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Leaning on Line Pack

Green energy mandates might overburden gas pipelines.

Fortnightly Magazine - January 2011

and balancing arrangement, quick-start units have no choice but to burn oil, almost always at a large cost premium relative to the delivered cost of natural gas.

Pipeline as Current

Rationalizing the use of pipeline line pack is a natural synergy between gas and electric stakeholders that can simplify and expedite the integration of wind resources. But exploiting this synergy will require a transaction structure that safeguards reliability objectives in both gas and electric markets. How best to rationalize a transaction structure that maintains gas and electric reliability objectives while providing appropriate compensation to market participants isn’t presently well understood.

The U.S. pipeline network is in effect a vast horizontal silo that’s packed and drafted daily to meet the scheduling requirements of gas utilities and power generators alike. In response to higher wind penetration rates, enhanced use of pipeline line pack—the volume of gas stored in a pipeline—across the consolidated network of pipelines and storage facilities can be used like a battery to hedge against wind intermittency. Line pack depends on the pressure levels in the pipeline, and it constantly changes as pressure is varied. Typically, pipelines build up line pack during periods of decreased demand and draw it down during periods of increased demand.

Heavy penalties for unauthorized overpulls hinder a generator’s reliance on line pack. ISOs are understandably cautious regarding the formulation of incentives that might paradoxically deplete line pack and cause pipelines to clamp down on a generator’s unauthorized use, not to mention the harm such depletion could cause to gas customers. In Britain, line-pack depletion during low wind periods was shown to limit the ability of the gas network to fully supply gas-fired generators. 15 The study modeled line pack in Britain’s pipeline system over a two-day period with a two-hour time step under three scenarios: base (2009), low wind (2020), and high wind (2020). The difference between high and low line pack was 6 million cubic meters (mcm) in the base and high wind scenarios, and 11 mcm in the low wind scenario.

Line pack is highly dependent on pipeline specifics, but another way to consider it involves the gas pull from the activation of gas turbines (GTs)—in this case GE’s LMS100. 16 Figure 3 summarizes the gas requirements of an LMS100 both at steady state and during the first 10 minutes of ramping. In a low wind situation, such as occurred in Texas in 2008, one to 10 LMS100-type gas turbines might be required to fill in between 100 MW and 1,000 MW of needed generation for one to two hours.

This withdrawal of roughly 16,000 MMBtu would be a worst-case scenario, since it’s unlikely that all the GTs would be located at one location on one pipeline. But such a withdrawal would be in addition to the present average daily line-pack swing and could challenge the system unless the event doesn’t coincide with the current maximum dip in daily line-pack fluctuations.

Depending on the length of the supply chain, it takes many hours—or even days—for natural gas to complete the journey from the wellhead to