Cost of electricity is a major share in the total operating cost of an enterprise. Ever rising energy bills and reduced availability, necessitates the need for efficient use and innovative techniques. A sizable portion of electricity (to the tune of 40 to 50%) is consumed by utilities like refrigeration and air conditioning alone, in most hotels, hospitals and commercial buildings.
However, some of the best practices in vogue in Indian industries have energy saving potential of the order of 20 to 40%. Practices like: incorporation of SCADA & BMS, use of Energy Optimus, Energy Savers for ACs, Variable Speed Drives (VSDs), Earth Air Tunnels (EAT), Waste Heat Recovery (WHR) etc., are few among them. The best practices discussed in this article have shown great promise for large scale adoption in the near future.
TR? Yes TR or Tons of Refrigeration is a commonly used and familiar term even by nontechies. It is something that even a housewife, barely initiated in technicalities would figure out and understand. However, it clues one, only to the capacity and size of an air conditioning or refrigeration system, but not its performance.
In industry on the other hand, where great footage is accorded to the quantum & quality of the refrigeration effect, and where the power consumption & efficiency of the system are crucial, the term kW/TR has mileage and greater relevance _ and is the more apt energy performance related indicator in use. It simultaneously reflects the quantum of power consumed (kW) per unit of refrigeration effect (TR) i.e., the specific power consumption for the refrigeration system or the machine, as the case may be. Moreover kW and TR in any facility are parameters that are not too difficult to measure.
Like any other specific power consumption indicator, kW / TR can be widely and conveniently used for comparison with bench marks, for intense comparison amongst a bank of machines, and for performance trend analysis. It speaks of the conversion efficiency in broad terms and an upward trend warns of bad performance. Timely intervention to curb the rising trend would be in order, perhaps by restoring to good maintenance & operating practices and / or incorporating appropriate efficient technological changes / retrofits.
To obtain kW / TR value it is obvious, that one needs to measure power consumption in kW and refrigeration effect in TR. Power measurements, i.e., the input power to the drives of the refrigeration compressor, chilled & condenser water pumps, cooling tower fan, are easy tasks involving use of a portable power analyser or panel mounted power/energy meters. In contrast, the TR of the refrigeration effect or load is a slightly more involved assessment in the sense that, chilled water flow measurements are to be undertaken.
TR in a basic reflects the amount of heat removed or the chilling effect, which would render 1 British Ton of water into ice in a period of 24 hours.
The TR effect can be calculated by the
relation: TR = Q X Cp X (Ti –TO) / 3024
- TR is the cooling duty
- Q is the mass flow rate of the chilled water/brine coolant in kg/hr
- Cp is coolant specific heat in kCal /kg.OC
- Ti is the inlet temperature of coolant to evaporator (chiller) in OC.
- To is the outlet temperature of coolant from evaporator (chiller) in OC.
Flow measurement is the most tricky and difficult part in the overall assessment of the ref. system. Generally, flow meters and indicators are not provided for in most of the cases. Only in recently installed systems, we find online flow meters. Ultrasonic flow meters are now available, which can be used for measuring liquid flows. Magnetic flow meters also find wide use, though these have to be installed in the pipe line. For measuring air flows, (in CTs and AC ducts) anemometers are good bet. A pitot tube can also be used for air flow measurements in AC ducts.
Once we have a handle on kW/TR values, then it can be expressed, either in terms of Coefficient of Performance (COP) or Energy Efficiency Ratio (EER), and other commonly used energy performance indicators.
The above figures includes compressor power alone. If one adds the chilled water / condenser water pumps’ power and also the cooling tower fan power, then these kW/TR values may slightly go up. One often encounters the reciprocating compression systems in the field. The typical benchmark overall kW/TR figures including compressor, condenser pump, chiller pump, cooling tower fan, could be 1 to 1.1 for air conditioning systems.
For Vapour Absorption Refrigeration (VAR), Systems the energy performance indicator is Kcal/TR instead of kW/TR. The typical Kcal/TR values are given in the next figure (below).
