Automation - OG inverters that resume without interaction after LVD/excessive loads/etc?

Re: Automation - OG inverters that resume without interaction after LVD/excessive loads/

"Fully automating" an off grid system is difficult and expensive--And sometimes of dubious value.

Many times, systems that are less complex will run more reliably and require less maintenance/easier to repair.

If you are looking for a system that will not destroy the battery bank if the sun goes dark--I have a few suggestions to think about.

One is to look at a battery monitor with remote alarm contacts. Victron makes a nice one. Set the alarm to turn on at 50% state of charge and turn off at 80% state of charge--And the battery bank will be pretty happy. Note that battery monitors only estimate the state of charge of the battery bank (by measuring the amp*hours into/out of the battery bank)--Which can drift over time. They also will reset back to 100% if the charging voltage is held high for over X hours of time. Us the battery monitor to turn off/off the AC inverters (or loads).

Lead acid battery have a complex relationship with state of charge and voltage/current/charging/discharging/age/time/temperature/etc... Just monitoring the battery bank voltage can lead you astray.

If the site is occupied but the people there are not technically inclined (parents living at cabin)--You probably can stop with a battery monitor and have them call when something goes wrong (i.e., call you/start the genset when the battery bank goes below 50% state of charge, turn off when the battery bank goes below 20% state of charge, etc.). You could even use a remote link (cell phone/Internet) to monitor your system state and go out when needed to bring back up (i.e. if something "breaks" like a solar array cable is chewed through by a squirrel, are you OK with the system being down for a month or two?).

Something else to think about--LiFePO4 batteries are much simpler to set min/max charging/discharging voltages. No maintenance, no worry about sitting around at 20% state of charge, etc... If you can have interruptions, a smaller LiFePO4 bank (to keep battery costs down) might be very nice (you have power when there is enough sun, you don't when the sun is blocked by clouds for a day, etc.). You just set a minimum bank voltage to cut off your loads and you are done.

-Bill

Re: Automation - OG inverters that resume without interaction after LVD/excessive loads/

It is difficult to make a system that will power from 100 to 6,000 watts that is very cost effective...

Perhaps, you can build out a first system with a pretty rugged 300 Watt 12 VDC TSW Morningstar inverter... Has "search mode" (lower power standby) and a remote on/off signal (you can connect your system save function here--And/or let people turn off inverter if not needed). Will power lighting, small electronics, cell phone chargers, etc. very nicely.

Throw on 2-4 golf cart batteries+solar+backup AC battery charger (if needed), possibly a battery monitor--And you will have a cheap and pretty rugged system. Change to AGM batteries if you don't want to have to check/add water (perhaps after the first set of "cheap" batteries are "murdered"). Would give you some real operational information on what is needed (bigger system needed? Will people really be able to adjust their usage to available sun, will cost of system be "worth it" to the users, etc.).

For poor weather--Look at a Honda eu1000i or eu2000i (900 to 1,600 watt) if you think they can manage pull start and the whole generator maintenance issues (you can even get propane conversions or tri-fuel generators--Propane runs cleaner (less oil changes?) and could be safer vs gasoline (bring 100 lb tanks, or if there is a large propane tank for cooking/heating).

And get a cheap (and noisy) 3.5 kWatt or even 7 kWatt Champion Genset from Costco (currently selling gas/propane model for ~$800 or so) for when they want to run 6 kWatt of tools at the same time.

Note that small gensets can run 4-10 hours or so with smaller loads (1,600 to 400 watt or less). However a "big" 7kWatt will run only ~1-2 hours on a gallon of fuel--So there is that whole issue (fuel storage, handling, who pays, oil changes, etc.).

Put everything on a small trailer--You can bring home/move to where needed and do the maintenance locally (if solar panels--The array may have to be fixed--portable arrays are difficult to justify--size/how rugged/etc.). Of course, anything on wheels may be "too mobile".

Do you have a good idea of what the "loads" look like (daily use peak watts, Watt*Hours per day, etc.)? I would really suggest that this be done as a split low power off grid solar and plan on genset for any substantial power usage for a first try at off grid solar for non-technical folks. If they mess up the solar--You are only talking about a few hundred dollars of batteries and a simple to repair system. Throw in a simple automatic transfer switch (AC inverter and AC generator to low power "permanent loads", plus wire up a simple Iota 20-45 amp 12 VDC battery charger)--There would be very little for them to do--Monitor Battery Monitor (check battery water levels, hydrometer if they are interested), and tell them to run the genset for heavy loads or if battery AH capacity falls below 50% (or give them some sort of voltage level to watch for).

