Tiny House Off Grid... Need help w/ system selection!

[J.P.M.]

Assuming I set up an evacuated tube type collector to heat water, it might end up being practice that the hot water tank gets cycled once a day on electricity if there's no sun using generator load support. I don't think this would end up being too costly with a small honda inverter setup.

I like the evacuated solar collector idea.

Why an evacuated tube collector ? Unless water temps. > ~ 70 C. are required, or the application is in a VERY cold climate, a simple flat plate collector system will produce as much or more hot water for a lot less $$. Reliability may well be higher for the flat plate as well.

[karrak]

Assuming I set up an evacuated tube type collector to heat water, it might end up being practice that the hot water tank gets cycled once a day on electricity if there's no sun using generator load support. I don't think this would end up being too costly with a small honda inverter setup.

I like the evacuated solar collector idea.

Thumbs up on the evacuated tube hot water heater. We have had a flat plate collector system for over twenty years and an evacuated tube systems for around six years. In cloudy weather the evacuated tube system if far more efficient than the flat plate system. Don't have an electric booster for the evacuated system and it would be only around ten days a year that the water is not hot enough for a shower.

Maybe J.P.M. could comment on the thermodynamics of tube versus tube systems in cloudy conditions

If you get one of the cheap Chinese tube systems make sure the tank is made from 316 grade stainless steel and that you change the sacrificial anode on a regular basis. Ours has to be replaced once a year.

Simon

Off grid 24V system, 6x190W Solar Panels, 32x90ah Winston LiFeYPO4 batteries installed April 2013
BMS - Homemade Battery logger github.com/simat/BatteryMonitor
Latronics 4kW Inverter, homemade MPPT controller

[karrak]

Have you looked at lithium ion batteries instead of lead acid?

Much smaller and lighter, can be kept inside, can sustain discharge rates of C/2 with ease, and are far easier and more efficient to charge

Simon

Off grid 24V system, 6x190W Solar Panels, 32x90ah Winston LiFeYPO4 batteries installed April 2013
BMS - Homemade Battery logger github.com/simat/BatteryMonitor
Latronics 4kW Inverter, homemade MPPT controller

[J.P.M.]

Thumbs up on the evacuated tube hot water heater. We have had a flat plate collector system for over twenty years and an evacuated tube systems for around six years. In cloudy weather the evacuated tube system if far more efficient than the flat plate system. Don't have an electric booster for the evacuated system and it would be only around ten days a year that the water is not hot enough for a shower.

Maybe J.P.M. could comment on the thermodynamics of tube versus tube systems in cloudy conditions

If you get one of the cheap Chinese tube systems make sure the tank is made from 316 grade stainless steel and that you change the sacrificial anode on a regular basis. Ours has to be replaced once a year.

Simon

Off grid 24V system, 6x190W Solar Panels, 32x90ah Winston LiFeYPO4 batteries installed April 2013
BMS - Homemade Battery logger github.com/simat/BatteryMonitor
Latronics 4kW Inverter, homemade MPPT controller

I'd be happy to comment, but please understand that the analysis does not involve much, if anything, in the way of Thermodynamics. It's about heat transfer, mostly of the convective and radiative types and the solar energy resource.

1.) For starters, when conditions are cloudy, the lower P.O.A. irradiance will mean less energy available for solar devices to collect/use. Evac. tube collectors may well collect more energy than other type of collectors under low irradiance (see below), but the total amount of useful energy collected will still be small relative to sunnier conditions. A larger amount of what's already a small amount available over the course of a day/month/year is still a small amount. Not a lot of energy is turned into heat by most any solar thermal device at low light conditions. the higher collection eff. of evac. tubes under low light is mostly a bogus deception, not unlike Sunpower's "most efficient" B.S.

2.) Since the goal of water heating is to provide the max. amount of energy at the REQUIRED delivery temp. at a competitive price to the available alternate means of meeting the duty - not necessarily the max. temp., that means that for DHW applications, flat plate collectors are usually a better fit for the duty when temp. requirements are moderate.

