Looking for feedback on my off-grid system design

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cdre
cdre Solar Expert Posts: 78 ✭✭
So I’m designing a whole-house system for my off-grid home in Belize. I’ll be working out of a home office, so IT stuff (satellite, network, computers) are a big power consumer. I’ve already got a smaller system on a parked trailer, and it works great, so I have a little experience at this point.

I’m looking at around 6.5 kwh daily consumption with refrigerator taking around 1.2kwh, IT stuff taking around 2.8kwh and the rest broken up between lighting, fans, water pump, ect.

I’d like to minimize generator run time, so I’m thinking I’d need around 3kw of panels assuming 4 hours of full sun at 50% total system efficiency. Panels won't be shaded and will be oriented south, angled at 19 degrees (Belize latitude). I like the Morningstar MPPT controller since it has an Ethernet connection for gathering data.

For charger/inverter, I haven’t totally decided, but I’m leaning toward an Outback FX2524T. Looking at running a 24v battery system. For batteries, considering 16 T-105s (or equivalents).

Thinking of going with a 6.5kw propane generator on an auto-start switch for when loads get heavy or cloudy day’s cause low PV generation…

Any suggestions/comments? Thanks!

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  • cdre
    cdre Solar Expert Posts: 78 ✭✭
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    Re: Looking for feedback on my off-grid system design

    Also, correct me if I'm wrong, but I should be able to get away with just one Morningstar 60 amp charge controller if I run my panels at 48v... Any disadvantage to higher voltages? The cable from panels to charge controller will be around 40' long.
  • stephendv
    stephendv Solar Expert Posts: 1,571 ✭✭
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    Re: Looking for feedback on my off-grid system design
    cdre wrote: »
    Also, correct me if I'm wrong, but I should be able to get away with just one Morningstar 60 amp charge controller if I run my panels at 48v... Any disadvantage to higher voltages? The cable from panels to charge controller will be around 40' long.

    The limiting factor will be the battery bank voltage. If you choose a 24V battery bank, then you're limited to 1.4kW of PV through a 60A charge controller. If you're not going to run anything on 24V DC, then why not go for a 48V system?

    A 2kW inverter charger like the outback could be a bit too small for a whole house system, think about starting surges on motor loads (fridge, vacuum cleaner etc). Recommend 3kW at least, if not 4.

    The loads for "IT stuff" seem a bit high too, you could try to economise by:
    - Using LED panels instead of LCD backlights
    - Use low power cpu's for all server type tasks, there are a bunch of atom based systems available at the moment.
    - consolidate all the "always on" functionality into a single low power system, instead of many bits and pieces
  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
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    Re: Looking for feedback on my off-grid system design

    Unlike so many, you are at least starting out with a reasonable expectation of size and loads.

    My first thought would be consider EVERY conservation mode you can think of, and then think of a few more. Reducing your loads 25% for example reduce your system costs a whole bunch.

    If you are fixed with the ~2.8 kw for the IT stuff, the place to begin to look is the household stuff. (assuming you can't move toward more efficient IT stuff).

    ~3.7 kw is a fairly big draw for a whole house in some respects. I would look at the fridge,, 1.2 kwh is a fairly large draw for a modern fridge might draw as little as .5 kwh/day, but in your case the warm climate might make that draw a bit more. You might also consider a Propane, or indeed a kerosene fridge. Propane might be very expensive as a fuel, but in your part of the world, kerosene might be relatively common. Dometic/Servell still make Kerosene fridges that burn very little kerosene, they burn clean and perform well. Depending on size and loading, a Lp fridge will burn ~ 1500 btu/hour, or ~ 1/4 of a gallon per day, a kerosene unit will use ~ 1 litre of kerosene per day.

    Another option would be, if you are going to have to run a generator on some regular basis, consider adding water tank capacity, and use the time when the genny is running to charge batteries to pump water at the same time. Also look to maximize the ef of your lighting and fans, as well as the rest of your loading.

    Just for comparison, we live off grid, we use .6kwh/day (pretty frugal!) and we do that with 4 T-105s (could do it with 2) and 400 watts of panels. This is a pretty good balance between Pv capability/battery capacity/loads. We fully charge each day, and have ~3 days of reserve. You are going to suffer (by comparison) because both your battery and PV temps are going to be higher than mine.

    So in your case, if you multiply our system by ~10, 4 kw of PV maybe 20 T-105s (or the equivalent of L-16s or something) you should be getting close.

    Good luck, welcome to the forum,

    Tony

    PS you might find these interesting:
    http://gasrefrigerator.net/gas_refrigerator_frostek.htm

    http://www.thenaturalhome.com/gasappliances.htm
  • cdre
    cdre Solar Expert Posts: 78 ✭✭
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    Re: Looking for feedback on my off-grid system design

    Thanks for the feedback!

