Recap: Greenhouse Effect

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1 Recap: Greenouse Effect Relies on fact tat glass (or plastic) is transparent to visible radiation but opaque to infra-red (IR) radiation. E,g. Car window closed visible radiation only transmitted. Car window open you absorb visible, IR and ultra-violet radiation get sun burnt! Greenouse: Glass traps air (and ence reduces convection). Te air is ten eated up by solar radiation Steps: 1.Visible ligt passes troug glass into greenouse and is absorbed by soil, plants etc. (IR is reflected). 2.Soil eats up and emits IR radiation. 3.IR radiation is reflected back into greenouse by glass walls and by te roof (its trapped!). 4.Radiation is absorbed by soil etc. and greenouse eats up (until balanced by conduction loss at wall etc.). 5.Can get very ig temperatures inside (on a sunny day) even if cold outside Open windows to let eat escape (car too!).

2 Atmospere: Global Warming Main greenouse gases Te CO 2 and H 2 O gases in atmospere are opaque to IR radiation and ence trap eat in lower atmospere. Carbon dioxide (CO 2 ) and water Trapped IR ligt Eart Visible ligt from Sun Carbon dioxide vapor (H 2 O) are in large quantities in te atmospere. CO 2 produced by volcanoes, burning fossil fuels etc. is moderated by plant absorption (and by oceans). Rise in CO 2 acts to trap eat wic in turn will create more H 2 O vapor and problem worsens! Produces overall increase in global temperature and muc more varied and potentially violent weater. Could result on melting polar caps and consequent sea level rise Also cange in salinity can cause deep ocean currents (e.g. Gulf stream) to stop e.g. causing Europe to freeze up in winter.

3 Heat Engines (Capter 11) Question: Wat is a eat engine? Answer: A device tat takes in energy from a warm source and converts a fraction of te termal energy into mecanical energy (i.e. work). Heat engines are essential for our everyday life: - Steam engine (power stations) - Internal combustion engine (automobiles) - Jet engine (aircraft transportation) - Rocket motor (spacecraft) - Nuclear power / solar power / geotermal Heat engines convert termal energy into mecanical energy. But not all of te energy can be converted to perform useful work.

4 A quantity of eat (Q ) is taken Hig temperature (T from ig temperature reservoir (T ). ) Some is converted into work (W) and Q te rest of te eat (Q c ) is released into lower temperature sink (T c ). Q c W Te ig temperature reservoir is eated e.g. by fuel combustion, Lower temperature (T c ) nuclear reactions, solar radiation. Te lower temperature sink carries off waste eat tat is not used for conversion into work (dumped into te environment). Example: Hot car exausts, power station cooling towers, cooling fins /radiators Qu: Wy don t we use te waste eat to do more work? Answer: Fundamental pysical laws governing conversion of eat to work tat require a fraction of eat from source to be rejected at a cooler temperature tan source! Tis means eat engines can never be 100% efficient!

5 Efficiency of a Heat Engine Efficiency (e) = Work done Heat input into system W = Work done by engine on its surroundings (+ve) Q = Quantity of eat taken from source to perform work. Engines usually function in cycles were te engine repeats te same process over and over. Efficiency is computed using te eat and work values for one (or several) complete cycles. Example: A eat engine takes 1500 J of energy from a ig temperature source in eac cycle and does 300 J of work in eac cycle. e = W = 300 = Q 0.2 or 20 % = 1500 J Q 1500 W = 300 J Result: Not very efficient (1200 J of energy lost) but tis is typical of many engines. = W Q

6 Internal Energy (ΔU) As an engine returns to its initial state at te end of eac cycle, its internal energy remains uncanged. i.e. te cange in internal energy (ΔU) over one cycle is zero. or ΔU = 0 However, te first law of termodynamics states tat te cange in internal energy of a system equals te amount of eat added minus te work done by te system: Q net eat ΔU = Q net W out flow For a eat engine ΔU = 0 (over 1 complete cycle). U W Tus: W = Q net or W = (Q ot Q cold ) Work done in one cycle equals te net eat flow into and out of engine (conservation of energy). W Q Q Q c Engine efficiency: e = = c or e = % Q Q Q

7 Carnot Engine Sadie Carnot, France (early 19 t century) developed an ideal eat engine (i.e. engine operation completely reversible). Carnot reasoned tat greatest efficiency of a eat engine is given by taking in all of te input eat at a single ig temperature and releasing all te unused eat at a single low temperature. Tis is analogous to a water weel operation. Carnot determined tat in an ideal eat engine te ratio of two energy terms is identical to te ratio of temperatures: Q cold T = cold were T is in Kelvin Q ot T ot Te efficiency of an ideal (Carnot) engine is: e c Q = c 1 or Q e c = 1 T T c

8 Results: Carnot Engine Te Carnot efficiency is te maximum possible efficiency for a eat engine. Remarkably, te efficiency only depends on te temperatures of te two reservoirs between wic te eat engine operates! To obtain ig engine efficiency it is terefore essential to operate wit a large temperature difference as possible. Consequences: Only a eat engine wose cold reservoir was at zero Kelvin or wose ot reservoir was infinite could operate at a 100 % efficiency.

