SHULMAN
LOW TEMPERATURE FLASHED STEAM POWER GENERATION
GARY SHULMAN
Geothermal Power Company, Inc., 1460 West Water
Elmira, New York 14905 USA
Key Words: low temperature flash steam, low temperature geothermal reservoirs, two stage separation,
dual flash cycle, low pressure flash plants, Brady Hot Springs Power Plant.
ABSTRACT
Flashed steam power generation can be operated economically with
geothermal reservoir temperatures below 177°C. A two stage flash
steam plant in Nevada. USA, is described, designed for an initial
reservoir temperature of 174°C. Reservoir temperatures as low as
and 130°C are analyzed for 10
power generation.
1. INTRODUCTION
The world's great geothermal resource was first used in electric power
generation at Larderello, Italy, in 1904, where boron was being mined
for boric acid production from fumaroles in the area. Electric power
generator driven by a one
was first generated by a small 10
cylinder engine, fed with pure steam raised in a boiler heated by
geothermal steam. From this early development with heat exchange
boilers, the state of the art has turned to direct use of the geothermal
steam in turbines.
The Lardarello wells produced dry steam with no brine, similar to
Cove Fort in Utah, USA, and
The Geysers in
Kamojang, Java, Indonesia. The first power production from a liquid-dominated hydrothermal resource began in Wairakei, New
Zealand in the early 1960s. Since that time, almost all geothermal
power has been produced by steam turbines, with a world installed
capacity of over six thousand megawatts.
Although current exploration has discovered large resource areas
the focus for geothermal power genwith temperatures below
eration has been on resource temperatures of 177°C or higher. This
is due to the fact that most current geothermal developers consider
177°Cas the lowest resource temperature for flash steam power generation. This paper will illustrate the use of lower temperatures for
this purpose and show that low pressure flash plants can compete
economically with more complex, higher cost binary plants for this
service, and should be considered favorably by developers and utilities.
In 1992 a discovery well drilled at Brady Hot Springs, Nevada, USA,
indicated a reservoir temperature of 174°C at a depth of 335 meters.
A power sales contract was secured by the developers from Sierra
Pacific Power Company, the nearest public utility, at approximately
per kwh. A geothermal power plant was designed and inUS
stalled for the generation of 26
This plant serves as an example of the utilization of a dual flash cycle for power production at
temperatures below 177°C.
2. PROCESS
There are three basic flash steam cycles to be considered in geothermal power generation. Listed in order of increasing cost and complexity they are:
single flash, non-condensing
singleflash, condensing
condensing
Resources with temperatures below
can use these cycles for
power generation. The cycles described herein are applicable to
both dry steam resources and hydrothermal resources, except for
dual flash which can only be done with hydrothermal systems. A
dry steam resource providing 131°C steam has been used in both
condensing and hybrid cycles at Cove Fort, Utah, USA. Experience has shown that most resources are hydothermal; therefore we
have elected to describe each cycle using that assumption.
Hydrothermal wells produce a two-phase mixture of steam and water
that must be separated prior to use in these cycles. Separators can
be located at the power plant or at each wellhead. These vessels
generally accept the two-phase flow at a tangential inlet, thereby
imparting a circular motion to the fluid in the vessel. Centrifugal
forces aid in the separation of dense liquid from light vapor. The
vapor is of course largely steam, but may also contain a small but
significant mixture of noncondensible gases. The steam may also
have small entrained water droplets which could damage turbine
blading over time. Demisters are used to collect and remove water
before steam enters the turbine.
2.1 Single flash, non-condensing
The single flash non-condensing cycle shown in Figure 1 is the simplest cycle, with steam exhausting to atmosphere through a diffuser
silencer after producing useful work in the turbine. Plants of this
type do not maximize the use of the resource, but they can be installed at a minimum cost. Their simplicity speeds the construction
and installation of skid-mounted turbine generator modules. Their
use as portable units for reservoir evaluation and early cash flow in
a project has been demonstrated. They may also be useful as a
power source for electric drill rigs, thereby reducing drilling costs.
Some geothermal resources have a very high noncondensible gas
content, which makes condensing cycles impractical. Such resources
may be best served with single flash non-condensing equipment.
2.2 Single flash, condensing
The addition of acondenser more than doubles the output of a single
flash plant. It also increases the cost and complexity of the plant,
but the cycle remains fairly simple as shown in Figure 2. In this
cycle, exhaust steam flows into a condenser after expansion through
the turbine. The condenser pressure is maintained at .07
to
to maximize the expansion of the steam in the turbine,
thus enhancing the power output. With low temperature resources,
over half of the power developed by the turbine comes from the
expansion of the steam below atmospheric pressure.
Condensers are usually fabricated from stainless steel. In a direct
2059
SHULMAN
Figure 1.
