Weak Solution and Stage Absoption Heat
To increase the performance of absorption heat transformers, there has been a continuous effort to obtain high temperature lift and COP by using the exhaust gas as the heat source. A two-stage absorption heat transformer (TAHT) is one kind of advanced heat transformer in which it is possible to achieve a higher absorber temperature. A recent study has presented a mathematical model of a two-stage heat transformer with a solution heat exchanger operating with water/sulphuric acid. Rivera et al. has proved that the double configuration can have a higher absorber-temperature, but the COP is less than that for the single-stage absorption heat transformer. Grossman developed a simulation program called “A user-oriented computer code (ABSIM)” for a multi-stage absorption system, which can be employed to investigate various cycle configurations with different working-fluids.
An exergy analysis and the concept of the second law of thermodynamics play an important role in evaluating the behaviors of thermal and chemical systems, because their applications lead to a better understanding of energy transformation processes. Energy utilization diagrams (EUDs) represent features of both the first and second laws of thermodynamics with respect to energy transformations. The pinches can readily be observed on these EUDs.
In the present paper, a modification of a two-stage absorption heat transformer with the incorporation of the latent and the sensible heat modes is implemented. We discuss the case in which the exhaust gas from a practically-operating gas-turbine plant is taken as the heat source for the TAHT. The results show that this TAHT is more efficient, the exergy efficiency is increased from 48.14 to 54.95% and the temperature lift is increased by 10.7 K.
Two-stage absorption heat transformers basically consist of two generators G1 and G2, two condensers C1 and C2, an evaporator E1, an absorber evaporator A1 and an absorber A2, as shown in Fig. 1. Heat from a source is divided into two streams. One of them is supplied to separate water from a water/lithium bromide mixture in generator G1. The vaporized water is condensed in the condenser C1 and then is pumped into evaporator E1, where it is vaporized at an intermediate temperature and pressure PH1. The other stream enters the generator G2. The generated water vapor is condensed in condenser C2; then the condensed water is pumped, at pressure PH2 which is slightly higher than PH1, and vaporized by receiving heat QA1. The vaporized water is absorbed in absorber A2 at a high temperature by the strong solution coming from generator G2. The weak solution, via the heat exchanger HEX2, enters generator G2, so repeating the cycle again.
Fig. 2(b) displays the arrangement of the generator in close-to-equilibrium operation. We keep the pressure nearly uniform and divide the generator into, say, four compartments. Because the temperature of the weak solution that comes from the solution heat exchanger is high, a large temperature-difference would exist. If this stream enters the first compartment (i) directly, a large exergy-loss will be generated. Hence, the weak solution stream releases heat during its passage through compartments (iv, iii, and ii) and finally flows into compartment (i). The superheated vapor evaporated from each compartment releases heat in the upper compartments, joins with the vapor stream from compartment (i), and then enters the condenser. Also the external heat-source gives evaporation heat to the weak solution in compartments (iv) to (i). In compartment (iv), the concentration of the weak solution reaches the required concentration and is fed through the heat exchanger into the absorber. By selecting proper temperatures for compartments (i) to (iv), close-to-equilibrium operation can be achieved. For comparison, we include a single-compartment generator in Fig. 2(a).
Fig. 3(b) shows the configuration of the absorber. We split the process into four absorption-processes. Namely, the saturated vapor is divided into four streams, one to be introduced to each compartment. Meanwhile, the strong solution is heated in compartments (iii) and (ii) and enters compartment (i). The heat released by absorption is accepted also by the feed water in compartments (i) to (iv). The strong solution passes compartments (i) to (iv), becoming dilute, and enters the solution heat exchanger. To compare this scheme with a single-compartment absorber, we introduce Fig. 3(a).
As can be seen in Fig. 4, we can compare the one-compartment absorber with the multiple-compartment absorber by introducing the Dühring diagram. To express clearly and simply, we explain this change in a single-stage absorption heat transformer (SAHT). Fig. 4(a) shows the Dühring diagram of the single compartment absorber. Because the temperature of the pure saturated vapor is low, the final point in equilibrium with the weak solution is not at point H, but is A, as shown in Fig. 4(a). As a result, point H is an imaginary state that cannot be attained. If we make the state point A close to point H, we can obtain a higher temperature-lift.
Based on this consideration, the multiple-compartment absorber in Fig. 4(b) is proposed on the basis of the sensible-heat mode. Its Dühring diagram is shown in Fig. 4(b). Here, the strong solution enters the absorber gradually by absorbing the split-saturated vapor at two different pressures. Adopting two different pressures, PH1 and PH2 in Fig. 1 can make the temperature of useful heat increase, namely, A1 approaches the imaginary point H much more closely.
Consequently, the strong solution passes through equilibrium points A1, A2, A3 and finally gets to point A in equilibrium with the weak solution. The highest temperature of the absorber is seen at state A1. A similar phenomenon also occurs in the multiple-compartment generator. We apply this multiple-compartment concept to absorber A2 and generators G1 and G2 in Fig. 1 (case II).
Fig. 5 shows the improved configuration of a TAHT. The water vapor, which comes from evaporator E1, is split into two streams, one flows into compartment (i) and the other enters compartment (ii) in absorber A1. The strong solution that comes from generator G1 flows into compartment (i), where it absorbs the vapor from evaporator E1, becomes dilute and then enters compartment (ii) of absorber A1. In compartment (ii), this solution absorbs the vapor from evaporator E1, becoming the weak solution, and enters the solution heat exchanger. The liquid water from condenser C2 is divided into two streams and is pumped to different pressures. The first stream is heated by absorption heat in compartment (i) and the other stream is heated in compartment (ii). Finally, these two saturated vapor streams are introduced into the four-compartment absorber, i.e. absorber A2.
We use the calculation assumptions in Table 1. The thermophysical properties of aqueous lithium bromide solution are based on the ASHRAE Fundamentals data. To evaluate the performance of the absorption heat transformer system, two criteria, namely the coefficient of performance (COP) and the exergy efficiency are applied.
The performances of basic TAHT (case I) and the two improved versions (cases II and III) are summarized in Table 2. It can be observed that the total exergy loss is 299.1 kJ/s and the exergy efficiency is given by 48.14% for the basic THAT. By introducing the improved versions, the exergy loss will be decreased to 251.1 kJ/s in case II and 239.2 kJ/s in case III. Simultaniously, the exergy efficiency is increased to 50.99% in case II and 54.95% in case III. In addition, the heat-source temperature can be extended to 331.0 K. However, the COP is slightly reduced, say, to 30.83% in case II and 29.34% in case III but 31.44% in case I.
Tags: weak solution
