How to keep a PV system simple and efficient?

I am an expat American living and working in Nepal.

Nepal has terrible grid power during the winter months – no melting glaciers to power the hydroelectric plants. We have rolling blackouts that progress from a mere two hours a day during the summer to a whopping 18+ hours a day during the winter. Fortunately, during the winter there is constant sunshine and temperatures are cool. I am in the middle of researching a way to affordably install a PV system for my family. We don't need very much power. The maximum would be 1200 watts for the microwave to make tea. There is a plan to dig a 100' well and install a pump that can run off the PV system as well; I don't know what the electrical requirements of that would be a this point. The real hog in terms of electrical use is the 400 watt iMac computer and hardware. The lighting is all CFL.

In a situation like this where we can at times use grid power to charge batteries and run our CFLs and appliances but at other times need solar to run them and charge the batteries what is the least expensive route to go?

Right now I have an Indian made 1400 watt sinewave UPS that runs off four 12 volt Exide 180AH batteries wired in series-parallel to create a 24 volt battery bank.

The country runs on 230V 50Hz but the computer gear is dual voltage. Some of my other electrical expat gadgets are 120V 60Hz.

Should I set up two systems? One for the computer using a pair of Morningstar SureSine inverters? The 1400 watt UPS to run the rest of the gadgets in the house and lights? How to feed the 24 volts into the SureSine?

Not sure how to show this, but would this work?

PV array>charge controller>transfer switch>battery bank>UPS>load
Grid power>UPS>transfer switch>battery bank>UPS>load


I could set a preference for the solar array in the transfer switch so I could use any power produced during the day.

Please any help would be greatly appreciated.

John

Comments

  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    Re: How to keep a PV system simple and efficient?

    If you have access to fuel, a small inverter generator 1Kw - 2Kw would be the ticket.
    An electric kettle is a better way to heat water than a microwave, kettle 95% of the power goes into the water, microwave, about 40% gets blown away by the cooling fan from the magnetron. An induction hot plate, with a ferrous pan is also 85% efficient getting power into cooking. Electric tea kettle can run on mod-sine wave. others need sine wave for best operation.

    The MS 300W Suresine is very efficient, but only 12V (AFIK)

    Solar is a good way to recharge, and with cold winter and sun reflections off snow, a MPPT charger could be used to good advantage.

    All depends on your budget. Inverter genset being least expensive, and then as much solar PV as you can afford. Battery upgrades from flooded to AGM can increase harvest/stroage by about 20% but final charge management is important.
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • jprokos
    jprokos Registered Users Posts: 8
    Re: How to keep a PV system simple and efficient?

    Fuel is the other problem, not reliable at all here.

    I have the panels, 1400 watts worth, I am not wanting to spend $5000 on an inverter, and a matching charge controler if I don't have to.
  • mike95490
    mike95490 Solar Expert Posts: 9,583 ✭✭✭✭✭
    Re: How to keep a PV system simple and efficient?

    Well, for a well pump, you'll need either a gronfos DC pump ($$$) or a common AC pump, with a BIG sine inverter that can start it up. The DC Gronfos pumps need about 150W @ 40V from PV, and a $ controller box. They pump slow (1-2 GPM, but a couple hours worth, and you have a lot of water). Charging, you should look at a MPPT controller. Maybe the Tristar 45A MPPT http://store.solar-electric.com/motr45ampmps.html nearly $500, but will harvest the most power for your batteries. Getting to the $3,000 bracket, you have the Xantrex XW inverter/charger, that can stand-alone supply 240V with a 10 sec overload to start big pumps. http://store.solar-electric.com/xaxwhyin.html - And charge batteries when power is present. it's all a trade, fast charge, or well pump, or long battery life....
    Powerfab top of pole PV mount | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
    || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
    || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

    solar: http://tinyurl.com/LMR-Solar
    gen: http://tinyurl.com/LMR-Lister ,

  • stephendv
    stephendv Solar Expert Posts: 1,571 ✭✭
    Re: How to keep a PV system simple and efficient?
    jprokos wrote: »
    Right now I have an Indian made 1400 watt sinewave UPS that runs off four 12 volt Exide 180AH batteries wired in series-parallel to create a 24 volt battery bank.

