Energy Simulations to Support Design of New Housing at Vanke Gardens

L. Caldas, J. Kaufman, and L. Norford

Introduction

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, fixed shades.

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 today. 

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.

Windows 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 temperature.

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.

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