Low-cost Sensors for Balancing Indoor Air Quality and Energy Usage

Saving energy and improving Indoor Air Quality (IAQ) and thermal comfort in buildings are traditionally competing goals. Facilities that manage one of these objectives well tend to compromise the other. Building Automation Systems (BAS) have been limited in their ability to sense these characteristics of a building and therefore cannot act on them. A majority of buildings define IAQ in terms of supply air temperature alone and manage energy consumption by scheduling. There is a large potential to improve both IAQ and energy consumption through innovative control strategies. These strategies are independent of developments in the energy conversion equipment itself by enhancing its control. Good control requires good feedback and feedback for IAQ is complicated by a lack of sensors able to be easily integrated into BAS that can record other contributions to IAQ such as CO2 and the presence of trace gases. Since BAS have not traditionally had access to this type of data, there is little experience of how to best apply this new information in a dynamic system to achieve reduced energy consumption while improving IAQ. This paper will discuss development and integration of low-cost microsensor arrays into affordable BAS. Multi-function sensor packages are in development that can measure CO2, temperature, humidity, room occupancy, and potentially other trace gases of interest. The sensor package can communicate with the BAS wirelessly or over existing building power wiring. Communications have been developed for a low-cost implementation with algorithms focused on security and robustness. INTRODUCTION Traditionally, saving energy, providing thermal comfort, and improving Indoor Air Quality (IAQ) are competing considerations. Why are these topics important in buildings? Residential, commercial, and industrial buildings are responsible for 43 percent of the US CO2 emissions [1]. Poor indoor air quality has been estimated to cost the U. S. annually (in lost production) between $17B and $48B and to affect as many as 80 million persons [2, 3]. Finally, various studies have indicated that introducing energy saving technologies and efficiencies can reduce energy consumption by 10% to 30% [4]. Facilities that manage one of these objectives (saving energy or thermal comfort or IAQ) will tend to compromise the others. There are several reasons for this tradeoff. Improved IAQ often means additional ventilation air as called out in ASHRAE Standard 62 Ventilation for Acceptable Indoor Air Quality which usually means raising outdoor air flow rates into the building. Typically this increased outdoor air must be conditioned, increasing energy use. The additional outside air also translates into humidity variations and control; again, potentially increasing energy consumption [5]. Additionally, building automation systems (BAS) do not have the ability to comprehensively and continuously assess IAQ and thermal comfort because low cost, easy to use sensors for carbon dioxide, humidity, air flow, and trace gas concentrations are not readily available. Also, since these measurements are not normally taken, little thought has been given to integrating their feedback into BAS or where are the best locations for placing these sensors. In the absence of reliable, quantitative measurements, the building designer is often left with the only option of providing a supply of fresh air that may or may not need to be conditioned and filtered before entering the building space. Having accurate and current IAQ thermal comfort data would allow the BAS to reduce energy consumption. Making significant progress towards least-energy IAQ requires efforts in both sensor technology and developing experience and logic that effectively utilize the sensor data. METHODOLOGY Acceptable IAQ and thermal comfort is typically not obtained through just one approach. It is achieved by applying several methods such as contaminant source control, adequate filtration, humidity management, and proper ventilation [6]. Much of the perceived conflict between IAQ and energy efficiency results from just two elements of an energy strategy-the tendency to minimize outdoor air ventilation rates and the willingness to relax controls on temperature and relative humidity to save energy. Energy reduction activities are generally recognized as having a significant potential for degrading the indoor environment and causing problems for the building owner and occupants [6]. To help achieve a balanced approach between IAQ, thermal comfort, and energy savings, numerous approaches have been tried or suggested. The goals of most of these include [4]: 1. Ensure that the energy conservation measures have no adverse influence on IAQ or thermal comfort. 2. Quantify the improvements in IAQ and the energy reductions resulting from implementation of the energy conservation measures. 3. Verify that selected IAQ parameters satisfy the applicable guidelines and standards. To verify that these goals have or have not been met will require measurements and analysis. To reduce the uncertainty to acceptable levels for the calculations and comparisons, numerous measurements will be needed. The economics demand that the measurements be made by low cost instruments/sensors and for the reduced uncertainties the measurement must be accurate and located in appropriate places. Some examples of improving thermal comfort or IAQ while providing energy conservation are described in the following and more details are contained in reference [4]. To ensure thermal comfort one would measure air temperature, mean radiant temperature, relative humidity, and air velocity at multiple heights in multiple workspaces. Instrumentation cost for one workspace would exceed $5k. This system could be moved from place to place providing only short-term data at each location. Monitoring and control of outside air ventilation rates which can directly influence energy costs and IAQ have often included measurements that account for natural and mechanical ventilation, outside air flow into air handlers, and in various ducts. If measurements can be made on a continuous basis, real-time ventilation control can be implemented. However, these measurements are often expensive, hard to use, and cost over $10k, in total. The final example is the real-time measurement of CO2. CO2 concentration measurements are often used to monitor occupancy rate and ventilation efficacy which may indicate other pollutants associated with occupancy. In some buildings this does not work well because of temporal variations in occupancy, uncertain rates of CO2 generation, and concentration changes are on different time scales than occupancy or ventilation changes. Typical cost for CO2 analyzers are $1k or more. Lower cost sensors are now available for ~ $300 that are reasonably accurate although baseline drift can be an issue. When attempting to achieve acceptable IAQ via ventilation, ASHRAE 62-2001 offers two methods for determining the amount of outdoor air required to properly ventilate indoor occupied spaces. The two methods are the Ventilation Rate Procedure