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