Solar Cottage on an Island

GaryO · May 2015

Hello all.

I'm about 75% complete in the construction of a 1200 SQFT cottage on an Island. Solar is my only practical power solution and I am getting ready to make some purchasing decisions.

I have calculated my power requirements to be approximately 3000Wh per day (not including HVAC). This is completely feasible to be 100% solar with a generator backup.

However, I want to install a mini-split system which will consume 1300W of energy (230VAC) by itself. I have settled on the Fujitsu mini-split systems since they seem to have the highest efficiency on the market (29-33 SEER, depending on the unit).

The first solar reseller I spoke with recommended I run the HVAC completely from the generator (starting the generator only when the HVAC is calling for it). However, this is not possible according to Fujitsu since their compressor unit powers the wall unit (there is no way to divorce the control power from the main power from what I can see in their schematics).

It seems like the best method would be to tie the HVAC into my off-grid solar inverter. The HVAC controller does have a contact to which activates when the system is running. I could use this to tell the generator to start. Ideally, the inverter could then use this opportunity to charge batteries while the generator handles the load for the duration of the call for HVAC. When the HVAC shuts off, the generator shuts off and the system returns to battery-only mode.

Does anyone here have any advice on how to accomplish this? Or a better way to accomplish this?

Gary

westbranch · May 2015

Wild eyed :cool: guesstimate based on our place... you will need well in excess of 2500W of panels to run those loads.

.. Where can you economize on power...???

GaryO · May 2015

Thanks for your reply, westbranch. I will be installing (12) 300W panels for my system. My question is more related to the best method to bring the generator online to power the HVAC demand. It seems unnecessary to size my solar/battery system to run this intermittent 1300W load. I am looking for a best practices for configuring a generator to cover this "peak" load while also charging my batteries when it does.

Sequence of operation:

1) Mini-split HVAC system is tied to inverter system drawing minimal power while it monitors room temperature and occupancy.
2) Mini-split HVAC system calls for cooling and commands the compressor to activate, drawing as much as 1300W. This event causes a contact closure on the HVAC.
3) Contact closure tells generator to start.
4) Generator takes over power delivery to the large HVAC load (and charges batteries).
5) Mini-split satisfies temperature requirement, shuts off compressor and contact closure opens.
6) Generator turns off and power is restored to battery-only mode.

The HVAC will draw between 500W and 1300W when active, and I don't plan to run this off of the batteries for more than a few seconds while the generator activates.

I appreciate any help that anyone can offer!

westbranch · May 2015

OK looks like you will need an inverter/charger like an outback or Schneider that allows what is know as Generator support and will turn the gen on and off automatically, based on load... pick a brand and we can go from there..

Vic · May 2015

GaryO,

Have you seen this Thread on this Forum?:
http://forum.solar-electric.com/forum/solar-electric-power-wind-power-balance-of-system/energy-use-conservation/23007-mini-split-update-for-offgrid

IMO, due to the way that Min-Split systems operate, looking to Generator Support for this function may not work well, because, generally the compressor can run for extended periods, but at fairly light loading. All depends on your situation. FWIW, Vic

GaryO · May 2015

Vic -

That's a great thread. Read the whole thing after you linked me. Some interesting stuff there on WIFI control features now offered by some of the manufacturers (in addition to a great supplier with some excellent pricing).

I'm going to have to look into this "Generator Support" function that westbranch mentioned. From discussing more with some of the solar suppliers, it sounds like the "normal" mode of off-grid option will have the inverter automatically moving over to generator supply as soon as it sees load on the generator terminals (Magnum inverters), simultaneously allowing the inverter to go into charge mode to fill the batteries while the generator runs (this is what I want).

However, the solar charge controller will probably sense this high voltage at the battery terminals and go into maintenance mode, essentially throwing away any power being generated by the array during that time.

Despite the fact that it sounds like my approach will work, it still feels like there may be a more efficient way to run this system. I will put a post into that thread and see how others are driving their HVAC system.

Vic · May 2015

OK< thanks Gary for the reply,

Not exactly to your question, directly, but more to the point of how many Mini-Split Inverter-type A/C units operate.

The Inverter allows the amount of cooling required to vary the output of the compressor. This often means that the compressor runs for a long period of time, but at light loading. This means that the power needed to run the A/C can be quite small. It would make little sense to run the generator when the loading is light, and the PV or other RE energy is sufficient for the task.

