Thermal energy storage in the ground of a greenhouse by the polypropylene capillary heat exchanger

American Journal of Applied Sciences, Dec, 2008 by M. Lazaar, S. Kooli, M. Hazami, A. Farhat, A. Belghith

INTRODUCTION

Our terrestrial system can absorb 3 billion tons of carbon per annum whereas we produce the double of this quantity. We will produce the triple in 30 years if the tendencies continue. Gerard Megie, director of CNRS, point out that the carbon dioxide concentration in the atmosphere passed from 280 to 365 ppm (left per million) in thousand years. The principal pollutant emissions of combustion are: CO, NO, SO2, SO3, C and the unburnt hydrocarbons. The carbon monoxide is the most pollutant emitted in the world, (149 million emitted ton which corresponds with 77% of the world pollutant emission. 92.4 million tons are emitted by North America, since it is the most industrialized countries. According to the OECD (organization for the development and the economic co-operation), North America is the origin of 62 % of the carbon monoxide emission, 60,6 % of nitrogen oxides emission, 50,9 % of sulphur oxides emission and 66,2% of particulate matter emission in 1980. Europe is the second polluting area in the world (31,4 % of carbon monoxide emission, 32,2 % of nitrogen oxides emission, 43,8 % of sulphur oxides emission and 28,8% of particulate matter emission).

The massive combustion of the fossil fuels and the biomass is at the origin of a strong production of carbon dioxide (CO2) which is responsible for an increase of greenhouse gas production.

This last conduit to a progressive climatic reheating which will lead to changes of balance (turnings into a desert, floods ...).

The modern greenhouses are large consumers of energy, mainly in the form of heat. Indeed, thermal energy can represent, in traditional installations, from 15 to 25 % of the production costs (1). In France (1993), according to the agency of the environment and control of energy A.D.E.M.E, the heating of the agricultural greenhouses consumes 3% of the electric power and 11% of fossil energies. This type of energy is responsible on the increase in the greenhouse effect, air pollution, pollution of the grounds, of water, acid rains. Moreover the stock of fossil energies is limited to one century for the gas and oil and to two centuries for coal. Reducing fossil consumption will make it possible to prolong their use and to reduce the heating loads. Thereafter the storage of the excess of energy under the greenhouse is the best solution. Different recuperation systems of solar energy and its storage were tested. The systems used were; the changing-phase materials (2), the water accumulators (3) and the ground. Some authors used a direct storage of the heat air into the greenhouse ground. This system consisted of an air extractor coupled with buried PVC exchangers. They saved 12 % of annual heating energy. But they underlined the insufficiency of data concerning geometry, necessary depth and thermal insulation to optimize the dimensions of their system. Some researchers showed that it is necessary to put the buried exchangers close to the ground surface to increase the storage and the direct energy restitution and to minimize the losses towards the deep ground. Other researchers (3) carried out energy storage in water accumulator. Then, an immersed exchanger was used to ensure the heat transfer between the greenhouse air and the accumulator. They reached an efficiency of 22 %, but this system is more expensive than direct storage in the ground. Boulard and Baille (4) simulated and analyzed the influences of some parameters on the performances of two thermal energy storage systems. These systems using PVC exchangers were; air-ground and water-ground system. These authors show that the heat storage by the air-ground system, called direct storage, was twice as significant as that stored by the water-ground system called indirect storage. They quantified the maximum daily heat quantity extracted in the first case of 40W.[m.sup.-2]. In spite of their diversity, these conditioning systems cannot ensure the autonomy of the traditional plastic tunnels for which the requirements in heating are significant.

This article describes the use of capillary polypropylene heat exchanger for a tunnel greenhouse conditioning. In Laboratory of Energetic and the Thermal Processes (L.E.P.T) of the Center of Energy Technologies and Researches from Tunisia, we installed two agricultural greenhouses (Fig. 1) of the same surface (100[m.sup.2]). One greenhouse is equipped with a heating system and the other not. The last is useful like a pilot greenhouse. Two exchangers coupled between them to manage thermal energy in a greenhouse compose the heating system used. The first system is a battery of plaits with capillary tubes buried under ground with a depth of 70 cm (Fig. 2). The second system is an air exchanger based on plastic black tubes known as agrotherms suspended with two meters and half height. This exchanger is always used for the greenhouse heating by the geothermic (Fig. 3). These two exchangers are coupled to ensure the autonomous greenhouse air-conditioning. We provide the exchanger in hot water by the hot water tank of the electro-solar power station of our laboratory. We feed the heating system by water at a temperature of 50[degrees]C.