Plant-assisted air-conditioning systems

for a better tomorrowDan Newkirk, J. Spencer Evans, Osama S. Alraddadi,

Caroline G. Kelemen, Reinhard Mietusch, Yu Xue, and Bhargav Rajkhowa

IEEE PotEntIals January/February 2015 n 110278-6648/15©2015IEEE

Elemental Engineering

Biofiltration to remove con-taminants from the air has a long history, dating to early research about how to safely maintain deep space flight. Recent-

ly, several research teams have used this body of work to apply biofiltra-tion to the heating, ventilating, and air conditioning (HVAC) systems of buildings. A plant-based air filtration system called the biowall has been developed as a method to maintain high levels of indoor air quality (IAQ) without losing energy eff iciency. In this design, various plants are inte-grated into the ventilation sys-tem. As a byproduct of plant growth, CO2 and volat i le organic compounds (VOCs) are removed from the air and oxygen is added. This system allows condit ioned air to remain in the building and avoids energy losses due to thermal mixing in convention-al ventilation systems.

Indoor air qualityConcerns about IAQ were first expressed in the 1950s when corre-lations were established be-tween polluted indoor air, allergies, and

other chronic illnesses (Randolph, Theron, and Moss, 1980). In the 1970s, with buildings becoming increasingly airtight and isolated from the outside environment, the problem of polluted indoor air became an even greater concern. The IAQ problem became more severe as

airflow rates were lowered to reduce energy use in buildings.

In the developed world, IAQ is ex-tremely important, as most individu-als spend up to 90% of their time in-doors (U.S. Environmental Protection

Agency, 1989). Toxic air that has re-cently been widely reported in China and India is a prime example of poor IAQ. In fact, the World Bank has de-clared indoor air pollution in develop-ing countries as one of the four most critical global environmental prob-lems. The Environmental Protection

Agency (EPA) has reported IAQ to be among the top five public health risks in the United States.

Poor IAQ has led to a number of health issues for building occupants. The symptoms include headaches,

Digital Object Identifier 10.1109/MPOT.2014.2358911 Date of publication: 8 January 2015

© can stock photo/yuryz

12 n January/February 2015 IEEE PotEntIals

dizziness, nausea, fatigue, irritation of the eyes, and general malaise. These illnesses have been defined as Sick Building Syndrome (SBS) by the World Health Organization (WHO). It was estimated in 2000 that the Unit-ed States lost US$125 billion annu-ally in decreased productivity due to poor indoor air quality.

Estimates indicate that the Unit-ed States can save US$40–200 bil-lion per year simply by improving its IAQ. Poor IAQ has also been linked to other severe illnesses such as can-cer, kidney and liver damage, nerve damage, and loss of coordination.

In addition to the effects of ill health, there is also the added con-sequence of additional energy con-sumption to maintain the IAQ. In

urban areas, this problem is further compounded as the outside air is usually highly polluted, and the cost of purifying and conditioning this air further increases the ventilation energy cost.

With these detrimental effects in mind, plants, with their natu-ral ability to purify air and remove pollutants, are increasingly being viewed as a solution to the problem. In addition, there are also the vi-sual and psychological benefits that plants have on humans. The possi-bility of reducing energy consump-tion by naturally cleaning indoor air has further attracted interest in this area, which is broadly classified as phytoremediation.

Phytoremediation historyAlthough receiving more attention recently, the concept of using plants to improve IAQ really began when humans first considered trav-eling to space. The fundamental issue that scientists questioned was how to create a closed system that could be self-sufficient indefi-

nitely in deep-space missions. Rus-sian scientist Konstain Tsiolkovsky, who imagined bioregenerative life support systems for space travel, first considered the theoretical solution in the early 20th century. Vladimir Vernadsky, who theorized mimicking the systems of earth’s biosphere to obtain life support, further developed the concept. The two are often credited by Russian scientists as the fathers of space travel (Salisbury, Gitelson, and Lisovsky, 1997).

It was not until the 1950s that experimental data were gathered to test these theories. It began with Clare Folsome creating small, closed ecosystems in flasks that functioned effectively until their

research ended in the 1980s. In the 1960s, two scientists, Yevgeny Shepelev and Gana I. Meleshko, at the Institute of Aerospace Medicine began studying the exchange of re-sources between humans and Chlo-rella, obtaining a sealed system for about a year (Salisbury, Gitelson, & Lisovsky, 1997).

