For the tropical surface ocean waters being the largest thermal solar collectors and reservoirs in the planet, a solution is proposed for inexhaustible supply of freshwater by combining two renewable energy routes through a hybrid offshore energy farm, ocean thermal energy conversion, and offshore wind power.

1. 1 23
Journal of the Brazilian Society of
Mechanical Sciences and Engineering
ISSN 1678-5878
J Braz. Soc. Mech. Sci. Eng.
DOI 10.1007/s40430-015-0383-8
Sustainable freshwater from the tropical
oceans
Vicente Fachina

2. 1 23
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3. TECHNICAL PAPER
Sustainable freshwater from the tropical oceans
Vicente Fachina1
Received: 26 February 2014 / Accepted: 28 May 2015
Ó The Brazilian Society of Mechanical Sciences and Engineering 2015
Abstract The northeast region of Brazil has gone through
severe droughts for many decades, which tend to get worse
due to the climate change scenarios in the twenty-first
century. The population has been migrating southbound,
which has brought about serious social imbalances. For the
tropical surface ocean waters being the largest thermal
solar collectors and reservoirs in the planet, a solution is
proposed for inexhaustible supply of freshwater by com-
bining two renewable energy routes through a hybrid off-
shore energy farm, ocean thermal energy conversion, and
offshore wind power. Preliminary feasibility studies are
carried out for such an energy farm to deliver freshwater,
and to be deployed along the coastline of the northeast
region of Brazil. It is calculated 0.43 USD/m3
as a mini-
mum levelized cost of freshwater, for which the total
investments, excluded the desalination units, the freshwater
pipelines and the electrical cables, amount to USD
28 billion. Other less capital intensive scenarios are shown,
with USD 16 billion investment for 0.57 USD/m3
levelized
cost of freshwater.
Keywords Ocean thermal Á Offshore wind Á Seawater
desalination Á Sustainable energy Á Sustainable freshwater
1 Introduction
This study proposes a hybrid energy farm for supplying
freshwater to the northeast region of Brazil, as a long-term
strategy against the chronic scenario of lack of freshwater
in this century [1].
The principle of the ocean thermal energy conversion
(OTEC) is conceptually similar to the geothermal one.
Both ones are characterized for utilizing Rankine-derived
power cycles. In the tropics, the heat source is the surface
ocean water, and the heat sink is the deep cold water from
below 800 m depth. Investments on OTEC are high for the
larger pieces of equipment needed to convert into elec-
tricity the low density thermal energy of the tropical
oceans. Nonetheless, there are two positive expectations
for economic consideration:
1. 90 % expected energy availability factor [2], for
OTEC does not present problems such as the inter-
mittency of direct solar energy or wind power or rain,
or seasonality in hydropower and bio-fuels, and;
2. Multi-product economics, with other deliverables
besides electricity, such as desalinated seawater and
cold water for air-conditioning systems or aquiculture.
Cornelia and Davis [3] carried out a comparison study
for the renewable energy supply options in the oceans, and
the conclusion was that the OTEC route is the best fit for a
scenario of high energy and carbon prices. Such expecta-
tions may provide competitive economic performance
indicators in locations with no access to hydropower, or
with high fuel costs for thermal power plants or freshwater
demand. Ocean islands or tropical coastal regions may
benefit from the OTEC deliverables. In case of possible
artificial islands or ocean cities in a distant future scenario,
OTEC may be one of the components in a renewable
Technical Editor: Fernando Antonio Forcellini.
& Vicente Fachina
vicentefachina@gmail.com
1
Petrobras, Rio de Janeiro, Brazil
123
J Braz. Soc. Mech. Sci. Eng.
DOI 10.1007/s40430-015-0383-8
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4. energy portfolio, together with offshore wind power, solar
power, and power from currents and waves.
2 OTEC: history summary and prospects
The OTEC route was introduced by the end of the
nineteenth century. The world wars and low oil prices
have hampered further commercial developments in the
twentieth century. Follow below the main achievements
to date.
