New Offgrid, DC or AC Coupled, and components

Getting into the nuts and bolts of wind and solar power components, installation and performance issues, ask technical questions, answers and tips

New Offgrid, DC or AC Coupled, and components

Postby dakiller » Fri May 25, 2018 1:38 pm

I'm currently working though the designs of a new home I'm going to be building in southeast Victoria and I'm wanting to go off grid solar to power it.

Family of 4, large 5 bedroom home with all the trimmings. Tank water, no gas, lots of insulation, etc.

I'm looking at having 20kW of solar and around 20-25kWh of battery storage (but maybe more if needed). I've designed the shed size to fit the array, facing due north and to mount the panels at 38* for optimum winter time generation. There will be a backup generator, that I would want to be needed for those 5-10 bad days in winter.

My idea is to somewhat reject the notion that you need 3 full days of autonomy as is traditional, but to have an array big enough that will pick up enough in all but the worst weather to get by, and to make use of what is there when the sun is shining. Panels are quite cheap now.

Budget is somewhat open, but I am aiming for mid $50k and would stretch it further if I saw the benefit.

Most of the loads required of the system is your normal household stuff, but the two main loads that will size this system is the induction cooktop (8kW) and the custom heatpump system that will do hydronic floor heating/water to air ducted cooling/hotwater/pool heating (yet to be fully specced out at this point, but probably a 10kW load at full steam, giving 30-40kW of heating/cooling capacity)

I plan on putting in a lot of automation in the system as well. The HVAC should be aware of how much power is being generated and to ramp up and down its use based on what power is surplus. Intellegently load shedding for when the inverter is starting to get overloaded.

Now, it is the finer parts and components that I'm not really sure on at this point.

For a DC coupled system -
I got my eyes on the Selectronic SP PRO 15 or 20kW model.
I just found out about the Winston and CALB LiFePo4's, that look pretty good value.
Needing a 120V battery pack, I haven't seen anything that could feed in that much power at 120v from a 20kW array to the batteries and inverter.

AC coupled looks a lot simpler, but you pretty much double up on your inverters, one from the array and one from the batteries. I don't like the idea of the double conversion of DC-AC-DC to store in the batteries to then go DC to AC when discharging.

I can see AC coupling being good for predominately daytime usage and to limit the size of the battery inverter, where you dump as much power during the day from the array's inverter into loads in the house, trickling into the batteries, and then cut back at night on the big loads. I still don't think I could get away with anything smaller than the 7.5kW SP Pro, but it does allow me to use a 48v battery bank instead of 120v.

Thoughts?
dakiller
Solar Supporter
Solar Supporter
 
Posts: 6
Joined: Fri May 25, 2018 11:57 am

Re: New Offgrid, DC or AC Coupled, and components

Postby davidg » Fri May 25, 2018 2:39 pm

I'd suggest based on experience of actually designing and installing systems, without doing a load profile that the budget is not enough, certainly using a 15kW+ inverter with an applicable automatic generator, that you most likely don't need 20kW's under normal circumstances.

I'll ignore exactly how the array layout. PV 15kW's should be enough in most cases. AC coupled would be abetter option since your using an Automatic Generator.

What on earth are you trying to power? Most systems I've done only need the 7.5kW SP Pro max. There is only one system I done not that loan ago that needed more inverter power, it is using a 3 phase water desal unit. For I used 3 SP-Pro inverters (3 Phase) and 2 of those were only to power the desal unit due to the 3 phase requirement.

That system due to the size of the battery bank/pack was a fair bit more.

Load profiling is the right place to start. A system is designed from hat which then sets the budget/cost, followed by naturally installing it. The process is much much more likely to produce a successful outcome that just works properly.
SPS Standalone Power System/Hybrid specialist - Store the Sun!
SELECTRONIC SPMC482-AU, 8.2kW's of Arrays
Selectronic Certified AC-Coupled Fronius 8.2kW Inverter
1900Ah 48V Bank

An OTT Genset for a house.
PVOutput Stats

Sparkys light up your life :)
User avatar
davidg
Solar Crusader
Solar Crusader
 
Posts: 2102
Joined: Sat May 05, 2012 7:38 pm
Location: Dandenong VIC- *Really don't like Power Retailers at all.

Re: New Offgrid, DC or AC Coupled, and components

Postby Gordon-Loomberah » Fri May 25, 2018 3:01 pm

Welcome to the Energy Matters Forums dakiller :)

dakiller wrote:I've designed the shed size to fit the array, facing due north and to mount the panels at 38* for optimum winter time generation.


