Cooling System Concepts
In a data center, the HVAC system energy consumption is dependent on three main factors: outdoor conditions (temperature and humidity), the use of economization strategies, and the primary type of cooling consider the following:
- The HVAC energy consumption is closely related to the outdoor temperature and humidity levels. In simple terms, the HVAC equipment takes the heat from the data center and transfers it outdoors. The higher the outdoor air temperature (and the higher the humidity level is for water-cooled systems), more work is required of the compressors to lower the air temperature back down to the required levels in the data center.
- Economization for HVAC systems is a process in which the outdoor conditions allow for reduced compressor power (or even allowing for complete shutdown of the compressors). This is achieved by supplying outdoor air directly to the data center (direct air economizer) or, as in water-cooled systems, cooling the water and then using the cool water in place of chilled water that would normally be created using compressors.
- Different HVAC system types have different levels of energy consumption. And the different types of systems will perform differently in different climates. As an example, in hot and dry climates water-cooled equipment generally consumes less energy than air-cooled systems. Conversely, in cooler climates that have higher moisture levels, air-cooled equipment will use less energy. The maintenance and operation of the systems will also impact energy (possibly the greatest impact). Related to the cooling system type, the supply air temperature and allowable humidity levels in the data center will have an influence on the annual energy consumption.
Cooling System Comparison Tables for HVAC Professionals
Chilled water plants include chillers (either air- or water-cooled) and cooling towers (if water-cooled). These types of cooling plants are complex in design and operation but can yield superior energy efficiency. Some of the current, highly efficient water-cooled chillers offer power usage that can be 50% less than legacy models.
Broadly speaking, cooling systems will connect to a central cooling plant that generates chilled water or condenser water for use in the remote air-handling units or CRAHs. The decision to use a central plant can be made for many different reasons: facility size, growth plans, efficiency reliability, and redundancy, among others. Generally, a central plant consists of primary equipment such as chillers and cooling towers, piping, pumps, heat exchangers, and water treatment systems. Typically, central plants are used for large data centers and have the capability for future expansion.
Cooling System Types Comparison
| System Type | Energy Efficiency | Climate Suitability | Complexity | Maintenance | Capital Cost |
|---|---|---|---|---|---|
| Water-Cooled Central Plant | Very High | Hot & Dry Climates | High | High | High |
| Air-Cooled Central Plant | Moderate | Cool & Humid Climates | Moderate | Moderate | Moderate-High |
| Direct Expansion (DX) | Lower | All Climates | Low | Low | Low-Moderate |
| Evaporative Cooling | High | Hot & Dry Climates | Moderate | Moderate | Moderate |
| Water-Cooled Servers | Very High | Any Climate | High | High | Very High |
Economizer Types and Applications
Indirect economization is used when it is not advantageous to use air directly from the outdoors for economization. Indirect economization uses the same control principles as the direct outdoor air systems. In direct systems, the outdoor air is used to cool the return air by physically mixing the two airstreams. When indirect economization is used, the outdoor air is used to cool down a heat exchanger on one side that indirectly cools the return air on the other side with no contact of the two airstreams. In indirect evaporative systems, water is sprayed on a portion of the heat exchanger where the outdoor air runs through. The evaporative effect lowers the temperature of the heat exchanger, thereby reducing the temperature of the outdoor air. These systems are very effective in a number of climates, even humid climates. Since an indirect heat exchanger is used, a fan is required to draw the outside air across the heat exchanger, sometimes known as a scavenger fan. This fan motor power is not trivial and needs to be accounted for in estimating energy use.
Economization Options
- There are several different approaches and technology available when designing an economization system. For indirect economizer designs, heat exchanger technology varies widely.
- It can consist of a rotary heat exchanger, also known as a heat wheel, which uses thermal mass to cool down the return air by using outdoor air.
- Another approach is to use a cross-flow heat exchanger.
