Passive Solar Shading Options

Posted October 1, 2008

In a passive solar design, you need to have shading on the south facing windows during the summer and have direct sun on the windows during the winter.  There are a few different ways to accomplish this.

One way is to plant deciduous trees and shrubs on the south, east and west side of the building.  During the summer the trees will shade the building and block the heat from hitting the house.  It also can have a cooling affect around the building due to the transpiration from the plants.  In the fall and winter, the trees will loose their leaves and will let the sun shine in, heating the house.  The advantage to using trees is that they closely track the temperature changes during the seasons, with the leaves budding in the spring once it has warmed up and falling off in the fall once the temperature has dropped.  The disadvantage to using trees and shrubs is that the woody mass of the tree will always shade the building, reducing the amount of sun in the winter.  The trees also take some time to grow to the point where they will be an effective shade.  They can also be a problem if solar panels are installed on the roof and are shaded by the trees.  This last problem can possibly be avoided by having the solar panels ground mounted beyond the shade of the trees.

A second way to shade the house is to use overhangs. You can calculate the depth of the overhang by finding the angle of the sun at the summer solstice – June 21 (90° – latitude + 23.5° = ss) and winter solstice – December 21 (90° – latitude -23.5° = ws).  Then take the distance from the bottom of the window to the bottom of the overhang (wh) and the distance from the bottom of the overhang to the top of the window (oh).  Then use the formula wh/tan(ss) to get the best overhang for the summer solstice and the formula oh/tan(ws) to get the optimum overhang for the winter solstice.  You want the overhang to be less than the winter solstice calculation and more than the summer solstice calculation.  For an example, I will use a house at 44°N latitude. The bottom of the window is 78″ from the bottom of the overhang and the top of the window is 18″ below the bottom of the overhang.

The angle at the summer solstice = 90° – 44° + 23.5° = 69.5°
The angle at the winter solstice = 90° – 44° – 23.5° = 22.5°

Summer overhang = 78″/tan(69.5°) = 78″/2.67 = 29.2″
Winter overhang = 18″/tan(22.5°) = 18″/.414 = 43.5″

So the overhang should be between 29.5″ and 43.5″.  Since you want shading for some time on either side of the summer solstice add about 6″ to the overhang.  In this case I would use a 36″ overhang, which would give complete about 6 weeks on either side of the summer solstice.  The disadvantage of a set overhang is that the temperatures are not the same the months before and after the summer solstice, which means there is more shading than you want before the solstice and not enough after the solstice.  The advantage is that you have the shading immediately after the house is built, and the shading is predictable.

Another option would be to build a trellis overhang, which would be a hybrid of the two above systems.  The trellis which would be built the same depth as an overhang would give some shading by itself, but if it is covered with a deciduous vine, such as grape, the leaves would give additional shading during the summer and in the fall the leaves would drop and give you more light before the winter solstice through the holes in the trellis.  You would have the added bonus of grapes to harvest.

Posted under Design

Ground Source Heat Pumps

Posted September 23, 2008

A ground source heat pump allows you to extract and dump heat into the ground.  In the summer the heat from the house is dumped into the ground, cooling the house.  In the winter the heat in the ground is extracted to heat the house.  In most areas of Canada, once get some distance underground, the temperature of the ground stays relatively stable at about 11C, and then slowly increases as you go deeper.  The ground source heat pumps move the heat to or from the house into a pipe in the ground.  There are two ways that the pipes can be installed, either vertically or horizontally.  In the vertical method, a well is drilled and the pipe is dropped down the well.  In the horizontal method, the pipe is laid in loops at the bottom of a trench that has been dug in the ground, usually 4-6 feet deep.  In the vertical systems, the loop can either be open or closed.  In the open loop systems, groundwater is extracted from the water table, the heat is added or extracted, and the water is then returned to the ground, either down a second well, or to an above ground body of water such as a stream or pond.  In the closed loop system, a U shaped tube is dropped down the well and the water or an antifreeze mixture is pumped down and then back up a connected pipe.

To understand how a ground source heat pump works, first a little bit of basic physics.  For a molecule of liquid to speed up enough to become a gas (vaporization) it has to absorb a lot of heat  This is called the latent heat of vaporization.  The same amount of heat is lost when a gas condenses to become a liquid.  To heat up 1 gram of water from 0C to 100C takes 100 calories, but to turn that same gram of water from a liquid at 100C to one gram of vapour at 100C takes 540 calories.  You can see that it takes a lot more heat to cause a phase change (liquid to vapour) than to heat a liquid to boiling.  When a gas is compressed, it heats up.  Image a gas in a cylinder with a piston.  When the piston raised into the cylinder, the molecules of gas start bouncing into the walls of the cylinder more rapidly since the molecules are travelling at the same speed, but in a smaller space.  This more rapid collisions translates into an increase in temperature.  One last bit of science.  As the pressure in a gas increases the temperature at which it condenses into a liquid also increases because the molecules of the gas are closer together at a higher pressure and so more easily gain the order required to condense into a liquid.

We will start the process where the refrigerant is a liquid.  The liquid refrigerant pumped into a device known as a heat exchanger and is brought into close proximity with the ground temperature water which has been pumped in from the underground loop.  The liquid refrigerant then boils to become a low temperature gas and absorbing a large amount of heat in the process (latent heat of vapourisation).  This low temperature gas is then sent to the compressor.  The compressor increases the pressure in the gas and in the process increases the temperature of the gas.  This high temperature gas is then passed into a condenser where cool air or water from the heating system is passed in close proximity to the gas in another heat exchanger and the heat from the gas is transferred to the air or water, causing the refrigerant to condense back into a liquid, and losing the heat of vapourisation.  The cooled liquid is then passed through an expansion valve lowering the pressure and therefore the boiling temperature of the liquid.  The liquid is once again passed by the ground water and since the pressure has been dropped the liquid can boil at a lower temperature and so absorbs the heat from the groundwater and the process begins again.  In the process of transferring heat to the refrigerant, the groundwater is cooled below the ground temperature, and it is then recycled to the ground loop where it heats up again.  During the cooling season, the process is reversed and instead of removing heat from the groundwater, heat is added to the groundwater.

In the process of moving the heat from the groundwater to air in the house, electricity is used to run the compressor and the water pumps.  The amount of electricity used, however, is less than if the electricity was used in a resistance heater.  This is called the Co-efficient Of Performance (COP).  A resistance heater is considered to have a COP of 1 and a ground source heat pump will have a COP between 2.8 and 5.2 depending on the type of loop and efficiencies of the pumps and compressors.  This means a ground source heat pump will deliver between 2.8 times and 5.2 times as much heat per kilowatt-hour of electricity as a resistance heater.

In my discussions with a heat pump installer, the vertical loops are recommended in Canada due to the more consistent and higher heat available deeper underground since the frost can reach up to 4 feet deep in the more populated areas of the country and even deeper further north.

Posted under Devices