Passive Solar Heater

Solar Heating Solutions

A Passive Solar Heater can heat a space even in the cold of winter. With heating costs on the rise, passive solar heat is an old idea that's making a hot debut.

Solar Heat

A passive solar heater captures and magnifies the thermal (heat) rays from the sun to become useable heat for a room or building.

Even in very cold climates passive solar heat can raise the temperature in a space to a comfortable degree.

Principles of Solar Heating

Think about a car on a sunny but chilly day. A car with a black interior and a sunroof. In the sun the black interior of the car abosrbs the thermal rays of the sun and the car's internal temperature becomes several degrees warmer than the exterior temperature.

With the windows and sunroof closed the inside remains hot as long as the sun shines. Opening the sunroof would allow the heat to rise out the opening - since heat's natural inclination is to rise. The hot air that escapes will be replaced with cooler air - again, a natural process, and as the new air heats up it rises out the top and more air is drawn in through the bottom creating a loop of "cool air in and hot air out".

Uncovered windows and sunroof will allow the heat to quickly escape once the sun sets but covering the windows with insulating material will capture the heat and, for a time after sundown, the car will maintain it's warmth.

Now you could park your car next to the south side of your house, add an air intake hose from the house to the car and an exhaust hose out the sunroof to the house and have a passive solar heater but really there are more practical ways to achieve the same thing. The basic principles, however, remain the same.

Here's what you'll need to make a passive solar heater:

• South facing wall or window that receives largely unrestricted sunshine.


• Start with a large rectangular open box of some sort to contain the collectors for your passive solar heater, the size will suit your needs but for the purpose of this example it'll be 3 feet by 6 feet by 6 inches.
  
  You may have to build one to suit your needs and it requires just a back and the 4 sides, the front will be covered with a transparent material discussed later. Plywood works and if you don't have the resources to cut the wood to the size you want, many lumber stores will do it for you.
  
  Although hard styrofoam is not as durable it works as well and has insulating properties so it can save you a step.


• Insulation for the inside of the box. This can be as simple as crumpled up newspaper or as complex as fiberglass.


• Reflective material like alumimum foil, mylar sheeting (space blanket), mirror, silver bubble insulation (those windshield reflectors that keep the heat out), etc. to magnify the sun's heating effect. This goes over top of the insulation inside the box.


• Collectors - this can be any empty vessel that is black or can be painted flat black (black attracts more heat than other colors). Be creative, you can use conduit, sewer hose, aluminum cans, plastic jugs, pvc pipe, etc.. lay them out in a single layer in the box ensuring that each line forms a single cylinder - what I mean here is that the heat must be able to flow upward from one container to the next and out the top, it won't work to have each aluminum can as a single unit but if you cut the bottom and top from each can (or container) and stack them atop one another it'll form a chimney. You can have as many chimney's as your box will hold side by side in a single layer. BBQ paint is made to withstand high temperatures and works well for this project. Each of your chimney's will need it's own air intake at the bottom of the unit (that brings in air from the room) and an air exhaust that spills the heated air into the room.


• Well insulated ducting for your intake and exhaust. You can use dryer hose or unused sewer hose or something like that and wrap insulation around it or spray it with expanding foam or whatever. You get the idea.


• Adding a piece of glass, plexiglass or even plastic over your passive solar heater keeps the captured heat inside the unit rather than allowing it to escape back into the atmosphere.


• A fan facilitates the movement of hot air into the space that you're heating.

An important consideration when choosing or building a solar space heater is that the air that's being heated is circulated from inside the dwelling rather than taken from outside. This will make your unit much more efficient since the air is already partially heated. Heating the outside air normally takes considerably more energy to bring up to the desired temperature.

Here are some videos of a passive solar heater created by a Canadian. The second video shows a fan system in the process of being refined. These'll give you a good idea of how the solar heater above is assembled but remember that this is a creative process and the number of revisions that can be made are limited only by your imagination and the resources available to you.

There are numerous projects on the web that incorporate these basic principles and an amazing array of creative methods can be used to make your own solar space heater.

Supplement your passive solar heater with another form of heat, such as pellet/wood burning stoves, highly efficient electric heaters, geothermal, etc., to ensure that your space maintains a comfortable temperature in even the most severe weather.

Getting the Most out of a Passive Solar Heater

Thermal Mass

The term thermal mass refers to materials such as concrete, brick, rock, adobe, water, etc. The density of these materials allow them to absorb and store heat when the sun falls on them.

Thermal mass can be incorporated in a number of ways and serves to stabilize indoor temperatures by absorbing and later distributing heat. The mass should be at least 4 inches thick and composed of a very dense material to achieve the best benefit. Cement works very well and can be made more attractive by adding a ceramic tile, marble or stone facade.

The composition of the cement will determine it's overall effectiveness. Concrete that contains a high percentage of stone is more efficient than if it is filled with less dense materials such as perlite, ash or fiber. The same is true of brick with the addition of magnesium. The thermal collecting properties can be more than triple that of ordinary brick.

In order to get the most benefit from thermal mass as a heating altenative you'll need to have between one half and two thirds of the elements in the room (floor, walls, ceiling) as thermal mass.

Insulation

Although outdoor temperatures can fluctuate dramatically a well insulated home will provide more constant temperatures in a living space.

Remember that although a large south facing window is ideal for collecting passive solar heat as long as the sun shines, an uncovered window after sunset will allow all of the heat to escape very rapidly. The trick is to let the heat in during the day and prevent it from migrating to the outside at night. Insulated window coverings trap the heat inside and slow down it's egress to the outside. Once the sun has set, close the window coverings and enjoy your solar heat for another few hours.

Trombe Wall

A trombe wall is a wall of thermal mass that is located very close inside a south facing window (north in southern latitudes) and is really a very large passive solar heater.

At the time of construction the wall is vented at the bottom and top to allow circulation of the cool air from the room and hot air from the space between the window and wall. The space between the window and wall can reach temperatures exceeding 100 degrees F (we'll refer to this space as the collecting space).

The vents allow for a transfer of air between the room and the collecting space. Because heat naturally rises, the upper vents allow the hot air to move into the room and cool air from the room replaces that air from the lower vents creating a circular flow from the collecting space to the room. At night the vents are closed (the collecting space will cool down without the sun falling on it) and by this time the thermal mass of the trombe wall will be sufficiently warm to radiate the passive solar heat it's collected throughout the day into the room and keep it warm for the evening hours.

Typically, a trombe wall is constructed of materials that readily absorb and store heat like masonry, adobe, stone and the like. Water can also be used and has the advantage of heating more rapidly and evenly than masonry. The water can be contained in a wall constructed for this purpose or contained in drums or large bottles (dark colored) secured on top of one another to produce a wall effect.

Construction methods

A rammed earth house that incorporates other passive solar principles is a great solution since the sheer mass of the building material creates an ideal thermal collection surface. We also like straw bale construction for it's insulation properties. Although straw bales do not provide the thermal mass that can be found in rammed earth construction the insulation properties are superior to most conventional building materials.

In conclusion

Incorporating solar principles into your building and renovation design is the most cost effective way of making use of the solar energy that strikes your property.

Purchasing or building a passive solar heater will save you money on your energy bill while reducing your carbon footprint. Adding thermal mass along with other passive solar principles will dramatically increase your savings and if you're in the home planning stage you can practically eliminate the need for central heating or fossil fuels in many places on the planet. Over the life of the building these principles will save you thousands of dollars, recovering the cost of the design in short order. For off grid living, combining solar with other low impact sources will often give you the best bang for your buck.

Here's a great DIY passive solar heater project with tons of comments to help you make alterations to suit your needs.

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