Influence of the new generation heating system on the greenhouse heat balance
Greenhouse production carried out in a closed cycle requires a significant amount of heat. Many authors state that, depending on the type of the crop being cultivated, the share of heat in the structure of production inputs may exceed 50%. Hence, a lot of attention is paid to this issue. The new generation heating elements introduced to the greenhouse market several years ago meet a number of expectations, both in terms of energy as well as health and quality effects. Heating elements with a star cross-section called the "Walczak" pipe have a number of positive features. They are characterized primarily by a small water capacity which makes the control easier and contributes to heat savings because they allow quick adaptation of the heating system to external conditions and allow maintaining optimal microclimatic conditions having a decisive impact on the quantity and quality of production. In addition to the mentioned above features, they have even a few times less heat capacity of the greenhouse heating comosition. Moreover, the "Walczak" pipes have a different temperature on the surface of the heating element, which allows for changing the form of heat transfer to the cultivated plants (share of radiation and convection). The form of energy transfer has an impact on the growth and physiological status of plants and their health (reduction of fungi growth - higher temperature of plants in relation to the environment). The microclimate, which is a derivative of the heating system, has a huge impact on production efficiency. Maintaining the right conditions in a greenhouse, especially in the morning and evening hours means both, energy saving and meeting growing conditions. The heating installation made of "Walczak" pipes meets many of the above-mentioned expectations, also it was noted during the preliminary tests, it fits well in air drying systems and in heat recovery and management systems, therefore it fits into the energy-saving systems of the economy.
New generation of
In recent years we have devoted more and more attention to issues connected with microclimate. Temperature, humidity, air composition are the factors which not only determine the generative effects, i.e. the quantity and quality of the crops we obtain, but also have an essential impact on the amount of energy input, in other words, the heat demand.
A couple of years ago steel heating pipes with star-shaped cross section (known as “rury Walczaka”) appeared on the market. It turns out that the effects of using them are impressive. Multiple reduction of the pipes’ capacity makes it possible to quickly adjust the pipes’ heat efficiency to variable environmental conditions. This feature, to a large extent, eliminates overheating of greenhouses and generates considerable savings in the amount of heat delivered to greenhouses. An interesting fact about the new generation of pipes is that heaters have varied temperatures on their surfaces. This property facilitates a change in the form of heat transmission. It matters whether heat is transmitted by convection or radiation. Heat provided in the form of radiation (which happens when temperature is higher) allows to generate higher temperature of plants in relation to the environment. This effect limits the occurrence of the dew point on the plants grown and, simultaneously, improves assimilation and significantly limits the occurrence of fungal diseases. Following a production process in which “rury Walczaka” are used, varied effects may clearly be observed. Publications and papers presenting “rury Walczaka” both in Poland and abroad have received a great deal of interest. It is a great pleasure to do research and relay the knowledge thus acquired when effects of the solutions applied are highly positive (Rutkowski, Knaga 2015).
Currently, the pipes are produced in a similar way to the above-mentioned pipes with star-shaped cross section, but this time they are made from polypropylene and polythene. Where does this idea come from? The protruding elements of the new generation heater (the shoulders of the star), similarly to the steel ones, have lower temperature than the hollows. During the process of vegetative heating, a plant clings to the cooler part of the heating element, thus the contact area does not pose a burn hazard. That is why such kinds of heating elements can be applied in vegetative heating. The hollows of the pipes allow to transmit energy of higher temperature which beneficially impacts plant development. Plastic wires do not corrode and have a lower flow resistance of circulating liquid. A downside of the pipes is that a greater number of hangers is required to mount them in plant vegetation zones. At this point we may ask a question: “Are the wires under research to be used only to heat greenhouses?”. The answer is that the wires, thanks to some of their features, can be used for other purposes. The shape of the star makes it possible to use them to heat plants in open-field cultivation during the rooting period because they are more resistant to damage caused by low temperature. The knowledge we have acquired and contacts with international companies let us claim that the pipes will be used not only in greenhouses. After all, our neighbors from the Czech Republic are widely recognized for producing heat exchangers whose elements are made from plastic and fulfill their function, simultaneously using much cheaper materials. Materials used for producing heating elements do not have a tendency to corrode, thus they can be used in highly humid conditions. It seems to be legitimate to use them as lower interchangers of heat pumps, as sealing elements during heat accumulation in the ground or while installing dehumidification systems in a greenhouse.