Jacques D’Arsonval [4] was the first to introduce the
harnessing of the stored thermal solar energy in the oceans
in 1881. Georges Claude [5], a D’Arsonval´s student, built
an OTEC unit onboard a 10,000 ton cargo ship anchored
close to the Brazilian coast in 1935, but climate conditions
and large waves had wrecked the ship before the OTEC
unit started delivering electricity.
In 1982 and 1983, the Japan Institute for Ocean Energy
Research operated a closed-cycle OTEC for scientific
research. Around 90 % of the electricity produced went to
pumping and other internal consumptions [6].
Between 1993 and 1998, a 1 MW open-cycle, land-based
plant was operated in Hawaii for power generation [6].
In 2013, a 50 kW OTEC pilot plant was launched in the
Kumejima Island, Japan [6].
Within this decade, and possibly over the 2020 one,
some prospects on OTEC comprise contracts for imple-
menting pilot plants ranging from 2 to 10 MW net power,
and also designs for 100 MW plants (commercial
references).
3 OTEC: estimation of energy resource
Ocean thermal energy stems from solar heating of tropical
surface ocean waters. The qualification of such energy
resource as reserves depends on natural, local, techno-
logical, and commercial conditions. Based on the total
area of the tropical oceans, 60 million km2
[2], there are
about 220 EJ/year (7 TW) theoretical energy resource [7].
In terms of energy density that averages out 0.12 MW/
km2
.
Fig. 1 Temperatures on the
surface ocean water [8]
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5. Figure 1 shows the mean temperatures of the surface
ocean water in the Brazilian coastline, which favors the
OTEC route.
Figure 2 shows a numerical simulation for the exergy1
of surface ocean water as to deep ocean water (the
reference state). At 299 K SOW (surface ocean water)
and 279 K DOW (deep ocean water) temperatures, the
SOW exergy is 2.8 MJ/m3
. Together with the exergy
efficiency of the power cycle, that value can be used to
estimate the SOW and DOW flows for a determined
electricity output.
4 OTEC: a power cycle
Figure 3 illustrates a concept design of a thermodynamic
power cycle with a binary ammonia–water working fluid,
based on research and development by Kalina [9], and
Uehara, Kegami and Noda [10].
The reasoning for utilizing a binary working fluid,
instead of a pure substance in a traditional Rankine cycle,
is to maximize energy performance through the energy
balance of such a mixture.
In the evaporator–separator system, a SOW stream
exchanges heat with an ammonia–water mixture: heat
vaporizes the ammonia component, which gets separated
from the water component through an endothermic reac-
tion. After the evaporator–separator system, vapor flows
toward the turbo-generator.
The turbo-generator converts a fraction of the vapor
enthalpy into electricity. After the turbo-generator, satu-
rated vapor is discharged in the absorber–condenser sys-
tem. In the absorber–condenser system, the water
component from the regenerator is mixed with saturated
vapor from the turbo-generator discharge: removal of heat
occurs in the absorber through an exothermic reaction, and
more heat is removed in the condenser. A sub-cooled liquid
is formed, which is then pumped to the regenerator.
In the regenerator, the water component from the sep-
arator preheats the sub-cooled liquid, which then flows to
the evaporator–separator system, and the cycle repeats
itself.
5 OTEC: thermodynamic modeling
The power cycle in Fig. 3 has been simulated by a specific
soft tool for thermodynamic cycles. The highlighted
numbers depict each system boundary for the energy bal-
ances. The electricity availability factor is 47 %, and the
exergy efficiency is 44 %, for an offshore 200 MW OTEC
asset, floating platform type. The system design and
operation ought to minimize exergy waste and destruction
along the process chain for electricity generation. The
assumptions for delivering freshwater are:
(a) Minimum energy consumption for the desalination
process: 7.2 MJ/m3
[11];
(b) Average pumping head for the freshwater pipelines:
5 m.
