There are many ways to take advantage of solar energy, some very simple and low cost and others relatively expensive.  At present solar electric is quite expensive, even when averaged over the lifetime of the system, but like any other electronic product engineering could bring the price down dramatically.  Meanwhile, passive solar design has become standard practice for many designers. Early passive solar houses used a large quantity of glass on the south side, creating somewhat of a space-age look.  It is now more common to use only a moderate amount of glass, allowing just about any style home to take advantage of passive solar energy.  Due to its complexity and cost, active solar has taken the backseat to passive solar, but is still the method of choice for solar water heating.

Due to its low cost and effectiveness in virtually all climates, every home should incorporate passive solar design to some degree unless shading makes it impossible.

Determining how much solar you have
The amount of solar energy striking your home at any given time is determined by the length of time the sun is up, the angle of the sun, the degree of cloud cover and the shading due to anything blocking the sun during the day.  The farther north you go, the less solar gain you have available in the winter when you need it, but luckily there is still usually enough.  Homes located on a north facing slope often have much less available gain due to the shading of the slope itself, and so the ideal location is either fairly flat or south facing.  Calculating your available gain involves looking up solar gain and weather data for your location in a chart, and is beyond the scope of this discussion.  (See references)

Thermal Mass: Storing heat for when the sun isn't out
It is almost impossible to think about solar energy without thinking about thermal mass because the sun is only out during the day when there are no clouds.  While it is always possible to resort to using burning fuel when the sun isn't available, even a small amount of thermal mass will allow us to store enough of a day's heat to reduce fuel use.

The two significant properties of a thermal mass material is how much heat it will store per cubic foot of space and how fast can be get heat in and out of the material.  The most common materials to store heat in are concrete, rocks and water, with concrete being by far the most common one because it can also be an integral part of the building.  Water stores the most heat per cubic foot and also transfers heat the fastest (due to convection within the water), but has the property that it tends to eventually leak and cause damage.  Concrete and rocks have almost as good a heat storage ability, but move heat very slowly, although this can be sped up by directly heating the stone (i.e. paint it dark color and let the sun shine on it directly), and by increasing the surface area to improve conduction.

In all cases, thermal mass will act to try to keep the house at temperature of the mass by either absorbing heat or giving off heat, limited only to the speed in which heat transfers in and out of the material (which itself is dependent on surface area).  The more thermal mass a house has, the harder it is to change it's temperature.  Think of thermal mass as a holding tank with a fixed size pipe going into it.  The properties of the mass are determined by the size of the tank and the size of the opening.  If a large thermal mass is used and house reaches a temperature outside the comfort range, its very difficult to get the house back to a comfortable temperature because the mass will resist any changes.

In order to store heat (or cool) a mass must be a different temperature than the air temperature, and the greater the difference the more heat it will hold, so there is a practical limit to how much heat the mass can store (due to a variety of practical reasons: e.g. in the case of storing heat in the floor, we don't want our floors too hot!)  Thermal mass is typically designed only to hold enough heat to get through a short cold period: anywhere from overnight to a couple of cloudy days, thereby avoiding the problems of not being able to heat or cool the house when for some reason it gets either too hot or too cold.  Even a small thermal mass system can take an uncomfortably long time to reach a comfortable temperature if allowed to get too hot or cold.

Methods of Collecting Solar Energy

Passive Solar
Passive solar refers to methods of collecting the heat energy of the sun without using any moving parts.  In its simplest form, passive solar is just a matter of orienting the house so that its longest side faces south, and putting most of  the windows on the south side, called a direct gain system.  As an alternative to having windows on the south side, a south facing greenhouse (sunroom) can be attached to the house, or other variety of indirect gain system could be built into the south wall.  Each method has it own set of advantages.

(typical room layout for passive)

(overhang diagram)

Direct Gain
In the simple direct gain system, the home is designed so that there is a long side facing south and put the majority of our windows there.  Because people generally want our bedrooms cooler and out public spaces warmer, the kitchen, dining and living room are put on the south side (see figure), although other layouts are certainly possible.  The south facing windows are then shaded by overhangs so that when the sun is high in the summer it is blocked by the overhang, but when it's low in the winter it passes below the overhang.  As a rule of thumb, the amount of south facing windows should equal about 10-15% of the floor area of the house.

It is difficult to use east or west facing windows for solar gain because the sun is always low in the sky no matter what the season, so you get too much heat in the summer.  In northern areas, the sun stays far the south all day, further reducing the amount of gain.