The MorningStar 300 Watt inverter can be set for 11.5 volt disconnect (better for batteries than 10.5 volt typical disconnect) and has auto reconnect (for low and high voltage disconnects).

And once you have a year or two (or seasons) of usage--Everyone will probably have a better idea of what is needed to run the site. And they will not have spent too much money learning.

-Bill

Re: Automation - OG inverters that resume without interaction after LVD/excessive loads/

treed678 wrote: »

I'm always open to things I haven't thought of or mentioned. Unfortunately the customer is requiring off grid solar only, so generators can't be part of the equation.

You are there and know the area and your customers better than I... However, if you can educate your customer that you probably have a 3-7 days of bad weather every so often during the winter. More or less, with lead acid batteries about 1-3 days with 50% maximum discharge is practical range of storage, and ~2 days of storage with 50% discharge seems to be pretty cost effective (balanced system design; battery ~4x daily loads).

I anticipate 75%+ laptops and phones. One requirement is 6 - 120VAC receptacles, so that's approximately 200w (laptop) X 6 = ~1,200w ...*typical* worst case scenario (I always assume it will be worse at some point).

Laptops--200 watts is pretty large power usage... You might average ~20-30 watts if the battery is full (i.e., running on AC power), and upwards of ~60-100 watts when recharging+running.

Conservation being our friend here... Either they want to run their systems (whatever they have), or they may want to look at getting something that will consume as little power as possible. Perhaps, if they have AC power there (or at home), you can give them a Kill-a-Watt type meter and tell them to plug the laptop(s) into the meter and run it for 24 hours (or more) and report back to you, the peak watts, kWH used, and hours of run-time on the meter. That will give you a lot more accurate estimate of power needs.

200 Watts * 10 hours per day = 2,000 WH per day
20 Watts * 5 hours per day = 100 WH per day

Either estimate is possible--But without details from your customer, you either oversize the system (and over price it), or undersize and they come back a month or so later complaining it never works.

I do think use will follow fairly closely weather and mostly day time hours (being outdoor), which will work well here with solar. In other words, more use when solar production is good ...very little in cold, wet winter weather (hopefully?). [Just pictured Chevy Chase in "Christmas Vacation." What have I gotten myself into?]

PV Watts for Huntsville AL, 35 degrees from horizontal (fixed array):

Month    Solar Radiation (kWh/m 2/day)
1      3.51     
2      4.40     
3      4.86     
4      5.57     
5      5.51     
6      5.74     
7      5.83     
8      5.84     
9      5.34     
10      5.46     
11      3.98     
12      3.27     
Year      4.94      

PV Watts has an "hour by hour" output based ~20 years of history--So you can do some looking around what a typical good day in winter vs a bad day in winter will do for you.

For example, a 1,000 Watt array and 52% efficient off grid system "bad" vs "good" December days:

1981, 12, 21, 08:00, 0
1981, 12, 21, 09:00, 1
1981, 12, 21, 10:00, 13
1981, 12, 21, 11:00, 52
1981, 12, 21, 12:00, 54
1981, 12, 21, 13:00, 50
1981, 12, 21, 14:00, 38
1981, 12, 21, 15:00, 12
1981, 12, 21, 16:00, 2
1981, 12, 21, 17:00, 0
...
1981, 12, 25, 07:00, 0
1981, 12, 25, 08:00, 4
1981, 12, 25, 09:00, 150
1981, 12, 25, 10:00, 347
1981, 12, 25, 11:00, 395
1981, 12, 25, 12:00, 460
1981, 12, 25, 13:00, 415
1981, 12, 25, 14:00, 416
1981, 12, 25, 15:00, 322
1981, 12, 25, 16:00, 90
1981, 12, 25, 17:00, 6
1981, 12, 25, 18:00, 0

You can look through the data in a spread sheet program (add up the WH per daylight period per day) and you can give them an "estimate" if what bad weather would do to their available power.

I think I'm going to push for more PV and take the chance with "murdering" a small battery bank. The panels can last "forever," basically, but the batteries will not, regardless. Maybe I can build the enclosure large enough to double or triple storage if necessary, and pitch the first bank to them as a "trial set." I don't want to skimp on the inverter or protection because that's a potential bottleneck from day 1 and likely subject to abuse. I'm more concerned that it can take some punishment, and that I can protect it, vs efficiency. Search mode to conserve energy would be nice though. Fairly certain I'll go with a remote at this point.

