INTRODUCTION
The research on solar collector has gained its popularity over the years due
to the increase in demand for the utilization of solar energy. Series of research
have been done to improve the means of converting solar energy to other forms
of energy that would meet the needs of human being. This as called for a concerted
effort of scientists and engineers, who came out with the classification, different
approaches of designing and evaluation of the efficiency of solar collector,
that would be economically viable and technically effective.
Energy is vital input for economic growth in agriculture and industry. Fossil
fuels are depleting at a faster rate due to over exploitation, besides increasing
in cost of environmental sustainability. Search for sources of green energy
and their technological development is of paramount importance to have a balanced
and buoyant environment for better quality of life (Kothari
et al., 2009).
Amer et al. (2012) proposed solar power and
wind energy as adjunct renewable energy sources for due to increase in consumption
of fossil fuel which poses a threat on environmental sustainability. Joseph
et al. (2011) emphasized the future of solar energy supplying the
whole energy needed in facilities such as building with cheap cost of production
and environmental friendly.
The single pass collector is also known as flat plate collector. It was said
by Garg and Parkash (2006) that it forms the hearts of
any solar collection system designed for operation in the low temperature range,
from the ambient to 60°C or the medium temperature range, from ambient to
100°C. A well engineered flat plate collector delivers heat at relatively
low cost for a long duration. The most important part of the collector is the
absorber plate along with the pipe or duct to pass air in thermal contact with
the plate to transfer heat from it.
The single pass flat plate collectors as shown in Fig. 1
have the following advantages over other types of solar energy collector: They
absorbed diffuse, direct and reflected components of solar radiation. Tracking
of the sun is not required but they are fixed in tilt and orientation. They
are easy to construct with cheap cost. They require low maintenance cost and
long life. They operate at high efficiency (Garg and Parkash,
2006).
It was emphasized by Sen (2008) and Karatasou
et al. (2006) that it is necessary to have tilted surfaces for the
maximum collection of solar energy but the angle of tilting depends on both
the latitude and day of the year.
An analytical program, suitable for use on a digital computer was developed
by Test (1976) for the purpose of studying the hourly
behaviour of a flat plate solar collector. The data for the past ten years were
studied to determine the average diffuse radiation on clear days, ambient temperature,
average cloudiness and effect of cloudiness on global radiation. The quantities
were then put in the form of empirical equations so that they would predict
the proper input for a computer programme. The results indicated that the energy
absorbed by a collector and its efficiency are strongly dependent on collector
temperature and cloudiness. Test (1976) warned that
a good report of collector efficiency must contains the conditions under which
a collector was operated and its geometry.
A theoretical model and experimental setup were carried out by Sopian
et al. (2009). It was confirmed that the effect of using porous media
in the lower channel of the solar collector provides a higher outlet temperature
compare to the conventional single pass collector. This improves the thermal
efficiency of the collector. The effects of mass flow rate, solar radiation,
temperature and variation of the depths of the upper and lower channels on thermal
efficiency were established. In addition, the heat transfer and pressure drop
relationships was developed for air flow through the porous media. It was concluded
that the typical efficiency of the double pass solar collector with porous media
is about 60-70%.
It was reported by Rabadi and Mismar (2003) and Chamolia
et al. (2012) that the instantaneous efficiency of collector is increased
when using porous media. Al-Hadidi and Ibrahim (2008)
divided the daily solar time into four T1 (between sunrise and 9.30
a.m.), T2 (9.30 a.m.-12.00 p.m.) T3 (12.00 p.m.-2.30 p.m.)
and T4 (between 2.30 p.m. and sunset).
The collector efficiency of upward type double pass flat plate solar air heater
with fins attached and externally recycle was investigated theoretically by
Ho et al. (2011). A double pass device was constructed
by inserting the absorbing plate into the air conduit to divide it into two
channels (upper and lower). The double pass device introduced in this research
was designed for creating a solar collector with heat transfer is double.
The mathematical and experimental study conducted by Ramani
et al. (2010) emphasized the inclusion of porous material in the
second air passage of double pass counter flow solar collector as a means to
improve the thermal performance. The mathematical model was based on the volumetric
heat transfer coefficient. It was revealed that the thermal efficiency of double
pass solar air collector with porous absorbing material is 20-25 and 30-35%
higher than that of double pass solar air collector without porous absorbing
material and single pass collector, respectively.
