1、Biowall for Improved IAQ in Residences Bhargav Rajkhowa William Hutzel, PE Student Member ASHRAE Member ASHRAE Reinhard Mietusch Osama Alraddadi ABSTRACT With the rise of energy efficient homes, indoor air quality poses a difficult challenge between balancing energy conservation and the need to main
2、tain a healthy indoor air environment. Maximizing house insulation and making a residence airtight are basic approaches to reduce energy consumption for heating and cooling. However, this can create stale air inside the house that needs to be supplemented with fresh air at regular intervals. ASHRAE
3、62.2 (Ventilation and Acceptable Indoor Air Quality in Low-Rise Residential Buildings) defines the minimum ventilation requirements based on a buildings square footage and the number of occupants. Natural ventilation, mechanical ventilation, and energy recovery ventilation are common technologies us
4、ed to introduce fresh air while conserving energy. A living plant-based filter is being developed and tested to complement these traditional ventilation or air cleaning strategies. The device has the potential to improve indoor air quality while reducing the quantity of air needed for ventilation, w
5、hich creates the potential for energy savings. INDOOR AIR QUALITY CHALLENGES Indoor Air Quality (IAQ) is still a serious issue in buildings. The environmental protection agency (EPA) classified the problem as one of the top health concerns in the U.S. (EPA 2009). According to William Fisk from Lawre
6、nce Berkeley National Laboratory, asthma, sick building syndrome (SBS), allergies, and other respiratory illnesses are highly associated with poor indoor environments. Health risks are not the only indoor problem because polluted indoor air environments were also found to be negatively affect produc
7、tivity, which costs the U.S. up to $150 billion a year (Fisk 2000). A significant contributor to the pollution in indoor air are Volatile Organic Compounds (VOCs) that are emitted from many household products and construction materials. Adhesives and paint can emit toluene and benzene while new clot
8、hing and carpets off-gas formaldehyde, and these are just some of the examples of VOCs found in buildings (Spengler and Chen 2000). Therefore, ventilation with outside air or indoor air-cleaning devices have become important in residential buildings, especially the airtight ones. OR-16-C002Figure 1
9、shows an airtight home which minimizes the interaction between cold outdoor air (OA) with warm indoor air (IA). But due to VOC emissions from various sources the IA between becomes stale and polluted. This is where biowalls (BW), with their potential for phytoremediation, could prove useful. A BW, w
10、hich would be installed in the return duct of the homes HVAC system, would clean the air and then send it back through the homes Air Handler (AHU). Figure 1 Biowall application in airtight buildings Botanical Air Filtration In the 1980s, Bill Wolverton tested a plants ability to remediate indoor air
11、 from VOCs, mainly for energy efficient homes and space stations (Wolverton et al. 1984). In a closed Plexiglas chamber where temperature and humidity were controlled, Wolverton showed that a variety of plants had the ability to remove formaldehyde from air at different rates and efficiencies. In th
12、e late 1990s, Alan Darlington and his team installed a large botanical air filter inside a building to investigate both positive and negative impacts of the botanical filter to the building. Darlingtons hydroponic system, which pulled air through both aerial parts and the root zone of the plants, pr
13、esented removal efficiency up to 70% of the total VOC without showing significant negative impacts. The system maintained acceptable IAQ despite using much less outdoor air than a similar building (Darlington et al. 2000). In 2010, Wang and Zhang researched another dynamic botanical air filter integ
14、rated into an HVAC system. They found that their botanical air filter could safely reduce the outside air ventilation up to 25% with a potential energy savings up to 15% based on the climate of Syracuse, NY (Wang and Zhang 2011). BIOWALL IN SOLAR DECATHLON In 2011, a team from Purdue University buil
15、t a net-zero home as a part of the Solar Decathlon competition, where Purdues BW was first presented. Figure 2 shows the external view of the home to the left and the installed BW to the right. As the main goal was to reduce the ventilation energy consumption while maintaining IAQ, the aeroponic BW
