9 Principles of Generation of Ocean Thermal Energy Conversion

Prof. Purnima Jalihal

1. Introduction

The most important factor to look for new sustainable energy sources is linked with the impending disaster of climate change. This and global warming are considered to be a result of anthropogenic green house gases with CO2 being the most important . Electricity production is responsible for producing the largest amount of CO2 emission. Ocean energy forms include wave, tidal, marine currents and thermal gradient. Surface sea waves are generally wind driven and depend on location and seabed profiles for intensity in terms of wave heights and periods. Tidal ranges are high only in certain specific locations. For instance in India, mostly around the peninsula coast, the tidal range is low but in places like Gulf of Khambat and Sundarban, it can be as high as 6-8m. Marine hydrokinetic turbines run using the water velocity. Water currents similar to waves, change direction and magnitude according to seasonal variations. The current turbines need to be designed for optimum speeds for maximum power output. The last type of energy is called ocean thermal energy conversion. The surface of the ocean has a higher temperature than if you go deeper down into the sea and especially in the tropical waters, the difference between temperatures at the surface and that at the bottom at around 1000m water depth it can be as high as 20oC. This difference in temperature or thermal gradient can be used to harness energy and is called OTEC in short. Thus we have seen various forms of possible renewable energies. Solar, onshore wind and biomass have progressed to a reasonable level of development. However, ocean energy is still in infancy. European countries have high wave climates and attempts to harness wave, tidal and marine current energy have been going on for a few decades but the impetus has now grown again from the perspective of climate change and future fuel deficit.

2. Why ocean energy for India?

India has about 7500km of coastline with a large Exclusive Economic Zone (EEZ) of around 2.1 million km2. The EEZ is the area of coastal water and seabed associated with a country within a certain distance of a country’s coastline. India also has the Lakshadweep group of islands in the Arabian sea and Andaman group in the Bay of Bengal. Since the area available is large, it can be gainfully utilized to put up ocean energy devices. It is commonly understood in India that wave, winds & currents intensities are low closer to the equator and hence harnessing them may lead to low powers. However, it may be noted that lower climatic conditions can make devices easier for design. Northern latitudes have designed and installed many devices and many of these failed simply due to the extreme weather conditions there. The challenge in the Indian coast is to make the devices self starting at low speeds and after technical viability, cost effectiveness needs to be achieved. Wind, waves and currents are variable by day as well as by season both in direction and magnitudewise. As against this, thermal gradient in India remains mostly constant throughout the year. This can be used to harness energy and is called Ocean Thermal Energy Conversion (OTEC). A typical temperature profile for Indian waters is shown in Fig.1.

3. Ocean Thermal Energy Conversion (OTEC)

3.1 What is OTEC?

The sun warms the surface sea water to an extent that all the energy is captured in a region upto 100m in thickness near the surface. This is called ‘mixed layer’ since wind and wave actions cause the temperature and salinity to be uniform in this layer. As we go deeper down into the ocean, the water becomes colder. A huge amount of cold water exists at depths around 1000m, which is due to accumulation of ice-cold water that has melted from the polar regions. The two bodies of warm water from the surface and cold water from the deep can be used to run the OTEC cycle for generating power. Essentially OTEC converts a low-grade heat source into electricity by using a thermodynamic cycle. The efficiency of the cycle is determined by the Carnot cycle and is generally around 7-8%, which is for an ideal reversible heat engine. However in reality heat exchangers, turbine, pumps, generator, etc. contribute to large losses and hence efficiency is much lower than the ideal one.

3.2 A brief history

In 1870, Jules Verne introduced the concept of OTEC in his novel ‘Twenty Thousand leagues under the sea’. But Jacques d’ Arsonval and his student Georges Claude were the pioneers of OTEC. Claude built the first onshore OTEC plant in 1930. The first successful offshore plant producing net power was mini-OTEC in Hawaii in 1979.

Subsequently several studies around the world have been conducted for the various complexities involved. Saga University in Japan has been working on the thermal cycles optimization in the laboratory for several years. Similarly US, Department of Energy has been funding activities on OTEC R&D with industry.

Currently plants which have been installed are :-

India had made attempts in as early as the year 2000 to set up a 1 MW floating barge mounted plant offshore. This project is discussed in a later section.

3.3 OTEC cycle and system

The OTEC cycle can be classified into two types:

(a) Open cycle :

In this, the working fluid is vented out after use. One way to use the open cycle is to run surface sea water through vacuum and the steam generated is used to run a turbine. The vapour is condensed using deep sea water to create fresh water.

(b) Closed cycle :

In this cycle a fluid with low boiling point like ammonia is used to rotate a turbine to generate power by using warm surface sea water to vaporize the fluid. Subsequently cold deep sea water condenses the vapour back to liquid state and this liquid is re-circulated in a closed loop to vaporize and drive the turbine. Hybrid cycles combine features of both open and closed cycle systems. A schematic closed cycle is shown in Fig.2.