An experimental study was performed by Karim and Hawlader
(2004) which shows that the efficiency of the collectors is a strong function
of air flow rate. Efficiency increases with flow rate and tend to be saturated
beyond a flow rate of about 0.056 kg m-2 sec. A flow rate of about
0.35 kg m-2 sec was considered optimal for solar drying of agricultural
products. Since the v-corrugated collector shows better efficiency in both single
and double-pass operation and is also structurally stable, this collector was
considered predominantly useful for drying applications.
It was recorded by Aldabbagh et al. (2010)
that the efficiency increases with increase in flow rate between 0.012 and 0.38
kg sec-1 was used and for the same flow rate the efficiency of the
double pass was found to be higher than the single pass by 34-35% and concluded
that the maximum efficiencies obtained for the single and double pass air collectors
are 45.93 and 83.65%, respectively for the mass flow rate of 0.38 kg sec-1.
The enhanced collector was reported by Herrero Martin et
al. (2011) that it increases the thermal efficiency value by 4.5%. The
double pass solar collector is shown in Fig. 2.
Attempt has been made in this paper to show the differences between the single
pass collector and the double pass solar collector with porous matrix based
on the material requirement, design and theory of heat transfer within the collector
system and its surrounding. The relationship between efficiency with reduced
temperature parameter (Tp-Tsrd)/G, power output with mass
flow rate and efficiency with increment in temperature were illustrated with
the aid of graphs.
MATERIALS AND METHODS
Analysis of various parts of the single pass collector and double pass solar
collector filled with porous media were considered and the heat transfer within
the system and its surrounding were looked into. This reflected the disparity
in heat loss of both systems which implies different performance efficiency
value when exposed to the same solar radiation.
Glazing with a plane glass: A plane glass is usually installed on the
single pass flat plate absorber while two planes are used on the double pass
solar collector with porous matrix. The glass material has transmission allowance
for the short wavelength radiation from the sun which range between 0.25 and
3.0 μm (Duffie and Beckman, 2006) and it disallows
the passage of long wavelength radiation. But the flat plate emittance is of
long wave length (Hollands et al., 2001). Therefore,
it seals the heat energy within the space between the plane glass and the collector.
This sealing is more effective in double pass collector with two planes as glazing
material.
However, part of the long wavelength radiant heat emitted by the absorber escapes
to the atmosphere, some are re-radiated onto absorber mostly double pass that
has double plane, while the rest are loss to the surrounding through convective
heat transfer. Therefore, a plane glass usually used for glazing of single pass
solar collector cannot eliminate the heat flow beneath the glass cover to the
surrounding. Hence, there is need for further reduction of heat loss to the
environment.
The following equations (Hollands et al., 2001)
are used to evaluate the values of absorptance α, reflectance ρ and
transmittance τ solar energy on a plane glass:
Two plane glasses: The two plane glasses are more efficient than one
glass. They are usually applied to double pass solar collector with porous media.
Since the glasses are made of the same material and assumed symmetry of each
plane, they are bound to have the same value of reflectance ρ and transmittance
τ. In this case there would be a pair of reflectance and one transmittance.
The following Equations are used to evaluate two plane glasses (Hollands
et al., 2001).
Therefore, the transmittance and the reflectance of centre glass would be estimated
by:
The following equations (Tiwari, 2002) reflect the heat
flow around the two plane glasses, in which four temperatures are very relevant
in the process of evaluating the quantity of heat flow within this region. The
temperatures are: Air temperature Tout, outer glass temperature Tg,out,
inner glass temperature Tg,in, inner air temperature Tin
and thermal resistance R. The Equations required are of forms:
where, qint is the quantity of heat transfer internally with two
the glasses.
The absorbed solar radiation per unit area of each plane glass, Sg,in
and Sg,out can be calculated from:
Flat plate solar collector: This is the energy conversion unit that
absorbs the solar radiation. It does not concentrate the incident radiation
energy before absorption. Therefore, it collects both the direct and diffuse
parts of radiation.
The following features distinguished the flat plate collector from others:
| • |
It has high transmission cover |
| • |
The absorber plate is usually coated with high solar radiation absorptance
and low infrared ray emittance |
| • |
It has high heat conductivity |
| • |
Ability to release the absorbed heat to the flowing fluid that has contact
with it |
At this juncture, a point of divergence set in for both the single pass and
double pass solar collector with porous matrix as far as flat absorber is concerned.
