The progress made by renewable generation over the past two decades has been nothing short of spectacular. But as more wind and solar generation has flooded onto the grid, it has had to face an endless refrain from opponents about variability. Attend any mainstream power industry conference and you hear speaker after speaker mention the fact that the sun doesn’t always shine or the wind doesn’t always blow as though it would be a “what do you know” moment for the gathered multitudes.
But they do have a point. Energy storage continues to be the Achilles Heel of wind and solar as witnessed by the fact that government agencies in the U.S. and Europe are throwing subsidies at the problem, seeking a viable way to solve riddles, such as how you harness wind energy generated at times when nobody wants it. California, for instance, mandated its investor-owned utilities install 1,325 MW of energy storage by 2021.
Many possible storage technologies have been proposed and some show promise. Yet cost remains a big issue, threatening to marginalize these solutions in a similar way that carbon capture and sequestration has remained a distant and expensive dream as a way to minimize coal emissions.
Yet could it be that the answer lies under our noses — or more accurately under our feet? The natural gas pipeline networks that crisscross the landscape may just provide the ideal method of harnessing off-peak wind and excess solar. And instead of involving government billions to sponsor technological development and maturation, our pipelines require a relatively inexpensive upgrade and retrofit to be ready to store renewable energy. Further, this approach could possibly even fund itself, thereby avoiding the need to pass the costs on to the consumer in terms of higher monthly energy bills.
Storage technologies, such as batteries, molten salt, pumped hydro, ice storage, power-to-gas (PtG) and compressed air energy storage (CAES), have been around for some time. Quite some development work is being invested in these fields yet none have emerged from the pack. They make fascinating magazine articles, but fail to gain momentum where it counts — in high-volume deployments in the field.
So what does the pipeline network have to offer? The UK pipeline network, for example, is operated by National Grid. It consists of 7,600 km of high-pressure pipelines and 26 compressor stations covering the major population centers of Scotland, England and Wales. While National Grid serves around 10.8 million British consumers, it faces declining natural gas usage as renewables capture a steadily rising market share. This decline is highlighted by the fact that some gas-fueled plants have been shuttered in recent years.
The German gas pipeline grid is even more extensive — 500,000 km of pipeline transports 1,000 billion kWh of energy (twice the quantity consumed by the German grid). Its underground gas reservoirs can store another 230 billion kWh. Similarly, large amounts of pipeline infrastructure exist in most European nations.
How could this be used as a storage mechanism? Gas doesn’t just flow around these pipelines and magically appear at the consumer end. It takes a sophisticated equipment network to take that gas from sources, such as North Sea reservoirs and liquefied natural gas (LNG) import terminals, process it, pressurize it and drive it in the desired direction toward the points of consumption.
Compressors do much of the work, maintaining the gas at high pressure and propelling it on its journey. These compressor stations generally contain several compressors for redundancy. If one fails, the others can pick up the slack. In the UK, they are normally driven by a gas turbine that extracts gas from the pipeline in order to operate. In Europe, gas turbines or electric motors are used by the compressors.
How does this tie into energy storage? Those compressors could be powered by unwanted wind energy during the night, for example. That energy would effectively be stored within the pipeline network and reclaimed as energy at the consumer end of the line. This serves the purpose of energy storage by transferring wind energy into energy in the form of compressed gas.
Power-to-compression (PtC) is quite different from PtG. The latter has long been considered a vital element of a renewable future. It harnesses surplus renewable energy using chemical electrolysis to produce hydrogen or substitute natural gas (SNG), which can either be utilized directly or transported and stored and then reconverted into electricity. However, heavy equipment costs and energy losses during its various processes have impeded adoption.
PtC, on the other hand, entails compressors utilizing renewable energy surplus for gas transportation and bypasses the need for chemical interaction. It can also be augmented by power-to-heat (PtH), which uses energy to preheat the gas prior to its pressure being brought down at pressure regulator stations.
The existing UK compressor station network is out of compliance with the UK Industrial Emissions Directive (IED) Compressor Strategy for the natural gas transmission system (NTS). Stricter requirements for carbon monoxide (CO) and nitrous oxide (NOx) mean that many of the gas turbines of the National Grid will have to be modified or replaced. A total of 17 gas turbines are affected by the IED and there is a commitment to investing in the needed changes in order to maintain the stability and flexibility of the nation’s gas supply.
Image: This dual-driven pipeline compressor arrangement by Energy Transfer is used in many sites in Texas.
Now that a large amount of cash is going to be spent in establishing a low carbon NTS, this is a good time to consider the best use of those funds. Certainly, new or upgraded, cleaner-burning gas turbines will play a major role in the network’s future — but to leave it at that is to miss the full potential of the pipeline network.
With extensive engineering work already going to be done at each station, a relatively simple reconfiguration could result in a massive PtC-based energy storage grid, pave the way to an even higher concentration of renewables, and offer a payback mechanism for the cost of the upgrade that would not involve upping monthly consumer bills.
