Modeling of Ocean Thermal Energy Conversion ...

Proceedings of the Proceedings ofASME ASME 2010 2010 4th 4th International International Conference Conference on on Energy Energy Sustainability Sustainability ES2010 ES2010 May May17-22, 17-22,2010, 2010, Phoenix, Phoenix, Arizona, Arizona, USA USA

ES2010-0 ES2010-90394 MODELING OF OCEAN THERMAL ENERGY CONVERSION (OTEC) PLANT IN REUNION ISLAND Franck Lucas Building Physics and Systems Laboratory (LPBS) – University of La Réunion Saint Pierre, Ile de la Réunion, France

Frantz Sinama Building Physics and Systems Laboratory (LPBS) – University of La Réunion Saint Pierre, Ile de la Réunion, France

François Garde Building Physics and Systems Laboratory (LPBS) – University of La Réunion Saint Pierre, Ile de la Réunion, France

ABSTRACT

NOMENCLATURE

Renewable Energy has a crucial interest for a remote area like Reunion Island. The supply of electricity based on renewable energy has many advantages but the major drawback is the production of electricity which varies highly according to the availability of the resource (wind, solar, wave, etc...). This causes a real problem for non interconnected electrical grid where intermittent renewable energies should be limited to a maximum of 30%. The Ocean Thermal Energy Conversion (OTEC) provides an alternative of electricity production from the available energy of the oceans present all the time. By using surface hot water and deep cold water from the ocean, it is possible to operate a thermodynamics cycle, which will then generate electricity. In this article, in the first part a literary and technological review is carried out in two areas: electricity production and cooling of buildings with deep water. This study establishes a knowledge base on thermodynamic cycles consistent with the OTEC and on dimensional and functional parameters associated with this technology. Steady state simulations are presented to understand the operation of the system. Steady state models will evaluate the potential of the OTEC in distributing base electricity. These simulations will help evaluating the potential for new thermodynamic cycles such as the Kalina cycle. With these tools, a sensitivity study will evaluate the influence of different parameters on the cycle.

A

Area, m²

Cp

Specific heat, kJ/kg-K

h

Specific enthalpy, kJ/kg

.

m

Mass flow rate, kg/s

NTU

Number of Transfer Units

P

Pressure, kPa or Pa

qv

Volume flow, m3/s

.

Q

Heat transfer rate, kW

s

Specific entropy, kJ/kg-K

T

Temperature, °C ou K

U

Global convection heat transfer coefficient, kW/m²-K

UA

Overall thermal conductance, kW/K

.

W

Power, kW

Greek symbols ε

Effectiveness

η

Efficiency

1

Copyright © 2010 by ASME

Indices a

actual

c

condensation or cold source

e

evaporation

h

hot source

in

input

is

isentropic

max

maximum

min

minimum

net

net output power

NH3

ammonia

out

output

pipe

pipe

p

pump

sat

saturation

tg

turbine - generator

1,2,3,4 State points

INTRODUCTION The oceans cover more than 70 percent of the Earth's surface. This makes them the world's largest solar energy collector and energy storage system. OTEC is an energy technology that converts solar radiation to electric power. The operating principle is based on the second law of thermodynamics and on a Rankine cycle. Indeed, it is possible to run an engine to produce work, using the temperature difference between water on the surface (hot source) and deep water (cold source) taken at a depth of 1000 m. The ultimate goal is the production of electricity. There are two major types of processes: the closed cycle and the open cycle. Moreover, an OTEC plant can be installed on land (onshore) and at sea (offshore). More details on the cycles are given in the Section on “ELECTRICITY PRODUCTION”. A summary of the history of OTEC through the ages developed by Avery and Wu [1] shows that it begins in the 19thC with the French physicist Jacques-Arsène d'Arsonval who gave the first correct formulation of this technology in an article published in 1881. Levrat [2] shows that it's one of his students, George Claude in 1928 who first materialized an OTEC plant. It was an open cycle which used as the hot spring water, the cooling water a blast furnace in Belgium, and as the cold source, the water of the Meuse. It produced 60 kilowattselectric (kWe). Between 1928 and 1950, other experiments followed such as installing a turbine in Cuba, creating the first

