Space Thermoacoustic Radioisotopic Power System, SpaceTRIPS: The Magnetohydrodynamic Generator

Round 1

Reviewer 1 Report

This paper presents a theoretical analysis of an induction MHD electrical generator coupled with a thermoacoustic engine for the conversion of thermal energy into electrical energy. The main purpose is to develop an engine that can supply electric energy in deep space exploration missions. The system is described in detail although the paper is focused mainly in the theoretical analysis of the MHD generator. The proposed device presents some advantages with respect to other concepts, particularly, because it has no moving parts and, since there is no contact resistance between electrodes and the liquid metal, it is possible to adjust the inductance of the coil and adapt the electrical current produced. An analytical one-dimensional model is developed that allows to characterize the behavior of the generator under different operation conditions. I consider that the paper is a valuable and original contribution in the field of MHD generation and deserves to be published in the Sustainability journal. However, I have some suggestions that the authors may consider to clarify some points in the manuscript:

-The role of mechanical friction and electromagnetic braking should be clarified. If I understand correctly, the friction coefficient K is related only with mechanical friction originated mainly in the boundary layer.  The authors mention that “the friction term can be assimilated to an electromagnetic force with an equivalent current...” Can this be understood as a “virtual electromagnetic friction” just for modeling purposes? What is the role of the electromagnetic braking force on the fluid? How does this force affect the performance of the device?

-As the proposed device avoid the presence of very high currents and low voltage as in systems where electrodes are present, it would be interesting to mention the range of variation of electrical currents and voltages delivered by the device.

-A comment about the stability of the system would be welcome. Are there particular values of the inductance and capacitance that may alter the stability of the system, for instance, that lead to exceeding the critical velocity?

-The authors mention that the explored conditions are not optimal, particularly the size of the device. Is it possible to use the theoretical model to determine the optimal size of the prototype?

-Although the main objective of the authors is to apply the proposed device for deep space exploration, they mention that it is also suitable for the use of renewable energies. Therefore, it could be interesting to mention what kind of technologies could be coupled with the thermoacoustic MHD generator, for instance, solar technologies. In the past, it was proposed the use of these devices with parabolic solar collectors. Is that a suitable option for the proposed device or is there another suggestion?

Author Response

The authors would like to thank the reviewers for their important contribution to the improvement of the paper through their valuable suggestions. 

Author Response File: Author Response.pdf

Reviewer 2 Report

The article is aimed to propose a mathematical model for MHD energy generator coupled with thermoacoustic engine, that feed from radio isotopic power element. The space trips are appointed as the main purpose for application. The model is presented and fully justified. The critical analysis of the results is made fairly and complete enough. 

In the same time, it must be noted that the authors have used the complete laminar flow approximation. This position allows them to apply the linear differential equations for the system exploration. However, I did not meet any proof of the allowance of this approximation. Moreover, it is definitely known that the flow between parallel plates can be unstable for the wide range of Reinolds and Hartman values. It seems to be relevant for the article to appoint that the considered system is laying in laminar area and to justify it. 

It seems to me that the citations of the recent papers concerning MHD generators and flows a little bit poor. In spite of the basic used ideas were cited properly, but there are a wide range of recent researches in this field connected to the subject.

The manuscript sounds scientifically and it is designed properly. The results are reproducible with respect of the notices made by authors in the Results section (concerning skin effect) and the stability reasons mentioned above.

The conclusions and discussion are clear and based on the content of the manuscript. 

Citation 9 seems to me incomplete: may be the access time should be appointed and the title of the site (not only the page).

Author Response

The authors would like to thank the reviewers for their important contribution to the improvement of the paper through their valuable suggestions

Author Response File: Author Response.pdf

Reviewer 3 Report

Good article. Some spelling errors should be corrected. For example in lines 368 to 371.

Author Response

The authors would like to thank the reviewers for their important contribution to the improvement of the paper through their valuable suggestions

Author Response File: Author Response.pdf