Performance investigation of a power, heating and seawater desalination poly-generation scheme in an off-shore oil field

• Recovery of prime mover exhaust thermal power using organic Rankine cycle.
• Polygeneration of electric power, process heat and desalinated seawater in oil field.
• Exergy analysis of poly-generation for four commercially-available organic fluids.
• Economic feasibility of poly-generation in the Middle East oil industry.

Hydrocarbon production fields are energy-intensive, with significant demands for on-site power, process heat and fresh water, particularly in arid climates. A poly-generation scheme based on the conversion of gas turbine exhaust thermal power into mechanical work to drive a seawater reverse osmosis unit and generate process heat in an off-shore oil field in the Arabian Gulf is evaluated thermodynamically and economically. The prime mover exhaust thermal power is recovered using a bottoming organic Rankine cycle (ORC), with four working fluids used in commercial ORC systems evaluated. The performance of the poly-generation system is assessed both on a yearly and a seasonal basis. The octamethyltrisiloxane (MDM) cycle yields 6 MW of net power output at ideal and overall exhaust gas heat recovery efficiencies of 14% and 10%, respectively, 37 MW of process heat, and 1380 m3/hour of permeate. The reverse osmosis unit operates at a specific energy consumption and exergy efficiency of 4.1 kWh/m3 and 29%, respectively. The exergetic efficiency of the poly-generation system is estimated at 32%, thereby enhancing the efficiency of the original gas turbine power generation system by 6%. The system becomes profitable after approximately three years for subsidized local water and natural gas prices.