9 Example: Maximum efficiency of a coal-fired power station? Carnot efficiency: e c = = T ot = 550 ºC (823 K) T cold = 27 ºC (300 K) or e c = 64 % Tus, 36% of eat is wasted by rejecting it troug te large cooling towers into surrounding atmospere. Tis represents maximum possible efficiency for tis temperature difference. In practice te real efficiency is somewat lower. Practical considerations: - limit maximum temperature (metal softening) - minimum temperature limited by nearby lake or river temperature.

10 Second Law of Termodynamics Developed by Lord Kelvin, U.K. (19 t century) based on Carnot s work. No engine, working in a continuous cycle, can take eat from a reservoir at a single temperature and convert it ALL to work. Tis is a re-statement tat no engine is 100 % efficient. Te 2 nd law also sows tat no engine can ave a greater efficiency tan a Carnot engine operating between two given temperatures. Tus, a eat engine wit an efficiency greater tan Carnot engine (for tat temperature difference) would violate 2 nd law of termodynamics. Te 2 nd law is a natural law (not derived from matematics). It cannot be proven but in time as sown itself to be an accurate statement on eat transfer, eat engines and eat pumps.

11 Heat Pumps (Refrigerators) A refrigerator is a eat engine running in reverse. A refrigerator keeps food cold by pumping eat out of its colder interior into te warmer room. A refrigerator warms te room considerably. A pump (electric motor) does work on te eat engine causing eat to be removed from te lower temperature reservoir and deposited in te iger temperature reservoir. In doing so a greater amount of eat (Q ) is released in te upper reservoir tan taken from te lower reservoir. Tis does not violate laws of termodynamics or conservation of energy (as diagram sows). i.e. ΔU = 0 and Q = Q c + W (work done ON system) T T c Q Q c W

12 Clausius Statement of 2 nd Law of Termodynamics Heat will not flow from a cooler body to a otter one unless anoter process (e.g. work) is involved. Heat pumps are remarkable devices as te amount of eat transferred (pumped upill ) can far exceed te work input (as Q = W + Q c ). Heat pumps can terefore deliver quantities of eat tat are several times te amount of electric energy supplied as work! Coefficient of performance (COP) is ratio of eat delivered (Q ) divided by work input (W): Q T COP = = W T (T is in Kelvin) T c Te COP is terefore iger wen T and T c are similar (i.e. ΔT is not large.) If (T T c ) is large requires more work to pump eat upill to te ot reservoir. (Heat pumps are not good in Uta.)

13 A eat pump uses two sets of coils for eat excange between outside and inside air. Inside: Electric motor compresses gas raising temperature and pressure. Cold air Gas ten condenses in eat excanger giving off eat to inside room. Outside: liquid vaporizes as it passes troug an expansion valve and takes in eat from surrounding air. Net result: Cool outside, warm inside Expansion valve Compressor Warm air Heat transfer (a refrigerator uses te same mecanism). If outside air temperature is too low (i.e. COP low) ten we can use ground temperature at a few meters dept (or a river) for te cold reservoir to improve COP. Heat pumps can also be used as air conditioners in summer (more complex arrangement). COP is typically 3 for a practical system (i.e. 3 times more eat transferred tan work done).

14 Power Plants Excluding ydroelectric and wind power, most electricity is generated from termal power sources tat use eat engines. Fossil fuel: power plant operates at ig temperature (~ 500 to 600 ºC) wit te lower temperature, below 100 ºC. Typical efficiency ~ 40 to 50 %. (So at best ~ 50% of fuel energy is lost to environment). Waste eat can be used for space eat (or agriculture) but not often as power stations distant from cities. (CO 2 waste). Nuclear power: less efficient ~ 30 to 40% (as upper temperature lower). No CO 2 emission but dangerous, longlived, radioactive waste instead! Geotermal power: muc less efficient as temperature difference smaller (typically 20 to 25 % efficient, ΔT ~ 150 ºC) but clean and free! Ocean currents: temperature difference only ~ 20 ºC. Results in very low efficiency (~ 7 %), but clean and free!