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Solar Energy

The sun, in most parts of the world is our best resource for renewable energy. The sun can provide us with electricity using PV (photovoltaic) panels and provide heat to warm our homes, warm our showers and even cook our food. The intent of this page is to give you a good overview of how you can take advantage of the sun. There are some great sites on the Internet that can provide you with the details of various technologies. Please spend some time reviewing the sites listed under Renewable Energy Links to get the details.

PV (photovoltaic)
PV technology has been around for a while now, even before the space program. Actually, it was in the 1800's that scientists first learned that they could generate electricity from the sun using selenium. But it wasn't until the space race that PV started to become a viable source of power. You can learn more about the history of PV's right here.

Today, PV is becoming more commonplace but still in most cases if you have the capability to tie into the grid (your friendly, local electric company), it is less expensive (but then you are using mostly non-renewable energy). For some people, the expense of tying into the grid is cost prohibitive, so PV is a solution. The PV cells are commonly wired into panels that are placed on house roofs or on electronic trackers that keep the panels pointed at the sun. The sun doesn't shine everyday and that in order to become totally energy self-sufficient requires the storage of electricity using a bank of batteries. The thought of using batteries is not entirely pleasing - they have to be maintained and they eventually will need to be replaced. Batteries also need to be vented and kept in a container that will prevent major accidents from occurring. (Batteries are extremely corrosive if they leak or "boil over".)

Up until this point, the electicity produced and stored is DC - Direct Current. In most places in the world, alternating current is used to power homes and businesses. This is because AC can be supplied much more efficiently than DC. So for the majority of applications, the DC needs to be converted to AC using an inverter. (You can find DC appliances on the market and it is possible to run a house on DC, but it's usually easier just to use AC by means of an inverter.)


Here's a block diagram describing the pieces to a photovoltaic system. A charge controller is required to prevent the batteries from being overcharged.

Solar Heating - Hydronic
If you have ever turned on a garden hose that was sitting in the sun, you will quickly realize how hot water can get heated by the sun. Even in the coldest of climates, all you need is a sunny day to take advantage of hydronic heat. Hydronic heat can be used to heat your domestic hot water and to heat your house.

Typically, water or an antifreeze is pumped through a series of tubes that are exposed to direct sunlight. These tubes are housed in either a highly insulated box or better yet, encased in a vacuum sealed compartment. During the 1970's energy crunch, quite a few of these systems were put into place across the United States. Today, these systems have become scarce due to the lack of maintenance to the systems and the fact that the United States no longer has an energy rebate program like they did in the 1970's. But for those who strive to become as self-sufficient as possible and through a little ingenuity, you too can take advantage of these systems.

The heat generated by hydronic panels can be stored in large tanks of water or in an insulated sand bed under the slab of a house. Just like a PV system, the trick is to have a large enough "battery" to store the excess heat. The heat is then transferred through a heat exchanger to heat the domestic hot water and pumped under a floor or radiated through a furnace to heat the house. There's quite a bit of plumbing involved in such a system, so you will probably want to work with someone who has installed these type of systems before.

Hydronic systems, when used for heating homes are the most productive in northern climates on sunny, cold days. With outside temperatures below zero, a house can be adequately heated with just the use of hydronic panels. It's a great source of "free" energy. "Free" of course means that an upfront investment has been made in the system. These systems can be quite expensive if all of the components are purchased new, but with a little digging you can find good used hyrdonic panels that can be purchased for at least 50% less than new ones.


A typical hydronic system can provide hot water heating for domestic hot water and radiant floor heating.

Solar Heating - Passive Solar
Passive solar heating is defined as using the sun's energy to heat an object without the use of any mechanical device. If you happen to live with a cat, you will quickly realize that cats know where to find passive solar heat. A south facing window is quite often occupied by our feline companions basking in the warmth of the sun. If a house is designed properly, you can take advantage of the sun's warmth on a winter's day and not feel the heat on a summer's day. There are a number of factors to consider when designing a passive solar house. The keys to designing a successful passive solar home are: location, calculating shadows, window glazing, collector material and thermal mass.

Location
The first key to a good passive solar design is location. It is imperative that the house site has good southerly exposure without a lot of shade. You can actually get by with having a few deciduous trees (trees that lose their leaves in the winter), but the less shade the better. Make sure you plan ahead with the building site too. You don't want to go through all the trouble of building a passive solar house to find out later that shade from another building is taking away your solar heat.

