The greenest solar power? Life cycle assessment of foam-based flexible floatovoltaics

Abstract

This study presents a life cycle analysis (LCA) of a 10 MW foam-based floatovoltaics (FPV) plant installed on Lake Mead, Nevada, U.S.

Full text

Preprint: Hayibo, K.S., Ma yville, P., Pearce, J., 2022. The Greenest Solar Power? Life Cycle Assessment of Foam-Based Flexible Floatovoltaics. Sustainable Energy & Fuels, 2022, 6, 1398 - 1413.
https://doi.org/10.1039/D1SE01823J
The Greenest Solar Power? Life Cycle Assessment of Foam-Based
Flexible Floatovoltaics
Koami Soulemane Hayibo,a Pierce Mayville,b and Joshua M. Pearce * abc
This study presents a life cycle analysis (LCA) of a 10-MW foam-based floatovoltaics (FPV) plant installed on Lake Mead,
Nevada, U.S. A material inventory of the flexible crystalline silicon (c-Si)-based module involved massing and determination
of material composition of the module’s encapsulation layers with ATR/FTR spectroscopy and electron microscopy. The LCA
was performed using SimaPro and the results were interpreted in terms of cumulative energy demands, energy payback
time, global warming potential, GHG emissions, and water footprint including negative values for reduced evaporation. A
sensitivity analysis was performed on the lifetime of the modules and the foam-based racking. The results show 30-year
lifetime foam-based FPV system have one of the lowest energy payback times (1.3 years) and the lowest GHG emissions to
energy ratio (11 kg CO2 eq/MWh) in c-Si solar PV technologies reported to d ate. In addition, the foam-based FPV system also
had 5 times less water footprint (21.5 m3/MWh) as compared to a conventional pontoon-based FPV (110 m3/MWh). The
lifetime of the foam-based racking does not affect the result, while the lifetime of the modules has a significant effect on
the lifecycle impacts of the foam-based FPV plant. Foam-based FPV has a net positive impact on the environment for CO2
emissions and energy consumption if its lifetime is above 7.4 years and the technology has the potential to become the
greenest c-Si-based solar PV technology if the lifetime of the modules can be guaranteed for at least 26.6 years. Future work
is needed to determine these lifetimes of these systems and expand them.
Broader Context
Sustainable solar photovoltaic (PV) technology can further
improve its environmental performance by reducing materials
needed for systems providing a given amount of energy or
providing other services. Recent work has shown floatovoltaics
(FPV) to be promising candidates for greener PV because of
their symbiotic relationship with water. A new type of FPV that
uses only foam as the racking material was shown in this life
cycle analysis study to be the greenest form of crystalline silicon
(c-Si)-based PV to date if its lifetime reaches 30 years. This is
because it has one of the lowest energy payback times (1.3
years) and the lowest GHG emissions to energy ratio (11 kg CO2
eq/MWh) of any c-Si PV technology. In addition, FPV saves
water from evaporating which could be critical in arid and semi-
arid regions. The foam-based FPV system also has 5 times less
water footprint (21.5 m3/MWh) as compared to even the water
resource benefits of a conventional pontoon-based FPV (110
m3/MWh). Overall, foam-based FPV has a net positive impact
on the environment for CO2 emissions and energy consumption
if its lifetime is above 7.4 years but future work is needed to
determine the lifetimes of different components of these
systems and optimize them to maximize the environmental
benefit.
Introduction
Converting solar energy directly to electricity with photovoltaic
(PV) technology is well-established as a green sustainable
solution to humanity’s energy needs [1]. This has been
determined by extensive life cycle analysis also known as life
cycle assessment (LCA) studies, which have been historically
primarily focused on land-based ground or roof-mounted PV
systems. LCA studies vary in terms of the system boundary, the
impact assessment methods as well as the study location. The
lifecycle impacts of PV systems vary rapidly with time because
of the continuous improvement in device performance,
manufacturing methods and material types [2]. For example, a
study conducted by Kreith et al. in 1990 investigated the energy
use and the greenhouse gases (GHG) emissions of a ground
mounted single crystalline (c-Si) PV system with a module
efficiency of 8.5% in the United States (U.S.) for a lifetime of 30
years. The cumulative energy demand (CED) and the GHG
emissions were estimated to 6,300 kWh/m² and 280 kg CO₂
eq/MWh, respectively [3]. In 2012, another study by Fthenakis
et al. was conducted in the U. S. using the same lifetime (30
years), and mounting system (ground-mounted) as Kreith et al.,
with substantially improved modules efficiency (20.1%).
According to Fthenakis et al., the system’s CED was 1,295
kWh/m² and the GHG emissions was estimated to 64.2 kg CO₂
eq/MWh [4]. This shows a respective reduction of 79% and 77%
in the energy use impact and GHG emissions impacts in 22 years
due to technology improvement. In the time separating these
two studies, other studies have examined the LCA of both c-Si
and multi-crystalline (mc-Si) solar PV systems. Between 1990
and 2000, the life cycle analysis of first-generation solar PV
systems resulted in GHG emissions values ranging from 50 kg
Co₂ eq/MWh [5,6] to 280 kg CO₂ eq/MWh [3,7] for c-Si
technologies, and 20 kg CO₂ eq/MWh [6,8] to 200 CO₂ eq/MWh
[7,9] for multi-crystalline systems. During the same period,
studies found that the energy payback time (EPBT) for c-Si PV
systems were comprised between 2.5 years [5–7] and 15.5
years [6,8]; and was in the range of 1.7 years [6,7,10] to 3.2
years [5–7,10] for mc-Si technologies. During the following
decade, with advances in life cycle analysis methodologies and
availability of solar PV technologies inventories due to PV
becoming a mainstream energy generation technology, more

