Introduction to the thermal-electrical analogy. PDF slides
Some examples on how to use the thermal-electrical analogy to model a variety of transfer phenomena. PDF slides
The heat flux per square meter of a wall is proportional to the temperature difference across the wall , and to its transmittance [W/(m.K)] :
The unit of this heat flux is [W/m]. The total heat flowing across a wall of area , in [W], is then:
We can also define a global heat loss coefficient such as [W/K]
The thermal resistance of a component of thickness and heat conductivity is:
A wall made of several material layers behaves like a series circuit of resistances: their values add up.
A building envelope made of several separated parts (opaque wall, windows, roof…) can be modelled as a parallel circuit of resistances: heat flows through each surface (m) simultaneously with more or less intensity. The heat loss coefficients of each surface add up:
|Thermal resistance||(m.K)/W||can be summed in series|
|Heat loss coefficient||W/K||can be summed in parallel|
Cette méthode peut être appliquée pour représenter les transferts à l’échelle de tout un bâtiment, y compris en incluant des transferts par renouvellement d’air (voir vidéo 2).
Calculate the heat loss coefficient of an insulated room.
The walls of a room are made of:
- 44 m of concrete wall ( cm, W/(m.K)) with an insulation layer ( cm, W/(m.K))
- 8 m of double glazing ( W/(m.K))
- The indoor heat transfer coefficient is (m.K)/W, the outdoor one is (m.K)/W
The room is ventilated with an air renewal rate of 9 m/h.
Calculate the heating power that should be prescribed to maintain an indoor temperature of 19°C, if the outdoor temperature is 2°C.
The total heat loss of the room is the sum of three parts: heat flux through the concrete+insulation wall, heat flux through the windows and air renewal.
1. Insulated concrete wall
The thermal resistance is the sum of each layer’s resistance, and the surface resistances:
The heat loss is the reciprocal of this total resistance, times the area of the wall:
The transmittance that describes the glazing probably doesn’t include the heat transfer coefficients and . They should be included in the equation of the total glazing heat loss coefficient:
3. Air renewal
If the air renewal rate is given in the unit (m/h), the heat loss associated to it can be calculated easily:
The total heating power lost by the room is the sum of these three types of heat loss, times the indoor-outdoor temperature difference: