Challenges with Hybrid Renewable Energy Systems

Though a hybrid energy system has a bundle of advantages, there are some challenges and problems related to hybrid energy systems that have to be addressed:

• 1. Most of the hybrid systems require storage devices for which batteries are mostly used. These batteries require continuous monitoring and increase in cost, as the batteries’ life is limited to a few years. It is reported that batteries’ lifetime should increase to several years before they can be effectively used in hybrid systems.
• 2. Due to the dependence of renewable sources involved in the hybrid system on weather results in the load sharing between the different sources employed for power generation, the optimum power dispatch and the determination of cost per unit generation are not easy. However, optimization of hybrid energy system is essential. This should be obtained with several end objectives.
• 3. The reliability of power can be ensured by incorporating weather independent sources like diesel generator or fuel cell.
• 4. As the power generation from different sources of a hybrid system is comparable, a sudden change in the output power from any of the sources or a sudden change in the load can affect the system stability significantly.
• 5. Individual sources of the hybrid energy systems have to be operated at a point that gives the most efficient generation. In fact, this may not occur, due to the fact that the load sharing is often not linked to the capacity or ratings of the sources. Several factors decide load sharing like reliability of the source, economy of use, switching require between the sources, availability of fuel, etc. Therefore, it is desired to evaluate the schemes to increase the efficiency to a possible higher level.

Going forward, all of these challenges will need to be addressed.

Elements of Hybrid Energy (Power) System

The discussion of hybrid energy system is complex because there are several facets to the concept of hybrid energy system. I introduced this concept in my previous book [2] on “Energy and fuel systems integration.” In this book various forms of hybrid nature of both fuels and energy systems are discussed. The book first considers the advantages of combining various fuel resources like coal, biomass, waste, and various forms of oils for downstream processes of combustion, gasification, liquefaction, and pyrolysis. The book illustrates that hybrid fuels (also designated as co-fuels in the book) offer many advantages to the downstream processes such as more favorable product slate, less impact on environment, more flexibility on scale, etc. The book also shows that co-digestion for anaerobic treatment of waste also improves methane and hydrogen production which are desirable for subsequent production of energy. For automobiles, hybrid fuels improve fuel flexibility and less environment impact. The book thus clearly articulates many advantages of hybrid fuels in energy industry.

Besides hybrid fuels (or as the book articulates as the integration of various fuels), the book also considers the role of combined heat and power (another form of hybrid energy system) for the nuclear industry. Cogeneration is another form of hybrid energy system that can be applied to both coal and gas power industries same as that for nuclear industry. The book also evaluates hybrid power generated from renewable sources. Finally, the book also considers the hybrid power used in the automobile industry.

Since the publication of the previous book [2]), the development and use of hybrid energy system has gained significant momentum and new advances. Besides the ones mentioned above, significant efforts are now made to develop hybrid power systems involving more than one energy source. The need for hybrid storage systems and hybrid grid transport systems has also been recognized, and these concepts are significantly advanced. Cogeneration has taken an alternate approach where waste heat is used to generate more power (like waste heat conversion to power by thermoelectricity) or even waste CO, to generate more power using fuel cell technology. Both hybrid and electric cars have made significant power electronics advances. New developments of utility grid, microgrid, nanogrid, and off-grid technologies and their interactions have allowed new possibilities for interconnections between centralized and distributed energy sources. These are also important for deeper penetration of renewable sources in the overall mix of the energy industry [9, 12-22, 31-34, 40]. They also make a hybrid energy system (or integrated energy system) a more sustainable strategy for the future.

The present book contains several elements of hybrid energy system. In literature, hybrid energy system (or more specifically hybrid power) is defined as the system in which more than one source of energy is used to generate energy (or power). As described earlier, while this definition makes most sense, it is not totally inclusive of all elements of hybrid energy (power) system. As discussed in this book, there are five basic elements of hybrid power system:

• 1. Hybrid power generated from more than one input source of energy like coal, oil, gas, biomass, solar, wind, etc. Energy storage can also be considered as one source of power. Going forward, the overall objective of centralized grid-connected energy is to (a) increase efficiency through cogeneration or conversion of waste heat to power by thermoelectricity, (b) reduce CO, emission by converting CO, to power via CO,-based fuel cell, or (c) increase renewable energy contribution in grid power. All of these strategies will make centralized power more hybrid.
• 2. Hybrid power is generated in the mobile industry to improve the efficiency of vehicles and reduce its impact on the environment. Unlike in stationary power generation, the end objective here is to generate more mechanical energy for motion.
• 3. As we increase the contribution of renewable energy in the overall energy mix, the use of energy storage becomes more important. Just like the source for power generation, no energy storage device completely satisfies the need for sustainable and high-quality power supply. The future for uninterrupt- able power supply with renewable sources for power will require hybrid energy storage (multiple storage) systems. Hybrid energy (power) storage systems will become an integral part of hybrid energy (power) system.
• 4. In order to capture distributed renewable and low-density energy sources, the grid transport will be modernized and become more hybrid. Hybrid microgrid will capture distributed energy sources at medium to low voltage in a more efficient manner. This hybrid microgrid either will be connected to macrogrid or can operate in islanded mode. When hybrid microgrid is connected to macrogrid, it can act as a backup in case of macrogrid failure.
• 5. In order to serve customers in remote and isolated areas, the off-grid hybrid energy systems are being developed. These systems include mini-, nano-, or picogrids and stand-alone systems. Unlike microgrids, these systems will not be connected to macrogrid. These systems are very useful for rural and remote area electrification.

The hybrid power system must consider all five elements of power generation, storage, and transport (grids) for both static and mobile applications.