An initial assessment of the energy efficiency of heating pipes made from PP with a star-shaped cross section (made in the process of extrusion) known as “rury Walczaka” was carried out by the Faculty of Production and Power Engineering at the University of Agriculture in Cracow.
The research was conducted in accordance with the guidelines concerning measurement conditions of thermal devices. In the first stage of the research, heat efficiency per unit was established at the ambient temperature of 15-22 OC. The ambient temperature of the chamber where the research was conducted results from the intended use of the thermal wire. Ultimately, the main purpose of the thermal wire is to heat tomato stems which are grown in the shelter. The heating medium (heat carrier) was water at the temperature up to 56OC. The research was conducted when all the parameters were stable, i.e. the ambient temperature in the research chamber, the temperature of the heating medium and the flow rate. The power value which equals to power supply was determined in isolated points on the basis of trials conducted at 30-minute time intervals.
The research involved a12-meter-long thermal wire with a loop which permits a change of length caused by temperature differences. The examined element was hung at the height of 0,5m and connected to the power leads separated by 6 meters. The unit of water capacity in a thermal wire equals 0,5 dm3. After converting it to the hydraulic diameter of circular cross-section, it corresponds to a diameter of 25,2 mm. It is an equivalent of a pipe 1” in a series of hydraulic elements’ diameters.
The temperature of both the medium flowing inside the thermal wire and the outside temperature was measured by means of paired sensors Pt100.
The results of the research are depicted in a chart (Figure 1), where heat efficiency is presented in relation to the unit value (the energy of one running meter of the heating element).
Research of the thermal wire was conducted at three values of the flow rate: 4dm3/min, 2.1 dm3/min and 1,1 dm3/min, which after conversion correspond to the average theoretical speed in the wire of 3,2 cm/s, 7 cm/s and 3,6 cm/s respectively. For such values, the characteristics of unitary heat efficiency were compiled depending on the temperature difference between the medium flowing inside the wire and its environment (fig. 1). As the chart indicates, there is a linear relationship between these characteristics. The chart also shows that as the flow rate increases, so does- to a small extent- the heat efficiency of the examined elements. What is more, we may clearly observe the growth of the unitary heat efficiency together with the temperature difference DT. An increase in temperature difference by 1OC leads to a respective growth of unitary heat efficiency of 0,912 W/mb for the flow rate of 4,0 dm3/min, of 0,893 W/mb for the flow rate of 2,1 dm3/min and of 0,761 W/mb for the flow rate which equals 1,1 dm3/min. On the basis of the relations presented in fig. 1 it can be claimed that when the flow rate is 4,0 dm3/min and the temperature difference between the factor in the thermal wire and its environment equals 30OC, heat output of 1,25 k can be obtained from a 50-meter-long wire. It has to be mentioned that the energy is obtained thanks to warming up the air as a result of processes such as convection, conduction and radiation. In order to better illustrate the unitary heat efficiency of thermal wires made from PP, this parameter was compared to a steel pipe of star- shape cross section and the resulting findings are presented in a chart (Fig.2).
In order to provide energy assessment of innovative thermal wires made from PP, a comparison between wires of new generation and steel pipes has been made. The heating elements under comparison have a star-shaped cross section. The steel pipe has an outer contour of 34,5 mm, which, when measured against the hydraulic diameter of a round pipe, equals 28mm. The hydraulic diameter of the pipe compared is slightly bigger than the diameter of the thermal wire. It has to be mentioned that while the research was conducted, a slightly bigger flow in the steel pipe was observed. It equaled 5 dm3/min. As the characteristics presented in Figure 2 show, the characteristics of the unitary heating efficiency of the thermal wire made from PP and “rury Walczaka” made from steel do not differ too much when the temperature difference is up to 15 OC,. However, when the difference in temperatures is from 15 up to 30 OC, the curves representing these characteristics clearly depart from each other and steel pipe achieves a greater efficiency.
It has to be remembered that these recommendations were formulated on the basis of the research of thermal wires in natural setting, i.e. the air, where convection, induction and radiation take place.
The opinion was formulated by:
Jarosław Knaga, Ph.D. Eng.
Kazimierz Rutkowski, Prof. Ph.D. Eng.