    ...and you already have the panels. So following your requirement to spend as little as possible, the only thing you need is a charge controller between the panels and the batteries. Then you have a working backup system: Both the UPS and solar panels would charge the batts and the UPS inverts to your 50hz house loads.

    The only thing missing from this equation is an inverter for your 60Hz 120Vac devices There are many many low power inverters available besides the suresine that will fit the bill, e.g.:
    http://www.victronenergy.com/inverters/inverter-12v-24v-48v-800va-3kva/ and
    http://www.studer-inno.com/?cat=sine_wave_inverters&id=430

    Regarding the pump, one more option is a 3 phase pump and a variable frequency drive (VFD). This solves the problem of a high startup current so that you don't need to buy such a powerful inverter.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,431 admin
    Re: How to keep a PV system simple and efficient?

    I would seriously look at energy conservation... Is there anyway you could use a laptop computer for much of your needs? 18-60 watts vs 400 watts... That would cut your computer loads to ~1/10th... Either allowing you to reduce your planned array/battery bank by a whole bunch--or the ability to run 10x longer on the planned/existing setup.

    While running a microwave or electric hot water pot is a serious load for a PV/Battery system--The length that you use the load can be more of an issue:

    Using the desktop computer for 10 hours a day at 400 watts:
    • 400 watts * 10 hours = 4,000 Watt*Hours (4 kWhrs)
    Using a microwave for 15 minutes per day:
    • 1,200 watts * 1/4 hour = 300 Watt*Hours (0.3 kWhrs)
    So, in this made up example, the computer would be using 10x the amount of battery/solar power vs the "big" microwave.

    And, a typical small laptop (don't know about Macs) use around 20 watts average:
    • 20 watts * 10 hours = 200 Watt*Hours (0.2 kWhrs)
    Other laptop advanatages--they have their own "UPS" (battery pack)--nice for iffy power situations.

    Regarding how to connect everything together... UPS's typically waste a fair amount of power... Not good for extended battery operation from a Back Up / Solar RE system.

    You can setup your system so that the Battery Bank is the "heart" of your design.

    On the output, you have the inverter(s) going to your dedicated loads that need 24x7 power (computers, communications, entertainment, etc.).

    In the input, you have your solar array + Charge Controler, and a good quality AC battery Charger that can both charge the battery bank and run the (smallish) DC/Inverter loads (basically, you have just made your own super-sized UPS).

    Then have your "big inverter" to power the loads when needed. A DC switch on the inverter input (or "inhibit" switch). To power your pumps, microwave, etc... For those loads, you can use a manual transfer switch (or relay) to swap those loads between utility power and backup power.

    The large inverter (depending on how much power it wastes, if you get one with a "standby/sleep mode" / etc.) could be left on and you use a relay powered by the AC mains. When AC mains fail, the relay switches over to the back AC inverter power.

    Regarding setting up your system to use multiple inverters (say one >1,400 watts MSW cheap monster for large loads like pumping, microwave and a small "efficient" 300 watt one for laptop, phone, charging appliances, etc.)... You are using 24 VDC--get two different sized inverter and connect them to the same battery bank. They will share the battery bank just fine (assuming it is large enough to handle the big loads) and you will have less to maintain and can "share power" (one day more pumping, another more computer work).

    Lastly, pumping--look at a small in-well pump (say 1 GPM) to a holding tank. Then use gravity or a small 24 VDC diaphram pump to local water pressure. The tank will provide the volume (5-10 GPM) when needed.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • westbranch
    westbranch Solar Expert Posts: 5,183 ✭✭✭✭
    Re: How to keep a PV system simple and efficient?

    another thought is a hybrid system, use a small genset as backup to charge your batt bank,and or pump water in emergency. Perhaps a cistern that is filled by mains with a couple or 3 days capacity? this would leave the solar to provide many more daily essentials. Make hay while the sun shines...

