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Energy for Sustainability Randolph & Masters, 2008 Chapter 7: Solar Energy for Buildings
Energy for SustainabilityRandolph & Masters, 2008
Chapter 7:Solar Energy for Buildings
Cooling energy loads are complicated
Ti qINT
TAMB
qENVELOPE
qSOLAR
Ventilation,Infiltrationdehumidification
qVENT
Air Conditioner
qA/C
PA/C
qREJECTED
Hot roof
AC Cooling Energy Load begins with CDD
CDDTb= HDDTb – 365 (Tb – Ta)
Cooling Degree Days (CDD):
CDD65= (Ta – 65) #days
Cooling Energy Load:
QA/C (Btu/yr) = 24 hr/day x (UAT) Btu/hroF x CDD oF-day/yr
QA/C = 24 (UAT) CDDSEER (Btu/Wh)= Annual cooling (Btu /yr)
Electrical input (Wh/yr)
Annual Envelope Cooling Electricity (Wh/yr)= 24(UA)CDD
SEER⋅ηDucts
Solar Gain and Cooling Energy
Even Smarter Windows
Conventional low-e blocks Far IR,keeps winter heat in house
Spectrally selective coating lets invisible but blocks Near IR from sunreducing solar gain and cooling load
Spectrally selective glass + sensor controlled lighting
Window low-e coating depend on local needs
Best for Passive Solar Heating:Hi SHGF, Hi R
Best for Reduced Cooling, Daylighting: Lo SHGF, Hi R
Cool Roofs
Summer Heat Gain through Roofs and Effect of Roofing Reflectance
Much of solar thermal energy in NIR, not visible wavelengths
Cool roofs have same visible color, but higher thermal reflectance
Green Roofs
• Saves on building cooling energy • Retains stormwater• Reducing urban heat island
Solar Thermal Applications
Solar angles, orientation, site surveys Incident Solar Radiation (insolation) data Passive Solar Heating Hot Water Heating Solar collectors, efficiency
Solar Path and Position
Sunrise
Sunset
Noon
S
E
W
= Altitude angle
= Azimuth angle (south = 0)
N 90 L
Effect of Sun’s Path on Incident Angles
90-L+23.5 o
90-L- 23.5 o
Dec 21
June 21
South South
Winter
Summer
(a) Altitude angle at noon (b) Designing an overhang
N 90 L
Effect of roof overhang on Solar Shading
P
Y
N
Y = P tan N
South
Noon Shadow line
Sizing a Roof Overhang to shade summer sun, admit winter sun
8 ft
P
P y
tan N
P 8
tan 76o 2.0 ft76o
Shaded at solar noon in June means it will be shaded all day long !
1.1 ft29o
JUNE 21 DEC 21
2 ft
Y = 2 tan 29o = 1.1 ft
Sunny at solar noon in Decembermeans it will be sunny all day long !
