Low e-coatings: Role of radiation loss

Transcription

1 Low e-coatings: Role of radiation loss To see if thermal radiation is even important, let s compare the heat lost through a window from convection vs. thermal radiation, assuming that all the thermal radiation is transmitted in both directions: Assume a double paned window with R value (RSI) =0.352 m 2 K/W (R2 in US units) Assume T out = 0 C and T in = 20 C Conduction: Q/(At)=ΔT/R=(20K)/(0.352m 2 K/W) = 56 W/m 2 Thermal Radiation: The house radiates outward at σt in 4 (power per square meter) The outside radiates back into the house at σt 4 out Net heat out through window is: σ(t in4 -T out4 ) =(5.67x10-8 Wm -2 K -4 )x((293k) 4 -(273K) 4 ) = 103 W/m 2 This is significant

2 Low emissivity windows A thin coating is applied to the inner surface of the window that is designed to reflect long wavelength radiation back to the inside We need a coating that will reflect λ=10 μm radiation Common choice = F-doped SnO (fluorine doped tin oxide) Coated glass Reflectivity Uncoated glass R value (ft 2 - Fhr/Btu) Double pane, 1/2" air space Double pane, 1/2" air space, low E glass Standard 2x4 wall, fiberglass insulation R value (RSI) (m 2 K/W)

3 Transportation: Roughly 1/3 of our current energy usage How can we reduce our fossil fuel usage in transportation? Gasoline powered cars are very inefficient Many factors work to reduce that actual overall efficiency measured at the driving wheels: -combustion losses ~60% (40% thermodynamic efficiency of combustion) -engine friction losses ~20% -transmission losses ~4% -accessories ~2% Net available power at drive wheels (road power) ~14% Next: Consider hybrids and electric vehicles

4 Hybrids: How they work Parallel hybrid: (Honda Insight). DC/AC Motor/generator Background: Gasoline engines have low torque at low RPM This means gasoline engines must be over-sized to give acceptable acceleration (poor fuel economy) Electric motors have high torque at low RPM and do not require a transmission (wide range of RPM) Parallel Hybrid Drivetrain: (gas tank) (gasoline engine) Uses highly fuel efficient Atkinson cycle gasoline engine Figs from Wikipedia: Two motors coupled through a common coupling. Power can be applied to the wheels by either motor at either time. Electric motor provides high torque for high acceleration. Energy is stored in battery. Allows use of a very efficient gas engine with a very narrow operating range. Regenerative braking recovers energy during braking (motor acts as a generator)

5 Series hybrid drivetrain (e.g. Chevy Volt): Motor/generator DC-AC converter Drive wheels are only powered by electric motor. Uses highly efficient gasoline engine which is only used to generate electricity. Electric motor can function as a generator during regenerative braking as before This simplifies the drive train by only having one engine coupled to the wheels Uses a larger battery than parallel hybrid Fig from Wikipedia:

6 Electric Cars Advantages: Electric motors have extremely high power to weight ratios Very low friction losses (no sliding pistons) large range in RPM => no transmission is required high torque (acceleration) at low rpm zero local pollution emissions quiet batteries can be used as grid storage reservoirs for intermittent renewable energy Disadvantages: Battery technology: cost, lifetime, range, charge time Electricity must be generated from fossil fuels in most places

7 Cost of fuel: gasoline engine vs. electric motor Question from Midterm 2 (modified) Q2: The Chevy Volt car is powered by a lithium ion battery module operating at 300V with a total capacity of 45 A-hr. (a) How much current does the battery supply at the peak rated power output of 111 kw? (b) How much energy can the battery store? Express in kw-hr and MJ. (c) If the car requires 2.4 kw-hr to travel 10 km, how far can it travel on a single battery charge with no gasoline backup? (d) If the energy for recharging a battery is purchased from the power company at $0.08/kW-hr, what is the cost of travelling 100 km? (e) If gasoline costs $1.30 per liter what is the cost of running the car on gas for 100 km, assuming a fuel consumption of 5 liters/100 km?

8 Solution: (a) P=IV. I=P/V = 111x10 3 W/300V=370A (b) Stored energy ΔU=QV = (45 A-hr)*(300V) = 13,500 W-hr=13.5 kw-hr In MW: (13,500kW-hr)*(3600s/1s)=48.6 MJ (c) 2.4 kw-hr to travel 10 km Total distance: (13.5kW-hr)/(10 km) = 56 km (d) Total energy = (2.4 kw-hr)/(10km)*(100km) = 24 kw-hr Cost = (24 kw-hr)*($0.08/kw-hr) = $1.92 (e) (5l/100km)*100km*$1.5/l= $6.50

9 The previous example assumed $0.08/kWh for electricity (BC) What if we just used oil to generate the electricity? 1 bbl oil ~ 5.8x10 9 J thermal equivalent Burned in a thermal power plant with 35% efficiency this gives: (5.8x10 9 J )x0.35 = 2.0x10 9 J In kw-hr: (2.0x10 9 J)x(1hr/3600s) = 564 kwhr At $100/bbl, this gives a fuel cost estimate of ($100)/(564 kwhr) = $0.18/kWh Cost to travel 100km: (24kWh)*($0.18/kWh)= $4.32 < $6.5 (gasoline engine)

10 Repeat calculation for natural gas: Natural gas units: 1 GJ ~ 26 m 3 by volume assuming typical energy content Current cost of gas delivered to residential user ~ $3.5/GJ Again, take 35% efficient gas turbine power plant: Natural gas prices GJ of gas gives 0.35GJ of electricity at a cost of $3.5 This gives (3.5x10 8 J)*(1hr/3600s)= (97.2 kwh) Cost = $3.5/97.2kWhr = $0.036 /kwhr Repeat previous calculation: Cost to travel 100km= (24kWh)*($0.036/kWh)= $0.86 i.e. much lower than gasoline ($6.5)

11 Carbon emissions/fuel consumption, automobiles Chevy Volt on electric power 24 kwhr/100km McKay, page 122

12 Energy consumption of various transportation modes: kwh/person/100km McKay, page 128