3.) Evacuated tube collectors have a more horizontal efficiency profile on a graph of efficiency vs. temp. That's because the evac. tube collector will have a lower overall heat loss coefficient because of little to no convective loss between the absorber plate and the glass tube. That's an advantage if high temps. are required for things like process, but comes at a price in initial cost and probably maint.

4.) However, the OPTICAL efficiency of an evacuated tube collector is always less than a flat plate collector at any temperature, and that will mean less heat collected when lower service temps. of the kind usually found in DHW applications are required. That lower optical efficiency is primarily due to two factors. The glazing of the evac. tube collector ( the evac. tubes themselves ) present a larger average angle of incidence to the sun and that increases surface reflectance, resulting in larger optical losses because of a decrease in the amount of solar energy that makes it past the outer glazing surface (the tube) to the absorber. Also, because of the geometry of the tubes and absorbers, less NET area as a % of the evac. collector's gross plane area is available to produce heat. A std. flat plate collector may have upwards of 93-94 % of its gross external dimension available as absorber surface. An evac. tube collector with tight packing may have about 85% or so of the gross collector dimensions available for absorber surface. Less absorber surface, less energy collected.

5.) Evacuated tube collectors have some open distance between the tubes, either a relatively small dimension of say 3/16 " or a bit more, or a larger dimension of the order of a couple of inches, often and usually with reflective materials behind the tubes either as planar reflectors or CPC (compound parabolic concentrators). The cheap evac. tube units use simple planar AL that seems to have a tendency to oxidize and lose reflectivity and thus lower efficiency even more. The first configuration usually reduces net to gross area ratios by about 15% or so. The second configuration usually halves the absorber area or more but makes up for some of that with the back reflectors. However, the back reflectors usually have about 20 % or more optical losses of their own and also have increased maint. requirements. Overall, evac. tube units lose about 15+ % of their available effective surface area due to configuration limitations.

6.) OG-100 collector rating numbers can be instructive.

Under sunny conditions of 2,000 BTU/ft.^2/day:

A typical well made evac. tube collector (SPP - 30A from Solar Panels Plus) will produce 36,000 BTU/day at a 90 F delta T., amb. to fluid temp. or (36,000 BTU/day)/(44.75 ft.^2 absorber surface) = 804 BTU/ft.^2 per day per ft.^2 of absorber area.

A typical well made flat plate collector (TRB - 32 from Sunearth) will produce 25,400 BTU/day at a 90 F. delta T, or 25,400/29.42 ft.^2 = 863 BTU/day per ft.^2 of absorber.

Furthermore, at most common DHW service temps., , say, 30+90 F. = 120 F., and using these fairly typical examples, typical flat plate heater heaters produce more bang for the buck: The Sunearth flat plate panel sells for about 60 % as much as the SPP 30 A but generates about 70% as much heat on a per panel basis.

For the same 90 F delta T. at 1,000 BTU/ft.^2 per day irradiance:

Evac. tube = 13,000 BTU/day = 291 BTU/ft^2/day.
Flat plate = 4,800 BTU/day = 163 BTU/ft.^2/day.

The point there is that if you live in a cloudy climate, while the evac. tube heater produces about 2X as much heat per ft.^2 as the flat plate, it's still a relatively small amount of heat for the price of either unit, probably making fossil fuel fired hot water a better choice than solar of either collector type. Hell, burning wood makes more sense at those prices. Solar water heating is not for every climate.

7.) Another thing to consider with respect to evac. tube units: shooting for higher operating temps. increases standby losses proportionately. One way to mitigate that is to increase the storage size and thus lower the average temp. at which the unit operates. However, that larger tankage costs money. It also means that any needed advantage of higher temps. made possible by the evac. tube will either be lost or diminished. Kind of counter productive.

