Single-stage LiBr Research Articles

Performance analysis and optimization of a trigeneration process consisting of a proton-conducting solid oxide fuel cell and a LiBr absorption chiller

In this work, the trigeneration system, consisting of a proton-conducting solid oxide fuel cell (SOFC–H+) and a single-stage LiBr absorption chiller, was proposed. The SOFC–H+ and single-stage LiBr absorption chiller models were developed through Aspen Plus V10. From the sensitivity analysis, the results show that increases in temperature and fuel utilization can improve the performance of the SOFC–H+. Conversely, the air to fuel (A/F) molar ratio and pressure negatively affect the electrical efficiency and overall system efficiency. In the case of the absorption chiller, the coefficient of performance was increased and made stable according to a constant value when the generator temperature was increased from 90 to 100 °C. When the optimization was performed, it was found that the SOFC–H+ should be operated at 700 °C and 10 bar with fuel utilization of 0.8 and A/F molar ratio of 2 to achieve a maximum overall efficiency of 93.34%. For the energy and exergy analysis, a combined heat and power SOFC–H+ was found to have the highest energy and exergy efficiencies, followed by the trigeneration process. This indicates that the integration of the SOFC–H+ and LiBr absorption chiller is possible to efficiently produce electricity, heating and cooling.

Multi-objective optimization of vapor absorption refrigeration system for the minimization of annual operating cost and exergy destruction

A low-grade energy source-driven thermal compressor-based vapor absorption refrigeration system is considered a better alternative for an electrically operated mechanical compressor-based conventional vapor compression refrigeration system. In this article, multi-objective optimization of a single-stage LiBr–H2O vapor absorption refrigeration system (VARS) and a modified VARS are explored by minimizing the total annual operating cost and exergy destruction of the system. Exergy destruction during the operation is inversely proportional to the total annual cost of the system. Multi-objective optimization can be implemented to optimize these conflicting objectives for both systems. The nonlinear multi-objective model based on the concept of exergy and economic performance are minimized using multi-objective genetic algorithm (MOGA), multi-objective particle swarm optimization (MOPSO), and multi-objective sanitized teaching learning-based optimization (MOs-TLBO) for both VARS and MVARS. The current study also compares the decision variables obtained for minimizing the total annual cost and exergy destruction in basic and modified VARS thermo-economic models. The comparative analysis suggests that MOGA, MOPSO, and MOs-TLBO can be successfully employed to achieve these objectives. MOGA determines minimum annual cost, whereas minimum exergy destruction is reported by MOPSO. On the other hand, when compared to the other two algorithms (MOGA and MOPSO), MOs-TLBO reports a minimum total annual cost of plant operation for the reported minimum exergy destruction by effectively tuning the operating temperature of the system components for both basic and modified VARS. The current study reports Pareto points, which offer an optimal annual cost and exergy destruction based on the requirements using multi-objective algorithms.

Effect of Cooling Water on the Performance of Lithium Bromide–Water (LiBr–H2O) Absorption Based Heat Pump

The need for extracting high heat from the industries has encouraged the development of Lithium Bromide-Water (LiBr-H2O) absorption based heat pump. This paper presents a simulation study of single-stage LiBr–H2O vapor absorption heat pump. The detailed thermodynamic analysis of the single-stage LiBr–H2O vapor absorption heat pump was carried out in this study. The validation of this model is performed by considering the values from the literature.The effect of cooling water on flow rates, COP (Coefficient of performance) and conductance are examined.

On-chip waste heat-driven absorption cooling system for sustainable data center environment: simulation

ABSTRACTDevelopment of waste heat-driven absorption-based cooling system is inspired for the need of removing high heat flux from the sustainable data centre environment. This paper presents a simulation study of single-stage Lithium Bromide–Water (LiBr–H2O) vapour absorption heat pump for chip cooling. In the present work, a complete thermodynamic analysis of the single-stage LiBr–H2O vapour absorption-based heat pump for chip cooling without a solution heat exchanger is performed and a user friendly graphical user interface (GUI) package including visual components is developed by using MATlab (2008b). The effect of chip heat flux on COP (Coefficient of Performance), flow rates and conductance is examined using the developed package. The model is verified using the data available in the literature which indicates that there is a greater reduction in the absorber load. The influence of chip heat flux on the performance and thermal loads of individual components are studied, and it is concluded that COP decreases from 0.7121 to 0.68924 with increase in chip heat flux.

