Replies

1. GBA Editor
   Martin Holladay | Sep 13, 2020 04:38pm | #1

   Mark,
   This article addresses everything you need to know, including advice on ventilation: "How to Design an Off-Grid House."

2. 
   mark_rob | Sep 13, 2020 05:37pm | #2

   Thanks Martin, very good article obviously based on years of experience.
   I should rephrase my question to:
   How can I efficiently temper incoming -40 air into the the off grid cabins. I have been considering 12 volt fans and earth tubes to warm the incoming air which could get the air to around freezing. I"m not aware of any 12 volt HRVs from the RV industry. I have read about home-made tube within a pipe heat exchangers, which could work, but I need something pretty robust and stand alone for the occupants.

3. Expert Member
   Akos | Sep 13, 2020 06:19pm | #3

   There is a passive system you can get from the other side of the pond:
   https://ventive.co.uk/products/

   You can probably come up with something similar using a fanless ERV/HRV unit driven by stack pressure. With carefully locating intake and exhaust, it can be made to work but will need some tweaking.

   Make sure you have a solid plan for air sealing the warm side of your SIPs and venting above the roof deck to avoid SIP rot.

1. 
   mark_rob | Sep 13, 2020 08:37pm | #4

   Thanks Akos
   I've spent some time on their website and if they still manufacture residential appliances it could be brilliant for a wood stove heated house. When the stove is running and the room is hot, the air exchanger exchanges more air. When the stove is off, and not much heat is being generated, the system slows down.
   Mark

4. Expert Member
   BILL WICHERS | Sep 13, 2020 10:29pm | #5

   You’ll probably find small muffin fans operating directly off of your battery buss voltage to be your most efficient option. Standard voltages for these fans are 12, 24, and 48 volts DC. The higher voltages will result in more efficient systems, so I’d try for 48v DC systems as much as possible. Many people try to limit themselves to “12 volts” because they’re used to that from cars, but there are many advantages to working with a higher DC buss voltage.

   An interesting fact about battery discharge is that you get more useful power out of a battery bank running at double the voltage and half the current and it’s not linear. You can actually extend your runtime using a higher DC buss voltage and small DC-DC converters if you want to stick with 12v lights.

   Bill

1. 
   mark_rob | Sep 15, 2020 06:27pm | #6

   Bill
   Thanks for the suggestions, especially on the voltage. I had a PV supplier suggest the same thing, but I'm 6 months away from starting on the off grid cabins and hadn't done any research on the voltage of the system yet. Your suggestion will be my starting point.
   Thanks
   Mark

1. Expert Member
   Akos | Sep 15, 2020 07:56pm | #7

   I would first look at how much power you need. Common low voltage lighting and fridges is either 12V or 24V, best to stick with those as they can run directly off the battery without any additional electronics/loss.

   With 12V you are generally limited to around 1500W of 120V AC. If you don't have a lot of AC loads, it is the simplest and cheapest.

   On 24V you can go to 4000W, which is enough to run a microwave or small stove.

   Higher than 24V makes sense for a full house where most of your loads are 120V/240V.

   One recommendation is to go with LiFePo4 batteries as these will last much longer. I have a set for my electric lawn mower that are 10 years old and still running strong. Iron Phosphate batteries don't have the issues with capacity loss at higher discharge rate or off gassing/maintenance of lead acid batteries. Just be careful as there are a lot of questionable products out there, I've use Valance U27-12XP before which work quite well.

1. Expert Member
   BILL WICHERS | Sep 15, 2020 10:17pm | #8

   Akos, you can convert from 48v to 24v at usually 88-94% efficiency with modern DC-DC converters. I agree that if the total load is light, 24v may be a good choice. I would not try to run 4kw on a 24v system though. I’d probably put the cutoff for 24v at maybe 1kw or so. Higher voltage systems have lower losses with less materials.

   Bioenno Power is another source for LiFePO4 batteries as well. They have a good reputation.

   Bill

1. Expert Member
   Akos | Sep 15, 2020 10:43pm | #9

   You can, but you add yet another box. These cost money, can fail and have a quiescent current. Off grid, all these power losses and draws add up. Plus simpler tends to be better.

   I have a 2.5kW 24V Outback, I think they also make a 3.5kW. Needs big wires but that is about it. Provides enough power to run a table saw.

5. 
   mountaincabin | Mar 15, 2021 03:49pm | #10

   Mark, I am also off grid and used the Lunos e2 and eGo. We're in the cascades in WA with still about 4 ft of snow on the ground so I understand you're worry about electrical usage in the winter. They are 1.4 w per unit so even with off-grid it's doable. I had the same thoughts of you but after air sealing our cabin for multiple reasons, this was the best option and is working out real nice. We have one paired e2 unit and two eGo for each bathroom for our 1800 sq ft place.

1. 
   mark_rob | May 05, 2021 12:29pm | #11

   Thank you very much, I'll look them up. My email is [email protected], if you don't mind I'd like to correspond further with you on you design, build and mechanicals.
   Mark

2. 
   jonny_h | May 05, 2021 01:02pm | #12

   Do you run these units on 120VAC from an inverter, or have you found a way to modify them for direct DC operation? From the looks of the product, I have to imagine that they're using brushless DC fans operating on some lowish DC voltage internally, so if the electronics are easiily accessible with a sensible intermediate bus voltage, they could run from batteries directly and eliminate a couple conversion steps.