• Cooling India
  • Jul 15, 2017

Energy Conservation
for Air Conditioner

In this article, authors have explained about the basic working mechanism of Air Conditioner and with the case study of real example of split units of AC have tried to explain the basic calculations, how to select the AC for home requirements. Moreover, the very important energy saving tips and BEE guidelines for star rating is also explained...

- Paresh  R. Modha, Rajan R Modha, 
  Minesh K.Joshi


 Air conditioners are used to vacate heat from specific area called controlled area. Chill water plant, central AC plant and package units are used for big premises. For home and office small units of capacity 1 to 2 tonnes are mostly used.

  Window units used earlier are now obsolete and split units are most common. Normally controlled area is closed and inside air is circulated in close loop. Here controlled area is room. Air conditioning units are used for small area like bed room, seating room, cabin, laboratory, etc.

Heat Transfer

  Heat flow from higher temperature to lower temperature in natural course. But in this case, heat is required to transfer from lower temperature area to comparative higher temperature area and therefore, has to be forced by some means. This is achieved by means of circulating refrigerant in closed path. By compression, refrigerant gets heated up to temperature higher than ambient and is sent to condenser where heat is thrown off to atmosphere by forced air by fan and the gas condenses to liquid. This liquid is allowed to expand (release pressure) when it cool down below the room temperature and sent to evaporator cooling coils.

  The air blower forces the air over cooling coils where refrigerant absorb heat from air and evaporate to gas whereas the air loose heat and cool down and is delivered in the room.

Analogy

  The whole process is similar to removing splashed water from the floor. House maid unfolds mop and spread over the water. Mop absorbs some water and then she takes it out and squeeze to release water. This process is repeated continuously so that water from floor is transferred outside. Refrigerant in air conditioner works as mop to absorb and release the heat.

Electricity used in A/Cs

  Electricity supplied to air conditioner for its functioning is principally consumed by following three motors.

1. Compressor Motor

• This motor is of highest capacity in the unit.
• Consume maximum energy during operation of air conditioner.
• Motor runs intermittently as per signal from thermostat.

2. Condenser Fan Motor

• This motor drives fan having three or four blades that forces air through condenser coils to discharge maximum heat of refrigerant.
• This fan works only when compressor is working.

3. Air Fan Motor

  This motor drives blower that circulates cold air in the room. Comparative hot air is drawn from the top and forced to pass through evaporator (cooling coils) and after cooling delivered in the room.

  This motor runs continuously till air conditioner in on.

Other motor :

• There may be very small motors for movement of louver for distribution of cold air as desired.
• These motors run as per requirement of operator but its energy consumption is insignificant.

Thermostat

  The function of thermostat control is very important. Desired temperature can be set by operator. When room temperature falls below the set value, the supply to compressor and condenser fan is cut off and further cooling is suspended. As result of stoppage of these two motors, power consumption is reduced.

  Afterward when room temperature rises above the set value, the compressor and condenser motors are again started.

Thermostat Setting

  Thermostat temperature setting has important role in economical operation of air conditioner. Start and stop of compressor and condenser motors are controlled by thermostat. When thermostat setting at lower temperature, compressor and condenser ON period is long and OFF period is short.

  Whereas when thermostat setting at higher temperature, ON period is short and OFF period is long. Power consumption of air condition unit depends on running period of these main motors. Higher the Run/Rest ratio of compressor, more is energy consumption. It is estimated that rising of temperature setting by 10C, results in reduction of about 2.5 % in energy consumption.

Comfort Zone in AC Environment

  Ambient temperature in hot zone is uncomfortable and so also temperature in cold zone is uncomfortable. Whenever temperature is abnormal, it can be brought in comfortable zone by operating air conditioner. The comfortable zone may be in the range of 220C to 280C depending on metabolic 28 C rate of user.

  Cooling down to very low temperature is uncomfortable as well uneconomical also. Hence, it is to be avoided. But practically some air conditioners are operating in cold zone as under.

Over Cooling in the AC Environment

  When person from outside hot atmosphere enters the room and start air conditioner, he feels very poor cooling. Actually, the body including cloths and accessories is at atmospheric temperature. Air conditioner has to draw out all this additional heat also and therefore, it may take little some time.

  But due to impatience and ignorance temperature setting is reduced to minimum. In fact, cooling cannot be fast by reducing temperature setting but it takes own time as per capacity of the unit. Eventually, lower setting continues and result in over cooling.

  Some air conditioners have facility of turbo cooling mode. This may serve the purpose for fast cooling but care has to be taken to switch to normal operation. As far as possible setting should not be disturbed and kept at comfort zone only.

Pre Cooling

  Other way is to start air conditioner in advance so that by the time of entry, room is already cool. This can be possible by instruction to watchmen/attendant/assistant to start the air conditioner before time of arrival.