Due to uncertainties as to the desalination and post-
treatment processes, the specific energy consumption for
delivering freshwater is taken as twice the value in (a),
290 291 292 293 294 295 296 297 298 299 300 301 302
1
1,5
2
2,5
3
3,5
4
4,5
TSOW [K]
e[kJ/kg]
277 K TDOW
279 K TDOW
Fig. 2 SOW exergy as to DOW
1
Exergy is the measure of energy as to an environmental ground-
zero energy reference. The model behind Fig. 2 is: e = h-
h0 - T0(s - s0), where e is specific exergy, h is specific enthalpy,
s is specific entropy, and T0 is the DOW absolute temperature.
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6. to comply with user regulatory specifications. The
energy consumption for pumping is 0.4 MJ/m3
, which
accounts for uncertainties as to pumping head and
losses.
As a rough calculation for a 200 MW OTEC asset,
1.57 MJ/m3
(56 % 9 2.8 MJ/m3
, Fig. 2) is the heat
rejected in the condenser. Assuming 1.4 m/s pipeline DOW
velocity, the internal diameter of the DOW pipeline shall
be 7.4 m. The challenge for constructing and deploying
such a huge pipeline in an open ocean environment has
been beaten already (commercial sources).
6 Economics of a first-generation OTEC asset
for electricity
In a first-generation energy asset, the premise of public
capital to participate becomes fundamental due to the high
risks involved. Nevertheless, the participation of private
capital is necessary to promote better business efficiency
by proper public regulation.
For a first-generation OTEC asset, this topic covers two
parts: an economic modeling, and a cost structure.
6.1 Economic modeling
By assuming an investment setup with public–private
partnerships, two discounted payback times are considered,
five or ten years, for a 200 MW asset as acceptable
thresholds for private capital. The assumptions for applying
the equation set 1 to 3 are:
• Useful life: 25 years;
• Capital costs: 10 %/year and 5 %/year;
• Carbon price2
: as of Fig. 4;
• Electricity price: as of Fig. 4;
• Energy availability factor: 90 %;
• All USD values as of 2010 baseline;
• Investments in the desalination units, electrical cables,
and the freshwater pipelines disregarded;
• Fiscal, finance, insurance or depreciation effects
disregarded.
As to the capital cost, two values are assumed: (a) 10 %/
year from the beginning of the concept design (on time
TG-1
U
Q
NET
POWER
REGENERATOR
ABSORVEDOR
CONDENSER
EVAPORATOR
SEPARATOR
NH4+
OH-
NH4
+
OH
-
QC
Wi
NH4
+
OH
-
TG-2
yw1
(1-y)w1
w1
1-w1
7
8
9
10
1
2
3
4
5
6
11
T5
T2
W
Fig. 3 Closed power cycle with ammonia–water working fluid
2
To use the emission factor 130 kgCO2e/GJ (468 gCO2e/kWh) for
natural gas [17].
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7. being -m) up to the 5th year from the beginning of
operation, to simulate higher uncertainties for lack of
detailed project information; (b) 5 %/year from the 6th
year onwards, to simulate lower uncertainties for better
project information.
Equation 1 is a goal function to represent a null present-
worth value, which is a condition for determining the
participation levels of public capital, Cp, to satisfy the
restriction of the payback time, t*, for the private capital.
The product prices, quantities and costs are pri, qi and c$,
respectively.
Equation 2 is the investment value, IOTEC, where w is
the net power, and a0, k are parameters. This model is
valid for first-generation OTEC assets only, as to Vega
[12].
Equation 3 represents an investment cash flow over a
time period, m, after which the operation phase begins. The
investment cash flow assumed is 8 % for concept design in
the 1st year, 70 % for EPC in the 2nd year, and 22 % for
commissioning and start-up in the 3rd year. This distribu-
tion is a reference only, for such project phases usually
overlap.
