Journal of Fundamentals of Renewable Energy and Applications
Open Access

Comparative Thermodynamic Assessment of Wind and Hydropower Systems in Renewable Energy Portfolios

Yasutada Uchida^{* *}Correspondence: Yasutada Uchida, Department of Renewable Energy, Kyoto University, Kyoto, Japan, Email:

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Description

The fundamental and indispensable role of thermodynamics in the design, analysis and optimization of renewable energy processes cannot be overstated and it is the bedrock upon which our pursuit of sustainable energy systems is built, dictating theoretical limits, revealing avenues for efficiency improvements and guiding the very architecture of a net-zero future. This profound scientific discipline, encompassing the First and Second Laws, provides the essential framework for understanding energy conversion, transfer and transformation across the entire spectrum of renewable technologies from the quantum mechanics of solar panel cells to the massive scale of hydropower generation. Crucially, the first law of thermodynamics, asserting the conservation of energy, mandates a meticulous accounting of all energy inputs, outputs, and losses within any renewable system, ensuring that no energy is created or destroyed, merely converted from one form to another.

However, it is the Second Law of Thermodynamics, with its unwavering dictate of increasing entropy and the inevitability of exergy destruction that truly illuminates the challenges and opportunities in achieving practical and efficient renewable energy conversion.

Wind energy, while seemingly distinct, is also profoundly governed by fundamental thermodynamic principles, particularly those relating to fluid dynamics and energy conservation. The relentless engineering efforts in aerodynamics and turbine design are thus an ongoing battle against these thermodynamic constraints and the myriad irreversibilities inherent in real-world operation. These include blade drag, wake effects that reduce the kinetic energy available to downstream turbines and the inevitable electrical and mechanical losses within the gearbox and generator, all contributing to entropy generation and exergy destruction.

Geothermal energy harnesses the earth internal heat, operating on thermodynamic principles remarkably similar to solar thermal systems but with a constant, naturally occurring heat source. The extraction of heat from geothermal reservoirs and its conversion into electricity via power plants (flash, dry steam, or binary cycles) is fundamentally constrained by the Carnot efficiency, much like CSP. The critical challenge lies in efficiently transferring this high-quality thermal energy to a working fluid and then through a thermodynamic cycle, such as the Organic Rankine Cycle (ORC) for lower-temperature resources, to maximize exergy extraction. Minimizing pressure drops, optimizing heat exchanger performance, and selecting appropriate working fluids with favorable thermodynamic properties are all direct applications of thermodynamic analysis aimed at reducing irreversibilities and maximizing the net power output.

Hydroelectric power, a basis of renewable energy, converts the gravitational potential energy of water into electricity, a process largely dictated by the first law and principles of fluid mechanics, though entropy generation remains a critical consideration. The potential energy of water stored at height is converted to kinetic energy as it flows through penstocks, then to mechanical energy via turbines, and finally to electrical energy by generators. While not directly a "heat engine" in the conventional sense, the efficiency of this conversion is limited by frictional losses within the penstocks, turbulence in the water flow and mechanical, electrical losses in the turbine-generator assembly, all of which represent exergy destruction and an increase in entropy. Maximizing the head and flow rate, while minimizing hydraulic losses, is a direct application of energy and momentum conservation, ultimately aimed at delivering the highest possible exergy to the grid. Biomass and biofuels directly combusted for heat and power, gasified into syngas or fermented into liquid fuels, the underlying principles involve the enthalpy of reaction, the heat of combustion and the inevitable entropy generation during highly irreversible chemical transformations.