It likely means the cyogenic system is purely theoretical and won't work in the real world. Exergetic destruction is alway positive as it is by definition, represents the irreversibility within a process, which is a measure of the inefficiencies and the loss of potential to do work. Irreversibilities, such as friction, heat transfer through a finite temperature difference, mixing, and chemical reactions, always lead to a positive value of exergy destruction.

What this means is that exergetic was either miscalculated, purely theoretical work without any fundamental thermodynamic basis or maybe the person who calculated it wrongly defined it as negative.

Dear Henrique,

In conventional exergy analysis, negative exergy destruction means that the thermodynamic process is impossible to occur. However, before concluding this, I recommend reviewing the calculations to ensure there are no errors.

On the other hand, in advanced exergy analysis, it is possible to have negative exergy destruction. For example, the exogenous-avoidable part can be negative. This means that there is room for improvement in other equipment within the system, and such improvement can lead to an increase in the exergy destroyed in the analyzed equipment.

Everything I mentioned applies to any thermal system, including cryogenics.

I also take this opportunity to share something that might be useful. If you aim to develop an exergo-economic or exergo-environmental analysis of the cryogenic system, I recommend looking into the UFS Model. This model was initially designed for refrigeration and heat pump systems.

Sincerely,

Atilio.

Caro, Atilio Barbosa Lourenço muito obrigado pela sua resposta. Vou pesquisar mais sobre essa parte "exogenous-avoidable part". Aproveito para lhe perguntar sobre eficiência exergética e destruição de exergia em dispositivos de expansão. É possível falar em eficiência exergética e destruição de exergia nesse tipo de componente? Pergunto pois vi textos com opiniões divergentes sobre o assunto.

Grato!

Dear Henrique,

I prefer writing in English so that other researchers can follow the message exchange.

In any process where entropy is generated, whether for a closed system or a control volume, exergy destruction will occur. This is known as the Gouy-Stodola theorem. Since the throttling process is internally irreversible, exergy destruction will always occur.

Dissipative equipment (or components) are those where exergy is consumed, but no exergy production occurs. An example of such equipment is the expansion devices in refrigeration systems.

Indeed, there are some approaches in the literature regarding the definition of exergy efficiency for such expansion devices.

The first approach is that, in fact, exergy efficiency cannot be defined. In this case, the purpose of the expansion device is to serve some productive equipment. For example, a thermostatic expansion valve that feeds an evaporator.

The second approach is to apply the input-output method. In this case, the fuel is the flow exergy upstream of the device, and the product is the flow exergy downstream. A criticism of this approach is that it does not account for the thermodynamic property changes that occur during the throttling process.

The third approach stems from the disaggregation (split) of physical exergy into its thermal and mechanical components. A criticism of this method is the arbitrariness in defining these two components when phase change occurs in the refrigerant fluid. Another criticism concerns its application limitations. Depending on the temperature and pressure levels, it may not be possible to define an exergy product. This can happen, for instance, in bypass valves of steam turbines.

Finally, the approach that many researchers, including myself, have adopted still involves the disaggregation of physical exergy, but into the components of the UFS and A&F Models. For a better understanding, refer to the open-access article published in the Entropy journal by Rodrigo Guedes dos Santos et al. If you would like additional reading suggestions, look for articles published by the Nucleus of Excellence in Thermoeconomics and Energy Sustainability (NETES).

Best regards,

Atilio.