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Passive Solar Heating

Passive Solar Heating Home Design

Your home's windows, walls, and floors can be designed to collect, store, and distribute solar energy in the form of heat in the winter and reject solar heat in the summer. This is called passive solar design or climatic design. Unlike active solar heating systems, passive solar design doesn't involve the use of mechanical and electrical devices, such as pumps, fans, or electrical controls to move the solar heat.

Passive solar homes range from those heated almost entirely by the sun to those with south-facing windows that provide some fraction of the heating load. The difference between a passive solar home and a conventional home is design. The key is designing a passive solar home to best take advantage of your local climate. For more information, see how a passive solar home design works.

You can apply passive solar design techniques most easily when designing a new home. However, existing buildings can be adapted or "retrofitted" to passively collect and store solar heat.

To design a completely passive solar home, you need to incorporate what are considered the five elements of passive solar design. Other design elements include:

  • Window location and glazing type
  • Insulation and air sealing
  • Auxiliary heating and cooling systems, if needed.

These design elements can be applied using one or more of the following passive solar design techniques:

  • Direct gain
  • Indirect gain (Trombe wall)
  • Isolated gain (Sunspace).

When incorporating these design elements and techniques, you want to design for summer comfort, not just for winter heating.

Your home's landscaping can also be incorporated into your passive solar design.

Five Elements of Passive Solar Home Design

The following five elements constitute a complete passive solar home design. Each performs a separate function, but all five must work together for the design to be successful.

Aperture (Collector)
The large glass (window) area through which sunlight enters the building. Typically, the aperture(s) should face within 30 degrees of true south and should not be shaded by other buildings or trees from 9 a.m. to 3 p.m. each day during the heating season.

The hard, darkened surface of the storage element. This surface—which could be that of a masonry wall, floor, or partition (phase change material), or that of a water container—sits in the direct path of sunlight. Sunlight hits the surface and is absorbed as heat.

Thermal mass
The materials that retain or store the heat produced by sunlight. The difference between the absorber and thermal mass, although they often form the same wall or floor, is that the absorber is an exposed surface whereas thermal mass is the material below or behind that surface.

The method by which solar heat circulates from the collection and storage points to different areas of the house. A strictly passive design will use the three natural heat transfer modes—conduction, convection, and radiation—exclusively. In some applications, however, fans, ducts, and blowers may help with the distribution of heat through the house.

Roof overhangs can be used to shade the aperture area during summer months. Other elements that control under- and/or overheating include electronic sensing devices, such as a differential thermostat that signals a fan to turn on; operable vents and dampers that allow or restrict heat flow; low-emissivity blinds; and awnings.

How a Passive Solar Home Design Works

A photo of the interior of a sun space constructed on the side of the house with sunlight entering through several windows and sky lights.

A sunspace or attached greenhouse relies primarily on convection to move heat from the sunny space to other adjacent rooms. Photo credit: Donald Aitken

To understand how a passive solar home design works, you need to understand how heat moves and how it can be stored.

As a fundamental law, heat moves from warmer materials to cooler ones until there is no longer a temperature difference between the two. To distribute heat throughout the living space, a passive solar home design makes use of this law through the following heat-movement and heat-storage mechanisms:

  • Conduction

Conduction is the way heat moves through materials, traveling from molecule to molecule. Heat causes molecules close to the heat source to vibrate vigorously, and these vibrations spread to neighboring molecules, thus transferring heat energy. For example, a spoon placed into a hot cup of coffee conducts heat through its handle and into the hand that grasps it.

  • Convection

    Convection is the way heat circulates through liquids and gases. Lighter, warmer fluid rises, and cooler, denser fluid sinks. For instance, warm air rises because it is lighter than cold air, which sinks. This is why warmer air accumulates on the second floor of a house, while the basement stays cool. Some passive solar homes use air convection to carry solar heat from a south wall into the building's interior.

  • Radiation

    Radiant heat moves through the air from warmer objects to cooler ones. There are two types of radiation important to passive solar design: solar radiation and infrared radiation. When radiation strikes an object, it is absorbed, reflected, or transmitted, depending on certain properties of that object.

    Opaque objects absorb 40%–95% of incoming solar radiation from the sun, depending on their color—darker colors typically absorb a greater percentage than lighter colors. This is why solar-absorber surfaces tend to be dark colored. Bright-white materials or objects reflect 80%–98% of incoming solar energy.

