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**EE580 – Solar Cells Todd J. Kaiser**

Lecture 09 Photovoltaic Systems Montana State University: Solar Cells Lecture 9: PV Systems

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**Several types of operating modes**

Centralized power plant Large PV system located in an optimum location, feeding into the grid Distributed Grid tied Small residential type systems Stand Alone systems No grid connection needed or wanted Montana State University: Solar Cells Lecture 9: PV Systems

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**Residential Side Mounted**

You loose as much as 50% of the power if one cell is shadowed Could have future issues when the tree matures and shadows PV system Montana State University: Solar Cells Lecture 9: PV Systems

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**Residential Stand Alone**

Montana State University: Solar Cells Lecture 9: PV Systems

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**Montana State University: Solar Cells Lecture 9: PV Systems**

Roof Mounted System National Center for Appropriate Technology Headquarters (Butte, MT) 60 Shell SP75 modules each rated at 75 Watts Peak electrical output of system is 4.5 kilowatts 48 volt system connected to utility grid with inverter Provides 15% of building electrical consumption This is the National Center for Appropriate Technology Headquarters. As you can see it is solar designed and has solar panels on the roof. Solar power fills some of the power needs at the headquarters in Butte, Montana. The photovoltaic system was installed on NCAT's roof by Solar Plexus of Missoula; it consists of 60 Shell SP75 modules. Each module is rated at 75 watts and is flush-mounted on the south-facing, third-floor roof; total module area is about 480 ft2. Peak electrical output of the system is 4.5 kilowatts. Thirty modules make up each of the east and west arrays. Each array is electrically connected to produce direct current (DC) at 48 volts. The output from each array can be disconnected for comparison purposes. The system's DC electrical output is connected to a Trace SW4048UPV utility intertie inverter. The inverter, connected to the utility grid, senses the utility's voltage and frequency and produces alternating electricity at the utility's voltage and frequency. The inverter is designed to operate only when utility electricity is available. Whenever the utility goes down, the inverter shuts down and will only turn back on when the utility line comes back on. This is for safety and now all modern inverters have this feature. The system provides about 15% of the NCAT building's electrical consumption. Sources: Montana State University: Solar Cells Lecture 9: PV Systems

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**Montana State University: Solar Cells Lecture 9: PV Systems**

Hybrid System Montana State University: Solar Cells Lecture 9: PV Systems

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**Montana State University: Solar Cells Lecture 9: PV Systems**

Mobile Systems Montana State University: Solar Cells Lecture 9: PV Systems

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**Montana State University: Solar Cells Lecture 9: PV Systems**

Simple Stationary Montana State University: Solar Cells Lecture 9: PV Systems

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**Montana State University: Solar Cells Lecture 9: PV Systems**

Emergency Montana State University: Solar Cells Lecture 9: PV Systems

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**Temperature Dependence ?**

Solar Cells loose efficiency with the increase in temperature Colder is better Montana State University: Solar Cells Lecture 9: PV Systems

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**Montana State University: Solar Cells Lecture 9: PV Systems**

Solar Heating Solar heating (70-90%) is more efficient than photovoltaic (15%-20%) but electricity generally is more useful than heat. Montana State University: Solar Cells Lecture 9: PV Systems

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**Montana State University: Solar Cells Lecture 9: PV Systems**

Solar Cell Basics Photovoltaic Systems Cell Panel Array Balance of System (BOS) Mounting Structures Storage Devices Power Conditioners Load DC AC PV Panel ~/= Battery Charge Regulator Inverter DC AC DC Load AC Load Montana State University: Solar Cells Lecture 9: PV Systems

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**Modularity: Solar Cell to Array**

Module or Panel Array Cell (c-Si 10×10 cm2 η=15% P=1.5Wp V=0.5V I=3A) Solar panel (36 c-Si cells P=54Wp I=3A V=18V ) Solar array Montana State University: Solar Cells Lecture 9: PV Systems

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**Specifications of PV Modules**

Type c:Si, a-Si:H, CdTe Rated Power Max: Pmax (Wp) Rated Current: IMPP (A) Rated Voltage: VMPP (V) Short Circuit Current: ISC (A) Open Circuit Voltage: VOC (V) Configuration (V) Cells per Module (#) Dimensions (cm x cm) Warranty (years) Montana State University: Solar Cells Lecture 9: PV Systems

