Energy Sustainability and Hybrid Energy Systems

There is a lot of discussion in both academic and industrial communities on sustainability of energy sources over a long period. There are strong reasons for this; some of which can be articulated as follows:

• 1. Energy is a commodity that everybody needs to improve their quality of life. There is only limited supply and all nations (developed and developing) want to consume more energy. This cannot happen over a long period of time without making some fundamental changes in the way we recover sources and use them. Also as mentioned earlier, no single source is ideal for sustained energy consumption.
• 2. In order to preserve sources of energy over a long period, attention to efficiency of recovery and conversion is very important. This can vary significantly from source to source. High efficiency reduces the energy and fuel consumption for the same benefits which are always desirable. Besides inefficiency associated with various conversion equipment such as PV cell, thermoelectric materials etc., thermodynamic limitations for power generation and waste heat generated in both static and mobile power generation as well as heat used in various manufacturing processes must be recovered to improve efficiency of fuel usage. All of these inevitably lead to the use of hybrid energy systems in various processes. Efficiency of energy consumption is going to become more and more important as the energy consumption in developing countries rapidly grow. While some aspects of energy efficiency are material (like PV and TPV cells, thermoelectric, etc.), equipment (used in various industrial processes), and process (like thermodynamic cycles used to convert thermal energy to electrical energy, fuel cell, etc.) related, in many cases, low efficacy results in significant amount of waste heat and materials. The improvement of efficiency will require extracting more power from waste heat or the use of waste heat by cogeneration. The conversion of waste material to more energy also means hybrid energy systems. Thus hybrid energy system by definition implies more efficient use of energy.
• 3. The cost of energy (both capital and operating) is very important because that partly determines the market it will capture. For new ventures, return on investment is also very important. Properly chosen, hybrid energy system allows better flexibility for the cost optimization. Careful optimization of hybrid sources allows each source to sustain longer.
• 4. Energy and environment protection must operate in harmony. There is a serious concern worldwide regarding the effects of carbon release to the environment by some sources and some technologies. This can seriously affect the sustainability of a particular source or technology over a long period. Carbon emission must be either prevented or treated. The treatment can lead to generation of hybrid power such as use of fuel cell to treat CO, by ExxonMobil gas-based power plants. Hybrid renewable energy system is well suited to reduce CO, emission. Environment protection will become increasingly more important for future energy usages. This means less use of fossil fuel and deeper penetration of renewable energies in the overall energy mix. Unlike fossil energy, renewable energy is in general of low density and time and location dependent (particularly solar and wind). This will force the use of multiple sources, including energy storage, to have steady and uninterrupted supply of energy which is particularly important for power generation.
• 5. To some extent, all ten sources of energy are competing with each other. Because of the rapid development of various energy technologies, it would be imprudent to rely on a single source over a sustained period of time. For example, the rapid development of solar and wind energy technologies have made them competitive in power production compared to gas- based power plants. Rapid development of fuel cell can further change this picture. Nanotechnology has further fueled this competition. In this fast changing environment, the use of hybrid energy system will ensure more sustainability.
• 6. Single source energy system generally has limited flexibility of scale and region coverage. Biomass power plants by itself cannot be on a very large scale. Solar and wind energy cannot be very large without access of large land. Since future dictates better balance between centralized and distributed energy systems, hybrid energy system can best facilitate achievement of this balance.
• 7. Hybrid storage systems and grid transport systems are necessities to counteract limitations of individual storage and large-scale grid transport systems.

8. Higher penetration of renewable sources in the overall energy mix is inevitable due to strong social and political interests in greening energy sources. This cannot occur in a flexible manner without the need for energy storage which results in hybrid energy system. Hybrid renewable energy sources are poised to best serve off-grid locations such as rural and remote communities.

The argument presented above indicates that sustainable future of energy industry requires hybrid sources, generation, storage, and transport to satisfy the energy need of both urban and rural or isolated environments. Hybrid energy sources can provide:

• 1. More options for local optimization of cost. Hybrid energy systems can be designed to achieve desired attributes at the lowest acceptable cost, which is the key to market acceptance.
• 2. Greater penetration of renewable sources in the overall energy mix. Hybrid energy systems can be designed to maximize the use of renewable resources, resulting in a system with lower emissions.
• 3. More efficient and less use of fossil fuels in the future.
• 4. More options for large centralized and small distributed operations. This will well serve both urban and rural and isolated communities. This will also allow better harvesting of distributed energy sources.
• 5. Less reliance on large-scale energy grids. Micro, mini, and nanogrids can operate independently or together with a utility grid. This along with standalone systems will help more than billion people who are currently without electricity.
• 6. More options for optimum uses of local energy resources, which can vary in different country. They provide flexibility in terms of the effective utilization of the renewable sources.
• 7. Greater role of energy storage to provide stable power and heating/cooling for all energy usages such as buildings, vehicles, district heating, industrial heating, etc.
• 8. Offsets negative aspects of many sources without their elimination from the overall mix.
• 9. Allow gradual transition to carbon-free energy world.
• 10. Less social and political tensions between available sources and need for energy for all parts of the world.
• 11. Better management of sustainable and high-quality power.
• 12. A hybrid energy system can make use of the complementary nature of various sources, which increases the overall efficiency of the system and improve its performance (power quality and reliability). For instance, combined heat and power operation, e.g., MT and FC, increases their overall efficiency or the response of an energy source with slower dynamic response (e.g., wind or FC) can be enhanced by the addition of a storage device with faster dynamics to meet different types of load requirements [,9, 12-22, 31-34.40].