Design Methodologyfor Identifying Optimum ... - IEEE Xplore

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Proceedings of the IEEE SoutheastCon 2015, April 9

-

12, 2015

-

Fort Lauderdale, Florida

Design Methodology for Identifying Optimum Photovoltaic System Configuration at UNC Charlotte Wesley Johnson, Mustafa Alshafai, Murtadha Alahmad, Joel Woods, Nabila A. Bousaba, Abasifreke Ebong William States Lee College of Engineering University of North Carolina Charlotte Charlotte, USA {wjohns56, malshafa, malahme2, jwoods23, nbousaba, aebongl} @uncc.edu Abstract-

Solar

(PV)

photovoltaic

systems

provide

an

environmentally friendly method to harvest energy. Incorporating a

PV

system onto a campus building would help offset energy

costs. UNC Charlotte identified the Colvard building roof as a promising candidate for solar panels. In this paper, the feasibility of placing a

PV

array on the roof is investigated. Six system

configurations were identified as: (i) micro-inverter and grid tie (ii) micro-inverter with grid tie and battery backup (iii) micro inverter and battery (stand-alone), (iv) central/string inverter with grid-tie, (v) central/string inverter with grid tie and battery backup, (vi) central/string inverter and battery (stand-alone). The designs were systematically eliminated based on the benefits of (a) micro-inverters, (b) battery bank, (c) AC coupling and (d) grid interconnections. Micro-inverters were ruled beneficial based on literature review and also the expansion opportunities they afforded. The battery bank was ruled unnecessary based on power reliability

data

obtained

from

Facilities

Management.

AC

coupling was also deemed an inefficient way to couple a battery bank based on both cost and losses in efficiency. A grid tie was chosen over a stand-alone setup due to the power reliability of on

ushering grid-tie systems. Before IEEE 1547, which was introduced in 2003, there were no industry standards for grid interconnected systems [31. With new standards in place, solar PV systems have become simpler to tie in with a grid and have seen more widespread use. Both businesses and homeowners have begun integrating PV systems to offset energy costs utilizing net metering schemes offered by utilities. Some universities have also utilized PV systems. Georgia Tech installed a 340kW system on the roof of its aquatic center in 1996 to heat the Olympic swimming pool. Over a ten year period, no signifIcant drops in effIciency or performance were observed [4] This case study demonstrates that a university is capable of maintaining a PV system. With greater awareness of environmental issues, UNC Charlotte is considering funding a large photovoltaic project for its own campus. Since there are many variables to consider, it is important to assess the feasibility and fInancial viability of integrating a solar PV system.

campus power and the unjustified cost a battery bank posed. Thus, the study found that a grid-tie system with micro-inverters is the most suitable configuration for the Colvard roof space at UNC Charlotte. The study also found that laying the

PV

panels

flat maximized yearly power output for the roof. A novel way to mount

the

panels

by

point-loading

was

devised,

allowing

additional weight to be added without compromising the roof's structural integrity. Keywords-Photovoltaic systems, Solar Energy, Cost benefit analysis,

Sustainable

development,

Renewable

energy

sources,

Green buildings,

I. LITERATURE REVIEW

A. Initiation ofStudy The solar photovoltaic (PV) cell is a technology that converts sunlight into electrical energy. The science of PV began with the observations of Henri Bequerel in 1839[11. However, the fIrst silicon solar cell was fabricated in 1954 in Bell Labs [II. The technology was fIrst used in space and later evolved to terrestrial applications [II. The technology began to take root in the 1970s for producing power. It was initially used in off-grid applications [21 and grid interconnection was limited to goverrnnent and utility demonstration programs [21. But as time progressed, almost all solar PV projects involved a grid tie-in. Between 1995 and 2010, the solar PV power that was grid interconnected went from 13% to 97% [21. IEEE 1547 has been a major part in

978-1-4673-7300-5/15/$31.00 ©2015 IEEE

II. ELECTRICAL TOPOLOGY In order to achieve the maximum possible power from the roof, six system confIgurations were evaluated. Each of these design confIgurations were developed from the benefIts of the individual factors below: Micro-inverter Battery bank AC coupling Grid interconnection Using these four factors, six design confIgurations were completed as seen in Figure 1 below. As each individual confIguration was researched and the best choice for each component was reached, the [mal design candidate was chosen using Figure 1. To determine the optimum design, the possible confIgurations were listed along with research areas to be examined as seen in Figure 1. As different options were compared, a [mal design candidate was identifIed. The methodology for arriving to the ideal confIguration seen in Figure 2 is described in sections II.B and II.C.

