BESS and HVAC loads

[Chamuit]

Question for the PV experts. How do you handle HVAC loads when you install a battery system along with solar? I've only seen single battery systems feeding a loads panel with a dozen 120V circuits so far.

Most inverters are rated at 9.6kW or less, so they wouldn't be able to handle the LRA of a majority of condensers.

Are you locking these out?

Using soft starts?

 

[jaggedben]

No one-size-fits-all answer.

For me, I have not done any soft starts because I don't want to both take the time to learn and then also be responsible for people's HVAC equipment that I don't normally sell or install. (So far I also haven't run across any HVAC people who even understand the issue, FWIW.) I know that the BESS mfrs recommend soft starts, but I don't see it making good business sense to mess with people's HVAC that way. Your milage may vary.

There's two other things.

One, even if the BESS can start the compressor, do you want the HVAC sucking down the state of charge in an hour to two? There's no substitute for having a conversation with the homeowner about expectations, and just leaving the compressor not backed up. That said, if your system supports a smart control to turn off the compressor when battery goes below a certain percentage, that's a good way to deal with it.

Last but certainly not least, start getting to know HVAC equipment and systems a bit more. 'Inverter Drive' or 'variable speed' compressors don't need high starting current. Get a DMM that does inrush and actually measure to find out if starting current is a problem for your system; sometimes it's not. Sometimes the airhandler is powered from the compressor and sometimes it's separate, meaning you can backup the fan without having to backup cooling or heating. (And sometimes the air handler had 8kW heating strip to worry about.) A little extra knowledge can go a long way toward having the right conversation with the HO.

 

[Chamuit]

@jaggedben, thanks for your response. I have only installed 7 battery systems in locations with no solar involved.

In a system designed for backup power, as a typical practice, do you install a loads panel and move the circuits you want to support during an outage?

What would you do if you have an installation where the HO wants to power an HVAC system?

We have tested a couple of soft starts here where I work. Using only a clamp-on ammeter, we have seen a 1/2 to 2/3 reduction in start-up amperage on condensers. Have not put them on any battery systems yet.

 

[jaggedben]

@jaggedben, thanks for your response. I have only installed 7 battery systems in locations with no solar involved.

In a system designed for backup power, as a typical practice, do you install a loads panel and move the circuits you want to support during an outage?

Yes, most often. Or backup an existing panel and re-arrange loads as necessary. Also coordinate with other contractors on new construction and remodels so it comes out right.

What would you do if you have an installation where the HO wants to power an HVAC system?

Tell them how much the extra battery power will cost or that they just can't do it. Let them decide.

 

[ggunn]

Yes, most often. Or backup an existing panel and re-arrange loads as necessary. Also coordinate with other contractors on new construction and remodels so it comes out right.


Tell them how much the extra battery power will cost or that they just can't do it. Let them decide.

Agreed. Show them the consumption numbers for their HVAC system and the capacity of their ESS, and how many hours (or minutes) they can run their HVAC on battery power, if they can run it at all.

Simple grid tied PV is pretty much set it and forget it. When batteries and grid outage backup come into the picture, the customer must gain some level of understanding of how it works and how to manage their loads when they are running on battery power. It is our responsibility to do all we can to make sure that their expectations are realistic.

 

[Chamuit]

@jaggedben @ggunn

I am trying to explain to Jr C-suite type folks what would be involved with an islanded or grid-down system with a BESS supporting (forget run time) an air conditioning system. They have seen the Ford Lightning advert showing the whole house lit up during an outage and, since they live in east Texas, believe that the truck can power the HVAC in conjunction with the lighting. Thus the question.

I have talked to several installers and can't get past the "we don't do that" in their thinking.

Would it be true to say that a system would need at least: 2 inverters, two batteries, and possibly an ATS? (and the associated costs)

 

[Electrical Geek]

We have done both load shedding and the soft-start. An HVAC system will definitely drain the batteries a lot faster, even with a soft-start. The system has to be designed to handle it if the HO wants them included.

