Why are we still talking about Passive Solar versus Active Solar instead of Efficiency versus Renewables?

In recent design team meetings, I’ve heard discussion about incorporating “passive solar” design strategies into a large commercial structure. As these conversations progress it turns out what is usually meant by passive solar is the inclusion of beneficial heat gain though windows and skylights. This is of course a commonsense approach for almost any design, but is it really passive solar?

When I first entered the architecture profession in 1976, solar design was a major topic of concern for architects. By the early 1980’s that had evolved into two different strands – passive solar and active solar. Architects of that era had a clear understanding of these two design approaches. A summary of strategies used in each is below.

Passive Solar strategies:

              Daylighting to reduce electrical lighting use

              Direct sunlight on thermal mass surfaces

              Thermal storage tubes

Earth cooling tubes

              Rock storage beds

              Roof pond

              Trombe wall

Active Solar strategies:

              Flat plate rooftop collectors for space heating

              Movable insulated shutters

              Flat plate collectors for domestic water heating

Today, most of these design options are obsolete, having been replaced with newer technologies. I believe that the terms passive solar and active solar should be replaced in our design conversations with the more accurate terms energy efficiency and renewable energy. Examples of these two would be:

Energy Efficiency strategies:

              Super insulation – walls, roof, floor, foundation etc.

              Advanced glazing – triple pane, dynamic glass, thin glass, vacuum glass, etc.

              Ventilation reduction - demand control, energy recovery, etc.

              Infiltration reduction

              Daylighting and sun-shading

              Heat pump heating/cooling

Renewable Energy strategies:

              Geo-exchange/Ground source heat pump

              Water-exchange heat pump

              Air source heat pump

              Photovoltaic panels

              Wind (only where the resource justifies it)

I hope that future design strategy sessions will refer to energy efficiency and renewable energy strategies along these lines, and it will be less confusing for all concerned.

The term Daylighting appears in both sets of lists, under Passive Solar and Energy Efficiency. Daylighting is not, however, the same strategy as it was in the heyday of passive and active solar. At that time, common electrical lighting power density (watts/s.ft.) was as much as 4.0, and an efficient lighting design was 2.0. As a result, replacing that much energy density with free and lower heat intensity daylight was a good investment. A well daylighted building in that era could reduce the portion of end energy use for lighting from 20% to 10% or less. When a typical office building may have had an Energy Use Intensity of 100 kBtu/s.ft./yr., that was a significant reduction.  Daylighting was therefore one of the most common energy conserving measures.

Today, lighting power densities below 0.5 watt/s.ft. are common. There is simply so little energy being used on interior lighting now that trying to replace it with well controlled sunlight is not always a good investment, from an energy payback perspective.  For that reason, daylighting should be used by architects to improve human productivity, satisfaction, and health, rather than as a primary energy conserving measure (ECM). A good daylighting solution is one that has a positive impact on the building’s occupants, without increasing the building EUI.

Bringing this analysis full circle, daylighting and daylight harvesting are still frequent topics in building sustainability discussions. The strategy is even embedded in Building Codes. That is fine, but designers should recognize that both are minimally helpful in making buildings more energy efficient and should focus their effort and attention on designing daylighting so well that it improves occupant health, comfort, and productivity in any weather condition at any time of year.

A final note on the use of the term passive solar is that Passive House (or Passiv Haus) has become a popular movement abroad and in the U.S. for very good reasons.  This design concept uses a combination of super-insulated envelopes with proper ventilation (often separating fresh air from heating/cooling) to achieve outstanding EUI’s for all types of buildings. Passive Houe was derived from those early Passive Solar buildings on the 1970’s and 1980’s in the United States. The Passive in Passive House refers to the super insulated envelope that consists of fixed components and features. There are no moving parts and no equipment needed to make that envelope function, except for window coverings.  But make no mistake, Passive House buildings are highly dependent on ventilation systems that include machinery to provide fresh air.  A better name would be Passiv/Activ Haus.

I hope that in your next design strategy meeting, you will use the terms efficiency and renewables, leaving Active and Passive Solar in the rearview mirror.