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Heat absorption and transfer - page 1

Choosing highly reflective building material is just one factor in the process of passive cooling. No surface has an 100% albedo and some of the sun’s radiation will get past this first line of defence and will heat up the roof, the bricks, the concrete path etc. To make matters worse, even highly reflective material can store heat more effectively than a dull object can.

To give you an example, let’s assume you had a barbecue yesterday and right now you are in the process of cleaning up the mess. On the lawn – in the sun - you find the missing steak knife. Despite being highly reflective, the blade of the knife is very hot. Ouch! the wooden handle, however’ as you would expect, and despite being dull and less reflective, is not hot at all. From painful experiences as a child, you know that you should pick up the knife at its wooden part.

This example tells you that different substances have varying capacities of storing and transferring heat. Scientists found many ways to express the thermal properties of substances. Formulas may describe the energy or time it takes for a substance to heat up, whereas others identify the time it takes for heat to travel a certain distance or to dissipate.

Two of these thermal properties are probably of most interest to you: the heat capacity, “…ratio of heat absorbed by a material to the temperature change,” and the thermal conductivity, “…transfer of energy (heat) arising from temperature differences,” as defined by the Encyclopaedia Britannica.

The definition for heat capacity is too general to be of any use. So, the scientists came up with a more specific solution and aptly named it the specific heat capacity or specific heat in short. It is the term for the amount of heat it takes to raise the temperature of a substance by one degree Celsius.

From the ‘barbecue’ example, we can safely assume, that the metal part of the steak knife requires less calories to heat up than the wood handle. Both received, however, the same amount of energy from the sun for the same time. So, we can further argue that the same amount of calories heated the metal blade much more rapidly and to a higher temperature than the handle. Since the specific heat of steel is about four times less than that of wood, the temperature of the steel blade will be approximately four times as high. No surprises here.

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Global Bioweather

Heat absorption and transfer - page 1

Heat Island

Urban