The intermittent production of renewable energy sources (RES) highly affects the performance of power generation systems. Hybrid power generation, which combines multiple sources, has resulted in more reliable and efficient energy systems, lower overall production costs, and greater flexibility in managing energy production and distribution.
This blog post, the first part of the series, The Subtle Art of Balancing the Grid, explains hybrid power generation as a strategy for managing smart microgrids (SMs). Yetu Smart Grids hydro-solar power solutions are used to give context.
Hybrid Generation
Before I delve into the nitty-gritty of why hybrid power centrals (HPCs) are critical when managing SMs, it is best that we understand the concept of hybrid renewable energy systems (HRES), focusing on hydro-solar HRES.
HRES comprises one renewable and one conventional energy source, or more than one renewable with or without conventional energy sources, and works in stand-alone or grid-connected mode. Think of an HRES as having one RES (solar PV, wind, or hydro) as the primary and backup energy source, either renewable or conventional.
The figure below shows a typical layout of a hydro-solar HRES.
Figure 1: Hydro-solar HRES
The hydro-solar HRES uses the natural flow of water to generate electricity through hydropower while utilising solar panels to capture the energy from the sun. If the hydro turbine is the primary source, solar PV would be the secondary source that supplies the deficit or caters to the transient load demands, or the other way around.
The combination of two RESs allows for a more stable and consistent power generation system. Additionally, these plants are designed to incorporate innovative technology, allowing for real-time monitoring and optimisation of energy production.
From Figure 1:
PCC represents the point of common coupling. It is the point where the renewable energy production system connects to the electrical grid. This is a critical location because it is the point where the system must meet all the regulatory requirements, including compliance with technical standards, safety requirements, and other regulations. The PCC is also important for monitoring and controlling the power flow between the system and the grid.
BESS stands for battery energy storage system. The BESS improves the reliability and stability of renewable energy production systems by storing excess energy generated during low-demand or high-production and then releasing it during high-demand or low-production periods.
PCS represents power conversion systems. PCS can be a mono- or bi-directional converter that controls the charging and discharging of BESS, serves as an interface between the DC bus and AC components, or supplies power directly to the load in the absence of a power grid. They include equipment such as inverters, DC-DC converters, transformers, etc.
Each distributed energy resource (DER), energy storage system (ESS), load, import from the grid, or feed for the grid in an HRES is monitored, controlled, and optimized by the energy management system (EMS). The PV, battery, hydro, load, and grid controls encompass the EMS. The EMS is a software-based system designed to ensure that the system operates efficiently while minimising waste and maximising output. It does this by collecting data on energy production and consumption, analysing it, and using it to make informed decisions about how best to manage the system.
The microgrid can be stand-alone or grid-connected—connected to a centralised grid or another microgrid.
HPCs: Strategy for Managing SMs
Hybrid renewable energy systems (HRESs) play a crucial role in managing smart microgrids (SMs). These systems combine different energy sources to harness their complementary advantages, allowing for the integration of one renewable energy source (RES) with another.
For instance, solar energy is only available during the day and varies throughout the year, while water levels in reservoirs for hydropower plants fluctuate depending on the seasons. Integrating these sources can help address the unpredictability of RESs, which may result in the oversizing of stand-alone systems to meet reliability requirements, making the design costly.
Considering Figure 2, the time distribution of demand is matched by implementing an HPC consisting of a hydro turbine that supplies the base load and a solar PV generator that caters for the deficit and the transient load demands—the same case as the energy storage system (ESS), depending on availability.
Figure 2: Balancing supply and demand using hydro-solar HRES
Implementing an effective HRES requires careful algorithm design, enabled by the EMS. A possible algorithm to manage a hydro-solar HPC could include the following steps:
Measure the energy demand from the loads. This is the demand to be met.
Measure the available supply from the hydro turbine and the solar PV generators. This is the available generator supply.
Compare the current energy shortfall or excess based on the monitored levels.
Calculate the cost of using each energy source to fill the shortfall or excess.
Prioritise using the energy source with the lowest cost to balance the system.
Adjust the energy output from each source accordingly to achieve balance.
If the renewable supply is greater than the demand,
Store the excess renewable energy in the ESS for future use.
OR
Run the hydro turbine and solar PV generators at a reduced rate to match the demand.
If the renewable supply is less than the demand,
Run the hydro turbine and solar PV generators at maximum output.
AND
Draw additional energy from the storage system to meet the shortfall.
Monitor the state of charge (SoC) of the ESS.
When it reaches a minimum threshold,
Reduce the load on the system by load shedding or other means.
AND
Continue to optimise the generator supplies to minimise the use of the storage system.
Continuously repeat this process, measuring supply and demand and adjusting the system to balance them as much as possible using the available renewable generation and stored energy.
To achieve a balance between supply and demand, the EMS will optimise generator supply to match demand as closely as possible, store excess energy during times of high production, and release it during periods of high demand. It also monitors storage systems to prevent exceeding the threshold limits. When the energy demand cannot be met, loads may be shed as a last resort.
In summary, HPCs play a critical role in managing SMs by balancing the intermittency of RESs, optimising the use of green energy, improving sustainability, and offering economic benefits, thus enabling a stable, reliable, and decarbonized power supply for the microgrid.
Case: HPC in real life
HRESs are becoming popular due to their ability to maximise energy output while minimising environmental impact and costs. One company that has successfully implemented an HPC is Yetu Smart Grids.
The Gitugu Hydropower Plant, recently launched, combines hydroelectric power with solar and energy storage systems to provide a reliable and sustainable energy source to local households and businesses within the region.
The system is designed to optimise energy production by using each power source to complement the others, ensuring a constant electricity supply. With the Gitugu HPC, Yetu Smart Grids has demonstrated the technological and economic viability of hybrid renewable solutions that provide clean, stable baseload power around the clock.