The Case For CO₂ DCV

Demand controlled ventilation (DCV) offers great potential for energy savings and better moisture control in buildings by modulating outside air ventilation to building spaces based on actual occupancy.  This technology offers great potential for savings in retrofit applications. For example a study of 100 commercial buildings by the US EPA found that the majority (83%) of buildings were ventilated well over the ventilation standard of 20 cfm/person with average ventilation rates being 29.8 cfm based on design occupancy (Ventilation Design And Performance In US Office Buildings, ASHRAE Journal, April 2005)

Use of CO2 sensors for ventilation control in these buildings would eliminate this over-ventilation  by providing a real time control feedback to ensure that the appropriate ventilation level in these buildings is maintained based on actual occupancy.

The savings delivered from DCV  can be significant, particularly for higher density applications with variable occupancy such as schools, retail stores and malls, hospitality venues, auditoriums and meeting spaces.  Airtest actually offers a free excel-based tool that allows you to determine potential CO2 DCV savings based on engineered weather data for over 200 locations throughout North America.  A number of utilities have actually used this program as a basis for calculating rebates for CO2 DCV.  The program is available here: https://airtest.com/support/energy-analysis/co2energyanalysis.php. Typical savings can be 15% to 40% of heating and cooling costs and can amount to thousands of dollars for many building types.

While CO2 transmitter cost is generally well below $200, the cost of installation in a retrofit application can be five to 15 times the cost of the sensor, all related to wiring and integration with the existing building control system. This can push energy savings paybacks out beyond four and five years for some applications.  With the elimination of most of the installation cost, there becomes a much greater opportunity for CO2 DCV retrofits.

A WiFi CO2 Transmitter Designed for Retrofit Applications

While the wireless application of a wide variety of sensors has occurred in the building control industry, the power consumption problem has held back CO2 sensing and control.  AirTest has a great interest in reducing the total installed cost for a CO2 sensor in order to allow building owners to reap maximum payback in energy savings.  As a result we have developed the TR9299-WiFi, the first battery powered WiFi CO2 and temperature transmitter using an  LED-based CO2 sensor platform that consumes less than 1% of the energy of a conventional CO2 sensor.  The incremental per unit cost of incorporating a wireless capability into the CO2 transmitterdesign, is a fraction of the wired installation cost per sensor, and allows integration of the sensor with the most widespread, well established, and dependable wireless technology used today… WiFi.  Soon to come are versions targeting Zigbee Pro and EnOcean communication platforms.

The TR9299 is designed to have a battery life of approximately three years on two AA lithium batteries (sampling every two minutes and broadcasting every 10 minutes).  For building operators who are not excited about changing out batteries ever, the transmitter can also be low voltage powered (5-24 VAC/VDC).  The TR9299 also has an on-board data logger capable of storing over 3,000 measurement points, which will make it ideal for some temporary measurement applications.  An on board visual and audio indicator can also be activated to warn of elevated levels if desired (i.e. LEED credits for IAQ monitoring).  The transmitter also features a background auto calibration function that can be activated via a plug-in PC user interface.  Further technical details on the product can be found here: http://www.airtest.com/support/datasheet/TR9299wifi.pdf.

Versions of the transmitter are also available to measure:

• Temperature and humidity (TR9399-WiFi:  www.airtest.com/support/datasheet/TR9399wifi.pdf) and,
  
• Temperature only for monitoring temperatures in freezers and coolers (TR9499-WiFi): www.airtest.com/support/datasheet/TR9499wifi.pdf).

All WiFi transmitters are designed to offer significant versatility on how the WiFi communication capability is used.

1. Data from the sensor can be directed to a website hosted by AirTest.  The website features data storage,  and the data can be viewed in a number of ways including graphing, setting alarm levels for each transmitter, activating email or text alarm, and requiring alarm acknowledgment including the provision of alarm escalation if an alarm is not acknowledged.  There is a nominal monthly fee for this web hosting service.
   
2. Data can be sent to any address on the Internet be it an SQL database, IP capable controller, or cloud based data collection platform.   All that is required is to write a small translation utility to decompress and interpret the data-packets sent from each sensor (protocol details are available at no cost).
   
