Passive Solar Design Principles - Design For Climate

Passive Solar Design—What is it?
Essentially, Passive Solar Design is all about appropriate design for the climate in which you live.

Rather than try and fight against the natural elements by heating and cooling your house with mechanical systems using fossil fuels, a passive solar house will use its own structure (or building envelope) to harness the potential of the sun and day/night temperature fluctuations.  By doing this, it can “passively” control the indoor climate and make it comfortable to live in

Can Passive Solar Design work in any climate?
Passive Solar Design is best suited to temperate and cool temperate climates;  fortunately for people living in the ACT and NSW Southern Tablelands, the climate here is perfect for  making use of passive solar design principles.  For Australian climates in the far north, coastal tropics and subtropics, passive solar design is still relevant,  but different design methodologies and construction approaches are required, with more emphasis on shading and ventilation.

How many climates are there in Australia?
The Building Code of Australia identifies 8 different climate zones (Figure 1) however, the Nationwide House Energy Rating Scheme (NATHERS) is much more fine-grained in its analysis and identifies 69 different climate zones.  The more accurate NATHERS system is used when you have your house star-rated by computer performance simulation.

 
Figure 1.  The 8 climate zones identified in the Building Code of Australia.   Figure 2.  The 69 climate zones identified in the Nationwide House Energy Rating Scheme.

 

What are the important climate characteristics in our region?
In the ACT and Southern Tablelands, we have a climate which requires much more heating energy than cooling energy to maintain comfort.  The winters are cold and dry, with the wettest months arriving in the spring.  Summers are generally mild with cool nights, however,  short periods of intense heat can be  encountered.  Throughout the year, the dry climate produces a very high diurnal range, often in the order of 200C.  Rainfall appears to have been consistently reducing in the past 10-15 years.

How should a house be designed for this climate?
A passive solar house in a cool temperate climate must perform three basic tasks to  achieve comfortable indoor conditions in winter:
· Collect broad spectrum radiation from the sun through its windows
· Convert the radiation into a more useful and stable form, then store it
· Re-generate the energy during the night when it is most needed
In order to perform these tasks, the house must:
· Have a correct overall proportion and orientation with respect to North
· Have a correct northern glass area to floor area (and enclosed volume) ratio to enable collection of radiant energy.
· Have a well insulated building envelope to prevent heat loss to the outside air (and to prevent heat gain in summer)
· Have sufficient thermal storage capacity inside the insulating envelope to convert and store the energy collected during the daytime heating period, and regenerate it in the correct cycle.

 

Figure 3.  The overall proportion of a passive solar floor plan should ideally maintain a length to width ratio between 1.5:1 minimum and 2:1 maximum, the longer direction running with the east-west axis.
Some other effective variations on this basic format are shown on the following sheet.
Larger plans can be split into smaller pavilions which individually maintain correct proportion.

 

Figure 4.  Linked rectangular pavilions forming a courtyard between them.  Each pavilion can maintain a correct proportion for passive solar collection.  Some degree of self shadowing is unavoidable, and active solar technology often needs to be employed to overcome the inherent inefficiencies which result from an increased wall area to enclosed volume ratio.

 

Figure 5. Curved and faceted north facades can effectively be used provided the angles don’t drift outside a 15-20 deg deviation from the true geographic north direction.  The closer to true north, the more efficient is the passive solar collection and rejection process.

 

Figure 6.  The collection façade (north) of the house should ideally lie between 15 deg. North and 15 deg. South of the east/west axis line.

Internal zoning; how the inside planning affects passive solar performance.
Overall proportion of a plan is only part of the orientation task in passive solar design.  To complete the picture, the internal zoning of the spaces must be correct with respect to the external building envelope.  In order to best utilize the energy admitted through the northern windows, the main living spaces should lie adjacent to the windows, and the spaces should not be too deep.

NatHERS identifies six main types of spaces which are categorized by use, these being:
a. Kitchen/Living
b. Living
c. Bedrooms
d. Other nighttime use (conditioned both day and night—e.g. study, ensuite)
e. Other daytime use (a service room not conditioned at night—e.g. laundry)
f. Garage
There are also special zones categorized for roof spaces and sub-floor spaces. 
For the purposes of simulating energy use, it is assumed that living spaces are occupied for the longest period, and thus require most energy for heating and cooling.  The next most important spaces are bedrooms and studies, and after that, the service rooms (bathrooms, laundries, passages etc.)


Figure 7.  Examples of effective internal zoning for best winter solar gain and summer heat exclusion

 


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