This approach would mean adding an additional electric motor driver to each compressor. With two drives (a gas turbine and an electric motor), it becomes possible to switch from one to another. Having both ensures independence from the electrical grid. In the event of a major shutdown, the gas turbine would continue to operate, thereby maintaining the nation’s gas supply. Alternatively, the electric motor is there to function whenever the gas turbine fails. Two drivers provide redundancy for the system.
Perhaps more importantly, an electric motor opens the door to running the entire pipeline network on surplus wind power during the night and early morning hours. Most graphs of wind production show high output during these periods and relatively low output during peak demand times. In some areas, consumers are paid to take this power or it is sold at very low prices. This adversely affects the economics of wind and feeds directly into the arguments of its detractors. But what if all that power was consumed by the pipeline grid for 50 percent or more of its operations? This might be possible, depending on economics.
Open Grid Europe, for example, examined this arrangement at the Krummhorn compressor station on the North Sea shore of Germany. With 15 MW of compression power required, it would be able to take advantage of the fact that nearby onshore wind parks are generating a significant energy surplus. It looked to be much simpler to add an electric motor driver at the site and therefore minimize the build out of the power distribution network needed to offload that power in major load centers to the south. In this case, any surplus power will be used to run the compressor. When not available, it would immediately switch back to gas turbine-based operation.
The planned gas turbine was a 20,000 hp two-shaft design by Solar Turbines with a modern lean-premix combustion system that would allow the compressor station to meet NOx and CO emission limits. The 4-pole induction motor, controlled by a variable frequency drive, would be connected to the compressor at the opposite end to the gas turbine connection. The motor would drive through a parallel shaft (speed up) gearbox and together be designed to replicate the power output characteristics of the gas turbine.
The changeover from one driver to another would be achieved using clutches on each driver for engagement or disengagement. This would be like a flywheel on a bicycle to smoothly engage one driver and disengage the other — the whole process takes place almost instantly and is automatic. The clutches would be supplied by SSS Gears Ltd. of UK. Known as self-shifting synchronous (SSS) clutches, they are a special type of overrunning clutch with a one-way torque transmission capability.
This dual-drive capability is commonplace in North American compression stations. In some cases, it is used for economic advantage via commodity trading. Gas and electrical energy contracts can be purchased and sold in markets based on short-term spot pricing and long-term ahead wholesale prices. Gas wholesale prices, in particular, experience volatility in day-ahead markets. By negotiating good long-term contracts, the compressor station operation gains the ability to switch from one driver to the other based on the most advantageous prices for electricity and gas.
Successful implementation of such a strategy could defray the cost of compressor station upgrades. In addition to gas and electrical contract trading, the stations also become prime candidates when the local grid must consider which electrical load must be reduced in return for high-value interrupt payments. Such mechanisms are essential to help prevent the undesirable effect of starting less emission friendly, older generation equipment.
It is interesting to note that as this article was being researched and written, the Krummhorn compressor facility elected to only install a dedicated electric motor drive and not the gas turbine option. Certainty, this decision will lower upfront costs. But while Krummhorn can use wind and solar energy to drive the compressor, energy costs will rise when renewable energy becomes unavailable. Equally, the possibility of electrical blackout must also be considered, and we can assume that the station, devoid of the gas-fuelled alternative driver, will not be in a position to receive high-value interrupt payments from the grid without incurring penalties associated with compression loss.
Energy Storage Reservoir
In the UK, as well as in other countries with national gas pipeline networks, those stations driven by gas turbines should consider the option of installing an additional capability to switch to electric motor drive. This adds flexibility, redundancy, an additional revenue source and most importantly, sets up the underground pipeline network as a vast energy reservoir, storing the electricity generated by wind during the night and making that energy available when it is needed by the various points of consumption. This is accomplished by storing the energy in the form of pressurized gas at night and providing the energy to the consumer when the pressure is released.
In addition, emissions levels are further reduced. While the latest gas turbines provide relatively low levels of emissions, switching to wind power for a substantial period would eliminate emissions almost entirely during those periods.
The National Grid in UK has until 2023 to upgrade its existing gas turbine compressor fleet. These machines are almost all older Rolls Royce RB211 machines. Their emissions levels fall far beyond today’s stringent limitations. By advocating this more environmentally friendly approach to operate compressor stations with both low-emission gas turbines and electric motors, and opening up the commodity trading side, compressor modernization and renewable energy storage could be brought forward by many years.
A further incentive to act more rapidly exists in the form of carbon levies. Every year that the RB211s remain in operation means much higher levies due to excessive levels of emissions. In the same way that a dual drive Prius pays a VED penalty that is far less than a gas-guzzling American muscle car, dual-driven compressor stations would enjoy the benefits of lower emissions, lower fuel costs and much smaller carbon levies.