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Passive Solar

Passive solar refers to methods of collecting the heat energy of the sun without using any moving parts.  In its simplest form, passive solar is just a matter of orienting the house so that its longest side faces south, and putting most of  the windows on the south side, called a direct gain system. In fact, EVERY BUILDING IS A PASSIVE SOLAR BUILDING, since every building has sun shining on it and windows in it: the ones we traditionally think of as passive solar take advantage of the sun, while for others the available sun is either squandered or a source of excess heat that must be removed. For a photo essay of buildings around Seattle, click here.

There are also other less direct methods of collecting solar energy, for example attached greenhouses, trombe walls, water walls etc that are somewhat more complex, but have their own set of advantages. As an alternative to having windows on the south side, a south facing greenhouse (sunroom) can be attached to the house, or other variety of indirect gain system could be built into the south wall.  Each method has it own set of advantages.

There are essentially five components to a passive solar building:

  • Orientation: the long axis faces with 15° of south1.
  • Windows: the right type, and amount of windows face south to collect sunlight
  • Overhangs: the solar collection windows are protected from the summer sun
  • Insulation: the building is superinsulated to keep the heat in
  • Thermal mass: optionally add some mass to hold heat for when the sun isn't out.

(diagram: passive solar components)

Orientation: generally, in all climates having the long axis in the east-west direction is the best,  whether the idea is to collect solar or avoid it.  This is because the south facing sun is high enough in the sky in summer that an overhang will keep it out, and low enough in the winter that the same overhang will have no effect; and in a hot dry climate a reasonable size overhang can be built to keep the sun out all year long.  In general, west facing solar gain is to be avoided in all climates because in order to get any in the winter, you get too much in the summer.  East facing solar gain is acceptable in cool summer climates, but can also lead to summer overheating.

In hot climates, a north-south orientation can be best if the entire building can be protected from the western sun (eg by recessing it in a east facing hillside, by earth sheltering in an east facing hillside, building a wide veranda, or otherwise nearly completely obscuring the western sun.)  In hot climates with frequent cooling breezes, it may be more important to orient to catch the breeze than face south.

When solar gain is desired, any orientation with 15 of true south will work.  Facing more to the east will catch more morning sun (but leave the west face more vulnerable to overheating), while facing more west will catch more afternoon sun (but leave the west face less vulnerable to overheating).

Windows: as a general rule, at least 7% of the floor area in windows is the minimum amount to get significant solar gain, and about 15% is about the most that generally makes sense.  Average monthly solar gain data is available for at least a hundred US locations, and using that a more sophisticated analysis could be done, but everything else dependent on weather conditions, you can only calculate best case, worst case and averages.  Unless night time thermal blinds/shutters are used, at some point the heat loss at night (and cloudy days) starts to outweigh the value of the solar gain during the day.  Note also that large amounts of glass tend to create large amount of glare, especially when the sun is all coming from one direction.  For buildings with large amounts of glass, glare is circumvented by collecting some solar away from the occupied floor, using light shelves to distributed the light better etc.

Windows facing other directions should be minimized.  When afternoon overheating is possible, west facing windows that aren't protected with a big overhang can be a large liability.  East facing windows are less of a problem, but can cause annoying heating on hot, sunny summer days.  North facing windows rarely collect heat (except around the summer solstice: the further north, the further sun travels north of west at the solstice, the more gain), but can lose a lot a heat during cold weather.

For solar gain, south facing windows should have a relatively high SHGC, .5 or above, except in cooling dominated climates, where all windows likely have a SHGC of .35 or less.  Windows facing other directions should generally have a low SHGC: in fact the availability of low-E windows with low SHGC values make east and west facing windows less critical.

In mild climates, or climates with reliable sun, clear glass has the highest SHGC, and so would be a good choice for south windows.  In other climates, the tradeoff is that by adding a low-E coating the window's R-value improves by about 50% (R2 to R3), but the SHGC typically drops from around .7 to around .5, so the best choice depends on the situation.

passive overhangs Overhangs:  The difference between summer solstice and winter solstice is approximately 46°, with the winter solstice sun being at (90-latitude-23), and the summer solstice sun being at (90-latitude+23).4  Using this, an overhang can be designed that lets in the winter sun, but excludes the summer sun.

