Thread Rating:
  • 0 Vote(s) - 0 Average
  • 1
  • 2
  • 3
  • 4
  • 5
Looking for a solar-battery-net metering sanity check
#1
Howdy; I live in Montana, where the utility is about to impose a demand charge for solar net metering customers. They will allow us to bank energy, but at a lower rate. I’ve been thinking of ways to use a battery to effectively lower my monthly billed demand by essentially changing our service from feeding off the grid to feeding 100% off the battery. Solar would charge the battery when it was available, and grid would charge the battery when no solar. But the goal with the grid charging is that I would like to essentially trickle charge the battery to minimize our demand charge. In essence some sort of grid charge controller that I could set up to be a trickle of energy that I would slowly increase to figure out what the minimum trickle capacity would need to be to maintain enough amp hrs in the battery to run the house. 

I’ve been watching Jehu Garcia’s YouTube videos on his diy powerwalls, and I would likely use his designs for the powerwall piece. But heck, I’m certainly open to suggestions on the powerwall piece. 

So I guess here are my questions:

1. Do grid charge controllers exist that would allow us to essentially trickle charge the battery in a manually adjustable fashion that would allow us to minimize our demand charge?

2. Are there any issues with pulling primarily from the battery 100% of the time? Longevity, degradation, practicality, safety, etc

3. If I was to use Jehu’s 18650 Lithium Ion battery-based powerwall (likely in a 48V configuration), what is the likely life of these batteries? The batteries are where the cost will be. How long will they likely last in the sort of operational configuration?

I’m sure I will have more questions as I continue to plod along here, but I’d like to start with getting some feedback on the above questions to help me build a foundation. 

Thanks to all for your time and thought and experience and willingness to pitch in and help out. I REALLY appreciate it.

Here is some more info:

Average Monthly kWh use: 2,655 kWh/mo
Average Daily kWh use: 87.3 kWh/24-hr day
Average Daily Wh use: 87,287.7 Wh/24-hr day
Average Daily Ah use: 727.4/24-hr day

I'll post up what I'm thinking on system size after I've done some high level calcs.
Reply
#2
You would want a Hybrid Inverter. It charges from the grid and/or solar. You can usually adjust the preferences for each. And if you loads are higher than what the battery/solar can supply, it'll auto pull from the grid the remaining power needed.

You never pull 100% from the battery unless there is no charging happening at all, or you are pulling more than what solar and battery can supply (so at night, you always pull 100% if not grid connected)

Longevity of cells (the battery is full assembly) varies depending on how much current you pull from them on a regular basis and what the voltage range you run at is. You can get 1000's of cycles if you only use the cells between 3.2V - 4.1V. But going from 2.9V - 4.2V you drop down to only 100's of cycles.

Please the read the FAQ, it is located on the main page. Since you are new to this, there are probably a lot of questions that are already answered there.


Deleted your other thread as it was a duplicate and this one is in the right location.
aventeren likes this post
Proceed with caution. Knowledge is Power! Literally! Cool 
Knowledge is Power; Absolute Knowledge is Absolutely Shocking!
Certified 18650 Cell Reclamation Technician

Please come join in general chit-chat and randomness at Discord Chat (channels: general, 3d-printing, linux&coding, 18650, humor, ...)
(this chat is not directly affiliated with SecondLifeStorage; VALID email req'd)
Reply
#3
87kWh a day is rather a lot - do you have lots of either roof or ground space available for panels, and do you have permission to install them, as you'll need a fair number in order to put a dent in that daily usage.
Korishan and aventeren like this post
Reply
#4
Agreed, 87kWh is a lot! That's almost 3x what I use. Altho I only run the A/C down to 74F, and only at night. During the day it's set at 78F. I only don't use electric range. My electric water heater only runs twice a day for 2 hours each.

aventeren: What are your loads? Is there a possible way of lowering your load requirements?
Proceed with caution. Knowledge is Power! Literally! Cool 
Knowledge is Power; Absolute Knowledge is Absolutely Shocking!
Certified 18650 Cell Reclamation Technician

Please come join in general chit-chat and randomness at Discord Chat (channels: general, 3d-printing, linux&coding, 18650, humor, ...)
(this chat is not directly affiliated with SecondLifeStorage; VALID email req'd)
Reply
#5
Reposting on this thread per request (forgive me if its wrong one Smile).

