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             Continental J. Renewable Energy 2 (2): 1 - 18, 2011                          ISSN: 2251 - 0494
             © Wilolud Journals, 2011                                  
                                      `         Printed in Nigeria


                                                 O. Ojike
  National Centre for Energy Research and Development, University of Nigeria, Nsukka, Enugu state. Nigeria

    A double glazed passive solar collector was characterized and its properties determined. It consists of
    four black rectangular steel boxes neatly laid side by side so that their top formed absorber plate with
    selective surface. The boxes which behave as thin fins were filled with each 12.7kg of RT60 paraffin.
    The space between the absorber plate and the inner glass serves as the air heater. The insulation is m
    of fibre glass of 0.06m thick at the bottom and sides of the collector. By natural convection, ambient air
    enters the air inlet header while the heated air leaves through the air outlet header. The collector is
    inclined towards the equator at 17oC. Energy balance equations were used to characterize the collector.
    The thermal response of the collector at Nsukka was tested with collector efficiency varying between
    28 and 56%. The collector’s average efficiency was 40%, which is within the published values for solar
    flat plate collectors operating under 100oC. The collector could be used as a heat source for various
    agricultural processing systems.

                                         absorber-plate, efficiency, heat
    KEYWORDS: passive, paraffin, energy, absorber

Flat-plate collectors are the simplest and most commonly used collectors for converting the sun’s radiation into
useful heat. A collector is a device for converting the energy in solar radiation into a more usable or storable
form. Flat plate collectors are designed for applications requiring energy delivery at moderate temperature (less
than 100°C) as in water and space heating (Duffie and Beckman, 1991, Agbo and Okoroigwe, 2007). They have
the advantages of using both beam and diffuse solar radiation, not requiring orientation t  towards the sun and
requiring little maintenance. They consist of (1) a dark flat plate absorber of solar energy, upon which solar
radiation falls and is absorbed, changing to heat energy (2) a transparent cover that allows solar energy to pass
                                                                                              heat-transport fluid
through but reduces the upward convection and radiation heat losses from the collector, (3) a heat
(air, antifreeze or water) to remove heat from the absorber, and (4) a heat insulating backing to minimize heat
losses by conduction. The absorber consists of a thin absorber sheet (of thermally stable polymers, aluminum,
steel or copper, to which a matt black or selective coating is applied).

Applications of solar energy in agriculture aside from growing crops include pumping water, drying crops,
brooding chicks and drying chicken manure (Leon and Kumar, 2006). To solve the energy supply problems
associated with poultry egg incubation, Irtwange (1992) developed a solar heated system with rock  rock-bed energy
storage (Enibe, 2002). Although his system could not operate due to faulty design features and improper
choice/location of the energy storage device, it suggested the possibility of developing solar collectors with heat
storage systems for agricultural purposes. Thus, in this study characterization of passive solar collector design as
a heat source for agricultural processes is done.

Solar radiation is a periodic energy resource with strong diurnal variation. Its use for agricultural purposes must
therefore incorporate a storage system to take care of the off-sunshine hours. Ways of storing solar energy can
be roughly classified into three types: sensible heat storage, latent heat storage and chemical reaction energy
storage (FAO, 1994). Several advantages of phase change material (PCM) energy storage stron     strongly suggest its
preference in solar egg incubation system. This includes a high energy storage density and the isothermal nature
of the heat storage and recovery process (Conti and Charach, 1996; Enibe, 2003).

