Page 208 - Low Temperature Energy Systems with Applications of Renewable Energy

Heating with geothermal systems 195

Table 5.4 World geothermal energy usage for greenhouses and aquaculture [2].

Greenhouse Aquaculture Greenhouse Aquaculture
Year Capacity, MWt Energy, TJ
1995 1085 1097 15,742 13,493
2000 1246 605 17,864 11,733
2005 1404 616 20,661 10,976
2010 1544 653 23,264 11,521
2015 1830 695 26,662 11,958

as functions of temperature. Lettuce does best at w13e14 C, tomatoes at w20 C,

and cucumbers at w27e28 C. On large farms, each crop can be confined to its
own building where ideal conditions can be maintained. Table 5.5 gives recommended
temperatures for a variety of crops.
Natural gas, where available, may be used to provide heat to greenhouses in cold
climates, but at the expense of the environment owing to the emissions of carbon di-
oxide, ironically called a “greenhouse gas.” Geothermal greenhouses often rely on
backup natural gas-fired systems in times of extreme cold weather. Carbon dioxide,
which typically is found dissolved in geofluids, is often separated from the geofluid
and used within the greenhouses to enhance crop growth.
Engineering design of a greenhouse begins with the application of the principles of
thermodynamics and heat transfer. Once the site and materials of construction (glass,
plastic, polyethylene, fiberglass, wood or aluminum framing) are decided, the analysis
of heat loss can get started. Heat losses are related to conduction, convection and ra-
diation, lumped together as transmission losses. Additional heating is needed to warm
outside air that is used for ventilation in accordance with requirements for air-change.
It is common to select as the design outdoor conditions not the coldest temperature
on record, but one that applies for all but about 22 days of the heating season [23]. This

Fig. 5.18 Growth curves for three crops versus temperature [22].