Later, Vic

westbranch · May 2015

Another thought just popped up and that is to consider an inverter type gen set to economize on fuel consumption if you are going to use an inverter type mini split that will have longer run times with lower power demands than a conventional A/C unit

GaryO · June 2015

Vic wrote: »

OK< thanks Gary for the reply,

The Inverter allows the amount of cooling required to vary the output of the compressor. This often means that the compressor runs for a long period of time, but at light loading. This means that the power needed to run the A/C can be quite small. It would make little sense to run the generator when the loading is light, and the PV or other RE energy is sufficient for the task.

Vic -

This is a good point. However, I believe upon arriving at the property the inside will be at a max temperature differential to the outside and both mini-splits will immediately turn on and go to maximum compressor output. This does beg the question for the control scheme. Maybe I need to bring the generator online only for the initial cool-down and then allow the battery system to take it from there. Perhaps I can monitor power to the mini-split and turn the generator on only when the loading exceeds X watts.

GaryO · June 2015

westbranch wrote: »

Another thought just popped up and that is to consider an inverter type gen set to economize on fuel consumption if you are going to use an inverter type mini split that will have longer run times with lower power demands than a conventional A/C unit

I will be using either the 15kW or the 6kW generac propane generator. Primarily because it makes for a nice looking permanent install that will not be as prone to theft and also because I am able to get about $2000 back (approx half the cost) on either of those through an incentive by the Propane council. I'm not a fan of propane, but I need it on site anyway for cooking and instant hot water.

Dave Angelini · June 2015

Use the 15KW generator as the sound will keep the sharks away. Oh, that is a fresh water island, I forgot. Well maybe the sound will keep the thieves away. They will know where you are and nothing is permanent as far as thieves go. I have seen them steal a septic tank.

Since this place is going to be unoccupied, I think you need to rethink some of this. Start small, what is the rush? Time can be a big help and providence often steps in and makes the thinking clear.

westbranch · June 2015

Also there are some propane conversion kits out there that will work on the Honda Eu3000i and others as well. And also autostart addons.
The H3000 series ,and up, are electric start

GaryO · June 2015

Dave Angelini wrote: »

Use the 15KW generator as the sound will keep the sharks away. Oh, that is a fresh water island, I forgot. Well maybe the sound will keep the thieves away. They will know where you are and nothing is permanent as far as thieves go. I have seen them steal a septic tank.

Since this place is going to be unoccupied, I think you need to rethink some of this. Start small, what is the rush? Time can be a big help and providence often steps in and makes the thinking clear.

The 6kW is rated for lower noise level than the Honda inverter generator of equivalent size. However, although it is rated 10dB louder at full load, the 15kW is also a variable speed unit which idles the engine down at low loads (the 6kW Generac is constant speed). I've been thinking about this for a long time. Problem is, I'm doing my electrical wiring now and decisions must be made for electrical systems before I put up my knotty pine paneling on the interior.

GaryO · June 2015

westbranch wrote: »

Also there are some propane conversion kits out there that will work on the Honda Eu3000i and others as well. And also autostart addons.
The H3000 series ,and up, are electric start

I really don't know why I would do this since the Generac 6kw is rated for the same noise level as the equivalent Honda, is made for off grid applications, looks nice for a pad mounted permanent install, has a huge advantage on oil change intervals and has remote start out of the box. Oh, and with the incentive, it costs half as much.

Mountain Don · June 2015

GaryO wrote: »

Generac 6kw is rated for the same noise level as the equivalent Honda, is made for off grid applications,

That is a nice generator. I have a friend with one. Running noise is not objectional as far as generators go. His operates on propane.

GaryO · June 2015

The frustrating thing about Generac is that I can't get an answer from them about the efficiency between their 6kW and 15kW unit. There are significant design differences. Their own specifications show that the 15kW uses 17% less propane at 7500W (half load) than their 6kW unit uses at full load because of the variable RPM design of the 15kW. This begs the question of the efficiency of both generators at small or no-load condition. I would also like to know the noise level of the 15kW as it idles down to no-load since it has variable RPM. Neither spec is available, but if the 15kW is at the same noise level and uses less propane than the 6kW when operating in the < 3000W range, it would be the smarter purchase.

Mountain Don · June 2015

How about the warranty? A few years ago when my friend bought his 6 Kw he was told it was the only one they would warrant when used off grid. His use may have been different though as because he is in a deep canyon he has virtually no solar ability and relies on the 6 Kw generator to keep the batteries up as needed.

westbranch · June 2015

Mountain Don wrote: »

6 Kw he was told it was the only one they would warrant when used off grid.

If you search here forGenerac, I beleive you will see comments about contiuous use and off grid use are not covered for most of their products...

Mountain Don · June 2015

OK good... I know that my friend John is glad he bought the model he did as Generac has had to repair a couple things and the warranty covered them.

GaryO · June 2015

westbranch wrote: »

If you search here forGenerac, I beleive you will see comments about contiuous use and off grid use are not covered for most of their products...

