Trigeneration with geothermal energy Potentials and pitfalls of combined supply with power, heating, and cooling

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Low temperature cycles Concepts and prospects
Felix Ziegler TU Berlin, Institut für
Energietechnik Fachgebiet Maschinen- und
Energieanlagentechnik Silke Köhler GFZ
GeoForschungsZentrum Potsdam, Germany Present
affiliation RWE Power AG, Germany
Production of Power Optimisation Other
options Basic thoughts
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Triangular Cycle
Fits in heat source / heat sink
characteristics Sets upper limit Realisation Gas
Turbine with Isothermally Cooled
Compressor Rankine Cycle Kalina Cycle (sorption
power cycle)
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COST
Specific heat turnover
Use of low temperature heat sources The heat
exchangers will cause a large fraction of the
first cost. The total turnover of heat may serve
as a partial cost indicator.
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Specific heat turnover
Power plant
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Rankine Cycle (Organic Working Fluid)
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Real Cycle (Organic Working Fluid)
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Heat Transfer Diagram ORC

• Constraints
• Brine Tb,in, mass flow rate, specific heat
  capacity
• Cooling medium TCW,in, TCW,out, specific heat
  capacity
• Free variables
• Working fluid
• Te
• DTmin,in, DTmin,out

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Return Temperature Brine, ORC Kalina
Optimised for work output
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Overall Efficiency ORC Kalina
Optimised for work output
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Conclusions of Comparison

• Both systems are suitable for power production
  from low enthalpy reservoirs
• With given constraints from heat source and heat
  sink
• ORC cool the brine more
• Kalina reach higher thermal efficiency
• High parasitic loads at ORC, especially for air
  cooling
• ORC are more sensible to changes of heat sink
• Suitability of the systems
• Kalina KCS34 up to 150 C brine or CHP
• ORC from 150 C brine temperature
• Improvements
• Supercritical ORC may improve thermal efficiency
• Other sorption power cycles may improve cooling
  of brine

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Additional options COGENERATION (HEAT AND
POWER) COLD PRODUCTION Goal Increase of
cost effectiveness
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Geothermal Heat and Power(no real
cogeneration!)
Serial

• brine temperature gt temperature for heating
  purposes
• Not necessarily simultaneous production

• Additional constraints due to heating demand!
• Outlet temperature brine
• Mass flow rate brine

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Cold production
Heat driven chiller
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Specific heat turnover
Chiller
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Conclusions

• Power production
• Free Variables
• Layout
• Working fluid (medium, composition)
• Upper process temperature
• More free parameters with Kalina

Cooling from low grade heat is also an attractive
option Sorption seems to be a key technology
Future requirements - Technical experience -
Cost Heat exchange
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Comparison of chiller with power plant
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Value Specific heat turnover s Specific heat turnover s
heat 5 ct/kWh 1 1
cooling energy 10 ct/kWh 2/COPc 2 5
electrical energy 20 ct/kWh 2/h - 1 20
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Kalina KCS 34 Layout
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Heat Transfer Diagram Kalina
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Neustadt-Glewe

• Constraints
• Brine temperature, mass flow rate
• Heat sink temperature
• Heating capacity in district heating system
• Free variable
• Portion of brine through plant / upper process
  temperature
• Objective functions
• Generator capacity Pmech
• Cooling of the brine Tb,out
• Resource Utilization Factor RUE (overall
  exergetic efficiency)

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Results of Optimisation

• Brine 35 kg/s, 98 C
• District heating system (assumed) 50 kg/s, 70/55,
  3.1 MWth
• Working medium power plant Perflourpentane, water
  cooling 15/20

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ORC Layout
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Heat Transfer Diagram Kalina

• Constraints
• Brine Tb,in, mass flow rate, specific heat
  capacity
• Cooling medium TCW,in, TCW,out, specific heat
  capacity
• Free variables
• Composition basic solution
• Mass flow rate basic solution
• Pressure desorption
• Pressure absorption

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Optimisation of Lorentz Cycle
h w Tb,r