The solar energy collected by the solar panels will be stored in a water accumulator. For this prototype we considered a tank made from 200 mm thick high density EPS blocks, coated inside with a watertight reinforced PVC bag. This tank, which will be partially buried underground, will have a useful volume of 8m3. Figure 4 shows a side-section of this accumulator. For a AT of 20°C, the amount of energy stored in this tank will be 8.000 x 20 = 160.000 kcal = 186 kWh, roughly equivalent to 19 liters of diesel.

This energy corresponds to six hours of energy consumption under local extreme temperature conditions (Tin = 14°C and Tout = -1°C) for the considered prototype greenhouse. It also roughly corresponds to the daily average solar collection with these collectors during the colder months (160 kWh/day in January). If the collectors are not tilted back during periods with higher solar incidence, the temperature gain in the tank may easily exceed 40°C in a day.

As the root zone heating comes from water circulating at 30°C to 40°C, this tank is apparently suitable for this collector area. Warmer water inside the tank, besides increasing heat losses, decreases collector efficiency.

4. Conclusions

From these results we can assume that, in our climate conditions — temperature and radiation — the energy gain from a set of 3 x (36 m2) solar thermal panels installed inside a simple 300 m2 greenhouse glazed with structured PC plates, can exceed its needs if thermal properties are optimized. Therefore, a single module like this can eventually supply energy to one or two more standard modules with equivalent area.

References

[1] SolTerm5 (2007) — Analise de Desempenho de Sistemas Solares Termicos e Fotovoltaicos, INETI,

Lisbon

[2] W. J. Roberts (1997, 2005). Environmental Control of Greenhouses, CCEA, Center for Controlled Environment Agriculture, Cook College, Rutgers University.

[3] Dr. Gene A. Giacomelli (2002). Considerations for Energy Management of Greenhouse Heating and Cooling, University of Arizona

[4] Online Conversion, Blue Sparks Network — www. onlineconversion. com

[5] Suntrek Industries, Irvine, USA — www. suntreksolar. com

[6] W. J. Roberts, D. R. Mears, J. C. Simpkins, J. P. Cipolletti (1981). Movable Thermal Insulation for Greenhouses, Biological and Agricultural Engineering Department, Rutgers University.

[7] William J. Roberts, D. R. Mears (1980). Paper n° 80-4027, Floor Heating of Greenhouses, Biological and Agricultural Engineering Department, Rutgers University.

[8] Steve Diver (2002). Root Zone Heating for Greenhouse Crops — ATTRA, U. S. Department of Agriculture

Aknowledgements

The authors thank Professor Dr John Rekstad, from the Physics Department of Oslo University, for his help in the project discussion.