Elvin Infante

The University of the Sunshine CoastElvin Infante: 1044435

Solar Collectors

TABLE OF CONTENTS

1 Introduction.................................................................................................................1

2 Aim..............................................................................................................................1

3 Theory..........................................................................................................................1

3.1 General.............................................................................................................................1

3.2 Solar Collector Design........................................................................................................2

4 Method........................................................................................................................3

5 Results and Discussion.................................................................................................4

5.1 Discussion of Results.........................................................................................................4

5.2 Possible Improvements.....................................................................................................6

5.2.1 Overall Experiment............................................................................................................6

5.2.2 Solar Collector....................................................................................................................6

6 Conclusion....................................................................................................................7

Appendix A........................................................................................................................8

Appendix B ...................................................................................................................9

1 INTRODUCTION

Over time there has been an increasing interest in renewable energy sources with much if this interest spurred

by the need for our society to move past fossil fuels and look to other sources that will help to achieve a more

sustainable way of life. One of these sources is solar energy. Solar collectors transform solar radiation from the

sun into heat energy of which is transferred to medium and converted to useable energy to be applied to any

number of everyday things. This report will discuss the results obtained from an experiment, which entails a

solar collector heating a piece of copper and will aim to relate the results to the fundamentals behind solar

radiation conversion.

2 AIM

The aim of this practical is to determine the heating characteristics of a dish type solar collector using a piece

of copper as the absorber. The experiment will focus on the maximum temperature and the power output as a

function of temperature to demonstrate the theories that have been learnt.

3 THEORY

3.1 General

In an ideal situation, it should be observed that at some point in time during the experiment, a balance

between the rate at which energy from the sun is absorbed by the copper, the rate at which the copper heats

up and the rate at which heat is radiated from the copper to the surrounding environment will be achieved.

This balance signifies that the system is working at its maximum efficiency where by the copper is being heated

effectively by the system. This scenario is expressed in the following equation:

As the system moves beyond this point, the overall efficiency should decrease and as a result the amount and

rate of change in temperature. As time elapses and the setup reaches its maximum temperature, it should be

apparent that the power absorbed by the copper equals that of the power emitted by the solar collector as the

temperature has plateaued thus indicating that no further heating is experienced by the copper.

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3.2 Solar Collector Design

In order to meet the specifications of this experiment, a dish type solar collector design was adopted. The dish

type solar collector is, as the name states, a disk that reflects the incoming energy to one point – its focal

point. Optimizing the design so that the focal point is focused on the absorber is key to reducing the amount of

wasted energy. Another crucial factor is the overall construction of the system, as a poorly constructed system

may result in the energy being reflected away from the absorber, thus it is critical that the curve of the dish is

smooth and is stays true to its intended parabolic shape. Figure 1 below shows the way in which a dish type

collector works, with Figure 2 showing the actual collector that was used.

Figure 1 - Typical Dish Type Solar Collector

Figure 2 - Solar Collector Used for Practical

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4 METHOD

The following points describe the procedure that was undertaken to achieve the goals of the experiment.

1. The focal point was determined by the use of a piece of paper that was held above the centre of the

dish and moved up and down. This indicated the size of the area of focus thus the optimal position to

position the piece of copper.

2. The piece of copper was mounted into position.

3. An ambient temperature reading was recorded in the shade prior to attaching the thermocouple to

the mounted piece of copper, making sure that it was in contact with the copper.

4. The solar collector was positioned in a central position beneath the heating lamp.

5. The heating lamp and timer were started instantaneously with temperature readings being recorded

at 15-second intervals.

6. The data was compiled and placed into tables of which were used to form graphs of the resulting data

and calculations.

There were two graphs formed of which included a ‘Temperature vs Time’ and ‘Efficiency (η) vs Temperature’.

In order to achieve efficiency PHEATING was first determined using the following equation:

(Eq.1)

where:

PHEATING is the heating power being experienced by the copper

m is the mass of the copper

c is the specific heat of the copper

dTdt is the instantaneous rate of change of temperature over time

This equation was then used to determine the solar collectors efficiency throughout the heating process with

the use of the following efficiency equation:

(Eq.2)

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where:

η is the efficiency at any point during the heating process

α is the heating coefficient

I is the intensity of the source

ACOLL is the cross sectional area of the piece of metal

In order to determine the gradients to the curve formed from plotting Temperature over Time, calculus was

used. Firstly an expression for the curve was derived using Microsoft Excel, following this the derivative to the

curve was solved and ‘x’ values were substituted which outputted the gradient of the curve for that value of

‘x’.

5 RESULTS AND DISCUSSION

5.1 Discussion of Results

This section discusses the results that were obtained as a result of conducting the experiment and delves into

the meanings of these outcomes and any unexpected occurrences. Table 1 below shows the variables used

through the practical.

Table 1 - List of Variables

Variables Used

Mass (m) 0.0112kg

Specific Heat – Copper (c) 386J/kg C °

Absorption Coefficient (α ) 0.95

Reflectivity (r) 0.9

Intensity (I) 800 W/m2

Area – Collector (A) 0.45m2

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Table 2 below summarises the results achieved from the experiment.

