Ocean Thermal Energy Conversion

Luis A. Vega, Ph.D. Hawaii Natural Energy Institute (HNEI)

[email protected] October 1, 2009

Mission

Facilitate development and commercialization of wave power (WP) devices and ocean thermal energy conversion (OTEC) systems

OTEC Primer

• Energy Consumption & Petroleum Resources

• OTEC Generalities

• US OTEC Program 70s, 80s & 90s

• Lessons we should have learned

• Environmental Impact Assessment

• Present Situation

• Next Generation Vega OTEC 3

Economic & Energy Indicators 4

Petroleum Resources

Resources per IEA; API; USGS: R (barrels)

Present Consumption: C (barrels/year)

R/C = 50 years

If China & India maintain Growth 30 years

diminishing resources price increases

OTEC Primer








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OTEC Visionary Perspective

• Solar energy absorbed by oceans is 4000 x humanity annual consumption;

• Less than 1 % of this energy would satisfy all needs. [@ thermal electric conversion 3 %]

Typical Temperature vs. Depth Tropical Oceans

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OTEC Engineering Perspective

• Two ocean layers with T: 20 °C to 25 °C in equatorial waters…

heat source and heat sink required to operate heat engine

• How to convert to useful form and deliver to user?

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Energy Carriers

OTEC energy could be transported via electrical, chemical, thermal and electrochemical carriers:

Presently, all yield costs higher than those estimated for the submarine power cable (< 400 km offshore).

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Open Cycle OTEC

*Surface (Warm) seawater is flash-evaporated in a vacuum chamber resulting low-pressure steam drives turbine-generator;

*Cold seawater condenses steam after it has passed through the turbine produces fresh water

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Closed Cycle OTEC

Warm (surface) seawater and Cold (deep) seawater used to vaporize and condense a working fluid, such as anhydrous ammonia, which drives a turbine-generator in a closed loop producing kWh

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Hawaii’s Ocean Thermal Resource: Truisms

• OTEC could supply all the electricity and potable water consumed in Hawaii, {but at what cost?};

• Indigenous renewable energy resource that can provide a high degree of energy security and reduce GHG emissions.

OTEC Primer








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US Federal Government (Rephrasing late 70’s to early 80’s OTEC Mandate)

By Year 2000 104 MW Installed

equivalent to 100 x 100 MW Plants (Capital > $40 x 10 9)

Therefore,

Must implement optimized designs and industrial facilities for plantships producing OTEC electricity or other energy carriers to be delivered to shore…

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US Federal Government OTEC Program (70’s –80’s)

Hindsight

should have used funds ($0.25 x 109) to build at least one “large” plant with off-the-shelve hardware…

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OTEC Assessment (‘90s)

Continuous (24/7) production of electricity and water demonstrated:

- MiniOTEC (Hawai’i)

- Nauru (by Japanese Companies under Tokyo Electric)

- OC-OTEC Experimental Apparatus (Hawai’i)

210 kW OC-OTEC Experimental Plant

(Vega et al:1993-1998)

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Desalinated Water

Production

(Vega et al:

‘94-’98)

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OTEC Power Output as Function of Control Parameters

• Open Cycle Control Parameters: Seawater Mass Flow Rates; Seawater Temperatures & Vacuum Compressor Inlet Pressure

• Closed Cycle Control Parameters: Seawater Mass Flow Rates; Seawater Temperatures ; NH3 Mass Flow Rate & Recirculation/Feed Flow Ratio

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OC-OTEC Power Output vs. Cold Water Temperature

1-minute Averages of 1-sec samples show:

Cold Seawater Temperature Oscillation as Signature of Internal Waves

( 3,500m; P 60 minutes; H 50 m)

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OC-OTEC Power Output vs. Warm Water Temperature

1-minute Averages of 1-sec samples show:

Surface Seawater Temperature Variation as Signature of Warmer Water Intrusion driven by Ocean Gyre shed from Alenuihaha Channel between Maui and Hawaii (Big Island)

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Development Barriers (Hawai’i)

Tech. Issues: Need to Build & Operate Pre-Commercial Size Plant

Cost Issues: Cost Effective for Size 100 MW

Enviro. Issues: Relatively Minimal

Political Issues: Need Federal Help… only Hawai’i “benefits” (1/300 citizens) ?

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OTEC-Vega 30

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Lessons Learned

• Life-Cycle Design

• Constructability

• System Integration

• Capital Cost

OTEC Primer








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Environmental Impact Assessment (EIA)

• OTEC can be an environmentally benign alternative for the production of electricity and desalinated water in tropical islands

• Potentially detrimental effects can be mitigated by proper design

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Temp. Anomalies & Upwelling

Sustained flow of cold, nutrient-rich, bacteria-free deep ocean water could cause: - sea surface temp. anomalies;

- biostimulation

If and only if resident times in the mixed layer; and, the euphotic zone are long enough

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Euphotic Zone: Tropical Oceans

• The euphotic zone: layer in which there is sufficient light for photosynthesis;

• Conservative Definition: 1 % light-penetration depth (e.g., 120 m in Hawaii);

• Practical Definition: biological activity requires radiation levels of at least 10 % of the sea surface value (e.g., 60 m in Hawaii).

