Need help designing new system

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kaipo_boy
kaipo_boy Solar Expert Posts: 143 ✭✭
in Off Grid Solar & Battery Systems #1
Complete newbie here, I've looked through some other similar topics and realized I've got a steep learning curve ahead of me here. Great forum guys, and my thanks ahead of time.

I live in Hawaii so solar makes emminent good sense here, the particular part of the Oahu that I live on is technically (measuring rainfall) a desert. We only have 1 power generation company here and they are hideously expensive, current rate as of 2014 is $0.37 per kw and I believe they are asking for another increase. Not being able to afford an entire PV array, I've decided to do a small, stand alone PV system that will supply only limited power to a specific portion of the home to be partially self sufficient. I plan on only 1 panel and a couple batteries and small inverter and charge controller for lights, fridge, computer, TV and vcr. Grid tie is out of the question as the local company requires 'approval' of course, and the line goes around the block several times; they aren't approving anything so have a stranglehold on PV installations.

The TV/vcr combo pull near 350w but would be on from near 5pm-10pm most nights. Not sure if 2 T-105s with a small 600w or 1000w inverter would handle that load. I plan on getting LED lights, currently I run 2 fluorescent tubes of 44w? each.

I just purchased (last week!) 2 small 2200w/1800w ryobi inverter generators as my first step, they are slightly noisier than the honda EU2000i but produce about the same power and cost about half as much. All reviews were favorable other than initial issues with QC. My experience with them is about a week old and so far good enough. I plan on building a sound isolation box though, in case I need to run one late at night to recharge batteries run down too much.

I need help with panel selection and sourcing them; (my initial impression was only a single ~250w panel and 2 T-105s to start and later as funds allow, add another identical system) battery selection and number, charge controller selection, inverter selection (pure sine only pls).

I know the fridge selection is critical as power consumption of refrigeration products is probably the largest outside of water heating. I am in the process of getting solar water to cut my power bill so that will get knocked out; for the fridge, I was going to go with a Sundanzer DC fridge, but recently stumbled across several websites which note nearly similar power consumption from a standard AC chest freezer with a controller assembly which cycles the power on and off to turn the freezer into a horizontal fridge by lowering the in cabinet temps only to about 35 deg F instead of the normal near 0 degree F in a regular freezer. I have ordered a digital Johnson controller which should be here next week, and a new 14.8 cu ft GE chest freezer which will serve as my fridge. The reported power consumption for setups like these is near 200wh per 24 hour cycle. I have a dedicated chest freezer in the garage for freezing, which will be replaced with a DC freezer powered by a single PV panel (of about 100w directly connected, no batt/no inverter). So really the only power considerations will be lights, computers/router/moden, TV/entertainment and stove (I am considering changing to inductive stove as well, was considering gas but in hawaii gas is super expensive as well).

Any help with system design, parts sourcing, and component selection greatly appreciated! I am conversant with most tools and have a full sized lathe and mini mill and TIG and MIG welders and know how to use all quite well. In fact, if any of you need small custom work done, feel free to drop me an inquiry, if its a small job and you pay shipping both ways, no charge for the work :), want to pay you guys back and help out around here.

email: taniw001@hawaii.rr.com

aloha,
walt
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  • AuricTech
    AuricTech Solar Expert Posts: 140 ✭✭
    #2
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    Re: Need help designing new system

    While I'm sure folks with far more knowledge will soon chime in, the first thing that jumps out at me about your planned system is that it has about half the PV capacity that your battery bank needs, so you'll be firing up your genset fairly often.
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  • solar_dave
    solar_dave Solar Expert Posts: 2,397 ✭✭✭✭
    #3
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    Re: Need help designing new system

    The first step in this is to measure your loads you want to replace. Get a Kilo-watt meter and plug the devices in to get a really good estimate for each device. Then you can make a plan.

    Really Grid tie is the most cost effective, but I understand the want to bypass the grid attachment requirements in Hawaii. You will find that off grid power will cost you about $1-$2 a kWh vs your utility $0.35 kWh. Grid tie can be done for for much less and is much more efficient, all the power gets to go someplace. Not the case with off grid, once the batteries are full the electrons just "fall on the floor". The key to off grid is to replace the usage every day and only discharge the battery about 25% so to have a couple days of running during poor solar conditions. THis usually means having more panels than a grid tie system would need. Of course you need to understand the impact of the utility net metering plan with a grid tie system.
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,447 admin
    #4
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    Re: Need help designing new system

    There was a poster here that did a lot of work to go off grid for his fabrication business. "Adas". You might want to read some of his old threads about how he did it in a very cost effective manner.

    http://forum.solar-electric.com/search.php?searchid=765974

    I think you are heading down the road for step one... Conservation. Get your power usage to the minimum needed for your life--And then look at designing an off grid system to support it. It will be time and money well spent.