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**Storage Devices (Batteries)**

Advantages Back up for night and cloudy days Disadvantages Decreases the efficiency of PV system Only 80% of energy stored retainable Adds to the expense of system Finite Lifetime ~ years Added floor space, maintenance, safety concerns Montana State University: Solar Cells Lecture 9: PV Systems

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**Power Conditioners (Inverters)**

Limit Current and Voltage to Maximize Power Convert DC Power to AC Power Match AC Power to Utilities Network Protect Utility Workers during Repairs Montana State University: Solar Cells Lecture 9: PV Systems

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**Montana State University: Solar Cells Lecture 9: PV Systems**

Simple DC Direct Powering of Load No Energy Storage DC Montana State University: Solar Cells Lecture 9: PV Systems

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**Montana State University: Solar Cells Lecture 9: PV Systems**

Small DC Home and Recreational Use Charge Regulator DC DC Load Single Panel Single Battery Montana State University: Solar Cells Lecture 9: PV Systems

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**Montana State University: Solar Cells Lecture 9: PV Systems**

Large DC Home and Recreational Use Industrial Use Charge Regulator DC DC Load Multiple Panels Multiple Batteries Montana State University: Solar Cells Lecture 9: PV Systems

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**Montana State University: Solar Cells Lecture 9: PV Systems**

Large AC/DC Both AC and DC loads DC Charge Regulator DC Load ~/= AC AC Load Inverter Multiple Panels Multiple Batteries Montana State University: Solar Cells Lecture 9: PV Systems

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**Utility Grid Connected**

No On-Site Energy Storage Inverter ~/= AC AC Load Multiple Panels Electric Grid Montana State University: Solar Cells Lecture 9: PV Systems

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**Montana State University: Solar Cells Lecture 9: PV Systems**

Hybrid System Supplement Generator DC Charge Regulator DC Load ~/= AC AC Load Inverter Multiple Panels AC Generator (Wind turbine) Multiple Batteries Montana State University: Solar Cells Lecture 9: PV Systems

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**Montana State University: Solar Cells Lecture 9: PV Systems**

PV System Design Rules 1. Determine the total load current and operational time 2. Add system losses 3. Determine the solar irradiation in daily equivalent sun hours (EHS) 4. Determine total solar array current requirements 5. Determine optimum module arrangement for solar array 6. Determine battery size for recommended reserve time Montana State University: Solar Cells Lecture 9: PV Systems

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Determining Your Load The appliances and devices (TV's, computers, lights, water pumps etc.) that consume electrical power are called loads. Important : examine your power consumption and reduce your power needs as much as possible. Make a list of the appliances and/or loads you are going to run from your solar electric system. Find out how much power each item consumes while operating. Most appliances have a label on the back which lists the Wattage. Specification sheets, local appliance dealers, and the product manufacturers are other sources of information. Your first step in designing your PV system is determining your load. Any device that consumes electrical power are called loads. You want to examine your own power consumption. Its important to note, that you want to reduce your power needs as much as possible. That is because it will reduce the size of your PV system and reduce your initial costs. You will want to make a list of things you will run on your system and try to reduce it as much as possible. For example, change your lighting from incandescent to CFL. So after you’ve found every means to reduce energy use, figure out how much power each appliance or load uses and how much time you are using it for. Most appliances have the power it uses on the back label. Another means of finding how much power a device uses, you can look off specification sheets, local appliance dealers, and the product manufacturers are other sources of information. Sources:

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**Power Consumption (DC)**

DC [W] Television 60 Refrigerator 60 Fan 15-30 Radio/tape 35 Lighting Bathroom 25-50 Bedroom 25-50 Dining room 70 Kitchen 75 Living room 75 Montana State University: Solar Cells Lecture 9: PV Systems

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**Power Consumption (AC)**

AC [W] Television 175 Radio 15-80 Lighting Bathroom 75 Bedroom 75 Dining room 100 Kitchen 100 Living room 75 Tools Saw circular Saw table Drill 240 AC [W] Appliances Refrigerator 350 Freezer Microwave oven Toaster Washing machine Coffee maker Air conditioner Montana State University: Solar Cells Lecture 9: PV Systems