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12, 2015

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AC Coupling Benefits

Figure 1: Design factors mapped to possible design options

YES

Critical Loads

Frequent Outages

NO

NO

No

Battery Bank

YES Figure 2: Grid-tie with micro-inverters was determined to be ideal configuration for UNC Charlotte project

A. Battery Option Evaluation To determine the necessity of the battery bank, data was obtained from Facilities Management. No unplanned campus wide outages had happened in the past 18 months. In addition, no nearby critical loads could be found. These two observations made the battery bank unjustifiable when taking into consideration the additional cost of adding a battery bank. If there had been frequent outages, one could then determine whether there are critical loads. If there are both critical loads and frequent outages, a battery bank would be justifiable.

Battery Bank Figure 3: DeciSIon dIagram for battery bank demonstratmg no battery bank should be used

B. Inverter Type Selection Typical solar PV systems use either a central/string inverters or micro-inverters. A central/string inverter involves connecting panels in series. The inverter may then perform maximum power point tracking (MPPT) on that string. In comparison, a micro-inverter mounts on the back of each individual panel and performs MPPT on that panel.

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One of the biggest differences between central and micro inverters is the lifespan that is covered under manufacturer warranties. Many micro-inverter manufacturers provide 25 year warranties with their products, which can be compared with the 10 year warranties that are often provided with central inverters [81. This difference provides a lower upfront cost per watt for a central inverter but a higher overall cost per watt when replacement costs are included. From this study, the micro inverter has a distinct advantage in all aspects including greater energy collection, higher system effIciency, and lower system lifetime cost. However, a micro-inverter setup would likely have a high initial cost.

Figure 4: Comparison of string inverter (left) and micro-inverter (right) showing redundancy of micro-inverter setup

Micro-inverters were found to provide approximately 5% additional power output as cited by two studies. In one study, a setup was made with central/string inverters, and another with micro-inverters. The paper cited a 5.6% power increase when using micro-inverters[51. Enphase conducted a study on its own products to confIrm that its micro-inverters yield a 5% power output gain when compared to a central inverter [61.

The Enphase M250 was selected as a micro-inverter. The cost of power (watt) over 30 years was estimated to be $l.27 USD using current pricing (as of 2015 [91). A micro-inverter setup is very simple and modular. The micro-inverters can be made into a three-phase setup, as seen in Figure 5 on several different strings of buses. A transformer can then be used to move the voltage up to the necessary voltage for grid interconnection.

3 - Phase 208VAC

The roof was examined, and several expansion opportunities were found. The north roof and the top of the building's atrium have expansion opportunities as seen in Figure 5. The ease of adding panels without resizing inverters makes micro-inverters highly advantageous over larger central/string inverters.

-"

Figure 6: Three-phase setup of Micro-inverters as specified by manufacturer (21)

. - . .---- -

Figure 5: Colvard roof (south roof on bottom)

A common concern with micro-inverters is the large electrolytic capacitors that are used to make them. A study by Enphase showed that the Enphase micro-inverter capacitors were adequate for longevity, and were estimated to last up to 30 years [71.

A micro-inverter setup was chosen based on the increase in power, lower lifetime cost per watt, higher effIciency and the ease of future expansions using the decision tree in Figure 7. The ease of expansion also helps to justify the use of micro inverters if the assumption of a lower cost per watt is incorrect. It was noted, however, that a micro-inverter setup would not be ideal for a battery bank setup (if chosen).

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YES

I EffiCiency I I Benefits I . .

Cost Benefits

NO

absorption of lower frequencies of light [Ill. Dirt accumulation of small amounts such as 4.25mg cm2 has been reported to reduce power output of amorphous and CdTe panels by 33%, and crystalline panels by 28.6% [II].

