Solar Heresy 101

Over 30 years ago I took a class in solar heating and cooling. At the time photo-voltaic systems were relegated to space exploration and locations where cost was not an issue. Off grid power was supplied primarily wind and petroleum generators, so solar orientation was relegated to heating concerns. Passive and active solar heating and cooling systems were the rage of the energy crisis of the 1970s. In addition, wood stoves were reinvigorated and to some degree, reinvented. At the time there was little to no awareness of global warming or carbon dioxide as a major contributor.

At any rate, the primary and sacred tenant that was taught and learned well, was that to intercept the most solar heat, a building in the northern hemisphere needed to orient it’s glazing (or panels) towards due south.  In the southern hemisphere, the sacred direction is north. east and west are not options, as they are not the sacred directions. Perhaps we should say equator facing rather than north or south.

So the first solar house I built faced due South. It had huge windows facing South and very little facing any other direction. I learned that for a few hours around noon the sun streamed into the house, warming it nicely. Unfortunately, that was maybe four hours out of eight (winter) or 16 (summer). The rest of the time the windows leaked more heat than they let in. It was a good thing that wood was plentiful in New England, because I had to burn about 10 cords of it to stay warm in the course of a winter. There have been improvements in windows since those days, but little improvement in the doctrine of solar house orientation. Unfortunately, there hasn’t been much improvement in wood stove design either.

Since that time I have designed other passive solar homes and installed photo-voltaic and wind electric systems. One thing that opened my eyes wide was a Zomeworks tracker. This delightful invention relies on freon in tubes that warms in the sunlight, boils and moves from one side of the tracker to the other, causing a weight imbalance which turns a rack of PV panels towards the sun. Throughout the day, barring clouds or strong winds, this simple device will track the sun, changing a 2 to 4 hour peak charging window into a 5 to 10 hour window, depending on the season. In Northern Colorado in late December, the tracker is fully turned towards the morning sun by 9:00 AM and follows it until it descends below the horizon around 4:30 PM.  If it was able to operate without the large shock absorbers that keep it stable in inclement weather, I’m sure it would turn towards the morning sun sooner, although too early and there isn’t much energy in the morning sunlight. There are other trackers on the market which use electric motors and LED sensors to track the sun, although I like the simplicity of the Zomeworks model. Http://www.zomeworks.com

Now while it has been done, it really isn’t practical for most of us to build a rotating house that is able to duplicate the actions of the PV tracking devices. If we design with long, straight flat walls, we have the same issue that solid mount PV panels have: a small window of opportunity for peak performance. There is some benefit that east and west windows provide, but the angle is too oblique to be much good in winter and in summer they tend to overheat the house. A perpendicular incidence of the sunlight is best for both producing electricity and for providing heat through a window. If we look at what nature provides, we find that the (Northern Hemisphere) winter sun’s path runs from South East to South West. It will take it’s lowest path on the winter solstice and it’s highest route on the summer solstice. In Winter it will rise and set at it’s furthest south (or north in the southern hemisphere). I’ll call it the equator facing rule. The idea is to give up the less intense morning and afternoon sun for the full blast of the noon sun, and the few hours either side of noon on the shortest day of the year. As a concept it works, mostly.

I decided to abandon the idea of straight walls and a 90 degree angle to solar noon as the best approach. Instead, I design the equator facing walls to gather the sun during all the hours available, not just around noon. Thus my new ideal wall shape is curved, with the convex side facing the equator. A concave surface facing the equator is a viable alternative. Although this seems wasteful of interior space to me, it does have interesting possibilities of allowing additional solar gain during morning and afternoon hours at both extreme ends of the building. In the drawing below, I show the solar incidence angles for several building shapes during morning, noon and afternoon hours. Note that when the angle of incidence is 90 degrees that the maximum solar gain is achieved. The further from 90 degrees, the less the solar gain.

Incidence of maximimum solar gain for different wall shapes

Incidence of maximimum solar gain for different wall shapes

I have not constructed samples for all these shapes with equal sized windows so as to quantify their relative efficiencies. However, to my way of thinking, the best overall shape is #2, the semicircle, followed by #3. the angled bow. #4, the concave structure has some interesting possibilities that I may have to look into. The least interesting to me is #1, the rectangle. I currently live in a round house. I can relate from non-scientific observation that it heats quickly in the morning and maintains a level of heat until late afternoon. If we graphed out the solar gain for the various models, I think we’d see something like the following. This follows my experience of living in both types of buildings for several years each.

Solar gain - flat wall vs curved wall

Solar gain - flat wall vs curved wall

The heat gain for the rectangle and semicircle type of surface is represented by the area under the curves. My assumption is that the surfaces are equal in area and if on a building, that there is equal glazing along the surface of the curve. I believe that the concave and angled bow shapes would be very similar to the semicircle if similarly graphed.

Should you decide to adjust the roundness of the semicircular wall, the curved surface line will come closer to the flat surface line as you flatten out the curve. Another way of looking at it is that you are increasing the radius of the semicircle but keeping the length of the segment the same. The smaller the radius is, the lower and longer it’s line will be since you are reducing the 90 degree surface area, but at the same time increasing the amount of time some surface area will be at 90 degrees to the sun’s rays. In addition, assuming this is a building wall, it is possible to increase or decrease the heat gained during a particular time of day by the addition or subtraction of window area where it aligns with the time of day. My current home has more window area on the morning side of the house. As a result, it gains heat rapidly in the morning and holds it throughout the day, sometimes cooling somewhat in the late afternoon. A house I build with lots of equator facing windows was quite warm between 11:00 am and 1:00 pm, but would take a while to warm up and cool off quickly afterwards.

As a point of reference, let me return to the Zomeworks solar tracker. A similar graph, again based on my experience with the amount of electric power I can get from PV panels on this device vs. panels mounted in a straight equator facing direction would show the following. Again we assume the same amount of surface area for both panel areas.

fixed panels vs solar tracker

fixed panels vs solar tracker

Note that in the flat vs round example the area under both curves was not too different, just arranged differently. The lower solar gain at noon of the curved wall was compensated for by the longer amount of time that solar gain was experienced. In the second case, the tracker maneuvers the entire panel surface into the direct sun early in the morning and tracks the sun throughout the day. This changes the curve from a classic bell curve to a very fat, almost table shape with a large increase in area under the curve (i.e. lots more solar gain). This is because the bell shape is the result of two factors, the angle of incidence of the panels to the sun and the actual available energy in the sunlight, which decreases as the sun approaches the horizon. Since the tracker minimizes the angle of incidence factor, the remaining factor (the proximity of the sun to the horizon) is what is primarily observed.

A solar absorption device, be in a PV panel or a window in a wall, is most effective when it is able to track the sun’s path throughout the day. This is demonstrably true of solar tracking devices like the Zomeworks PV panel trackers. On a fixed object like a building I see great benefit to shaping the equator facing wall so it directly faces the sun for more than just a few hours. There is room for additional research and lots of fun building structures to explore this area.

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