Thermodynamic performance assessment of ocean thermal energy conversion based hydrogen production and liquefaction process

In the proposed study, the thermodynamic performance assessment of ocean thermal energy conversion (OTEC) based hydrogen generation and liquefaction system are evaluated. In this context, the energetic and exergetic analyses of integrated system are conducted for multigeneration. This integrated process is consisted of the heat exchangers, turbine, condenser, pumps, solar collector system, hot storage tank, cold storage tank and proton exchange membrane (PEM) electrolyzer. In addition to that, the impacts of different design indicators and reference ambient parameters on the exergetic performance and exergy destruction rate of OTEC based hydrogen production system are analyzed. The energetic and exergetic efficiencies of integrated system are founded as 43.49% and 36.49%, respectively.     

Exergetic performance assessment of a stenter system in a textile finishing mill

Drying of textiles is one of the energy‐intensive unit operations and stenters are the most widely used drying machines in textile finishing mills. This study reveals energetic and exergetic analysis of a stenter system in a textile finishing factory based on actual operational data. The system includes a stenter along with its circulating and induced draft fans, a hot oil boiler and an oil circulating pump. The exergy destructions in each of the components of the overall systems were determined for average values of experimentally measured parameters. Exergy efficiencies of the system components were determined which help in assessing their performance and to establish strategies for improvement. The exergetic efficiencies of the stenter and hot oil boiler were found to be 28.7 and 34.7%, respectively, while the overall exergy efficiency of the system was obtained to be 34.4%. Copyright © 2007 John Wiley & Sons, Ltd.     

Modeling and analysis of energetic and exergetic efficiencies of a LiBr/H<Subscript>2</Subscript>0 absorption heat storage system for solar space heating in buildings

The development of efficient long-term heat storage systems could significantly increase the use of solar thermal energy for building heating. Among the different heat storage technologies, the absorption heat storage system seems promising for this application. To analyze the potential of this technology, a numerical model based on mass, species, energy, and exergy balances has been developed. The evolution over time of the storage imposes a transient approach. Simulations were performed considering temperature conditions close to those of a storage system used for space heating coupled to solar thermal collectors (as the heat source), with ground source heat exchangers (as the cold source). The transient behavior of the system was analyzed in both the charging and discharging phases. This analysis highlights the lowering of energetic and exergetic performance during both phases, and these phenomena are discussed. The thermal efficiency and the energy storage density of the system were determined, equal to 48.4 % and 263 MJ/m³, respectively. The exergetic efficiency is equal to 15.0 %, and the exergy destruction rate is 85.8 %. The key elements in terms of exergy destruction are the solution storage tank, the generator, and the absorber. The impact of using a solution heat exchanger (SHX) was studied. The risk of the solution crystallizing in the SHX was taken into account. With a SHX, the thermal efficiency of the system can reach 75 %, its storage density was 331 MJ/m³, and its exergetic efficiency and exergy destruction rate was 23.2 and 77.3 %, respectively.     

Exergetic performance coefficient analysis of a simple fuel cell system

In this paper, a fuel cell power generation system fed by hydrogen is analyzed by different performance criteria over the entire range of potential operating conditions. First law efficiency and net power output are considered for conventional energetic indices of performance, and exergy destruction rate is taken into consideration as an exergetic performance criteria. A new exergetic criterion called the exergetic performance coefficient (EPC) is introduced and is applied to the system model based on zero-dimensional approach. The system model consists of the following components: fuel cell stack, afterburner, fuel and air compressors, and heat exchangers. The effects of the operating conditions on the system performance are studied parametrically. The obtained results based on the exergetic performance coefficient criterion are compared with first law efficiency, power output and exergy destruction rate. Results show that design insights of fuel cell systems can be considerably improved when conventional energetic analyses are supplemented with EPC criterion.     

