CO2 as an energy-saving control

Supplying sufficient fresh air to a building for maximum occupancy is wasteful of energy if it is not fully occupied. Harry Swinburne explains how monitoring CO2 levels can reduce that energy consumption.Rising energy costs have raised interest in carbon-dioxide (CO2) control to save energy and reduce the running costs of new and existing HVAC installations. HVAC systems are designed to provide fresh air to the maximum design occupancy, but the space will not always be fully occupied, This is true in all buildings, especially offices, schools, theatres, cinemas, supermarkets, health centres and conference centres. The cost of supplying fresh air to buildings is high owing to the enthalpy (heat content) of the exhaust air and also the air transport energy. Energy can be saved by reducing the ventilation rate according to actual occupancy, using CO2 levels. Optimising ventilation in this way saves energy by reducing both the heat wasted by exhausting too much air and the energy used to move the air. Despite suggestions from as early as the year 1916 that CO2 control should be used to maximise HVAC system efficiency, the technology had tended to be expensive, while energy costs were relatively low. Now it is highly cost-effective, due to more accurate and more cost-effective sensing technology, coupled with high energy costs. Air-quality monitoring is encouraged by the new European standard EN 13779, which is based on the Energy Performance of Buildings Directive (EPBD) and aims to raise the energy efficiency of HVAC systems. CO2 sensor The latest designs of CO2 sensor use an optical principle based on the absorption properties of CO2. A sensor comprises three components (Fig. 1). • An infrared source that emits energy through a patented waveguide.
• An optical filter that passes only the required wavelength.
• A detector to measure the amount of infrared energy. The Command family of wall modules from CentraLine includes models for CO2 sensing and control. The greater the level of CO2 in the chamber, the less infra-red energy reaches the detector. This has been found the most accurate measuring method, the CO2 being selectively filtered among all other influences. There is no need for calibration. Sensors may also contain a proportional controller or/and a simple limiting switch; small applications can then be handled directly by the sensor/controller. The sensors provide a linear output signal (0 to 10 V or 4 to 20 mA) representing the CO2 concentration in parts per million (ppm). The signal can be scaled for different measuring ranges, depending on the sensor accuracy and application needs. In general, sensors should have a range of 0 to 2000 ppm CO2. In large, open buildings such as cinemas and theatres, the sensor should be in the exhaust-air duct. In other buildings it is better to have a wall-mounted unit for individual control in every room. Applying control The design of the CO2 control system depends on the HVAC system. In new systems, the controls are designed to provide a fresh-air supply at the minimum fan speed. If the minimum speed is insufficient for air quality, heating or cooling, the fan speed increases. Where existing systems are to be adapted for CO2 control, the best of several solutions should be selected. • In small installations it may be acceptable to switch the fan off and on according to the sensor information. There must be a limiting switch in the sensor.
• Systems with mixing chambers can be extended with CO2 proportional control and a means to select the maximum signal from the existing temperature control and the new air-quality control. This extension kit is therefore independent of the existing building automation system.
•Fresh-air systems can only be extended with air-quality control output to the fan frequency converter. In some cases the fan motor must be replaced to apply a frequency converter (as the insulation class will be too low). The controls will comprise a CO2 sensor, a proportional controller and a maximising device.
• In the case of systems with a mixing chamber and a frequency converter, only the building-automation application can provide the necessary functionality, so it may be necessary to upgrade the complete air-conditioning control. Air-quality sensors based on oxidixable gases, such as odours and carbon monoxide, can be used where CO2 is not the primary control variable — such as restaurants and changing rooms in sports facilities. The percentage energy savings achieved by applying air-quality control in a HVAC control system are typically in the double-digit range. The controls also increase HVAC plant life and reduce equipment noise. Building managers should now seriously consider CO2 control of HVAC systems now that it has become a highly cost-effective means to save energy and prolong the life of HVAC plant. Harry Swinbourne is UK business manager for CentraLine by Honeywell.

Related links:

• www.centraline.com/energy

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