2015–

2016

Solar Array Installation in Chaska, Minnesota

Prepared byBen Colbert, Juer Liu, Mitchell Nommensen, Sohan Phadke, and Zi Yao Ngai

Students in BBE 5733: Renewable Energy TechnologiesCollege of Science and Engineering | University of Minnesota

Instructor: Min Addy

Prepared on Behalf of City of Chaska

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Solar Array Installation in Chaska, MN Ben Colbert, Juer Liu, Mitchell Nommensen, Sohan Phadke, Zi Yao Ngai

In conjunction with: The Resilient Communities Project December 11th 2015

BBE 4733, Professor: Min Addy

Introduction: The City of Chaska has its own municipal electric utility, which has provided safe and

reliable service to customers since 1899. The city desires to build on its green energy portfolio by incorporating solar (photovoltaic) installations in the community. The City is interested in installing a small solar array in the community through the Minnesota Municipal Power Agency Hometown Solar Grant Program.This program is determined to educate the future leaders of Minnesota about the benefits of renewable energy by subsidizing small solar arrays in local community buildings and schools. Because the solar array will be used as an educational asset, the solar array will be constructed on the ground rather than on the roof to make it more viewable. This group has been tasked in conjunction with the Minnesota Resilient Communities Project to assess the viability of the project as well as optimize the design for maximum community value and electricity generation. The first step of this assessment was selecting the location for the array that will provide the maximum community exposure and solar yields. After determining the optimal location, a technical analysis on the expected solar yields was performed to determine the required size of the array and determine optimal geometry of the panels. The basic irradiance data gathered was then used to design the array including comparisons of products from several vendors in both efficiency and price. Finally an economic analysis was performed to determine the overall economic feasibility of the project including fixed costs, variable costs and expected payback period. The recommendations were originally presented to the Chaska electric utility on December 8th, 2015. This report provides further detail into the assessment of the project and calculations done to determine the final recommendations. Determination of Optimal Location: (Juer) Selection of a suitable site was based on a set of criteria mainly dependent on the conditions of the surrounding environment. While the average insolation data offers an insight into solar energy potential on a regional scale, locally relevant conditions such as terrain may significantly influence the solar energy potential in a specific site. In addition, some economic and social criteria should also be taken into consideration during the decision making process. Among the 16 candidates located in the City of Chaska, 3 sites were selected for evaluation of their potential of site­suitability by considering the potential audience, and optimal location was determined from there.

Site I sits between four of the proposed sites­ Chaska Middle School West, Chaska Middle School East, Chaska Community Center and Chaska Elementary School. For its location, the large, diverse audience of students makes this a critical educational site, which could provide far­reaching informational purpose. Site II sits outside Jonathan Elementary School, with a close proximity to the school that makes the solar array fully accessible to students. Additionally, the easy access to a major road allows it to be viewed by many motorists daily. Site III sits near Clover Ridge Elementary School. As it is adjacent to residential area, this site could provide good exposure to the local community. Moreover, the local athletic fields could provide a large number of potential audience as they come to view sporting events.Site I embraces plenty of open field space which would be an optimal landscape for solar arrays, and all the buildings in this areas use well above the projected electric output at all times. Therefore, no energy waste would likely occur nor would energy storage be necessary. A few open fields located in site II and III also allow for enough solar exposure to the array.

As the sun irradiation figure shown that all 3 of the locations are among the most optimal in Minnesota. While there is very little difference between locations, it appears that the site II seems to have slightly lower solar irradiances on average than the other two potential locations.

As shown above that all three locations receive a large amount of sun annually. Specifically, Location III receives significantly lower sun hours per year which might inevitably lower yields of the installation. Looking at optimal utility and community value, Site I becomes the expected optimal site, as it achieves the best solar yield of all three locations and its proximity to 4 of the proposed sites ensures maximum exposure to the community. Additionally, by being displayed to students of several ages, the students will hopefully see the use of renewable energy resources as applicable in their daily lives. Lastly, the athletic events held in nearby sports fields will draw in even more viewers, which shapes the location as the ideal choice. Analysis of Solar Yields: (Ben)

In order to estimate the electricity generation provided by the solar array and determine the optimal geometry of the solar panels, a solar yield analysis was done on the selected location. The photovoltaic cells used in these solar panels generate electricity through the use of silicon based p­n junctions. A simplified diagram of these devices can be seen below.

