"I went to the woods because I wished to live deliberately, to front only the essential facts of life, and see if I could not learn what it had to teach..." - Henry David Thoreau, WaldenLast night I had released the output of my model for a zero-energy, off-grid cabin designed for upstate VT. I've been thinking a little more about it and have made some updates. I've also torn a page from Thoreau and decided some conclusions drawn from the exercise might be worth going over.
1) I haven't given adequate consideration to ventilation. Without good ventilation, issues from general stuffiness to mold can occur: not good things. Running the numbers I was surprised how much it mattered. I used some of the passive house standards, which rely on air-to-air heat exchangers to reduce heating loads. To comply with 0.6 air exchanges/hr with an 80% eff HX, I need to add 5.5 kWh/day to my heating load (10x my previously calculated heat loss). I also need to change the layout of my floors. Previously the only floor-to-floor ventilation was through a small 3'x2.5' ladder opening. I've set back the 2nd floor and apex floor to also permit a 6'x3' opening from ground floor to roof. Such an arrangement allows for better floor-to-floor ventilation and, if the top windows can open, a passive cooling structure powered by the morning sun as well.
|Set-back 2nd and apex floors for ventilation, plus proper orientation.|
3) Now for some general lessons:
- The biggest drivers were heating: incoming air for ventilation and hot water for cleaning. I find that pretty remarkable. With respect to heating air, the amount of energy is dependent on number of air-exchanges/hr, which is entirely determined by the volume of your house; heat loss through insulation can be made marginal, but air-exchanges are a big sink. Smaller house -> lower energy consumption. Period. With respect to heating water, it's all about consumption; using less water means having to heat less water. A real win-win. We really should do what we can to cut back on hot water consumption.
- As I mentioned before, the hardest challenge was water consumption. I really think this is telling as I believe that our future will be water constrained more than power constrained. In this example, if I needed more power, I had plenty of room for more solar panels; it would just cost a little more. If I needed more water, I was in trouble. Water conservation is a serious thing. I'll discuss more about it in another post.
- All in all, the exercise wasn't that hard; in a day I managed to put together a design for a small cabin with rainwater collection, greywater recycling, solar thermal heating, and PV-sourced electricity that featured all the comforts of modern living: heat, hot water, internal plumbing, TV, ventilation, oven, refrigerator, dishwasher, washer and dryer. This cabin could comfortably house 4-6 people over the weekend (giving time to bank water and power resources through the week), or 2 people continuously. By my estimates, it costs about $200K: same as the median house price in the US. In short, it's a pretty serviceable home, implying I don't think that zero-energy homes are some future fantasy, but a realizable present option should people want to pursue them.
I really liked this exercise. I'm surprised-but-kinda-not that I was able to design a comfortable home that was completely off-grid. I'll see what I can do to upload the model for review.