off shore factory in Brazil or a proposed OTEC plant in Abidjan. The problems were that Claude had underestimated the power of the wave at the laying of pipes and to maintain them for weather events (storms, hurricanes). As he worked with his own funds, it was difficult for him to set up projects and keep them running. Marchand [3] shows that other projects were studied in the 50s, but were not completed because of their low profitability. In 1979, the first 50 kWe closed-cycle OTEC demonstration plant went up at the Natural Energy Laboratory of Hawaii (NELHA) [4]. The plant was mounted on a converted U.S. Navy barge moored approximately 2 kilometers off Keahole Point. The plant produced a net power of 15 kWe . In 1981, Japan showed an onshore closed-cycle plant of 100-kWe in the Republic of Nauru in the Pacific Ocean [5]. Freon was the working fluid, and a titanium shell-and-tube heat exchanger was used. The plant surpassed engineering expectations by producing 31.5 kWe of net power during continuous operating tests. Since the 90s, experimental projects were conducted by the NELHA with an open cycle plant of 100 kWe of net power or on Sagar Shati, an Indo-Japanese research ship (NIOT-IOES) with 1 MWe of net power. In the vast areas of the tropical ocean, the temperature difference between warm water on the surface and deep sea water exceeds 20 °C. This natural phenomenon can be used by OTEC. The resource is widely available, stable and available at all the time. The exploitation of natural resources using OTEC is that the world ocean warmed by the sun absorbs heat. The flow of heat absorbed is estimated at 52.4 PW or 456,106 TWh / year. A value of Technically Exploitable Potential (TEP) of the global resource is given by Avery and Wu. According to these experts, it is possible to extract 0.19 MW of solar energy captured per 1 km2 of ocean surface located in the tropical zone most conducive to the OTEC holding. This zone extends over 60 million square kilometers. It is a TEP of 12 TW of electricity which represents 100 000 TWh / year by considering either onshore or offshore OTEC. The main advantages of OTEC are • A free resource, renewable and available 24/24 • The use of cold water for other activities other than electricity • The use of OTEC produces neither waste nor residue, toxic to the environment

2


Th,out

Th,in Evaporator

2 Feed pump

3 .

Turboalternator

1

W

4 Condenser

Tc,out

Tc,in

Figure 1. Schematic representation of the closed cycle

ELECTRICITY PRODUCTION There are two types of cycles for electricity production with OTEC, the closed cycle and the open cycle. In the closed-cycle OTEC system (Figure 1), the warm seawater vaporizes a working fluid, such as ammonia, flowing through a heat exchanger (evaporator). The vapor expands at moderate pressures and turns a turbine coupled to a generator that produces electricity. The vapor is then condensed in another heat exchanger (condenser) using cold seawater pumped from the ocean's depths through a cold-water pipe. The condensed working fluid is pumped back to the evaporator to repeat the cycle. It remains in a closed system and circulates continuously.

Other cycles have been developed to improve the production of electricity. The Kalina cycle, developed by Russian engineer Alexander Kalina, is derived from the Rankine cycle. It does not use pure ammonia but a mixture of two or more fluids. Generally, the mixture used is water / ammonia and has a large temperature gradient. Indeed, the existence of this shift in temperature reduces the temperature difference between the gas stream cooling in the steam generator and the working fluid. The internals irreversibilities due to the temperature gradient are therefore reduced. [6] A basic Kalina cycle is represented in Figure 3 and includes three main elements: • A steam generator with a reheater • A turbine • A distillation and condensation system Uehara cycle was developed by Professor Dr. Haruo Uehara. It was developed in order to optimize the power cycle generation using ocean thermal energy. Professor Dr. Haruo Uehara demonstrates this new cycle as an improvement of Kalina cycle. The main feature is about changing the composition of the ammonia-water mixture by using a staged expansion with sampling [7]. As for the Kalina cycle, the aim of this cycle is to replace the evaporation and condensation at constant temperature with working fluid through changes with temperature gradient, and thus reduce the irreversibility of the system