As shown on the last page, the p­n junctions rely on photons from the sun to excite electrons in the panel and generate electrical currents. Therefore the electricity generation of the panel is based upon the amount of solar irradiance that the panels can absorb. The total solar radiation can be broken down into 3 major types, direct radiation, sky diffuse radiation and ground diffuse radiation. A simplified figure explaining these three types of radiation is shown below.

The University of Minnesota has recently completed a database of solar irradiance data

for the entire state of Minnesota through the Minnesota Solar Suitability Analysis. This database was used to determine the optimal site location in the previous section and was further analyzed to approximate the incident direct normal radiation used in the calculations. A simplified plot of the interpolated solar irradiance at the selected site I is shown below.

As mentioned previously the electricity generation of the panels is dependent on the

intensity of solar radiation as well as the sun's position relative to the panel. A simplified equation for calculating the solar irradiance of the panel is shown below.

The angle θ shown above describes the angle between the sun and the panel and is known as the angle of incidence. This value is dependent on solar altitude (sun's angle above the horizon, ß), the solar azimuth (sun’s angular position in the sky relative to true north, Φ), the surface azimuth (panels’ angular position relative to true north, ψ) and the tilt of the solar panels, ∑. The equation used for calculating the angle of incidence is shown below.

The Department of Energy has a public database that contains hourly solar positioning values for the city of Minneapolis that were used to approximate the sun's position for the selected site. These solar positioning values can also be used to calculate the sky and ground diffuse radiation shown below. The sky and ground constants shown below were determined using monthly values provided to us by the course instructor.

Using the irradiance data from the Minnesota Solar Suitability Analysis, along with the solar positioning data from the Department of Energy, the anticipated solar radiation for multiple panel geometries was determined and is shown below.

By analyzing this data our team discovered some important trends in solar radiation that helped to optimize the design of the solar array. Because South facing solar panels performed optimally, the projections shown above are all for South facing panel arrays. As seen in the table, higher tilt angles performed better in the winter months due to the fact that the sun is lower in the sky, while conversely the lower tilt angle panels were able to take in the most radiation during the summer months. Beyond fixed angle designs, two sun tracking models were analyzed to determine the effect of sun tracking on efficiency. The tilt tracking did little to improve performance for the expected increase in cost, however the biaxial tracking shows great potential , as it greatly improves the solar irradiance that can be captured even beyond that of the indirect normal radiation, particularly during the summer months when the sun is in the sky longer and has a wider azimuth range. As far as fixed angle designs, the 30 degree angle tilt performed the best, as it is optimized for summer conditions when the solar irradiance is at its highest. It is important to note that the values shown in the table were modeled for optimal irradiance. Derate factors due to weather and panel efficiency were included in the economic analysis, along with a cost benefit analysis of the different designs. Technical Design: (Mitchell, Zi Yao)

The technical design phase of this project focused on the implementation of solar power system selection. In this phase, we reviewed main components of the solar array, and provide recommendations with explanations of our choices. Selecting the best modules for the installation at location I in this project can maximize the technical yield of an array and minimize the payback period of the solar power system. Our group investigated the key components of a solar power system: solar panel, solar inverter, and mounting system. These components are crucial in a solar power system for the generation of optimum electricity. Our group began by first considering the solar panel. This is important because the solar panel is the most important component in a solar power system. Solar panels, which are also called photovoltaic modules, absorbs the sun’s rays to provide a DC power output. There are 3 main types of photovoltaic cell types on the market today: monocrystalline silicon cells, polycrystalline silicon cells, and amorphous silicon cells. In this project, only monocrystalline silicon cells and polycrystalline silicon cells were considered due to their generally higher efficiencies over amorphous silicon cells. 2 types of solar panels from 2 different companies that were chosen to be considered in this project are shown in the figures below:

