Chapter 5 Summary Review & Problems
ECET 521 – Solar Energy Systems
Paul Lapsansky
In chapter 5 the author discusses how the most popular application for solar energy systems is
for water heating. This is based on fact that it is a relatively simple system that involves only
low-temperature heating. Presently, the world’s commercial low-temperature heat
consumption is estimated to be around 10 EJ per year for hot water production which is only
2.3% of the potential for solar water heating. A solar water heater is a combination of a solar
collector array, an energy transfer system and a storage tank. The main component in the
solar water heater is the solar collector array which absorbs solar radiation and converts it to
heat. The heat is then absorbed by the transfer fluid which is usually either water or some
none freezing liquid or air that passes through the collector. The heat produced can then be
stored or use directly. Because parts of the system will be exposed to weather conditions they
must be protected from freezing or overheating.
There are two types of solar water heating systems available:


Direct or open loop system, where water is heated directly in the collector.
Indirect or close loop system, where water is heated indirectly by the heat transfer fluid
that is first heated in the collector.
Systems also differ in respect to how the heat transfer fluid is transported throughout the
system mainly either by natural (or passive) means or by forced circulation (or active) means.
Passive systems include both thermosiphon and integrated storage systems. Both systems
utilize natural convection as the means to circulate the fluid in the system. The integrated
collectors storage system (or ICS) uses hot water storage as part of the collector. The main
disadvantage to this design is the high thermal losses from the storage tank to the
surroundings.
In active systems the heat transfer fluid is mechanically pumped through the collectors. The
systems are usually more expensive than and not as efficient as the passive systems particularly
if any freezing measures are required. There are five types of systems that belong in this
category: direct circulation systems, indirect water heating systems, air systems, heat pump
systems and pool heating systems. The direct circulation systems, the system pump is used to
circulate water from storage to the collectors when there's enough available solar energy
available to increase water temperature and then returns the heated water to the storage tank
until it is needed. The advantage to this system is it can be used with water supply from a coldwater storage tank or connected directly to the city water. The direct circulation system can
only be used were freezing temperatures are infrequent.
With indirect water heating systems, a heat transfer fluid circulates through the closed collector
loop to a heat exchanger where heat is transferred to water. The most common heat transfer
fluid is a water- ethylene glycol solution. The benefit to this system is better freeze protection
due to the water-ethylene glycol solution.
Air water heating systems are similar to indirect water heating systems except that air is used
instead of fluid for circulation through the collector via ductworks to an air-to-water heat
exchanger.
Heat pump systems use mechanical energy to transfer thermal energy from a lower
temperature source to a higher temperature. Solar pool heating systems require no separate
storage tank because the pool acts as the storage tank and in most cases the pool filtration
pump is used to circulate the water through solar panels or plastic pipes.
HEAT STORAGE SYSTEMS
Thermal storage is one of the main parts of solar heating, cooling and power generating
systems mainly because approximately half of the year any location is without sunlight during
the night. Heat storage is necessary if the solar system must operate continuously during the
day and night. For some systems such a swimming pool heating, intermittent is acceptable but
this is not the case for most systems.
The most common storage media for air collector systems are rocks or other building material
such as gravel mainly because of its abundance and low cost. Liquid system uses two type of
water storage, pressurized and unpressurized. Pressurized systems are preferred for smaller
service while unpressurized is better suited for systems greater than 30m³.
MODULE AND ARRAY DESIGN
A module design is a group of collectors that can be grouped in parallel and combined in seriesparallel. Parallel is more frequently used due to its balanced design, low pressure drop and
because it can be easily drained.
Array design includes groups of collectors called modules that provide the necessary flow
characteristics. Two types of systems can utilized: direct return and reverse return.
DIFFERENTIAL TEMEPERATURE CONTROLLER
The Differential temperature controller is one of the most important components of an active
solar energy system because it is responsible for maintaining adequate temperature control.
