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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
Louvers on a roof
Solar PV farm

Passive temperature control in buildings

To minimise energy consumption in our buildings we should make the greatest possible use of passive, rather than active, control of temperature as far as possible.

In particular the passive cooling of buildings by nocturnal radiation has great unused potential.

Heating and cooling by controlling solar radiation onto the roof during the day and radiation from the roof at night are the main subjects of this page.

Written 2011/01/19, modified 2018/09/30
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 rock for unconventional gas 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.
 

Cool morning air in valleys

You might have noticed that in the early morning temperatures in valley bottoms, following a calm clear night, are noticeably cooler than higher up nearby slopes. This is also due to nocturnal radiative cooling.

On a clear, calm night heat radiates from the surface of the land into space, cooling the land. The cool air is heavier than warm air, so it flows down-slope to the nearest low ground, building up in the valleys.

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.






Louvers on a roof for controlled passive heating and cooling

A black roof, perhaps concrete slab, could be combined with white insulated louvers to make very effective use of controlled passive heating and cooling of a building. The louvers would rotate to allow maximum sunshine onto the black roof when solar heating is required, closed when insulation is required and rotated to the vertical position when maximum cooling by nocturnal radiation is needed. The concrete slab would serve as a thermal bank.

 

Latitude, orientation, building height

The proportion of sunlight impinging on the walls rather than the roof of a building will depend mainly on three things:
  1. The latitude of the location, which correlates with the height of the Sun above the horizon during the day;
  2. Whether the building is single story or multiple story;
  3. The orientation of the longer dimension of the building, whether facing toward the sun in the middle of the day or some other direction.

Solar PV electricity generation

A controlled passive temperature regulation system such as the one envisioned here would clash with the generation of electricity using a solar PV system. One possible way around this would be having solar PV surfaces on both the louvers and the slab beneath the louvers, but obviously this would increase costs, complexity of design and impact the efficiency of the passive heating and cooling system.
The louvers would need to be as thin as possible consistent with a sufficiently high level of insulation to maximise sunlight reaching the slab in the heating phase. There would be a compromise between:

  • Maximising the insulation of the louvers;
  • Minimising their number so as to minimise shading when they were in the open position;
  • Minimising their thickness;
  • Minimising the height of the louvers when in the fully open position (to minimise wind stress and optimise visual aesthetics).
If the louvers ran in an east-west direction across the roof they would not need to have their angle changed to follow the Sun's movement during the day. The angle would need to be adjusted to allow for the Sun's movement north and south during the year.

The angle of the louvers would need to be computer controlled based on variables including:

  • Ambient outside air temperature;
  • Slab temperature;
  • Amount of solar radiation;
  • Angle of the Sun at the zenith;
  • Whether heating or cooling was required.

The cooling of the space beneath the slab could take place by natural convection, but heating would probably require forced air circulation.

Challenges:

  1. Achieving a good seal when the louvers were closed would be of critical importance; unwanted leakage of air from beneath the louvers to the outside environment would greatly compromise the effectiveness of the system;
  2. Accurate and reliable automatic control of the angle of the louvers would be important.
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What effect might a solar PV farm have on temperatures?

On the morning of 2018/02/06 I chatted with a bloke in The Bakehouse, Clare, where my wife and I were having coffee. He was concerned that a proposed 1000 acre solar PV farm would lower the temperature causing more frosts on his vineyard, which is adjacent.

The vineyard owner's reasoning was that the solar farm would absorb energy, convert it to electricity, and send it off. Less, energy, lower temperatures; makes sense.

So I looked up the relevant research:

  • A paper titled, "Larger solar power plants increase local temperatures", written by Greg A. Barron-Gafford, Rebecca L. Minor, Nathan A. Allen, Alex D. Cronin, Adria E. Brooks and Mitchell A. Pavao-ZuckermanNature;
  • A paper titled "Researchers discover solar heat island effect caused by large-scale solar power plants", written by Graham Binder and published in Phys.org.
Both the above papers sugested that a large-scale solar PV farm would increase the local temperature.

It seems that because solar PV panels are much darker in colour than farmland covered with dry grass they absorb much more of the energy from the sun and get hot during the day, while the pale dry grass reflects the light and heat back out into space.

However, this might not be the whole story. Dark things are better radiators than are light-coloured things – so the solar panels will be more efficient at radiating heat into space overnight than will soil covered with pale coloured dry grass. Of course the panels will cool quickly, while the soil beneath the grass will take much longer to cool. Will the increased rate of radiation from the solar panels lower the night-time temperatures making frosts more likely after all?

(As I write, at 1353 hours, 2018/02/06, the official temperature in Clare, South Australia is 35°C; according to my infra-red thermometer the underside of my solar panels are at 70°C, and there is patchy cloud; on a similar day without cloud they would most likely be hotter. There was very little wind at the time.)

Seeming to contradict the two above references, a paper written by Alona Armstrong, Nicholas J Ostle1 and Jeanette Whitaker and published on IOPScience reported an observed cooling of up to 5.2°C during summer in the UK.

Reading all three references suggested, as one might perhaps expect, the situation was not simple enough to say that the solar farm increased or decreased the temperature; it depended also on time of day and time of year, and also on exactly what the solar PV farm was compared to.

(And, of course, vineyards also are likely to have some impact on temperatures, they are dark coloured, so absorb heat. On the other hand, they transpire water, so cooling themselves.)

So what's the answer? I don't know enough to say.






Index

Adding evaporation to increase the cooling
Desirable gain or loss of heat
Introduction
Louvers on a roof for passive temperature control
Movement of heat
Passive cooling by nocturnal radiation
Thermal mass
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Undesirable gain or loss of heat
What effect might a solar PV farm have on temperatures?
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