by Roland V. Wahlgren

Summary (full text is available for purchase)
Deliverable 1 of the CIDA INC Contribution Agreement with Batavia Greenhouse Builders Ltd. for the Viability Study—WaterProducer-Greenhouse™—Turks and Caicos Islands is this Technical Feasibility Report.

WaterProducer-Greenhouse™ conceptual drawing

Our Greenhouse for subtropical Grand Turk (see conceptual drawing) obtains its water from the humid ambient air. Twenty-seven exhaust fans force outdoor air at 1.52 m/s (300 ft per minute) through a 3,500 m² (0.87 acre) greenhouse. There is one air change every minute. Cool (15°C; 59°F), salty (36 ‰) groundwater, from four wells 400–500 m (1,300–1,650 ft) deep, is pumped through 27 water-cooled coils. Total flow is 256 L/s (4,050 US gal per minute) The aluminum-finned copper-nickel coils (total face area 2,700 ft²) are colder than dew-point. Airflow moisture (11–28 g/m³) turns into pure fresh water droplets collected in a reservoir for crop irrigation. Thermodynamic modeling shows that any crop mix among tomatoes, cucumbers, melons, squash, runner beans, peppers, and eggplants, would use less than 5% of the 200 m³ (52,800 US gal) fresh water produced daily by the greenhouse. About 190 m³ (50,200 US gal) fresh water is pumped each day to storage tanks. This extra water is allocated according to demand for drinking, beverage manufacturing, food-processing, livestock, and irrigation of nursery stock.

Side elevation of the proposed greenhouse showing air flow through the building (© 2002–2003 Batavia Greenhouse Builders Ltd.)

The salty groundwater is returned, unchanged in salinity, to the environment via disposal wells on the proposed Greenhouse site at Crisson Plantation.

View of Crisson Plantation from the east (photograph by R. V. Wahlgren)

Evaporative cooling pads soaked with saltwater from the condenser maintain a year-round temperate summer climate inside four main growing zones in the greenhouse so that a variety of high-value vegetables and fruits can be grown and sold. A special feature of our design is a fifth 840 m² (0.21 acre) zone, the cool zone. Although not used for water production, it maintains coolness for lettuce and strawberries. Modeled energy consumption is just over 5200 kWh daily, based on a power requirement of about 220 kW. Fresh water energy cost of 25.1 kWh/m³ is slightly higher than for reverse osmosis—but our water would be pure with no dissolved solids. Cooling efficiency of the simulated greenhouse space has a coefficient of performance (COP) ranging from 2 to 17 depending on weather conditions—better performance than electrically powered refrigeration or air-conditioning. Approximate capital cost with wind-diesel power supply would be USD 4.9 million with less than 10 years payback for the power installation. By postponing power autonomy and connecting to the Grand Turk power grid, capital cost is reduced to USD 3.5 million but annual energy costs will continue at a rate of USD 650,0000. The Greenhouse is shown, through extensive tests of the design by a thermodynamic model, to be technically feasible. The Greenhouse could produce commercial volumes of water, vegetables, and fruit for Grand Turk residents and tourists.

Approved by CIDA-INC July 2002. Technical Feasibility Study Team: Roland V. Wahlgren*, Atmoswater Research, North Vancouver, BC, Canada; Bob Crocker, Site Specific Structures, Langley, BC, Canada; and Aar Koeman, Batavia Greenhouse Builders Ltd., Aldergrove, BC, Canada.

*Author of report, to whom correspondence should be addressed. E-mail: atmoswater@shaw.ca