Design and Build of a Solar Energy System for Personal Implementation

Design and Build of a Solar Energy System for Personal Implementation by Daniel J. Desaulniers An Engineering Project Submitted to the Graduate Facult...
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Design and Build of a Solar Energy System for Personal Implementation by Daniel J. Desaulniers An Engineering Project Submitted to the Graduate Faculty of Rensselaer Polytechnic Institute in Partial Fulfillment of the Requirements for the degree of MASTER OF ENGINEERING IN MECHANICAL ENGINEERING

Approved: _________________________________________ Sudhangsu Bose, Ph. D., Engineering Project Advisor

Rensselaer Polytechnic Institute Hartford, CT April 2012 (For Graduation Dec 2012)

© Copyright 2012 by Daniel Desaulniers All Rights Reserve

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CONTENTS LIST OF TABLES ............................................................................................................. v LIST OF FIGURES .......................................................................................................... vi NOMENCLATURE ........................................................................................................ vii ABSTRACT ................................................................................................................... viii 1. Introduction.................................................................................................................. 1 1.1

Alternate Energy Systems .................................................................................. 1

1.2

Solar Cells .......................................................................................................... 1

1.3

Solar Energy System .......................................................................................... 4

2. Methodology ................................................................................................................ 5 2.1

Solar Irradiance Analysis ................................................................................... 5

2.2

Solar Panel Analysis ........................................................................................ 10

3. Solar Panel ................................................................................................................. 11 3.1

3.2

Solar Panel Assembly ...................................................................................... 11 3.1.1

Panel Design ........................................................................................ 11

3.1.2

Material Selection ................................................................................ 13

3.1.3

Panel Construction ............................................................................... 14

Solar Panel Testing .......................................................................................... 21

4. Results and Discussion .............................................................................................. 24 4.1

Estimated Solar Panel Results.......................................................................... 24

4.2

Solar Panel Testing Results.............................................................................. 27

4.3

Solar Panel Data Comparison .......................................................................... 28

5. Conclusion ................................................................................................................. 29 6. References.................................................................................................................. 30 7. Appendix A: Solar Panel Construction Drawings ..................................................... 31 8. Appendix B: Solar Cell Specification Sheet .............................................................. 39 9. Appendix C: Solar Panel Estimation Program Data Tables ...................................... 41 10. Appendix D: Solar Panel Estimation Program Yearly Data Tables .......................... 45 iii

11. Appendix E: Solar Panel Testing Data Tables .......................................................... 55

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LIST OF TABLES Table 1: Parameters Used to Estimate Solar Radiation Intensity [2] .................................. 8 Table 2: Solar Panel Material List ................................................................................... 14

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LIST OF FIGURES Figure 1: Solar Energy System .......................................................................................... 1 Figure 2: Shockley-Queisser Limit Curve[1] ...................................................................... 4 Figure 3: Earth's Declination Angle [2] .............................................................................. 5 Figure 4: Solar Angles with respect to the Earth’s Surface [2]........................................... 6 Figure 5: Solar Panel Angles[2] .......................................................................................... 7 Figure 6: 1.8 Watt Polycrystalline Solar Cell .................................................................. 12 Figure 7: Solar Panel SolidWorks Model ........................................................................ 13 Figure 8: Solar Cell Tabbing ........................................................................................... 15 Figure 9: Corner of Assembled Frame ............................................................................ 16 Figure 10: Solar Cells Series Soldering ........................................................................... 17 Figure 11: Solar Panel Ready for Encapsulation ............................................................. 18 Figure 12: Positive End with Diode................................................................................. 19 Figure 13: Solar Panel Terminal Box .............................................................................. 20 Figure 14: Solar Panel Test Setup ................................................................................... 21 Figure 15: Test Setup Panel Rotation .............................................................................. 22 Figure 16: Test Setup Panel Support ............................................................................... 23 Figure 17: Solar Panel Testing ........................................................................................ 24 Figure 18: Estimated Panel Output on April 2, 2012 ...................................................... 25 Figure 19: Azimuth Angle Comparison .......................................................................... 26 Figure 20: Estimated Yearly Panel Output ...................................................................... 27 Figure 21: Measured Panel Output on April 2, 2012....................................................... 28

