Climate stabilization (at 450 ppm CO2) requires global emissions to peak by 2015and to fall to ~80% below 2000 levels by 2050 Slide: Jim Williams Source: Intergovernmental Panel on Climate Change, Climate Change 2007: Synthesis Report
Physical Meaning of Energy:Energy = the ability to do work Force distance Work = Force · distance
Energy = the ability to do workPotential energy = mgh(mass, gravitational acceleration, height) velocity Kinetic energy = ½ mv2 (mass, velocity)
Examples of EnergyNatural gas in the pipeline (chemical)Gas flame on my kitchen stove (chemical to thermal)Hot water in the kettle (thermal)Electricity in the wall outlet (electrical)Spinning blade of the coffee grinder (mechanical kinetic)Pancakes & maple syrup (chemical)Vase sitting on top shelf (mechanical potential)Vase falling down to floor (mechanical kinetic)Radioactivity (nuclear to radiant)Sunshine (radiant to thermal)Wind (mechanical kinetic)
Because a measurable quantity of energy is conservedduring any conversion of one form to another,it makes sense to give a single name to that quantity.
Matter and Energy Resources“High Quality” means High quality energy: concentrated mechanical, electrical, radiant pure easy to use in an orderly state Medium quality energy: nuclear, chemical“Low Quality” means dispersed impure more difficult to use Low quality energy: disordered thermal (heat)
2nd Law requires:Some of the chemical fuel energy will be degraded into heat.The amount of mechanical work or electricity produced will be lessthan the fuel input.
Basic lesson:Use energy sources matched in quality with end use needs.
Units of energy: Units of power:calories calories per hourkilocaloriesjoules joules per second = wattskilowatt-hours (kWh) kilowatts (kW)British Thermal Units (BTU) BTU per hourtherms (105 BTU)quads (1015 BTU) Power = energy per unit time
Electric usage 232 kWh $0.11/kWhGas usage 52 therms $0.71/thermConversion factors: 1 therm = 100,000 Btu = 105 Btu 1 kWh = 3,413 BtuQuestions:• Which is my greater energy consumption – electricity or gas?• Which is more expensive per unit energy – electricity or gas?
Electric usage 232 kWh $0.11/kWhGas usage 52 therms $0.71/thermConversion factors: 1 therm = 100,000 Btu = 105 Btu 1 kWh = 3,413 BtuConvert 232 kWh into therms by multiplyingby the conversion factors (3,413 Btu / kWh) and (1 therm / 105 Btu):232 kWh x (3,413 Btu / kWh) x (1 therm / 105 Btu) = 7.9 therms → I use 7.9 therms worth of electricity
$ 0.115 / kWhPG&E electric rates have stayed about the sameover the past five years
$ 1.04 / therm $ 0.92 / thermPG&E gas rates have gone up from $0.70 / therm
Electric rate $ 0.115 / kWhGas rate $ 0.92 – 1.04 / thermWhich is more expensive, gas or electricity?Conversion factors: 1 therm = 100,000 Btu = 105 Btu 1 kWh = 3,412 Btu$0.115/kWh x (1 kWh/3,412 Btu) x (105 Btu/therm) = $3.37/therm→ electricity is over three times as expensive as natural gas
Passive Solar Design Principle #2:Remember conduction, convection and radiative heat transfer.
Q = m c ∆TQ is amount of heat storedm is massc is specific heat∆T is temperature difference before/after
Outside and Inside Temperatures 100 90Degrees Fahrenheit 80 70 60 50 40 30 TEMP OUSTIDE 20 TEMP INSIDE 10 0 12:00 6:00 AM 12:00 6:00 PM 12:00 6:00 AM 12:00 6:00 PM 12:00 6:00 AM 12:00 6:00 PM 12:00 6:00 AM 12:00 6:00 PM 12:00 6:00 AM AM PM AM PM AM PM AM PM AM 06/27/01 to 07/01/01
Passive Solar Design Principle #3:Store warmth or coolth in thermal mass.
R-value: thermal resistanceU-value: thermal conductance, R = 1/U
Heat flow example:R-20 wallU = 0.05 Btu/h-ft2-oFArea = 100 ft2∆T = 30oFWhat is the rate of heat loss?Q = U A ∆T = (0.05 Btu/h-ft2-oF) × (100 ft2) × (30 oF) = 150 Btu/h
Note:U-value is weighted average of framing and area between framing.Any air gap between insulation & framing ruins the insulating effect.
Ballpark value for residential building envelope:UA = 500 Btu/h-oFHow much heating energy does it take?Convenient characterization of heating climate:“Degree-days” DD actually oF-d or ∆T-days
443 Degree-Daysin San Francisco for themonth of January3001 Degree-Daysfor the whole yearFor example:300 days of ∆T = 10oF
UA = 500 Btu/h-oFHow much heating energy does it take?San Francisco heating climate: 3001 DDQ = U A ∆T-days × hours/day = (500 Btu/h-oF) × (3001 oF-d) × (24 h/d) = 36 million Btu = 360 therms
Passive Solar Design Principle #4:Insulate well.
SHGC: fraction of solar gain admittedthrough windowPerformance trade-off with U-valuefor solar heatingU-value: thermal conductanceU = 1/R0.35 Btu/hr-ft2-oF ≈ R-3
Passive Solar Design Principle #5:Be smart about windows.
Passive Solar Design Principle #1:Think about where the sun is going to be.#2Remember conduction, convection and radiative heat transfer.#3Store warmth or coolth in thermal mass.#4Insulate well.#5Be smart about windows.
Heat loss by conduction, convection and infrared radiationHeat gain bysolar radiation Building envelope
Heat gain by conduction, convection and infrared radiationHeat gain bysolar radiation
Heat loss by conduction, convection and infrared radiationHeat gain from naturalgas via hydronic floor
Question: Should I turn the heater off Heat loss by conduction,while I’m gone? convection and infrared radiation YES! Driven by temperature difference between inside and outside Replaces heat lost through envelope Heat gain from natural gas via hydronic floor
Basic principle for smart energy use in any building:Think of heat flow through the envelope.
South Texas Project, Photo: www.nielsen-wurster.com
CCS: Carbon Capture and Storage or Carbon Capture and SequestrationProblematic issues:• sheer quantity of carbon• no inherent performance incentive• verification• permanence of disposal
Nuclear energyProblematic issues:• “vulnerability to human frailty, incl. stupidity and malice” (John Holdren)• slow, committing infrastructure investment• ethical concerns
Portfolio of renewable energy resourcesProblematic issues:• spatial and temporal constraints on energy availability• requires sophisticated, integrated planningIn my opinion, these are the most readily solvable problems.
Exclusion zone radius 18 km, area 109 m2Incident solar radiation 1000 W/m2at conversion efficiency 0.1could generate 108 kW or 100 GW of solar powerat capacity factor 0.2 would produce 5% of U.S. electric energy