Abstract
Heating the plant root zone can significantly improve crop growth and quality while reducing fuel consumption in greenhouses. Therefore, providing an efficient heating system is one of the most critical requirements of greenhouses during cold seasons. In this study, a hybrid under-soil heating system assisted by solar energy was theoretically investigated for a greenhouse located in Shahrekord (32° N latitude and 51° E longitude). The proposed system employs a linear Fresnel concentrator to supply the required thermal energy during daytime to maintain the root-zone temperature at 26 °C.
The results indicate that variations in pipe diameter, pipe spacing, and the area of each section significantly affect the amount of heat supplied by the solar collector. Specifically, increasing the pipe diameter from 6 to 9 mm and the pipe spacing from 0.2 to 0.3 m leads to an increase in the required heating capacity of the system. Additionally, the total area of the reflector field required for January, the coldest month of the year, was estimated to be 32 m².
Introduction
Root temperature is an important ecological factor that affects plant growth. Low soil temperature near the root not only directly affects the functions of root hormone absorption and metabolism, but also affects the photosynthesis and respiration of plants, resulting in slow crop growth and low yield. Therefore, maintaining the appropriate temperature is important to improve crop growth. Root-Zone system technology is a way to reduce energy consumption and increase plant productivity. Typically, this system heats the soil bed using non-renewable energy. Solar water heating is a very good technology that can reduce fossil fuel consumption and carbon dioxide emissions. Considering the shortage of non-renewable energy sources, the production of pollutants, and the high cost of this source, as well as the advantages of using the Root-zone system in a greenhouse, the aim of this research is to investigate a greenhouse solar heating system equipped with a linear Fresnel concentrator, an auxiliary source, and a heat storage tank in Shahrekord. Equipping the Root-zone system with a linear Fresnel collector, in addition to reducing fossil energy consumption, increases agricultural production. It should be noted that the linear Fresnel has better solar energy absorption than other solar collectors, so that it can heat water up to 100 degrees.
Material and Methods
A combined solar heating system for a greenhouse was theoretically investigated in Shahrekord (32 degrees latitude and 51 degrees longitude). The system in question has a linear Fresnel concentrator that provides the heat required to bring the roots to a temperature of 26 degrees during the day.In this system, a tank for heat storage, a linear Fresnel collector (heat generation unit), and a pump are used to heat the roots of the plants. The hot fluid inside the linear Fresnel collector is forcibly moved using a pump. Hot water pipes are placed on the bed with uniform heat distribution, and after heating the roots of the plants, they return to the tank and are placed on the collector path, where they are heated again and placed on the path. On cloudy and winter days, the auxiliary heat source heats the tank water to prevent a possible drop in temperature.
Results and Discussion   
In designing the Root-zone system, an attempt is made to ensure that the lowest temperature is required for the roots to reach the desired temperature. Some researchers have sought the ideal root temperature for other crops. In general, most growers have introduced an average temperature of 26 degrees Celsius in the rooting medium as the optimal condition. The heat required by the Root-zone system is affected by parameters such as the number of pipes under the root (N) and the area of each section (A). In the first stage, the area (A) and diameter of the system pipes (d) and its effect on the required heat and temperature are examined. In the second stage, based on the heat required by the system, the number of linear Fresnel glasses is examined in Shahrekord according to the radiation angle.
-    The area change should be done according to w and N to meet the plant's needs, because for seed germination or other processes that require high temperatures, the distance between the tubes should be reduced to prevent uneven seedling growth.
-    As the diameter increases, the amount of heat required by the system increases. As the diameter of the Root-zone pipes increases, the amount of temperature and heat required to bring the root to the desired temperature increases. According to Newton's law of temperature, as the diameter of the pipe increases, the amount of heat required to reach the desired surface to the desired temperature increases.
-    To minimize heat loss from the Pagrama pipes, we will insulate them with 3 cm thick rock wool.
-    Temperature and solar radiation are important factors for determining the number of solar panels and heat supply. Based on this value, the thermal efficiency and the amount of radiant energy per square meter of collector were calculated. 
-    To provide the necessary heat for a 100 m greenhouse, 5898.07 heat is required. Results show that the total mirror field area for December, the coldest month of the year, is 32 m2. The Fresnel reflector in question has 29 rows of mirrors, 3 × 0.36m. Results show the required mirror field area for 12 months in Shahrekord. By combining the heating system, i.e. the solar collector and the auxiliary source, the number of solar panels installed can be reduced.
Conclusions 
The results of the analyses performed are as follows:
-    As the area of each section increases, the required temperature for it increases. 
-    As the diameter of the fluid-carrying pipes under the soil bed increases, the required temperature for the Root-zone system increases. The increase in diameter should be made according to the increase in fluid velocity in the pipe. 
-    This system requires an auxiliary source to prevent pressure drop on cold nights and days.
-    Most greenhouse owners were dissatisfied with the high energy consumption and production of pollutants by the Root-Zone system. With the necessary studies and installation of solar collectors, the cost of energy consumption and production of pollutants can be minimized.
-    The developed model with the obtained results is applicable to different linear Fresnel reflectors. This model can be used for fast and accurate evaluation of linear Fresnel reflectors. In addition, it can be used for optimization using the initial relations as optimization variables.
-    The system has an average thermal efficiency of 56 percent, which is a suitable value and competitive with other similar technologies. While the construction and maintenance costs are much lower than those of competitors.
Author Contributions
M. Karimi: Investigation, Data Curation, Writing – Original Draft.
A. Maleki: Methodology, Supervision, Writing – Review & Editing.
Sh. Besharati: Review & Editing.
Data Availability Statement
All information and results are presented in the text of the article.
Acknowledgements
The authors thank Shahrekord University for providing laboratory facilities and technical support.
Ethical Considerations
The authors have observed ethical principles in the preparation and publication of this scientific work, and this is confirmed by all of them