Calculation of CO2 concentration

Since people and ventilation rate are already entered in the usage profiles for the thermal simulation, the CO2 content of the indoor air can be calculated without any further input.

The only sink is the ventilation. Outdoor air is assumed to be a constant 415ppm CO2 (about average 2020, increasing by over 2ppm per year). Indoor plants are negligible - they absorb small amounts of CO2 during the day and release it at night, only during growth the uptake is predominant.

The only assumed source are the people resulting from the usage profiles. Pets or indoor burning (candles, gas stove) would have to be entered as people to account for both heat and CO2.

For the heat and moisture emission of persons, we use the data from VDI 2078, which divides the energy turnover from the activity levels of DIN EN 13779 (I:100W, II:125W, III:170W, IV: 210W) into sensible and latent heat, depending on the temperature. The CO2 emission is proportional to the energy conversion. As a proportionality factor, we use 240g of CO2 per kWh (at a basal metabolic rate of 80W, a person at complete rest would thus produce 168kg of CO2 per year). Depending on the activity level, a person thus emits 24 (at I), 30 (II), 40.8 (III), or 50.4 (IV) gCO2/h.

The change of CO2 concentration dc [g/m³] results from source and sink, the mass balance is V*dc=dt* (P - L * (c - c_out) ) with the indoor air volume V [m³], the input by persons P [g/h] and the ventilation volume flow L [m³/h] and the time step dt [h].

The average CO2 concentration in the time step is c = c0 + dc = c0 + dt/V*(P - L * (c - c_out) ), resolved to c thus

c = ( c0 + dt/V * (P + L * c_out) ) / (1+dt/V*L)

The conversion from g/m³ to ppm (i.e. CO2 molecules per million air molecules) is done with constant 1ppm=1.83mgCO2/m³, which approximately holds for air density at 20°C and 1bar. At lower air density, the CO2 content would actually be somewhat higher.