Analysis of Air-Side Economizers in Terms of Cooling-Energy Performance in a Data Center Considering Exhaust Air Recirculation

Overview

In data centres with high-temperature IT equipment, enormous cooling systems operate year-round; studies have aimed to improve the cooling efficiency of server-room units, but cooling-energy analysis considering exhaust air (EA) recirculation had not been researched sufficiently. This study analysed the cooling-energy saving effect considering EA recirculation and supply-air (SA) temperature conditions when direct and indirect air-side economisers were applied to a data centre in Korea; the existing system consumes 69,121 MWh/yr.

At 13.5 °C supply air with no recirculation, the direct airside economiser had 4536 h/yr of outdoor-air availability and 43.8% energy savings versus the existing system. Raising supply air to 18 °C and 22 °C raised direct-economiser outdoor-air hours to 5665 h and 6838 h, with energy savings of 54.9% and 67.2%. The indirect airside economiser (70% HX effectiveness) had 6988 h at 13.5 °C, 8206 h at 18 °C and 8707 h at 22 °C, with savings of 32.3%, 44.1% and 55.3%.

At 15% exhaust-air recirculation, annual cooling energy rose about 6.1% for the direct economiser and about 9% for the indirect economiser, and indirect-economiser savings fell from 32.3% to 26.3%. With a mixing-ratio approach on fixed outdoor-air intervals, supply-air temperature fell outside the ASHRAE recommended range, so the OA fraction must be controlled to the target supply condition. The authors recommend placing exhaust-air grilles so recirculation is below 5%, or handling it in the envelope plan at design stage.

Limits: a single Korean data centre and a dynamic simulation, so results depend on that climate and facility. Recirculation of hot exhaust cuts the hours when outdoor air can be used and erodes savings, while raising supply-air temperature greatly increases economiser hours.

Key points

1. At 13.5 °C supply air with no recirculation, the direct airside economiser had 4536 h/yr of outdoor-air availability and 43.8% energy savings versus the existing system (69,121 MWh/yr).
2. Raising supply air to 18 °C and 22 °C raised direct-economiser outdoor-air hours to 5665 h and 6838 h, with energy savings of 54.9% and 67.2%.
3. The indirect airside economiser (70% HX effectiveness) had 6988 h at 13.5 °C, 8206 h at 18 °C and 8707 h at 22 °C, with savings of 32.3%, 44.1% and 55.3%.
4. At 15% exhaust-air recirculation, annual cooling energy rose about 6.1% for the direct economiser and about 9% for the indirect economiser; indirect-economiser savings fell from 32.3% to 26.3%.
5. The authors recommend placing exhaust-air grilles so recirculation is below 5%, or handling it in the envelope plan at design stage.
6. With a mixing-ratio approach on fixed outdoor-air intervals, supply-air temperature fell outside the ASHRAE recommended range; the OA fraction must be controlled to the target supply condition.

Using it in design

• In airside economiser design, plan exhaust grilles and the building envelope so recirculation stays below 5%; 15% recirculation raised energy about 6.1% for direct and 9% for indirect.
• Consider raising supply-air temperature (13.5 to 22 °C) as the main lever for economiser hours and savings (43.8% to 67.2% for direct).
• Control the outdoor-air fraction to the target supply condition rather than splitting it into fixed intervals; otherwise supply temperature can leave the ASHRAE recommended range.

Who should read it

Cooling engineers designing airside economisers and assessing the energy effect of exhaust recirculation.

Document contents

1. 1. Introduction
2. 2. Cooling Load and Energy Consumption of a Conventional Data Center (2.1 Overview, 2.2 Boundary, 2.3 Devices: cooling tower, cooling coil, fan and pump components; 2.4 Cooling Load and Energy Consumption)
3. 3. Monthly Analysis and Case Study (3.1 Case Study, 3.2 Direct Air-Side Economizer, 3.3 Indirect Air-Side Economizer, 3.4 Cooling Energy Consumed by Air-Side Economizers)
4. 5. Results (annual analysis)
5. 4. Conclusions

Related standards

• ASHRAE TC9.9 Thermal Guidelines

Read the document

Publication details

Authors

Seonghyun Park; Janghoo Seo

Publisher

Energies (MDPI AG)

Year

2018

Volume

11

Issue

2

Pages

444

Document language

English

Source type

Scientific

Access

Open licence — downloadable

DOI

10.3390/en11020444

Licence

CC BY 4.0 — share with attribution

Licence evidence

www.mdpi.com

Citation