Power management and temperature control off-grid

It’s been a remarkably cool not-especially-hot summer so far, what with Monsoon having gotten such an early start. So it’s not that big an issue so far that the Secret Lair can get awfully hot and stuffy in the afternoon and evening. But it’s still summer in the high desert so the Lair gets kind of hot and stuffy in the afternoon and evening.

I handle this in the old school manner: Open windows, ceiling fans, a window fan sucking air inside when the sun goes down, and a sit-down with a beer on the covered porch. The Lair’s battery bank is quite small, four T-105s, so leaving electrical draws on all night is not a thing I’m all that willing to do. Fortunately the 12-volt bedroom ceiling fan draws so little juice on lower settings that I can make an exception in its case, and that has made a huge difference in sleeping comfort. Plus a stick-built cabin just naturally cools off overnight anyway. Even if it’s an exceptionally warm evening, sooner or later it’ll cool off enough that you can get some sleep.

Not so Ian’s Cave.

Thermal mass, they said. It’s your friend, they said. And we went with that, back in 2009, without sufficiently thinking things through. If we’d really seriously planned the project we’d have paid a lot more attention to ventilation.

Ian’s Cave is a concrete dome buried under I don’t know how many tons of sand. It holds heat very well in the winter, and takes a couple of months to get uncomfortably hot in the summer. But once it does, there you are. The good news is that the first couple of months of winter won’t pose much of a heating problem. But in mid-summer…

…my thoughts always turn to “how can I cool this bitch down, just a bit?” I have to work in there. I deeply regret that we didn’t put some ventilation ducts and fans through the back of the dome before we shotcreted and buried it. Too late to worry now.

I can get the overall internal temperature and humidity down a couple of points by running that big fan on a timer overnight. But I have a strong emotional resistance to doing things like that. And Ian’s batteries…

…while much more substantial than mine, are also ancient by battery standards. At twelve years old, they’ve already lived more than twice the standard lifespan. I attribute this entirely to clean living: They have a caretaker who takes loving care, and they’re seldom stressed. Basically, most of the time they run a refrigerator and that’s all.

So when I start doing things like running big fans overnight, I always feel like I’m committing at least a venal sin and I pay attention to what it’s doing to the batteries.

And I always – reluctantly – come to the conclusion that I’m not really breaking any rules. The bad thing about a big battery bank is that replacing it costs multiple thousands of dollars. The good thing about a big battery bank is that it’s pretty hard to really stress if you’re not neglecting it.

In this case, every single morning without the fan running the battery charge sits pretty at 90%. Every single morning with the fan running, it’s at 82%. Which tells me I should really just relax.

But I’m deep in the ranks of the poors, and relaxation isn’t my default state. Basically I treat batteries the same way I treat puppies – I’m something of a worrywart.

About Joel

You shouldn't ask these questions of a paranoid recluse, you know.
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16 Responses to Power management and temperature control off-grid

  1. Mike says:

    Joel, it’s surprising to learn about this issue with the thermal mass of Ian’s abode. Has Ian ever considered borrowing a backhoe and adding more thermal mass to the roof?

    If Ian is contemplating new batteries, you may want to tell him about Lithium iron phosphate batteries. They have better aging and cycle-life characteristics than Lithium-Ion batteries, have a reduced risk of fire, and the price is comparable to other batter types.

  2. Anonymous says:

    Can you run a trench feet deep out the front and install pipe that vents cold air.

  3. Joel says:

    It already has windows in front. If I were rebuilding it, I’d replace one of the front windows with a smaller one – or maybe bash a hole in the block wall – and install an air intake, then run ducting across the top of the dome to the rear.

  4. Mark Matis says:

    As I had previously discussed, there are low power air conditioners that will work on 120 VAC.

  5. For curiosity’s sake, what sort of batteries are in Ian’s bank?

  6. Joel says:

    Rolls Surrette.

  7. malatrope says:

    I have exactly the same batteries, and for flooded lead acid they are great. However, if you accidentally discharge them too much, just like every other FLA battery their lifetime is drastically reduced. I finally replaced mine with LiFePO4 like Mike suggested — but I sure don’t know where he can get them at the equivalent price. Unless you get the worst of the Chinese garbage, they are twice the price per kilowatt hour.

  8. RCPete says:

    I sort of like Rolls Surrette, but… Twice the capacity of T-105s, 3 times the cost. I have Rolls in one system, and Trojans in the newer one. Had to have the Rolls shipped, while I could get the others locally.

    (Downside to shipping; they managed to get electrolyte all over the batteries. Ruined some clothing unpacking and cleaning the things. That and dealing with a palette of 4 RS batteries made it not much fun.)

  9. Ben says:

    If I had to buy batteries today, I would probably go with Trojan T105 again. In today’s market, I think that they are the sweet spot in price/longevity, plus they are available at any golf cart dealer. That said, II’ll be happy to switch to lithium when (on a cost/longevity basis) they become even close to competitive.

  10. Joel says:

    Yeah, that’s my read. RS batteries are impressive but frankly not worth the price. If I have to replace my Trojans twice as often I’m still money ahead – and anyway I’d probably have to update/expand my solar panels to be sure of keeping RS batteries charged. And on my budget Lithium is just a pipe dream.

  11. Anonymous says:

    I still think passive earth tubes could be installed

  12. I don’t know if the prices I found are representative, but even given the capacity numbers (which are generally very impressive), I think you could do better with LiFePo batteries. You get a far superior ability to handle deep drains and you don’t have to service the things. Of course, lithium batteries are subject to certain problems with fire that might make them less than suitable for some uses. And as you say, they are outside your price range. Maybe not outside of Ian’s though. 🙂

    Guy named John Daniels on YouTube has done some pretty amazing “solar on the cheap” work. I modeled my system (400 watts, 200 ah in batteries, for emergencies and to run my ham crap during non-emergencies) after one of his designs. Of course, I didn’t follow his advice on batteries and went with LiFePo. But I was able to source 2 good ones for cheap. Well, inexpensive, anyway.

  13. anonymous says:

    Heat rises, Is there any point on the top of dome which can be penetrated with a PVC pipe, painted black. Low close able vents (maybe on lower door panel ?) to allow air flow to push interior air out through pipe. I’m not an engineer, just spit balling some thoughts to help.

  14. Stanley says:

    Anon at 2108 (above) raises a point I’ve wondered about for years – the disappearance of cupolas. Way back when, (some) buildings had skylights, usually over central stairwells, that could be opened for ventilation. I remember a discussion with a tehnician who serviced lighthouse equipment commenting that when the glass surround at the top was open there was a very strong breeze inside the structure, strong enough that closing the entrance door was problematic. I’d think that a cupola with openable windows could perform much the same function.

    Coupled with deeply buried intake pipe, and perhaps some means of dehumidification, it would seem that at least some amount of hot weather cooling could be provided at minimal levels of energy consumption.

  15. boynsea says:

    Anon and Stanley are on the right track here. Heat rises. Provide a vent at the top of the dome (with a raincap, or one of those turbine vent things and a way to seal it up for winter, and an intake low down in the structure. The hot air escaping will draw in air from the intake, hopefully cooler air. No energy needed for this system. Will not be a spectacularly effective system, but would help.

  16. The reason earth tubes aren’t a thing is the mold problem. You’re sucking ambient air in, cooling it a bit, and if there’s enough humidity (like the monsoon season) you get condensation. Water plus darkness equals mold.

    I would suggest extending the shade out a bit, it looks like direct sunlight is hitting the front of the cave. The support structure is there for it. Perhaps put a tarp or two up and see if that makes a difference.

To the stake with the heretic!