Energy Saving Options Electricity

Electricity Generation

Electricity Generation During Electricity Generation, thermal energy is converted to work. The process is subject to the Carnot principle

Carnot Principle Heat Supplied, Qs at Ts W = Qs -Qr Efficiency = W/ Qs (Q = m cp T) So, Efficiency = Ts -Tr Ts Work, W Heat Rejected, Qr at Tr (Temperatures in degrees K)

Efficiency: Heat to Work If Ts = 600oC (steam) = 873 K and Tr = 350 oC = 623 K Efficiency = 873 - 623 873 = 28.6% Thus, 71.4% of the heat energy is rejected to the environment Heat Supplied, Qs at Ts Work, W Heat Rejected, Qr at Tr (Temperatures in degrees K)

Supply Capacity

The maximum kVA that can be taken from an electricity mains Supply Capacity The maximum kVA that can be taken from an electricity mains supply cable is limited by a fuse. Utilities charge a fixed amount monthly for each kVA of available supply capacity

Maximum Demand

The kVA being taken by the supply cable is measured continuously, Maximum Demand The kVA being taken by the supply cable is measured continuously, or at given intervals. The user pays a premium to the electrical utility according to the maximum value of kVA (the maximum demand) which occurs during the month

The Reduction of Maximum Demand The user should monitor the kVA readings to ascertain when the peak occurs and its magnitude

The user will then be in a position to reduce maximum demand charges by identifying activities that contribute to maximum demand rescheduling activities that occur at maximum demand time (peak lopping) staggerong start-up times using stand-by generators to peak-lop maximising power factors switching-off plant when not required

The Reduction of Maximum Demand It should be noted that each kW saved by electricity conservation, also saves 1 kVA of maximum demand charges

Electric Motors and Variable Speed Drives

Electric Motors ac Both the efficiency and the power factor of an electric motor vary with the load The largest potential savings with electric motors is to match motor to load so that the motor runs at maximum efficiency ac

Electric Motors ac A survey of all motors at a site should be made and efficiencies estimated. Those motors which are too large should be changed. Many times this involves swapping motors around the plant and buying only a few new motors ac

Variable Speed Drives ac In situations where the load on the motor fluctuates, the use of variable speed drives should be considered to avoid large heat losses at lower loads ac

Fans and Ducts DUCT AIR FAN

Fans and Ducts Power = flow rate of fluid x specific volume of fluid x pressure rise across fan /fan efficiency ac

To Reduce Power Requirements Fans and Ducts To Reduce Power Requirements 1. Reduce flow-rate of fluid 2. Decrease pressure drop in the duct ac

Flow Control can be achieved in two ways: Fans and Ducts Flow Control can be achieved in two ways: 1. A damper can be used to restrict the flow 2. The fan speed can be altered ac

Reducing Head Losses (Pressure Drops) restrictions, filters, dampers Fans and Ducts Reducing Head Losses (Pressure Drops) Minimise bends and elbows restrictions, filters, dampers and frictional forces

Pumps and Pipes PIPE LIQUID PUMP

Pumps and Pipes feed water pumps chilled water pumps condensate return pumps oil pumps process fluid pumps cooling tower water pumps

Pumps and Pipes Power = flow rate of fluid x specific volume of fluid x pressure rise across pump / pump efficiency ac

To Reduce Power Requirements Pumps and Pipes To Reduce Power Requirements 1. Reduce flow-rate of fluid 2. Decrease pressure drop in the pipe ac

and similarly, Flow Control can be achieved in two ways: Pumps and Pipes and similarly, Flow Control can be achieved in two ways: 1. A flow control valve can be used to restrict the flow, just as a duct damper 2. The pump speed can be altered just as the fan speed ac

Pumps and Pipes Considerations of selecting optimal pumps and speeds to match pipe flow rates are exactly as for fans and ducts.

Reducing Head Losses (Pressure Drops) Pumps and Pipes Reducing Head Losses (Pressure Drops) Minimise bends and elbows restrictions, orifices, valves vertical rises frictional forces

Compressors

Compressors Work in = pressure energy + change in internal energy m cp dT = pV + m cv dT m cp dT = m R dT cp = R + cv for air, 1005 = 287 + 718 J/kg K m = mass of gas cp,cv = specific heats at constant pressure and volume p = pressure dT = change in temperature R = characteristic gas constant

Compressors Thus 1005 units of work are required to produce 287 units of pressure energy, even at 100% efficiency of compression. Furthermore, the work (electricity) has been produced in the first place in the conversion of heat to work at 30% efficiency.

So it requires at least 3350 units of heat energy to produce 287 Compressors So it requires at least 3350 units of heat energy to produce 287 units of pressure energy, or 11.7 units of heat for 1 unit of pressure energy.

Compressors Compressed air is the most expensive energy commodity and should only be used as a last resort. Question every use for compressed air. It certainly should not be used for swarf blowing and cleaning purposes.

Compressors check conditions of plant check efficiency check position of inlet duct check maintenance procedures check control arrangements check the amount of compressed gas supplied check delivery temperature and pressure check for leaks

Compressors check uses for compressed gas check pressures at points of use reduce generating pressure to minimum consider interstage cooling consider interstage bleed-off at different pressures consider the use of localised booster compressors switch off compressors when not in use

Compressors consider the introduction of compressed gas accumulation so that off-peak electricity can be used or to peak lop maximum demand use outside air/water for cooling/intercooling recover heat from cooling and intercooling

Compressors avoid condensation and concomitant blockage of pipelines reheat compressed gas to increase discharge pressure meter compressed air usage look for heat recovery opportunities

Lighting

Lighting check zonal lighting requirements zone lighted areas check that parts of the building are not being lit unnecessarily use infra-red detectors/time switches check the maintenance procedures replace lamps when their efficiency drops check lighting controls use automatic controls

Lighting challenge the need for large areas of glazing eliminate glazing obtain economic balance of artificial lighting and day-lighting use separate circuits for day-lighted peripheries use separate circuits for use outside working hours

Lighting check colours of room surfaces check conditions and cleanliness of luminaires and windows keep windows and roof-lights clean avoid dark background colours never use low-efficiency filament lamps use low-energy fluorescent or discharge lamps look for heat recovery opportunities

Refrigeration and Air Conditioning

Refrigeration and Air Conditioning check maintenance and operating procedures evaluate load patterns and operating cycles check conditions of plant and equipment check for and seal leaks check insulation levels and conditions of insulation consider the use of thermal (cold) storage

Refrigeration and Air Conditioning check operation of condenser fans check cleanliness of heat transfer surfaces check water treatment systems check cooling tower operation check control arrangements check operating temperatures and pressures look for heat recovery opportunities

Refrigeration and Air Conditioning Select proper design points - temperature and humidity - thermal comfort chart Select minimum airchange rates - vent off pollutants at source check control arrangements check that heating and cooling systems cannot conflict reduce heating and cooling loads check zonal requirements

Refrigeration and Air Conditioning check for unoccupied areas consider zoning, partitioning, false ceilings and destratifiers minimise infiltration look for opportunities for heat recovery check insulation levels isolate system from the surroundings

On-site Cogeneration

On-site Cogeneration Examine the annual heating and power requirements for the site and consider whether the on-site generation of power with use of the heat generated might be an economic option. Such a system could also produce refrigeration via an absorption refrigeration system using the heat rejected from the power generator