Innovation in Global Power - Parsons Brinckerhoff

Renewables – The Risks, Concerns and Potential
the end of 2010. Their longer term objective is for a rapid
development of the resource to 500MW generating capacity.
PB was hired by Geodynamics to evaluate alternative power
production technologies and prepare a budget costing for a
proposed modular power plant. To prove the geothermal
resource, we designed a test facility and a small (1.2 MW)
pilot-scale power plant using Habanero 3 and Habanero 1
as production and reinjection wells.
We considered various options to maximise the potential
power generation capacity based on 100 percent return of
geothermal brine to the deep aquifer without depressurisation
of the geothermal brine or the consequent release of the
dissolved non-condensable gases, such as methane and carbon
dioxide. Options evaluated were:
• Kalina cycle. This technology uses a varying mix of
ammonia and water around the cycle to maximise the
energy recoverable. The boiling water and ammonia mixture
can closely follow the brine temperature in the high-pressure
heat exchangers.
• Organic Rankine cycle. This technology uses an organic
fluid, such as pentane, which is preheated and then vaporized
in a series of heat exchangers. The high-pressure vapour
can then be used to drive an expander in a manner similar
to a conventional steam turbine plant.
• Flash steam cycle. We developed this new concept after
early modelling showed that a conventional steam power
plant in which the water is preheated and then boiled in a
series of heat exchangers could not realise the full potential
for heat recovery from the brine. Under the flash steam
cycle, circulating water is heated so that there is a constant
temperature differential between the brine and the
circulating water. The high temperature clean water can
then be flashed down in two stages to produce steam that
passes to a conventional steam turbine. The exhaust from
the turbine is then cooled in an air cooled condenser, and
this condensate is returned to the circulating water flow.
A summary of the performance and strengths and
weaknesses of the various options is shown in Table 1.
Based on a detailed investigation of power plant options, the
client elected to proceed with the proposed flash steam cycle.
Figure 1: Summary of Performance Strengths and Weaknesses
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Power Plant Modular Approach
Preliminary modelling has indicated that the production and
reinjection wells should be spaced approximately 1000 m
(3,200 feet) apart to achieve good long-term output from
the geothermal resource. The cost of surface piping was
found to be significantly higher than the marginal additional
cost of deviated (directional) drilling, so we developed a
configuration of five production and four reinjection wells
that could be drilled from a common pad.
The expected flows from the five production wells total
approximately 500 kg/s (1100 lb/s) of brine, which will yield
an average net power output of about 48 MW. This output
is toward the upper capacity limit available from a standard,
two-cylinder, low-pressure steam turbine. The deviated
drilling limitations and steam turbine size limitations both
suggested that this would be a convenient module size for
development of the geothermal resource.
High Pressure Heat Exchangers
The brine heat exchangers have a very high design pressure
that is in the range used for super-critical boiler feed
pre-heaters. Unlike these heaters, however, the brine heat
exchangers will require full access on the tube side so the
tubes can be mechanically cleaned to remove scale that can
result from the brine flowing through.
We have had considerable difficulty in finding heat exchanger
vendors who have the technology and experience in such
high pressure construction and are willing to make an offer to
design and build. It would seem that there is easily sufficient
work available for the few experienced vendors available that
they do not need to pursue comparatively small “one off”
jobs. To overcome this problem, we have undertaken the
thermal and mechanical design of the high pressure heat
exchangers so that otherwise competent vendors are able
to offer a construction only service. The design that we
developed uses heavy forged tube plates and channels with
pressure seal style, flat plate closures to the channels.
Reinjection Pumps
A review of available pumping technology for reinjection of the
high pressure brine revealed that the multi-stage centrifugal
pumps used in the oil industry represent the best available
technology. These pumps have been developed to handle
gassy hot fluids contaminated with sand, and they have given
very good service.These pumps are offered conventionally in
a submersible pump configuration that makes the seal design
comparatively simple. For ease of servicing, however, our
installation required a horizontal surface mount pump with
conventional motor drive.
PB Network #68 / August 2008 52