More Power Without Additional Emissions
Many pipeline booster compressors are driven by gas turbines operated in the simple-cycle mode, which is a classicsolution to limit plant costs, simplify operation, reduce maintenance requirements, etc. But this also means operating the system with a lower efficiency.
With the increase in fuel costs and concerns about greenhouse emissions, specifically CO2 production, a solution to this challenge was needed. With an eye toward solving it, GE Oil & Gas has introduced ORegen, a system that generates electricity from the waste heat of gas turbines operating in simple-cycle mode. ORegen was certified as part of GE’s Ecomagination program implemented in 2005.
Converting a simple-cycle turbine into a combined cycle is possible but costly, complicated and requires water, which is often not available, especially along pipelines.
According to GE Oil & Gas, ORegen increases the overall efficiency of the system by producing electric power without additional fuel or need for water. The system is very flexible, can be installed within a small footprint, requires very low maintenance and is designed for unmanned operation.
ORegen is a thermodynamic superheated cycle recovering waste heat from the gas turbine exhaust to convert it into electric power. This thermodynamic cycle is based on an organic rankine cycle (ORC) that uses a closed diathermic oil loop which, in turn, is used to heat an organic fluid loop. The low temperature heat from the gas turbine exhaust is then converted into useful work, or in this case, electric power.
This ORC works with a hydrocarbon based fluid in place of water. The system is similar to a conventional steam bottoming cycle except for the organic fluid that drives a turboexpander coupled to a generator. Both the diathermic oil and the organic fluid allow a low-temperature heat source to be efficiently exploited to produce
The ORC applied to ORegen is a closed-loop system. The organic working fluid is vaporized and pressurized in the evaporator using the gas turbine exhaust heat. Produced this way, the vapor is then expanded in the turboexpander and condensed by an air-cooled heat exchanger. The condensate is pumped back to the evaporator, thus closing the thermodynamic cycle. The fluids are sealed inside their loop and there is no need to replace them and no worry of them becoming tapped.
The heating and cooling sources are not in direct contact with the working fluid nor with the expander. The characteristic of the thermal oil depends on the temperature level of the application. When ORegen is applied to high-temperature systems, a regenerator is added to further improve the cycle performance.
The selection of the working fluid is key to the Rankine Cycle. The main characteristics are a low freezing point and high temperature stability, high heat of vaporization and density, low environmental impact, no additional EHS consideration and availability at low cost.
In practice, the ORegen system can be applied to the entire range of GE mechanical drive gas turbines of the heavy-duty and aeroderivative families to produce up to 16 MW of electric power without using additional fuel, thus in an environmentally friendly way. The overall efficiency of the system is over 51% when ORegen is applied to aeroderivative gas turbines and over 40% when first generation heavy-duty gas turbines are used.
The ORegen system has been primarily engineered for the oil and gas industry, but is applicable in general when you need to augment simplecycle gas turbine output without burning additional fuel