Elsevier

Solar Energy

Volume 108, October 2014, Pages 576-591
Solar Energy

Multi-objective optimization of a novel solar-based multigeneration energy system

https://doi.org/10.1016/j.solener.2014.07.022Get rights and content

Highlights

  • Comprehensive thermodynamic modeling of a multi-generation system is reported.

  • New solar based multi-generation systems are proposed for environmentally benign applications.

  • Apply a multi-objective optimization technique based on a code developed in the Matlab software.

  • A sensitivity analysis to see the effect of design parameters on objective functions is conducted.

Abstract

A new multigeneration system based on an ocean thermal energy conversion system and equipped with flat plate and PV/T solar collectors, a reverse osmosis desalination unit to produce fresh water, a single effect absorption chiller and a PEM electrolyzer is proposed and thermodynamically assessed. Energy and exergy analyses are employed to determine the irreversibilities in each component and assess system performance. A multi-objective optimization method based on a fast and elitist non-dominated sorting genetic algorithm (NSGA-II) is applied to determine the best design parameters for the system. The two objective functions utilized in the optimization are the total cost rate of the system, which is the cost associated with fuel, component purchasing and environmental impact, and the system exergy efficiency. The total cost rate of the system is minimized while the cycle exergy efficiency is maximized using an evolutionary algorithm. To provide further insight, the Pareto frontier is shown for a multi-objective optimization. In addition, a closed form relation between exergy efficiency and total cost rate is derived, and a sensitivity analysis is performed to assess the effects of several design parameters on the system total exergy destruction rate, total cost rate and exergy efficiency.

Introduction

Global electrical energy generation capacity is expected to grow by 70% from 2010 to 2030, with renewable electrical energy generation capacity increasing from 162 GW to 1019 GW worldwide over this time period (Martinot et al., 2007). Renewable energy, derived from such natural resources as sunlight, wind, rain, tides, waves, geothermal heat and biomass, are naturally replenished when used. About 16% of global final energy consumption is associated with renewable energy resources, with 10% of this amount from traditional biomass (mainly used for heating) and 3.4% from hydroelectricity (Dresselhaus and Thomas, 2001). Other renewable energy forms (small hydro, biomass, wind, solar, geothermal) account for approximately 3% of the total but their use is growing. A large amount of solar energy is stored as internal energy in the surface waters of the world’s oceans, providing a source of renewable energy. Ocean thermal energy conversion (OTEC), a method for harvesting energy from the oceans, is a process for harnessing this renewable energy in which a heat engine operates between the relatively warm ocean surface, which is exposed to the sun, and the colder (about 5 °C) water deeper in the ocean, in order to produce electricity.

OTEC usually incorporates a low-temperature Rankine cycle which boils a working fluid such as ammonia to generate a vapor, which rotates a turbine to generate electricity. The working fluid then is condensed back to a liquid in a continuous process. 80% of the energy that is received from the sun by the earth is stored in the world’s oceans (Tchanche et al., 2011, Faizal and Rafiuddin Ahmed, 2011), and many regions of the world have access to this OTEC resource. OTEC can produce fuels by using the generated electricity to produce hydrogen, which can be used in hydrogen fueled cars as well as in the development of synthetic fuels. For a small city, millions of tons of CO2 are generated annually through fossil fuel use while with OTEC the value is zero or almost zero, during the operation of devices. OTEC has the potential to replace some fossil fuel use, perhaps via OTEC ships travelling the seas of the world.

An OTEC system utilizes low-grade energy and typically has a low energy efficiency (approximately 5–10%). Therefore, achieving a high electricity generating capacity with OTEC requires the use of large quantities of seawater and, correspondingly, a high pumping power. These factors detract from the cost-effectiveness of this technology, making it non-viable commercially today. In order to improve the effectiveness and economics of OTEC cycles, it is proposed to integrate them with industrial operations so that, apart from generating electricity, they can be used for fresh water production, air conditioning and refrigeration, and hydrogen production (Tchanche et al., 2011). Potential sites for OTEC have been identified, mainly in the Pacific ocean, and about 50 countries are examining its implementation as a sustainable source of energy and fresh water, including India, Korea, Palau, Philippines, the U.S. and Papua New Guinea (Meegahapola et al., 2007). In 2001, as a result of cooperation between Japan and India, a 1-MW OTEC plant was built in India (Meegahapola et al., 2007), and others are planned to be constructed in the near future (Esteban and Leary, 2012).

