Module 8

Solar Gain

Module 8 Solar Gain

• On completion of this module learners will be able to:

– Relate sun position to building orientation for different

times of the year

– Describe the impact of solar radiation on opaque

elements and windows

– Discuss methods to control the impact of solar

radiation

• 8.1 Solar geometry

• 8.2 Impact of solar radiation

• 8.3 Controlling the impact of solar ration

For countries in

the northern

hemisphere the

sun follows an arc

rising in the east

and setting in the

west. The sun is

lower in the sky in

winter months

and higher in the

sky in summer

months

December

September

June

8.1 Solar Geometry

Source: www.coltinfo.co.uk

South facing

• In summer the sun is high

in the sky so horizontally

fixed shading or deeply

recessed windows will

reduce solar gain.

• In winter horizontal

system will not work.

However the intensity of

solar radiation is less in

winter months and

generally solar gain is

beneficial in winter.

South facing window

Source: www.coltinfo.co.uk

East facing

East facing windows are affected by direct solar

radiation from morning to midday. Horizontal shading

does not really become effective until after 9.00 am

East facing affected by

morning sun

Source: www.coltinfo.co.uk

West facing

West facing windows are affected by direct solar radiation

from afternoon to evening. Horizontal shading may be

effective up to 3.00 pm in the afternoon depending on the

time of year. From late afternoon to evening

the altitude of the sun is

lower allowing solar

radiation to enter on a more

horizontal path. The

intensity of solar radiation in

the summer months at these

times is still high and it

coincides with peak outside

air temperatures

West facing affected by afternoon &

evening sun

West Facing

Vertical shading on west

facing windows can

provide shading from

solar gain from afternoon

to evening for all months

of the year.

West facing affected by afternoon &

evening sun

Source: www.coltinfo.co.uk

8.2 Impact of Solar Radiation

A. Opaque elements e.g wall, roofs

– Insulated • As the external structure heats up the insulation

layers will prevent/slow down transmission of the energy through the structure helping to maintain the internal environment cool

• One problem with highly insulated structures is that solar radiation entering through windows that cannot be absorbed by internal structural elements (thermal mass) which tends to produce a rise in internal air temperature. (air has a low specific heat capacity which means a small amount of energy input can produce a large change in temperature)

A. Opaque elements e.g wall, roofs

– Uninsulated • If the element is thermally massive, it will absorb

the solar energy striking it and delay the entry of this energy into the room for a number of hours. If there is sufficient thermal mass it may delay the entry of energy until night time when cooler outside air can be used to cool the structure.

• If the element is thermally light the solar energy heats up the external surface and this energy is quickly transferred through the structure resulting in rise in internal air temperature.

B. Transparent elements – windows

Solar radiation enters through glazing by direct, diffuse and reflected paths

Reflected

Diffuse

Direct

Direct - from line of

sight with the sun

Diffuse - background

light from the vault of

the sky

Reflected - direct and

diffuse reflected from

surrounding surfaces

• Solar radiation enters the window as long wave

radiation.

• It is absorbed by elements in the room which heat

up.

• Energy is re-admitted as short wave radiation.

• Low-e coatings prevent this from passing back

out through the window.

• Good in winter but may lead to overheating in the

summer

• Low e coatings normally

applied to surface 3

• Hard coat emissivity 0.15

to 0.2

• Soft coat emissivity 0.05

to 0.1

• The lower the emissivity

the less radiation emitted

from the window and the

better the U-value.

• Emissivity coatings

slightly reduce the

amount of light entering

the room

Fig. 1 page 7

CE66 Energy

Savings Trust

– Different glass types reflect, absorb and transmit

different amounts of light

– Light transmittance

• The proportion of light that is transmitted by the

glass

– Solar transmittance

• The proportion of solar radiation transmitted

through the glass by direct transmission and by

radiation which is absorbed by the glass and then

re-emitted into the room.

– The number of panes of glass, internal coatings,

colour tints, gaps between panes, gasses between

panes all affect light transmittance and radiation

transmittance.

– For example, double glazed argon filled, low e soft

coat may have 78 % light transmittance and 79 %

solar transmittance.

– Tinted solar control glazing can have as little as 41 %

light transmittance and 44 % solar transmittance.