  Some air conditioners have facility of remote operation through device like mobile. This can be used for advance starting for pre-cooling. However, care should be taken not to start much in advance because it may be wasteful act.

  Almost similar situation occurs in the case of bed room. At the time of entry in the room cooling seems to be insufficient. As usual temperature setting is kept minimum for fast cooling.

  At late night temperature drops and has to use blanket. Similar observation may be in air conditioned train. At the start, few passengers complain to attendant for insufficient cooling but at late hours most of the passengers are using blanket.

  Does it not seem to be ridiculous that on one hand to spend money for over cooling and on other hand face shivering and use blanket for protection?

  Such situations can be solved by pre cooling the bed room and using sleep mode that raise the setting in steps at late night.

Ignorance

  Some users just start the air conditioner when required but unaware of significance and implementation of temperature setting. So, air conditioner runs on default minimum setting.

Important Tip

  Increasing setting from 22°C to 26°C results in 10% saving of energy. In many cases, setting is at 18°C or 20°C. If this is revised to 26°C or 28°C, there may be energy saving of 20%. So, the first and foremost tip is to have proper temperature setting.

Other energy saving tips:

During the Installation:

a. Internal unit

  Internal unit should be installed in such a way that air flow is not obstructed. Air is taken in the unit from the top. There should be sufficient space at top and there should not be any decorative hanging etc in the air path that obstructs the air circulation. Cooled air is delivered from bottom. There should not be any cupboard, curtain, etc under the unit that obstruct the air path.

  Any obstruction can reduce air flow and result in less cooling. Ultimately to reach set temperature, compressor has to work for long time requiring more energy. Also fan load increase due to obstruction in the air path and consume more energy.

b External unit

  External unit should also have the similar arrangement for free air flow. There has to be sufficient space on back and top of the unit.

  Location of the unit should be such that it may not be in direct sun rays during period of operation. Condenser unit has to reject the maximum heat of compressed refrigerant in atmosphere. Direct sun rays heat the unit and condensing may be partial. Mostly, air conditioner is operated in afternoon and night and therefore, external unit should be on east side. However, due to practical constraint, it is on west side, it should have shed to protect from direct sun ray in such way that air flow is not blocked.

Heat Ingress

  Air conditioners are used when outdoor is very hot. If this external heat enters the room, the machine has to draw out this additional heat also to attain temperature as per setting. This results in longer running of compressor consuming more energy. Therefore, all efforts have to be made to restrict the entry of heat in the room. From Terrace, If the room is on top floor, the terrace is heated by sun ray and this heat is conducted to the room. This can be avoided or minimized by painting terrace with white glossy paint so that sun ray is reflected.

  Alternatively for better and lasting solution, white glassed tiles or mosaic with pieces can be fixed in terrace. Additionally, shedding green net can be installed at about two feet above the terrace which may prevent sun ray striking and heating the terrace. The air trapped between terrace and net act as insulation.

  Ceiling of the room can have thermocol lining as heat insulator or false ceiling made of plaster of parish or gypsum board. Air trapped between ceiling and false ceiling act as insulator.

  From Walls, where the wall of the room is facing the west, it is heated by sun ray in the afternoon. This heat is conducted in the room and increase the heat load of air conditioner. The outer side of the wall can be painted with light color glossy paint so sun ray is reflected and heating is minimized. Wooden or thermocol lining can be provided on inside of the wall.
Window of the room having glass shutters and facing west can be covered with sun ray control film. Double glass (inside and outside) shutters will be better as the air trapped between glasses will act as thermal insulation. Additionally light colored cotton curtain on window is also helpful. Light color reflects the sun ray and air between shutter and curtain act as insulator.

Intermix

  Care is required to avoid intermix of cold air in the room and hot air outside. Any vent or gap in door/window/partition should be sealed to avoid transfer of air. Generally, there is gap between bottom of door and floor. Door Seal Strip available at hardware shop should be fixed. Door and window should remain closed when air conditioner is operating. Door closer should be installed to avoid instances of open door by mistake. Air curtain or automatic door control (open close device) can be installed where frequent entry / exit is expected. Air buffer is better solution where feasible. It is intervening closet having two doors, one to the room and other outward. Doors are interlocked not to open simultaneously and may have door closers.

Words of Caution - Bed Rooms

For bed room condition is different

  Long period of about 6 to 8 hours is passed continuously sleeping. Person is inattentive to any abnormalities in the room. Air conditioner running with minimum setting and without sleep mode may cause over cool during last segment of the night. Blood may have comparative higher viscosity at low temperature requiring more pressure for circulation. Also due to breathing of occupant oxygen is consumed and carbon dioxide is released. In totally, sealed room proportion of oxygen reduce and that of carbon dioxide increase. Condition may be worst during last segment of the night and may be harmful for occupant. Therefore, intentional slit has to be provided to maintain proportion of oxygen and carbon dioxide by diffusion through it. This is more relevant to those having cardiac and breathing problems. Doors and windows of the car have rubber lining and hermetically sealed. The volume of air in car is very less as compared to room. Number of occupants may be up to 5 as against 2 in bed room. Air conditioner is operated in air circulation mode and windows are closed for better cooling.