detailed and elaborated LCA were conducted on solar PV
systems [11,12]. Studies conducted from 2001 to 2010 have
evaluated the GHG emissions of single crystalline PV systems
between 29 kg CO₂ eq/MWh [6,7,13] and 671 kg CO₂ eq/MWh
[6,7,14] while multi-crystalline had emissions of 12 kg CO₂
eq/MWh [6,7,15] to 80 kg CO₂ eq/MWh [7,16]. In terms of
energy consumption, the EBPT during the same period was 1.75
years [6,7,13] to 8 years [6] for c-Si systems while mc-Si PV
systems had an energy payback time between 0.8 years [7,17]
and 7.5 years [7,18]. The last decade (from 2010 to 2020) has
seen a plethora of LCAs performed [11]. For example, the GHG
emissions for both c-Si and mc-Si PV systems were estimated as
lo w a s 1 2 k g CO ₂ e q/M Wh [7,19–21], and the highest value were
67 kg CO₂ eq/MWh [22] and 88.7 kg CO₂ eq/MWh [7,23] for c-
Si and mc-Si PV systems, respectively. On the other hand, the
EPBT in that period was between 0.91 years [7,19] and 4.65
years [6,7,21,24] for c-Si, and mc-Si PV systems had an energy
payback time ranged from 1.01 years [7,21,25] to 6.05 years
[26]. It should be noted that amorphous silicon-based (a-Si:H)
PV has even superior environmental performance, but has not
gained market share because of lower efficiencies than c-Si and
mc-Si that demands higher costs in the balance of systems
[27,28].
In order to keep global temperatures on the planet from
increasing over 2°C from preindustrial levels it is crucial to
transition towards renewable and sustainable energy sources
[29] since coal and other fossil-fuel-based energy sources are
known to worsen climate change that is at the core of global
temperatures rise [30,31]. One such energy source is solar PV
that has become widely spread, accessible an d has the potential
to meet worldwide energy use by scaling [1,32]. Although bett er
than coal technologies in terms of land occupancy when carbon
emissions mitigation is considered [30], PV itself demands large
surface areas to power society. This may cause a land use
conflict with feeding an increasing world population. One
method growing in popularity to alleviate land use conflicts is
floating photovoltaics (or floatovoltaics (FPV)). FPV is not only
deployed on un-used surface areas, but it also enjoys two
primary synergies. First, water cools the PV increasing their
power conversion efficiency [33–41] and second, the FPV
reduces water evaporation, which can be extremely valuable in
semi-arid and arid regions [38,42–44]. There are already some
indications that FPV is environmentally superior to conventional
land-based PV [45,46]. One method of improving
environmental performance is to simply use less materials. This
is observed in the PV field, where for example, frameless PV
modules outperform framed modules in LCA studies [25,47].
The only study found in the literature that investigated the LCA
of a conventional pontoon-based FPV has shown a high return
on financial investment as compared to other PV systems [46].
A new type of FPV has been developed that uses foam racking
and completely eliminates the need for the conventional
racking infrastructure in pontoon-based FPV [41,48]. This has
already been shown to be economically advantageous as with
conventional land-based PV systems, the racking material
makes up 8% of the total system cost for utility scale PV in the
U.S. [49,50] and a far higher percentage for smaller PV systems.
Furthermore, racking in conventional ground-mounted PV
represents 8 to 23% of the total environmental impacts [15,51].
This offers the possibility that foam-based FPV are the greenest
potential source of solar PV electricity. The environmental
impacts of the newly-developed foam-based FPV system is not
known, so this study aims to determine the environmental
impacts and to explore the possibility of foam-based FPV
becoming the greenest PV system to date. Specifically, this
study will use a cradle to grave LCA on a 10-MW foam-based
solar FPV plant located on Lake Mead, Nevada, U.S. Detailed
technical analysis including both spectroscopy for material
identification and electron microscopy for material volumes
were conducted on the flexible PV modules to determine the
overall material makeup. Then, energy payback time, carbon
dioxide (CO2) payback time, and a detailed water fo otprint using
the water scarcity indicator (WSI) method, were calculated for
the 10-MW foam-based FPV plant. A sensitivity of the lifetime
of foam-backed FPV is run as the technology is newer than the
expected lifetimes. The results are compared to prior LCAs for
other PV technologies and discussed in the context of the
overall environmentally superior technology. Finally, guidance
is provided for improving the environmental performance
further.
Material and Methods
Life Cycle Analysis
Life cycle assessment (LCA) is a scientific tool that is used to
evaluate the environmental impact of a product system
throughout the entirety of its lifecycle [24,52]. The lifecycle of a
product system is made of several steps including the extraction
of raw material, the manufacture stage, the use stage, the
disposal or end-of-life stage of the product system, and all
transportation that are needed between the different stages.
An analysis that covers the entirety of the lifecycle of a product
is called a cradle-to-grave analysis [52]. The LCA framework
used in this study follows international LCA standards ISO 14040
and ISO 14044. According to these standards, an LCA study need
to include a definition of the goal and scope, an inventory
analysis, and an impact assessment and interpretation [52,53].
SimaPro 9 [54,55] has been used to execute the LCA simulation
in this study.
Goal and Scope
In the present study, an LCA is performed on the after-market
assembled foam-based FPV module that was proposed by
Mayville et al. [41,48]. The flexible PV with mounting holes is
specifically intended for marine applications and eliminates the
need for a nylon tarp-based material to connect one module to
another as was the case in the original study. The cradle-to-
grave LCA covers the complete life cycle of the modules, the
floating racking (foam, adhesive, and zip-ties), the inverters and
the electrical installation. The life cycle stages that are

investigated are: the manufacture stage, the use stage, as well
as the end-of-life stage.
The functional unit was chosen as 650.3 GWh delivered to the
grid. This correspond to the amount of energy generated by a
10-MW solar FPV plant, with a 30-year lifetime operation, using
foam-backed flexible SunPower [56] solar PV modules that are
installed on the surface of Lake Mead, Nevada in the United
States. This functional unit is obtained using an open-source
energy production calculation sheet of a foam-based FPV
developed in a recent study [41]. The model uses an empirical
temperature model that i s tailored to foam-based FPV modules,
and accounts for losses as well as the degradation rate of the
modules.
The system boundary in this study begins with the extraction of
raw material for the manufacture of the flexible modules, the
floating system, the inverters, and the cables for the electrical
installation. The analysis also covers the operation of the plant
and ends with the disposal of the equipment at the end of
service life. The lifetime of the module used for the LCA is 30
years. According to the Solar Energy Industry Association (SEIA),
a solar module’s lifetime ranges from 20 to 30 years [57], and
according to the flexible module’s manufacturer, the modules
can perform up to 40 years [58] (although it should be noted
that the warranty for the flexible modules sold for marine
applications is only 5 years [56]). The system boundary starts
with the raw material extraction. The assessment covers the
manufacture and assembly of the plant’s equipment, the
operation of the plant, and the decommissioning at the end-of-
life. The system boundary ends when the energy is transferred
to the electricity grid as shown on
Figure 1
.
Table 1. Desi gn parameters of a 10-MW foam-based solar floatovoltaic plant lo cated on
Lake Mead, Nevada, United States.
* A sensitivity is run on the lifetime of FPV from 5-30 yrs in 5-yr spans
Parameters
Value
Power Rating of the Plant
10 MW [59]
Installation Location
Lake Mead
Module Make and Model SunPower SPR-E-Flex-110 [56]
Module STC Power 110 W [56]
Number of Modules 90,910 [59]
Module Degradation Rate 0.50% [60,61]
Average PV Efficiency Year 1 20.90% [59]
Average Annual Energy Production
21.7 GWh/year [59]
Lifetime of the System
30 years* [57]
Lifetime energy Production
650.3 GWh [59]
Projected Yearly Water Savings
115,000 m
3
[59]
Projected Lifetime Water Savings 3.4 million of m3 [59]
Figure 1. Syste m boundary diagram of the foam-based FPV plant.

