Grid-tied solar

[dave1949]

Not workable as a capacity extender IMO due to the variable output, just as a gas saver. My thought is that the fuel savings would be a bonus of having a grid tie system.
On the "backfeeding the inverter issue" I suspect that as long as your total load is low enough that the genset can handle it, the genset will just see a reduced load when the solar panels are producing power.


Most grid tie systems (from what I understand) will not "fire" the inverter and supply power until they see 120VAC @ 60hz, thus only one lockout is needed.

Aaron Z

If the genset has the capacity (and price) to handle the load alone, then money-wise all that can be gained is whatever fuel is saved during a grid outage, and that's only when the solar system is making power. Assuming the solar system is there anyways twiddling its thumbs , that is something but probably not a lot.

You are right about the interlock, only the generator breaker would need to be involved.

I Googled backup generators in grid-tied systems thinking maybe someone has designed a generator to work in that situation. It doesn't look like anyone does.

Some useful articles on the topic:
Generators in Grid-Tied Systems | Home Power Magazine


Q & A: Interaction of Grid-Direct Inverters with Backup Generators | SolarPro Magazine
An excerpt:
But what if an electrical engineer wants to go in a totally different direction? Can the PV system connect to the backup circuit? Will the grid-tied inverter synchronize with the generator output and help support the backup loads? The answers to these questions are complex and entirely dependent on the quality of the generator’s ac output.

It is possible for a PV inverter to interconnect with the stable output of a high quality generator. The inverter monitors the generator’s voltage, frequency and waveform. If the ac waveform is grid quality, the inverter will attempt to synchronize with the generator. If the ac input to the inverter does not dip, sag or surge when the PV system comes on line and starts delivering power to the system, the inverter will remain on line. If the building loads and the generator output are both large relative to the output of the PV system, everything will work satisfactorily as long as the generator does not go out of UL voltage and frequency. However, if the generator voltage or frequency increases beyond the UL 1741 limits, even for a short time as might happen when the PV system comes on line or with a large load shift in the building, then the PV inverter will go off line.

But what if the instantaneous power output for the PV system is larger than the building loads? If there is more power available from the PV system than the total load can consume, the inverter will attempt to put the extra power somewhere, raising the ac line voltage, for example. In this case, when the PV system comes on line, as soon as the voltage or frequency reaches the maximum UL 1741 limit, the inverter will trip off. Unless the total load demand exceeds the instantaneous power output of the PV system at all times, the grid-tied PV system has no chance of staying connected to the generator output.

 

[dave1949]

A question concerning home solar power: A bit off thread but thought someone here might know the answer.

This is about OFF grid solar and not an invitation to debate ... please.

Is off-grid solar susceptible to EMP? If so, is it only specific components at risk?

Yes, it would be susceptible to EMP. Off-grid or grid-tied utilize electronic components and the solar panels themselves can be damaged by EMP.

https://www.heatmyhome.co.uk/solar-panels/will-electromagnetic-pulse-affect-solar-panels#.VJZSDV4AA

 

[KiotiKowboy]

Dave - Thanks for the link.

If the author is to be believed, an off-grid setup would most likely survive a "solar" event but not a nuclear event. Hey, that's one out of two!

I'm building in the spring and will have some type of solar on the home. I'm trying to weigh the benefits/cost of being on or off the grid. My home will be more than 1/4 from the grid, so cost will be high to get connected. If I skip that, I can shift those costs into paying for an off-grid system.

 

[dave1949]

Dave - Thanks for the link.

If the author is to be believed, an off-grid setup would most likely survive a "solar" event but not a nuclear event. Hey, that's one out of two!

I'm building in the spring and will have some type of solar on the home. I'm trying to weigh the benefits/cost of being on or off the grid. My home will be more than 1/4 from the grid, so cost will be high to get connected. If I skip that, I can shift those costs into paying for an off-grid system.

Yes, you could shift the funds, they are going out the door anyways. I would get an estimate on how much the 1/4 mile will cost though. And check if there are 3rd party line installers used in your area. There are here and they are a bit cheaper than the utility doing the poles and lines.

The caveat would be with an off-grid system you are signing on to monitor and maintain a battery bank. Big, expensive batteries. It usually means checking the specific gravity of each cell like we used to do with automobile batteries. You need to monitor how deeply they are being discharged. Know when to get out the generator if the battery charge is too low. Battery cells need to be put through a de-sulfating process occasionally too. The other system components are very reliable, but like anything they can fail.

Maybe newer charge controllers do some of the technical work for you, I haven't kept up with that. Anyways, all cool stuff for guys to mess with but if you pass away, you will you know :laughing:, it falls to your perhaps non-technical or uninterested wife. Then the question becomes, is there someone you trust to do that or can you/could she hire someone to do it? You are the power company from start to finish--that's the good news and the bad news.

If you reserve space in your new house floor plan for the battery bank, you can keep your options open while you study the issue. The battery bank should be located in an area that stays around 72F for best performance and life. Space requirements vary, but something like 3'x12' should do it. Choose a location that gives easy access for a sturdy 2-wheel hand truck.

You should also select a complement of appliances that make sense for solar power. A gas range-oven, a gas clothes dryer, and a gas or solar hot water assisted water heater are the items to focus on. Plus high efficiency for everything electric. Off-grid systems are not cheap. Offsetting system size with propane use usually makes sense.

