Please evaluate my proposed system

flash
flash Registered Users Posts: 8
This is my first post and I must say I am very pleased to have found this site. I got interested in solar a few months back and set off to learn as much as I could.

My objective is to have an emergency off-grid solar back up system capable of providing at least 1600 watts of power per day. I took heed of suggestions on this site that it was more cost effective to conserve and minimize the power demand. The emergency I expect is for a few hours, a few weeks and perhaps a few months.

I’m located in Southern California with good sun exposure.

Here are my bare bones daily needs: CPAP 800 watts, Lights 160 watts, TV 200 watts and a 7 cu ft freezer conversion 400 watts. The CPAP runs for 8 hours at 100 watts/hr, the lights are 20 watt CLFs, the 42 inch LCD flat screen TV will be used to catch the news for 1 hour/day and the GE freezer if used as a freezer would require 800 watts per day but I intend to run it with my Johnson Controls thermostat and estimate as a refrigerator is will use about 400 watts per day.

At the very beginning not knowing which end was up I purchased 2 Harbor Freight 45 watt panel kits a 750 watt Harbor Freight inverter and two 115 AH EverStart 12 volt marine batteries wired in the diagonal. Each kit came with a charge controller rated at 5 amps. The 750 watt inverter with a light load only put out 90 volts or so. The wattage increased with a larger load. It did run my lights and the TV. I replaced it with an Ames 1250 watt/3100 watt peak inverter. The Harbor Freight kits would be suitable to run lights or a TV and I look back at them as part of the learning curve.

I have a Coleman Powermate 5000 watt generator and 10 gallons of gas stored. When funds permit I would sell and replace it with a Honda 1000 or 2000 watt generator.

To the Ames inverter and the two batteries I would like to add the following: A Morning Star 45 amp HTTP charge controller, 3 Grape 250 watt panels (8A at 30 volts) and two more of the 115 AH EverStart 12 volt batteries. My situation is such that the panels will not all be mounted in the same location. My first option is to mount one of the panels and use it to keep the batteries fully charged like a trickle charger. The other two panels would be mounted on portable frames and erected as needed. Linking all three panels to up the voltage would involve considerable wire so I may just stick with a parallel hook up. When the EverStart batteries are used up I will consider other battery and voltage options.

Using information I found on this site I would have a total of 460 AH in the batteries. 12 X 46 / .77 = 716 watts for the size of the panels so I would up the panels to 750. At 4 hours of sun that yields 1500 watts, 5 hours yields 1875, 6 hrs yields 2250 watts and 8 hrs yields 3000 watts.

What do you think of my proposed system?

Charlie

Comments

  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Please evaluate my proposed system

    This thread seems to have gotten lost. In case the questions weren't answered elsewhere I'll toss a little general knowledge at it.

    First, terminology: what you want is 1600 Watt hours per day. This can be done, but it won't be from a small set of panels like those HF 45 Watt units. They are a waste of money.

    Second, some of that equipment (like the refrigeration) may not like the output of that crappy AIMS inverter. It will want pure sine for best results.

    Third, your power estimates are probably optimistic. That 42" TV will take 200 Watts, less if you can turn down the brightness to minimum. The 'frige will have a pretty sharp start-up demand for its motor too.

    But if you go by your estimated 1600 Watt hours per day, convert that to DC (1800 - depending on the conversion efficiency of the inverter) and add in the inverter's own 24 hour consumption you are probably over 2kW hours DC per day. If you can count on 4 hours equivalent good sun that would be around 600 Watts of panel needed.

    However, when you charge/discharge batteries the efficiency gets worse. In reality you're looking at 52% over-all efficiency between the panel rating and the AC Watt hours available for use. So that 1600 becomes: 1600 / 0.52 = 3077 / 4 = 769 Watt array. Your 750 Watts might just do it if the daylight is a bit better than expected and the loads can be kept under control.

    From a battery point of view you could do this with a minimum of 400 Amp hours @ 12 Volts, so 460 is not unrealistic. You might want to consider the "golf cart battery solution": 450 Amp hours @ 12 Volts or better yet, 225 @ 24 Volts with a decent inverter.

