Overview
Steamfield
Steam Plant
Organic Rankine Cycle (ORC) Plant
Direct Use
Geothermal plant design
Geothermal plant design overviewDetail design
Geothermal plant design overviewThermodynamic limits
• Conversion of geothermal heat to electricity
• W is work (rate of work = Power). Q is heat flow. T is temperature
• Wmax = Q1 (1 - T0 /T1).
• Wmax is called Exergy. Note ratio of temperatures
• Geothermal plant T1 is low, therefore heat rejected to the surroundings is very high – large cooling systems
Geothermal
Fluid
Surroundings
Po
we
r P
lan
t
Geothermal plant design overviewGeothermal system limits
• Practical limitation on how much heat we can extract
from the geothermal fluid
Silica scale Wmax = m (h1 –h2 ) – m T0 (s1 –s2 )
SYSTEMMass flow in
Mass flow out
Geothermal plant design overviewExergy and its use
• Exergy is the energy that is available to be converted into electricity
• Throttling, flashing to lower pressure, heat transfer and mixing of
different temperature fluids involve no energy loss but they all
degrade the ability of the energy to generate electricity
• Exergy analysis can identify the reason for efficiency differences in
alternative power cycles
• An exergy balance can highlight the areas of energy degradation
throughout the steamfield and power plant systems
Geothermal plant design overviewSankey diagram
EXERGY
(Relative to
Surroundings
Temperature of
20°C)
SEPARATORS AND
FLASH PLANTS
IP STEAM WELLS
36% (157 MW)
IP STEAM FROM 2-PHASE WELLS
29% (128 MW)
ILP STEAM
11% (47 MW)
IP STEAM
70% (304 MW)
IP 2-PHASE WELLS
63% (274 MW)
ILP 2-PHASE WELLS
1% (5 MW)
WATER (REINJECTION LINE) 9% (38 MW)
WATER TO SURFACE
10% (45 MW)
EXERGY TO
REINJECTION
9% (38 MW)
NET
ELECTRICAL
OUTPUT
38% (164 MW)
EXERGY LOST IN CONDENSER
29% (128 MW)
68
% (2
97
MW
)
Rese
rvo
ir Exe
rgy W
ithd
raw
al 1
00
% (4
36
MW
)
SteamfieldPressure optimisation
• Steam field pressure optimisation – balances
effect of pressure on well deliverability, makeup
well drilling and field long term performance vs
steam field piping costs and turbine costs and
output• Use Net Present Value (or
cost) as the optimisation target – gives a single number combination of capital costs plus on going drilling costs against plant net generation over the project lifetime
SteamfieldSeparators
• Separators – separate the steam
and liquid phases with small
inherent pressure drop
• Use cyclone separator principle
• 2nd flash separators typically
designed on same principle, throttle
brine to lower pressure to flash
proportion to steam prior to
separation
SteamfieldSteam quality
• Solids and easily dissolved impurities
follow the liquid phase. NCG’s and
volatiles follow the steam phase
• High separation efficiency minimises
impurities in the steam
• Scrubbers next line of defence
Separation
Breakdown
90
95
8
5
100
5 1
0
2
0
3
0
4
0
5
0
Separation
Breakdown
Spiral
Inlet TypeTangential
Inlet Type
Seperator DIA 1372mm
SteamfieldSteam scrubbing
• Drop pots, scrubbers
and demisters remove
impurities in the
condensate
• Geothermal steam
deposits are quite
corrosive, important to
keep air out, minimise
shutdowns
• Pipeline steam scrubbing – condensation nucleates on
impurities
• Steam washing – inject de-oxygenated water to promote
condensation
Steam power plantOutline
• Commonly used world wide, proven commercial technology
• Unit sizes typically 20 MW to 140 MW
• Following slides cover key geothermal specific components -
• Types of steam plant
• Steam turbine
• Direct contact
condenser
• Cooling water circuit
and cooling towers.
• NCG gas extraction
system
Steam power plantTypes of steam plant
• Dry steam or single
flash
• Double flash
Steam power plantSteam turbines
• Dissolved gases in
condensate are corrosive
– stress corrosion
cracking
• Entrained particles are
erosive to inlet valves,
nozzles and blades
• High quality rotor forgings
to limit impurities and
reduce likelihood of SCC
• HVOF alloy or ceramic
blade coatings to resist
erosion.
• Single or Double flow in single casing designs
• Multiple casings for multiple exhaust flows to reduce exhaust losses
• Need to be very robust machines for the steam conditions
Steam power plantSteam turbines
• Impulse or reaction blading
• Special design features due to saturated steam
inlet conditions. Exhaust very wet, require high
levels of inter-stage condensate drainage.
• Latest designs have drain removal grooves on
trailing edge to promote removal of large drops
from steam path
• Integral blade snubbers at the outer edge that
bear on each other to damp out blade
vibrations
• Loose sleeve and forged lug at blade mid span
on long blades to further improve damping
• Stellite on leading edges of last rows due to
moisture impingement
Steam power plantDirect contact condensers
• Thermal (heat transfer) performance drives condenser vacuum
• Two zones – condensing
and gas cooling zone.
