Chapter 18, Problem 96PP

Energy from me Ocean

Whenever two objects are at different temperatures, thermal energy can be extracted with a heat engine. A case in point is the ocean, where one “object” is the warm water near the surface, and the other is the cold water at considerable depth. Tropical seas, in particular, can have significant temperature differences between the sun-warmed surface waters, and the cold, dark water 1000 m or more below the surface. A typical oceanic “temperature profile” is shown in Figure 18-37, where we see a rapid change in temperature—a thermocline—between depths of approximately 400 m and 900 m.

Figure 18-37 Temperature versus depth for ocean waters in the tropics (Problems 93, 94. 95. and 96)

The idea of tapping this potential source of energy has been around for a long time. In 1870, for example, Captain Nemo in Jules Verne’s Twenty Thousand Leagues Under the Sea, said, “I owe all to the ocean; it produces electricity, and electricity gives heat, light, motion, and, in a word, life to the Nautilus.” Just 11 years later, the French physicist Jacques Arsene d’Arsonval proposed a practical system referred to as Ocean Thermal Energy Conversion (OTEC), and in 1930 Georges Claude, one of d’Arsonval’s students, built and operated the first experimental OTEC system off the coast of Cuba.

OTEC systems which are potentially low-cost and carbon neutral, can provide not only electricity, but also desalinated water as part of the process. In fact, an OTEC plant generating 2 MW of electricity is expected to produce over 14,000 cubic feet of desalinated water a day. The governments of Hawaii, Japan, and Australia are actively pursuing plans for OTEC systems.

96. •• A commercial OTEC system may take in 1500 kg of water per second at 22 °C and cool it to 4.0 °C. How much energy is released in one second by this system? (For comparison, the energy released in burning a gallon of gasoline is 1.3 × 10⁸ J .)

1. A.     2.5 × 10⁷ J
2. B.      1.1 × 10⁸ J
3. C.      1.4 × 10⁸ J
4. D.     l.6 × 10⁸ J