SolarWorld Pro SW 315 XL Mono Canadian Solar CS6P­250P

SolarWorld Pro SW 315 XL13 Mono was the first solar panel examined. This solar panel is manufactured by SolarWorld in Germany. It contains monocrystalline silicon cells which have an efficiency of 16.16%. It has a 25­year linear performance guarantee and an extension of product warranty to 10 years.13 The system design of this solar array needs 16 modules to add up to a 5kW array in total. The total array area for 16 modules will be around 31.5m2. This solar panel has a unit price of $323.82 per panel and the total array cost is calculated to be approximately $5180. The second type of solar panel that was considered was the Canadian Solar CS6P­250P12. This panel contains polycrystalline cells with an efficiency of 16.13%. This panel has a 6 year product warranty and a 25 year module power output warranty.12 The system design of this Canadian Solar panel array needs 20 modules for a 5kW array. The total array area for 20 modules is approximately 31 m2. This solar panel has a unit price of $252 per panel for a total of $5040 for 20 modules. In general, both solar panels have an efficiency at around 16% and a similar installation area of 31m2. However, the price of the Canadian Solar panels is slightly lower than that of the SolarWorld panels.

The solar inverter is another key component that was considered in the solar power system selection. Solar inverters convert the direct current (DC) output of photovoltaic solar panels into utility frequency alternating current (AC). There are 3 types of solar inverters: string inverters, micro inverters, and power optimizers. The advantages and disadvantages of the three solar inverters were considered. Among these 3 types of solar inverters, string inverters have the lowest cost, while micro inverters have the highest cost. Micro inverters and power optimizers can mitigate the impacts of shading, while string inverters can do nothing to combat shading. A large drawback of micro inverters and power optimizers are that they are required for each panel in the array, whereas only one string inverter would be needed for this entire array. Moreover, string inverters are more suitable for ground mounting (compared to being more suitable for rooftop installation for micro inverters and power optimizers). The ABB POWER­One Aurora PVI 5000­TL­OUTD(S)11, a string inverter, was chosen as the solar inverter for this project. ABB POWER­One Aurora PVI 5000­TL­OUTD(S) is shown in the figure below:

This solar inverter has a 5000W AC rated power and a 4800W DC rated input power,

which is compatible for a 5kW array. With a 97% maximum inversion efficiency, only 3% of

power would theoretically be lost during the conversion from DC to AC. This solar inverter has a 5 years warranty at cost of $1,250.

When installing solar panel arrays, there are three common routes one can go when choosing a mounting system; Fixed ground mount, single­axis sun tracking, or bi­axial tracking. In the solar yield analysis for this project, all three options were considered but was decided that on this scale, a single row, fixed ground mount system was most effective. All systems considered are offered by Patriot Solar Group based in Albion, Michigan. All mounts from Patriot Solar Group are constructed with G­90 galvanized steel and come with a 10 year mount warranty and a 2 year electrical warranty.

First considered was the fixed ground mount system4. This array can be post driven as a permanent solution or ballasted. The cost for an array of this scale will cost roughly $1200. The main benefit of a ground mount system is its simplicity. The 3 major components include C­channel posts, roll­formed trusses, and roll framed rails. Despite being a fixed angle system, the trusses are marked for angles between 5 and 40 degrees that can be manually adjusted. A single row array can easily accommodate for the 16 to 20 panels that this project will require. Because of the mount’s simple design, the array can easily be expanded without extensive electronic work. The disadvantage in this type of system comes back to its simplicity. Compared to the other two systems that will later be detailed, a fixed mount system will collect the least amount of solar irradiance because it simply cannot absorb beyond its fixed position. In addition, if the system was to be optimized for the summer and winter months, each mount would have to be manually adjusted. In this system it is not a large task, but with multiple mounts this could become costly.