After the environmental impacts of the 10-MW foam-based FPV
plant have been studied, and quantified on a per MWh basis, it
is compared to a conventional pontoon-based FPV analyzed by
Cromratie Clemons et al. [46]. The pontoon-based FPV
components are the modules, the mounting structures, the
pontoon floating system, the anchors, the connection cables
and the inverters [46]. Finally, the environmental impact of the
two FPV systems are then compared to standard ground
mounted fixed-tilt solar PV systems from the literature.
10-MW Foam-Based FPV Plant Design
The floating PV system in this study was designed using the
open-source foam-based FPV temperature model proposed in a
recent study [41,62]. The spreadsheet was adapted to evaluate
the energy production and the water conservation potential of
a 10-MW foam-based FPV plant installed on Lake Mead during
its entire lifecycle [59]. The cooling effect of the water on the
foam-based FPV is included in the temperature model. The data
used for the simulation is historical hourly data collected in
2018 on Lake Mead by the United States National Oceanic and
Atmospheric Administration (US NOAA) [63] and satellite data
obtained from SOLCAST [64]. The annual PV degradation rate
was applied to the energy produced during the first operation
year to estimate a realistic total energy production of the
system throughout its lifecycle. The water preservation
potential of the foam-based FPV was estimated first on an
annual basis, and extended to the lifetime of the plant. It should
be noted that this is a conservative assumption due to the
trajectory of global temperatures created by anthropogenic
climate change [65–69]. The system parameters used in the
study, the energy production, as well as the quantity of water
saved are summarized in Table 1.
Life Cycle Inventory Analysis of a 10-MW Foam-Based FPV Plant
Table 2. Life cycle in ventory of the LCA performed on the FPV s ystem during its entire
lifecycle.
Foam-Based FPV Modules.
The foam-based FPV module
proposed by Mayville et al.
[41,48]
is made of a flexible
SunPower SPR-E-Flex module
[56]
, at the back of which
polyethylene foam has been attached to ensure buoyancy of
the module on the water surface. The modules were adhered to
the foam surface using a rapid action waterproof polyurethane
sealant. The modules are secured together by using stainless
steel zip ties. The inventory used for the LCA of the 10-MW
foam-based FPV system is shown in Table 2.
The flexible solar PV module consists of SunPower single
crystalline silicon-based solar cells [56]. The cells have an STC
efficiency of 23%. The chain of production of the solar module
spans across three different locations. The manufacture of the
solar modules begins by the production of single crystalline
silicon wafers. The wafers are obtained through two
transformation of metallurgical grade silicon. Metallurgical
grade silicon is transformed into solar grade silicon using the
modified Siemens process which is less energy intensive as
compared to the regular Siemens process [6]. The solar grade
silicon is further purified through the Czochralski process to
yield single crystalline silicon ingots suited for photovoltaic cell
manufacture [6,70,71]. The single crystalline silicon ingots are
cut into wafers. The location considered for the production
chain of the wafers in this study is China because China holds
the largest share of the worldwide silicon production as of 2020
[72]. The wafers undergo a metallization process to obtain
single crystalline Si solar cells [73]. In the case of SunPower
flexible solar modules, the wafers are cut thinner (150 μm) [74]
than wafers used in rigid single crystalline solar modules (170
μm) [75]. The thinness of the wafers enables the modules to
have a flexibility of 30°. The flexible solar module used in this
study are assembled in France [56]. The inventory data used for
the manufacture of the flexible solar module originates from
the 2020 report of the International Energy Agency (IEA) on the
life cycle inventory data of solar PV systems [75]. This report
updates a previous report that was published in 2015 [71]. The
new data is used to update the existing inventory in SimaPro.
Transportation between the different process locations, and
Material
Quantity
110 W Flexible single crystalline photovoltaic modules 90,910
Mass of Solar PV Modules 181,820 kg
Polyethylene foam 10,710 kg
Polyurethane marine sealant 25,567 kg
Stainless steel zip-ties 982 kg
Anchoring concrete 450 kg
Metal chain
450 kg
Electric installation for 570kWp open ground PV system
18
500 kW Inverters
40