Some research has been directed to the development of OTEC recently. Uehara (Uehara and Nakaoka, 1984, Uehara and Ikegami, 1990, Uehara et al., 1996) conducted numerous theoretical and experimental studies on the major components of an OTEC plant, and showed that ammonia is a suitable working fluid for an OTEC plant employing a closed organic Rankine cycle (ORC). The energy efficiency of the Rankine cycle in an OTEC plant is usually no more than 5%, due to the small temperature differences between surface waters and deep waters of the ocean. Thus, in order to improve the efficiency of OTEC, other thermodynamic cycles are being considered such as the Kalina and Uehara cycles, which use an ammonia–water mixture as the working fluid (Yamada et al., 2009); these cycles are reported to have higher energy efficiencies than a Rankine cycle operating with the same temperature difference (Yamada et al., 2009). Increasing the temperature difference between the heat source and the colder heat sink can improve the efficiency of OTEC plants, as can the integration of OTEC with other energy technologies. Saitoh and Yamada (Yamada et al., 2009) proposed a conceptual design of a multiple Rankine-cycle system using both solar thermal energy and ocean thermal energy in order to improve efficiency.

The overall efficiency of conventional power plants that use a fossil fuel with a single prime mover is usually less than 40%, although advanced plants can achieve higher values. That is, more than 60% of the heating value of the fuel entering a conventional power plant is lost. Multigeneration energy systems, which in general produce several useful outputs from one or more kinds of energy inputs, can provide many benefits, including increased efficiency and correspondingly reduced thermal losses and wastes, reduced greenhouse gas emissions, reduced operating costs, shorter transmission lines, fewer distribution units, multiple generation options, increased reliability, and reduced likelihood of grid failure.

Ahmadi et al. (2014) carried out an exergy-based optimization of a multigeneration energy system, containing a gas turbine as a prime mover, for electricity generation, heating, cooling and domestic hot water production. They applied a multi-objective evolutionary based optimization to find the best design parameters of the system, considering exergy efficiency and total system cost as the two objective functions. Dincer and Zamfirescu (2012) performed energy and exergy analyses of renewable energy-based multigeneration, considering several options for such products as electricity, heat, hot water, cooling, hydrogen and fresh water. Ozturk and Dincer (2012) conducted a thermodynamic analysis of a solar-based multigeneration system with hydrogen production. The solar-based multigeneration considered for this analysis consists of four main sub-systems: Rankine cycle, organic Rankine cycle, absorption cooling and heating, and hydrogen production and utilization. The exergy efficiencies and destructions for the subsystems and the overall system show that parabolic dish collectors have the highest exergy destruction rate of the components of the solar-based multigeneration system.

Ahmadi et al. (2013) assessed and optimized a novel integrated multigeneration system for residential buildings. The proposed multigeneration system included a heat recovery unit, an organic Rankine cycle, an ejector refrigeration cycle, a domestic water heater and a proton exchange membrane (PEM) electrolyzer, and the application was a two-story residential detached house in Brampton, Ontario, Canada. Ozturk and Dincer (2013) performed a thermodynamic assessment of an integrated solar power tower and coal gasification system for multigeneration, including a parametric study to determine the effect of design parameters on energy and exergy efficiencies of the system.

The main purposes of using multigeneration often are to increase efficiency and sustainability and to reduce environmental impact (including global warming) and cost, and the research reported to date suggests that multigeneration can support these purposes.