However, the first major break-through in bioregenerative life support systems came with the con-struction of BIOS–1 in 1965. It was quickly realized that higher plants were required for their ability to fil-ter air, and when chambers were added to accommodate these plants the project moved to BIOS–2 in 1968. After some preliminary analy-sis, opportunities were found to bet-ter recycle mass within the system, specifically water and the project moved to BIOS–3 in 1972. BIOS–3 housed humans with minimal out-side input for over a year and is still conducting research. Since its con-struction, the NASA program has also begun work in bioregenerative systems in projects like Bio-Plex,

Biosphere 1 and Biosphere 2 (Salis-bury, Gitelson, and Lisovsky, 1997).

nasa researchOver 35 years ago, when high con-centrations of pollutants were found in closed space vehicle environments NASA researchers started investigat-ing the use of plants to improve IAQ in these airtight compartments so as to maintain a healthy breathing zone. Their studies showed that plant leaves, though useful in reduc-ing the pollutant levels, were not sufficient considering the surface area required. At the same time, they also investigated the potential to amplify plant cleaning capacity by capitalizing on the productivity of micro-organisms in the root region of the plants.

In additional research conducted at NASA’s National Space Technolo-gy Laboratories (NSTL) in Southern Mississippi by NASA scientist Bill Wolverton, phytoremediation proved successful in treating domestic sew-age and removing toxic chemicals including radioactive elements from soil and wastewater. In 1984 and 1985, Wolverton also demonstrated the first evidence that house and of-fice plants and the microorganisms associated with their roots were ca-pable of removing indoor air pollut-ing chemicals such as VOCs, alco-hols, aromatic hydrocarbons, and human bio-effluents as well. Wol-verton claimed that the air-cleaning capacity of common plants can be amplified by 200 times through the use of hydroponic systems, high-efficiency carbon filters, and a root-level circulation system.

Following the precedent of Wolver-ton, Alan Darlington combined bio-filtration with the phytoremediation process to improve the IAQ by remov-ing both VOCs and CO2 from the air as it passes through the wall into a building’s office space. Biofiltration is defined as the process of drawing air in through organic material (such as moss, soil, and plants), resulting in the removal of organic gases (volatile organic compounds) and contami-nants (Aydogan, 2012).

In the developed world, IAQ is extremely important, as most individuals spend up to

90% of their time indoors.

IEEE PotEntIals January/February 2015 n 13

In light of these discoveries about phytoremediation, vegetation systems are increasingly being in-corporated in buildings to help im-prove the IAQ. Currently, there are three types of vegetation systems that are used.1) Passive vegetation systems—sys-

tems where outdoor plants are brought into the indoor environ-ment for their physiological and psychological benefits and ability to produce oxygen and generate humidity.

2) Active vegetation systems—sys-tems added to the indoor environ-ment to utilize the passive air cleaning capacity of plants.

3) Building-integrated active vegeta-tion systems—a system that is incorporated into the HVAC system of the building and actively helps to remove indoor air pollutants and lowers energy consumption at the same time. Purdue’s biowall is an example of a building-inte-grated active vegetation system.

system designFigure 1 shows a biowall that is integrated with the central heating and cooling system of a home. It leverages the natural ability of plants to metabolize airborne con-taminants to improve IAQ. Contami-nated air from the building passes through the biowall before being routed by the return air ductwork to the fan/coil unit of the air condi-tioner. As the air passes through the filter, a portion of VOCs present are trapped and absorbed by plants. These VOCs are then broken down further by beneficial microbes in the root zone into CO2 and nitrogen.

Experimental designAn experiment was designed to test the ability of different plants in the biowall to remove gaseous contami-nants from air in real time. The basic methodology was adapted from earlier biofiltration research, where a known contaminant was introduced into a test chamber and the exponential decay was moni-tored (Wang and Zhang, 2011). For

this research, approximately 20 ml of toluene was introduced into a test chamber. The concentration of tolu-ene was measured over a 300-min period. After data collection, the tol-uene values were normalized based on the peak concentration and an exponential decay curve was fit in order to calculate a time constant.