In an open-cycle OTEC system (Figure 2), warm seawater is the working fluid. The warm seawater is "flash"-evaporated in a vacuum chamber to produce steam at an absolute pressure of about 2.4 kPa. The steam expands through a low-pressure turbine that is coupled to a generator to produce electricity. The steam exiting to the turbine is condensed by cold seawater pumped from the ocean's depths through a cold-water pipe. If a condenser is used in the system, the condensed steam remains separated from the cold seawater and provides a supply of desalinated water.

Steam generator

.

Turboalternator Sepa rator

Distiller

Reheater

Th,out

1

Turboalternator Tc,in

Evaporator Th,in

W

Feedwater Heater

Condenser

Boiler feed pump

.

W

Reheater 2

Tc,out

Condensate pump Throttle Condenser

Freshwater

Absorber

Figure 3. A simplified Kalina Cycle [8]

Figure 2. Schematic representation of the open cycle

3


REFRIGERATION AND AIR-CONDITIONING The cold seawater (4 to 6°C) from an OTEC system creates an opportunity in providing large amounts of cooling to the operations nearer to the plant. The cold seawater delivered to an OTEC plant can be used in chilled-water coils to provide airconditioning for buildings. Sea Water Air Conditioning (SWAC) is composed of a heat exchanger and two loops: the primary loop for deep water and the secondary loop cooling water which is used to provide power for buildings. Such projects were carried out in Stockholm under study in Hawaii [9]. The Intercontinental Hotel is the first private building to be fitted with a SWAC system which has been working since 2006 and the characteristics of the plant are given in Table 1 [10]. SWAC features Length of the pipe Diameter of the pipe Depth pumping Cold water flow Cooling power installed Pumping power

•

η is ,tg = .

h3 − h4 h3 − h4,is

Table 1. SWAC features of the Hotel Intercontinental in Bora Bora

STEADY STATE SIMULATION The study of the cycle is based on a Rankine cycle with ammonia as working fluid. This is one of the best fluids for low temperatures used by the cycle. Moreover, the pressure of the ammonia fluid in the cycle is not very high. . In this paper, the hot source is the sea water of temperature of 28 ° C taken at 40 m deep. The cold source is cold water thoroughly taken at 1000 m at 5°C [11]. The mass flow rates are adjusted to the surface of exchange and power provided. The following general assumptions are made in the thermodynamics analysis: • The heat and friction losses are neglected in the cycle. • The pressure losses in the heat exchanger are also neglected. • The ammonia at the condenser exit is a saturated liquid. More detailed assumptions on heat exchangers and sea water pumps are given later. The steady state simulation is performed with EES (Engineering Equation Solver) [12]. This software has many advantages for thermodynamics analysis: a solver for non linear equation and a fluid properties library [13]. The modeling cycle uses these equations taking consideration the assumptions listed above [14]:

(1)

.

W tg = m NH 3 (h3 − h4 ) •

(2)

Pump :

η is , p , NH 3 = .

v1 (Pe − Pc ) h2 − h1

(3)

.

W is , p , NH 3 = m NH 3 (h2 − h1 ) •

2300 m 400 mm 900 m 270 m3/h 1.5 MWf 15 kW

Turbo-alternator:

.

(4)

Condenser: .

Qc = m NH 3 (h4 − h1 ) • .

(5)

Evaporator: .

Qe = m NH 3 (h3 − h2 )

(6)

For heat exchanger, in steady state condition, the NTU-ε method of heat exchanger analysis can be used [15]. With this method, it is possible to determine the outlet temperature of the heat exchanger with these equations: •

maximum heat transfer:

Q max = m NH 3 Cp min (Th,in − Tc,in ) .