  In long run oxygen and carbon dioxide proportion is disturbed due breathing of passengers. Even though comfortable temperature and seating condition, passengers may feel weary.

  Therefore, while on long run it is important to change from air circulation mode to fresh air mode for one minute at every hour.

Capacity of AC

  Capacity required for air conditioner depends on following:

• Air volume (room size)
• Numbers of occupants (persons in the room)
• Heat load (heat ejected by gadgets in room)
• In case of capacity is less than required, desired cooling is not achieved and compressor runs continuously without rest and chances of failure.
• Similar condition occurs where room is open to kitchen, other room or up stair.

Maintenance of AC

• Air filters of internal unit should be cleaned periodically.
• Reverse air blow through condenser and evaporator coils by vacuum cleaner is required to remove any deposition and blocking.
• Any blocking may restrict air flow resulting in less heat transfer and lesser cooling and longer compressor operation.
• Check gas pressure. Add gas if low.
• Check for gas leakages. Block any leakage.

Avoid Waste By

• Occupancy sensors are useful to avoid wasteful running of air conditioner in absence of occupant.
• Iron or hot plate / stove should not be used in the air conditioned room. It has dual disadvantage.
• It adds undue heat load increasing energy consumption.
• Part of the heat generated by iron or hot plate is taken away by air conditioner and hence iron or plate consumes more energy.
• Stove consumes oxygen and emits carbon dioxide in the closed room.

Vast Scope for Energy Saving can be done by:

  Primarily, above measures are useful for energy saving and cut in electricity bill. At the same time, this will indirectly control emission of green house gases and ultimately global warming. Air conditioner is the high energy consuming gadget compare to geyser and fridge. Up to 30% energy saving is possible by implementation of applicable measures from the above.

Star Rating of AC

  Now as per rule star rating is compulsory for air conditioners. Regulating authority is Bureau of Energy Efficiency (BEE). The star rating is related to Energy Efficiency Ratio. Higher the star rating, higher is the EER and lower the power consumption. High star rated unit may be costlier.

  Case Study for Air Condition Selection (For Split Units)

Cost-Benefit Analysis

Price of higher star rated AC is more but the additional cost is recovered as saving in electricity bill.
Comparison of 1.5 ton, 5 star and 1 star air conditioner
One ton equals 3,515 watts cooling capacity.
Average Power = cooling capacity ÷ EER
For 1.5 ton 5 star unit:
Power = 3515 × 1.5(ton) ÷ 3.3(EER) = 1598 Watts = 1.598 Units per Hour
For 1.5 ton 1 star unit
Power = 3515 × 1.5(ton) ÷ 2.5(EER) = 2109 Watts = 2.109 Units per Hour
Price difference of same type 1.5 ton, 5 star and 1 star A/C is taken as Rs 8000/-
Annual variable cost on account of interest, depreciation and maintenance taken as 20% of capital cost.
Annual variable charges for additional cost = 8000 × 20 = 1600/-
Energy charge average Rs 6.5 per unit.

Case A for Regular Bed Room

Normal usage period : Noon From 12 Hrs to 17 Hrs = 5 Hrs
Night From 23 Hrs to 07 Hrs = 8 Hrs
Total usage hours per day = 13 Hrs
Annual utilization about 140 days (includes summer days and period between end monsoon and start winter.)
Annual saving on energy bill = (2.109 – 1.598) × 13 Hrs × 140 days × 6.5 Rs/Unit = Rs 6045 per year.
Annual Net Saving = 6045 – 1600 = Rs 4445 per year
Additional cost of Rs 8000 is recovered almost in 2 years.
Hence, it is acceptable.

Case B for Guest Room

Normal usage period
Night From 23 Hrs to 07 Hrs = 8 Hrs
Annual utilization about 25 days (includes summer days and period between end monsoon and start winter.)
Annual saving on energy bill = (2.109 – 1.598) × 8 Hrs × 25 days × 6.5 Rs/Unit = Rs 664 per year.
Annual Net Saving = 664 – 1600 = Rs -936 per year is negative
Additional cost of Rs 8000 could not be recovered.
Hence, it is not acceptable.
High star rated A/C is better where long running hours.

Conclusion

  In this article, authors have tried to cover the basics of Air Conditioner and done case study (For Split Units). The case study clearly indicates how to select the AC for the room. From the case study it can be clearly found that, high star rated AC is better where long running hours. Authors have also shown the updated BEE Star labeling scheme and list out the points by which energy conservation can be done effectively. By this way, the energy can also be saved and individual can save money too.


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