10 Thermodynamic Analysis of Activated Carbon–Ethanol …
197
Figure 10.6 shows the effect of varying condensing temperature (T csat ) on COP,
keeping all other parameters constant i.e. R m = 2.5, T c = 400 K (activated carbon–ethanol), T c = 450 K (Zeolite–water), T a = 300 K, and T esat = 273 K. Condensing temperature has an adverse effect on COP, with increase in T csat value
of COP decreases monotonically. It can be explained physically as well as using
Eqs. 10.1 and 10.8–10.10. From Eq. 10.1 it is clear that higher the T csat higher will
be the x c and hence lower the difference x b − x c , this will decreases the value of SCE
and COP.
Figure 10.7 shows the effect of varying evaporating temperature (T esat ) on COP,
keeping all other input parameters constant, i.e. R m = 2.5, T c = 400 K (activated
carbon–ethanol), T c = 450 K (Zeolite–water), T a = T csat = 300 K. Evaporating
temperature has positive impact on COP, means increase in T esat increases the value
of COP. It can be explained the same way as the effect of condensing temperature.
Figure 10.8 shows the variation of SCE with respect to maximum desorption
temperature (T c ) at R m = 2.5, T a = T csat = 300 K, and T esat = 273 K. SCE
increases with increase in maximum desorption temperature (T c ).
Figure 10.9 shows the effect of variation of maximum desorption temperature on
second law efficiency for two cycles under consideration. From Figs. 10.3, 10.4, and
10.9, it is clear that maxima of COP and maxima of second law efficiency (η I I ) are
not in the same range. That is why there needs to be a trade off in choosing T c in
the best possible way such that neither COP nor η I I is sacrificed too much in the
interest of improving the individual entities. In order to obtain a common scale for
COP and η I I both the parameters are normalized within a range of zero to one by
dividing COP and η I I with corresponding maximum values C O P max and η I I max
respectively. C O P/C O P max and η I I /η I I max are then plotted simultaneously for
varying T c values (Fig. 10.10). The intersection point of C O P/C O P max versus T c
plot with η I I /η I I max versus T c plot provides the required optimized values of COP,
η I I , and T c .
From Figs. 10.3, 10.4, 10.5, 10.6, 10.7, 10.8 and 10.9, it is evident that activated
carbon–ethanol working pair has higher values of COP, SCE, and η I I as compared
to zeolite–water. From Fig. 10.2, we can observe that maxima for activated carbon
ethanol occurs in the range 360–375 K, whereas for zeolite–water maxima occurs
around 445–460 K. So zeolite–water working pair can be used at higher desorption
temperatures while activated carbon–ethanol is suitable for lower desorption temperature. Typical COP values heat recovery cycle for both the working pairs for air conditioning applications (T esat = 283 K) at desorption temperature of 100
◦ C (373 K)
and at condensing temperature of 27, 37, and 47
◦ C are given in Table 10.3.
197
Figure 10.6 shows the effect of varying condensing temperature (T csat ) on COP,
keeping all other parameters constant i.e. R m = 2.5, T c = 400 K (activated carbon–ethanol), T c = 450 K (Zeolite–water), T a = 300 K, and T esat = 273 K. Condensing temperature has an adverse effect on COP, with increase in T csat value
of COP decreases monotonically. It can be explained physically as well as using
Eqs. 10.1 and 10.8–10.10. From Eq. 10.1 it is clear that higher the T csat higher will
be the x c and hence lower the difference x b − x c , this will decreases the value of SCE
and COP.
Figure 10.7 shows the effect of varying evaporating temperature (T esat ) on COP,
keeping all other input parameters constant, i.e. R m = 2.5, T c = 400 K (activated
carbon–ethanol), T c = 450 K (Zeolite–water), T a = T csat = 300 K. Evaporating
temperature has positive impact on COP, means increase in T esat increases the value
of COP. It can be explained the same way as the effect of condensing temperature.
Figure 10.8 shows the variation of SCE with respect to maximum desorption
temperature (T c ) at R m = 2.5, T a = T csat = 300 K, and T esat = 273 K. SCE
increases with increase in maximum desorption temperature (T c ).
Figure 10.9 shows the effect of variation of maximum desorption temperature on
second law efficiency for two cycles under consideration. From Figs. 10.3, 10.4, and
10.9, it is clear that maxima of COP and maxima of second law efficiency (η I I ) are
not in the same range. That is why there needs to be a trade off in choosing T c in
the best possible way such that neither COP nor η I I is sacrificed too much in the
interest of improving the individual entities. In order to obtain a common scale for
COP and η I I both the parameters are normalized within a range of zero to one by
dividing COP and η I I with corresponding maximum values C O P max and η I I max
respectively. C O P/C O P max and η I I /η I I max are then plotted simultaneously for
varying T c values (Fig. 10.10). The intersection point of C O P/C O P max versus T c
plot with η I I /η I I max versus T c plot provides the required optimized values of COP,
η I I , and T c .
From Figs. 10.3, 10.4, 10.5, 10.6, 10.7, 10.8 and 10.9, it is evident that activated
carbon–ethanol working pair has higher values of COP, SCE, and η I I as compared
to zeolite–water. From Fig. 10.2, we can observe that maxima for activated carbon
ethanol occurs in the range 360–375 K, whereas for zeolite–water maxima occurs
around 445–460 K. So zeolite–water working pair can be used at higher desorption
temperatures while activated carbon–ethanol is suitable for lower desorption temperature. Typical COP values heat recovery cycle for both the working pairs for air conditioning applications (T esat = 283 K) at desorption temperature of 100
◦ C (373 K)
and at condensing temperature of 27, 37, and 47
◦ C are given in Table 10.3.