Wednesday, May 7, 2014

Zero-Energy Cabin: Update

"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, Walden
Last 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.
2) I've also updated my water usage requirements. Turns out my first consumption estimate was too high. This might be filed under TMI, but it can't be as bad as other "living as nature intended" blogs I've read or come across. My shower estimate assumed 10min of water use at 2gal/min. I timed myself this morning before work just to see. I take "naval showers" (water on -> wet up -> water off -> lather up -> water on -> rinse off -> water off) not for environmental considerations, but out of preference; I'm not a small guy and my shower stall reminds me of that on a daily basis. That said, having timed my morning shower, it's a comfortable 5 min of "water on" time, thus cutting my consumption in half. I've updated my water consumption model to reflect that, and found that I could squeeze 4 naval showers with a low-flow shower head (4x10gal), 1 load of dishes in an EnergyStar dishwasher  (4 gal), and 1 load of laundry in an EnergyStar washing machine (40 gal!!!) per week. This is perfectly sufficient for 4 people for the weekend or 2 people living continuously. I was actually a bit surprised by this, as water use seemed to have been the hardest constraint to satisfy.

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.

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