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NOMENCLATURE δ

Solar Declination Angle (Degrees)

N

Day Number in a Year (Dimensionless)

ω

Hour Angle of the Sun (Degrees)

θZ

Zenith Angle (Degrees)



Local Latitude (Degrees)

α

Solar Altitude (Degrees)



Solar Azimuth (Degrees)

β

Solar Panel Tilt Angle (Degrees)

θ

Incident Angle (Degrees)

γ

Solar Panel Azimuth Angle (Degrees)

A

Apparent Extraterrestrial Solar Intensity (W/m2)

B

Extinction Coefficient for Solar Radiations (air mass)-1

C

Monthly Average Ratio of Diffuse to Direct Normal Radiation (Dimensionless)

IDN

Direct Normal Irradiance (W/m2)

ISC

Solar Irradiance Constant (W/m2)

I0

Apparent Solar Irradiance (W/m2)

P

Location Pressure (Dimensionless)

x

Altitude above Sea Level (ft)

Idirect

Direct Radiation Flux (W/m2)

/Po

Idiffuse Diffuse Radiation Flux (W/m2) ρ

Environment Reflectivity Factor (Dimensionless)

Iglobal

Total Radiation Flux (W/m2)

η

Solar Panel Efficiency (Dimensionless)

Pm

Total Solar Panel Output (W)

Ap

Total Area of Solar Cells (m2)

P

Estimate Panel Output(W)

I

Current (A)

V

Voltage (V)

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ABSTRACT With the new interest in alternative energies, the move of many home owners toward the use of solar power to subsidize their dependency on fossil fuels to create electricity has never been more prevalent. While the desire to switch is overwhelmingly there, the major issue holding most back is the extremely high initial cost of such systems and long payback periods. In this project an affordable homemade solar energy system was designed, constructed and tested. The system consisted of a solar panel, charge controller and 12 volt battery bank. For the 64.8 Watt panel that was built the total cost was $184.87, which gave a watt to dollar ratio of 2.85. Using a developed solar program the estimated yearly output of the system was predicted. With a constant panel angle of 40° the average 24 hour output of the panel for the entire year was 16.7 Watts per day, which includes down time during night. Comparing estimated performance to measured performance for the unloaded panel, an average increase in power of 54% was found during testing. Differences in results are potentially due to the irregular environmental parameters and the highly temperature dependent solar cells.

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1. Introduction 1.1 Alternate Energy Systems With the recent push in energy independence, alternate energy systems haven’t been more prevalent. From solar to wind, these renewable energy systems are slowly making their way into households across the world. However However, there is one major issue that is stunting what could be wild growth, cost. While the desire desir is there, the overwhelming initial cost cannot be justified by the long term payback period. But, if one were to build their own energy system, the low cost could be easily warranted. Solar power is a renewable energy which can bee harnessed through the t use of solar panels. anels. Once integrated with the addition of a battery, as seen in Figure 1, it can be used to provide electricity to many common household items. Whilee solar power is dependent on the sun and can only charge during the day, they are quiet, as opposed to wind systems, and are easy to install with relative low yearly maintenance.

Figure 1: Solar Energy System

1.2 Solar Cells Solar cells are the building block of solar panels and understanding how the cells work must be discussed to understand how a solar panel can produce electricity. electricity Silicon based cells ells work based on the properties held within silicon. When silicon crystallizes it creates a special crystalline structure where a single silicon atom will bond with four 1

