Jökull - 01.12.1961, Page 61

the individual houses. The water is wasted after
use. Without boosting the temperature o£ the
supply is 75° C to 80° C, whereas the waste
generally is at 30° C to 50° C, depending sub-
stantially on the load. One of the main diffi-
culties of the system is the fact that the temp-
erature of the waste increases with increasing
load ancl the efficiency is at minimum at the
maximum load.
The remainder, 15%, is built as a double-pipe
return system where the thermal water is circula-
ted. A part of the fresh thermal water supplied
by the wells is circulated through the double-
pipe system and returned to the main supply-
line of the single-pipe system. The outgoing
temperature to the district is kept at around
80° C and the return at 40° C to 50° C. The
double-pipe system simply acts as a gate in
the main supply-line where the temperature
drops by the amount corresponding to the
heating load of the double-pipe district.
Based on present building cost, the total
cost of heat supplied by the R. M. D. H. S.
averages approximately 4.00 $/Gcal. This is
only about 60% of the heating cost based on
oil. This ratio illustrates the economical import-
ance of the R. M. D. H. S.
Other district heating systems in Iceland are
designed in a similar way as the single-pipe
part of the R. M. D. H. S. An exception is the
system at Hveragerdi in the southern part of
the Hengill area. This system draws on wells
supplying a mixture of steam ancl water. The
thermal water contains considerable quantities
of silica and the system is therefore designed
as a double-pipe closed system with indirect
heating.
(ii) Development in New Zealand. Kerr et
al. (G/52) give a detailed description of the
use of geothermal energy for heating purposes
at Rotorua in New Zealand. They furnish
examples of heating systems in an animal hus-
bandry, hotel, hospital, forest service station and
a high-school. The systems applied in Zealand
are in many ways similar to those built for
similar conditions in Iceland. There are, how-
ever, some differences.
The thermal water ancl steam supplied by
the wells at Rotorua have a temperature con-
siderably above 100° C. The water is relatively
impure. This has led to the use of indirect heat-
ing by means of heat exchangers. The house
systems are closed and operate at a temperature
above 100° C. Drawings of the various systems
are furnished by the authors (G/52).
(b) Green-house heating.
Green-house farming by geothermal energy
is of economic importance in Iceland. The main
crops are tomatos, cucumbers and flowers. The
total floor-area of all green-houses in Iceland
is about 95,000 square meters. Some details are
given in the paper by Lindal (G/32).
(c) Corrosion and scale forming in heating
systems.
Sigurdsson (G/45) describes briefly corrosion
and scale forming in the Reykjavik Municipal
District Heating Service. l'he report states that
oxygen-free thermal water is not corrosive to
steel or concrete and does not precipitate scale.
However, traces of oxygen absorbed by the
water initiate corrosion and scale forming in
pipes and radiators. As it is difficult to keep
the circulating thermal water completely free
of contact with air, a dilute solution of sodium-
sulphite is added to the water in order to reduce
traces of oxygen. The thermal water is corro-
sive to copper alloys.
ICerr et al. (G/52) report that the thermal
water at Rotorua is not suitable for direct use
in heating systems. Inclirect heating is the rule.
Difficulties are reported with steel or cast iron
thermostatic valves resulting from impurities
in the water. Non-ferrous type fittings corrode
rapidly.
The present writer (Bodvarsson, 1950) has
carried out a brief stucly of corrosion and scale
forming in various locations in Iceland where
thermal water has been utilized for space heat-
ing for two or three decades. In all systems in-
vestigated the thermal water was usecl directly
in the radiators. The systems generally consist
of ordinary steel pipes and cast iron radiators.
The results are as follows.
Thermal water issued by springs and wells
does generally not contain oxygen. There are
three temperature ranges with different pheno-
mena. First, water procluced by springs or wells
at a temperature below 60° C is not harmful
even if it has had contact witli air ancl absorbed
some oxygen. A slight scale may be precipitated
in radiators after a decade or two.
Second, water issued in the range 60° C to
100° C generally causes very little corrosion and
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