Effect of reflectivity on building energy consumption

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Hi Nimesh,

You haven't fully described your goal/purposes for this case study... but here are a few thoughts that might help you and others performing similar case studies:

1. If you wish to compare 3 alternatives for exterior envelope materials/finishes, representing a wide range of absorptance values, your study may be more accurate to first define those material options (i.e. brick vs. glass vs. aluminum...) and then consider modeling distinct emissivity values for each. Emissivity is a surface property for eQuest (separate from construction). Suggested reading attached - check out the different discussions for interior & exterior emissivity for examples. There's also a large table of Absorptance values for different materials you may find helpful for your present study.

2. You described your model as a "simple room," suggesting this is a more abstract study & not specific to an actual project. From experience, selecting a "consumption savings" metric for your results, while perhaps easiest for you initially, can be misleading for such abstract cases and cause you more work in presentation. If such results are extrapolated or assumed to apply directly to a specific building, with different system or different physical proportions (window/wall/roof area ratios), there's a good chance the results of a study of this nature can mislead. The general caution here is to consider your audience first (how do they want to interpret/use this information?), and present your results in a fashion that clearly explains what the results do and do not represent. Choosing to report with a different metric, perhaps specific to envelope load reductions, might make that task easier.

3. Regarding installed interior lighting: thermal envelope performance and lighting design are two separate topics that require distinct tools/software/approaches to analyze. If you wish to evaluate the effects of interior finish reflectances (if any) on installed wattage, you are better off engaging a lighting designer or undertaking your own photometric study to produce those results. eQuest can of course incorporate such results after the fact if desirable. This particular query is inherently space/project specific however so it may not be appropriate to incorporate into a purposefully abstract study. In short, for an abstract model that's not intended to be project-specific, you are probably better off holding installed watts constant for lighting. Otherwise, incorporating the effects on installed watts should involve a separate evaluation to determine what extent the finishes will affect installed watts. It may be helpful to note eQuest has capacity to model daylight harvesting system behavior for simple space/fenestration configurations provided sufficient inputs (GIGO). Interior reflectances have a more direct effect on such systems, but that's also a project-specific study unto itself.

Ultimately the model is in the hands of the modeler, and these are your considerations & decisions to make. I hope this provides some food for thought!

[cid:489575314 at 22072009-0ABB]


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