NO

A. Angled Design

YES

I I

Expansion Opportunities

Equation 1 below was used to determine the space required for the panels and Equation 2 was used to calculate the number of panels that could be mounted on the roof. Tables 4 and 5 show the results of this calculation. A standard panel size of 1m x 1.24m was chosen for the calculations.

INO I

Central/String Inverter

Micro-Inverter . .

Figure 7: DecIsion tree for Inverter selection shows micro-inverter as Ideal setup

In the case of a battery-based system being requested, it was determined that a micro-inverter setup would not be ideal for used with a battery bank. This is especially an issue with stand alone setups. There would be significant losses from the hybrid inverter to the battery and from the battery back through the hybrid inverter. In addition to this issue, the hybrid inverters that are commercially available are prohibitively expensive and would raise the LCOE.

Aangled

Panels

f-

MicroInverter

f-

1

-00

Battery Bank

LShadow)

•

R is the row height in m

•

WPer_Panel is the width per panel in m

•

Lshadow is the shadow length in m

(1)

A..oof

N angiedA =

(2)

angled

Where: Nangled is the number of angled panels, Aroor is the area of the roof in m2

•

Aangled is the area per angled panel in m2•

Table l' Area per Angled Panel Input Data

Load

Additional Loss

+

Aangled is the area per angled panel in m2

•

•

HybridInverter

R * (WPer _Panel

Where:

•

Additional Loss

=

INPUT

Panel Height (m)

Panel Width (m)

Latitude (deg.)

Space factor

Roof Length (m)

Roof Width (m)

1

1.24

35.2

2.3

68

33

Figure 8: Losses in AC coupled setup are shown to be excessive

Table 2: Calculated Number of Ang Id e Pane s OUTPUT

III. MOUNTING OPTIONS An angled panel type mounting system can be placed on roofs, and also the ground. Since the angle can be controlled by angle mounting, the system is not limited to the angle of the roof which may not coincide with the proper angle for maximization of light [101. The downside to an angled design for mounting is that the panels will cast a shadow which requires the rows of panels to be offset. A flat design would allow more roof space to be covered in PV since there will be no shadowing. In theory, panels could be placed without any spacing. But panels need to be accessed. A method to be able to access and remove panels without spacing was devised as discussed in Section III.D. It should be noted that the panels should not be completely flat (0°) to prevent dirt accumulation. A slight angle will allow water runoff to take away dirt. Dirt accumulation prevents

Row Height (m)

Width per panel

Length shadowed (m)

(m) 0.8171

0.5764

1.8794

Area per panel

Usable Roof Area

(m2)

(m2)

Number of Angled Panels

2.85

1885

661

B. Flat Design The building is square, allowing the initial estimated area to be the length multiplied by the width as seen in Equation 3. The actual usable area was calculated by subtracting the unusable area from Arooc. The estimated number of panels was incorporated as seen in Equation 4. One should note that additional space was reserved for a few walk spaces with the flat design since there are no spaces between panel rows.

A =L*W -'7oof

(3)

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Where: •

Aroof is the area of the roof in m2

•

L is the length of the roof in m

•

W is the width of the roof in m

N

jlm

=

A,..,of

(4)

A

Where:

jlm

•

Nflat is the number of flat panels

•

Aroof is the area of the roof in m2

•

Allat is the area of a single flat panel in m2

mounting system called Snap'N'Rack [121. The mounting platform is raised off of the ground by 3 feet to allow for maintenance of the underside of the panels where the micro inverter would lay. The design can be seen in Figure 10. This design allows the access and removal of panels from the underside of the mounting structure. This design also allows for the removal of the majority of access walkways that are normally necessary.

Table 3' Calculated Number of Flat Panels Panel Height (m)

Panel Width (m)

Area per panel (m2)

Usable Roof Area

1

1.675

1.675

1698

f-I .......

Number of Flat Panels

- 32.0

--

(m2) 1054 r--C.

24.0

SAM Simulation

Simulations were run in the System Advisor Model (SAM) to find which setup maximized the power output from the roof. The flat design was compared to the angled design as seen in Figure 9 which shows the monthly output of the panels and a significant power output gain from laying more panels flat than fewer at an optimum angle. The optimum angle used in the SAM simulation was equal to the latitude at the university. The SAM simulations revealed that the angled design with 661 panels would have a yearly output of 241,000 kWh while the flat mounting option with 1054 panels produces 368,590 kWh each year. 45000 •

40000

Flat

Figure 10: Point-loading mounting scheme with top view (left) and side profiles (right).