The bottom line: Evacuated tube collectors have their place. Their lower heat loss coeff. make them well suited for medium to high temp. applications. But, for residential DHW applications which are usually classified as low temp. applications, it's overkill. Evac. tube units can produce quite high temps. Point is, the high temps that can be attained are not needed in a well designed DHW system so that capability is wasted. However, because it's wasted doesn't mean you don't pay for it. You do.

For particulars about evac. tube and flat plate design particulars, see Duffie & Beckman.

[Mike90250]

The large "house" sized inverters have generator load support The Schneider Conext XW series does. (24 & 48V models, but they are not cheap)

[karrak]

J.P.M.,thanks for the information, very informative, backs up my observations.

Interestingly, the evacuated tube hot water system was cheaper than I could get a flat plate system.

Simon

[J.P.M.]

J.P.M.,thanks for the information, very informative, backs up my observations.

Interestingly, the evacuated tube hot water system was cheaper than I could get a flat plate system.

Simon

You're welcome. Pricing could be different for many reasons that are site and situation specific. Every application is somewhat unique. Perhaps the flat pate quote was for larger than needed capacity. Vendors have been known to take advantage of user ignorance to sell excess surface.

The bottom line in life is as long as your happy. Although for a lot of folks, when it comes to energy use, it does seem that ignorance is bliss fits what I've observed in behavior over the years..

[Sunking]

Get lost Karrak, you have no biz here.

[John Galt 1]

Most people who live off grid use propene for heating their water, perhaps you should too. It would also make running a dish and clothes washer on sunny days much more doable. While I've never checked the power draw my washers use it isn't very high. roughly 750 watt hours for each cycle.. On sunny days your batteries will go into absorb before lunch most days and much to the power your panels could make will not be passed through the controller after the bulk phase ends.

Magnum supports auto generator start. http://www.solarhome.org/magnumme-ag...rkversion.aspx As your loads draw your battery down the voltage drops. If the voltage drops below the set point for more than a set amount of time (set time is to avoid a sudden high amp draw such as a motor starting tripping the auto start) then the AUX function on the inverter or controller can start the generator.

[J.P.M.]

Most people who live off grid use propene for heating their water, perhaps you should too. It would also make running a dish and clothes washer on sunny days much more doable. While I've never checked the power draw my washers use it isn't very high. roughly 750 watt hours for each cycle.. On sunny days your batteries will go into absorb before lunch most days and much to the power your panels could make will not be passed through the controller after the bulk phase ends.

Magnum supports auto generator start. http://www.solarhome.org/magnumme-ag...rkversion.aspx As your loads draw your battery down the voltage drops. If the voltage drops below the set point for more than a set amount of time (set time is to avoid a sudden high amp draw such as a motor starting tripping the auto start) then the AUX function on the inverter or controller can start the generator.

Depending on climate and how low one can get the DHW use, propane may well be a better alternative than solar water heating, especially if propane is used for other tasks like space heating or cooking. But, I guess if I was off grid with propane, and something of a DIY'er, I'd consider something like a breadbox water heater as a preliminary heat source in front of other water heating sources. Those bread box heaters are pretty dumbed down, dummy proof and can be quite effect from a cost and efficiency standpoint, especially in a milder climate that gets some sun. The drawback is you and any neighbors probably need a bit of a sense of humor about them when it comes to appearance. See builditsolar for ideas.