from the place of production (France) to the place of installation
(Nevada, US) are included in the LCA.
PV Module Encapsulation Layers Analysis.
The layout of the
flexible module used in this study is made from top to bottom
of a transparent layer, an ethylene vinyl acetate (EVA) layer,
single crystalline solar cells, another layer of EVA, and a white
layer; as shown on Figure
2. Flexible single crystalline solar PV
modules are a rising technology, therefore there is no existing
literature on the material inventory of the top and bottom
layers of the module. In this analysis, the layers of the module
have been examined using two different spectrometer
techniques to acquire material data.
An attenuated total reflection (ATR) characterization has been
run on a sample of the front transparent and back white layers
in which the module is encapsulated
.
An iS50R (Thermo
Scientific) Fourier transform infrared (FTIR) spectrometer was
used for the analysis and was calibrated with the following
parameters: 256 scans, 4 cm-1 of resolution, and a wavenumber
bandwidth of 4000 - 650 cm-1 [76,77]. The resulting spectra
were compared to possible matches in the iS50R software
database. The front transparent layer belongs to the
polyurethane rubber family and the back white layer was
identified as polyethylene terephthalate (PET) polymer as
displayed on Figure 3 and Figure 4. Polyurethane rubber (PUR)
was proposed in the literature as a polymer that can be used for
the front transparent surface of solar modules [78].
After the material composition of the two layers were
determined, an FEI Philips XL 40 [79] environmental scanning
electron microscope (ESEM) was used to measure the thickness
of each layer as shown on Figure 5. The thickness of the
materials is combined to the dimensions of the module (116.5
cm x 55.6 cm) [56] to calculate the volume of each material
required for the assembly of a single module. The mass of the
material used in the life cycle analysis is found by multiplying
the volume of each material to the density of the material.
The life cycle inventory data of the module referenced in the
international energy agency report describe the manufacture
input for rigid single crystalline PV modules. The inventory data
is updated using the measured data form the flexible module.
The front glass, the aluminum frame, and the polyvinyl fluoride
(Tedlar) used in a rigid solar PV module are replaced by
polyurethane and polyethylene terephthalate. Additionally, the
corners and the side of the module are pierced and hold
stainless-steel grommets to simplify the installation procedure.
The characteristics and mass balance of the different parts of a
flexible solar module are shown in Table 3.
Foam-Based FPV Racking.
The racking used for the foam-based
FPV modules is made of foam, marine sealant, and zip ties. The
foam and the marine sealant are applied after the acquisition of
the solar PV module as shown on Figure 6.
The mass of all the
Figure 5. ESEM view of the th ickness of the layers of the flexible module enc apsulation.
Figure 2. Explode d diagram of after-ma rket modified foam-based floa ting solar PV
module.
Figure 3. Spec tral compar ison of polyu rethane rub ber with the to p clear layer of the
flexible module.
Figure 4. Spectra l comparison of polyet hylene terephthalate a nd the back white laye r of
the flexible module.

Table 3. Characteristics and mass balance of the different layers of the flexible PV
module.
Material
Thickness
(μm)
Density
(kg/m3)
Mass
(g)
Polyethylene Terephthalate
(Bottom Layer)
253 1380 [80,81] 226
Ethylene Vinyl Acetate
(Solar Cells Encapsulation)
1330 948 [82,83] 817
Polyurethane Rubber (Top
Layer)
527 1210 [78,84] 413
Monocrystalline Silicon
Solar Cells
150 - 224
Grommets - - 4
Junction box, cables, and
electronics
- - 316
Total
-
-
2000
FPV racking components was determined with an open source
digital scale with a precision of 0.05g [85].
The foam used to make the modules float on the water surface
are made of polyethylene (PE) 1.2 lb ½” (12.7 mm) and was
assumed to be manufactured in the U.S. For each module, 40
pieces of foam were used, each being 50 mm by 240 mm,
resulting in a volume of PE of 6,096 cm3 to ensure the floatation
of a single module [41]. The density of the PE is 19.22 kg/m3
[48]. The total mass of PE needed for each module is 118 g.
The foam pieces are adhered to the back of the module by a
polyurethane marine sealant [86]. The quantity of marine
sealant used for a single module was weighted and 281 g of
adhesive were needed for each module. Polyurethane adhesive
is manufactured by mixing in equal parts methylene-diphenyl-
diisocyanate and polypropylene glycol [87]. These two materials
were used in SimaPro to create the manufacturing process of
the sealant.
Stainless steel zip-ties are used to limit the relative movement
between neighboring modules and secure the modules on the
water surface. Each module requires 10 zip-ties, two per corner
grommet and one per side grommet. A single zip-tie weighs 0.82
g, therefore 8.2 g of zip-ties are needed for one module. The zip-
ties have been assumed to be manufactured through metal
casting.
Electrical Components and Anchors.
The electrical components
of the plant that have been considered in this study are the
inverters and the electrical installation of the cables. The native
cables of the modules are waterproof because the modules are
designed to be used in a marine environment. The native cable
inventory is included in the module’s inventory. Open ground
installation inventory was used for the rest of the electrical
installation connecting the string of modules to the inverter
because there is no need to dig trenches for cabling in water.
The open-ground electrical installation inventory that was used
encapsulates the cables as well as the installation process. The
material inventory used for the electrical installation and the
inverters are found in SimaPro and originated from the
Ecoinvent database [88]. The lifetime of the electrical
installation was assumed to be 30 years and the lifetime of the
inverters was assumed to be 15 years [71]. Therefore, the
inverters need to be replaced halfway through the lifecycle of
the system, and this was included in the LCA.
The anchoring of the system was considered to be a
combination of concrete blocks and metal chain. The ratio of
the anchor weight to the floating system weight is nearly 1:500
as found in boat anchors [89].
End-of-Life Scenario of the 10-MW plant
The end-of-life of the different equipment that went into the
assembly of the foam-based FPV plant was factored into the life
cycle assessment. The disposal of the inverters and the
electrical installation is included in their respective life cycle
inventory. The default waste treatment process included in the
inverters and the electrical installation profile is incineration
[54].
The current recycling process of crystalline silicon rigid solar
modules consists of dismantling the modules and recovering
material such as glass, aluminum, and copper while the rest of
the material are landfilled or incinerated [90]. The end-of-life
process of rigid crystalline solar PV modules is adapted to the
flexible modules. In the case of flexible modules, the recovered
materials are the copper from the wiring as well as the stainless
steel from the grommets. The PUR is landfilled because
landfilling remains the most common way to dispose of
polyurethane [91] while the EVA, the PET, the solar cells and
other electronic components are incinerated.
Regarding the foam-based racking of the FPV module, the
polyethylene foam waste and the zip-ties are separated from
the waste stream and recycled [92,93] while the polyurethane
sealant is landfilled.
Impact assessment methods
Figure 6. Afte r-market modification of a flexible solar PV module by taping PE f oam on
the back layer of the modules using polyurethane marine sealant