Typical Temperature vs. Depth Tropical Oceans

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EIA

Can OTEC have an impact on the environment below the oceanic mixed layer (sea surface to 100 m) and, therefore, long-term significance in the marine environment?

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OTEC Return Water

• Mixed seawater returned at 60 m depth dilution coefficient of 4 (i.e., 1 part OTEC effluent is mixed with 3 parts of the ambient seawater) equilibrium (neutral buoyancy) depths below the mixed layer;

• Marine food web should be minimally affected and sea surface temperature anomalies should not be induced.

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CO2 Outgassing

• CO2 out-gassing (per kWh) from the seawater used for the operation of an OC-OTEC plant is < 0.5% the amount released by fuel oil plants;

• The value is even lower in the case of a CC-OTEC plant.

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Present Situation

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Cost of Electricity Production

COE ($/kWh) = CC + OMR&R

+ Fuel (for OTEC zero)

{+ Profit – Env. Credit}

CC = Capital Cost Amortization

(Note.- much higher for OTEC)

OMR&R = Operations + Maintenance + Repair + Replacement

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Case Studies: Hawai’i

Kwajalein (RMI) American Samoa

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Hawai’i Assessment (4Q/07)

Presently, Avoided Energy Cost in SOH 0.15 to 0.20 $/kWh [was < 0.06 $/kWh in 90’s]

HECO 0.147 (composite values) MECO 0.198 HELCO 0.193

Therefore, OTEC > 50 MW is cost competitive

in Hawaii

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Hawai’i: 100 MW OTEC Plant

• Floating platform stationed 10 km offshore, delivering: 800 million kWh/year to the electrical grid 32 million-gallons-per-day (MGD) of water

• Up-to-date cost estimates yield electricity produced at a levelized cost below current avoided cost in Hawaii

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Hawai’i: 100 MW OTEC Plant (’07)

• A PPA from the utility at 17 c/kWh includes ample return-on-investment

• In addition, at $2 per-thousand-gallons sale price to the Board of Water Supply, revenue is equivalent to a reduction of 3 c/kWh in the cost of electricity production.

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Hawaii: Updated Assessment

• Securing financing , without operational records, remains a daunting challenge;

• Reactivate the OTEC Federal program with specific goal of designing and operating a scaled version of a commercial size plant ( 5 MW over a 5 year period with annual budgets of $25M)

• Federal Program would show equipment suppliers potential market for the technology, and should lead to design refinement.

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Kwajalein Atoll (Marshall Islands)

According to USN:

COE (May’05-June’06)

10 MW Capacity (diesel gensets)

COE ($/kWh) : [0.16 + 0.05] = 0.21

[fuel + OMR&R]

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Kwajalein Atoll (Marshall Islands)

• USN willing to issue Power-Purchase-Agreement if COE reduced by at least 10% ( 0.9 x 0.21 = 0.19 $/kWh)

Not feasible with 10 MW OTEC

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American Samoa

• ASPA records indicate: Annual Consumption 148.8 million kWh, equivalent to 17,000 kW (17 MW) firm capacity

• Fuel Cost of electricity

0.1847 $/kWh

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American Samoa

• ASPA interested in 35 MW “future” additional capacity

• Can OTEC produce electricity at a cost comparable to the present Fuel Surcharge of 0.1847 $/kWh ?

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35 MW OTEC COE ($/kWh)

Capital Cost

[$/kW]

Loan Term COE

[$/kWh]

12,000 ± 20% 8%

15 years

0.21

{0.18 to 0.25} Note: 80% CC

“ 4.2%

20 years

0.15

{0.13 to 0.18} Note: 70% CC

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Samoa: 35 MW OTEC Plant

• Floating platform stationed 3 km offshore Fatuasina Pt. , delivering: 280 million kWh/year to the electrical grid 11 million-gallons-per-day (MGD) of water

• Cost estimates yield electricity produced at a levelized cost comparable to ASPA’s current Fuel Surcharge

50 MW OC-OTEC Plantship

• 414,400 MWh/year

• 118,400 m^3/day (desalinated water)

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OTEC Primer








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Energy Carriers

Two to three decades from now, would it make sense to produce H2 or NH3 in floating OTEC plantships deployed along Equator?

Presently, would need barrel of petroleum fuel

at least 7x higher ($400) to be “cost effective”

Education

Ocean Thermal Energy Conversion