Flexible gas engine concepts support renewable energy in power systems

July 14th, 2014, Published in Articles: Energize

 

As the amount of variable renewable generation increases, in order for production to meet demand, sufficient balancing capability is needed. This requires greater flexibility in the power system, and generation technology based on internal combustion gas engines is one of the evident alternatives. Modern gas engine power plants with multi-engine concepts have numerous combinations for arranging a solution to meet specific generating needs.

South Africa, through the state utility, Eskom, has in the past managed to provide a reliable supply of electricity through the extensive use of coal based power generation. However, as South Africa seeks to diversify their energy mix, and reduce CO2 emissions, alternate energy sources and technologies will be required to satisfy these objectives. In response to this, the South African Department of Energy in 2011 released an RFP for 3700 MW of renewable energy (REIPPP) and to date, this successful programme has seen the construction of 3922 MW across 64 projects [1]. The impact, however, of rapidly increasing the renewable energy penetration on a power system does present the system with a number of challenges for which one should recognise and plan for today.

Fig. 1: Operational flexibility with multiple gas engines.

Fig. 1: Operational flexibility with multiple gas engines.

When introducing renewable generation based on wind and photovoltaic (PV), production variations in the grid increase. Even though wind forecasting is fairly good today, and is likely to get even better, there will always be forecasting errors causing uncertainty for grid operators. The optimal solution to balance rapidly developing variations and uncertainty in renewable generation would be with reservoir hydropower. However, such an option is not available in many locations, and thus the most feasible way to cope with rapid changes is with fast and efficient, very flexible gas fired power generation.

Modern gas engines employ a series of techniques and features that allow them to offer superior simple-cycle efficiency combined with fast loading capability. Embedded engine automation with individual cylinder control ensures very stable operation, and at the same time, defines the essence of fast loading, load following and unloading features (Fig. 1). This power generation technology, which reaches synchronisation readiness in 30 s from standstill (hot standby), represents a true option for a non-spinning secondary reserve. When considering its proven capabilities of reaching 100% load in less than 5 min, the technology is a perfect tool for enabling a major introduction of variable renewable generation [2].

Fig. 2: Example of the development of the Inertia Constant for one European country. It shows the current situation and the forecasted situation for the year 2020, with high and low wind renewable energy production not contributing to system inertia.

Fig. 2: Example of the development of the Inertia Constant for one European country. It shows the current situation and the forecasted situation for the year 2020, with high and low wind renewable energy production not contributing to system inertia.

Such fast start and ramping capabilities should be recognised and rewarded by the markets, since total power system savings could be achieved by reducing the use of secondary reserves or part load balancing requirements. For instance, transmission system operators (TSO) could procure faster tertiary reserves (or some other balancing reserves) through reducing the amount of spinning reserve for renewable energy sources.

Frequency stability with combustion engines

The introduction of renewable energy sources is advancing rapidly, and some countries are targeting 50% of their electrical energy need from renewable power by 2020 [3]. According to the South African Integrated Resource Plan 2010, the current plan is to reach approximately 10% by 2030 but given the recent success of the REIPPP, these ambitions are likely to be increased.

Fig. 3: Primary reserve frequency graph for a 2020 high wind scenario in a European country, where the blue line is business usual and the red line indicates that 50% of the conventional generation, except that of nuclear and CHP, is replaced with engines.

Fig. 3: Primary reserve frequency graph for a 2020 high wind scenario in a European country, where the blue line is business usual and the red line indicates that 50% of the conventional generation, except that of nuclear and CHP, is replaced with engines.

While the environmental benefits of these sources are obvious, the intermittency and unpredictability of their power output may pose a challenge for grid frequency stability. This is further exaggerated by the fact that renewable sources do not always contribute to stability services and system inertia, where they are replacing power plants that were contributing to those services. With even moderate penetration levels, the vulnerability of the system may increase during disturbances such as a sudden loss of production.