Solar-Powered Air Conditioning for Buildings in Hot Climates: Desiccant Evaporative Cooling vs. Absorption Chiller-based Systems

In many countries, solar cooling systems are one of the best candidates to tackling global warming and summer peak loads of building air conditioning systems. In this work, based on the collaboration with the Saudi research institute “King Abdullah City for Atomic and Renewable Energy”, two types of solar driven cooling systems are investigated and compared: the open-circuit Desiccant Evaporative Cooling technology and the system based on single-stage LiBr absorption chillers. A computer code has been developed in Trnsys® to simulate a well-insulated single-family residential building, starting from a 3D model of the building architecture, and the solar cooling systems. The simulations have been carried out for two different locations: one in a dry desert area (Riyadh, KSA), the other one on the seaside (Abu Dhabi, UAE). The results show that absorption chillers need a very effective heat rejection system to work properly, whilst DEC systems exhibit a dramatic performance reduction in wet climates.

A simulation study on performance evaluation of single-stage LiBr–H2O vapor absorption heat pump for chip cooling

The growth of Lithium Bromide–Water (LiBr–H2O) absorption based heat pump is encouraged for the necessity of extracting high heat from the electronic chips. This paper presents a simulation study of single-stage LiBr–H2O vapor absorption heat pump for chip cooling. In this study, a detailed thermodynamic analysis of the single-stage LiBr–H2O vapor absorption heat pump for chip cooling in the nonexistence of solution heat exchanger was performed and a user-friendly graphical user interface (GUI) package including visual components was developed by using MATlab (2008b). The influence of chip temperature on COP (Coefficient of Performance), flow rates and conductance was examined by using the developed package. The model is validated by using the values available in the literature and indicates that there is a greater reduction in the absorber load. The influence of chip temperature on the performance and thermal loads of individual components was studied and it was concluded that, COP increases from 0.7145 to 0.8421 with an increase in chip temperature.

A novel renewable polygeneration system for a small Mediterranean volcanic island for the combined production of energy and water: Dynamic simulation and economic assessment

This paper investigates the integration of solar and geothermal energy in a novel polygeneration system producing simultaneously: electricity, thermal energy, cooling energy and fresh water. The polygeneration system under analysis includes concentrating photovoltaic/thermal solar collectors (CPVT), a Geothermal Well (GW) a multi-effect distillation (MED) system for seawater desalination, a single-stage LiBr–H2O absorption chiller and additional components, such as: storage tanks, heat exchangers and balance of plant devices. The CPVT produces simultaneously electrical energy and thermal energy, at a maximum temperature of about 100°C. The electrical energy is delivered to the grid, whereas the thermal energy can be used for different scopes. First, the thermal energy can be used for heating purposes and/or Domestic Hot Water production. As an alternative, solar thermal energy can be used to drive an absorption chiller, producing chilled water for space cooling. Finally, solar energy, in combination with the thermal energy produced by low-enthalpy (about 80°C) geothermal wells, may be used by the MED system to convert seawater into desalinated water. Geothermal energy is also used to produce Domestic Hot Water at 45°C. The system is dynamically simulated by means of a zero-dimensional transient simulation model. The simulation model also includes detailed control strategies, for the management of the different technologies included in such a complex system. The system is assumed to be operated in some of the several small volcanic islands in the Mediterranean Sea, assuming Pantelleria (Trapani, Italy) as main case study. Here, the availability of solar and geothermal energy is high whereas the availability of fresh water is scarce and its cost consequently high. Results show an excellent energetic performance of the system under investigation. From the economic point of view, the profitability of the system dramatically increases when user Domestic Hot Water demand is high.