Condense 90% steam first,
at almost constant
temperature. Gas cooling
zone, 10% of heat,
temperature reduces
• Gas cooling section
condenses vapour from
NCG, counterflow section
so the NCG temperature
approaches cold water
temperature
Steam power plantDirect contact condensers
• Direct contact condensers key heat transfer parameters
• Droplet size – smaller greatly improves heat transfer rate. Drops grow as steam condenses
• Reducing drop size increases condenser pressure drop
• Transit time – time droplet takes from nozzle to floor
• Computation fluid dynamics cost effective way to model the thermodynamic and fluid dynamic performance
Steam power plantGas extraction systems
• Multi-stage compression used to allow use of inter stage cooling to minimise size and steam/power use
• Multiple trains can be used for flexibility to allow matching of gas extraction system capacity to actual steam NCG content – eg. 40%, 60%, 80% trains
• Choice of mechanical compressors, steam ejectors or hybrid steam ejectors and liquid ring vacuum pumps (LRVPs)
• Optimum choice depends on NCG content of steam, roughly >5-6% implies mechanical compression, 2-6% hybrid ejectors+LRVP, <2% 2 or 3 stage steam ejectors
• Considerable overlap depending on detailed optimisation
Steam power plantGas extraction systems
• Mechanical compressors • high speed centrifugal compressors
with intercoolers. Need fast acting anti-surge valves between each stage and a sophisticated control system
• efficient, much less heat load on cooling system. But costly, mechanically complex, no redundancy
• Steam ejectors
• lower cost, reliable but use large amount of steam
• Performance of intercondensers critical to good performance. Check interstagegas temperatures are close to design to verify
• Hybrid steam ejectors + LRVPs
• 1st 1 or 2 stages has to cope with high volumetric flow, so use steam ejectors
• Final stage LRVPs, slow speed, reliable, reasonably efficient
Steam power plantCooling towers
• Evaporative cooling devices – water approaches ambient wet bulb
• Two main types, crossflow and counterflow
• Crossflow can be lower cost to construct but counterflow are lower height
– less pump power
• Elevation of basin needs to be set considering cooling water pressure drop
to condenser and highest operating condenser pressure
• Evaporation produces plume, can be controlled if required with dry section
– hybrid tower
Steam power plantCooling towers
• Fan noise can be a major issue
that needs to be controlled via fan
design and operating speed
• Dissolved H2S oxidises to
sulphur, builds up as toxic sludge
in the basin that needs to be dug
out and disposed of as hazardous
waste, usually annually
• Cooling tower fill maximises water/air interface
• Splash fill to produce water drops and has least clogging
• Film fill much more efficient, water forms a thin film on the fill - is available in low clog form for geothermal applications
Organic rankine cycle (ORC) plantLiquid phase geothermal fluid
• Heat exchangers transfer heat from geothermal fluid to working fluid
• Radial expanders (turbines)
• Closed cycle with cycle pump
• Air cooled condensers
• Pinch point
occurs where the
organic fluid
vaporises
Organic rankine cycle (ORC) plantTwo phase organic rankine cycle
• Uses steam and brine and discharges both at reinjection temperature
• High pressure steam, such as from a steam cap can pass through a back pressure turbine before entering binary plant
• Using steam in the vaporiser and condensate + brine in the pre-heater gives an efficient design
• Geothermal steam is condensed just above atmospheric pressure – NCG’s can be vented
• A similar arrangement can be used without back pressure turbine on lower pressure steam
Organic rankine cycle (ORC) plantTwo phase organic rankine cycle
• High efficiency comes from matching the steam latent heat
of condensation with the heat of vaporization of the organic
fluid.
• Matching the
temperature profiles
in the preheater of
the organic fluid and
the brine +
condensate is also
effective
• Recuperators take
out superheat from
turbine exhaust and
significantly
improve cycle
efficiency
Organic rankine cycle (ORC) plantOrganic working fluid
• high molecular weight, lower boiling point than water, is above
atmospheric pressure at near ambient temperature
• select organic fluid to match the heat sources and heat sink
• latent heat at both condensing and evaporating pressure and the
specific heat in the pre heater section
• the kind of expansion (wet, dry or neutral) determines if
superheating is required and the need for recuperators
• fluid properties impact on the main
component design, dimensions and
performances
• has impact on cost, performances and
reliability
• ƒprice and availability on the market
• ƒ environmental impact, toxicity and
safety
Organic rankine cycle (ORC) plantAdvanced high temperature binary power cycle
• Binary + two phase expanders
• Secondary fluid is
heated as a liquid
and not vaporized
• Fluid is expanded
to two phase in
the expander
• Expander consists of a set of fixed
nozzles and an axial impulse rotor
• Improved net power
Direct use
• Direct use can make better use of geothermal heat by avoiding
the inherent losses associated with conversion to electric
power
• Examples of Direct Use
• Clean steam production
for various industrial
plant
• Timber drying kilns
• Pool heating
• Glass house heating
• Hotel and hospital
heating
Direct useNTGA Clean Steam Plant
• Kawerau plant that generates 16 bargclean steam suitable for use in paper machines
• Key design requirements
• Reliability, operates 363 days per year
• Sustainable, key requirement for owner and user
• High steam quality, key is high feed water quality
• Plant characteristics
• Uses 22 barg separated geothermal steam to generate 16 barg clean steam
• Generates its own feed water by treating geothermal condensate
• Separated brine is flashed and sent to a stripping column for treating
Direct useNTGA process flow diagram
• Simplified process flow diagram for clean steam
Geothermal plant designTake home messages
• Geothermal power plants are custom designed to the field
• Geothermal power plants reject large amounts of heat for their size
• Separator efficiency is key to steam quality
• Steam plants are proven technology
• ORC plants can better utilise low temperature sources and sources with high NCG content
• Improved ORC cycles are in development
• Direct use can make maximum use of the geothermal heat resource and be very commercially attractive
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