Tool validation

The sizing and design tool developed is validated against EnergyPlus [1] simulations.

‘Tool Step 1’ is validated against ‘pseudo’ steady state Airflow Network simulations with fixed boundary conditions, whereas ‘Tool Step 2’ is validated with dynamic design day boundaries according to § 5.3.3. Care was taken to set identical input and climate information for both the developed ‘High-Vent’ tool and EnergyPlus.

Steady state (‘Tool Step 1’)

To validate ‘Tool Step 1’ of the developed design tool, based on electric analogies against the airflow network of AIRNET (integrated into the EnergyPlus environment), a simple EnergyPlus simulation model is created for fixed boundary conditions.

As the internal and external temperatures in each simulation are stable throughout the whole run period (defined in the E+ design day object), different ventilation strategies can be analysed as if ventilation was decoupled from the building. This was realised first for the environment by setting up a design day object and then for the internal zones via ‘ideal loads’ – heating and cooling system objects with equal fixed setpoint temperatures.

The unchanging external wind velocities at the meteorological station are set 5,24 m/s, 2,11 m/s, and 0,85 m/s (design day values from § 5.3.3). The external air temperature is chosen as 25 °C. Including the atmospheric variation over height, the local air temperature at the inlet opening of the segment (49,75 m above ground level) is 24,68 °C. Adding the design temperature difference between inside and outside, the storey zones’ air temperature is 27,68 °C and the temperature in the exhaust chimney is 28,68 °C.

To simplify the validation process, some model adaptations are equally made for both the electric analogy model and the AFN validation model. All other specifications are consistent with the design application scenario defined in § 6.3.

• Only one zone is simulated for each storey with two linkages to the inlet and the exhaust chimney. The internal flow resistance is set to zero to combine the core and the office zones into a single zone for each storey. In the electric analogy model, the factor K (resistance from office to core of each storey) was set to 1000, resulting in resistance values very close to zero. In the AFN model, the internal opening is simply not present.

• The inlet height from the supply chimney to the storeys is 0,50 m above the floor level height instead of 0,00 m to easily set up the model geometries without horizontal openings (i.e., with only vertical openings).

• The outlet opening from the storeys to the exhaust chimney are set to 3,00 m above the floor level height instead of 2,66 m.

Figure 3.30: View from the inlet side on 12 zone AFN model representing one building segment.

Table 3.2: Deviation from 10 h^-1 design air change rate (+ % indicates higher values from the AFN).

cell (storey) scenario 1 scenario 2 scenario 3

index I vmet = 5,24 m/s vmet = 2,11 m/s vmet = 0,85 m/s

H 0,1% 3,3% 14,3%

G -0,1% 1,8% 3,9%

F -0,1% 0,8% 1,5%

D -0,3% 0,2% 0,3%

C -0,4% -0,3% -0,4%

mean deviation -0,2% 1,2% 3,9%

The model developed fits reasonably well, especially for combined wind and buoyancy driven flows with high wind velocities as in the case of scenario 1. For buoyant flow as the main driving force, and with low exhaust stack heights for the upper storeys 4 and 5 of the segment, it was figured out that there were some problems within the EnergyPlus AFN buoyant model. Together with the EnergyPlus support team and the responsible person for the AFN⁵, it was concluded that the buoyancy forced calculations were partly buggy, and right now buoyancy alone may not always provide meaningful solutions. More precisely, a warning was given from the EnergyPlus support, that the model in some cases just gave a rough estimation.

Further compounding the problem, the author of this thesis found a bug in the code.

The AFN does not calculate the local temperature at the opening node height;

instead, the local temperature is taken from zero height (ground level). To overcome this problem, temperature in the inlet chimney is reduced by a system of ideal loads to virtually adapt the inflow external air temperature towards the temperature at local

height. All other E+ modules except the AFN seem to consider the temperature variation over height. The EnergyPlus support is slated to fix the bug in a future E+

version.

Dynamic design day (‘Tool Step 2’)

To validate ‘Tool Step 2’ of the tool developed, another EnergyPlus simulation – a three zones, one-storey model (3^rd storey of a segment at 51 m start height), is created for dynamic design day boundary conditions (from § 5.3.3) with unchanging wind velocity. More details of the simulation setup can be found in § 6.3.1.

In the first assessment, external conditions used in the developed tool are compared to the design day inputs used in the EnergyPlus simulation. As they are both based on the SWMD-approach developed before, they show identical properties as shown in Figure 3.31.

Figure 3.31: Tool validation dynamic environmental input parameters for Istanbul extreme summer design day.

The outcome of the most important test is shown in Figure 3.32. The comfort related parameters are compared to each other. Especially during occupancy, a good agreement was reached between the EXCEL-tool based simulations and EnergyPlus simulations. For Istanbul design day conditions, the most crucial parameter for system sizing, the peak internal operative temperature, has an aberration smaller than 0,1 °C. Also, the internal humidity fits well, especially during the building

Figure 3.32: Tool validation dynamic internal output for Istanbul extreme summer design day and naturally ventilated base-case scenario.

Finally, the dynamic electric analogies model is validated against the airflow network of AIRNET. A good agreement was achieved here as well, as shown in Figure 3.33.

Figure 3.33: Tool validation dynamic air change rate for Istanbul extreme summer design day.