The difficult condition is that there is the same amount of sun on Mar 21st as there is on Sept 21st, and in most climates Sept 21 is markedly warmer than Mar 21st.  In warmer climates it may be best to extend the overhang some so that on these equinoxes, some, or even quite a bit of sun is avoided.  In some, some form of movable exterior shading may be used seasonally when necessary, and in others the night is cool enough by Sept 21st that shading isn't an issue.

Another alternative is to use deciduous plants on a trellis: in March they have no leaves, so let in the sun, while in April, they still have their leaves and block the sun.  The downside is that even with the leaves off, some gain is still blocked in March, and that plants have a habit of growing wild and can cause damage to the house  if allowed to grow anywhere.

Insulation:  if you going to bother to collect solar heat, you want to be able to hold onto it in the evening and on cloudy days.  Using greater insulation means that less heat is needed on cold days, and less cooling on hot ones, making the amount of solar gain and nighttime cooling closer the actual demand.

Thermal mass: When there are not that many south facing windows (say less than 7%), there is generally not enough solar gain to cause the building to overheat, but as the south glazing goes above 7%, mass is needed to absorb the additional heat.  The mass serves not only to keep the afternoon temperature comfortable, but to release this extra heat during the night when the sun isn't out. In order to be particularly effective, in storing solar gain, the sun should shine directly on the mass, and the mass should be a dark color so it absorbs it all (since the suns energy is all radiant).  If the mass isn't directly in the sunshine, the air temperature will climb and the mass will absorb heat by conduction from the air, but this process is very slow, especially if the mass is on the floor, because solar gain can easily create highly stratified air.

Most materials store very little heat per unit volume, the best ones are stone and concrete (including brick etc), which store around 28Btu/°F/ft3 and water which stores about 62Btu/°F/ft3 of heat2.  Heat moves quite slowly in and out of stone, and much faster in and out of water (due to internal convection), making water a much more ideal storage medium, but because leaks are so disastrous and since concrete and stone are common building materials, they are much more commonly used as thermal mass than water.  Heat moves so slowly thru stone that it could easily take all day for heat to move thru an 8" concrete wall, and as a result, the first 4" of concrete is more effective on a daily basis than the next 4".

Thermal mass is a double edged sword in that its tends to keep the building at whatever temperature it current is.  That means that if its allowed to cool off (say a winter vacation), it could take a couple of days to heat back up, and if its allowed to heat up, it likewise could be days till it cools off, even after the air has cooled.  The more mass the larger the effect.  Also with any significant amount of thermal mass, night time thermostat set back will result in the heat released from the mass and so temperature drop will be minimized.

Direct Gain Systems

These are the simplest systems, using all of the above design guidelines.  The simplest of all systems are sun-tempered, meaning they have smaller amount of glass and minimal or no thermal mass.  Almost equally simple buildings have a bit larger amount of glass and thermal mass in a standard place, typically a slab on grade floor.  These buildings easily supply 25% of the heating needs even in a maritime climate like Seattle, and more than 50% in sunnier climates.  The biggest selling point of these systems is not only are they relatively simple to design and build, but the require little interaction from the occupants.

Although it is possible to get over 90% of the heating requirements met by passive solar, doing so often, but not always, requires more glass, more mass and typically more work on the part of the occupants.  Insulated shutters must be closed on cold nights, external shading may need to be put up in the fall, and nighttime ventilation may need to be done by opening and closing windows.  Most of this can be automated, but doing so requires more equipment and hence more up front cost.  One sure thing, as the reliance on solar heating goes up beyond what is easily available, the more complex the design has to be to compensate for the extremes of weather: essentially you can get quite a bit easily, but getting more gets increasingly difficult.

Few buildings use even the simplest passive solar techniques, even when the site has good solar access.  Unfortunately, during the 70s and 80s, the more complicated passive solar buildings became the hallmark of the method, leaving a negative impression on much of the public. It is probably best to stick to using the simple passive solar system, then consider active solar, or even a source of renewable electric with a heat pump, along with the other refinements on the basic passive solar approach.

Window sizing: Rather than using the rules of thumb to determine the amount of south glass, it is possible to calculate the needed amount of gain and compare that to the expected amount of gain for the amount of glass chosen.  To do so, first do a heat loss calculation for the building for a typical day of each month of the heating season.  Then on the generation side, subtract the internal gain of the building (this is essentially the amount of electric used) from this amount.  The result is how much heat is needed.  Based on tables and a selected window amount, see how much heat that window amount generates compared to the need.  The remaining heat will need to be generated by a back up heating system (which is any of the standard HVAC systems).  Note that as the window size increases, not only does the solar gain increase, but so does the heat loss.