If you want to avoid a solar noon peak in output from the PV array, have a think about facing some panels to the East and West, see this thread:
solar-wind-gear/topic5064.html

You will probably find that the battery cant accept 20kW, even at 120V, so best not to try to force all that current in at once ;)

My idea is to somewhat reject the notion that you need 3 full days of autonomy as is traditional,


I know some commercial systems still seem stuck on that idea, but I rejected it years ago, as it makes no sense with panels being relatively cheap cf batteries these days.


Needing a 120V battery pack, I haven't seen anything that could feed in that much power at 120v from a 20kW array to the batteries and inverter.


Outback FM80s (and no doubt some other CCs) will do 60V, so charging each half of the battery separately, and using balancers may work. You need to be sure the arrays are in the same orientations and unshaded etc though. There may be other solutions, although it's not something I have looked into.
http://gunagulla.com Loomberah weather and astronomy including live solar radiation intensity and UV + Gunagulla aquaponics, organic eggs and cherries
User avatar
Gordon-Loomberah
Community Moderator
 
Posts: 5762
Joined: Thu Sep 24, 2009 9:41 pm
Location: Loomberah NSW Australia

Re: New Offgrid, DC or AC Coupled, and components

Postby davidg » Fri May 25, 2018 8:57 pm

Gordon-Loomberah wrote:Outback FM80s (and no doubt some other CCs) will do 60V, so charging each half of the battery separately, and using balancers may work. You need to be sure the arrays are in the same orientations and unshaded etc though. There may be other solutions, although it's not something I have looked into.

Charging each half is a quite messy option. AC coupled resolves this in very clean efficient way, provided there is an automatic generator in the system, there are very good reasons for having or need a suitable generator or grid.

20kw's of array/s is not an issue for a properly designed AC coupled system it will do this or even higher if needed, no issue, however there has to be point to doing big arrays and it's not always for to push into storage. For instance it maybe because there is a much bigger demand for power during daylight hours that is much higher than during non solar periods (dusk to dawn).

Most of the time batteries unless it's BIG pack will never accept amount of current input generally, for instance 120V @ 166A would need a pack in size of at least 120VDC 1500AH @ C10, to be able take the current input, however the charging will not continue at that for the whole charge cycle. An SP-Pro correctly programmed will control the array/s output to provide the maximum or whatever is left over for the batteries while supplying other loads at the sametime, the reaction times it runs and uses is staggeringly fast controls all voltages rock stable, completely transparent to the the user/s of power, it truly is managed exceptionally well by SP-Pro.

BTW there is more than one way of designing a system, some of which are not documented but are also great solutions for SPS use.

The question is really do you really need that much?

A proper load profile would set a maximum demand need, that controls design, which then controls cost and installation. 120VDC is great for reducing current demand on batteries if needed or keeping it realistic but is it really required, for battery packs particularly reducing continuous high current demands for lengthy periods in non solar times is a reason to consider them, a split, 2 or 3 phase system maybe another.

Oh and I still think PB is better for Off-grid / SPS. I understand why LifePO4 looks attractive but sometimes it's not always the best option. I have used and installed LiFePO4 for systems, there is still a big QM over the longevity. I know another system manufacturer that use other brands of equipment to produce "boxed" solutions and have done many, many more LiFePO4 systems than I have, they have come the same conclusion for SPS and longevity nothing beats PB done well, particularly with the new generation of carbon based PB cells. Yes they are heavier yes they take up more space yes the charge cycle maybe a bit less efficient. My own system the battery pack is 25+ years and still going remarkably well.

my 20cents input. to make it all muddy :)
SPS Standalone Power System/Hybrid specialist - Store the Sun!
SELECTRONIC SPMC482-AU, 8.2kW's of Arrays
Selectronic Certified AC-Coupled Fronius 8.2kW Inverter
1900Ah 48V Bank

An OTT Genset for a house.
PVOutput Stats

Sparkys light up your life :)
User avatar
davidg
Solar Crusader
Solar Crusader
 
Posts: 2102
Joined: Sat May 05, 2012 7:38 pm
Location: Dandenong VIC- *Really don't like Power Retailers at all.

Re: New Offgrid, DC or AC Coupled, and components

Postby dakiller » Sat May 26, 2018 11:13 am

I've done some more looking, and I've come to the conclusion that DC coupled doesn't work nicely with large PV arrays. You just don't have big enough controllers available to do it.

So I'm weighing up AC coupled into a 7.5kW SP PRO and I just picked the Fronius inverters to start with (I get the need for the SP PRO needing to manage the other inverters, so that they back off feeding in power when all demands are met.)

