Civil Engineering Reference
In-Depth Information
stones. Of course there are many agencies and standards that regulate how and
under what conditions, manufacturers have to test and represent their product per-
formances to the market. These products then will be labeled as products complying
with the standard and therefore maintaining reliable performances.
But even these regulations cannot guarantee the exact product characteristics as
what has been advertised due to the allowable tolerances in testing procedures. It is
customary for all the testing and certifying standard agencies to allow some degree
of deviation from the represented performance levels. For example assume AHRI
550 which is the standard for testing vapor compression chilling machines allows a
test tolerance of 7.5 % for certifi ed tested chillers at full load. Then a chiller manu-
facturer can represent its chillers with a specifi c effi ciency of fi ve tenth of kilowatts
per ton (0.5 kW/t) (at full load), if they follow the required test procedure and the
effi ciency of their tested chillers fall within the acceptable range of 7.5 % of the
advertised performance at full load (similar conditions are true in part load condi-
tions). This means even a chiller with effi ciency of up to 0.53 kW per ton can be
qualifi ed and labeled as a chiller with 0.5 kW per ton effi ciency. Similar tolerances
are allowed by different agencies for every other equipment and material utilized in
the building construction and operation as well. Such deviations from the exact
characteristics of the used materials and equipment effi ciencies in the building from
what has been used as input to the energy model provide ground for what is called
uncertainty in energy modeling simulation output.
As I explained earlier one of the early steps in preparing an energy modeling
simulation for a building is to enter the size and heat transfer characteristics of the
building different envelope elements into the model, such as
U
-values of walls and
roofs and
U
-value and shading coeffi cient of different building glazing elements. Of
course the
U
-value of each wall or roof is calculated by combining the
U
-values of
the material that are used to construct the wall or roof layer by layer, such as bricks,
sheet rocks, and concrete. This data then along with other input data will be used by
the energy modeling simulation software to calculate the hourly energy required for
cooling or heating of the building throughout the year.
Since 1980s comprehensive academic efforts have been done to quantify the
effects of uncertainty in the building energy modeling specifi cally on a ground of
uncertain envelope element's heating characteristics of the buildings.
In 1990, Clarke, Yaneske, and Pinney in their paper “The harmonization of ther-
mal properties of building materials” (Clarke et al.
1990
) represented data regarding
the variations of the building construction material properties with moisture con-
tent, and also the resulted uncertainties in the manufacturer represented characteris-
tics of different building materials. They showed that the range of thermal
characteristics of the construction material which are really used in construction of
the building can swing in considerable ranges from what has been selected during
the design time. For example conductivity of calcium silicate cellular glass with
density of 136 kg/m
3
when it is tested in different temperatures between 10 °C and
37.7 °C, can swing in a range of 0.047-0.051 W/m K. and copper conductivity as its
density changes between 8,600 and 9,000 kg/m
3
changes in a range from 200 to
384 W/m K. This showed that the characteristics of the ultimately used material in
