Technology

Introduction

Our oceans are vast, untapped and renewable collectors of heat from the sun (Solar thermal energy). 1/7000ths of the surface of our seas contains enough solar thermal energy to provide, through OTEC, for our entire planet’s current energy needs, renewably, and sufficient desalinated water for all water uses of a population of 7 billion (at 2.5 tons/person/day, provided it can be distributed).

Ocean Thermal Energy Conversion (OTEC) works in tropical and sub-tropical seas where surface waters are over 25°C and water at 1,000m depth is at 5°C, so that there is a difference in temperature of at least 20°C throughout the year.
It is this difference in temperature that powers the OTEC process. Various modes are now being researched, by allowing a liquid working fluid to be vaporised by the warm surface water, powering a turbine, producing electricity, the vapour being condensed by cold water at 5°C and then re-used as a liquid to be vaporised again to complete the power generation cycle.

For the power cycle, seawater can be used as the working fluid, vapourised in a vacuum chamber to create a saltless vapour. This can power the turbine, the saltless vapour being condensed on heat exchangers to give desalinated water.

For every MW of power generated, 2.36m litres of desalinated water will be created every day. An OTEC plant can be seen as a combined power plant and desalination plant. Its value is thus increased.

Unlike most other forms of renewable energy sources, such as photovoltaic, wind and wave energies, which vary in output according to night/day cycles of weather conditions, OTEC can produce power and desalinated water 24 hours a day and uninterrupted all year around.

Additionally and where applicable, the cold water can be used as an inexpensive form of air conditioning, and the high-grade phytoplankton, drawn up during the process, as feed for aquaculture/fish farms surrounding the facility.

History

OTEC was invented and patented by the French engineer Jacques Arsène d’Arsonval in the 1880s. It was later pioneered by Georges Claude, another French engineer who had to his credit the invention of liquefied air and the neon tube. In the 1930s, he personally spent $10m on the technology, but at a time when technical ability was limited. Following the oil-crisis in the 1970s, America, under President Carter, invested over $200m in OTEC, but investment was then suspended when the price of oil dropped.

Since then, only one open-cycle project has been realised and run successfully: A 210kWh facility off the island of Hawaii, designed, built and run for a period of 5 years, from 1993-1998, by Dr. Luis Vega, a member of the Energy Island Group.

How OTEC works

Very generally, in what is known as Open Cycle OTEC, solar heated tropical warm seawater is flash evaporated in a vacuum chamber, the resulting low-pressure steam, driving a turbine-generator, producing electricity.
Cold seawater drawn up from the depths condenses the steam on cold heat exchanger plates after it has passed through the turbine, producing saltless water, the salt being separated out by the evaporation process.

In Closed Cycle OTEC, warm surface seawater and cold deep seawater are used to vapourise and condense a working fluid, such as anhydrous ammoniac, which drives a turbine generator in a closed loop producing only electricity.

In Hybrid OTEC, various permutations of these cycles are used, either to produce desalinated water only, or electricity only by the closed cycle, or electricity by the closed cycle and desalinated water by using the “spent” cold water to desalinate separately produced steam, with the purpose of creating greater amounts of electricity and desalinated water, at greater overall efficiencies as may be required, and achieved by greater system complexity.

In hybrid systems, for every MW of electricity produced, you can obtain 2.36 million litres of desalinated water per day.

75 MW plants are now being planned, enough to bring basic electricity demands of 1 kW to 75,000 households. The water produced would be 750 litres per household, 7.5 times the requirement of 5 people using 10 litres/day. The water can be used for local agriculture and industry, any excess water being given or sold to neighbouring communities.

Advantages & Limitations

• No land costs or local/community planning issues.
• Base loaded, consistent renewable outputs (24hrs/day)
• Potential to match gigawatt scale of nuclear power but from a safe and renewable source.
• Multiple outputs:
  1. Renewable Electricity
  2. Desalinated Water
  3. Carbon Credits
  4. Phytoplankton/micro-organisms as food for fish-farming
• Constituent parts exist ‘off-the-shelf’, improving costs and timings.
• Avoids use and release of chemicals (and burning of fossil fuels) involved in established desalination technologies.

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• Costs currently prototypical (and, there being more than one output, are not easily compared with established renewable energy and desalination technologies).
• Technical challenges inherent in structures at sea
• No facility over 210kWh has yet been built and run and that was by Dr.Luis Vega, a member of the Energy Island Group