Thermal Analysis in dynamic response - Basic workflow for massive building in mediterrean climate

Thermal analysis basically means using a manual calculation or computer program to mathematically model the interplay of thermal processes within a building. There are a wide range of mathematical models used for this purpose, all of which vary significantly in both ease of implementation and comprehensiveness. Whilst none of the available models fully represent all aspects of the real world situation, there are persuasive arguments that they do not need to:

Simulation is the process of developing a simplified model of a complex system and using the model to analyse and predict the behaviour of the original system. Why simulate? The key reasons are that real-life systems are often difficult or impossible to analyse in all their complexity, and it is usually unnecessary to do so anyway. By carefully extracting from the real system the elements relevant to the stated requirements and ignoring the relatively insignificant ones (which is not as easy as it sounds), it is generally possible to develop a model that can be used to predict the behaviour of the real system accurately.

Aburdene, M.F. Computer Simulation of Dynamic Systems

In order to obtain this kind of analysis I’ve to choose an appropriate tool.

OpenStudio

“OpenStudio is a cross-platform (Windows, Mac, and Linux) collection of software tools to support whole building energy modeling using EnergyPlus and advanced daylight analysis using Radiance.  OpenStudio is an open source project to facilitate community development, extension, and private sector adoption. OpenStudio includes graphical interfaces along with a Software Development Kit (SDK).”

( http://openstudio.nrel.gov/ )

EnergyPlus is the merger of two older and well established tools, DOE2 and Blast. Basically it performs hourly simulations of buildings, air handling systems and central plant equipment to provide mechanical, environmental and architectural engineers with accurate estimates of a building's energy needs. The zone models of BLAST (Building Loads Analysis and System Thermodynamics), which are based on the fundamental heat balance method, are the industry standard for heating and cooling load calculations. BLAST output may be utilised in conjunction with the LCCID (Life Cycle Cost in Design) program to perform an economic analysis of the building/system/plant design. Developed by the US Department of Energy.

For more information about thermal analysis and EnergyPlus calculation see links below:

http://wiki.naturalfrequency.com/wiki/Thermal_Analysis_Methods

http://apps1.eere.energy.gov/buildings/energyplus/energyplus_testing.cfm

http://apps1.eere.energy.gov/buildings/energyplus/energyplus_research.cfm

We have to understand if Openstudio it’s the right tool for our purpose. We need to know if the software take in account the contribution of a mass layer (internal side of the wall) to reduce the use of active air-condition system.

First of all, I have modeled a simple unit, that i’ve called “Learning Cube”. It’s composed by 4 face with a shaded window in the south one and a unshaded window in the north face.

Let's start from the beginning. Create with Sketchup tool the plan of our cube.

Then select the geometry and use the Openstudio Create space from diagrahm to extrude the volume.

and set the high (unfortunately in inch, 10’ it’s ok)

by double clicking the volume we can edit it.

To create windows we’ve two ways. By drawing, with SketchUp tools or by applying a glazing % on the selected wall. Using the SketchUp tools it’s a better idea to have more control on the geometry.

I create two windows, one on the south wall and the other one on the north wall, to study after the different effect of the solar radiation on the glazed surface.

The advantage of using Openstudio consists in being able to model the geometries of the shading systems faithfully , instead of applying a percentage to the glass. A next step will be to see if the shading of windows, if the form is changed , produces real differences in the results.

This helps us to find out if solar gains are reliable and how they affect the temperature of the air and surfaces. We also need to check if the program takes into account the variations of air temperature and surface temperature in different parts of the room or if it's just an average .

We know that Transys instead calculates an average surface temperatures and then gives as homogeneous throughout the thermal zone.This behavior is not the real one. It would be interesting to find out if, with the use of Openstudio however, we can document the differences in air temperature between different parts of the room, and the differences in surface temperatures between different surfaces.

This would be useful for us to document the influence of internal mass , not only for the purpose of operating temperature, but also the feeling of comfort due to the surface temperature.

To model the shading device, first of all, we have to select the Add Shading Device tool.

And click in a corner of the window. Will appear blue semi transparent cube on which we have to right-click and select edit group.

Now we can model the shading device geometry with SketchUp tool. Double-click twice out of workspace to exit the editing of the group.

Ilaria Montella, Vincenzo Panasiti