XtÃ
1
X
i
priqiÀc$ qð Þi
" #
t
xÀt
À
X0
Àm
1 À Cp
À Á
ImxÀm
¼ 0
tà 2 5; 10f g
0Cp1
x 1 þ j
)t 2 Àm; þ5½ Š ! j ¼ 10 %
)t 2 þ6; þ25½ Š ! j ¼ 5 %
ð1Þ
IOTEC ¼ a0w1Àk
ð2Þ
IOTEC ¼
X0
Àm
nm Á Im
)
X0
Àm
nm ¼ 1
n [ 0
ð3Þ
7 An OTEC cost structure
Table 1 shows a cost breakdown for a first-generation
200 MW OTEC [12]. Three quarters of the costs com-
prise the naval structures (22 %), the heat exchangers
(21 %), the process plant (19 %), and the DOW pipeline
(13 %).
Submarine electrical cables shall transport electricity
from the offshore energy assets to the desalination units
located onshore.
-30%
-20%
-10%
0%
10%
20%
30%
40%
50%
60%
70%
80%
20 40 60 80 100 120 140 160 180 200
Publiccapital
$CO2 [USD/t] (2010base)
200 BRL/MHh @ 5 ROI 400 BRL/MWh @ 5 ROI
600 BRL/MWh @ 5 ROI 200 BRL/MWh @ 10 ROI
Fig. 4 Level of public capital
on a first-generation OTEC
asset
Table 1 Cost breakdown in USD million for a first-generation OTEC
asset
Scope item % USD 9 106
Defining installation location 2 23.2
Heat exchangers 21 243.8
DOW pipeline 13 150.9
Mooring 5 58.0
Electrical cabling 9 104.5
Process plant 19 220.6
Naval structures 22 255.4
Installation 5 58
Commissioning 4 46.4
Total $1,160.9
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8. The desalination units and the freshwater pipelines are
out of scope of this cost structure.
Nonetheless, actual cost data shall stem from research
and development on OTEC through a 3 MW pilot plant for
the Fernando de Noronha Island in Brazil (author´s next
work) to gradually build technological and commercial
maturity for larger undertakings.
8 Economics of a hybrid energy farm
for freshwater
As a long-term solution for the increasingly supply risk of
freshwater on the planet [1], it is proposed a concept design
of a hybrid offshore energy farm, comprising both OTEC
and offshore wind power units, to supply freshwater to the
northeast region of Brazil for human consumption only.
Neither OTEC nor offshore wind power alone may
suffice, that is why a hybrid setup may comply with the
projected freshwater demands, and with minimum Leve-
lized Cost of Water (LCOW).
Equation 4 models an energy density as the ratio
between the net power, w, and an installation area of
diameter /.
p ¼
4w
pu2
ð4Þ
Four variables are to be optimized for a minimum total
LCOW: (a) Quantity of OTEC units; (b) Quantity of off-
shore wind power units; (c) OTEC net power; and (d) Wind
net power.
Equation 5 is a goal function for the total yearly cost of
freshwater, C, which is based on the LCOW for each route,
c, and the freshwater supply, d, for each route. The total
freshwater supply shall meet the demand, B. Also, the
grand total investment (both OTEC and offshore wind
power) shall be within a capital constraint, Imax.
The variables involved in this function shall be con-
strained as shown. For example, the nominal power values
and quantities ought to be within defined ranges, out of
which their implementations are not feasible, either tech-
nically or economically. Also, the upper thresholds for the
quantities of units can be estimated by either economic or
environmental constraints. All the variables are described
in the glossary section.
Equation 6 models the freshwater supply, d, for each
route. The product between the number of units and the net
power in this equation entails solving a non-linear goal
function (Eq. 5). This implies there being no global opti-
mum solutions, but local ones only. Therefore, stricter
restrictions on the gross power ranges do help out.
Equation 7 models the levelized cost of freshwater for
each route, c.
Equation 8 models the unit nominal power, w0
, for each
route.