    Inside a home, infrared radiation occurs when warmed surfaces radiate heat towards cooler surfaces. For example, your body can radiate infrared heat to a cold surface, possibly causing you discomfort. These surfaces can include walls, windows, or ceilings in the home.

    Clear glass transmits 80%–90% of solar radiation, absorbing or reflecting only 10%–20%. After solar radiation is transmitted through the glass and absorbed by the home, it is radiated again from the interior surfaces as infrared radiation. Although glass allows solar radiation to pass through, it absorbs the infrared radiation. The glass then radiates part of that heat back to the home's interior. In this way, glass traps solar heat entering the home.
  • Thermal capacitance

    Thermal capacitance refers to the ability of materials to store heat. Thermal mass refers to the materials that store heat. Thermal mass stores heat by changing its temperature, which can be done by storing heat from a warm room or by converting direct solar radiation into heat. The more thermal mass, the more heat can be stored for each degree rise in temperature. Masonry materials, like concrete, stones, brick, and tile, are commonly used as thermal mass in passive solar homes. Water also has been successfully used.

Passive Solar Home Design - Roof Overhangs

In passive solar home design, exterior roof overhangs provide a practical method for shading building elements such as windows, doors, and walls.

How They Work

Overhangs are most effective for south facing elements (in the northern hemisphere) and at midday. If the building element bears more than about 30° off true south, the effectiveness of an overhang, as with any solar feature, begins to decrease significantly. Overhangs usually only affect the amount of direct solar radiation that strikes a surface. Diffuse sky and reflected radiation gains are not often directly affected by overhangs.

The higher overhead the sun is, the shorter the shadow a person will cast on the ground. However, the short brim of a baseball cap can create a long shadow across the body of a standing person. The same concept applies in designing overhangs for buildings. The higher, or more vertical, the arc of the sun, the longer the shadow that the building overhang generates along the face of the wall. Summer shadows extend down walls the furthest, winter shadows the least. Sites closer to the equatorial path of the sun have deeper-extending wall shadows than ones farther from the equator, assuming the same overhang length.

A well-designed overhang may be all that is necessary
to shade a vertically sloped interior sunspace wall.

Types of Overhangs

Overhangs may be solid, louvered, vegetation-supporting, or a combination of all of these aspects. Some shutters, eaves, trellises, light shelves, and awnings serve the same purpose as an overhang.

Overhangs may also be fixed, operable, and/or removable. Examples include roof eaves, awnings, and Bahama shutters (top-hinged louvered shutters typically propped open with wooden dowels) respectively. Fixed overhangs offer perceived longevity and low maintenance at the expense of flexibility or the ability to adjust to site-specific factors. Although adjustable devices allow the user to fine tune the amount of shade or direct sunlight, they require more maintenance. Removable fixtures generally provide flexibility and longevity plus some personal involvement with installation and removal.

Openings, such as windows, do not always require fixed overhangs. A fixed overhang designed for optimal shading on the autumnal equinox (September 21) casts the same shadow on the vernal equinox (March 21). While northern-hemisphere shading may be welcome in September because of the heat, shading in March may be undesirable. Vegetation, on the other hand, can follow the climatic seasons. Vines that shed their leaves for winter usually leaf out about the time shading is needed. Movable shading devices, while adjustable, often become maintenance problems.

Sizing Overhangs

Unfortunately, there is as yet no universally simple formula for sizing overhangs. While one overhang methodology works well for some locations, it can be completely inappropriate for others. For example, there are a limited number of overhang-sizing guidelines acceptable for buildings located in southern states, particularly hot-humid climates. Guidelines acceptable for the high plains of Montana are unlikely to work for a site in Florida.

Due to the varying microclimate conditions encountered across the United States, the methods presented here are general in scope. Anyone seeking a more specialized analysis should seek professional advice from an architect trained in passive solar design.

Every climate requires special design attention. The following general guidelines may be useful in determining a suitable overhang design. The guidelines are listed by climate type, for solar noon, when the sun reaches its maximum altitude for a given day. Solar noon is very rarely the same as noon in local standard time.

Cold climates: above 6,000 heating degree days (HDD)* (at base 65°F [18°C])
Locate shadow line at mid-window using the June 21 (summer solstice) sun angle.

Moderate climates: below 6,000 heating degree days (HDD)* (at base 65°F [18°C]) and below 2,600 cooling degree days (CDD)* (at base 75°F [22°C])
Locate shadow line at window sill using the June 21 (summer solstice) sun angle.