The low variable costs of wind and solar powered generation, and the political incentives for priority of their dispatch, provide means to displace conventional generation in power systems. This so called “merit order” effect is expected to lead to lower load factors, lower revenues, higher price volatility and increased cycling, including starts and stops for conventional generation. This can also lower the systems’ resiliency to disturbances in two ways.

Fig. 4: Secondary reserve frequency graph for a 2020 high wind scenario in a European country, where the blue line is business as usual and the green line indicates that all secondary control response comes from stand-by generation.

Fig. 4: Secondary reserve frequency graph for a 2020 high wind scenario in a European country, where the blue line is business as usual and the green line indicates that all secondary control response comes from stand-by generation.

The needed generation reserve in the systems has to cope with not only the normal stochastic system variations, such as electricity production trip, but also with the variability and unpredictability of renewable power production. This will increase the need for system reserve capacity and its deployment. Many studies have been made to evaluate the impact of renewable generation on the amount of reserve where the absolute numbers of potential additional reserve requirement vary [4], and this will not, therefore, be further elaborated upon here.

The first effect originates from the fact that currently renewable power generation units may not be required to contribute to system stability services nor to system inertia. This is based on the simple condition that the eventual reserve provision and response of renewable generation is at least to some degree subject to the natural variability and unpredictability of the units’ respective power source. Thus, they are not currently often required to participate in system services in the form of primary and secondary reserve provision. In addition to the above mentioned, there is also a desire to use renewable generation to its full extent.

Fig. 5: Power plant SCADA (WOIS) historical trend screenshot from standstill to 100% load, 46,7 MW in 4 min 17 s (the x-axis trend sliders show the times for the start and 100% load reached).

Fig. 5: Power plant SCADA (WOIS) historical trend screenshot from standstill to 100% load, 46,7 MW in 4 min 17 s (the x-axis trend sliders show the times for the start and 100% load reached).

The second effect arises from the way many of the renewable generation units are connected to the power system. Many modern renewable generation units are connected to the system indirectly; that is through a converter or a frequency drive and do not, therefore, contribute to system inertia.

This loss of inertia through the displacement effect, may lead to a higher rate of change of frequency and eventually to a lower absolute frequency drop in the case of system disturbances. This has spurred TSOs to evaluate a request for renewable generation units to provide so called “synthetic” inertia, whereby indirectly connected renewable generation units would mimic the inertia response of conventional generation technologies [5].

Fig. 6: The loading rate of one Wartsila 50SG generating unit is 400 kW/s. This corresponds to 24 MW/min.

Fig. 6: The loading rate of one Wartsila 50SG generating unit is 400 kW/s. This corresponds to 24 MW/min.

The decreased inertia, and the potential for reduced availability of system services with increasing wind generation, may lead to a greater requirement to ramp conventional generation units up and down. This is not only due to the natural variations of their power source, particularly if rising demand coincides with falling wind generation, but also indirectly through the above mentioned displacement effect.

Combustion engines are very suitable for system balancing services, since they have the inherent capabilities as described earlier to compensate for the natural variability of renewable generation, for the indirect effect of decreased system stiffness, and for the reduced availability of system services (Figs. 2, 3, 4).

Fig. 7: Multiple units in a single power plant.

Fig. 7: Multiple units in a single power plant.

A large study on the frequency stability capabilities of combustion engines performed by KEMA [6], shows that for compensating the reduced system stiffness, the natural ability of combustion engines to provide a high loading capability helps power systems to enhance system performance. This furthers the introduction of more sustainable power generation capacity without endangering system performance.

The quick start and rapid loading capability of combustion engines further makes system services, such as secondary control (upwards), available from stand-by condition. The possibility for having rapid secondary power replacement from combustion engines from stand-still in the case of a production shortfall enhances overall system performance capability.

This capability compensates for the potential lack of rapid response service capability from other conventional plants, particularly if they would be forced to cease running due to the high availability of renewable power production output.

Fig. 8: Computer generated image of a typical 560 MW combined cycle gas engine power plant.