The primary objective of the present research is to improve understanding of multigeneration, by performing thermodynamic modeling and exergy and economic analyses, of an integrated solar and OTEC based multigeneration system based on flat plate and PV/T solar collectors, a reverse osmosis (RO) desalination unit to produce fresh water, a single effect absorption chiller, an OTEC cycle and a PEM electrolyzer that produces hydrogen. Such multigeneration system can be complicated and expensive, but it may nevertheless be beneficial for a coastal area where the needs of several useful commodities exist simultaneously largely because it is based on renewable energy. Therefore, considering environmental costs is necessary when comparing this integrated energy system with a natural gas fired combined cycle power plant. In addition, the exergy efficiency of this system is higher than that for the natural gas-based system, as it generates electricity with a low temperature. As a consequence, this study stresses the importance of integrated renewable-based energy systems for sustainable development. The main steps of this study are:

  • To thermodynamically model this integrated energy system.

  • To conduct exergy and exergoeconomic analyses of this integrated system.

  • To apply a multi-objective optimization technique based on a code developed in the Matlab software program using an evolutionary algorithm.

  • To propose a new closed-form expression for the exergy efficiency in terms of total cost rate at the optimal design point.

  • To derive an equation for the Pareto optimal points curve that can serve as an aid for designing optimal multigeneration plants.

  • To perform sensitivity analyses of the variation of each objective function with the main design parameters of the system.

  • To select the final optimum design point using a decision-making method.

Section snippets

System description

Fig. 1 shows a schematic of an integrated OTEC system equipped with flat plate and PV/T solar collectors, a reverse osmosis (RO) desalination unit, a single effect absorption chiller and a PEM electrolyzer. This integrated system uses the warm surface seawater to evaporate a working fluid (ammonia in this study, with the thermodynamic properties listed in Table 1), which drives an ORC turbine to produce electricity, which in turn is used to drive a PEM electrolyzer to produce hydrogen. The

Thermodynamic modeling and analysis

For thermodynamic modeling, the multigeneration system considered here (Fig. 1) is divided into five sub-systems: (1) OTEC system, (2) solar systems, (3) single-effect absorption chiller, (4) proton exchange membrane (PEM) electrolyzer, and (5) reverse osmosis desalination unit. We determine the temperature profile in the multigeneration plant, input and output enthalpies, exergy flows, destructions and efficiencies, and environmental impacts. The relevant energy balances and governing

Exergy analysis

Exergy analysis can help develop strategies and guidelines for more efficient and effective use of energy and material, and can be utilized for many processes, including power generation, CHP, trigeneration and multigeneration. The exergy of a substance is often divided into four components. Two common ones are physical and chemical exergy. The two others, kinetic and potential exergy, are assumed to be negligible here, as elevation changes are small and speeds are relatively low (Kotas, 1986,

Economic analysis

The renewable energy-based multigeneration system in Fig. 1 has various components. In this section, the purchase cost functions of each subsystem is explained.

Multi-objective optimization

A multi-objective optimization method based on an evolutionary algorithm is applied to the multigeneration system for heating, cooling, electricity, hot water, fresh water and hydrogen to determine the best design parameters for the system. Objective functions, design parameters and constraints, and overall optimization are described in this section.

Results and discussion

The genetic algorithm optimization is performed for 300 generations, using a search population size of M = 100 individuals, a crossover probability of pc = 0.9, a gene mutation probability of pm = 0.035 and a controlled elitism value c = 0.55. The results of the optimization are given and described below.

Conclusions

The thermodynamic modeling and multi-objective optimization reported here of a multigeneration energy system provides useful information. A calculus-based optimization approach using evolutionary algorithms (i.e. genetic algorithms) allows multi-objective optimization of the multigeneration plant. Fitting a curve on the optimized points provides a closed form equation. The results show that system performance is notably affected by the mass flow rate of warm ocean surface water, solar radiation

Acknowledgement

The authors acknowledge the support provided by the Natural Sciences and Engineering Research Council of Canada.

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