A novel renewable polygeneration system for hospital buildings: Design, simulation and thermo-economic optimization

This paper presents an analysis of a possible energy retrofit of an existing University Hospital District, located in Naples (Italy), by using an innovative renewable polygeneration system. This system integrates both Concentrating PhotoVoltaic/Thermal collectors (CPVT) and Solar Heating and Cooling (SHC) technologies. The CPVT parabolic trough collectors are equipped with triple-junction PhotoVoltaic (PV) cells: this technology usually shows ultra-high energy conversion efficiencies. The main components of the system are: CPVT collectors, a single-stage LiBr–H2O absorption chiller, storage tanks and balance of plant devices. The system is assumed to be installed at a University Hospital District located in Naples (Italy), equipped with a gas-turbine cogeneration system. The data regarding cooling, heating and electrical demands and productions are measured for a one-year operation. The CPVT produces electrical energy, which is consumed in part by the system parasitic loads, whereas the eventual surplus is fed in the electrical grid. Simultaneously, the CPVT provides heat, used for space heating, for domestic hot water and/or to drive the absorption chiller, which produces cooling energy. The system is designed and dynamically simulated in TRNSYS environment, including detailed and validated mathematical models for the simulation of all the components. The results are analysed from both energy and economic points of views, using parametric analyses and thermo-economic optimizations. The energy performance of the system is excellent since all electrical and thermal energies produced by the renewable system are consumed by the user. The economic results show that the system can be profitable (pay-back period around 12 years) even without any public funding. In case of feed-in tariffs, the system becomes extremely profitable from the economic point of view. The thermo-economic optimization, based on a mixed heuristic/deterministic algorithm, shows that the system profitability can be further improved, increasing solar field area and decreasing storage specific volumes for m2 of collectors installed.

A novel solar trigeneration system integrating PVT (photovoltaic/thermal collectors) and SW (seawater) desalination: Dynamic simulation and economic assessment

The paper investigates the integration of renewable energy sources and water systems, presenting a novel solar system producing simultaneously: electrical energy, thermal energy, cooling energy and domestic water. Such system is designed for small communities in European Mediterranean countries, rich in renewable sources and poor in fossil fuels and water resources. The polygeneration system under analysis includes PVT (photovoltaic/thermal solar collectors), a MED (multi-effect distillation) system for SW (seawater) desalination, a single-stage LiBr–H2O ACH (absorption chiller) and additional components, such as storage tanks, AHs (auxiliary heaters) and BOP (balance of plant) devices. The PVT produces simultaneously electrical energy and thermal energy. The electrical energy is delivered to the grid, whereas the thermal energy may be used for space heating and/or domestic HW (hot water) production. As an alternative, the solar thermal energy can be used to drive an ACH, producing CHW (chilled water) for space cooling. Finally, the solar energy, in combination with the thermal energy produced by an auxiliary biomass-fired heater, may be used by the MED system to convert SW into potable water. The system is dynamically simulated by means of a zero-dimensional transient simulation model. A thermo-economic analysis is also presented, aiming at determining the optimal values of the most important design variables.

Thermoeconomic analysis of storage systems for solar heating and cooling systems: A comparison between variable-volume and fixed-volume tanks

The paper investigates different control strategies for the thermal storage management in SHC (Solar Heating and Cooling) systems. The SHC system under investigation is based on a field of evacuated solar collectors coupled with a single-stage LiBr–H2O absorption chiller; auxiliary thermal energy is supplied by a gas-fired boiler. The SHC is also equipped with a novel thermal storage system, consisting in a variable volume storage tank. It includes three separate tanks and a number of mixers and diverters managed by novel control strategies, based on combinations of series/parallel charging and discharging approaches. The aim of this component is to vary the thermal storage capacity as a function of the combinations of solar radiation availability and user thermal/cooling energy demands. The system allows one to increase the number of active tanks when the time shift between solar energy and user demand is high. Conversely, when this time shift is low, the number of active tanks is automatically reduced. In addition, when the solar energy in excess cannot be stored in such tanks, a heat exchanger is also used in the solar loop for producing DHW (Domestic Hot Water).The analysis is carried out by means of a zero-dimensional transient simulation model, developed by using the TRNSYS software. In order to assess the operating and capital costs of the systems under analysis, an economic model is also proposed. In addition, in order to determine the set of the synthesis/design variables which maximize the system profitability, a parametric analysis was implemented. The novel variable-volume storage system, in both the proposed configurations, was also compared with a constant-volume storage system from the energy and economic points of view. The results showed that the presented storage system allows one to save up to 20% of the natural gas used by the auxiliary boiler only for very high solar fractions. In all the other cases, marginal savings are achieved by the variable-volume storage tanks system.