Fig. 8: Computer generated image of a typical 560 MW combined cycle gas engine power plant.

Firm and flexible capacity

The ability of a power generation solution to reach full output within only 5 min from the start button being pressed is clearly a desirable feature for system stability, especially with the introduction of renewables with inherent volatility. Four-stroke, lean burn combustion gas engines are very well suited for demanding operating profiles, since the operating temperatures offer the engine manufacturers a wider selection of materials that are not sensitive to cycle fatigue.

The capability to reach 100% load from standstill in less than 5 min has already been tested and validated in several gas engine power plants. One example of such is the Lea County Electric Cooperative (LCEC) generation plant, located in New Mexico, USA. The power plant consists of five Wärtsilä 34SG generating sets, with the ground breaking ceremony being held on 26 April 2011, and the plant commissioning in January 2012. These dates indicate how quickly these modular multi-engine power plants can be delivered and installed.

On 4 January 2012, performance tests were performed with the customer witnessing the time needed to reach plant full load of 46,7 MW. Fig. 5 shows the result of the tests where the engines are ready for loading in roughly 30 s from the start button being pressed. The loading to 100% takes place simultaneously for the five generating gas engine sets at a ramp rate of 25% per min, and the full plant load is reached in only 4 min and 17 s (refer to Fig. 5).

The use of multiple gas engines to build up larger power generation units offers huge possibilities for operational flexibility. There are no limitations in switching from one operation strategy to another from season to season, based on production forecasts, on daily variations, or whenever the market situation presents an opportunity.

The demonstrated fast ramping capacity, from standstill to 100% output in 5 min, is suitable for secondary reserves. And, by keeping a few engines running while others are stopped, it is also possible to have a “non-spinning spinning”, i.e. stand-by spinning reserve capacity available. For example, if one engine out of five is running at 50% load and the rest are stopped, it is still possible to ramp up from 10% to 100% load within 5 min with a constant load increase of 18%/min. The benefit of having a minimum number of units in production at a fairly high load level is that the heat rate will not suffer from low load spinning, while simultaneously the minimum production load can be kept at a very low level. Energy from the cooling systems of the running engines can also be utilised for keeping other engines pre-heated, thereby reducing their own consumption and further improving the total net efficiency.

Engine units can be started, stopped, loaded, and unloaded simultaneously or as separate units totally independent of each other, to give the optimal operation profile, supporting the needs of the grid and giving optimal operation economy. The only limitations are that the engine, as mentioned earlier, requires a short time (about 30 s) for starting and synchronisation if not in operation, and when stopped it requires typically about 5 min for safety functions before it again can be reloaded.

The loading capacity is limited by the engine temperature; if the stand-by temperature is kept low (50 to 55°C), a few extra minutes will be required to reach full load. Correspondingly, the loading capability of an engine that has reached its working temperature can even exceed 100%/min (Fig. 6). By starting and stopping units according to demand, an overall heat rate close to the theoretical optimum can be maintained most of the time. Furthermore, the part load efficiency with gas engines is excellent. High part load efficiencies, in combination with fast ramp capabilities, reduce fuel consumption and the CO2 emissions caused by cyclic operational demands significantly compared to any other technology based on fossil fuels, and thus make these engines the perfect partner for enabling a high degree of renewable power generation.
The multi-unit solution also provides extremely good availability, as maintenance can be scheduled and performed on one unit without disturbing the availability of the other units in the plant (Fig. 7).

Flexible gas engine plants can also be combined with steam generation and steam turbines as combined cycle solutions, or with district heating connections where the cyclic operation pattern can be evened out by heat storage. In situations with excess renewable power in the grid, the same storage can work as an energy dump for cheap electricity.