Table 2 - Summary of Results

Temperature

(C ° )

Gradient Change in Gradient P-Heating

(W)

Efficiency

27 0.526 2.2845 0.0071

47.6 0.473 -0.053 2.0543 0.0063

61.1 0.423 -0.05 1.8370 0.0057

73.1 0.376 -0.047 1.6330 0.0050

83.7 0.331 -0.045 1.4376 0.0044

93.0 0.289 -0.042 1.2552 0.0039

101.1 0.25 -0.039 1.0858 0.0034

118.8 0.149 -0.101 0.6471 0.0020

128.6 0.072 -0.077 0.3127 0.0001

132.6 0.019 -0.053 0.0825 0.0001

Overall the solar collector performed quite well as it met the brief due to its ability to focus the energy that

was emitted from the source to the copper and was able to raise the copper’s temperature as is clearly evident

in both Table 2 and Figure 4. Observing the gradients over the heating period in both Table 2 and the curve

characteristics of Figure 4, it can be said that the temperature increase experienced by the copper was at a

much greater rate at the beginning of the testing than that of the tale end. This result is consistent with the

previously mentioned theory as it is clearly evident that as the setup reaches its maximum temperature the

heating power decreases. This is also demonstrated in the incremental changes in the raw temperature data

shown in Appendix A, as the changes are quite significant up until 133° with jumps of 4° - 21°with the larger

changes at the beginning. These changes decrease after this point with the data showing a steading

temperature increase of 1° - 2° until a maximum temperature is reached signified by the data showing a

consistent reading of 158° . This lack of change is a major indicator that there is an absence of PHEATING thus

further satisfying the theory initially stated.

It is also apparent that the PHEATING value never reaches levels close to that of the source (800 W/m2). Though

there have been a number of assumptions adopted during this experiment that could explain this, this lack of

reflected power can also be attributed to the solar collector itself. The most prominent issue that was

observed was the focus area of the solar collector’s focal point. When tested the focal area was much larger

than that of the copper thus leading to believe that a portion of the power reflected from the source missed

the copper completely. This effect was increased as time elapsed as the solar collector began to deform due to

the temperatures with the aluminum foil wrinkling, the cardboard bending and the glue bonds breaking. This

change in structure and shape meant that the power from the source was now being reflected away from the

copper thus reducing the solar collector’s effectiveness.

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Another aspect worth highlighting is the

efficiency readings that are

displayed in Appendix B. The trend of efficiency that was obtain did not meet expectations as it was expected

that the efficiency would begin at a low level with an increase experienced of which would reach a peak, then

fall to a lower level. This behavior should be been expressed through a curved similar to that of a negative

parabola with the peak indicating the systems maximum efficiency or a balance between the power absorbed

and the heating power with the power emitted. When referring to the results expressed in Figure 5 in

Appendix B, it may be assumed that the data is indicating that the system began at temperatures beyond its

efficiency thus the resultant curve was achieved.

5.2 Possible Improvements

This section discusses the possible improvements that may have been implemented to improve certain aspects

of the experiment.

5.2.1 Overall Experiment

The most pressing improvement would be to have the experiment conducted outside on an optimal day with

no cloud cover and limited breeze. This was unattainable at the time that this experiment was conducted due

to the nature of the weather. Conducting the experiment in these conditions would provide more accurate

results. In addition to this, access to results achieved by other designs could also be helpful as it provides

further insight to the advantages and disadvantages of build quality and design.

5.2.2 Solar Collector

It is believed that the most influential change that could be made to the solar collector that was used for this

experiment was the material that was used both for its construction and for the reflective component.

Improved construction materials would reduce the deformation of the structure, thus reducing the amount of

power being reflected away from the copper. Improved reflective material has the potential to increase the

system’s operating capacity thus also increasing the system’s efficiency.

Figure 3 – Deformation of Panels and Aluminium Lining

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6 CONCLUSION

The solar collector that was constructed for the purpose of demonstrating the theory behind simple solar

collection performed adequately and was able to produce results that met expectations on the most part with

the system able to achieve a maximum temperature of 158 C° over a 12.5min period, achieving a maximum

efficiency of 0.007. The results displayed the relationship between the power absorbed, heating power

experienced and the power emitted by the piece of copper and successfully showed that as the system

reached its maximum temperature, the heating power experienced by the copper was less apparent.

Improvements to both the experiment; by using the sun as the source and the solar collector; with

improvements to the materials used could have attain better results. With this said, the overall experiment

was enough to allow for the understanding of underlying components relating to solar collection.

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APPENDIX A

Table 3 – Raw Data Collected from Practical

Raw Data

Temperature (degrees)

Time (seconds)

27 0

48 30

62 60

74 90

86 120

95 150

103 180

110 210

115 240

120 270

125 300

129 330

133 360

135 390

137 420

139 450

141 480

143 510

145 540

147 570

149 600

151 630

153 660

155 690

157 720

158 750

158 780

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APPENDIX B

Figure 4 – Temperature over Time Graph

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Figure 5 – Efficiency Over Temperature Graph

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