Theorycrafting Help

DRickey

A few years back, I used a couple of 235W panels from NAWS and 4 size 24 deepcycle batteries from Walmart to turn an old Rialta RV into a self-contained geek pod. One of the things I tried to do was to run a small window AC unit from battery power. I almost made it work, in that I could get 4-5 hours of the AC if I started from full batteries and had good sun (400W or so actual output). Then I would have to start the generator or the engine.

I figured out that my inability to make it work came from 3 basic sources:

1) Not enough solar. The draw of even a 5000btu AC was 600W. There was only room for a maximum of 3 full size panels, and that would have required blocking the roof vent. There are more efficient AC systems, but they need a minimum of 1000W (12,000 btu).

2) Not enough insulation. The Rialta had roughly 1 inch of styrofoam in the walls, R factor of roughly 4. Even after building my own insulated window covers, painting the exterior arctic white, and otherwise retrofitting with foamboard under the carpet, fiberglass under the bed and a "soft bulkhead" between the cab and the "house", a small AC simply couldn't keep up (which meant no downtime, and even more power draw as the compressor struggled).

3) Not enough battery. 12v isn't adequate when you're drawing more than a couple of hundred watts, even with monstrously thick wiring, and I was short storage capacity by at least a factor of two.

So I am now working out "Geek Pod Mark 2". The plan is to build a custom RV on a large truck, with a roughly 20 x 8 foot box (as compared to the 6 x 14 of the Rialta). Minimum of 1500W of solar panel (6 panels, potentially 8 and 2000W+), and 900-1300 pounds of battery (8-12 4D AGM batteries in paired 24v). R factor of 24+ (6 inch Structural Insulated Panel all around). 3000 watt Schneider/Xantrex inverter. No windows, only one door (I'd have a breakaway/pop-out emergency exit at the other end).

For climate control I was planning on using a dual-hose portable AC (12000 btu, 1000W), and a D2 or D4 Airtronic cab heater (diesel furnace, no significant power draw). Isotherm Slim 25 water heater (might swap out the standard 750W element for a 500W 24v version). A laptop computer, a 700W microwave and a standard residential refrigerator would finish out the major power draws (I'd have LED lights and some flat-panel displays, but those are almost rounding errors in comparison).

One of the questions I am still mulling is the generator. Like everything else, I want it to be diesel. I could go cheap, get a Chinese built residential backup power model. Or expensive, an Onan RV model. Or *really* expensive (but really reliable), a Whisper Power model originally designed for marine applications. If I go that way, I could get their 24V mini genset (150 amps), but the cost would be insane (roughly $14K).

Another option would seem to be to roll my own, get one of the little Kubota diesels the Whisper is based on and pair that with a big 24v alternator. Of course, with 24v, I am completely dependent on the inverter and the batteries, I lose a fallback option (cut all the batteries and solar out of the circuit and run from the generator alone).

Any thoughts? Something I missed?

mike95490

Putting the standard fridge in the house envelope, means it's heat load will have to be pumped out by the house AC.
Maybe a custom built fridge like marine units, with remote compressor and condenser would dump the heat elsewhere (pre heat your hot water with the fridge waste heat)
Look for a surplus Lister Alpha gen set, rugged and compact and maintainable. Or APU from a 18 wheeler

vtmaps

DRickey said:

One of the questions I am still mulling is the generator. Like everything else, I want it to be diesel. I could go cheap, get a Chinese built residential backup power model. Or expensive, an Onan RV model. Or *really* expensive (but really reliable), a Whisper Power model originally designed for marine applications. If I go that way, I could get their 24V mini genset (150 amps), but the cost would be insane (roughly $14K).

Another option would seem to be to roll my own, get one of the little Kubota diesels the Whisper is based on and pair that with a big 24v alternator.

Something I missed?

You didn't say what capacity you want for the generators.  You may have missed these:
http://www.hardydiesel.com/diesel-generators-7-33-kw.html

--vtMaps

DRickey

Mike: The APU is something I hadn't thought of, I didn't know they had that much generator/alternator power (looks like most are in the 125-150 amp range). I just want something to fill the gap for hot and cloudy days, or if I get too carried away cooking or something. If I need it regularly, I didn't plan this out right or execute it properly. Since I don't really care about the heating or cooling functions, it looks like I could get the engine and alternator from a used one pretty cheap.

