Ultra-Low SWaP CO2 Sensing for Demand Control Ventilation

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Ultra-Low SWaP CO2 Sensing for Demand Control Ventilation

PARC has been developing a low size, weight, and power (SWaP) printed CO2 sensor system for occupancy detection to enable Demand Control Ventilation (DCV) on a per-room basis. Printed sensors consume microwatts of power and are compatible with integration into existing building management system hardware such as wired smoke alarms or energy-harvesting wireless nodes. The device aims to measure 0-5000 ppm CO2 concentration. The printed sensor comprises a temperature sensor utilizing a CO2-sensitive carbon-based sorbent. CO2 sensing is based on the temperature change induced in the sorbent from the heat of adsorption of CO2 (~25 kJ/mol)1 along with the corresponding change in mass. We have demonstrated that we can measure the heat of adsorption when the sorbent is exposed to changes in trace concentrations of CO2. Laboratory experiments involve flowing of air with a controlled concentration of CO2 through an environmentally sealed measurement chamber which accommodates the printed boards containing a sorbent-buried thermistor and a reference thermistor. The change in the sorbent temperature resulting from the exposure to CO2 is measured by monitoring the resistance of the thermistors. Negative temperature coefficient thermistors provide predictable change in their resistance inversely proportional to small changes in temperature and allow us to detect temperature changes in the mK range. We have screened wide range of solvent/binder pairs and additive materials for developing sorbent ink formulations, using Differential Scanning Calorimetry (DSC) measurements to evaluate the degradation of CO2 absorption capacity of the carbon-based sorbent when exposed to solvents, which would deteriorate the CO2 measurement sensitivity. Experimental results show that deposition shape and mass affect both the response magnitude and time of the sensor to CO2. We have developed sensor circuitry and sensor interface boards with minimal crosstalk between the sensing and reference thermistors. This allows us to discriminate changes in CO2 concentration down to 50 ppm resolution. Several methods for converting dynamic to absolute CO2 concentration measurements have been investigated and will be also described. 1- Hornbostel, M. D. et al. Carbon N. Y. 56, 7785 (2013).

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