wintersummer
Sun Path Diagram
Solar Site Survey
Shows time of day and months of year when obstructionswill shade the point at which survey is made
Interpreting Solar Site Survey
Think about application: Space Heating (need in heating months) Water Heating and Photovoltaics (need all year) Pool Heating (need late spring to early fall)
Survey gives months and time of day site is shaded Need to protect 10:00-2:00: % of daily insolation
37½ N Lat: 73% Dec, 72% Jan/Nov, 62% Feb/Oct Better to protect 9:00-3:00
37½ N Lat: 93% Dec, 92% Jan/Nov, 84% Feb/Oct
Creating a Sun Path Diagram
Plot average monthly solar altitude and azimuth angles for your latitute
Go to http://solardat.uoregon.edu/SunChartProgram.html
Incident Solar Radiation(Insolation)
January Hourly Clear-sky Solar Insolation (Btu/ft2-hr)
SOLAR TILT ANGLESTIME 0 20 30 40 50 60 90
LATITUDE 35 N (Btu/ft2-hr)7, 5 0 0 0 0 0 0 0 8, 4 45 77 90 101 109 113 107 9, 3 105 158 178 193 202 206 18010, 2 152 217 241 257 266 268 22511, 1 182 254 279 297 305 305 251
12 192 266 292 310 318 317 259Btu/ft2-d 1158 1679 1870 2006 2083 2099 1784
LATITUDE 40 N (Btu/ft2-hr)7, 5 0 0 0 0 0 0 0 8, 4 28 54 65 74 81 85 84 9, 3 82 134 155 171 182 188 17210, 2 126 193 218 238 250 254 22411, 1 154 229 257 278 290 294 254
12 163 241 270 291 304 307 264Btu/ft2-d 942 1461 1660 1811 1908 1950 1733
LATITUDE 45 N (Btu/ft2-hr)7, 5 0 0 0 0 0 0 0 8, 4 12 27 33 39 44 47 48 9, 3 59 107 127 143 155 162 15610, 2 99 165 191 212 227 234 21711, 1 125 201 230 253 269 276 251
12 133 212 243 267 283 290 262Btu/ft2-d 722 1212 1408 1562 1671 1731 1604
DAILY CLEAR SKY INSOLATION (Btu/ft2-day) LATITUDE 40 N
AZIMUTH: HORIZ SOUTH FACING EAST, WESTTILT: 0 20 30 40 50 60 90 20 30 40 50 60 90JAN 942 1461 1660 1811 1908 1950 1733 910 885 845 809 752 536FEB 1346 1828 1993 2100 2145 2128 1715 1293 1240 1186 1112 1032 707MAR 1827 2191 2280 2304 2261 2153 1481 1748 1674 1580 1471 1352 900APR 2289 2450 2427 2334 2182 1968 1031 2184 2077 1952 1805 1634 1058MAY 2554 2550 2440 2261 2037 1756 717 2428 2299 2142 1959 1759 1101JUN 2622 2556 2414 2206 1958 1657 603 2488 2349 2179 1984 1777 1092JLY 2526 2516 2406 2228 2006 1728 706 2401 2272 2115 1933 1734 1082AUG 2239 2389 2364 2272 2123 1914 1006 2134 2028 1904 1757 1588 1021SEP 1786 2122 2202 2219 2173 2066 1414 1707 1633 1538 1429 1310 865OCT 1291 1743 1897 1997 2039 2022 1632 1238 1183 1130 1056 976 663NOV 916 1411 1601 1744 1837 1876 1667 884 859 818 783 726 515DEC 785 1287 1486 1641 1747 1803 1652 758 741 711 681 633 456
kBtu/ft2-yr 643 746 766 764 742 700 466 614 586 551 511 465 304
Daily averageClear-sky Solar Insolation – Btu/ft2-da
Actual Average Insolation, Btu/ft2-dayLOS ANGELES CA LATITUDE 33.93 N
TILT JAN FEB MAR APR MAY JUN JLY AUG SEP OCT NOV DEC YEARLAT - 15 1204 1426 1743 2028 2028 2028 2250 2155 1870 1585 1331 1141 1743
LAT 1395 1585 1807 1997 1933 1902 2092 2092 1902 1712 1490 1331 1775LAT + 15 1490 1616 1775 1870 1712 1648 1838 1902 1807 1743 1585 1426 1712
90 1299 1299 1204 1046 792 697 761 951 1141 1331 1363 1299 1109