other silicon atoms thus sharing electrons and filling its outer shell to maintain balance. Since the structure is balanced, this makes for poor electrical conducting conditions. To remedy this problem, an impurity (phosphorous) is added to the silicon, which disrupts the harmonious environment. Phosphorous atoms have almost the same electronic outer shell arrangement as silicon; however they have an additional electron. When energy is added to the cell, extra electrons from the phosphorous atoms are easily knocked free. The electrons then drift around looking for openings to bond with, while simultaneously creating an electrical current. But, since the silicon and phosphorous bonds are equal, it is a rare event if the extra electrons find an opening. The siliconphosphorous system then maintains “floating” electrons and becomes negatively charged and is otherwise known as N-type silicon. The same process can be completed to develop P-type silicon, or positively charged silicon. To do this, boron is added to the silicon as opposed to the phosphorus. Boron, however, has one less electron in its outer shell than silicon, and this loss in electrons results in a positive charge. When the N-type and P-type silicon materials are separate, they remain electrically stunted, but when combined, like in a solar cell, they develop an electrical field. At the contact point, the free electrons on the N-type rush to fill in the all the openings on the P-type. As more and more openings are filled, equilibrium is achieved acting like a barrier which prevents any additional electrons from flowing to the positive side. This difference in charge produces the electrical field which then operates as a diode, only allowing electrons to flow from the P-type to the N-type, however not the other way around. Introducing light to the solar cell then creates disruption in the equilibrium. Photons in the light break apart the electron-hole pairs that were previously developed when the materials were originally combined. With the electron-hole pairs beginning to break down, the free electrons are sent back to the N-type while the free holes build up on the P-type. This difference in charge creates voltage and if, through external means, the N side is connected to the P side, electrons will begin to flow producing current. With voltage and current one now has power.

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There are three types of solar cells; monocrystalline silicon, polycrystalline silicon and ribbon silicon. The monocrystalline silicon cells are the most expensive. However, they are also the most efficient. Polycrystalline cells are less expensive than the monocrystalline as well as less efficient. Lastly ribbon cells, take on the same declining trait, cheaper to make but far less efficient. Selecting the right cell should come down to economics. While one could have a small highly efficient system with monocrystalline cells, this most likely would be very expensive. On the other hand you could have a very large system with ribbon cells but at a lower cost. This balance between cost and size should play a major role in the selection process. One specific characteristic of solar panels that can be very problematic is their dependency on temperature. This dependency is due to the band gap which is a property all materials have. The band gap is a term which refers to the difference between the bottom of the conduction band and top of the valence band. Electrons are allowed to move from one band to the other if a certain amount of energy is applied. Based on the size of the band gap, a material’s electrical designation can be determined; insulator, semiconductor or conductor. Large band gaps will require a large amount of energy to move electrons, therefore relate to insulators. A small band gap requires less energy thus present in semiconductors. Lastly, tiny or no band gap is present in conducting materials. Since the silicon in the solar cells is a semiconductor, it has a relatively small band gap, requiring only sunlight to move electrons. The band gap of solar cells is what will drive its overall efficiency. Relating the two, a curve can be produced known as the Shockley-Queisser limit as seen in Figure 2[1]. This limit refers to the maximum theoretical efficiency of a solar cell. It can be seen that as the band gap is either reduced or increased the efficiency of the cell drops. This change in band gap directly correlates to change in temperature. As temperature increases, the band gap will decrease. Therefore large fluctuations in solar cell temperatures will cause the cell’s band gap to move out of its optimum operating range causing a drop in performance.

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Figure 2: Shockley-Queisser Limit Curve[1]

1.3 Solar Energy System A solar panel becomes a solar energy system with integration of an energy storing device such as a battery. While during the day the panel may be producing power, there needs to be a location where power can be stored for use during night time or other times es when power production is limited. Utilizing common 12 Volt batteries, such as those found in your car, is a great way to store energy. It is preferred however to use deep cycle batteries and not car batteries in particular as these are designed to deliver short, high current bursts bursts. Deep cycle batteries, such as those found on boats boat and RVs, are designed to output lower sustained amps such as those ddesired esired for constant power flux, making them much more equipped for the job. When the battery is not being used, and the solar panel has charged it to its limit, the threat of a meltdown exists. To prevent any damage to the battery, battery due to an overcharge, a charge controller must be added. The charge controller is a device which prevents the threat of overcharg overcharging ing the battery; in essence it is a relief valve. As power is supplied from the solar panel panel, the current first passes through the charge controller. Once the battery reaches its 12 Volt limit limit, the charge controller then diverts power to a “dummy” load to alleviate lleviate the threat of a battery melt down. The charge controller is an essential component in a system that will protect the large investment made on batteries. Besides the charge controller and battery, one optional component that can be added to the system stem is an inverter. The inverter takes the system from 12 Volt Direct

