Gas Fired Back-up Power – Back-up power for wind: Striking the right balance

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Balancing power in a grid that has a significant amount of wind capacity is a challenge for any utility. Some utilities in the USA are finding that installing reciprocating gas engines for ‘wind firming’ can be a good solution.

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Mikael Backman, Wärtsilä, Finland

Utilities are increasingly faced with the challenge of incorporating intermittent generation such as wind and solar into the grid. It is a challenge that is particularly significant in the USA, which installed around 6000 MW of new wind power generation in 2009.

The 115.6 MW Plains II gas fired plant in Arvada, Colorado, which uses 14 Wärtsilä 20V34SG engines to provide back up power for wind farms

This growth in wind power is resulting in an increasing number of flexible generating plants based on reciprocating gas engines, which are able to provide support or ‘wind firming’ and other ancillary services in addition to the traditional intermediate or peak power production.

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WIND FIRMING

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Xcel Energy is a USA utility that operates across eight states. The utility has 3.4 million electricity customers supplied by a mix of generating assets. According to the American Wind Energy Association (AWEA), the utility is the largest investor-owned wind power provider in the US, with more than 3000 MW of wind power generating capacity within its eight-state service territory ” a figure it is planning to increase to 7000 MW by 2020.

Such a massive amount of wind has seen the utility employ flexible generating plants. In Colorado, for example, Xcel Energy has more than 50 power generating sites operated by its subsidiary Public Service of Colorado (PSCo), but it still had a need for more flexible sources of generation in its system.

Cogentrix, a large US-based independent power producer, therefore commissioned a power plant in Arvada to deliver power and ancillary services under a long-term contract with PSCo. Known as Plains End I, the 111 MW plant began commercial operation in 2002.

The value of this plant was confirmed when a similar order for a second plant was awarded in 2006. Plains End II is located adjacent to Plains End I. This 115.6 MW plant, which uses 14 Wärtsilä 20V34SG engines, began commercial operation just over 12 months ago.

PSCo does not operate in a deregulated market, but it does have a control area in which it needs to balance load and supply. This control area has to have sufficient spinning reserve. When PSCo needs power quickly, the Plains End plants are started and operated until a baseload plant can be brought online. Since their start-up, Plains End I and II have proved their worth. It may be hard for some utilities to justify investing in a reciprocating engine plant purely for the purpose of wind firming.

Plants like Plains End I and II are therefore purchased with the aim of providing a combination of services. They are used to provide peaking or intermediate generation, as well as system support and firming of any form of intermittent generation. Plains End I and II can be operated according to the wind output. As a flexible, quick and generating assets in the PSCo system, Plains End I is used to balance the power dispatched as wind power production varies.

When the load reduces in the evening and at night, flexible assets are dropped off in favour of less flexible baseload assets such as coal. However, the wind typically blows at these times, essentially hitting at the worst time of the day in terms of system stability. This is where flexible reciprocating engine-based plants can be useful.

These flexible gas engine plants can start-up in ten minutes to provide non-spinning reserve, and could in the future be tuned to start up in as fast as five minutes, if the requirements for this product change.

In addition to the ten minute non-spinning reserve, there is also a need for plants that react immediately to varying market demand. Therefore some plants have to be kept running at partial load to provide spinning reserve. If, for example, a plant is kept ‘spinning’ at 50 per cent load, this power can be sold. But the capability to quickly ramp-up to provide an additional, for example, 30 per cent capacity can be sold in the ancillary service market. Plains End II was primarily installed to provide generating capacity and black start capabilities to the local grid and has the capability to provide all of these generation requirements.

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Reciprocating benefits

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As well as having a superior full load efficiency, gas engines are much more efficient in running at part load compared to large combustion turbines.

According to AWEA, the US broke all previous records by installing nearly 10 000 MW of new wind capacity in 2009

A plant with multiple gas engines gives plant owners the capability to operate at an extremely low minimum plant output with all of the gensets in operation. This feature increases the number of megawatts of spinning reserve available to the customer, consequently increasing the value of the plant.

Wärtsilä spark-ignited gas engines have a typical heat rate of 8600 Btu/kWh, higher heating value (HHV) and stays below 10 000 Btu/kWh, HHV even at 50 per cent load. With a gas turbine, the heat rate falls dramatically at partial load.

The Wärtsilä 34SG is a medium-speed, spark-ignited gas engine operating according to the Otto process and the lean-burn principle. The engine has ported gas admission and a pre-chamber with a spark plug for ignition.

The engine runs at 720 rpm or 750 rpm for 60 Hz or 50 Hz applications and produces 8440 kW to 9000 kW of mechanical power, respectively. The efficiency of the Wärtsilä 34SG at 46.5 per cent, is said to be the highest of any spark-ignited gas engines today.

The natural gas fuelled, lean-burn, medium-speed engine has high reliability, high efficiency and low emissions. The engines can provide rapid response to system frequency variations.