Design and dynamic simulation of a novel solar trigeneration system based on hybrid photovoltaic/thermal collectors (PVT)

In this paper, a Solar Heating and Cooling (SHC) system including photovoltaic/thermal (PVT) collectors is considered, implementing a novel polygeneration system producing electricity, space heating and cooling and domestic hot water. In particular, PVT collectors operating up to 80°C are considered. A case study for a university building located in Naples (Italy) is developed and discussed. The system is mainly composed by: PVT collectors, a single-stage LiBr–H2O absorption chiller, storage tanks and auxiliary heaters. The system also includes additional balance-of-plant devices: heat exchangers, pumps, controllers, cooling tower, etc. The PVT produces electricity which is utilized in part by the building lights and equipments and in part by the system parasitic loads; the rest is eventually sold to the grid. Simultaneously, the PVT system provides the heat required to drive the absorption chiller. The system performance is analyzed from both energetic and economic points of view by means of a zero-dimensional transient simulation model, developed with TRNSYS. The economic results show that the system under investigation can be profitable, provided that an appropriate funding policy is available. In addition, the overall energetic and economic results are comparable to those reported in literature for similar systems.

Energy analysis of single-stage LiBr–water vapour absorption refrigeration system using artificial neural network approach

In this study, artificial neural network (ANN) has been used as a new approach for carrying out the energy analysis of a single-stage absorption refrigeration cycle with water–lithium bromide as the working fluid pair. Energy analysis of an absorption system is a very complicated process mainly because of the limited experimental data and analytical functions required for calculating the thermodynamic properties of fluid pairs, which usually involves the solution of complex differential equations. Instead of complex differential equation and limited experimental data, faster and simpler solutions were obtained by using equations derived from the ANN model. As seen from the results obtained, the calculated thermodynamic properties are within acceptable results. Thermodynamic properties of each point in the cycle are calculated using related equations of the state. Heat flow rate of each component in the cycle and some performance parameters are calculated from the first law analysis. The results show that a high coefficient of performance value is obtained at high generator and evaporator temperatures and also at low condenser and absorber temperatures.

Transient simulation of polygeneration systems based on PEM fuel cells and solar heating and cooling technologies

This paper presents a dynamic simulation of an innovative polygeneration system based on solar heating and cooling and PEM fuel cells technologies. The polygeneration system is based on the following main components: evacuated solar collectors, single-stage LiBr–H2O absorption chiller and a PEM fuel cell. The fuel cell operates at full load producing electrical energy which is in part consumed by the building lights and equipments. The fuel cell is grid connected in order to perform a convenient net metering. Finally, the system also includes heat exchangers producing domestic hot water in case of scarce space heating/cooling demand. The analysis was carried out by means of a transient simulation model, developed using TRNSYS software and includes the investigation of the dynamic behavior of the building, developed in TRNBUILD. A small university hall, including also a fitness center, was adopted as test case. Energetic and economic models were also developed, in order to assess primary energy savings and the operating and capital costs of the systems under analysis. The results of the case study were analyzed on monthly and yearly basis, paying special attention to the energetic and monetary flows. The results are excellent from the energy saving point of view. On the other hand, the pay back periods can be profitable for the final user only in case of significant public funding.

Transient analysis and energy optimization of solar heating and cooling systems in various configurations

In this paper, a transient simulation model of solar-assisted heating and cooling systems (SHC) is presented. A detailed case study is also discussed, in which three different configurations are considered. In all cases, the SHC system is based on the coupling of evacuated solar collectors with a single-stage LiBr–H 2O absorption chiller, and a gas-fired boiler is also included for auxiliary heating, only during the winter season. In the first configuration, the cooling capacity of the absorption chiller and the solar collector area are designed on the basis of the maximum cooling load, and an electric chiller is used as the auxiliary cooling system. The second layout is similar to the first one, but, in this case, the absorption chiller and the solar collector area are sized in order to balance only a fraction of the maximum cooling load. Finally, in the third configuration, there is no electric chiller, and the auxiliary gas-fired boiler is also used in summer to feed the absorption chiller, in case of scarce solar irradiation. The simulation model was developed using the TRNSYS software, and included the analysis of the dynamic behaviour of the building in which the SHC systems were supposed to be installed. The building was simulated using a single-lumped capacitance model. An economic model was also developed, in order to assess the operating and capital costs of the systems under analysis. Furthermore, a mixed heuristic-deterministic optimization algorithm was implemented, in order to determine the set of the synthesis/design variables that maximize the energy efficiency of each configuration under analysis. The results of the case study were analyzed on monthly and weekly basis, paying special attention to the energy and monetary flows of the standard and optimized configurations. The results are encouraging as for the potential of energy saving. On the contrary, the SHC systems appear still far from the economic profitability: however, this is notoriously true for the great majority of renewable energy systems.

