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Practical tips to make ACs more efficient and save energy
Technical Articles- Feb 01,18
An AC system approximately consumes 40 to 50% of the total connected electrical power. D Nirmal Ram provides practical tips to reduce energy consumption of AC and save environment and costs.
An AC system approximately consumes 40 to 50% of the total connected electrical power. D Nirmal Ram provides practical tips to reduce energy consumption of AC and save environment and costs.
Air-conditioning is no longer a luxury. The selection of the air-conditioning system will have an optimum first cost for a lower operating and maintenance costs. As such energy saving on air-conditioning (AC) system is of paramount importance. An AC system approximately consumes 40 to 50% of the total connected electrical power. As such emphasis is made on energy conservation on air-conditioning system.
Energy efficiency measures can be instituted in buildings without reducing comfort. In fact many energy efficiency measures result in a more pleasant indoor environment while reducing operating and first costs. Even small energy savings have incremental effects on the environmental and cost savings.
Many energy efficiency measures do not require additional first costs. Those measures that do result in higher first costs often create savings realised from lower energy use over the building lifetime, downsized equipment, and reduced mechanical space needs and utility rebates. Many businesses are now actively leveraging their facilities as a strategic tool to attract and retain their employees.
Strategies
There are three fundamental strategies to increaseenergy performance: Reduce demand, harvest site energy and maximise efficiency. Demand reduction is accomplished by challenging initial use assumptions and by reducing internal loads and gains through the shell and lighting improvements.
Harvesting site energy includes using free resources such as daylight, ventilation cooling and solar heating to satisfy needs for space air-conditioning. Finally the efficiency of the HVAC system should be maximised.
Demand reduction is accomplished through design strategies such as reducing the overall building foot print so as to reduce the total space that require air-conditioning; relaxing the temperature design criteria to allow for a wider acceptable indoor temperatures and utilizing occupancy sensors to automatically turn off equipment when building occupants are not present.
Optimise HVAC system efficiency by avoiding over-sizing of plant and equipment. All components should be sized appropriately and take into account all the energy performance measures.
Orientation
The primary function of an air-conditioning system is to maintain inside design conditions as required for human comfort or required by a product or process. The AC capacity is based on the actual instantaneous peak load conditions and can be estimated. We have to make a comprehensive survey for the accurate estimation of the load. The heat gain through the exterior walls particularly the west puts a considerable chunk of heat load in the cooling load estimate. As such avoiding the exposed exterior walls or having larger span of exterior walls towards north could reduce the AC capacity.
Similarly, air-conditioning spaces can be located avoiding the roof exposed to sun and in such cases where this cannot be avoided suitable thickness of roof insulation has to be contemplated. Exposed glass windows are to be shaded with venetian blinds, sun films. Proper choice of building materials will play a vital role towards this concept. The newly launched ECBC (Energy Conservation Building Code) highlights the heat transfer coefficients for many wall, ceiling, roof and glazing required for the buildings resulting in saving of the HVAC operating cost.
Ventilation
The introduction of outdoor air for ventilation of conditioned spaces is necessary to dilute the odorous given off by people, smoking and other internal air-contaminants so as to conform to the IAQ (Indoor Air Quality)sStandards. The amount of ventilation required varies primarily with the total number of people and their activity. A demand controlled ventilation using carbon-di-oxide sensors controlling the dampers which in turn controls the variable frequency drives for changing the speed of the blower will result in higher energy efficiency.
For higher outdoor air quantity, heat recovery by means of heat recovery wheel, run around coils and heat pipe will result in lesser system capacity.
In certain climates, economiser cycles can be used to take advantage of free cooling using outside air within appropriate temperature ranges. Outside air load can be reduced by treating the air using earth tunnel system or with a two stage system with sensible and adiabatic cooling.
Inside design conditions
Comfortable temperature is a relative figure and depends on outside temperature and humidity. Selecting a lower temperature than comfortable temperature is direct waste of energy. So compromise the need to use low temperature and humidity.
Equipment selection
Generally, it is found that equipment selected on the actual instantaneous peak heat gain is oversized and therefore capable of maintaining much lower room conditions than the original design. One reason for such is the non-simultaneous occurrence of the peak in the individual loads (diversity). The actual cooling load is generally considered below the peak instantaneous heat gain thus requiring smaller equipment to perform a specific job. In addition, the air quantities and/or water quantities are reduced, resulting in smaller overall system.
Refrigeraton system & allied equipments
The efficiency of a compressor and motor is highest when operating at the full rated capacity. As such a plant has to be properly sized with multiple systems to take care of partial load conditions. A proper sizing of water, refrigerant pipes, and valves will call for smaller equipment. When practicable it is desirable to use multiple towers instead of large single cell tower. For areas with high ambient conditions, adiabatic cooling of the air around the chiller may be an option.
Variable refrigerant systems can circulate refrigerant according to the demand and results in energy saving. Star rating is available for unitary air-conditioning systems.
Hybrid chilled water system
A hybrid chiller option with a combination of air cooled screw chillers, water cooled centrifugal chillers and absorption chiller with heat recovery boilers from DG sets will enable efficient operation. Water cooled chillers will be running during the period of high ambient conditions using recycled water and air cooled chillers can function during nights/lower ambient conditions whereas the absorption chillers can run using the heat recovered from the DG flue gases when the DG sets are in operation.
