Passive solar Heating and Cooling

If you have windows in your home then to some degree you are already taking advantage of passive solar energy.  The sun comes through the windows and heats your home. That being said, the odds are that the solar energy striking your home is probably not being leveraged as effectively as it could be. Sometimes the rooms get too hot and you have to kick in air conditioning.  At other times of the year they may be shaded and let in too little heat.  Passive solar design is all about creating the optimal use of the sunlight that strikes your home.

If you already have a home then realistically there are limits to how much you can retrofit your home to take advantage of the sun.  Nonetheless most people find they can considerably reduce their energy costs by leveraging some passive solar design principles. If you are building a new home then there is a great deal a talented architect can do to take full advantage of the power of the sun.

In order to determine if a passive solar approach will work in your home begin by looking at home much sunlight strikes your house during the heating season. In the US that season falls somewhere between October through April.  The south facing wall of your home will get the most direct sunlight.  Most that sunlight will fall between 9:00 AM in the morning to 3:00 PM in the afternoon.  In a home designed for leveraging solar energy usually the house is designed so that the longest wall (assuming a rectangular construction) is facing south and has windows to capture the energy striking the south wall. 

When looking at passive solar design it is important to think of the balance of winter sun to summer sun.  You can't simply put a lot of windows on the south facing wall because while that will provide lots of energy in the winter it will also provide too much energy in the summer causing you to have to overuse your air conditioning and defeating the purpose altogether.  One way of getting around this problem is to use a combination of windows and awnings.  The windows let in the sun in winter because the sun is low on the horizon, but in summer the awnings block too much sunlight from coming in because the sunlight is coming in at a higher angle. 

As a general design role, solar designers suggest that the window surface in the south-facing wall not constitute more than 7-12% of the square footage of your home.  North  and east facing windows should not constitute more than 4% of the total square footage. West facing glass should not be more than 2% of the footage.

All of this sounds well and good if you are building a new home but what about existing homes.  Retro-fitting an existing home to leverage passive solar energy is a bit more challenging.  One simple approach is to add new windows to your south facing wall.  Another option, one that is often less costly, is to make the existing windows larger.

If you live in a warm climate energy issues may be more about air conditioning than heating.  In these cases a solution may be to add overhangs or awnings to windows that are getting to much energy coming through them in the summer months. Adjustable awnings which can be rolled out during the summer months to provide shade and then rolled up during the winter months to allow for maximum sunlight can be very effective.

Another approach for getting more solar energy into your home is to build small additions, sometimes referred to as sun rooms, which are designed to maximize solar intake. If the ventilation for these additions is properly designed the incoming energy not only provide heat to the sun room itself, but can be extended via convection or forced air systems to help heat the rest of your home. There are a number of kits for building greenhouse additions to your home that can not only provide a beautiful gardening space but improve your homes energy efficiency at the same time.

As with any passive solar approach be conscious of the need to maintain balance between winter heating and summer overheating.  Often all glass sunroom or greenhouse kits can produce overheating.  This is not only bad for your energy bill, it is not too great from the plant's perspective either.  With some greenhouse kits the interior temperatures can get up over 90 degrees even in winter. As with other approaches the use of adjustable awnings or shutters in the sun room can make it easier to adjust the green house for the different seasons.  Also, make sure that there is good adjustable ventilation paths between the sun room and the adjoining room or you will not be able to fully leverage the energy it is receiving.

One option for getting the most out of a sun room is to use a small fan driven by solar panels to help circulate the air.  The fans can be thermostatically controlled so that they only kick in when the temperatures in the sun room are higher than the rest of the house.  When the sun goes down the fan turns itself off so that it doesn't drive colder air into the home. 

In talking about energy coming in through windows we have mostly been dealing with what is know as Direct Gain.  There are also other approaches to leveraging solar energy that are sometimes referred to as Indirect Gain.  One approach which leverages indirect gain is to use what is called a Trombe wall.  A Trombe wall is nothing more than a wall built from a solid material which has the ability to absorb a and hold a lot of thermal energy.  This can also be referred to as solar mass.  Thermal storage walls are usually placed parallel to the south facing wall so that they receive maximum sun energy. 

Timing is part of what makes a Trombe wall work.  During the day the wall receives the light coming in through the windows and begins to store heat.  In the evening, when the sun goes down, the wall continues to radiate that heat into the adjoining room.  You can also put an interior window in the wall so that you get some light energy during the day and still plenty of energy in the evening. In addition, well designed walls usually also provide ventilation openings in the top and bottom to maximize convection currents within the home.  The vents at the top of the wall allow the rising hot air to ventilate into the adjoining room and the vents in the bottom of the wall allow the cooler air at floor level to circulate back through. 

One challenge in using a thermal storage wall is simply their sheer weight. The types of materials that tend to hold solar mass such as concrete, masonry and bricks are often too heavy for existing foundations.  Make sure that you have an architect who understands the foundation of your home look at this issue before attempting to put in a thermal wall yourself.