1
Physics 100
Lecture 19
Solar Thermal Energy
April 10, 2017
2
Class Quiz: Which of the following is the
most important factor in understanding
the “carbon cycle” of the Earth?
A. Absorption of carbon by
plants
B. Absorption of carbon by
the ocean
C. Production of carbon by
plants
D. Production of carbon by
polar glaciers
83%
13%
4%
A.
B.
C.
0%
D.
3
Class Quiz: Which of the following is the
most important factor in understanding
the “carbon cycle” of the Earth?
A. Absorption of carbon by
plants
B. Absorption of carbon
by the ocean
C. Production of carbon by
plants
D. Production of carbon by
polar glaciers
See pp. 287-288
of the text.
Reminder of the hazards of
carbon-based energy
Citation: Physics Today 69, 11, 48 (2016); full article
Each row of the diagram represents a carbon reservoir, with the amount of carbon
in each reservoir in 1870 shown in the middle column in petagrams (1 Pg = 1015 g).
The boxes and arrows to the left and right depict the carbon budget16 in petagrams
for the period 1870–2014.
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5
Some good news
https://www.eia.gov/todayinenergy/detail.php?id=30712
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Alternatives to fossil fuel energy

Nuclear energy
• No emissions, but
safety and waste
disposal problems

Renewable energy
• Solar thermal
• Solar electric
• Wind
• Biomass
• Geothermal
• Nuclear fusion
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www.eia.gov/totalenergy/data/monthly/pdf/sec1_6.pdf
see also www.eia.gov/totalenergy/data/monthly/
Renewable energy overview
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Renewable energy trends
Doubling time:
2.95 y
9.12 y
https://www.eia.gov/outlooks/steo/report/renew_co2.cfm?src=Environment-b1
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The Solar Constant




S = 1354 W / m2
Fluctuates from 1321 (in July) to 1412 W / m2
(in January) due to slight change in distance
from Sun
Seasons are due to tilt of the Earth’s axis
relative to orbital plane, not the variation in
the solar constant
Includes infrared, visible, and ultraviolet light
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Insolation

The amount of solar radiation that
reaches the Earth’s surface is called
insolation (incident solar radiation)
(not to be confused with insulation, a material
that impedes thermal conduction)

Insolation is affected by many factors…
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Atmospheric absorption

The surface never gets more than 1050 W/m2
(on a clear day) due to atmospheric absorption
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Energy Balance for the Earth

On average (including
cloudy days), only 50%
of solar radiation
reaches the Earth’s
surface.
Figure 6.4
page 156
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A closer look
−23%
−60%

Atmosphere:
+217% −217% = 0% net

Earth’s surface:
+23%
+48%
+163% −160% = 3% net
+33%
(in reality this is very
close to zero, the
numbers were rounded)
+113%
−105%
−29%
−8%
−33%
+8%
+21%
+29%
+105%
−119%
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The tilt of the Earth’s Axis

Changes how much sunlight is received
by a location on the Earth throughout the
year
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Seasonal change to insolation

The Sun’s angle above
the horizon changes
with the seasons
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The sun doesn’t shine all day

Stevens Point gets about 1250 Btu/ft2/d
17
Map of average solar insolation
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12
14
14
16
16
12
10
12
10
14
16
18
10
12
<10
10-12
12-14
14-16
16-18
18-20
20
22
24
14
26
20-22
22-24
26
24
14
20
22
24-26
18
16
26-28
>28
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Stevens Point gets about 1250 Btu/ft2/d of
insolation. Convert this number to kWh/m2/d
A.
B.
C.
D.
96%
0.253 kWh/m2/d
1.25 kWh/m2/d
3.94 kWh/m2/d
236 kWh/m2/d
0%
A.
4%
B.
0%
C.
D.
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Stevens Point gets about 1250 Btu/ft2/d of
insolation. Convert this number to kWh/m2/d
2
1250 Btu 1055 J
1 kWh
3.28 ft