In their endeavour to retain their competitive edge in the fiercely competitive market place, most of the business enterprises are readily adopting best practices for energy cost reduction. Some notable ones in the cooling/refrigeration field are discussed hereafter. Recent technologies, best practices, systems that have been successfully adopted and implemented are:
- SCADA system for Air Conditioning Plants
- Energy Optimus for Energy savings
- Optimised running of cold & hot H2O pumps
- Optimised running of cooling tower fans
- Optimised running of Air Handling Units (AHU’s) through PID controls & VFDs
- Installation of VAR systems in process industries including hotels and hospitals
- Adoption of air ambiator for low cost air cooling requirements
- Replacing old efficient window A/C’s with the latest energy efficient A/Cs
- Use of hybrid water sink energy efficient window AC
- Use of Energy saver for Window A/c’s
- Traditional cooling with Wind Towers
- Use of underground Earth Air Tunnel (EAT) to supply pre-cooled air
- Generation of hot water by waste heat recovery through de-superheating
SCADA system for air conditioning plants
Supervisory Control and Data Acquisition (SCADA) system, is a sophisticated automation, data acquisition and data logging tool as well as a control option during operation, thus facilitating operators to effect parameter changes based on actual real time data available at his finger tips.
The facility to view key process parameters like, temperature, pressure and flow of chilled water, cooling water and air, and input power consumption of compressors, pumps, fans, allows the operating personnel to analyse and instantaneously take corrective action. The optimised utilisation of equipment running time manifests as reduced energy consumption. The real time consumption trend, by individual departments, helps in overall optimisation, leading to energy savings. Several pharmaceutical and dairy units have implemented SCADA and have achieved overall energy savings of the order of 5 to 10%. Many commercial buildings are also going for Building Management Systems (BMS).
Energy optimus can save energy to the tune of 25 to 30%. Energy optimus can be used in high energy consuming equipments like chillers, HVAV, pumps, air compressors…
Optimised running of water pumps
Very often we find that the cold well and hot well concept being followed, in chilled water / brine centralised systems. Invariably, users encounter the nagging problem of hot water mixing into the cold water by overflow, thus killing the advantage of refrigeration, and losing energy in the process. To overcome this wasteful phenomenon, the speed (RPM) of the cold and hot water pumps can be varied as per need, to control flow and pressure, by installing PID controllers and variable speed drives. Motor speed is varied based on level in the cold/ hot well, and also the pressure in the cold water header.
The power savings that have been achieved by some of the dairy industries are worth more than Rs 2 lakhs annually, with an investment of Rs 4 lakhs _ i.e., pay back within 2 years.
Optimised running of cooling tower fans
CT fan operation can be optimised by installing temperature controllers for the cooling tower and Variable Speed Drives (VFDs) for the cooling tower fans. The controller provided in the cooling tower pump house keeps track of the temperature of cold water in the header through the sensor provided in the header, and accordingly VFD varies the speed of the CT fans motor. This automatically optimises the CT fan operation and results in sizable power savings, especially for a 24 x 7 type of operation. A majority of modern dairies and pharma industries have implemented this type of system. A good number of industrial units have also implemented ON/OFF controls _ actuated based on cooling tower sump water temperature as a low cost solution. The power savings thus achieved by one of the modern dairy, in cooling tower fan alone was around Rs.8 lakhs annually.
Optimised running of AHUs
Optimisation of Air Handling Units (AHUs) can be achieved by installing controllers and Variable Speed Drives for the AHU blowers. The controller installed in the AHUs continuously tracks and monitors the temp. inside the air conditioned area, and accordingly, the speed of the blower motor is varied by the VFDs resulting in lower power consumption.
These types of controls and VFDs have become common nowadays in most of the commercial buildings like hotels and hospitals. In industries also, they have potentials to save energy to the tune of 15 to 20%.
