Using Batteries as an MPTT source to offset ON PEAK demand needs

[cinoaz]

Theory and Seriousness, all a learning process.

I have a Huawei 10K inverter, SUN2000 - 3.857.691011.4KTL-USL0 (10K model) that has 3 PV input lines (for 3 PV arrays) each rated at 15amps. I'm only using 2 at the moment, with two arrays of 15 panels each array with Power Optimizers on each panel (QPLUS_L_G4_2_335-345_2017-01 panels). I'm on a GRID based "On Demand" plan, which, tracks the highest demand between 4pm and 7pm and uses that value to determine the electric bill. If you can limit your total DEMAND rate during this time, electricity is very, very cheap at ALL times, even during PEAK. It's that damn DEMAND multiplier.

In the real world, there's Dinner to be made (ovens and such), clothes to be washed, dishes to be cleaned, TVs to be powered on, Computers to be powered on, all at times right at peak, 4pm to 7pm when everyone is getting home.

If, somehow, you had a "buffer" of stored energy that can be tapped into during this window, you could limit the GRID demand load. Just enough to prevent the PEAK spikes during ON PEAK hours.

All 3 inputs of my GTI have a minimum of 360v with a max of 600v with a 15amp limit. If I can control the 3rd input so voltage is only available on that 3rd input between 4pm and 7pm, that means, I can connect my battery bank to the 3rd input and it only draws for that time, the batteries can be used to supplement the solar to handle the boost in Demand for that time frame, thus reducing my DEMAND charge from the GRID, lowering my entire bill.

Outside those hours, the battery bank would be disconnected from the GTI via relay. The Batteries are connected to individual 2A chargers plugged into my Electric Panel via relay as well. I use GRID power to charge the batteries back up before the next cycle. The Relays are inverted to each other (either or), so they can never be ALL OFF or ALL ON, they are always opposite. Again, electricity is CHEAP but it's that DEMAND multiplier is the counter for that.

The issue isn't HOW much electricity you use, it's how much you pay for each KW used. The ON DEMAND rate is SUBSTANTIALLY cheaper than any other rate per KW from the GRID because they know they are going to make up for it using the DEMAND modifier. If you can make your peak DEMAND remain close to 0 or 1, your bill is next to nothing. I can use 1,000Kw and my bill is like 20 bucks (exaggerated yes).

Yeah, you can shut off appliance, don't do this, turn this off, etc, etc. However, if you didn't want to force changes in your daily routine, this seems to be the ticket. I don't want a Battery Backup, I want a Battery Filler.

A slew of questions comes pouring in.

1. WiIl the sudden "inrush" of voltage and current" blow through the inverter?
2. Does the Inverter limit the input to 15amps or will the Inverter start sucking in AS MUCH as the battery can give until either the Battery or Inverter goes BOOM? The manual says each PV array is 15A max, but, it doesn't say you have to configure panels to stay under that. I mean, if you add more panels to adjust for "loss" or similar shading, I have to beleive even if the PV array can exceed 15A the Inverter will run away with it?
3. Finding anything that gracefully handles 480V, even tho limiting to 15amps, is a challenge. There is a relay, HDD-6V20E-HS1.0U , https://www.power-io.com/products/hdd.htm, that handles it, but, there's still the inrush?
4. Since the GTI is an MPTT device, I think it's looking for a gradual "ramp" of voltage. Again, finding something that can handle 480v DC is akin to Frankenstein's Castle laboratory.


Granted, 480V DC from a bank of batteries is some serious stuff, but, why should the GTI care if the Voltage is coming from Batteries, a PV Grid Array, a Windmill, a water wheel, or some kid peddling a bike with a generator on it? All it cares about is converting DC Voltage to AC, and if you can get DC Voltage to it's input, it will convert it?

Help?

 

[daklein]

I do that type of thing with microinverters, from 10s 36v & 14s 48v lithium battery banks. Much lower voltage though.

