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.
MAJOR COOLING SYSTEM EQUIPMENT TYPES
Central cooling plants
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.
Water-cooled plant equipment
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.
Air-cooled plant equipment
Similar to the water-cooled chiller plant, the air-cooled chiller plant can be complex, yet efficient. Depending on the climate, the chiller will use more energy annually than a comparably sized water-cooled chiller. To minimize this, manufacturers offer economizer modules built into the chiller that uses the cold outside air to extract heat from the chilled water without using compressors. Dry coolers or evaporative coolers are also used to precool the return water back to the chiller.
Direct expansion (DX) equipment
DX systems have the least amount of moving parts since both the condenser and evaporator use air as the heat transfer medium not water. This reduces the complexity but it also can reduce the efficiency. A variation on this system is to water cool the condenser, which improves the efficiency. (Water-cooled CRAC units fall into this category.)
Evaporative Cooling Systems
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.
Water can be used for many purposes in cooling the data center. It can be chilled via a vapor-compression cycle and sent out to the terminal cooling equipment. It can also be cooled using an atmospheric cooling tower using the same principles of evaporation and used to cool compressors or, if it is cold enough, it can be sent directly to the terminal cooling devices. The goal of a water economization strategy is to use mechanical cooling as little as possible and rely on the outdoor air conditions to cool the water to what is required to generate the required supply air temperature. When the system is in economizer mode, only air-handling unit fans, chilled water pumps, and condenser water pumps will run. The energy required to run these pieces of equipment should be examined carefully to ensure the savings of using water economizer will not be diminished by excessively high motor energy consumption. Data centers that use water-cooled servers, such as in a high-performance computing facility, can use much warmer water due to the server’s ability to maintain internal temperatures using water that is at a much higher temperature than what is typically seen.
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.
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.
- 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.
Within these options, there are several sub-options driven by the specific application that ultimately will inform the design strategy for the entire cooling system.