Thermoeconomic assessment of an absorption refrigeration and hydrogen-fueled diesel power generator cogeneration system

The thermoeconomic assessment of a cogeneration application that uses a reciprocating diesel engine and an ammonia–water absorption refrigeration system for electrical power and cold production from hydrogen as fuel is presented. The purpose of the assessment is to get both exergetic and exergoeconomic costs of the cogeneration plant products at different load conditions and concentrations of hydrogen–diesel oil blends. The exhaust gas of the reciprocating diesel engine is used as an energy source for an ammonia–water absorption refrigeration system. The reciprocating diesel engine was simulated using the Gate Cycle™ software, and the ammonia–water absorption refrigeration system simulation and the thermoeconomic assessment were carried out using the Engineering Equation Solver software (EES). The results show that engine combustion is the process of higher exergy destruction in the cogeneration system. Increased hydrogen concentration in the fuel increases the system exergetic efficiency for all load conditions. Exergy destruction in the components of the ammonia–water absorption refrigeration system is increased with increasing load due to the rise of heat transfer. At intermediate and high loads energy efficiency is increased in the power system, and low values of unit exergetic cost and competitive specific exergoeconomic costs are noticed. The cogeneration system operation at intermediate and high engine loads was proven to be feasible.     

Thermoeconomic analysis method for optimization of combined heat and power systems—part II

In this paper, a thermoeconomic functional analysis method based on the Second Law of Thermodynamics and applied to analyze four cogeneration systems is presented. The objective of the developed technique is to minimize the operating costs of the cogeneration plant, namely exergetic production cost (EPC), assuming fixed rates of electricity production and process steam in exergy base. In this study a comparison is made between the same four configurations of part I. The cogeneration system consisting of a gas turbine with a heat recovery steam generator, without supplementary firing, has the lowest EPC.     

An investigation of a household size trigeneration running with hydrogen

This study examined the performance and emission characteristics of a household size trigeneration based on a diesel engine generator fuelled with hydrogen comparing to that of single generation, cogeneration using ECLIPSE simulation software. In single generation simulation, the engine genset is used to produce electricity only and the heat from the engine is rejected to the atmosphere. In cogeneration and trigeneration, in addition to the electricity generated from the genset, the waste heat rejected from the hot exhaust gases and engine cooling system, is captured for domestic hot water supply using heat exchangers and hot water tank; and a part of the waste heat is used to drive absorption cooling in trigeneration. Comparisons have been made for the simulated results of these three modes of operation for hydrogen and diesel. The results prove that hydrogen is a potential energy vector in the future which is a key to meeting upcoming stringent greenhouse gases emissions. The study show that hydrogen has very good prospects to achieve a better or equal performance to conventional diesel fuel in terms of energetic performance, and a near zero carbon emission, depending on the life cycle analysis of the way the hydrogen is produced. The results also show enormous potential fuel savings and massive reductions in greenhouse gas emissions per unit of useful energy outputs with cogeneration and trigeneration compared with that of single generation.     

Parametric study of exergetic efficiency of a small-scale cogeneration plant incorporating a heat pump

A comparative parametric analysis is carried out of a small-scale combined heat and power plant incorporating a heat pump and the conventional system in which heat is produced in a hot water boiler and electrical energy is drawn from the power grid. Relative exergetic efficiency is defined as the quotient of exergetic (rational) efficiencies of the cogeneration plant and the related conventional system. Dependence of this efficiency on the power-to-heat ratio for chosen values of parameters characterizing the compared systems is calculated and shown pictorially.     

Comments on derivation of an index for evaluating economics of cogeneration systems and its applications

Industrial cogeneration systems usually must satisfy a power load and heat loads at different temperatures. The limitations of the economic index proposed by Pak and Suzuki for such cogeneration systems is discussed in this paper. The importance of a rational exergetic basis for evaluation of different grades of energy is emphasised. Thermodynamic criteria, e.g. the exergetic efficiency, relative fuel savings and fuel chargeable to power, are shown to provide useful information regarding cogeneration options. Any assessment scheme for cogeneration schemes must incorporate thermodynamic criteria in addition to economic criteria.     

