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close this book National design handbook prototype on passive solar heating and natural cooling of buildings
close this folder V. Basic design principles and strategies
View the document A. Climates
View the document B. The sun's movement
View the document C. Orientation for solar access
View the document D. What is solar access?
View the document E. Solar energy collection
View the document F. Energy storage (heat)
View the document G. Heat retention
View the document H. Heat distribution
View the document I. Passive solar heating strategies
View the document J. Natural cooling strategies

H. Heat distribution

7. Thermo-circulation

As air is heated it becomes less dense and floats upward to be replaced by cooler air. Research by Balcomb and others has demonstrated that this effect can generate considerable energy flows from a double height sunspace into a two-storey building behind. This heat distribution effect is due to the relatively high temperatures achieved at the lower levels of a well-designed sunspace. In taller buildings the flow will be even greater. The flow due to the vertical distance between openings can be calculated and is described later in chapter VII.

2. Mechanical circulation

Whilst natural air movement Is desirable it is often more effective to use mechanical means to circulate warm air (air + energy) from one place of collection to the place needed. When used to move air at low flow rates, fans can be very effective and economical to operate.

Simple exhaust fans (with or without ducting depending on the application) can be used to move warm air that collects at the top of high volumes to occupied spaces at lower levels. They can be used to move warmed air from a sunspace to a cooler non-sunlit space behind.

The rate at which air moves with mechanical devices depends primarily on the fan design and the power of the motor. Such information can be found in design guides for air handling equipment or sometimes In the manufacturers literature.

Worked example No.2

To calculate the daily heat gains or losses through 1 m of north-facing window In Sydney during July, select the following information from the climate section:

Mean daily ambient temperature

To = 11.7°C

Assume mean internal temperature = 21°C

Solar heat gain (SHGF) = 1 2.2MJ/m² (from Sydney insolation tables)

To calculate or select the U-value for glass = 6 W/m².degC: assume glass 3mm clear shading coefficient A = 1.00

To calculate daily heat loss by conduction:

H

= A.U (Ti-Ta) x 24 x 3.6 x 10-3

 

= 1 x 6 x (2.72) x 24 x 3.6 x 10-3

 

= 4.8 MJ/m2.day

To calculate daily heat gain by radiation:

H

= SHGFx SC

 

= 12.2x 1.00

 

= 12.2 MJ/m2.day

The total daily heat gain as a result of conduction and radiation exchanges is 12.2MJ radiant gain less 4.8 MJ conductive loss/m².day

= 7.4 MJ gain per m of window per day.

If curtains are drawn at night then it is possible to use a modified U-value (Um). Assume curtains to be lightweight with restricted air circulation. Refer to section H.1 above. The modifier for light drapes with restricted air circulation is M = 0.6. Assume also that curtains are closed for 13 hours per day (say 6 p.m. to 7 a.m.).

Therefore:

Um

= U/24 x ((M x Hd) + 24 - Hd)

 

= 6/24 x ((Q.6 x 13) + 24 - 13)

 

= 4.7 W/m2.degC.

The revised daily heat loss by conduction is therefore:

H;

= A.Um (Ti - Ta) x 24 x 3.6 x 10-3

 

= 1 x 4.7 x 9.3 x 24 x 3.6 x 10-3

 

= 3.8 MJ/m2.day

The revised total daily heat gain is now (12.2 - 3.8)

= 8.4 MJ per m² of window per day.

Now consider windows on other orientations and see why the north windows are so important for winter heating.