WUFI's equations
Posted: Wed Feb 25, 2015 10:52 am -1100
Dear Sirs,
I have one question regarding WUFI's method. I understoond that the main equation of the dynamic simulation is an enthalpy balance, with two different components: the first is the Fourier equation plus the vapour's diffusion in the building's component (dependent on the water's evaporation enthalpy). So, when I perform a "Only thermal calculation" I am just considering the Fourier equation (so no evaporation/condensation phenomenons); instead, the "complete dynamic simulation" adds the second member of the enthalpy conservation.The thermal calculation can be compared to what Trnsys/Energy Plus do, which is good if you are not considering the humidity fluctuations and you just use an average value for the inside.
Now, when I perform an hygrothermal simulation my heating/cooling demands grow, because of the latent heat. I've observed that my heating demand is 580 kWh bigger then the thermal calculated one and my cooling demand just 180 kWh. So 400 kWh are missing:
- considering that the demands represent (I think) the heat I should give to/take from my envelope to balance the heat that has been taken by water for its enthalpy variation (more enthalpy=the water frees energy VS less enthalpy=water absorbs energy)
- considering that WUFI is solving the differential equations in fixed points of the building's components (the grid),
is it correct to think that the missing heat is due to vapour/water migration from the structure components to the boundary conditions (outer space/internal space)?
In other words, considering an hermetic space, if 100 kg of water evaporate 100 kg should condense (for mass conservation), and this corresponds to equal latent heat flows (their sum is zero). But if I have an unsealed space (as an external wall), I could have mass migration, so the "latent balance" is not achieved. Am I thinking correctly?
I have one question regarding WUFI's method. I understoond that the main equation of the dynamic simulation is an enthalpy balance, with two different components: the first is the Fourier equation plus the vapour's diffusion in the building's component (dependent on the water's evaporation enthalpy). So, when I perform a "Only thermal calculation" I am just considering the Fourier equation (so no evaporation/condensation phenomenons); instead, the "complete dynamic simulation" adds the second member of the enthalpy conservation.The thermal calculation can be compared to what Trnsys/Energy Plus do, which is good if you are not considering the humidity fluctuations and you just use an average value for the inside.
Now, when I perform an hygrothermal simulation my heating/cooling demands grow, because of the latent heat. I've observed that my heating demand is 580 kWh bigger then the thermal calculated one and my cooling demand just 180 kWh. So 400 kWh are missing:
- considering that the demands represent (I think) the heat I should give to/take from my envelope to balance the heat that has been taken by water for its enthalpy variation (more enthalpy=the water frees energy VS less enthalpy=water absorbs energy)
- considering that WUFI is solving the differential equations in fixed points of the building's components (the grid),
is it correct to think that the missing heat is due to vapour/water migration from the structure components to the boundary conditions (outer space/internal space)?
In other words, considering an hermetic space, if 100 kg of water evaporate 100 kg should condense (for mass conservation), and this corresponds to equal latent heat flows (their sum is zero). But if I have an unsealed space (as an external wall), I could have mass migration, so the "latent balance" is not achieved. Am I thinking correctly?