How to design and size a solar battery system

welcome to the solar eclipse video series covering the basics of solar photovoltaics or solar pv my name is drew shavon and i'm an extension specialist with the university of maryland in previous videos we explored several load assessment strategies to determine our daily energy requirements as well as how to size solar photovoltaic arrays and inverters based on that energy demand and you can review those previous videos for more information but in today's video we're starting a new series exploring the integration of battery backup including how to determine the appropriate size battery for your system so in today's video we'll discuss some of the considerations that go into sizing a battery bank and then we'll calculate the battery capacitor battery size that will meet the energy loads for your particular application and we'll start by discussing the basics of battery integration into a solar electric system a battery bank is simply some number of batteries that may be connected in series or in parallel that provides power whenever the sun isn't shining so a properly sized battery bank will ensure that you have sufficient power to run your electrical loads whenever your solar panels have no sunlight on them while there's many battery technologies currently available on the market the most common batteries are lead acid which includes flooded lead acid gel and agm or absorbed glass mat as well as lithium-ion batteries the performance of most lead acid batteries are fairly comparable to one another but are widely eclipsed by the performance of lithium-ion batteries while much could be said on the differences between lead acid and lithium-ion batteries we'll only touch on a few of the key differences in this video lithium ion batteries can store almost four times more energy than lead acid batteries with energy densities around 150 watt hours per kilogram compared to lead acid batteries around 40 watt hours per kilogram but these batteries also differ in terms of their usable energy it's recommended that you use around 85 percent of a lithium ion battery's capacity or only 50 percent of a lead acid battery dropping below these recommended thresholds could damage or significantly shorten the lifespan of the batteries but we'll consider these factors when sizing battery banks later in this video the lifespan of a battery is typically rated by the number of cycles in which the battery is charged and discharged when properly maintained lead acid batteries will typically last between 7 to 10 years or around 1 000 cycles lithium ion batteries on the other hand may last around 20 years or between three thousand to five thousand cycles so the longer lifespan of lithium-ion batteries could largely offset their cost difference which is typically two and a half to three times more than traditional lead acid batteries with lithium-ion batteries needing to be replaced less often a lithium-ion battery could be a good option considering its total lifespan and now that we've covered some of the basics of battery technologies we'll consider some of the information that's needed to size the battery bank for a solar electric system now we'll briefly review how to determine your power and energy requirements but please review our previous video on load assessments for more information if you recall the simplest method for grid tied systems is to look at your electric utility bill to see how many kilowatt hours or kwh are used monthly in this example we can see that 617 kilowatt hours of energy were used in the current billing period and adding up the energy over the last 12 months results in an annual energy use of 5592 kilowatt hours so this is a rather simple method for determining your energy use in kwh now we've also considered how we can account for individual electrical loads using an electrical loads list and this would be a particularly useful strategy for off-grid mobile or any battery integrated system if you recall we simply record the power rating of each device in watts as well as the number of hours that we intend to operate the device you may want to review our previous video on load assessments to discover some of the various methods for assessing the power rating of a device these methods include voltage and current measurements using a multimeter referencing the manufacturer's data sheet or the owner's manual or using an electric power meter plugged into the circuit itself for an example we'll consider a small motor having a power rating of 1100 watts and we anticipate using this motor about 15 minutes each day or about a quarter of an hour each day so multiplying the power rating of the motor 1 100 watts by its daily use 0.25 hours provides a daily energy consumption of 275 watt hours and you'll simply repeat this process for any additional electrical loads paying particular attention to any device having a duty cycle that would be something like a refrigeration system or an air conditioner that cycles on and off throughout the day in those instances you could measure the device's baseline during periods of low energy consumption and its maximum demand whenever the compressor is active and running another approach to addressing duty loads is to divide the device's estimated annual energy use by 365 days dividing the annual energy use for this refrigerator for example 640 000 watt hours by 365 days provides a daily energy use of about 1 753 watt hours now it's also important to account for any seasonal loads to ensure that the battery bank is sufficiently sized to supply power during higher energy use periods of the year but once we've listed the daily energy consumption for each device in watt hours we'll add all the values to determine our total daily energy use in this case we obtain a daily total energy use of 19 528 watt hours which represents the amount of energy that our battery bank must provide each day now before we determine the size of the battery bank that's needed to power these electrical loads we'll estimate any system losses that might be experienced in our previous video on pv system design and array sizing we considered some of the common system losses on the generation side of a solar electric system but in today's video we'll focus primarily on load subsystem efficiency including round-trip battery efficiency wiring loss and conversion efficiency since no battery is 100 percent efficient we commonly use a round-trip efficiency factor to measure the energy retention of a battery after it's been charged you see there's always a loss of power during charging and discharging the round trip efficiency for a lead acid battery is commonly around 85 percent while lithium-ion batteries are commonly around 95 percent and these loss estimates are simply based on the chemistry of each battery with lead acid batteries typically operating with lower electrical currents and taking longer to charge but for this example we'll plan on using a lead acid battery having a round trip efficiency of 85 percent which corresponds to a factor of 0.85 now we'll consider any wiring losses that may occur as the electricity flows between the battery bank the conversion equipment and the electrical loads as discussed in a previous video the voltage drop across a wire largely depends on the length of that wire but we typically want less than a two to three percent voltage drop across the circuit so we'll assume a wire efficiency of 97 percent in this example or a factor of 0.97 next we'll consider the conversion efficiency of a battery inverter to raise the relatively low voltage of a battery bank up to the standard ac voltage of 120 or 240 volts battery inverters are typically less efficient than solar inverters with efficiencies generally ranging between 92 to 94 percent depending on the load level so in this case we'll assume the battery