2
6
2
ft  d
1 Btu 3.6  10 J
1m
kWh
 3.94 2
m d
2
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How much water can you heat from 20ºC to
50ºC per day using a 10 m2, 50% efficient solar
collector in Stevens Point, on average?
A.
B.
C.
D.
92%
2260 kg
565 kg
150 kg
3.94 kg
8%
0%
A.
0%
B.
C.
D.
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How much water can you heat from 20ºC to
50ºC per day using a 10 m2, 50% efficient solar
collector in Stevens Point, on average?
kWh
2
3.94 2  10 m  0.50  19.7 kWh/d
m d
6
kWh 3.6  10 J
7
19.7

 7.09  10 J/d
d
1 kWh
7
Q
7.09  10 J/d
m

 565 kg
c T  4190 J/kg  °C  30°C  about 147 gallons!
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Solar Thermal Facts





Water heating is the most common
application for solar energy
Domestic hot water (DHW) systems are
being sold with a 5% annual growth (14 year
doubling time)
95% of solar heaters are for swimming pools,
only 5% for DHW
About two-thirds of a household’s DHW load
can be economically met by solar
EIA web site
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Solar Thermal Facts



Estimated 35,464 new DHW
systems installed in 2010,
and 29,540 pool systems
Total US capacity is over 815
MWTh (megawatts thermal
equivalent)
Today there are an estimated
790,000 solar pool heating
systems installed in the U.S.,
more than any other solar
technology application.
SEIA Solar Industry Data
graphs from 2013 report
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Batch water heater
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Flat-plate collector
US Solar Energy Year in Review 2008
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Flat-plate collector
US Solar Energy Year in Review 2008
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Right Here at UWSP

There is a solar water
heating system on the roof
of many residence halls
and the HEC

Energy production used to be
tracked on the web. There is a
document about performance
history of the HEC system and the
eventual removal of the HEC panels in
July 2016.

The UWSP Sustainability
site has some additional
information
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Solar Space Heating



Three components:
•
•
•
Insulation
Solar collection
Thermal storage
Can reduce heating
costs up to 80%
Not as popular due
to costs.
Compare 2008 installations: 139 MWTh hot water,
762 MWTh pool heating, 21 MWTh space heating
US Solar Market Insight
SEIA Solar Heating & Cooling
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Approximately what maximum percentage
of the 40,000 Btu/h home heating needs
can be met by a 700 ft2, 50% efficient
collector if the insolation is 1700 Btu/ft2/d?
A.
B.
C.
D.
93%
25%
35%
60%
75%
4%
A.
4%
B.
0%
C.
D.
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Approximately what maximum percentage
of the 40,000 Btu/h home heating needs
can be met by a 700 ft2, 50% efficient
collector if the insolation is 1700 Btu/ft2/d?
Btu
h
Q  40,000
 24  960,000 Btu/d
h
d
Btu
Esolar  1700 2  700 ft 2  0.50  595,000 Btu/d
ft  d
Esolar 595

 100  62%
Q
960
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Concrete has a density of 2400 kg/m3 and
a specific heat of 750 J/kg/oC. What
volume of concrete can store as much
heat as 10 m3 of water (42 MJ/oC)?
A.
B.
C.
D.
4.29 m3
12.1 m3
18.5 m3
23.3 m3
81%
15%
0%
A.
4%
B.
C.
D.
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Concrete has a density of 2400 kg/m3 and
a specific heat of 750 J/kg/oC. What
volume of concrete can store as much
heat as 10 m3 of water (42 MJ/oC)?
A.
B.
C.
D.
4.29 m3
12.1 m3
18.5 m3
23.3 m3
Q  mcT and density  
m
V
Q
 mc   V  c solve for V :
T
Q T
42 106 J/ C
V

c
 2400 kg/m3   750 J kg C 
 23.3 m3 or about 12.7 tons of concrete!
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National Solar Thermal Test
Facility – Albuquerque, NM


Can generate
about 1.5 MW
of electricity
Used for
research and
development
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Solar Energy Generating Systems (SEGS)
Second largest solar
energy facility in the
world, with 354 MW
installed capacity
(California Mojave
Desert)
Aerial view showing portions of four of the five SEGS III–VII plants located at Kramer Junction
Entire image courtesy Wikipedia
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Ivanpah Solar Electric Generating System

Commissioned 2014
CA Mojave desert

Currently world’s
largest solar
thermal facility
at 392 MW

List of worldwide
solar thermal
power stations