Less directly, the ocean also plays a part in producing hydropower. Water constantly moves through a vast global cycle, evaporating from lakes and oceans, forming clouds, precipitating as rain or snow, and then flowing back down
to the sea.


The water (hydrologic) cycle

The energy of this water cycle, which is driven by the sun, can be tapped to produce electricity or for mechanical tasks like grinding grain. Hydropower uses a fuel—water—that is not reduced or used up in the process. Because the water cycle is an endless, constantly recharging system, hydropower is considered a renewable energy.



This open-cycle ocean thermal energy conversion demonstration in Hawai‘i used the heat stored in the ocean to generate electricity.

The ocean that surrounds Hawai‘i has long sustained its people, and today new technologies are being developed that will enable us to tap the ocean as a source of energy for future generations. As the world's largest solar collectors (covering 70% of the Earth's surface), oceans absorb thermal energy from the sun. They also exhibit kinetic energy with tides, currents, and waves. Even though the sun affects all ocean activity, the gravitational pull of the moon primarily drives the tides, and the wind powers the ocean waves.

There are different types of ocean energy that can be applied in various ways. Learn about the following technologies on the U.S. Department of Energy Web site:



At the Natural Energy Laboratory of Hawai‘i Authority, cold seawater (43°F) is pumped from 2,000 feet below the ocean's surface and then used to cool laboratory buildings.

Of the various forms of ocean energy, ocean thermal resources and wave energy are the most promising due to the abundance of the resource.  Hawai'i's tides and ocean currents, however, are not as promising, though technological improvements to  ocean generator designs may make these resources competitive in the future.

The Hawai'i National Marine Renewable Energy Center at the University of Hawai'i Hawai'i Natural Energy Institute is a federally-designated research facility emphasizing OTEC and wave energy development.



Ocean thermal resources--solar-heated surface waters and/or cold deep-sea waters--can be used to either replace conventional air conditioning or to generate electricity.

In experimental deployments, cold water from the ocean depths has shown promise as a cost-effective alternative to traditional air-conditioning systems. The Natural Energy Laboratory of Hawai‘i Authority (NELHA), a state agency dedicated to energy and ocean-related research, education, and environmentally sound commercial activities, has experienced the economic benefits of this technology firsthand. Since NELHA began using a small-scale deep seawater air-conditioning (SWAC) system to cool three of its research compound buildings, it has reportedly saved up to as much as $4,000 a month in energy costs.  Larger SWAC systems have been proposed for urban districts in Honolulu.

The ocean’s deep, cold water, coupled with warm, surface water, has also been used in numerous demonstrations and experiments at NELHA to generate electricity through the Ocean Thermal Energy Conversion (OTEC) process.  Among the many OTEC milestones achieved at NELHA were the “Mini-OTEC” barge, which in 1979 demonstrated the world’s first production of net electrical power via at-sea closed-cycle OTEC.  Other noted achievements include the floating OTEC-1 plant, shore-based biofouling and corrosion research, and a 250-kW open-cycle OTEC demonstration, the largest such plant ever put into operation, which continued into the 1990s.  In addition to OTEC and SWAC, the cold water has many commercial uses in aquaculture and agriculture.

Additional OTEC research, demonstration and pre-commercialization activities continue in Hawaii, with power plants as large as 100 MW proposed.



Hawai‘i’s ocean wave resource, particularly on the windward and northern coasts of the island chain, is among the strongest in the U.S. A wide variety of wave energy devices are being demonstrated and tested worldwide; several international companies have visited Hawai‘i to explore the potential for deployment here.

Since 2004, Ocean Power Technologies (OPT), a company based in New Jersey, has been testing its PowerBuoy design in the ocean off Kane‘ohe in a program managed by the U.S. Navy.  In 2010, OPT announced that their 40kW buoy had been connected to the electrical grid—a first for the U.S. 

Another company with definite plans to demonstrate its technology in Hawai‘i is Oceanlinx, based in Australia.  Working with Maui Electric Company, Oceanlinx hopes to install a 500-kW oscillating water column device off Pa‘uwela Point on Maui’s northern coast.  Oceanlinx has demonstrated several modifications of its device near Sydney, Australia.



Hydropower is using moving water to power machinery or make electricity. When flowing water is tapped for electricity generation, it is called hydroelectric power or hydropower. There are several types of hydroelectric facilities, all powered by the kinetic energy of flowing water as it moves downstream. Turbines and generators convert the energy into electricity, which is then fed into the electrical grid to be used in homes, businesses, and industry.

To learn more about hydropower technology, visit the U.S. Department of Energy Web site:



In Hawai‘i, hydropower is not new. Early power plants in Nu‘uanu and Hilo, starting as early as the 1880s, were hydroelectric.  Today, on Hawai‘i's Big Island, the Wailuku River hydropower plant near Hilo, operating since 1993, is the largest hydro plant in the state. There are also smaller plants on Hawai‘i's Big Island, owned by the utility, as well as others on Kaua‘i and Maui. Although these hydropower plants are small in comparison to many mainland facilities, they have furnished power to sugar mills and to the three island utility companies for many years. There are also a few in-line hydro plants operating, which generate electricity from water flowing in pipelines. Additional hydropower projects on Hawai‘i's Big Island, Kaua‘i, and Maui are under discussion.

An additional application of hydroelectricity is pumped hydro, which is essentially a method of storing energy. An intermittent energy resource such as solar or wind, or low-cost off-peak power, could be used to pump water uphill to a holding reservoir.  The water can then be released when needed from the upper reservoir through a generating turbine.  Potential pumped hydro projects have been investigated and proposed in various locations throughout Hawai‘i.



Hawaii Clean Energy Initiative