Feasibility Study of the Bare-Photovoltaic-Tether Concept: Prototypes and Experimental Performance Evaluation of the Photovoltaic Tether Segment

Round 1

Reviewer 1 Report

This paper explores the feasibility of the Bare-Photovoltaic-Tether (BPT) concept for deorbit devices by combining the bare tether electron collection with a tether segment coated with thin-film Copper Indium Gallium Selenide solar cells to harvest additional power for the cathodic contact. The authors present the prototype of the photovoltaic tether segment and then assess the suitability of the concept for a Low Earth Orbit (LEO) environment. Overall, the paper presents valuable insights into the feasibility of the BPT concept for deorbit devices and provides a foundation for further research into overcoming the challenges associated with the BPT concept.

1. In Figure 7, the authors seem to forget to put the units.

2. In Figure 10, please remove the brackets because you are using dimensionless parameters.

3. Line 441. What do you mean by "A significant increase in the current of the submodules was observed, while the voltage remained unchanged"? You referred to Table 5 here, but no Table 5 was shown in your manuscript. Only two Table 4. Please revise it.

4. Figure 19, please remove the brackets because you are using dimensionless parameters. I found too many such mistakes in your manuscript. Please revise them throughout the paper.

Author Response

A major revision of the language was made.

 

Comment 1:

In Figure 7, the authors seem to forget to put the units.

Answer: The axes of the plot indicate the dimensionless position of the measurement points. The 8 cm x 8 cm area was divided in 64 equidistant squares. The intensity measurements were performed at the center of each square. We agree that using square brackets without units [ ] is confusing. The square brackets in Figure 7 were removed and a sentence, indicating the meaning of the axes, was added to its caption (Line  340).

Comment 2:

In Figure 10, please remove the brackets because you are using dimensionless parameters.

Answer: Figure 10 was removed from the article, due to the feedback of another reviewer.

Comment 3:

Line 441. What do you mean by "A significant increase in the current of the submodules was observed, while the voltage remained unchanged"? You referred to Table 5 here, but no Table 5 was shown in your manuscript. Only two Table 4. Please revise it.

Answer: Table 5 is now correctly labeled. In addition, we referenced the numbers from the table in the text to underline and make clear the statement of the “significant increase in the current” (Line 502). We also changed the electrical terms to “short circuit current” and “open circuit voltage” to be in line with Table 5.

Comment 4

Figure 19, please remove the brackets because you are using dimensionless parameters. I found too many such mistakes in your manuscript. Please revise them throughout the paper.

Answer: This figure is now labeled as Figure 18. We removed the square brackets in this figure. (Line 625)

Reviewer 2 Report

Comments for author File: Comments.pdf

Author Response

Comment 1:

Discussion in Page 4, lines 138-142, is (maybe) too lengthy: it can be understood that twisting can lead to average performance without discussing in detail (quantitatively and qualitatively).

Answer: We changed the wording to be less lengthy and repetitive (Line 158).

 

Comment 2:

Page 6, line 204: “due to the creation of rejects” would require a further explanation.

Answer: We added an example of the reject rate to the text, also defining the criteria of “reject” in this context: a significant deviation of either short circuit current or open circuit voltage from the envisaged values. (Line 227)

 

Comment 3:

Page 11, line 344, for the second time, it is said that: “lead to a shading of 50 % in any case”, and in the previous line: “the twisting (...) is expected to be evenly distributed”. The reviewer is not sure whether this speculative discussion helps to explain the complexity of the required analysis for simulating (expected) actual operational conditions.

Answer: We agree that this section is speculative and oversimplifies the actual conditions. We therefore deleted the corresponding sentences. We now use more tentative language to express the assumption of a evenly twisted tether: “If the surface of the PTS is assumed to be uniformly twisted..” (Line: 379)

 

Comment 4:

Figure 10 is maybe not needed (representation of cosine function at different frequencies). In the text, the references to Fig. 10 are just explanatory.

Answer: We agree, that Figure 10 is not crucial for the article and it’s sufficiently described in the text. We therefore removed Figure 10.

 

Comment 5:

In Page 13, line 401, it is said that “the offset (...) is estimated to be + 5 K” and it is explained why from a qualitative, but not quantitative point of view.

