Thermoeconomic analysis of a cogeneration system of a university campus
Introduction
The successive energy crises have stimulated the study of more efficient ways for the use of the available energy in fuels. As consequence new technical plants have been conceived seeking the primary energy conservation. Cogeneration may be defined as the simultaneous production of electrical or mechanical energy and useful thermal energy from a single energy source, such as oil, coal, natural or liquefied gas, biomass, or solar. By capturing or applying heat from an effluent energy source that would otherwise be rejected to the environment, cogeneration system can operate at efficiencies greater then those achieved when heat and power are produced in separate or distinct processes.
This paper presents the application of a methodology for the thermoeconomic feasibility study of the replacement of a gas turbine cogeneration plant that exists on a university campus. This system in addition to generating electricity allows the recuperation of residual heat that is utilised as a source of energy for the production of steam in a recuperation boiler.
Section snippets
The energy requirements and the cogeneration system
This study analysed the case of San Diego State University (SDSU) in the State of California, USA. San Diego State University utility plants operate 24 h a day, every day of the year. They purchase electricity and natural gas from San Diego Gas and Electric, and purchase their water and sewer services from the City of San Diego. Through campus utility plants managed by Physical Plant, they generate their own steam, chilled water, and a significant portion of their electricity with their
Energy analysis
The following equations are based on the thermodynamic principals according to the indicated procedures by Wu [1] and Taki et al. [2]. The equations are valid for any situation described in the previous items.To calculate the recovered heat flux in the form of steam Eq. (12) may be used.
Energy-economic analysis
The investment decisions are usually based on capital costs and on the payback period. The costs of electricity and steam production can be determined from the , , , [4]. These equations consider all the pertinent aspects for the construction and operation of a cogeneration plant, since they do not fasten important parameters such as the total plant capital cost including capital taxes and insurance (IPL) or interest rate (r).
Exergy-economic analysis
The development of design techniques for an energy system with minimized costs is a necessity in a world with finite natural resources and the increase of the energy demand. The presented method combines the Second Law of Thermodynamics through the exergy concept, associated to an economical approach of the thermal system. For the analysis of the cogeneration system in question, the following steps were taken:
- 1.
identification of the system functions of cogeneration as a whole and each unit
Conclusions
The feasibility of the replacement of the gas turbine system in a gas turbine cogeneration system existing on a university campus has been shown. The system design and operational parameters are important to evaluate cogeneration systems. The energy-economic study shown that the best system, based on pay-back period, was the system that used the gas turbine “M1T-06” of Kawaski Heavy Industries followed by “M1T-03” of Kawasaki Heavy Industries. The best system, based on the maximum savings (in
Acknowledgements
The authors wish to express their thanks to support of FAPESP (Fundação de Amparo a Pesquisa do Estado de São Paulo––Brazil, Process number 99/08851-0).
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