L. Vanoli

Micro-combined heat and power in residential and light commercial applications

Abstract The cogeneration is worldwide considered as the major option to achieve considerable energy saving with respect to traditional systems. This paper deals with the application of micro-cogeneration (electrical power

One-Dimensional Model of a Tubular Solid Oxide Fuel Cell

In this paper, a detailed model of a solid oxide fuel cell (SOFC) tube is presented. The SOFC tube is discretized along its longitudinal axis. Detailed models of the kinetics of the shift and reforming reactions are introduced in order to evaluate their rates along the SOFC axis. Energy, moles, and mass balances are performed for each slice of the components under investigation, allowing the calculation of temperature profiles. Friction factors and heat-exchange coefficients are calculated by means of experimental correlations. As for the SOFC overvoltages, the activation overvoltage is calculated using the Butler‐Volmer equation and semiempirical correlations for the exchange current density, Ohmic losses are evaluated introducing an appropriate electrical scheme and material resistivities, and concentration overvoltage is calculated by means of both binary and Knudsen diffusion coefficients. On the basis of this model, a case study is presented and discussed, in which temperatures, pressures, chemical compositions, and electrical parameters are evaluated for each slice of the SOFC tube under investigation. Finally, a sensitivity analysis is performed, in order to investigate the influence of the design parameters on the performance of the system. DOI: 10.1115/1.2784296

Direct and indirect measurement of WBGT index in transversal flow

ISO 7243 considers the WBGT (Wet Bulb Globe Temperature) index as an index of thermal stress in hot environments. It can be evaluated directly by measuring the globe temperature, the natural wet bulb temperature, and the air temperature (only outside in the presence of solar radiation) or, indirectly, by measuring the air temperature, the air velocity, the humidity and the mean radiant temperature. The globe and natural wet bulb temperatures can only be estimated empirically, because they are not thermodynamic properties. The direct method, thus, permits neither very accurate measurements nor direct traceability. The aim of this study is to compare the above-mentioned methodologies experimentally. This analysis was carried out by varying the thermal and fluid dynamic parameters for transversal flow in the common measurement range.

Simulation Model and Analysis of a Small Solar-Assisted Refrigeration System: Dynamic Simulation and Optimization

In this paper, a complete zero-dimensional transient simulation model of a solar-assisted refrigeration plant is presented. In addition, a case study is discussed, aiming at determining the optimal configuration of the system, from the energetic point of view, in a specific application. The system under analysis consisted of several components: evacuated solar collectors, circulation pumps, variable speed pump, water storage tanks, auxiliary heater, single-stage H2 O-LiBr absorption chiller, cooling tower, feedback controller, on/off hysteresis controller, single lumped capacitance building and controllers. The simulation was performed using the TRNSYS environment which is provided by a large component library. This software also includes a detailed database with weather parameters for several European cities. The system and the building were simulated using TRNSYS built in models. The system was simulated using specially designed control strategies and varying the main design variables. In particular, a variable speed pump on the solar collector was implemented in order to maximize the tank temperature and minimizing the heat losses. Finally a sensitivity analysis was also performed in order to calculate the set of synthesis/design parameters that maximize the total system efficiency.Copyright

Optimizing the position of the tube casing slotted section for geothermal wells with a downhole heat exchanger

The use of downhole heat exchangers (DHE) in the exploitation of geothermal resources is characterized by an absence of mass withdrawal from the aquifer. Although this peculiarity reduces installation costs it also imposes limits on the heat flow withdrawable (generally less than 100 kW), and, therefore, on the use of DHEs in small applications such as greenhouses, small buildings or thermal baths. For this reason DHEs are mainly used in superficial geothermal aquifers (up to 30 m depth), usually with liquid-phase water at temperatures greater than 60°C. A study has been made of the influence of the position of the casing slotted section within an aquifer on the heat withdrawal rates using DHEs. This study numerically simulates an aquifer using the finite-element method to determine the heat flow that can be withdrawn by the DHE when the slotted section position is varied within a geothermal aquifer. The simulations carried out also enable us to determine the influence of the main characteristics of the aquifer and the extraction plant on the design of the tube casing slotted section. On the basis of the numerical results obtained, a particular configuration of slotted section is proposed where this is subdivided into different sections, one placed in the lower part of the aquifer and the other in the upper part. The results obtained have shown that this configuration optimizes the heat flow drawn by the DHE from the geothermal aquifer.

