Balanced Generation

Tackling grid integration challenges

By M. Rajagopalan, Market Development Director, Middle East and Asia, Wärtsilä India Private Limited

India has embarked upon an ambitious programme to install 100 GW of solar photovoltaic (PV) and 60 GW of wind capacity by 2022. However, such a large-scale addition of variable or intermittent generation in a system packed with inflexible baseload coal plants, which are expected to total around 250 GW by 2022, will pose a challenge to grid management. Tackling these will require some planning and course correction.

Is there a precedent of any other country tackling such a high percentage of renewables in its overall mix? Let us consider the examples of Germany and China. In Germany, which has invested heavily in wind and rooftop solar PV plants, there is a wide gap between the renewable output and the power demand. There are times when renewable energy generation exceeds the total demand, and times when it slackens considerably. Due to the feed-in rights of renewable generation, it gets despatch priority and baseload coal plants are turned down proportionately. If there is still excess supply, it is “dumped” on the larger European grid, causing serious discomfort to the neighbouring markets. Meanwhile, when renewable energy generation is inadequate, baseload plants are ramped up suitably. Thus, in Germany, flexibility is achieved by ramping up/down the coal plants. Paradoxically, this has increased the country’s net carbon emissions, thus defeating the purpose of adding renewable energy. The overall cost to the consumer has also increased.

In China, over 100 GW of wind turbines have been installed, particularly in the northern provinces, which also, incidentally, have large coal reserves and a large number of pithead coal plants. As load centres are far away in the east and the southeast, power is transmitted through high voltage direct current lines. When the winds are high and the demand is low (at night time or early morning), wind generation has to often be curtailed as it is the only way to manage the excess given that coal plants need to be kept operating at a minimum load due to technical restrictions or because they need to provide heat. According to Bloomberg data, over 21 per cent of wind generation in 2016 was curtailed, again defeating the purpose of adding renewable power plants, even as coal plants recorded an abysmal annual load factor of 41 per cent.

India seems to be heading the China way. Due to its overreliance on coal plants, which have a rather inflexible operating profile, the variability of renewable energy generation needs to be tackled either by curtailing it (as is happening in Tamil Nadu) or absorbing it by cycling down coal-based plants to a technical minimum level with low efficiency. Coal plants are already operating at a poor load factor of 51 per cent and renewable addition will only make the problem worse. A report released by the Ministry of Power on renewable integration April 2016 acknowledges these issues and emphasises the need for more flexible generation in the system. In this context, “flexibility” means a plant’s ability to start and ramp up to a desired load within a few minutes (when renewable energy generation drops); stop or reduce generation instantly (to fully absorb renewable generation when it picks up); start/stop any number of times without a maintenance penalty; and operate efficiently at all loads.

Hydroelectric plants are ideal for this application. However, all states do not have hydro potential and even if they do, it is not an all-season solution as it is rain dependent and irrigation linked. Fast-response gas-based generation is also a good alternative. Modern plants, especially those based on combustion engines, have all the desired characteristics, with all-season despatchability and the ability to ramp up to full load within five minutes. These are different from the many plants in the country that are based on combined-cycle gas turbine technology, and are not flexible enough for renewable balancing, and are being operated suboptimally as baseload plants.

But isn’t gas-based generation expensive? It is, if viewed in a narrow sense of cost per kWh at the plant level. However, if viewed holistically at a system level, it helps in reducing the overall generation cost per kWh by allowing coal plants to operate at their best load and efficiency without the need to cycle down or incur more maintenance costs. In addition, it helps absorb cheaper and cleaner renewable energy fully without being burdened by its variability. It also reduces the need for water, a precious commodity, and investment costs as the capex is far lower and the commissioning faster, thus reducing the overall cost. The cost of gas has also come down considerably in recent months.

Thus, flexibility needs to be built in by a  planning modelling exercise and optimising the generation mix suitably. The addition of coal power plants can be limited to meet baseload requirements and allow hydro and flexible gas plants to complement renewable energy. With this approach, we can meet our  renewable generation and carbon emission targets without impacting the reliability of the system.

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