Comparison between Air-Exposed and Underground Thermal Energy Storage for Solar Cooling Applications

Round 1

Reviewer 1 Report

The paper is interesting and I really would lie to recommend this article for publication. However, before that they need to address the following issues.

 

What would be the cost of digging the ground? Investment cost?

 

Line 45-46: “"thermal tanks" do not generate any negative environmental impact.” This statement could be a bit far-fetched. In some regions of the world (especially in cold regions) they could cause permafrost melting. Permafrost melting has a catastrophic environmental impact in these regions. These regions have especially long summers with lots of solar energy which makes sense for storage use in winter. As mentioned above permafrost melting is the side effect. The authors therefore should not generalize.

 

The literature review seems kind of stating some statements without being critical. For example lines 51 to 57:

“Some authors have studied the implementation of two thermal energy storage tanks [2–4] with the purpose of storing fluid at low temperature and feed it into the solar collectors to increase its energy content and store it in another high temperature tank.  They have also implemented storage with packed beds to minimize the thickness of the thermocline and separate the stratification zones without physical barriers [5–7]. Other authors have chosen to implement phase change materials because they offer high thermal storage density with moderate temperature variation [8,9].”

 

When you quote “Some authors have studied the implementation of two thermal energy storage tanks [2–4] with the purpose of storing fluid at low temperature and feed it into the solar collectors to increase its energy content and store it in another high temperature tank. ”

Questions arise: “Did they achieve it? Was it worth it, etc.” Could you comment on what they did? This would, in my view, enrich the discussion/presentation of the literature review.

How is the soil temperature shown in Fig. 3 measured?

 

Line 212-213: ““Type 41” was configured to turn off the absorption cooling system during weekends, holidays and vacations”

That is great! Which days are holidays? Which days are weekends?

Line 213: “The solar thermal energy collection and storage sub-system pump is turned off during vacations only” Why? I mean different days than Type 41.

 

Line 224-225: “The experimental data were measured with HOBO model TMCx-HD temperature sensors placed at a depth of 2 m in a vertical well drilled at the Puertecitos school.”

Fig. 2 shows soil temp at 0m and 1m as well. How were those measured?

Line 229: “The linear fit ..” I am really confused here. Linear fit? Fig. 5?

Minor thing: Equation numbers position is not consistent.

What is D in equation 1? Is that the storage cylinder diameter?

Fig. 7: The x-axis last number is 309, I think it should be 3,096

Author Response

Please see the attachment, thanks

Author Response File: Author Response.pdf

Reviewer 2 Report

Thank you for inviting me to review this paper. The paper points out an interesting concern on the novel methods for storing thermal energy – air exposed and underground thermal energy storage. Authors performed simulation studies for the proposed system assessment using TRNSYS, while the simulated results were calibrated against the measured data accordingly.

I will only have a minor concern on the solar thermal storage system proposed. Basically, it is not feasible to connect solar thermal collector or similar to the refrigeration cycle directly without accommodating a heat exchanger. Can authors explain this further?

Additionally, authors need to illustrate how to accommodate the underground tank and get it connected to the whole system.

Author Response

Please see the attachment, thanks.

Author Response File: Author Response.pdf

Round 2

Reviewer 1 Report

I recommend the paper for publication. Minor edits may be necessary, for example in the sentence below (line 321 "for" may need to be changed to "to".

“storage subsystem started operating at the weekend (2,929-2,976 h) in order to reach the required temperature (≈90 °C) for activate the absorption cooling system”.

I recommend the paper for publication. Minor edits may be necessary, for example in the sentence below (line 321 "for" may need to be changed to "to".

“storage subsystem started operating at the weekend (2,929-2,976 h) in order to reach the required temperature (≈90 °C) for activate the absorption cooling system”.