To correctly determine the amount of windows needed, a heat loss calculation should first be done to determine how much heat is needed, the subtract off that amount the internal gain of the house (the waste heat produced by appliances) to determine how much heat is needed.  Using a chart (available in most books on passive solar, see references),  then determine how much energy is reaching the house in each month (taking into account the shading we add to the windows).  Dividing the total needed by the amount by the amount reaching the house in a day gives the total amount of solar needed.  Often a house is designed so that it gets most of its heat from the sun on a average winter day, but requires burning some fuel on colder or cloudier days.  Determining the exact performance of the house is difficult, and often done with the aid of a software package. 

There are often seasonal difficulties in direct gain systems, because the sun is at the same place in the sky at both equinoxes, on Mar 21 and Sept 21, but the weather is much colder in March than it is in September.  There are two good ways to deal with this:  using deciduous plants (such as trellis with grapes on it) or an exterior movable screen that increases the overhang.  The degree that extra gain is a problem depends on the climate and the amount of solar gain the house has.

Solar gain is also affected by the angle the sun strikes the window and the properties of the window itself.  Ideally the house should face directly south, but in practice anywhere with 10-15 degrees of south gets good results.  In the past solar houses were designed with windows tilted off vertical so that they would be exactly perpendicular to the winter sun, but a normal vertically mounted vertical window will perform nearly as well.  The most significant effect is that of the kind of glass itself: if Low-E glass is used, it should be of the variety that has a high solar heat gain coefficient (see windows)

In cities where existing lots have not been laid out with solar in mind, direct gain solar systems can be difficult of impossible.  The most desirable lots have no obstructions of the sun and have streets running in the E/W directions so that either the front or the back of the house faces south.  When the neighborhood isn't designed for solar, the south face of the house may not have privacy, and so putting windows there can be a problem.

(figures: trombe wall, greenhouse, thermosiphon trombe, thermosiphon below to rock bin)

Indirect Gain
In an indirect gain system, the sun passes through glass and heats an air space that is connected in some way to allow heat to pass into the house.  In a Trombe wall (named after French inventor Felix Trombe) or water wall, the a large mass is placed very close to the glass (either concrete or barrels of water) and the sun then heats the mass which then radiates its heat through the other side to the house.  In a greenhouse the sun heats the air in a room and the air is then moved into the house either via convection or a fan. Typically a greenhouse is attached to the main house via an exterior door, so that the room can be closed off to prevent heat loss at night and on cold cloudy days.  A variation on both systems is a thermosiphon wall where a small air space is heated by the sun and the rising warm air moved by convection into the house (see figures).

All indirect gain systems have  the advantage of separating the solar collector from the homes windows, allowing flexibility in design for locations that aren't well suited to direct gain.  Because the collector is all in one spot, it takes less effort to control both overheating and cold weather heat loss because only one set of doors/shutters must be closed, although most people do not want to have to open and close shutters on a regular basis.

In a mass wall system like the Trombe wall, the wall is a very poor insulator and so is a liability during prolonged cold cloudy periods and on cold nights.  It also has a long time lag, it may take hours before the sun hitting the outside of the masonry reaches into the home (ideal for mild, sunny winters but otherwise a liability).  Likewise, a greenhouse requires owner intervention daily and must be highly vented in the summer to prevent overheating.  Thermosiphon systems are easier on the homeowner, but care must be taken to prevent reverse thermosiphoning, where the heat of the house is removed by convection at night. As in all systems, better engineering can conquer many of the problems of each system, and no system is prefect.

Designing Passive Solar Homes
Passive solar homes are conceptually easy to design, but getting the details right can be quite difficult.

• Determine your system type - depending on your lot, climate, budget and personal preferences, pick which kind of system you want to use.
• Home orientation - orienting your home to the south makes life easier in all solar collection systems, but all that is critical is that the solar collection area be facing south.
• Floorplan - determine how your solar collection will integrate into the rest of the home.  Its is generally more important to have warm common spaces than warm bedrooms.
• Overhangs & Overheating - design overhangs to avoid overheating in the summer, which still allowing for solar collection in the spring.  In many climates and external screen will also be necessary.
• Daylighting - beware of excessive daylight created by a large solar collection area.
• Thermal mass - depending on how cloudy your climate is, design at least one full days worth of heat storage into the home, but beware of putting too much heat storage in.
• Sizing your system - determine the amount of glass you need by know your heat loss, internal gain (from appliances), and average amount of solar gain available. Since weather varies significantly, you will often design for the typical conditions, using backup heat during periods of extreme cold.

