Investigating the Effect of a North Wall on Energy Consumption of an East–West Oriented Single Span Greenhouse

Abstract

Introduction Greenhouse is a structure which provides the best condition for the maximum plants growth during the cold seasons. In cold climate zones such as Tabriz province, Iran, the greenhouse heating is one of the most energy consumers. It has been estimated that the greenhouse heating cost is attributed up to 30% of the total operational costs of the greenhouses. Renewable energy resources are clean alternatives that can be used in greenhouse heating. Among the renewable energy resources, solar energy has the highest potential around the world. In this regard, application of solar energy in greenhouse heating during the cold months of a year could be considerable. The rate of thermal energy required inside the greenhouse depends on the solar radiation received inside the greenhouse. Using a north brick wall in an east-west oriented greenhouse can increase the absorption of solar radiation and consequently reduces the thermal and radiation losses. Therefore, the main objective of the present study is to investigate the effect of implementing of a north wall on the solar radiation absorption and energy consumption of an east-west oriented single span greenhouse in Tabriz. Materials and Methods This study was carried out in Tabriz and a steady state analysis was used to predict the energy consumption of a single span greenhouse. For this purpose, thermal energy balance equations for different components of the greenhouse including the soil layer, internal air and plants were presented. For investigating the effect of the north wall on the energy consumption, the Ft and Fn parameters were used to calculate the radiation loss from the walls of the greenhouses. These factors were determined using a 3D–shadow analysis by Auto–CAD software. An east-west oriented single span greenhouse which has a north brick wall and is covered with a single glass sheet with 4 mm thickness was applied to validate the developed models. The measurements were carried out on a sunny winter day (November 30, 2015). The hourly variations of solar radiation on a horizontal surface were measured to calculate the total solar radiation received by the greenhouse using the Liu and Jordan equations. For heating of a greenhouse in nighttime, an electrical heater was used while an additional required energy was measured using a single phase meters. The inside and ambient temperatures of the air were recorded using SHT11 temperature sensors. A computer-based program of EES (engineering equations solver) was developed to solve the energy balance equations. Different statistical indicators were used to predict the accuracy of the presented models. Results and Discussion The obtained results showed that in winter months the greenhouse without the north brick wall can receive 14% more solar radiation than the greenhouse with a north brick wall. On the other hand, the use of a north wall in the greenhouses can reduce the radiation and thermal loss from north wall. To maintain the temperature at 25 °C in day-time and 15 °C in night-time, the additional required energy was calculated for greenhouse with and without north brick wall. The results indicated that the total energy requirement to keep the plants warm was 313.8 MJ in greenhouse without north brick wall and 210.8 MJ in greenhouse with the north brick wall. In other word, use of the north brick wall in the greenhouse can contribute to reduce energy consumption by 32%. Comparisons between the predicted and measured results showed a fair agreement for greenhouse energy requirements. The correlation coefficient and mean percentage error for this model were determined to be 0.79 and -2.34%, respectively. Due to the small values, the radiative exchange within greenhouse cover and the sky was neglected. Therefore, the results of the presented model showed fewer values in comparison with the experimental results. It can be concluded from the final results that a considerable amount of the incident radiation has been lost to the ambient by convection from the cover of the greenhouse (glass walls and north walls). Conclusions In the present study, the effect of north brick wall on solar radiation absorption and energy consumption of a single span greenhouse located in Tabriz was investigated. Results showed that use of north brick wall in an east-west oriented single span greenhouse leads to a reduction of 14% in solar radiation absorbed by the greenhouse. The results indicated that use of the north brick wall in the greenhouse can decrease energy consumption by 32%. There was a fair agreement between the experimental and theoretical results with the calculated correlation coefficient and mean percentage error of 0.79 and -2.34%, respectively.

Keywords

• energy consumption
• north wall
• single span greenhouse
• solar radiation
• steady state model