A single­axis sun tracking mount system6 could be used. This system comes with an increase in performance, but also an even greater increase in cost. From analysis results, this system can increase annual energy amounts by 20­25%. However, an array capable of holding 16 panels will cost roughly $6,000, or a 400% increase over the fixed mount system. In the analysis for the single­axis mount we chose to use tilt tracking, rather than azimuth tracking. In this design, the panels will be south facing and tilt up and down as the sun moves across the horizon.

A bi­axial mounting system8 is by far the most technologically advanced and efficient option of the three. In this system, the array is programed to track both the tilt angle and azimuth of the sun, giving the greatest degree of precision. By tracking both variables, the array is able to follow the exact position of the sun throughout the day. This increase in technology can be seen in its performance gain. When compared to a fixed angle mount, a gain in annual energy generated by the bi­axial system is 40% on average. This system comes at a much larger cost as one would expect, around 625% more costly than the fixed angle system ($8700 vs $1200). This increase in cost can be associated with the complexity of the design. In addition to the original fixed ground mount components; this system requires a dual­axis controller, PLC controller, dual axis limit switches, and gear driven motors. The benefit of this type of system is the density of panels in an array. For a biaxial mount, between 16 & 24 panels can fit on one mount. If the

system were to be expanded, a larger amount of panels could reside on a set area of land than either of the other two systems could.

The fixed angle ground mount was chosen in this design largely because of the cost associated with each system. The added cost for both the single­axis and biaxial tracking systems were not able to be justified by the increase in performance that each provided especially at the current scale of this project. Economic Analysis: (Sohan)

To determine the optimal design elements and select the best combination, an economic analysis was conducted to compare the payback periods of each combination. The payback period was defined as the amount of time required for the solar array to overcome its cost of construction and maintenance and start providing a net profit. Power generation for each racking system and solar cell, in addition to all costs and projected maintenance costs were factored into the final calculation. The payback period compared the money required to construct the array to the money saved if electricity from the solar cells had instead been purchased from the city.

The formula for the payback period was found by equating the sum of fixed and variable costs for the array to the cost of purchasing any electricity generated from the city of Chaska directly. The following equation was the initial equation used:

Total Fixed Costs + (Total Variable Costs per year * Time) = Price of electricity per year * Time When the two are equal, the time will be equal to the payback period. Solving for the

payback period gives: Payback Period = (Total fixed costs) / (Price of electricity per year ­ Total Variable Costs)

The city of Chaska charges two separate rates for electricity for different months of the year ($0.06181 per kWh between June and September, and $0.05666 per kWh between October through May)10. Using the efficiencies for each of the solar panels, (16.16% for the SolarWorld panels and 16.13% for the Canadian Solar panels)12,13, the total irradiance data calculated above for multiple tilt angles/tracking methods, and the Chaska city electricity rates, both the total energy and total cost of electricity was calculated.

The formula used to calculate energy was: (Sum of monthly irradiances for that period * panel efficiency * 0.8 derate factor*array area) = Energy generated for that period

A derate factor of 0.8 was used to account for the 97% max efficiency of the inverter11 as well as account for any deviations from expected irradiances including weather and shading.

The formula used to calculate price was: (Energy generated for that period * price per kWh of electricity for that period) = Price of purchasing electricity from the city for that period

Repeating the calculations for the other time period and summing them gave the total energy generated and total price of purchasing electricity from the city.

The above calculations were repeated using the different monthly irradiance values that had been calculated for fixed 30 degree tilt, single axis tracking, and bi­axial tracking. The tables are included below.

