About

In an effort to maximize the utilization of The Prairie, so my parents and relatives that arnt able to do traditional camping or don't enjoy camping can enjoy the prairie too. I've decided a tiny home would be a good way to do that.

This page functions as an easy way to share the ideas with others as well as a central place for me to keep my notes.

Structure

Since there's 3 hours worth of driving between me and the location this will be built, I've attempted to leave nothing up to chance and instead, fully lay everything out in CAD before we even pick up a shovel.

Legally this building is too small to be considered a home of any sort, so it's instead classified as a Personal Storage Facility. I'll elaborate on why that's interesting in the future.

Electrical

All appliances (water heater, HVAC, cooking, etc) will be electric. This simplifies the user experience when staying at the tiny house: if batteries are dead, nothing works; otherwise everything works. None of that checking if the propane tank is low or having to leave to get fuel for a generator or anything.

Power Requirements

Heating the tiny home during the dead of winter will be the toughest energy challenge, so this is what we'll size the solar system to. The sun is low, weak, frequently occluded by clouds/precipitation, and only shines in a useful manner for 5ish hours per day. Meanwhile, the tiny home has to keep itself above freezing during the coldest weather of the year.

My highest gas bill (used almost exclusively for heating) for my house last winter showed 734 cubic feet of natural gas per day. The U.S. Energy Information Administration says each cubic foot contains 1057btu of heat. My house is 2050sqft, so divide the energy usage by 10 to get something close to representative for the 200sqft tiny home. HeatingEnergy = 734cf/d * 1057btu / 10 = 77583btu/d => 22.7kWh/d. So 22.7kWh is an incredibly high number, so high that I can't even dream of getting that much solar in the dead of winter with the poor solar conditions.

Luckily the energy usage for heating the house is based on keeping the inside at 20C, while the tiny home only has to be at 3C to weather the winter. Weather Underground has historical data for the Pontiac airport in January, which shows the average temperature was -3C. A quick subtraction shows the energy the house used was to keep it 23C warmer than outside, while the tiny home only needs to keep it 6C warmer than outside, which means the new heating energy estimation is now 22.7kWh/d * 26% = 5.9kWh/d. This is still quite a lot, but much closer to feasable.

Turns out there's a scientific way to estimate how much heating and cooling is required, known as Block Load. By punching in the data for the tiny home, the calculator spits out these results. Much to my suprise the estimated heating load (1.7kWh) is much lower than my guestimation based on my home's gas bill.

Photovoltaics Location

Everything being electric obviously puts a pretty large load on the electrical system. The first thing to consider is solar harvesting, if we can't collect enough sunlight, it doesn't matter what else we do, nothing will work. The inital plan was load the roof up with solar panels, but after some quick napkin math it was clear the roof isn't nearly big enough for that. Now the plan is a ground mounted array, essentially a big grid of panels on stilts to keep them above the natural wild grass height.

Figuring out where to put the panels isn't too tough, just a matter of measuring the height of me (1.8m), taking a photo of me in front of the biggest tree south of where I want the panels (blue dot in below image), then calculating the height of the tree from the photo (14.4m). Next was determining how far back to set the panels so they dont get blocked by the trees. The following paragraphs use 15° as the target angle, so we'll solve for that while keeping in mind the panels are 1.8m off the ground. Setback = 12.6m / tan(15°) = 47m. This location is marked with the orange box. Orange box is to scale assuming the panels are layed flat on the ground in two rows of three lengthwise, leading to a rectangle roughly 5m long and 2m wide.

For the rest of the calculations, I'm going to use LG360N1C-N5 solar panels as an example, I likely will use a different panel because what's available at the local solar store changes over time, but all panels this size are relatively similar. Using these 360W panels and referencing the Solar Atlas, and we position the array at 20° to collect direct sunlight when the sun is ≥15° in the sky, a 2kW array will only collect 2.3kWh of energy over those 4 hours. To get 2kW worth of panels we'll need 6 360W panels. The bonus here is when the array is adjusted to 50° these same panels will generate 9kWh from March to August, which covers prime camping time. That huge amount of power helps justify the investment to survive the winter; if the array is large enough to survive the winter then it'll also be large enough to keep the tiny home uncomfortably cold all summer.

