The aim of this work is to analyze the thermal performance of a greenhouse exposed to semi-arid conditions by investigating experimentally the heat transfers occurring through the walls and ground. A closed unheated Venlo greenhouse without crop in the area of Batna (southern Mediterranean basin) was considered. The study focuses on the effects of (i) the thermal inertia of the soil, (ii) the radiative losses through the cover, and (iii) the convection mode and flow regime on the heat transfer coefficients. Experiments were conducted from January to March 2008 under clear or cloudy skies, and low or high wind velocities. From the results, it is concluded that the heat stored in the ground of the greenhouse represents a significant heat source which can compensate the energy losses through the walls, especially during a night preceded by a high diurnal insulation. This process can maintain an average inside - outside temperature difference during the night within the range [1.59-2.81] K. Results also show that the radiation losses are the main component of the energy losses of the greenhouse, mainly through the outside wall surface of the cover. Conversely, the radiative heat exchange along the inner wall represents the main heat supply to this wall. The convection mode inside the greenhouse induced by the air movement seems to play a significant role on the convective exchange coefficient inside the greenhouse. These coefficients both inside and outside the greenhouse were estimated and analyzed, and a good agreement with the models reported in the literature was found. This study could help growers define and adapt their heating strategy to avoid undesired low temperatures which may damage their crops at night.

Semi-arid regions are frequently subject to major temperature changes during a 24 h period, which may drastically affect greenhouse indoor climates. In order to improve energy management of these buildings, numerical tools have been developed to predict the evolution of the inside climatic conditions. However, most of the available models neither take account of the transmittivity variation through the day nor of differences between wall temperatures. In the present paper, a model for predicting the thermal and water behaviour inside an unheated agricultural green-house is presented. The energy balance method is applied to each element: cover, indoor air and soil surface. Specific modules have been developed to calculate heat transfer coefficients for the cover of the greenhouse as well as heat transfer through the subsoil. These modules have been integrated in the TRNSYS environment. Radiative transfers and view factors were also calculated. The simulations predict two main parameters under transient conditions: the indoor air temperature and the indoor humidity in response to the outside conditions. These parameters were validated with fair agreement from experiments conducted in a monospan greenhouse located in Batna (6.11° E, 35.33° N). Based upon the results of the simulations and the measurements it was also concluded that firstly, the transmittivity was not constant in time and varied with surface orientation; and secondly, vertical surface temperatures were different during the daytime while the temperature difference between roof surfaces remained insignificant. The evolution of humidity was not correctly reproduced by the model, probably because the effects of condensation and variation of soil water content were not properly included in the equations.