[PNPmacnab]

Wanting to live off grid in the middle of nowhere, you already have think differently. I have the same number of panels at my camp in the Adirondacks I stay at 4 months in the summer. All my hot water for several years has been from excess solar power. I average over 2KWH excess dump each day after running everything. I have a 9 gal and 20 gal in series. The smaller one heats first and feeds the cold water side of the shower. The hot water side is fed by a 6 gal propane water heater.So I have the best of both worlds, Just turn the handle to the side that has the hot water. Last couple of years I have never had to turn on the propane. I have a 7CF chest freezer with a large amount of water to store cold. It only operates during the day at very close to 33F. That gets me through the night till it starts again in the morning. Need to do shopping in the morning, can't load up with groceries late afternoon and no frozen foods (they only make you fat anyway). Of course the whole system goes to crap about the middle of October when the dreary days begin of massive cloud cover. It is surprising how well the system does with only one car battery. If I had more I would only become lazy like everyone else. I would go heavier on the panels. That gets you through the bad days and is far cheaper to do than batteries. Learn how to program those little $3 micros to schedule and set priorities. Necessity is the mother of invention. This next season I will be adding a modified dishwasher that will do time slice operation.

Whatever you do don't buy 24V heater elements. 2,000W 120V elements are readily available and cheap. They work fine with a 36V string (50V) and even better with a 48V panel system. It is quite easy to operate them at power point directly off the panels.

[engage1000]

Hey guys,

Merry Christmas and Happy New Year! Just touching base, thanks so very much for all your ideas and input! This thread has been a huge help and as a result of the discussions on here...

-I've abandoned the idea of heating the 6 gallon tank with electric, and will implement a solar thermal system and will experiement with one of those Espar hydronic heaters and make use of the hot water tank's built in exchanger. Saved 1500 watts/day (estimated). Ka-ching.

-Fridge, Yeah, the bar fridge. As soon as funds allow, within a year, it will be turfed for a Novakool that rings in at 400ish watts per day on DC from the factory, and if additional insulation is added which I totally will do, then 200-250 watts is apparently common from what I've read.

All of a sudden the measly 400 amp planned bank should shoot for two days autonomy no sweat. And I promise to have strong panels.


sunking,

I've had a couple weeks to digest your informative grounding lesson, thanks so much for the time and effort you spent to write it.

Grounded Equipment vs Grounded System... check... I think. Magnum inverter manual calls for "grounded system", I think, in that the literature specifically spells out a scenario of grounding negative DC to trailer frame and have fuse on POS only. AC ground is tied to this as well (but not neutral, as this is handled by the mobile inverter, sensing whether there's a bond upstream of itself or not, I think). It's nice to understand a bit more about why that is.

I'm losing you however when you start talking about said system floating, needing two OCPD's, and then start talking about ground fault in a DC system. I don't know enough about DC systems to know how ground faults work (I assume it's different than a short circuit), and how do two fuses in a floating system help you from not getting shocked?

Above and beyond that, I'll reference the inverter manual again in that it doesn't state specifically to ground the grounded trailer frame to the ground with a rod, there's no mention of it. I assume this is because they assume it's a mobile platform (which in my case it pretty much is not, it will get moved once or twice maybe).

What are the potential problems of ground the system to frame, but not to earth???

Last question, based on your graphps, the only apparently physical difference between a floating system and a grounded system is tying battery NEG into the ground bus. Is this true?

And final last question... if you did that, does one need to match the ground gauge with the gauge of the negative battery cable? Does one really need a 4/0 DC ground to frame??

Thanks!

[Sunking]

Sunking,

I've had a couple weeks to digest your informative grounding lesson, thanks so much for the time and effort you spent to write it.

You are more than welcome and my privilege to help. Since you took the time to read and dig to ask very good questions, I will do the same in return and help you a smuch as I can. Just do not get to hung on on details and make i tmore complicated than it really is.

Grounded Equipment vs Grounded System... check... I think. Magnum inverter manual calls for "grounded system", I think, in that the literature specifically spells out a scenario of grounding negative DC to trailer frame and have fuse on POS only. AC ground is tied to this as well (but not neutral, as this is handled by the mobile inverter, sensing whether there's a bond upstream of itself or not, I think). It's nice to understand a bit more about why that is.