AIMS Pure Sine Wave Inverters allow for grid charging of batteries and transfer switch to run off the batteries. The AIMS has an adjustment to set the amps of charging from max to min. There are many factors here and I'm not pushing AIMS, just pointing out that a charge/transfer-switch/inverter unit might be a way to go. You can still have a MPPT controller to charge batteries from PV array at same time the AIMS does.
Reply
#6
(06-22-2019, 06:11 PM)aventeren Wrote: So I guess here are my questions:

1. Do grid charge controllers exist that would allow us to essentially trickle charge the battery in a manually adjustable fashion that would allow us to minimize our demand charge?

2. Are there any issues with pulling primarily from the battery 100% of the time? Longevity, degradation, practicality, safety, etc

3. If I was to use Jehu’s 18650 Lithium Ion battery-based powerwall (likely in a 48V configuration), what is the likely life of these batteries? The batteries are where the cost will be. How long will they likely last in the sort of operational configuration?

I’m sure I will have more questions as I continue to plod along here, but I’d like to start with getting some feedback on the above questions to help me build a foundation. 

Thanks to all for your time and thought and experience and willingness to pitch in and help out. I REALLY appreciate it.

Here is some more info:

Average Monthly kWh use: 2,655 kWh/mo
Average Daily kWh use: 87.3 kWh/24-hr day
Average Daily Wh use: 87,287.7 Wh/24-hr day
Average Daily Ah use: 727.4/24-hr day

I'll post up what I'm thinking on system size after I've done some high level calcs.

1) No, charge controllers don't work like that.  They charge the battery until it's full then taper charge.  They don't care about demand charge (nor should they.)

2) Sure, you can pull 100% from the battery - just shut down the AC input and the solar charge controller.  But why on earth would you do that?

3) Using a standard 18650 you'll get about 500 cycles.  That's about a year and a half at full depth cycling.

Here's what you probably want to do:

1) Get a big hybrid inverter like a Radian.  Add storage (48V) and a solar charge controller.

2) Set the inverter to "mini grid" mode - this will use AC from the utility only when needed to charge the battery.

Be aware that you are looking at something like $100K for this system.  (You are going to need something like 20kw of solar, and battery backed solar runs about $4-5/watt without batteries.)  Also be aware that if you are building the pack yourself you are going to need around 25,000 cells.
Reply
#7
(06-22-2019, 10:19 PM)aventeren Wrote: Here is some more info:

Average Monthly kWh use: 2,655 kWh/mo
Average Daily kWh use: 87.3 kWh/24-hr day
Average Daily Wh use: 87,287.7 Wh/24-hr day
Average Daily Ah use: 727.4/24-hr day

I'll post up what I'm thinking on system size after I've done some high level calcs.
I have an operating system that produces 10,000 kwh / year off-grid.  That's 833/mon which is *1/3 of your goal*.   
To produce 10,000kwh/year I have
- 24 panels that total 6,885kwh
- 2 x Midnite Solar Charge Controllers
- 40kwh 18650 Battery Bank (5,340 cells)
- AIMS 12,000 watt Inverter - e.g. 240v@50amps
- Several automatic transfer switches and a couple of APC UPSs to feed it all into the house
Cost - US Tax Credit brings the bill to $26K.   My electricity is .12 = $1,200 savings/year = 22yr ROI and that doesn't include replacing equipment/batteries over 22yrs.

SO - I agree that with previous post that you're talking over $100K outlay.  

