In previous article, we discussed about the importance of energy efficient cooling system. We mentioned about energy saving measures that can be applied in a chiller plant and discussed some of them briefly. Today we will continue our discussion on measures to save energy from a chiller plant and also for individual AC unit.
- Condenser and evaporator heat transfer efficiency
The heat transfer efficiency of evaporator, condenser and condenser heat rejection equipment decreases as a result of fouling. This leads to reduced capacity and increased energy consumption. So, measures should be taken to maintain heat transfer efficiency by improving maintenance performance to reduce fouling. Condenser tube watersides, evaporator tube watersides need to clean on a regular basis. Cooling water need to be tested and treated on a continuous basis. On a wet cooling system, bleed rate need to be adjusted to maintain proper water conditions with minimum water consumption. Where possible, automatic monitoring and cleaning system can be installed to reduce the dependency on chiller operator which may lead to human error.
- Pump energy consumption
Pump modifications and operating practices may reduce the energy required to pump chiller water and condenser cooling water. Adjustment of pump discharge to match the flow and/or pressure requirement is very important. Usually we select pump considering the peak load. But most of the time pump is running at reduced load. So, for constant speed drive system, extra energy is required during low load condition. In that case, impeller diameter can be trimmed or pump discharge valve can be throttled. As an alternative variable speed drive/variable flow system can be used that will adjust the speed/flow based on the requirement. And similar to chiller efficiency, pump motors having the highest economical efficiency should be installed.
Most of the energy that is consumed in cooling system goes into compressors or actually, into motors that drive compressors. There are huge differences in efficiency in different types of compressors. Also there are large differences in efficiency in different models of same type of compressor. Compressors available today are significantly more efficient than the best models available a few years ago. So, when compressor fails, it needs to be replaced with a more efficient one. Also, inefficient compressor even it is running well needs to be replaced with efficient one. In centrifugal system, variable speed system can be installed.
- Refrigerant condition
Refrigerant management is the most important routine maintenance for keeping the equipment at maximum efficiency. Chiller system need to be checked for leaks and repaired on a regular basis. Proper amount of refrigerant need to be charged, neither too little nor too much. We also need to make sure that nothing but refrigerant enters into the system. Water from idle condensers and evaporators need to be drained regularly.
- Equipment scheduling and operating practice
Saving energy is easier than producing energy. So, turning off equipment when it is not needed is the most powerful principle of energy conservation. Measures in this section recommend operating practices and automatic controls that limit operations of chiller and its major component to conform to need. If we have more than one chiller of different sizes, we should distribute the load in different chiller in a manner that minimized the total plant operating cost. Automatic controls can be installed to start/shut down the chiller plant automatically according to the load, to limit the operation based on temperature and enthalpy of outside air and to alert the person to start manually where automatic start is not a viable option. Heat rejection equipment should be turned off when the corresponding chiller turns off.
- Thermal storage
Thermal storage is a form of storage that stores energy (cold or hot) for uses at a later time. In cooling thermal storage systems, cooling is performed during non peak period and stores it to use during peak period. As the tariffs differ greatly in on and off peak, thermal storage system enable the building owner to save a significant amount of money. Moreover, when the chiller capacity is less than the building peak demand, thermal storage system can be used to meet the extra demand during peak time.
- Improvement of individual AC unit
This category of equipment includes window air conditioners and packaged terminal air conditioners. The units typically are installed in an opening in the building envelope and include all their components in a single package. There are also some other air conditioning units which have outside heat exchangers and are separated from inside equipments. Usually, they are larger than the former type. This type of equipment operates on the principle of refrigeration cycle. Following measures can be taken to improve the efficiency of these two types of independent cooling units:
- Clean, adjust, lubricate and repair air conditioners at appropriate intervals
- Install high-efficiency fan motors
- Seal through-wall conditioning units to prevent outside air infiltration
- In units that provide outside air ventilation by exhausting conditioned air, abolish that feature
- Similar to chiller, install air conditioning units with highest practical efficiency
An efficient energy management system can help you to run your air conditioning safely and efficiently.
Benchmark Solutions energy audit team has AEE (Association of Energy Engineers) accredited Certified Energy Auditors (CEB) who are capable of review your cooling system and controls as well as your overall energy management system and make specific recommendations regarding viable cooling efficiency improvements. These improvements will help to run your system safely, reliably and in cost efficient way. Five (5) to twenty five (25%) percent fuel savings in air conditioning systems is often achievable.
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