BOULDER CO LATITUDE 40.02 NTILT JAN FEB MAR APR MAY JUN JLY AUG SEP OCT NOV DEC YEAR
LAT - 15 1204 1458 1712 1933 1965 2092 2092 1997 1870 1616 1268 1109 1712LAT 1395 1616 1775 1902 1870 1933 1933 1933 1902 1775 1458 1331 1743
LAT + 15 1521 1680 1775 1775 1648 1648 1680 1743 1838 1807 1521 1426 168090 1426 1458 1363 1141 887 824 856 1014 1268 1458 1395 1363 1204
BOSTON MA LATITUDE 42.37 NTILT JAN FEB MAR APR MAY JUN JLY AUG SEP OCT NOV DEC YEAR
LAT - 15 951 1204 1458 1648 1807 1902 1902 1807 1585 1299 887 792 1426LAT 1078 1331 1490 1585 1680 1743 1775 1743 1616 1363 983 919 1458
LAT + 15 1141 1363 1458 1490 1490 1521 1553 1585 1553 1395 1046 983 139590 1078 1236 1173 983 887 824 887 983 1109 1141 951 919 1014
ATLANTA GA LATITUDE 33.65 NTILT JAN FEB MAR APR MAY JUN JLY AUG SEP OCT NOV DEC YEAR
LAT - 15 1078 1331 1616 1902 1965 1997 1933 1870 1680 1553 1204 1014 1585LAT 1204 1458 1680 1838 1838 1838 1807 1807 1712 1648 1331 1173 1616
LAT + 15 1299 1490 1616 1712 1648 1616 1585 1648 1616 1680 1553 1236 155390 1109 1173 1109 951 761 697 697 856 1014 1268 1204 1109 983
Generating Solar Insolation Data
1. NREL site: http://rredc.nrel.gov/solar/old_data/nsrdb/redbook/sum2/
2. Find location
3. Download the data to a text file
4. Open the file into Excel (delimit by comma)
5. Isolate the data you want by deleting unwanted rows and columns
6. Convert data to desired units.
NREL Insolation Data Website
Roanoke Actual Avg Insolation DataBtu/ft2-da
Roanoke VA Lat 37.5TILT J AN FEB MAR APR MAY J UN J UL AUG SEP OCT NOV DEC Year
0 730 984 1301 1650 1840 1967 1872 1745 1428 1142 793 635 1341Lat - 15 1047 1269 1523 1777 1840 1904 1872 1809 1618 1460 1111 920 1512
Lat 1174 1364 1586 1745 1745 1777 1745 1745 1618 1555 1237 1079 1531Lat + 15 1237 1428 1555 1618 1555 1523 1523 1586 1555 1555 1301 1142 1465
90 1142 1174 1142 984 793 730 761 888 1047 1237 1142 1047 1007
Passive Solar Heating
Definition and components Building Orientation Calculating Solar Gain Sun-tempered House Effect of Thermal Mass Passive Solar Design Types
Passive Solar Buildings Heating of houses is the biggest single category of energy demand in
the entire building sector. So, why not let the sun do some of that job? There are two ways to try to do that:
Passive solar is based on just letting the sun pass through windows and other solar apertures to provide needed heat. Passive solar systems are simple, cheap and reliable.
Active solar approach uses special solar-thermal collectors to collect heat, then moves it to storage and distribution systems using pumps and blowers. They provide greater control of heat flow, but their high cost and uncertain reliability have led to less widespread acceptance.
The basic design guidelines for passive solar are pretty simple. 1. Maximize envelope efficiency2. Orient the building along an East-West axis to control solar gains3. Provide south-facing glazing systems to admit solar energy4. Design proper overhangs to protect south-facing windows in the
summer5. Provide sufficient thermal mass to absorb solar energy in excess of