 A phase change material is a solid which stores energy by melting upon the application of heat. The stored
energy is recovered upon solidification of the liquid. Many latent heat materials have been reviewed (Enibe
2003; Agyenim et al., 2010; Alkilani et al., 2009). These are mainly hydrated salts (such as Glauber’s salt),
paraffins, non-paraffins and fatty acids (Enibe 2002; Agyenim et al., 2010). Many low-   -cost paraffins are now
available for use as PCMs at different temperatures up to 110oC (Veraj et al., 1997). PCM energy storage

                         O. Ojike: Continental J. Renewable Energy 2 (2): 10 - 18, 2011

devices have been developed for space applications (Fatah, 1994), domestic hot water systems, greenhouse
heating and solar power plants (Conti and Charach, 1996). Good operating efficiencies have been reported.
The use of phase change materials for energy storage is not without its troubles. For salt hydrates, the major
drawback is that of crystallization and segregation of the salt during repeated cycles of heat charge and
discharge (Rabin et al., 1995). Another problem, common to all PCMs, is the low thermal conductivity of the
material. The problem may be overcome by the use of fins of various configurations (Veraj et al, 1997; Lacroix,
1993; Shatikian et al, 2008).

The output useful energy from a flat-plate collector depends on the thermal and optical losses occurring within
the system. Thus, the performance of the collector can be optimized if the losses are reduced minimally. Several
works (Yeh et al., 2003; Mumah, 1995; Eisenmann et al., 2004; Agbo and Unachukwu, 2006; Agbo and
Okoroigwe, 2007) have been done in the area of design, performance evaluation and optimization of the
collectors. Evaluation of the collector losses had been carried out by most of these authors using the Klein
model of the loss coefficient. Pillar and Agarwal (1981) had reported on the optical and thermal losses of the
flat-plate collector as a function of the number of glazing cover, plate emittance, wind velocity and the ambient
temperature using the Klein model. Agbo and Okoroigwe (2007), utilized the Malhotra et al. (1981) model for
the collector overall heat-loss coefficient to investigate the effect of wind speed, number of glazing cover,
ambient temperature, gap spacing between collector plate and the glazing cover, collector tilt angle and the plate
emissivity on energy losses in the collector. This model is simpler than the Klein’s and can also be used to study
the effect of collector tilt angle which is not reflected in the Klein’s model. The choice of a theoretical model for
scientific evaluations is cost-effective and tremendously reduces the empiricism associated with system’s
designing and performance evaluation.

The aim of this study, is to determine the properties of a double glazed passive solar collector using the
Malhotra et al. (1981) model for the collector overall heat-loss coefficient. The properties to be determined are
the output useful energy from a flat-plate collector and the system efficiency.

Description of the Passive Solar Collector
The passive solar collector was developed at the National Centre for Energy Research and Development,
University of Nigeria, Nsukka. It is of natural convection type as shown in Fig 1 and 2. The solar collector is
always tilted and oriented in such a way that it receives maximum solar radiation during the desired season of
use. Okonkwo (1993) suggested that a practical approach to flat plate collector is to tilt the collector along
north-south direction at an angle from the horizontal to the local latitude plus 10o to 15o. The best stationary
orientation is due south in the northern hemisphere. Therefore, the solar collector in this work is oriented facing
south and tilted at 17o to the horizontal. This is approximately 10o more than the local geographical latitude
(Nsukka is 6.8oN), which is the best recommended orientation for a stationary absorber (Adegoke and Bolaji,
2000). The major components of the collector as shown in fig. 1 are four thin rectangular steel (1.1m by 0.5m by
0.03m) black boxes neatly laid side by side so that their top formed the absorber plate (Bt) of (2.2m by 1m)
2.2m2 area. The boxes were filled with each 12.7kg of RT60 paraffin (P). It has two glass covers, Gout and Gin.
The insulation is made of fibre glass of 0.06m thick at the bottom and sides of the collector. In the evenings, the
solar collector may be covered with an opaque screen to minimize the night-time heat loss coefficient.

Fig. 1. Photograph of the solar collector assembly with energy storage and air-heating chamber.

                         O. Ojike: Continental J. Renewable Energy 2 (2): 10 - 18, 2011

                           Fig. 2. Schematic diagram of the collector

The thermophysical properties of the PCM is shown in Table 1.