Figure 2 identifies the major fac-tors that impact VOC experiments. Surface deposition and air leakage, the outer rings of the image, are functions of the test chamber. Ad-

sorption and absorption (the middle ring) are a function of the filter me-dia. The “bull’s-eye” of Fig. 2 is air-cleaning by microbes that are pres-ent in the root structures of plants. This was the basic mechanism tar-geted for study during this research.

Data and findingsThe initial phase of biowall research in a controlled laboratory environ-ment has been completed. Prelimi-nary results relating to IAQ and the potential for energy savings are reported here.

Improving indoor air qualityFigure 3 shows the IAQ data for the generation and decay of toluene. The normalized toluene concentration within the chamber is shown on the y-axis, starting from 70%. The dura-tion of the test on the x-axis is expressed in minutes. In one trial the biowall with plants was present in the chamber and air was pulled over the plants. In the other trial the plants were absent. In both cases the curves peak around 100 min and fol-low one another until 180 min, at which point the decay rates deviate. Both decay scenarios were character-ized by an exponential decay.

The difference between these two curves provides insight on the effect the biowall has at improving IAQ.

fig1 plenum for Biowall. (a) Isometric view. (b) cross-section view.

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fig2 the research targeted IaQ im-provements due to microbes.

14 n January/February 2015 IEEE PotEntIals

Based on the exponential decay con-stants, one can claim that the pres-ence of a biowall can help break down VOCs three times faster than the nat-ural decay rate.

Data was also gathered regard-ing the biowall producing any mold, spores, or other pathogens using a particulate sensor. Nothing unex-pected was found in the air stream.

Reducing energyAdditional data was collected to develop a model of the biowall’s per-formance in an energy-efficient resi-dence. Figure 4 is a simple cross section of the biowall showing the path for air. Some air passes through the growth medium and some is bypassed. Despite the bypass, almost 100% of the air mol-ecules eventually run through the biowall due to the constant circula-tion. Temperature, relative humidity, and airflow data were collected before and after the biowall. Not sur-prisingly, passing air over the filter decreased the temperature and increased the relative humidity.

The electricity consumed by the biowall was also monitored for use in the thermodynamic model. This experiment used electricity for light-emitting diode grow lights, a fan to pull air across the biowall, and a conden-sate pump for removing excess water. The average power for all components was approximately 100 W. The results of the energy modeling shows that the biowall is beneficial for reducing ener-gy consumption in all climates zones,

particularly when used in conjunction with a energy recovery ventilator.

summary and advantages

Improved human healthThe biowall has been shown to improve IAQ and have a positive impact on human health. Using tol-uene, a known neurotoxin found in paints, adhesives, and personal care products, the biowall showed a three-fold faster decay than not hav-ing a biowall with a statistically sig-nificant data set. This filtration would improve the life of the home-owner through increased productivi-ty and decreased respiratory illness. Furthermore, the biowall has been shown not to produce additional allergens into the system.

Economic viabilityTo compete with other sustainable technologies, the biowall must also be economically viable. The biowall created to conduct the experiment cost approximately US$2,500. With more work in this area, it is believed that the material costs could be cut

in half. With added installation fees, a biowall would cost the homeowner around US$2,000. To become an economically viable option in the resi-dential sector, the simple payback period should be fewer than ten years. This would mean an annual energy savings of US$200 per year.

Although not quantified for the initial experiment, it is also impor-tant to consider the life cycle en-vironmental cost impacts. In this case, the upfront cost of the device is the environmental impact of the materials and methods used in manufacturing. The annual savings would be from both the energy sav-ings (reduced natural resource con-sumption) as well as the diversion of additional air pollutants into the atmosphere due to the plants’ filtra-tion abilities. For annual costs, there would be the water and fertilizer used to keep the plants alive.

Reduced natural resource consumptionTaking the biowall’s energy impact one step further, a thermodynamic model was created to assess its per-formance in an energy-efficient resi-dence. The characteristics for this model home were based off Purdue University’s 2011 submission to the U.S. Department of Energy’s Solar Decathlon Competition. This ther-modynamic model was then evaluat-ed using typical meteorological year data for outside conditions.

