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Started by woodweasel, October 20, 2013, 10:43:56 AM

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maple flats

Until a year ago, I was off grid at my sugarhouse (maple syrup). I had 1480 watts of panels, an FX 80 outback charge controller (CC) and a 240 AH 48V battery bank (too small but will not upgrade until that bank needs replacing) and a Xantrex 6048 inverter. It was OK for my needs but we wanted more. We went grid tied and net metered. We added 4840 more watts solar but this was grid tied only. However, we can charge the batteries if needed with the grid or a separate generator. In reality I only have 1 circuit on the battery back-up, it runs 300 watts of heat in the heavily insulated Reverse Osmosis room. I also have lights that can be switched from one source to the other if necessary. When my 240 AH 48V Trojan battery bank needs replacing I will be going to Rolls Surette batteries and likely will triple the AH.
I like having the solar and would do it again if needed. I did have to make one change when I went grid tied, I had to replace the Outback CC with a Xantrex CC, they had to be the same brand to talk to each other so I could maintain full charge on the batteries and still sell excess to the grid thru my net meter.
logging small time for years but just learning how,  2012 36 HP Mahindra tractor, 3point log arch, 8000# class excavator, lifts 2500# and sets logs on mill precisely where needed, Woodland Mills HM130Max , maple syrup a hobby that consumes my time. looking to learn blacksmithing.

Thehardway

Woodenhead, that's a nice setup you have.  I have a couple questions. 

1.  Why did you not use DC power for your lighting needs?  Cost, convenience, etc.  Since you have PV/batteries, it should have been more efficient than running through inverter.  Especially with LED lighting now available.

2.  What is your average monthly power production in Kw/h over the course of a year?

3.  What is your estimated total investment/installation cost in alternative electricity (wind/solar/batteries/inverters/backup generators/wiring)?

4. What is your average annual cost for propane?

5. How many occupants living in your home?

6. How do you pump your water and why not use water pump as your dump load?

7. if you get 3 or 4 rainy or snowy days with little sun do you have to resort to generator?


I understand the desire for AC and it makes sense in Ontario as you likely only need it in the middle/end of the summer when you have surplus PV power anyway.  In FL, not so much.  Have you considered tracking arrays since your arrays are ground mount?  Might be an easy way to get an additional 15-20% out of your arrays with less investment.
Norwood LM2000 24HP w/28' bed, Hudson Oscar 18" 32' bed, Woodmaster 718 planer,  Kubota L185D, Stihl 029, Husqvarna 550XP

John Mc

Tracking arrays generally do not make economic sense, at least not in my part of the world.  Back in the days where the PV modules were much more expensive, it made sense to do what you can to get the most out of them. At today's prices, however, it's generally cheaper to just add more modules if you need more power. (Not to mention that a tracker adds moving parts to a system that otherwise has none. This adds maintenance and repair hassles).

I don't know if they make sense in other parts of the country where they get more sun, and it makes a wider swing across the sky, but here in VT, every analysis we've done has shown that the end user is better off with non-tracking arrays.
If the only tool you have is a hammer, you tend to see every problem as a nail.   - Abraham Maslow

Thehardway

John Mc,

I agree, in most cases it is cheaper to just add more panels.  There are a few cases where I can see tracker making sense.  (Ie. extrememly limited space and panels)

Was just thinking it would not take that much to put them on a home brew turret style track with some rollers. Additional moving parts/maintenance are a pain though. If you are home all day you can do a manual rotation every couple hours.  Have seen some old c-band satellite dish mounts recycled as low cost tracking mounts as well. Can usually be had for free if you take them down.
Norwood LM2000 24HP w/28' bed, Hudson Oscar 18" 32' bed, Woodmaster 718 planer,  Kubota L185D, Stihl 029, Husqvarna 550XP

John Mc

Thehardway -

There are places where trackers can make sense.  A tracker can take up less space than the equivalent output of fixed panels, if you are only putting up one.  There is a limit to how many panels you can put on a single pole mount tracker.  If you are doing multiple trackers, the spacing needed to avoid having one tracker shade the next one ends up taking up more space than just putting in the equivalent output fixed array.

We've also seen some rather inflated/misleading figures claimed for how much more output you can get from a tracker vs the same modules mounted in a fixed array. The sales folks and literature from one well-known company in our area claims that their trackers will put out "up to 45% more output.  When the typical homeowner hears that, they assume that's what there results will be over the course of a typical year.  In our area, it's closer to about 30% extra output, and that's only if you have wide open solar exposure. Given the mountains and the forests of Vermont, that wide-open exposure is not a given for a lot of installations.

