What are the capabilities

SystemSystem Posts: 2,511 admin
Is it possible for solar panels to push a 40 hp sub irrigation pump (either 1 or 3 phase pumps) long enough to possibly cut irrigation cost in half? I have a center pivot that covers 210 acres and right now it takes 4 40 hp electrical pumps to run the pivot. I was looking at using solar energy to help run the pivot.


  • BB.BB. Super Moderators, Administrators Posts: 33,325 admin
    Re: What are the capabilities

    Yes--anything is possible... The devil is in the details.

    Some real quick assumptions... Assume you are around Charlotte N. Carolina. And that 1HP = 1,300 watts of energy (losses). And use the PV Watts website (set to 1kW of solar panels, everything else is default). Assume you will irrigate around 6 month of the year and that you pump 8 hours per day (hey, I am not a farmer--I don't know your pumping schedule ;) ). Assume that you can get 1 year net metered power (excess used in summer for irrigation can be "recovered" during winter as a credit to your bill).
    [FONT=Fixedsys]Results fixed array 1kW Charlotte N. Carolina
    Solar Radiation (kWh/m2/day)
    AC Energy (kWh)
    Energy Value ($ 8.5 ¢/kWh)
    1      4.04          95        8.07   
    2      4.46          94        7.99   
    3      5.30          120        10.20   
    4      5.89          127        10.79   
    5      5.79          125        10.62   
    6      5.77          118        10.03   
    7      5.55          116        9.86   
    8      5.59          118        10.03   
    9      5.26          108        9.18   
    10      5.23          115        9.78   
    11      4.25          94        7.99   
    12      3.74          87        7.40  
    Year    5.07          1318        112.03   [/FONT]
    1kW of solar panels will generate an average of 1,318 kWhrs per year for Grid Tied system.

    40 HP * 1.3 kWatts / HP * 6 months * 30 days per mnth * 8 hrsperday / 1,318kWhr per year per 1kW = 56 kWatts of solar panels

    Assuming you can install a Grid Tied system for $7 per Watt ($7,000 per kW) of solar panels. Assume 20 year system life (panels will last longer, electronics will probably need replacement once or twice in that time period--assume $1,000 per kWatt for maintenance). Excluding any cost of money, taxes, etc.:

    56kW of solar panels * $7,000 per kW to install = $392,000 (very rough number)

    56kW * ($7,000+$1,000 per kW) / (1,318kWhr per kW per year* 56kW * 20 year) = $0.30 per kWhr

    You can get a 30% tax credit, and there may be some state credits. And since you are a business, you can probably deduct / depreciate the cost of the system (don't know the details). But since you are a business, you may hit some issues with your utility (may not have net metering--but some other credit for offset of power generated or something--maybe 1/2 your kWhr rate is generation costs, the other 1/2 is distribution costs which the utility may still want to charge).

    Should be enough here for you to plug in your actual numbers (look at your power bill for kWhrs per month/year instead of my estimate).

    The above is just a very rough, back of the envelope calculation. I am not a solar installer or a tax guy--hopefully the above can help you set your course of how you want to proceed.

    In general--if there are things you can do to reduce your power usage (more efficient pumps/motors, lower pressure irrigation, etc.) will be the best place to put your money. Possibly looking at alternative power sources (propane, natural gas, etc.) may also be worth looking at. Co-generation (local generator, use waste heat for process--such as crop drying, etc.).

    Your thoughts?

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • System2System2 Posts: 6,290 admin
    Re: What are the capabilities

    I'm not sure if I understand you right but it would take a solar panel that could push 56kWatts to run (1) 40 hp pump. It takes me (4) 40 hp pumps to run 1 pivot so therefore I would have to have (4) 56kWatts panel setups? Also the pivots run 24 hrs a day during this time of year so would that change the Watts? That means I would have to have a crap load of batteries and invertors to run this thing.
  • BB.BB. Super Moderators, Administrators Posts: 33,325 admin
    Re: What are the capabilities

    You will want a Grid Tied system---Unless you have some very special off grid application where it is, literally, worth $1.00-$2.00 per kWhr (or 10-20x your grid power costs).

    Grid Tied Inverters... Just connect to a solar array, and pump the power into the utility lines. The power your pumps uses just comes from the utility grid (as far as you are concerned). The electric meter will either spin forward or backwards, depending on how much power you are generating and using (generate more power than you use, the meter turns backwards; generate less power than you use, the meter turns forwards).

    In a way--the Utility is behaving just like an (almost) unlimited AC battery bank when you install a Grid Tied / Utility Interactive Inverter.