Reasonable assumptions about the performance of a commercial bio-wall had to be made to complete this thermodynamic analysis because our Phase I research results were limited in scope. We only measured the in-stantaneous contaminant removal capacity of plants by themselves. An EPA Phase II project is needed to eval-uate plants when they are coupled with a carbon filter. The carbon filter will capture and bind the contami-nants while the plants slowly metabo-lize them over time.

We relied on data from a biofil-tration research project at Syracuse University that achieved Clean Air Delivery Rates (CADR) as high as

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fig3 the toluene decay rate with and without the Biowall.

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fig4 Biowall energy considerations.

IEEE PotEntIals January/February 2015 n 15

1200 cfm using a carbon-based fil-ter as a growing medium (Wang and Zhang, 2011). For the purposes of our model, a modest clean air deliv-ery rate of 30 cfm was used, which is about half of the outside air require-ments for achieving good air quality in a house.

The biowall was also evaluated under a number of different sce-narios. These scenarios include combinations with and without an energy recovery ventilator (ERV) and a biowall. Three different geo-graphic locations were also tested to evaluate the biowall’s impact in dif-ferent climate zones. The results are included in Fig. 5, where the sce-nario is listed on the x-axis and the building energy intensity is listed on the y-axis. The cooling and heating columns have been stacked to give an idea of how they affect the total energy consumption.

A number of conclusions about energy reduction can be reached.■■ The combination of a biowall and

an ERV used the least energy.■■ The choice between an ERV and

a biowall depends on climate.■■ A biowall saves energy as com-

pared to a home with no energy-saving technologies.

Future workThe biowall project is an example of new technologies that are part of a broader movement toward achieving both sustainability and energy effi-ciency in the built environment. High-performance buildings are being designed and constructed that are extremely energy efficient, some-times even net zero in terms of their energy requirements from the utility grid. However, it is widely recognized that these innovative living and working spaces won’t become the norm unless they are also comfort-able and healthy for the occupants.

activated carbon filterActivated carbon is a porous materi-al that has a large surface area to volume ratio. A pound of highly acti-vated carbon has a surface area of approximately 140 acres. As con-

taminated air passes through the carbon filter, the molecules of con-taminants will be trapped in the pores in the interior of the material.

By combining the activated car-bon filter with the current biowall, we expect to see an increase in in-stantaneous absorption rate because activated carbon has the ability to trap the contaminant as soon as it passes through the filter. The current idea is the use of plants to rejuvenate the filter, as plants have the ability to slowly remove the trapped molecules.

system optimizationA single control system for all the physical supplies including lights, water, and a fan will be developed. The material used in the watering system will also be upgraded and, because it is a highly pressurized environment, both the pipeline and the nozzle need to be reinforced, so as to minimize the risk of leaking.

The growth condition of plants will be re-evaluated. The harshness of the environment on the biowall made the establishment of plants

slow and hindered. In phase II, the intention is to narrow down the number of species.

ControlsOne of the essential components of today’s efficient residences is an energy management system (EMS). The energy manager monitors, con-trols, and manages energy use of homeowners on a demand basis to reduce the energy consumption based on the available energy resources at the residence. EMSs

also provide real-time energy con-sumption, a control for homeowners over their energy usage, and various types of alarms. The biowall’s com-ponents, on the other hand, will be integrated to the EMS to control irri-gation and lighting systems and to regulate and monitor relative humid-ity (RH) and temperature levels inside the residence.

Irrigation and lighting systems are vital in the biowall to maintain plant growth and to keep plants alive and healthy. Both systems consume

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fig5 Biowall’s impact on annual hVac energy use.

Based on the exponential decay constants, one can claim that the presence of a Biowall can

help breakdown VOCs three times faster than the natural decay rate.

