Power Barges: Tools for Progress
The power barge market seems to be moving in several different directions depending on the specific needs of the potential customers and the business model of the various suppliers.
There are complex, high tech projects that are aiming for higher and higher levels of efficiency and gains in scale economies. These projects are often slated for areas without access to low cost sources of clean fuel.
On the other end of the spectrum are plans for series production of simple cycle gas turbine power barges. These will have low capital costs, a quick delivery schedule and limited thermal efficiency.
The first combined cycle power barges entered service within the last two or three years. These plants often require two or more barges to accommodate the required equipment.
One example plant that is rather typical of the types of projects that are being done in this area is the Smith Cogeneration/Enron plant located in Puerto Plata, the Dominican Republic.
The plant has a net capacity of 185 MWe, produced by a 76 MW GE Frame 7 gas turbine on one barge and a 118 MWe steam turbine on the other barge. (The plant actually can produce more than 185 MW during cool weather, but the 185 figure includes the inevitable capacity degrade in hot climates.)
The gas turbine barge is a 72 by 250 foot barge converted from another use. The other barge is a specially designed 100 foot by 278 foot platform that provides space for the boilers, the steam plant and its auxiliary systems.
(As an interesting side note, the gas turbine barge was small enough to be transported to its destination at 12 knots by loading it onto the deck of a heavy lift ship called the Super Servant IV. The larger barge required the service of a semisubmersible seagoing barge with a transit speed of about 5-6 knots.)
Approximately 45 MW of the steam for the steam turbine is produced by recovering the heat from the exhaust of the gas turbine using a three stage heat recovery steam generator. The rest of the power is produced in ordinary steam boilers by burning residual fuel oil. The output of the steam turbine is adjusted to follow the load, thus limiting the rate at which the fuel oil is burned.
The gas turbine is designed for dual fuel capability( natural gas and distillate oil), but is currently supplied with oil because of the lack of a suitable gas supply in the Dominican Republic. The plant became fully operational in January, 1996.
A much larger, 450 MWe plant destined for Port Qasim, Karachi, Pakistan is being planned by a consortium that includes Raytheon Engineers and Constructors and Westinghouse. Thought to be the largest current power barge project in the world, the conceptual design calls for six Westinghouse 50 MW gas turbines and three 50 MW steam turbines. There are no plans for supplemental boilers. The completed project will require six barges, three with two gas turbines each and three with one heat recovery steam generator and one steam turbine each.
The main goal of the combined cycle plants is to improve efficiency and lower the cost of electricity to the point where it is competitive with diesel engine based options. Several diesel engine barges have recently entered commercial service in Honduras and Jamaica.
So far, no economic data has been released that allows one to conclusively determine which option is better, a 45% efficient diesel engine or a gas turbine combined cycle whose overall efficiency approaches 55% in cool weather.
In some areas, there is a sufficient supply of low cost gas to allow simple cycle gas turbines to be economically competitive. This situation exists in areas like Trinidad, Southeast Asia and parts of Indonesia where huge gas fields exist. In some cases, the gas is almost given away since it cannot be readily moved to a place of high demand without major capital investment in either pipelines or liquefaction facilities. When fuel is cheap, system efficiency plays a lesser role in overall economics.
One company that has decided to design power barges specifically aimed at areas with low fuel costs is Houston based Stewart & Stevenson. This firm has been packaging General Electric aeroderivative gas turbines for rapid land based installation for several years, so moving toward barge mounted systems is a natural extension of their existing business.
Unlike the above combined cycle projects, which were custom engineered to meet specific site requirements, Stewart & Stevenson has decided to produce a series of production plants aimed at what they consider to be the broadest market.
The barges will be supplied with two GE LM6000 aeroderivative gas turbines each, for a generating capacity of 80 MWe. The goal of the series production is to minimize capital cost and project lead time. By producing the barges in a factory setting, often on speculation that a purchaser will be found for an already constructed machine, the firm intends to apply lessons that they have learned in their land based power generator business.
All power barge projects have certain inherent limitations. They must be protected from the effects of storms, they can consume valuable waterfront docking areas, and they can cost more than plants built on solid, but inexpensive land. Because of the space limitations of the barge, the designs are often compromises rather than being optimized for efficiency or ease of maintenance.
Fossil fueled power barges have additional disadvantages. Their fuel consumption requirements can add a significant burden to already strained transportation systems, their emissions in port areas can add to air and water pollution problems in densely populated areas, and their economic value can be quite vulnerable to unexpected variations in the cost of the high quality fuel that they require.
One major tragedy of the oil price increases during the 1970s was the arrested development in areas with no indigenous sources of fuel or alternative power supplies.