Energy Simulations to
Support Design of New Housing at Vanke Gardens
Caldas, J. Kaufman, and L. Norford
We have used the VisualDOE building-energy
simulation program to explore how a typical building at Wanke
Gardens can be improved. The VisualDOE program runs on a
Windows personal computer and incorporates the DOE2.1E code,
as developed at Lawrence Berkeley National Laboratory. Some
of the strategies identified in this study are already incorporated
into the new building that MIT has designed. The MIT team
is now considering other measures as well.
We have simulated a 10x12 m, five-story,
four-unit, free-standing building. Building materials, the
size and orientation of windows, and wall and roof construction
are realistic, to the extent that we have accurate information
about the building. The base case has an air-infiltration
rate of 1.5 air-changes per hour (ACH). We use an hourly
weather file for Beijing.
Our simulations have focused primarily
on summer conditions. As simulated, the building has no air-conditioning.
Our goal is to minimize the number of hours over the year
when the indoor temperature exceeds 29.4 oC. (This
temperature value is automatically selected by VisualDOE as
the lower bound of very hot temperatures and corresponds to
85 oF.) We also characterize each run by plots
of hourly indoor and outdoor temperature over the course of
a single July day.
We have conducted two series of simulations
and report on the first set at this time. First, we have
examined reasonable alterations to the building:
Internal versus external insulation for exterior walls;
Mechanical ventilation of the building (15 ACH);
Shading of south-facing windows, with exterior, horizontal,
Second, we have examined alterations that
would be difficult to achieve in practice but which point
toward substantial improvements, even if only partially realized:
No absorption of solar radiation by the roof and exterior
walls (absorptivity of 0);
No transmission or absorption of solar radiation by windows
(shading coefficient of 0.01);
No internal gains.
Series 1 Simulations
Results for the first set of simulations
are shown in Figure 1. For the base case, indoor temperatures
for about 50% of the year are in the lowest temperature bin.
The heating system, as simulated, maintains the building at
18 oC in winter and does not allow the temperature
to fall to a lower value. There are 1733 hours per year in
the hottest indoor-temperature bin, above 29.4 oC.
This is about 20% of the year, a substantial fraction of time
when indoor conditions are quite uncomfortable. In practice,
the number of hot hours may be even larger, because occupants
often enclose their balconies to make a sunspace, thereby
eliminating the benefit of the balcony floor as a shade for
windows and balcony doors below it. In all of our simulations,
we strive to reduce the number of hours in the highest temperature
bin, the black bar at the far right for each case.
Placing insulation on the exterior rather
than interior surface of exterior walls promotes the flow
of heat into and out of building mass. This may reduce indoor
temperatures during very hot weather but will also reduce
the benefit of thermostat set-backs, if used. We note that
the there was no winter thermostat set-back in our simulation
and that summer temperatures were allowed to float, so effective
building thermal mass should in principle be of some benefit.
Figure 1 shows that the benefit of external insulation, with
no other measures, is extremely modest. The location of insulation
should therefore be made on the basis of other factors, including
ease of construction, the elimination of thermal bridges,
and, as we shall see, the presence of substantial ventilative
cooling. Thermal bridges can occur where interior and exterior
walls join. We have not simulated their impact.
The third simulation shows that the number
of hot hours, indoors, is reduced by 40% by around-the-clock
ventilation at 15 ACH. We also explored sealing the building
during the day and ventilating only at night, but this strategy
led to higher indoor temperatures because there were both
solar and internal heat gains during the day.
The fourth simulation shows the impact
of adding shading to south facing windows that did not have
it. Most of the windows were already shaded, but adding shading
to the remaining ones further lowers the number of hot hours.
The fifth simulation combined exterior
insulation with mechanical ventilation. With increased airflow
to take away heat stored in exposed building mass, there is
a further reduction in the number of hot hours.
The sixth and final run of the first series
adds shading for south-facing windows. Now, the number of
hot indoor hours is reduced to 669, a drop of more than 60%
from the base case.
Figures 2 and 3 show diurnal
temperature plots for the base case and for the sixth simulation.
For the base case, indoor temperatures exceed outdoor temperatures
throughout the 24-hour period. For the sixth simulation,
indoor temperatures are lower, particularly in the evening
hours, and are approximately equal to outdoor temperatures
when outdoor temperatures are at their peak afternoon values.
Thermal comfort has been improved.
Series 2 Simulations
These simulations have been performed
but not fully analyzed. We provide a brief summary and will
post additional results, including figures, as soon as possible.
The second series of simulations is summarized
in Figure 4. Figure 4 is similar in concept to Figure 1 but
starts where Figure 1 stops. That is, the first case in Figure
4 is essentially the same as the sixth case in Figure 1.
Figure 4 shows that the number of hot indoor hours can be
reduced again, by either reducing the absorptivity of exterior
surfaces to zero or by eliminating internal heat gains. Neither
is fully achievable, of course, but careful specification
of materials and finishes and the use of aggressive shading,
including vegetation, may substantially reduce solar heat
absorbed by walls and the roof. Internal heat gains can be
lowered through use of low-wattage lights and selection of
high-efficiency appliances and household electronics. Of
the two measures, drastically reducing internal heat gains
does more to reduce internal temperatures than does eliminating
absorbed solar energy.
A simulation that combines the elimination
of absorbed solar radiation, the elimination of internal heat
gains, and the elimination of solar radiation through windows,
via transmission or absorption, produces the best results,
as expected. Now there are only about 300 hours in the highest
indoor-temperature bin, less than 20% of the number of hours
in the base case that represents a Wanke Garden house as built
Figure 4 also shows that sealing
the building during the day, by turning off the fans and closing
windows, does not reduce the number of annual high-indoor-temperature
hours. This is true even when internal and solar heat loads
are reduced. However, as will be shown in an update to this
presentation, turning off the ventilation fans and closing
the Figure 5. Hourly internal temperatures for different
design and operation strategies.
will reduce indoor temperatures when outdoor temperatures
are at their mid-summer peak. On hot afternoon hours
during these days, it is advisable to minimize the flow of
heat into the building. Sealing the building can reduce indoor
temperatures to about 30 oC, below the outdoor
Our simulations indicate that the base-case
building, which represents current design and construction
practice at Wanke Gardens, will be very hot indoors for about
20% of the year. The number of hot hours can be reduced by
about 60% by three easily achieved measures: mechanical ventilation,
exterior insulation, and shading of south-facing windows.
The number of hot hours can be reduced
by another 50%, to less than 25% of the original total, by
eliminating the impact of the sun on windows, exterior walls,
and the roof and by eliminating internal heat gains. Even
modest efforts to reduce solar and internal gains will reduce
the hours when the building is hot or when occupants would
want to use air conditioning.
back to top