Next is finding out how tall this array is going to be. So the plan is a two position array; a summer and winter position, naturally. During summer we'll set the panels to be perpendicular to the sun at an average elevation of 50°; similarly, winter will be 20°. To calculate the height of the top edge of the solar array during summer, height = sin(angle)*widthOfArray + heightOffGround = sin(40)*2m + 1.6m = 3.1m; similarly, winter = 3.7m.

Next consideration is wind load, since these panels are essentially a big huge sail. This is pretty easy. arrayArea = 2m*5m = 10m², windForce = 0.5ρ * v² * A = 0.5*1.2kg/m³ * (40m/s)² * 10m² = 9.6kN = 2158lbf. That calculation assumes the panels are vertical and facing directly into the wind. 40m/2 seems like a lot, but according to the nearest airport with weather data, which does happen. Gunna need some strong stilts.

Energy Management

The current plan for energy storage is a bank of 48v lithium batteries. Which capacity batteries depends on which size will physically fit in the cabinet. That will be determined later.

For charging the batteries, a Victron MPPT charger will do the trick. With this charge controller I'll be able to run the solar panels in series, up to 150V, which will help minimize transmission losses in the wires between the panels and the tiny home. The current plan is two strings of 3 panels, resulting in Impp = 20.6A and Voc = 124.8V, well within the capabilities of this charge controller. The batteries themselves are rated for 100A charge current, so no worries there. Connecting the panels to the tiny home will be the job of two 4AWG wires, totalling 36m round trip, aka 0.03Ω, so we'll lose about 0.6V ⇒ 12W ⇒ 0.6% of the generated energy in the wires, that's within tolerance.

To convert the battery power into useful power, a Victron MultiPlus-II 120V inverter/charger will be used. This will be connected to the batteries and will feed 120VAC into a 60A electrical panel in the tiny home. From the panel, it's just normal house wiring to the outlets and switches in the building.

Energy conservation will be the job of, I hate to say it, but smart switches. Energy is a precious resource, so all devices will be controlled by an army of TOPGREENER switches/outlets. Unfortunately, this is the easiest way to make sure things don't get left on when nobody's there. If I find a way to get internet to the tiny house, this could have the added benefit of allowing me to turn on the HVAC, water heater, fridge, etc a few hours before I head up north so everything's ready to go when I arrive.

Plumbing

In the spirit of off grid, the goal is to use rainwater as the sole source of water for the tiny home. Rainwater is close enough to clean that we can use it without any sort of filtering. Roof runoff, on the other hand, is what we're actually collecting and that is not even close to clean. Lucky for me, Dave has done a bunch of work on crafting slow sand filters to clean roof runoff and we can use his designs to get clean water from the sky.

The main gotcha with building a private storage facility, is it can't have a shower per code. Fourtunately, sinks and toilets are legal though.

Possibly the feature that makes this most similar to a normal house is the septic field. There's just no good way around having a septic field unfortunately, both code and practicallity require it.

To estimate the sewage flow from the tiny home, table 1 from the Michigan Criteria for Subsurface Sewage Disposal gives us some estimates, and assuming two people in the tiny home, we can estimate between 200 (if using a single space in a mobile home park as a representive building) up to 300 g/d (if using a luxury residence as a representitive building), not sure why standard dwellings are not listed in the table.

The tricky part is getting all of this to survive the winter. Winterization is the absolute last resort, I'd rather keep a small automatic generator on site to run a space heater all winter than winterize the plumbing in the fall. The current plan is to keep the inside of the building above freezing, ≥3C to protect all the indoor plumbing. For the sand filter that's outside, I haven't settled on a plan. The two leading ideas are heat tape wrapped around the pipes and barrels, or having a second pipe from the water heater with a solenoid to dump hot water into the barrels periodically to keep them warm. Both ideas require the barrels and pipes to be insulated, but neither of the ideas are elegant. If you have any ideas, please let me know.