Single
Steam Plant Schematic
gas from the noncondensible gas stream. This scrubbing tower
poses the hydrogen sulfide to a chemical such as sodium hydroxide
or sodium hydrochlorite, creating salt water and non-hazardous sulfur salts such as sodium sulfate or sodium sulfite. After treatment,
the remaining noncondensibles are usually piped to the cooling tower
for dispersion in the cooling tower plume. Recent efforts by some
operators to compress and reinject the noncondensible gases containing hydrogen sulfide have met with some success.
2.3 Dual flash, condensing
contact type, the steam is condensed by cooling water spray nozzles
located within the vessel. Cooling water is circulated between the
the steam which condenses with this large infusion of cooling water
serves as make-up water for the cooling tower.
A two-stage set of steam ejectors is used to remove the noncondensible
gases from the condenser. These gases consist primarily of carbon
dioxide, with traces of hydrogen sulfide. Ejectors are venturi devices which use high pressure steam to create a vacuum, which draws
the noncondensible gases from the condenser. The ejector steam is
then condensed from the mixture in small stainless steel vessels called
inter- and aftercondensers.
If necessary, the stream of noncondensible gases can be treated for
hydrogen sulfide removal. In many cases, a simple counterflow
chemical treatment tower can be used to remove hydrogen sulfide
Figure 2 9.7
Figure 3 shows a dual flash condensing cycle. It is a simple extension of the single flash cycle which makes use of the energy remaining in the separated brine. By directing this brine to a low pressure
separator, additional steam can be generated which can increase the
total power generated by more than 50%. This additional power
generation is limited by the low pressure flash separation pressure,
which is generally maintained above atmospheric. There have been
recent investigations into lowering the pressure to less than atmospheric, thereby providing additional steam for turbine expansion
entirely within the vacuum range. Although technically feasible, this
has not been done commercially to date.
separators,
pres-
sure separation.
Hot
Dual Flash Geothermal Power Plant
An example of two-stage, dual flash geothermal power generation is
the 26
power plant at Brady Hot Springs, Nevada, USA. This
plant used (5) production wells plus (3) reserve wells, each having
an installed
horsepower line shaft pump to produce 7570 liters
per minute of brine per well. The wells were drilled to an average
depth of 460 meters. The brine from these wells is pumped to two
first stage separators, in which the steam is separated from the brine
The steam from each separator is then
at a pressure of 4.22
Single Flash Condensing Steam Plant Schematic (9.0
net)
Aftemndem
2060
SHULMAN
piped to the inlet of a condensing turbine at a pressure of 3.87
and condensed at a pressure of
to generate 9
in each of the two turbine generator sets. The brine at 4.22
from the first stage separators is then admitted to a second stage low
for further separation of brine and
pressure separator at 2.11
steam. The steam from this unit is piped to a third turbine generator
set with an inlet pressure of 1.76
and condensed at
with a generator output of 8
The brine from the separator at
is piped to reinjection wells approximately 1.5 kilometers distant from the production wells.
making a power supply to
The total parasitic load is about 5
This plant has been in operation
the transmission line of 21
for two years at 95% availability.
Figure 3. 15.1
The use of geothermal reservoirs with temperatures of
and 130°C are listed below with pumped well flow requirements to
of electric power generation using steam turbine
support 10
inlet pressures, at atmospheric pressure, of 1.033
and condensing to a vacuum of
Power Output
10,000
Reservoir Brine Required
976,270
150°C
1,380,300
130°C
2,329,740
Low temperature geothermal reservoirs well below 177°C can be
operated by flash steam power generation using a dual flash cycle to
significantly increase power output over single flash for a given resource flow rate. As with any other geothermal power technology,
the resource requirements increase with lower resource temperatures.
At lower resource temperatures, requiring lower separation pressures,
steam turbines must be designed or modified to handle larger specific volumes of steam.
This can be accomplishd by two stage flash power generation with
two stage steam separators, and'also with steam separators near
Flash Steam Plant Schematic (14.0
3. USE OF LOW TEMPERATURE RESERVOIRS
Reservoir
170°C
4. CONCLUSION
Net)
mospheric pressure. In the latter case almost all power is generated
in the vacuum phase of steam. In this manner, the simple, low cost
design, ease of operation and maintenance, and ability to generate
their own cooling water makeup, provide flash plants with a significant economic advantage compared with binary technology for low
temperature power generation.
REFERENCES
DiPippo, R. (May 1988). International Development in Geothermal
Power Production. Geothermal Resource Council, Bulletin. Vol. 17,
Kestin, J. (1980). Sourcebook on the Production of Electricity from
Geothermal Energy. United States Department of Energy. Brown
University.
2061