Modeling and experimental validation of an Ocean Thermal Energy Conversion system in Reunion Island Frantz SINAMA1, Franck LUCAS1, François GARDE1, Driss STITOU2, Sylvain MAURAN2 1 - Laboratoire de Physique du Bâtiment et des Systèmes (L.P.B.S) Université de La Réunion – 40 Avenue de Soweto - 97410 Saint Pierre Ile de la Réunion – France 2 –Laboratoire PROMES-CNRS Tecnosud - Rambla de la thermodynamique 66100 Perpignan - France Email:
[email protected]
Abstract
Renewable Energy has a crucial interest for Reunion Island. The supply of electricity based on renewable energy has many advantages but the major drawback is of being a fatal energy where the production of electricity 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. By using surface hot water and deep cold water of the ocean, it is possible to operate a thermodynamics cycle, which will then generate electricity. In this article, in the first part a literature and field review is carried out, to make a point on existing technologies by targeting two areas: electricity production and cooling of buildings. This study establishes a knowledge platform on thermodynamic cycles consistent with the OTEC and on dimensional and functional parameters associated with this technology. Steady state and dynamic simulations are presented to understand the operation of the system. Steady state models will evaluate the potential of the OTEC in distributing base electricity. Dynamic models will assess possibilities of electrical power modulation output for the distribution of electrical power for semi-base or peak. These simulations will help evaluating the potential for new thermodynamic cycles such as the “Capili Cycle”, a modified Carnot cycle operating with a liquid piston. Thanks to these tools, a feasibility study provides the main characteristics of a future experimental facility and the necessary elements for the drafting of specifications.
Principle and applications of OTEC systems
OTEC systems have many applications : electricity production, water desalinization, aquaculture, refrigeration and building air-conditioning, etc … (Figure 1). Electricity production with the closed-cycle OTEC system: warm seawater vaporizes a working fluid, such as ammonia, flowing through the evaporator. The vapor expands at moderate pressures and drives 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. The condensed working fluid is pumped back to the evaporator to repeat the cycle. Sea Water Air Conditioning (SWAC) : The cold seawater made available by an OTEC system creates an opportunity to provide large amounts of cooling of buildings to operations that are related to the plant •
Hot water Input 26 °C
TECin
Evaporator
Cold water Output
W
TEHin
TCCout
9 °C
TCHin
TECout
Condenser
TCHout
Figure 1 : Example of OTEC plant - Source OCEES
Objectives
This study is carried out in the frame work of the DEEP BLUE project which aim is to promote the development of OTEC technologies in Reunion Island. The first objective is to develop numerical decision making tools for OTEC installation feasibility studies. Then a pilot plant will be installed onshore (in Saint Pierre) to validate the models and assess the performance of the cycle.
Hot water Output
Cold water Input
Turbine and generator
23 °C
TCCin
5 °C
•
mWF
circulation pump
Steady state simulation
The aim is to simulate an OTEC closed cycle to evaluate the potential for electricity production. These models are based on a simplified model of ideal Rankine cycle adapted to the conditions of OTEC (Avery and Wu, 1994). The simulation tools used are Engineering Equation Solver (EES) and Thermoptim. Example of figures of the components parameters for the closed cycle are given in table 1. Models for the condenser and the evaporator : heat exchanger are modeled thanks to the LMTD method. • Q ,heat flux from the hot source (W) ( ) ( ) T − T − T − T EHout ECout EHin ECin Evaporator model : QH = U EV ⋅ S EV ⋅ U , overall heat transfer coefficient
TEHout
Figure 2 : Example of operation of closed cycle
Working Cold Water Hot Water Evaporator Condenser Evaporator Condenser Fluid Mass Mass Flow Mass Flow Area Area Pressure Pressure Flow Rate Rate Rate
T/G efficiency
•
( TEHout − TECout ) ln (TEHin − TECin )
•
•
Model for the Turbine : W = mWF ⋅ (htin − htout )
H
EV
(W/m²K) SEV , evaporator area (m²)
1 m²
1m²
htin, turbine inlet enthalpy (J/kg) htout, turbine outlet enthalpy (J/kg), determined through the T/G efficiency
800 kPa
600 kPa
0.8 kg/s
200 kg/s
200 kg/s
56%
Table 1: Component parameters
Model for the pump : Work from the pump is due PV work : hPout − hPin = vWFC ⋅ (PCout − PCin ) hPin, pump input enthalpy (J/kg) hPout, pump output enthalpy (J/kg) vWFC, Specific volume for saturated ammoniac at PCout (m3/kg)
PCin, turbine inlet pressure (Pa) PCin, turbine outlet pressure (Pa)
TEHin − TCCin η= The Carnot efficiency is defined by the following equation : TEHin
The output values of the model are Carnot efficiency and gross power of the turbine. In our case, the Carnot efficiency is about 3 %. The gross power output depends on the temperatures of the sources. If the temperature difference between the sources increases by 1 ° C, the power increases by 10% (Figure 3).
Dynamic simulations
The aim is to simulate an OTEC system to assess possibilities for the distribution of electrical power for semi-base or peak electricity production of electrical output power modulation. This model allows taking into account transient behavior of components such as heat exchangers and turbine. It is then possible to consider grid electricity demand and simulate the start-up or shut down of the cycle.
Future work
The model in steady state has shown the potential of the OTEC. To better understand the phenomena involved, several avenues are being explored : Sensitivity analysis to understand the influence of different parameters on the cycle. Evaluate the potential for new thermodynamic cycles such as the Kalina or Capili cycle
Conclusion
Gross Power (kW) 36 34 32 30 28 26 24 22 20 20
21 22 23 24 25 Input Working Fluid Temperature Condenser(°C)
26
Figure 3 : Evolution of gross power output
This works gives an initial approach to systems based on OTEC system. As part of this work and "Deep Blue“ project, a feasibility study is underway for the establishment of a pilot plant with an industrial. This experimental facility will provide elements of validation for our models. The development of these models will help us to predict the behaviour and the performance of the installation.