and the IAQ Procedure. The Ventilation Rate Procedure prescribes both the quantity and the quality of the ventilation air necessary to assure adequate dilution of contaminants generated in the occupied space. A three-step approach is described [6]: • First, determine the quality of the outdoor air. The Standard cites the levels for particulate or gaseous contaminants. Particulate or gaseous filtration is necessary if outdoor air exceeds the threshold levels referenced. • Second, if the outdoor air is unacceptable, the Standard advises that it be cleaned or filtered. • Lastly, determine the amount of outdoor air required in each space. Standard 62 states: “IAQ shall be considered acceptable if the required rates of acceptable outdoor air in Table 2 are provided for the occupied space.” Table 2 lists the amount of ventilation that must be supplied by room type and by volume per person or by volume per square foot. Although the Standard 62 is prescriptive, concise, and complete on most issues, it is unclear on others. For instance, in unoccupied space, contaminants could accumulate and the Standard 62 does not call for ventilation unless it is needed to prevent contaminants that are injurious to people, contents, or structure. Many contaminants are difficult to measure so the designer may need to do routine ventilation, when not really needed. The IAQ Procedure is a performance-based method for obtaining IAQ by setting limits on contaminants. In particular, the procedure sets the limits for 10 contaminants. Tables are given that list the levels that must not be exceeded; however, acceptable IAQ is not necessarily assured. “Tables B-1 and B-3 do not include all known contaminants that may be of concern, and these contaminants limits may not, ipso facto, ensure acceptable indoor air quality with respect to other contaminants.” (Section 6.2.1 of ASHRAE Standard 62-2001) This procedure also addresses odor specifically. The designer cannot comply with the IAQ Procedure without a subjective evaluation of the completed system. In summary, the Standard provides an alternate method for obtaining acceptable IAQ through direct contaminant control. This would seem to be the preferable method from an energy conservation perspective; unfortunately, the Standard only includes a short list of contaminants and requires post-design, subjective evaluation for odors. The requirements to ensure IAQ for contaminants and odors that are not specified and often difficult to measure (and health limits may not be established) leave the designer without a clear definition of acceptable indoor air quality. Consequently, most designers prefer the Ventilation Rate Procedure because of its more prescriptive and clearer requirements, even if it may result in more energy consumption. When applying either of these two methods, complications arise when there are multiple-space systems, occupied and unoccupied spaces, intermittent occupancy, building use changes, and other dynamic situations. The ASHRAE Standard may not require measurements; however, the dynamics of providing building ventilation tend to make continuous measurement essential to real-time control strategies. What are the common indoor air pollutants? The list includes: Formaldehyde Asbestos Radon Tobacco Smoke Combustion by-products Household chemicals Pesticides Allergens & mold The concentration that may cause health effects vary with each pollutant type and can range to < 1 part per million (ppm) to a few percent [7]. As buildings have become better constructed and better sealed, air changes per hour have been reduced significantly; as much as 200%. Some research has shown that for each 30% reduction in air exchange, a 43% increase in indoor air contaminants resulted [7]. Because most buildings based the ventilation requirements on air flow per person per type of room, designs are usually based on minimum level of outdoor air ventilation required for the designed occupancy level. It has been argued that the CO2 levels in a room are a good indicator of the occupancy in that space and can be used for controlling ventilation. Numerous studies and reviews have been conducted for CO2-based demand-controlled ventilation [4, 6, 8, and 9]. Various methodologies have been applied for different spaces from homes to schools to hotels to offices. These efforts have included field experiments and computer modeling and simulations. General consensus is that CO2-based ventilation control is effective for public buildings, educational facilities, and retail establishments [8]. CO2-based ventilation control also appears to be effective in the office environment if there is: • unpredictable variation in occupancy • heating and cooling required most of the year, and • low pollutant emissions from nonoccupant sources Energy savings were documented or estimated to range from 5% to over 25% with active demand ventilation control. The exception for this control methodology is where nonoccupant contamination sources are significant. These sources could include building materials, humidity control effects (mold), and outdoor sources. Without sensors and measurements, maintaining a baseline ventilation rate at all times has shown to be adequate and reduce the amount of energy savings [8]. Significant energy savings has been demonstrated using CO2 as an indicator of the number of occupants in large rooms [10]. Approximately 35% savings were found in airconditioning costs and 25% 50% on overall HVAC costs were estimated if implementation was completed correctly. This system included on-demand ventilation; however, no mention was made concerning the rooms’ IAQ. Once again the balance between IAQ and energy savings is highlighted as well as the need for sensors. Two major issues for CO2 instrumentation are performance (accuracy, reliability, stability) and location. Not much detailed work has been done in determining the proper location for sensors, experimentally or by the creation of predictive tools [11]. There is evidence that CO2 sensors are adversely affected by humidity and temperature (resulting in baseline drift) and calibration issues. These types of concerns have been raised for humidity sensors as well. An excellent survey on sensor and measurement methods is contained in the Indoor Air Quality Handbook [11]. In 2001, finding measurement technologies that were well suited for monitoring IAQ was difficult. The situation has improved some but is still far from ideal. There are several reasons for this: 1. For the most part there is a lack of regulations in place for IAQ. 2. This couples with the lack of a large market so that the incentives for significant R&D investments are not there. The improvements in performance and the reduction in cost have progressed slowly for IAQ instrumentation. Individual gas sensors have dropped from in the $1k-$2k range in the 1970’s to $500 each in the 1990’s to $250 range in the early 2000s (Table 1). Power consumption is often too high for battery use (10s of milliwatts) or scavenging power from the environment (microwatts) TABLE 1. COMPARISON OF CURRENT SENSOR TECHNOLOGIES Sensor Power Cost Size Humidity <1 mW $10 Micro