                  Table 1: Thermophysical     properties of the phase change material

                           Source: Enibe, 2003

Basic Flat-Plate Energy Balance Equation
In steady state, the performance of a solar collector is described by an energy balance that indicates the
distribution of incident solar energy into useful energy gain, thermal losses and optical losses. The solar
radiation absorbed by a collector per unit area of absorber S is equal to the difference between the incident solar
radiation and optical losses. The thermal energy lost from the collector as shown in fig 3 to the surroundings by
conduction, convection, and infrared radiation can be represented as the product of a heat transfer coefficient UL
times the difference between the mean absorber plate temperature Tp and the ambient temperature Ta.

Fig. 3: Energy balance of a Flat-plate collector (Agbo, and Okoroigwe, 2007)

The useful energy output of a collector of area Ac is given as:
             Qu = Ac [S - UL (Tp - Ta)]                                                     (1)

                             O. Ojike: Continental J. Renewable Energy 2 (2): 10 - 18, 2011

               =( )                                                                (2)
(   )  is average transmittance-absorptance product which according to Duffie and Beckman (1991) is
             ( ) ≌ 0.96( ) .                                                             (3)
(   ) is transmittance-absorptance product for beam radiation and is gotten from Fig. 4.

Fig. 4. Typical ( )/( ) curves for 1 to 4 covers (Duffie and Beckman, 1991).
  Ѳ      Angle of incidence, the angle between the beam radiation on a surface and the normal to          that surface.

For collector facing directly south
Cos Ѳ = sin δ sinФ cos β − sin δ cosФ sin βϓ + cos δ cosФ cos β cosω + cos δ sinФ sin β ϓcosω                     (4)
      Latitute, the angular location north or south of the equator, north positive; -90 ≤ ≤90o
        Declination, the angular position of the sun at solr noon (i.e., when the sun is on the local meridian)
with respect to the plane of the equator, north positive -23.450 ≤    ≤ 23.450                              =
23.45sin (360       )                                                            (5)
n        day of the year. 1 ≤ n ≤ 365
    Slope, the angle between the plane of the surface in question and the horizontal; 0 ≤      ≤ 1800
         Hour angle, the angular displacement of the sun east or west of the local meridian due to         rotation     of
the earth on its axis at 150 per hour, morning negative, afternoon positive.

Collector overall -heat loss coefficient, UL: The collector overall heat loss coefficient is the sum of the top, edge
and bottom loss coefficients.

              (     )(                 )      (     )
          =                                                                                         (6)

                         (       )
          ℎ =                                                                                 (7)

Where UT, and UB are the top loss and bottom loss coefficient, εg is the transparent cover emissivity, εp the
absorber plate emissivity, Tg is temperature of the inner glass cover, while glass, absorber plate, and radiation
heat transfer coefficients are h1, h2 and hr respectively.

Following the basic procedure of Hottel and Woertz, Klein developed an empirical equation for the top loss
coefficient, UT as (Duffie and Bechman, 1991; Agbo and Okoroigwe 2007):

                         O. Ojike: Continental J. Renewable Energy 2 (2): 10 - 18, 2011


where f = (1 + 0.089hw-0.1166hw εp) (1 + 0.07866N), Cair = 520 (1-0.00005β2), β is the collector tilt and σ is the
Stephan Boltzmann constant.


The convective heat-transfer coefficient hw, for air flowing over the outside surface of the glass cover depends
primarily on the wind velocity, v and can be determined from (Duffie and Beckman, 1991):

              hw = 2.8 + 3.8V                                                             (10)

Where V is the wind velocity in m/s

The bottom loss coefficient, UB derives from the thermal conductivity, KS and the thickness, Ls of the bottom
insulator as:

The air mass flow rate was calculated from the expression
               ṁ = ρaVAa                                                                             (12)

with Aa as air inlet area and ρa calculated for any given temperature Ta by considering air as a perfect gas at
constant pressure.

Thus, the useful instantaneous collector’s efficiency is given as:
                 =                                                                          (13)

For solar collectors with storage system, the cumulative efficiency,    is a more useful measure of performance
than the instantaneous efficiency, (Enibe, 2002).

