heating systems. Approximately 70 % of the heat flux can be achieved in the most up-to-dated vegetable growing structures by using vegetation heating only.

Circulating heating water can be performed by either forced or natural circulation. Pressure difference (with the value of ) forces the fluid to flow in the pipelines considering natural circulation. This pressure difference can be derived either from r density difference affected by temperature alteration or by water surface level differences. In regard to plant growing structures, there is just a slight difference in height level, and moreover the length of the heating pipeline system is too big. It follows from this that recirculation pumps have to be applied in the systems, realizing forced flow this way. Usually radial pumps are used for this purpose, which are installed following either the so-called distributor pipe or the heat rejection units (before the reduced temperature water would enter the collector pipe). The pump is built in the discharged side (the side with positive pressure) in the first case while it is mounted in the suction side (side with negative pressure) in the second one. The standard temperature of the pump installed to the suction side cannot exceed 70 °C, which is favorable considering dripping loss. However, the so-called WILO pumps (waterproof devices in which both the rotor and the pump house are placed into a casting and adapting synthetic resin to avoid water leakage into the coils of the electric motor) are capable to operate a discharged system safely.

Energy saving has gained importance recently which makes temperature regulation for an important task to deal with. One of the technical solutions is the two port valve which delivers the 90 °C inlet water to the heat rejecters in order to hold the level of a certain indoor temperature (t_i) at a constant outdoor one. The range of the internal temperature fluctuation becomes too wide at the use of this device, which can exceed 3 - 5 °C as well. Another option is supplying the heat flux as a function of external temperature by water with the temperature of 20 – 80 °C, as it follows:

ill.

where: k_hőleadó – heat transfer coefficient of heat rejecter [Wm^-2K^-1]

A_fűtő – surface of heat rejecters [m²]

average temperature of water flowing into the heat rejecters (at t_be temperature) and out of that (at t_ki temperature) [°C]

Water flow with temperature of t_be can be provided either by three or by four port valves. By the use of these valves some portion of the water circulating back to the furnace is mixed to the one flowing from the heat rejecters. Any temperature between 90 °C and t_i can be adjusted by varying of the mixing ratio. The temperature fluctuation range can be reduced to one-third part by using three or four port valves, which fall has a very positive effect on plant growing. Water flows with big temperature fluctuation can damage the furnace which chance can be reduced by adapting four port valves (Fig. 12.10.) instead of three port ones. Furthermore this installation can reduce both the impact of sulfur corrosion and the chance of furnace cracking caused by cold water.

Fig.12.10. Three port valve

12.5. Options of Air Conditioning in Summer

The balance equation of plant growing structures with good approximation:

where: fraction of luminous flux passing through the external shading device and the total incoming luminous flux. If no shading screen exists then 1,

intensity of solar radiation incident on the ground W·m^-2

rate of light transmission of the cover of plant growing structure

floor surface of plant growing structure m².

By means of the equation above, the actual internal temperature (t_i) with no heating can be determined, which value is proportional to the increase of solar intensity. The value of solar radiation in Hungary can reach 600 W/m² which can raise the internal temperature to more then 50 °C. The plant is not photosynthesizing during that period of time rather tries to emit the absorbed energy towards its environment by evaporation for example. Water flow required for the evaporation cannot be provided by the xylem tracheids only. Unfavorable processes take place inside the plant in this case due to the decrease of turgor pressure which is the reason for lowering the internal temperature. Several options are available to achieve this goal.