Remote vs Search Mode--You should take into account the inverter's tare losses (idle power usage)--A small inverter may take 6 watts 24x7, a larger inverter may take 20-40+ watts... For small systems, the tare loss can be significant--And I would suggest the search/auto off usage (or even a "timer" of some sort)--Manual switches tend to be forgotten and can kill the battery bank if system is left on but no loads. Larger systems, the tare losses are usually lost in the overall power usage.

Are there 1kw+ inverters that are sealed or designed to work in wide temp & humidity environments, below $750? $500 or less would be even better. I hope that isn't asking too much. I'd look at Outback specialty inverters, but I know that's $1,000+ right out the gate, and AC charging won't be utilized.

You could always install a ventilated inverter in a larger weather proof box (then you have to look at heat exchange rates, possible addition of interior/exterior heat slinks, etc.). Then you could use an Exeltech, Samlex, etc. inverter. Samlex has a few remote inverter controls for some inverter series.

-Bill

Re: Automation - OG inverters that resume without interaction after LVD/excessive loads/

LiFePO4 would probably be of interest if there is a "no generator" rule... Lead acid batteries sitting around at less than 75% state of charge really start to sulfate faster (the lower the charge, faster the sulfation--As I understand).

If you can run the LiFePO4 batteries between 20% and 80% state of charge--They can work real well for this application. If if they sit for 10 days at 20% SOC--No problem with early life failures.

The reason I do not like to put too much "fudge factors" in to over size the system is that it is very costly to do this (larger batteries, more solar panels needed, more batteries to replace when they eventually fail).

And while it is not as expensive to over-size the inverters (higher tare losses)--The over-sizing of the battery bank is too be avoided. It just is not very cost effective.

Also, when you start with the 100-150-200 watt dance--It still does not really help size the battery bank (other than to supply average power / maximum surge current ratings). A typical laptop may use around 400 Watt*Hours per day... But that usage is highly variable. If these folks are using fast computers all the time with back lighting on full, it could get upwards of 1,000 kWH per day. If they are just running smaller laptops to log data at the end of the day--You could be looking at 50 WH per day. That is a 20:1 ratio of power usage. The chances you will hit their sweet spot is pretty remote. And that wide of energy range will really kill you if the LiFePO4 battery is too small or too large for their needs.

What are they doing for power now? Can they simulate power at home/office? Or could they run a Honda eu2000i for a week with a Kill-a-Watt meter to measure their actual loads--Then you put together a system in X working days for their installation and use?

I still do not see how they can use a relatively large off grid power system and both need a good size amount of power during sunny days, but be willing to "shut down" for 10 days of bad weather. Most people tend to simply plug in and use the batteries until dead, and when there is a little bit of sun, they are right back there plugging in to get power--With Lead Acid batteries, not a good plan.

If they are willing to watch the weather forecast and if there is bad weather predicted, they are willing to shut down the power system when it gets to 75% SOC and simply wait until sunny weather returns--It can work (I am pretty pessimistic about human nature).

A smaller LiFePO4 battery bank (call it 60% or so the size of the lead acid AH capacity for similar system) would give them a day or so of backup power, and can sit discharged until the sun returns (assuming you have a good LVD signal/control for the inverter). It would seem to be more rugged and probably cost effective. I would at least look at the Li battery bank as an option. They could always add a genset of some type if bad weather power outages turn out to be not acceptable (i.e., leave room for a battery charger and AC power connection in your system--Or even a smaller Outback Inverter-Charger with generator support).

-Bill

Re: Automation - OG inverters that resume without interaction after LVD/excessive loads/

From what I have read (mostly here)--If they are cycling between 50-80% state of charge--That is much better than just sitting for days/weeks at 50% SOC.

In fact, running your batteries between 50-80% SOC with a charge back to >90% SOC once a week is supposed to be just fine or even "good" for the battery bank (different vendors will tell you different things, and even the vendor changes their recommendations over time). Lead Acid batteries are more efficient charging in this range (less gassing and less heat). And spend less time charging at higher SOC/Gassing/Temperatures.

-Bill

Re: Automation - OG inverters that resume without interaction after LVD/excessive loads/

I really cannot answer that question--I don't know. But if you are "bouncing around" that area (charging / discharging) during that time--I guess you are OK. But, I do not know.

It may also depend on the battery design/mfg. too. Different plate materials can have different reactions to charging/discharging cycles.

-Bill