16 n January/February 2015 IEEE PotEntIals

energy in the biowall, and therefore they need to be monitored and con-trolled. Almost all the demands for water and light for indoor plants are documented in horticulture hand-books, which provide a controlled measure for the energy consumption of lighting and watering. However, as the biowall is integrated into an HVAC return duct, the air passing through the growing medium will dry the roots of the plants faster than the obtained measurements. Consequently, mois-ture sensors will be installed on every module to ensure that the moisture level in the growing medium meets the plant’s demand for water. By hav-ing all the previously mentioned in-puts from watering demand, lighting demand, and sensors, algorithmic code is written on the EMS to manage the supply of water and light to the plants in an energy-efficient manner while maintaining the growth and health of plants. The EMS will have two digital outputs. One output signal will go to a solenoid valve connected to the irrigation lines to control water-ing. The other binary output will turn the lights either on or off.

The air passing through the biow-all, when it is returned to the HVAC system, will certainly cause changes in the RH and temperature levels. As the growing medium is moist af-ter being watered, the air will carry moisture on its way back to the air handling unit and will increase the RH level in the environment. This has been identified and proven in the pre-vious versions of the biowall (Newkirk, 2014). One way to avoid the increase in RH is using a drip irrigation sys-tem to provide an optimum amount of water based on the demand of plants. The other way is to incorporate RH sensors, temperature sensors, and heating and cooling demand into the control strategy for watering. For example, watering the plants will be avoided as much as possible during the time of operation of the HVAC sys-tem. Additionally, the RH and temper-ature sensors will be used as alarms whenever changes in RH and temper-ature are detected from the biowall. Thus, RH and temperature variables

and HVAC demands are added as in-puts to the control strategy of water-ing the plants in the biowall.

In brief, incorporating the irriga-tion and lighting of the biowall into the EMS will assist in conserving water, reducing energy consumption, controlling RH levels, and maintain-ing plants’ growth. The control strat-egy has various inputs such as sen-sors. The controlled output are the watering and lighting times.

Modular designThe biowall performed well and increased IAQ like expected, but sev-eral problems arose. The water-tight-ness and necessity of changing plants were a challenge.

As a result, the goal for the next generation is to develop a modular, watertight, and dimensionally op-timized design. In phase II, a long-term test is intended in an energy-efficiency optimized house to show the possibility of integrating and op-erating an easy-to-maintain biowall in a house in Indiana.

Figure 6 shows a vertical cut through a potential biowall design. Some new features include ■■ the three-step module change

ability (1)■■ the quick connect (2) and the

module-integrated irrigation sys-tem using the in-house water line (3)

■■ a design that enables water re-use (4).

The idea is to prepare modules besides the biowall and change them easily if it is required.

Actual investigations are about creating a test bench to■■ optimize dimensions of biowall

modules ■■ evaluate resistances of different

types of growing media■■ determine ideal size and location

of bypass air flow and define optimal amount of water.

Furthermore, a design model will be generated that enables us to deter-mine a biowall for a given environ-ment by using the test data.

Growing media, irrigation, and plant healthThe previous design used a spray irrigation system to water biowall plants. Although the delivery was effective, there were multiple prob-lems with this approach.■■ Distribution was uneven and dif-

ficult to control, with the majori-ty of water eventually ending up at the lower portion of the wall due to gravity.

■■ The moist environment encour-aged a large amount of algae

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fig6 the updated modular Biowall cross section.

IEEE PotEntIals January/February 2015 n 17

growth to occur on the filters and biowall frame, specifically where light intensity was greatest.

■■ Spray irrigation lines require higher pressure, increasing ener-gy use, and risk of line bursts. The previous design also had is-

sues with overall plant health. After a short period of acclimation, plants were transplanted into pockets cut into the filters. Rockwool was used to help the plants take root and es-tablish themselves. This environ-ment proved too harsh for most of the plants chosen to truly thrive and therefore reduced the efficiency of the biowall overall.

In our revamped design, we will be using a lightweight growing media mixed with activated carbon pellets. This will allow air to pass through without the use of filters while trap-ping VOCs near the root zones. Indi-vidual drip lines will be fed to each plant, therefore reducing water waste and algae build up. Different plants will be tested for their heartiness as well as their remediation capabilities. Finally, a new nutrient solution will be incorporated into the water line to ensure optimal plant health. This hydroponic approach, once properly configured, should allow for health-ier plants, larger root zones and a more effective biowall overall.

Real-world applicationThe biowall will be evaluated in a household environment in two years. A custom version of biowall will be built specifically for that house, based on the interior volume, HVAC perfor-mance, temperature, and humidity. The performance of the biowall will be monitored and recorded, and the data will be used to improve the future implication of biowall.

