KTH ROYAL INSTITUTE

OF TECHNOLOGY

Hybrid system for free cooling from

compressor refrigeration systems Björn Palm

Overview

• Introduction

• The problem to be solved

• The possible solutions

• Examples of similar solutions

• Current status

• Continued work

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Basic facts about the project

• Time period: 2015 08 01 – 2018 06 30

• Total Budget: 850 000 sek

• Main applicant: Björn Palm

• Organization: KTH/Energy Technology/Applied

thermodynamics and Refrigeration

• Co-financing company: Dantherm AS

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Typical example of application for Dantherm: Cooling of a shelter for electronics

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Possible cooling systems for mobile base stations

Vapor Compression system Thermosyphon Direct cooling loop

Existing AC system from Dantherm

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Existing thermosyphon cooler from Dantherm

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How to combine AC with thermosyphon?

Compressor cooling is often running even when the ambient

temperature is relatively low.

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Alternative 1: Bypass compressor and expansion valve

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Challenges:

• The optimum amount of refrigerant in the two systems (AC

and Thermosyphon) are different.

• Thermosyphon (evaporator) does not work very well when

oil is in the system (based on Dantherm experience).

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Alternative 2: Same as 1 but with pump circulation

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This method is already used by NTT in Japan with success

Alternative 3: Two separate loops, with common heat exchangers

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Pros and cons:

• No mixing of the refrigerants of the two systems

• The evaporator and condenser divided into two parts

• Two systems are working individually and all the previous

problems (oil in the system or different amount of

refrigerant) will be solved as a result

• Drawbacks: complicated piping and design

• This solution may be less expensive than the other two

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Examples from the literature

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Vapor compression + Direct cooling loop

Examples from the literature

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Vapor compression + Thermosyphon

Linjun Han, Wenxing Shi, Baolong Wang, Penglei Zhang, Xianting Li, Development of an integrated air

conditioner with thermosyphon and the application in mobile phone base station,

International journal of refrigeration 36 (2013) 58–69

• The refrigerant charge is presented as the ratio of total charge to the maximum charge of the evaporator at 20 C.

Dantherm experiments with combined systems

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© Dantherm A/S

Power consumption vs ambient temp for sample system from Dantherm

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Possible solutions for improvement of

a hybrid cooling system Using nano/micro porous structure evaporator:

Using a nano/micro porous structure on the surface of the evaporator of the thermosyphon can provide us two benefits.

• Increasing the boiling rate by increasing nucleate site density

• Starting optimum performance of the thermosyphon at higher ambient temperature (Δ𝑇𝑡𝑠 is shifted to the right)

Rahmatollah Khodabandeh, Richard Furberg, “Heat transfer, flow regime and instability of a nano- and micro-porous structure

evaporator in a two-phase thermosyphon loop”, International Journal of Thermal Sciences, International Journal of Thermal

Sciences 49 (2010) 1183-1192

Unit to be the base for the prototype

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Unit to be the base for the prototype

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Previous and ongoing activities

Patent review report (internal) finalized

Decision about layout of new prototype system taken

Components of system under selection

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Thank you for your attention!

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