.

•

(7)

effectiveness: .

ε=

Qa

(8)

.

Q max Where Q a = m NH 3 Cp h (Th,in − Tc ,out ) = m NH 3 Cpc (Tc ,in − Tc ,out ) .

.

.

The effectiveness can be defined using the NTU:

NTU =

UA .

(9)

m NH 3 Cp min In our case, for evaporation and condensation, the temperature phase change is constant; the effectiveness can be reduced to:

ε = 1 − exp(− NTU )

4

(10)


Figure 4. Evolution of the gross power based on the performance of the turbine

RESULTS AND DISCUSSION In this section, the results of a parametric study for an ammonia closed cycle using the previous model are presented. For each case presented, one parameter becomes a variable while all others stay constant. The varying parameters concern the components of the cycle. They are as follows: • for the turbo-alternator: the pressures and the efficiency • for the heat exchangers: the area • for the pumps: the flow rate To analyze the cycle, each input variable is compared to a single output variable, the turbine’s power, which is the gross power of the cycle. The cycle parameters that stay constant are as follows: • Thermodynamic properties of fluids • The temperatures of the sources • The pressures at input and output of the pipes of seawater • The depth of the inlet water for the two sources • Global convection heat transfer coefficient of the heat exchanger

Figure 5. Evolution of the gross power based on the evolution of the turbine inlet pressure

The second parameter studied is the pressure: at the inlet and outlet of the turbine. When the inlet pressure of the turbine is increased, an increase of the gross power may be observed as shown in Figure 5. Indeed, the pressure increases up to a limit. This value corresponds to a critical point for the working fluid from the temperature of evaporation of the fluid itself and the efficiency of the turbine. Figure 6 shows how the global efficiency decreases when the output pressure of the turbine increases. It is therefore preferable to work with low pressure so that the working fluid condenses at a low temperature. This pressure corresponds to the pressure of condensation and is determined by the condenser's size. Note also that the inlet temperature of cold water limits the minimum temperature of condensation. The two results above show that the applied pressure at the turbine affect the global efficiency of the cycle. Indeed, to obtain the gross power sighting, a sufficient pressure difference must be maintained between the terminals of the turbine.

This will be a first approach on the influence of these parameters on the overall behavior of the cycle. The first parameter analyzed is the efficiency of the turbinegenerator. The gross power depends on the turbo-alternator efficiency (Figure 4). This variation is linear and increases with the efficiency. It seems logical that the cycle is more efficient when the performance of the components increases. The value of this efficiency will depend on the overall performance desired and on the component used. In fact, it helps to obtain a minimum value of performance to be achieved for a better selection of components.

Figure 6. Evolution of the gross power based on the evolution of the turbine outlet pressure

5


Figure 8: Evolution of the gross power based on the area of the evaporator

Figure 7. Evolution of the gross power based on the evolution of the mass flow of ammonia.

The third parameter studied is the mass flow rate of the working fluid, considering this influence on the gross power. Figure 7 shows how the gross power increases as the mass flow rate of ammonia rises. The more the working fluid flow rate increases the better the heat exchange will be and the work of the turbine. This is also due to the fact that the cold and hot temperatures are fixed, and therefore the evaporating and condensing temperatures are fixed. The mass flow rate should be adjusted according to the required power. Note that it will depend in on the size and the cost of the turbine. The last parameter studied is the surface exchangers. The exchange area has minimum influence on gross power as shown in Figure 8 for the evaporator for example. Indeed, it may be seen that when the exchange area is doubled, the performance slightly increases. It may be observed that when the exchange surface was determined (or UA), it had minimum influence on the performance of the cycle. Another step will be to model more improved exchangers in order to test multiple technologies to evaluate which is the most suitable for an OTEC. The parametric study showed which parameters have a significant impact on the performance of the system. It helps to size the best possible system based on expected performance.

