Question 12.3: Ideal Combined Cycle We will assume the same operating condi......
Ideal Combined Cycle
We will assume the same operating conditions as in Example 12.2 with the high source temperature of the power cycle being 457°C. Compare the COP of the ideal refrigeration cycle and that of the combined power-refrigeration cycle.
Given: T_{l} = 273 + (−10) = 263 K, T_{a} = 273 + 22 = 295 K, T_{h} = 273 + 457 = 730 K
Figure: See Figure 12.4.
Assumptions: Reversible Carnot cycle
Find: COP_{Carnot,c} and COP_{Carnot,comb}
Directly using Equation 12.4 yields
COP_{Carnot,c} \equiv \frac{Q_{1}}{W_{net}} = \frac{T_{l}}{T_{h} – T_{l}} (12.4)
COP_{Carnot,c} = \frac{T_{l}}{T_{h} – T_{l}} = \frac{263 K}{(295 – 263) K} = 8.22Also, from Equation 12.6
COP_{Carnot,comb} = \frac{T_{l}}{T_{h}} \left\lgroup \frac{T_{h} – T_{a}}{T_{a} – T_{l}} \right\rgroup = \frac{263}{730} \frac{(730 – 295) K}{(295 – 263) K} = 4.90Comments
The ideal COP of the combined cycle is much lower (about 60%) than that of the vapor compression cycle. This example serves to illustrate the large differences to be expected in energy efficiency studies that deal with either site energy or source energy. The second law efficiency is a better indicator in both instances, but especially so for the latter.