Question

A scientist designed a gas turbine based-ocean thermal energy conversion (OTEC) system. Is it a wise idea to do so, if not why, suggest an alternative way? Calculate the Carnot (ideal) efficiency of a common OTEC system.

Answer 1

OTEC system makes use of the temperature difference between the warm surface seawater and deep cold water. In a gas turbine-based OTEC system, the working fluid is evaporated by the warm water, expanded by gas turbine and then condensed by the cold ocean water.

In order to determine the Carnot efficiency, the temperature difference is critical. As a rule of thumb, a temperature difference of 20°C is necessary to sustain the viable operation. Using the surface temperature, T1=298K and cold deep water temperature asT2=277K in the formula for Carnot cycle efficiency, we get the efficiency to be 7.04%.

In general, common viable gas turbine-based OTEC systems have Carnot Efficiency in the range of 6-8%. The high setup cost and low Carnot efficiency are potential limitations of conventional OTEC systems. However, using gas turbines are pretty standard.

To come up with an alternative design to gas turbines, Stirling Engine can be implemented. In an OTEC system, the working fluid is ammonia or R-134a. The OTE-SPP (Ocean Thermal Energy Stirling Power Plant) system can use expanded ammonia to run multiple pistons. These pistons can turn the generator and produce energy. Using multiple pistons in a with a common piston head in a single chamber can reduce the dead volume and increase efficiency in a Stirling engine. Stirling engines based OTEC systems have theoretically better efficiency (as high as 60%) than gas turbines based OTEC systems.