Stepping outside the box for thoughts on alt designs. Single voltage inverters, transformers, etc

[ggunn]

Pretty much other than I find many grid tie inverters to lack many features for generator support, priorities, etc. There are very few that will actually run a generator, sync to it, and use it only as a supplement to the solar when solar is made a priority.

I am open to hear from others here on options but consider you have a 5kw load, and running only a 2kw generator. If the inverter does it's job, it will pull 2kw from the generator and 3kw from the solar system for the load. Once things are well sustained, the generator can either push charging to a storage pack or just throttle back.

It can be done, but the issue is that you cannot backfeed a generator like you can the grid, so in an off grid situation when the PV output exceeds the load and the storage is full, you have to be able to either throttle back or turn off the PV. PowerWalls can munge the frequency of their inverters, and some inverters can recognize the change and throttle back accordingly.

Generators are demand sources, so they do throttle their input back when demand is lower, but they cannot have a negative output. Maybe some commercial ones can, but regular consumer level generators can't. That's why a common solution is to interconnect the PV outside the ATS for a grid tied service with generator backup. Batteries are really not a player because the case where they are full (and therefore virtually not present) needs to be taken into account.

 

[jaggedben]

This topic comes up a lot. If I have PV, BESS, and a generator why can't they all work together in a small residential off-grid system? The reason is no one makes the equipment for that. It's more expensive than just switching between PV and the generator. Since off-grid systems are expensive to start with people are attracted to anything that brings the cost down. There are also design issues with sizing the generator that tends to make it more efficient to fully load the generator when it is on and not share the load with the PV system.

I think the Schneider Conext and Sunny Island support it. But those are 15-20 year old products, which tells you something about the lack of demand for innovation in this part of the market. The money isn't in small purely off-grid applications.

 

[tortuga]

But those are 15-20 year old products, which tells you something about the lack of demand for innovation in this part of the market.

I am no expert like you Ben but could be if the product works why re-design it? I'd say lots of the electrical products I install every day have not changed significantly in the last 20 years. I remember being really impressed with the sunny island especially as a frequency control, I think SMA still sells them but they are not compatible with all newer inverters.

 

[ggunn]

I am no expert like you Ben but could be if the product works why re-design it? I'd say lots of the electrical products I install every day have not changed significantly in the last 20 years. I remember being really impressed with the sunny island especially as a frequency control, I think SMA still sells them but they are not compatible with all newer inverters.

For one thing, the Sunny Island is 120V; you'd need two of them or an autotransformer to interface with a 240/120V inverter.

 

[tortuga]

For one thing, the Sunny Island is 120V; you'd need two of them or an autotransformer to interface with a 240/120V inverter.

Yeah every install I have seen with them had two. I don't think that was even an issue. My only issue with them is they only support 48V battery banks. But your not really limited much as to battery type or chemistry. One install that I have seen was not even connected to PV it was being used as a UPS in a commercial building with two generators and a battery, I did not work on it just saw it.

 

[fastline]

I tried and tried to get engineering level tech questions answered from Schneider yrs ago and gave up regarding how they integrate the generator in an off-grid.. I believe all they are doing by using 'generator support' coupled with battery power is running the generator through the DC charger so the inverter side really knows no different. It is pulling power from the battery bank so PV and generator would support the loads through the DC side. That is a lot of 'converting' though.

Though it is probably possible to feed AC directly from a generator, plus the inverted AC, I can see a list of concerns or scenarios that could be an issue. I have paralleled synchronous generators in the past and once coupled, they are magnetically coupled, but you have to get frequency and timing just right before flipping the switch. I think this is how inverter generator paralleling works though I have never verified. I think if they are receiving AC when they fire up, they look at that sine wave and simply sync to it. We have had 3 hondas sync'd before.

 

[pv_n00b]

The generator support systems are relatively easy to build if you don't mind running the generator all the time and having a small PV system. For instance, say you have a 5kVA load that never goes under 1.5kW during the day. You can support that load with a 5kVA generator and 1kWacp PV system. Odds are good the PV and generator will play well together with no additional controls. You probably want something that drops out the PV system if the load falls under 1kW. You can save some on generator fuel, but the generator will always be running when you need any power.

