Techno-Economic Analysis of Waste Heat Utilization in Data Centers: Application of Absorption Chiller Systems

Overview

Recovering the decentralised, low-quality waste heat of data centres is hard because of many technological and economic hurdles, and absorption chiller systems are a promising technology for waste-heat recovery at ultra-low temperatures. The authors derive closed-form equations for total cooling, hybrid electricity use and percentage savings in terms of compression and absorption chiller COPs. Hybrid electricity saving is given as 1 - COP_CC/(COP_CC + COP_AC × (1 + COP_CC)), expressed in %.

Calculations assumed at most 15 kW of waste heat per rack, water as coolant at 0.3 to 0.6 l/s, and a design outlet water temperature of 70 °C (below the typical 85 °C because of intermediate heat-exchanger losses). The equations were applied to 4.5 MW and 13.5 MW data centres. The system saves 4,340,000 kWh/year in the 4.5 MW data centre and 13,025,000 kWh/year in the 13.5 MW data centre.

These savings correspond to annual reductions of 3068 and 9208 tons CO2-eq respectively. Simple payback is estimated at 2.76 years for the 4.5 MW data centre and 2.56 years for the 13.5 MW one. On-site absorption cooling is thus a quick-payback reuse path when rack water leaves at around 70 °C.

Limits: the analysis rests on liquid-cooled racks and 70 °C outlet water and does not apply to air-cooled halls; simple payback is a basic economic measure, not detailed financial modelling.

Key points

1. Hybrid electricity saving is given in closed form as 1 - COP_CC/(COP_CC + COP_AC × (1 + COP_CC)), in %.
2. Calculations assumed at most 15 kW of waste heat per rack, water as coolant at 0.3 to 0.6 l/s, and a design outlet water temperature of 70 °C (below the typical 85 °C because of intermediate heat-exchanger losses).
3. The system saves 4,340,000 kWh/year in the 4.5 MW data centre and 13,025,000 kWh/year in the 13.5 MW data centre.
4. These savings correspond to annual reductions of 3068 and 9208 tons CO2-eq respectively.
5. Simple payback is estimated at 2.76 years for the 4.5 MW data centre and 2.56 years for the 13.5 MW one.

Using it in design

• Evaluate an absorption chiller driven by roughly 70 °C water from liquid-cooled racks to cut compression chiller load; calculate the hybrid saving from the two COPs with the closed-form formula.
• Make waste heat per rack (at most 15 kW), water flow (0.3-0.6 l/s) and outlet water temperature (70 °C, with intermediate heat-exchanger losses) design inputs.
• Report payback as a simple measure (2.56-2.76 years) and verify with a detailed financial model.

Who should read it

Engineers assessing absorption cooling from waste heat in liquid-cooled data centres.

Document contents

1. 1. Introduction
2. 2. Methodology (heat recovery and absorption chiller equations)
3. 3. Results and Discussion (3.1 Energy Performance: 4.5 MW and 13.5 MW, 3.2 Economic Performance, 3.3 Environmental Performance)
4. 4. Conclusions

Related standards

• ASHRAE TC 9.9 thermal guidelines (A1-A4 classes)

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Publication details

Authors

Leyla Amiri; Edris Madadian; Navid Bahrani; Seyed Ali Ghoreishi-Madiseh

Publisher

Energies (MDPI AG)

Year

2021

Volume

14

Issue

9

Pages

2433

Document language

English

Source type

Scientific

Access

Open licence — downloadable

DOI

10.3390/en14092433

Licence

CC BY 4.0 — share with attribution

Licence evidence

www.mdpi.com

Citation