SOLAR WATER HEATER PERFORMANCE EVALUATION
Five international standards on solar domestic water heater performance have been published.
These five standards fall within three broad categories: rating test, black box correlation
procedures and testing computer simulation.
PRACTICAL DESIGN CONSIDERATIONS
Installation requires the consideration of various other components that make up a solar water
heater system including: pipes, supports and insulation, pumps, valves and instrumentation.
EXERCISES
5.7
Aa = 2m²
Β = 45°
L = 35°
N = 258 days (Sept 15)
δ = 2.22
For t = 8am
h = -.25(60x8) = -60
cos θ = sin(L-β)sin(δ)+cos(L-β)cos(δ)cos(h)
= sin(-10)sin(2.22)+cos(-10)cos(2.22)cos(-60)
= (-.1736)(.0387)+(.9848)(.9992)(.5)
= .4853
θ = 61°
K = 1 - .12[1/cosθ – 1]
= 1 – 0.12[1/.4853 – 1]
= .873
Qu = AaGt[.76k – 5.6(Ti – Ta)/Gt]
= 2(250)[76(.873) – 5.6(37.6 – 28)/250]
= 224.25W x 3.6 = 807.3 kJ
Ts-n = 37.6 + 1/(100x4.18) [807.3 – 12(3600/1000)(37.6-28)] = 38.5 °C
ΔT/Gt = (38.5 – 28) / 250 = .042 °C-m²/W
Time
6
7
8
9
10
11
12
13
14
15
16
17
18
Ta
25
26
28
30
32
34
35
34
32
30
28
26
25
It
360
540
900
1440
2160
2880
3420
2880
2160
1440
900
540
360
Ti
40
39
38.5
39.6
42.3
46.8
54.9
64.1
69.1
69.8
65.3
60.7
58.4
ΔT/Gt
.150
.087
.042
.024
.017
.016
.021
.038
.062
.1
.149
.231
.334
ϴ
90.4
75.6
61
46.3
32.3
19.3
12.2
19.3
32.3
46.5
61
75.6
90.3
5.9
Ac = 25 m²
Fr = .91
Ul = 6.3 W/m²°C
(mcp)c = 3150 W/°C
5.10
𝐹𝑟
25𝑥. 91𝑥6.3
3150
= [1 +
(
− 1)]
𝐹𝑟
3150
. 65𝑥3150
−1
= .976
𝐹𝑟
25𝑥. 91𝑥6.3
3150
= [1 +
(
− 1)]
𝐹𝑟
3150
. .95𝑥3150
−1
= .998
K
0
.636
.873
.947
.978
.993
.997
.993
.977
.946
.873
.636
0
Qu
0
0
807.3
1715.5
2854.1
3803.8
4442.7
3201.3
1799.8
538.7
.012
0
0
Cc = .85 x 3150 = 2677.5 W/°C
Cs = .65 x 4180 = 2717 W/°C
C = 2677.5 / 2717 = .99
NTU = 5500 / 2677.5 = 2.05
−2.054(1−.99 )
1−𝑒
Ɛ=
= .68
1 − .99𝑒 −2.05(1−.99)
𝐹𝑟
60𝑥3.25
2677.5
= [1 +
(
− 1)]
𝐹𝑟
2677.5 . 68𝑥2677.5
−1
= .97
5.14
Ts-n = Ts + (1/500 x 4.18)[AcFr[S-Ul(Ts-Ta)-Ɛl(mlcp)(Ts-Tmu)-(UA)(Ts-Ta)]
Hour
7-8
8-9
9-10
10-11
11-12
12-13
13-14
14-15
15-16
16-17
17-18
S
0
.35
.65
2.51
3.22
3.56
3.12
2.61
1.53
.66
0
Ta
12.1
13.2
14.1
13.2
14.6
15.7
13.9
12.1
11.2
10.1
9.2
Ts
Qu