Thermal Performance of a Solar Heat Storage Accumulator Used For Greenhouses Conditioning

Abstract

The use of solar energy for greenhouse heating has gained an increasing acceptance during the last years. Active solar systems applied to greenhouses can supply a significant part of the heating requirements. However, there are some problems related to the cost of the heat collection unit and the heat storage methods. In this context several techniques were born. The most famous of these techniques is the seasonal storage of thermal heat in soil. The objective of our work is to study a system of thermal energy storage conceived in our Laboratory (LEPT, Tunisia). The system is composed of a vat having a large dimension (6 m 3) filled with fin sand. Inside the vat three batteries of capillary exchangers are buried at three different levels. To heat the accumulator soil, we use a solar collector with a surface equal to 6 m 2 . In order to size the heat accumulating system, a numerical study is started. It allows evaluating the soil temperature as well as the energy cumulated inside the accumulator during the charging and the discharging period. INTRODUCTION The primary objective of greenhouses is to produce agricultural products, outside the cultivation season. So it is necessary to heat the greenhouses, particularly during the cold seasons. However even for the countries in the process of development heating cost exceeds 30% of the overall greenhouse cost (M. Santamouris et al [1]). In this context several techniques were born. The most important technique consists in seasonal thermal heat storage in the soil characterized by a very significant heat-storage capacity. Kurata. K et al [2] assumed a system composed of solar collectors connected to a tubular heat exchanger, buried in the greenhouse soil. Another set of heat exchanger were suspended inside greenhouse. A numerical study showed that the efficiency of the seasonal storage, as well as the daily storage, depends on the system configurations, the climate conditions and soil thermal characteristics (Bejan. A et al [3]). During the day, the excess of the solar heat is collected for short-or long-term storage and it is recovered at night in order to satisfy the greenhouses heating needs (Oztûrk. H et al. [4]). In their approaches, Oztûrk. H et al. contemplate a daily solar energy store inside greenhouse soil by the use of a solar collector. Numerical results permit to evaluate the effect of thermal conductivity, the capacity of heat and the water content on soil temperature and on thermal energy storage. Several tests, for the evaluation of the soil thermal conductivity effect on the thermal heat were carried out. The improvements of the method for such study consist in the evaluation of the temperature, moisture profiles and the cumulated energy in the deep soil (between 20, 40 cm) (Reuss. M et al. [5] Lamard. E et al. [6]). The influences of the input climatic parameters on the ground surface tempertaure were investigated using a neural network approach. Starting with the calculated surface temperature, the ground temperature can be estimated (Ogée. J et al. [7]). They used the energy balance equation to predict ground surface temperatures. A number of investigations which aimed at improving the greenhouse thermal performance have been done by some researchers in the design, modelling and testing of solar collector adopted for greenhouses heating. Arinze. E et al. [8] developed a dynamic model for predicting temperatures and moisture levels in greenhouse soil. Willits.D .H et al. [9] investigated factors affecting the performance of rock storage as solar energy collection/storage systems for the greenhouses. They took the data from two similar rock-storage systems of slightly different design attached to two different greenhouses. Other researchers studied the phenomenon of water evaporation from the ground (Sifaoui. M.S. [10]). In his work Sifaoui. M.S has studied the surface and the deep soil evaporation by analysing the assessment of energy such as the sensible heat flow, the heat flow absorbed by the soil and the latent heat flow. He noticed that if the soil surface is exposed to an intense flow of heat solar radiation, a superficial dry layer will be formed. He also noticed that wet soil temperature is lower than the dry soil for only 20 cm of depth. For the other depths the temperatures are almost the same. As an alternative to the analytical approach, several numerical simulation models based on finite differences have also contributed to characterize diffusive heat exchangers. Some of them are limited to the description of the soil behaviours if we use an only tubular heat exchanger buried in the