    Also there has not been any discussion about the amount of your average daily solar insolation... you do not want to draw the bank down below 70% capacity (avg) to ensure reasonable life expectancy. Rule of thumb here is 3 days reserve capacity from the bank... so figure out 3 days consumption and double that figure to calculate the amount of PV panels needed to keep you going.

    In some ways living on or with solar power is a way of living, living around, or in tune with:D, when you get the power and saving what you need for the night.

    HTH
    Eric
     
    KID #51B  4s 140W to 24V 900Ah C&D AGM
    CL#29032 FW 2126/ 2073/ 2133 175A E-Panel WBjr, 3 x 4s 140W to 24V 900Ah C&D AGM 
    Cotek ST1500W 24V Inverter,OmniCharge 3024,
    2 x Cisco WRT54GL i/c DD-WRT Rtr & Bridge,
    Eu3/2/1000i Gens, 1680W & E-Panel/WBjr to come, CL #647 asleep
    West Chilcotin, BC, Canada
  • jprokos
    jprokos Registered Users Posts: 8
    Re: How to keep a PV system simple and efficient?

    Thank you for your suggestions - still mulling over it all. Agreed, conservation is best in a situation like mine and in general as well.

    With gas powered anything I worry –don't want to buy into a dying technology. Seems horribly inefficient, more so than solar. What are the conversion ratios for gas? I know it depends on the generator, but in general how many watts of stored electricity for each liter of gas spent? What about the cost of the generator depreciated over it's lifetime?

    The nearest gas dealer is about ten miles away, but since we have to go by motorcycle or bus it's a huge ordeal. This is Nepal folks…

    I checked the insolation at NASA's Atmospheric Science Data Center. For the months where we would be relying heavily on solar generated power the values are excellent: Oct 6.27, Nov 6.84, Dec 6.67, Jan 6.36, Feb 6.80, Mar 6.90, Apr 6.78, May 6.65. Those months are very dry here and very sunny. We live in a bit of a valley so get fewer sun hours, especially around winter solstice, but our latitude helps – 27N

    Turns out, after checking with an ameter, that the computer only draws about 200 watts during general use, the 400 watt rating must be it's absolute maximum. I use the computer to generate income so it's a necessity.

    Questions for Bill:
    Is it possible to connect two inverters to one battery bank if they have different volt ratings? I have four 12 volt batteries wired in parallel and series to give 24 volts but could I connect a 12 volt inverter to one side of the bank? Guess that would cause the batteries to age differently - and thus faster? Still thinking of the Morningstar SureSines which are only 12 volts.

    I have contacted Iota Engineering about a transfer switch and they don't have an export model any more. I would need one to transfer loads between grid power and the battery power wouldn't I. I will be needing a 230V50Hz model, any suggestions? Make my own?

    Keep in mind that when I go to the supermarket I don't buy the milk chocolate mixes that have sugar in them - I buy the cocoa powder only and add my own sugar - cheaper that way. :blush:

    I put together a 12 volt battery equalizer with a bare power transformer (12-0-12-15A) and 6 diodes. Cost me very little and works great when equalizing the battery bank once a week.

    Again, thanks for all the suggestions. It's just a matter of going over it all and doing the math.

    John
  • Dave Angelini
    Dave Angelini Solar Expert Posts: 6,730 ✭✭✭✭✭✭
    Re: How to keep a PV system simple and efficient?

    In my opinion and experience the best way to really keep an offgrid system efficient is to enable and use search mode on your inverter/charger. It will force you to do all the things that save energy. There are challenges but they can be overcome. The starter system that I design is based on it and most people can handle it if they are proactive. It will save you between 1/2 and up to a full KWH per day. Many find that they can use it during the winter when it really counts. Large systems easily overcome these losses but require the cash outlay!

    Good Luck!
    "we go where power lines don't" Sierra Nevada mountain area
       htps://offgridsolar1.com/
    E-mail offgridsolar@sti.net