daytime needs
Solar Window Orientation
SEW Morning
sun
AfternoonsunNS
0
200
400
600
800
1000
1200
1400
1600
1800
1 2 3 4 5 6 7 8 9 10 11 12
Daily Inso
lati
on (
Btu
/ft2
-day)
Jan Feb Mar Apr May Jun Jly Aug Sep Oct Nov Dec
South-facing
East, West-facing
Latitude 40 N
Clear sky
Building Orientation
SunsetWinter
SunriseWinter
SunsetSummer
SunriseSummer
S
EW
N
S
EW
BAD
N
plantsplants
GOOD
Calculating Solar Gain
S
= 1204 Btu/ft 2day x 0.71 = 855 Btu/ft 2day
= UA T x hours= 0.50 Btu/hr .ft2.oF x (70 - 30) oF x 24 hr/d= 480 Btu/ ft2day
SHGC = 0.71
U = 0.50 Btu/hr-ft 2
Ta = 30oF Ti =70oF
1204 Btu/ft 2day
Solar Gain:
Thermal Loss:
= (855 - 480) = 375 Btu/ft 2day Net Gain:
BUILDING EFFICIENCY: (UA)-VALUE AND THERMAL INDEXHouse #1: SUNTEMPERED with 100 ft2 of south window out of total 250 ft2
Component Area (ft2) Insulation R U=1/R* UA* % of Total*Ceiling 1500 R-30 #13 30.3 0.033 49.5 12%Non-S Windows 150 dbl Al #21 1.4 0.714 107.1 26%Doors 60 No storm #19 2.6 0.385 23.1 6%Walls 970 R-21 #3 19.2 0.052 50.5 12%Floors 1500 R-21 #7 29.3 0.034 51.2 12%
ACH Volume (ft3) EfficiencyInfiltration 0.6 12,000 0 129.6 32%Ventilation 0 12,000 70% 0 0%
TOTAL (UA)-value = 411 Btu/hroF
Note: *Columns are calculated values. All other information is data entry
ANNUAL FUEL CONSUMPTIONFuel Price $12 per million Btu (Table 6.12)Furnace Efficiency 80%Distribution Efficiency 75%Internal gains, qint 3000 Btu/hrIndoor set point, Ti 70 oFHDD65 5052 oFd/yr for Blacksburg, VA (Table 6.11)Tdesign -5 oF (Table 6.11)Furn. pick-up factor 1.4
Calculations:* Balance Point Temp 62.7 oF Tbal = Ti - qint/(UA)* HDD @ Tbal 4546 oFd/yr HDDTb=HDD65-(0.021*HDD65+114)(65-Tbal)* Qdel 44.8 Million Btu/yr Qdel=24(UA)HDD* Qfuel 74.7 Million Btu/yr Qfuel=Qdel/(furn eff x dist eff)* Annual fuel bill 897$ per year $/yr=Qfuel x fuel price
* House without solar 1,108 per year using all 250 ft2 of windows Note *Rows are calculated values. All other information is data entry
Assume south windowsNet zero heat loss
Thermal mass needed if excess solar gain
Total househeat loss rate
q (Btu/hr)
MN Noon MN
Solar gains withlarge windowarea
Need thermal storage, Qs
Solar gains withmodest windowarea
Effect of Mass on indoor temperature
0
10
20
30
40
50
60
70
80
90
100
6 12 18 24 30
Tem
pe
ratu
re (
oF)
6am Noon 6pm MN 6am
14% So. glass, no extra mass
14%, extra mass
Ambient
7% So. Glass, no extra mass
Passive Solar Heating Types
Air Circulation in Passive Solar
Convective Loop or Envelope House
Effect of Passive Gains on Auxiliary Thermal Index (Btu/ft2-oF-da)
Estimating Solar Performance:Load Collector Ratio (LCR) method
p
ST
A
UALCR
24
SSF 0.18 0.3log10 LCR2 LCR log10 LCR2 LCR5
QDEL (Btu/yr) = 24 (UA)ST HDD65 (1-SSF)
LCR2=LCR@SSF=20%; LCR5=LCR@SSF=50%; DGB1, SSB7, TWB2 are design type
LCR ExampleA house in Denver with a total (UA)-value of 400 Btu/hroF has 200 ft2 of U-0.50 direct
gain windows (with suitable mass). If the furnace and ducts are each 90% efficient and the fuel is natural gas at $1.20/therm, estimate the fuel bill. Denver has HDD65 = 6016 oF-day/yr.