In other parts of the country, a tracker may beat 30%, but generally, how they get away with the claim of "up to 45%" is by looking at daily output - on the summer solstice (the longest day of the year, when the sun makes the biggest arc across the sky), they may get 45% - again subject to geographic location and specific site conditions.
If the only tool you have is a hammer, you tend to see every problem as a nail.   - Abraham Maslow

maple flats

My system produced just over 6000 KWH in it's first year (6.32 KW system) in central NYS. The first winter was a little low because I had some hardwood trees partially shading the array in late Nov-late Jan. Those trees have since gone to either the sawmill or the firewood pile and the array now gets full sun until about 4:00pm in the winter. Cold weather, bright sun and snow on the ground makes for some high outputs from the array.
logging small time for years but just learning how,  2012 36 HP Mahindra tractor, 3point log arch, 8000# class excavator, lifts 2500# and sets logs on mill precisely where needed, Woodland Mills HM130Max , maple syrup a hobby that consumes my time. looking to learn blacksmithing.

Thehardway

John Mc,

Yes, I've had your opinion confirmed elsewhere as well that tracker improvements are over hyped.  I use the conservative estimate of 15-20%.  Based on this figure, if you had a 5 panel array, it would be the equivalent of adding 1 panel or roughly 200W -300W peak. 

I have heard a lot off folks say it is better to just buy more panels than a tracker.  Out of curiosity, does your experience take into account PV-inverter ratio?  Doesn't adding extra panels either place you operating out of the efficiency curve and having either stranded costs on panels or buying more inverter than you can consistently use?  Would this be a good case for using micro-inverters?   I think the best argument, if any, for trackers is they keep everything operating closer to its peak efficiency for longer periods of time thereby eliminating stranded costs.  Of course this is all theory and real world conditions/ experience always trump theory. 

I am planning to install a  fixed mount 20-25 panel 6-7KW grid tie system on the roof of my garage in the near future.  I am leaning towards using Enphase micro-invertors.  It would meet 120-150% of my electrical needs in the summer and about 50% in the winter.

I like to ask questions to find out the "why" of peoples systems to avoid pitfalls/mistakes when installing my own, there is no intent to question anybody's intelligence or imply they made a mistake.  Most systems are very subjective to time of use, individual needs, and therefore site specific from what I gather.
Norwood LM2000 24HP w/28' bed, Hudson Oscar 18" 32' bed, Woodmaster 718 planer,  Kubota L185D, Stihl 029, Husqvarna 550XP

Ianab

The number of panels shouldn't affect the amount of batteries or the inverter needed.  The "curve" of the power going into the batteries would change, and be more of a peak in the middle of the day, and start/stop faster at the ends of the day. But as long as you don't exceed the capacity of the charge controller and battery charge rate this wont be an issue.

Why they have those "ratios" is so you get a sensible ratio. Too many panels for the battery and you end up wasting power (the batteries can't hold all the charge). Not enough panels and you can't get the battery charged properly before the end of the day. Both those have to approx match the load you are going to draw (Inverter size)  It's not an exact science as loads, amount of sun etc vary, but you want to get the numbers "sensible" to have the system run efficiently.

The inverter size is determined by the peak load you need to draw from the batteries. Putting in a 10kW inverter when you are only planning to draw 3 kW peak is where you may affect efficiency (and waste $$)

There might be a case there for running several inverters off the battery bank? Circuits that aren't needed 24/7 could then be powered down totally. So you only switch on power to the workshop when you are in there for example.

Ian
Weekend warrior, Peterson JP test pilot, Dolmar 7900 and Stihl MS310 saws and  the usual collection of power tools :)

John Mc

Thehardway -

We really like the Enphase micro-inverters. They're efficient, and come with a 25 year warranty (string inverters generally have a 10 year warranty).  They also have an advantage if your array gets partial shading at times. With a micro-inverter, only the module that is shaded suffers. With a string inverter, every module in that string suffers if one module is shaded.

You do need to match the module size to the inverter. For example, in our area, we generally use a 250 W module with the Enphase M215 microinverter.  Enphase did recently come out with the M250 inverter. It can handle higher wattage modules (but make sure the type of module you get is compatible). However, the higher wattage modules and the larger microinverter both cost more per watt of capacity - so if you've got the space, it may be cheaper to go with the smaller inverters & modules (depending on what you are looking at for installation costs).

