Basic Energy Units, Terms and Calculations.ppt

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1、Basic Energy Units, Terms and Calculations,War Eagle!,Roll Tide!,Energy Units,Need to distinguish between energy and power Common energy units: Btu (British Thermal Unit)- energy required to heat one lbm of water one degree Fahrenheit 1 Btu = 778.16 ft-lbf = 1.055 kJ = 0.252 Cal Commonly used measur

2、e of fuel energy, heating and cooling quantities 1 MBtu = 1000 BTU; 1 MMBtu = 106 Btu 1 Quad = 1015 Btu (billion 109, trillion 1012, quadrillion) 1 Q = 1018 Btu,Energy Units,kJ- standard SI-mks energy unit 1 kJ = 1000 J = 1000 N-m kJ used to measure fuel energy and heating and cooling quantities kWh

3、- used to measure electrical energy 1 kWh = 3412 Btu = 3600 kJ = 860 Cal Calorie- used to measure food energy, technically should be called a “kilocalorie.” Chemists “calorie” (lower case c) is the energy needed to raise 1 g of water 1 degree C. 1 Cal = 1000 cal = 4.186 kJ = 3.968 Btu,Power Units,Ho

4、rsepower- used to measure rate of mechanical work 1 hp = 2545 Btu/hr = 0.746 kW kW- SI power unit used for both work and heat transfer. Sometimes see “kWth” for thermal kW. 1 kW = 3412 Btu/hr = 1.34 hp = 0.2843 Tons Ton- American unit of cooling rate commonly employed to measure air conditioning cap

5、acity 1 Ton = 12,000 Btu/hr = 3.517 kWth,Performance Measures,The following measures are used by energy engineers to rate equipment performance: Mechanical efficiency is used to measure efficiency of transforming one form of work to another. The theoretical limit is 100%.,Performance Measures,Therma

6、l efficiency measures efficiency of converting heat to work (heat engines). The upper limit is fixed by heat engine Carnot efficiency.Heat rate is the reciprocal of thermal efficiency, and represents fuel energy in Btu required to produce a kWh of electricity.,Performance Measures,Annual fuel utiliz

7、ation efficiency is the average fraction of fuel energy converted to useful heating by a furnace or hot water heater. AFUE 1.Coefficient of Performance is the dimensionless measure of effectiveness of delivering a desired heat transfer rate (heating, cooling) with an electrical power input. The maxi

8、mum COP is limited by Carnot refrigeration cycle efficiency.,(DOE),Performance Measures,Energy efficiency ratio is the same as COP used for cooling except it has dimensions of cooling in Btu/hr per W of electricity input. Typically the seasonal EER SEER is used to rate air conditioners.,(DOE),Perfor

9、mance Measures,Heating Seasonal Performance Factor is the same as COP used for heating except it has dimensions of cooling in Btu/hr per W of electricity input. HSPF is the heating equivalent of SEER and is used to rate heat pumps in heating mode.,(DOE),Energy Calculation Examples,A typical home nat

10、ural gas furnace is rated at 75,000 Btu/hr. A typical electric space heater ($20 at Wal-Mart) is rated at 1500 W. About how many electric space heaters would be required to provide the heat output of the gas furnace? A coal-fired powerplant produces 1200 MW of electricity using 380 tons/hr of coal w

11、ith a heating value of 14,000 Btu/lbm. Find the thermal efficiency and heat rate of the plant.,Energy Calculation Examples,A 3.5 Ton air conditioner with an SEER of 10 Btu/W-hr runs 20% of the time over a 30 day period. If electricity costs 8/kWh, how much does it cost to operate the A/C over this p

12、eriod? A natural gas water heater has an AFUE of 80%. Over a month the water heater heats 10,000 gallons of water from 60 to 130F. Natural gas has a heating value of 1000 Btu/ft3 and costs about $1 per CCF (1 CCF = 100 ft3). What is the water heater operating cost?,Energy Calculation Examples,A $150

13、 garage door opener has a hp motor with a motor efficiency of 20%. Motor efficiency is the same as mechanical efficiency because it represents the fraction of electrical work input converted to mechanical shaft work output. An open/close cycle takes 20 s and electricity costs 8 kWh. A car at idle uses about 10 hp with thermal efficiency of 15%. Gasoline has a heating value of about 125,000 Btu/gal, and costs about $1.40/gal. If the car operates 40 s longer for the driver to manually open the garage door, how many open/close cycles are needed to pay the costs of the garage door opener?,