The single pass flat plate has a good insulating material at the lower side
of the plate. The insulation of the single pass collector usually consists of
a reflective foil with a fiberglass material that will not allow out-gas at
elevated temperature. Though, polyester fiber material is becoming popular due
to health and safety concerns with regard to usage of fiberglass.
Heat loss from a plane glass: The Eq. 12 below reflects
the summation of radiation and convective heat transfer between the absorber
plate and the cover:
where, h1 and h2 are the inner convective and plate to
cover radiative heat transfer coefficients, εp and εc
are the wavelength of the plate and cover emissive radiations and the temperatures
of the plate and cover are Tp and Tp, respectively.
The convective heat transfer correlation at tilt angle up to 60° is stated
below:
where, Cp, β, μ and k are fluid properties at mean temperature
0.5 (Tp, Tp), θ is angle of inclination of collector,
L is distance between absorber and glass.
Heat loss from double plane glass: The convective heat transfer correlation
is of the same form with the single plane except the constant 1.466 that changed
to 1.7 as shown below:
Heat loss from glass: The sum of convective and radiative heat loss
from glass cover of a single pass collector to the surrounding on a clear day
is depicted in the following Eq. 4.
where, Tsrd is the temperature of the surrounding.
The ambient and surrounding temperatures are the two vital functions that determine
the effective radiation heat transfer coefficient (h4):
Heat loss from lower side of the collector: This heat loss from the
lower side of the absorber is a major deficiency of the single pass solar collector.
The working fluid that passes through the porous material at the lower channel
of the double pass has saved heat loss. The heat transfer coefficient for loss
of heat through the lower channel from the collector plate 13 is as stated in
Eq. 19:
where, hout is the external heat transfer coefficient tlow
is the thickness of the insulator at lower channel and klow is the
conductivity of the insulation.
Similarly, loss at the edge of the collector is as shown in Eq.
20:
where, tedg and kedg are the thickness of the insulation
and the conductivity of the insulation, respectively.
Heat loss from upper side of the collector: In order to simplify the
equation involved, the ambient temperature Ta is assumed to be equal
to the surrounding temperature (Tsrd).
Therefore, the heat loss coefficient can be valued in terms of collector plate
and ambient temperature difference:
Therefore, the summation of heat losses is UT 4:
Factor of heat removal by fluid: The working fluid circulates round
the collector system as it pass through the absorber plate, it gains heat from
the collector. The loss of heat from collector earlier discussed is to the surrounding
but this is crucial, since it is the determinant of the collector efficiency.
Therefore, the heat gain factor is calculated in the following form Eq.
19:
where, G is solar radiation intensity, A is area of plate:
The heat removal factor FR is low in value for single pass collector
when compare with double pass solar collector because of porous media and additional
pass of the fluid below the absorber.
Effects of porous material: Porous media are solid materials that have
good absorptivity and emissivity properties. They are loosely filled into the
lower channel of the double pass solar collector. This reduced the porosity
of the lower channel and the flow rate of the working fluid also reduced. The
solid matrix acts as energy storing material. The energy that would have crossed
the boundary of the system is retained. This is dissipated to the working fluid
flowing across it. Hence, it improves the efficiency of the collector system
as reported by Sopian et al. (2009) and Rabadi
and Mismar (2003). Cheap material such as rock samples and metal chips are
used as porous media.
Working fluid path: The channels of flow of working fluid in a single
pass solar collector are of two types:
| • |
The collector in which the working fluid flow past the top
surface of the absorber |
| • |
The collector that working fluid flow beneath the flat plate solar collector |
However, the working fluid flows past through the both sides of the double
pass solar collector, this make it more efficient because of increase in fluid
rate of heat removal from the absorber compare to the single pass solar collector
that has contact with one face of the absorber only. Through, the flow direction
may be reversed in double pass by starting of fluid flowing from lower channel
to the upper surface of the collector.
RESULTS AND DISCUSSION
Here, we used the literature values to show major differences between the single
pass collector without porous media and double pass collector that has gravels
at the lower channel of the absorber plate.
