Tuesday, May 6, 2014

I Got Bored So I Designed an Off-Grid Cabin...

It's always been a dream of mine to have a cabin in the woods as Henry David Thoreau did...though perhaps more of a comfortable ski cabin in VT than a small shack on a lake in MA...so I guess not as much as Thoreau as I'd like to let on. Either way, the thought stuck with me so unrelentingly today I had to put it all down on paper.

First, let's identify our constraints. In building his cabin, Thoreau scavenged for his building materials. I'm looking for something a bit nicer, so the option is to either build it on-site (difficult to do in remote locations), or build it pre-fab and ship it. Being more a fan of the latter, you can't just slap on any size house on a truck; they're usually pre-fab modules less than 15' wide and 30' long. So let's choose a footprint 15'w and 25'l. Second, let's assume we're remote enough such that we don't have access to power, water, or sewer connections.This is where things get interesting... In designing zero energy homes, two goals must be simultaneously achieved: maximize power production and minimize energy consumption. Maximizing energy production in our remote area will be achieved by solar energy (PV and flat plate thermal), and minimal energy consumption is done chiefly through heavy insulation and good resource (namely water) management. Since we're relying on the environment for our water and power, we have to take into consideration our area's solar insolation and average rainfall. I've chosen upper VT with data from the new US Climate Data website and NREL [1] [2]. Finally, some aesthetic considerations. I've always liked A-frame houses; I think they're well designed for low-energy homes in wintry climates, so let's use that as our model.

My model outputs a 575 sq. ft. cabin with room for 3 queen beds and 1.5 bathrooms, a large 10x7 kitchen, and plenty of space space in the wings for infrastructure. Using some estimates from David MacKay's amazing book Sustainable Energy without the Hot Air, I've estimated a 10kWh/day electricity usage and 14kWh/day thermal usage (almost all of which is in warming up water). To satisfy that demand, 33m2 of 10% solar panels in VT's 3 kWh/day/m2 solar flux would do it, same with 7m2 of 65% efficient flat plate water heaters for the thermal load. Those areas may sound like a lot, and they are, but another reason for my choosing an A-frame house is the very large roof area to work with.
Roughed out section and floor plan. Things are about to scale.
Because of the large roof space, power isn't a problem; we're taking up just over 60% of the south-facing roof. The high-sloped roofs and small footprint do resort in another problem though: water availability. Remember, I'm leaning on rainwater collection instead of a city water connection. I've calculated that only about 90 gal/week of water is available during the dry winter months due to lack of rain fall, rising to just over 225 gal/week in the summer. 90 gal/week is nothing; it would be the equivalent of 4x10min showers with a regular high-volume shower head. It certainly isn't much beyond a weekend cabin during the winter. I'm trying to stretch that out with low-flow shower heads, a recycled greywater system for flushing toilets using sink/shower/dishwasher effluent, and large cisterns to hold about a month's supply of water. 
My best guess as to what a low water-intensity plumbing system would look like. It should be noted, I am not a plumber...
I did learn a couple of things, especially about plumbing. I originally thought water pressure was going to be a huge problem, and started to work on a fancy gravity-based system, but I learned that there exists simple automatic pumps that turn on and deliver 40+psi pressure when they sense an open valve down the line. I also learned that while rainwater can be held for a while, greywater must be disposed of no later than 72 hours or else the organic content in it turns foul. You can also apparently run a septic system in freezing climates, provided you just dig down far enough below frozen ground (+18"), hence my choice of septic system vs. composting toilets.

So there it is, a zero-energy, off-grid weekend ski cabin with all the comforts of home. Based on upper bounds of modular home estimates, I've pegged the cost of this cabin to be $200K with all the PV and energy storage costs included (I think Thoreau said he paid $28 in 1845 for his). One day I hope to actually build this. Maybe I'll write about it then...

[1] http://www.usclimatedata.com/climate/vermont/united-states/3215
[2] http://rredc.nrel.gov/solar/pubs/redbook/PDFs/VT.PDF
[3] http://www.withouthotair.com/Contents.html


  1. You forgot to leave room for a wood stove and chimney. Remember, neither a PV system nor a solar thermal system will provide heat for an off-grid house in northern Vermont in December.

    1. Quite right, Martin; most of the off-grid homes I've seen use a wood stove. In this case though I've envisioned a second hot water tank to store thermal energy for hot water baseboard heaters. Based on my model, daily demands should be met based on average solar insolation, and the 100gal hot water heater should be enough for 3 days of storage with 10% loss if necessary, which should be sufficient given winter has 1/3 sunny days as summer in that location. Nice thing is it should all use the same hardware. Just a different thought as to how things could be done.