3Electricity Generation During Electricity Generation,thermal energy is converted to work.The process is subject to the Carnotprinciple
4Carnot Principle Heat Supplied, Qs at Ts W = Qs -Qr Efficiency = W/ Qs (Q = m cp T)So, Efficiency = Ts -TrTsWork, WHeat Rejected, Qr at Tr(Temperatures in degrees K)
5Efficiency: Heat to Work If Ts = 600oC (steam)= 873 Kand Tr = 350 oC= 623 KEfficiency =873= %Thus, 71.4% of the heatenergy is rejected to theenvironmentHeat Supplied, Qs at TsWork, WHeat Rejected, Qr at Tr(Temperatures in degrees K)
7The maximum kVA that can be taken from an electricity mains Supply CapacityThe maximum kVA that can betaken from an electricity mainssupply cable is limited by a fuse.Utilities charge a fixed amountmonthly for each kVA ofavailable supply capacity
9The kVA being taken by the supply cable is measured continuously, Maximum DemandThe kVA being taken by the supplycable is measured continuously,or at given intervals.The user pays a premium to theelectrical utility according to themaximum value of kVA(the maximum demand)which occurs during the month
10The Reduction of Maximum Demand The user should monitor the kVA readingsto ascertainwhen the peak occursand its magnitude
11The user will then be in a position to reduce maximum demand charges by identifying activities that contribute to maximum demandrescheduling activities that occur at maximum demand time (peak lopping)staggerong start-up timesusing stand-by generators to peak-lopmaximising power factorsswitching-off plant when not required
12The Reduction of Maximum Demand It should be noted that each kWsaved by electricity conservation,also saves 1 kVAof maximum demand charges
14Electric Motors ac Both the efficiency and the power factor of an electric motor vary with the loadThe largest potential savings with electricmotors is to match motor to load so that themotor runs at maximum efficiencyac
15Electric Motors ac A survey of all motors at a site should be made and efficiencies estimated.Those motors which are too large shouldbe changed.Many times this involves swapping motorsaround the plant and buying only a fewnew motorsac
16Variable Speed Drives ac In situations where the load on the motor fluctuates, the use of variable speeddrives should be considered to avoidlarge heat losses at lower loadsac
23Pumps and Pipes feed water pumps chilled water pumps condensate return pumpsoil pumpsprocess fluid pumpscooling tower water pumps
24Pumps and Pipes Power = flow rate of fluid x specific volume of fluid x pressure rise across pump/ pump efficiencyac
25To Reduce Power Requirements Pumps and PipesTo Reduce Power Requirements1. Reduce flow-rate of fluid2. Decrease pressure drop in thepipeac
26and similarly, Flow Control can be achieved in two ways: Pumps and Pipesand similarly, Flow Control can be achieved in two ways:1. A flow control valve can beused to restrict the flow, justas a duct damper2. The pump speed can be alteredjust as the fan speedac
27Pumps and PipesConsiderations of selecting optimal pumps and speeds to match pipe flow rates are exactly as for fans and ducts.
28Reducing Head Losses (Pressure Drops) Pumps and PipesReducing Head Losses (Pressure Drops)Minimisebends and elbowsrestrictions, orifices, valvesvertical risesfrictional forces
30Compressors Work in = pressure energy + change in internal energy m cp dT = pV+ m cv dTm cp dT = m R dTcp = R + cvfor air,1005 =J/kg Km = mass of gascp,cv = specific heats atconstant pressureand volumep = pressuredT = change intemperatureR = characteristic gasconstant
31Compressors 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 beenproduced in the first place in the conversionof heat to work at 30% efficiency.
32So it requires at least 3350 units of heat energy to produce 287 CompressorsSo it requires at least 3350 unitsof heat energy to produce 287units of pressure energy,or 11.7 units of heat for 1 unit ofpressure energy.
33CompressorsCompressed 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 swarfblowing and cleaning purposes.
34Compressors check conditions of plant check efficiency check position of inlet ductcheck maintenance procedurescheck control arrangementscheck the amount of compressed gas suppliedcheck delivery temperature and pressurecheck for leaks
35Compressors check uses for compressed gas check pressures at points of usereduce generating pressure to minimumconsider interstage coolingconsider interstage bleed-off at different pressuresconsider the use of localised booster compressorsswitch off compressors when not in use
36Compressorsconsider the introduction of compressed gas accumulation so that off-peak electricity can be used or to peak lop maximum demanduse outside air/water for cooling/intercoolingrecover heat from cooling and intercooling
37Compressorsavoid condensation and concomitant blockage of pipelinesreheat compressed gas to increase discharge pressuremeter compressed air usagelook for heat recovery opportunities
39Lighting check zonal lighting requirements zone lighted areas check that parts of the building are not being lit unnecessarilyuse infra-red detectors/time switchescheck the maintenance proceduresreplace lamps when their efficiency dropscheck lighting controlsuse automatic controls
40Lighting challenge the need for large areas of glazing eliminate glazingobtain economic balance of artificial lighting and day-lightinguse separate circuits for day-lighted peripheriesuse separate circuits for use outside working hours
41Lighting check colours of room surfaces check conditions and cleanliness of luminaires and windowskeep windows and roof-lights cleanavoid dark background coloursnever use low-efficiency filament lampsuse low-energy fluorescent or discharge lampslook for heat recovery opportunities
43Refrigeration and Air Conditioning check maintenance and operating proceduresevaluate load patterns and operating cyclescheck conditions of plant and equipmentcheck for and seal leakscheck insulation levels and conditions of insulationconsider the use of thermal (cold) storage
44Refrigeration and Air Conditioning check operation of condenser fanscheck cleanliness of heat transfer surfacescheck water treatment systemscheck cooling tower operationcheck control arrangementscheck operating temperatures and pressureslook for heat recovery opportunities
45Refrigeration and Air Conditioning Select proper design points - temperature and humidity - thermal comfort chartSelect minimum airchange rates - vent off pollutants at sourcecheck control arrangementscheck that heating and cooling systems cannot conflictreduce heating and cooling loadscheck zonal requirements
46Refrigeration and Air Conditioning check for unoccupied areasconsider zoning, partitioning, false ceilings and destratifiersminimise infiltrationlook for opportunities for heat recoverycheck insulation levelsisolate system from the surroundings
48On-site Cogeneration Examine the annual heating and power requirements for the site and consider whether the on-sitegeneration of power with use of the heat generatedmight be an economic option.Such a system could also produce refrigerationvia an absorption refrigeration system using theheat rejected from the power generator