A metrological analysis of a (Direct Digital Control) DDC-based air conditioning system

Although (Direct Digital Control) DDC-based air conditioning systems can potentially carry out efficiency checks, the instrumentation installed in these systems is not always suitable for making reliable plant status estimations. In order to verify the metrological performances of these systems and to define how reliable measurements can be made the authors set up a small-sized HVAC (Heating Ventilating and Air Conditioning) system provided with standard HVAC type sensors. The measurement instrumentation was verified by means of calibration and field tests, and the operating conditions in which it is possible to estimate in a sufficiently reliable manner the performances of each system component using the same instrumentation were determined experimentally. This analysis has shown that the normally used sensors are often highly dependent on operating and installation conditions and, consequently, the efficiency estimation of each component is very unreliable using uncalibrated sensors.

Multi-Point Energy and Exergy Analysis of a 1.5 MWe Hybrid SOFC-GT Power Plant

This paper presents a multi-point energy and exergy analysis of a hybrid SOFC–GT power plant. The plant layout consists of the following principal components: an internal reforming SOFC, a steam-methane pre-reformer, a catalytic burner, a radial gas turbine, a centrifugal air compressor, a centrifugal fuel compressor, plate-fin heat exchangers, counter-flow shell and tube heat exchangers, and mixers. The partial load performance of the centrifugal compressors and radial turbine is determined using maps, properly scaled in order to match required mass flow rate and pressure ratio values. The plant is simulated on the basis of a zero-dimensional model discussed in previous papers. Two different partialization strategies are introduced in order to assess the partial load behavior of the plant. Results show that the plant achieves the best partial load performance for the case when both air and fuel mass flow rates are simultaneously reduced.Copyright

Influence of contact mechanics in the prediction of the effective thermal conductivity of spheroid packed beds

Thermal contact conductance is an important parameter in a wide range of thermal phenomena, and consequently a large number of experimental, numerical and statistical investigations have been carried out in literature. In the present paper an analysis of thermal contact resistance is carried out to predict heat transfer between spherical rough surfaces in contact, by means of a statistical approach. The micro-geometry of the surface is described through a probabilistic model based on the peak height variability and invariant asperity curvature radius. The numerical model has been applied to evaluate the effective thermal conductivity of packed beds of steel spheroids and validated through the comparison with the experimental data obtained by means of an apparatus designed and build up for this purpose.Copyright

5.18 Energy Management in Geothermal Energy Systems

Authors propose a thermoeconomic analysis of a renewable polygeneration system producing power, desalinated water, heating and cooling and connected to a district heating and cooling network supplying a small district. A comparison between two layouts is performed, a hybrid (solar and geothermal) and a geothermal one, by performing a 1-year dynamic simulation and processing the results on different time bases. A parametric analysis is carried out to assess system performance and its capability to match the time-dependent energy demands. The hybrid configuration provides the best thermodynamic and environmental performances; conversely the geothermal one provides the highest economical profitability, achieving a much lower simple payback time, averaging 4.3 years instead of the 8.4 years of the hybrid configuration.

Assessment of Renewable Energy Expansion in the Java-Bali Islands, Indonesia

Many developing countries face a dilemma between meeting the intensive growth in electricity demand, broadening an electricity access, as well as tackling climate change. The use of renewable energy is considered as an option for meeting both electrification and climate change objectives. In this study, long-term forecasting of electricity supply for the Java-Bali power system – the main power system in Indonesia – is presented. The forecasts take into consideration the Indonesian government policy of increasing the share of new and renewable energy in the national energy mix up to 23% by 2025 and 30% by 2050. After a systematic review of energy system models, we perform the analysis of the Java-Bali power system expansion using the Long-range Energy Alternative Planning system (LEAP) model. Three scenarios are developed over the planning horizon (2016-2050) including the business as usual scenario (BAU), the renewable energy scenario (REN) and the optimization scenario (OPT). The results of the three scenarios are analyzed in terms of the changes in resource/technology deployment, CO2 emissions and total costs.