An experimental study on the heat recovery from the exhaust air of stenters with a packed bed column

Stenters are the dryers that are commonly used in textile finishing mills. The exhaust air of the stenters is of great potential of energy saving via heat recovery mainly using the latent heat of condensation of the water vapour involved. This study reveals the usage possibility of a packed bed column for the heat recovery from the exhaust air of the stenter. An experimental investigation was carried out with a laboratory‐scale counter flow packed bed column. Additionally, an exergetic evaluation of the system was performed. Water inlet temperature and relative humidity ratio of the exhaust air are the main factors that affect the water outlet temperatures and efficiency of the system. It was observed that a decrease in the water inlet temperature increases the recovered exergy; however, water outlet temperature is not significantly affected by the inlet temperature of the water. The exergy efficiency of the system increases with the increase in the relative humidity ratio of the exhaust air. Copyright © 2007 John Wiley & Sons, Ltd.     

A new integrated renewable energy system for clean electricity and hydrogen fuel production

In this paper, a new renewable energy-based cogeneration system for hydrogen and electricity production is developed. Three different methods for hydrogen production are integrated with Rankine cycle for electricity production using solar energy as an energy source. In addition, a simple Rankine cycle is utilized for producing electricity. This integrated system consists of solar steam reforming cycle using molten salt as a heat carrier, solar steam reforming cycle using a volumetric receiver reactor, and electrolysis of water combined with the Rankine cycle. These cycles are simulated numerically using the Engineering Equation Solver (EES) based on the thermodynamic analyses. The overall energetic and exergetic efficiencies of the proposed system are determined, and the exergy destruction and entropy generation rates of all subcomponents are evaluated. A comprehensive parametric study for evaluating various critical parameters on the overall performance of the system is performed. The study results show that both energetic and exergetic efficiencies of the system reach 28.9% and 31.1%, respectively. The highest exergy destruction rates are found for the steam reforming furnace and the volumetric receiver reforming reactor (each with about 20%). Furthermore, the highest entropy generation rates are obtained for the steam reforming furnace and the volumetric receiver reforming reactor, with values of 174.1 kW/K and 169.3 kW/K, respectively. Additional parametric studies are undertaken to investigate how operating conditions affect the overall system performance. The results report that 60.25% and 56.14% appear to be the highest exergy and energy efficiencies at the best operating conditions.     

EXERGETIC EVALUATION OF GAS TURBINE COGENERATION SYSTEMS FOR DISTRICT HEATING AND COOLING

A cogeneration system (CGS) generating both power and heat for district heating and cooling is required to be able to cope efficiently with its heat demand change. In this paper, two types of gas turbine CGSs were investigated: (1) a CGS using a dual fluid cycle; and (2) a CGS using a combined cycle. Exergy flows at various points of each CGS have been evaluated when its heat demand is changed. The following have been shown through simulation studies: (a) the higher the heat supply ratio, the higher the exergetic efficiency of the dual fluid cycle CGS; (b) the lower the heat supply ratio, the higher the exergetic efficiency of the combined cycle CGS; and (c) the highest exergetic efficiency of the dual fluid cycle CGS at the maximum heat supply operation is higher than that of the combined cycle CGS; and the exergetic efficiency of the combined cycle CGS at the minimum heat supply operation is higher than that of the dual fluid cycle CGS. A simple criterion has also been derived for determining which type of CGS has higher average exergetic efficiency for a specific district when its heat demand characteristics are known. © 1997 by John Wiley & Sons, Ltd.     

Energy and exergy analyses of an ice-on-coil thermal energy storage system

In this study, energy and exergy analyses are carried out for the charging period of an ice-on-coil thermal energy storage system. The present model is developed using a thermal resistance network technique. First, the time-dependent variations of the predicted total stored energy, mass of ice, and outlet temperature of the heat transfer fluid from a storage tank are compared with the experimental data. Afterward, performance of an ice-on-coil type latent heat thermal energy storage system is investigated for several working and design parameters. The results of a comparative study are presented in terms of the variations of the heat transfer rate, total stored energy, dimensionless energetic/exergetic effectiveness and energy/exergy efficiency. The results indicate that working and design parameters of the ice-on-coil thermal storage tank should be determined by considering both energetic and exergetic behavior of the system. For the current parameters, storage capacity and energy efficiency of the system increases with decreasing the inlet temperature of the heat transfer fluid and increasing the length of the tube. Besides, the exergy efficiency increases with increasing the inlet temperature of the heat transfer fluid and increasing the length of the tube.     