When required, the spark gas engine technology can supply grid voltage support through the generation of reactive power and offers black start capability at a very low capital cost.

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Growing market

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Gas engine plants typically run in two modes. In spinning reserve mode, all the engines will be turned down, ready for ramping up as needed. In energy production mode for peaking only as many engines as needed to deliver the required power output are used, but each can be run at full load if necessary. This allows optimum plant efficiency to be maintained at any plant output.

These capabilities have resulted in a fast-growing market for reciprocating gas engine plants, especially in the USA where ancillary services such as spinning reserve, frequency regulation and black start capability can all be sold in the market.

Wärtsilä ran a model in the Electric Reliability Council of Texas (ERCOT)area in Texas to study the dollar value of having a flexible Wärtsilä power plant. The model was run for a year, based on actual 2008 data. The results showed that a generator could substantially increase revenues if the plant was run for ancillary services as well as energy production, as opposed to just pure power production.

The majority of flexible utility plants run between 1500-4000 hours per year. Some of Wärtsilä’s installations have been installed to run for more than 4000 hours but the additional hours would be to provide extended intermediate load generation as well. Wärtsilä currently has approximately 1600 MW either installed or under construction in the USA. About half of this is dedicated to flexible utility generation.

The engine hall at STEC ‘s Pearsall power plant, which represents Wärtsilä’s largest installation in the state of Texas to-date

At the beginning of 2008, South Texas Electric Cooperative (STEC), a non-profit generation and transmission cooperative headquartered in Nursery, Texas, awarded a contract to Wärtsilä to build the $100 million, 202.5 MW gas fired Pearsall power plant.

This is Wärtsilä’s first large installation in the state of Texas. The plant will provide the generation needed for the region’s growth, as well as the grid stability required for the increasing influx of wind power.

Pearsall power plant is located about 80.5 km southwest of San Antonio, in Pearsall, on an existing brownfield site. When complete it will be equipped with 24 Wärtsilä 20V34SG engines. Currently, STEC serves eight distribution cooperatives, which provide services to over 170 000 customers in 65 South Texas counties.

The plant will be connected to the ERCOT grid and supply power and ancillary services to STEC’s cooperative members, helping to serve their 750 MW peak load. The plant is anticipated to run close to 4000 hours per year.

The first 75 MW of the Pearsall power plant entered commercial operation in late 2009 while the remaining 128 MW will start-up at the beginning of this year.

In its most recent order from the US, Wärtsilä has been contracted to supply a 170 MW gas fired power plant to be built at yet another site in Texas. The Antelope Station, near Abernathy, is to be located close to a significant wind farm generation resource, and will serve to stabilize the grid when the output from the wind farms change unexpectedly.

The contract was signed in November 2009 with Golden Spread Electric Cooperative Incorporated (GSEC), a consumer-owned public utility providing power to 16 member distribution cooperatives that serve more than 200 000 retail consumers. The quick-start and rapid response characteristics of this plant will assist in assuring high grid reliability, while providing competitive energy to GSEC members.

The growing summer electricity demand and the large amount of wind generation located in the region were the main drivers in GSEC’s choice of quick-start generation technology.

The new power plant will incorporate 18 Wärtsilä 20V34SG generating sets, along with mechanical, electrical and control auxiliaries, switchgear and exhaust emission controls. Additionally, by adding 18 separate units, GSEC reduces its risk from unit failures.

The power plant, which is expected to achieve commercial operation in early 2011, will generate approximately 170 MW; enough to meet the peak load requirements of 55 000 homes.

As the growth of wind continues Wärtsilä expects to see more of these plants being installed by utilities in the US and the rest of the world.

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System balancing

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There is a great deal of work being undertaken by the North American Electric Reliability Corporation (NERC) and other groups in the USA on how to integrate variable generation into the grid.

The flexibility of reciprocating engine-based plants with multiple units is extremely useful for wind integration. Plants have been sold recently in areas of the USA where there is massive wind power build. Texas, for example, currently has around 8-9 GW of wind power capacity.

In addition to being variable, the best wind resources are often far away from the load. This means long transmission lines have to be built from the wind farms to the urban areas in central and eastern Texas.

This is not cost-effective for a wind farm that has a relatively low load factor and will not utilize the transmission system fully. A wind firming plant can improve cost-effectiveness by using this transmission capacity when the wind plants are not running.

When integrating wind into a grid, the first thing to do is to aggregate it over the largest possible area. In ERCOT for example, the entire system is operated as a single entity where the wind can be aggregated over a large area. This makes balancing the system much easier because the net variability is reduced.

However, there are still occasional large ramps especially at times of minimum load at nights when large baseload plants are running. Since these large plants can only be turned down to 50 per cent, at best, of their maximum output, there is the possibility that any excess power from wind would have to be wasted. A flexible asset, which can be turned on and off in response to these ramps, can be very useful in this scenario in enabling the full utilization of the wind energy.

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