Passive solar design
I want to talk about what is a passive solar design and why is important too. I think is the best way to start building in a sustainable manner. Instead of just using renewable energies to obtain the power necessary for the building we can think how to reduce this quantity of power beforehand. Moreover the buildings, must currently achieve some comfortable standards. This is why a lot of energy is used to keep the correct temperature and humidity.
Minimize the energy use in a building is the main point of a passive construction. A well designed home reduces the heating and cooling loads necessaries to keep comfortable spaces. In that way, the energy used and CO2 emissions are also reduced.
A passive solar home collects heat as the sun shines through south-facing windows and retains it in materials that store heat, known as thermal mass. The share of the home's heating load that the passive solar design can meet is called the passive solar fraction, and depends on the area of glazing and the amount of thermal mass. The ideal ratio of thermal mass to glazing varies by climate.
Well designed passive solar homes also provide daylight all year and comfort during the cooling season through the use of night time ventilation.
Different sources I have read highlight the necessity of the following five elements to design a good passive space / building. I want to use them to explain more clearly which factors we must take into account to build these kind of spaces.
1) Collector: Is the surface of glass which allows the entrance of the sunlight.
2) Absorber: Is the surface which could be wall, floor or water container sits in the sunlight paths. Sunlight hitting the surface is absorbed as heat.
3) Thermal mass: The material that store the heat produce by sunlight. While the absorber is an exposed surface, the thermal mass is the material below and behind this surface.
4) Distribution: Method in which the solar heat circulates from the collector and store material to different areas of the building. A strictly passive design use the three natural heat transfer modes such us, conduction, convection and radiation.
5) Control: Elements to allow or restrict the heat flow. Roof overhangs can be used to shade the aperture area during summer months. Other elements that control under and/or overheating include electronic sensing devices, such as a differential thermostat that signals a fan to turn on.
The choice of one approach versus the other will be determined by a number of factors, including site location, climate, budget, etc. In choosing a particular design approach you need to evaluate site and climate conditions carefully to determine the best approach or combination of approaches. For that reason, as well as these last five points, to achieve a comfortable temperature and humidity we need taking into account three elements more. They are defined in how the solar gain is achieved:
1) Control the amount of direct solar radiation depending on the position of the windows and the orientation of the building.
The simplest passive home is a direct gain design. South facing glass admits sunlight to the interior space and virtually all of it can be converted into thermal energy. The walls and floor provide solar collection and thermal storage. At night, when outside temperatures drops and the interior space cools, the heat flow reverses and heat is given up to the interior space until the thermal mass and room reach a temperature equilibrium.
The Advantages of Direct gain system are:
- Low cost to build.
- Provides direct heating to the living space.
- South facing windows provide natural daylight and outdoor views.
The Disadvantages of Direct gain system are:
- It can overheat if the balance of windows and thermal mass is not correct.
- Large areas of windows cause loss of privacy.
- You can not cover thermal mass by carpet or block it.
- South facing windows should have summer shading.
- In direct gain systems heat will be lost quickly.
2) Control the indirect solar radiation gain which is captured by a part of the building envelope and then transmitted to the different spaces by conduction, convection and radiation.
An indirect gain system uses the basic elements of heat collection and storage in combination with convection. For example, you can place a dark-colored thermal storage wall behind a south facing windows and in front of the living space (As is seen in the following picture). Sunlight enters through the glass and the surface immediately absorbs it.
The Advantages of Indirect gain system are:
- The thermal mass prevents extreme temperature swings.
- The floor and wall area can be used more flexibility than in a direct gain system.
- A second wall between the space and the facade provides greater privacy of the house.
The Disadvantages of Indirect gain system are:
- The south facing view and natural daylight are lost.
- Furniture and objects placed againts the Trombe wall (wall between the space and the facade) affect its efficiency in heating the space.
- This second wall heats only the room which it is connected, so construction costs maybe high relative to the contribution it makes to the overall heating needs of the house.
3) Control the isolated gain which involves passively capturing solar heat and then moving it around the spaces. Sunspaces, greenhouses and solar closets are alternative ways of capturing isolated heat from which warmed air can be taken.
With isolated systems, hot air can also be converted to hot water via an air-to-water heat exchanger. The hot water can then stored in an insulated tank. Hot water storage has the advantage that water has a high heat capacity (4X concrete), permitting it to store and release large amounts of stored heat energy over longer periods of time. Water from the tank can be circulated for direct heat if the house has a radiant floor system.
Another approach to an isolated gain system is known as an attached sunspace or attached greenhouse. The sunspace is a separate room on the south side of the house with a large glass area and thermal storage mass. When you design a sunspace with a sufficient mass, it can provide thermal performance better than that of Trombe walls and direct gain systems. Moreover, using awnings to block the sun the heat can be reduced in summer. This following picture shows an isolated sunspace that can be added to any south wall of a building. The space stores heat in a masonry wall in the form of 10" concrete blocks. With a forced convection system, you can use the empty center of the blocks as concrete ducts to circulate air through the entire mass. And if you make the glazing removable for the summer months, the mass can also provide cooling since it will store cooler night time air.
The Advantages of Isolated gain system are:
- Sun spaces increase the resale value of a home.
- Heat storage can be located in a basement or underground
- Solar thermal panels can be easily incorporated into any architectural style.
The Disadvantages of Isolated gain system are:
- Heavy furnishings and rugs must be avoided to prevent shading of he thermal storage mass.
- Shading and venting are important to avoid overheating during summer time.
Three different ways can be choose to design a building which take advantage of the solar energy and in that way is more sustainable. For that reason, I believe the design phase is the most important part to build a house which can reduce the quantity of power needed. We should decided which options are the best in the first phases of the project to reduce the costs and possible problems during the construction.
