Calculation of cooling exergy efficiency
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The following data were obtained through HYSYS simulations: LNG is heated from -155°C to 0°C at a flow rate of 90 t/h; concentrated CaCl2 brine is cooled from 5.3°C to -42°C at a flow rate of 539 t/h. The exergy efficiency of the cooling process as a whole was calculated. Assume that the exergy at the inlet of LNG is Ec1 and at the outlet is Ec2; the exergy of the CaCl2 brine at the inlet is Eh1 and at the outlet is Eh2. Then the exergy efficiency is: (Eh2-Eh1) / (Ec1-Ec2). The exergy values for each stream can be found in HYSYS as follows (unit: kJ/kg): Ec1 = 895, Ec2 = 527; Eh1 = 2.5, Eh2 = 161. By multiplying each stream’s value by its flow rate and then calculating the exergy efficiency, we obtain: 2.57 > 1. Why is the exergy efficiency greater than 1?
What character is used for exergy? There might be some energy that hasn’t been taken into account here; in a+b=c+d, only a and c are considered, while some value of b or d is not taken into consideration
Exergy is \"yong\", which refers to what is known as useful energy; since this character cannot be written, it is written separately. Mine is just a simple heat exchange between two streams; nothing else.
I feel that this involves an issue related to the reference state; the way in which the HYSYS software selects the reference state is different from the standards specified in the guidelines.
I’ve tried it; the reference state is 25°C and 100 kPa, which is consistent with the reference state we selected by hand.
Under what conditions are the values you obtained?
Ec1 895 (7.9 MPaA, -157°C), Ec2 527 (7.1 MPaA, 0°C); Eh1 2.5 (1.2 MPaA, 5.3°C), Eh2 161 (0.4 MPaA, -42°C). The cold fluid is LNG, while the hot fluid is concentrated calcium chloride brine
The standard exergy is 0; whether Ec1 and Eh2 should be negative depends on the choice of the standard state.
This post was last edited by jinjinaaaaaaaa on 2017-12-26 at 17:11. Thank you for the replies. Following your approach, I did some rough calculations and found that the exergy efficiency is 68.8%. However, I think this value might be a bit high! Question: 1. The exergy value obtained in Hysys is positive ； 2. The standard state is 25°C and 100 kPa. If this is taken as the zero point for exergy, then each of the values mentioned above will be negative! As a result, the exergy efficiency is again greater than 1
Another issue is your exergy efficiency expression; please refer to the specifications for that.
This post was last edited by jinjinaaaaaaaa on 2017-12-27 08:59. Thank you very much; I have also been thinking about the expression for exergy efficiency. Could you please tell me what the standard number is? I’ll download it and take a close look. . . I feel that the purpose exergy efficiency should be similar to my previous calculation formulas; the general exergy efficiency doesn’t seem very reasonable. I seem to have seen these formulas in some literature before. Thank you