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Sun IV - Shades




Introduction

In this post, we let away solar panels. Instead, we discuss about optimizing energy input This can be done using optimal house orientation or using the shade of other elements. Tree are almost free (but take time to grow), so they are good natural candidate to this function.

If you haven’t read them, you can check the other articles of this series:

Shade from the Trees

In summer, the sunlight provides a lot of energy, which is often converted into heat. To avoid getting too much energy into a house, there are multiple strategies:

  • thermal insulation
  • optimal orientation
  • shade

Thermal insulation is widely discussed in many books, blog, forum, so we won’t talk about it.

House orientation and shading are two strategies that cost nothing (almost), and allow to reduce the amount of sunlight received by a house in summer.

Getting Sunlight Energy when needed

By oral knowledge, you may know that the best orientation of a house is facing south. However, a house has one roof, one floor, and 4 faces, so all houses are facing south unless for terraced houses.

What we really mean by “south” is one of the two largest faces must be south-oriented. The other largest face is then facing north, and the building is along an East-West line.

To understand why the configuration with a large south face is interesting, we have to go back to our sun angles:

  • In summer, the sun strikes the East face in the morning, and the west face in the evening. Therefore, energy is accumulated by theses faces. To minimize the energy received, we need to minimize the area of these faces, and to get a house with a reasonable surface area, maximize the south face size. At mid day, the sun is high, on top of the roof, so there are no difference between E-W or N-S configuration.
  • In winter, the sun doesn’t illuminate the east and west sides that much (because it rises SE and sets SW), so the gain are minimals if these faces are large.

It is interesting to know about this fact, before buying/building a house, or orienting your van. However, when built, you cannot turn your house 90°.

Planting Trees

When you cannot turn your house, you can create shade. Trees take time to growth, but they are almost free to run.

Trees must not be too close from the house, otherwise roots may affect the basement, and leaves may clog the rain gutters. And not too far, otherwise you don’t get the desired effect.

So, I created a simulator: given the sun angle, tree height (check on books the typical height of the tree you want to grow), wall height, you can find the optimal distance of the tree. This simulator can be used both ways:

  • Or you want shade from the tree, so you want them close, but not too much because of the roots;
  • Or you have solar panels, and you don’t want the tree shade to be on your house (but you want to take advantage of tree for other reasons).

Two angles to look at

Remember the first post, there are two angles:

  • Hour Angle (direction of the sun over North-South / East-West axis)
  • Elevation angle (height of the sun in the sky)

Your house will be affecte by different shade at different moment.

For instance, the shade of tree T1 will only benefit in the morning, where the sun has a specific angle, which is different from T2 or T3. For each tree, we need first to look at the positional angle of the tree in the earth plane, with the house as the center, to check when the shade would benefit, and then, given this hour, check the sun angle in summer.

Trees planted in the north do not provide any shade, so they have no impact. Trees directly on the south are not recommended because even if they don’t have leaves in winter, they would provide shade in winter, reducing sunlight exposure.

To know when a tree can produce a shade on your house, you need measure the HRA angle of the tree for finding the hour when the shade may coverthe house:

\[HRA(h) = \omega(12 - h)\]

with \(\omega=15°/\text{hour}\) and \(h\) the current time in hour.

Next, knowing the hour at which the tree can make a shade, you need to look at the elevation angle:

Sun angle over a day.


Now, you just have to play with this tool given the elevation angle to know if the tree will shade your house:

Tree shade on the house.


Note: It is commonly said that the sun rises in the east and sets in the west. This is an average. In summer, the sun rises N-E and sets N-W, while in winter rises almost S-E and sets S-W, unless in the equator.

Other Benefits of Trees

If you just want shade, you can also build a wall or make a giant sun-shield.

Trees have other benefits:

  • Trees are a kind of chemical factories: they transform the light into sugars. Therefore, the energy is partially converted into heat. Comparing to tarmac that absorb all energy in the form of heat, the temperature during the night will be cooler thanks to tree energy conversion.
  • Next, trees participate to the soil ecosystem: over time, leaves are transformed into mulch reducing soil evaporation which helps other plants to develop, and consequently allows to convert more sunlight into sugars thanks to photosynthesis.
  • Last, if tree are numerous, they allow to locally reduce wind speed, lowering damages during storms.

Sources


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