    The inverter itself is just a conversion device... Whether it is solar DC PV array, a Wind Turbine, or even Hydro power--the operational result is the same. To efficiently convert the input solar RE power into utility compatible output power (voltage, current, frequency, and safety isolation/shutdown circuitry).

    A large system like this, with seasonal usage (I assume that there are 3+ months you may not irrigate near as heavily), it becomes as much as a MBA/Business discussion as it does an engineering one.

    First... Your billing. You will need to work with your local utility... Many of the smaller utilities (and co-ops) don't even support Net Metering or Solar Grid Tied connections.... So, that is one issue.

    Another is the billing. Many states have passed laws that say the utility has to support "Net Metering"... Basically, we buy power at retail, and if the meter turns backwards, the utility buys power back at retail. At the end of the month (or the year--depends on plan), you pay your reduced bill, or if you generated more than you used, your get a credit (which can last up to 1 year, or simply set to zero if 1 month Net Metering).

    Next, there is the question of time of use metering. For me, I pay $0.30-$-.60 per kWhr for noon-6pm peak during the "summer" 6 month period. And I pay $0.09-$0.44 per kWhr for off-peak (I have tiered rates--the more power I use, the more I pay. These are residential tiers so don't have any relashionship to what commercial/ag may pay).

    There is even an issue with Power Factor for many utilities... Inductive Motors can have a very bad power factor--the typical solution is to install motor run capacitors to bring the PF from 0.5-0.6 to near 1.0 (your utility may charge you a penalty if you have poor power factor).

    And--even more--At least in California for commercial power, we have "reservation charges"--basically, the peak 15 minute power usage is "reserved" for your consumption (and you pay a fee every month, which is, very roughly, about 1/2 of the total energy bill--the other half being the kWhr generation charges).

    So--you need to determine how much power you need to generate and if you can spread the power "pay back" over 12 months, or only month by month. And how much you will pay/save depending on the details of your utility's policies (net metering, or simply buy power back at wholesale).

    So--once you have some estimates of how your power will be billed/credited--you can go through the "engineering side" of the system.

    For example, assume you want to offset the power month by month (i.e., 100 kWhrs generated, 100 kWhrs used) for 6 months of the year.

    From the previous chart I generated... You can see that about 110 kWhrs per months minimum (per 1kW of solar panels).

    Notice, that your pumps operate 24 hours per day, but most of the energy is generated 6 hours per day... So, for 6 hours, your meter will be turning backwards (lots of solar production relative to pump usage), and the other 18 hours a day, the meter will be spinning forward (little/zero solar power, pumps still running). At the end of one month, ideally, the meter has turned forwards and backwards the exact same amount and you owe zero dollars.

    Notice--that, very roughly the solar array running 6 hours per day to offset your 24 hour per day pumping means that the solar array will pump out ~4x the peak wattage (middle of the day) vs your pumping operation (4x mid-day power - 1x pumping =~3x heavier transformer/utility connection to support your large solar arrays vs just supporting the pumps alone).

    So--In California Land, one could see the reservation charges increased by a factor of 3x, and their electrical kWhr billing charges reduced to zero... And, after all is said and done--the actual bill could double after the solar system has been installed.

    Yes--you could see your utility bill go up in our regulatory climate unless there are PUC regulations that specifically provide offsets and regulatory relief for Solar GT power (which there may well be). In our area, systems that are over 10 kW to over 100 kW come under other regulations--which need a lot of research to understand how they work.

    The pencil work for how big the array needs to be is pretty simple:

    4x 40hp * 1.3kW * 30 day * 24 hr per day / 110 kWhrs per 1kW of panel = 1,361 kWatts of solar panels

    1,362 kWatts of solar * $7,000 per kW installed price = roughly $9,500,000 (assuming $7,000 per kWatt installed)

    This is probably the worst case cost--you may get better pricing, you may get lots of Federal, State, local credits, depreciation, green credits, etc...

    But, no matter how you slice this--this is a huge solar array (a 1+ MW solar plant would probably get in your newspaper and on the local evening news).

    But, without working hand in hand with your utility and an Engineer with experience in solar and regulatory/tax implications--you will probably not be able to make any cost effective decisions with just the information I have supplied here.

    You can review this government website for laws and rebates for your state/local area.

    I am not trying to blow you off here--but we are way out of my depth and other than some back of the napkin calculations--there are so many other variables--that anything more I type here is not going to really help you.

    But, do continue to ask questions--once you understand how the basic system works and how it may interact with your utility billing--you will be well prepared to continue your research with the professionals. Taking the time to understand the basics for Grid Tied power and how it works will help you to follow the discussions and ask good questions.