Ok that makes life easier on you because you have no choice to make. You do not have the luxury of choosing between a Grounded or Un-Grounded system. The manufacture made al lthose decisions for you already and you have no choice but to comply and use a Grounded System. See that is not complicated, just go with it. You have no choice.

I'm losing you however when you start talking about said system floating, needing two OCPD's, and then start talking about ground fault in a DC system. I don't know enough about DC systems to know how ground faults work (I assume it's different than a short circuit), and how do two fuses in a floating system help you from not getting shocked?

OK let's clear this up real fast and simple. Makes no difference if it is AC or DC. Both work exactly the same. Short circuits and Faults are the same thing. To operate a OCPD requires it to have more current than the device rating. In fact to operate effectively 6 times the device rating. So for a 20 amp breaker 120 amps fault or short circuit current. Only difference between th eAC and DC is the operating voltage, and since DC is much lower voltage means it must have much lower Resistance. Both must obey Ohm's Laws. Current = Voltage / Resistance. Quick lesson and Q/A. If we have a 120 VAC circuit with a 20 amp breaker what must be the total Fault or Short Circuit resistance to induce 6X fault current. Do it again for 12 volt DC.

I will do the AC one for you, then you tel me DC. Yu should not even need a calculator or pencil to do this. For 120 VAC we need 120 amps right for a 20 amp Breaker. 120 volts / 120 amps = 1 Ohm. So the Ground Wire must be half of that Resistance because it is half of the total circuit resistance. So tell me what it takes at 12 volts DC?

See how easy that was? It takes 05 ohms at 12 volts. Understand?

What are the potential problems of ground the system to frame, but not to earth???

Excellent question and this should help turn the lights on for you, or knock them out if you do not understand. Electrocution and/or Death if you fail to bond your trailer frame to earth. Here is what I mean. Does not matter if your System is Floating (un-grounded) or Grounded. You are still required to Ground the equipment to earth or something in place of earth if nominal system voltage is 50 volts or greater. Don;t get hung up on your low voltage battery, you have a 120 volt AC Inverter. Forget the low voltage battery.

On a Floating system as you noted both Polarities have OCPD. If one touches something like ground or metal nothing happens. Ground th eothe rpolarity and sparks will fly. A ground system only one polarity, AC or DC, has a OCPD. On a Grounded System only one polarity, the un-grounded polarity has the OCPD and the other is BONDED.

Last question, based on your graphps, the only apparently physical difference between a floating system and a grounded system is tying battery NEG into the ground bus. Is this true?

And final last question... if you did that, does one need to match the ground gauge with the gauge of the negative battery cable? Does one really need a 4/0 DC ground to frame??

Thanks!

That is two questions.

Technically it can be either the positive or negative polarity. You have no choice because the manufacture already made it for you. Negative.

Last is an excellent question and should connect the dots. First a question for you. What does it take to make a OCPD work? Remember 6 times rating?

So how do we do that? We have to use a Planned Path. We use a GREEN OR BARE WIRE as a planned path back to the SOURCE OF POWER we call ground to confuse the Communist and DIY's. Dirt,, panels and Inverter are not your source of power. The BATTERIES ARE THE SOURCE. We cannot use dirt because the resistance is way too high. Look again and see what is going on. See dirt anywhere in the path back to the battery?

What size wire? What size is the largest wire on the battery Term Post is the answer? Assuming a 24 volt battery, short distance to the Inverter or load. and a 2000 watt Inverter wil require at least a 2 awg to all equipment connected to the battery Including the Main Bonding Jumper between the Battery Term post and Trailer frame. Even the wire from your trailer frame to dirt has to be the same size even though it never ever carries any load current. Al the earth wire does is prevent you from shocking yourself if outside standing on the dirt. Ground is ONLY a reference Point in time and space. Where is that in a plane or your car> It ain't dirt.

Hope that helps.

Final comment I am writing a Ground Tutorial for the Forum and should publish soon. Right now I am on vacation back in the states.

God Bless and Happy New Year.