I'm not sure how heating and AC is best done in your location. Having visited my brother in SE Kansas a few years back when it was 108F, my guess is AC will be an energy challenge. If you are seriously looking at off-grid, you obviously need to talk to someone more knowledgeable than me and nail down the details--soon because it rolls into your house design.

 

[KiotiKowboy]

All good info and I appreciate your thoughts.

My new home is actually being designed by a company that specializes in energy efficient homes. Solar is their specialty. So far, however, most of our effort has gone into the structure and we are only now getting ready to discuss the solar options.

The good news for now is they are offering 25% off their solar setups, and there is a 30% fed rebate. Hopefully this will make the cost of off-grid possible.

 

[dave1949]

All good info and I appreciate your thoughts.

My new home is actually being designed by a company that specializes in energy efficient homes. Solar is their specialty. So far, however, most of our effort has gone into the structure and we are only now getting ready to discuss the solar options.

The good news for now is they are offering 25% off their solar setups, and there is a 30% fed rebate. Hopefully this will make the cost of off-grid possible.

Sounds good. You have the opportunity to build in energy efficiency from the ground up; that doesn't come around very often in a lifetime. I would utilize all the solar energy you can that makes sense. You won't regret it comfort-wise and it does wonders for your future energy costs.

 

[farmer2009]

Thanks Dave. That really sums it up. As you couldn't run a down sized genset you would need a full sized one and then IF the PV system could stay on line with it then it might save some fuel. But really would need a battery bank or some kind of dump for extra power. I was just thinking out loud about could it work. But it looks like there would be to many could, might, and maybes involved.

 

[SirReal63]

I'm building in the spring and will have some type of solar on the home. I'm trying to weigh the benefits/cost of being on or off the grid. My home will be more than 1/4 from the grid, so cost will be high to get connected. If I skip that, I can shift those costs into paying for an off-grid system.

We were in a similar situation, when you look into the actual cost of a 10+kW off grid system the costs can be staggering. We bit the bullet and paid to have the electricity run and will do a smaller 3.3kW system on the smaller guest house. I hope to begin the solar install in the next week or two. The system is a "grid assist" setup that will use grid power to supplement battery charging but will not feed back into the grid. I am using AGM batteries, though a lot more expensive they take some of the headache out of using wet cell batteries. A/C and heat will be provided by a HotSpot Energy 48 volt mini split, which is great because it runs only on 48v DC and is very energy efficient. All lighting is with LED, LP gas will work the stove and the on demand hot water heater. A wood burning stove (Jotul F118CB) will provide heat in winter when it is typically more cloudy to aid in heating.

This will be my first solar install so it will be a huge learning experience.

 

[dave1949]

The December, 2014 results.

Below the model but consistent with the two previous Decembers. The last couple days of the month saved it from being really low.

We passed 12 AC megawatt-hours generated since install this month. The break down since install:
12.031 mwh generated
10.896 mwh in from the utility
8.337 mwh out to the utility

Here are some numbers that show how generation, usage and net billing works with my system.

Net usage since install is: 12.031 generated + 10.896 in - 8.337 out = 14.59 mwh
14.59 / 29 months since install = .5031 (503.1 kwh avg. per month)

The total billed kwh since install are: 10,896 kwh in - 8,337 kwh out = 2,559 kwh
2,559 kwh / 29 months = 88.241 kwh avg. per month billed.

Looking at it another way:
With no solar generation usage would be: 12,031 kwh generated - 8,337 kwh out + 10,896 kwh in = 14,590 kwh
14,590 kwh / 29 months = 503.1 kwh avg. per month billed.

Average difference in billed kwh over the 29 months to date:
503.1 kwh/month (no solar) - 88.2 kwh/month (with solar) = 414.9 kwh/month utility power replaced with solar.


Month--NREL Model (AC kWh)--2012 Actual---2013 Actual---2104 Actual

Jan.----388----------------------NA--------------440-------------360
Feb.----412----------------------NA--------------375-------------446
Mar.----509----------------------NA--------------462-------------577
Apr.----374----------------------NA--------------487-------------525
May----353----------------------NA--------------375-------------361
Jun.----308----------------------NA--------------379-------------390
Jul.-----341----------------------NA--------------377-------------395
Aug.----383----------------------461-------------470------------453
Sep.----394----------------------481-------------476------------489
Oct.----369----------------------378-------------453-------------380
Nov.----283----------------------400-------------414------------295
Dec.----331----------------------297-------------278------------299

 

[tcartwri]

They installed a monitoring meter to the right of the inverter which shows total production - not really necessary, but this is their "standard practice" also I understand this can be used to track SREC but currently we don't have a purchase program in NH...

It is actually extremely beneficial to install an in house monitor so that you can see what's going on at all times. Running out to the array or the inverter loses it's appeal pretty quickly and you have no idea what it's been doing for long periods of time. I was able to diagnose a problem with one of my inverters that was shutting down during peak production times, without throwing error codes. I never would have known I was down in production otherwise.

I was also immediately alerted to a blown panel, that could have been a fire or shock hazard.

Mine sits in the kitchen above the sink and I check it at least twice a day.