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**Determining your Loads II**

Calculate your AC loads (and DC if necessary) List all AC loads, wattage and hours of use per week (Hrs/Wk). Multiply Watts by Hrs/Wk to get Watt-hours per week (WH/Wk). Add all the watt hours per week to determine AC Watt Hours Per Week. Divide by 1000 to get kW-hrs/week Then you need to calculate your AC and DC loads. Sometimes you have more complicated systems that are half AC and half DC. But here we are going to work with just the AC systems. So first you want to list your AC loads, like your hair dryer, refrigerator, computer. Then you would want to list the wattage and hours used per week. For example, say I use my 250 Watt computer for an hour a day every week day, that adds up to five hours a week. Then I would multiply 250 Watts by 5 hrs a week and I would get 1250 Watt-hours per week. Next I would divide by 1000 to get the kW-hrs/week. As you can see kW-hrs is how the electric companies measure your energy use and that is what you pay for on your bill. The other thing to note here is that, for a refrigerator, it is not running all the time so you should not calculate it running 24 hrs/ 7 days a week. For devices like this the manufacturers will have a good estimate of how much energy it uses per year. Sources:

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**Determining the Batteries**

Decide how much storage you would like your battery bank to provide (you may need 0 if grid tied) expressed as "days of autonomy" because it is based on the number of days you expect your system to provide power without receiving an input charge from the solar panels or the grid. Also consider usage pattern and critical nature of your application. If you are installing a system for a weekend home, you might want to consider a larger battery bank because your system will have all week to charge and store energy. Alternatively, if you are adding a solar panel array as a supplement to a generator based system, your battery bank can be slightly undersized since the generator can be operated in needed for recharging. Now you need to determine the batteries needed for your system. Now if you are planning on doing a grid-type system, you can skip this step. But if you need battery back for places such as hospitals, or you are not tied to the grid, you need to first determine how long you want the electricity to be provided if there is no sunlight. This is usually expressed as “days of autonomy" because it is based on the number of days you expect your system to provide power without receiving an input charge from the solar panels or the grid. You also need to consider usage pattern and critical nature of your application. If its not very important to you to have electricity every single day, then its okay to use a small number for the days. If you are installing a system for a weekend home, you might want to consider a larger battery bank and a smaller PV array because your system will have all week to charge and store energy. Alternatively, if you are adding a solar panel array as a supplement to a generator based system, your battery bank can be slightly undersized since the generator can be operated in needed for recharging. Sources:

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Batteries II Once you have determined your storage capacity, you are ready to consider the following key parameters: Amp hours, temperature multiplier, battery size and number To get Amp hours you need: daily Amp hours number of days of storage capacity ( typically 5 days no input ) 1 x 2 = A-hrs needed Note: For grid tied – inverter losses Once you have determined your storage capacity, you are ready to consider the following key parameters. So after you know how many days you want to go just off the battery, you want to determine the number of daily amp hours. You do this by taking the number of days of storage capacity and multiply it by the daily amp hours and then you get the total amp-hours needed for your battery. Note if you are installing a grid-tied system you will need to take into account the efficiency of the inverter to convert battery loads into AC loads. Sources: Photovoltaics Design and Installation Manual --SEI

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**Temperature Multiplier**

Temp oF 80 F 70 F 60 F 50 F 40 F 30 F 20 F Temp oC 26.7 C 21.2 C 15.6 C 10.0 C 4.4 C -1.1 C -6.7 C Multiplier Batteries work better at lower temperatures than at higher temperatures so you want to select the closest multiplier for the average ambient winter temperature your batteries will experience. Sources: Select the closest multiplier for the average ambient winter temperature your batteries will experience.

