Mixed mode Cooling

In mixed-mode buildings, natural ventilation is the primary means of cooling, but when this is inadequate to provide comfort conditions, active cooling is introduced. The Center for the Built Environment (CBE) describes mixed mode buildings as follows.

 

Mixed-mode buildings employ a hybrid approach to space conditioning that combines operable windows and mechanical cooling. By taking advantage of the strengths of both systems, well-designed mixed-mode buildings can be more comfortable and use less energy. People who have more control over their indoor environments are typically more satisfied with temperature, air movement, and air quality. Mixed-mode buildings give occupants control while still allowing central control to keep the building near the comfort zone. The three basic categories of mixed-mode buildings describe how natural ventilation and mechanical cooling operate at different times and places. Concurrent systems allow open windows and air conditioning at the same time in the same place; Changeover systems allow them at different times in the same places; Zoned systems allow them at the same time in different places.

 

Mixed Mode Schematic from the CBE website

 

DesignBuilder can model all three types of mixed mode system.

 

  1. Concurrent (same space, same time)
    Concurrent mixed-mode operation is the most prevalent design strategy in practice today, in which the air-conditioning system and operable windows operate in the same space and at the same time. The HVAC system may serve as supplemental or “background” ventilation and cooling while occupants are free to open windows based on individual preference. Typical examples include open-plan office space with standard VAV air-conditioning systems and operable windows, where perhaps perimeter VAV zones may go to minimum air when sensor indicates that a window has been opened.

  2. Changeover (same space, different times)
    Changeover designs are becoming increasingly common, where the building switches between natural ventilation and air-conditioning modes on a seasonal or even daily basis. The building automation system may determine the mode of operating based on outdoor temperature, an occupancy sensor, a window (open or closed) sensor, or based on operator commands. Typical examples include individual offices with operable windows and personal air conditioning units that shut down for a given office any time a sensor indicates that a window has been opened; or a building envelope where automatic louvers open to provide natural ventilation when the HVAC system is in economizer mode, and then close when the system is in cooling or heating mode.

  3. Zoned (different spaces, same time)
    Zoned systems are also common, where different zones within the building have different conditioning strategies. Typical examples include naturally ventilated office buildings with operable windows and a ducted heating/ventilation system, or supplemental mechanical cooling provided only to conference rooms. For many mixed-mode buildings, operating conditions sometimes deviate somewhat from their original design intent (e.g., a building originally designed for seasonal changeover between air-conditioning and natural ventilation may, in practice, operate both systems concurrently).

Note: Concurrent and Zoned systems can be modelled simply by including natural ventilation with HVAC systems without using DesignBuilder mixed mode control. In DesignBuilder, the term "mixed mode" refers specifically to Changeover systems where the HVAC and the natural ventilation do not take place at the same time.

 

 

DesignBuilder supports mixed mode in all combinations of natural ventilation and HVAC model options.

 

The steps to setting up a change-over mixed mode model in DesignBuilder are:

 

  1. Include active cooling and natural ventilation by switching on the Natural ventilation on and Cooled options on the HVAC tab.
  2. Also, include mechanical ventilation if there is outside air introduced through the HVAC system. Note that Detailed HVAC AHUs allow Mixed mode controls to be defined to model optimal interaction between the natural ventilation system (Scheduled or Calculated) and the AHU.
  3. Ensure that the setpoints for natural ventilation and cooling operation are correct. The natural ventilation (window/vent opening) setpoint should ideally be 2°C lower than the cooling setpoint to ensure maximum use of natural ventilation and to avoid the active cooling system being used unnecessarily and also 2°C higher than the heating .temperature. Typical values might be heating setpoint temperature = 21°C, natural ventilation setpoint temperature = 23°C, cooling setpoint temperature = 25°C. Heating should ideally be switched off in the summer to avoid unnecessary heating caused by natural ventilation overcooling the space.
  4. See the mixed mode section on the HVAC help for detailed information on the data that can be set, including conditions that require windows and vents to be closed such as rain, wind and low outside air temperature.

Note: "Mixed mode" means the same as the term "Hybrid ventilation" used in the EnergyPlus documentation.