Panel Tilt

Total Irradiance for Jun­Sept (kWh/m^2)

Total Panel Area (m^2)

Panel Efficiency

kWh gen Jun­Sept

Price for Jun­Sept ($/kWh)

Total Price Jun­Sept ($)

SolarWorld 30 Deg 331.18 31.5 0.1616 1348.67 0.06581 88.76

Canadian Solar 30 Deg 331.18 31 0.1613 1324.80 0.06581 87.19

SolarWorld Tilt Track 334.57 31.5 0.1616 1362.48 0.06581 89.66

Canadian Solar Tilt Track 334.57 31 0.1613 1338.36 0.06581 88.08

SolarWorld Biaxial 664.21 31.5 0.1616 2704.88 0.06581 178.01

Canadian Solar Biaxial 664.21 31 0.1613 2657.00 0.06581 174.86

Panel Tilt

Total Irradiance for Oct­May (kWh/m^2)

Total Panel Area (m^2)

Panel Efficiency

kWh gen Oct­May

Price for Oct­May ($/kWh)

Total Price Oct­May ($)

SolarWorld 30 Deg 325.08 31.5 0.1616 1323.83 0.05666 75.01

Canadian Solar 30 Deg 325.08 31 0.1613 1300.40 0.05666 73.68

SolarWorld Tilt Track 350.03 31.5 0.1616 1425.43 0.05666 80.77

Canadian Solar Tilt Track 350.03 31 0.1613 1400.20 0.05666 79.34

SolarWorld Biaxial 588.61 31.5 0.1616 2397.01 0.05666 135.81

Canadian Solar Biaxial 588.61 31 0.1613 2354.58 0.05666 133.41

Panel Tilt Total kWh Generated

Total Price ($)

SolarWorld 30 Deg 2672.50 163.76

Canadian Solar 30 Deg 2625.20 160.87

SolarWorld Tilt

Track 2787.91 170.43

Canadian Solar Tilt

Track 2738.56 167.41

SolarWorld Biaxial 5101.88 313.82

Canadian Solar Biaxial 5011.58 308.27

Biaxial tracking clearly provides the best energy generation, while single axis tracking provides a marginal energy production increase.

The next step was to combine the array costs, mounting system costs, and electricity costs together to calculate the payback period.

The total array fixed costs were found by summing the total cost of the panels, a $1250 inverter cost, and a $750 installation labor cost.

The total mounting system fixed costs were taken from estimates provided by Patriot Solar Group (which included labor and equipment) and adding a $300 permit fee.

Total variable costs for the array were minimal (the panels rarely require maintenance besides washing). To estimate the array variable costs, it was assumed that one panel would need to be replaced every five years, likely as a result from weather such as hail, stray baseballs hitting it, etc. The the array variable costs were the price of 1 panel divided by five to get a price per year value.

The mounting system variable costs (also minimal) were set to a fixed $15 per year, likely the cost of redoing some small wiring as a result of cold Minnesota winters.

The results of the price calculations (array, mounting system, and electricity) are summarized in the table on the next page:

Array/Mounting Sys

Array Installation Cost ($)

Array Maintanence Cost ($/yr)

Ground Mount Installation Cost ($)

Ground Mount Maintanence Cost ($/yr)

Total Fixed Cost ($)

Total Variable Cost ($/yr)

Total Price Electricity ($/yr)

SolarWorld Pro SW 315 XL Mono/Patriot Solar Ground Mount 7181.12 64.77 1168 15 8349.12 79.77 163.76

SolarWorld Pro SW 315 XL Mono/Patriot Solar Single­Axis Tracker 7181.12 64.77 6084 15 13265.12 79.77 170.43

SolarWorld Pro Sw 315 XL Mono/Patriot Solar Dual­Axis Tracker 7181.12 64.77 8718 15 15899.12 79.77 313.82

Canadian Solar CS6P­250P/Patriot Solar Ground Mount 7040 50.40 1168 15 8208 65.40 160.87

Canadian Solar CS6P­250P/Patriot Solar Single­Axis Tracker 7040 50.40 6084 15 13124 65.40 167.41

Canadian Solar CS6P­250P/Patriot Solar Dual­Axis Tracker 7040 50.40 8718 15 15758 65.40 308.27

Lastly, the payback period must be calculated. The formula derived earlier was used, with one modification. A $7500 reduction in total fixed costs was included as part of a grant award consistent with the Minnesota Power ­ SolarSense Solar Rebate Program9. The reason this was included was firstly because the purpose of the Hometown Solar Grant Program is to provide grant money to build the renewable resource, and secondly because without it, all of the payback periods far exceeded the expected service life of the installation.