    You could also put the three panels in series on an MPPT controller and overcome most problems with long wire runs from the array. You might want to look at the MidNite Classic Lite controller too: http://www.solar-electric.com/misoclli150m.html About $100 more than the MS, but a better controller over-all.
  • niel
    niel Solar Expert Posts: 10,300 ✭✭✭✭
    Re: Please evaluate my proposed system

    it seems he has posted about this proposed thing elsewhere.
    http://forum.solar-electric.com/search.php?searchid=163673

    so i wonder if this thread should continue or not. being you replied coot i will leave this decision to you on how you wish to handle it.
  • flash
    flash Registered Users Posts: 8
    Re: Please evaluate my proposed system

    Cariboocoot thank you for your informative response. I have learned quite a bit from other posts you have made.

    Yes it is 1600 watt hours per day that I estimate I will want in case of an emergency. If I had to cut back the TV would be the first thing to go.

    I have made the assumption that a 7 cu ft freezer converted to a refrigerator would use about 400 watt hours per day. Is this in the ball park?

    Is it feasible to turn off the inverter during the best sun hours to allow a full charge to go to the batteries? That would mean that the freezer/fridge would off for 6 to 8 hours. There is very little need for power during the daylight hours.

    I’m at latitude 33.6 degrees N and the solar radiation average is 5.46 kwh/m2/day with a high of 6.29 and a low of 4.21. How do these numbers relate to hours of good sun?

    I would like to add two more 115AH EverStart Marine batteries to the two I have and wire them up at two strings of 24 volts each. Ordinarily the system will be unused most of the year so is it better to have the batteries at full charge or to use them occasionally? When the EverStarts are done it will be time for golf cart batteries.

    I will look into the MidNite Classic Controller. On the controllers do you set a switch etc to match the nominal voltage, like 12, 24, 48 volts, expected or does the controller take care of this automatically?

    On the inverters is one locked into one voltage or are adjustable?

    My plan is to mount one panel on the roof and the other two will be on portable stands set up when needed. The roof panel, 8A @ 30 volts, will be used to keep the batteries fully charged. To avoid complications with voltage, when the other two panels are used, would it be ok even though it will involve more wire to wire the panels in parallel? Would the controller nominally see 32A @ 30 volts with this set up?

    Charlie
  • vtmaps
    vtmaps Solar Expert Posts: 3,741 ✭✭✭✭
    Re: Please evaluate my proposed system
    flash wrote: »
    Is it feasible to turn off the inverter during the best sun hours to allow a full charge to go to the batteries? That would mean that the freezer/fridge would off for 6 to 8 hours. There is very little need for power during the daylight hours.

    Its feasible, but not wise. When you power up the freezer at the end of the day it will make up for its missing daytime power by drawing more. What you are suggesting is that you take the daytime power and put it in the battery and then draw down the battery to power the freezer. That is less efficient than just sending the power directly to the freezer during the daytime.

    --vtMaps
    4 X 235watt Samsung, Midnite ePanel, Outback VFX3524 FM60 & mate, 4 Interstate L16, trimetric, Honda eu2000i
  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Please evaluate my proposed system
    flash wrote: »

    I would like to add two more 115AH EverStart Marine batteries to the two I have and wire them up at two strings of 24 volts each. Ordinarily the system will be unused most of the year so is it better to have the batteries at full charge or to use them occasionally? When the EverStarts are done it will be time for golf cart batteries.


    Charlie

    I have no personal experience, but I have seen reports that leaving some FLA batteries on constant float charge can promote growth of metallic lead tendrils from the plates which can cause internal shorts.
    A brief high current discharge from time to time supposedly burns these out/melts them back.
    SMA SB 3000, old BP panels.
  • flash
    flash Registered Users Posts: 8
    Re: Please evaluate my proposed system

    vtMaps:

    Thanks for clearing that issue up.

    Charlie
  • Cariboocoot
    Cariboocoot Banned Posts: 17,615 ✭✭✭
    Re: Please evaluate my proposed system
    flash wrote: »
    I have made the assumption that a 7 cu ft freezer converted to a refrigerator would use about 400 watt hours per day. Is this in the ball park?

    I would not make any assumptions about refrigeration power use. Get a Kill-A-Watt meter and measure it for real. I've tested quite a few 'friges and freezers and the results are all over the scale.
    Is it feasible to turn off the inverter during the best sun hours to allow a full charge to go to the batteries? That would mean that the freezer/fridge would off for 6 to 8 hours. There is very little need for power during the daylight hours.

