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Introduction
Movement of heat
Undesirable gain or loss of heat
Desirable gain or loss of heat
Passive cooling by nocturnal radiation

Passive temperature control in buildings

To minimise energy consumption buildings should use passive, rather than active, control of temperature as far as possible.

Written 2011/01/19, modified 2017/02/11
Contact: email daveclarkecb@yahoo.com
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Introduction

In an age when we are running out of conventional, easy to get, petroleum, when the dangers of fracking are becoming clearer, and when the importance of minimising our climage changing and ocean acidifying greenhouse gas production is obvious to all but the stupid or intentionally blind, minimising our wastage of energy is all-important.

Passive temperature control uses the environment and the properties of the building to control temperature within the building. Active temperature control uses energy consuming methods such as heating by burning fossil fuels and cooling with air conditioning to control the tempertures in the building. Passive temperature control requires thought and an understanding of the science involved, but uses little energy. While to fully utilize passive temperature control extra effort at the construction stage may be needed, some can be achieved just by minor changes to the running of a house and more by making relatively minor changes to a home.

 

Thermal mass

Substances vary in the amount of heat needed to change their temperatures. In general, heavy (massive) things like stone, bricks and steel have high thermal mass, it takes a lot of heat to raise the temperature of a stone wall for example. On the other hand, once warm, a stone wall can provide heat to slow the cooling of a room for a long time. But weight-for-weight, very few substances have a thermal mass as great as that of water.
There are several fundamental features of passive temperature control:

Insulation;
Insulation limits the amount of heat that unintentionally enters or leaves the building by conduction.

Thermal mass;
Thermal mass provides some stability to the temperature in the building.

Controllable ventilation;
If ventilation can be controlled then it may be possible to use the cool of the night to reduce the temperature in the building, or the warmth of the day to increase the temperature, whenever outside temperatures are suitable. Undesirable ventilation from gaps needs to be minimised.

Control of the entry of sunlight.
Stopping sunlight from getting into the building in summer, but allowed to enter in winter (supposing that the sun is shining), can give added control of temperatures without using energy.

Using radiation
Radiation moves heat from one place to another. Control of when and where radiative heat transfer takes place can be used to our advantage.
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Movement of heat

 
Heat map of room
 
Thanks to Australian Consumer's Association (Choice) for this illustration. It represents a room in cross section.

"This heat map of a test room clearly shows the stratification effect created by a convection heater when there's little air movement in the room: the yellow bar at the ceiling represents about 22°C, the purple bit (where your cold feet would be) about 14°C."

It is important to consider where the heat from a heater will go; warm air rises, cold air falls.

It will also help you to understand any sort of temperature control if you can get a grasp of how heat can move from place to place. There are three main ways in which heat can be transferred from place to place:
Conduction;
When you touch a hot pot on a stove top heat is transferred to your hand by conduction.

Convection;
A fluid (liquid or gas) that is warmed will (in almost all cases) expand and become less dense than the surrounding fluid; it will then rise while the surrounding fluid falls to take its place. Applying heat to the bottom of a pot warms all the water in the pot much more effectively than by applying heat to the top of the pot would.

Radiation;
The heat that you feal on your skin when you expose it to direct sunlight is moving by radiation. Radiation is most important when the radiating body is very hot (a red-hot radiator-type of room heater operates at about 900°C, the surface of the Sun is about 5500°C); although it can still be significant if the radiating body is large (for example, the ceiling of a room), but only a few degrees above the temperature in the room. It is the only way that heat can move through the vacuum of space.
Heat can also be transferred from a fluid that is forced to flow past an object (for example, from air that blows through an open window, or from heated water that is pumped from a boiler in a central heating system).