12.5.1. Shading

This method can be accomplished in several ways. The shading surface can be placed outside the plant growing structure (this installation is called external shading shown by Fig. 12.11.) and it can be located in the plant growing area, too. A third option is called smudging which means the reduction of the surface of the cover of the plant growing structure intentionally by having effect on the value of light transmission ability (). Scattering clay powder on the cover has been the simplest method since the birth of plant growing structures in order to decrease the light transmitting ability. A large-scale method also exists to achieve this goal, of course. An appropriate composition of a special fluid is sprinkled on the light transmitting surfaces (Fig. 12.2), which is removed by a dissolvent fluid in autumn. Both the use of an external shading system and the adaptation of smudging have an effect on internal temperature. There is no factor in the relation above which could describe the indoor shading systems (Fig. 12.13) since the substation part of luminous flux is absorbed in the material of the shading screen and turned into heat energy. The indoor air mass heated by the absorption of luminous flux radiates that range of electromagnetic waves in which both the glass and most of the polymer derivatives are opaque. This phenomenon is called greenhouse effect.

Fig. 12.11. External shading device Fig. 12.12. Applying smudging on the coverage

Fig. 12.13. Internal shading system

12.5.2. Air Ventilation

The internal air temperature can be reduced by increasing the value of so-called basic air exchange rate. For determining the actual value of air exchange rate we need to know the value of air flow per time unit generated inside the plant growing structure. The methods used for air ventilation can be classified in two groups: natural way (ventilation arises due to temperature difference) and forced way (air flow is generated by devices, namely by fans).

12.5.2.1. Natural Ventilation

Considering this ventilation method a given amount of air flows inside the plant growing structure through the surfaces opened on coverage () while the same amount of air leaves through the remaining part of the surface of openings. The average air velocity is determined partly by density difference derived from temperature difference of indoor and outdoor air masses:

kg·m^-3

where: density of outdoor air mass [ kg·m^-3 ]

cubical thermal expansion coefficient of air with the value of 273^-1 K^-1 approximately

Differential pressure is also generated as the consequences of differential density affected by height difference in the center of gravity of intake and exhaust ventilation openings (), and gravitational acceleration (g = 9,812 ms^-2) as follows:

Not only the value of differential pressure but the pressure energy content of the air mass alters, too. In pursuance of Bernoulli-equation it transforms to kinetic energy. Based on this and after rearranging the equations we get the average velocity of the air flow:

m·s^-1

where: weighted average of the values of contractions coefficient of the openings

In accordance with all the aboves the value of air exchange rate can be calculated since the volume of intake and exhaust air masses per hour are:

m³·h^-1

The determination of the mathematical relationship above that ideal case was considered when 50% of ventilation opening surfaces were used for intake air flow while the remaining 50% of the surfaces were used for exhaust one. The values of coefficient of contraction have to be altered as long as the conditions of air flow vary substantially from the situation above. The value of air exchange rate is in view of ventilation air flow:

h^-1

Considering both the air exchange rate determined in this way and the balance equation at the beginning of the main chapter, the actual value of internal temperature can be calculated. Vents with given area located on the surface of the plant growing structures result in a higher value of air exchange rate (according to the equations above) if difference in levels between the center of intake and exhaust air flow is as high as possible. For this reason it is appropriate to classify air vents into two groups. The so-called bottom air vents can be established on the surface of:

• gable (Fig. 12.14.) or

• side walls (Fig. 12.15.).

Fig. 12.14. Vents on gable surfaces Fig. 12.15. Sidewall vents

Top, open ridge air vents as :

• roof vents- (Fig. 12.16.),

• pit vents - (Fig. 12.17.),

• chimney-style vents (Fig. 12.18.)

can be established as an escape route for the exhaust air.

Fig. 12.16. Roof vent design Fig. 12.17. Pit vent design

Fig. 12.18. Chimney vent design

The discrimination of bottom vent installations can be justified by the name of certain parts of the device. In contrast with that top vent designs can be distinguished by these criteria having effect on the value of air exchange rate. After analyzing each design it can be stated that the position of center of air vents can be lifted up maximum to the ridge height by adapting top air vent designs. However, referring to pit vent installations the center of the air vent is already higher at the moment of opening than the ridge is and it can elevate further to the level of roof edge set upright. In case of adapting chimney designs the half of the distance between the chimney closing lid and actual value of the extracted chimney height will become the value of height of the center of exhaust air flow.