Installation of VAR Systems
Vapour Absorption Refrigeration (VAR) or Vapour Absorption Machine (VAM) are being used by many process industries like pharma, rayon, textiles, fertilizers, refineries and power plants, where steam or waste heat is available
In one of the process industries in India, They’ve replaced the existing single effect VAM to double effect VAM for cost savings. Recently, one of the leading rayon industry has replaced the existing centrifugal chillers by installing 525 TR single effect vapour absorption chillers. The reported cost benefits are as follows.
Towards energy cost reduction, many people are going for direct fired VAR system, where steam is not available. One of the innovative methods adopted by a multispecialty hospital (Vadodara, Gujarat) is use of solar energy for air conditioning. By generating 3 kg/cm2 steam by using solar concentric panels, they are able to run the VAR machines and reduce their high power bills.
Adoption of air ambiator technology
We can reduce our air conditioning energy cost by 50 to 60% by adopting air ambiator technology. If you are able to compromise little on humidity, we can go for air ambiator system. We can achieve temps much below the wetbulb temp. using 2-stage cooling. The capital cost of the this system is also less, and this system can be used for outpatient wards of hospitals, canteens, cinema halls, etc., the table (next page bottom) gives the capital cost, operating energy cost of different systems i.e., vapour compression and the air ambiator.
Replacing old inefficient window ACs
The old window ACs are bound to consume more energy. The window ACs, which are designed about 10 years back, the specific energy consumption is around 2 to 2.5 kW/TR. The present genre state-of-the-art, window ACs with scroll compressors are more efficient, and are designed to consume about 1.2 to 1.4 kW/R. With the implementation of standards and labeling programme by the Bureau of Energy Efficiency (BEE), the user has a choice to go for energy efficient window ACs (including split ACs).
Use of hybrid water sink energy efficient window AC
New type of energy-efficient air conditioners are available today, that are more energy efficient (at least 30% when compared to 3-star rated ACs). The fluid that collects heat at the evaporator and releases it at the condenser is called refrigerant. A pump, called the compressor, forces the refrigerant through the circuit of tubing and fins in the coils. Air moves through the tiny spaces between the fins and is cooled by the refrigerant in the coils. This cycle is called vapour compression cycle. The Hybrid AC uses the same compression cycle by adding another stage in condensation. A unique condenser and evaporator design, allowing for faster condensation and evaporation, makes the cycle more efficient than the conventional vapour compression cycle. In most of our installations, we have been able to achieve average power consumption of 0.7 kW/ton.
Energy saver for window ACs
MAS is a programmable micro-processor based energy saver window AC. The demonstration at different locations have indicated a savings of 20 to 25%. This is suitable for both window AC and split AC.
Traditional cooling with wind towers
By constructing wind towers and drawing fresh air intake through the towers and spraying water, similar to air coolers, we are able to reduce ambient air temperature by 5 to 7OC. By passing this cool air through the chiller water fan coil in the AHUs, we can reduce the air conditioning load of the building by 2 to 3%. This is being practised at the CII-Godrej Green Business Centre at Hyderabad, the first building outside USA to get platinum rating by LEED, USA.
Use of underground Earth Air Tunnel (EAT) to supply pre-cooled air
The principle of the tunnel is to take advantage of consistency in temperature through out the year at certain depth below ground. At a depth of four metre below the ground, the temperature remains constant, round the year and is equal to the annual average temperature of a place. For instance, Delhi this temperature is between 25 to 26OC. So, if air is passed through such earth tunnel, before funneling into a room, we can expect it to be cool in summer and warm in the winter. In this system air is passed through the underground pipes and then circulated in the rooms by AHU’s to reduce heat load. However the tunnels cannot remove the excess humidity from the air during monsoon, humid summers. So, additional chillers have to be installed to achieve the required comfort levels. The additional investment required to construct the Earth Air Tunnel can be paid back within a year. The TERI’s Gual Pahri campus near Delhi has incorporated this type of system.