I'll have to look at building my own fridge again, I get the theory but I'm not sure I can do the detail work and seal it properly.

Vtmaps: I'm looking strictly at the smallest stuff available, in addition to not needing more, the weight is a concern. Even the lightest diesel gensets run 350 pounds, and more than that gets to be an issue for securely mounting (and balancing the load).

DRickey

So after some more research, I am coming to the conclusion that a used Carrier or Thermoking APU (roughly $3-4k) is the best solution to my generator problem, and the cooling units that go with them don't suck. In fact, they're good enough I might want to use it as my primary AC (fewer holes in my insulated box, the better). Makes sense that they'd be good units, both have been making mobile refrigeration systems forever. Not sure I would trust their inverters over the Schneider, but they'd be decent as a backup.

They don't charge directly to 24v, but I can live with that (whole point of the Schneider is that it can charge from shore/generator power and invert). I'd have to get a welding shop to work up a mount, but I was going to need one to build the battery boxes and reinforce the deck under the generator anyway.

Still not sure about building my own refrigerator. I can see how it could be much more efficient, especially with being able to dump the heat outside and make the wall considerably thicker than any standard fridge. But getting the sealing right, especially for the doors, seems like it could be a real PITA.

mike95490

Look to boat yards for building fridges, they have compressor / evap / condenser kits for building fridges in all sorts of strange nooks & crannies of a boat.

DRickey

Yeah, it's looking more an more tempting, the cost is not minor (roughly $1250-1500 for each, refrigerator and freezer), but the efficiency gains aren't, either (especially with the advanced controls that take advantage of a DC compressor's capabilities for variable RPM). I'm thinking that with lots of insulation, I can build a 16/12 cubic foot refrigerator/freezer setup that will draw considerably less power (by about a factor of 4) than anything remotely equivalent in a residential or commercial system. And I can build the whole thing to match my personal ergonomics (no bending over, or losing stuff in the back corners of the fridge). All of it running off the 24v, so that the inverter and generator aren't in the failure chain.

The only drawbacks are that it won't be frost-free (which means having to waterproof everything and allow for drainage) and it won't freeze/cool large amounts of stuff in a hurry.

I'm getting seriously tempted by that 30 SEER Mitsubishi split A/C, as well. Even with the losses for conversion (first to 115v AC and then stepping up to 230v) and parasitic loads, air conditioning on as little as 100 watts (probably closer to 150 with parasitic loads and conversion losses) is huge. With the sheer amount of insulation I'm planning, it's possible that I could really take full advantage of the 100w/2800btu mode of that model.

Did realize I had a significant flaw in my plan: If I use SIP for the ceiling, I don't think I can count on it staying structurally sound over the long term (essentially, all that would be holding up the ceiling is the bond between the particle board and the foam, and this application would shake it around a *lot*). Probably have to rafter the ceiling with something like aluminum lighting truss. This model appears to be closest to what I need.

vtmaps

DRickey said:

The only drawbacks are that it won't be frost-free (which means having to waterproof everything and allow for drainage)

Seafreeze makes Danfoss compressor custom fridge units that catch the condensate in a small bottle inside the fridge.

--vtMaps

DRickey

Figured out a solution for the overhead strength issue: Raycore panels (not technically SIP's, but very similar) have wooden boards integrated into them. They aren't normally used this way, but what is? Anyway, they should be much better for my needs, the boards slightly compromise the insulation quality but made the whole panel much more reliably rigid. Roughed out a layout sketch for the whole thing, but it isn't on my tablet. I can post it later if anyone cares to see it.

vince

You might be interested in my write-up of my tiny house that is completely self contained. I describe the design consideration in detail and use a split level ac. Been very happy this summer with the performance. Run the ac most of the day and at 6 pm still have a floating system. vincentdummer.com/wp

DRickey

That is interesting, I see that you used 6" Iso foam, how is it doing (with the minisplit) at climate control? Do you run the heat pump all the time? Do you have any backup heat source (if it got below the range for the heat pump)?

What about water, do you have any source/storage besides the rain barrel? What do you do with the sewage? Is that a macerating toilet?