Three major indicators have been investigated in this study: i)
the energy payback time (EPBT), ii) CO2 payback time (CO2PBT),
and iii) the water footprint.
When performing the LCA of an energy production system, it is
important to evaluate the energy break-even time or energy
payback time of the system. The energy payback time (EPBT) is
defined as the period of operation time during which an energy
production system will generate the same amount of energy as
the primary energy that is required to manufacture, install,
maintain, and decommission the system [5,24,25,71]. The EBPT
in the case of the foam-based FPV module is calculated by
dividing the total energy consumed during the lifecycle of the
module by the annual energy production of the module as
shown in Equation (1)
 () =  () /  (/) (1)
Where:
Econs
is the total energy consumed during the entire
lifecycle of the floating FPV plant, from manufacture
to disposal.
Ean
is the average yearly energy production of the
plant
The cumulative energy demand (CED) impact assessment
method evaluates the direct and indirect energy consumption
throughout the entire lifecycle of a product or process [55,94].
The CED has been used in this study to evaluate the energy
consumption of the foam-based floatovoltaic plant.
Similarly, to the EPBT, the CO2 payback time (CO2PBT) is used to
evaluate the number of operation years needed for a system to
offset the total CO2 emission during its lifecycle by its annual CO2
emission reduction potential. It should be noted that
substantial amounts of emissions occur at the end of the life
cycle so care must be taken when using the CO2PBT to do
dynamic carbon emission analysis [95].
 ()=    (   )
   (  /) (2)
The CO2 emission reduction potential is location specific and
depends on the electricity grid mix of the location of interest. In
this study, the CO2 emission reduction potential is calculated by
multiplying the annual energy production of the power plant by
the U.S. grid mix CO2 emissions of 2019 (0.41 kg CO2/kWh [96]).
The life cycle CO2 equivalent (CO2 eq) emissions of the system is
assessed by using the global warming potential over 100 years
(GWP 100) method. The GWP 100 quantifies the effect of
different greenhouse gases such as carbon dioxide (CO2),
nitrous oxide (N2O), methane (CH4), and chlorofluorocarbons
(CFCs) in terms of kg of CO2 equivalent over a time period of 100
years [24,52,55,97–99]. It provides a uniform way of evaluating
the global warming potential of the different greenhouse gases
that are released during the lifecycle of a product or process
[55,97].
Several studies have shown that floating solar PV modules have
the potential to prevent water evaporation from lakes and
reservoirs [38,42–44,100]. In order to identify how the water
evaporation mitigation factors into the environmental footprint
of the foam-based FPV module, the water scarcity indicator
(WSI) proposed by Hoekstra et al. has been used to quantify the
water footprint of the module [55,101]. This method has been
preferred because it analyzes safe water sources depletion by
combining socioeconomic and hydrological data [55,101] and
the FPV module is intended to be installed on a lake surface.
After the water footprint of the module is determined, it is
compared to the water saving potential.
Sensitivity Analysis
Two major assumptions were made during the LCA of the foam-
based FPV system: the lifetime of the flexible modules and the
lifetime of the polyethylene foam. It is therefore crucial to
analyze how the variation of these two parameters affects the
life cycle impacts of the system. In the main analysis the
lifetimes of both components were set to 30-years. The
warranty provided by the module manufacturers on the flexible
modules, however, is 5 years [56], even though conventional
solar PV modules are known to last well-beyond 30 years
[57,102]. It important to point out that the specific modules
used in the FPV experiments in this study only had a 5-year
warranty. There are, however, flexible PV modules in the same
class that have more industry-standard 25-year warranties. For
example, Renogy offers a long warranty based on output. It is at
a 5 year/95% efficiency rate, 10 year/90% efficiency rate, 25-
year/80% efficiency [103]. Here a 30-year timeline is used to
represent the realistic lifetime of the modules and accounted
for an 0.5% drop per year.
On the other hand, polyethylene is known to be able to remain
intact in a marine environment for up to 15 years before the
start of its degradation process [104], and water is known to
accelerate the degradation as compared to air [105]. Therefore,
the lifecycle impacts have been reassessed by varying the
lifetime of the modules and the lifetime of the PE foam with a
5-years increment. In the case of the flexible modules, their
replacement is always also accompanied by the replacement of
the marine sealant. The sensitivity analysis is performed
independently between the lifetime of the modules and that of
the PE foam. For each iteration of the lifetime of the PE foam, a
simulation is run over the lifetime range of the combination of
flexible modules and sealant while the lifetime of the PE foam
is maintained at a constant value. When the modules are
replaced after a short period of time, their degradation rate is
reset. This reset provides a slight boost in the energy production
and has been factored in the sensitivity analysis. For each
iteration of the lifetime, the average annual energy production
as well as the lifecycle inventory quantities for the modules, the
sealant and the foam are displayed in Table 4. The stainless-
steel zip-ties are releasable and reusable [106], therefore their
inventory as well as the inventory of electrical installation and

Table 4. Life cycle inventory of the flexible mo dules, mari ne sealant, and foam for
different lifetimes ranging from 5 to 30 years.
inverters remained the same as in Table 2 throughout the
sensitivity analysis.
Results
In all the three impact categories, the contribution of the
concrete anchors as well as the metal chains were negligible
compared to the other equipment of the system.
Energy Payback Time (EPBT)
The total energy consumption of the 10-MW floatovoltaic plant
during its 30-years lifecycle (from natural resources extraction
to disposal) is 28 GWh. The manufacture of the flexible solar PV
modules has the greatest energy consumption and amounts to
85.4% (24 GWh) of the total energy consumption of the plant.
The second highest energy intensive part of the plant is the
lifecycle of the inverter with a total energy consumption of 2.64
GWh. The electrical installation as well as the foam-based
racking accounts respectively for 3.01% and 2 .36% of the overall
energy consumption. The energy consumption of the disposal
scenario is negative (-50 MWh) because of the negative
allocation of the energy collected during the incineration of the
equipment at the end-of-life (See Figure 7). Additionally, Figure
7 shows a detail view of the energy consumption of the foam-
based racking. The total energy use of the foam-based racking
is 663 MWh, of which the manufacture of the polyurethane
uses 55% while the polyethylene foam and the zip-ties use
respectively 41% and 4%. The results of the EPBT calculation
show that the foam-based FPV plant will offset its lifetime
energy consumption in 1.3 years.
Co2 Payback Time (CO2PBT)
During the lifetime of the system, the total GHG emissions
amount to 7,403 metric tons CO2 eq. The life cycle stage that
emits the most greenhouse gas is the manufacture, assembly,
and transportation of the flexible solar PV modules. The GHG
emissions of the modules are 6,230 metric tons CO2 eq or 84%
of the total GHG emission of the plant during its lifecycle as
shown on Figure 8. The inverters have the second highest
emissions (572 metric tons CO2 eq) followed by the end-of-life,
the electrical installation and the foam-based racking which
respectively contribute 337; 161; and 104 metric tons CO2 eq to
the total GHG emission of the system. Figure 8 also displays a
detailed emissions contribution of the different parts of the
foam-based racking. Of the 104 metric tons CO2 emitted by the
racking, the poly urethane sealant accounts for 60% while the PE
foam and the zip-ties respectively contribute 33% and 7%. The
calculation of the CO2PBT shows that the system can offset the
total GHG emissions in 0.82 years.
Water Footprint
The water footprint simulation has shown that the total water
usage during the life cycle of the system is 14 million m3. The
manufacture of the flexible PV leads the water consumption of
the entire lifecycle (11.5 million m3). The other notable
components that consume a significant amount of water during
its lifecycle are the inverters with a water use of 2.3 million m3
as displayed in Figure 9. Figure 9 also shows the detailed total
water consumption of the foa m-based racking (190,000 m3) and
the manufacture of the zip-ties is leading the water use (83%).
Lifetime (years) 5 10 15 20 25 30
Flexible PV Modules
(metric tons)
1,091
545
364
273
218
182
Marine Sealant
(metric tons)
153 77 51 38 31 26
PE Foam (metric tons)
64
32
21
16
13
11
Average Annual Energy
Generation
(GWh/year)
23.1
22.8
22.5
22.2
21.9
21.7
Figure 7. Detailed e nergy use results (in G Wh) of the life cycle assessment of the 1 0-MW
FPV plant usi ng the CED meth
od.
Figure 8. Detailed GHG emissions (in metric tons C O
2
eq) of the 10-MW foam-based F PV
plant using t he GWP method.