Looking though the documentation on it, I see tables on the minimum battery capacity V's AC Coupled inverter power.

http://download.selectronic.com.au/docu ... 0Notes.pdf

Image

Does that really mean, that a SPMC482 cannot have more than 15kW of solar feeding it? It says you need a minimum of 20kWh of battery to do it, which is fine here, but you can't do over 15kW?


The application of my system is a new home I'm building. It is a large 50 square house with pool. I'm designing a custom heat pump based HVAC system to do hydronic floor heating, ducted cooling, domestic hotwater and pool heating all of the one heat pump system. The compressor load for this will be about 10kW when run at 100%, but it will be able to be run at lower power levels when required. I had the opportunity to go either single phase or 3 phase for this, and will be going with single phase for more simplicity and not having to balance the rest of the homes loads across 3 circuits. The HVAC would be set to run predominately off the solar alone and to not be used much on battery.

The home will have an induction cooktop, which is an 7.2kW unit. I would absolutely like to be able to use this on battery.

There will be the standard home loads in the mix as well, but I don't think they matter in sizing inverters much in this instance. If you do start loading the system up, the peak capacity of the inverter will take the short term load, and if it doesn't let up, then it will tell the HVAC to ramp down for that time, or get to the point that alarms start to sound.

My desired 20kW array was a somewhat arbitrary choice when I picked it at the start, but my justification is that the panels are cheap, and an extra 5kW of panels isn't that much more compared to the overall system cost, and it allows for more generation in cloudy and poor weather and less demand on the batteries.
dakiller
Solar Supporter
Solar Supporter
 
Posts: 6
Joined: Fri May 25, 2018 11:57 am

Re: New Offgrid, DC or AC Coupled, and components

Postby LeighC » Sat May 26, 2018 1:06 pm

Hi, have you checked out the passive house build & specs yet?
I went to an introductory course the other week & I was surprised by using their principles & building an energy efficient home you may get away with a lot less loads & power usage than you think.
I'm not affiliated with them in any way but it was a real eye opener.
I am starting a build before long & it has really changed my thinking a great deal.
Cheers - Leigh
LeighC
Solar Supporter
Solar Supporter
 
Posts: 23
Joined: Sun Jun 13, 2010 10:34 pm
Location: Northern Victoria

Re: New Offgrid, DC or AC Coupled, and components

Postby Privatteer » Sat May 26, 2018 2:00 pm

I'm still looking into it myself and it may not suit your requirements but the LG version of batteries may suit.
There is the LG RESU 10H (9.8kWh storage, 3.5kW continuous power output) which can be DC coupled to a solar array at 350 - 450V. As I understand it as far as the solar inverter is concerned its a solar panel so getting double use of the inverter while also protecting against brief dips in output from clouds etc.

They also do a LG RESU 10 (9.8kWh, 5kW continuous power output) which is the more normal 48V version that can be stacked to at least 2 from my reading. Can be used with the selectronic to create the AC grid..
Stage 1:
Fronius Primo 5 + 6.6kW Canadian Solar
Privatteer
Solar Crusader
Solar Crusader
 
Posts: 476
Joined: Sat Nov 19, 2011 12:00 am
Location: South West WA

Re: New Offgrid, DC or AC Coupled, and components

Postby davidg » Sat May 26, 2018 7:39 pm

Privatteer wrote:There is the LG RESU

I have a trick up my sleeve for an OTT design of over 120kWh's of them, using 2 x SP-PRO SPMC-482AU
Which would also support upto 30Kw's of PV. #just saying
SPS Standalone Power System/Hybrid specialist - Store the Sun!
SELECTRONIC SPMC482-AU, 8.2kW's of Arrays
Selectronic Certified AC-Coupled Fronius 8.2kW Inverter
1900Ah 48V Bank

An OTT Genset for a house.
PVOutput Stats

Sparkys light up your life :)
User avatar
davidg
Solar Crusader
Solar Crusader
 
Posts: 2102
Joined: Sat May 05, 2012 7:38 pm
Location: Dandenong VIC- *Really don't like Power Retailers at all.

Re: New Offgrid, DC or AC Coupled, and components

Postby davidg » Sat May 26, 2018 9:16 pm

dakiller wrote:I've done some more looking, and I've come to the conclusion that DC coupled doesn't work nicely with large PV arrays. You just don't have big enough controllers available to do it.

Not quite true however there are reasons not to do it, rather than for doing it, it depends on what is needed and why. Both DC and AC coupled have there pluses and minuses, the trick is to understand what is the advantage to each and why.

dakiller wrote:So I'm weighing up AC coupled into a 7.5kW SP PRO and I just picked the Fronius inverters to start with (I get the need for the SP PRO needing to manage the other inverters, so that they back off feeding in power when all demands are met.).