For this reason both top and bottom air vents have to be installed on plant growing structures. The amount of air exchange performed by air vent designs is determined both by geometric parameters, shape and by the difference in level of height of the center of intake and exhaust air flow. For example the height of an air vent installed just on the sidewall is not totally indifferent because the half of this parameter gives the value of difference in height level of intake and exhaust air flow. This also confirms the desirability of installing the air vents in pairs (one on gable, one on sidewall) with the same area if it is possible, forasmuch it can function as a technical solution for approximating (or exceeding in certain cases) the value of the difference in height level() to the average height of plant growing structures. In accordance with increasing the value of floor area of a plant growing structure, it is hard to provide the equality in the area of top and bottom air vents. This can result in local temperatures of the gutter-connected structures, which value can even exceed 3-5 °C.

Air intake vent established on sidewall is preferred to one on gable, because the center of vent nozzles are located higher (e.g. because of the doors) than in the case of locating aside. Air ventilation ability of the chimney-style vents are the most appropriate though its installation cost and shading impact on the cultivation are both at high level. This also results in a non-negible loss of revenue especially during the winter months which are poor in sunlight. This is the reason why this design is not adapted often in Hungary.

It should be noted that the dimensions of air vents compared to the boundary surface are defined and limited by structural properties and costs. By taking the above into consideration it can be stated that up to 30% of the area of the boundary surface can be used for air vent installation purposes.

Heat energy left by air exchange – in accordance with the second term of right side of the balance equation at the beginning of chapter 12.5. – can be divided into two parts: internal energy change of the dry components of air (as oxygen, nitrogen noble gases) in the first place, which is proportional to the value of , and heat flux needed for increasing air humidity in the second place. The gradient of increasing of this last one is inversely proportional to the increase of air flow. It occurs partly because there are not enough available surface and even time for evaporating water with the amount of . One of the reasons is the lack of sufficient amount of water for evaporation, unless the ground is irrigated continuously, of which water demand is 0.6-0.8 per unit of floor area per hour. Therefore, if an air exchange rate greater than 10 is performed inside a plant growing structure then the approximation of should be adapted. To improve the safety of calculation the air exchange rate formula can be substituted in the balance equation. After rearrangement we receive the following function:

where:

; ;

;;

It follows from the cubic function above which has just one real root that due to the heating effect of solar radiation (mostly in summer) the outdoor temperature can be approximated – by value from above – to the indoor one. This difference will become 0 just by infinitely large air exchange rate. Adapting air exchange (ventilation) to keep the internal temperature on a certain value in case of the outdoor air temperature is below the indoor one by the value of at least. The temporal fluctuation of the internal temperature can be achieved by setting the cross section area of air vents. Because of this reason we can find devices varying the size of cross section area steplessly. If outdoor air temperature approaches or even exceeds the indoor one (t _i) then we can reduce the rate of increase compared to the outdoor temperature only. It can be stated that if the best available technology and the most eligible size and design are adapted then the air exchange rate neither can exceed the value of 40. In case of adapting this air exchange rate the internal average air temperature can exceed the outdoor one even by 12 °C in the periods of summer with the highest solar radiation intensity.

12.5.2.2. Forced Air Ventilation

If a plant growing structure has to be operated in summer then forced ventilation has to be adapted to reduce the heat energy surplus by the correlation above.

In case of applying forced ventilation the units (which usually are axial fans) creating air flow are installed either on gables or on side surfaces. By applying axial fans it is expedient to install the ones which rotor diameter equal with the size of gutter height (Fig. 12.19.) since the required volume flow can be achieved in this way by the smallest rotation speed. The intention for minimizing the speed of rotation is justified by the noise impact which is proportional to the fourth power of RPM. A fall in rotation speed has also effect on the structure solidity and resonance phenomena. A substantial part of gable or sidewall surface facing the axial fans placed with the triple spacing of the gutter height is enabled to open. In accordance with the direction of rotor speed either negative pressure (air flows outside from the growing room) or positive one (air flow directs from outside to inside) prevails in the growing room compared to the outside pressure. Both modes can create the same temperature field distribution, theoretically. However, the gradient of enthalpy increase of air flow is smaller by applying negative pressure modes, hence smaller energy is needed for upholding the process. This fact proves why the this process is preferred to the positive pressured ones.