    On the size of the inverter, I'll have few "electric motor" type surge loads, so for now I'm going to start with a smaller unit. If I need to in the future, I can always add another inverter.

    On battery bank voltage, I was originally thinking of going with a 24v water pump to minimize conversion loss, but given the relatively low consumption of the pump, I don't think it's worth stressing. I'll not have a well pump, rather I'll be harvesting rainwater for everyday tasks and buying drinking water. Two strings of 8 T-105s in 48v might be the ticket.

    For the refigerator, I'm calculating based on an energy star GE unit with estimated annual consumption of 463kwh -- comes out to 1.26kwh daily.

    On the IT equipment, I'm looking at the following:
    Sat Modem, router, hard drive - 60w x 24hr = 1.44kwh
    Laptop - 50w x 10hr = .5kwh
    Desktop - 200w x 4hr = .8kwh

    I'll try to minimize my IT draw as much as possible with energy efficient monitors and using the laptop when the desktop is overkill.

    I'm looking to go electric with the fridge, as it is a permanant residence (not just a couple weeks a year) and fuel costs in Belize are absurd as it is. Currently I run a propane dometic fridge on butane and put around $20 a month into it. That's a relatively small RV unit. For the cost of a standalone propane or kerosene unit, I'm willing to bite the bullet on buying some extra panels for electric. If you know of any units that are much more efficient, I'm definitely interested.

    That said, assuming my consumption is approximately correct, do my proportions between load, panels, and batteries look about right?
  • icarus
    icarus Solar Expert Posts: 5,436 ✭✭✭✭
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    Re: Looking for feedback on my off-grid system design

    Once again, at a quick glance, I think the batteries are about right. You would have a couple of days reserve. I think you are light on the PV, but if you are prepared to run the gennie on some regular basis you probably will be ok. (That said, I would certainly invest in a good battery monitor like the Bogart Tri-metric.

    At 48 vdc your PV might charge ~ 50 amps If my math is close, the batteries (48 vdc) would have an ah capacity of ~450ah, so you are in the 10% range which is ideal.

    By my rule of thumb, 3000 watts of Pv might deliver, on average:

    3000/2*4=6kwh/day

    Bottom line is, if you can drop your loads a few hundred wh/day you are going to be getting pretty close.

    Good luck and keep in touch.

    Tony
  • BB.
    BB. Super Moderators, Administrators Posts: 33,479 admin
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    Re: Looking for feedback on my off-grid system design

    A cheap and dirty refrigerator is to convert a chest freezer into a refrigerator... You may get usage down below 0.3kW hours per day for such a setup (with a properly configured inverter--search mode enabled to cut standby losses).

    The big issue is finding the right refrigerator thermostat and how you control the AC/DC power (inverters running without load waste a lot of power).

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • audredger
    audredger Solar Expert Posts: 272 ✭✭
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    Re: Looking for feedback on my off-grid system design

    I have an off grid house, we use ~6Kwh a day with 3150 watts of PV and a 4000 watt 24 volt inverter. I'm not sure that 8 T-105's will be enough. I use 24 L-16H's.
    I too would recommend a 48 volt system and say a 3648VFX inverter. by my calculations; 3000 watts @ 48 volts is 62.5 amps. I think if you want the morning star controller then you need two or, one Flexmax 80.

    As a point of consideration; you winter sun should be better than mine in Northern California and you may not need all 3000 watts.
  • AntronX
    AntronX Solar Expert Posts: 462 ✭✭
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    Re: Looking for feedback on my off-grid system design
    cdre wrote: »
    For the refigerator, I'm calculating based on an energy star GE unit with estimated annual consumption of 463kwh -- comes out to 1.26kwh daily.

    This guy's fridge uses only 37 KWh per year: http://mtbest.net/chest_fridge.html
    Read details on his .pdf file: http://mtbest.net/chest_fridge_1.pdf
    cdre wrote: »
    On the IT equipment, I'm looking at the following:
    Sat Modem, router, hard drive - 60w x 24hr = 1.44kwh
    Laptop - 50w x 10hr = .5kwh
    Desktop - 200w x 4hr = .8kwh

    Replace your IT equipment's power adapters with DC-DC converters connected directly to your battery bank. This way you can avoid having to use inverter, and energy losses that would come with it. There is absolutely no reason why you should be using AC current to power your computer gear. It only makes sense if you are on the grid. The energy savings would be immense. Your laptop, desktop, sat modem, router and hard drive all can be powered by DC. The only thing that may be difficult to get working on DC would be your LCD monitor, since most use built in PSUs. For those small loads that must run on AC, get a small inverter, with maximum power rated no more than double of your average load. For example, I use 600W Exceltech inverter that is running very efficiently at low power between 75 - 300 Watts. And it only uses 6 Watts when idle. Look at Outback VFX inverter's idle power load, and it's efficiency at very low power. It will not be good. That's where VFX sized inverter will waste a lot of your energy. You will still need large inverter for large loads, but you can keep it in stand by mode, or turn in on manually when needed.
    cdre wrote: »
    I'll try to minimize my IT draw as much as possible with energy efficient monitors and using the laptop when the desktop is overkill.

