• Energy Research
• Closed Access close
• PT close
• Applied Thermal Engineering close

Abstract The use of CO2 as a working fluid in power generation and storage applications has experienced a significant boost in recent years, based on its high-performance characteristics in power generation or heat pumps. This work proposes a novel combined use of transcritical CO2 cycles as an energy storage system and carbon dioxide storage inside geological formations. In this work, the layouts for concept integration were developed. They were adapted to operate under different scenarios and operation modes based on storing energy from renewable sources or storing energy to capture CO2. The preliminary results show these cycles as promising energy storage technologies, with a high potential to compete in terms of electric to electric storage efficiencies (42–56%) and costs (70–120 USD/MWh). Besides, results show that more than 1 Mton/year of CO2 could be additionally stored with this renewable energy storage concept depending on the conditions. These results show the opportunity for the concept as an energy storage system, with special interest when combined with carbon-intensive industries as cement or chemicals.

Abstract We have investigated numerically the dependence of the temperature span and optimum operating frequency of solid state magnetic refrigerators on the most important physical properties of magnetocaloric materials. We concluded that the temperature span depends inversely on the thermal conductivity of the magnetocaloric material and is proportional to the square root of its density, heat capacity and maximum adiabatic temperature variation upon the application/removal of a magnetic field ( Δ T a d m ). Also, and contrary to general belief, the increase of the thermal conductivity is not the only factor able to increase the optimum operating frequency. Reducing the density, heat capacity, Δ T a d m or broadening of the Δ T a d ( T ) curve can result in larger operating frequencies. Our results show that a compromise between the maximum achievable temperature span and optimum operating frequency must be reached, as the increase of one reduces the other.

Abstract A modular framework to model the steady off-design behavior of micro-CHP natural gas boilers based on Rankine technology is presented. The system charge integration into the model eliminates the use of any assumptions (i.e. subcooling, superheating, condensing pressure, etc.) which makes the presented model completely predictive. It is illustrated in the modelling a micro-CHP that satisfies the hot waters and central heating domestic needs (35 kWt) and produces electricity (≤1.5 kWe). A library of sub models of components with empirical (rotary vane pump and vapor scroll expander), semi-empirical (compact plates condenser) and spatially detailed physical (gas burner and evaporator) models is used to construct a model, using R245fa as thermal fluid. The model is calibrated and validated in tests in which 0.1 kg/s of water was heated from 20 °C to 30–36 °C, and 80–500 W mechanical power was delivered at the expander shaft, sweeping restrict ranges of three control variables: burner thermal power of combustion (10–14.5 kWt), pump (500–740 rpm) and expander (2500–2750 rpm) rotation speeds. The model predicts most output variables with acceptable errors, e.g., less than ±10% for the expander outlet pressure (190–220 kPa, abs) or the temperatures at the outlet of the evaporator (80–150 °C) or the expander (60–120 °C).

The expert system for fouling assessment development is one of the possible approachs to the mitigation of deposit formation on boiler heat transfer surfaces. This paper presents the evaluation of the diagnostic variables and their domain based on the criteria for the fouling assessment. In agreement with the boiler furnace design, respective fields for the deposit control are selected to be instrumented with the +clean' radiation heat flux and 'not-cleaned' radiation heat flux for the on-line reading. The ratio of the 'clean" and 'not-clean" radiation heat flux is proved to be a sensitive diagnostic variable for the deposit thickness. The rate of change of radiation heat flux ratio, together with the efficiency of the heat transfer surface, are diagnostic variables used for the design of the expert system knowledge base. The knowledge base structure is based on the object oriented concept. In this respect the FOULING object is defined. Its structure is composed of the respective attributes and values linked with the number of situations to be selected for the assessment.The examples given are related to a 300 MWe power-plant boiler furnace. It includes the number of situations reflecting different deposit formation situations: total and upper level of side surface of fouling deposit. Copyright © 1996 Elsevier Science Ltd.

Abstract The ETS-10/water pair was explored for the first time for cyclic adsorption heating purposes, with modelling and simulation studies. Measurements of water adsorption equilibrium properties were carried out, and, for the first time, the effective thermal conductivity and specific heat capacity of ETS-10 were measured. The experimental results were used for the modelling and simulation of an adsorption heating unit. A model was developed, which contemplates adsorption equilibrium, one-dimensional heat and mass transfer in the bed, heat transfer in the external film, and intraparticle mass transport. From the numerical simulations, the coefficient of performance (COP) and specific heating power (SHP) were calculated, which allowed evaluating the heating performance of the adsorption unit. The bed thickness, adsorbent regeneration temperature, and heating thermal fluid temperature influence considerably the cycle time and cyclic adsorption loading swing, thus impacting on COP and SHP. For three simulated cycles differing in bed thickness, COP values in the range 1.36–1.39 were obtained, which are close to the estimated ideal value of 1.41; the corresponding SHP ranged from 934 to 249 W kgs−1. Based on sensitivity studies, a good compromise is required between the bed thickness, regeneration temperature, and the heating fluid temperature in order to meet superior performances of the system.

Abstract A two-dimensional CFD computational model has been developed to describe the gasification process of coffee husks within a fluidized bed reactor. The Eulerian - Eulerian method is used for both gas and solid phases to provide an explanation for mass exchange, energy, and momentum. The results were obtained after comparing both the numerical model and the experimental data for validation. The current model also predicts the effects of equivalence ratio and moisture content on gasification temperature, and provides sensitive analysis of the model of the produced syngas composition in addition to the higher heating value and cold gas efficiency. The simulated syngas composition was found to be in good agreement with the experiment. The high moisture content of coffee husk has negative effects on cold gas efficiency and HHV, an effect that decreases as the equivalence ratio increases.

Abstract In the present work a simplified mathematical model for a condensing gas water heater appliance was built to evaluate the impact of relative humidity (RH) on the appliance. Validation was performed considering experimental results from condensing and non-condensing appliances. The mathematical model was solved using an improved Broyden method. Analytical results are in good agreement with experimental results.

Abstract In this work, a comprehensive two-dimensional computational fluid dynamics model was used in order to assess the potential of syngas produced from gasification of Portuguese miscanthus by using a semi-industrial gasification plant. An Eulerian-Eulerian approach is used in the fluent environment to describe the transport of mass exchange, momentum and energy for both solid and gas phases. The results were obtained after comparing both the numerical model and the experimental data for validation. The simulated syngas composition was found to be in good agreement with the experiment. The effect of equivalent ratio (ER), temperature and steam-to-biomass ratio (SBR) on products of gasification and their concentrations were assessed. The ER has some negative effects on syngas quality (decrease of fuel gases and lower heating value (LHV)) and some positive effect on the carbon conversion efficiency (CCE) and on the reduction of tar content. The reactor temperature has a positive effect on syngas quality (increase of fuel gases, lower heating value and decrease of tars). High temperatures also favor the gasification efficiency. The SBR has some positive effects on syngas quality (increase of H2 content and reduction of tars) and some negative effect on syngas LHV and there are some particular SBR that maximizes the CCE and CGE.