Its important to note that the size your talking about - e.g. 72panels + 36,000watt Inverter + x, y, z - is quite large in todays home solar.  You might have non-technical issues such as neighbors / city that object to 72panels.   Bottom line - its a significant undertaking to do the scale and quite serious level of power you list and you might do well to get some professional consultations.
Reply
#8
Are you sure it'll apply to you?
"NorthWestern spokeswoman Jo Dee Black said the new charge would ensure net-metering customers pay their fair share of service costs. Customers with solar panels already in place would be exempted."
https://www.apnews.com/f5e7fbca9e0c4bd1b4cc6b0236655585

Your 87kWh daily use is rather high. A review of what's using power _AND WHEN_ will be crucial to plan a battery system.
Shifting your loads (eg. hot water system) to run only when you have solar power will significantly reduce the required battery size.
And most conservation measures will likely be much cheaper than a battery system.
From my experience, replacing small electric space/oil heaters with heat pump systems made a huge difference.
Converting night time lighting to LED had the biggest bang for bucks&effort.
Modular PowerShelf using 3D printed packs.  30kWh and growing.
https://secondlifestorage.com/t-AJW22-s-...PowerShelf
Reply
#9
Sorry I haven't gotten back to everyone--had family in town.

I own an energy company that will allow me to install the solar project for cost. System size will be 19.6kW-DC, and we have plenty of room. Expected annual production is 25,885 kWh.

We just moved into the house in Nov '18, so we have not yet been through a whole year of energy use--and the previous home owners did not operate the house like we are (we have kids, they didn't, etc). My energy use thus far has been (in kWh):

12/6/18 3,142
1/7/19 2,828
2/6/19 3,100
3/6/19 3,742
4/4/19 3,142
5/6/19 2,555
6/5/19 563

Average Monthly kWh is therefore 2,655--but our peak use is 3,742 kWh, which is 123 kWh/24-day. So 87.3 kWh/24-hr day AVERAGE energy requirement thus far, which I would expect to continue to go down through the summer and then start to climb back up in the fall/winter. House is a combination of electric, natural gas and wood heat--and the electric heating is where the bulk of our energy use comes from in the winter months. All lighting is LED already.

Prior to Northwestern Energy adding a demand charge to solar net metering customers, just installing the solar system was an absolute no brainer for us, as our heat would largely be taken care of via the solar electricity, which we would then bank in the summer and draw back in the winter. But the demand charge has totally changed that for us, and we are now looking at how to drastically reduce our demand seen by the utility by installing a battery between our loads and the meter--in essence using the battery as a demand limiting system. We could look at reducing electric demand by converting heat away from electric, but we have kids and we're trying to do our part to not ruin the climate for them.

Thanks for all of the responses above. I am going to reply with my follow up questions in those threads.
Reply
#10
Your prio 1 is to look into why and towards what you consume all that energy.
I went from 30 to 16kwh in avg per day doing that work and that saved me alot in terms of solar and battery installment. Thats the biggest save you can do.
aventeren likes this post
The Ultimate DIY Solar and build place
YouTube / Forum system setup / My webpage  Diy Tech & Repairs

Current: 10kW Mpp Hybrid | 4kW PIP4048 | 2x PCM60x | 83kWh LiFePo4 | 10kWh 14s 18650 |  66*260W Poly
Upcoming: 14S 18650~30kWh | Automatic trip breakers, and alot more
Reply


Who read this thread?
49 User(s) read this thread:
daromer, Sean, mike, Majorphill, Korishan, PAF, jdeadman, wim, watts-on, Hanssing, Stefaan De Ridder, BlueSwordM, Geek, Oz18650, AndyMc1970, djuro, Rookie, sturgeo, Headrc, completelycharged, Ibiza, Oliman, Bubba, CBlack, 100fuegos, Jim Jr., Katarn_89, w0067814, chuckp, Beholder, Lekr, Generic, Solardad, stevelectric, ajw22, RikH, kevinjaye, OffGridInTheCity, Richard, ma3006he, juanjo57, ben teker, aventeren, billvon, Emiel, kimbo, Mlr112, piglet, krobertson

Forum Jump:


Users browsing this thread: 1 Guest(s)