Thermoeconomic analysis and optimization of high efficiency solar heating and cooling systems for different Italian school buildings and climates

The paper investigates the energetic and economic feasibility of a solar-assisted heating and cooling system (SHC) for different types of school buildings and Italian climates. The SHC system under investigation is based on the coupling of evacuated solar collectors with a single-stage LiBr–H 2O absorption chiller; auxiliary energy for both heating and cooling is supplied by an electric-driven reversible heat pump. The SHC system was coupled with different types of school buildings located in three different Italian climatic zones. The analysis is carried out by means of a zero-dimensional transient simulation model, developed using the TRNSYS software; the analysis of the dynamic behaviour of the building was also included. An economic model is proposed, in order to assess the operating and capital costs of the systems under analysis. Furthermore, a parametric analysis and a subsequent mixed heuristic-deterministic optimization algorithm was implemented, in order to determine the set of the synthesis/design variables that maximize system profitability. The results are encouraging, as for the potential of energy saving. On the contrary, the SHC economic profitability can be achieved only in case of public funding policies (e.g. feed-in tariffs), as always happens for the great majority of renewable energy systems.

Maximization of primary energy savings of solar heating and cooling systems by transient simulations and computer design of experiments

In this paper, the simulation of the performance of solar-assisted heating and cooling systems is analyzed. Three different plant layouts are considered: (i) the first one consists of evacuated solar collectors and a single-stage LiBr–H 2O absorption chiller; here in order to integrate the system in case of insufficient solar radiation, an electric water-cooled chiller is activated; (ii) configuration of the secondly considered system is similar to the first one, but the absorption chiller and the solar collector area are sized for balancing about 30% of the building cooling load only; (iii) the layout of the thirdly considered system differs from the first one since the auxiliary electric chiller is replaced by a gas-fired heater. Such system configurations also include: circulation pumps, storage tanks, feedback controllers, mixers, diverters and on/off hysteresis controllers. All such devices are modelled for maximizing the system energy efficiency. In order to simulate the systems’ performance for dynamic heating/cooling loads, a single-lumped capacitance building is also modelled and implemented in the computer code. A cost model is also developed in order to calculate the systems’ operating and capital costs. All the models and the relative simulations are carried out by TRNSYS. A design of experiment procedure is also included. By such tool the effects of the system operating parameters’ variation on the relative energy efficiency are analyzed. In addition, the set of synthesis/design variables maximizing the system’s energetic performance can be also identified. The annual primary energy saving is chosen as the optimization objective function, whereas collector slope, pump flows, set-point temperatures and tank volume are selected as optimizing system design variables. A case study was developed for an office building located in South Italy. Here, the energetic and the economic analysis for all the three considered system layouts are carried out. The simulations’ results are referred to both the initial and the optimized systems configurations. The highest primary energy saving vs. the reference traditional HVAC system is reached by the first considered system layout. The economic performance of the investigated solar heating/cooling systems is still unsatisfactory. The economical profitability of the considered solar heating and cooling systems can be improved only by significant public finding. From this point of view, the best results were achieved by the second above mentioned system configuration.

Comparison of the solar-operated two-stage and single-stage LiBrH2O absorption cycles

Analysis of a solar operated two-stage LiBrH 2 O cycle is presented. The cycle has the advantages that it can operate at a higher ambient temperature and at a lower generator temperature than those required by the single stage cycle. The performance of the cycle is predicted and compared to the performance of the single stage cycle at various design conditions. Four design parameters are identified to control the performance of the cycle. These parameters are the heat sink temperature, the evaporator temperature, the generator temperature, and the intermediate pressure. Numerical correlations are developed to specify the minimum allowable sink temperature, the minimum and maximum allowable generator temperature and the coefficient of performance of both the single-stage and the two-stage cycles. By defining a system thermal ratio for the combined absorption machine and a flat plate collector, the performances of the integrated systems are evaluated and compared when using both cycles. Conditions at which the performance of the solar operated two-stage cycle is superior to that of the solar operated single stage cycle are defined.