Thermodynamically, for converting thermal energy to mechanical energy, cycles used generally are Rankine and Kalina cycles. The Uehara cycle developed in Japan also is important from an academic perspective.

3.4 Components of an OTEC System

The major components of an OTEC system are:-

1. Heat Exchangers
2. Turbine
3. Seawater Pumps
4. Cold water conduit
5. Platform
6. Station keeping / mooring
7. Data acquisition & control systems
8. Startup and shutdown procedures & safety.

Before we discuss individual components it is important to understand the other parameters required for the design and installation of an OTEC plant.

These include:-

(a) Bathymetry at desired location

To draw cold water we need to access deep waters around 1000m below sea surface. Some locations like islands have deep water very close to the shore like the Lakshadweep islands in India. In India around the coast the deep water is available 40-50 km from shore on the east coast and even further on the west coast. This necessitates a floating offshore plant. Thus the bathymetry on the depths of the seabed around the desired location is very essential. Some locations are such that shallow waters extend to some distance and sudden deep depths occur at the end of this plateau. Plants can be located at such sites and are called shelf-mounted.

(b) Environmental Parameters

For a floating / fixed platform mounted plant in open sea the design should cater to the maximum possible wind, wave and current conditions. The cold water conduit design will greatly be influenced by the environmental parameters.

Generally for round the clock operations the design may need to cater to 50 year and 100 year storms. Hence data is required for long term prediction via modeling. Many of these models cannot be validated due to absence of actual data. However design should follow rigorous analysis considering all loads caused by environmental forces.

(c) Other issues

Seabed and soil information may be needed for moorings or fixed structures in the sea. Site specific issues like fishing zones and naval bases may also need to be considered for finalizing location. Certain locations are prone to seismic activity and hence this may need to be taken into account in the analysis.

3.4.1. Heat Exchangers

Before we talk about heat exchangers let us understand process parameters which go into heat exchanger design. Firstly a closed cycle needs a working fluid. The basic requirements for a candidate fluid are:-

(i) Low boiling point

(ii) Low volume of the fluid per kW of power produced. (iii)High heat transfer characteristics

(iv) Environmental acceptability

(v) Acceptable cost.Most refrigerates could be used in an OTEC cycle. However in general ammonia is popularly considered for a closed loop OTEC cycle.

Heat exchangers are of shell and tube or plate type. Generally plate heat exchangers are more suitable for this application, since they are more compact than shell and tube exchangers. Apart from designing for as high a heat transfer coefficient as possible, the material used is important. Once of the sides has seawater and the other the working fluid. Hence the plate material needs to be compatible with seawater as well as working fluid for ammonia and seawater, possible materials are stainless steel and Titanium. The parameters influencing heat exchanger design include temperature of intake water and that of the water leaving the heat exchanger. The losses inside the heat exchanger also contribute towards the performance. The constraint is the small temperature gradient of 20-12oC which is available for the complete OTEC cycle.

3.4.2 Turbine

Generally axial and radial turbines of single or multiple stages are used [3]. The pressure ratio across the turbine is dictated by the working fluid and speed of the turbine is important from the generator’s perspective. The efficiency of the turbine – generator plays an important role in the overall efficiency of power generation.

3.4.3 Sea Water Pumps

Large volumes of sea water are required to be pumped up for the OTEC cycle. Generally the losses especially in the long cold water conduit are mostly due to frictional losses. Thus the pumps required are high discharge and low head pumps. The power requirement of the pumps governs the net power generated in the OTEC cycle.

3.4.4 Cold Water Conduit

The most complex and challenging component is the cold water conduit. An offshore floating plant will need to be positioned in deep waters with a long conduit hanging vertically down or supported in some configuration to draw cold water continuously from depths of around 1000m.

The conduit has to be designed for loads due to waves and currents and also for installation and deployment scenarios. As the rating becomes larger and larger, the size of the conduit becomes very big and is known to be the single most complex unit in the entire system.

3.4.5 Platform

When land based or shelf mounted plants are not possible, a floating platform is required to support the process equipment as well as the conduit to draw cold water. Various platform configuration are being studied the world over including barges, semi-submersibles, spars, etc. The platform design is critical because it has to be an all weather one and if it is directly supporting the conduit, forces induced by it due to environmental conditions on the conduit, can lead to high stresses in the conduit.

3.4.6 Station keeping and mooring

A floating platform needs to be kept in position or at station else the cold water conduit connected to it may start drawing warm water due to vessel drift. Moorings have to be designed to be all weather for this purpose. Current offshore practice has codal requirements for moorings in the oil industry however long term moorings for OTEC are yet to be attempted.

3.4.7 Data acquisition and control

The dependence on flow and temperatures of cold and warm sea water necessitate the measurements of these parameters. Hence a data acquisition system coupled with suitable instruments like pressure and temperature transmitters, flow meters, etc. are essential. Automatic control may also be required in the working fluid regime in case of temperature fluctuations.