The AC output of each micro at 240v is under an amp, and I use wifi relay boards to connect or not one of the AC legs when desired. A RPi watches what's going on and decides how many micros to use. The main battery inverters SI6048US have a separate FLA battery that is mostly kept floating, and the battery microinverters AC couple to it and effectively run the house loads directly at night. Daytime there is AC coupled solar. The SI inverters are mostly just hanging out (using standby loss energy but oh well) and ready if any big load comes on. It works fine. The DC input to the micros from the battery is left always on (via a manual DC breaker for overcurrent protection), and they should be precharged. I have a separate manual breaker with light bulb to precharge the microinverters if restarting them.

Remember it's not good to connect a whole DC PV string to an inverter while in full sun. MAybe it would work fine if there was a precharge circuit used. Probably blow up if not.

MAybe it would be desireable to control what the GT inverter takes from the battery. If there was an export limiting feature that might do it?

 

[cinoaz]

MAybe it would be desireable to control what the GT inverter takes from the battery. If there was an export limiting feature that might do it?

Yeah, there was another thread that was being debated. Do you need to have some "current limiter" inline to keep the "available" amps the inverter sees to the maximum it can handle, or, will the inverter just use the amps within its max limit on that line (i.e. self limit).

Lots of options when talking 12, 24, 36 and 48v systems, a dime a dozen. When talking about 360volts, I'm shopping the same stores as the Electric company and those solutions are outside my ballpark in terms of cost.

 

[Riwall]

I've been experimenting with feeding a small 500w grid tie inverter from the 12s 4.2kWh NMC battery in my lawn mower. Figured it would be nice to get some peak offset using a battery that mostly just sits there. The inverter has no issues with drawing excessive current. It simply draws enough to hit 500w. I did install a fuse just in case.

 

[Ampster]

@cinoaz
Welcome the forum. I think you will find this place more receptive than that other forum. Especially after Sunking showed up. I can't offer much help on this topic. I agree with some of the other posters that battery input to an MPPT controller can be tricky.
If I understand you correctly that you are on a Demand plan not a Time of Use rate. Do you know what your peak demand is between 4 and 7? Even if you could get it to work, based on a rough estimate the most you could get out of one MPPT at 400 volts and 15 Amps is 6kW and there may be limits per each MPPT controller based on the total inverter capacity of 10kW. That is a key question I asked on the other forum and I think it would be important in order to know if you could get any usable kWs from this idea? It might only be one third of the 10kW inverter capacity, which would be 3.3kW.
Have you costed out how many kWhs of batteries you would need to cover that period? Could a simple off grid inverter and some batteries cover your peak without risking blowing up your existing inverter?
I have a demand plan on a commercial building and the demand charges occur any time of day and add $50 per month to my bill, even though I have solar to cover the kWh charges. The kWh credits to not go against the demand charges. That demand is caused by a tenant who charges and EV. He is paying the demand charges but long term the solution is another meter dedicated to EV charging which would not have demand charges. If I can get a separate meter installed for less than $3000 I would have a five year payback and could offer EV charging to my other tenants. I did the math on a battery system and it did not have a good rate of return.

 

[Ampster]

I did a little Googling and discovered that some of those Huawei inverters have battery input for an LG RESU high voltage battery. Other than the cost of the LG RESU that would be the simplest and you could use the inverters TOU settings to discharge during your 4-7 demand charge window. You would also get the full power of the inverter versus any limitation of the capacity of a single MPPT channel.

 

[daklein]

If the HV battery/solar inverter talks to BYD HV batteries, this may be of interest. https://github.com/dalathegreat/BYD-Battery-Emulator-For-Gen24

 

[HarryN]

One possible idea is to add (or move) some solar panels to face the sun during this peak power time period.

It isn't completely obvious, but if you can find a vertical wall that faces the sun @ ~ 18:00 hrs in August, it is nearly ideal for generating power during this critical time period.