Power from the Slope?
Perhaps electricity production for L48 would be viable alternative to gas line
With open seasons having come and gone for both of the projects aimed at building a pipeline to ship natural gas from the North Slope, the jury is out on the question of whether the gas industry thinks that there is a viable market for Arctic gas. But, since power generation in the Lower 48 states appears likely to be a major source of future North American gas demand, why not simply generate the power on the North Slope and ship the power rather than the gas to market?
A 1983 study, carried out for the Alaska Power Authority (the predecessor to the Alaska Energy Authority), done in parallel with studies into the viability of hydropower generation on the Susitna River or Lake Chakachamna, concluded that the generation of power on the North Slope for transmission to Southcentral Alaska would be technically feasible. The study estimated a total capital cost of $4.2 billion in 1982 dollars for the project, but did not express any opinion on the economic viability of the concept.
A power transmission line through Canada to Lower 48 markets would be much longer than a line into Southcentral. And conventional wisdom says that the cost of building a major transmission line more than 2,000 miles in length, coupled with energy losses along that line resulting from the inefficiencies of power transmission, would render a North Slope power generation and transmission option impractical.
HVDC a game changerHowever, new power transmission technologies involving the use of high-voltage direct current, or HVDC, could prove to be a game changer by reducing the line losses to an acceptable level.
Shawn Freitas, a project engineer who investigated the possibility of power transmission from the North Slope as part of a master’s program at the University of Alaska Anchorage, explained the technical issues to Petroleum News. Traditional power transmission systems of the type envisaged in the 1983 Alaska Railbelt study involve the carriage of what is termed alternating current, or AC, on power lines slung above the ground between tall pylons. Power losses along a line result from the electrical resistivity of the copper wiring of the line, magnified somewhat by the alternating nature of the electrical current.
In a DC line, because electrons are only flowing in one direction, the effective resistance to the electrical current is less and so the power is carried more efficiently, Freitas said.
“When you’re moving electricity through an AC line your line losses are enormous ... there’s a cost to energy alternating like that, whereas with DC there is no alternation,” he said.
Line loss as a percentage of the total power moved through the wiring in a low-voltage DC circuit is still substantial, because of the inherent resistivity of the wire, but by using high voltages, say in the range 600,000 to 800,000 volts, line losses become very low. In fact Freitas has calculated that for a transmission line, say from the North Slope to Calgary in Canada, the loss of energy along the line would be comparable to the energy loss from a gas line along the same route — gas-line energy losses primarily result from the use of gas to power the compressors that propel gas through the pipeline.
“If I can move enough electricity I’m just about even with compressed gas,” Freitas said.
Buried lineIn addition, unlike an AC transmission line, an HVDC line can without any loss of efficiency be buried in the ground, or even run under water, rather than having to be slung in mid air. That would greatly reduce construction costs and perhaps make an HVDC line practical from maintenance and environmental perspectives in Alaska’s remote and often ice-laden territory.
Moreover, locating a major power plant on the North Slope to generate electricity for the Lower 48 would present the advantage of proximity to aging oil fields for disposal of exhaust carbon dioxide, perhaps using the carbon dioxide for enhanced oil recovery, Freitas pointed out. Exhaust heat from the plant might find application in methane hydrate or heavy oil production, he added.
“You have an opportunity to produce zero-emissions power, which is a really big deal if you’re going to send electricity to western markets,” Freitas said.
The abundant North Slope coal resources could also be a potential future fuel for power generation on the Slope, Freitas has suggested.
Proven technologyAnd HVDC is no science fiction fantasy. China is in the process of establishing an HVDC power transmission grid, including part of the network that transmits power from the huge Three Gorges Dam, Freitas said. And HVDC systems have been in use for a number of years in other parts of the world, often for carrying power from hydroelectric power plants to electricity load centers, he said.
According to the web site of Swiss-Swedish corporation ABB, the company that has spearheaded the use of HVDC technology, the world’s longest HVDC line runs between Xiangjiaba and Shanghai in China, a distance of about 2,000 kilometers. A 2,500-kilometer line is under construction in Brazil, the website says.
10-gigawatt plantIn his research at UAA Freitas investigated the potential cost of building a 10-gigawatt, combined-cycle, gas-fired power plant at Prudhoe Bay, coupled with a 2,300-mile, 800-kilovolt, underground transmission line following the shortest road route to Calgary. The power plant would use 20 conventional gas turbine power units, with the modules for the plant having to be barged to the North Slope. And the transmission line would require substations at either end.
Using publicly available information, and upping the numbers to allow for uncertainties in that information and to accommodate project contingencies, Freitas came up with a total construction cost of $12.4 billion for the power plant and $5.6 billion for the transmission line. That compares with the $30 billion to $40 billion cost that has been bandied around for a gas line and an associated North Slope plant for removing carbon dioxide from the gas.
“If you’re just looking at transmitting the power, it’s way cheaper,” Freitas said.
And the gas-line cost obviously does not include the cost of building Lower-48 power plants, were the North Slope gas to be used for new Lower-48 power generation capacity.
Gas treatmentOn the other hand, Freitas’s cost estimates do not include gas treatment on the North Slope. Unlike a gas line, the power generation option would not require the removal of carbon dioxide from the North Slope gas. However, to bring the gas up to “turbine spec” at the power station, natural gas liquids and hydrogen sulfide would have to be removed, with the cost of that treatment perhaps being substantial, Freitas said.
“High-end turbines … have no tolerance for high quantities of NGLs or sulfur,” Freitas said.
And the world-record length of the transmission line would present some significant technical risk for the project, he said.