SK

[engage1000]

You are more than welcome and my privilege to help. Since you took the time to read and dig to ask very good questions, I will do the same in return and help you a smuch as I can. Just do not get to hung on on details and make i tmore complicated than it really is.

Hope that helps.

Final comment I am writing a Ground Tutorial for the Forum and should publish soon. Right now I am on vacation back in the states.

God Bless and Happy New Year.

SK

Hi Sunking,

Sorry it has taken me this long to revive this thread and get back to you guys. I really appreciate the information in the above grounding lesson, and would enjoy reading that tutorial if you happened to have a chance to write one up!

After a few financial setbacks and having to put the project on short-term hiatus, I am pretty much ready to go over the next couple weeks to get the equipment I need. However, I need some advice on a few things and was hoping you guys could help me out!

The other week I picked up the first piece of the puzzle, a nice new Honda Eu3000is. Photo here:
IMG_3731.jpg

My questions are as follows, and I'm sure I'll have a few more as time goes on. In return for your advice I promise to take lots of pictures and post regular updates as I put everything together!

1. I would ideally like to have a transfer switch of some kind set up to direct generator power directly to the inverter or directly to the power panel in the event of inverter failure of some kind. What do you figure is the best way to do this? Just a standard 30amp transfer switch and backfeed the panel with a 30amp 2 pole breaker? Legally I would need a lockout on the panel I guess.

2. The inverter will support the entire power panel, which is just a 100amp panel with some circuits on it. Again, I won't pull more than a couple thousand watts out of the inverter at any given time. Inverter manual calls for a 60amp breaker (the magnum MSH4024M is a single phase). Can I just buy a 60amp main for my square D panel and use that to supply both buses? Will that main breaker be an issue with it being 2 pole for what is really a single pole setup?

3. Leaning towards a Midnite Mini Disconnect to tie the DC stuff together with breaker, spot for shunt, some breakers for other DC, etc. I'm a bit tight on space and thought about going the route of using a Marine disconnect switch, class T fuse, and individual components. Any thoughts on this from someone who's done it that way?

4. Charge Controller. Have plans on going for a Midnite Classic 150 as the price is right and I hear good things, it also has arc fault built in which I believe is code requirement here in Canada. However, is it worth the extra to get the Magnum charge controller? It seems to be new with not much info. I guess the benefit would be inverter/charge controller talking to each other?

5. Solar Disconnect. Any advice on a solar combiner that can disconnect would be great. Will be using six of these 280watt panels ... https://www.canadiansolar.com/filead...M_en-v5.54.pdf

I have also attached the solar workup sheet from midnite. Midnite String Sizing REsults.pdf

Would this work? http://www.midnitesolar.com/productP...tOrder=2&act=p

But I should be able to get away with this one? http://www.midnitesolar.com/productP...tOrder=4&act=p It's technically disconnecting, just not with one handle though.

6. Lastly, charge rate. Worksheet says I'll be dumping 58.3 amps at 28.8 volts into the battery bank. Bank will be four 6v L16 415AH AGM. Would that be considered a C7 charge rate? Is it too much?

Thanks !

John

[sensij]

But I should be able to get away with this one? http://www.midnitesolar.com/productP...tOrder=4&act=p It's technically disconnecting, just not with one handle though.

6. Lastly, charge rate. Worksheet says I'll be dumping 58.3 amps at 28.8 volts into the battery bank. Bank will be four 6v L16 415AH AGM. Would that be considered a C7 charge rate? Is it too much?

That combiner looks ok. With just two strings, you don't technically need overcurrent protection, just a two pole DC disconnect. However, putting breakers into that box works fine too (assuming that satisfies whatever code requirement you might have for a disconnect).

The Classic has the ability to limit charge rate, so even if the panels are capable of putting out 58.8 A, or 70 A, if the voltage is lower, you can set the limit in the controller wherever you want and it will regulate the array to that.