Closed-System Greenhouse

Based on the experience with the simple solar water desalination systems a closed – system greenhouse with integrated solar water desalination was developed and evaluated first in a small greenhouse (Strauch 1985a, b), and then in a pilot plant designed in Germany, and erected and evaluated at the University of Adana, Turkey (Baytorun et al. 1989; Meyer et al. 1989). The greenhouse was designed to fulfil the following demands:

• Plant production in arid regions in a controlled environment with protection from wind, dust and low air humidity.

• Inside air temperatures which do not exceed suitable conditions for plants, even at high outside radiation and temperature.

• Reduction of water use by decreasing transpiration rate, and reduction of humidity losses through air exchange by making the greenhouse water-and airtight.

• Minimizing energy and water consumption by not using artificial cooling and heating.

• Recollection of condensed water from interior greenhouse surfaces.

0

0.0 2.0 4.0 6.0 8.0

Global radiation (kWh/m2day)

Independent water supply by solar desalination.

Independent energy supply by solar cells.

Construction with commercial greenhouse elements to reduce investment costs.

Figure 15.10 shows the cross-section of the greenhouse with a water desalination system at the southern side wall. Figure 15.11 shows the north and south view of the greenhouse. The special characteristics of the greenhouse were:

Fig. 15.9 lar desalination system with two glass panes

Fig. 15.10 Cross-section of the closed-system greenhouse, type Hannover

• The shape of the northern roof was designed to reflect the main part of the global radiation during high position of the sun.

• The southern roof was covered by normal glass. A special glass that absorbed a high amount of the incoming near infrared radiation to reduce the heat genera­tion inside brought no advantage. Because of its own high temperature, the inside temperature was increased. Better would be a near infrared reflecting glass, but this was not available some years ago.

Fig. 15.11 Closed-system greenhouse type Hannover

• A movable lamellar outside shading system could be closed to reduce the incoming outside solar radiation at the southern roof.

• All trusses were designed outside the cladding material to prevent them warming up and heating the inside atmosphere. This construction is very recommendable for arid regions.

• A security or emergency low-capacity forced ventilation was installed at the northern side wall that could be used to circulate inside air only or to ventilate with outside air. The air exchange capacity of the fan was 6 (1/h).

• Solar panels were able to provide the necessary electricity power.

• Side wall and roofs were equipped with condensed water collection gutters to recollect evapotranspirated and condensed water at the inside surface.

The greenhouse was specially sealed, and could be kept closed as long as possible to reduce the requirement for desalinated irrigation water.

• A desalination unit was located at the southern side wall of the greenhouse. Because it was difficult to seal the system sufficiently at the site during mount­ing, it was recommended to separate greenhouse system and simple desalination system.

A tomato crop was grown in the greenhouse. Figure 15.12 shows an example of temperature course inside and outside with high global radiation and with climate control measures: ventilation, V+ and V— on and off, as well as shading, S+ and S— open and closed. Measurements demonstrate the possibility of keeping the inside temperature near the outside temperature.

The inside temperature could be kept near the outside temperature thanks to the outside shape, outside shading, outside trusses and low ventilation rate. The incom­ing solar radiation was mainly converted to latent heat by the evapotranspiration, and by preventing the conversion to sensible heat in the construction components which are located outside.

The CO2 concentration decreased down to 150 Vpm in the closed atmosphere, which limited growth, but the security fan was able to keep the CO2 concentration at a sufficient level. An CO2 enrichment would be preferable. Too high humidity can be prevented by ventilation, but forced ventilation should be restricted to avoid too much humidity loss. The temperatures at night remained 1-3°C above outside temperature.

These measurements showed that plants can be grown without artificial cooling during a winter period in arid regions.

The mean irrigation water requirement for the middle period of tomato crop growth was 0.51 l/day plant or 1.17 l/m2 day from April to beginning of June. That was less than the requirement in normal greenhouses.

The desalination productivity of the integrated desalination was about 3 l/m2day, and thus less than expected in comparison to the experimental results of the separate desalination systems (see Sect. 15.1). Reasons were leakage in the structure and the internally mounted construction components in the desalination system in contrast to the greenhouse structure.

From the reduced irrigation water requirement in the closed system and the yield of desalinated water, one can expect a realistic area relation of desalination system to greenhouse floor area of 23-37% (Strauch 1985b). The clear desalination water can be mixed with salty water for irrigation.

The recollection of condensed water for irrigation depends on the tightness of the greenhouse structure. The amount of water recollected by condensation inside was about 20% of the irrigation water in the completely closed and sealed system, but would be reduced by the climate control measures of ventilation.

Summarising the results, the design of recollection systems inside and the corresponding water recollection was not recommendable for an economical use in practice.

Summarising all results, the following proposals for improvements were made (Meyer et al. 1989):

1. The desalination system should be designed as a separated passive system

(Fig. 15.8).

2. The greenhouse system for arid regions could be a more simply designed one

(Fig. 15.13) with the following characteristics:

Fig. 15.13 New proposal for a closed-system greenhouse in arid regions

• Through north roof, with an inclination such that the main part of solar radiation around midday will be reflected.

• South roof cladding with normal glass, or near infrared reflecting material which is available today.

• Movable reflecting outside shading.

• All trusses outside the cladding material.

Such a greenhouse can help to solve some of the problems of crop growth in arid regions.

Updated: July 1, 2015 — 7:18 am