in several ways. In one method, a reservoir is created behind a barrier, or barrage, and tidal waters are allowed to pass through turbines in the barrier to generate electricity. This method requires mean tidal differences greater than 4 metres and favourable topographical conditions to keep installation costs low. The Gulf of Khambhat and the Gulf of Kutch on India's west coast, with maximum tidal ranges of 11 m and 8 m, respectively, and an average tidal range of 6.77 m and 5.23 m, are promising sites for this type of technology. The Ganges Delta in the Sundarbans, West Bengal is another possibility, although it offers significantly less recoverable energy; the maximum tidal range in Sunderbans is approximately 5 m with an average tidal range of 2.97 m. It is estimated that barrage technology could harvest about 8 GW from tidal energy in India, primarily in Gujarat. The barrage approach has several disadvantages, however, one being that a badly engineered barrage can have significant negative effects on migratory fishes, marine ecosystems and aquatic life. Integrated barrage technology plants can also be expensive to build. In December 2011, the Ministry of New & Renewable Energy, Government of India and the Renewable Energy Development Agency of West Bengal jointly approved and agreed to implement India's first 3.75 MW Durgaduani mini tidal power project. Another tidal wave technology harvests energy from surface waves or from pressure fluctuations below the sea surface. A report from the Ocean Engineering Centre, at the Indian Institute of Technology Madras estimated the annual wave energy potential along the Indian coast is 5 to 15 MW/metre, suggesting a theoretical maximum potential for electricity harvesting along India's 7500-kilometer coastline of about 40 GW. However, the realistic economical potential is likely to be considerably less than this. A significant barrier to surface energy harvesting is that its equipment may interfere with fishing and other sea-bound vessels, particularly in unsettled weather. India built its first surface energy harvesting technology demonstration plant in Vizhinjam, near Thiruvananthapuram. The third approach to harvesting tidal energy is ocean thermal energy technology. This approach harvests the solar energy trapped in ocean waters. Oceans have a thermal gradient, the surface being much warmer than the deeper levels of the ocean. This thermal gradient may be harvested using the modified Rankine cycle. India's National Institute of Ocean Technology (NIOT) has attempted this approach without success. In 2003, NIOT attempted