Drexel University

HVAC Systems

Passive Solar Effects  

 

System Description:

Passive solar heating is a system in which natural heat from the sun’s rays are utilized to maintain a comfortable environment within a building using little or no mechanical parts or the input of energy other than what occurs naturally by the sun.  Any time you feel the heat of the sun passing through a window or the warmth of a floor as the sun beats down on it, you are experiencing passive solar heating.  Passive solar systems only affect temperature in a space.  Then do not typically effect ventilation and filtration.  They also cannot change the moisture content of a space but humidity can rise or fall as a secondary effect depending on how passive solar changes the temperature of a space.  Passive solar heating effects virtually all building designs and almost always needs to be taken into consideration when designed an HVAC system. 

            

Passive solar systems can consist of many subsystems or components.  One of the most important components that contributes to passive solar heating is glazing, or windows.  To utilize the sun’s rays glazing should be placed on the south side of a building where the sun can strike it.  During the winter months when the outdoor temperature is cold and the sun is lower in the sky, south-facing windows can capture the heat from the sun’s rays by allowing the light waves to pass in and out of the glass while trapping the heat waves within like a greenhouse.  The heat can then be absorbed by the floors, walls or ceilings that the sunlight directly strikes.  This is known as direct gain.  These objects that absorb, store, and re-radiate heat from the sun are called thermal masses

 

Image found: http://www.eere.energy.gov/de/images/illust_passive_solar_d1.gif

Passive Solar Heat Diagram

 

Thermal Mass:

Thermal mass plays a key role during nighttime hours when the sun is no longer available to maintain comfortable room temperatures.  Materials with a low thermal conductivity and a high specific heat such as adobe, concrete, stone masonry and brick are great materials to use for solar mass.  This is because they are very dense which allows them to absorb a great amount of solar heat which is then dispersed slowly and consistently throughout the night into the cooler rooms.  Concrete and masonry are also very convenient to use because they may already be incorporated in a building’s design as structural components. 

 

 

Thermal Storage Properties of Materials

Material

Conductivity (k)

Specific Heat (Cp)

Density (p)

 

Btu hr/ft2* °F/ft

Btu/lb* °F

lb/ft

Concrete (dense)

1.00

0.20

140.0

brick (common)

0.42

0.20

120.0

brick (magnesium additive)

2.20

0.20

120.0

Adobe

0.30

0.24

106.0

 

 

Constraints:

Concrete and masonry do have there limitations also.  Floors, ceilings and walls made of concrete or masonry can be a minimum of 4” thick to be considered to have sufficient amounts of solar mass, but some passive solar designs may recommend greater thicknesses as much as 8 to18 inches.  This may not be an ideal thickness for some designs that need to be more lightweight.  Also, concrete and masonry absorb heat energy very slowly throughout the day so there is a point when adding thickness to a wall no longer increases the amount of absorption.  This is why it is more beneficial to increase the surface area of the solar mass that is hit by the sun rather than increasing the thickness, which may not always be easy to do based on a building’s particular design constraints.  To maximize the amount of heat absorbed, light should be dispersed in such a way that for every 1 square foot of light passing through a window it should strike 9 square feet of thermal mass

 

Image found: http://www.azsolarcenter.com/technology/pas-2.html

Absorption, Storage, and Re-radiation of Thermal Mass

 

Effects of Finishes:

The use of colors can also aid in amount of heat absorbed by thermal mass.  Since lighter colors reflect the sun’s rays, they can be used on surfaces that are non-thermal masses to reflect the sunlight onto surfaces that are.  In the same way, darker colors can be used on thermal mass surfaces to absorb the heat more efficiently.  It is recommended that the color of a thermal mass surface directly exposed to sunlight is not too dark or it may result in having a surface that is uncomfortably hot.  A glazed finish on thermal mass surfaces can help to disperse the light rays and distribute the thermal energy more evenly throughout the material. 