Fig. 12.19. Axial fans accomplishing forced air ventilation

Air enters the greenhouse through the openings then its temperature increases by during flowing parallel to the ground and finally it leaves the growing room in the plain of fans. Considering natural ventilation, the same temperature increase can occur in the vertical plain. Variable temperature distribution along the horizontal plain hinders the same level growth of the specimens of cultivation. Air exchange rate is increased in order to decrease the rate of inequality in temperature distribution. However, this results in the growth of the value of average air flow velocity:

m·s^-1

where: n – number of operating fans

flow rate of a fan [m³·s^-1]

height of cross section of air flow [m]

If fans are placed on gables then the sum of gutter

and ridge height has to be considered

L – length of that wall where fans are located [m]

Based on observations plants has no particular reaction to the air velocity up to 0.5 m/s. But the gradient of their growth falls back in an air flow being more intensive than this value. There are crops which are sensitive to air flow velocity. For example, cucumber cultivation may perish in a constant air flow with the velocity of 1 m/s, while 1.5 m/s is the threshold of air velocity for all the other cultivated plants in Hungary. For this reason 0.5 m/s represents the threshold in air velocity adapting forced ventilation.

We can execute the calculations based on both the relationship above, the threshold of air flow velocity and the quantified dimensions of plant growing structures (e.g. gutter and ridge height, gable and side length) can be determined in order to assign the position of fans, ventilation openings (doors and windows). We can also examine on which side these devices should be installed in order to increase air exchange rate. If the values of side and gable length roughly equal then one gable wall for the fans and the other on opposite for vents have to be chosen.

Inappropriate temperature distribution caused by forced air ventilation cannot be reduced at low price in all cases. In case of polyethylene coated plant growing structures the installation of large number of fans on the much longer curved side surfaces would be more expensive than adapting on with great output at the top section of gable wall (avoiding blocking the roadways). Therefore, the temperature distribution inside this kind of plastic tunnels is not as preferable as it is in glass coated structures.

Considering the geometric parameters of plant growing structures in Hungary, the maximum value of air exchange rate does not exceed 70. Electric energy consumption has to be also regarded due to the operation of fans, of which rate is 5% of the value of internal energy increment of air flow.

12.5.2.3. Adiabatic cooling

The cubic function at the end of the chapter of natural ventilation has been enabled by the acceptance of simplification of the equation by . But energy which is needed for increasing absolute air humidity and proportional to the value of can become a decisive parameter. This was the reason for being out for creating a device which enables to evaporate 10 m³ of water per hour per hectare without increasing air velocity over 0.5 m/s. For this reason a so-called “cooling wall” has been placed inside a plant growing structure installed with forced air ventilation (Fig. 12.20.). The role of the cooling wall is to evaporate water droplets as many as possible by the air flow passing through its surface. It can be achieved by applying porous materials with significant active surface loaded between two sheets of metal wire mesh. Wood chip used to be applied but nowadays either pieces of unglazed tiles or compressed vulcanized fiber sheets (Fig. 12.21.) are used for this purpose. The air flow entered then accelerated through the gaps of boundary surface is forced to change direction continuously and tears out droplets from the water film trickling on the surface of the load from top to bottom.

Fig. 12.20. Cooling wall

The rate of air flow is determined by both the power output of fans and the size and air resistance of area of opened cross section. As a result of change of the value of heat flux abstracted from the growing room, hence vary of air flow can be achieved by the next methods:

• increasing the number of operating fans,

• varying the rpm value of fans (adapted seldom),

• varying the air resistance of cooling wall.

