Page 228 - Geothermal Energy Renewable Energy and The Environment

216 Geothermal Energy: Renewable Energy and the Environment

10.0

8.0

Liters/second 6.0

4.0

2.0

0.0
20.0 40.0 60.0 80.0 100.0 120.0
Temperature (°C)

FIGUre 11.8 Flow rate, in l/s, required to replenish heat lost from a pond, as a function of the inflow tem-
perature. The pond temperature is assumed to be 27°C.

2004) where make-up fluid is added to replenish heat lost through the natural processes we have
been considering.
This type of direct use application must also take into account the chemical effects of the water
that is being used. Sensitivity of the animals and plants to nutrient and trace metal concentrations,
pH, and dissolved gases must be evaluated in terms of the local water chemistry before direct injec-
tion of geothermal fluids into the pond can take place. In instances where the chemical composi-
tion of the geothermal fluid is inappropriate for the animals or plants that are to be produced, heat
exchangers can be easily installed to accommodate the needed heat transfer.

dryInG

Drying commodities of various kinds is a well-established use of geothermal heat. Installations
exist around the world for drying onions, garlic, coconut, meat, fruits, lumber, potatoes, spices,
sugar, concrete blocks, and a range of other products. The advantages of using this heat source are
elimination of fuel costs to fire boilers and heaters and reduction of risk of fire by eliminating the
need for combustion.
The intent in all of these applications is to reduce the water content of the commodity of interest.
In the case of many vegetable and lumber products, the requirement is to reduce the water content
from about 50–60%, to as low as 3%. The actual end point to be attained depends on the nature of
the product and the anticipated shelf life.
Drying vegetables is usually accomplished by having a geothermal fluid with temperatures
between ~110°C and ~170°C pass through a hot-water-to-air heat exchanger. The heated air is then
blown through drying ovens with perforated stainless steel conveyer belts, or in heating cabinets. In
many applications, the drying will be accomplished by having the vegetable products pass through
multiple drying stages at different temperatures, which allows the most efficient use of the available
heat. The end product usually has moisture contents of 3–6%. The geothermal fluid exiting the heat
exchangers is generally in the temperature range of 30–50°C.
Lumber drying requires lower temperature geothermal fluids, generally in the range of 93–116°C
(Lineau 2006). This lower temperature reflects the fact that the drying process for lumber requires
more time (days to weeks), slow water extraction to prevent damage to the lumber, and higher