Hybrid Energy Systems Defined

Hybrid energy systems can be defined in a number of different ways. Hybrid energy system as defined here is an umbrella of systems which include multiple sources of energy and multiple storage devices and systems with hybrid energy processes. These systems can be connected to utility grid, microgrid, or they can be off-grid (like mini grid, nanogrid, or stand-alone systems). The pros and cons of hybrid energy systems and related issues and challenges depend on further details on the contents and the methods adopted for their use.

Multiple sources of generation in a hybrid energy system can be nonrenewable- nonrenewable (like coal and gas), nonrenewable-renewable (like coal-solar), renewable-renewable (like solar-wind), nuclear-nonrenewable (like nuclear-gas), or nuclear-renewable (like nuclear-solar). Each of these systems has its own pros and cons and challenges and issues. For renewable sources, further breakdown in nondis- patchable (like solar and wind) and dispatchable (like biomass, geothermal, hydro, etc.) is required. This differentiation also requires different strategies for implementation, particularly due to intermittent nature of solar and wind energy. All of these combinations are unique and situation dependent and are aimed toward reduction of carbon emission with different degrees of success. Multiple sources also include considerations of process-generated sources like waste heat or waste product. The present book considers all of these options.

Storage is an example of another method of energy generation when it is required. As pointed out in my previous book [3], there are multiple devices for storage, each with its own pluses and minuses. Many energy applications require multiple storage devices for sustainable and high-quality energy (particularly power) use. The present book includes multiple storage devices as part of hybrid energy systems. Finally, the details of implementations and challenges faced on the use of hybrid energy systems depend on whether these systems are connected to utility grid, microgrid, or they are part of off-grid operations (like minigrid, nanogrid, or stand-alone systems). Hybrid energy systems include both power and heat (cool).

Decarbonization of energy industry essentially means less and more efficient use of fossil fuels (particularly coal, oil, and gas in that order). This requires two-prong strategies: (a) improve efficiency of all energy conversion processes and reduce the unnecessary need for the energy consumption. This can be largely achieved by conservation measures and building more hybrid energy systems like cogeneration and carry out better integration and hybridization of raw materials and process steps so as to reduce material and energy need which are dependent on fossil fuels and (b) replace the use fossil fuel by renewable or nuclear energy sources for all required process energy. This can be done by using hybrid energy systems involving multiple sources of generation with or without storage. In essence the task for energy industry is to move from right to left in Figure 1.1 to the extent possible. It should be pointed out that decarbonization is also possible if produced CO, is captured and sequestered or converted to other useful materials. However, as pointed out by national academy report [5], this is a difficult task and significant more R&D is needed. As mentioned earlier, this subject is also examined in details in my next book on treatment strategies for carbon emissions.

General Electric (GE) defined hybrid power as: “Hybrid power plants usually combine multiple sources of power generation and/or energy storage, and a control system to accentuate the positive aspects and overcome the shortcomings of a specific generation type, in order to provide power that is more affordable, reliable and sustainable. Each application is unique, and the hybrid solution that works best for a specific situation will depend on numerous factors including: existing generation assets, transmission and distribution infrastructure, market structure, storage availability and fuel prices and availability” [3]. Similar concept in a broader term

Comparison of as-published life cycle greenhouse gas emission estimates for electricity generation technologies [6,7]

FIGURE 1.1 Comparison of as-published life cycle greenhouse gas emission estimates for electricity generation technologies [6,7].

of hybrid energy systems (which include both power and heat) is adapted in the present book.

There are numerous examples of hybrid energy systems that are worth noting. GE definition of hybrid power includes multiple sources of power generation (like wind and solar) with energy storage with and without grid. The district heating with multiple renewable sources of heat with and without storage is another example of hybrid energy system where power may not be involved. A zero-energy building with solar energy to generate power and heat and geothermal energy for HVAC system with or without storage is also another example of hybrid energy system. Here both power and heating and cooling are parts of hybrid energy system. Cogeneration (combined heat and power) is another example of hybrid energy system to improve energy efficiency. In this case process generates second source of energy (i.e., waste heat). Exxon-Mobil and Fuel Cell energy partnership to generate power by burning coal or natural gas and use fuel cell to generate more power from waste C02 is another form of hybrid energy system where power is generated in two different ways within a process. Using multiple sources (including renewable and nuclear) to generate heat with and without thermal storage for industrial processes (such as glass making operation) with conversion of waste heat to generate electricity by the process of thermoelectricity is another form of hybrid energy system where industrial heating results in the waste heat to generate power. This is a reverse cogeneration process once again used to improve energy efficiency. Hybrid vehicles can also generate power by internal combustion (IC) engine, fuel cell, or battery along with power generated via thermoelectricity of waste exhaust heat.Electric vehicle (EV) with energy storage (one or more) is also hybrid operation. In all the cases mentioned above there can be different level of integration of components of hybrid energy systems. Wastewater treatment with combined physical and chemical process is a hybrid energy system. Desalination can be carried out with multiple options of hybrid energy systems all designed to be more efficient and less energy consuming.

The book makes the case that hybrid energy system is an important strategy for decarbonization of various industries. Hybrid energy systems are (a) more reliable and flexible, (b) more efficient, (c) more affordable, (d) more environment friendly, and (e) ultimately more durable and sustainable over long term compared to the single component energy sources or process. Hybrid energy systems are essential for the deeper penetration of renewable and nondispatchable energy (particularly solar and wind which are intermittent by their nature) sources in overall energy mix in order to reduce carbon emission to the environment. Hybrid energy systems are also important for better use of nuclear heat and suitable power generation by a combination of nuclear and renewable sources. The advantages of hybrid storage and hybrid grid transport for power industries are described in detail in my previous book [3].

The important defining components of hybrid energy systems are thus:

  • 1. sources of power generations
  • 2. nature of storage device used
  • 3. nature of connections of hybrid energy systems to the grid
  • 4. nature of use: power, heat (or cool) or both
  • 5. end applications: building, vehicle, manufacturing industry, etc.

Previous book articulated more details on these components with focus on power. The present book goes one step further and articulates strategy of hybrid energy systems for decarbonization of broader industrial applications which include both power and heat (cool). Because hybrid energy systems are so varied, the book articulates how these systems can be effectively used in each industry. Unlike previous book, this book puts less emphasis on the grid structure used and more emphasis on how different industries use hybrid energy systems to decarbonize the particular industry. Both power and heat (cool) are considered.

There are also various ways the components of hybrid energy systems can interact with each other. Besides components outlined above, it is the nature of interactions that sets different hybrid energy systems apart from each other. As shown in detail in Chapter 6, the nature of interactions will dictate the details of the hybrid energy implementations.

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