The last, frequently adapted process involves a shutter system located in front of the cooling wall. Air exchange rate can be regulated easily by varying infinitely the angle of shutter units, which can be made from transparent matter (e.g. from glass shown in Fig 12.22).The position of shutter units does not have an effect on the consumption of electric motors running the fans. Just that amount of water in form of droplets has to be involved by the air flow which can be evaporated into the air in its way to the fans. In case of maximal solar radiation intensity, more than 1 liter of water per square meter is required hourly for the evaporation process, so the charge of water demand by operating this scale of air cooling system is not irrelevant either. Therefore, water trickled down on the surface of load is gathered in a trough at the bottom of the cooling wall and after filtration it is recirculated to distributor pipe located above.

Fig. 12.21. A piece of cooling pads Fig. 12.22. Cooling wall bounded by glass shutters

Heat energy needed for the evaporation of water droplets getting into the air flow is supported by internal energy (or the temperature) loss of the air. Taking into account of having no heat exchange between the flowing air mass and its environment, hence this air cooling procedure can be named adiabatic cooling.

The internal temperature may be reduced by 7-8 °C compared to the outdoor one by adapting the process of adiabatic cooling in case of 25-30% of outdoor relative air humidity and presuming the exhaust air flow becomes saturated. This can be traced by i-x diagram for air moist and quantified by the given air exchange rate. The following formula shows the enthalpy increase of 1 kg air mass which energy growth by radiation has to be eliminated by cooling:

Wh·kg^-1

Finally, it is worth making a review from real cooling systems. Devices (as absorption or compressor refrigerators) that may perform heat reduction by accomplishing cooling thermodynamical cycles can be used in plant growing structures, but their expensive use does not allow to lead crop production economically. Therefore, those are used for plant growing with experimental purposes (e.g. in phytotrons). High operational costs also exist in this case, but we can save time by adjusting the temperature independently of outdoor climatic features.

12.6. Moderating The Value of Illumination

In accordance with the spectral distribution, almost 50% of solar radiation incoming to the ground from our natural light source galls into the wavelength range of infrared radiation (), while 48% of the total radiation falls into the visible range of wavelength (). The remaining 2% includes the range of ultraviolet radiation () partially. These values depend on the geographical location, of course, because the ratio of UV radiation increases with the altitude. The actual values can be influenced by the environment as well, since the level of UV radiation is higher than at places with lack of water. The actual value of illumination in a plant growing structure is determined by the electromagnetic radiation fallen into the range of bandwidth of 380-760 nm:

[lx]

where: the angle between the normal vector of the illuminated surface and a straight line drawn from any spot of this surface towards the Sun

luminous flux of Sun

The growth of plants is affected by both the values of intensity and timing of illumination. Regarding most plant type the light compensation point falls into the range of 1.5-2 klx and below this value plants lose carbon instead of building that in. Plants can be divided in two groups by their light requirements: low light plants (3-5 klx) and high light plants (5-8 klx).

12.6.1. Reduction of Luminous Flux

The other part of absorbed radiation increases the average temperature of the cultivated plants. Plants have to perform evapotranspiration proportionately to the intensity of solar radiation in order not to allow their overtemperature to increase proportionately to outdoor temperature. This gradually intensed evapotranspiration can lead to the cessation of photosynthesis which results in launching unfavorable processes within the plant as the liberation of built in energy via molecular bindings. This also proves the need for decreasing the value of illumination especially in the summer months by using the process of shading. In accordance with regulating the internal air temperature of plant growing structures detailed in chapter 12.5.1, the installation of shading units can be sorted by their location:

• indoor units (locating inside the plant growing structure),

• outdoor units (locating outside the plant growing structure)

and as an intermediate solution of:

• smudging the surface of structure (e.g. by using whitewash), or technical solutions resulting in the reduce of light transparent ability with the value of

The rate of illumination can be determined by the following formula:

[lx]