Sensitivity results
The sensitivity analysis has shown that the lifetime of the foam
does not have a significant effect on the three impact categories
considered in this study. Figure 10 shows the results of the
sensitivity analysis where the variation of the lifecycle metrics
investigated in this study (GHG emissions, energy use, and
water footprint) are plotted. The variations of the three metrics
are plotted in groups, each group representing the effect of the
lifecycle of the PE foam on the metric. Inside, each group is the
variation of the metric regarding the lifecycle of the modules.
As shown on Figure 10, there is no visible change from one
group to the other due to the foam lifecycle, while the lifecycle
of the modules greatly impacts the metrics inside each group.
As an example, for a 30-year lifecycle of the modules, the GHG
emissions are 7,480 metric tons CO2 eq, the energy use is 28.3
GWh and the water footprint is 14.1 million m³, when the
lifetime of the foam is 5 years. On the other hand, when the
lifetime of the foam is 30 years, and the lifetime of the modules
is maintained at 30 years, the GHG emissions are 7,400 metric
tons CO2 eq, the energy use i s 28.1 GWh and the water footprint
is 14.1 million m³. Figure 11 displays a detailed result of the
effects of the lifecycle of the flexible modules on the GHG
emissions to energy ratio (kg CO₂ eq/MWh), the energy use
(GWh), the final water footprint (m³/MWh), the CO₂ payback
time (years), and the energy payback time (years). In Figure 11,
the sensitivity results of the effect of the module lifetime are
shown for a PE foam lifetime of 15 years. According to the
results, the lifetime of the modules has a significant influence
Figure 10. Sensitivity analysis results of lifecycle metrics. The metrics results are shown in groups for variation of PE foam lifetime ranging f rom 5 to 30 yea rs. Inside each group,
the metric res
ult is shown for modules l ifetime var iation ranging from 5 to 3 0 years. (a) – Energy Us e result using the CED method. (b) –
GHG emissio ns results using the GWP
method. (c)
– Water footprint results using the WSI method
Figure 9. Detailed water footprint (in m3) of the 10-MW PV foam-based FP V plant
using the WSI method.

on the life cycle impact. When the lifetime of the modules is set
to 30 years, the EPBT is 1.3 years, the CO₂PBT is 0.83 years, the
final water footprint is 16 m³/MWh, and the GHG emissions to
energy ratio is 11 kg CO₂ eq/MWh. When the modules are
disposed of and replaced every 5 years, the EPBT is 6.51 years,
the CO₂PBT is 4.29 years, the final water footprint is 105
m³/MWh, and the GHG emissions to energy ratio is 59 kg CO₂
eq/MWh.
Discussion
The life cycle analysis performed in this study has shown that
over the course of 30 years, a 10-MW foam-based floating PV
plant installed on the surface of Lake Mead would require a
total energy input of 28 GWh, would emit 7,403 metric tons CO2
eq of greenhouse gases, and would use up 14 million m3 of
water when the lifetime of the flexible modules are 30 years. At
the same time, the system generates 650.3 GWh of clean
electricity while preventing the evaporation of 3.4 million m3 of
water from the Lake reservoir.
Consequently, a foam-based FPV system installed on a water
surface located in a tropical or subtropical climate zone, such as
the climate zone where Lake Mead is located, would require 43
MWh of primary energy for every 1 GWh of clean energy
generated, or an energy ratio between its primary energy use
and its actual energy generation of 43 kWh/MWh. The energy
payback time is evaluated at 1.3 years. This indicates that the
foam-based FPV system will generate up to 23 times the energy
it consumes during its entire lifecycle. Recent LCA studies during
the past decade have shown that the EPBT for rigid c-Si solar PV
plants ranged between 0.91 years to 6.05 years depending on
the location and the encapsulation technology used for the
modules [7,19,21,24,25,107,108]. The EPBT obtained in the
current study (1.3 years) is located on the lower spectrum of
these values even though the tilt of the modules considered is
not optimal for the specified location. The foam-based modules
have a tilt angle of 0° since they are lying flat on the surface of
the water, whereas the optimal tilt of a PV system located near
Lake Mead in Nevada would be 30° (latitude of 28°). The foam-
based FPV modules have an EBPT close to that of conventional
rigid modules made with aluminum back surface field solar cells
(1.11 years [25]) because they benefit from an energy boost
provided by the cooling effect of the water surface. Even though
the energy production of foam-based solar PV modules located
far away from the equator is hindered by the inclination factor,
a recent study has shown that they can produce 3.5% more
energy than expected from a module with an inclination of 0° at
higher latitudes [41].
Similarly, for each GWh of clean energy generated, a foam-
based solar FPV system located on Lake Mead will have a
lifetime greenhouse gas emission of 11.38 metric tons CO2 eq,
corresponding to a GHG emission to energy generation ratio of
11 kg CO2 eq/MWh. The CO2PBT is less than a year being
estimated at 0.82 year. This indicates that the foam-based FPV
plant will offset 36 times the amount of CO2 it generates during
its lifetime. For comparison, the values of greenhouse gas
emissions in the recent literature for rigid crystalline silicon
modules were comprised between 12 kg CO2 eq/MWh eq and
88 kg CO2 eq/MWh [20,21,23,25,47,107–109]. It is important to
point out that in the study where the lower value of 12 kg CO2
eq was obtained, the authors did not perform a detailed life
cycle assessment and the system boundary was not clearly
specified [20]. Also, a study by Kim et al. has estimated the CO2
payback time between 1.53 and 2.53 years which remains high
compared to the result obtained in this study. Thus, the value
of GHG emissions obtained in this study is the lowest value
found in the literature to date. This indicate 30-years lifetime
foam-based flexible solar FPV is the greenest crystalline based
solar PV system to date when the flexible modules lifetime is 30
years. Cromratie Clemons et al. have estimated the CO2
emissions of a pontoon-based FPV installed in Thailand to 73 kg
CO2 eq/MWh [46]. Several factors influence the GHG emissions
of the foam-based FPV modules that were considered in this
study. The mass of solar cells used to manufacture the flexible
modules is lower than the mass required to manufacture a rigid
module because the cells are cut thinner (150 μm for flexible
module [74], 170 μm for rigid modules [75]). Also, the
manufacture process of the flexible modules does not involve
the use of glass or aluminum. Moreover, the simplicity of the
racking used in the case of a foam-based FPV system is a key
factor in the reduction of the GHG emissions. There is no
concrete foundation or metal support involved in the
manufacture of the foam-based floatation supports. All these
factors result in a lower material use by the foam-based FPV
system as compared to conventional PV systems, therefore,
contributing to a reduction in the carbon emissions from the
foam-based FPV system.
According to the simulation results, the water used by a 10-MW
foam-based FPV plant located on the surface of Lake Mead in
Nevada during its entire lifecycle is 14 million m3. Therefore, the
water footprint of a foam-based FPV plant installed on a water
Figure 11. Detailed results of the impact of the variation of the modules lifetime (5 to
30 years) on the system lifecycle impact categories, for a PE foam lifetime of 15 years.