IV. PANEL SELECTION There are three types of panel currently available: monocrystalline, polycrystalline, and thin film. The earliest technology was the monocrystalline and polycrystalline silicon solar cells. In the 1970s, the efficiency of cells were in the range of 8-10%, but they have increased to 15-18% in recent years [131. Monocrystalline and polycrystalline are currently the most mature technologies and have the highest efficiencies. Individual panels can be evaluated from a dollar per watt or an area per watt basis. The individual benefits of each is detailed below.

a) Monocrystaline Silicon

35000

•

30000

Angled

Monocrystalline panels have the highest efficiency, but also the highest cost. Each solar cell is a single crystal.

...c 25000

�

b) Polycrystalline Silicon

20000 15000 10000 5000 o

I

Jan Feb Mar AprMav Jun Jul Aug Sep Oct Nov Dec

Figure 9: Flat and angled design were compared in SAM

D. Structural The team decided to point-load the roof on the columns supporting the roof to minimize the impact to the roofs structural integrity. Referencing the buildings drawings, a 32 foot by 24 foot square mounting frame with four supporting legs was drawn to determine the feasibility of the design. This design is intended to be a base for a commercially available

Polycrystalline panels tend to be slightly less efficient than monocrystalline. Polycrystalline are also slightly lower cost than monocrystalline panels. Each cell has multiple crystal structures.

c) Thin-film Thin film panels have the lowest efficiency and lowest cost. Some varieties are flexible, which may be advantagous for certain setups. There are many materials being researched, but among commercially available panels CdTe and amorpheous silcon are common.

A. Bifacial Solar Panels Another technology is bifacial panels. Bifiacial panels absorb sunlight on both sides of the panel, which increases the energy harvest per unit area of panel. However, the panels are designed

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to be at an angle with row spacing. Therefore, bifacial solar panels should only be used with angled designs. Because the top of the roof of the Colvard building is a smooth white surface, bifacial solar panels have been marked as a topic for further investigation. According to a list compiled by the California Energy Commission and California Public Utilities Commission, one of the few companies making bifacial panels for commercial sale in the U.S.as of January 2015 is a company called Prism Solar Technology [141. The company cites up to 35% power increase with its bifacial technology [lSI. It should be noted that the bifacial panels, such as the Prism Solar B260, will require a larger inverter than the Enphase M250.

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12, 2015

Table 4' Panel Selection Breakdown $/Watt

in2/Watt

(Normalized)

(Normalized)

30 Price/ 70 Size

SW 315 Sliver Mono Pro XL

0.6241

0.8823

0.8048

LG 300 Black Mono X NeoN LG300NIC

0.8593

0.7731

0.799

SW 275 Silver Mono Plus

0.6719

0.8144

0.7717

Astroenergy 310 Silver Poly

0.5247

0.8869

0.7782

Astroenergy 255 Silver Poly

0.5075

0.9105

0.7896

SW 145 Silver Poly

1

1

1

Sunpreme GX300 300W Double-Glass

0.638

0.8958

0.8185

240W Sanyo HIT240S-BL Panel

0.9622

0.7424

0.8084

Panasonic HIP-195 Double Panel

1.7347

0.8795

l.l36

Prism Solar B260 (no gain)

0.8056

0.8859

0.8618

Prism Solar B260 (20% gain)

0.8056

0.7386

0.7587

Prism Solar B260 (35% gain)

0.8056

0.6565

0.7012

Panel Type


configuration is the SW 275 Silver Mono Plus panel, because it has the best weight for 30% price / 70% size next to the bifacial panels. Also, if the weight changed to 50% price / 50% size or 70% price / 30% size, the chosen panel will still be one of the top performers. The bi-facial panel was found to be highly competitive and should be considered for angled designs V. BENEFITS One of the hurdles when designing a PV system is weighing the costs and the benefits that come with the system. This includes a preliminary cost breakdown for the system as well as the total monetary benefit over its life. Three areas were investigated to determine the benefits: Incentives Levelized Cost of Electricity (LCOE) Payoff period

B. Survey of Commercially Available Panels Several different PV technologies were researched. The research included monofacial and bifacial technologies in addition to polycrystalline and monocrystalline panels. After finding the best commercially available panels from several sample groups, a comparison between the efficiency and the cost for all the panels had been made to choose the best panel that can maximize the roof space. Table 4 below shows how all of the sample panels were compared. The values used in this comparison were obtained from commercial vendors and the data sheets provided by the manufacturers.

