Free Cooling from a 10 MW Supercritical CO2 Turbine: How Much and What Is It Worth

Free Cooling from a 10 MW Supercritical CO2 Turbine: How Much and What Is It Worth

Executive summary (assumptions up front)

• Plant size: 10 MW_e supercritical CO2 power block

• Turbine inlet temperature (TIT): ~500 C

• Cycle efficiency used for heat-rate estimate: ~45% (representative for well-designed sCO2 RCBC at 500 C)

• Heat rate: ~7,582 BTU/kWh

• Turbine mass flow (engineering estimate): ~66.7 kg/s (consistent with ~150 kJ/kg turbine specific work)

• Post-turbine pressure letdown for cooling: from ~8 MPa to ~1 MPa via throttling (Joule–Thomson)

• Joule–Thomson cooling for CO2 near ambient: ~0.8 K/bar → ~56 K drop for ~70 bar letdown

• Specific heat (near ambient): cp ≈ 0.9 kJ/kg-K

These are standard, conservative design-basis values to yield order-of-magnitude results. Actual numbers depend on your exact pressures, recuperation, setpoints, and available ∆p.

Cooling power available from the pressure drop

Using isenthalpic throttling (Joule–Thomson) across a suitable valve on (a) the low-temperature, high-pressure side or (b) a dedicated sidestream post-turbine:

Temperature drop (∆T): ~56 K

Mass flow (ṁ): ~66.7 kg/s

Cooling capacity:

Q ≈ mcp ΔT≈ 66.7×0.9×56 kW ≈ 3,360 kW

In BTU/hr: 3,360 kW × 3,412 ≈ 11,470,000 BTU/hr

In refrigeration tons:

3,360 kW / 3.517 ≈ 956 tons

Range note: If your actual ∆p is lower or state point differs (e.g., slightly different temperature or pressure), expect roughly 2.5–4.0 MW of free cooling as a realistic band.

What is the COP?

Key point: This cooling comes from a pressure letdown that the cycle already needs; there is no additional electric compressor as in a chiller. In that sense, the effective COP is extremely high (cooling as a byproduct of a power cycle).

Two useful COP views:

Lower-bound COP (very conservative): Attribute all main-cycle compression work to this cooling function (even though you would incur that anyway to run the power cycle).

Representative sCO2 compression work ≈ 40 kJ/kg

For 66.7 kg/s → ~2.67 MW compressor power

COP LB ≈ Qcool/Wcomp ≈ 3.36/2.67 ≈ 1.26 COP

This is an intentionally conservative, worst-case accounting.

Effective COP (process viewpoint): Since the letdown is inherent to the power process, additional electric input for the cooling is near zero, so the effective COP is very large (practically free cooling). Many operators simply treat it as cooling available at negligible marginal electrical cost.

Dollar value: power generated and cooling avoided cost

10 MW electrical generation value (at $0.10/kWh)

Per hour:

• 10,000 kW×$0.10=$1,000/hr

• Per day (24 h): $24,000/day

• Per year (8,760 h): $8.76 million/yr

Cooling avoided cost (vs. running a chiller at $0.10/kWh)

To supply the same ~3.36 MW of cooling with a conventional chiller, the electric draw depends on chiller COP. Two common reference COPs:

COP = 3 (air-cooled typical):

• Electric power needed = 3,360/3≈1,121 kW

• Cost avoided: $ 112/hr or $2,690/day or $980,000/year

COP = 5 (good water-cooled):

• Electric power needed = 3,360/5≈672 kW

• Cost avoided: $67/hr, $1,610/day, $590,000/yr

If your plant’s chiller COP differs, plug in your COP:

Avoided cost per hour ≈ 3,360 kW / COP × $.10

Context on the heat rate at 500 C

With an assumed ~45% cycle efficiency at 500 C TIT (representative for modern sCO2 recompression Brayton designs), the heat rate is:

3,412 / 0.45 ≈ 7,582 BTU/kWh

and thermal input for 10 MW_e is about 22.2 MW_th.

Practical takeaways

• A 10 MW sCO2 unit at 500 C can furnish on the order of ~3.3 MW (≈11.5 MMBTU/hr, ~950–960 tons) of low-temperature “free” cooling by exploiting the Joule–Thomson temperature drop across a designed pressure letdown.

• From an accounting perspective, that cooling has an effective COP that is extremely high, because it piggybacks on pressure changes already inherent to the power cycle.

• In dollar terms, the cooling avoids roughly $67–$112 per hour of chiller electricity (depending on whether your baseline chiller COP is ~5 or ~3), in addition to the $1,000 per hour of avoided electricity purchase from generating 10 MW.

Notes for detailed design

• The exact cooling temperature level and capacity depend on where you take the sidestream, recuperator pinch points, and the allowable ∆p.

• You can increase or stage the effect with multi-step letdown or by routing a fraction of flow to match a particular cooling duty and temperature.

• Always confirm Joule–Thomson coefficients and real-gas properties at your precise state points (NIST REFPROP or similar), and verify materials/phase behavior if approaching the saturation dome.

TEL: 1-608-238-6001 Email: greg@infinityturbine.com

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