Passive Solar Gain by the (PHPP) numbers

It has been a long time since my last blog entry.  Mark Porter and the crew of Porter and Sons Construction have been building a beautiful and what will be an ultra energy efficient home.  I'll be posting pictures of my first site visit in near furture posts.  This post will discuss some findings of the energy modeling effort.

The Passive House Standard requires the use of  the Passive House Planning Package (PHPP) an extensive energy and thermal building performance modeling tool.  It has proven very accurate in predicting home energy use and thermal building performance.   In an earlier post, I presented some basic guidelines for good passive solar design.  This post looks at how home orientation has played a key role in reducing the overall energy demand, annual space heating demand, and carbon footprint of this project. 

Our project has very good solar access, no south shading objects, and the long axis of the home is oriented just 10 degrees east of true south.  How much does passive solar gain contribute to the energy balance and carbon savings?   What would the energy demand and carbon footprint cost look like if the project house was rotated 180 degrees placing the current south windows to the north face?   Would the house still meet the stringent Passive House Standard  requirements for annual heat demand, a  90% reduction in heating compared to a code built home?  How would these scenarios change the carbon footprint annually?

A few basic parameters of the current house design.  South windows are properly shaded during the summer.

• 63% of the window area faces south
• 19% of the windows area faces north
• 13% of the window area faces east
• 5% of the window area faces west

Here are the passive solar gains according to PHPP energy modeling for the two scenarios.

Scenario 1. The current house design as built.  

• Passive solar gains account for 11,642 kBTUs/yr of free energy.  This amount of renewable energy migitates roughly 1,740 lbs of carbon annually.   The annual heat demand is 4.56 kBTUs/sqft per yr, which meets the 90% reduction in heating demand required for Passive House Energy Standard.

Scenario 2.  So what do the numbers look like when I spin the house 180 degrees to the north?

• As one might expect, the solar gain contribution drops dramatically to 6,268 kBTUs/yr and the carbon migitation is reduced to 460 lbs annually.  Based on this example, passive solar design saved nearly 4 times the carbon compared to the home that was spun 180 degrees to the north.

This example illustrates that by taking advantage of the sun through properly placed windows you can greatly reduce the energy demand and carbon footprint of your home.