The cumulative heat gain from start up to any particular time is obtained by integrating the total useful heat gain
for the period. Thus,
                  =                                                                        (14)

The cumulative efficiency at any time may be obtained by dividing the cumulative
useful heat gain from the start by the cumulative irradiation, giving
                 =                                                                         (15)

Solar Radiation
Fig. 5 gives a graphical representation of the hourly solar radiation of December 10, 2010 which is the
recommended average day for the month of December (Duffie and Beckman, 1991) in which the evaluation was
done. The graph equally shows the variation of lowest and highest solar radiation days. The lowest and highest
solar radiation days were December, 12 and 2 respectively.

                                                    O. Ojike: Continental J. Renewable Energy 2 (2): 10 - 18, 2011


                          Solar Irradiance (W/m2)
                                                     400                                                      Slr
                                                     300                                                      02/10Slr
                                                     200                                                      12/10Slr
                    Time (hour)
                  Fig.5 Solar Irradiance against Time

where Slr, 02/10Slr and 12/10Slr are solar irradiance on the 10th, 2nd and 12th December, 2010 respectively.
From Fig. 5 it is observed that the hourly solar radiation is maximum at 11:00 am and 2:00pm while lowest at
early morning and late evening hours of the day respectively.

Temperature of the Passive Solar collector/Heat Storage System
Fig. 6 shows the variation in temperature of the (outer and inner glass medium, and absorber plate) collector and
storage (paraffin) system.

                     Temperature (oC)

                                                            80                                                       Amb

                                                            60                                                       P

                                                            40                                                       Bt

                 Time (hours)
Fig. 6. Temperature readings of the solar collector against Time of 10th December, 2010
where amb is ambient temperature.

From the figure, minimum temperatures of 24o, 36o, 42o, and 45o were attained at 4:00am for outer glass, inner
glass, absorber plate and paraffin respectively. After attaining the minimum values at 4:00am the temperatures
for the entire collector continue to rise until they reach maximum temperatures of 34o, 75o, 96o and 106o for
outer glass, inner glass, absorber plate and paraffin respectively at 2.00pm from then they begin to drop again
till they reach the minimum values around 4.00 am the following day and the circle continues.

Comparing figures 5 and 6 it can easily be observed that the values in figure 5 are directly affected by solar
radiation available at any given time. During the day, that is sunshine period; the absorber plate absorbs solar
radiation and converts it to heat energy. As air flows across the absorber plate, it absorbs heat from the plate and
flows out through the air outlet. However, as solar irradiance starts reducing from the peak period less heat
energy is available for storage medium and increasing quantity of stored energy is absorbed by the moving air

                         O. Ojike: Continental J. Renewable Energy 2 (2): 10 - 18, 2011

into the incubating chamber. This continues till the minimum value is attained around 4:00a.m when solar
radiation starts heating up the system again.

                                     Fig. 7. Collector Efficiency

Figure 7 shows the useful collector efficiency plotted as a function of total hourly insolation. The average
efficiency is about 40%, which is well within the useful efficiency of flat plate solar collectors operating below
100oC (see for example, Duffie and Beckman, 1991; Enibe, 2002). The scatter observed in the plots is due to
other climatic factors which influence collector efficiency, such as ambient temperature and diffuse fraction of
incident radiation. The latter, as observed by Ezekwe and Ezeilo (1981), can be up to 40% on some overcast

The passive solar collector with paraffin as a storage medium can conveniently be used for various agricultural
processing purposes which include heating systems and drying of various crops. This can be done by attaching a
drying chamber to the air outlet of the collector. Its advantage over ordinary passive solar dryers is that the heat
storage medium helps drying to continue even during off sun-shine periods especially during the night hours.
Thereby ensuring faster drying process. It can equally be attached to an incubating chamber to serve as a heat
source for poultry egg incubation. In this way, energy needs of an average farmer can be solved to a large

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Received for Publication: 05/09/2011
Accepted for Publication: 04/11/2011


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