designing an off-grid solar pv power system for office Buildings. the peak load is 30kw

ersamta77ersamta77 Registered Users Posts: 2
Dear all,
          i am writing to seek technical advise  on the subject mentioned above.
i have been asked to design and make presentation to the senior management a solar power system to cut electric bills during daytime.
And, in the evening the load switch back to the utility grid.  My chief instructed me to design without  battery and it should no be a grid tie system .  Can someone help my out on this . all suggestions ate welcome.



  • BB.BB. Super Moderators, Administrators Posts: 32,639 admin
    edited June 2020 #2
    Where will the system be installed? Mogadishu Somalia?

    What are your loads (Watt*Hours/kWH per day)? What type of loads? Computers, lights, elevator, water pumps, Air Conditioning, etc.?

    How will your system/building respond when a flock of birds, cloud, or aircraft flies over the array and kills the power for a few seconds?

    Your solar array (without any storage) has to supply all the power needed for every moment during the day... It probably needs to be 2-4x larger than needed to account for cloudy weather, etc. And during the morning and evening, when the harvest is less, do you plan on reducing the power needed by the building/tenants, or do you plan on 2x or larger array (over the original 2-4x larger) to support power to the building at other than a few hours in the middle of the day?

    There are things you can do with your loads... For example, you could use laptop computers which have their own internal batteries, or use a UPS (uninterruptible power supply) system (batteries+diesel backup genset) for days when the sun is not present... Of course, you have just pushed the "storage problem" elsewhere (into the building / computer / desktop / UPS center).

    Is this article from four years ago still accurate?

    Are you on the coast/northern part of the country, or in the south/inland part--Where the solar harvest is 1/2 that of the "sunnier" regions?

    You can use PVWatts program and download a spreadsheet file that has hour by hour solar harvest for 365 days (real data based on "real day" for each date)--So you can see the variability of the harvest:

    There are other ways to "extend hours of harvest" (mechanical tracking arrays, having 1/2 the array face (roughly) east, and the other half facing west, to increase harvest towards sunrise and sunset.

    Make the energy assumptions, and run the math, and you can create a system that will do 80% of what you want.

    But your real answer is Utility Power + GT Solar (utility power is your backup), or look into Diesel Genset(s) and a version of GT solar that is "compatible" with the gensets (not a trivial issue).

    The other, village level solution... Yes, a big old battery bank, and use Solar+Diesel (as needed). Run the village from batteries during off peak hours (nights, mornings), and run the diesel for mid-day through evening loads (A/C, cooking, work).

    Reliable power is not cheap, and off grid solar is no different (for home sized systems with solar+battery+inverter+backup genset), the full costs (assuming 5 year battery life, 10+ year power electronics life, and 20+ year solar panel life, plus fuel costs in the $4-$8 per gallon range), runs something like $1.00 to $2.00+ per kWH--And you pay at least 1/2 of those costs (power generation over 20 years) up front to design and build the system. Add the cost of training/staffing to run/manage the power system, etc... The solar solution is not going to be cheap (plus security to prevent solar panels/etc. from walking off???).

    In general, to save money--Starting point is conservation. Finding the most energy efficient devices to do the job (i.e., low power laptop computer at 30 watts vs a desktop computer at 300 watts), use traditional cooling methods common in many hot areas in the region (ground level construction, shade/awnings/insulation/air circulation towers/etc.) and limit energy usage (no multi-story buildings with elevators, probably no central A/C, etc.).

    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • mcgivormcgivor Solar Expert Posts: 3,854 ✭✭✭✭✭✭
    First off, designing a solar system relies on one factor, which is the availability of the sun itself, in a perfect solar world there would never be any clouds with  only clear obstructed access to the sun, unfortunately that isn't the case in 99.9% of cases.

    There has to be, even for a momentary situation, where the sun pauses behind a cloud for example, this would need to be addressed by some form of backup, be it grid or battery. Battery support is most likely the most inefficient in terms of economics, blending grid with solar using the grid to support loads for these short, hopefully short, periods would be the best choice.

    The equipment needed to support such a large load would require the assistance of an electrical engineer, likely involving hybrid and or AC coupling which would involve the grid to some degree, if battery support is not an option.

    Additionally solar is not a linear energy source, there is only a small window of opportunity which varies depending on geographic location, usually around noon, to support loads greater than the target value, outside this window of opportunity would require an array of significantly larger capacity, or a means to store the energy when needs are not supported, choose one or the other.  
    1500W, 6× Schutten 250W Poly panels , Schneider MPPT 60 150 CC, Schneider SW 2524 inverter, 400Ah LFP 24V nominal battery with Battery Bodyguard BMS 
    Second system 1890W  3 × 300W No name brand poly, 3×330 Sunsolar Poly panels, Morningstar TS 60 PWM controller, no name 2000W inverter 400Ah LFP 24V nominal battery with Daly BMS, used for water pumping and day time air conditioning.  
    5Kw Yanmar clone single cylinder air cooled diesel generator for rare emergency charging and welding.
  • ersamta77ersamta77 Registered Users Posts: 2
    thanks very much for all the experts advise on this issue raised.  I will explain  this to my chief and recommend the Grid tie as advised. from my analysis based your advise, the grid tie will be the cheapest option for my organisation.
  • BB.BB. Super Moderators, Administrators Posts: 32,639 admin
    edited June 2020 #5
    If grid tied is allowed by your energy supplier, and only works if your utility power is running.
    If utility power is down, GT solar is down too.
    In the USA, the maximum (less paper work) is 10,000 Watt system (10 kW).
    Over that amount, more paperwork and more requirements, and usually less"friendly to the customer" rate plan.
    To be honest, you need to talk with your power utility. Their requirements will usually make or break a GT system.
    Normally, utilities do not like or want GT solar power systems. It does not make technical or economic sense for them.
    In USA and such, the government forced GT for a green agenda, and the utility and most of their customers (95%) are forced to subsidize GT solar installations and their rate plans for the 5% of the customer base with GT systems.
    In a less regulated environment, your utility may have zero interest in your GT solar power connection.
    It is possible to do a solar/no battery system. Pumping water to cisterns, freezing water to chill air/reduce A/C usage... You are really using an "alternative" method of storing and using energy. While interesting, may not be practicable in your region.
    Also, you not only need peak power (30 kWatts), you need to define your energy usage (Watts*Hours per day "by season").
    Peak Watts define the size of the genset and W*H per day defines the size of your Diesel fuel tank (using an analogy).
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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