The efficiency of outdoor and indoor shading (marked by ) has to be meant in the same way as has been defined at the beginning of chapter 12.5 because the location of shading unit compared to the cover is irrelevant. The formula refers to darkness which differs from the definition of darkness in technical sense (). However, darkness has importance on cropping technique since the growth of the plant can be influenced by accomplishing eligible photoperiodicity (e.g. force plants upon blooming). This is the reason why both tunnels and shading screens adapted in plant growing structures are produced in black color (Fig. 12.23.). We must lay emphasis on shading screens not to be mistaken by energy screens used for reducing heating energy consumption. Densely woven materials have the best features for reducing energy consumption although their light transparent ability exceeds the permitted level of minimal shading. The rate of illumination can be set by closing the darkening screen unit partially.

Fig. 12.23. Darkening screen Fig. 12.24. Setting the value of illumination by the close of darkening screen

In every case when plants are unable to produce by photosynthesis should be considered dark in the aspect of plant. Being unable to absorb for the process of photosynthesis, the electromagnetic radiation has to be considered dark, since the plant reflects whole amount of radiation which falls in with its color. It is also approved by Fig. 12.25.

Fig. 12.25 Diagram of spectral sensitivity of a green colored plant

Therefore, it is good to know the rate of electromagnetic waves, which matches the color of the plant. This rate can be measured by using an appropriate color filter, and also explained the fact why green shading nets (as so-called Raschel mesh nets in Fig. 12.26) or screens are adapted in practice.

Fig. 12.26 External design of shading by using Raschel mesh

12.6.2. Increasing Luminous Flux

On certain days the intensity of illumination lays claim to enable to regulate dormancy terms (or also known as photoperiodicity) of the plants grown in plant growing structures by increasing both the period and the intensity of the adaptation of substitutional artificial light sources. This kind of need mostly arises in wintertime in Hungary. The requirements for illumination of the cultivated plants must be known for specifying the value of specific output power of the light sources. Sizing processes of output power of the lamps claims to know the values of luminous efficacy of light sources in addition to consider the classification above and the requirements for illumination. The value of luminous efficacy specifies the ratio of input energy converted to visible electromagnetic radiation with wavelength of 380-760 nm. It is appropriate to get acquainted with the light source adapted in plant growing structures.

12.6.2.1. Edison Bulbs

If a resistor wire made of alloys containing high melting point metals (most frequently tungsten) placed either in vacuum or in inert gas and apply voltage across that then it results in “I” electric current on the resistor in accordance with Ohm’s law (). As a result of this electric current with the value of , electric power transforms to heat or rather radiation energy flux. In vacuum, due to the released heat energy the metal resistor wire starts to glow. The spectral distribution diagram of this radiation concerning the actual temperature falls a slight part into the range of sight but rather it is masked by the bandwidth of IR radiation. This rate can be upgraded by increasing the glowing temperature. This process is accomplished by halogen lamps but the melting point of the resistor wire limits sets limit for the temperature rise. For this reason the luminous efficacy of these light sources is no more than 3-5% which is too poor to come into view for the adaptation in plant growing structures.

12.6.2.2. Low Pressure Discharge Lamps

In a view of these designs an electric discharge is generated between two electrodes inside a tube charged with mercury (Hg), sodium (Na) and noble gases at the pressure of less than 1 kPa. The flow of the electrons between the electrodes is provided by gas space is plasma phase. Meanwhile, the natrium and sodium atoms get into excited phase by the electron flow, and after the relaxation Hg emits UV and bluish electromagnetic waves while sodium radiates yellowy ones. For limiting the rate of generated current electrical ballast is needed. Furthermore, high-voltage impulse is required for the ignition (converting the gas load to conduct) of the discharge provided by a starter unit. The spectral distribution of this kind of light sources is not continuous but consists of lines only. Just two different models remained in use by these days:

a./ fluorescent lamps: low pressured Hg gas is located in a straight tube with standardized length. Due to mains voltage and frequency the excited electrons with alternative motion emit UV radiation by colliding with the fluorescent powder on the inner surface of the tube. The composition of this powder is indicated by the characters standing behind the letter “F” written on the lamp glass. This may contain 1, 2 or 3 characters depending on the number of decisive spectrum lines of luminous flux affected by UV radiation. The value of luminous efficacy is 10-12%. Since the horizontal projection of installed light source may shade even 20% of the growing surface, hence it is rather adapted both in phytotrons and for in researches, but this kind of light source is used seldom in plant growing structures (Fig. 12.27.)