These ECOGEN products are specifically made for off-grid and they are warranted for that. However, they do NOT want you to use it for primary (continuous) power - only as part of an alternative energy off-grid system like solar or wind.

Dave Angelini · June 2015

Mountain Don wrote: »

OK good... I know that my friend John is glad he bought the model he did as Generac has had to repair a couple things and the warranty covered them.

And that is why the Honda make just keeps running for decades! Change the oil and that is about it.

GaryO · June 2015

Dave Angelini wrote: »

And that is why the Honda make just keeps running for decades! Change the oil and that is about it.

I think the Honda wants an oil change at 50 hrs which isn't practical in my case at all. I could run enough for an oil change over a 3 day weekend!

Vic · June 2015

Gary,

Hope that you have the option to add considerably more PV to the system. Also hope that you are considering, carefully, the battery capacity for the system -- it is often difficult to just add some more batteries to a system, without tossing the first set.

It is just a personal taste thing, but, L O N G generator run-times can be very tedious. Realize that humidity may be an issue in your area, but expecting a genset to cycle a number of times per day can create maintenance issues.

Also, even it the 15 KW genset that you are considering is an Inverter model, this is still a large genset, and still seems over-sized, to me. Many inverter-generators are not really Variable Throttle designs, but really Stepped Throttle units, which reduces the of fuel savings.

Propane is not an efficient fuel for engines, but it is convenient, as it ages well, and can be delivered in large quantities ... wonder, on your island, just how the Propane will get into your tank ... does the Propane truck come over on a Ferry ?

Opinions, FWIW, Vic

BB. · June 2015

Look at the fuel flow rates for your genset. I try to recommend as small as genset as practical for folks. Or even two gensets, one small for keeping battery bank charged/running in bad weather and a second large genset for larger (shop, etc.) loads and as backup for the small genset.

For example:

http://gens.lccdn.com/generaccorporate/media/library/content/all-products/generators/home-generators/ecogen-series/15kw-6103/0k8466-c-15kw-ecogen-hsb.pdf

15kWatt Generac on Propane:

Liquid Propane ft3/hr (gal/hr) [l/hr]
1/2 Load==47.61 (1.32) [4.99]
Full Load==104.4 (2.87) [10.86]

That is 1.32 Gallons per Hour for 50% to (roughly) 0% load... Can you "consume" ~7.5 kWatts from your genset for hours on end for fuel efficient operation? If you you only need a few kWatts, that is a lot of fuel for little amount of power.

-Bill

GaryO · June 2015

BB. wrote: »

That is 1.32 Gallons per Hour for 50% to (roughly) 0% load... Can you "consume" ~7.5 kWatts from your genset for hours on end for fuel efficient operation? If you you only need a few kWatts, that is a lot of fuel for little amount of power.

-Bill

Bill:

I've been looking at these specs for some time. Why do you believe that this unit consumes 1.32g/h all the way down to 0% load? The 15kW is a variable RPM generator. I'm not an expert on these, just don't know how you jump to that conclusion.

I completely agree that buying the smallest generator would be the intuitive right choice for economy, but the 6kW Generac consumes significantly more propane at 6000W than this one does at 7500W. Having said that, The only time I will need this kind of power is when the battery charger is set to maximum charge level. Otherwise, I could get by with something less than 3000W including the HVAC.

- Other contributing factors to considering the 15kW: 1) It's nice to have the extra capacity when you need it one day. 2) The 15kW comes ready to supply 230V, while the 6kW requires a conversion kit - actually making the 6kW more expensive. 3) The 15kW is a newer design than the 6kW.

GaryO · June 2015

Vic wrote: »

Gary,

Hope that you have the option to add considerably more PV to the system.

Propane is not an efficient fuel for engines, but it is convenient, as it ages well, and can be delivered in large quantities ... wonder, on your island, just how the Propane will get into your tank ... does the Propane truck come over on a Ferry ?

Vic-

I am the delivery mechanism for all things island related - including the propane. I will have to bring it over in 100# canisters (they hold about 24g of propane each). There will always be one in use and one on deck. Since I need propane for the stove and instant hot water heater, it seems like the obvious choice for a generator that won't have to run full time.

My usage on the island will be periodic. There for the weekend, gone for the weekdays, typically.

Sadly, even though PV systems are a perfect match for my needs, my 1.25 acre island is heavily wooded. I won't have perfect sunlight for the PV array. I'm even half considering putting my panels out over the water to escape the tree line.

Gary

Vic · June 2015

OK, Gary,

It is hard for the rest of us to picture all of the details of anyone else's site.

A 15 KW genset is very large, compared to the rest of your system.

