Comparison Of Aluminum Alloys And Manufacturing Processes Based On Corrosion Performance For Use In OTEC Heat Exchangers
Authors: Eldred Michael , Landherr Adam , Chen In Cheih 
Published: July 2010

Ocean Thermal Energy Conversion (OTEC) is a process that produces electricity from the temperature differences in tropical ocean waters. In this process, seawater flows through heat exchangers which transfer heat to and from a working fluid in a Rankine power cycle. Seawater has very corrosive properties, and therefore the selection of material and method of fabrication is critical in order to achieve the long lifetime that is required to make the OTEC power plant economical. Previous corrosion studies have shown that titanium performs very well in seawater, however the cost and availability of this material outweighs the benefits that are gained from a long lifetime of operation. Studies on the corrosive properties of various aluminum alloys have also shown that they perform very well, albeit not as well as titanium. Nevertheless, aluminum is drastically cheaper and more readily available in the quantities that are needed by an OTEC industry, so the shorter expected life may be justified by the cost savings. This OTEC heat exchanger development and testing program is funded by the Naval Facilities Engineering Command (NAVFAC) and the Office of Naval Research (ONR). The initial results from corrosion testing of six aluminum alloys (Al 5052, 3003, 6063, 1100 and two proprietary alloys) in warm surface and cold deep seawater sources at the Natural Energy Laboratory of Hawaii Authority (NELHA) on the Big Island of Hawaii will be presented. A comparison of these alloys containing various joining methods, such as a braze joint and friction stir weld joint, as well as various surface conditions, such as rolled, machined and extruded, will be discussed.

Introduction

Ocean Thermal Energy Conversion (OTEC) is a technique to utilize the deep cold ocean water and the immense solar collector of the surface water of tropical oceans to provide continuous renewable energy. In the past, OTEC has struggled with high capital costs in a world of cheap energy. However, the present high cost of oil and the realization that oil and natural gas is limited has renewed interest in OTEC. Rising electricity costs, increased concerns for global warming, and a political commitment to energy security have made OTEC economically attractive in tropical regions where a high percentage of electricity production is oil based. OTEC power plants operating a few miles from the coast could furnish baseload electrical power, at a predictable cost with no significant air pollution or CO2 emissions. It is an environmentally sustainable option with large power producing potential.

One of the most critical components in an OTEC plant is the heat exchangers. They are the single most expensive component and have a large effect on the overall efficiency of the plant. A 100 MW OTEC power plant would house over 200 individual heat exchangers, each larger than a 20' shipping container. Small changes in heat exchanger performance have significant economic consequences and any failure from corrosion or fouling would be catastrophic. The heat exchangers require very careful design and thorough testing and evaluation prior to incorporation into a billion dollar OTEC plant. All aspects of the heat exchanger design, such as material selection, method of fabrication, water channel and ammonia channel shapes must be addressed to determine an optimal, and thus economic, solution so the OTEC plant is efficient and has a long lifetime of operation.