Any science available on the energy loss associated with deviations from the ideal house orientation?
mikeysp
| Posted in Energy Efficiency and Durability on
Hi.
I am building a home in Mixed-Humid Climate Zone, 4a, (Nashville) and want to know what happens to kwh usage as the orientation deviates from the ideal.
I have read repeatedly the “ideal” orientation is east/west with few or no windows on east/west sides and most windows facing south.
What happens as this orientation deviates from the ideal by 10, 20, 45 degres, etc…?
Are there any numbers to see how much it matters?
Thank you.
-Mike
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This computer program will model your house with any orientation and window to wall combination using your local weather and. You will need about 30 hours to watch the training videos and learn to use the model.
https://beopt.nrel.gov/
Walta
Thank you Walter.
I may need to go that route; but, I was hoping that someone wrote an article demonstrating what happened as he turned the model by degrees to show the implications.
-Mike
I've read that you can deviate from due south up to 15 degrees east or west with a minimal loss of solar gain but you should also pay special attention to the sizing of your overhangs. This online calculator will help you design proper overhangs.
https://susdesign.com/overhang/
Thank you for that link Scott!
My situation is depicted in the image. I had planned to orient as shown in yellow; however, after reading for many hours on this site, I realized I need to change the orientation of my house. It just is a bit of a challenge, since the driveway has already been put in and I have cleared sites for the house and barn. Septic is also in. I needed to discover GBA before I even found my land, but here I am. Trying to correct the situation as best I can.
Mike,
A well insulated and air-sealed building envelope with an appropriate amount of glazing on each face is much more important than the orientation of the house. If you had good architectural reasons for where you situated it, there isn't any compelling building science reason to move it.
As an owner of a Passive House, I will say that south orientation is a "nice to have" , not a be all end all. Same thing with windows. Definitely put your biggest windows facing south, but only if it makes sense considering other factors (view, aesthetics, etc). Minimize windows or window size on the east and west, but absolutely have some windows there. Just make sure you have a way to shade them. The primary function of the windows is to let you see out, and allow light into the house. Make sure you will be happy with the windows, irrespective of Passive solar performance. Depending on the windows you get, even with prime orientation they might be net energy losers. The key is to get as much energy back as you can while fulfilling the primary purpose.
I ran my BEopt model from east to west. Due east cost $24 dollars per year more to operate than dew south and dew west cost $22 more than west.
Walta
Walter that is a very helpful report even knowing many factors can change that. What zone did you do that in? I tried to blow the image large enough to read; but, the image becomes blurry before it becomes large enough to read.
Mike,
Here are links to two relevant articles:
"Reassessing Passive Solar Design Principles"
"Study Shows That Expensive Windows Yield Meager Energy Returns"
Martin Holladay
Thank you for the links, I had read them last week, but it was good to reread them.
In my inaugural question on GBA I asked "how do I learn green building quickly?" and you responded with:
"Mike,
You're not going to learn everything you need to know in a few weeks. You're going to make mistakes -- but if you're lucky, many of the mistakes will be fixable. Hard-to-fix mistakes include:
1. Orienting the building the wrong way.
2. Making the foundation too close to grade.
3. Building a house with low ceilings."
Can you elaborate on what you meant by point #1?
Thank you. -Mike
Mike,
In my article titled "Reassessing Passive Solar Design Principles,", I made the following point (with a tip of the hat to Dana Dorsett): "While the passive solar principle favoring east-west orientation may be hard to defend from a space heating perspective, most new homes should, if possible, include a roof that is optimized for the installation of a PV array. An east-west orientation makes this possible."
I think most observers would agree with my analysis that, compared to other mistakes made by owner-builders, the wrong orientation is hard to fix.
Excellent! Thank you for the clarification. I need to make the decision of using a tracker or the roof then. I like the tracker idea as logic leads me to believe it will capture the a significant amount more of energy. I have the space for it. Just need to answer the aesthetics question.
Best to use PVWatts.
I'd suggest running some calculations. I had a similar thought, but discovered that it's not as significant as I thought. In most cases, the tracker will not pay for itself. They add a lot of cost.
Even for solar panels, a moderate deviation from due south makes a surprisingly modest difference on energy capture. Assuming you optimize the tilt angle accordingly, deviations up to 40deg can result in a degradation of only about 10%. There are calculators online for this, but I can't recall the one I used.
I found similar results when looking into the orientation for my house with solar panels in mind. I'm attaching a chart that shows in the percent solar loss from each degree deviation from solar south in case it's helpful. My understanding is that this would be different by location and I'm a good deal further north than Mike (I'm in Southern NH around 43*N), but the general principle should apply. For my purposes I considered anything within 25* of solar south to be effectively no loss at all (less than 2.5%) from a solar panel production standpoint, though that's something of a judgement call.
Walter, at what power rate were those prices calculated?
Are you planning (now or in the future) any rooftop solar PV for the house?
If you are, then having the designated roof space as close to due south is important, both for maximum generation and qualifying for things like rebates/etc. At least, that was the case with our house in zone 6 NH.
Brian,
I found out yesterday that I can send power into the grid, so it will probably be on my menu to make the house net-zero down the road; however, I think I might put a tracker, to follow the sun, since I have the space.
Brian, must the rebate be predicated by the roof of the house or could you have installed it separate from the roof of the house, say on a tracker pole in the yard, and still have qualified? -MIke
Any type of ground mount wasn't possible for our property, so our system had to be rooftop. It's been 3.5 years since our install, so I don't remember specifics, but I do remember that in order to get rebates from the state and/or our utility, the system had to meet certain efficiency specs. I would imagine they wouldn't want to provide rebates on a per watt basis if you orient the panels in a direction too far off due south.
I would recommend checking with local solar installers before you build to get an idea of what works best for your property and house. Some planning ahead of time can make the install much easier, whether it's a ground or roof system.
The first run BEopt default selection happened a 78% gas furnace.
I ran it again with a 17 seer heat pump at .1025 per kwH
The deference between E and S came out at $27.00 pet year.
I used Jefferson City MO location and weather data.
With lot of glass on the south wall.
If your goal is to truly get to net zero in Missouri and I think most other states your will need to fool the utility into thinking you will use 20% more power than you end up using. As by state law they will not grid connect any solar system that is predicted to produce over 80% of the estimate. So if they get actual meter reading you will never get a 100% system connected. So your only hope is preconstruction when they will guess at your power consumption X number of watt per square foot. There business model does not work if you are not buying any of their product.
Walta
Walta, that is most insightful. I am in TN, and my approving authority is the TVA, not my local power company. The Tennessee Valley Authority (TVA) is a federally owned corporation in the United States created by congressional charter on May 18, 1933, to provide navigation, flood control, electricity generation, fertilizer manufacturing, and economic development to the Tennessee Valley. I will need to answer this question of 80% maximum production asap; because, I will not be able to spring the money for a PV array for a year or two after construction.
Thank you.
-Mike
Who knows what future policies will be? In general, net metering charges/credits are shifting towards more accurately reflecting the actual economics of residential scale solar.
Consider shifting your focus from the largely irrelevant net-zero to cost-effective and green.
Net zero is cost effective and green in a lot of situations. Which methodology do you feel would be more green and cost effective?