Fig. 12.27. Fluorescent lamp lighting in growing room

b. /compact fluorescent lamps: light source containing one or more U-shaped tubes instead of straight ones. The inner surface of the tube or tubes are covered by a special composition of fluorescent powder – so called “light powder” – which composition is indicated by the characters standing behind the letter “F” as well. The method of light emission is similar to fluorescent lamps’ except that the frequency of electric current falls into the range of 20 – 30 kHz instead of 50 Hz. The value of luminous efficacy can be 16 – 18 %. The cost of so-called high frequency electrical ballast, which converts mains frequency at the most effective way installed within the lamp socket, increases the commercial price of this device. This is the reason why it is usually not adapted in plant growing. The rarely used “energy saving bulb” is a miscall deriving from its principle of operation.

12.6.2.3. High Pressure Discharge Lamps

These lamps have similarity in the method of operation with low pressure discharge lamps but the pressure of metal vapor can be compared to the atmospheric pressure. An electrical ballast is needed for the operation, and also a lamp starter unit referring to sodium and metal halide lamps. Types of high pressure discharge lamps used in plant growing structures:

a./ Mercury-vapor lamps artificial light source equipped with double glass cover. Quartz arc-tubes are used inside due to high operating temperature, which contains mercury vapor at atmospheric pressure. The inner surface of the external bulb is coated by “light powder” since this matter is responsible for converting electromagnetic radiation. After excitation the atoms in mercury vapor emit UV electromagnetic radiation with wavelength of 220 nm which is transformed into the range of visible wavelengths (alias into the range of light). By the proper powder composition both the value of luminous efficacy can be increased to 14-16% and color rendering of the lamps can be upgraded. The white pale hue of luminous flux generated by mercury-vapor (Fig. 12.28.) lamps has a beneficial effect on protein synthesis, vitamin and carotene production of plants, ferments and growth substances. The strike of the lamps requires a warm up period which takes a few minutes, while mercury in bulb inside transforms into vapor. The turned-off lamp cannot be restriked just after the bulb cools down.

Fig. 12.28. Mercury-vapor lamps inside the growing room

b./ Sodium-vapor lamps: sodium vapor, mercury and a small amount of xenon is located. Resonance spectral lines of natrium broaden considerably to the range of 550 – 770 nm at atmospheric pressure. Color correction can be achieved by the excitation in mercury and xenon gas charge in order to make this light source type eligible illuminating interior spaces. As far as crop production is concerned However, it supports to increase both leaf surface area, gradient of growth of stem and dry matter content in general referring to plant production. The rate of photosynthesis is the most intense in this bandwidth of electromagnetic radiation. For this reason it is used expansively at the field of vegetable growing (Fig. 12.29.). The value of luminous efficacy of this type and mercury vapor lamps is close equal. The internal discharge tube is made from ceramic due to its high-temperature and -pressure operation as well as aggressive features of sodium vapor.

Fig. 12.29. Sodium-vapor lamps inside the growing room

Checking questions :

1./ Sort the plant growing structures by their structure!
2./ What kind of structural and environmental factors determine the heating output of a plant growing structure?
3./ What sort of heating systems can be installed in plant growing structures?
4./ Describe the technical solutions and limitations of natural ventilation applied in plant growing structures!
5./ Describe the technical solutions of forced air ventilation applied in plant growing structures!
6./ What type of additional lighting devices can be applied inside plant growing structures?
7./ Describe the technical solutions of shading devices installed in plant growing structures!