Open-Cycle OTEC

Abstract

The historical development leading to the proposal by Claude to generate power by producing steam in flash evaporation of warm seawater has been discussed in Chapter 2. In this chapter, the thermodynamic fundamentals of the open-cycle concepts are discussed, leading to a detailed review of state of the art and commercial prospects of the process. There are several variations on the standard OTEC open-cycle (OC) system. The three major variations are “hybrid cycle” (Bartone, 1978), “mist lift cycle” (Ridgway, 1977), and “foam lift cycle” (Beck, 1975; Zener et al., 1975). These are advanced concepts that offer certain attractive features and are being investigated. The three cycles will be discussed in Sections 5.3, 5.4, and 5.5, respectively. The standard OTEC open cycle is discussed in the following. The modest but nearly steady temperature difference that exists between the warm surface water and the much colder water at great depth in some tropical regions of the world has attracted the attention of many thermodynamicists from the time that these temperature differences were first observed. From the thermodynamicist’s view, any significant temperature difference can be used to produce power. The open or Claude cycle is the forerunner of various OTEC cycles. The open cycle refers to the use of seawater as the working fluid. A schematic diagram of the system, which comprises a flash evaporator, vapor expansion turbine and generator, steam condenser, noncondensables-removing equipment, and deaerator, is shown in Fig. 5-1 (Chen, 1979). The cycle is a basic Rankine cycle for converting thermal energy of the warm surface water into electrical energy. In the cycle, the warm seawater is deaerated and then passed into a flash evaporation chamber, where a fraction of the seawater is converted into low-pressure steam. The steam is passed through a turbine, which extracts energy from it, and then exits into a condenser. This cycle derives the name “open” from the fact that the condensate is not returned to the evaporator as in the “closed” cycle. Instead, the condensate can be used as desalinated water if a surface condenser is used, or the condensate is mixed with the cooling water and the mixture is discharged back into the ocean.

Choosing the Most Suitable Working Fluid for a CTEC

“…It was also found that steam generation was not constant over time, as the heater did not maintain a stable power level throughout the duration of the test. Based on the analysis of Figures 8 and 9, and the previous studies of Avery et al [4], the differences in the cooling relationships of each working fluid can be seen. In the case of R134a, the turbine inlet temperature must be 20 • C, and reduced in the condenser to 10 • C for condensation.…”

“…Based on the analysis of Figures 8 and 9, and the previous studies of Avery et al [4], the differences in the cooling relationships of each working fluid can be seen. In the case Theoretical Rankine cycles were developed using thermodynamic equations [31] incorporating R134a and ammonia (see Figures 9 and 10).…”

“…The sizing parameters (point six) use a factor, B, with the units kPa M kJ/kg. This is inversely proportional to the mass flow, which determines the lowest mass flow for the operation of the plant [4].…”

“…A CTEC is similar in principle to the more well-known ocean thermal energy conversion devices (OTECs), but would use the thermal gradient from the cooling process of the condenser of a thermoelectric plant with cold water from greater depths to generate energy via a closed Rankine cycle. CTECs would obviously be located on the coast, near to a thermoelectric power plant [4]. Currently, there are no OTEC or CTEC plants operating commercially, but they have been widely studied in recent years.…”

Economic Viability Analysis for an OTEC Power Plant at San Andrés Island

“…It can be seen that the maximum width is around 4 km. Based on the climatological data, the San Andrés Island has an ideal temperature to implement an OTEC plant, in addition to the other necessary characteristics, such as a short distance between 1000 m deep and the coast, with a steep slope and a seabed without many reliefs, the location must have low waves (height less than 3.7 m), and ocean surface currents less than 1.5 m/s, and a low incidence of natural phenomena such as storms, earthquakes, hurricanes, among others [54].…”