Final Report: Ultra-low Power CO2 Sensor for Intelligent Building Control

EPA Contract Number: EPD13026
Title: Ultra-low Power CO2 Sensor for Intelligent Building Control
Investigators: Carter, Michael T.
Small Business: KWJ Engineering, Inc.
EPA Contact: Manager, SBIR Program
Phase: I
Project Period: May 15, 2013 through November 14, 2013
Project Amount: $80,000
RFA: Small Business Innovation Research (SBIR) - Phase I (2013) RFA Text |  Recipients Lists
Research Category: Small Business Innovation Research (SBIR) , SBIR - Green Buildings

Description:

The purpose of this Phase I SBIR program was to demonstrate that an advanced, ultralow power, microfabricated thermal conductivity detector (TCD) could be used to accurately measure carbon dioxide (CO2) in the air for the purpose of controlling room ventilation in demand-controlled ventilations systems (DCV).  Demand-controlled ventilation, in which fresh air is supplied in response to occupancy and rising ambient CO2 concentration, is emerging as an energy-efficient means of controlling room comfort. Current ventilation for CO2 level is accomplished with nondispersive infrared (NDIR) sensors, which are costly to use in large numbers, such as in a large office building. The goal of Phase I was to demonstrate that a microfabricated TCD device, which can be manufactured for a small fraction of the cost of NDIR sensors, could be used to monitor CO2 accurately in the air over ranges of temperature, atmospheric pressure and relative humidity encountered in real conditions, and that CO2 could be measured at concentrations and with resolution relevant to DCV ventilation control.

Summary/Accomplishments (Outputs/Outcomes):

MEMS thermal conductivity sensors for CO2 were fabricated by standard microfabrication methods. The sensors were characterized in CO2 monitoring over a wide range of CO2 concentrations, 0–5000 ppm CO2. Temperature, pressure and relative humidity were varied over wide ranges expected for real environments, with this data collected by auxiliary sensors, and then used to apply calibration corrections to the CO2 concentration. The calibration correction method was validated and improvement for Phase II were identified. The detection limit and resolution of the CO2 measurement were related to the other environmental variables.
 
CO2 was found to be measurable in the air with about 350 ppm resolution, which was close to the 100–200 ppm target range that we believe will be adequate for the first functional systems. KWJ Engineering found that CO2 could be very effectively compensated for the effects of variable temperature, pressure and relative humidity and elucidated an approach by which these variables can be used in real time to correct CO2 measurements.

Conclusions:

The thermal conductivity sensor, in miniaturized MEMS form, is capable of extremely fast and stable CO2 measurements in air. The microsecond response time of the MEMS device allows thousands of individual measurements to be performed and signal averaged, resulting in high signal to noise, before a conventional CO2 sensor has made a single measurement. The CO2 concentration can be accurately calibrated for changes in temperature, pressure and relative humidity on a continuous basis.

The principal application of the CO2 sensors and methods developed in this program is expected to br in building ventilation control. However, CO2 is used extensively in many industries, including the medical, agricultural and food and beverage industries and there are applications of our CO2 sensor in these large business segments as well. The MEMS CO2 device is expected to be considerably less costly and more capable than conventional CO2 sensing technology.

Supplemental Keywords:

sensor technology, Demand Controlled Ventilation, CO2 sensor