 

[pv_n00b]

I tried and tried to get engineering level tech questions answered from Schneider yrs ago and gave up regarding how they integrate the generator in an off-grid.. I believe all they are doing by using 'generator support' coupled with battery power is running the generator through the DC charger so the inverter side really knows no different. It is pulling power from the battery bank so PV and generator would support the loads through the DC side. That is a lot of 'converting' though.

Schneider is not very good about providing information on off-grid design with the XW and a generator. I would not use it if they are not willing to describe the functionality in detail. Hinting that it can work is not good enough.

 

[jaggedben]

I am no expert like you Ben but could be if the product works why re-design it? I'd say lots of the electrical products I install every day have not changed significantly in the last 20 years. I remember being really impressed with the sunny island especially as a frequency control, I think SMA still sells them but they are not compatible with all newer inverters.

Well, I've worked a little bit with a Conext as a side project and it reminded me of programming MIDI keyboards in the 90s. Maybe they'd have more uptake if they had modern user friendly UIs that didn't take half-day to figure out how to perform one function (because each step is a puzzle and if you mess it up you have to start the whole process all over). Not saying that tips the balance with respect to your question, but it's one point in favor.

....

From a conceptual standpoint, DC generators with grid forming inverters are probably the way to go here. I believe Honda makes some small ones, someone could partner with them or develop their own. I believe Generac's former Pika stuff does this too? There's no engineering reason, for example, that Enphase couldn't make a DC generator with some of their IQ8s inside that would seamlessly integrate.

But maybe part of the issue here is people who work with solar not being crazy about fossil fuels and vice versa. Me included, tbh. Consumers and manufacturers alike. A lot of people who go with solar and batteries in off-grid are doing it because they don't want a generator.

 

[fastline]

But maybe part of the issue here is people who work with solar not being crazy about fossil fuels and vice versa. Me included, tbh. Consumers and manufacturers alike. A lot of people who go with solar and batteries in off-grid are doing it because they don't want a generator.

That is usually the first hurdle with people that want an off-grid setup. In many cases it has to do with complexity or cost to get grid connected, but some still seem to think it is "free energy".... As someone that runs off-grid, I can assure that without a generator, you can the words reliable power at the doorstep. There are days that sun does not shine. Those are usually days you will need some power too.

As well, I find that people get it all wrong with generators. They want to wait until batteries are depleted, then 'charge them'. Wrong approach if you want batteries to last.

And......I keep waiting for induction motor controls to evolve with soft start as the norm. There is just no reason with current tech that we need all motors going across the line. It causes drama if not grid connected. Not that I advocate for off-grid, but some folks don't have a choice and off-grid has opened the door for real remote living.

 

[jaggedben]

And......I keep waiting for induction motor controls to evolve with soft start as the norm. There is just no reason with current tech that we need all motors going across the line. It causes drama if not grid connected. Not that I advocate for off-grid, but some folks don't have a choice and off-grid has opened the door for real remote living.

'Variable Speed' or 'inverter drive' is basically soft start and in my area I'd say 4/5 of the new HVAC compressors I see are such. Sump pumps not so much.

 

[pv_n00b]

That is usually the first hurdle with people that want an off-grid setup. In many cases it has to do with complexity or cost to get grid connected, but some still seem to think it is "free energy".... As someone that runs off-grid, I can assure that without a generator, you can the words reliable power at the doorstep. There are days that sun does not shine. Those are usually days you will need some power too.

Hey, some people like the soft warm glow of a kero lamp during the week their PV system is flat out during the winter.
Client: I don't want an expensive, hard to maintain, generator
Contractor: So you are okay with being without any electrical power during a week-long storm in the dead of winter? Did I say dead, because it will be the dead of winter?
Client: So those generator maintenance plans make it pretty carefree, right?

 

[ggunn]

Hey, some people like the soft warm glow of a kero lamp during the week their PV system is flat out during the winter.
Client: I don't want an expensive, hard to maintain, generator
Contractor: So you are okay with being without any electrical power during a week-long storm in the dead of winter? Did I say dead, because it will be the dead of winter?
Client: So those generator maintenance plans make it pretty carefree, right?

Living off grid with just PV and batteries is not for the faint at heart.