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Current (VDC) up to 120 Volt Alternating Current (AC), allowing the system to be compatible with most home appliances.

2. Methodology 2.1 Solar Irradiance Analysis Solar panel performance can be estimated through the understanding of solar radiation and earth’s celestial habits. Using predetermined equations and existing solar activity data, the intensity of the sun’s radiant energy, or irradiance, on the earth’s surface can be calculated thereby allowing one to accurately predict the power output of a solar panel. Earth’s orientation, otherwise known as declination, is defined in Equation 1. Depicted in Figure 3, the declination angle, expressed in degrees, is a function of N, the day number in the year. This equation is an approximation and will return slightly varied declination angles over the course of a four year period, which is corrected with the resulting leap year. Equation 1

 = .  ° 

 

 ∗ [2]

Figure 3: Earth's Declination Angle [2]

In conjunction with the earth’s declination angle is the earth’s rotation, shown in Equation 2. The earth’s rotation is expressed in degrees and differentiates morning as a negative number and afternoon as a positive.

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=−

Equation 2

 

 − [2]

Using the declination angle and the earth’s rotation, Equation 3 is used to determine the zenith angle for any coordinate of latitude. As show in Figure 4, the zenith angle is the angle between the sun and a line normal to the earth’s surface. ! "# = ! $ !  !  + $  [2]

Equation 3

Figure 4: Solar Angles with respect to the Earth’s Surface [2]

Additionally the zenith angle can be used to find the solar altitude as seen in Figure 4, and defined in Equation 4. Equation 4

& =   ! "'  [2]

From the solar altitude, the solar azimuth is seen in Equation 5. The solar azimuth is also shown in Figure 4, and is measured in degrees away from due south where the value is negative in the morning and positive in the afternoon. Equation 5

! ( =

 & $ [2] ! & ! $

However, because the solar azimuth equation utilizes cosine, all values returned will be in the incorrect coordinate system. Therefore to correct this, all azimuth values are subtracted from PI. In doing so the system is rotated 180°, now making solar noon the equivalent to 0°. Equations 1 thru 5 are used to describe the solar angle with respect to earth’s horizontal surface; however solar panels are almost never mounted at 0°. Therefore to 6

account for the panel’s tilt angle, Equation 6 is used. This equation calculates the incident angle, which is the angle between the direct radiation beam from the sun and line normal to the panel, where β is the panel tilt angle. Figure 5 depicts the panel in relation to the sun. Equation 6

! " = ! $ − ) !  !  + $ − ) [2]

Figure 5: Solar Panel Angles[2]

In some cases, to increase the amount of power the solar panel may achieve throughout the day, the solar panel will “track” the sun by varying its azimuth angle, γ, as seen in Figure 5. Equation 6 assumed the panel was facing south (north if located in the southern hemisphere), Equation 7 accounts for the varying panel azimuth angle. The azimuth angle can be calculated, based on the earth’s rotational rate, to rotate 180° thus tracking the sun, sun up to sun down. Equation 7

! " = ! & ! ( − * ) + & ! )[2]

