Hello,
I am new to eQuest. I am trying to model the effects of thermal mass
in the external walls on space conditioning efficiency. Does eQuest take
into account solar gain on the building walls? Why I ask is that I've
noticed that increasing thermal mass in the walls, (while maintaining the
same R-value), has no effect on the heating load and very little on the
heating demand. I also noticed that there doesn't seem to be a way to
specify external shading from trees, adjacent buildings, etc.
Peter
External shades can be modelled as ?Fixed Shades? in the detailed mode.
Solar gains are indeed modelled. Have a look at the summary view for a wall construction to see the impact of changing thermal mass:
Peter,
There are two ways to model walls (and roofs) in eQUEST. To capture thermal mass effects you have to use the layer-by-layer input. Using resistance only will not account for thermal mass.
Keith Swartz, PE
Senior Energy Engineer | Seventhwave | Madison.Chicago.Minneapolis
(formerly Energy Center of Wisconsin)
608.210.7123 | www.seventhwave.org
Hi Peter,
Thermal mass is the ability of the wall or roof to store thermal energy.
.................................................................................................................
Let us have a review of few equations:
1. The equation for thermal mass is:
Thermal mass= mass of the wall * specific heat capacity of wall
.*(equation-1)*
2. It can be further deduced to:
Thermal mass= thickness * area of wall * density of wall * specific
heat capacity of wall .* (equation-2)*
3. Stored energy in wall = Thermal mass * delta T (change in wall temp)
4. R value is the reverse of conductance.
5. The heat transfer is dependent upon the major formula
Heat transfer (Q) = Conductance (1/R) * Area of wall * delta T.(temp
diff between outside and room temp)
*(equation-3)*
..................................................................................................................
Thermal mass is responsible for the thermal energy stored and temperature
rise of walls or opaque surfaces.
Hence the major criteria to be supposed for major dependency of heating
loads or demands is R value or U value as the stored energy due to the
thermal storage is not that huge to make a feel on demands or loads.
One will have to do a drastic change in any of the factor like changing
thickness, density or specific heat of opaque surfaces to have some effect.
..................................................................................................................
To see the effect of thermal mass let us suppose two cases:
1. High specific heat: Then the stored energy will be huge but the heat
transfer after that will be less.The time taken will be large
.
2. Low specific heat: Then stored energy will be less and the amount of
heat transfer is less in quick span of time.
So rounding off these two cases results in less heat transfer per unit time
due to thermal storage.
This is an indirect way of heat transfer as compared to R value method as R
value method is applied for every second.
Hope this can be helpful .For your case you will have to be sensitive about
the actual inputs and thermal conditions.
Thanks,
Sharad.Kumar.
Green Horizon Consulting LLP
Gurgon
India.
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