What's so special about this passive solar home.

So I thought it would be important to describe what makes this home so special and give an overview of the things used to create an efficient home. To build a solar home it takes a lot of careful planning that starts long before the construction starts. The design phase of a solar home project is critical in making it all work. It starts with a solar site survey that will determine the amount of solar energy available at a given site. This is done with a tool called the solar pathfinder which can define the actual amount of energy available, it accounts for shading at the site and uses 30 years of weather data to calculate the total energy available for given date, month or year. It can then create a report for solar electrical power, thermal power for solar hot water (SHW) and solar energy striking the south windows. It gets a little complicated after that but this is used to determine the size of the solar systems that can be used and how much you should expect to gain from that energy. For this home we are using the solar energy to heat the home during the winter, create hot water , and aide in cooling the home with the use of a solar chimney.
        Another of the important elements of this home is the efficiency of the building envelope. For solar energy to create the energy needed for the home, the home has to use less energy than a normal home. We are doing this with the use of Structural Insulated Panels as the shell of the home. These panels have an insulation value 30-40% better than code built homes ands creates an air tight envelope, that reduces air infiltration below what you can do with traditional construction. They are also pre-fabricated to speed the construction and reduce the waste on the job, they come in  large sections 4' wide and up to 24' long.
       Many of the traditional passive solar design principles, that have been used for centuries are used but in a totally unique way. Large southern expose, proper orientation, thermal mass and shading are all things that need to be considered. Here is where this house differs from tradition and what makes this home so unique.

Passive solar Window system   Breaking the Mold

             Windows have long been the nemesis of passive solar design. To get a window that allows a lot of solar energy to enter the home you had to sacrifice the insulation value of the window. This made them a leak in the envelope when the sun wasn't shining. Our windows allow for 72% of the solar energy to enter them and insulates them 50% better than energy star rated windows. SHGC.72   Uvalue.2 This is done with a unique combination of LowE surfaces and a particular type of glass. That sounds great but how to you shut off the energy when it is not needed? Traditional design has been to create overhangs that shade the sun during the summer months. While this works well, if you don't mind the look of large overhangs, it can never give you total control over the sun. The low sun angles during the morning and afternoon hours will still strike the house and create unwanted heat from entering during the summer and swing months. To fix this this, these windows have an exterior shading system integrated right into them. A retractable exterior shade that hides itself into the head of the window and can be controlled by a thermostat and override switch. Giving the homeowner total control over the energy entering the windows and their home. 
         That all sounds good right? but how do you then store this energy? Once again here is how these windows differ from tradition. Typically passive solar homes had large amounts of thermal mass (concrete,stone,tile) to capture and store the energy. This works but the downfall is that all of that energy needs to come through the windows warm the space and then strike the floor. This creates a problem with overheating and way to much light coming into the home. Our windows have a clear gel encapsulated between two of the four layers of tinted glass, that captures this energy in the window. The gel is now your thermal mass, no more need for concrete or tile floors and you no longer have to wear your sun glasses in your home. Capturing this energy at the window is also far more efficient than in the floor.
         These windows being developed by Wendell Colson, VP of research and development for Hunter Douglas are not yet commercially available but will hopefully begin entering the market sometime next spring. They made there first public debut at the AIA national convention in Miami in April, before that little was known about them. I have been working with Wendell for several years and have had the opportunity to use his previous version of the windows as he has been working out the bugs. One project that was completed in 2009 had one such system and the homes annual heating costs, here in MA were $341 for the entire winters months. Here is a one video showing the previous version of the windows reaching 112 degrees on a cold winters day.

 
             
The new windows are far more refined, easier to install, cost less and are expected to out perform this previous version. This home will have 21 of these windows along the south side of the home. While wendell has tested these for over a year now in his test huts, this will be the first large scale deployment in a real home. From his research he has determined that every sq/ft of window you will be able to contribute 100,000 Btu's. Wendell has tested these for over a year now in his test huts, this will be the first large scale deployment in a real home. With 233 sq/ft of these windows is this home and the combination of the efficient envelope, the windows should be able to provide 65-75% of the total energy needed to heat this home. The additional energy will be provided with a wood stove and a very small mini-split duct heat pump. Whats really cool about these windows is that a 11pm on a cold winters night your windows will still be warmer than the room temperature. have you ever felt a window on a cold winters night? The gel that has stored this energy over the course of the day releases this into the house at night, from past experience actually raising the room temperature by a degree or two until 10-11pm before leveling off and maintaining the house through most of the night. The reach there lowest temperatures around 7am the next morning and if the sun comes up, starts the process all over again. As the project continues I will post pictures and videos of the windows going in and how well they perform.
        Solar chimney for cooling? Most people have no idea of what a solar chimney is or does. I'll try not to get to technical but it helps to understand a little bit of building science and how heat creates pressure within your home and how that effects your home. The term used in the energy field is the stack effect, what it means in simple terms is that warm air in your home wants to rise and escape from your home, creating high pressure at the highest point of you home. For most homes we want to stop the stack effect, in this home we are using it to help cool the home. The solar chimney is nothing more than a simple framed box on the roof. This box should be designed to heat up as much as possible, dark metal siding works great. The top of the box is built up with enough room for air to escape but not enough to get water in. You then place a well insulated door that can be opened from within the living space into the chimney. When the door is opened The heat creates a natural draft which wants to draw air out of the house. We strategically located windows along the north side of the house, low to the ground that can be opened to allow for air to enter. This natural ventilation will dramatically cool the house under some conditions. The whole house fan system can be turned on to assist in this. If you still need some cooling the mini-split duct heat pump can provide that in a very efficient way.
         we are of course using a solar hot water system to provide a portion of the hot water, a Heat recovery ventilation system to ensure indoor air quality while saving the energy. They may even some day put PV panels on the roof, an option they are seriously considering. All energy star rated appliances. The home is hoping to meet Energy Stars highest rating for home efficiency.

The home will be open for tours on Saturday October 2 for 9am-4pm as part of the North East Sustainable Energy Associations green building tour and the national solar tour.

http://www.nesea.org/greenbuildings/

Stay tuned
Tom Pittsley