Training Geothermal Energy & Organic Rankine Cycles Binary ORC Working Fluids & Exergy Analysis
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Binary ORC Working Fluids & Exergy Analysis

60 min Geothermal Energy & Organic Rankine Cycles

Binary ORC Working Fluids & Exergy Analysis

Binary (ORC) power plants use a secondary working fluid with a lower boiling point than water to recover heat from low-to-medium temperature geothermal brines that cannot flash sufficient steam. The thermodynamic performance, environmental impact, and safety profile of the working fluid are all critical design criteria. Second-law (exergy) analysis quantifies where irreversibilities occur and guides component optimisation.

1. ORC Working Fluid Selection Criteria

An ideal ORC working fluid for geothermal applications should have:

  • Critical temperature $T_c$ slightly above the heat source temperature (allows supercritical operation or efficient subcritical evaporation)
  • Dry or isentropic saturation curve: $ds_g/dT \geq 0$ (positive or vertical slope in T-s diagram). This avoids condensation during turbine expansion — eliminating the need for a superheater.
  • Low GWP (global warming potential): $< 150$ is the EU F-gas regulation threshold; future regulations targeting $< 10$.
  • Zero ODP (ozone depletion potential): mandated globally since the Montreal Protocol.
  • Low flammability/toxicity: ASHRAE A1 classification preferred for unmanned plants.

Common choices: R245fa ($T_c = 154°$C, dry, GWP=1030), isobutane/R600a ($T_c = 135°$C, dry, GWP=3), pentane ($T_c = 197°$C, dry, GWP=~5, flammable), ammonia/R717 ($T_c = 133°$C, wet, GWP=0, toxic).

2. ORC Cycle Components & Thermodynamics

The ideal ORC cycle (1-2-3-4 on T-s diagram):

  1. 1→2: Pump (liquid compression, isentropic) — $w_p = v_1(P_2 - P_1)$
  2. 2→3: Evaporator (heat addition at $T_H$, isobaric)
  3. 3→4: Turbine (isentropic expansion) — $w_t = h_3 - h_4$
  4. 4→1: Condenser (heat rejection at $T_C$, isobaric)

Net work: $w_{net} = w_t - w_p$. Thermal efficiency: $\eta_{th} = w_{net}/q_{in}$. For a well-matched working fluid at $T_H = 150°$C, $T_C = 35°$C, practical $\eta_{th} \approx 0.10{-}0.14$.

3. Second-Law (Exergy) Analysis

The exergy destruction in each component measures irreversibility. For the evaporator (brine → ORC fluid):

$$\dot{E}_{dest,evap} = T_0 \dot{S}_{gen,evap} = T_0\left(\dot{m}_{ORC}(s_3-s_2) - \frac{\dot{Q}_{evap}}{T_{brine,lm}}\right)$$

where $T_{brine,lm}$ is the log-mean brine temperature. The largest exergy destruction in ORC systems typically occurs in the evaporator (temperature mismatch between brine and working fluid) and the condenser (heat rejection at $T_C$). Recuperators (internal heat exchangers) reduce evaporator irreversibility for dry fluids by preheating the liquid before evaporation.

4. Organic Rankine Cycle vs Steam Rankine for Low-Temperature Sources

Water has a very high latent heat and evaporation temperature (100°C at 1 atm), making it poor for low-temperature sources — the evaporator temperature mismatch is enormous. ORC fluids with $T_{boil} = 20{-}80°$C at moderate pressures (2–10 bar) match brine temperatures much better, achieving $\varepsilon_{exergy} = 50{-}60\%$ vs $30{-}40\%$ for steam cycles from the same brine.

Worked Example — R245fa ORC at 140°C Source

Brine at 140°C, condenser at 35°C. Carnot: $\eta_C = 1 - 308/413 = 25.4\%$. R245fa: $T_{boil}$ at 8 bar ≈ 100°C; turbine inlet $h_3 = 476$ kJ/kg, $s_3 = 1.736$ kJ/kg·K. Isentropic expansion to 35°C condenser ($P_4 \approx 1.5$ bar): $s_4 = s_3$, $h_{4s} \approx 451$ kJ/kg. $w_t = 476-451 = 25$ kJ/kg. $q_{in} \approx 200$ kJ/kg. $\eta_{th} = 25/200 = 12.5\%$. Exergetic efficiency $= 12.5/25.4 = 49.2\%$. ✓

ORC Exergy Analyser — Working Fluid Comparison
Carnot η =?%
ORC η (approx) =?%
Exergy efficiency =?%

Practice Problems

1. Compare the saturation curves of R245fa (dry) and ammonia (wet) on T-s diagrams. Explain why ammonia requires superheating before turbine entry while R245fa does not. What is the turbine outlet state for each fluid at the same expansion ratio?
2. An ORC plant has turbine work of 45 kJ/kg, pump work of 3 kJ/kg, and heat input of 320 kJ/kg. The heat source is brine at 120°C and dead-state is 20°C. Compute: (a) thermal efficiency, (b) specific flow exergy of the brine, (c) exergetic efficiency.
3. Explain the concept of the "pinch point" in the ORC evaporator heat exchanger. Why does the pinch point limit the maximum brine outlet temperature and hence the extractable heat? How does working fluid selection affect pinch-point temperature difference?
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