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ASME FEATURE Table 1 – Heat Transfer and Pressure Drop Correlations WHRU


Fin side heat transfer


Fin side pressure drop


Tube side heat transfer


Tube side pressure drop


Condenser Single phase heat transfer


Condensing heat transfer


Single phase pressure drop


Condensing pressure drop


Recuperator and IHRU Single phase heat transfer


Single phase pressure drop


Table 2 – Bottoming Cycle Process and Component Design Point Parameters Component



WHRU (exhaust/CO2) UA [kW/K]



WHRU (exhaust/hot fluid) UA [kw/K]



Recuperator UA [kw/K]



Process heat generator (CO2/hot fluid) UA [kw/K]






Max pump/compressor outlet pressure [bar] Pump/compressor efficiency [%] Expander efficiency [%] Motor/generator efficiency [%]

Nomenclature P R T h s v η

Pressure (Pa) Volume flow ratio (-) Temperature (K) Specific enthalpy (J/ kg) Specific entrophy (J/kg K) Tip speed velocity ratio (-) Efficiency (-)


DP Design point in Inlet out Outlet


is available anyway. If the heat demand is increased further, the power output drops relatively steadily. For the IHRU system, there is a significant power drop from 0-5 MW heat production. This is mainly due to the control strategy applied. The mass flow and pressure levels are controlled to heat the hot fluid to 170°C. For moderate process heat production, the mass flow of CO2 is not optimized for power production. The mass flow is increased to lower the CO2 temperature at the turbine inlet such that hot fluid is produced at 170°C. It would be beneficial to produce the hot fluid at a higher temperature so that the CO2 mass flow is optimized for power production. The hot fluid could then be reduced to the desired temperature by mixing. For heat production between 15 and 20-25 MW, the IHRU system performs equally or better than the dual WHRU system. For the Northern case, the power from the IHRU system drops quite drastically for heat production above 20 MW. To make more heat available for the hot water, the work extracted from the expander must be reduced. This is done by reducing the pressure ratio. The pressure ratio was decreased by reducing the heat uptake pressure (as condensing pressure is controlled by heat exchange with the cooling water). The high pressure is an important parameter for cycle efficiency. For the Southern case, operating as a Brayton cycle, the power production reduction is less drastic. Here the low pressure is free to increase, resulting in a reduced pressure ratio without decreasing the high pressure as much. The 10 MW heat production case is used as the design case. The resulting expander efficiency is shown in Figure 5. The expander efficiency is relatively constant for all cases, except for the IHRU system at the highest heat production case. The low efficiencies experienced here indicate that the operating conditions are outside the range of the expander.

Conclusions Compact CO2 cycles could be an interesting alternative for additional power generation on platforms equipped with gas f 80 turbines [4]. c 85 On many installations, both power and process heat has to be 95 provided from the fuel burned in the gas turbines. A bottoming cycle added to increase power production would have to be controlled such that the heat demand also is satisfied. The ratio of power to heat demand is expected to vary during operation, which adds complexity to the operation of the bottoming cycle. Advanced models able to provide realistic off design calculations for two bottoming cycles have been implemented. The calculations have shown that both proposed CO2 processes are able to produce both heat and power, both in Northern and Southern climates in a wide range of power to heat demand ratios. Initial evaluations indicate that the expanders are able to operate in a large range of conditions and are able to handle large variations in the ratio of power to process heat demand. f


Acknowledgments This publication forms a part of the EFFORT project, performed under the strategic Norwegian research program PETROMAKS. The authors acknowledge the partners

ASME Power Division Special Section | APRIL 2015

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