16th Sep 2020
As building design & HVAC system operation continues to react to the virus responsible for the COVID-19 (SARS-CoV-2) pandemic, there has been an increased focus on the simulation of ventilation strategies that can reduce risks for the transmission of infectious diseases. New research is being published daily; general guidance is refining and building simulation specialists have an increased demand for the simulation of HVAC systems that promote the health & safety of building occupants meanwhile accurately sizing and effectively simulating the energy performance of those systems. This article highlights some of the most common ventilation system strategies being simulated by practitioners in 2020.
In the model highlighted below, common filter locations in a Dedicated Outdoor Air System (DOAS) is shown below. A High Efficiency Particulate Air (HEPA) Filter is located downstream of the Supply Fan and Return Fan. Many high Minimum Efficiency Reporting Value (MERV) filters are being specified for filtering viruses and bacteria, e.g. MERV 14-15, and HEPA filters are increasing in popularity due to the millions of people that are using filtration to reduce smoke ingression from annual wildfires. The example below shows a common maintenance schedule, the 99.97% efficient HEPA filter is due to be cleaned every 90 days, at which point the additional static pressure of the system resets to a lower value. Increased static pressure caused by dirty filters causes a penalty on the fan energy as shown (4 spikes). This can make compliance of some Building Codes & Standards more strenuous.
ASHRAE have published documentation stating “the most unfavorable survival for microorganisms” is when the Relative Humidity (RH) is between 40-60%. With temperature setpoints included, the target comfort range for a room or zone during occupied hours is shown below in the psychrometric chart. Many ApacheHVAC components can be used to achieve the desired indoor temperature and RH, provided the correct controlled variables are assigned, see Table 6-1. A tight comfort range in the psychrometric chart can cause energy penalties, so it is recommended to evaluate Airside Heat Recovery options, especially those with 0% risk of return air mixing with supply air.
Another ventilation strategy option is with Outside Air Dilution. Three common approaches to increased outdoor air include:
During a pandemic, ~600 ppm CO2 is recommended in some transportation buildings, which can be relaxed to ~1,000 ppm during recovery periods. As shown below, target CO2 ranges can be assigned to airside controllers in ApacheHVAC and also to operable window actuators in MacroFlo.
For any given naturally ventilated space, it is important to understand the impact of operable windows for a range of conditions within the space. It is recommended to use CFD simulation to understand the air circulation patterns in order to discover potential stagnant zones or supplement the passive strategy with an exhaust fan or ceiling fan for increased air circulation. Patterns for air circulation require user-interpretation and typical CFD metrics may include air velocity, CO2 concentration and Local Mean Age (LMA) of air.
Ultraviolet Germicidal Irradiation (UVGI) can physically be located in the 'upper-room' which can provide higher effective air changes of disinfection or it can be located in the HVAC unit (in-duct or AHU). UVGI is modeled as a custom room Gain/Load with a sub-hourly variation profile that follows the HVAC Schedule (System extended hours). As a rough estimate, the UVGI electric demand is 0.02 W/sf, (0.2 W/m2), based on 1 cfm/sf supply air for an all-air system.
The principle is simple: air should move from clean to dirty. From an energy simulation perspective, there are options available to model the transfer air from System A to System B. Supplemental conditioning to the air or filtration may be necessary to maintain space conditions.
The hospital patient room is a good example of a negatively pressurized bathroom; a positively pressurized corridor with an undercut door, and wind-driven infiltration. All such factors should be included when considering air pressurization.
Social distancing and reduced occupancy levels, at least in the short term, in buildings should also be considered. The impact of part-load performance in existing systems, as well as the impact on how best to size select and operate systems can be performed using Diversity Factors based on the changing scientific advice. Assessing occupancy movement in order to assess applicability of one-way movement strategies throughout buildings or how to implement suitable distancing measures could also be addressed, again assessments being based on the current scientific advice.
COVID-19, and the increasing frequency of wildfires, has brought into sharp relief the additional strategies that designers need to consider in building and system design and performance evaluation when indoor air quality is paramount. To reduce risk of the transfer of infectious diseases scientific advice on social distancing and variable occupancy levels have also introduced further considerations that need to be taken into account for performance based simulation and analysis.
“Minimize COVID-19 Airborne Transmission in Buildings”. Professor Peng Xu, Tongji University