Min ! C ¼ chdh þ cede
dh þ de ! B
X
I Imax
0 nh nmax
h ; nh 2 N
0 ne nmax
e ; ne 2 N
wmin
h w0
h wmax
h
wmin
e w0
e wmax
e
nhw0
h Ahph
new0
e Aepe
ð5Þ
dh ¼
nhwh
bh
; de ¼
newe
be
ð6Þ
ch ¼ bheh; ce ¼ beee ð7Þ
w0
h ¼
wh
ah
; w0
e ¼
we
ae
ð8Þ
The projected demand of freshwater for human consump-
tion only by 2025 in the northeast region of Brazil is
198 m3
/s [13]. An average demographic rate increase of
1 %/year is assumed for the 2025–2050 timeframe, after
which it becomes flat due to energy efficiency investments.
Therefore, 254 m3
/s projected demand results for 2050
onwards.
9 Results and discussions
9.1 Economics of a first-generation OTEC asset
for electricity
For a 200 MW OTEC asset, about 16 million USD/year
operational and maintenance costs are estimated [14]. The
following prices are assumed: 50 USD/t CO2, and 65 USD/
GJ (BRL 400/MWh) electricity. Together with the total
investment as of Table 1, the equation set 1–3 yields
26 USD/GJ (162 BRL/MWh) Levelized Cost Of Energy
(LCOE). The level of public capital is 41 % for a 5-year
ROI.
Figure 4 shows the level of public capital as a function
of the CO2 price, and of the electricity price for either
5-year or 10-year ROI. For instance, with 600 BRL/MWh
electricity price at 5-year ROI, about 120 USD/t CO2 price
comes into being a breakeven point, for which all capitals
can be private. Negative values mean 100 % private capital
at less than 5-year ROI or 10-year ROI.
Therefore, high enough energy and carbon prices are
key enablers for OTEC to gain good economic
attractiveness.
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9. 9.2 Economics of a hybrid energy farm
for freshwater
Table 2 shows inputs and outputs from the equation set
4–8. The numbers of OTEC and offshore wind power units
are those which minimize the goal function given by Eq. 5,
together with its constraints. Table 2 shows a base-case
scenario, upon which the numbers of OTEC and offshore
wind power units are not constrained by capital shortages.
Figure 5 projects other scenarios.
The grand total investment, both public and private,
boils down to USD 28 billion, excluding the desalination
units, the freshwater pipelines, and the electrical cables.
That value is the sum of two parts: (a) the number of
OTEC units multiplied by the total investment on a first-
generation OTEC asset (Table 1), and then taking two-
Table 2 Simulation results for
a hybrid freshwater energy farm
(base-case scenario)
Unit Offshore OTEC Offshore wind
Inputs Electricity availability factor, a – 47 % 95 %
Unit gross power, w0 MW 200 9.9
Total gross power, nw0 MW 6400 800
LCOE, e USD/GJ 26 39
Specific energy for freshwater, b MJ/m3
14.8 14.8
LCOW, eb USD/m3
0.39 0.58
Available area, A km2
100 K 100 K
Energy density, p MW/km2
0.12 4.0
Available capacity, Ap GW 11.6 400
Min gross power, wmin
MW 50 8
Max gross power, wmax
MW 200 10
Unit min distance km 32.1 1.7
Installation perimeter km 1000 1000
Max. no. units, nmax
– 32 579
Freshwater supply per route m3
/s 203.2 50.8
Number of units, n – 32 80
Unit net power, w MW 94 9.4
For OTEC, 0.39 USD/m3
LCOW stems from multiplying the 26 USD/GJ LCOE by the specific energy
consumption for freshwater, 14.8 MJ/m3
. The specific energy consumption for freshwater is obtained by
adding the specific energy for desalination, 14.4 MJ/m3
, and the specific energy for pumping freshwater,
0.4 MJ/m3
. The same procedure goes for the LCOW by wind power
For offshore wind power, 39 USD/GJ LCOE [15]. The energy density for the offshore wind power, 4 MW/
km2
, is taken as twice the value from the onshore wind power [16]
0
0,1
0,2
0,3
0,4
0,5
0,6
0
10
20
30
40
50
60
80 159 287 377 407
LCOW[USD/m3]
No.OTECunitsorCapital[USDbillion]
No. offshore wind power units
Capital OTEC LCOWFig. 5 Scenario analysis of the
freshwater energy farm
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10. thirds of this product; and (b) 4 million USD/MW off-
shore wind power unit investment [15], multiplied by the
total gross power for wind (Table 2). The two-thirds term
in bullet (a) is to simulate a possible economics of scale
on building that amount of OTEC assets during an
installation program.
