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Wed Mar 15, 2017 10:47 am

Overview building models
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The main aim of all models is to specify required demand in order to design, size and optimize energy generating/distribution system and not to optimize the building itself.

A number of building models were implemented in Polysun. They differ by complexity and required inputs. A model choice is depend on the case and available input data. Some typical simulation cases are: retrofitting/optimizing of the energy system for the existing building and design of the energy system for a new building

It exist 2 levels of complexity inf Polysun, it means comprehensive or simple building model.
The main difference between this level of complexity is that in the “comprehensive ” (physical) model Polysun based on some physical parameters of the building calculates heating/cooling demand which shall be covered by the system. In the “simple” model building demand is defined by the user. In figure 1 it is shown how models can be switched in the building window. If “Energy demand known?” is set to “No” then the physical model is chosen.

Physical Model
The physical model is the most complex building model. It takes in consideration many building characteristics: size, UA-value (heat loss – building enclosure area produkt), solar transmittance of windows, building orientation, ventilation and infiltration losses, gains from equipment, lighting and people, window shadings, heat recovery of ventilation and thermal mass of the building. These data allow to build heat balance of the building each timestep to find instantaneous demand as difference between total losses and total gains
Phys. 2.png
Choice of model (simple, comprehesive)
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In the building heating/cooling balance losses from heat generating/distributing equipment can be counted. For each thermal component (e.g. storage tank) place of installation can be chosen. The user can chose between ambient environment, heated area (building itself) and unheated area (a part of the building without set point temperature requirements, e.g. garage or storage room). If the unheated area (see figure 1) is chosen, the user can define which part of heat losses goes to the building.

User-defined demand
Often in the practise building demand is known before designing or opimpizing energy supply system. It can be estimated based on standards and norms, own experience or measured data. There are three building models in Polysun which can be set with demand values, namely quasi-dynamic, degree-hours and based on hourly energy demand inputs (see figure 2). The models differ by required inputs and demand distribution through the year.
figure 2 Building.png
Figure 2: choise of simple model in Polysun GUI
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Quasi-dynamic and degree-hour models have four different options how building demand can set: annual values, based on fuel consumption, maximum power demand and monthly values (see figure 3). Moreover some of these input types have sub-types depending on the model choice. The degree-hour model enables laso cooling demand calculation. The main difference between quasi-dynamic and degree-hour models is that the quasi-dynamic model counts some dynamical effect (such as instantaneous solar gains) while distributing annual or monthly values to hourly values. In the same time the degree-hour model obtains hourly value based on so called “balance” temperature of the building.
figure 3.png
Choice of demand definition for degree hours model in GUI
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figure 4; hourly demand input.png
Hourly energy demand input
figure 4; hourly demand input.png (66.72 KiB) Viewed 26218 times

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