SOLUTION: The sun-tempered (UA) value subtracts those solar windows(UA)ST = 400 Btu/hroF – 200 ft2 x 0.50 Btu/hrft2oF = 300 Btu/hroF
The Load Collector Ratio is:
From Table 7.6, LCR2 = 68 and LCR5 = 18, so using (7.6) we get
Using (7.5) givesQDEL = 24 hr/day x 300 Btu/hroF x 6016 oFday/yr x (1-0.324) = 29 x106 Btu/yr
The fuel bill will be
LCR 24 UA ST
Ap
24 300
20036
SSF 0.18 0.3log10 68 36 log10 68 /18
0.324
Fuel 29x106 Btu yr
0.90x0.90
$1.20
105 Btu$430 / yr
Hot water heating
Cost of Hot Water
Example: 64 gal/da, Th = 130oF, Tc = 55oF
Q = 64 g/d x 365 d/yr x (130 – 55)oF x 1 Btu/lboF x 8.34 lb/gal = 14.6x106 Btu/yr
Cost 14.6x106 Btu yr
0.57
1 therm
100,000Btu
$1
therm$256 / yr
Q = #gal/da x 365 da/yr x (Th- Tc)oF x 1 Btu/lboF x 8.34 lb/gal = Btu/yr
$ Cost natural gas = Btu/yr x 1 therm x $ EF 100,000 Btu therm
Typical Energy Factor and Levelized Cost of Different Hot Water Heaters
1st yr LevelizedEnergy Installed Annual energy Life Cost
Water Heater Type Factor Cost energy cost (years) $/yrGAS FIRED Therms
Conventional storage 0.57 380$ 256 256$ 13 351$ High-efficiency storage 0.65 525$ 225 225$ 13 329$ Instantaneous demand 0.70 650$ 209 209$ 20 243$ High-efficiency pilotless demand 0.84 1,200$ 174 174$ 20 255$ Solar (SF = 0.7) with gas back-up 0.57 2,500$ 77 77$ 20 271$
ELECTRICITY kWh Conventional storage 0.90 350$ 4758 428$ 13 557$ High-efficiency storage 0.95 440$ 4508 406$ 13 539$ Instantaneous demand 0.95 600$ 4508 406$ 13 556$ Electric heat pump 2.20 1,200$ 1947 175$ 13 340$ Solar (SF = 0.7) with electric back-up 0.90 2,500$ 1427 128$ 20 318$
Assumptions: 64 gpd demand, 75oF delta T, natural gas @ $1/therm, electricity @ 9¢/kWh, 3% fuel escalation, 5% discount rate, solar savings fraction 70%.
Solar Hot Water
Tin
Tout
Absorber plate
Insulation
Glazing
Flow
Frame
Header
Flow tubes
T
Controller
Pump
Cold
Hot
FLAT-PLATE COLLECTOR SYSTEM DIAGRAM
The Flat Plate Collector
qDEL= qABS - qLOSS
qDEL = IA(τα) – ULA (TP – TAMB)
qDEL
qINCIDENT
IA UL A TP TAMB
IA UL
TP TAMB
I
()
Tp Tamb
I
1
UL
X = Tin Tamb
I
X = “inlet parameter”
“y-axis intercept” FR()
“slope factor” F R UL
Efficiency based on Plate Temperature Efficiency Based on Inlet Temperature
a. b.
Unglazed,pool collector
Single-glazedDHW collector
Double-glazedSpace heating
Tin..for average Tamb and I80o 110o 140o
80o
High High UL
Medium Medium UL
Low Low UL
Pool DHWSpace Heat
110o 140o
Collector Efficiency
__
FR K FRUL
T__
in T__
amb
I__
__
0.750.93 1.18Btu hroFft 2 100o 80o F
166Btu hrft2
0.55 55%
A ft2 gal /day 8.34 lb gal1Btu lboF T oF
Insolation Btu / ft 2day Collector 1 losses
648.34 140 60 19970.550.85
46 ft2
Solar Water Heating Analysis
Evaculated Tube collectors
Batch Water Heater: Integrated Collector and Storage (ICS)
0
20
40
60
80
100
120
140
160
6 9 12 15 18 21 24 27 30
Morning
Evening
Hot Water Btu/day ICS % of LoadLoad Pattern Supplied Delivered Efficiency 40gal @120FEvening load 40 g @ 123F 20,904 42% 104%Morning load 40 g @ 84F 8,015 16% 40%
Tamb
I/10T(F
) or
Inso
l (B
tu/f
t2hr)
/10
Thermosiphon Water Heater