inverter in our ac coupled system has an efficiency of 92 percent now we'll combine all of these system losses into a single factor in this case we'll multiply 0.95 for the round-trip efficiency by 0.97 for the wiring losses by 0.92 for the conversion efficiency and this provides an overall load subsystem efficiency of 85 percent or 0.85 now that we've estimated our total daily energy use and the total system losses will calculate the size of our battery bank first we'll choose the system voltage most off-grid battery banks are either 12 24 or 48 volts 12 volt systems are generally used with smaller maximum demands while 24 and 48 volt systems provide the widest options while higher voltages are available we'll consider these smaller system voltages in today's video since we have a relatively small energy demand of about 19 and a half kilowatt hours per day but how do you actually decide what voltage to use in your own system well the first consideration may be what voltage your electrical loads are using if you're directly powering a dc load having a specific voltage like 12 volts then you'll need a 12 volt battery bank higher voltage systems like a 24 volt system can't run 12 volt appliances without a converter that may increase the cost of the system if you're using an inverter on the other hand to convert from dc to ac then your inverter may require a certain voltage coming from the input and higher outputs from the inverter generally require higher dc inputs a 2000 watt inverter for example may be available in 24 volt dc while a 6 000 watt inverter may require 48 volt dc your solar array must also produce a higher voltage than your battery bank a 12 volt solar panel for instance generally can't be used with a 24 volt battery solar panels can however be connected in series with one another to obtain higher voltages so long as the solar charge controller can handle that higher voltage it may be worth noting that a 24 volt charge controller could be half the size of a 12 volt controller which may represent additional cost savings the distance between your solar panels and your battery bank should also be considered since lower voltage levels require thicker more expensive copper wiring so if your solar panels are relatively far from the battery bank then you could reduce the gauge of your copper wire by using a higher voltage this is because higher voltage levels result in a lower electrical current so if you're pushing large loads and have a 24 or 48 volt system then you could use smaller less expensive wire to reduce the cost of the system with these considerations in mind we'll consider a nominal 24 volt battery bank in this video and now we need to determine the number of days that we plan on running our electrical loads from the battery bank during periods of low solar irradiance in other words how many days do we anticipate having no or only partial sunlight and this is what's called days of autonomy and this will help to ensure that we have enough power to run our electric loads even with reduced solar output resources on how to compute the days of autonomy will be included in the description of this video but a good figure is typically three to five days with that said there are a few factors that you may want to consider in deciding on the days of autonomy for starters you may want to consider the historical weather data for your location you might find that your site receives three four or even five days of inclement weather at a time and that would be a good starting point to determine the days of autonomy you may need another power source like a generator if you choose a fewer number of days but if you choose more days of autonomy on the other hand then the cost of your system may be much higher in this case we'll just select three days of autonomy which should provide sufficient reserve power for our application and location today so in this case we could run our electrical loads for three days without recharging the batteries while batteries are typically rated at 77 degrees fahrenheit or 25 degrees celsius their capacity will typically drop at lower temperatures we can account for this using a battery temperature compensation factor which may be found in the manufacturer's specification or sometimes in other resources including ieee standards which will be linked in the description below for this example we'll consider storing our lead acid battery in a room having an average winter temperature of 50 degrees fahrenheit this temperature corresponds to a temperature correction factor of 1.19 next we'll consider the depth of discharge for our battery bank which describes how far down the battery can be drained batteries are not designed to be discharged completely and they'll actually live longer the less that you drain them while battery manufacturers often supply a depth of discharge chart for each battery it's generally accepted that lead acid batteries have a 50 maximum depth of discharge while lithium ion batteries can be discharged to about 85 percent of their rated capacity without significantly degrading their lifespan a 1000 watt lead acid battery for example should deliver around 500 watt hours while a lithium-ion battery would be able to deliver about 850 watt hours for this reason you may need a larger number of lead acid batteries to achieve the same capacity but there's also some trade-off between the potential longevity of a battery and the size of the battery a larger battery may be more expensive but could actually last longer since a smaller percentage of its capacity is depleted in this video we'll assume a maximum depth of discharge of 50 percent or a factor of 0.5 for some lead acid batteries now that we've selected some of the parameters for our battery bank we'll use a simple calculation to estimate its size so 19 528 watt hours per day from our loads list times three days of autonomy times 1.19 as a temperature compensation factor divided by the 24 volt battery system voltage that we selected for this application divided by 85 percent or 0.85 for the load subsystem efficiency losses that we calculated earlier divided by 0.5 to account for the 50 maximum depth of discharge for our lead acid battery equals a total usable battery capacity of 6835 amp hours and since battery capacities are typically rated in amp hours by the manufacturer we can easily determine how many batteries we need for this application in this case we might select some 12 volt 215 amp hour flooded lead acid batteries in which case will divide the total battery capacity of 6835 amp hours by the rated capacity of each battery 215 amp hours and so this tells us that we need about 32 of these batteries to power all of the electrical loads with three days of backup and that could be rather expensive now with that said you probably want to avoid over sizing the battery bank since batteries are significantly more expensive than solar panels so for this reason you might opt to add additional solar capacity to your system as opposed to adding battery capacity additional solar capacity may also help your battery bank recover more quickly following any inclement weather well i hope this video is provided to you with understanding of how to estimate the size or amp hour capacity of a battery bank that can supply power to your electrical loads in upcoming videos we'll consider other aspects of battery integrated solar electric systems including battery configurations and solar charge controllers you can subscribe to this channel to stay connected on upcoming episodes of this solar eclipse video series but in the meantime please visit our website for more information on solar photovoltaics and other energy related topics

Channel: EnergyUME

Views: 126,886

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Id: FtO1sunTHOE

Length: 15min 42sec (942 seconds)

Published: Sun Jun 12 2022