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**Determining Battery Size**

Determine the discharge limit for the batteries ( between ) Deep-cycle lead acid batteries should never be completely discharged, an acceptable discharge average is 50% or a discharge limit of 0.5 Divide A-hrs/week by discharge limit and multiply by “temperature multiplier” Then determine A-hrs of battery and # of batteries needed - Round off to the next highest number. This is the number of batteries wired in parallel needed. Next you want to determine the discharge limit for the batteries. This is usually between Normally for acid batteries you never want to discharge them completely. So an acceptable discharge is somewhere around 50 percent. So you divide the number of amp-hrs/week by discharge limit and multiply by “temperature multiplier” that we received from the last slide. If you end up with a fraction – round up to the next whole number of batteries to deliver the necessary current. Sources:

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**Total Number of Batteries Wired in Series**

Divide system voltage ( typically 12, 24 or 48 ) by battery voltage. This is the number of batteries wired in series needed. Multiply the number of batteries in parallel by the number in series – This is the total number of batteries needed. Now to get the total number of batteries, you will be needing to think about the voltage. Remember, Power = Voltage x Current. You take your system voltage and divide by the battery voltage. For example you have a 48 volt system and you divide by 12 voltage battery, you get that you need 4 batteries. So now you need to wire 4 batteries in series and then you multiply the number of batteries in parallel by the number in series and this is the total number of batteries needed. Sources:

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**Determining the Number of PV Modules**

First find the Solar Irradiance in your area Irradiance is the amount of solar power striking a given area and is a measure of the intensity of the sunshine. PV engineers use units of Watts (or kiloWatts) per square meter (W/m2) for irradiance. For detailed Solar Radiation data available for your area in the US: After you have figured out your batteries or you have skipped the other steps because you are already attached to the grid, you can then determine the number of PV modules you will need. First you want to find the Solar Irradiance in your area. Irradiance is the amount of solar power striking a given area and is a measure of the intensity of the sunshine. PV engineers use units of Watts, or kiloWatts, per square meter (W/m2) for irradiance. And if you want your irradiance for any location in the US, go to this website:

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**How Much Solar Irradiance Do You Get?**

You can get the basic idea of the average irradiance over the United States in W-hrs per square meter per day from this map, which is the average daily solar radiation from It should be noted that this map is meant to give you a feel for solar irradiance. The best data and easiest to use to design a system is available in table/spreadsheet form. For non-tracking arrays, you want the global insolation on a surface tilted at the latitude facing south (for northern hemisphere). Sources:

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**Calculating Energy Output of a PV Array**

Determine total A-hrs/day and increase by 20% for battery losses then divide by “1 sun hours” to get total Amps needed for array Then divide your Amps by the Peak Amps produced by your solar module You can determine peak amperage if you divide the module's wattage by the peak power point voltage Determine the number of modules in each series string needed to supply necessary DC battery Voltage Then multiply the number (for A and for V) together to get the amount of power you need P=IV [W]=[A]x[V] Next you want to calculate energy output of a PV Array. You have already determined the total number of Amp-hrs per day and you will want to increase this by 20% for battery losses then divide by “1 sun hours”, from the map on an earlier slide, to get total Amps needed for array. Then divide your Amps by the Peak Amps produced by the solar module you have chosen to use. You will need to pick out a certain type of solar module and determine the peak amperage if you divide the module's wattage by the peak power point voltage. The power (P) equals the current (I) times the voltage (V). Dividing P by V gives you the current in Amps. Next, determine the number of modules in each series string needed to supply necessary DC battery Voltage. For example, if you need 12V and your peak power voltage is 4V you need 3 panels in series. Finally you can determine the total power for your PV system by multiplying the number of amps and the number of volts together. P=IV [W]=[A]x[V]. Sources:

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Charge Controller Charge controllers are included in most PV systems to protect the batteries from overcharge and/or excessive discharge. The minimum function of the controller is to disconnect the array when the battery is fully charged and keep the battery fully charged without damage. The charging routine is not the same for all batteries: a charge controller designed for lead-acid batteries should not be used to control NiCd batteries. Size by determining total Amp max for your array If your system has a battery, you’ll need a charge controller. Charge controllers are included in most PV systems to protect the batteries from overcharge and/or excessive discharge. The minimum function of the controller is to disconnect the array when the battery is fully charged and keep the battery fully charged without damage. The charging routine is not the same for all batteries: a charge controller designed for lead-acid batteries should not be used to control NiCd batteries. To determine the size of charge controller use the total Amp maximum for your array. The basic criteria for selecting a controller includes the operating voltage and the PV array current. Controllers are critical components in stand-alone PV systems because a controller failure can damage the batteries or load. The controller must be sized to handle the maximum current produced by the PV array. There are two types of controllers: series and shunt. Series controllers stop the flow of current by opening the circuit between the battery and the PV array. Shunt controllers divert the PV array current from the battery. Both types use solid state battery voltage measurement devices and shunt controllers are 100% solid state. Sources:

