Articles
ADDITIVE COMMENTS IN SOLAR ENERGY RECOVERY WITH RADIANT MULCH FILM TUBES UNDER PLASTIC GREENHOUSES
During the day water circulated in this system is warmed up and stored in a pool and during the night the heated water is recirculated for heating the greenhouse.
During the month of junuary and under optimal conditions for solar recovery (sunny days and low plant development) about 800 kcal/m2 can be stored if using a 70% covering of the ground, which allows a heating capacity during the night about 60 kcal/m2 .h.
In this optimal conditions a raise of the night temperature with about 3.5– 4°C is fittable.
Sevaral conditions doesn’t allow to abtain this optimal solar recovery such as (more or less cloudy sky, plant development, utilisation of 50% covering of the soil in order to facilitate training), so that in general temperature raise isn’t higher than 2–2.5°C.
In order to obtain better results cheap outdoor captors (about 20% of the greenhouse suface) are needed for which we can use the same mulch film tubes or 12.5 cm diameter PE monoducts under small tunnels.
An other important inconvenient is the high storage capacity needed which is of about 601/m2 at least.
Storage in the ground could be interesting if using long covered channels, because we can have always a warm and cold side of this storage channel fovaring energy exchanges.
Nevertheless during the whole winter period at least 30 days black out of a solar energy are existing under Tunisian or south mediterranean conditions and this necessits the use of a vapour condensing boiler system, which is the most efficient heating system of water.
With a burner capacity of about 150.000 kcal/h one should be able to maintain the water of the pool for 10–12 greenhouses at a sufficient level (not lower than 18–20°C) during extreme cold nights.
The solar captor we used was in his first version of black PVC with 350μ of thickness and a longevity of 3 years but now the radiant mulch is a black PE of 150μ contraining 30% linear PE. This film is usuable one season at least.
The cost price of the solar energy recovery system is:
| * PE captor – distributor mulch film tube | 0.650 dollar /m2 |
| ( 5°% covering ) | |
| * distribution system (PVC tube Ø 63 PVC tap Ø,63 | |
| PVC distributor, PE or rubber tube Ø 18 mm ) | |
| 0,390 dollar/m2 each year | 0.092 dollar/m2 |
| * electric pump (1 per greenhouse each year ) | 0.260 dollar/m2 |