    If you already have LCD monitor, it is already as efficient as it gets at this point. Don't bother. You can measure it's power use to be absolutely sure. It should not be more than 25 - 30 Watts.
  • stephendv
    stephendv Solar Expert Posts: 1,571 ✭✭
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    Re: Looking for feedback on my off-grid system design
    AntronX wrote: »
    Replace your IT equipment's power adapters with DC-DC converters connected directly to your battery bank. This way you can avoid having to use inverter, and energy losses that would come with it. There is absolutely no reason why you should be using AC current to power your computer gear. It only makes sense if you are on the grid. The energy savings would be immense.

    I don't agree with this, the losses in a DC-DC converter are not inconsequential. In fact, most of them that I looked at had an efficiency of between 82 - 92%, which is worse than most inverters. Granted if you go DC-DC you also save in the power supply conversion, but we're not talking big numbers here. IMO an inverter and a switch mode power supply for DC appliances would come very close to the efficiencies of a DC-DC setup, and is much easier to work with.
    AntronX wrote: »
    If you already have LCD monitor, it is already as efficient as it gets at this point. Don't bother. You can measure it's power use to be absolutely sure.

    Got to take you up on this one too ;) Some LCD panels now come with LED backlighting which is 40% more efficient than stardard LCD panels, have a look at:
    http://www.lg.com/uk/it-products/monitors/LG-led-monitor-W2486L.jsp
  • cdre
    cdre Solar Expert Posts: 78 ✭✭
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    Re: Looking for feedback on my off-grid system design

    I took a quick look at the chest fridge, and it looks like a great idea.

    With IT being such a heavy load, I'm guessing a DC to DC converter would definitely make sense in terms of saving some of the conversion loss. Great idea.

    I may shift over to an FM80 and use one of the software packages to track charge/load/batt status data. The Flexpower 1-2 at 48v seems like a pretty good all around solution... Any other system recomendations?
  • stephendv
    stephendv Solar Expert Posts: 1,571 ✭✭
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    Re: Looking for feedback on my off-grid system design
    audredger wrote: »
    by my calculations; 3000 watts @ 48 volts is 62.5 amps. I think if you want the morning star controller then you need two or, one Flexmax 80.

    A 60A charger would be fine for 3000Wp, bare in mind that you would always charge at a higher V than 48V and that after you've derated the 3000Wp array, you won't be getting 3000W from it anyway.
  • BB.
    BB. Super Moderators, Administrators Posts: 33,479 admin
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    Re: Looking for feedback on my off-grid system design

    Working with DC for larger installations has its own issues... Getting DC compatible supplies (tend to be more expensive) and what voltage do you standardize on?

    12 volts is difficult to send more than a handful of feet from the battery bank--and the currents are pretty high... Just for a 400 watt power supply:
    • 400 watts * 1/10.5 volts min * 1/0.80 eff * 1.25 NEC safety rating = 60 amp circuit/fuse/breaker rating
    Sending power around the house/cabin is much easier at 120 VAC vs 12 DC...

    48 VDC equipment is a possibility (very common in telecommuncations)--but still probably not as cheap and easy as 120 VAC to find supplies and wire up.

    I tend to be in the camp of let the 120 VAC inverter manage your power conversion and just use efficient AC appliances... The price of the inverter will probably be a wash or even a savings by the time you use 120 VAC common equipment/wiring/breaker boxes/etc. everywhere.

    Plus a lot of 12 VDC equipment is intended for automotive use... They are designed to operate from 12 to 14.5 volts or so... Some of these car adapters have problems running over 10.5-15.5 VDC that a deep cycle battery system can operate under.

    -Bill
    Near San Francisco California: 3.5kWatt Grid Tied Solar power system+small backup genset
  • AntronX
    AntronX Solar Expert Posts: 462 ✭✭
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    Re: Looking for feedback on my off-grid system design

    Bill... you are such a pessimist, LOL :)

    Each device will use it's own small dc-dc converter in place of 120v ac psu. 48V dc will run to the point of use, not whatever dc-dc converter outputs. What I am trying to avoid is 48V dc > 120V ac > xxV dc conversion losses. Biggest one will be 48V to 120V when the OP is only using a laptop with sat modem. Overtime, the energy wasted to keep VFX inverter running (while consuming only 50 Watts ac on the output) will be massive.

