Energy Efficiency MeasuresDISTRICT COOLING - CENTRAL PLANTSDistrict cooling systems distribute chilled water to multiple buildings. Chilled water can be used for process cooling or heating and most commonly is generated from a chilled water plant. Great amounts of discussion and debate have gone into designing and optimizing chilled water systems. Overall, the goal is always the same: to provide adequate cooling while minimizing the total energy of the system which includes energy users such as chillers, cooling towers, and pumps. The same principles can be applied when modifying an existing system to improve energy efficiency. Opportunities can be broken into three portions, generation, distribution, and end use. Keep in mind that a modification to one of these three areas effects the others; they work together as a system. The main goal should be to reduce overall energy costs. Generation: Typically chilled water generation systems have three components which should be optimized in coordination with each other. The key parts are the chiller, the heat rejection system, and the pumping system(s). Chiller efficiency is not a constant (nameplate) efficiency, but is dependent on condensing temperatures, chilled water temperatures, and part load capacity. Changing temperature setpoints, flowrates, and chiller staging all effect energy efficiency. For heat rejection, Utah has a favorable climate for evaporative cooling. By taking advantage of the low wet-bulb temperatures of our climate, cooling towers can often provide cold enough water to cool a building or process without the use of a chiller. This strategy is often referred to as “Free Cooling,” and although it requires pumping and fan power, it avoids the energy and demand expenses of compressor-based cooling. Adjusting approach temperatures and reducing fan energy are examples of control strategies that minimize energy required for heat rejection. Pumps are also a significant energy user in cooling systems and should be included in energy efficiency plans. Variable speed pumping is a common way to save pump energy and demand costs. Distribution: Proper insulation minimizes losses. Minimizing pressure drop and reducing pumping energy can have significant savings. Keys are to keep the pipe diameter sufficiently large for the flow rates needed, minimize bends (particularly right angled bends) and design layouts to minimize overall piping. End-use: Maintaining heat transfer surfaces and valves reduces wasted energy. Proper water treatment can minimize scaling which decreased heat transfer efficiency. A leaking or stuck valve wastes chilled water, overcooling a process or space that in turn must be heated to maintain the desired temperature. Both cooling and heating energy are wasted. While proper maintenance and operating strategies can minimize wasted energy, changes to system control sequences or changes in the system configuration or equipment need to be designed by a professional. A design engineer with experience with district cooling systems can provide the best overall solution for an owner, optimizing all of the components of the system. For detailed technical information on designing a chilled water system for optimum performance, the American Society of Heating Refrigeration and Air-Conditioning Engineers (ASHRAE) is a great resource. Resources: |
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