Undesirable gain or loss of heat

 
Blinds reduce heat gain or loss
Blind
This (Luxiflex) blind reduces heat transfer by radiation (the sun is shining on its outside) and it contains air 'pockets' that are effective in reducing heat transfer by convection.
Unwanted heat may get into a building by:
Sun shining through windows; or onto blinds, warming the blinds, then heating the room by convection;
This can be minimised by:
  • Ideally, stopping the sun shining on the window (by use of a shutter or similar on the outside of the building);
  • Stopping the heat that gets through the window from getting into the room by the use of blinds or curtains.

Pasing through walls, roofs or floors by conduction; the unwanted heat that is conducted through the ceiling will then get to the rest of the room by convection and radiation.
This can be minimised by the use of insulation.

Coming in with hot air that blows through gaps under doors etc.
This can be minimised by blocking the gaps.
Heat may be lost from a building mostly by conduction through walls, floors and roofs by conduction and by air that blows through gaps; less is lost by radiation through windows because the radiating bodies are not greatly warmer than the objects outside the windows.

Desirable gain or loss of heat

Entry of sunlight into a building can be controlled by various means. Cooling or warming air can be allowed, when desired, to enter through open windows on one side of a buiding and out on one of the other sides; having open windows on two or more sides is much more effective than having several windows on one side.
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Passive cooling by nocturnal radiation
Radiative cooling

Temperatures fall at night, but in the height of summer in my location even minimum overnight air temperatures can be too warm to be comfortable.

 

Temperature of what?

The word temperature is often used losely. Most of the time what we measure is the temperature of our thermometer. If the thermometer is carefully placed in a well ventilated place out of direct sunlight and protected from other major sources of thermal radiation the temperature of the thermometer can be close to the air temperature, which is what we are aiming to measure when we want to talk about things like "how hot is it today?". If we use an infra-red thermometer we can, to some extent, measure the temperature of whatever we are pointing the thermometer at.
 
The slab of rock
Slab (and dog)
Socrates kindly offered to stand there to give the scale.
But it is possible to passively cool things to temperatures substantially below air temperature. For example, yesterday I placed a slab of rock 380mm × 380mm × 100mm thick weighing about 35kg on top of three smaller stones so that there was an air-gap between the slab and the soil. This morning, around sunrise, I measured the air temperature as 23°C and the temperature of the top face of the slab as 15°.

How did this happen? At night, especially when there is a clear sky (as there usually is in the summer where I live) heat is radiated away into space. A slab of rock is quite a good radiator, and can lose heat by radiation faster than the surrounding air can warm it. (The bottom of the slab, which received radiation from the soil below, was at 16°.)

How can this be used to advantage?

A cellar, or for that matter a room, could be built with a concrete (or stone) slab roof. During the day the roof could be covered with insulation to stop it being warmed by sun light or the warm air; at night the insulation could be moved away so that the slab could radiate heat away into space.

The experiment with the stone slab suggests to me that it would be possible to cool the roofing slab to around seven degrees below the minimum air temperature on a cloudless night. (The roof slab of my cellar is 150mm thick, so would be slower to cool than the 100mm stone slab of the experiment; on the other hand, the air beneath the roof slab would probably be cooler than the air beneath the stone slab.)

How would you make the insulated covering easily moved? Perhaps it could be made into a rigid 'slab' which could be rolled on or off the cellar on rails? Alternatively the cover could be folded up in the manner of the covers of a ship's hold.

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I don't need this on my cellar; it is sufficiently cool even in the middle of summer (no more than 23°) without this form of additional cooling, but in an area of higher temperatures it could be valuable.

Adding evaporation to increase the cooling

If a shallow layer of water was to be ponded on top of the concrete slab this could increase the cooling effect even more.

The layer of water could be kept topped up using a float valve. It would be necessary to consider the effect of strong winds on the layer of water; perhaps a single layer of gravel could be used to stop the water from being excessively moved about by strong winds.






Index

Adding evaporation to increase the cooling
Desirable gain or loss of heat
Introduction
Movement of heat
Passive cooling by nocturnal radiation
Thermal mass
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Undesirable gain or loss of heat
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