Thermal gasification or direct combustion? Comparison of advanced cogeneration systems in the sugarcane industry

We compare different cogeneration system scenarios for efficient energy production from bagasse fuel in an Indonesian sugar and ethanol factory. These scenarios include the use of condensing-extraction steam turbines, variable speed electric drives for process equipment, measures to reduce low pressure steam demand for process needs, and two advanced cogeneration systems. One advanced system includes an 80 bar high pressure direct combustion steam Rankine cycle (advanced SRC), while the other uses a biomass integrated gasifier combined cycle (BIGCC); both utilize fuel dryers. Using steady-state thermodynamic models, we estimate that the net electricity generation potentials of the BIGCC and advanced SRC systems are approximately seven and five times the potential of the existing factory, respectively. The maximum net electricity generation potentials for the respective systems are 170 kWh/tc (BIGCC) and 140 kWh/tc (advanced SRC). However, the BIGCC system needs a bagasse feed rate that is 50 percent higher than the advanced SRC system to satisfy the factory low pressure steam demand for sugar and ethanol processing, which may affect its ability to provide steam and electricity during the off-season. For the Indonesian sugar factory, the annual revenue potential of the BIGCC system is US$14 million per year, approximately 50 percent higher than that of the advanced SRC system (electricity sale rate: US$45/MsWh; carbon credit price: US$13.60). BIGCC technology is still in an early stage of development and there are no commercial systems in sugar factories, so an advanced SRC system may be a more suitable option in the near future.     

Investigation of a new integrated system for multiple outputs with hydrogen and methanol

In this study, an integrated multigeneration system that can produce hydrogen, electricity, heat, and methanol simultaneously is thermodynamically investigated. This integrated multigeneration system consists of three subsystems, namely: (i) electrolyzer, (ii) thermal power plant; and (iii) methanol production reactor. Energy and exergy analyses of all system components, as well as the sustainability analysis of the whole system, is performed thoroughly. The integrated system's thermodynamic performance is thoroughly investigated by changing some critical operational and environmental parameters in parametric studies. Based on the results of this study, recommendations for better energetic, exergetic, and environmental performance are presented for better sustainability. The results of this study show that the integrated multigeneration system is capable of producing hydrogen, heat, electricity, and methanol with overall energetic and exergetic efficiencies about 68% and 47%, respectively.     

Combined heat and power generation via hybrid data center cooling-polymer electrolyte membrane fuel cell system

Although there has been a lot of waste heat utilization studies for the air-cooled data center (DC) systems, the waste heat utilization has not been studied for the liquid-cooled DC systems, which have been rapidly gaining importance for the high-performance Information and Communication Technology facilities such as cloud computing and big data storage. Compared to the air-cooled systems, higher heat removal capacity of the liquid-cooled DC systems provides better heat transfer performance; and therefore, the waste heat of the liquid-cooled DC systems can be more efficiently utilized in the low-temperature and low-carbon energy systems such as electricity generation via polymer electrolyte membrane (PEM) fuel cells. For this purpose, the current study proposes a novel hybrid system that consists of the PEM fuel cell and the two-phase liquid-immersion DC cooling system. The two-phase liquid immersion DC cooling system is one of the most recent and advanced DC cooling methods and has not been considered in the DC waste heat utilization studies before. The PEM fuel cell unit is operated with the hydrogen and compressed air flows that are pre-heated in the DC cooling unit. Due to its original design, the hybrid system brings its own original design criteria and limitations, which are taken into account in the energetic and exergetic assessments. The power density of the PEM fuel cell reaches up to 0.99 kW/m² with the water production rate of 0.0157 kg/s. In the electricity generation case, the highest energetic efficiency is found as 15.8% whereas the efficiency increases up to 96.16% when different multigeneration cases are considered. The hybrid design deduces that the highest exergetic efficiency and sustainability index are 43.3% and 1.76 and they are 9.4% and 6.6% higher than exergetic and sustainability performances of the stand-alone PEM fuel cell operation, respectively.     