Generation of hot water by waste heat recovery through de-superheating
We are more familiar with waste heat recovery from furnaces, boilers, DG sets etc. Waste heat recovery from refrigeration and Air conditioning system has become a reality. By de-superheating the refrigerant from the discharge of compressor, before sending to the condenser, we are able to produce hot water at 55 to 60OC. As a thumb rule, we can generate 20 lit/TR hot water with a ΔT of 30OC. This type of system is more useful for hotels.
A check list for energy saving in AC systems and buildings
- Tune up the HVAC control system
- Consider installing a Building Automation System or Energy Management System or restoring an out-of-service one
- Balance the system to minimise flows and reduce blower/fan/pump power needs
- Reduce reheat as much as possible
- Use appropriate HVAC thermostat setback
- Use morning pre-cooling in summer and pre-heating in winter (i.e., before electrical peak hours)
- Use building thermal lag to minimise HVAC equipment operating time
- In winter during unoccupied periods, allow temperatures to fall as low as possible without freezing water lines or damaging stored materials
- In summer during unoccupied periods,allow temperatures to rise as high as possible without damaging stored materials
- Improve control & utilisation of outside air
- Use air-to-air heat exchangers to reduce energy requirements for heating and cooling of outside air
- Reduce HVAC system operating hours (e.g., computer rooms)
- Provide dedicated outside air supply to kitchens, cleaning rooms, combustion equipment, etc to avoid excessive exhausting of conditioned air
- Use evaporative cooling in dry climates
- Reduce humidification or dehumidification where possible
- Use atomisation rather than steam for humidification where possible
- Clean HVAC unit coils periodically and comb mashed fins
- Upgrade filter banks to reduce pressure drop and thus lower fan power requirements
- Check HVAC filters on a schedule (at least monthly) and clean/change if appropriate
- Check pneumatic controls air compressors for proper operation,cycling, and maintenance
- Isolate air conditioned loading dock areas and cool storage areas using high-speed doors or clear PVC strip curtains
- Install ceiling fans to minimise thermal stratification in high-bay areas
- Relocate air diffusers to optimum heights in areas with high ceilings
- Consider reducing ceiling heights
- Eliminate obstructions in front of radiators, baseboard heaters, etc.
- Check reflectors on infrared heaters for cleanliness and proper beam direction
- Use professionally-designed industrial ventilation hoods for dust & vapour control
- Use local infrared heat for personnel rather than heating the entire area
- Use spot cooling and heating (e.g., use ceiling fans for personnel rather than cooling the entire area)
- Purchase only high-efficiency models for HVAC window units
- Put HVAC window units on timer control
- Don’t oversize cooling units, (oversized units will “short cycle” which results in poor humidity control)
- Install multi-fueling capability and run with the cheapest fuel available at the time
- Consider dedicated make-up air fir exhaust hoods. (Why exhaust the air conditioning or heat if you don’t need to?)
- Minimise HVAC fan speeds
- Consider desiccant drying of outside air to reduce cooling requirements in humid climates
- Consider ground source heat pumps
- Seal leaky HVAC ductwork
- Seal all leaks around coils
- Repair loose or damaged flexible connections (including those under air handling units)
- Eliminate simultaneous heating and cooling during seasonal transition periods
- Zone HVAC air and water systems to minimise energy use
- Inspect, clean, lubricate and adjust damper blades and linkages
- Establish an HVAC efficiency-maintenance programe. Start with an energy audit and follow-up, then make an HVAC efficiencymaintenance programme a part of your continuous energy management programe
The energy bill towards refrigeration and air conditioning alone is contributing to 50 to 60% in majority of the industries such as hotels, hospitals and commercial buildings. The air conditioning loads are increasing day by day _ and creating burden on our already overloaded electricity grid. By adopting best practices that are given in this article you will be able to save energy to the tune of 10 to 20% _ depending on the application. I remind, the best practices discussed in this article have already been implemented at may places in India, and they show great promise for large scale adoption in the near future.