Free Cooling for Saving Energy: Technical Market Analysis of Dry, Wet, and Hybrid Cooling Based on Manufacturer Data

Overview

In light of energy and climate targets, free cooling unlocks a major resource-saving potential compared with refrigeration; to quantify it, the study specifies the physical and technical limits of cooling tower applications and provides comprehensive data on electricity and water consumption. Six types of package-type cooling towers are distinguished: dry, closed wet, open wet and three hybrid systems (wetted, sprayed, wetting-mat). One generalised system is defined for all types to enable comparability, and data were collected from the data sheets of 6730 system models of 27 manufacturers.

Median approach is 10 K to ambient dry-bulb for dry coolers, 5 K to wet-bulb for open wet towers and 6.5 K to wet-bulb for closed-circuit wet towers; wet cooling also needs about a 5 K safety gap from freezing. All tower types show a typical coolant range of at least 5 K, so tower free cooling is mostly not feasible if the outlet must be less than 5 K above the inlet.

Specific electricity demand ranges from 0.01 to 0.06 kWel/kWth across types; open wet towers are lowest at about 0.012-0.021 kWel/kWth, and refrigeration would use roughly ten times more electricity per unit cooling. At 32/27/21 °C, interquartile specific water consumption was 3.32-4.60 l/h/kWth for sprayed hybrid and 2.06-2.69 l/h/kWth for wetting-mat hybrid coolers; dry coolers use 0. For Stuttgart, with a required coolant temperature of 25 °C, dry cooling covers about 72.8% of annual hours versus 98.0-99.6% for wet and hybrid cooling; at 20 °C, 52.2% versus 79.2-87.9%. Minimum outlet temperatures with evaporative cooling are up to 16 K lower than with dry cooling.

Limits: data come from manufacturer data sheets, not measured field performance, and values at standard operating points can differ from real operating conditions. The study gives benchmark data for modelling dry versus evaporative heat rejection in data centres and industry.

Key points

1. Median approach is 10 K to ambient dry-bulb for dry coolers, 5 K to wet-bulb for open wet towers and 6.5 K to wet-bulb for closed-circuit wet towers; wet cooling also needs about a 5 K safety gap from freezing.
2. All tower types show a typical coolant range of at least 5 K, so tower free cooling is mostly not feasible if the outlet must be less than 5 K above the inlet.
3. Specific electricity demand ranges from 0.01 to 0.06 kWel/kWth; open wet towers are lowest at about 0.012-0.021, and refrigeration would use roughly ten times more electricity per unit cooling.
4. At 32/27/21 °C, interquartile specific water consumption was 3.32-4.60 l/h/kWth for sprayed hybrid and 2.06-2.69 l/h/kWth for wetting-mat hybrid; dry coolers use 0.
5. For Stuttgart at a required coolant temperature of 25 °C, dry cooling covers about 72.8% of annual hours versus 98.0-99.6% for wet and hybrid; at 20 °C, 52.2% versus 79.2-87.9%.
6. Minimum outlet temperatures with evaporative cooling are up to 16 K lower than with dry cooling.

Using it in design

• Check free-cooling feasibility from the required coolant outlet temperature, the tower approach (dry 10 K, open wet 5 K, closed wet 6.5 K) and the at-least-5 K coolant range.
• When choosing between dry and wet heat rejection, compare annual free-cooling hours (72.8% versus 98-99.6% at 25 °C) together with specific water consumption (0 versus 2-4.6 l/h/kWth).
• Use free cooling's electricity advantage (0.012-0.06 kWel/kWth, roughly one tenth of refrigeration) as an input in the energy model.

Who should read it

Cooling designers and energy modellers selecting dry, wet and hybrid heat-rejection equipment.

Document contents

1. 1. Introduction (1.1 Previous Research, 1.2 Study's Objective)
2. 2. Materials and Methods (2.1 System Definition, 2.2 Data Acquisition, 2.3 Statistical Evaluation)
3. 3. Results (3.1 Limits of Cooling Tower Use, 3.2 Electricity and Water Consumption, 3.3 Exemplary Application)
4. 4. Discussion
5. 5. Conclusions and Outlook

Read the document

Publication details

Authors

Paula M. Wenzel; Marc Mühlen; Peter Radgen

Publisher

Energies (MDPI AG)

Year

2023

Volume

16

Issue

9

Pages

3661

Document language

English

Source type

Scientific

Access

Open licence — downloadable

DOI

10.3390/en16093661

Licence

CC BY 4.0 — share with attribution

Licence evidence

www.mdpi.com

Citation