My design target was, as I said above, a self-contained geek pod. It tends to be a given that I have to move every year or two for work, and I've really gotten tired of the hassles. Not just the packing and unpacking (although that certainly sucks), but all the trivial crap like getting utilities started, then getting them shut down, turning in TV equipment, phone service, closing out a lease and getting a new one, and on and on and on. When I was paring my stuff down to what could fit into the Rialta, I realized that I had tons and tons, literally, of crap that had gotten thrown in boxes 3, 4, 5, even 10 moves ago, and carted across the country sight unseen, over and over, for years. That I literally hadn't thought about in that entire time.

In living in that little space for a year, I realized that most of the things that go along with a permanent address are simply not necessary. For the last 20 years, I didn't bother getting to know my neighbors, because I wouldn't be around long. For furniture, I only needed a comfortable bed, a comfortable chair, and a decent place to work at my computer. I did miss having a shower I could actually use (the Rialta shower is 18 x 30 inches, you have to reconfigure the RV interior to use it, and of course there is not a lot of water in the tank) and the ability to fix my own dinners, store leftovers, and eat ice cream that I didn't buy in the last five minutes. And although most of that crap I carted across the country several times was completely disposable, I did still have several large totes worth of stuff with emotional value I wanted to hang onto (and leaving it at the mercy of friends or relatives long term would be begging future problems).

So that was my target: Everything I needed to be comfortable, no more and no less. All the basics of shelter and sustenance requiring no more thought or management that they would in a more normal living arrangement. No provisions for guests, or entertaining anyone but myself. Transportation for errands and such by scooter. No commuting to speak of, I'd park near work (if not literally *at* work).

And as much as possible, completely stealth. I spent most of that year in the San Diego area, and specifically near La Jolla, literally the worst place to be an urban boondocker when it comes to law enforcement. It is literally illegal to sleep anywhere in the San Diego area outside of a permanent dwelling or designated campground, and the police routinely roust anyone near the ocean in an RV, or anywhere in the county if they get calls from citizens. I eventually learned tricks for avoiding all of that, but the biggest factor was when I repainted the Rialta completely white, with black running boards and bumpers, and hung a ladder on the side. Making it look like a service/commercial vehicle cut my issues with law enforcement and nosy/paranoid homeowners to zero (it helped that the Rialta doesn't *look* like an RV, with that paint job it looked like some kind of weird cargo van).

So from the outside, I want it to look like just another generic truck. No windows, no RV-style doors, no obvious giveaways like rooftop vents and AC units. Just one more forgettable, ignorable truck, that I can park in any commercial or industrial area in the country and people literally edit it out of their visual field. Since I don't want to just survive, but live and be comfortable with all of my electronic gadgets, I've been putting a lot of thought and research into the generation and use of electricity.

And not that anyone actually asked, but here's the current layout plan, assuming a 8 x 26 foot box (if I could get one with a 8.5 foot wide box, I'd probably make the shower bigger).

vince

I have radiant floor heat and only hot water onboard. I have a propane 6 gallon water heater, that will also heat with 110V and a thermal solar panel will pre-heat the water. The split level heat pump is on-demand when batteries are full as the floor heating is the primary heating. Silent, comfortable and dependable while taking advantage of available solar energy. The toilet is a composting toilet so no water use and no black waste tank. Daily water use is about 5 gallons, mostly from shower, so a bucket outside will collect it. Water purification system allows for using collected rain water. Only warning, my fridge is louder then I like so at night I turn it off and have blue ice in freezer compartment to maintain temperature is much as possible (a timer will start the fridge in the morning).

DRickey

Well, because you put me onto researching hot water as a heating method, I found out that this was a thing: 17k BTU diesel fired hot water heating for boats. More expensive and less efficient than the Airtronic, but...effectively unlimited hot water (it can be set up to maintain hot water tanks as well as multi-zone heating), 17K BTU is equivalent to 5000 Watts of resistance heating, meaning effectively instant hot water on demand, and the elimination of a major electrical drain.

It's tempting, even if it would cost about $2500 (compared to $1K for the Airtronic).

vince

My space requires about 2500-3500 BTU to maintain the temperature, To heat it up or cool it down after it has not been occupied takes of course more energy, but it should be fairly economic to run that heater.