A good rule of thumb is to size the windows so that they supply all (or most) of the heat needed on a typical winter day, but backup heat will be needed other times.

(diagram: example passive solar calculation)

Passive solar homes typically have room layouts that take the sun into consideration.  People generally desire more heat and light in public areas (living, dining etc) than private ones (bedrooms), so public spaces are usually on the south and private on the north3.  These layout choices can apply even in hot climates since the north side is still generally cooler.  What is most critical isn't that a specific rule is followed, but that room are put in place with the sun in mind, rather than ignoring it.

Solar gain is also dependent on the angle of the window.  A window that faces the sun directly will get the most gain, while one at a sharp oblique angle will get the least.  Skylights, especially on south facing sloped roofs, tend to collect too much heat in the summer due to facing the sun directly, and so generally should be avoided.  Clerestory windows, or small dormers with sized overhangs work much better, but admittedly have aesthetic architectural implications.

(diagram: solar gain & window angle)

Indirect Gain Systems

In an indirect gain system, the sun passes through glass and heats an air space that is connected in some way to allow heat to pass into the house.  In a Trombe wall (named after French inventor Felix Trombe) or water wall, the a large mass is placed very close to the glass (either concrete or barrels of water) and the sun then heats the mass which then radiates its heat through the other side to the house.  In a greenhouse the sun heats the air in a room and the air is then moved into the house either via convection (or a fan, making it an active solar system). Typically a greenhouse is attached to the main house via an exterior door, so that the room can be closed off to prevent heat loss at night and on cold cloudy days.  A variation on both systems is a thermosiphon wall where a small air space is heated by the sun and the rising warm air moved by convection into the house (see figures).

All indirect gain systems have  the advantage of separating the solar collector from the homes windows, allowing flexibility in design for locations that aren't well suited to direct gain.  Because the collector is all in one spot, it takes less effort to control both overheating and cold weather heat loss because only one set of doors/shutters must be closed, although most people do not want to have to open and close shutters on a regular basis.

In a mass wall system like the Trombe wall, the wall is a very poor insulator and so is a liability during prolonged cold cloudy periods and on cold nights.  It also has a long time lag, it may take hours before the sun hitting the outside of the masonry reaches into the home (ideal for mild, sunny winters but otherwise a liability).  Likewise, a greenhouse requires owner intervention daily and must be highly vented in the summer to prevent overheating.  Thermosiphon systems are easier on the homeowner, but care must be taken to prevent reverse thermosiphoning, where the heat of the house is removed by convection at night. As in all systems, better engineering can conquer many of the problems of each system, and no system is perfect.

Passive Solar Energy Modeling

(Future: extend the model from the heat loss page to include internal and solar gain for various glazing amounts and various climates.)

Resources

Passive Solar Energy, Second edition, Bruce Anderson & Malcom Wells, Brick House Publishing 1994.
A good introductory guide.

The Solar Home Book, Bruce Anderson & Michael Riordan, Cheshire Books, 1976
Out of print, but still available used, this is a good in-depth book.

The Passive Solar Energy Book, Edward Mazria, Rodale Press, 1979
Possibly out of print, but generally available as a used book.

The Passive Solar House, James Kachadorian, Chelsea Green, 1997.
Although emphasising only one kind of solar system (designed by the author), the basic principles are there also.

http://www.energysavers.gov/your_home/designing_remodeling/index.cfm/mytopic=10250
Web site from the department of energy, covering about the same material as is covered here.

www.susdesign.com Many passive solar calculators..sun angle, shading and solar insolation data. 


Notes

1: apologies to people in the southern hemisphere, where south means north.

2: These values are ballpark, based on 120-140Lb/cu ft for concrete and a specific heat of.22Btu/lb, and 62lb per cu ft for water, and a specific heat of 1Btu/lb.  Density and specific heat of stone like materials will vary somwhat.

3: the exception is bathrooms, which often end up on the north side, but people usually like them warm for bathing purposes, so supplemental heat is often needed.

4: this angle of the sun is at solar noon (ie standard time, adjusted for the exact longitude).  Since the sun travels in a arc across the sky, at all other times it will be lower.