I'm sure you are aware of this, but with a grid-tie system, the monthly/seasonal output is generally less important than the total annual output. This of course depends on how your state's net metering laws are set up. (Here in VT, we can carry over a surplus from month to month for up to a year.)
If the only tool you have is a hammer, you tend to see every problem as a nail.   - Abraham Maslow

tmarch

This month was my systems 2 year anniversary and I did some quick figuring. It's produced 17983 KWH which is 7-8% more than predicted and it's saved me $2928.56 which is roughly 11% of the total investment and 17% of the investment after tax credits. Much better than my .08% IRA.
With the 2 raises my utility has taken in that time I anticipate a better return in the future. Still planning an additional 4-5K off grid system to help in the winter months and power outages.
Retired to the ranch, saw, and sell solar pumps.

Thehardway

Ianab,

In an off-grid or battery backup type power situtation, you are correct, the invertor size should be based on the largest load you plan to place on the system and the ampacity of the battery bank, not the peak output or the number of panels.  Even a small number of panels can charge the battery bank with the correct charge controller, it may just take a long time.

The same does not apply for a grid-tie system.  In a Grid-tie system, the inverter should be sized by the peak output of the panels as you can draw no more than the panels are capable of putting out and the inverter is capable of supporting. You want to be able to draw every amp your panels put out as that is what you paid for or are getting money back from the utility for,  and there is no storing it up for later use as there is no battery bank or charge controller.  For this purpose, in grid tie systems, it is best to match number of panels to inverter size as closely as possible to avoid stranded costs.

Example.  Say you have a 230W panel on a battery backup system.  The charge controller will charge the battery bank until the battery is completely full.  A smart person then has a beneficial load to handle spill over such as a water pump with storage or a water heater element and storage capacity.  You can then use the power stored in the battery at will with whatever size inverter you install.  You don't need to use all 230W at once.

Apply that same 230W panel to grid tie.  You can only use all 230W at peak if you have  a 230W inverter.  The 230W of power is either running something on site or being sent to your utility to turn meter backwards.  If you have less than a 230W inverter, you lose that potential output.  Of course that is peak output of the panel so the average output of the 230W inverter will be much less than 230W, hence stranded costs.   


Grid tie is inverter is best sized by input, off-grid is best sized by output at least that is the way I was taught and understand it.
Norwood LM2000 24HP w/28' bed, Hudson Oscar 18" 32' bed, Woodmaster 718 planer,  Kubota L185D, Stihl 029, Husqvarna 550XP

John Mc

TheHardWay gives a good descrition of rid tie vs off-grid considerations. 

To add another wrinkle:  It's not unusual (at least in my area), to put a larger module on an inverter than what the inverter is rated for.  For example, we often put 250 watt module on a 215 watt rated micro-inverter. 

We do lose a bit of the module's peak output on bright, sunny days, but make better use of the inverter when the sunlight is less than peak. It ends up being more cost effective in the long run.  How you match will depend on your location and the technology in use.  For example, a 2 axis tracking system in Arizona would have a different optimum module vs inverter rating than a fixed mount in Vermont.
If the only tool you have is a hammer, you tend to see every problem as a nail.   - Abraham Maslow

Ianab

QuoteGrid tie is inverter is best sized by input, off-grid is best sized by output at least that is the way I was taught and understand it.

100% agree with your there.

I was talking about an off grid system with storage batteries.
Weekend warrior, Peterson JP test pilot, Dolmar 7900 and Stihl MS310 saws and  the usual collection of power tools :)

Thehardway

Quote from: Ianab on February 24, 2014, 03:45:28 PM


100% agree with your there.

I was talking about an off grid system with storage batteries.

I suspected that was the case but just wanted to make sure we didn't confuse anyone :-[  I could smell the lovely smell of circuit boards burning... :laugh:

Now the important question... what in the world is "Marmite"?  Is that what the Brits call Marmalade or we Yankees call jam? Sounds like some rare metal from outerspace that powers superheroes.
Norwood LM2000 24HP w/28' bed, Hudson Oscar 18" 32' bed, Woodmaster 718 planer,  Kubota L185D, Stihl 029, Husqvarna 550XP

WoodenHead

Quote from: Thehardway on January 13, 2014, 11:24:12 AM
Woodenhead, that's a nice setup you have.  I have a couple questions. 

1.  Why did you not use DC power for your lighting needs?  Cost, convenience, etc.  Since you have PV/batteries, it should have been more efficient than running through inverter.  Especially with LED lighting now available.