    Much depends on which inverter you get. Some have a "search/standby" function which shuts down power until a load is sensed and then turns the inverter on full. Even among those there are differences in terms of power used and load sense needed to start. An Outback inverter uses about 20 Watts constant, or 6 Watts in search mode. The large Samlex inverters consume more (over 30 Watts) when running and have higher current requirements to trigger start-up. You have to weigh the amount of power saved leaving the inverter off against the need for the power availability. If the duty cycle on the refrigerator is 50%, that's one thing. If it's 30% - it will need to start more often and you may not save that much power over-all.
    I’m at latitude 33.6 degrees N and the solar radiation average is 5.46 kwh/m2/day with a high of 6.29 and a low of 4.21. How do these numbers relate to hours of good sun?

    To get some numbers for your area, try out PV Watts: http://www.nrel.gov/rredc/pvwatts/grid.html It's meant for grid-tie systems, but it will tell you the relative amount of sun available throughout the year. From your numbers it looks like you'd be good for 5 hours on average.
    I would like to add two more 115AH EverStart Marine batteries to the two I have and wire them up at two strings of 24 volts each. Ordinarily the system will be unused most of the year so is it better to have the batteries at full charge or to use them occasionally? When the EverStarts are done it will be time for golf cart batteries.

    Don't be too quick to do this. Adding new batteries to old isn't such a good idea; the more 'aged' the batteries (from time and/or use) the lower their capacity. New ones will be dragged down by the load of trying to bring the old ones up, shortening their lifespan. The 'marine' batteries aren't really good for this application. You would be better off planning out your 24 Volt system wholly, saving up the money, then buy it all at once.
    I will look into the MidNite Classic Controller. On the controllers do you set a switch etc to match the nominal voltage, like 12, 24, 48 volts, expected or does the controller take care of this automatically?

    Lots of programmability with the MidNite Classic controllers. You will set nominal system Voltage, charge set points, Absorb time, et cetera. They are not simple, which is why they work so well. You can download and read the manual (as well as other data sheets) here (under 'details'): http://www.solar-electric.com/mnclassic.html
    On the inverters is one locked into one voltage or are adjustable?

    Inverters have a fixed nominal Voltage. This is why it's necessary to pick the right system Voltage to begin with; the only way to upgrade is to replace (unlike charge controllers).
    My plan is to mount one panel on the roof and the other two will be on portable stands set up when needed. The roof panel, 8A @ 30 volts, will be used to keep the batteries fully charged. To avoid complications with voltage, when the other two panels are used, would it be ok even though it will involve more wire to wire the panels in parallel? Would the controller nominally see 32A @ 30 volts with this set up?

    Charlie

    Here you may have another problem. MPPT type controllers don't like having panels in different locations. If you add two to one for charge purposes they should all be pointing the same way. Otherwise the controller may not pick the right power point and you will miss out on some Watts. Using something like the MidNite to handle one panel most of the time is somewhat overkill, but the odd Vmp necessitates an MPPT controller. Using only one of those panels may not work well even for 'Float' charge on a 24 Volt system due to the Vmp being so close to the battery charge Voltage: add some hot weather and the panel may not produce the minimum Voltage required.

    Otherwise what you'd get is about (240 Watts * 3 = 720 Watt array * 0.77 typical efficiency / 24 Volts) 23 Amps peak charge current. That is just shy of the 8 Amp rating times 3 (the Vmp of the panels being a bit low there is not as much 'extra' Voltage available for conversion to current.

    You might also consider the Rogue 3024 controller for this application: http://www.roguepowertech.com/products/mpt3024.htm
    Less money than the MidNite and perfectly suitable, but no room for expansion beyond your existing set-up (without adding another controller).
  • inetdog
    inetdog Solar Expert Posts: 3,123 ✭✭✭✭
    Re: Please evaluate my proposed system
    Here you may have another problem. MPPT type controllers don't like having panels in different locations. If you add two to one for charge purposes they should all be pointing the same way. Otherwise the controller may not pick the right power point and you will miss out on some Watts.

    To clarify that statement for those interested: The MPPT CC will pick the "right" power point for the two strings panels in parallel, as long as the different strings of panels are identical. If the panels are different, the CC may make a really poor decision. If the temperatures of the two strings are significantly different (one gets morning sun and heats up first, for example) then the controller could also pick a poor power point for the combined system.
    But even when the CC will pick the best power point for the combined (paralleled) system, that still may not be as good as running each separate string at its individual MPP (at different voltages) and then combining the controller outputs. Some MPP controllers or inverters have more than one array input for these two reasons.
    SMA SB 3000, old BP panels.
  • flash
    flash Registered Users Posts: 8
    Re: Please evaluate my proposed system

    Thanks to all for some really good information and suggestions.

    Charlie