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Wiring Selecting the correct size and type of wire will enhance the performance and reliability of your PV system. The size of the wire must be large enough to carry the maximum current expected without undue voltage losses. All wire has a certain amount of resistance to the flow of current. This resistance causes a drop in the voltage from the source to the load. Voltage drops cause inefficiencies, especially in low voltage systems ( 12V or less ). See wire size charts here: V=IR or R = V/I Wiring is also important. You want to select the correct size and type of wire to really make your PV system work properly. If your wiring is too small, you are going to lose a lot of your electricity to resistance losses. The size of the wire must be large enough to carry the maximum current expected without undue voltage losses. All wire has a certain amount of resistance to the flow of current. This resistance causes a drop in the voltage from the source to the load. Voltage drops cause inefficiencies, especially in low voltage systems. You can go to this website for sizing charts for wires based on the size of your PV array. The voltage (V) drop is given by the resistance (R), measured in Ohms in the wire times the current (I) flowing through the wire. V=IR is called Ohm’s Law – one of the most useful rules in electronics. Sources:

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Inverters For AC grid-tied systems you do not need a battery or charge controller if you do not need back up power –just the inverter. The Inverter changes the DC current stored in the batteries or directly from your PV into usable AC current. To size increase the Watts expected to be used by your AC loads running simultaneously by 20% For AC grid-tied systems you do not need a battery or charge controller if you do not need back up power, just the inverter. The Inverter changes the DC current stored in the batteries or directly from the PV into usable AC current which is the most common type used by most household appliances and lighting. To size increase the Watts expected to be used by your AC loads running simultaneously by 20%. You most likely will not have all of your appliances running continuously at the same time but this will account for surges in the system. Sources:

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Inverters For AC grid-tied systems you do not need a battery or charge controller if you do not need back up power –just the inverter. The Inverter changes the DC current stored in the batteries or directly from your PV into usable AC current. To size increase the Watts expected to be used by your AC loads running simultaneously by 20% For AC grid-tied systems you do not need a battery or charge controller if you do not need back up power, just the inverter. The Inverter changes the DC current stored in the batteries or directly from the PV into usable AC current which is the most common type used by most household appliances and lighting. To size increase the Watts expected to be used by your AC loads running simultaneously by 20%. You most likely will not have all of your appliances running continuously at the same time but this will account for surges in the system. Sources:

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Books for the DIYer If you want to do everything yourself also consider these resources: Richard J. Komp, and John Perlin, Practical Photovoltaics: Electricity from Solar Cells, Aatec Pub., 3.1 edition, (A layman’s treatment). Roger Messenger and Jerry Ventre, Photovoltaic Systems Engineering, CRC Press, (Comprehensive specialized engineering of PV systems). In addition, if you want to do everything yourself also consider these resources: Practical Photovoltaics: Electricity from Solar Cells for a laymen’s treatment of the subject and Photovoltaic Systems Engineering for a comprehensive specialized engineering account of PV systems.

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**Photovoltaics Design and Installation Manual**

Photovoltaics: Design & Installation Manual by SEI Solar Energy International, 2004 A manual on how to design, install and maintain a photovoltaic (PV) system. This manual offers an overview of photovoltaic electricity, and a detailed description of PV system components, including PV modules, batteries, controllers and inverters. Electrical loads are also addressed, including lighting systems, refrigeration, water pumping, tools and appliances. There is also an excellent manual by SEI – Solar Energy International. This textbook manual is on how to design, install and maintain a photovoltaic (PV) system. This manual offers an overview of photovoltaic electricity, and a detailed description of PV system components, including PV modules, batteries, controllers and inverters. Electrical loads are also addressed, including lighting systems, refrigeration, water pumping, tools and appliances. The manual includes chapters on sizing photovoltaic systems, analyzing sites and installing PV systems. The manual also includes detailed appendices on PV system maintenance, troubleshooting, and insolation data for over 300 sites around the world. It is used as the textbook in SEI's PV Design and Installation Workshop. Source:

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