3. Control Solutions Inc. has developed an inexpensive gateway product (BB2-7010-06): www.AirTest.com/support/datasheet/BB2-7010-06, that when connected to a local WiFi network can provide a BACnet/IP client and server, Modbus TCP client and server (master and slave), SNMP client and server (agent), and Modbus RTU master and slave.  The BB2-7010-06 includes a WiFi sensor server, which collects data via a WiFi access point from AirTest WiFi transmitters.  Up to 200 transmitters can be connected via one gateway.
   

A Commissioning Friendly CO2 Transmitter

We have been selling CO2 sensors and transmitters for over 10 years and one of the biggest concerns we heard recently is that CO2 transmitters are not “Commissioning Friendly”.  Basically, sensors have been designed to be factory calibrated and end users have been discouraged from trying to adjust the sensors in the field because field calibration can be subject to all kinds of variables that could cause significant error in the calibration process.  This has caused a significant problem for commissioning agents, who are responsible for verifying sensor operation and readings.  It is often also a headache for contractors who need the commissioning agent’s report to get paid.  Because the sensor elements are embedded in larger enclosures there is no easy way to verify readings except by comparing with a calibrated portable device in the immediate area of the sensor.

It is important to note that the TR9299 does have a background calibration feature that can be activated and even customized by the user. User inputs include the ability to set the background concentration used for calibration, the interval that should be used to sample background concentrations, and the ability to program a quick initial background calibration that occurs soon after initial power up.

In designing the TR9299-Wifi thought carefully about how to make commissioning fast, non-complicated and accurate.  We have developed two approaches that we think considerably enhance the commissioning process.

1. Ambient Air Calibration:  CO2 DCV control is based on measuring the difference between inside and outside concentrations.  A very effective way to calibrate sensors is to expose them to outside air concentrations and assume a baseline value such as 400 ppm.  With the battery operated TR9299 the transmitter can easily be removed from the wall and placed outside for 10-15 minutes.  A small magnet that is held on the back of the sensor can be removed and placed on the outside of the front cover of the transmitter to initiate the ambient calibration.  A flashing light on the cover of the transmitter indicates when the calibration is complete.
   
2. Using A Known Concentration Of Gas:  One of the reasons for the circular shape on the front of the transmitter is to fit a calibration cap that allows the flow of a known gas to the sensor in a controlled way that will ensure maximum accuracy.  Using a plug-in PC interface developed by AirTest, the user can compare or actually adjust calibration of the sensor based on flowing a known concentration of gas, or flowing nitrogen to confirm the zero point, or simply comparing the sensor reading to a recently calibrated hand held device.  While performing the calibration, readings can also be graphed on the interface and logged to a PC with time/date indicators creating a permanent record of the commissioning or calibration process.

Summary

[an error occurred while processing this directive] CO2 Demand Controlled Ventilation can save considerable energy in buildings by regulating fresh air ventilation based on actual occupancy.  However, the payback of installing these sensors can be considerably lengthened in retrofit applications where the cost of wiring and integrating CO2 can be 5 to 15 times the cost of the CO2 Transmitter.  Due to a breakthrough in the design of infrared LED technology, the power consumption of CO2 sensors can be reduced by up to 99% making battery powered, wireless sensors possible. Temperature generation by the sensor that can affect the readings of nearby humidity and temperature sensors is also eliminated.

The AirTest CO2 transmitter design also provides a “Commissioning Friendly” feature set that allows for a wide variety of methods to quickly and accurately verify or perform a calibration.  Following the production roll out of the TR9299-WiFi transmitter in May of this year will be versions designed for other wireless protocols including ZigBee Pro and EnOcean.

About the Author

Mike Schell is VP of Marketing and Business Development for AirTest Technologies Inc. a Canadian company that offers a wide variety of sensor technologies for buildings. Mike has extensive experience in sensor technologies and building control, and was a co-founder of the first company to market low cost CO2 sensors for Demand Controlled Ventilation.  Mike was instrumental in ensuring the acceptance of this innovative technology by today’s codes and standards.   He has authored over 70 articles and papers on indoor air quality, building science and environmental control.

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