The modified payback period equation is now: Payback Period = (Total fixed costs­$7500) / (Price of electricity per year ­ Total Variable Costs)

The relevant quantities and calculated values are summarized below. The panel/mounting system combinations are listed by increasing payback period.

Summary Table Total Fixed Cost ($)

Total Variable Cost ($/yr)

Total Price Electricity ($/yr)

Payback Period (Years)

Canadian Solar CS6P­250P/Patriot Solar Ground Mount

8208.00 65.40 160.87 7.4

SolarWorld Pro SW 315 XL Mono/Patriot Solar Ground Mount

8349.12 79.77 163.76 10.1

SolarWorld Pro SW 315 XL Mono/Patriot Solar Dual­Axis Tracker

15758.00 65.40 308.27 34.0

SolarWorld Pro Sw 315 XL Mono/Patriot Solar Dual­Axis Tracker

15899.12 79.77 313.82 35.9

Canadian Solar CS6P­250P/Patriot Solar Single­Axis Tracker

13124.00 65.40 167.41 55.1

SolarWorld Pro SW 315 XL Mono/Patriot Solar Single­Axis Tracker

13265.12 79.77 170.43 63.6

The fixed angle ground mount (the cheapest of the three) was the clear winner, as it’s

payback times were approximately half the expected service life. Unfortunately, even with the large increases in irradiance capture from the bi­axial tracking mount, the high installation cost could not be overcome, and left it with a payback period of greater than 30 years. The fixed tilt mounting system was the worst of the three options. This was consistent with the pricing and irradiance data. A tilt­tracking solar panel captures marginally more solar irradiance while requiring a much higher installation costs.

Within the same mounting system, in all three cases the Canadian Solar array was the winner, due to its lower installation costs and lower maintenance costs.

Final Recommendation:

Our recommendation is an array of Canadian Solar CS6P­250P Panels with a Patriot Solar Group Post­Driven Ground Mount at a fixed tilt angle of 30 degrees, located at Site I. This set­up will provide the optimal power per price, has a reasonable payback time given enough grant funding, and is made of materials that can last for an expected 20 years. Locating the array at Site I provides high instructional outreach capacity to the target audience of elementary and middle schoolers.

This set­up will provide up to 2625 kWh/year of clean energy, and can be paid off in 7.4 years with the assistance of the grant. Such an installation at this location will allow the array to serve as a strong educational asset and a positive community centerpiece, all while providing best power generation based on price. Sources: 1. Minnesota Solar Suitability Analysis Project Solar Maps 2. Dept of Energy National Solar Radiation Database 3. https://www.energysage.com/solar/101/string­inverters­microinverters­power­optimizers 4. http://patriotsolargroup.com/products/post­driven­ground­mount­300/ 5. http://www.firstsolar.com/Home/Technologies­and­Capabilities/Mounting­Systems/The­

First­Solar­Tracker 6. http://patriotsolargroup.com/products/single­axis­tracker/ 7. https://www.energysage.com/solar/101/string­inverters­microinverters­power­optimizers 8. http://patriotsolargroup.com/products/dual­axis­tracking­mount­2kw/ 9. http://programs.dsireusa.org/system/program/detail/1092 10. https://mn­chaska.civicplus.com/292/Rate­Schedule 11. http://new.abb.com/power­converters­inverters/solar/string/single­phase/pvi­5000kw­600

0kw 12. http://www.civicsolar.com/product/canadian­solar­cs6p­250p­250w­poly­blkwht­solar­pa

nel 13. http://www.civicsolar.com/product/solarworld­pro­sw­315­xl­mono­315w­mono­slvwht­

solar­panel