Solar Space Heating Panels
   This article is open to additional information being added later as I can manage the time. I would want to detail as much useful information as possible, however for today what we have here is a good start.

   In this discussion we will be looking at solar space heaters of the air kind although solar collectors that heat liquid for hot water heating are also used for heating floors.  Solar space heating panels are sized according to the average solar Btu derived from software plots for a particular site.  The plots show Btu data based upon a specified square area of window or solar space heating panel.  The plotted solar Btu data is in the form of radiant energy input upon the specified surface area computed before losses.  For use with sizing solar space heaters, the average efficiency of the solar space heater is used which represents it s efficiency after losses where such panels are rated to be 48% efficient.  This might not seem very efficient however the solar space heating panel is considerably more efficient at utilizing solar energy than a solar PV panel.  Also it is best to buy such panels from those who have been in the business awhile and who have run analysis on their product and can provide information on its performance. You may view a video demonstration of how the solar space heating p...

   Before we look at how we utilize the computed data, there are several things we might want to take into consideration when we size a solar space heating panel system.  One major consideration is the coldest average day of the year and the losses of our building at that temperature.  If the sun happens to be shining well on such a day we would want our solar space heating panels to replace the losses of our building during the daylight hours.  We will have to also look at any south facing windows and calculate how much solar gain they contribute.  The second thing that we would want to consider is whether or not we would want to use passive or active solar space heating panels.  Passive panels use hot air convection currents to move air into and out of the panel while active panels utilize a thermal switch which activates a blower in the duct work when the panel is hot enough for providing heat, in the later case a wall mounted thermostat within the home can be used to prevent the panel from coming on when not needed and hence prevent over heating the home.  The later case is the wiser choice since this would help in managing the thermal energy within a home.  If you use an active solar space heating panel you have the option of doubling up on the heat gathering capability to make use of partially cloudy days by adding an extra solar space heating panel or two to the scenario.  Hence there are options depending upon your vision or that of a home owner.
   In this plot provide for a site by Solar2 software, the raw data for the solar Btu for the site is provided for a solar space heating panel of approximately 4' x 8'.  Referencing 12:00 PM noon of each day in January we can see that the solar panel reaches its peak output in Btu terms.  For instance on a nice sunny day in January the output at noon is 80.7 x 100 = 8070 Btu.  In adding up the sum for a day in January we have 54,310 Btu daily before losses (15.9 kW).  Now since the average efficiency of an air solar space heating panel is 48% the total amount of Btu thermal energy that we can effectively use is 26,068.8 Btu daily.  At 12:00 PM noon we can utilize (80.7 * 100) * 0.48 = 3873.6 Btu.  Now notice the lack of solar heat during April through to August, this is due to the shaded over hang of the roof above the south facing wall.  Adjustment of the over hang of the roof in the software plots, can help one to tweak the solar gain in the summer months to be near to nothing, and this also means that the thermal mass of the exterior on the south side will be shaded and stay cool all summer not adding much in terms of heat gain to the interior.  Hence our software plots can help us to do two things, compute the desired winter time solar gain and the summer time shading.  Thus our software plots also helps us to keep our structure cool in the summer time.

   The mere use of one solar space heating panel would  mean that the building design has very low losses such as would be the case of a "Passivhaus" design.  Green built homes can have their losses reduced to the same extent or such that they might require 2 solar space heating panels.  However as mentioned earlier we have to also take a look at the solar heat gain from the south facing windows and add that thermal energy to the sum total of heat input during a typical winter's day.  Still further one may elect to double up on the solar space heating panels to make use of solar radiation on partially cloudy days in which case the active kind of panel would be preferred since it can be electrically controlled.

  To use Solar2 you have to input the latitude and longitude of your building site as well as few bits of data about the size of the building, and its roof over hang; also you can add in solar shading fins to the design if you want.  The only thing about this simple software is that you can only plot the gain of one window or one solar panel at a time.  However since the windows and panels are all of the same size you will most likely only need to plot the gain of one of each example and the rest you can calculate with pen and paper.

  Computing the solar gain of windows: this is pretty much the same as with solar space heating panels however windows do come with specifications so here are some things to consider, such as toggling the design between low e and high e window types.

  Windows selected for radiant solar heating on the south facing wall of a home will have a high solar heat gain coefficient (SHGC).  For triple glazing, consider a whole-window SHGC in the range 0.33 - 0.47; for double glazing consider a SHGC in the range 0.42 - 0.55 (the higher the better).  Double glazing will be the most used type of window for passive solar heating scenarios.  The efficiency of the window will be comparable to the average efficiency of a solar space heating panel or within this relative range.  To use the SHGC of a window with solar Btu calculations multiply the raw solar Btu data plotted for the window size in square feet times the SHGC decimal rating of the window.  If you do not have the SHGC data for a double glazed window then the average SHGC of 0.485 (48.5%) will suffice for our calculations.  Notice this is relative to our solar space heating panel efficiency and hence both are within the ball park for general calculation purposes.

  Remember that Low E glass windows are used to block solar radiant heat and to reflect some of the heat losses through a window back into a the interior of a home. When used on the south facing walls, they are primarily used for natural lighting while blocking excessive solar heat gain.  This would be the case in homes designed for lots of natural lighting while controlling the gain, while select windows on the south side will not be the Low E type to allow a specified amount of heat gain into the home.  However since most solar heating designs will include a shaded over hang to block summer time sun (as can be seen in the photograph below), most of the south facing windows are the high emission double glazed type that come into play in the winter time in terms of heat gain.  Whereas the low e types of windows will be used on the east, west and north sides to lower interior heat losses, as well as to block ambient infra red heat around the home in the summer from entering the windows, even solar reflection off of clouds in the summer time can enter in through north facing windows and contribute to solar heat gain this way.  ~ Solar heated home designs in the northern latitudes where it is much colder, would make use of more triple glazed windows to improve the insulation factor.

  As can be seen in some passive solar home designs, the south facing wall can be all windows however to do this triple glazed windows would be used to lower the heat gain (unless you want to make up for losses from an on grade slab) while some of the windows are selected to be low e types and others are not. The SHGC figure would need to be around 0.40 on average and by analysis of the losses of the home design, as mentioned some of the windows would need to be low e types.  Homes that have allot of windows on the south side are more suited for the higher latitudes to increase the solar gain where solar radiant heating by latitude begins to diminish in degrees as the site location moves northwards on the map.  The efficiency of the windows have to be considered in the colder latitudes and hence triple glazed windows in such locations will be the norm if you want allot of windows as seen in the above photograph.  Near the equator lots of windows can again come into play, however such home designs will have solar shading features to block the sun around all of the exterior walls of the home.  Just to make mention of it here, homes on the equator would benefit mostly from solar shading of all four exterior walls with lots of natural ventilation being used in the over hangs.  (We do not see enough mention of how to make homes more energy efficient in the hotter latitudes, however such locations would be prime for experimenting with ways to make use of passive geo thermal cooling applications to lower the cost of cooling.  To do this we would think in terms equivalent to the cool atmosphere of root and wine cellars.  Split level and partially in ground designs would be the thing in hotter climates.)

  Here we can see that by way of software analysis and careful selection of windows, and solar space heating panels, as well as such things as thermal mass or phase change thermal mass and solar shading, we can design a smart home that behaves in a smart way all of its own, without being run by computers or any sort of electrical power to achieve things that are built into the design.

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Tags: solar_Btu, solar_heating_panels


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