– Solar transmittance through the glazing can be

considered instantaneous. If the internal elements

surrounding the room are thermally massive they can

absorb the radiation and delay when it appears as a

rise in air temperature in the room. If the elements are

thermally lightweight the solar radiation appears as a

heat gain very quickly.

8.3 Controlling the Impact of Solar

Radiation

Objective

– Maximise solar gain in winter to offset heat

loss

– Limit solar gain in summer to prevent

overheating

– Minimal interference with natural daylight

– Minimal interference with natural ventilation

Two approaches to controlling the impact of

solar radiation

•Minimise entry of solar radiation

– Primarily through shading

•Minimise effect of solar radiation that does

enter

– Use thermal mass to absorb the energy and

night cooling to remove the energy

– If there is insufficient thermal mass, the air

temp will rise so increase the ventilation rate

to remove the warm air

Benefits of shading

• Shading reduces cooling load – Prevents solar transmittance through windows.

– Shading of un-insulated opaque structures can significantly reduce surface temperature and heat transfer through the structure.

• Shading can reduce glare – Shutters and blinds can reflect light up onto white

ceilings which provides diffuse uniform lighting deeper into the room. Direct sunlight on surfaces produces glare. Dark areas coupled with areas of glare tends to result in people turning on lights to achieve a more uniform distribution of light.

• Shading and comfort – Sitting in strong direct sunlight causes thermal

discomfort.

• Shading systems

– Urban design

• Shading by proximity of neighbouring buildings

• Narrow streets may lead to reduced ventilation on

calm, warm days

• Sloping land

– Vegetation

• Strategic planting of tress and shrubs. Trees also

shade opaque surfaces.

• Pergolas and vines

– External devices • Typical examples

– Overhangs

– (tend not to give good protection against low angle sunlight for east and west facades)

– Awnings

– Shutters

– (horizontal slats block direct and diffuse sunlight but allows reflected sunlight to be directed into the room. Also allows ventilation)

• Very effective at reducing heat gain because they intercept the solar radiation before it enters the room space

• Can be difficult to maintain

• Impact on aesthetics of the building

– Internal

• Typical example

– Roller or venetian blinds

– Curtains

• Not as good as external devices because energy

has already entered the room

• Very good at controlling glare

Thermal Mass and Ventilation

– High thermal mass elements can absorb thermal

energy.

– This thermal energy can be in the form of solar

radiation or elevated internal air temperatures due to

warm summer air entering the dwelling.

– During the day increasing the ventilation rate will

produce a cooling effect on occupants due to

increased air flow.

– At night ventilation will be required to circulate cool

night air over the thermally massive elements to help

dissipate the stored heat from the structure.

The following have a number of photographs of shading systems taken from the Energy Savings Trust publication “Reducing overheating – a designers guide”

27

Movable shading showing % of solar

radiation transmitted

Analysis of the performance of blinds and

shutters

– European Solar Shading Organisation

commissioned a report on the effects of

blinds/shutters.

– It considered various combinations of building

type, building orientation, glazing type and

shutter position for four locations: Rome,

Stockholm, Brussels and Budapest.

Conclusions of the report:

• For small cooling loads blinds and shutters can

make active cooling systems superfluous

• Shutters can contribute to a decrease of the

heating demand by up to 10%

• Shutters can contribute to a substantial

decrease in the cooling energy demand (16 to

97%) in the modelled scenarios

• External & internal shutters have the same effect

on reducing heat energy demand

• External blinds are more effective for reducing

cooling load demand.

Summary:

– Countries in the northern hemisphere are

affected by solar gain primarily on east, south

and west facades.

– Modern air tight, highly insulated buildings will

be prone to overheating due to solar gain

unless this risk is addressed at design stage.

– Effects of solar gain can be minimised by

attention to such things as building position

and orientation, shading affects of trees and

shrubs, overhangs, awnings, shutters, blinds,

thermal mass and ventilation.

Suggested reading:

CE129 “Reducing overheating – a designers guide”,

Energy Savings Trust, www.est.org.uk/bestpractice

CE66 “Windows for New and Existing Housing”, Energy

Savings Trust, www.est.org.uk/bestpractice

“Energy Saving and CO3 Reduction Potential from Solar

Shading Systems and Shutters in the EU-25 ,

(ESCORP-EU25)”, http://www.es-so.com