Airhandling units
Choice of air handling units with supply and return/exhaust air fans will result in energy saving. During ‘free cooling’ conditions when the ambient temperature is comfortable, the supply fan will be drawing all outside air, whereas the return/exhaust fan will be exhausting
the air from the conditioned area. The air handling unit can have dual speed motor so as to meet the varied air quantities for various seasons. This can work in tandem with a variable frequency drive. The function of the dual speed motor is to ensure that the drives do not call for a very low operating frequency leading to harmonics.
Toilet exhaust fans can be hooked with an infrared sensor/timer which will facilitate operation only during occupancy. Server rooms and data center which are required to function round the clock can be contemplated with dual fluid precision units. When the chiller is in operation the unit will be running in chilled water mode and will shift to DX mode when the chiller is not functional.
Chilled beams
Chilled beams offer a quiet indoor air free from draught. In a typical chilled beam, the air is cooled by means of chilled water and the supply air flow rate is dimensioned to meet the Indoor Air Quality requirements. It can be used for regulation of the temperature for an individual comfort.
Variable airvolume units
Cabins and conference areas which have partial usage can be provided with variable air volume units so that they can be operated based on the usage which will result in energy saving. Variable air volume units coupled with motion sensors will enable closure of the units to the minimum levels will result in energy efficiency.They can be actuated by a motion sensor so that the VAV will close to the minimum position in the un-occupied areas. Variable air volume unit will further facilitate change of temperature set points during various seasons.
Ducting
Size ductwork appropriately and install balancing dampers to reduce velocity losses. Ducts with larger cross sectional areas have much lower resistance and can reduce fan energy significantly.
Thermal storage
Power tariffs and rationing during peak load hours - This becomes a potential tool for use by the designer to harness the sleeping giant of thermal storage. Thermal storage systems become handy in areas wherein due to water shortage the usage of water cooled chillers are limited. The air-cooled chillers can run during nights when the ambient temperatures are comfortable. Thermal storage systems will be an economical option where there are dual tariffs in the electrical power.
Vapour absorption system
Alternate sources of energy particularly waste steam/heat can be used for refrigeration. One such system uses water as the refrigerant and lithium bromide as the absorber. It has less moving parts and as such it results in lower power consumption. Vapor absorption system can use waste heat from the diesel generating sets using heat recovery boilers. Heat recovery from solar heat is also a possible option.
Variable speed drives
Variable frequency drives are used for energy saving. Variable frequency drives can be used for the primary and secondary chilled water circulation pump sets by sensing the temperature and pressure differential in the chilled water lines. Two way motorized valves in the air handling units can be actuated by a thermostat which will vary the flow according to the loads resulting in pressure changes which can be sensed and used for changing the speed of the pump sets.
Variable speed drives are used for the air handling units. Variable air volume units will regulate the airflow for various zones based on the occupancy and temperature by a variable air volume unit, which will give a pressure signal for the VFD to change the speed of the air handling unit.
A demand controlled ventilation system uses a variable speed drive operating based on the opening and closure of the fresh air dampers controlled by the carbon-di-oxide sensors. A typical basement exhaust system can use a variable speed drive controlled by carbon-mono-oxide sensors Centrifugal/screw chillers with variable speed drives are also available.
Electronic controls
Usage of accurate/sensitive controls can save power by 10 to 20%. It has been found that by maintaining lower inside conditions than the ones designed result in higher power consumption at the cost of occupants comfort levels.
Building automation system
The main objective of the building automation system is to reduce the running and energy costs, improve the quality and supply of information on the air-conditioning system. The system can establish basis which will be good bench mark for energy efficient operation subsequently.
Good installation practices: A good equipment will not serve its purpose if it is not installed properly. Good installation practices with stringent quality control measures will result in easy maintenance and will pave a way to energy saving. Air leakage if any in the ducting system will result in serious energy loss.
Commissioning and handing over: Commissioning is a systematic process to ensure that the air-conditioning system performs according to the design intent and the owner’s operational needs. Commissioning maximizes energy efficiency and thereby minimizes environmental impacts associated with energy production and consumption. Energy Conservation reduces the need for natural resource extraction, improves air quality and reduces global warming process.
A commissioning plan shall be created as early in the design phase. The readings shall be logged and tabulated properly.
Training shall be performed by qualified personnel for sufficient duration to ensure that the facility staff has all the information needed to optimally operate, maintain the air-conditioning system. Operation and maintenance manuals with as built drawings shall be obtained from the contracting agency.
Operation and maintenance
A well drawn-out diligent operation and preventive maintenance schedule really saves energy. Proper maintaining of inside design conditions does not mean that the system is working efficiently as peak load will not exist throughout the year and the plant is designed based on the peak load. As such proper preventive maintenance is to be performed for proper upkeep of the system to save electrical energy. Operation shall be focussed only in areas, which can result energy saving without compromising the design intent. When there is a compromise, it is not a saving but a faulty operation!
Saving is only a relative term. Improvement is possible at every stage on continuous basis and there is no limit for energy saving and as such air-conditioning is no longer a luxury!
D Nirmal Ram is the Chairman of the organising committe of ACREX India 2018, an event for HVAC (heating, ventilation, and air conditioning), refrigeration and building automation systems. ACREX India 2018 will be held in Bengaluru from February 22-24, 2018.
AC
AC System
HVAC System
Ventilation
Refrigeraton System
Hybrid Chilled Water System
Airhandling Units
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