Hybrid Cooling Saves Water, Reduces Energy Use
Demands for evaporative cooling equipment that uses less water and energy influence the development of hybrid cooling technologies.
Several global trends are contributing to the desire for cooling system solutions that can help companies use less energy and water for cooling. Heavy flooding in some regions, drought conditions in others, and extreme temperatures and weather events help drive the demand for such solutions. In addition, population increases — due to migration from rural areas to cities as well as emerging markets — drive water demand. Many companies, especially in California and the western United States, seek solutions that reduce water demands while maintaining the benefits of evaporative cooling equipment.
Because energy and water production are interrelated, it is important to select equipment that appropriately balances energy and water usage. Although air-cooled and water-cooled systems each have their tradeoffs, engineers often design water-cooled systems for lower energy consumption and a smaller footprint than air-cooled equipment. Water-cooled systems continue to free power grids from excessive strain while reducing water usage at power plants. Some applications, however, can benefit from additional flexibility to balance energy and water savings.
The hybrid heat transfer technology is intended for applications where water is scarce or expensive. One example of hybrid technology uses different operation modes to reduce water and energy consumption.
Hybrid cooler technology can help achieve energy efficiency in addition to providing water savings. The combination of a high dry switch-point and redundancy features contributes to consistent year-round operation. Features such as a crossflow design help reduce maintenance and operating costs while providing layout flexibility.
Comparing Water and Energy Use in Air- and Water-Cooled Systems
There are two types of cooling systems: air cooled and water cooled. The difference between these two systems is related to design temperatures.
Air-cooled systems are designed based on the ambient dry-bulb temperature; by contrast, water-cooled systems are designed based on the ambient wet-bulb temperature. For example, the design wet-bulb temperature in Baltimore is typically 78°F (26°C), which is significantly lower than the design dry-bulb temperature of 95°F (35°C). Because water-cooled systems are designed around this lower temperature, water-cooled equipment has a smaller footprint and uses significantly less energy than air-cooled systems.
Energy savings and footprint reduction vary due to several factors, including the amount of heat to be rejected, geography and jobsite conditions. It is, therefore, important to understand application needs and jobsite constraints in order to select the system type for your application.
Figure 1. The hybrid cooling system can operate in three modes: combined wet/dry, adiabatic and dry.
Hybrid Approach Optimizes Water and Energy Savings
For those applications that can benefit from more closely controlling resource utilization, companies look for a balance between energy use and water consumption, especially if access to water is limited. Hybrid heat transfer technology offers an option for applications — for instance, manufacturing, process, industrial, power or data centers — where water is scarce or expensive.
Hybrid cooling technology can offer both energy efficiency and water savings. The hybrid approach involves using different operation modes to reduce water and energy consumption. The option to use dry operation, and the crossflow design, can reduce maintenance costs compared to conventional evaporative fluid coolers. A key part of the maintenance cost savings is reduced chemical costs because the equipment can operate dry throughout much of the year.
Figure 2. A higher dry switch-point minimizes water and chemical use.
One hybrid cooler with a high dry switch-point uses significantly less water than other evaporative cooling equipment designs. Providing energy-efficient cooling while maximizing water savings, the hybrid system also can use less energy than air-cooled equipment.
The hybrid design includes two coil sections: a prime surface coil section and a finned coil section. These heat transfer sections operate in three modes:
- Combined wet/dry.
This means that users can balance water and energy savings based on specific application needs.
With the hybrid cooling system, the combined wet/dry mode offers the most efficient heat transfer method and flow control. Initial cooling occurs in the finned coil as air passes over the finned coil and precools the internal fluid. As spray water is distributed over the prime surface coil, primary evaporative cooling occurs. Using both sensible and evaporative heat transfer, the combined mode results in water savings compared to conventional evaporative units. (The sensible cooling of air is the process in which only the sensible heat of the air is removed so as to reduce its temperature, and there is no change in the air’s moisture content.) The combined wet/dry mode reduces water use by around 25 percent water. The dry finned coil is always sensibly cooling approximately 25 percent of the heat load.
Adiabatic mode occurs when the process fluid flows through the finned coil only and bypasses the evaporative prime surface coil. Water continues to evaporate over the fill, precooling the incoming outside air before it reaches the finned coil. This greatly increases the rate of sensible heat transfer. With reduced water consumption while maintaining the low fluid design temperatures required to maximize system efficiency, adiabatic mode requires up to 75 percent less water.
Operating in adiabatic mode has another benefit. It reduces the visible plume that occurs when the air is fully saturated, especially in colder and damper conditions. Many operators want to reduce or eliminate the plume for aesthetic reasons.
With the dry mode, both coils are used to maximize the heat transfer surface area. The spray water pump is turned off, and no water is consumed. The fluid to be cooled passes from the finned coil into the prime surface coil, maximizing the heat transfer surface available. In dry mode, plume is eliminated. The unit runs dry to get the greatest water savings possible, and it can achieve higher dry switch-points when running at partial load. In the dry mode, all heat is rejected sensibly.
Figure 1 illustrates the three available operation modes. Figure 2 shows an example of how the hybrid technology design can save water compared to finned-coil products. The hybrid technology has a lower operating weight than the traditional model but an 11.4°F higher dry switch-point. The higher dry switch-point minimizes water and chemical use.
Figure 3. The combined flow design fully wets the coil tubes in the hybrid cooler.
Maintenance Costs and Uptime
Hybrid cooling technology can also reduce maintenance costs. A crossflow design eases access to key components such as the coil, motors, fill and spray branches, and water-collection basin. In one hybrid cooling system design, the spray-distribution system and coil sections are accessible and can be quickly inspected during operation. The combined-flow design allows for evaporation in two areas:
- Prime Surface Coil. Water flows in the same path as the air along with evaporation, fully wetting the tubes and helping to prevent scale buildup.
- Fill Section. Up to 70 percent of the evaporation occurs in the fill section.
The fully wetted tubes and reduced evaporation over the coil section help ensure higher system uptime. Figure 3 illustrates how the combined flow design fully wets the coil tubes when compared to conventional counterflow designs that can leave dry spots and result in scale buildup.
The hybrid technology offers a compact footprint that can be a factor in achieving layout flexibility (figure 4). In this example, both units are 12’ by 12’ with a total fan horsepower of 40 hp. A typical counterflow design requires clearance on all four sides due to the induced-draft design. By contrast, the hybrid design has a single-side air inlet; in addition, 30 percent of the air enters from above the wet coil. This means the hybrid design requires only one side of clearance.
Water consumption in cooling systems is, in some ways, industry specific. For example, data centers use massive amounts of energy and often select evaporative cooling because they want to reduce their energy costs. At the same time, they also require redundancy in the event of water loss or siting requirements that demand accommodating water restrictions.
Figure 4. Hybrid technology offers a compact footprint, which can be a factor in installations requiring layout flexibility.
One hybrid cooler design recently was been selected for a large liquefied natural gas facility and its process cooling loop. The main drivers for use of the hybrid cooler on this project were plume abatement and water savings. Additionally, due to the remote location of the facility, the hybrid cooler’s modular design allowed for minimized site labor and less installation time compared to field-erected equipment.
In conclusion, conventional evaporative heat rejection remains the go-to solution for low system energy use and reduced footprint. As demand grows for evaporative cooling equipment with operating flexibility, however, hybrid technologies offer equipment options to balance energy and water use based on specific application needs. To select evaporative cooling technology for a specific site, the equipment buyer should consider geography, application, jurisdiction and regulations. Taking such concerns into account will help ensure they select the right cooling system for their projects. PC