As BB Bill mentioned, it is very nice to have several genset options -- believe that was your plan, anyway.

We do not know about the rest of your system, like just what is your inverter, planned battery etc, and therefore the charger and charging needs that the genset will need to supply.

Again, the Inverter gensets that I have looked at are NOT really Variable Throttle -- they are Stepped Throttle, with about four plateaus. This somewhat reduces the fuel savings at partial-load on the genset.

You have the data, and you are the fuel transporter, and the endurer of any generator noise.

Gen Support would drive me NUTSo, but you know your sensitivities, and seems that, given the location/setting of the site, you do not have too many options.

Opinions, FWIW, Good Luck. Vic

BB. · June 2015

Here is a graph of a typical 7 kWatt propane generator fuel usage from (fixed RPM unit, I believe):

http://www.centralmainediesel.com/order/7000-watt-propane-generator.asp?page=P7000

Attachment not found.

I agree your inverter/generator should use less fuel than a fixed RPM unit at lower power usage... Interestingly, at least for the smaller Honda eux000i family, they are probably a bit less efficient at full loads (the wild AC to AC inverter fixed frequency output conversion losses) vs fixed rpm units.

I wonder if Generac will send you a fuel consumption graph/data?

I just try to be very conservative--If the Generac 15kWatt is fuel efficient down to 25% of rating--That would be 3.75 kWatts...if you can maintain that as an average load--Then it may be a good match for you.

In any case is ~0.7 gph hour of propane "OK" for your needs?

-Bill

Dave Angelini · June 2015

I remember how great it was to have a saltwater watermaker on board my boat. I did not have carry/Lug/ jerry jug water anymore. Even at 1.3 gallons an hour it was the most important piece of gear on board beside the GPS and autopilot. I hope you really love this place as it is quite a challenge just to think about your constraints, let alone do them.

GaryO · June 2015

Vic wrote: »

OK, Gary,

We do not know about the rest of your system, like just what is your inverter, planned battery etc, and therefore the charger and charging needs that the genset will need to supply.

Again, the Inverter gensets that I have looked at are NOT really Variable Throttle -- they are Stepped Throttle, with about four plateaus. This somewhat reduces the fuel savings at partial-load on the genset.

You have the data, and you are the fuel transporter, and the endurer of any generator noise.

Gen Support would drive me NUTSo, but you know your sensitivities, and seems that, given the location/setting of the site, you do not have too many options.

Opinions, FWIW, Good Luck. Vic

I do hate the generator noise, however it seems like 53dB (the volume of a quiet conversation) would be tolerable, but I won't really know until I get there.

The solar system being recommended to me is: 12 panel system capable of 3,360W. 48V, 415Ah AGM battery bank and Magnum 4400W inverter which can do 230VAC.

I am in an area with about 4.2 sun hours which will be further reduced by the trees (which I don't want to cut down since they provide privacy and shoreline erosion protection).

The periodic use makes me hesitant to go crazy with a huge battery array (which I know I will have to replace in 5 years). I like the generator because I can start with that (alone) and then add the solar and convert the generator to serve as the backup system when batteries deplete.

Again, I'm here for the advice and other options from people who have done this already. My ideas may be completely misguided!

GaryO · June 2015

BB. wrote: »

Here is a graph of a typical 7 kWatt propane generator fuel usage from (fixed RPM unit, I believe):

I agree your inverter/generator should use less fuel than a fixed RPM unit at lower power usage... Interestingly, at least for the smaller Honda eux000i family, they are probably a bit less efficient at full loads (the wild AC to AC inverter fixed frequency output conversion losses) vs fixed rpm units.

I wonder if Generac will send you a fuel consumption graph/data?

In any case is ~0.7 gph hour of propane "OK" for your needs?

Bill:

The 6kW Generac is actually FIXED RPM. I'm guessing it follows your curve fairly closely. The 15kW is variable RPM from 2700-3600 RPM.

Generac will not provide ANY data not in their spec sheet. I tried everything - even calling their manufacturing site to speak with a design engineer. Their minions who answer the tech support line simply read the spec sheets back to you which isn't very useful...

If I do as I originally stated and use the generator simply to run the HVAC when it calls for operation, the generator will supply 1300W plus whatever the battery charger wants. My understanding is that the battery chargers will allow you to set a charging level based on the size of generator you have (so I assume I could get faster battery charging from a higher capacity generator).

Since I will have canisters of 24g of propane, 0.7g/h of propane would allow that canister to last for 34 hours of continuous runtime. I can probably live with that (but, of course, will not feel as happy about it when I have to lug another 100# tank onto the boat to replenish it!).

Gary

Solar Cottage on an Island

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