    Oh, and small and 90 - 95% efficient dc-dc buck converters can be had for $30 - $50 at places like Mouser.
  • AntronX
    AntronX Solar Expert Posts: 462 ✭✭
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    Re: Looking for feedback on my off-grid system design
    stephendv wrote: »
    I don't agree with this, the losses in a DC-DC converter are not inconsequential. In fact, most of them that I looked at had an efficiency of between 82 - 92%, which is worse than most inverters. Granted if you go DC-DC you also save in the power supply conversion, but we're not talking big numbers here. IMO an inverter and a switch mode power supply for DC appliances would come very close to the efficiencies of a DC-DC setup, and is much easier to work with.

    Your assumption is inaccurate at few points.

    First, dc-dc converter power output rating needs to be sized where it's peak efficiency "sweet spot" is exactly what each IT device draws. Example: My DSL modem pulls 1A at 12V dc. The buck dc-dc converter rated 2A max continuous power output has 90 - 95 % peak efficiency at 1A load. Look at bare board converters that mouser.com sells. There are bunch of them that are pretty efficient and small.

    Second, you assume that inverter will run at it's advertised efficiency at any load. Not true. The conversion efficiency can drop to about 20% at very low load levels.

    Then another benefit is not having a cheapo wall-wart psu wasting another 20 - 30% of energy. The benefits add up, and can allow someone to greatly reduce size of PV array requirement.
  • cdre
    cdre Solar Expert Posts: 78 ✭✭
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    Re: Looking for feedback on my off-grid system design

    Came across some 48v ATX PC power supplies as well.. a little pricey at 200, but an option...

    http://www.powerstream.com/DC_PC.htm
  • AntronX
    AntronX Solar Expert Posts: 462 ✭✭
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    Re: Looking for feedback on my off-grid system design
    cdre wrote: »
    Came across some 48v ATX PC power supplies as well.. a little pricey at 200, but an option...

    http://www.powerstream.com/DC_PC.htm

    I use 24V version on my desktop. It pulls around 2 watts on standby. I have it hardwired to the battery bank. It's like having a virtual UPS system. I have 2.5 Ghz AMD Dual core CPU with mini ATX board with built in DVI graphics and 500 GB WD Black drive. It only uses 55 Watts DC on idle. Plays 720p mp4 video flawlessly, and 1080p playback if fine as long as the file is mpeg2 or something light. I opted for built in graphics because PCI video cards are power hungry. My Nvidia 7900 pulls 45 Watts on idle.
  • chevenstein
    chevenstein Solar Expert Posts: 100 ✭✭
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    Re: Looking for feedback on my off-grid system design

    Do the IT loads really have to be on around the clock? I used to keep a network of several servers and other equipment on 24/7 when I was on grid but now I run only a laptop and 20" LCD display when I'm actually working and have offloaded all of my 24/7 infrastructure needs to inexpensive virtual machines hosted somewhere else. Granted, I no longer maintain a file server for home entertainment purposes (which did need to be on 24/7 to be useful) but all of my systems involved in consulting were easily collapsed and outsourced for a fraction of my old commercial cable connection and power charges. Plus I don't have to worry about half a rack of mission critical gear in my laundry room needing repairs, periodic upgrades, or possibly suffering catastrophic loss due to the house flooding or catching fire.
  • cdre
    cdre Solar Expert Posts: 78 ✭✭
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    Re: Looking for feedback on my off-grid system design

    I think I can pare back my use quite a bit... I've worked with a netbook and 2nd monitor before, and it works great for everyday tasks while using minimal power. Need sat modem, router, and VOIP phone on for at least 18 hrs a day for business. Estimated draw for those 3 is 30w. I think I can pare back to around 1.3kwh on IT, bringing me just under 5kwh daily consumption...

    That said, if I bring my array down to 2.5kw and stick with 16 T-105's at 48v, is my system still relatively well balanced?
  • stephendv
    stephendv Solar Expert Posts: 1,571 ✭✭
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    Re: Looking for feedback on my off-grid system design
    AntronX wrote: »
    Your assumption is inaccurate at few points.
    The benefits add up, and can allow someone to greatly reduce size of PV array requirement.

    Antronx, good points, I didn't consider those. Still, I think there's a point where 120VAC everywhere is a more convenient and simpler choice - if less efficient - , particularly for whole house systems.