surface located in a tropical or subtropical climate zone is
evaluated at 21.5 m3/MWh. In comparison the water footprint
of a pontoon-based FPV has been estimated to 110 m3/MWh by
Cromratie Clemons et al. [46], making foam-based FPV 5 times
less water-intensive than pontoon-based FPV systems. On the
other hand, a recent study has demonstrated that covering the
surface of Lake Mead with foam-based floating PV has the
potential to prevent water evaporation [41], specifically when
the lake surface is covered by a 10-MW foam-based FPV, the
system is able to save 3.4 million m³ of water from evaporating.
In terms of quantity of water saved per energy generated, the
plant has the potential to prevent the evaporation of 5.2
m3/MWh. This potential water saving offsets the water
footprint of the system by reducing it from 21.5 m3/MWh to
16.3 m3/MWh. One advantage foam-based FPV assuredly has
over conventional PV and pontoon-based FPV is the
suppression of water usage during the operation phase of the
system. A comparison of the water footprint from the
manufacture phase of a flexible PV module and a rigid PV
module performed in SimaPro using the water scarcity index
method of Hoekstra et al. [101] has shown that the water
footprint of rigid PV modules (568 m3/m2 of module) is 2.9 times
greater than the water footprint of a flexible module (196
m3/m2 of module). Combining the fact that flexible modules
have a lower water footprint than rigid modules, the fact that
foam-based FPV systems do not consume water during their
operation phase, and the fact that foam-based FPV substantially
offsets its own water footprint by about 25% via reduced
evaporation from the host water body, indicates that foam-
based FPV have the best water footprint to date among
crystalline silicon-based PV systems when the lifetime of the
modules is 30 years. Because the foam-backed modules are in
direct contact with the water surface, it should be mentioned
that the reduced evaporation and change in albedo due to the
FPV could contribute to increased heating of the lake. This
calculation is not straightforward because it depends on the
surface albedo of the lake and the absorption coefficient of the
water throughout the year and future experimental work is
needed to quantify these variables. It should be pointed out
that roughly a fifth of the energy that would normally be
absorbed by the lake water is instead extracted via conversion
to electricity, so even if the surface albedo decreased because
of the FPV leading to local increases in surface temperature, this
effect is dampened by the electrical energy reduction.
The sensitivity analysis has shown that even though the results
are not affected by the lifetime of the PE foam, the lifecycle
impacts of a flexible FPV plant are sensitive to the lifetime of the
modules. When the lifetime of the modules was varied down
from 30 to 5 years with a 5-year decrement, the range of the
impacts was 1.3 to 6.5 years for the EPBT, 0.83 to 4.29 years for
the CO₂PBT, 11 to 59 kg CO₂ eq/MWh for the GHG emissions to
energy ratio, and 16 to 105 m³/MWh for the final water
footprint. When the flexible modules have to be replaced every
5 years, corresponding to the manufacturer’s warranty, the
EPBT (6.5 years) is higher than the value (4.65 years) reported
by Kim et al. using single crystalline silicon rigid modules [24].
This indicates, however, that the foam-backed-FPV modules are
not advantageous from an energy perspective although they
are from GHG emissions stand point. For the foam-based
flexible FPV system to be at least as EPBT-efficient as the value
of 4.65 years, the lifetime of the modules needs to be greater
than 7.4 years. Nevertheless, even when the lifetime of the
flexible modules is set to 5 years, which is the warranty offered
by the manufacturer, they appear to have a lower GHG
footprint (59 kg CO₂ eq/MWh) than the high value reported in
the literature (88 kg CO₂ eq/MWh [23,108]). Furthermore, a
reverse analysis on the GHG emissions shows that the foam-
based FPV system would emit as much greenhouse gases as the
lowest value found in the literature (12.3 kg CO2 eq/MWh [20])
when the lifetime of the panels is set to 26.6 years. This analysis
shows that for foam-based FPV to be assuredly the greenest
FPV to date, the lifetime of the modules needs to be at least
26.6 years.
The study has covered the life cycle analysis of a 10-MW foam-
based FPV plant and has shown that foam-based FPV has the
potential of becoming the greenest type of solar PV in an
industry already well-respected for being one of the greenest
forms of electricity production. By reducing the components
involved in the balance of system as well as by using lighter solar
PV modules, the energy requirement and GHG emissions
associated with crystalline solar PV are greatly reduced. It is
challenging to exactly quantify by how much foam-based FPV
improve the environmental impacts of the solar PV industry
because the results of a life cycle assessment vary depending on
the location. The focus of the current study has been to
determine the life cycle inventory of the newly proposed design
of a foam-based FPV system and compare that with existing
values from the literature for land-based solar PV systems and
pontoon-based FPV systems. It should be noted that these
studies have different system boundaries and locations.
Therefore, future work is needed to run a complete
comparative life cycle assessment of a foam-based flexible FPV
system, a pontoon-based FPV system, and a land-based PV
system using the same system boundaries; ideally using all
experimental values as inputs. A particularly interesting case to
examine is the comparison of foam-based flexible FPV and land-
based PV using flexible modules and alternative racking system.
Further studies are also needed to evaluate the impact of the
location and the recycling process on the different PV systems.
Another challenge that needs to be addressed by future studies
is the testing of the durability of the foam-based racking in
different aquatic environments. As FPV systems are a relatively
new technology, there is little information regarding the
maintenance process of the system [110]. This is particularly
true for foam-based FPV because this technology is still at the
research phase. Nevertheless, as any other energy production
system, FPV systems must undergo preventive and corrective
maintenance during their operation phase. Pontoon-based FPV
systems require cleaning during the operation phase, but in the
case of foam-based FPV, the modules are semi-submerged,
therefore cleaning is not an issue. The most important
maintenance aspect in the case of foam-based FPV would be to
ensure the integrity of the foam, the anchoring system, as well