Hot climates: above 2,600 cooling degree days (CDD)* (base 75°F [22°C])
Locate shadow line at window sill using the March 21 (vernal equinox) sun angle.

*(HDD and CDD data is available from local weather services.)

Overhangs and Zoning Ordinances

Overhangs may be inappropriate for sites with restrictive regulatory guidelines. For example, your calculations indicate your house needs a three foot (~1 meter[m]) overhang on the front. The local zoning ordinance restricts eave extension to two feet (0.61 m) beyond the front yard setback. If your house will be or is located precisely on the setback, you must do one of following: relocate your house at least one foot (0.3 mm) back from the front building setback; redesign your building fenestration (windows, doors, grilles, vents, and other openings); redesign your overhang; or apply for a variance (an exception to the ordinance).

This home in Golden, Colorado uses a liquid-based solar system for space and water heating.
Photo credit: Warren Gretz

Liquid-Based Active Solar Heating

Solar liquid collectors are most appropriate for central heating. They are the same as those used in solar domestic water heating systems. Flat-plate collectors are the most common, but evacuated tube and concentrating collectors are also available. In the collector, a heat transfer or "working" fluid such as water, antifreeze (usually non-toxic propylene glycol), or other type of liquid absorbs the solar heat. At the appropriate time, a controller operates a circulating pump to move the fluid through the collector.
The liquid flows rapidly through the collectors, so its temperature only increases 10°–20°F (5.6°–11°C ) as it moves through the collector. Heating a smaller volume of liquid to a higher temperature increases heat loss from the collector and decreases the efficiency of the system. The liquid flows to either a storage tank or a heat exchanger for immediate use. Other system components include piping, pumps, valves, an expansion tank, a heat exchanger, a storage tank, and controls.
The flow rate through the collector should be between 0.02 and 0.03 gallons per minute per square foot of collector when water is the heat transfer fluid (0.82 to 1.22 liters per minute per square meter of collector). Other flow rates apply for different heat transfer fluids. The total flow rate, used to size the collector pump, is the product of the above flow rate times the total collector area.

Storing Heat in Liquid Systems

Liquid systems store solar heat in tanks of water or in the masonry mass of a radiant slab system. In tank type storage systems, heat from the working fluid transfers to a distribution fluid in a heat exchanger exterior to or within the tank.

Most storage tanks require 1–2 gallons (3.8–7.6 Liters) of water for each square foot (0.093 square meter) of collector area. Tanks are pressurized or unpressurized, and the type used depends on the overall system design. Before choosing a storage tank, you should consider several factors, including cost, size, durability, where to place it (in the basement or outside), and how to install it. You may need to construct a tank on-site if a tank of the necessary size will not fit through existing doorways. Tanks also have limits for temperature and pressure, and must meet local building, plumbing, and mechanical codes. You should also note how much insulation is necessary to prevent excessive heat loss, and what kind of protective coating or sealing is necessary to avoid corrosion or leaks.

Specialty or custom tanks may be necessary in systems with very large storage requirements. They are usually stainless steel, fiberglass, or high temperature plastic. Concrete and wood (hot tub) tanks are also options. Each type of tank has its advantages and disadvantages. All types require careful consideration for their location, due to their size and weight. It may be more practical to use several smaller tanks rather than one large one. The simplest storage system option is to use standard domestic water heaters. They are designed to meet building codes for pressure vessel requirements, are lined to inhibit corrosion, and designed so it is easy to attach pipes and fittings.

Distributing Heat for Liquid Systems

There are different ways to distribute the solar heat: with a radiant floor, with hot water baseboards or radiators, or with a central forced-air system. In a radiant floor system, a solar-heated liquid circulates through pipes embedded in a thin concrete slab floor, which then radiates heat to the room. Radiant floor heating is ideal for liquid solar systems because it performs well at relatively low temperatures. A carefully designed system may not need a separate heat storage tank, though most systems do for temperature control. A conventional boiler or even a standard domestic water heater can supply backup heat. The slab is typically covered with tile. Radiant slab systems take longer to heat the home from a "cold start" than other types of heat distribution systems. Once they are operating, however, they provide a consistent level of heat. Carpeting and rugs will reduce the system's effectiveness. See radiant heating for more information.

