Page 209 - Low Temperature Energy Systems with Applications of Renewable Energy

196 Low-Temperature Energy Systems with Applications of Renewable Energy

Table 5.5 Temperature requirements for typical greenhouse crops, C [23].
Day Night
Vegetables
Peppers 18e29 16e18
Tomato 21e24 17e18
Cucumber 24e25 21
Lettuce (hydroponic) 24 18
Flowers
Roses 16e17 17
Poinsettias 21e27 18e22
Easter Lilies 16 n.a.
Carnations 24 10
Geraniums 21e27 (max) n.a.
Fuchsia 21 (min) 18 (min)

means that some auxiliary heating may be needed for a small period of time but pre-
vents an over-designed system for most times. A similar selection may be made for
winds as they are a strong determinant of convective losses. For example, a wind of
13 m/s creates about 50% higher heat loss than still air.
Transmission losses are proportional to the exposed surface area, whereas the venti-
lation losses are proportional to the volume of the building. Generally the heating is done
by means of floor coils and fan coils; the former may be used to heat the soil for the
crops, while the latter is used to maintain the air temperature at desired levels. One prob-
lem with buried heating coils in soil is that a temperature gradient is set up whereby
some plants see soil hotter than the optimum while others see lower temperatures,
and only some see the ideal temperature. This can create uneven growth conditions.
A generalized schematic of the geothermal heating system is shown in Fig. 5.19.The
temperatures in the circuit will be tailored to the conditions in the geothermal reservoir
and the needs of the crops. In most cases disposal is by means of an injection well.

Fig. 5.19 Simplified generalized arrangement for a geothermal greenhouse.