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A. Incentives DSIRE is an organization that is dedicated to locating and summarizing renewable energy grants and incentives [161. The database tabulates several incentives offered by the U.S. Federal Government for solar PV projects. Most are in the form of tax incentives. Since universities do not pay federal taxes, they will not benefit from these incentives. The U.S. Federal Government is not the sole source of incentives. Many states and local jurisdictions have programs. In North Carolina, the NC GreenPower initiative incentivizes solar PV systems both with businesses and non-profits such as universities. NC Greenpower funds systems that have a purchase agreement with a utility[16] Because the UNC Charlotte project is to be connected to the campus grid and will not feed to a utility, this particular option will not benefit UNC Charlotte. However, programs like NC Greenpower could potentially benefit other universities.

B. Levelized Cost ofElectricity The Levelized Cost of Electricity (LCOE) is a measure of the cost of electricity commonly used for renewable energy installations. The high upfront cost of a solar PV system can be seen to be balanced out by the long life that is achieved. Equation 5 below can be used to calculate the LCOE for the proposed system. A system net cost estimate was completed with a quote from a local solar installation company [17]. The total system cost including materials, installation, and maintenance reserve is $935,000. This net cost was used to calculate the payoff period in Section V.c. LCOE C.

Using the normalized comparisons above the weights chosen to select the panel type were 70% for size maximization and 30% for the cost of the panel. The chosen panel for the flat

=

Total Life Cost / Total Life Energy Production (5)

PayoffPeriod

The payoff period for a PV system greatly varies as the price of electricity changes year to year. This calculation is used to calculate how much time it would take to pay off the initial costs of the system. It takes into account the rising cost of electricity, inflation, panel degradation and the annual cost of

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maintenance. As seen in Figure 11 below, the payoff period for this system is around 23 years and will create a total lifetime profit of $500,000. This estimate uses a cost of electricity increase of 2.6% per year[18], an inflation rate of 2.38%[19], panel degradation of 0.5%[20] and a 30 year life for the system. The main reason for this long of a payoff period is that the University is a non-profit organization and cannot claim any Federal or State income tax credits that are associated with solar electricity. Included in the quote obtained for the installation of this system was potential tax credits of $775,000 which would bring the payoff period down to 6 years if they could be used[)7].

---

2

Total Cost

-

[4]

Begovi6,Miroslav,Ghosh,R. Seema,and A. Rohatgi. "Decade Performance of a Roof-Mounted Photovoltaic Array". Georgia institute of Technology. 2006.[Online]. Available: http: //hdl.handle.netlI8S3I2S920.[Accessed: Jan. 12,201S]

[5]

Kim,Yong Sin,and W. Roland. "Power conversion in concentrating photovoltaic systems: central,string,and micro-inverters". Progress in Photovoltaics: Research and Applications. 22 (9): 984-992. 2014.

[6]

"BUSINESS." Brand Edgar Online.[Online]. Available: http: //google.brand.edgarani ine.comlEFX_dIl/EDGARpro.dll?FetchFilingHtmlSectionI?Section lD=8114489-296717-383638&SessionlD=Bt7XFv7tKXijQR7. [Accessed: Nov. 6,2014.]

[7]

S. Shaffer. "Evaluation of Electrolytic Capacitor Application in Enphase Microinverters." Enphase. March 31, 2009.[Online]. Available: http: //enphase.com/globallfiles/Electrolytic_Capacitor_Expert_ReporLp df[Accessed: Nov. 6,2014.]

[8]

S. Harb,M. Kedia,H. Zhang,and S. Balog. "Microinverter and string inverter grid-connected photovoltaic system - A comprehensive study".

-Total Benefit

1.5

Cotiference Record of the IEEE Photovoltaic Specialists Cotiference.

