Suitability of wetland macrophyte in green cooling tower performance
The objective of the research paper is to estimate the thermal tolerance of
macrophytes that are used in heat rejection systems-“thermoGreenWalls” (tGW)
designed to reject thermal energy from the surface of an integrated plant-porous
material, leading to a residual cooling effect. tGW combine the functionality of
cooling towers with green walls technology.
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).
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.
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.
Heat rejection (HR)
=
f (Twin-Tout, flow rate)
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.
Plant sample: – Iris versicolor (L.), Scirpus cyperinus (L.) and Carex lurida.
Hydrophonics Design: Three thermally insulated cylindrical PVC tubes each having
12 plant insertion openings that contained four replicates of I. versicolor, S. cyperinus,
and C. lurida each. Two of the 3 pipes were maintained at 35oC and 40oC respectively
using a water heater. The third pipe was maintained at ambient temperature (25oC).
Fresh biomass
Biomass
Response
Plant Response
Measurement
Oven dry biomass
(dried at 68oC)
Stomatal Conductance GS
Physiological
Response
Leaf Temperature Tl
Photochemical Efficiency FvFm-1
(Measured in terms of Cholophyll
Fluorescense)
PHYSIOLOGICAL RESPONSE
OBSERVATIONS:
S.No Physiological Response
1
Leaf Temperature Tl
2
Photochemical Efficiency FvFm-1
3
Stomatal Conductance Gs (mmol
m−2 s −1)
Response Expected Response
under Stress
Observed
Decreasing Tl
No significant
change in Tl.
Plants exhibited
homeothermic
behaviour.
Sustained decrease Our results show a
in dark‐
relatively steady
adapted Fv/Fm and FvFm-1 (regardless
increase in Fo to
of temperature
indicate the
treatment), an
occurrence of
effect also
photo inhibitory
indicative of plant
damage in
tolerance or low
response to high
sensitivity to water
temperature
temperatures at 35
°C and 40 °C.
Increasing
transpiration rates,
resulting in
increased stomatal
conductance
The maximum
stomatal
conductance, Gs,
was attained at
week 5, with a
value Gmax that
depended on the
water temperature
and plant species.
The values of Gs
for a particular
species, in all the
other weeks were
almost same
regardless of the
water temperature.
After week 5, the
peak stomatal
conductance
declined to its
initial value for all
temperature
treatments and
species. This
suggests a possible
acclimation period
of 5 weeks for all
the macrophyte
species.
CONCLUSIONS:
Variation of physiological indicators like: Gs , Tl and Fv Fm −1 of macrophytes
exposed to continuous recirculating water at different temperature levels is
insignificant which demonstrates that the three wetland species can tolerate
temperatures 10 °C above ambient temperature.
Since the total HR depends on heat and mass transfer from porous media and
evapotranspiration from leaves, species that maximize heat and mass loss at higher
recirculating temperatures are ideal for tGW. Among the species studied, C lurida met
these criteria, attaining a maximum Gs at Twin = 40°C.
BIOMASS RESPONSE
OBSERVATION:
The mean fresh total biomass was highest at 35 °C and consistent across all the
species under observation.
The effect of elevated water temperatures on the shoot part of the plant was more
pronounced in I. versicolor at 35 °C, which constitutes ∼65% of the plant mass.
Under tGW conditions, allocation that favours evapotranspiration such as high shoot
biomass is desirable.
CONCLUSION
We found that the plant biomass increased in all cases above ambient water
temperature.
Higher fresh biomass at 35 °C observed across species might indicate a better resource
partitioning strategy enhanced by increased water temperature at 35 °C within the
rhizosphere.
SUMMARY:
We find no significant statistical difference between the control response (25oC) 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.