Question

The oceans in the tropics have substantial surface-to-deep-water temperature gradients. Depending on location, relatively constant temperature differences of 15 to 25oC can obtain for depths of 500 to 1.000 m. This provides the opportunity for using cold (deep) water as a heat sink and warm (surface) water as a heal source for a power cycle. The technology is known as "OTEC (Ocean Thermal Energy Conversion).
(a) Consider a location where the surface temperature is 27oC and the temperature at a depth of 750 m is 6oC. What is the efficiency of a Carnot engine operating between these temperature levels?
(b) Part of the output of a power cycle must be used to pump the cold water to the surface, where the cycle hardware resides. If the inherent efficiency of a real cycle is 0.6 of the Carnot value, and if 1/3 of the generated power is used fur moving cold waiter to the surface, what U the actual efficiency of the cycle?
(c) The choice of working fluid for the cycle is critical- Suggest some possibilities. Here, you may want to consult a handbook, such as Perry 'x Chemical Engineers * Handhold,.

Answer 1

TH := (27 + 273.15)K

TC := (6 + 273.15)K

 

a.        

 

b.        

 

c.         The thermal efficiency is low and high fluid rates are required to generate reasonable power. This argues for working fluids that are relatively inexpensive. Candidates that provide reasonable pressures at the required temperature levels include ammonia, n-butane, and propane.