Saturday, February 11, 2017

Design - Maximizing Passive Solar Gain - An Overview

A recent re-read of Mazria's book, The Passive Solar Energy Book, made me think that it might be time to pause and revisit the subject of passive solar energy for space conditioning.  Our house is designed around harvesting passive solar energy year-round that will eventually provide all of the heating and cooling we will need.  In winter, the energy will come through lots of south-facing windows, which is nothing more than typical for passive solar.  In summer, it will come from the Annualized GeoSolar system (AGS), which is not typical. (For info on AGS, click on "Featured Post" in the left column.) Harvesting heat from the summer sun instead of depending solely upon the whims of the winter sun, makes AGS a perfect adjunct to classical passive solar design -- or the other way around since we consider AGS as our primary energy source. However, I realize that only a handful of dwellings utilize AGS.  In fact, Wikipedia's description of AGS references only two sites in North America one of which is our project.

So, for all practical purposes, passive solar for conditioning means using the winter sun for heating.

But maximizing solar gain, whether from the winter sun or the summer sun, is more than solar collectors and windows. In order for passive solar to work, or at least work efficiently and economically, the building has to be designed for retaining and distributing solar energy.  (Hence the advantage of designing passive solar into new builds having sufficient thermal mass, air-sealing and insulation, as opposed to expecting a decent pay-off from passive solar for leaky, under-insulated existing structures with limited thermal mass.)  

A Little History
It is clear from the literature that passive solar has had its ups and downs.  It was boosted by the energy crisis in the mid-70's, especially it seems with respect to earth sheltering and greenhouses. (There are at least eight earth sheltered houses within a 30 mile radius of Collinsville, all but one built after the early 70's.)  In the '80s, energy prices came down long enough and low enough to dampen enthusiasm for passive solar. Then, after the turn of the century, the talk of "peak oil" and the reality of global warming combined with rising oil prices, emerging alternative energy and mainstreaming of energy certifications, such as Energy Star and Leed, heightened awareness for energy conservation.   But it does seem that, as building green is brought to scale, the emphasis is on reducing fossil fuel costs and preserving of finite resources.  Passive solar as the primary means for space conditioning seems to have been relegated to parts of chapters in the newer books on green building if mentioned at all.  However, judging from an occasional new book on, and the amount of online chatter about, natural green building, there are still quite a few purists building houses with straw bales, adobe, cob, rammed earth and rammed earth tires. And earth sheltering is still often combined with the natural modalities, e.g., Earthships that are earth homes built with rammed earth tires.

Maximizing Passive Solar
So maybe its time to revisit passive solar using Mazria's book and our project as bases for discussion.  Any building that is solely or largely dependent on solar for space heating and cooling must first maximize solar gain.  Then, because BTUs from solar are harder to come by than BTUs from carbon, it must be even more efficient than most green buildings at keeping BTUs where they can do the most good -- in during the winter and out during the summer.  

A way to look at passive solar is that it involves three phases -- harvesting, storing and distributing.  My blogs posts to date have co-mingled the three phases to the extent that maximizing solar gain per se has been lost in the discussion.  This series of four posts focuses just on maximizing passive solar gain. 

Suggested Reading
While I am using Mazria's book as a handy blueprint, a lot of information comes from a variety of other sources, principally the internet and the books I own or borrowed from the public library over the years. And I can recommend the reference list in Mazria as an excellent resource for hardcore passive solar literature. His book was published in 1979 and most of the references were published in the '60s and '70s with most in the '70s.

Earth sheltering and passive solar are complimentary modalities but one only has to peruse downloadable pictures of green buildings to realize that  passive solar without earth sheltering is the new norm. Nevertheless, while I am suggesting references, let me mention three go-to sources for information on earth sheltering: (1) the content and annotations in Don Stephen's paper on Annualized GeoSolar, (2) the well-annotated book produced by the University of Minnesota Underground Space Center titled Earth Sheltered Housing Design and (3) John Hiat's self-published book, Passive Annual Heat Storage, Improving the Design of Earth Shelters, that is invaluable for both its practical content and short bibliography. Unfortunately, both books are out of print but used ones are available online.  All three eclipse the methods that were popular in the '70s.  For a look back at those modalities, the classic is Rob Roy's Earth-Sheltered Houses, How to Build An Affordable Underground House.

Types of Solar Heating
There are three approaches to solar power for heating. The first and most popular is direct gain whereby the living space becomes solar collector, heat storage and distribution center all in one.  It is characterized by south-facing glass (northern hemisphere) and enough thermal mass properly positioned for absorbing, storing and radiating solar energy. 

The second approach to passive solar heating is indirect gain whereby thermal mass is positioned between the sunlight and the living space.  The mass absorbs solar energy and converts it into thermal energy for heating. The Trombe Wall, the
most famous iteration of indirect gain, is a masonry or water wall set just inside the south-facing windows. 

The third approach is known as isolated gain, so called because the collector and thermal storage system are isolated from the living space.  A classic example of this approach is a flat plate collector, whereby sunshine passes through glass (that is not window glass for the living area) and heats a thermal mass.  The heat is transferred via natural convection from the mass into the living space.  

Isolated gain
Of course, passive solar means that, regardless of approach, there are no mechanical assists like fans, blowers or pumps.

Annualized GeoSolar does not fit nicely into any of the three types of passive solar heating.  It has attributes of both direct gain and isolated gain depending on the season. For summer, there is flat plate collector but it is located 20' in front of the house (isolated gain) and the distribution system and thermal mass are located under or adjacent to the living space (direct gain). For winter, it is all direct gain -- south windows and interior thermal mass.

The Merits of Passive Solar Conditioning
The advantages of passive solar are........
     -  Simplicity:  design, operation
        and maintenance 
     -  Feeling of comfort at lower room
     -  Warmer floor
     -  Endless supply of free energy
The biggest disadvantage:  lack of control

Design Considerations for Passive Solar
Following is a list of the design considerations that receive the most attention by the references that I consult.  This is the first of four posts on passive solar.  Three forthcoming posts will enlarge on selected items from the list and touch on ancillary topics.

          -  Located for maximum exposure to winter sun
          -  Shape
          -  Orientation
          -  North side considerations
      Room arrangement
          -  Active living spaces
          -  Spaces requiring less heat and light
      Protected entry
          -  Location 
          -  Ratio of glass to floor area
          -  Transparent vs. translucent
          -  Exterior shading from summer sun
          -  Interior thermal shades for nighttime and gray days
          -  Operable windows for warm weather cooling
       Heat storage
          -  Masonry
          -  Adobe
          -  Water walls
          -  Size and location relative to windows
      Surface colors
          -  Thermal mass
          -  Wood frame walls and ceilings
          -  Exterior surfaces
        Supplemental heat source

The next post on passive solar will focus on the first three major items in the list -- building, room arrangement and protected entry.