Air-conditioning systems used in industrial, commercial and institutional buildings can account for a large fraction of the total electricity consumed. In Bangladesh where the climate is warm and humid for 9-10 months in a year, cooling and dehumidifying may account for more than 50 percent of a facility’s energy cost. Therefore, it is important to understand how cooling systems work so that they can be installed and operated as efficiently as possible. Many improved technology made it possible to build new facilities with improved energy efficiency. There are also many techniques available to improve the efficiency of existing air-conditioning system. These range from simple adjustments which pay back quickly to expensive retrofitting. Air conditioning system can be divided into two groups: Central air conditioning system (chiller) and individual AC unit. Measures to save energy in air conditioning system include but not limited to:
- Selection of best system from various options
- Chiller efficiency and life cycle cost
- Optimum operating temperature
- Sizing and configuration
- Condenser and evaporator heat transfer efficiency
- Pump energy consumption
- Refrigerant condition
- Equipment scheduling and operating practices
- Thermal storage
- Measures for improvement of individual AC unit
All of the measures are for chiller plant except the last one which is for individual ac unit. All of measures may not apply to all systems. So, chiller operator should understand his system first. He needs to go through all of documents supplied by manufacturer with chiller system, understand the operations and maintenance and apply the best measure for his system. Today we are going to discuss briefly about first four measures mentioned above. We will be discussing about other measures in our next article.
- Selection of best available system – Water-cooled versus air-cooled chillers:
From the refrigeration cycle of an air conditioning system, the heat removed is released in the condenser in which heat exchange occurs either with water or ambient air. Chillers are classified according to whether the condenser is water-cooled or air-cooled. In air-cooled chillers, there is a finned heat exchanger in which a single fan or a number of fans are used to blow ambient air through the condenser. On the other hand, Water-cooled chillers have shell and tube heat exchanger, where the heat is released into the condenser water. Water cooled chiller operates at low condensing pressure than air cooled chiller. The low condensing pressure is because of release of heat to water which is first cooled by the cooling tower to a temperature a few degrees above the wet-bulb temperature. In air cooled chiller heat is released to air and heat transfer depends on dry-bulb temperature. The low condensing pressure leads to lower differential pressure between evaporator and condenser which results in lower power consumption by the compressor. Therefore, water cooled chiller is far more energy efficient than air cooled chiller. Though the capital investment of water cooled chiller is higher than air cooled chiller, due to their superior energy efficiency, this extra initial cost will be paid back within few years. However, during evaluating of benefits of water-cooled chiller, the additional cost associated of cooling tower, condenser water piping system and water treatment need to be considered. Besides, Air cooled chiller is used where constant source of water is not available and where space is limited.
- Chiller efficiency and life cycle cost:
Usually high efficiency chiller tends to be more expensive than that of lower efficiency chiller. Although the initial cost of high efficiency chiller is high, because of their lower operating cost, the extra capital cost is normally paid back within few years. The financial benefits of high efficiency chiller will be confirmed if we do the life cycle analysis of a chiller which considers both the capital cost and operating cost throughout the expected lifetime which is usually 10 to 15 years. The operating cost should include all of forms of cost such as electricity cost, maintenance cost, refrigerant cost etc.
- Optimum operating temperature:
Optimizing the operating temperature is one of the important aspects of efficient operation. The efficiency of any cooling machine depends on the differential temperature against which it operates. Usually, in most cases this value is kept higher than it needs to be. So, we can reduce this differential to make the system more efficient. We can reduce this differential in two ways: a) by increasing the evaporator/chilled water temperature and b) decreasing the condensing temperature. The appropriate temperature depends on chiller design, cooling load, weather temperature and conflicts of efficiency of other equipments in cooling system due to change in differential temperature. The choice of selection of optimum temperature is very challenging as weather conditions and loads vary widely throughout the year. So, it’s better to use the automatic control to adjust the temperature on a continuous basis.
- Sizing and configuration of chiller:
Chiller efficiency varies with cooling load. Most of the chillers operate at high efficiency from 60 to 100% of its capacity while the peak efficiency is at 80%. And efficiency declines below 60% of capacity. Chillers are usually oversized for new buildings because of unavailability of appropriate load estimation tools which leads to a high safety factors in chiller design. Again, in most cases chiller is installed considering the peak cooling load of the building. But most of the time of the day/year (during night and winter) chiller is operated at partial load which leads to waste of energy. So, sizing and configuration of chiller need to be done using appropriate estimation tools for new building and proper cooling load profile can be used during retrofitting for existing building. If chillers of different capacities are available, different combinations of chillers can be used at different times of a day to match the building cooling load. This will ensure that chillers are operating within their best efficiency range at all times.
In our next article, we will be discussing about other measures than can be taken to make the chiller system efficient and also other individual air conditioning unit.
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.