Hot-water baseboards and radiators require water between 160° and 180°F (71° and 82°C) to effectively heat a room. Generally, flat-plate liquid collectors heat the transfer and distribution fluids to between 90° and 120°F (32° and 49°C). Therefore, using baseboards or radiators with a solar heating system requires that either the surface area of the baseboard or radiators be larger, that the solar-heated liquid be heated more with the backup system, or that a medium-temperature solar collector (such as an evacuated tube collector) be used.

It is possible to incorporate a liquid system into a forced-air heating system, and there are different options for doing so. The basic design is to place a liquid-to-air heat exchanger, or heating coil, in the main room-air return duct prior to the furnace. Air returning from the living space is heated as it passes over the solar heated liquid in the heat exchanger. Additional heat is supplied as necessary by the furnace. The coil must be large enough to transfer sufficient heat to the air at the lowest operating temperature of the collector.

Ventilation Preheating

Solar air heating systems use air as the working fluid for absorbing and transferring solar energy. Solar air collectors (devices to heat air using solar energy) can directly heat inpidual rooms or can potentially pre-heat the air passing into a heat recovery ventilator or through the air coil of an air-source heat pump.

Air collectors produce heat earlier and later in the day than liquid systems, so they may produce more usable energy over a heating season than a liquid system of the same size. Also, unlike liquid systems, air systems do not freeze, and minor leaks in the collector or distribution ducts will not cause significant problems, although they will degrade performance. However, air is a less efficient heat transfer medium than liquid, so solar air collectors operate at lower efficiencies than solar liquid collectors.

Although some early systems passed solar-heated air through a bed of rocks as energy storage, this approach is not recommended because of the inefficiencies involved, the potential problems with condensation and mold in the rock bed, and the effects of that moisture and mold on indoor air quality.

Solar air collectors are often integrated into walls or roofs to hide their appearance. For instance, a tile roof could have air flow paths built into it to make use of the heat absorbed by the tiles. Air entering a collector at 70°F (21.1°C) is typically warmed an additional 70°–90°F (39°–50°C.). The air flow rate through standard collectors should be 1–3 cubic feet (0.03–0.76 cubic meters) per minute for each square foot (0.09 square meters) of collector. The velocity should be 5–10 feet (1.5–3.1 meters ) per second.

Most solar air heating systems are room air heaters, but relatively new devices called transpired air collectors have limited applications in homes.

Room Air Heaters

Air collectors can be installed on a roof or an exterior (south facing) wall for heating one or more rooms. Although factory-built collectors for on-site installation are available, do-it-yourselfers may choose to build and install their own air collector. A simple window air heater collector can be made for a few hundred dollars.

The collector has an airtight and insulated metal frame and a black metal plate for absorbing heat with glazing in front of it. Solar radiation heats the plate that, in turn, heats the air in the collector. An electrically powered fan or blower pulls air from the room through the collector, and blows it back into the room. Roof-mounted collectors require ducts to carry air between the room and the collector. Wall-mounted collectors are placed directly on a south-facing wall, and holes are cut through the wall for the collector air inlet and outlets.

Simple "window box collectors" fit in an existing window opening. They can be active (using a fan) or passive. In passive types, air enters the bottom of the collector, rises as it is heated, and enters the room. A baffle or damper keeps the room air from flowing back into the panel (reverse thermosiphoning) when the sun is not shining. These systems only provide a small amount of heat, since the collector area is relatively small.

Transpired Air Collectors

Transpired air collectors use a simple technology to capture the sun's heat to warm buildings: The collectors consist of dark, perforated metal plates installed over a building's south-facing wall. An air space is created between the old wall and the new facade. The dark outer facade absorbs solar energy and rapidly heats up on sunny days—even when the outside air is cold.

A fan or blower draws ventilation air into the building through tiny holes in the collectors and up through the air space between the collectors and the south wall. The solar energy absorbed by the collectors warms the air flowing through them by as much as 40°F. Unlike other space heating technologies, transpired air collectors require no expensive glazing.

Transpired air collectors are most suitable for large buildings with high ventilation loads, a fact which makes them generally unsuitable for today's tightly sealed homes. However, small transpired air collectors could be used to pre-heat the air passing into a heat recovery ventilator or could warm the air coil on an air source heat pump, improving its efficiency and comfort level on cold days. However, no information is currently available on the cost effectiveness of using a transpired air collector in this way.

U.S. Department of Energy - Energy Efficiency and Renewable Energy

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