as the modules in a marine environment. In this study, the
flexible PV, the polyethylene foam, the polyurethane marine
sealant, as well as the stainless-steel zip-ties have been
assumed to have a lifetime of 30 years and the results have
shown that for foam-based FPV to be the greenest crystalline
silicon based solar PV system to date, the lifetime of the
modules needs to be at least 26.6 years. Therefore, the
durability of these core components, especially the solar
modules, needs to be tested experimentally and the impact of
using different type of sealant, foam, or zip-ties can be explored
as well as encapsulation methods for the flexible PV modules.
Future study is also needed to develop a complete
maintenance profile for FPV systems.
Because foam-based flexible PV modules are a new technology,
there is no available data from the module manufacturers
regarding the life cycle inventory, which have been adapted
from the recent values of the life cycle inventory of rigid
crystalline silicon modules. This assumption is the core
assumption in the article, but it should be noted that foam-
based FPV have a net positive impact on the environment in
terms of EBPT and CO2 emissions as long as they last even a
year, which has already been experimentally verified in the
course of seasonal deployments when obtaining the
experimental data to do this analysis. In, future studies, along
with testing for the durability of the flexible modules,
consultations with flexible module manufacturers would be
beneficial to get a more in-depth understanding of the life cycle
inventories of the modules, and the foam-based FPV system as
a whole. Not only do foam-based FPV appear to be the greenest
crystalline silicon-based PV to date, when the lifetime of the
modules is at least 26.6 years, as shown by the results of this
study, a recent study has also determined that using foam-
based racking could reduce the cost of racking by $0.37–
$0.61/W as compared to pontoon-based racking or land-based
PV racking [48]. Consequently, future work is needed to
experimentally scale up foam-based FPV to investigate the real
economic costs and viability of the system to find out the
return-on-investment period of a foam-based FPV plant, as well
as to compare the value of solar (VOS) [111] of the system to
that of conventional PV systems.
To further improve the environmental performance of the
foam-based FPV system, future studies need to perform
accelerated testing of the system to demonstrate the overall
lifetime, therefore reducing the material input for longer
lifetimes. Also, the investigation of the recycling processes of
the different plastic components that goes into the system
design, as well as the use of recycled material for foam
manufacturing are key factors to reducing the lifecycle impacts
of the foam-based FPV system. Additionally, FPV systems are
known to pair well with aquaculture to form aquavoltaic
systems [112–114]. Future work is needed to investigate the
LCA of a foam-based aquavoltaic system. The life cycle impacts
of any solar PV system are location dependent because of the
nature of the solar resource. This is even more applicable to FPV
systems because they can only be installed at specific locations
that have a water surface. Different locations worldwide
depend on different energy production systems. Therefore,
future studies should focus on the impact of the geographic
deployment optimization impact on foam-based FPV systems
by assessing the local water conservation needs as well as the
type of energy sources that feed the local electricity grid.
Conclusions
This study has shown that the base case results for a foam-
based FPV system, where the lifetime of the modules was 30
years, is one of the lowest energy payback times (1.3 years) in
c-Si solar PV technologies reported to date. It also represents
the lowest GHG emissions to energy ratio (11 kg CO2 eq/MWh)
to date among the same type of PV material technologies. In
addition, the foam-based FPV system also had 5 times less
water footprint (21.5 m3/MWh) as compared to a conventional
pontoon-based FPV (110 m3/MWh). The lifetime of the foam-
based racking does not affect the result while the lifetime of the
modules has a significant effect on the lifecycle impacts of the
foam-based plant. Theref ore, foam-based FPV has a net positive
impact on the environment in terms of EBPT and CO2 emissions
if its lifetime is above 7.4 years and the technology has the
potential to become the greenest crystalline silicon-based solar
PV technology if the lifetime of the modules can be guaranteed
for at least 26.6 years. Future work is needed to determine the
lifetimes of the system’s components.
Author Contributions
Conceptualization: KSH, JMP; Data curation: KSH, PM; Formal
Analysis: KSH; Funding acquisition: JMP; Investigation: KSH;
Methodology: KSH; Resources: JMP; Supervision: JMP;
Validation: KSH, PM, JMP; Visualization: KSH, PM; Writing –
original draft: KSH, JMP, Writing – review & editing: KSH, PM,
JMP
Conflicts of interest
There are no conflicts to declare.
Acknowledgements
The authors would like to acknowledge technical support from
K. Perrine. This w ork was supported by the Witte a nd Thompson
Endowments.
Notes and references
All the data used in the study, including the life cycle inventory, the
electrical design parameters and the water saving calculations, are
stored in an Open Source Framework repository available for
download here: https://osf.io/qt6mx/
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