Read more about it • A. Aydogan, “Building-integrated

active modular phytoremediation sys-

tem,” Ph.D. dissertation, Dept. Architec-

tural Eng., Rensselaer Polytechnic Inst.,

Troy, NY, May 2012.

• W. Chen, Z. Gao, J. S. Zhang, D.

Kosar, C. E. Walker, and D. Novosel.

(2006). Reduced energy use through

reduced indoor contamination in resi-

dential buildings, National Center for

Energy Management and Building Tech-

nologies. [Online]. Available: http://

w w w . n c e m b t . o r g / d o w n l o a d s /

ncembt-061101.pdf

• D. Newkirk, “Improving indoor air

quality through botanical air filtration

in energy efficient residences,” M.S.

thesis, Dept. Mech. Eng., Purdue Univ.,

West Lafayette, IN, 2014.

• S. Brasche, M. Bullinger, H. Geb-

hardt, V. Herzog, P. Hornung, B. Kruppa,

E. Meyer, M. Morfeld, R. V. Schwab, S.

Mackensen, A. Winkens, and W. Bischof,

“Factors determining different symptom

patterns of sick building syndrome—

Results from a multivariate analysis,” in

Proc. 8th Int. Conf. Indoor Air Quality and

Climate, Edinburgh, Scotland, 1999.

• U.S. Environmental Protection

Agency, “Executive Summary and Rec-

ommendations,” in Environmental Pro-

tection Agency Report Congress Indoor

Air Quality, Tech. Rep. EPA/400/1-

89/001A, Washington, D.C., 1989.

• B. C. Wolverton, “Houseplants,

indoor air pollutants and allergic reac-

tions,” NASA-TM-108057, NAS

1.15:108057, Natl. Aeronaut. Space

Admin., Natl. Space Technol. Lab., Han-

cock County, MS, Tech., 1986.

• B. C. Wolverton, “Foliage plants for

improving indoor air quality.” NASA-

TM-108055, NAS 1.15:108055, Nation-

al Foliage Foundation Interiorscape

Seminar. Hollywood, FL. : Natl. Aero-

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1988.

about the authorsDan Newkirk (dannewk@gmail.com) graduated from Purdue Uni-versity with a master’s in ecological sciences and engineering. He worked under Prof. Hutzel to lead the biowall project in the 2013–2014 school year. He currently works for Alpha Controls and Services in Champaign, Illinois, making build-ings across Illinois more comfort-able, secure, and efficient.

J. Spencer Evans (evans100@purdue.edu) is a senior at Purdue

University obtaining dual degrees in sustainable agriculture and environmental and ecological en-gineering. He joined the biowall research team in May 2014 after participating in past research on food security issues in western Ke-nya and phosphorus remediation in aquatic systems.

Osama S. Alraddadi (osa@purdue.edu) completed an associate’s degree in mechanical engineering technol-ogy (MET) from Yanbu Industrial College in 2010 and a bachelor’s of science degree in MET from Purdue University in 2014. He is currently pursuing a master’s degree in engi-neering technology with a focus on sustainable energy systems.

Caroline G. Kelemen (ckelemen @purdue.edu) is a senior in envi-ronmental and natural resources engineering at Purdue University. She originally joined the biowall research team in April 2013 but re-cently returned from a semester at the University of Canterbury in New Zealand.

Reinhard Mietusch (r.mietusch@gmail.com) is a mechanical engi-neering student in his final year of studies at the Technical University of Dresden, Germany. His field of ad-vanced study is energy management with a specialization in refrigeration and energy machines.

Yu Xue (xue22@purdue.edu) is a senior in botany and biochemistry at Purdue University. He joined the bio-wall research team in August 2013 and participated in the P3 compe-tition Expo in Washington, D.C., in April 2014.

Bhargav Rajkhowa (brajkhow@purdue.edu) completed his bachelor’s degree in mechanical engineering from The University of Mumbai. He is pursuing his master’s degree in eco-logical sciences and engineering at Purdue University through the College of Technology. He joined the biowall re-search team in August 2014 and will be assisting with the next phase of re-search and development.