PsyChart error

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Permalink Reply by Abraham Yezioro on September 7, 2017 at 12:35pm

Thanks Chris,

On the contrary. Thank you for having the patience to deal with this.

I get and accept your explanations. make sense and are clear. I can use your arguments to explain myself.

Another (maybe last) couple of questions.

You have some rule of thumb (or source) for input values for the LB_solarHeatCapacity_? The default values of 50 W/m2 is for a small passive solar heated space (full glazed facade).But i'm changing the default to relatively low values and it is not affecting the results.

I have a feeling that the temperature checking you are doing for the Thermal Mass is a bit tight. Is very hard to improve. Changing the timeConstant_ in the StrategyPar affects a lot the PSH but very little the ThM. The reason i'm asking is because for some locations i know that the thermal mass has a big influence. Though, the chart shows a small one as a total value but also looking at the 3DChart plot.

Thanks again!!

-A.

Permalink Reply by Abraham Yezioro on September 7, 2017 at 1:02pm

Chris,

I think i'll drop my question about the ThermalMass. It is a bit ambiguous, specially looking at various cities results. They can make sense. Just one location makes me wonder (Jerusalem).

-A.

Permalink Reply by Chris Mackey on September 9, 2017 at 8:33pm

Abraham,

I went and checked the thermal mass polygon in a couple of climates where I know it has a big effect and I got results that made sense to me.  Specifically, in the San Fernando Valley of Los Angeles, which has 12% of the year made comfortable, and Shiraz, Iran, which also has 12% comfortable (assuming default parameters).

Jerusalem also makes sense to me.  There is only a maximum possible 9% of the year that is inside the polygon (you'll see this if you set the timeConstant to a very high number).  The default strategyPar makes 6% of these hours comfortable and 3% without cool enough temperatures in the previous hours.  This seems reasonable to me.

I could be convinced to change the default time constant to 12 hours (instead of 8) as I know that 12 is the default of climate consultant but that seemed really idealized in my opinion.  You'll need really high exposed mass and insulation without much internal heat gain to make conditions stable for more than 8 hours in my opinion.

As for the solarHeatCapacity, I get changes when I drop it down to 10 W/m2 or boost it up to 100 W/m2.  It's definitely a parameter that operates on an "order of magnitude" scale and little tweaks to it won't change it too much.  You can think of this number as representative of a lot of other physical properties: most notably the depth of the space being passively heated and the thermal mass of that space's materials that participate in heat exchange over the time constant.  Climate consultant uses a default assumption of 30 W/m2 but, from my calculations, this is likely assuming a space that has a facade to floor area ratio that is greater than 1.  If we say that we need to raise the temperature of 10 cm of an exposed concrete floor for passive heating purposes, and we have a facade-to-floor area ratio of 1:

Required solar flux = ((1 facade-to-floor ratio) x (0.1 m3 of concrete) x (2400 kg/m3 concrete density) x (880 J/kg-K concrete specific heat capacity)) / 3600 seconds/hour

This lands you with a required solar flux of 58 W, which is almost twice the 30 W climate consultant default. While me might say that not all 10 cm of concrete participates over the course of a default 8-hour time constant (most of the action is probably within the first 5 cm), we also have to account for things like transmittance of solar though the window, which, for triple pane, is probably only half of the incident solar.  So 50 W seemed to be a more reasonable rule of thumb from my perspective, essentially assuming a facade-to-floor ratio of roughly 1 with 5 cm of concrete participating in an 8 hour heat exchange and a little more than half of solar heat getting through a fully glazed window.

Let me know if that makes sense or if you have any suggestions,

-Chris

Permalink Reply by Chris Mackey on September 9, 2017 at 8:56pm

Also, I have a small correction to my post. In the formula, I should be dividing by the facade-to-floor area ratio (not multiplying. So a larger facade-to-floor area means a smaller solar requirement.

Permalink Reply by Abraham Yezioro on September 10, 2017 at 1:37am

Hi Chris,

Thanks for  your detailed answer. Clear to me that all these checking are just a first approach to understand the potential of the location.

Your explanation for the time constant makes sense and i get it. If you have time check the attached papers we published a while ago in relation to the contribution of thermal mass in the reduction of temperature in residential buildings. See the nice contribution of the heavy TM or the lower one for light TM.

As for the solarHeatCapacity, your description (of the 50W) is derived on a 1 Facade/Floor ratio and fully glazed. The only way to reduce it is to increase the ratio (bigger facade area). Which is not recommended (energy losses), but this is a different issue. So, roughly, we can say that 50 is the lower value. If i have less glazing area this number will be higher (right?)

I want to define a value list of "architectural situations", so it is easy to explain and understand. One situation can be:

"Ratio facade/floor 1 & Fully glazed" = 50

"Ratio facade/floor 1 & Half glazed" = 75

"Ratio facade/floor 1.5 & Fully glazed" = 30

"Ratio facade/floor 1.5 & Half glazed" = 50

"Ratio facade/floor 0.75 & Fully glazed" = 70

"Ratio facade/floor 0.75 & Half glazed" = 90

Makes sense for you something like this?

I also defined a value list for the timeConstant like this:

Light Building (Mobile home) = 1
Medium-light building (Cement tiles on floor) = 4
Semi Heavy Building (Concrete floor + Tiles) = 8
Heavy Building (Concrete floors/ceilings + Heavy external and internal walls) = 12

As for the first 5-10 cm effective TM in general my assumption is that you take half of the mass to your space and half to the space above/below you. Will be interesting to do a parametric study on just the thermal mass, uninsulated and insulated to see what the depth limits effectivity will be. Interested in doing such a study together? Can be a nice work even for publishing.

Thanks a lot ... again,

-A.

Attachments:

• THERMAL MASS_article.pdf, 127 KB
• published.pdf, 747 KB

Permalink Reply by Chris Mackey on September 10, 2017 at 2:45pm

Abraham,
That is a nice paper on thermal mass and the results make sense to me. The effect of thermal mass on the peak temperature is usually huge and is overlooked by simple psych chart studies like this one that focus on the % time that you can use a certain strategy fall inside a comfort polygon (rather than how much more comfortable the worst-case scenario is).

Your rules of thumb for solarHeatCapacity look good and the only thing that I would clarify when you use them is that they assume clear double pane glazing without a low-E coating, which is normal for a project that is trying to maximise passive solar heating on a southern exposure.

As for the time constants, they also look good, although I would add that the time constant is a function of BOTH thermal mass and thermal resistance (insulation). In the most extreme case of a high mass passivehaus, you would have a time constant around 36-48 hours. Most code compliant buildings in the US with exposed concrete floors would have a time constants around the default 8 hrs. 12 hrs seems to be a good value for a building that is a bit better than code with exposed massive floors and walls.

-Chris

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