ABSTRACT

National Institute Ocean Technology (NIOT) has established its first ever Low Temperature Thermal Desalination (LTTD) plant using ocean thermal gradient in the year 2005 at Kavaratti Island of Union Territory of Lakshadweep. Subsequently, five more plants have been put up based on the experience gained. All these plants draw power from local grids driven by diesel generators and hence it is important to find ways to provide this energy using a renewable form. Efforts at NIOT are now focused on powering desalination using Open Cycle Ocean Thermal Energy Conversion (OC-OTEC) principle. This paper compares technical parameters of LTTD and OC-OTEC powered desalination plants.

INTRODUCTION

Energy and water are the basic requirements for any nation to grow faster. Industrialization, rapid growth of population, over exploitation of limited natural resources available on earth, failing monsoons, drought and climate change issues led to unexpected depletion of both energy and water resources across globe; thus, everybody's focus has been shifted towards exploring oceans as a source of much needed energy and water requirements of both domestic and industrial segments. Being a tropical nation, India receives a normal solar insolation of around 4.5-6 kW/m2 per day and this energy is stored in ocean waters; hence, the surface seawater is warm. As India has more than 7500 km long coast line and also around 20°C temperature gradient available between the surface seawater and the deep sea cold water at 1000 m depths, this thermal gradient can be harnessed by the Open Cycle Ocean Thermal Energy Conversion (OC-OTEC) principle, proposed by the French inventor Georges Claude, for powering desalination plants to mitigate India's water scarcity issues (Claude, 1930; Jalihal et al., 2021). Typically, a 1 MW OC-OTEC powered desalination plant will produce around 1700 m3 fresh water per day.

Vega(2010) presented the preliminary designs for a CC-OTEC plantship and an OC-OTEC plantship sized at approximately 50 MW net power output. In this design 21.5°C temperature gradient was considered and obtained desalinated water production rate of 118,400 m3/day in the OC-OTEC plantship. Bharathan et al (1990) presented the conceptual design of an OC-OTEC plant with a 40 kWe net power output. In this design, a Direct Contact Condenser (DCC) was considered for the vapor condensation after the turbine expansion and a small fraction (10 percent) of the steam produced was diverted to a surface condenser for the production of desalinated water. The operational status report of the plant revealed that the OC-OTEC process is technically feasible for the production of base load electricity and desalinated water (Vega, 1995). However no comparative data available for the LTTD and OC-OTEC for selection of desalination plant for a small Island community of around 10000 people population. This paper compares the technical parameters of 100 m3/day capacity Low Temperature Thermal Desalination (LTTD) plant commissioned at Kavaratti Island (Abraham, 2007; Rognoni et al., 2008) with the 100 m3/day capacity OC-OTEC powered desalination plant under implementation in the same island. This paper also discusses the prospects and challenges in implementation of the OC-OTEC plant.

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