Active Solar
In an active solar system we separate the solar collector from the house, and use electricity to move the heat into the house.  The collector consists of a sheet of glass covering an airspace in which a specially coated black metal surface collects upward of 90% of the solar energy hitting it.  Inside the collector, the black metal either heats the air itself, or a fluid (typically including anti-freeze for cold nights). In the more efficient versions, the metal is water running through it and the air is removed, significantly reducing the heat loss back outside.

(picture: active solar collector & pumps)

Once the solar is collected it can then either be distributed directly to the rooms in the house or stored in a thermal mass such as a rock bin or a radiant slab (see heat distribution).  Due to the cost and complexity of active solar, it has less popular than passive, although it is still the only method of choice for water heating.  Compared to passive methods, active solar allows much more architectural flexibility due to only needing to find a place for the collection panels.   Often the solar collectors are on the roof, but they can also be built into a south facing wall or even detached somewhat from the house (although distance will create extra avenues for heat loss).

One interesting variant of active solar is to use a very large (and also very well insulated!) rock bin and store heat in it all summer long to be used as reserve heat in the winter. Such a system is ideal for locations where winter has extended periods of cold and clouds, but summer is warm and has ample sunshine.  Another variant stores heat in a much thicker than normal slab, but uses air instead of water to distribute the heat (thereby avoid any potentially water problems).  Active solar systems are also easy to back up with various furnaces and heat distribution systems.

Solar electric systems come in two basic varieties: those connected to the power grid and those not connected (off-grid).  The off-grid systems are the currently the major supporters of solar electric because the initial cost of connecting to the power grid is often higher than installing solar electric.   Grid connected systems are becoming more common as states and utilities offer both rebates to help cover the initial cost and net metering laws, in which utilities buy back excess electric generated by your solar electric system, making it so you can use the power grid as a battery.   Note that the utilities will not actually pay you for excess electric, but simply subtract it from the total you use, with any excess beyond your month's usage going to the utility for free. Check with your utility to find its exact rules.

Both kinds of systems consist of three components:

• Solar cells, which turn sunlight into direct current electric, typically 12 volts, the same as car batteries.  Solar cells come in three varieties themselves, single crystal, polysilicon, and amorphous, in general order of decreasing both the electric conversion efficiency and cost.  The power generation ability of a solar cell is given in watts, and is the power it can generate at maximum sunlight, and often only as long as the cell itself stays under a maximum temperature. Solar cells are usually sold in panels and rated for their maximum output, so for the most part the type of solar cell can be ignored.  Lower efficiency cells will require more panels, and hence more roof area to generate the same power, and so may not be cheaper per generated watt than a higher efficiency unit.
• Inverters turn the low voltage DC electric generated by the solar cells and convert it to the 120 Volts AC that is the standard plug power in the United States.  In doing so, the inverter uses some of the power generated by the solar cell to run itself, and so lowers the overall efficiency of the system.  Some people circumvent this problem by buying special 12Volt DC appliances and running special heaving gauge wire required by 12 Volt DC wiring.  Since both are more expensive, there is a point where it is cheaper to just buy extra solar cells, and as the cost of those cells goes down, the use of 12 volt DC appliances is likely to disappear altogether.  Since many appliances are only available in 120 Volt AC versions,  virtually every system will have an inverter. For those who are connecting to the power grid, a special inverter is used.
• Batteries store excess power for use when there is no other.  In an off-grid system, this is anytime the sun isn't shining, and so those systems usually need a large (and expensive) bank of batteries.  In a system connected to the power grid, batteries aren't needed at all, and are only used for those who want a backup power source during power outages, which can be a significant feature if your home experiences frequent power outages.  Typical batteries are just an improved version of car batteries, and are both expensive, short lived (5-8 years) and easy to ruin.  There are unfortunately no good batteries out there, and none are likely to be available in the near future.

In addition to these main components, a system requires racks to hold the solar panels and wiring to link it all together. The overall efficiency of the system can be improved somewhat by installing the solar cells on tracking racks, which follow the sun during the day.

How much electric you generate will depend on the same criteria that all solar power does: how many clear days you get, how long the sun is up, whether the solar cells avoid shaded and whether they are facing the sun directly (with true south being the best if tracking mounts are not used).  Solar electric systems are rated by their maximum output, and are typically somewhere between a half a kilowatt to six kilowatts, with systems in the 1-2 kilowatt range being generally adequate for a house with a reasonable amount of electric appliances.  For example a 1kw system that receives 5 hours of full sunshine a day will generate 5kwh of electric in a day, while a 2kw system getting the same 5 hours will generate 10kwh a day.

