The Sun: Passive Solar Heating

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The Sun: Passive Solar Heating

The Sun: Passive Solar Heating

The Joy of Living in a Desert


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)


Solar Radiation

The amount of

sunlight incident on

the earth’s atmosphere

is the solar constant

(in W/m 2 or Btu/ft 2 )

• Only about 50% of the

sun’s light reaches the

surface of the earth


The Reason for the Seasons

• In the summer, the northern hemisphere is tilted

toward the sun

• In the winter, it is tilted away from the sun


Season and Height of the Sun

The height of the sun

above the horizon

depends on the time of

day and the season

• At noon, the sun is

higher in the sky in

summer than in winter

The sun is never

directly overhead in

Stevens Point


Taking Advantage of the Sun’s

• In winter, solar

radiation stores heat in

a concrete floor

• In summer, little solar

radiation enters the

house because of the

eaves

Height


The Components of Solar

• On a sunny day, we

receive mostly direct

solar radiation

• Diffuse radiation is

scattered from clouds

and the atmosphere

• Reflected radiation is

self-explanatory

Radiation


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/ft 2 /day


Example

• How much heat could you obtain with a 30

x 50 ft solar collector in Stevens Point

(assume 30% efficiency)

• In Stevens Point, the average insolation is

1200 Btu/ft 2 /d

• Area x Insolation = Btu’s per day

• Including efficiency: 0.30x(30x50)x1200

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


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

• Heat from solar

radiation is stored in

water drums or

concrete floor

• Convection circulates

heat to rest of house


Constructing a Passive Solar

House

• Windows: Install windows on the south side

of the house

• Heat Storage: Allow light entering through

the windows to fall on cement, rocks, water

drums, etc. which will store heat for use at

night (high specific heats are better)

• Use dark colors to absorb sunlight

• Cover windows at night to prevent heat loss


Direct and Indirect Gain

• 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


• Trombe walls are an

indirect gain system

because the heat from

the wall is circulated

to the house through

convection

Trombe Walls


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


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


A Solar Refrigerator?


Passive cooling

• Roof ponds

Harold hay invented a roof pond system that

provided both passive summer cooling as well

as winter solar heating.

Its consist of a single story building with a level,

un insulated, metal pan structural roof.


Roof ponds summer


Roof ponds winter


Evaporative cooling

• What is Evaporative cooling?

evaporative cooling is the exchange of

sensible heat in air for the latent heat of

water droplets on wetted surface.


Direct evaporation

• Direct evaporative cooling occurs when relatively

dry air is blown over a wetted surface .


Direct evaporation


Swamp coolers

• Between world war I and II

direct evaporative cooling

units became popular.

• Because of the motor this

design is considered hybrid

rather than strictly passive.

The major disadvantage is

their inability to cope with

higher bulb temps.


Indirect evaporation

• Open loop which sprays

exhaust air from the room,

cooling it by evaporation.

• Closed loop

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