Answer: The quantitative evaluation of this offset was difficult due to the radiatively dominated environment under the Solar Simulator. A basic experiment was performed, measuring the temperature on the front side and the backside of the PTS, which gave a difference of + 2 K. The thermocouple on the front side was applied using a 3M Black adhesive tape. Since this tape does not have the same thermal radiative properties (α/ε = 1.05) as the CIGS cells (α/ε = 3), the measurement was highly distorted. We tried to satisfy this by multiplying the measured difference by a factor of 2.5, which gave +5 K. We added a paragraph explaining this approach. (Line 448)

 

Comment 6:

In Table 4 (Page 14), the second column (σ) is not defined. There are two Table 4 (in the second the standard deviation is defined).

Answer: The definition of the standard deviation was added toTable 4 (Line 478). Table 5 is now labelled correctly (Line 509).

 

Comment 7:

It is worth discussing is the atomic oxygen fluence is representative for operating conditions in Section 3.2,Page 15, line 460.

 

Answer: The atomic fluence was oriented towards the fluence of the 100 days E.T.PACK in orbit demonstration mission, which was calculated using BETsMA v2.0. We added a sentence and the corresponding references (Line 489).

 

Comment 8:

In Page 20, line 630, it is said that the “maximum angle of light incidence available (...) varies during an orbital period and over the year”. The sentence is correct and consistent with the previous explanation. However, the sentence suggests that the main periods to be taken into account are the orbital period and the solar year, whereas the relevant frequencies are the orbital and the one related to the J2 effect on the drift of the orbital plane.

Answer: We changed the relevant section and implemented the reviewer’s comment on the J2 perturbations. (Line 524)

Reviewer 3 Report

It would be reasonable to add estimation of lifetime, power in the begining and in the end of life and mass of typical SC electric power system based on Photovoltaic Tether Segment.

Author Response

Comment 1:

It would be reasonable to add estimation of lifetime, power in the beginning and in the end of life and mass of typical SC electric power system based on Photovoltaic Tether Segment.

Answer:

Begin of Life (BOL) and End of Life (EOL) estimated power

For a deorbit kit based on Electrodynamic Tethers (EDT), only a short operation time is required. (e.g. E.T.PACK Fly: 100 days). Before operation, the EDT and the PTS are stored inside the spacecraft, which provides complete protection from UV-radiation and mediocre protection from atomic oxygen.

As we show in our study, Atomic Oxygen is a problem for the silver contacts of the PTS, which can be replaced by copper contacts or protected by coatings, which also improve the thermal behavior of the PTS. These coatings, however, are likely to suffer a loss in transparency once they are exposed to the ionizing UV-radiation and particle radiation in space.

As observed onboard the Tsubasa MDS-1 satellite, CIGS solar cells have a very high robustness against high energy particle radiation. This is explained by a thermal annealing process, in which the CIGS cells are degraded and regenerated simultaneously in space.

We therefore conclude that in the frame of a deorbit mission (100 days), there won’t be a significant degradation of power over time (below 10 %). However, using the PTS in an EDT-System for station keeping or collision avoidance requires longer operation periods (up to 20 years), in which a significant power decrease is expected. As the present study was oriented towards a deorbit device and due to the lack of long-term degradation data, the authors think that this discussion is out of the scope of the article. But it is important and must be considered in the future.

We added a paragraph, covering these thoughts (Line 713).

Mass of the Electrical Power System (EPS) based on the Photovoltaic Tether Segment (PTS)

The idea is to connect the PTS directly to the existing EPS of the spacecraft, so that no additional mass is required for power processing and distribution. However, this is only feasible as long as the EPS already features solar panels (for the required maximum power point tracking) and as long as the PTS power is not too high. We added a sentence, discussing this point. (Line 669)

 

For the mass of the PTS itself, the measured value of 3.3 g/m is provided (Line 664). It has to be noted, that this number only refers to the additional mass of the photovoltaic stack on top of the aluminum tether. The aluminum tether itself, which is necessary in any case, has a mass per length of 2.7 g/m, so that the overall mass of the PTS is 6 g/m (Line 196).

Round 2

Reviewer 1 Report

I agree to accept this paper.