 

[wwhitney]

When a BESS supports both grid-interactive mode and island mode, then the required ATS-like device is called an MID, or microgrid interconnect device. The main difference from an ATS is that the BESS is always connected, so rather than switch to an alternate power source, the MID just disconnects the grid.

As for running HVAC or other large motor loads from BESS while islanded, there's obviously the issues of power and energy. Namely whether you have enough inverter power for the starting surge required, and then whether you can store enough energy to run the load an appreciable period of time, given the load's average power draw. Which for HVAC will depend on the outdoor/indoor temperature delta.

Having inverter driven (variable speed) HVAC equipment helps with both of those. More so with the power question, as I understand that the startup surge basically goes away, so you just need to size the inverters to the nameplate power rating. And somewhat with the energy question, as they are somewhat more efficient, so they move more BTUs with less kWhs.

The following information is specific to Tesla Powerwall, not Ford's F150 solution, but it discusses some of the limits, including running conventional (non-inverter) HVAC equipment off batteries:

What Can Powerwall Back Up | Tesla Support

Learn more about the variety of home appliances and EV charging equipment that Tesla Powerwall can backup.

www.tesla.com

Cheers, Wayne

 

[ggunn]

They have seen the Ford Lightning advert showing the whole house lit up during an outage and, since they live in east Texas, believe that the truck can power the HVAC in conjunction with the lighting. Thus the question

What does living in east Texas have to do with it?

 

[Chamuit]

Everyone has air conditioning. When you tell someone, "I can power your whole house," they immediately assume you mean everything.

 

[Chamuit]

When a BESS supports both grid-interactive mode and island mode, then the required ATS-like device is called an MID, or microgrid interconnect device. The main difference from an ATS is that the BESS is always connected, so rather than switch to an alternate power source, the MID just disconnects the grid.

As for running HVAC or other large m...

Cheers, Wayne

Thank you. I have seen the Tesla information. It says you need two+ Powerwalls to run HVAC equipment. I talked to a Tesla training instructor last week, and he confirmed the 2-unit minimum. Although, he would push for three if the customer wanted HVAC support.

 

[wwhitney]

Thank you. I have seen the Tesla information. It says you need two+ Powerwalls to run HVAC equipment

That's a bit of a simplification, and not what the link says. It says that for conventional (fixed speed) equipment, you should look at the LRA (Locked Rotor Amps), and that the LRA of the largest motor needs to be less than 106A * # of PWs. Here LRA is being used as a proxy for the magnitude/duration of starting surge, and 106A corresponds to the maximum one PW can do. Now if the most common case is that there's one HVAC unit with an LRA between 107A and 212A, that would correspond to a 2 PWs rule of thumb.

Also, this only covers the case of starting the motor when it is the only load present; with other loads, I would think that an allowance needs to be made for the background load present when the motor starts. For the case of other HVAC equipment, the link suggests adding the RLA (rated load amps) of the other HVAC equipment to the largest LRA, and to compare that to 106A * # of PWs. But that again doesn't consider non-HVAC loads, and doesn't check the steady state demand, just the startup surge case.

Cheers, Wayne

 

[Chamuit]

I am shooting for a general rule of thumb instead of system-specific requirements.

 

[jaggedben]

...

Would it be true to say that a system would need at least: 2 inverters, two batteries, and possibly an ATS? (and the associated costs)

No. You need to get the LRA off the compressor label, and/or (better yet) an on-site measurement of the inrush current, and size your system accordingly depending on the product your selling, using the 'surge' or 'peak' specification from your inverter or ESS datasheets.

You will need an MID to run any system off-grid as Wayne explained.

I am shooting for a general rule of thumb instead of system-specific requirements.

Then you'll be shooting yourself in the foot.

 

[analog8484]

3 Powerwall's is the more realistic "rule of thumb" for running central A/C off grid.

 

[Chamuit]

No. You need to get the LRA off the compressor label, and/or (better yet) an on-site measurement of the inrush current, and size your system accordingly depending on the product your selling, using the 'surge' or 'peak' specification from your inverter or ESS datasheets.

You will need an MID to run any system off-grid as Wayne explained.


Then you'll be shooting yourself in the foot.

I'm not designing, bidding, or selling a job.