Why is a tree less desirable in the south and have negative energy and CO2 costs?

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azelaya
i-Tree Team
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Joined: Fri Jun 27, 2008 10:50 am

Why is a tree less desirable in the south and have negative energy and CO2 costs?

Post by azelaya » Tue Apr 23, 2019 3:46 pm

Why does the program indicate that a tree is least-desireably planted to the south of the house?
One of the things about Design to keep in mind is that the colored grid or zone is showing combined cooling (kWh) and heating (Therm) $ benefits as a way to measure “desirable”. This can be confusing as most people think of tree related energy effects as – mostly summer shade cooling effects where east and west are typically the best location for trees. However, Design is also considering winter effects – wind block from the north and also negative or lower winter impacts if a tree blocks the sun. The sun’s trajectory as it travels from east to west in the winter is more in the south and lower in the sky. Therefore, trees (evergreen and deciduous) located in the southern zones in Design tend to have greater negative winter heating impacts as they block or prevent winter sun from warming a home. This is why you tend to see greener zones – higher combined benefits in the northwest and north zones in Design for northern cities. This is also dependent on tree size, distance and direction from a home, and the price for heating gas vs electricity in a region.

The ArborDay foundation has a good graphic that helps illustrate tree energy effects for winter at the following link https://www.arborday.org/trees/climatec ... warmth.cfm .

2) Why does the program assign negative “winter savings” to a tree? Seems to me that 0 savings should be the minimum, resulting when there’s no tree. To my understanding a negative value implies the tree is sucking energy from the house somehow

Yes, Design is in fact telling you that a tree is sucking energy from the house. Thus, a tree can have a negative impact on energy costs for the homeowner because the tree is blocking winter sun and therefore causing a need for more home heating than without the tree in that location. So, a tree can cost a homeowner more money (negative effect) depending on factors. Furthermore, energy costs for winter heating can be more than electricity costs for air condition in the summer. As such, winter negative effects can offset summer cooling benefits in some cases with an overall negative effect.

3) Why does the program assign a negative CO2 value to the tree when the tree is planted in the south in a northern city? If this is a measure of carbon sequestration, seems to me that the minimum should be 0, when there’s no tree.

This is the same as above as Design is saying there is a negative CO2 effect for having a tree that is providing a negative energy effect. You are using more heat and this requires more fuel consumption resulting in increased CO2 release from home heating emissions. There may also be a secondary negative impact of CO2 emissions release from the power plant due to increased energy demand - heat consumption.

Like all models, there are limitations to Design as it is only using species and diameter and tree height is an import variable that the model has to provide based on limited information. There are more sophisticated models like i-Tree Eco which require people to enter tree height for energy effects but Design is intended to be accessible. Design can’t account for individual homeowners preferences for setting their thermostats for heating and cooling.

For more details on how the energy models work you can check out the “Community Tree Guides” on this page, http://www.itreetools.org/resources/archives.php. See the i-Tree Streets Reference City Community Tree Guides section for individual regional guides.
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