This study will allow cost evaluation to obtain better performance. For example, an increased flow rate of hot water can help to achieve better results on the thermodynamic efficiency, but will affect the cost of the pump and power and thus the overall performance of the cycle. This may applied for the length of the column suction for cold water intake. In fact, from a temperature of 5 to 4 ° C, the efficiency increases by 15%. The system will limit factors in the quest for maximum power; the parametric study will help to find what those elements are. Since April 2009, under an agreement with the regional council, a French company achieves a feasibility study on implementing a demonstrator of an OTEC power plant of 1 MW net power offshore the Reunion Island [17]. It represents a gross power of 1.6 MW. Through the parametric study, this power was sighted to get an idea on the input and output parameters. A simulation was conducted with these parameters and results are given in Table 2. Input Parameters Hot water temperature Cold water temperature UA of heat exchanger Ammonia mass flow Turbine efficiency Pressure turbine inlet Outlet pressure turbine Feed pump efficiency Sea water pump efficiency Output Settings Cycle efficiency Gross Power Output Temperature hot water Cold Water Temperature

28 °C 5°C 300 kW/K 40 kg/s 0.85 950 kPa 660 kPa 0.65 0.75 3% 1.6 MW 24.5°C 10.5 °C

Table 2. : Parameter for a 1.6 MW OTEC plant.

CONCLUSION The ocean thermal energy conversion has real potential in the world. The ocean is a nearly inexhaustible resource and it should be exploited to its full extent. The technology applied has been known for years and has been used in other areas. Many studies have been conducted like Avery or Takahashi [18] and shows the multiple benefits of the OTEC, which main advantages are described throughout this article. Many projects have been studied worldwide, but few were successful. The main obstacle was the high cost of installation compared to a conventional system using fossil fuels. But the current situation in the world, announced the increase of the cost of a barrel of oil, now shows the competitiveness of the OTEC.

6


In Reunion Island (an island depending on fossil energies) the energy situation and its geographical position encourages the use of renewable energy. Using OTEC would be an additional source of basic energy in the energy mix of the island. The simulation in steady state has provided an initial approach to better understand and control the operation of the plant as a source of basic energy. The simulation in dynamic state would be an interesting thing to perform. .The behavior of the OTEC plant will be seen as a source of extra energy (for peak consumption). But this model will also be coupled to external elements for related activities such as air conditioning with SWAC or conduct annual reviews of electricity. The parametric study has allowed first optimization for a closed cycle. The results clearly show that the optimization parameters can improve the performance of the cycle. However, an exergy analysis using the Gibbs method, on each component of the cycle, will be made to better optimize the cycle. Other optimizations may be considered after testing other new fluids, new cycle as the Kalina cycle, or increasing the temperature difference (eg increase in temperature of hot source with thermal solar panels), to improve performance and therefore electricity production. Under the offshore demonstrator project, a plan for minimizing risks is made by this French company itself. Part of this plan is to establish with the University of La Reunion (LPBS), the regional council and this French company, an agreement for the design and installation of onshore demonstrator, the "Prototype à Terre ETM" (PAT) . This demonstrator will be installed on the site of the University in Saint Pierre. In theory, it will provide a power of 15 kilowatts. Studies have already begun and the commissioning of this demonstrator is planned for the second half of 2011 at the University. The objectives of the PAT are: • Tool validation for models • Justification for choice of the offshore demonstrator cycle (Rankine, Kalina, ...) • Validating the behavior of different technologies of heat exchangers • Validating the use of ammonia in all life’s cycles of the process ACKNOWLEDGMENTS The authors acknowledge the support of Professor Bernier, M, Ecole Polytechnique Montreal for the steady state simulation. REFERENCES [1]

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Gautret, L. 2009, “ OTEC Launch in Reunion Island ”, ARER, http://www.arer.org/news/affiche_news.php?article=36 9 Takahashi, P. and Trenka, A, 1996. “Ocean thermal energy conversion”, John Wiley, p. 89, Vol. UNESCO energy engineering series.
