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System integration of variable renewables - focus on supply side flexibility

Posted: 14 November 2017 By: Stephen Mills

Held at Ministry for Economic Affairs and Energy (BMWi), Berlin, 1-3 November 2017


Flexible operation is an area that operators of coal-fired power plants and technology suppliers now have to take seriously. Many such plants were designed and built to operate predominantly on base load, but this mode of operation is declining in many power fleets around the world. Increasingly, start-stop, load following, and low load operation have become the norm. For many, the ‘luxury’ of operating solely on base load has long gone. Today, many plants have no option but to cycle regularly, largely in response to the increasing amount of electricity generated by intermittent renewables such as solar and wind power. Both of these are essentially fuel- and emissions-free, but they are also weather-dependent – output can change significantly and quickly. In order to respond to these variations, thermal power plants feeding into the same grid often need to ramp up and down in order to provide back up when input from renewables is lacking. 

A lot of coal plants were not built with this type of operation in mind and some certainly find it challenging. Especially for older units, warm-up times can be lengthy, response times sluggish, and low load operation difficult to maintain effectively. However, there are technical solutions and alternative ways of operating that can help plants to operate more flexibly in the face of the variable input from intermittent renewables. This was one of the main themes explored by the workshop and study tour organised and hosted by the German Federal Ministry of Economic Affairs and Energy (BMWi), held in Berlin between the 1- 3 November. This examined some of the efforts being made around the world to ensure that power systems and plants are able to operate more flexibly, especially as large increases in intermittent renewables are planned. For example, Germany has an ambitious target of producing 50% of its electricity from renewables by 2050. This will bring further challenges for thermal power plants – to survive, these will need to remain economically viable even when operating for fewer hours and not simply on base load. Flexibility will be a key factor in terms of grid and demand side operation, as well as power plant performance. 


Discussions underway at the German Ministry for Economic Affairs and Energy
This issue was explored via a series of presentations and discussions focusing on economies that included India, China, Germany, Denmark, Japan and Brazil. Understandably, the situation varies– each country has its own combination of energy resources, energy mix, and power fleet. Each also has different aims and aspirations for installing more renewables. Some are quite ambitious. For example, by 2022, India plans to develop 100 GW of solar, 60 GW of wind, 10 GW of biomass, and 5 GW of small hydro - a total of 175 GW. Alongside this, 22.7 GW of old coal-fired plants will be retired. A task force comprising utilities, government agencies, and technology providers is currently pondering the issue of flexibility. 

Germany has already introduced various measures to allow some of its coal-fired plants to operate more flexibly. When market conditions dictate, and a high level of input is coming from wind and solar, lignite-fired plants tend to switch to low-load operation. Plants firing hard coal often shut down, as do gas-fired units. In the face of this, measures have been taken to allow coal plants to remain on line (even at low load), reduce start-up times, and increase ramp rates. This has been achieved via technical modifications (such as advanced instrumentation and control systems – I&C), as well as changes in operating procedures. Some plant improvements achieved so far have been significant. For example, the 630 MW lignite-fired Neurath plant has reduced its minimum load from 400 MW to 270 MW and increased its ramp rate from 5 to 15 MW/minute. It can now reach ~70 MW in 15 minutes. Most of this has been achieved through the installation and optimisation of upgraded plant I&C systems.  Similar advances have been achieved at other plants such as the 3 GW lignite-fired Jänschwalde Power Station and the 800 MW Unit 7 of the Heilbronn cogeneration plant.  Improvements to I&C systems are considered to be ‘low hanging fruit’- they can make a big difference at a limited cost. 

Several studies considered the role of market design as a means to incentivise the flexible operation of assets and unlock investments in flexibility, as well as how to increase flexibility in regulated market environments.  Depending on the individual circumstances, there is a wide array of possibilities for increasing the flexibility of system operations. However, increasing the flexibility of power plants is one of the most cost effective. Technical solutions are available to accomplish this, although to achieve the greatest potential, the correct market conditions must also be in place. 

Day 2 took delegates (from United Arab Emirates, USA, Germany, Morocco, Italy, Thailand, Russia, India, Spain, China, Mexico, the UK, Japan, Argentina, Switzerland, Denmark and Norway) on site tours of the Moorburg CHP plant on the outskirts of Hamburg (see separate blog) and the North German regional energy supplier WEMAG’s battery storage facility in Schwerin. The latter was built in 2014 and enlarged in 2017 to its current capacity of 14.5 MWh. The battery power is currently used largely for grid frequency stabilisation and to compensate for short-term fluctuations in input from wind and solar power sources. The unit is now Europe’s largest hybrid lithium-ion battery network. Phase 1 was equipped with 25,600 Samsung lithium-manganese-oxide cells and five medium-voltage transformers. The 10 MW of power in Phase 2 comprises a network of 53,444 lithium-ion batteries in 215 battery cabinets, 18 inverters, nine transformers and a medium-voltage system. The facility is fully automated and is normally unmanned. 

Part of WEMAG’s battery storage facility in Schwerin
Day 3 included a visit to the large and impressive Siemens turbine factory in Belin. The site, one of Siemens’ oldest, employs ~3700 people and produces gas turbines for the Siemens Power and Gas Division. In 2016, the 1000th gas turbine was shipped from the site. Turbines have been supplied to 65 countries around the world. Delegates heard how Siemens have developed techniques that allow their gas turbines to respond faster and operate more flexibly to compensate for the variable input from renewables. In addition, steam power plants using Siemens steam turbines have also been similarly modified so that minimum load operations have been improved, ramp rates increased, and cycling enhanced. Even coal-fired plants originally designed solely for base load operation have been adapted to allow for greatly increased flexibility through combinations of technical and operating software upgrades. 

All in all, this was an interesting and timely workshop and tour. It focused on how the electricity market is changing (and will continue to change) in many countries as intermittent renewables feed even greater amounts of electricity into national and regional grids. The topic of enabling coal-fired power plants to increase their flexibility and reduce emissions whilst remaining economically viable is one that the Clean Coal Centre has addressed on a number of occasions in recent years. As technology and electricity markets continue to evolve, it seems likely that it’s one we may revisit again at some point in the future.     


 
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