How to Choose Energy Efficient Data Center Cooling

Choosing energy-efficient cooling for a data center cuts electricity costs and reduces carbon footprint while maintaining safe operating temperatures. This guide walks you through the key decisions—from assessing your heat load to selecting between air, liquid, and free cooling—so you can pick equipment that matches your actual needs.

What You’ll Need

• Current or projected IT load in kilowatts (kW)
• Facility floor plan and ceiling height
• Local climate data (average annual temperature and humidity)
• Existing power and cooling infrastructure details
• Budget for capital and operating expenses
• Understanding of your availability requirements (e.g., Tier level)

Step-by-Step

1. Calculate your actual heat load. The cooling capacity you need is roughly equal to your IT load (1 kW of IT power ≈ 1 kW of heat). Measure or estimate the total power draw of servers, storage, and network gear. Add about 10% for ancillary loads like lighting and people, but don’t oversize—oversized systems run inefficiently.
2. Determine your target inlet temperature. ASHRAE recommends server inlet temperatures between 18°C and 27°C (64°F–80°F) for most equipment. Higher inlet temperatures allow more efficient cooling and more hours of free cooling. Check your equipment’s manufacturer specs to confirm the allowable range.
3. Evaluate free cooling potential. Free cooling uses outside air or water to remove heat without compressors. If your climate is cool for a significant part of the year, consider air-side economizers (direct or indirect) or water-side economizers (cooling towers or dry coolers). Calculate the annual hours where outdoor conditions are below your target inlet temperature—this directly impacts operating cost.
4. Choose between room-based and row/rack-based cooling. For high-density racks (above 5–10 kW per rack), room-based CRAC units often cause hot spots and require excess airflow. Row-based or rack-based cooling (e.g., in-row coolers, rear-door heat exchangers) puts cooling closer to the heat source, improving efficiency and allowing higher inlet temperatures.
5. Compare air cooling vs. liquid cooling. Air cooling is simpler and cheaper to install, but liquid cooling (direct-to-chip or immersion) can handle much higher heat densities and reduces fan energy significantly. If your racks exceed 15–20 kW, liquid cooling may be the only energy-efficient option. For most traditional data centers, air cooling with hot/cold aisle containment is still effective.
6. Implement aisle containment. Whether you choose room or row cooling, physically separate hot and cold aisles using doors, curtains, or hard panels. This prevents mixing of hot exhaust with cold supply air, allowing you to raise the supply temperature and reduce fan speeds, cutting energy use by 10–30%.
7. Select efficient fans and compressors. Look for equipment with EC (electronically commutated) fans that vary speed based on demand, and variable-speed compressors or digital scroll compressors. These modulate output to match load, avoiding the wasteful on/off cycling of fixed-speed units.
8. Consider a control system with smart sensors. A building management system (BMS) or dedicated DCIM (data center infrastructure management) tool should monitor temperatures, humidity, and pressure, and adjust cooling dynamically. Look for features like setpoint optimization and fault detection.
9. Calculate total cost of ownership (TCO). Compare not just upfront price, but annual energy consumption (kWh), maintenance, and expected lifespan. Use the PUE (Power Usage Effectiveness) metric to estimate operational impact—a lower PUE means less energy wasted on cooling.
10. Plan for redundancy and scalability. Choose modular units that can be added as your load grows. Ensure that your cooling design meets your availability target without running all units at low load—redundant units should be able to run at efficient capacity when needed.

Common Mistakes

• Oversizing cooling equipment. Buying a 100-ton unit for a 50-ton load causes short cycling and poor humidity control, wasting energy.
• Ignoring local climate. Installing water-side economizers in a hot, humid region yields little benefit; air-side economizers in a dusty area require heavy filtration.
• Setting inlet temperatures too low. Keeping the room at 18°C when equipment allows 27°C wastes 10–20% of cooling energy.
• Neglecting airflow management. Without containment, cold air bypasses equipment and hot air recirculates, forcing lower supply temperatures.
• Forgetting about humidity. Over-humidification or dehumidification can consume as much energy as cooling itself. Most equipment tolerates a wide range (20–80% RH).

Tips

• Use ASHRAE’s thermal guidelines as a starting point, but verify with your specific hardware.
• Consider using a CFD (computational fluid dynamics) simulation to model airflow before purchasing equipment.
• Look for ENERGY STAR® rated CRAC/CRAH units where applicable (though most data center cooling equipment is not rated).
• Check if your utility offers rebates for energy-efficient cooling upgrades.
• Start with a pilot row to test new cooling technology before retrofitting the entire data center.

Troubleshooting

• Hot spots persist despite new cooling: Check for blocked perforated tiles, missing blanking panels, or unsealed cable openings. Use temperature sensors to identify recirculation paths.
• PUE does not improve as expected: Verify that your cooling setpoints are actually being used. Many systems default to conservative settings; adjust them gradually and monitor equipment inlet temperatures.
• Free cooling not working: Ensure the economizer controls are configured correctly and that outside air dampers or water valves are opening. Check for sensor errors.
• Humidity too high or low: If you are using air-side economizers, humidity swings are common. Consider a desiccant wheel or adjust the enthalpy switchover point.
• Liquid cooling leak: Use leak detection cables and pressure sensors. Have a shutoff procedure and keep spare connectors.

Frequently Asked Questions

Q: What is a good PUE for a data center?
A: A typical PUE is 1.5–2.0; efficient facilities can achieve 1.2–1.3 with free cooling and containment. Lower is better, but 1.0 is the theoretical minimum.

Q: Can I use outside air directly for cooling?
A: Yes, if your climate is cool and dry enough. Direct air economizers bring outside air into the data center, but they require filtration and humidity control. Indirect systems use a heat exchanger to avoid contamination.

Q: How do I know if liquid cooling is right for me?
A: If you have high-density racks (over 15–20 kW) or are building new, liquid cooling is more efficient and can reduce fan energy. For existing low-density facilities, air cooling with containment is usually more cost-effective.

Q: What is the difference between CRAC and CRAH units?
A: CRAC (Computer Room Air Conditioner) uses a compressor and refrigerant, while CRAH (Computer Room Air Handler) uses chilled water from a central plant. CRAH units are often more efficient when a chilled water system is already in place.

Q: How much energy can aisle containment save?
A: Typically 10–30% of cooling energy, depending on the current setup. It allows higher supply temperatures and reduces fan speeds.

Q: Should I set my cooling to follow the IT load?
A: Yes, using variable-speed fans and compressors that modulate with load is key. Running cooling at full capacity when the load is low wastes energy.

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