Performance and Evaluation of a Greenhouse Solar Collection with Underground Heat Storage System

Abstract

A simple steady-state and one-dimensional model has been developed in order to analyse the performance of a greenhouse solar collection with underground heat storage system which is being predominantly adopted in Japanese protected cultivation. The objective of this system is to store solar heat during the daytime by circulating the warm greenhouse air through the pipes buried in the greenhouse soil, and to gain heat from the soil through the pipes during the nighttime.However, the problem is that the frequent attack of energy crisis on Japanese protected cultivation has forced the farmers to adopt this system without any quantitative analysis. Of course, some experimental results are available but theoretical analysis has not been conducted as far as the authors know.The purpose of the present study is, therefore, to check the performance of the system as well as to evaluate the parameters involved in the system. The effectiveness of the temperature increase in the greenhouse is found to be evaluated by the ratio of the overall heat transfer coefficient around pipes times the total pipe surface area plus the overall heat transfer coefficient through the greenhouse soil surface times the greenhouse floor surface area to the overall heat transfer coefficient at the greenhouse cover times the greenhouse surface area (see Fig. 3). It is also apparent from this ratio that the heat release from the greenhouse soil surface is rather significant in this system and in general it would be 20-30% of the heat gain through the buried pipes.The effect of the material difference of the pipes has been examined and it is concluded that the effect on the greenhouse air temperature is within 0.2°C among PVC, polyethylene and steel (Table 3). From the evaluation of the pipe dimensions, it is concluded that the difference of thickness is not significant and the optimum diameter is 11cm as far as the greenhouse air temperature is concerned (Table 4).In order to select an adequate fan for the system, the friction loss due to the plenum duct made of concrete can not be neglected. It is found that the adoption of smooth ducts could reduce the friction loss of this part significantly and the consumption of electricity, consequently the greenhouse air temperature is increased (Fig. 4). The same tendency is clear in the case of two way air flow direction system against one way (Fig. 5).