Off Grid project questions

Discussion in 'General Electronics Chat' started by poopscoop, Mar 14, 2015.

  1. poopscoop

    Thread Starter Member

    Dec 12, 2012

    I have an assignment for school which involves notionally designing an off-grid power system. I've decided to add an element of realism by gearing it towards a very small local farm.

    This farm has a stream situated about 80 meters below (vertically) the main house and fields. I'm toying with the idea of using solar panels to power the home, and using excess power to run a pump to raise the water the 80m. When there is a lack of sun, the water can flow through the motor and generate power. Basically pumped hydro storage.

    I'm fairly certain pumped hydro will not be the most cost effective option. In the context of this assignment its irrelevant, as long as I make sure to note that it is less cost effective than batteries.

    Currently I plan to evaluate a diesel generator as a back-up power source, unless the teacher specifies that maintaining a grid connection is permissible (Which I doubt, because it makes the assignment very easy). I figure the generator can be a bit undersized in the peak load department, and have the surge currents compensated for by a small battery bank.

    Peak loads will be carefully controlled by inexpensive micro controllers and relays. Simple logic will shutdown water heaters, dryers, and ovens when the A/C wants to kick in, and restore them as feasible when surge currents pass. I don't have to actually design the logic, but I will emphasize that the system would have priority load shedding defined by the home owner.

    There a few things I don't know anything about, and I'm betting someone (nsaspook, for one) would know quite a bit about.
    1. What do batteries cost when you buy them in the quantity and type for an off-grid system? Are the prices similar to what I see for standard deep-cycle?
    2. Cables? What do they cost? Aluminum or copper?
    3. Are DC or universal motor water pumps available? Can I generate power with a universal motor? Or, better question, if I generate power with an induction motor, will the rectifier and inverter care that the current and voltage are out of phase? Should I add some sort of power factor correction?
    4. Realistic peak loads on induction motors. So given a list of motor parameters I can calculate it, but I know the blower and compressor on an A/C are not so nicely labeled. Is there a good rule of thumb I can use?
  2. nsaspook

    AAC Fanatic!

    Aug 27, 2009
    1. One type of battery for long term off-grid use that's also fairly inexpensive are golf-cart batteries. Available just about everywhere and are designed for abuse, high charge and discharge rates. The 6 Volt GC-2 (215 Ah 700 cycles) is a common type that will last years while a 'standard' deep-cycle battery might only last a few hundred cycles if you're lucky. A true RE battery like the T-105 will easily last 1000 cycles if properly maintained and if you look at the total storage Ah lifetime cost might be cheaper.

    2. If you design the system with a 48volt (using a series connected bank of 6V batteries) energy bank the requirements (for less than a 5% voltage drop) for large gauge cables for the DC side will be reduced from the solar panels, into the charge controller and inverter/batteries.

    3. IMO small pumped hydro is a loser when you already have batteries for energy storage so I really don't have much information that can help you there.

    4. What you want for an AC is a mini-split with a soft-start inverter compressor.
    Most appliances are not designed to be nice with surge limited power. If the label has 500W you might need to provide 2000W stable for the inverter to start most induction motors without soft start devices.
  3. poopscoop

    Thread Starter Member

    Dec 12, 2012
    One more question about batteries:

    I know you don't want to discharge a battery beyond a certain voltage if you want to use it again (for lead acid, anyways), but how does that work with the rated capacity? Is a 215Ah battery actually good for 215 hours at 1 amp (more or less).
    Last edited: Mar 18, 2015
  4. Kermit2

    AAC Fanatic!

    Feb 5, 2010
    aim for 50% discharge to keep the lifetime of the battery in a reasonable range. a 12 volt lead acid battery is 50% discharged when the voltage under loaded condition is around 12.0 to 11.9 volts