Calculating Shadows at High Noon
The roof overhang is critical so that your south facing windows receive maximum sun during the cold months and no sun during the warm months. We'll use the cordwood house that we are building near La Crosse, Wisconsin as an example. Here's the formula from The New Solar Home Book for figuring this kind of stuff out:

O = H/(tan A - tan B)

S = O * (tan B)

Where as: A = Summer Noon Declination, B = Winter Noon Declination, O = Calculated Overhang, H = Length of the shadow in feet during the summer noon declination, S = Length of the shadow in feet during the winter noon declination. Before we begin, we need a declination table:

Month (Day 21) Declination
December -23
January/November

-20

February/October -10
March/September 0
April/August +11.6
May/July +20
June +23

The first formula that's listed is good to use if you want to figure out what kind of overhang you may want to have. But for simplicity sake, we will use a fixed overhang value. For our house in La Crosse, Wisconsin we decided to build our cordwood house with a 3' overhang for both floors. Based on this, I can calculate how long the shadow will be on solar noon for the dates listed above in the table. The formula is as follows: S = O * (tan X)

S is the length of the shadow in feet, O is the overhang in feet, X is the calculated declination based upon your latitude. We will calculate this for the first day of all four seasons.

The first thing you need is the latitude of your site. A simple atlas will tell you your approximate latitude. In our case our latitude is 44 degrees north.

Calculate value for the first day of summer. X = 90 - L + D or 90 - 44 + 23 = 69. Using S = O * (tan X) our summer shadow is 3' * (tan 69) or 7.8'

Calculate value for the equinox. X = 90 - L + D or 90 - 44 + 0 = 46. Using S = O * (tan X) our fall and spring shadow is 3' * (tan 46) or 3.1'

Calculate value for the first day of winter. A = 90 - L + D or 90 - 44 -23 = 23. Using S = O * (tan X) our winter shadow is 3' * (tan 23) or 1.3'

What we have learned by this is that the maximum shadow is 7.8 feet and the minimum shadow is 1.3 feet. We now need to do a little tweaking with the 7.8 foot number. Why? Because no one is going to install a window right off of the bottom of a wall. Windows are typically installed anywhere from 18" to 30" from the floor. Our house will have the bottom of the windows starting at 18" from the floor and our walls are 8' high.

If you wanted a window to be in total sun on December 21st and in total shade on June 21st, the window could be up to 5' (7.8' - 1.3' = 6.5' - 18" = 5') tall and be placed 1.3' down from the overhang. From this you could also calculate that a 5' window would have a 1.8' (3.1' - 1.3') shadow on the first day of spring or autumn.

The purpose of all of this is to give you some idea as to where your shadows will fall on your house and to at least make some adjustments to your overhang, window size and placement to maximize passive solar gains in the months that you need it the most. If you already have a house that has a small overhang, consider using awnings that you can retract in the winter and extend in the summer.

Glazing
There's quite a bit of debate over what type of glass is the most beneficial for maximum heat transfer and minimum heat loss. The glass that gives you the most heat transfer in the winter, can be a nemesis during the summer months unless shaded properly. Also, the placement of windows in a house can make a big difference in your heating bill. Windows are the most energy inefficient part of your wall. Typically, windows have an R (insulative) value of 2 to 8. (Walls without windows typically have an R value of 19 or better for comparison.) So, you want to make sure that the majority of your windows are on the south side of the house and the least number on the north side. For the south side of your house, two clear panes of glass, low-emissivity double-glazed units (with the special coating on the outer surface of the inner pane), or anti-reflective triple or quadruple-glazed units, are generally recommended for windows used for solar heat gain in the winter.

Collector Material
Collector material is the material that the sun hits as it shines through your windows. Dark colors and non-reflective materials are the best. Ceramic tile that has a mat finish that has a dark color is a good example. Not only does it collect the sun's radiant heat it stores it as well. Which brings us to the next key point, thermal mass.

Thermal Mass
(No, thermal mass does not mean to going to a church service in Thermal, California.) Remember in the "battery" concept used for other solar methods? The same applies to passive solar technology. There needs to be some place to store heat and slowly give it back. Thermal mass is a major attribute to any energy efficient home - it stores heat. A good example in nature of thermal mass is the earth itself. The earth's surface takes a much longer time to cool off than the air temperature. That's why the average temperature in autumn in the northern hemisphere is warmer than the spring in the northern hemisphere.

What constitutes good thermal mass qualities in a house? A few examples are an insulated concrete floor and/or ceramic tile. Anything that is stone or ceramic is good. Wood has some thermal mass, but not much. Walls that are brick, plaster or a wall of cordwood masonry have lots of thermal mass. Remember that the sun's rays are going to be striking your floors and walls, so design your house to store heat where the sun shines. Some people have opted to used large clear plastic cylinders filled with water. Water has excellent thermal mass properties too.