1Reminders…• Inquire/Inform Assignment #1 due today in class• Finish reading textbook Chapter 3 and start Chapter 4• Homework #5 due next Wednesday• Research Project #1 details and topics given out on MondayWe will discuss about how the University of Colorado generates electricity and also heats the buildings.Co-GenerationSounds easy, but it is not….“The Cogeneration plant currently meets all the electrical requirements for the campus. Additionally, it exports an average of 8 MW of electricity to PSCo. The plant is operated and maintained by the staff within Facilities Management.” (2003)What fraction of CU electricity and heat come from the cogeneration facility burning natural gas in 2008?A) 100%B) 80%C) 50%D) 20%E) 0%The turbines in the cogeneration facility have been turned off. CU estimates that this saves $2M/year due to the rising cost of natural gas.Instead electricity is purchased from Xcel (mostly coal burning). Existing boilers for heat that were used as a supplementary heat system, now heat all the buildings.CU by state regulation is supposed to reduce emissions by 20% by 2020.The key question is what is the baseline year. I believe it is 2005 when the cogeneration facility was being run intermittently.We should check on what the University commitment is? And how it is stated?Note that the Universities energy usage per square foot is going down, but emissions are up. Burning natural gas in a cogeneration facility has much lower emissions than burning natural gas.Purchasing wind power is a minor perturbation. “In Sweden, nearly half the population lives in houses heated by ''district heating'': heat distributed from co-generation electricity utility plants. Unfortunately, the climate in most of our country dictates that peak electricity production occurs in the summer, because of widespread air-conditioning, and so most waste heat is produced in the summer, when it is not needed. Our most successful on-site cogeneration plants are in heavy industries, such as pulp and paper, where there is a steady year-round demand for heat to operate a process.”NYTimes2* Over 75% efficient !What if we do not want work (e.g. electricity), but fundamentally we want heat? Or we want things cool? (e.g. air conditioning, refrigerator,..)We need a “heat pump”.Heat EngineQ (hot)Heat energy inputQ (cold)Heat energy outputWork extractedHeat PumpQ (hot)Heat energy inputQ (cold)Heat energy outputWork input* Notice the change in direction of the arrows!If you have a hot region and a cold region, does heat energy naturally flow from cold to hot?If you put an ice cube into this hot coffee, does the coffee get hotter?Basically, a heat pump is like a heat engine run backwards (sometimes literally).In order to move heat energy from a cold region to a hot region, one must do work on the heat pump.Coefficient of Performance (COP) = Qhot/ WorkThe more heat you put into the hot reservoir per amount of input work energy, the better your COP.There are limitations given by conservation of Energy and Carnot (Thermodynamics):hotcoldQQW =+hotcoldhotcoldTTQQ=Coefficient of Performance (COP) = Qhot/ WorkcoldhothotcoldhothothotTTTQQQWQCOP−=−==That is the best you can ever get. Real COP is always less than this maximum.Apparently heat pumps work best if T(hot) is close to T(cold) (the opposite of heat engines). This makes sense since it is easier to heat a house up by a degree or two rather than by a large amount. The hotter you want the house the tougher it will be!3Heating your house with a heat pump.Or run in reverse to cool the house in summer.Note that you can instead use electrical heating. The electrical work done in the wire goes 100% into heating of your home. So an electrical heater Q(hot) = Work, Q(cold)=0, COP = 1.If you are using a heat pump to heat your house and T(hot) = 23 Celsius (room temperature) and T(cold) = 0 Celsius, what is the COP?12)0273()23273()23273(=+−++=−=coldhothotTTTCOPThis seems like a very good deal (at maximum). However, a typical commercial heat pump has COP ~ 3-4. Once the T(cold) is < 15 Fahrenheit they are not so good. Note they are also more expensive and harder to maintain. Note that for a refrigerator you are not so interested in Q(hot) / Work.You are interested in Q(cold) / Work. How much heat energy can you remove from inside the fridge for a given cost in electrical energy (Work).Energy Efficiency Rating (EER)= “What you want” / “What you pay for”coldhotcoldcoldhotcoldcoldTTTQQQWQEER−=−==“What you want” in this case is to remove heat energy from the cold region (inside the fridge)!RefrigeratorWarm (outside)Cool (inside)LiquidGasLiquid flowGas flowCompressorGas to liquidQLQHEasy to liquefyEvaporates at cold tempExpansionvalveThe preferred temperature is somewhere between 35 and 38 degrees F (1.7 to 3.3 degrees C). Anything higher and foods will spoil too quickly (it also presents food poisoning problems). Anything lower and freezing becomes a problem. If room temperature is 23 degrees C, what is the EER?15~coldhotcoldTTTEER−=coldhotcoldcoldhotcoldcoldTTTQQQWQEER−=−==Note that this EER is unitless. However, at a store they will quote you EER in (BTU/hour) / (Watts)Appliance Demand Electrical Power Input4Solar EnergySolar Energy:Good News: Completely renewable ☺At least for a few billion more years…Bad News: Finite rate available locally (“diffuse”) Most “renewables” are solar in origin (wind, biomass, hydro,…).To start we will focus on the use of direct sunshine.“Passive”: Use sun for heating.“Active”: Convert solar energy to other forms of energyExample: Photovoltaics (PV) electrical energySolar Powered Oil RigHow much of US annual energy consumption comes from renewables? How much from solar (directly)?A: Renewables ~ 15% and Solar ~ 5%B: Renewables ~ 10% and Solar ~ 2%C: Renewables ~ 7% and Solar ~ 2%D: Renewables ~ 7% and Solar << 1%E: Renewables ~ 2% and Solar <<1%Note that “<“ means less than and “<<“ means much less than.Answer = D. Renewables is mostly hydro, and amounts to about 7% of our energy. But Solar alone is extremely small still 0.06%. Clicker
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