Equations 1 thru 7 are used to describe the relationship between the solar panel and the sun, and are essential in predicting the solar panel’s performance. However, not only are the sun’s physical behaviors and the panel’s orientation required, but also the sun’s irradiative properties. Table 1 identifies three coefficients, A as solar intensity, B as the extinction coefficient and C as the monthly average ratio of diffuse to direct normal irradiance for clear skies, which are critical in understanding solar irradiance. These coefficients are required to calculate the direct normal irradiance, IDN, defined as the amount of energy the sun will emit on the earth’s surface. Values in Table 1 are average values measured 7

for that specific month. These values vary from day to day, however it was assumed these values remain constant throughout the month to simplify the intended analysis. Table 1: Parameters Used to Estimate Solar Radiation Intensity [2]

Day Declination Nominal Date Number Degrees January 21 February 21 March 21 April 21 May 21 June 21 July 21 August 21 September 21 October 21 November 21 December 21

19.85 54.06 80.00 110.47 140.15 172.50 201.84 232.49 265.00 292.34 324.20 357.50

-20.0 -10.0 0.0 +11.6 +20.0 +23.45 +20.6 +12.3 0.0 -10.5 -19.8 -23.5

Parameter A B C Btu/hr ft² W/m² Air Mass ¯1 Dimensionless 390 1,230 0.142 0.058 385 1,215 0.144 0.060 376 1,186 0.156 0.071 360 1,136 0.180 0.097 350 1,104 0.196 0.121 345 1,088 0.205 0.134 344 1,085 0.207 0.136 351 1,107 0.201 0.122 365 1,151 0.177 0.092 378 1,192 0.160 0.073 387 1,221 0.149 0.063 391 1,233 0.142 0.057

Irradiance is dependent on the solar constant, ISC, the radiant energy measured at the average earth-sun distance outside the earth’s atmosphere, of 1353 W/m2 ±1.5%

[4]

.

However, as seen in Table 1, the values for solar irradiance vary based on time of the year. During the winter months, measured between the autumnal and vernal equinoxes, the average solar irradiance is greater than what is measured during the summer months. This is due to the path in which the earth orbits around the sun which is not perfectly circular, but more of an oval shape. In January the earth enters its closest point to the sun at 147 million km, while in July the earth will pass through its furthest point at 152 million km. Therefore the governing characteristic which determines the change in the seasons is the earth’s physical orientation in space. The location of the earth with respect to the sun causes an “apparent” solar irradiance, I0, which can be approximated using Equation 8 Equation 8

. 

+ = +,-  + .  !   [2]

The extinction coefficient is a value which is a measurement of the absorption of light in the atmosphere. Like solar intensity, it varies over the year. However, it

8

increases in the summer and decreases in the winter. Using the solar intensity, extinction coefficient and zenith angle, Equation 9 can be used to calculate the direct normal irradiance. Equation 9

+. = /0

1

2 3 4 ∗ 2 ! "# [2]

Also required to calculate the direct normal irradiance is the pressure ratio which is solved using Equation 10. The pressure ratio will adjust the irradiance level based on the altitude above sea level, x. Higher altitudes will allow for higher irradiance values. Equation 10

2

2

= 0.∗5 [2]

To account for a tilted solar panel, the direct normal irradiance can be adjusted by the incident angle as seen in Equation 11. Equation 11

+67089 = +. ! "[2]

While the direct normal irradiance contributes close to 90% of the energy which would be absorbed by a solar panel, the diffuse irradiance is also factor. Diffuse irradiance is the result of reflected direct normal irradiance, whether off the ground or dispersed from passing clouds. The diffuse irradiance can be calculated through Equation 12. Coefficient C from Table 1 represents, as previously defined, the monthly average ratio of diffuse to direct normal irradiance for clear skies. The environment reflectivity factor covers average reflectivity values of ordinary ground or vegetation (0.2), snow cover (0.8) and gravel roofs (0.15)[4]. It is seen that the reflectivity from the environment will have no effect if the panel were to be horizontal. Equation 12

+67::; ! ) 

+ ?- + &

 ! ) [2]  

Summing both the direct normal and diffuse irradiances, the global irradiance can be found as seen in Equation 13. Equation 13

+@ABCDA = +67089 + +67::;

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