Look at this fundamental equation again, that is the rate of energy consumption due to sensible heating/cooling of atmospheric air used a space conditioning media!
It’s a POWER term which is the rate of energy consumption which inherently excludes the time dimension, this is instantaneous power demand from a space conditioning system to heat or cool the outdoor air to bring it to a desired temperature (to be supplied to indoors without compromising the thermal comfort of the occupants).
Qs is dependent on the “Q” term: air flow rate so it is also connected to the amount (volume) of outdoor air accepted to indoors in unit time (m^3/sec). Correlated to the indoor air quality expectations.
Qs is dependent on the “Rho” term which is the air density (kg/m^3). Air is a “gas” therefore its density is also highly affected by its temperature and the atmospheric pressure (and the altitude). Therefore, Rho is not a constant term. Calculations would be different for Denver, CO vs. Miami, FL.
Qs is dependent on the “cp” term. The specific heat of air: how much energy (in Joules) is needed to rise the temperature of 1 kilogram of air by 1 degree Celsius. Cp of air is very low (as compared to some other space conditioning media: such a water). Is it good news or bad? Cp also tells you how much energy a unit mass (kg) of air can hold during the transfer (from outdoors to indoors). And very low levels of Cp means, we need (relatively huge amounts/volumes) of air to carry heat or cold from one place to another. That’s why air ducts are much thicker than water pipes.
Qs is also dependent on the Delta T. The temperature difference between outdoors and indoors (or air intake and air supply or it can be the difference between supply air and return air). Is temps are the same (~deltaT = 0), there’ll be no thermodynamic process. Qs = 0 W and your job will be done here! However, there’ll always be DeltaTs (To-Ti). If To > Ti, then delta T >0 or if To<Ti then delta T < 0. This means that Qs can be positive or negative depending on your viewpoint these plus/minus signs can signify a heating or a cooling load. It turns out that as we expect a uniform and stable indoor temps Ti won’t change a lot over the seasons but To is highly dynamic and it depends on the characteristics of a certain location and climate.
Over which terms of the Qs equation, the human dimension of building performance modeling and simulation can manifest itself? The answer is “Q” (air flow rate) and Delta-T. But how? How can I define these parameters as inputs to a building energy model?
Over which terms of the Qs equation, the climatic characteristics of a building location can manifest themselves? The answer is “Rho”, “Delta-T” (particularly the To). But how? How can I define these parameters as inputs to a building energy model?
Some designers talk about the MASS FLOW RATE? What is that? Mass flow rate (kg/sec) is just the multiplication of “Q” (m^3/sec) and “Rho” (kg/m^3), just look at the units of measurement, things will reveal themselves. Here is an interesting take on this: at high altitudes like Denver, Co, the atmospheric pressure is low (since you climb up to gain elevation and there is lesser amount of atmosphere above your head) and under low pressure a given gas will expand so does the air and when you have unchanged mass occupying a larger volume (kg/m^3), the air density (Rho) will be smaller. First of all, smaller “Rho” will decrease your sensible heating/cooling load (provided that all other terms are unchanged) and similarly, your mass flow rate (kg/m3) will be smaller (since mass flow rate = Rho x Q). In sum, folks living in Denver, Co require fewer air mass to heat up or cool down a building space with the same temperature difference and even with the same outdoor air flow rate requirements. Wait a minute, do I need to update the “Rho” in the simulation model whenever I change the location? How this is handled by the building energy simulation program?
Omer T. Karaguzel, PhD
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