Spark-ignited internal combustion gas engines can operate on low pressure pipeline gas or on re-gasified LNG (liquid natural gas) without any need for compressors; they are also relatively insensitive to derating due to altitude, ambient temperature, and humidity variations. Simple-cycle solutions are as default dry solutions where all cooling is taken care of by closed loop cooling radiators. This enables a solution with close to zero water consumption, which is especially suitable for places where ambient temperatures can drop below zero, clean treated water is either scarce or a cost intensive resource, water effluent handling is challenging, the load pattern is irregular, or where cooling towers are banned due to microbiological risks, noise restrictions or plume visibility.

The engine’s performance

Wärtsilä has introduced the largest gas combustion engine existing on the market. It meets current and future requirements for overall cost of ownership, with very high simple and combined cycle efficiency. It is designed for easy maintenance and long periods of maintenance-free operation.

The maximum electrical power outputs are 18,3 MW at 50 Hz. The Wärtsilä 50SG engine runs at 500 rpm and has an efficiency of over 50% at the generator terminals. When compared to all other combustion gas engines and gas turbines on the market, it is evident that this efficiency is the highest existing today among all simple-cycle power plants.

The spark-ignited gas engine has been developed in response to the increasing market need for larger gas engines to run power plants with outputs of up to the 600 MW range, and beyond. For a 500 MW simple-cycle power plant, 28 engines of the Wärtsilä 50SG size are required. When closing the cycle by adding a steam turbine, the power plant output increases to 560 MW (Fig. 8) with an efficiency of 55% [1].

In addition to high efficiency and power output, another key benefit of the 50SG, as with all Wärtsilä engines, is its ability to run up and down in load without affecting the maintenance schedule. This is beneficial for peak applications, or in markets where there is a significant amount of wind power on the grid, since it can reach full power in minutes from standstill in the event of a sudden drop in wind capacity. The engines can also be stopped in one minute and reloaded in just five min.
The loading rate of a power plant in operation with 50SG generating units is remarkable. A 500 MW simple-cycle power plant or a 580 MW combined cycle plant, both consisting of 28 engines, has a loading rate capability up and down of 672 MW/min as all units can be loaded and unloaded simultaneously, and the loading rate of one engine is 24 MW/min.

Conclusion

The increasing levels of intermittent renewable generation can pose a challenge to power system reliability during disturbances. Fast and flexible generation based on combustion engines can cope with these challenges and support power system stability. The demonstrated very fast loading from standstill to full load in less than 5 min, and the possibility of having the generation synchronised within  30 s, provide the means for a non-spinning secondary reserve. This means greater possibilities to balance systems with high levels of intermittent renewable generation more economically. A secondary reserve with neither fuel used nor emissions generated is a unique option for the future energy mix featuring a high penetration of renewable generation.

The new engine is the largest gas combustion engine with the highest simple-cycle efficiency existing on the market. Even though this engine allows plant sizes up to 600 MW, the multi-engine features mentioned in this article are inherent to the solution and, furthermore, step-wise extensions minimise the risk when the future operational pattern may become increasingly uncertain.

For South Africa to achieve its future renewable energy targets, the enabler is power generation based on gas engines with the unique combination of valuable features and multiple operation modes with clear benefits for power system operators and power producers.

References

[1]    A Eberhard, J Kolker, and J Leigland: “South Africa’s Renewable Energy IPP Procurement Programme: Success Factors and Lessons”, World Bank Group Public-Private Infrastructure Advisory Facility (PPIAF), May 2014.
[2]    J Klimstra, and M Hotakainen, “Smart Power Generation”, www.smartpowergeneration.com
[3]    “Ambitious energy policy towards 2020”, www.energinet.dk/EN/OM-OS/Nyheder/Sider/Ambitioes-energipolitik-frem-mod-2020.aspx
[4]    EWEA Powering Europe: “Wind Energy and the electricity grid”, November 2010.
[5]    ENTSO-E: “Draft Network Code for Requirements for Grid Connection applicable to all Generators”, January 2012.
[6]    G Dekker and J Frunt: “Frequency stability contribution of Wartsila combustion engines”, 2012.

Contact Wayne Glossop, Wartsila SA, Tel 011 881-5953, wayne.glossop@wartsila.com

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