full2011_inter.pdf - page 214

2011 International Conference on Alternative Energy in Developing Countries and Emerging Economies
- 214 -
In this study, control of solar collector mass flow rate
of a solar water heating system having a temperature-
stratified water storage was undertaken so that the
collector outlet temperature was higher than the
temperature at the top of the storage tank. Then the
system got more heat from the solar radiation input.
III. S OLAR HOT WATER S YSTEM MODEL
A. Solar Collector
The solar collector model following ASHRAE
STANDARD 93-77 [6] is undertaken when the mass
flow rate of solar collector is 75 liter/m2 h (1.2
liter/m2 min) for water as a working fluid. The collector
efficiency and the useful heat gain from the collector are
ߟ ൌ ൣܨோሺ߬ߙሻ ௘ െ ܨோܷ௅ ൫ܶ௙௜ െ ܶ௔൯Ȁܫ் ൧ (1)
ܳ௨ ൌ ܣ௖ൣܨோሺ߬ߙሻ௘ܫ் െ ܨோܷ௅ ൫ܶ௙௜ െ ܶ௔൯൧ (2)
When the mass flow rate deviates from 75 liter/m 2h,
gave a method to adjust the values of the solar thermal
characteristics, ܨோሺ߬ߙሻ and, ܨோܷ௅ as
ܳ௨ ൌ ݉ܥ௣ ൫ܶ௖௢ െ ܶ௙௜ ൯
ൌ ܣݎ௖ ൣܨோሺ߬ߙሻ௘ܫ் െ ܨோܷ௅ ൫ܶ௙௜ െ ܶ௔ ൯൧ (3)
where:
ݎ ൌ ܨோሺ߬ߙሻȁ௨௦௘ ܨோሺ߬ߙሻȁ ௥௘௙ ൌ ܨோܷ௅ ȁ௨௦௘ ܨோܷ௅ ȁ௥௘௙
ൌ ݉ܥ௣ ሶ
ܣ௖ܨᇱܷ௅ ቀͳ െ ݁ݔ ݌൫െܣ௖ܨᇱܷ௅Ȁ݉ሶ ܥ ௣൯ቁቤ௨௦௘ ݉ܥ௣ ሶ
ܣ௖ܨᇱܷ௅ ቀͳ െ ݁ݔ ݌൫െܣ௖ܨᇱܷ௅Ȁ݉ሶ ܥ ௣൯ቁቤ௥௘௙
(4)
The solar collector is used to generate heat to a storage
tank of which the temperature is stratified. In this study,
three nodes model of the storage tank is considered as
shown in Fig. 1.
Fig. 1. Solar water heating system having a three nodes stratified
storage tank.
The storage tank is assumed to be adiabatic the energy
equation of each node of the storage tank could be
Node 1 :ܥܯ൫௣ ൯ଵ ௗ்ೞభ ௗ௧ ൌ ݉ሶ ௪ ܥ௣ ሺܶ௖௢ െ ܶ௦ଵሻ െ ݉ሶ ௅ܥ ௣ሺܶ௦ଶ െ ܶ௦ଵሻ (5)
Node 2 :ܥܯ൫௣ ൯ଶ ௗ்ೞమ ௗ௧ ൌ ݉ሶ ௪ ܥ௣ ሺܶ௦ଵ െ ܶ௦ଶሻ െ ݉ሶ ௅ܥ ௣ሺܶ௦ଶ െ ܶ௦ଷሻ (6)
Node 3 :ܥܯ൫௣ ൯ଷ ௗ்ೞయ ௗ௧ ൌ ݉ሶ ௪ ܥ௣ ሺܶ௦ଶ െ ܶ௦ଷሻ െ ݉ሶ ௅ܥ௣ ሺܶ௦ଷ െ ܶ௅௜ሻ (7)
The conditions of pump are
ܲݑ݉݌ ݋݊ ǡ ܫ் ൐ ܫ௖௥௧ ܽ݊݀ ܶ௖௢ ൒ ܶ௦ଵ ൅ ͳ
ܲݑ݉݌ ݋݂݂ ǡ ൜ ܫ் ൑ ܫ௖௥௧ ܫ் ൐ ܫ௖௥௧ ܽ݊݀ ܶ௖௢ ൏ ܶ௦ଵ ൅ ͳ
ܫ௖௥௧ is critical solar radiation which is defined as
ܫ௖௥௧ ൌ ܨோܷ௅ ൫ܶ௙௜ െ ܶ௔ ൯
ܨோሺ߬ߙሻ (8)
Fig. 2 shows the calculation steps to evaluate the mass
flow rate of the solar collector of which the collector
outlet temperature is higher than the water temperature at
the top of the storage tank.
Fig. 2. The calculation steps to find out the suitable mass flow rate of
solar collector.
TS2
TS3
TS1
1
.
Sp L TCm
Li p L TCm .
3
.
Sp c TCm
CO p c TCm .
2
.
, S L Tm
3
.
, S L Tm
1
.
, S c Tm
2
. , S c Tm
WD
RF
aT fiTLURF
crt I ) (
fi a TT , T , I : Input
Start
No
Yes
Yes
No
ctwm Input ,
.
2 1
.
1
.
1
1 S Sp L S CO pw S p T TCm T TCm
dt
dT MC
3 2
.
2 1
.
2
2 S Sp L S Cpw S p T TCm T TCm
dt
dT MC
Li Sp L S Cpw S p T TCm T TCm
dt
dT MC
3
.
3 2
.
3
3
1
1 t s co T T
coT Calculate
liter/min1.2= wm
Input
crt T I I !
3 2 1 , ,
C
s s s T T T
alculate
End
off pump mw ,0 .
alculate C