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Definition of energy demand in buildings

asked 2015-08-07 03:56:24 -0500

AleMac's avatar

updated 2020-01-25 18:17:34 -0500

Dear all, I have created a very simple ideal source of energy for building energy simulations, and I was now wondering: how would you define the energy required by a building when an ideal source of energy is responsible for heating/cooling of outlet water to reach the inlet water temperature? Basically, this ideal source of energy would just use the following expression: Q=m_dot x cp x (T_in - T_out) for heating Q=m_dot x cp x (T_out - T_in) for cooling

Could I define this as energy demand? Thank you in advance for your attention! Alessandro

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My question refers only to the terminology, not to the modeling itself. What I would like to know is how you would call the energy input for this ideal source of energy. I will try to explain myself with an example. Let's assume I have a thermal zone served by a radiator that has to meet the set-point temperature in the room. The radiatior is connected to a boiler that heat up return water from the room. The energy consumed by the boiler depends by the efficiency of the boiler. If the boiler has efficiency 0.9, a certain amount of energy will be consumed.

AleMac's avatar AleMac  ( 2015-08-08 03:53:02 -0500 )edit

If the efficiency is 0.8, more energy will be consumed. But what if this boiler has an ideal efficiency of 1? This is basically what I modelled in my system. How would you call that energy? Ideal energy consumption?

AleMac's avatar AleMac  ( 2015-08-08 03:54:25 -0500 )edit

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answered 2015-08-07 07:59:36 -0500

updated 2015-08-07 11:15:22 -0500

You describe only 1 form of energy required to condition the interior space of a building, in this case the sensible cooling or heating energy required to maintain a desired indoor temperature (Qhvac, sensible). This equation is used to calculate the HVAC energy. A more accurate equation for space conditioning would be Q_hvac = m_dot x (H_in - H_out) which would include the contribution of moisture in the equation.

A building would also required other forms of energy to provide a source for lighting and equipment. For lighting, this may be entirely provided by daylighting during the day, however, some energy may be required after sundown if the building required lighting. The energy sources for lighting and equipment would eventually result in a change in the requirement for space conditioning compared to a building without lighting and equipment. There may also be a need for heating water. Occupancy will also add energy in the form of heat and moisture. Additionally, if there are openings in the building shell, such as windows, solar gains (Q_solar) would also contribute to the building loads.

The equation shown (Q_source=m_dot x cp x deltaT) is used to calculate the source energy used to sensibly condition a building. However, you only know 3 of the 5 variables in this equation, T_in, T_out, and cp. To identify the fourth variable, modelers typically use Q_shell=UA(T_in-T_out) to define the building shell load. Knowing this value, your equation could then be used to determine the air mass flow rate required to condition the interior space of a building.

Bringing all possible building loads together yields:

Q_load = Q_shell + Q_solar + Q_lighting + Q_equipment + Q_occupancy + Q_waterheating + ?

The energy balance equation would then be:

Q_hvac, total = Q_load = m_dot x (H_in - H_out) for total energy

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Thanks for the answer. I should probably add that I have also implemented two other ideal sources for heating and cooling coil in the AHU. The AHU is connected to two secondary water loops, one for heating and one forr cooling. Therefore, in total, I have three ideal energy sources: 1) water circuit for both sensible heating and cooling 2) heating of air 3) cooling of air. In conclusion, the energy required by these three ideal energy sources to condition outlet water temperature and reach the inlet value can be considered as the total energy demand of the building.

AleMac's avatar AleMac  ( 2015-08-07 08:58:58 -0500 )edit

However, since these three sources are not real machines (e.g. bolier, chiller etc), but are just ideal machines, I was wondering how I could call the required energy PS: at this stage I am not considering electricity for lighting and equipment.

AleMac's avatar AleMac  ( 2015-08-07 08:59:58 -0500 )edit

I'm not clear on what you are asking.

Are you writing software and are wondering how to call the subroutines? (wrong forum) Are you asking how to determine the building load to use in your equation? (see previous answer) What software are you using?

rraustad's avatar rraustad  ( 2015-08-07 11:42:34 -0500 )edit

I will try to be more clear. First of all I am using Dymola (Modelica) as software. My question only refers to the terminology, not about the modeling itself. Imagine that in my model I have a thermal zone served by a radiator that has to meet the set-point temperature in the room. Let's assume that in this simple example there is no any ventilation system. Therefore we only have to meet the sensible heating loads. The radiatior is connected to a boiler. This bolier will use energy to heat up the water, and the energy consumed depends by an effciency curve.

AleMac's avatar AleMac  ( 2015-08-07 14:44:49 -0500 )edit

Usually, in literature, the energy used by the boiler is called "energy consumption". But what if I replace the boiler with an ideal source of energy that just use the expression I showed in my first message (basically a machine with efficiency 1)? In this case it is not really "energy consumption", but I would say it is a kind of "ideal energy consumption". I hope I was more clear.

AleMac's avatar AleMac  ( 2015-08-07 14:47:07 -0500 )edit

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Asked: 2015-08-07 03:56:24 -0500

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Last updated: Aug 07 '15