    And, we would be very interested to hear what your investigation returns.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • BB.BB. Super Moderators, Administrators Posts: 33,325 admin
    Re: What are the capabilities

    I guess, for completeness, I should add that installing batteries (for a system completely removed from the grid) would cost you ~2-4x as much (the $9.5 million I guessed at) for the same power output.

    And you have more maintenance costs (batteries have limited cycle life times). Storing that amount of power locally would require a huge, and probably leading/bleeding edge technology to be cost effective. Plus you would need the grid or backup generator station if you wish to continue pumping during cloudy weather.

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • System2System2 Posts: 6,290 admin
    Re: What are the capabilities

    What if I didn't want to remove the system from the grid and only use the solar power as a supplement, would you need a bunch of batteries?
  • Solar GuppySolar Guppy Solar Expert Posts: 1,989 ✭✭✭
    Re: What are the capabilities

    Start with the basics ... what are you paying for electricity now to power the pumps, then ask what would it cost for solar supplement on a kWh basis.

    I can tell you now, without knowing anything about where you are, what your rates are that your not going to add solar unless you want to, not because it will save any money

    Your obviously deep into the commercial rates, which typically get power in the 4-5 cent range per kWh of electricity, maybe even less if your on the TVA or other hydro based generation system. Solar grid-tie will be 4X that ( no batteries ) and off-grid will be about 20X that ( typically 1 dollar kWh of generation ability )

    So, start with the most basic question, how much energy does your pumps now use and what is that energy costing?
  • BB.BB. Super Moderators, Administrators Posts: 33,325 admin
    Re: What are the capabilities
    What if I didn't want to remove the system from the grid and only use the solar power as a supplement, would you need a bunch of batteries?

    All of the basic calculations I did where assuming you were doing Grid Tied and no batteries are involved (other than the 10-20x costs for power for Off-Grid systems).

    Off Grid would probably never make sense for a large pumping operations like yours...

    There are small off-grid capable pumping systems (put a 1kW of solar panels on 900 watt AC/DC capable in-well pump) that pumps during the sunny time of the day (typically into a holding tank or pond)... Those can be reasonable for off-grid use when they are remotely located (no power lines nearby). If you can figure out a system of wells, ponds, pumping that would help you be more efficient (say you used solar to pump from wells/aquifer/irrigation district supply up to an above ground/hill based storage systems that could then be used to feed your main pumps...

    However, nothing that I can think of that would run your size pumping operation 24 hours per day other than some sort of Solar or Wind grid tied system...

    And, frankly, the only way those make sense are because of government subsidies and regulatory offsets. There is no way that any solar RE power would make economic sense for your requirements.

    That is why it is so important to find out from your utility and local government officials what they can "offer to you" to make such a system viable for your needs.

    In the end, working with your pump/irrigation supplier/contractor to work on efficiencies for existing pumping system might save you 10-35% (SWAG) on your existing power costs...

    And, if you can obtain increased pumping efficiencies--this is still money well spent if you ever do decide on alternative power generation... Reduced power needs will reduce your capital/maintenance/tax requirements on any future power system.

    Here is a very simple diagram detailing the different options.
    1. Grid Tied--cheapest, most efficient, least amount of maintenance
    2. Grid Tied/Hybrid--Like a giant UPS... Operates as Grid Tied, but if grid fails, can transfer to battery/solar/generator power for backup.
    3. Off Grid--Totally removed from grid. Most costly, least efficient regarding power generated from solar panels and money required to build and maintain system. Usually only makes sense when it costs $10,000's of thousands of dollars to run utility lines to a remote home/cabin.

    PS: Grid Tied can be any size--You can start with a small system that only partially offsets your power needs, add to it, and eventually size it to provide 100% of your power needs (or more--if it makes economic sense).

    For example, some wind turbine projects get up to $0.45 per kWhr because of all of the tax breaks and subsidies... So, in some cases, the wind turbine companies actually pay to put their power on the grid (negative bills) because it makes sense for them to pay $0.0x to the grid operator because they are getting $0.45-$0.0x from other "sources".
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • icarusicarus Solar Expert Posts: 5,436 ✭✭✭✭
    Re: What are the capabilities

    The reality is, if you want to save money, pray for rain. As has been so well said, if you are looking to save money solar isn't going to do it. Here is where I would push for conservation but I can't see a lot of places you can cut your energy use, except to cut down on the water usage. My guess is that your current pump strategy has been honed as well as it can since this comes at such a significant cost.

    Goo luck,

  • nielniel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: What are the capabilities

    i think you're on the right track tony. conservation by maybe more efficient pumps if any exist in addition to trimming water use.
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