For 254 m3
/s freshwater demand, the total cost is 3.4
billion USD/year, which means about 0.43 USD/m3
LCOW, 80 % of which from the OTEC units. Installation
sites should be located alongside a 1000-km-long perime-
ter, between 10 and 50 km distance to coast, in the
northeast region of Brazil.
If capital is constrained, which is business as usual, the
outputs from Table 2 change accordingly, thus favoring the
less capital intensive scenarios; nevertheless, the more
constrained the capital gets, the farther away the levelized
cost of freshwater (LCOW) gets for the same demand.
Figure 5 shows several scenarios for capital thresholds
of 50, 25, 20, 18, and 16 billion USD. On the base-case
scenario (Table 2), with USD 50 billion cap, the fresh-
water energy farm comprises 32 OTEC units and 80
offshore wind power units to have 0.43 USD/m3
LCOW.
As long as the capital caps get smaller, less OTEC units
and more wind units are needed to have higher LCOW.
For the lowest capital cap, USD 16 billion, the fresh-
water energy farm comprises one OTEC unit only and
407 offshore wind power units to have 0.57 USD/m3
LCOW.
The actual scenario might happen somewhere in the
middle, between the lowest and highest LCOW, to depend
on the investment time horizon, and economical, political
frameworks. The assumptions for the equation set 1–3 are
quite distant from the action.
10 Conclusions
The proposal of the ocean thermal energy route dates back
the end of the nineteenth century, and it has gained new
momentum in the twenty-first century due to security
concerns as to energy and freshwater. As to the wind
power, it is far older, dating back the middle age. Never-
theless, offshore wind power farms are new, twenty-first
century achievement.
A hybrid energy farm comprising both ocean thermal
and offshore wind power units has been modeled to pro-
vide freshwater for human consumption in the northeast
region of Brazil. The economic figures are USD 28 billion
investment for a minimum 0.43 USD/m3
levelized cost of
freshwater, excluding the desalination units, the freshwater
pipelines and the electrical cables.
Other less capital intensive scenarios are possible, with
USD 16 billion investment; nonetheless, that investment
amount brings about higher LCOW, 0.57 USD/m3
.
Glossary
A Available installation area [km2
]
a0 Parameter in the Vega model
B Desalinated seawater demand [m3
/s]
c$ Product cost
c Levelized cost of desalinated seawater [USD/m3
]
C Total levelized cost of desalinated seawater
[USD/s]
Cp Participation level of public capital in an
investment project
d Desalinated seawater supply [m3
/s]
DOW Deep Ocean Water
e Subscript for wind power
EPC Engineering–Procurement–Construction
h Subscript for OTEC
I Investment value
LCOE Levelized cost of energy
LCOW Levelized cost of water
m Investment period
n Number of units
nmax
Maximum number of units
OTEC Ocean thermal energy conversion
p Energy density [MW/km2
]
pr Product price
q Demand quantity
ROI Return over investment
SOW Surface Ocean Water
USD US dollar
w Net power of each unit [MW]
w0
Nominal power of each unit [MW]
wmax
Maximum nominal power of each unit [MW]
wmin
Minimum nominal power of each unit [MW]
a Electricity availability factor
b Specific energy consumption for freshwater [MJ/
m3
]
e Levelized cost of electricity [USD/MJ]
k Parameter in the Vega model
/ Diameter of the installation area [km]
n Investment cash flow distribution.
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Author's personal copy