How thermal energy gets outside
Thermal energy travels by convection, radiation and conduction.
To understand the phenomenon of heat transfer (or thermal energy), it is necessary to admit a model of representation of the matter which says that this one is constituted of tiny invisible pieces which moves randomly.
- In the case of solid bodies - like metal, wood, glass or PVC - these pieces move while keeping an average position constant. They are said to viber.
- In the case of gas and liquids, these pieces move, while travelling globally according to a certain direction.
The vibrations of thes microscopic pieces of matter represent the thermal energy of the body in question. The higher the amplitude and the frequency of the vibrations, the more energy the body contains, the hotter it is.
Convection is the displacement of heat by the global displacement of an amount of a hot body. For instance, if you draw a bucket of 10 liter of water at 60°C from you water heater which is supplied with cold water at 10°C, to wash your floor and then you throw this water on your lawn, you lost by convection :
(60-10)×10×1.16=580 Wh, which is about 1/2 kWh of electricity.
Or if you aearte a 30 m3 room and you replace air at 25°C by air at 10°C, you lose by air displacement, thus by convection :(25-10)×0.3×30×1.16=157 Wh , which is about 1/6 kWh .
The coefficient 1.16 which we will meet in the calculations represents the value of the kilocalori in Wh, like 6.55957 reprensents the value of the Euro in frances. It allow to convert heat consumed per hour into paid kWh !!! It is worth : 1000cal×4.18/3600=1.16Wh.
But there is a very more harmful convection : it is the one that occurs on the whole surface of the house which is in contact with the ambient air outdoor, that is to say the whol surface of the outside walls plus the openings giving to the outside.
Thermodinamics courses call α in W/m2/°C the coefficient of transmission of the heat by convection, that is to say between a fixed body and moving matter. It depends on the nature of the surface, the orientation of it and the ambient conditions.
Indeed, heated and so dilated air in contact with some surface became lighter and goes up to be replaced by, say, new air, and hence colder. But that renewal depends on the orientation of the surface and on its state. It also depends on the global movement of the air, that is to say the wind. When there is wind, we take shelter to be less cold.
It is then necessary to bring two corrective coefficients, hence in first approximation :
α=2×(ΔT)1/4×σ×(1+V3/4)
σ is worth about 1.5 for a smooth surface, such as clean sheet metal, and 3 for a rought surface such as stone, concrete or roughcast.If we consider an average house having about 110 m2 of lateral surface and 110 m2 of ceiling, this gives, with a wall at 23°C and an outdoor ambiance at -7°C with no wind :
Q = α×S×(t2-t1) = 2×(30)0.25×3×30×220 = 93 kW
with a good wind, a breeze, of 10 m/s
Q = 614 kW
That is enormous. On the other hand if the surface of the wall is at -6°C we have respectively 1.32 kW and 8.74 kW.
That is still a lot. We observe that in order to avoid loss by convection, the thermal resistance of the wall must be very high so that the temperature of the surface of the wall is practically equal to the temperature of the outside air. Any calorie that arrives at the surface of the wall is irremediably condemnedt to fly off into the atmposhere.
We can realize it when we are shivering of cold with wet clothes. Wet clothes are good conductors of our heat, and the convection of the air grab every calorie. Before the rainstorm, the clothes being dry, we are not cold.
Clothes are like the insulation of the wall, the are the barrier to the loss of heat.
It exists also others loss by convection : these are the leak of air via the joint under the doors, around the doors and the windows, via the air intakes for the essential renewal of the air breathed in the house which must be about 25 m3 per hour and per person. We will come back on this point subsequently.
Loss by radiation are again more impalpable than the loss by convections, because contrarily to the draft under the aerator or close to the switchs, we do not feel them.
Losses by radiation of a body submerged in an ambiance, and small with regards to it, are given by the formula :
q (W/m2) = σ×[(T1/100)4-(T2/100)4] with T in °K
σ is a coefficient which characterizes the material. For stone or concrete, it is worth about 4.If the house, with a surface wall plus ceiling plus floor of S = 330 m2, was alone in the universe this would give :
Q = S×q = 330×4×(296/100)4 = 101 kW
If we admit that the house is on Earth in an ambiance at -7°C with its walls not insulated at 23°C, we obtain, without taking the floor into account :Q = 220×4×[(296/100)4-(266/100)4] = 220×4×(81-53) = 20.8 kW
But the surface of the wall, we saw this, will go down to -6°C. The losses by radiation will then be :Q = 220×4×[(267/100)4-(266/100)4] = 220×4×[(1/100)4(4×2663)]
= 220×4×(1/100)×4×2,663 = 0.662 kW soit 2/3 kW.
(for the pernickety, I apply : a4-b4=(a-b).(a3+a2.b+a.b2+b3)).
Pizzas, bread, are cooked by thermal radiation of heat stored in the vault.....
The last sort of propagation of heat is conduction ou conductivity.
If we consider a rod of metal submerged by one end in ardent ember, the infrared radiation of the ember will excite the atoms of iron which will viber more strongly. The will hit the neighboring atoms which will be excited in turn and so on step by step until the other end. This one will warm up and you will burn yourselves when you will want to grab it.
The denser a body is, that is to say the heavier, the more conductive it is.
The less conductive body is vacuum : its conductance is null, its thermal resistance is infinite. That is why themos bottles from the last century, with double-partition filled with vacuum kept coffee hot for 24 hours, while today's thermos keep it for hardly 3 to 4 hours. This proves that polystyrene is clearly less insulating than vacuum.
Then comes the air, with the condition of being dry and still. What insulates with expanded polystyrene, it is not the polystyrene itself but the bubbles of air that are imprisoned inside. What insulates in a diving suit, it is not the rubber but the bubbles of air that are emprisoned inside. And this is why it is necessary to wear a lead belt to go down to the abyss.
This has been said previously, water is a good conductor of heat, wit a λ=0,65, 15 times more than polystyrene, and also it stores well the heat, that is why it can be used in accumulation water heat.
Any materials used for insulation insulates thanks to bubles of air that they contains.
When you touch polystyrene, it seems warm to you, while it is at ambient temperature, at -15°C if it is in you freezer, because it is a bad conductore and it takes no heat from you. If you touch a stone or a piece of metal in the shadow, it seems cold because, good conductor, it take you much more heat. The feeling of cold is the loss of heat.