Thursday, September 23, 2010
A Bioneer's Greenhouse Part 5 - Solar
Sunlight is going to be an interesting concern in my greenhouse. Immediately to the east, I will have a screened porch. That means that I won’t get much sun first thing in the morning. Immediately to the west, I have a large juniper tree. That will steal a lot of my evening sun. Since I am facing 22 degrees west of due south, though, I think I should get some really good sun through the rest of the day.
Due to cost considerations, I will most likely have to forego installing the glazing for several years. When I do, though, I have a new set of concerns. I have specifically designed the house for passive solar design. In the summer, I am protecting the house from the sun as much as possible by providing as much overhang and shading as I can to the southern wall. In fact, if you notice, on the house itself, the second floor hangs 2’ over the lower floor. In addition to that, I have a set of wooden slats that give additional shading. During the winter, the sun comes in at a much lower angle, allowing the sun to come in the windows and warm the house. Once I get the glazing up for the greenhouse, I have another source of heat: the greenhouse itself. In the winter, the sun will warm up the greenhouse, producing heat. I will open the upper and lower windows from the house into the greenhouse. As the air heats up, I will rise and enter the upper windows. That will pull air in from the lower windows. This creates a convection current that heats the house. I will also have high and low windows on the glazing for the greenhouse so that I can use the same convection current to dissipate heat to the outside in the summer.
Thermal mass is another big consideration in my greenhouse. Thermal mass is a large, dense structure, such as a body of water or masonry that has the ability to absorb heat. As the day heats up, the thermal mass slowly absorbs the heat, tempering just how hot it can get. By the end of the day, the thermal mass is warm, but the outside temperature starts to cool down. The thermal mass will slowly release that warmth back out. In a house, a thermal mass is stored in the exterior walls, absorbing the heat from the outside and releasing it back to the outside in the summer. Sometimes thermal masses are used on the inside of houses in the form of concrete floors or walls, usually set back enough that they get little to no summer sun, but lots of winter sun.
I will be taking advantage of a high thermal mass in the greenhouse. My raised beds will be constructed of cinderblock. I am also considering using river rock and mortar to create a more aesthetically pleasing façade to the cinderblock. All of this creates a good thermal mass to help the greenhouse hold its heat on cold winter nights. I will also have the water feature in the corner that will provide a lot of thermal mass. I probably won’t want to let the greenhouse suck the heat out of the house on winter nights, so it will need to have a way to keep itself warm. A good thermal mass should do that, releasing the heat close to the plants. I must say, though, that I will be seeing how well this works before putting any temperature sensitive tropical plants in there. In the summertime, I need to find a way to keep the thermal mass from becoming a liability. I am hoping that I can use vines for this. The soil in the path will allow me to grow peas or pole beans or something similar and let it grow up the walls, thereby shading the walls from the sun. Cucurbits growing on top and cascading over the walls would also work. This should help limit the thermal gain in the summer. Also, the sun will be striking the walls at a more oblique angle, which should also help. I will probably use some sort of trellis to protect the side of the house as well.
All in all, I think that the greenhouse will use its sun efficiently and help keep my house warm in the winter. My only worry is that it will also help keep it warm in the summer. I need to prove to myself (and more importantly, my wife) that it will work as needed to keep things from getting too hot in the summer. I think I will probably end up installing an evaporative cooler on the side of the greenhouse as a backup plan. I have also considered having some sort of system that will allow me to have roll-up blinds that I can use to shade the inside of the greenhouse at certain times, or even partially shade it more often. If I get something that is cloth, it could be used as a shade to keep heat out during the summer days and a blanket to keep heat in on winter nights.