P120 Exam I 2008Avg: 86.2/110 = 78%

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Series1

Infiltration

Q = 0.018 Btu/ft3.hr.Fo V K 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 airis necessary (you need to breath!), and this is not readily apparent in this figure.

Some typical R valuesMaterial Thickness R (ft2.h.oF/Btu)

Hardwood 1” 0.81

Concrete block 8” 1.25

1-pane window 0.125” 0.88

2-pane window 0.5” air 1.72

Fiberglass 7” 21.8

Polyurethane 1” 6.3

Nylon carpet 1” 2.0

Wood siding 0.5” 0.81

Plywood 0.5” 0.627

Plasterboard 0.5” 0.45

Steel 1” 0.0032

Degree-Days Heating/Cooling

http://www.ersys.com/usa/18/1836003/wtr_norm.htm

Indianapolis

Price of Natural Gas(dollars/MBtu wholesale I believe)

http://futures.tradingcharts.com/chart/NG/W

“Low-e” (emissivity) coatings on windows

Conducting oxide

Double metal layerSingle metal layer

Phys. Today Nov. 2000

Basic Heat Engine

Efficiency = = W/QH = (QH-QC)/QH

Recall that this works because the entropy increase at Tc is greater than the entropy decrease at TH.

Heat Pumps (one system: heating and cooling)

As we saw with heat engines, the second law of thermodynamics puts limits on howmuch cooling you can get for a given amount of work (entropy of the Universe mustnot decrease!! => Heat out to warm reservoir must exceed heat in from cold res.)

Heat output is greaterthan work in!

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)

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 energybudget (a 15% increase in four years).

http://eosweb.larc.nasa.gov/EDDOCS/images/Erb/components2.gif

Solar Energy basics

Components of solar Energy on Earth

H&K fig 6.7

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: ??

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).

Insolation (Btu/ft2.day)Horizontal surf.surf. at = latitudeMean 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

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

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)

Typical Passive Domestic solar heating systems

H&K 6.26 “Trombe” WallH&K 6.24

Typical Active Domestic solar heating system

E.G. Domestic hot-water system

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?

Components of solar Energy on Earth

H&K fig 6.7

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), T=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]

Insolation (Btu/ft2.day)Horizontal surf.surf. at = latitudeMean monthly T (oF)

H&K Appendix D

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?

Focusing collectors(parabolic)

National Solar (thermal) test Facility (Sandia New Mexico)

http://www.sandia.gov/Renewable_Energy/solarthermal/nsttf.html

5 MW of thermal power for (with 222 “heliostats”)$21M (1978 $’s)

SolFocus (California startup)

http://www.solfocus.com/product.php?pid=4

5 MW of thermal power for (with 222 “heliostats”)$21M (1978 $’s)

Uses high-efficiency (40%) solar cellsOver a small area combined with Focusing elements)

6.2kW panel

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?