Three main solar energy systems are available for household energy needs.
1-Photovoltaic power generation systems are designed to produce electricity when sunlight hits photovoltaic panels usually installed on roof slopes with southern exposure.
Those systems may cost up to $100 000.00 for an average size home to meet its energy needs. Performance drops sharply to negligible level for cloudy days.
For properties off the grid, it may be cheaper to install panels and batteries instead of connecting to power line miles away. Check for availability of government grants and loans in your area.
To determine feasibility of a solar electric system for your home, consult sunlight performance logs taken over many years closest to your building location.
2-Solar water heating produces hot water for showering, washing dishes and even heating your home!
Passive water heating systems rely on convection or thermo-siphoning to circulate water from absorber tube panels to a hot water tank above. This system is used for summer cabins and although inexpensive, is less efficient than an active system because of the slow water flow rate.
An active system uses pumps to move hot water to storage tank which can be installed below and away from collectors.
An open loop active system pumps water through the collector panels down to the holding tank and/or directly to be used in the house. This system is for summer use or works in areas where it does not freeze.
A closed loop active system pumps glycol water antifreeze through the solar panels to a heat exchanger. The heat is then transferred to the potable water. It can be used in freezing weather.
In many locations, solar water heating may not warm your hot water enough to run a dishwasher properly or for a scalding hot shower. Using solar water heating in conjunction with a gas fueled tankless water on demand will considerably lower operating costs.
3-Passive solar heating is about designing your log home to capture sunlight using south facing windows thus converting the sun’s energy to heat the inside of your home.
A/ Maximizing solar exposure at your building site.
The best orientation for your passive solar log home is true south (northern Hemisphere). Within 10 degree to east or west, solar gain is close to 100%. A common solar home footprint shape is usually a long rectangle with a long south facing wall, including lots of windows to collect the sunlight. Kitchen is usually located on the north side and at the north east corner to enjoy the morning sun at breakfast time. Two story homes are more energy efficient and comes at a lower construction cost.
Although winter solar gain only decreases to 92% at 22 1/2 degrees off true south and respectively down to 70% at 45 degrees, summer solar gain is greater the further the house solar collectors (window and door glazing) orientation deviates from true south, thus creating serious overheating in the summer months.
B/ Glazing: For optimal solar performance, locate most of the house glazing on the south wall. However beware off over glazing, a common mistake of the past. Have only minimal windows on the north side. Windows on the east and west side should also be minimal to control summer solar heat gain unless you need light or have a view to consider.
Use low-e glass (low heat emissivity) whenever you expect unwanted solar heat transfer in or out of a particular window in the house design.
Interior thermo-shutters are a great way to dramatically reduce window heat loss at night. A common design consist of bifold foam panels sandwiched by 1/4” plywood sheathing. Curtains can be attached to the interior facing side for a pleasing decorative finish.
You may plant deciduous trees to block the summer sun, install exterior shutters, roll blinds, sunscreens at those windows. Better, build covered decks/patios on east and west sides or add arbor to grow vines to protect the log walls from weathering.
Evergreen trees on the north side is a great winter wind break.
C/ Roof overhang is the fixed projection of the roof beyond the exterior wall and its length must be carefully calculated to block the high summer sun from entering the house thru the south glazing. However the roof overhang is short enough to allow the low winter sun to flood the inside of the house.
The amount of roof overhangs is proportionate to the height of the window opening and location on the wall and a factor of the latitude of the building site.
Some design flexibility is required to account for different daily solar gain by season for a same latitude.
A log home situated in the pacific north west will likely receive little sun in the middle of the winter because the sky is mainly overcast. In this case roof overhangs are reduced to allow sun heat in the fall and early spring.
Montana receives much more sunlight in the winter and roof overhangs will likely be a bit longer to block the sun in the fall and early spring, to prevent overheating.
Roof overhang calculations are site specific and should be handled by a knowledgeable designer/architect.
Architectural computer programs using 3D modeling of your log home can precisely calculate the right amount of overhangs needed at your site for each window openings, taking in account your local heating requirements and seasonal sunlight availability.
D/ The thermal mass of your log home are the exterior and interior log walls, masonry fireplace, concrete floors…
The solar heat that enters the house during the day must be stored to be slowly released the following cold winter night, like a thermal battery.
The Trombe wall is a solar collector including a south facing glass wall with an air space between it and a dark concrete wall inside. When the sun heat passes thru the glass, the concrete wall stores the heat and radiates it back to the inside of the house.With vents installed at top and bottom of the wall, warm air rises between the glass and the wall, then flow in the living space from the top vents.
Unfortunately the reverse airflow occurs when the sun is gone, sucking the heat out of the living space. To stop this loss, exterior insulated panels would need to be deployed every time the sunlight is not hitting the glazing. This is obviously not a viable option. As well, building a concrete wall on the south facing side of your home instead of enjoying view and day light is hard to imagine for most of us.
The solar slab is a more sensible idea engineered by James Kachadorian in the late 1970′s. The sunlight would hit the south facing side of an insulated concrete slab with vents and fans circulating air under the slab from north to south of the building. However moisture can easily build up in those buried vents creating a mildew and mold problem, thus contaminating the indoor air supply of the house.
A simple well insulated concrete slab for a walk out basement floor can act as a thermal storage collecting sun heat from south facing glazing. As well a light weight concrete in-floor heating slab over a framed main floor system can also collect that sun heat and add more thermal mass storage to the house.
Much has been debated about the thermal mass of log walls.
About two years ago I visited a large log home in Arizona that RCM CAD designed many years ago.
The owner had installed an expensive air conditioning system for his log home and never used it as he was able to keep the house cool all summer long by simply running an exhaust fan upstairs at night. The day heat never had a chance to heat the logs to the point where the heat would penetrate the house as he was able to take advantage of the cool night air to cool off the log walls from within. The thermal mass of the exterior log walls worked well to avoid using the expensive air conditioning system.
Thermal mass slows down heat transfer and is proportional to the thickness of the log walls.
Providing it is a clear day, the winter sun can warm the logwork in the day, releasing that stored heat back inside the home when the sun is gone.
The R value of wood hovers between R 1.2 and R 1.5 per inch of thickness, depending of wood specie. D-fir being a denser wood has a lower R value and Western Red Cedar being a lighter wood has a higher R value.
Handcrafted log walls about 13” (33cm) diameter midspan are comparable to 2×6 frame walls with (R 19) fiberglass insulation.
Wood acts as a solid insulation and does not require any plastic vapor barrier on the inside or moisture barrier on the outside as frame walls do.
Log walls also store heat from sun light and from interior radiant heat source like wood stoves or masonry stoves, creating an even temperature interior environment, day and night.
A masonry stove is a great back up heat in solar home design and when placed where the winter sunlight can hit it, they act as another massive solar storage for your home.
Last word…
Passive solar heating and cooling your log home follows simple principles that have been discovered centuries ago by our ancestors.
However designing your solar log home requires precise calculations for sizing roof overhangs and adequate thermal mass for your specific building site.
Exposure of the main glazing side of your log home should be to the South to maximize winter solar gain and keep the sun heat away from entering the house in summer thanks to adequately designed roof overhangs and use of shutters, sunscreens, covered decks and landscaping for example…