Figure 3 shows the Single Pass Solar Collector (SPSC) and
Double Pass Solar Collector with porous matrix (DPSC) in which various efficiencies
in relation with the reduced temperature parameter (Tp-Tsrd)/G.
It can be deduced from the graph that single pass is effective at low value
of reduced temperature parameter but totally inactive at higher value. When
the efficiency is 40%, the value of reduced temperature parameter is about 0.03
and 0.08 for single pass and double pass collector with porous media, respectively.
The temperature parameter indicates that the temperature within the confine
of absorber is higher in double pass with porous media. This behaviour of the
collectors was confirmed by Duffie and Beckman (2006)
and Tiwari (2002).
|
| Fig. 3: |
Typical solar collector efficiency characteristics, (Tp-Tsrd)/G:
Temperature difference function of the collectors |
The output power of the solar collectors against the mass flow rate of the
fluid is shown in Fig. 4. It can be seen from the graph that,
when the mass flow rate is 0.1 kg sec-1 about 340 Watt of power can
be generated from single pass while double pass reflected close to 880 Watt.
This shows clearly that the output of the single pass solar collector is low
compared with the double pass, even increase in fluid mass flow rate cannot
make any meaningful improvement in the power out-put of single pass solar collector.
The thermodynamic efficiency against mass flow rate has the same pattern of
curves in Fig. 4 as reported by Sopian
et al. (2009).
The ambient temperature is the temperature in the neighborhood of flat plate
collector. Therefore, when the increment on ambient was about 15°C, 49 and
67% was recorded for both single and double passes, respectively.
|
| Fig. 4: |
Effect of mass flow rate on thermal output power |
|
| Fig. 5: |
Variation of performance efficiency with increment in ambient
temperature |
It can also be deduced from Fig. 5 that the efficiency of
the single pass solar collector may not exceed 52% efficiency despite increment
of about 20°C from the ambient temperature of 31°C. But double pass
solar collector can reach around 70% of efficiency within this range of increment
in temperature.
This is due to the fact that the double pass collector retains some of the
heat loss by single pass solar collector through the cover, absorber and insulating
material. Furthermore, the presence of heat retaining material, the porous media
has added more to the efficiency of the double pass. This is similar to the
report of Aldabbagh et al. (2010).
Future development: It was observed that improvement can be made on
the solar collectors if the heat loss can be reduced or brought to negligible
value. This is the idea that is behind the study of a collector in which the
working fluid would be able to have more than two pass called multi-pass as
reported by Karim and Hawlader (2004).
CONCLUSION
The study has tried to reflect the basic differences between the conventional
single pass solar collector with a plane of glass as a glazing material with
the double pass solar collector with porous media, gravel at the lower channel
of absorber plate. It was shown that enormous amount of heat is loss at various
boundaries that surrounded the absorber plate. Therefore, heat loss is greatly
reduced in the double pass and the gravel assisted in heat retention. Suggestion
of multi pass solar collector was also made for further improvement on performance
efficiency.
ACKNOWLEDGMENTS
The conducive environment that aids research activities established by PETRONAS
and the authorities of the Universiti Teknologi PETRONAS are highly appreciated.
NOMENCLATURE
| α |
= |
Absorptance |
| ρ |
= |
Reflectance |
| τ |
= |
Transmittance |
| Cg,in |
= |
Inner glass cover |
| Tg,in |
= |
Temperature of the inner glass (°C) |
| Tsrd |
= |
Temperature of surrounding, sky (°C) |
| h |
= |
Convective heat trans. coefficient(W m-2 °C-1) |
| q |
= |
Heat transfer (W m-2) |
 |
= |
Mass flow rate (kg m-2 sec-1) |
| A |
= |
Absorber surface area (m) |
| η |
= |
Collector efficiency (%) |
| t |
= |
Thickness (mm) |
| Nu |
= |
Nusselt number |
| L |
= |
Distance between the glass and plate (m) |
| FR |
= |
Heat removal factor from plate |
| UT |
= |
Plate total loss coefficient (W m-2 °C-1) |
| k |
= |
Conductivity (W m-1 °C) |
| Cp |
= |
Specific heat capacity at constant pressure |
| β |
= |
Tilt angle, vol. heat expansion coefficient |
| θ |
= |
Angle of incidence, collector tilt angle |
| G |
= |
Solar radiation intensity (W m-2) |
Subscribe Today
