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Second law analysis of HVAC systems for office buildings
Published in Paul Fazio, Hua Ge, Jiwu Rao, Guylaine Desmarais, Research in Building Physics and Building Engineering, 2020
The second law of thermodynamics analysis is based on exergy, referring to the portion of energy being able to do useful work. Exergy is a very useful index to evaluate the quality of energy from different sources. The exergy analysis allows for identifying where inefficiencies may occur in a system (Bridges et al. 2001). Wepfer et al. (1979) calculated the exergy efficiency of typical processes encountered in HVAC applications, such as the process of heat transfer and mixing. Tsaros et al. (1987) developed a computer model applied exergy analysis to a residential heat pump and showed the breakdown of inefficiencies due to components. They concluded that the compressor contributes to about 50% of the total exergy destruction. The second largest contributor to the exergy destruction is the fan. By using the exergy analysis, Bridges et al. (2001) analyzed the energy performance of air conditioners and domestic refrigerators, and outlined that the most inefficient components were the compressor in the case of the refrigerators, and the evaporator in the case of the air conditioners. Those results can draw engineers’ attention toward components where the most availability is being destroyed and quantify the impacts of modifications to those components.
Exergy analysis
Published in Kornelis Blok, Evert Nieuwlaar, Introduction to Energy Analysis, 2020
Kornelis Blok, Evert Nieuwlaar
In many facilities, exergy losses occur through the production, transfer, and utilisation of heat. As discussed in the previous section, these can include both internal and external exergy losses: An example of internal exergy loss is the exergy loss caused by heat transfer in a heat exchanger (see Figure 4.1). The energy loss caused by a heat exchanger is generally very small. However, the exergy loss can be very substantial: the higher the temperature difference between the media in the heat exchanger, the higher the exergy loss. Note that in a heat exchanger a certain temperature difference is always necessary as the temperature difference is the driving force for the heat exchange.External exergy losses are caused by all kinds of waste heat streams.
Hydrostatically Compensated Energy Storage Technology
Published in Jacqueline A. Stagner, David S-K. Ting, Green Energy and Infrastructure, 2020
M. Ebrahimi, D. S.-K. Ting, R. Carriveau, A. McGillis
Exergy is the portion of energy that can be used to do work and is defined as the difference between the total supplied energy and the portion that is not converted to work. Exergy analysis is a powerful tool that improves the evaluation and analyzation of thermodynamic processes by identifying their imperfections due to irreversibility. Since HC-CAES technology relies on the compression and expansion of air, thermodynamics plays a vital role in the performance of the system. Exergy analysis is one of the most powerful tools that can be applied to develop insights into the thermodynamic processes of an HC-CAES system and aid in performance modeling efforts. It effectively highlights the flow of energy in the system, areas of loss, and potential areas to focus on to improve system performance.
Research on the usability of various oxygenated fuel additives in a spark-ignition engine considering thermodynamic and economic analyses
Published in Biofuels, 2023
Murat Kadir Yesilyurt, Battal Dogan, Abdülvahap Cakmak
Exergy is the maximum useful work that can be achieved until a system reaches equilibrium, and is based on the second law of thermodynamics. The exergy balance of the test engine is given below: where the exergy of fuel is described as exergy of air as exergy of exhaust gases as cooling water exergy as exergy of thermal losses from the engine to the environment as and exergy destruction as Engine power is considered in the exergetic study.
Thermodynamic sustainability assessment for residential building heating comparing different energy sources
Published in Science and Technology for the Built Environment, 2022
Sustainable development requires not only usage of sustainable energy resources, but also the efficient use of these resources (Rosen et al. 2008). Exergy analysis is directly linked to sustainability and environmental impact of energetic processes. Hepbasli (2016) emphasized the importance of exergy analyses in his study and suggested novel exergy management approach instead of energy. In thermodynamically ideal, reversible process there is no exergy loss, the exergy efficiency has value 1, or 100%, and negative impact on the environment does not exist, in other words the process would be completely sustainable. Actual processes are irreversible and exergy destruction and losses exist. As exergy efficiency approaches value 0, sustainability approaches zero and environmental impact approaches infinity. Increasing exergy efficiency in utilization contributes to development over a longer period of time, decreasing the impact on environment. As exergy efficiency approaches 100%, environmental impact approaches zero and sustainability approaches infinity, because exergy is converted from one form to another without losses, and process approaches reversibility.
Analysis of a combined proton exchange membrane fuel cell and organic Rankine cycle system for waste heat recovery
Published in International Journal of Green Energy, 2021
Can Liu, Guokun Liu, Yanzhou Qin, Yuan Zhuang
Exergy is the maximum useful work that the system can perform when its state is brought to equilibrium with its surroundings which can be expressed as Ex. Exergy analysis is a method to evaluate the system performance based on the second law of thermodynamics and it can reveal the energy conversion quality. The exergy can be divided into energy flow exergy and mass flow exergy. As for energy flow exergy, there are exergy of heat and exergy of work. As for mass flow exergy, there are physical exergy and chemical exergy, which reflect the imbalance of physical state and chemical composition between the fluid and the environment, respectively. The specific expression of the four exergies can be found in Equations (13)–(16).