Suitability of wetland macrophyte in green cooling tower performance
Graphical abstract
Introduction
The exploration of plant functional traits for applications beyond food and biofuel production continues to draw research interest (Moses and Goossens, 2017) in an effort to promote a green sustainable earth (Shenzhen Declaration Drafting Committee, 2017). Such interest includes temperature influence on plant-mediated oxygen transfer for waste water treatment (Stein and Hook, 2005). In this study, thermal tolerance in heat rejection systems to mitigate urban heat island. Urbanization, constitutes a major anthropogenic signature of global-scale climate change (Alberti, 2015). In cities, increased short-wave solar radiation (due to reduced evaporation) is converted to long-wave thermal and sensible heat that elevates surface temperature, causing heat islands (Schmidt, 2016). Heat island can cause cities to have temperatures 10 °C or more above their surrounding areas (Tam et al., 2015). Urban heat islands and other environmental challenges associated with urbanization are increasingly mitigated using vegetation, integrating ecological design (Schmidt, 2016) and adopting processes such as canopy, shade, or water features and wetland functions.
Constructed wetlands typify ecological adaption, by providing low cost and environmentally sustainable green infrastructure that improves water quality (Moshiri, 1993, Shutes, 2001) and waste treatment (Calheiros et al., 2012, Kivaisi, 2001, Vymazal, 2011). Vertical-flow constructed wetlands (VFCW) are relatively new technologies that further capitalize on the benefits of wetland plants in engineered systems mostly for water quality treatment (Stefanakis et al., 2014). We introduce “thermoGreenWalls™” (tGW) as a model green cooling tower (see Fig. 1) –a vertical-flow constructed wetland, designed to reject thermal energy from the surface of an integrated plant-porous material, leading to a residual cooling effect (Axley and Felson, 2015).
tGW combine the functionality of cooling towers (Bernier, 1995) with green walls technology (Zia et al., 2013, Pacheco-Torgal et al., 2015, Manso and Castro-Gomes, 2016) to yield a hybrid technology that can be scaled down for smaller domestic buildings and up for larger industrial infrastructures. In addition to heat rejection, tGW provide several “green wall” co-benefits including aesthetic, urban agriculture (Sheweka and Magdy, 2011), mitigation of urban heat islands (Eleftheria Alexandri, 2008), bio-filtration (Hopkins and Goodwin, 2011), and carbon sequestration (Charoenkit and Yiemwattana, 2016). Wetland plants, or macrophytes, form the vegetation component of tGW. They perform the climate moderating function of wetland vegetation by regulating the air temperature through evapotranspiration (ET) (Kadlec and Wallace, 2009, Pokorný et al., 2016). The use of macrophytes in tGW raises the question of plant health. First, such macrophytes will be exposed to recirculating water at elevated temperatures essential to optimize heat rejection. Second, the rhizosphere of tGW plants lack a natural growth medium, soil that serves to improve aeration, allow selective capillary filtration, and provide a large surface area for desired evaporation (Ottele et al., 2011). Instead, tGW substrate is made of a polyethylene terephthalate material as the porous growing medium.
Third, to maximize heat rejection, the recirculating water temperature must be higher than the temperature at growth conditions native to macrophytes, but low enough to minimize thermal stress on the plants.
The design and concept of the tGW (Fig. 1) are defined by a governing thermodynamic energy balance:where heat flow into the system, Qin equilibrates heat transferred from the system, Qout at steady state.
Thermal performance is determined by heat rejection as a function of heat loss to the surrounding ambient environment and by the flow rate of recirculating water;
Here, an operating input water temperature, Twin and output water temperature, Twout define both the thermal boundaries of the biological component and the HR capacity of the system. In our study, we performed a hydroponic experiment with three wetland macrophytes submerged at the rhizosphere in different re-circulating water temperatures. Our goal was to determine the optimal temperature, Twopt tolerable by macrophytes that we might select to serve as Twin for a hypothetical green cooling tower, with the tGW to be designed for maximum heat rejection. It is hypothesized that macrophytes, under hydrothermal stress, will attempt to eliminate excess water and heat by increasing transpiration rates, resulting in increased stomatal conductance and carbon assimilation.
Section snippets
Materials and methods
To investigate the effects of elevated temperatures on wetland plants, a hydroponic system was constructed with three temperature treatment tubes, each having 12 plant insertion openings that contained four replicates of I. versicolor, S. cyperinus, and C. lurida in a controlled environment. The length of the tubes was chosen to minimize the temperature variation within them.
Results
The main objective of this research is to determine the physiological response of I. versicolor, S. cyperinus and C. lurida to elevated temperatures, with the goal to find an optimal temperature for operating the tGW at high HR capacity.
Discussion
This study investigates the tolerance of wetland plants to elevated water temperatures for their potential use in tGW. We find no significant statistical difference between the control response and the responses at 35 °C and the 40 °C-pulse treatment for several indicators. Further, we find no statistical difference among the responses across different species, which indicates that all these species are suitable for HR in tGW. Our results for the cumulative and temporal responses are summarized
Conclusion
Using three physiological indicators, we evaluated the tolerance of I. versicolor, S. cyperinus and C. lurida rhizospheres to recirculating water temperatures of 25 °C, 35 °C, and 40 °C-pulsed treatment. We found that the plant biomass increased in all cases above ambient water temperature. The maximum stomatal conductance, Gs, was attained at week 5, with a value that depended on the water temperature and plant species. After week 5, the peak stomatal conductance declined to its initial value
Acknowledgements
We acknowledge NSF-CBET-1438564, USA an environmental sustainability grant. We also thank James Axley for his critical role in the co-development of the project. Christine Weiss and Chris Bolick of Yale Botanic garden for valuable input in plant preparation and maintenance prior and during the study. We also thank the many students involved in the project. We thank anonymous reviewers for the constructive criticism and review.
Contributions
Felson worked with Axley and Atta-Boateng with support from Samuel Kaufman-Martin in setting up the experiments. Felson and Atta-Boateng ran the experiments and collected the data. Atta-Boateng processed the data and performed the statistical analysis. All authors contributed to the writing and editing of the article.
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