16、was installed in one of the air return ducts of the HVAC system of the house. The test was conducted in West Lafayette, IN for a week in August. The BW performance inside the house was evaluated with and without an ERV in operation. Admittedly though these tests were run in a fully operational home
17、and without benefit of experimental controls for weather, occupancy, etc. (Rodgers et al. 2012). 2016 ASHRAE Winter ConferencePapers 2Figure 2 External view of net zero energy home and a biowall in the home Integrating the BW into the HVAC system showed a possible reduction in ventilation energy con
18、sumption. Taking the electric power of the HVAC system into account, the combination of an ERV and a BW showed the potential for a ventilation energy saving of up to 35%. According to Rodgers, these savings were scaled to one summer month of HVAC system operation. A potentially higher saving could b
19、e achieved if winter conditions were incorporated in their estimation (Rodgers et al. 2012). INITIAL LABORATORY TESTING FOR IAQ AND ENERGY SAVINGS After the BW was presented at the competition, the botanical filter was moved into an Environmental Chamber (EC) to optimize its design. The second BW it
20、eration had 12 different plants and was placed inside the EC to test the removal rate of toluene from air in a more controlled environment. Pull-down tests were performed where a known amount of toluene was introduced inside the chamber and its time dependent decay was monitored. The first test moni
21、tored the decay inside the chamber without the BW while the second test was done with the BW inside the chamber. (Newkirk et al. 2014). The two decay rates were compared using a t-test that showed a statistical significance (P0.01) in the ability of the BW to improve IAQ. Newkirk also created an ene
22、rgy model to estimate the potential energy savings of the BW for residential energy efficient buildings on an annual basis. The energy model estimated up to 30% potential energy savings in the operation of the HVAC unit. Although this initial EC testing produced BW results that were more repeatable
23、than testing in a full scale house, there were still concerns about some of the testing procedures and instrumentation used. The researchers also concluded that the EC, while useful for BW design, would never come close to the level of scientific testing achieved in laboratories where gas phase filt
24、ers are evaluated according to ASHRAE Standard 145.2 (ASHRAE 2011). IMPROVED BIOWALL TEST APPARATUS In the fall of 2014 a new BW test apparatus was built and used to carry out a series of tests to help refine the design of the BW. It allowed researchers to observe the interaction between the plants/
25、growth media and the ventilation air for a house using a single vertical plenum with plants placed in a horizontal box with growth media. Figure 3 is a schematic showing the test apparatus set-up in the environmental chamber. It consists of a growth media box (A) where various growth media and plant
26、s can be tested, (B) sensors to measure temperature, relative humidity (RH) and CO2 in the EC and BW to record the BWs impact on the ECs characteristics. The apparatus also consists of a bypass tube (C) in order to vary the airflow across the BW and a variable speed fan (VSF) (D) which was used to e
27、valuate the air flow characteristics of the BW. Adequate light and water were provided manually. 2016 ASHRAE Winter ConferencePapers 3Figure 3 Test apparatus flow chart and position in the environmental chamber. (A) Growth media box. (B) Temperature, RH and CO2 sensor. (C) Bypass tube. (D) VSF. Air
28、Flow Characteristics After reviewing various hydroponic growth media, the BWs plants were placed in a hydroponic filter medium consisting of coco coir (CC) and growstone/activated carbon (G/AC). CC is a natural fiber extracted from the husk of coconuts. Growstone is a plant substrate made from recyc
29、led glass. AC is a form of carbon processed to have small, low-volume pores that increase the surface area available for adsorption or chemical reactions. In order to obtain the pressure drop across the BW for various test velocities tests were carried out using different filter media compositions.
