For those interested, my reply to Fernando Cabral about solar water heating is attached, or can be seen below. One thing I forgot to say is that when you have one collector facing East toward the morning sun, if the panels are placed in series, the heat collected by the East panel will be radiated out by the West panel facing away from the sun. If they are in parallel, the same problem is experienced, but it is the heat the "hot" panel has put in the tank which is radiated by the "cold" panel. The only way to avoid this problem would be to make a special plumbing system which made the circulating water go through only the East panel in the morning, shutting off flow through the West one, and reversing this arrangement in the afternoon, a complex solution and the reason all installations have all panels facing the same direction.
An extreme version of this problem was tested (against our protest that the test was not worth doing) by FSEC in the late 70s. It consisted of a cylindrical hot water tank, painted black all around, and covered by a cylindrical transparent glazing all the way around. We pointed out that the sides of this tank in shade would be radiating while the sides in sun would be collecting. The collector was tested for the inventor anyway, and he was very disappointed with the poor results. Another solar collector tank, similar to the one described, was slightly taperd at the top, and we called it the "suppository solar collector." Another one we tested was a fairly conventional flat plate collector in an insulated box and the inventor had a good idea of putting folded transparent plastic in the space between the glass and the blackened metal fin and tube absorbing plate. Solar water heating collectors have to be tested both in simulated actual opertion, with water in them, and empty, dry. The latter test is to make sure they won't have any problems when sitting in the sun, on the roof or ground below, while the plumbing and wiring are being done. This collector failed the dry, "stagnation," test when the plastic inside melted, ran through some cracks in the case, and pooled on the ground below. Ross McCluney, Cocoa, FL Message-ID: <[EMAIL PROTECTED]> Date: Fri, 15 Jan 1999 10:00:38 -0500 From: Ross McCluney <[EMAIL PROTECTED]> Organization: Florida Solar Energy Center X-Mailer: Mozilla 4.04 [en] (Win95; I) MIME-Version: 1.0 To: [EMAIL PROTECTED] Subject: Re: Best angle to catch sun light - off topic References: <[EMAIL PROTECTED]> Content-Type: text/plain; charset=us-ascii Content-Transfer-Encoding: 7bit Fernando Cabral wrote: > One thing I have noticed is that no one plans the roof in order to take > advantage of the sun in an optimized way. They build their house and > than the best place for the panel is found. Usually this just means > deciding > which side of the house is facing North. That where the panel is placed. > > Also, the angle the panel makes with the horizon is just an accident: > whatever > the inclination of the roof is. > > Now I am planning to build a house for a small farm I have. I've been > thinking > on how to take the best advantage of the solar power. This includes > where > to have a garder with a nice sundial and where to place the solar panels > for water heating as well as (perhaps) electricity (at least in Brazil > solar panels for electricity are very expensive). > > At 19 37' 57" S, it is clear that the panel should be facing North. > But what is the best angle with the horizon. And, if I can have several > panels, is there a practical to calculate the best angle of each > so as I can guarantee the highest possible insolation level? > > Say, if I have three panels, is it best to place them side by side, with > > the same inclinatation and declination? Perhas if one is a inclined > towards > the East with a certain angle and the other to the West with a proper > angle I can capture more light? I suggest that you visit our web site and explore the available options for getting information from our documents section. Most of our publications are free. Though oriented mainly for northern latitudes in the U.S., I think you should be able to translate the recommendations to your location. A couple of conclusions I remember from years ago when I did a little work in this field: Solar water heater collectors, and solar photovoltaic cells for generating electricity, are not overly sensitive to small changes away from the optimum direction for them to face. Normally we recommend pointing the normal (perpendicular) of the flat plate collector surface due south (north in the southern hemisphere) and at an elevation from the horizon that is half way between the sun's solar noon summer and winter solstice positions, putting this direction close to the solar noon equinox position. Since summer months are longer than winter months, and winter months are colder, some people alter this direction, facing the collector closer to the lower-in-the-sky winter solstice, by 10 to 20 degrees, depending upon what is desired, to slightly improve performance in the winter at the expense of summer output. Furthermore, if you want to accentuate afternoon performance over morning performance, you can change the azimuth as well, but don't overdo it, because these alterations probably will reduce the total amount of solar energy collected over the course of a year slightly, or a lot if the direction is too far away from the primary recommendation. I say "probably" because so far I haven't considered the effect of clouds, haze, and other atmospheric variables. If your site has more sun in the morning than the afternoon, due to afternoon cloud buildup, for example, you can increase overall annual performance by rotating the collector slightly to the East. The above recommendations apply to all the solar panels. They should all face the same way--be parallel to each other. If you face one more toward the East and another more toward the west, you are in some respects shooting yourself in the foot. The East one will do better in the morning, but the west one will do worse, probably a lot worse, and vice versa in the afternoon. I know of no solar installation anywhere that has its solar collectors in different directions, at least on purpose and for optimum performance. The problem of facing buildings in the wrong direction is a very very common one. It might be forgiven for relatively uneducated people, but for architects it is inexcusable, but alas common. The problem is not as crucial for solar collectors for heat or electricity, since they are generally somewhat independent from the building. It just means that the support structure holding the solar panels has to be more complex, and probably unsightly, in order to face them in the proper direction on a roof facing the wrong direction. My field is the daylight illumination of building interiors--daylighting, and in this case mis-orientation can be devastating. In hot climates, such as found within 20 or 30 degrees of the equator, facing windows west without adequate shading produces horrible afternoon heat loads, localized thermal discomfort, and terrible glare. We recommend avoiding facing windows in this direction, or shading them with good exterior shading devices. Visit our fenestration web site for more information about this, shown below. You can use the main screen menu or just add "/~fen" at the end of the site address to go directly to the proper page. > I know this information can probabily be found in some book about > photometry or perhaps archtecture/engineering. I couldn't find > them... Look for books on solar energy or solar engineering. The "bible" is a book by Duffie and Beckman called "Solar Energy Thermal Processes" or something like that, but there are others. If it is photometry that interests you, try my book, listed below. I hope this helps. -- Ross McCluney, Ph.D. Principal Research Scientist Florida Solar Energy Center, 1679 Clearlake Rd., Cocoa, FL 32922-5703 Voice: 407-638-1414 Fax: 407-638-1439 e-mail: [EMAIL PROTECTED] Florida Solar Energy Center: http://www.fsec.ucf.edu Sundials: http://www.sunpath-designs.com Introduction to Radiometry and Photometry: http://www.artech-house.com --------------------------------------------------------------
