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Direct Use of Geothermal Resources 221

5.0

4.0 100 l/s

10 l/s 8 l/s
3.0
J/s (millions) conditions 6 l/s
Canby
operating
50 l/s
2.0
4 l/s
2 l/s
1.0 25 l/s
1 l/s

0.0
0.0 20.0 40.0 60.0 80.0 100.0
Delta T (°C)

FIGUre 11.13 The amount of useful heat extracted from geothermal fluids as a function of the temperature
difference between the supply and return fluids, contoured in terms of the flow rate, in l/s. The range of operat-
ing conditions for the Canby direct use cascaded system are enclosed within the shaded area. The dashed line
indicates the design load for the district heating system.

have been required to satisfy the peak demand. However, such periods are short-lived and recovery
from these demands is rapid.
Figure 11.13 shows how much energy per second can be obtained from geothermal fluids for a
given flow rate and temperature drop across the system. Also shown is the range of operating con-
ditions that the Canby district heating system has encountered. The dashed line is the design load
requirements. The fact that operation of the system falls below the design load requirements for all
but a small fraction of the time has allowed additional, cascaded direct use applications to be added.
A clothes washing and drying facility with five high efficiency dryers and washing machines
was added to the system. The dryers were retrofitted with increased air-flow passages and hot
water-to-air heat exchangers. The hot water supply is provided by a third heat exchange unit
through which the geothermal fluids heat system water. The air for the dryers is heated to between
62°C and 71°C. The temperature range reflects the fact that the ambient outside air that passes
through the water-to-air heat exchanger can vary significantly with the seasons.
The geothermal hot water cascades from the laundromat facility to a snow melt system installed
in attached paved areas, which keeps open areas free of snow during winter months.
Discharge to the local river must occur at temperatures no higher than 27°C. To remove sufficient
heat from the geothermal water in order to meet this temperature goal the fluid is cascaded to a 270 m
2
greenhouse facility. The function of the greenhouse, in addition to cooling the geothermal water, is to
provide the local community with fresh vegetables and a potential commercial product. The entering
fluid temperature fluctuates between 26.6°C and 54.4°C, depending on the load from the district heat-
ing system and other facilities in the system. Heat is transferred to the soil in the greenhouse, and to
the air space to maintain controlled warm air conditions year round. The fluid leaves the greenhouse
at temperatures low enough to assure it is discharged to the river well below the 27°C constraint.
The entire system is backed-up by a propane-fired boiler, in case of disruption to the heat supply
or if heat from the geothermal supply was inadequate. To date, the back-up system has been used
once, when the down-hole pump failed and had to be replaced. Otherwise, the backup system has
not been needed because the geothermal supply has been adequate to meet all load demands.