30、Figure 4 illustrates the relationship between the pressure drop per centimeter of thickness for the various filter medium compositions used. The vertical axis is the pressure drop per centimeter of thickness of filter medium and the horizontal axis is the composition of the growth medium used. The f
31、ace velocities that were used for testing are also used shown in the graph. A volumetric water content (VWC) of 20% was maintained for each of the tested mixtures. All growing media mixtures had a constant volume, which meant that the total thickness was kept constant. The different growing media we
32、re mixed as homogeneously as possible to ensure a uniform composition. A D B C 2016 ASHRAE Winter ConferencePapers 4Figure 4 Pressure drop per cm of thickness for various filter media compositions. VWC = 20% During testing the air velocity was changed and the pressure drop for each new speed was rec
33、orded. The pressure drop per centimeter of thickness for various face velocities was then calculated. Figure 4 clearly shows the expected result, namely that the pressure drop across the BW increases with airflow, which also increases the fan power consumed. Knowing the pressure drop across the BW b
34、oth for its thickness and filter media composition should allow appropriate sizing of the filter bed and also aid in the selection of a filter medium with the lowest possible pressure drop. Based on this data, an appropriately sized BW fan can be selected. BIOWALL ENDURANCE TESTING In the spring of
35、2015, a series of endurance tests were conducted using the BWs test apparatus in the EC. The plant species tested during these endurance tests were golden pothos, philodendron, spider plant, spearmint and thyme. The golden pothos, philodendron and spider plant have the ability to endure dry climates
36、 and the spearmint and thyme, in addition to acting as air-fresheners, were used to explore the BWs ability to grow food. The purpose of these tests was to collect information about the BWs impact on the temperature and RH for airflow across the BW. The tests also allowed observations of the impact
37、of the moisture content in the on pressure drop. This will allow the development of a control strategy to maintain acceptable RH levels in the residence while ensuring that the plants have enough water to stay healthy. 2016 ASHRAE Winter ConferencePapers 5Procedure Three tests were carried out in th
38、e EC. Each test was six to eight days in duration. In order to carry out these tests, the test apparatus fan was run on a duty cycle using a timer. The duty cycles chosen were 25%, 50% and 75%. A 25% duty cycle meant that the fan was ON for 15 minutes (min.) an hour (h), 50% was 30 min. /h and 75% w
39、as 45 min. /h. The initial fan speed was adjusted to maintain 0.1 m/s (20 ft/min). The BW has an approximate area of 0.453 m2 (5 ft2) which defines the total flow across the BW as 0.0471m3/s (100 ft3/min.). A duct traversal was carried out to ensure that the fan was running at the appropriate speed.
40、 The parameters monitored during each of these tests are listed below: 1. Pressure drop across the BW (Pascals/inches H2O).2. Weight measurements (kg.) at regular intervals to determine moisture loss.3. Temperature and RH in the BW.4. Temperature and RH in the EC.In the first series of tests, the te
41、st apparatus fan was turned ON for the selected duty cycle and the data was recorded. As the duty cycle increased, plant watering also increased. The effect of the increase in supplied water on the pressure drop across the BW was also recorded at regular intervals. The pressure drop across the BW wa
42、s found to be inversely proportional to its water content. The health of the plants was also monitored daily. Figure 5 shows the changes in RH for the 75% duty cycle test. The RH is plotted on the Y-axis and time is plotted on the X-axis. The red line is RH after the biowall, the yellow line is RH b
43、efore the biowall, and the green line is the difference (BW-EC). The spikes on the graph occurred as the de-humidification system cycled on and off. In addition, the RH across the BW increased significantly after the addition of water. Figure 5 Endurance RH data for test 3 2016 ASHRAE Winter Confere
44、ncePapers 6An air handler on a set schedule was used to maintain the temperature in the EC. It ran from 6AM to 9PM every day and circulated the air in the EC to maintain the temperature. Though temperature was fairly constant, this schedule did have a noticeable effect on relative humidity. Once the
45、 fan turned OFF the RH both across the BW and the EC increased and then went back down once it turned ON again. During the next phase of testing, the air handler was programmed to maintain a constant temperature and RH, and the moisture loss across the BW for each duty cycle was recorded to develop
46、an effective moisture control strategy. Other important parameters such as temperature, RH and pressure drop were also recorded. The data collected from these tests was used to develop a control algorithm for watering the plants to ensure that the plants received sufficient water while still maintai
47、ning comfortable RH levels in the house. AIR CLEANING EVALUATION A new test procedure is being developed to more accurately measure the BWs role in removing VOCs and improving IAQ. These tests will be conducted in two steps. The first step will introduce a particular VOC (toluene) into the EC and ci
48、rculate contaminated air through the BW apparatus. The rate of VOC removal will be determined by monitoring the VOC level in the EC over time and characterizing the performance in terms of an exponential decay. Then the VOC test will be repeated a second time without the BW present. The difference i
49、n the two tests (with and without BW) will be used to quantify the impact of the Biowall. Figure 6 illustrates components of a new data acquisition and control apparatus for VOC testing that has been developed. The dispersal rate and overall concentration of the toluene are controlled by a programmable syringe pump and introduced into the EC via an atomizing nozzle fixed to the syringe. The conditions in the EC will be monitored, displayed, and recorded using a data acquisition platform developed specifically for this purpose. All components of the gas dis