'"

o .-4 X

o

[9]

1

http: //www.wholesalesolar.com/products.folder/inverter-foIder/M250enphase-energy.htmUAccessed: December 19,2014]

VI

:::)

0.5 o o

2

4

6

8 10 12 14 16 18 20 22 24 26 28 30 32 Years

Figure 11: Break even graph for system payoff period

VI. CONCLUSION Six electrical topologies and two mounting configurations were systematically eliminated to one design, by evaluating the cost and efficiency of each individual component. The study showed that a flat configuration using micro-inverters without a battery bank would be appropriate for the project. A survey of commercially available components and their cost is also considered. The results indicate that the system would take � 23 years to pay for itself without financial incentives. However, with the use of incentives, the system will take only 6 years to pay off. ACKNOWLEDGMENT

The authors wish to thank the UNC Charlotte Energy Production and Infrastructure Center (EPIC) for their financial support for the study. The authors also wish to thank the UNC Charlotte Facilities Management team for providing valuable technical data and feedback.

[I]

[2]

[3]

288S-2890. 2013. "Enphase Energy M2S0." Whole Sale Solar. [Online]. Available:

REFERENCES A. Goetzberger,and U. Volker Hoffinann. Photovoltaic solar energy generation. Berlin: Springer,200S.[Online]. Available: http: //site.ebrary.comlidll0142944.[Accessed: Jan. 12,20151 Mints,Paula. "The history and future of incentives and the photovoltaic industry and how demand is driven." Progress In Photovoltaics 20,no. 6: 711-716. 2012. Environment Complete, EBSCOhost Basso,Thomas S.,and Richard DeBlasio."IEEE IS47 Series of Standards: Interconnection Issues". iEEE Transactions on Power Electronics. 19 (S). 2004.

[10] ] R. Mayfield. "Rack & Stack - PV Array Mounting Options." Home Power. April I,2008.[Online]. Available: http: //www.homepower.com/articles/solar-electricity/equipment products/rack-stack-pv-array-mounting-options.[Accessed: Oct. 17, 2014] [11] Qasem,Hassan,B. Thomas R.,M. Harald,A. Hassan,and G. Ralph. Dust-induced shading on photovoltaic modules". Progress in Photovoltaics: Research and Applications. 22 (2): 218-226. 2014. [12] "Solar Racking and Mount Systems." SnapNrack. January 1,2014. [Online]. Available: http: //www.snapnrack.com.[Accessed:Nov. 7, 2014] [13] W. Hoffinann. The economic competitiveness of renewable energy: pathways to 100% global coverage.

2014,pp. 233. [14] "Incentive Eligible Photovoltaic Modules in Compliance with SBI Guidelines." Go Solar California.[Online]. Available: http: //www.gosolarcalifornia.ca.gov/equipmentlpv_modules.php. [Accessed: December 19,2014] [IS] "Products." Prism Solar Technologies.[Online]. Available: http: //www.prismsolar.com/products.php.[Accessed: December 19, 2014] [16] "NC GreenPower Production Incentive." Database of State Incentives for Renewables & Efficiency.[Online]. Available: http: //www.dsireusa.org/incentives/incentive.cfin?Incentive_Code=NCO SF&re=O&ee=O. [Accessed: December 19,2014] [17] Verner,Chris. (20IS,Jan. 16) Solar Quotes[Online]. Available email: [email protected] Message: Solar Quotes [18] "North Carolina Electricity Profile." U.S. Energy Information Administration.[Online]. Available: http: //www.eia.gov/electricity/statelNorthCarolinal�[Accessed: December 19,2014] [19] "U.S. Inflation Rate." USlnflation.org.[Online]. Available: http: //usinflation.org/us-inflation-rate/�[Accessed: December 19,2014] [20] C. Jordan. Kurtz,and R. Sarah. "Photovoltaic Degradation Rates - An Analytical Review." National Renewable Energy Laboratory. June 2012.[Online]. Available: http: //www.nrel.gov/docs/fyI20sti/SI664.pd( [Accessed: December 19,2014] [21] "M-250 ]nstallation and Operation Manual." Page 17. Enphase Energy. [Online]. Available: http: //enphase.com/globallfiles/M250-lnstallation-and-Operation Manual-60-Hz.pdf.[Accessed: December 19,2014]