The interface of CoolVent was designed to be simple. In a two-step process, the user can define the input parameters: those characteristics of the building that will mostly influence the effects of natural ventilation, without requiring more detail than what the early design of a building can provide.
The simulation requires less than a minute to run. Results are presented in a format that enables one to clearly visualize the temperature and airflow in each zone of the building. Although the main output interface is visual (i.e. color-based), the user can also access detailed data for each simulation, and store it as a text file.
Two sets of data are required to run a CoolVent simulation: general and detailed building information. The general information about the building and the terrain properties of its location is entered by the user on the following window:
Simulation type The simulation can be run for a 24-hour period (transient case) or for an instant in time (steady case). The transient model calculate the performance of a building using a weather data file, and allows to model the effects of thermal mass. The steady-state simulation allows the user to define the free stream wind speed and its direction (N, NE, E, SE, S, SW, W, NW), as well as the ambient temperature and air humidity. In this mode, an initial building temperature must be defined, in order to initialize the calculations.
Building type Four predefined building types (ventilation strategies) can be modeled: single-sided ventilation, cross ventilation, central atrium ventilation and side atrium ventilation (Figure 2). These geometries represent the most common shapes in newly-built naturally-ventilated buildings. Single-sided ventilation only accounts for the air flow driven by buoyancy forces in a single zone. Cross-ventilation, on the other hand, only addresses the effects of wind. Finally, both central and side atrium designs include a combination of wind- and buoyancy- generated forces. The former involves an occupied zone at each side of the atrium, while the latter has only one occupied zone on the side of the atrium (similar to a solar chimney or wind scoop design).
Occupancy heat loads The type occupancy determines how much heat is being generated inside the building. In CoolVent, the user can define the occupancy type as residential, office, or educational (predefined heat gains per unit area), or by an arbitrary heat load density. These heat gains represent occupancy, lighting and equipment loads. It is assumed that the building is occupied from 8 am to 7pm. In non-occupied hours the occupancy heat loads are reduced to 20\% of the total. The use of an occupancy schedule will soon be implemented.
Terrain information The profile of the wind enveloping the building greatly depends on the terrain information. It is thus important for the user to define the type of terrain (urban, rural, or airport) and the average height of the surrounding buildings.
Building orientation The user can choose between eight building orientations (N, NE, E, SE, S, SW, W, NW).
Transient simulations are run using weather data. CoolVent provides instant feedback regarding the relevant weather parameters (wind speed, direction, air temperature and humidity) to allow the user to make an informed decision regarding building geometry and orientation. In transient mode the building orientation is selected in the same section where feedback on weather is provided, to promote the best orientation based on wind direction.
Simulations are run for a specific month, using monthly-averaged weather data. Any weather data file in .epw format can be used (if your city is not in the default menu, select “Other” in the “Select a City” menu). Dry bulb temperature, air humidity, and solar radiation data is averaged hour by hour for the selected month. The wind direction, subdivided into eight dominant directions, is selected based on the highest frequency for each hour, and wind speed corresponds to the average speed for that specific direction.
Detailed Building Information
Once the general building information has been specified, more precise parameters about the building dimensions must be specified.
Building dimensions The building is characterized according to the following parameters: number of floors, floor-to-ceiling height, floor-to-floor height, floor length and width, and roof height and atrium width (for central and side atrium building types only).
Glazing/opening dimensions In CoolVent, the window properties are divided into glazing and opening parameters. Glazing properties determine how much solar heat load is allowed into the building –independently of whether the windows are open or not –, while opening properties directly affect the air flow rate in or out of the building. The user must specify first the areas for glazing and window openings, and secondly, depending on the building type, the vertical location of the openings (single-sided ventilation), roof opening area (central and side atrium type), and internal door/partition free area (cross ventilation). Click on the calculator for advanced inputs.
Opening locations Users can specify the vertical location of the opening(s), which, in single-sided ventilation mode, is critical to define the flowrate. The window elevation is also an important factor for the prediction of the thermal stratification in the space. Click on the calculator for advanced inputs.
This section allows modeling the effects of thermal mass and of using fan for assisted ventilation on the building airflow physics. Modeling the effects of thermal mass can only be done when performing transient calculations. When 24-hour simulation is selected, the following parameters can be specified. Assisted ventilation can be modeled in both steady and transient simulations.
Thermal mass description Users can characterize the thermal mass of the building by defining: slab thickness, surface (expressed as a percentage of the occupational floor area), building material (concrete, brick or steel), a floor type (exposed, carpeted, raised), and a ceiling type (exposed or suspended). Including the effect of thermal mass in the simulation is optional.
Night cooling In the transient calculation mode night cooling can be selected, where the windows can be closed to 90\% of their area during a period of the day –ideally when the external temperature is the highest.
Window control strategies For simulations in winter conditions, CoolVent offers the possibility to a) close the windows if the ambient temperature drops below a user-specified temperature; and/or b) close the windows and turn on the heating if the inside temperature of any of the zones drop below a user-specified temperature.
Fan settings (assisted ventilation) When buoyant/wind forces aren’t enough to drive fresh air through the building, CoolVent allows modeling the use of assisted ventilation. Based on a fan size selected by the user, CoolVent will estimate the annual energy consumption related to using such fan (see Fan results in the results section). Advanced users can also specify any particular fan using fan curves.
Thermal Comfort Conditions
In order to present the results in terms of thermal comfort, the user may select standard ASHRAE comfort limits, or define a custom range of air temperature and humidity. More details on these calculations can be found in Rich 2011 (see CoolVent Literature).
The simulation can be run once all the input parameters have been specified. Running the calculations takes less than a minute. Results (zone temperatures, airflows, and thermal comfort conditions) can be presented in three different formats: as visualization, as data plots, or as text files.
The following image shows the output of a simulation for the building at a specific instant (screenshot). Each zone of the building is colored according to its temperature, based on a chromatic scale (darkest blue and red for lowest and highest temperatures, respectively). Colored arrows indicate the direction and temperature of the air flow into and out of each zone, and numerical displays provide the magnitude of the airflow rate, in cfm or L/s. A list of the temperatures (in °C or °F) for each zone can be found on the lower part of the visualization window.
Transient simulations are visualized as an animation, with an adjustable time interval between each screenshot. Steady simulations are presented as a single screenshot.
Daily Temperature Variation of a Zone
For transient simulations, it is possible to view in plots the temperature variation of the building over 24 hours. Each plot contains two curves: one representing the temperature variation of a specific zone, and the other showing the change in ambient temperature, over time.
Selecting any zone will provide information regarding the thermal stratification in the space — if available — based on Menchaca 2012 (see CoolVent Literature). While the zone temperature provided in the main visualization window corresponds to the exhaust temperature of the zone based on a energy balance, visualizing the stratification profile increases the user’s insight regarding the temperature in the occupied zone. Due to the limitations of the prediction model, thermal stratification can only be visualized when thermal mass is not modeled.
Based on the comfort conditions specified in the inputs section, CoolVent estimates what percentage of the day the zone is within the acceptable comfort range. Additionally, temperature plots per zone allow visualizing the comfort conditions using an adaptive thermal comfort model.
Based on the fan defined in the inputs section, CoolVent provides its expected electric consumption (hourly in kW and integrated daily in kWh) as well as its operational efficiency.
Temperature, airflow, fan and thermal comfort results can be exported into a text file, separated by zone and time of day.