2.  What is your average monthly power production in Kw/h over the course of a year?

3.  What is your estimated total investment/installation cost in alternative electricity (wind/solar/batteries/inverters/backup generators/wiring)?

4. What is your average annual cost for propane?

5. How many occupants living in your home?

6. How do you pump your water and why not use water pump as your dump load?

7. if you get 3 or 4 rainy or snowy days with little sun do you have to resort to generator?

I understand the desire for AC and it makes sense in Ontario as you likely only need it in the middle/end of the summer when you have surplus PV power anyway.  In FL, not so much.  Have you considered tracking arrays since your arrays are ground mount?  Might be an easy way to get an additional 15-20% out of your arrays with less investment.

1.  It was cheaper and simpler to wire the house for 120/240AC than for both AC and DC.  We can still use LED lighting.  However, you are correct that it is more efficient.  My particular off-grid inverters are about 90% efficient at their best.

2.  We presently average about 12-15kWh/day I would say.  Multiply by 30 to get a monthly figure.  This is about twice as much as I originally designed our house for.  But we started out with one child and now have six.   :D  Two have Autism and love to keep the lights on and the TVs running constantly.  That has increased our needs considerably.

3.  Total cost is somewhere around $50,000-$60,000.  Equipment was purchased when solar panels cost 8x they do now.  Batteries are more expensive now and inverters slightly less.  This includes the cost of a 7kW, 1800rpm diesel genset for backup.

4.  My annual cost for propane has been about $1800 to $2000 up until this year.  Prices went stupid this year, so my wife agreed to a woodstove.  We burn the slab wood that the mill produces.

5.  Me, my wife, and six kids (ages 4 months to 10 years).

6.  We use a standard 3/4HP submersible well pump (240VAC).  Again it was a matter of cost.  There are expensive DC pumps and you dream up all kinds of ways to store water.  Yes, water storage can be used a dump load.  However, hot water heating is still the most effective use of excess energy for us.

7.  Ideally your battery bank should be sized for between 3 to 5 days worth of storage.  Since our needs have increased, my battery bank is now a bit too small.  Eight years ago we could go for 7 months straight without starting the generator.  And we might use the generator for about 100 hours per year to carry us through the dark days of November, December and January.  Now the generator runs more often.  I'm working on adding a second array of 2.76kW of panels.  This should decrease generator usage again.  My battery bank is still too small though.   :-\

There are far fewer headaches increasing the size of your array than going with tracking.  Right now it is generally cheaper the add a few more panels and fixed racking than implement tracking.  Most people who have owned a tracking system for some time will tell you they have had problems within the first 5 years, particularly in our northern climate.

As for micro inverters, I've sold hundreds if not thousands of them.  Unfortunately I've replaced dozens and continue to do so.  The warranty originally was 15 years full replacement, but now is a limited 25 year pro-rated warranty.  People seem to have problem with them in areas where the utility voltage tends to be a bit high.  Urban areas have been a bit better than rural.

I've replaced two string inverters out of the dozens of those that I have sold.  And they were replaced because of issues right from the start.  They have been around much longer and seem to be more robust from my experience.

As for array sizing, for off-grid systems it is sized based on demand (kWh/day).  The off-grid inverters are sized for peak loading (kW).  For net-metering systems (grid tied) array size is often also based on demand (kWh/year).  However, as stated, the array size and inverter size go hand in hand for maximum efficiency.  Here in Ontario we have simplified grid tie connection rules for systems with name plate capacity less than 10kW AC.  It is often advantageous to over-size the array (20% and in some cases more) to make the most of the simplified connection requirements.

     

Thehardway

Thanks Woodenhead,

That's great info.  I find real life use is much better than all the fancy calculators etc. for planning purposes.  I am planning to install a grid-tie system hopefully next spring with battery backup shortly thereafter.   I'm finishing the garage right now and the panels will be roof mounted on the garage when complete.  Hopefully about 8250W array (rated) if all goes as planned.  33 panels @ 250W

In the winter I will likely owe the power company a little and summer I will produce more than I can use.

Had a power outage last week and was able to run off a 5500 Watt gen (continuous rate 7Kw peak) set as long as I didn't run pump and water heater simultaneously.
Norwood LM2000 24HP w/28' bed, Hudson Oscar 18" 32' bed, Woodmaster 718 planer,  Kubota L185D, Stihl 029, Husqvarna 550XP

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