The ocean thermal energy conversion (OTEC) system is a promising solution to provid e stable electricity supply. Al though the available temperature difference in OTEC systems is small, an ammonia/water mixture as working fluid is expected to decrease irreversible losses in the heat exchangers and to improve system performance. However, in actual heat exchangers, an adequate temperature crossing does not occur in the condenser but in the evaporator. Therefore, clarification of this characteristic is important. To date, the logarithmic temperature difference (LMTD) method is used in performance evaluations of OTEC heat exchangers. This method is of limited use if physical properties of fluids vary. A generalized mean temperature difference (GMTD) method is introduced to perform this evaluation. As changes in fluid property values can be considered in the GMTD method, method dependencies on heat exchanger characteristics, effectiveness, and system characteristics can be studied. In particular, GMTD and LMTD using a pure substance were found to be almost equal. Mean temperature differences using mixtures as working fluid were higher in the evaporator, but lower in the condenser, from the GMTD method than from the LMTD method. For higher ammonia concentrations in ammonia/water mixtures, the mean temperature differences from both methods are different.
Energy and environmental problems are becoming more critical; the development of natural energy is desired. Ocean thermal energy conversion (OTEC) using temperature differences between warm surface seawater and cold deep seawater can supply stable electrical power [
Research is continuing on the Rankine cycle which is the conventional closed-cycle process exploiting lowboiling-point working fluids such as ammonia. Using a pure substance as working fluid, the thermal efficiency of the OTEC cycle increases with decreasing irreversible losses in the cycle or increasing effective temperature difference, specifically the difference between evaporating and condensing temperatures of the working fluid. For that reason, it is necessary to decrease energy losses in the heat exchangers, namely to decrease the temperature difference between seawater and working fluid. This is done by increasing the heat transfer area or the overall heat transfer coefficient. Using non-azeotropic mixtures such as ammonia/water as working fluids in the OTEC cycle was proposed by Kalina in 1982 [
To enhance the ammonia/water mixture cycle, clarification of the heat exchanger performance is extremely important. The traditional and effectual evaluation method of heat exchanger performance is the logarithmic mean temperature difference (LMTD) method but application of this method is limited to heat exchangers using working fluids having constant heat transfer coefficient and thermal properties. The generalized mean temperature difference (GMTD) method however enables variations in thermal properties in heat exchange processes to be included. Utamura et al. [
In this paper, clarification is obtained on the characteristics of heat exchanger using ammonia/water mixtures in improving cycle performance, achieved with the available methods of evaluation heat exchangers. A comparison is made between LMTD and the recently developed GMTD calculation methods as performed on the heat exchanger using ammonia/water mixture and the applicability of the GMTD method is assessed.
In Figures 1(a) and (b), the distributions of the fluid temperature within the heat exchanger is given as a function of distance (Δx) and heat flow rate (ΔQ), respectively. The temperature variation of the ammonia/water mixture changes in the heat exchanger owing to the large difference between its boiling point and its dew point. Consequently, an increase in available exergy of the system is expected with this mixture as working fluid. From
temperature within the heat exchanger is given as a function of distance (Δx) and heat flow rate (ΔQ), respectively. The temperature variation of the ammonia/water mixture changes in the heat exchanger owing to the large difference between its boiling point and its dew point. Consequently, an increase in available exergy of the system is expected with this mixture as working fluid. From
The LMTD method has traditionally been used to evaluate the temperature difference between the heat source and the working fluid in a heat exchanger. The LMTD method is used only when the thermophysical properties of the local fluid in the heat exchanger and the overall heat transfer coefficient are constant. Hence, the fluid in the heat exchanger undergoes temperature changes as shown in
where THI, THO, TLI and TLO are the high and the low temperature fluids heat exchanger inlet and outlet temperatures.
For an improved evaluation, it is necessary to consider variations in the thermophysical properties that better reflect conditions within actual heat exchangers. Therefore, the calculation results of LMTD are compared with those of GMTD and assessed. Using
where Ui represents the overall heat transfer coefficient for the ith segment, mass flow rate of the warm or cold source, dAi is the heat transfer surface area, and ΔT is the temperature difference between highand low-temperature fluids. Integrating equation over all segments of the exchanger yields
Then, the GMTD ΔTGMTD is defined as follows [
The temperature integral is approximated as:
Therefore, the GMTD is calculated by: