Passive solar

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(Redirected from Passive solar heating)

Passive solar design is a broad category of solar power techniques and strategies for regulating a building's indoor air and domestic water temperatures, using climate, site features, architectural elements, and landscape materials. The goal is typically to increase the comfort, efficiency and reliability of a building, while reducing its operating costs and dependence on other sources of energy for heating and cooling.

In new United States residential construction, properly-designed passive solar heating and cooling is surprisingly cheap to construct, and commands premium prices. In areas with more than two weeks of frost, passive solar heat adds about 15% to the cost of new construction. In areas with fewer frosts, it has no extra construction costs for heating. Passive annualized solar heating shifts heat from one season to another, and these systems can reduce cooling costs as well.

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History

The ancient Greeks and Romans used solar design features in their housing, but the first passive solar houses of the modern era were built in Germany after the first world war, when the Allies occupied the Ruhr area, including most of Germany's coal mines. These designs were studied in the United States, but had little influence on builders.

The first passive solar house in the US was designed in 1940 by George F. Keck for a Chicago area real estate developer named Howard Sloan. Keck had designed an all-glass house for the 1933 Century of Progress Exposition in Chicago and was surprised to find that it was warm inside on sunny winter days, even though the furnace hadn't been installed yet. Keck was not aware of the research being done elsewhere on solar architecture, but he gradually started incorporating more south-facing windows into his designs for other clients, and by 1940 he had learned enough to design a passive solar house for Sloan.

Sloan built a number of passive solar houses in the 1940s, and his publicity efforts influenced a number of other builders during the postwar housing boom (Sloan is also credited with popularizing the term "solar" to describe his houses). But some builders of that era didn't realize that the houses were designed to face south, and many were built facing other directions, which hurt their reputation. Critics also pointed out that windows and doors weren't always properly sealed. Public interest declined by 1950 due to cheap oil and general prosperity, until it was revived after the 1973 oil crisis.

This is discussed in A Golden Thread: 2500 Years of Solar Architecture and Technology by Ken Butti and John Perlin (1980) ISBN 0442250058, ISBN 0917352076

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Types

Pragmatic: 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 with 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.

Annualized: Historically, most "passive solar" approaches have depended on near-daily solar capture and storage, only expected to maintain temperatures through a few days and nights. These are now termed "short-cycle passive solar". More recent research has developed techniques to capture warm-season solar heat, convey it to a storage mass, and still have heat available six months later, during the cool or cold season. This is referred to as "annualized passive solar." This requires large amounts of thermal mass. One technique buries water-proof insulation in 7-metre skirts around the foundation, and buries loops of plastic pipe or ducts under the foundations and slab. The "skirts" of insulation prevent heat leaks from weather or water.

Minimum machinery: A "purely passive" solar-heated house would have no mechanical furnace unit, relying instead on energy captured from sunshine, only supplemented by "incidental" heat energy given off by lights, candles, other task-specific appliances (such as those for cooking, entertainment, etc.), showering, people and pets. Some designers use convection for circulation, and some of their houses even lack fans. Some designers have used quite exotic aerodynamic modeling software (adapted from aircraft design) to prove these designs correct before starting construction.

Systems sometimes use limited electrical and mechanical controls to operate dampers, insulating shutters, shades or reflectors. Some systems enlist small fans or solar-heated chimneys to start or improve convective air-flow. A reasonable way to analyse these systems is by measuring their coefficient of performance. A heat pump might use 1 J for every 4 J it delivers giving a COP of 4, a system that only uses a 30W ceiling fan to heat an entire house with 10kW of solar heat would have a COP of 300.

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Techniques of collection

Passive solar designs ordinarily use one or more of three techniques to assure solar gain:

  1. Direct Gain - where sunshine is allowed to pass directly through windows and/or skylights into the living spaces themselves, and to warm the air and surfaces there. A home with sun-facing windows and a high-mass floor is a short-cycle example of this and John Hait's "Passive Annual Heat Storage" (PAHS) method is an example of an annualized solar approach primarily using this path.
  2. Indirect Gain - Where sunlight strikes an intervening material (such as water or a solid mass behind glass, and then arrives at the living spaces indirectly, after being captured, stored and re-released by that material. 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.
  3. Isolated Gain - Here the solar heat is passively captured and stored in isolated places or devices and then only allowed to move passively into the actual living spaces when and if desired. Examples of this are partitioned away sun-spaces, greenhouses, "solar closets" and thermosyphon flat-plate collectors. 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.

Direct and Indirect gain systems suffer because we have no reasonably priced transparent thermally insulating materials with R-values comparable to standard wall insulation. Aerogel is a promising, though expensive technology that might solve this. In practice the simplicity of isolated gain design, combined with the good long term performance and low cost make this the most practical method. To understand this design, consider a hypothetical house based on the work of Barra.

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A house based on the Barra design: an example

To convert the suns light into heat we use a separate, insulated space on the sunny side of the house walls. Looking at the outside, and moving through a cross section we see an outside clear layer. This was traditionally built using glass, but with the advent of cheap, robust Polycarbonate glazing most designs use twin- or triple-wall polycarbonate greenhouse sheeting. Typically the glazing is designed to pass visible light, but block IR to reduce losses, and block UV to protect building materials.

The next layer is an absorption space. This absorbs most of the light entering the collector. It usually consists of an air gap of say 10cm thickness with one or more absorption meshs suspended vertically in the space. Often window fly screen mesh is used, or horticultural shade cloth. The mesh itself can hold very little heat and warms up rapidly in light. The heat is absorbed by air passing around and through the mesh, and so the mesh is suspended with an air gap on both the front and back sides.

Finally a layer of insulation sits between the absorption space and the house. Usually this is normal house insulation, using materials such as polyisocyanurate foam, rock wool, foil and polystyrene.

This collector is very agile - in the sun it heats up rapidly and the air inside starts to convect. If we directly connected this to the house with a hole near the floor and a hole near the ceiling we would get an Indirect gain system. One problem with this that like Trombe walls, the heat would radiate back out at night, and a convection current would chill the room during the night. Unlike in a Trombe wall, however, we can stop this simply by stopping the air movement. Two common methods for this are automatic dampers, similar to those used for ventilating foundation spaces in cold climates and plastic film dampers, which work by blocking air flow in one direction with a very lightweight flap of plastic. The addition of the damper makes the design an efficient isolated gain system.

When the sun goes down at night we have stopped heat leaking back out into the collector, but we still have heat leaking through the walls. We need a store for each night. The Barra system suspends a slab of concrete as a ceiling to store heat. This is fairly expensive and requires strong support. We can instead use water, which can store 5 times as much heat for a given weight. A simple, cheap and effective way to store water here is to store the water in sealed 100 mm diameter PVC storm pipe with end caps. Whether we use water or concrete, the heat is transferred from the air in the collector into the storage material during the day, and released on demand using a ceiling fan into the room at night. The ceiling also heats the house by radiation. Some people have built houses with louvres (similar to those used on satelites) to adjust the radiation transfer.

If the climate were sunny every day when heating is required, we would have a complete design. Besides some locations in a desert, cloudy days are common. Whatever the location, cloudy days tend to have an exponential distribution for likelihood. In most places a system designed for 5 successive days of no sun provides enough storage for all but a few days in a hundred years.

We can store heat over a number of days using a large container of water. A 8 foot cube of water in the basement might store 15 kL of water, which is heated using a copper tube with fins in the collector. We can improve the performance of this cloudy day collector by putting the finned tube inside another layer of glazing at the back of the main collector, allowing the temperature to build up more than the surrounding air stream. On cloudy days the heat is transferred back out of the store to heat the house.

Barra's are said to be a lot more comfortable than other passive solar houses, with more uniform north-south temp distribution. His "spancrete" ceiling slabs in single and multistory buildings let hot air-heater air thermosyphon through the slab tunnels from south to north, where it exits and travels back north through the bulk of the room to the air heater inlet near the floor. No fans, and no selective surface beneath, but the hot air store lots of heat in the slabs. Lots of successful systems were built in Europe, but Barra seems fairly unknown in the US.

The (Italian) Horazio Barra system is described on pages 169-171 and 181 of Baruch Givoni's Climate Considerations book (Wiley, 1998.) The basic reference is Barra, O. A., G. Artese, L. Franceschi, R. K. Joels and A. Nicoletti. 1987. "The Barra Thermosyphon Air System: Residential and Agricultural Applications in Italy, UK, and in the Sahara." International Conference of Building Energy Management. Lausanne, Switzerland.

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Elements of design

The thermal storage time of a building is given by the product of the thermal mass (measured in say J/K) and the thermal resistance, or insulation (measured in say K/W) to get seconds. To give a house a long thermal storage time we need to maximise this product.

Materials used for thermal mass include stone, concrete, adobe and water. Of these, water has the highest thermal storage, both by mass (5 times) and by volume (3 times). Water has several advantages for thermal storage: it is very cheap, it can be moved around very efficiently in pipes and it is easier to move heat in and out using radiators. Solid materials on the other hand can be used as part of the structure, and have no danger of leaks or biological activity. Getting heat in and out of masonry is quite difficult and many designs use air ducts, or even water to move the heat in and out of storage. Some authorities express concern about preventing mold growth in the ducts. Annualized passive solar often uses the earth under, around or over the building.

Insulating materials such as rock-wool, foam and straw bales help slow heat loss through the wall better than wood, brick, natural stones or concrete.

Other design elements used in passive design include:

  1. Building orientation: The building is oriented to maximize solar capture, often with the long axis of the building running east to west.
  2. Window placement: Windows are typically minimized on the poleward side(s) of the building. Windows on the sunward side(s)are often made larger in order to receive optimal sunlight during the winter. Windows must be carefully sized for the site and size of building. In higher latitudes, excessively large, sunward-facing windows can become uncomfortably bright at certain times of the year and can accelerate fading of furnishings. Overly large windows can also make a room gain too much heat during the day and lose too much at night.
  3. Shade: Sunward-facing windows are shaded during summer months. Because the sun is at a lower angle in the sky during winter days than summer days, shades can be sized and shaped to provide shade during the summer yet allow the sun to shine in through windows in the winter. Landscaping may also play a part in this: deciduous trees may be planted on the window sides. In summer, these trees will shade the house, cooling it, while in winter when they do not have leaves they will permit the influx of sunlight. Trellises with plants that only grow in warm seasons can provide shade in the summer while allowing light through the winter. The trellis pieces can be angled to increase winter light. The same seasonal variation is achieved by mounting solar collectors on walls rather than roofs.
  4. Daylighting: Windows and internal reflecting areas are placed to maximize the sun's light for daytime interior lighting. Glazing is a very poor insulator, so recently small windows with optimal sun collection are advocated in preference to large areas of glass. Sunlight is around 200 times brighter than a brightly lit office.
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Related methods and techniques

If such a house also uses other alternative energy techniques like solar cells (PVs), hydronic solar thermal energy, wind energy, biofuel, compost-generated or an earth-heat exchanger, it might be called an energy-plus-house. If the emphasis is on ending dependence on social networks, it might be an autonomous building.

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See also

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External links

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