2011 International Conference on Alternative Energy in Developing Countries and Emerging Economies
- 213 -
Increase of Daily Performance of Solar
Hot Water Heating with a
Temperature-Stratified Storage by
Controlling Mass Flow Rate
S. Polvongsri
*
and T. Kiatsiriroat
**
.
*
Graduate School, Chiang Mai University, Chiang Mai,
(Thailand)
**
Department of Mechanical Engineering, Faculty of Engineering Chiang Mai University,
(Thailand)
Abstract
--Control of solar collector mass flow rate
to get a high temperature at the collector outlet,
especially at the low solar radiation level, of solar hot
water heating system having temperature-stratified
water storage tank was studied. With this technique,
it could be seen that more daily solar heat and higher
water storage temperature and higher daily system
efficiency could be compared with that of the
standard flow at 75 liter/m
2
h.
A case study was performed on 3 consecutive
cloudy days for a nursery building installed in Chiang
Mai, Thailand. With the standard mass flow rate, the
average daily useful heat from the solar collector was
17.30 MJ and the average daily auxiliary heat was
22.77 MJ while for the controlled mass flow rate, the
average daily useful heat was 24.89 MJ and the
average daily auxiliary heat was 16.81 MJ.
Index Terms
—
Solar water heating system, Temperature-
Stratified Storage, Controlling Mass Flow Rate, Daily
Performance
I. N
OMENCLATURE
ܣ
Solar collector area
(m
2
)
ܥ
Heat capacity of water
(J/kg
o
C)
ܨ
ோ
Collector Heat removal factor
ܫ
்
Solar radiation (W/m
2
)
ܫ
௧
Critical solar radiation (W/m
2
)
݉
ሶ
Water mass flow rate (kg/s)
ܳሶ
Heat rate from solar collector
(W)
ܳሶ
௨
The useful heat gain from the collector (W)
ܶ
The ambient temperature
(
o
C)
ܶ
Outlet temperature (
o
C)
ܶ
Inlet temperature (
o
C)
ܷ
Overall Heat loss (W/m
2 o
C)
ሺ߬
ߙ
ሻ
The transmittance- absorptance product
______________________
This work was supported by the Commission on Higher Education,
Thailand under the program Strategic Scholarships for Frontier
Research Network for the Ph.D. Program Thai Doctoral degree.
II.
I
NTRODUCTION
At present, solar hot water system is normally used in
hotels and hospitals including domestic hot water heating.
A general guideline for the water mass flow rate in solar
collector is
70-75 liter/m
2
h (1.2 liter/m
2
min) and the solar
thermal characteristics,
ܨ
ோ
ሺ߬
ߙ
ሻ
and,
ܨ
ோ
ܷ
are tested at
this condition under the solar radiation level over
790
W/m
2
(ASHRAE STANDARD 93-77). For a forced
convection system, a pump is used to circulate water
between a solar collector and a water storage tank. The
pump is on when the temperature of hot water leaving the
solar collector is higher than that of the storage tank and
off otherwise. In practice, especially for the temperature-
stratified storage tank, when the solar radiation level is
not high enough if the flow rate is kept constant as the
guideline value, then the temperature of water at the
collector outlet might be less than that of the water at the
top of the storage tank. Then the pump is off and the
system could not get the benefit from the sun even the
solar radiation level is higher than the critical solar
radiation,
ܫ
௧
.
If the mass flow rate could be controlled to
generate the outlet temperature be higher than that at the
storage tank, more solar heat could be utilized which
results in higher daily efficiency.
Huang
et.al.
[2] used Transient System Simulation
Program
(TRNSYS) to evaluate thermal performance of a
solar hot water system having a stratified water storage
tank when the solar collector mass flow rate was lower
than the recommended value.
The system efficiency was
increased when the unit mass flow rate was between
0.035-0.005 kg/(s-m
2
)
but the performance decreased
when the flow rate was less as 0.005
–
0.001 kg(s-m
2
).
The system efficiency was in a range of 0.47-0.54.
Badescu
et.al.
[3] and Farahat
et.al.
[4] reported that
when the solar collector mass flow rate was less the
exergy rate obtained from the solar collector was higher
than that at the recommended flow rate due to the higher
outlet temperature. The optimal mass flow rate also
depended on the solar radiation level, the ambient
temperature and the inlet temperature. Morison
et.al.
[5]
founded that the solar collector efficiency varied when
the mass flow rate deviated from the recommended value.
A correction factor of the solar collector efficiency was
also recommended.