3.4.8.Startup and Shutdown

Any OTEC system needs proper startup and shutdown procedures to be defined systematically a priori before installation. These procedures govern opening and closure of critical valves and commencing or stoppage of flows. Especially in case of working fluids like ammonia, which can have risk associated, the procedures need to be laid down properly from safety perspective.

4. Indian efforts on OTEC

 National Institute of Ocean Technology (NIOT) under the Ministry of Earth Sciences set up a 1 MW floating OTEC plant in 1000 m water depth inTuticorin in 1998. The essential components of the power module for producing electrical energy from the heat energy were an evaporator, turbine- generator, condenser and pumps for circulating the working fluid and sea water. The liquid ammonia was to be stored in a specially designed storage tank. A mist eliminator after the evaporator ensured that dry saturated vapor entered the turbine.

The power plant used titanium plate heat exchangers for the evaporators and condensers. These heat exchangers were the largest in size ever used for such an application. The evaporators were low chevron angle plates having a special steel coating on ammonia side to enhance the nucleation of liquid ammonia. The ammonia turbine was a 4-stage, axial flow, horizontal axis turbine working with a low pressure ratio of 1.4 and temperature range of 10o C. Computational Fluid Dynamics (CFD) analysis was done to determine the blade profiles and it was predicted to operate at 87% adiabatic efficiency, which is significant for the OTEC power cycle. The power system flow rates and the net power are dependent on the turbine efficiency. Any fall in adiabatic efficiency adversely affects the performance of the OTEC power cycle. Vertical turbine pumps of low head were used to pump sea water. All the components were assembled on a floating barge.

The major challenge was the design of the platform and the cold water pipe. A non self-propelled barge was designed to suit the purpose with special features like three moonpools and a retractable cold water sump to suit the NPSH requirements of the pumps. The final configuration of barge with pipe / mooring is shown in Fig.3.

As part of the commissioning activities, various subsystem qualification tests were carried out on shore as well as in shallow waters. The OTEC barge SagarShakthi was berthed near the port and many subsystems trials were carried out. Several trials in shallow waters also were carried out and subsystems qualified successfully.

Finally the 1000m long pipe of 1 m diameter was towed 40 km to the desired site. Sufficient offshore handling facilities were not available on the eastern coast of India, hence the deployment had to be carried out with serous limitations and the project could not be completed. Later the same barge was used for mounting desalination equipment and fresh water was first generated in shallow water. The learning was used for setting up desalination plants using thermal gradient successfully. India has thedistinction of setting up a low temperature thermal desalination plant inKavaratti, Lakshadweep for the first time ever in 2005. The plant is running successfully using thermal gradient in the ocean even today in 2016 generating 100,000 litres per day. Subsequently two more plants have been set up in islands of Agatti and Minicoy. A barge mounted plant of capacity 1 million litres per day was also demonstrated offshore on the same barge built for OTEC. Fig.4 shows the Barge mounted desalination plant of capacity 1 million litres per day.

Thermal Desalination expertise has now been developed along with offshore experience for deploying pipes for drawing cold water. NIOT is now setting up a laboratory to run the hybrid cycle of OTEC and desalination. Efforts are now on to power desalination using OTEC.

5. Summary

OTEC as an ocean energy resource is important from the country’s energy needs. Being tropical and with a long coastline, India needs to attempt harnessing OTEC power. The challenges in offshore implementation are many and still need research and development. Spinoffs like desalination along with the power generated from the OTEC cycle may help alleviate both power and water issues in developing countries.

References

• Trevor M. Letcher, “Future Energy – Improved, Sustainable and Clean Options for our Planet, Elsevier, 2014.
• William H. Avery and Chih Wu, “Renewable Energy from the Ocean – A Guide to OTEC’’, Oxford University Press, 1994.
• Nithesh, K.G., D Chatterjee, Purnima Jalihal, “Design and Numerical prediction of a radial-inflow turbine for OTEC application’’, presented in 5th International Symposium on Fluid machinery and Fluids Engineering (ISFMFE 2012), October 2012, Jeju, Korea.
• P. Jalihal, V. Jayashankar, L.S. Nair, M. Ravindran, J. Van Ryzin and P. Grandelli, “Analysis of Integrated CWP/Barge System for 1MW OTEC Plant”, Presented at IOA 99 Conference, Imari, Japan, October 1999.
• V. Jayashankar, P. Jalihal, M. Ravindran, H. Uehara, Y. Ikegami, T. Mitsumi, “The Indian OTEC Program”, Offshore Technology Conference, Texas, USA 4 -7 May 1998, pp 769-776.
• Purnima Jalihal, “Renewable Energies from the Ocean”, chapter in John Wiley Publication “Encyclopedia of Water”, March 2005.
• Purnima Jalihal, Raju  Abraham,  M.A.  Atmanand,  “Energy  and  Fresh  water  from  ocean Thermal Gradient – Indian Experience’’, presented at International Symposium on Deep Ocean Water Application for Food, Energy and Water 2012, Korea, Nov 2012.