Solar Heating Systems

Solar Heating Systems

Solar Heating SystemsA New Idea?

• The Greeks faced severe fuel shortages in fifth century BC, resorting to arranging their houses so that each could make maximum use of the sun’s warming rays. A standard house plan emerged, with Socrates noting, “In houses that look toward the south, the sun penetrates the portico in winter.”

• The Romans picked up on this technique, and improved it by adding windows of mica or glass to better hold in the heat.

• In the Americas, the Anazazi took advantage of solar insolation in their cave dwellings in 1220AD

Early Passive Solar Designs

• Montezuma Castle, Arizona, 1200AD

• Direct gain construction

• South facing• Overhang• Stone is a good

thermal mass

Passive and Active Solar Heating

Passive Solar Heating– The sun’s radiation heats a house without having to do

any work

Active Solar Heating– Work is used to pump solar heat into a house (usually

with a pump or a fan)

Direct Gain Solar Heating– Incoming sunlight is used to heat the floors of the actual

living space directly

Indirect Gain Solar Heating– Incoming sunlight is converted to heat and circulated to

the rest of the house through convection

Elements of Passive Solar Design

Elements of Passive Solar Design 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.

• The amount of solar gain transmitted through glass is affected by the angle of the incident solar radiation.

• Sunlight striking glass within 20 degrees of perpendicular is mostly transmitted through the glass, whereas sunlight at more than 35 degrees from perpendicular is mostly reflected.

Elements of Passive Solar Design Aperture (Collector)

• Low-emissivity (Low-E) coatings are microscopically thin, virtually invisible, metallic oxide layers deposited on a window surface

• Low-E coatings are transparent to visible light, and opaque to infrared radiation.

• In typical insulated glazing, the low-e coating is found on one of the interior faces of the glass.

• A simple low-e coating helps to reduce heat loss but allows the room to be warmed by any sunshine.

Elements of Passive Solar DesignAbsorber

• 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.

Elements of Passive Solar DesignThermal 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.

• Masonry materials, like concrete, stones, brick, and tile, are commonly used as thermal mass in passive solar homes. Water also has been successfully used.


Distribution • 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.

• An active design uses fans, ducts, and blowers may help with the distribution of heat through the house.


Control • 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.

Elements of Passive Solar DesignLandscaping

• Evergreen trees planted in back (North Side)• Deciduous trees planted in front (South Side)• Partial earth sheltering in back

Modern Passive Solar Design

• Note Evergreen trees and partial earth sheltering.

• What side (north or south) of the house are you looking at?

Modern Passive Direct Gain Solar Design

Modern Passive Direct Gain Solar Design

• South Facing, double pane windows serve as the aperature

• Ceramic floor tile acts as the absorber and thermal mass, storing solar heat

Trombe Walls

• Trombe walls are an indirect gain system because the heat from the wall is circulated to the house through convection

Trombe Wall - Outside View

Trombe Wall – Inside View

Trombe Wall

• The Trombe wall distributes or releases heat into the home over a period of several hours.

• Solar heat migrates through the wall, reaching its rear surface in the late afternoon or early evening.

• When the indoor temperature falls below that of the wall's surface, heat begins to radiate and transfer into the room.

• For example, heat travels through a masonry wall at an average rate of 1 hour per inch. Therefore, the heat absorbed on the outside of an 8-inch-thick concrete wall at noon will enter the interior living space around 8 p.m.

Thermosiphoning Air Panels• Panels are attached to

wall that allow air to be heated by sun

• Indirect gain because air is circulated to house by convection

Solar Heating with Greenhouses

• Heat from solar radiation is stored in water drums or concrete floor

• Convection circulates heat to rest of house

The Greenhouse Effect

• Glass will transmit visibly light but not infrared light (i.e. the radiation given off by room temperature objects)

• Solar radiation enters, but heat cannot escape as infrared radiation

• Heat is trapped and temperature rises

Solar Heating with Greenhouses

Passive Solar Water Heaters I

• A batch solar water heater consists of black water tanks set in the sunlight

• Glazing (glass panel) partially prevents heat from escaping

• Must be covered with insulation at night

Passive Solar Water Heating II

• In a thermosiphoning solar water heater water is circulated through a solar collector by natural convection

• Tank must be placed above the collector

Thermosiphoning Hot Water System

Active Solar Heating• In active solar heating, a fluid or air is first heated by the sun• Pumps or fans are used to distribute heat to storage or direct

use

Flat Plate Collectors

• Most common type of domestic solar collectors

• Solar radiation is absorbed by a metal plate

• Glass covers prevent heat from escaping


• Heat from a flat plate collector is exchanged with fluid in metal tubes

• Water can also trickle down corrugated metal sheets

Solar collector forheating water

A home in California in 1906


• Collectors mainly used to heat water.

• 5% are used for Domestic Hot Water.

• 95% used to heat water for swimming pools.

Orientation of the Collector Plates

• Since more heat is required in winter, collector plates should face more towards the winter sun

• A good rule is to angle the plate halfway between the noon height of the sun in fall and winter

Calculating the Tilt of a Collector Plate

• The optimum angle in spring or fall is

anglefall = your latitude

• The optimum angle in winter is

anglewinter = your latitude + 23.5°

• Thus optimum angle = (anglefall + anglewinter)/2

Calculating the Tilt of a Collector Plate

• What is the optimum angle for a solar collector in Peoria (40° N)?

– anglefall = 40°

– anglewinter = 40° + 23.5° = 63.5°

– optimum angle = (40° + 63.5°)/2 = 51.7°

51.7°

Size of Collector

Q = I x x AI = insolation = efficiency

A = area of collector

Insolation

• Insolation is the amount of useful radiation that can be collected on a horizontal surface

• Insolation can be increased by tilting a surface towards the sun (i.e. south)

Here insolation is in Btu/ft2/day

Calculating the Size of Collector

• How large a collector is required to heat a home that requires 100,000 Btu/hr? Assume the insolation is 1300 Btu/hr/ft2 and the collector is 50% efficient.

• Answer: Q = I x x AI = insolation, = efficiency, A = area of

collector

Rearrange equation to obtain A = Q/(I x )= 100,000/(1300 x 0.5) = 154 ft2

Calculating the Size of Collector

• How much heat could you obtain with a 30 x 50 ft solar collector in Peoria (assume 30% efficiency)

• In Peoria, the average insolation is 1200 Btu/ft2/d

• Area x Insolation = Btu’s per day• Including efficiency: 0.30x(30x50)x1200

= 540,000 Btu/day = 22,500 Btu/hr

Domestic Hot Water Systems• Some active systems are used only to heat water• Antifreeze solution is pumped through the collectors (to

prevent freezing in winter)• Heat from the antifreeze is exchanged with water

Domestic Hot Water Systems

Domestic Heating Systems• Active systems can be used to heat the house as well• Some heated water is circulated through pipes in the

floor, which heats the house

Domestic Heating Systems

Hot Air Heating• Air is heated in a flat

plate collector and circulated with fans

• Some heat from the air is stored in a bin full of rocks, the rest is used to heat the house

• Cold water is heated by circulating through the heated rocks

Education

Solar Heating Systems