Elsevier

Solar Energy

Volume 112, February 2015, Pages 290-299
Solar Energy

Energy and exergy analyses of a solar-biomass integrated cycle for multigeneration

https://doi.org/10.1016/j.solener.2014.11.027Get rights and content

Highlights

  • New integrated multi-generation system using biomass and solar energy is proposed.

  • Energy and exergy analyses are conducted for assessment.

  • Energy and exergy efficiencies of the multi-generation system and its subsystems are studied for comparison.

  • A parametric study is carried out.

Abstract

A biomass and solar integrated system for multigeneration of useful outputs, in which two renewable energy sources are combined to produce multiple outputs (e.g., power, cooling, hot water, heated air), is developed and presented. Energy and exergy analyses are used to assess the performance of the cycle, and the effects of various system parameters on energy and exergy efficiencies of the overall system and its subsystems are examined. The overall energy and exergy efficiencies of the system are found to be 66.5% and 39.7% respectively. Furthermore, the effect is also investigated of reference-environment temperature on energy and exergy efficiencies for the system, when operated only on biomass and solar energy.

Introduction

Energy usually plays a vital role in the development of a nation. With the gradual depletion of conveniently available fossil fuel reserves, people are seeking other energy sources which are long lasting as well as environmentally benign, like renewable energy. The main renewable energy sources in use nowadays are solar, geothermal, wind, biomass and hydro (Ahmadi et al., 2013). Biomass is mainly derived from living or dead matter present on earth (Cohce et al., 2011). Solar energy can be collected in various ways, e.g., concentrated solar panel and heliostat. In this study, a heliostat field and central receiver are used.

Energy challenges can, to some extent, be overcome by using energy resources more efficiently, and sometimes this can be achieved via trigeneration and multigeneration (Chicco and Mancarella, 2008). Another way of efficiently using energy resources is to integrate them in such a manner that the deficiencies of one energy source are overcome by the other, often leading to the better utilisation of the energy resources.

A number of studies have been reported on trigeneration and multigeneration. Using energy and exergy analyses on a waste heat recovery-based trigeneration system, Khaliq et al. (2009) examined the effect of exhaust inlet gas temperature on the energy and exergy efficiencies and found that energy efficiency increases with increasing exhaust inlet gas temperature while exergy efficiency decreases. Malico et al. (2009) developed a trigeneration system to meet the demands of a hospital for electricity, heating, cooling and hot water, and performed an economic feasibility analysis. An exergy analysis of a Gas Turbine for trigeneration by Khaliq (2009) determined that the maximum exergy destruction occurs in the combustion chamber and the steam generator. Al-Sulaiman et al. (2012) performed energy and exergy analyses of a biomass trigeneration system using an organic Rankine Cycle, and found that the maximum exergy efficiency of the organic Rankine Cycle increases from 13% to 28% when they switch from single generation to trigeneration. Ozturk and Dincer (2013) studied a solar-based multigeneration system and found its exergy efficiency to be about 57%. An analysis by Dincer and Zamfirescu (2012) of renewable energy-based multigeneration systems demonstrated that the exergy efficiencies vary from 55% to 65%, depending upon the degree of cogeneration used. Ozturk and Dincer (2013) showed that, through the integration of various systems, multigeneration increases energy and exergy efficiencies. Dincer and Zamfirescu (2011) determined that renewable energy-based multigeneration reduces fuel prices and harmful pollutant emissions, compared to conventional systems. The above studies indicate that multigeneration via the integration of two renewable energy sources can be beneficial.

The specific objectives of this paper are to propose and to assess with energy and exergy analyses a new integrated multigeneration system using biomass and solar energy, including the determination of overall energy and exergy efficiencies of the multigeneration system and its subsystems; and to carry out a parametric study to determine the effects of various parameters on the overall energy and exergy efficiencies of the multigeneration system and its subsystems.

Section snippets

System description

The proposed multigeneration system (see Fig. 1) uses two renewable energy sources: solar and biomass. It contains two Rankine and two Gas Turbine Cycles, as well as a absorption cooling cycle. The main outputs of the proposed system are electric power, cooling, hot water and hot air. The most significant part of this system is concentrated solar collector. The solar energy from the sun is collected through a heliostat tower. The heliostat tower reflects the incident solar energy to the central

Thermodynamic analysis

Energy and exergy analyses are performed for the proposed multigeneration system, in order to provide the information about its performance, efficiency and emissions.

The following assumptions are made for the analysis of the integrated system:

  • The reference-environment state has a temperature T0 = 298 K and a pressure P0 = 100 kPa.

  • The changes in kinetic and potential energy and exergy terms are negligible.

  • The turbines and pumps are adiabatic.

  • The isentropic efficiencies of turbines and pumps are 85% (

Results and discussion

In performing the energy and exergy analyses of the solar biomass integrated multigeneration system, values of mass flow rate (kg/s), temperature (K), pressure (kPa), specific enthalpy (kJ/kg) and specific exergy (kJ/kg) are determined for the each state of the system (Table 2). The reference-environment conditions are taken to be the ambient conditions, for which the temperature and pressure are 298 K and 100 kPa, respectively. Thermodynamic values are calculated using Engineering Equation

Conclusions

A new renewable energy-based multigeneration system has been developed and investigated using exergy and energy analyses. The overall energy and exergy efficiencies respectively of the developed system using biomass and solar energy are 66.5% and 39.7%. However, the energy and exergy efficiencies respectively are only 64.5% and 37.6% when the biomass system operates alone, and only 27.3% and 44.3% when the solar system operates alone. These results demonstrate the system’s efficiencies are

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1

Department of Mechanical Engineering, KFUPM, Dhahran 31261, Saudi Arabia.

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