    Using a chest freezer as a refrigerator--It should work well for you (250 Watt*Hours per day or less than 1/4 that of a standard energy start fridge/freezer), unless your spouse does not like moving racks of food around to get at the daily needs.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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  • AuricTech
    AuricTech Solar Expert Posts: 140 ✭✭
    #5
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    Re: Need help designing new system
    solar_dave wrote: »
    Really Grid tie is the most cost effective, but I understand the want to bypass the grid attachment requirements in Hawaii. You will find that off grid power will cost you about $1-$2 a kWh vs your utility $0.35 kWh. Grid tie can be done for for much less and is much more efficient, all the power gets to go someplace. Not the case with off grid, once the batteries are full the electrons just "fall on the floor". The key to off grid is to replace the usage every day and only discharge the battery about 25% so to have a couple days of running during poor solar conditions. THis usually means having more panels than a grid tie system would need. Of course you need to understand the impact of the utility net metering plan with a grid tie system.

    One way to maximize an off-grid system's overall efficiency is to plan for "opportunity loads" that can be run once the batteries are full. This includes things such as doing the laundry or performing other tasks that are not time-sensitive. Using opportunity loads will minimize the extent to which "electrons just 'fall on the floor'" (an interesting metaphor for any unused power produced by a PV system 8) ), thus maximizing efficiency.
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  • kaipo_boy
    kaipo_boy Solar Expert Posts: 143 ✭✭
    #6
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    Re: Need help designing new system
    AuricTech wrote: »
    While I'm sure folks with far more knowledge will soon chime in, the first thing that jumps out at me about your planned system is that it has about half the PV capacity that your battery bank needs, so you'll be firing up your genset fairly often.

    Thanks for the replies. Yes, I have a lot to learn. But the above is of particular concern... so a single GC2 or T-105 provides enough capacity that will normally require about 1 PV panel of near 250w?
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  • BB.
    BB. Super Moderators, Administrators Posts: 33,447 admin
    #7
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    Re: Need help designing new system

    We really need actual numbers here... A typical "golf cart" battery is usually 6 volts and 220 AH--So you need 2x for 12 volt, 4x for 24 volt, etc. battery bank.

    For a small but capable system, lets look at 4x 6 volt @ 220 AH batteries. These can be arranged in 2x2 for 12 volts @ 440 AH or 1x4 for 24 volts at 220 AH (amount of stored energy is the same).

    Say 24 volt system (if you have a larger off grid power system, 24 or 48 volt systems make the most sense, unless you have a lot of "native" 12 VDC loads).

    First, we figure out the amount of solar panels needed for 5% to 13% rate of charge... For seasonal/weekend use, 5% rate of charge can work OK. For full time use, then really need 10-13%+ rate of charge (makes for less "battery charging/load management" issues):
    • 220 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.05 rate of charge = 414 Watt minimum array
    • 220 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.10 rate of charge = 829 Watt array nominal
    • 220 AH * 29 volts charging * 1/0.77 panel+controller derating * 0.13 rate of charge = 1,077 Watt "cost effective" maximum
    Then there is sizing the array based on the amount of sun you get... First, how much power will a 24 volt @ 220 AH battery bank supply. Assuming 2 days of storage and 50% maximum discharge (a good off grid home sizing):
    • 220 AH * 24 volts * 0.85 inverter eff * 1/2 days storage * 0.50 maximum discharge = 1,122 WH per day (1.1 kWH per day)
    And to recharge that amount of power based on PV Watts, fixed array, for Honolulu, tilted to 21 degrees from horizontal:
    Month    Solar Radiation (kWh/m 2/day)
1      5.01     
2      5.59     
3      5.89     
4      5.79     
5      6.00     
6      5.91     
7      6.02     
8      6.25     
9      6.41     
10      5.72     
11      5.16     
12      4.92     
Year      5.72

    Say we pick 4.92 hours minimum (long term average) of sun per day:
    • 1,122 WH per day load * 1/0.52 system efficiency * 1/4.92 hours of sun per day = 439 Watt array minimum based on loads
    If you uses the 10% minimum rate of charge sizing of the array:
    • 829 Watt array * 0.52 system derating * 4.92 hours of sun = 2,120 Watt*Hours of AC power per day (December)
    Use 1kWH during the day and 1kWH at night/cloudy weather.

    Note, at this point, it is all math... If you decide you need 3.3 kWH per day of power, multiply the above numbers by 3x -- etc.

    Once you know your planned loads and how you want to use the system, you get the sizing. Then, you can start picking specific configurations and hardware.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
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