Does that really mean, that a SPMC482 cannot have more than 15kW of solar feeding it? It says you need a minimum of 20kWh of battery to do it, which is fine here, but you can't do over 15kW?

SPS at the type of power demand for things you've suggested it be much more storage capacity than that in reality, however those numbers are the 100% capacity not the usable capacity.

Also it's of Grid type Inverters not actual PV which could be somewhat bigger array's. But the single SPMC-482 pretty much yes, the reason for the technical limitation and it could handle it in terms of management, however there are actual electrical components internally that are rated to about that. Hence the 15kW's = 65A @ 230VAC.

A design using 2 x 482's and would offer upto 30Kw's of inverters managed, All as Single Phase only system, NOT using LG RESU ideally. Other options abound, plus a reasonable Single phase generator.

dakiller wrote:The application of my system is a new home I'm building. It is a large 50 square house with pool.

Ideally don't use the SPS system as primary power for the pool there are other options for this, that are Solar driven and pretty much remove that load from an SPS system along with Solar pool heating, design this to store excess heat elsewhere maybe, rarely considered worth thinking about though. If you have the land space then this not should be that hard to do and it appears you will be rural or at least have some reasonable acreage.

dakiller wrote:I'm designing a custom heat pump based HVAC system to do hydronic floor heating, ducted cooling, domestic hotwater and pool heating all of the one heat pump system. The compressor load for this will be about 10kW when run at 100%, but it will be able to be run at lower power levels when required. I had the opportunity to go either single phase or 3 phase for this, and will be going with single phase for more simplicity and not having to balance the rest of the homes loads across 3 circuits. The HVAC would be set to run predominately off the solar alone and to not be used much on battery.

EG. Solar Tubes to preheat liquid during the day to large storage tanks if it needs to be boosted in temp do it during daylight hours when you have the power, in fact using the actual ground for heat storage as well is very useful indeed you can add months of heat during the summer then draw it back in the winter.

I spent a long time designing and working on something similar along with using a commercial sized cool room and freezer room for operating completely off grid not using generator and not requiring large battery packs. Simply using lots of power at all times is not the best way for heating, cooling or freezing.

(Farm House)It took several iterations of design for heating, cooling our house design, a bit smaller than yours 40 odd squares and developed a design using a combination of thermal mass (The entire building concrete slab for overnight stability) and Solar during the day that uses a fraction of the power of a compressor based system uses, I started there too, I wound up designing not using Hydronic Heating there are other options. For a Green field house, provided your prepared to get serious with insulation not the silly little amounts even the so called heavily insulated is often a joke that are normally installed into a house in Australia. The demand for power during non-solar periods falls by a truly massive amount, likewise this has a massive impact for cooling as well. I saw passive house design mentioned well worth reading up on, while our is not truly passive it certainly has aspects of it in our design, the impact on energy use is substantial and the long term maintenance cost is also heaps less.

So now pretty easy to cater for the likes of induction cooktops, pyrolytic ovens, and similar.
I've Done SPS system's that support these appliances. #just saying

dakiller wrote:The home will have an induction cooktop, which is an 7.2kW unit. I would absolutely like to be able to use this on battery.

I don't see that as an issue.

dakiller wrote:There will be the standard home loads in the mix as well, but I don't think they matter in sizing inverters much in this instance. If you do start loading the system up, the peak capacity of the inverter will take the short term load, and if it doesn't let up, then it will tell the HVAC to ramp down for that time, or get to the point that alarms start to sound.

Actually that should NEVER occur unless there is no generator or grid backup in the mix.

dakiller wrote:My desired 20kW array was a somewhat arbitrary choice when I picked it at the start, but my justification is that the panels are cheap, and an extra 5kW of panels isn't that much more compared to the overall system cost, and it allows for more generation in cloudy and poor weather and less demand on the batteries.

That's fine you want another 5kW's or even 10kW's that's not impossible on a single inverter even an SPMC-482AU if it's really needed, BUT it might NOT be the BEST option.

(Farm House)My wife insisted on a commercial cooking range 8 burners, 2 ovens (ZANUSSI) I had it converted to Bottled Gas but that's the only gas, apart from a possible emergency which an instant gas hot water unit if we run out of solar hot water 700 litres of hot water storage, but it could happen :lol:

Tank water, septic system/s, pumps are not an issue.


BTW. There areas in Vic connected to the grid where the houses are only provided with a maximum supply of 40A maybe if your lucky 63A single phase and if you want more you have to pay for three phase, which gets expensive due to all power being under ground. I know of another instance in TAS where the distributor would only provide 15A and that was it period.