 

Water as Thermal Mass:

Since the speed at which concrete and masonry absorb heat does have its limitations, a non-structural material has been introduced as a thermal mass in some passive solar systems.  This material is water.  Water has a slightly higher but still low thermal conductivity which allows it to absorb thermal energy more quickly than concrete or stone masonry, which in turn allows it to absorb more energy throughout the day releasing more at night.  Some energy efficient homes may incorporate the use of a wall filled with water called a water wall.  Water walls are typically built with a metal or plastic shell to allow heat to pass through more readily into the water contained inside.  Water walls are non-structural so they cannot be used to support any loads.

 

Image found: http://www.azsolarcenter.com/technology/pas-2.html

Diagram of a Water Wall

 

Other Components:

There are all kinds of different methods and components used in passive solar heating systems.  The use of clerestories and skylights can be used to provide sunlight to rooms that do not have an exterior wall.  Blinds and shutters can be closed to block the summer rays when it is too hot or to keep thermal heat from escaping during nighttime hours.  Overhangs can be used as shading to only block the more vertical rays of the summer sun from hitting a window while still allowing the more horizontal rays of the winter sun to enter.  Deciduous trees can also have the same affect.  Vents and fans, along with natural convection currents due to hot air rising, can also help to move heat from exterior sun exposed rooms to other rooms of a house. 

 

System Limitations, Advantages and Applications:

The function of some buildings may prohibit the ability to use a practical passive solar heating system.  For example, in a building where natural lighting must be kept to a minimum such as some art galleries or museums where sun light may be damaging to objects on display, or in a building where lighting must be controlled to produce a specific effect, such as a theatre or concert hall, or a laboratory of some kind. 

 

In conclusion passive solar heating is a cost effective, low maintenance heat provider.  It takes little to no energy to run, and is extremely environmentally friendly.  Passive solar heating is a factor in virtually all forms of construction whether intentionally or inadvertently.  In either case, the effects that the sun has on any type of construction should always be considered in a design, whether it is to be utilized or protect from. 

 

 

Image found: http://www.azsolarcenter.com/technology/pas-2.html

Diagram of Clerestory and Skylight

 

Image found: http://www.azsolarcenter.com/technology/pas-2.html

Vents used to create heat flow to other rooms by natural convection currents

 

Parameters for Efficient System Usage:

 

Solar Constant(average rate of solar energy arriving at the outer edge of the earth's atmosphere)= 429.2 BTU per hour per square foot.

Ave. Rate of Solar Energy (At sea level)= 320 BTU per hour per square foot.

 

Direct gain system rules of thumb:

Do not cover thermal mass floors with wall to wall carpeting; keep as bare as functionally and aesthetically possible.

Use a medium dark color for masonry floors; use light colors for other lightweight walls; thermal mass walls can be any color.

For every square foot of south glass, use 150 pounds of masonry or 4 gallons of water for thermal mass.

Fill the cavities of any concrete block used as thermal storage with concrete.

Use thermal mass at less thickness throughout the living space rather than a concentrated area of thicker mass.

The surface area of mass exposed to direct sunlight should be 9 times the area of the glazing.

 

Indirect gain system rules of thumb for thermal mass walls

The exterior of the mass wall (toward the sun) should be a dark color.

Use a minimum space of 4 inches between the thermal mass wall and the glass.

Vents used in a thermal mass wall must be closed at night.

A well insulated home (7-9 BTU/day-sq. ft.-degree F) will require approximately 0.20 square feet of thermal mass wall per square foot of floor area or 0.15 square foot of water wall.

If movable night insulation will be used in the thermal wall system, reduce the thermal mass wall area by 15%.

Thermal wall thickness should be approximately 10-14 inches for brick, 12-18 inches for concrete, 8-12 inches for adobe or other earth material and at least 6 inches for water.

 

Isolated Gain rules of thumb for sunrooms:

Use a dark color for the thermal wall in a sunspace.

The thickness of the thermal wall should be 8-12 inches for adobe or earth materials, 10-14 inches for brick, 12-18 inches for (dense) concrete.

Withdraw excess heat in the sunroom (if not used for warm weather plants) until the room reaches 45 degrees and put the excess heat into thermal mass materials in other parts of the house.

For a sunroom with a masonry thermal wall, use 0.30 square feet of south glazing for each square foot of living space floor area. If a water wall is used between the sunroom and living space instead of masonry, use 0.20 square feet of south facing glass for each square foot of living area.

Have a ventilation system for summer months.