Blog 7 - Comparison between traditional and renewable energy sources and impacts on power system
Zehua, 19 January 2024
The replacement of conventional generation (synchronous machines) with renewable energy sources poses few challenges to power system operation. Let examine three of these aspects in this blog.
1. Power Supply Security
Photovoltaic (PV) power generation typically operates at full power for only 1,000 to 2,000 hours annually, roughly equivalent to 2 to 6 hours per day. This means that even if we were to expand its installed capacity to 5 to 10 times its current size, it might still fall short of meeting the current energy demand. For example, as of October 2021, a province in China had 23 GW of renewable energy installed, yet during rainy weather, the minimum output plummeted to just 0.16 GW. Therefore, ensuring a reliable power supply during periods of limited renewable generation and consuming energy when generation scales up is growing challenge in the foreseeable future.
In the scenario above, if we incorporate energy storage and assume a 20 GW load in a specific area, there would be approximately a 10 GW shortfall with a 50% contribution from renewable sources. In a situation lasting for 3 days, energy storage would be required to provide around 720 million kWh of electricity. At the current energy storage price, the cost would be about 110 billion GBP. Even if the price of energy storage drops to one-tenth of the current rate in the future, it would still require 11 billion GBP.
In contrast, traditional power generation can more effectively support power supply security. Overcoming the fluctuations in renewable energy generation and achieve low-cost energy storage is an urgent issue to be solved.
Flow diagram of a thermal power plant
(Source: https://www.quora.com/What-is-the-process-of-a-thermal-power-station)
2. Frequency Regulation
As shown in the figure above, the turbine coupled with traditional synchronous generators functions as a storage of steam energy, with primary frequency regulation harnessing residual energy in the steam collector for electricity generation. The boiler contributes to secondary frequency regulation by adjusting fuel supply. Each added synchronous unit in the system thererfore acts as a distributed energy storage component, leading to an overall increase in both generating capacity and regulation capability of the entire power system. This creates a self-expanding positive system.
Renewable sources currently lack the sufficient capacity for primary frequency regulation compared to traditional units. Achieving secondary frequency regulation with renewables demands additional energy storage. Consequently, the detailed exploration of how to utilize reserved capacity from renewable energy generation for future frequency regulation remains an important consideration.
3. System Inertia
System inertia refers to the resistance against alterations in the voltage and current frequency of the ac grid, and in the case of conventional synchronous machine-based systems, essentially the inertia that prevents changes in the rotor speed of a rotating machine.
Each synchronous generator adds inertia to the system. On the contrary, renewable sources, interfaced by means of power electronics do not contribute to system inertia. With growing numbers of renewable energy sources, the power system is progressively transitioning into a low inertia system. Frequency changes in low inertia systems are rapid and can lead to frequency flicker similar to voltage flicker due to the fast action of the power electronics.
To summarise, while renewable energy generation brings us cleaner energy, it also confronts us with many new problems to be solved.
Considering this perspective, one cannot help but wonder: Is it viable to directly integrate renewable energy with energy storage into the existing ac-based power system, endowing them with characteristics akin to traditional synchronous machines? Alternatively, is it conceivable to fundamentally reshape the future power grid, perhaps transforming it into numerous small microgrids, each strongly or weakly linked to the main grid, or operating independently? However, in the latter case, addressing the current extensive scale of the power system's asset poses yet another challenge.
Contact: zehua.tang@kcl.ac.uk