To determine how much electric you need, look at how much electric you use from a current electric bill and divide by the number of days to get the amount of power used in one day. If your house has electric hot water, and electric stove or other large power users, you will find that even if you are very conservative, you will use a very large amount of electric.   For example it takes about 3.5kwh to heat 30 gallons of water, which is a typical amount of hot-water to use in a day (although an experienced water miser can do much better!).

For a more conservative number, take the power rating of all your appliances ignoring hot water, electric stoves and electric heaters and multiply each one's power rating by how many hours a day that appliance will be used to get the total power  used (in kilowatt-hours).  For example a 800watt blow dryer used 5 minutes a day uses only about .07kwh, while a top energy star refrigerator uses about 1.2kwh a day, and a 200w computer on for  four hours a day uses .8kwh a day.  When they are all added up, it is often the case that a 1-2kw system producing between 4 and 10kwh a day is of adequate size.

Cost
Solar system costs can vary much, and like any other product, the costs change over time.  As a ballpark estimate, a solar electric system costs anywhere from $6000 to $10,000 per 1kw installed, with grid-connected systems being at the much cheaper end due to having no or very few batteries.  Larger systems lower the cost somewhat, although the cost of the solar panels is fixed.

Lifetime & Reliability
While single crystal and polysilicon cells should last a long time-at least 30 years, amorphous cells have a tendency to lose efficiency over time resulting in lower power output.

BIPV Systems
Building Integrated PhotoVoltaic systems integrate the solar cells into a building material, thereby lowering the overall cost somewhat.  While this technique is generally used in large commercial construction, there are two roofing products intended for home use, both using amorphous cell mounted on a roofing material: metal roofing in one case, and asphalt composition roofing in the other.  Anyone considering these systems should check their expected lifetimes with the manufacturer.

Solar Electric & Sustainability
Most solar electric systems are sized to be the minimum acceptable due to the initial high system cost, forcing homeowners to use other methods for hot water, cooking and space heating and cooling.  With combinations of passive and active solar, most space heating and cooling as well as hot water can be provided, but often some kind of backup is needed, and just as often that backup is either natural gas or propane.
Historically, this compromise has been acceptable, but as the price of solar cells comes down, it begins to make more sense to use solar electric as the backup power, because unlike fossil fuels it can be produced sustainably.  (But of course sustainability is always a complex analysis: we must include the energy to produce the solar cell, and we can't ignore alternative ways of producing "natural gas", for example a bio-gas generator using cow manure as a fuel is certainly sustainable).

Passive Cooling
Passive cooling is not really related to solar power, but is generally grouped with it because it is a technique that uses no fuel.  In passive cooling, the principle of hot air rising is taken advantage of by putting vents as high in the house as possible (the higher the vent, the larger the air flow), often in an opening skylight.  As long as the night time air is cooler than the house temperature this technique works very effectively. By designing good cross ventilation, further cooling can be achieved if there is at least a light breeze outside.

If periods of hot weather are short, the thermal mass of the house will absorb some of the extra heat, but beware of letting a house with thermal mass get too  warm as it will not cool off quickly.  Since the ground temperature in most areas stays relatively cool (typically below 60 degrees), we can use the ground as a thermal mass also by using a pipe under the house to draw cold air out of the ground.  Transferring too much heat will cause heating of the ground around the pipe, rendering it as ineffective as any other thermal mass, so the usual cautions apply.

When the relative humidity is low, passive cooling can be increased by evaporating water (such as the cooling towers used in the middle eastern countries), which takes advantage of the additional heat absorbed by the evaporating water.  Obviously if water is scarce this would be a bad technique. Sometimes no passive technique works and we have to resort to air conditioning.

See Also:

Passive Solar Energy, Second edition, Bruce Anderson & Malcom Wells, Brick House Publishing 1994.
A good introductory guide.

The Solar Home Book, Bruce Anderson & Michael Riordan, Cheshire Books, 1976
Out of print, but still available used, this is a good in-depth book.

The Passive Solar Energy Book, Edward Mazria, Rodale Press, 1979
Possibly out of print, but generally available as a used book.

The Passive Solar House, James Kachadorian, Chelsea Green, 1997.
Although empasising only one kind of solar system (designed by the author), the basic principles are there also.

The New Solar Electric Home, Joel Davidson, AATC publications 1987.

http://www.eren.doe.gov/erec/factsheets/passive_solar.html
Web site from the department of energy, covering about the same material as is covered here.

http://www.consumerenergycenter.org/homeandwork/homes/construction/solardesign.html
The California Energy commission solar design tutorial, also covering similar material to this one.