An SPS system you can have almost any option, capacity etc, which sometimes are not available at all connecting to the grid, even if you are prepared to pay for it.
SPS Standalone Power System/Hybrid specialist - Store the Sun!
SELECTRONIC SPMC482-AU, 8.2kW's of Arrays
Selectronic Certified AC-Coupled Fronius 8.2kW Inverter
1900Ah 48V Bank

An OTT Genset for a house.
PVOutput Stats

Sparkys light up your life :)
User avatar
davidg
Solar Crusader
Solar Crusader
 
Posts: 2102
Joined: Sat May 05, 2012 7:38 pm
Location: Dandenong VIC- *Really don't like Power Retailers at all.

Re: New Offgrid, DC or AC Coupled, and components

Postby dakiller » Sun May 27, 2018 12:01 pm

Both DC and AC coupled have there pluses and minuses, the trick is to understand what is the advantage to each and why.

High power consumption during the day when solar is high and low consumption during the night, makes sense to go AC coupled, 100% SPS.

davidg wrote:
Privatteer wrote:There is the LG RESU

I have a trick up my sleeve for an OTT design of over 120kWh's of them, using 2 x SP-PRO SPMC-482AU
Which would also support upto 30Kw's of PV. #just saying

Anything that you are willing to share in detail here? No need to hold back anything technical, I've got an EE background.

davidg wrote:SPS at the type of power demand for things you've suggested it be much more storage capacity than that in reality, however those numbers are the 100% capacity not the usable capacity.

Also it's of Grid type Inverters not actual PV which could be somewhat bigger array's. But the single SPMC-482 pretty much yes, the reason for the technical limitation and it could handle it in terms of management, however there are actual electrical components internally that are rated to about that. Hence the 15kW's = 65A @ 230VAC.

A design using 2 x 482's and would offer upto 30Kw's of inverters managed, All as Single Phase only system, NOT using LG RESU ideally. Other options abound, plus a reasonable Single phase generator.


I still don't get why that is the case. If you are AC coupled, then the inverters from the PV are just tied into the AC bus, none of that power needs to pass though the 482. The 482 just needs to communicate to the other inverters to make sure that excess power isn't provided that overloads the system. I presume this is done by sensing the AC bus voltage and regulating it to its fixed nominal level.


davidg wrote:Ideally don't use the SPS system as primary power for the pool there are other options for this, that are Solar driven and pretty much remove that load from an SPS system along with Solar pool heating, design this to store excess heat elsewhere maybe, rarely considered worth thinking about though. If you have the land space then this not should be that hard to do and it appears you will be rural or at least have some reasonable acreage.

EG. Solar Tubes to preheat liquid during the day to large storage tanks if it needs to be boosted in temp do it during daylight hours when you have the power, in fact using the actual ground for heat storage as well is very useful indeed you can add months of heat during the summer then draw it back in the winter.

The pool is going to be part of the system as a whole and will be used as a store of thermal energy.

When the house is in cooling mode in the summer time, the heatpump will heat the pool with a heat recovery loop in the refrigerant cycle, essentially getting a 2 for 1 use of the power. Likewise in winter, heat can be extracted from the pool in the months that you wouldn't consider swimming in, or if there was an abundance of solar available, just heat the pool as a means to store excess.

I'm not considering solar tubes, it will be all PV powered.

Along with the pool, the house slab is another form of heat storage with the hydronic heating on during the day and to passively radiate though the house overnight.


davidg wrote:(Farm House)It took several iterations of design for heating, cooling our house design, a bit smaller than yours 40 odd squares and developed a design using a combination of thermal mass (The entire building concrete slab for overnight stability) and Solar during the day that uses a fraction of the power of a compressor based system uses, I started there too, I wound up designing not using Hydronic Heating there are other options. For a Green field house, provided your prepared to get serious with insulation not the silly little amounts even the so called heavily insulated is often a joke that are normally installed into a house in Australia. The demand for power during non-solar periods falls by a truly massive amount, likewise this has a massive impact for cooling as well. I saw passive house design mentioned well worth reading up on, while our is not truly passive it certainly has aspects of it in our design, the impact on energy use is substantial and the long term maintenance cost is also heaps less.

Insulation is well up in the list of priorities for this build. One of them is to ask our truss and framing designer about going with 120mm exterior frame over 90 for the added insulation.
dakiller
Solar Supporter
Solar Supporter
 
Posts: 6
Joined: Fri May 25, 2018 11:57 am

Next

Return to Components, Installation And Performance

Who is online

Users browsing this forum: No registered users and 4 guests

new solar power specials