- Heat pipe technology can also be incorporated in an indirect economization strategy.
| Economizer Type | Operating Principle | Efficiency Gain | Best Climate Applications | Limitations |
|---|---|---|---|---|
| Direct Air | Uses outside air directly | 25-40% | Temperate, Low Humidity | Air Quality, Humidity Control |
| Indirect Air | Heat exchange without mixing airstreams | 15-30% | Wider Range of Climates | Additional Fan Power Required |
| Water-Side | Cooling tower water for free cooling | 20-35% | Moderate to Cool Climates | Water Treatment Needs |
| Rotary Heat Exchanger | Thermal mass transfer via wheel | 60-80% effectiveness | All Climates | Moving Parts, Maintenance |
| Heat Pipe | Passive heat transfer | 45-65% effectiveness | All Climates | Fixed Capacity |
| Cross-Flow Heat Exchanger | Air-to-air heat transfer | 50-70% effectiveness | All Climates | Space Requirements |
HVAC Energy Consumption Factors
| Factor | Impact Level | Optimization Strategy | Potential Savings |
|---|---|---|---|
| Outdoor Temperature | High | Economization, Night Cooling | 15-35% |
| Outdoor Humidity | Medium-High | Indirect Cooling in Humid Areas | 10-25% |
| Supply Air Temperature | High | Raise Set Points Where Possible | 2-4% per °C increase |
| Economization Strategy | Very High | Climate-Appropriate Selection | 20-50% |
| Equipment Efficiency | High | Modern High-Efficiency Equipment | 30-50% vs. Legacy |
| Maintenance | High | Regular Preventive Maintenance | 10-30% |
Evaporative Cooling Performance
Evaporative cooling uses the principle that when air is exposed to water spray, the dry-bulb temperature of the air will be reduced to a level that is close to the wet-bulb temperature of the air. The difference between the air’s dry bulb and wet bulb is known as the wet-bulb depression. In climates that are dry, evaporative cooling works well, because the wet-bulb depression is large enabling the evaporative process to lower the dry-bulb temperature significantly. Evaporative cooling can be used in conjunction with any of the cooling techniques outlined earlier.
| Climate Type | Wet-Bulb Depression | Cooling Effectiveness | Energy Savings vs. Conventional |
|---|---|---|---|
| Hot & Dry (e.g., Phoenix) | 11-22°C | 70-90% | 60-80% |
| Hot & Humid (e.g., Miami) | 1-5°C | 20-40% | 5-15% |
| Temperate (e.g., San Francisco) | 6-10°C | 50-65% | 30-50% |
| Cold & Dry (e.g., Denver Winter) | 8-15°C | 60-75% | 40-60% |
| Cold & Humid (e.g., Chicago Winter) | 1-3°C | 15-30% | 10-20% |
Water Economization Temperature Ranges
Direct economization typically means the use of outside air directly without the use of heat exchangers. Direct outside air economizer systems will mix the outdoor air with the return air to maintain the required supply air temperature. At outdoor air temperatures that range from that of the supply air temperature to that of the return air temperature, partial economization is achievable, but supplemental mechanical cooling is necessary. Evaporative cooling can be used at this point to extend the ability to use outside air by reducing the dry-bulb temperature, especially in drier climates. Once the supply air temperature can no longer be maintained, mechanical cooling will start and begin to cool the load. After the outdoor dry-bulb and moisture levels reach acceptable limits, the supplemental cooling equipment will stop and the outdoor air dampers will open to maintain the temperature. For many climates, it is possible to run direct air economization year round with little or no supplemental cooling. There are climates where the outdoor dry-bulb temperature is suitable for economization but the outdoor moisture level is too high. In this case, a control strategy must be in place to take advantage of the acceptable dry-bulb temperature without risking condensation in the data center or unintentionally incurring higher energy costs.
| Application | Water Supply Temp | Economizer Activation Point | Annual Economizer Hours (Temperate Climate) |
|---|---|---|---|
| Traditional CRAH Units | 7-12°C | <18°C Wet-Bulb | 3,000-5,000 |
| Elevated Temperature CRAH | 15-20°C | <24°C Wet-Bulb | 5,000-7,000 |
| Water-Cooled Servers | 25-35°C | <32°C Wet-Bulb | 7,000-8,500 |
| Rear Door Heat Exchangers | 18-22°C | <27°C Wet-Bulb | 5,500-7,500 |
