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Infiltration

Infiltration. Q = 0.018 Btu/ft 3 .hr.F o V K D T. Here K is the number of “Air exchanges per hour” and V is the interior volume of the house/building. Note: some exchange of air is necessary (you need to breath!), and this is not readily apparent in this figure.

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Infiltration

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  1. Infiltration Q = 0.018 Btu/ft3.hr.Fo V K DT Here K is the number of “Air exchanges per hour” and V is the interior volume of the house/building. Note: some exchange of air is necessary (you need to breath!), and this is not readily apparent in this figure.

  2. Some typical R values

  3. Degree-Days Heating/Cooling Indianapolis http://www.ersys.com/usa/18/1836003/wtr_norm.htm

  4. Price of Natural Gas(dollars/MBtu wholesale I believe) http://futures.tradingcharts.com/chart/NG/W

  5. Double metal layer Single metal layer Conducting oxide “Low-e” (emissivity) coatings on windows Phys. Today Nov. 2000

  6. Basic Heat Engine Recall that this works because the entropy increase at Tc is greater than the entropy decrease at TH. Efficiency = h = W/QH = (QH-QC)/QH

  7. Heat Pumps (one system: heating and cooling) Heat output is greater than work in! As we saw with heat engines, the second law of thermodynamics puts limits on how much cooling you can get for a given amount of work (entropy of the Universe must not decrease!! => Heat out to warm reservoir must exceed heat in from cold res.)

  8. Coefficient of Performance • As the outside temperature goes down, the performance of a heat pump also goes down. You can, however, design the system to exchange heat with the earth instead of the air (geothermal systems sold locally)! COP = |Qc|/|Work done| = |Qc|/(|QH|-|Qc|) (clearly a term originally defined for refrigerators)

  9. RENEWABLE ENERGY(US distribution in 2007) http://www.eia.doe.gov/fuelrenewable.html Note: the text has this figure from 2003. In four years Hydro has gone down from 45 to 36%, Biomass has grown from 47 to 53% and wind from 2 to 5%. Overall, renewables have grown from 6% to 7% of the total national energy budget (a 15% increase in four years).

  10. Solar Energy basics http://eosweb.larc.nasa.gov/EDDOCS/images/Erb/components2.gif

  11. Components of solar Energy on Earth H&K fig 6.7

  12. Spectrum of Solar radiation at the Earth’s surface H&K fig 6.2 SOLAR CONSTANT: 1360 W/m2 or 450 Btu/ft2.hr (at the top of the atmosphere.) Recall, per capita energy consumption is: ??

  13. Spectrum of Solar radiation at the Earth’s surface H&K fig 6.2 • SOLAR CONSTANT: • 1360 W/m2 or • 450 Btu/ft2.hr • (at the top of the atmosphere.) • Recall, per capita energy consumption is: 11kW • 8m2/person or 2500 km2 (i.e. a square 50 km on a side) could supply us all (in principle, but there are some practical problems, which make the actual area needed more like 230x230km).

  14. Insolation (Btu/ft2.day) Horizontal surf. surf. at q = latitude Mean monthly T (oF) H&K Appendix D • Only about ½ of the incident sunlight reaches the surface • The energy per unit area depends on the viewing angle, which changes daily and seasonally • Weather, elevation, humidity all can have an impact on the energy at the surface

  15. H&K fig 6.8, 6.9 & 6.32 Clear Day Insolation as a function of collector angle

  16. Fundamental components(any solar energy system) • Solar collector • Storage system of some sort (to account for night and cloudy days). • Energy transfer fluid (which could be air, as in some systems we have seen, water, antifreeze, or even electrons) • Auxilliary/backup system (typically)

  17. Typical Passive Domestic solar heating systems H&K 6.26 “Trombe” Wall H&K 6.24

  18. Typical Active Domestic solar heating system

  19. E.G. Domestic hot-water system

  20. Fundamental components(any solar energy system) • Solar collector • Storage system of some sort (to account for night and cloudy days). • Energy transfer fluid (which could be air, as in some systems we have seen, antifreeze, or even electrons) • Auxilliary/backup system (typically) • Q: Does solar energy go to zero on cloudy days?

  21. Components of solar Energy on Earth H&K fig 6.7

  22. H&K Problem 6-4 • What size flat plate solar collector is needed to supply a family’s domestic hot water (DHW) needs in March in Denver CO? Assume 80 gal/day are needed (1gal=8.3 lb), DT=70Fo for water, and that the collector-heat exchange system has an average efficiency of 40%. The collector tilt angle is set to the latitude of Denver (39o 10’). [DVB: Recall the specific heat of water is 1BTU/lb.Fo]

  23. Insolation (Btu/ft2.day) Horizontal surf. surf. at q = latitude Mean monthly T (oF) H&K Appendix D

  24. Typical collector design(fig 6.18) Can we understand the design criteria for each of these components? What happens if you run such a collector too hot?

  25. Focusing collectors(parabolic)

  26. National Solar (thermal) test Facility (Sandia New Mexico) 5 MW of thermal power for (with 222 “heliostats”) $21M (1978 $’s) http://www.sandia.gov/Renewable_Energy/solarthermal/nsttf.html

  27. SolFocus (California startup) Uses high-efficiency (40%) solar cells Over a small area combined with Focusing elements) 5 MW of thermal power for (with 222 “heliostats”) $21M (1978 $’s) 6.2kW panel http://www.solfocus.com/product.php?pid=4

  28. H&K Problem 6-6 • A cubic foot of water stores about 62Btu/Fo.ft3. Rocks have a much smaller specific heat (per unit mass), but a much greater density. For rock with a specific heat of 0.2Btu/Fo.lb and a density of 170lb/ft3, how many ft3 are needed to store the same energy as could be stored in 10ft3 of water?

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