Environmental Engineering Reference
In-Depth Information
geothermal fluid with foreign waters is prevented, geothermal fluid used as an
energy carrier will only be subject to iron precipitations. Sulphate, carbonic,
and silicate mineral precipitations within the aboveground geothermal fluid cir-
cuit are not expected. But to achieve this, the overall geothermal fluid system
must by all means be guarded against the penetration of oxygen; this helps ad-
ditionally also to prevent corrosion. Therefore the overall system has to be
maintained under permanent excess pressure during system operation, as well
as in idle mode. For this purpose a highly sophisticated gas shield system has to
be installed. Moreover, chemical changes due to microbiological activities
within the geothermal fluid circuit have been observed which may also cause
built-up of particulate matter (thus precipitations). In particular, bacterially in-
duced formation of hydrogen sulphide (H 2 S) will result in pH-changes, and
thus in sulphide precipitations. Also, bacteria are organic material and will
therefore enhance particulate matter contamination. Moreover, during start-up,
traces of oils and greases (e.g. from the borehole motor pump) may enter the
geothermal fluid circuit; which has to be prevented by appropriate measures.
Filtration of re-injected geothermal fluid. In spite of all process control preven-
tive actions, the re-injection reservoirs must nevertheless continuously be pro-
tected by a filtration of the injected geothermal fluid. Appropriate filters im-
plemented within the aboveground part of the geothermal fluid circuit ensure
good to excellent separation results within the predefined particulate matter
categories, and thus a good purification.
Heat transfer. The heat contained within the produced geothermal fluid needs to
be transferred to a heating system or a heating water circuit as efficiently and cost-
effectively as possible. For this purpose mainly screwed plate heat exchangers are
used which offer the following benefits:
low temperature differences of up to 1 K between the two media,
high heat transmission coefficients,
low building volume and mass,
sufficient pressure resistance for common geothermal pressures of up to 16 bar,
low water contents to be processed or disposed of in case of damage or service,
easy maintenance of the plate surface thanks to easy disassembly.
Mainly, heat exchangers made from titanium are used, as this material is highly
resistant to the frequently highly corrosive geothermal fluid.
In order to avoid the possibility of geothermal fluid entering the heating water
circuit (e.g. in the case of leakages), the heat exchanger is operated by overpres-
sure referring to the heating system side. Whenever this is not possible or ineffi-
cient other measures need to be taken (e.g. permanent monitoring of heating water
by measuring the electrical conductivity, installation of double-wall heat exchang-
ers between the two media; however, this will considerably impair heat transmis-
sion properties).
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