Finite time thermodynamics study and exergetic analysis of ammonia–water absorption systems

This paper presents an optimization study of a single stage absorption machine operating with an ammonia–water mixture under steady state conditions. The power in the evaporator, the temperatures of the external fluids entering the four external heat exchangers as well as the effectiveness of these heat exchangers and the efficiency of the pump are assumed fixed. The results include the minimum value of the total thermal conductance UA_tot as well as the corresponding mean internal temperatures, overall irreversibility and exergetic efficiency for a range of values of the coefficient of performance (COP). They show the existence of three optimum values of the COP: the first minimises UA_tot, the second minimises the overall irreversibility and the third maximises the exergetic efficiency. They also show that these three COP values are lower than the maximum COP which corresponds to the convergence of the internal and external temperatures towards a common value. The influence of various parameters on the minimum thermal conductance of the heat exchangers and on the corresponding exergy efficiency has also been evaluated. From an exergetic viewpoint it is interesting to reduce the temperature at the desorber and at the evaporator and to raise the values of that parameter at the condenser and the absorber. However these changes must be accompanied by an important increase in the total UA if it is desired to conserve a constant COP. The internal heat exchangers between the working fluid and the solution improve both the overall exergy efficiency and the coefficient of performance of the absorption apparatus.     

Partitioning of MCHP‐TES system run time to incorporate transient system behaviour: a comprehensive exergy‐based analysis using simulation

By combining heat and power generation, mini‐combined and micro‐combined heat and power systems (MCHP) provide an efficient, decentralised means of power generation that can complement the composition of the electricity generation mix. Dynamic tools capable of handling transient system behaviour are required to assess MCHP efficiency beyond a mere static analysis based on steady‐state design parameters. Using a simulation of a cogeneration system, we combine exergetic definitions for different operational system states to quantify the overall system efficiency continuously over the whole period of operation. The concept of exergy allows direct comparison of different forms of energy. A sensitivity analysis was performed where we quantified the effect on MCHP overall performance under varying engine rotational speed, thermal energy storage size and fluid storage temperature in a range of MCHP simulations. We found that the exergetic quantity of natural gas used by the MCHP decreased slightly at higher engine speeds (?2% to ?4%). While the total amount of electricity generated is almost constant across the range of different engine output, more thermal exergy (up to +21%) can be recovered when the engine is operating at elevated speeds. Furthermore, selection of specific optimal thermal storage fluid temperatures can aid in improving system efficiency. Copyright © 2016 John Wiley & Sons, Ltd.     

Improving the energy efficiency of conventional tumbler clothes drying systems

This paper discusses the drying processes of four different designs of household clothes tumbler dryer using electric power input. The relative advantages and disadvantages and the energy analyses of the dryers are presented. A parametric study was conducted to assess the effects of different variables on energy efficiency and specific moisture extraction rate. It is shown that the conventional tumbler dryer technology can be improved significantly by the use of heat‐recovery heat exchangers. Copyright © 2001 John Wiley & Sons, Ltd.     

Exergetic and engineering analyses of gas turbine based cogeneration systems

This paper presents exergetic and engineering analyses as well as a simulation of gas turbine-based cogeneration plants consisting of a gas turbine, heat recovery steam generator and steam turbine. The exergy analysis is based on the first and second laws of thermodynamics. The engineering analysis is based on both the methodology of levelized cost and the pay back period. To simulate these systems, an algorithm has been developed. Two cogeneration cycles, one consisting of a gas turbine and the other of a gas turbine and steam turbine and process to produce electricity and process heat have been analyzed. The results showed good agreement with the reported data.