Passive Heating

From Antivist

Passive solar heating uses the heat of the sun directly rather than converting it using, eg, photovoltaic cells.

A house can easily achieve 30% or better cost reductions in heating expense without obvious changes to its appearance, comfort or usability. This is done using good siting and window positioning, small amounts of thermal mass, with good but conventional insulation and occasional supplementary heat from a central radiator connected to a water heater. Sunrays may fall onto a wall during the daytime, which will radiate heat in the evening.

The beautiful thing about passive solar heating is that you also achieve longer periods of natural lighting within your home. This also reduses the amount of energy that is consumed through lighting appliances as well.

As far as passive solar, make sure that when you build or modify your home, that their are elements in your home that will absorb heat during the day and will release it slowing during the night. Concrete flooring is probably one of the best ways of acheiving this.

The downside to concrete flooring is that during the manufacturing of the concrete utilises alot of energy. CF is a good alternative to using hardwood flooring which is expensive and kills alot of trees that take alot of time to grow and thus renew.

Contents

Direct solar gain

Direct gain involves using the positioning of windows, skylights and shutters to control the amount of direct solar radiation reaching the interior spaces themselves, and to warm the air and surfaces within the building. The use of sun-facing windows and a high-mass floor is a short-cycle example of this. John Hait's "Passive Annual Heat Storage" (PAHS) method is an example of an annualized solar approach primarily using this path.

Direct solar gain systems suffer because historically there were no reasonably priced transparent thermally insulating materials with R-values comparable to standard wall insulation. This is now changing in Europe, where superinsulated windows have been developed and are widely used to help meet the German Passive House standard.

Indirect solar gain

Indirect gain, in which solar radiation is captured by a part of the building envelope designed with an appropriate thermal mass (such as a water tank or a solid concrete or masonry wall behind glass). The heat is then transmitted indirectly to the building through thermal conduction|conduction and convection. Examples of this are Trombe walls, water walls and roof ponds. The Australian deep-cover earthed-roof, innovated by the Baggs family of architects, is an annualized example of this path.

In practice indirect solar gain systems have suffered from being difficult to control, and from the lack of reasonably priced transparant thermally insulating materials.

Isolated solar gain

Isolated gain, involves passively capturing solar heat and then moving it passively into or out of the building using a liquid (for example using a thermosiphon solar space heating system) or air (perhaps using a [[solar chimney), either directly or using a thermal store.

Sun-spaces, greenhouses, and "solar closets" are alternative ways of capturing isolated heat gain from which warmed air can be taken. In practice it has been found that some owners use these structures as living spaces, heating them with conventional fuels and therefore significantly increasing, rather than reducing, the environmental impact of the building.

Don Stephens' "Annualized Geo-Solar" (AGS) heating is an annualized example of this option, which offers the advantages of preventing over-heating when living spaces are already deemed warm enough, and of extending time-delays until such heat will be desired.

Other passive solar design techniques

  • Building position - Based on the local climate and the sun's positioning (determined using a heliodon), the entire building can be positioned and angled to be oriented towards or away from the sun (according whether heating or cooling is the primary concern), overshadowing from other structures or natural features can be avoided or used, and the building can be set into the ground using earth sheltering techniques.
  • Building properties - The shape (and consequently the surface area) of the building can be controlled to reduce the heating or cooling requirement, and the use of materials properties to reflect, absorb, or transmit energy (for example using visible colour) is also a consideration.
  • External environment - Energy-efficient landscaping materials, including the use of trees and plants can be selected to reflect or absorb heat, create summer shading (particularly in the case of deciduous plants), and create shelter from the wind.

Although not classified as a passive solar technology, the use of thermal insulation or superinsulation is invariably employed to reduce heat loss or unwanted heat gain.

Trombe Wall

A Trombe wall is a sun-facing wall built from material that can act as a thermal mass (such as stone, concrete, adobe or water tanks), combined with an air space, insulated glazing and vents to form a large solar thermal collector.

The idea popularized in the 1960's was just the glazed, heavy wall. During the day, sunlight would shine through the insulated glazing and warm the surface of the thermal mass. At night, heat would escape from the thermal mass, primarily to the outside. Because of the insulating glazing, the average temperature of the thermal mass can be significantly above the average outdoor temperature. If the glazing insulates well enough, and outdoor temperatures are not too low, the average temperature of the thermal mass will be significantly higher than room temperature, and heat will flow into the house interior.

Modern Trombe walls have vents added to the top and bottom of the air gap between the glazing and the thermal mass. Heated air flows via convection into the building interior. The vents have one-way flaps which prevent convection at night, thereby making heat flow strongly directional. This kind of design is an isolated passive thermal collector. By moving the heat away from the collection surface, it greatly reduces thermal losses at night and improves overall heat gain. Generally, the vents to the interior are closed in summer months when heat gain is not wanted.

Nighttime thermal losses through the thermal mass can still be significant. The modern design can be still further improved by insulating the thermal mass from the collection surface. The insulation greatly reduces nighttime heat losses at the cost of smaller reductions in daytime heat gain.

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