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International audience; A dynamic model using the bond graph formalism of the expansion cylinder of an open Joule cycle Ericsson engine intended for a biomass-fuelled micro-CHP system is presented. Dynamic phenomena, such as the thermodynamic evolution of air, the instantaneous air mass flow rates linked to pressure drops crossing the valves, the heat transferred through the expansion cylinder wall and the mechanical friction losses, are included in the model. The influence on the Ericsson engine performances of the main operating conditions (intake air pressure and temperature, timing of intake and exhaust valve closing, rotational speed, mechanical friction losses and heat transfer at expansion cylinder wall) is studied. The operating conditions maximizing the performances of the Ericsson engine used in the a biomass-fuelled micro-CHP unit are an intake air pressure between 6 and 8 bar, a maximized intake air temperature, an adjustment of the intake and exhaust valve closing corresponding to an expansion cycle close to the theoretical Joule cycle, a rotational speed close to 800 rpm. The heat transfer at the expansion cylinder wall reduces the engine performances.

An Ericsson engine is an external heat supply engine working according to a Joule thermodynamic cycle. It is based on reciprocating piston-cylinder machines. Such engines are especially interesting for low power solar energy conversion, micro-CHP from conventional fossil fuels or from biomass and waste heat recovery. An innovative configuration has been designed where the mechanical pistons are replaced by liquid pistons. The engine is composed of two U-shape tubes partially filled with water, one for the compression spaces and one another for the expansion spaces. A thermodynamic model of the engine is developed. The model takes into account the dynamics of the liquid columns. It is shown that the engine indicated efficiency decreases when the rotation speed increases. The sign of the instantaneous torque on the flywheel changes four times per cycle, calling for a heavy flywheel. When the rotation speed increases above the natural frequency of oscillation of the liquid columns, the influence of the liquid pistons inertia is growing. Its impact is globally opposite to the one of the pressure in the cylinders. So, the effect of the inertia of the liquid columns is to reduce the amplitude of the torque variations.

Investment per specific electric capacity of power plants and combined heat and power plants operating exclusively according to the Joule cycle (Simple Cycle Power Plants) is more than two times lower compared to combined gas-steam systems. This paper discusses a system comprising a turboexpander with a compressor and an air heater installed instead of a steam turbine. The results of thermodynamic calculations carried out for many alternatives gas turbine designs, including ones with and without heat regeneration and for hierarchical gas-gas system are analyzed. Using energy balances, the energy efficiency of various configurations of systems was calculated. In the presented economic calculations, aspects related to the ratio of the annual electricity production to the annual heat production play the role of important factors, as technology applied in combined heat and electricity generation, system energy efficiency, specific investment and sales price of the generated electricity determine the results. From the calculations we can see that the gas-gas system is more economically feasible than the gas-steam system in terms of heat production. With regard to electricity production, the gas-steam system is slightly more profitable than the gas-gas system. This advantage decreases along with the decrease in the gas price.

With the help of Domenicali's equation and the heat flux equation, a finite difference method is directly used to determine the temperature and heat flux distribution profiles in one dimension of a thermoelectric generator simultaneously including the Peltier, Fourier, Joule, and Thomson effects. This calculative method is also used to evaluate the performance of a thermoelectric generator, based on experimental data of thermoelectric materials PbTe 1−y I y and PbTe: Na/Ag 2 Te. The efficiency of the thermoelectric generator as a function of the electric current is calculated and the effects of structure parameters on the performance of the thermoelectric generator are revealed. The method proposed here offers a simple way to properly choose n-type and p-type thermoelectric materials of a thermoelectric generator.

Thermoelectric self-cooling systems hold good prospects for the future, since they improve the cooling of any heat-generating device without electricity consumption. The potential number of applications seems to be enormous, hence the necessity of a specific model to simulate this type of thermoelectric application. This paper presents a computational model for thermoelectric self-cooling applications, capable of simulating both the steady and the transient state of the whole system. Supported by fluid-dynamics software and based on the implicit finite differences, the model solves the system of equations composed of the Fourier's law and the thermoelectric effects Seebeck, Peltier, Joule and Thomson, including temperature-dependant properties. Furthermore, the model architecture allows the inclusion of new analytical expressions and/or procedures in order to simulate any component with higher accuracy or include more complex ones. Statistical studies indicate ±12% of maximum deviation between experimental and simulated values of the main outputs. Furthermore, the model simulates the performance of the system under abruptly changing conditions. In conclusion, the computational model turns out to be a powerful tool that will play a key role in the design and development of thermoelectric self-cooling applications.

An improved model to analyze the performance of solar plants operating with cylindrical parabolic collectors and atmospheric air as heat transfer fluid in an open Joule–Brayton cycle is presented. In the new model, the effect of the incident angle modifier is included, to take into account the variation of the optical efficiency with the incidence angle of the irradiance, and the effect of the reheating of the fluid also has been studied. The analysis was made for two operating modes of the plants: with variable air flow rate and constant inlet temperature to the turbine and with constant flow rate and variable inlet temperature to the turbine, with and without reheating of the fluid in the solar field. When reheating is used, the efficiency of the plant is increased. The obtained results show a good performance of this type of solar plant, in spite of its simplicity; it is able to compete well with other more complex plants operating with different heat transfer fluids.

A performance analysis of innovative solar plants operating with cylindrical parabolic collectors and atmospheric air as heat transfer fluid in an open Joule–Brayton cycle, with and without intercooling and regeneration, is presented. The analysis was made for two operating modes of the plants: with variable air flow rate and constant inlet temperature to the turbine and with constant flow rate and variable inlet temperature to the turbine. The obtained results show a good performance of this type of solar plant, in spite of its simplicity; it seems able to compete well with other more complex plants operating with different heat transfer fluids.

One can measure “energy quantities” – e.g. joules, BTU, quads – but only at a given scale and within a specific narrative about energy conversions at the time. Therefore, at the moment of generating aggregate indicators, the arithmetic summing of assessments of energy quantities referring to different scales and narratives is meaningless. This paper addresses epistemological problems typical of energy accounting, which are at the moment tackled by acknowledging the existence of unspecified “qualitative differences” among different energy forms – e.g. a joule of electricity has more “value” than a joule of coal. Three energy forms referring to different scales and narratives about energy conversions are relevant for national accounting: Primary Energy Sources (PES), Energy Carriers (EC), and End Uses (EU). We critically examine the usefulness of current energy statistics in relation to this distinction. The conventional linear representation – flow chart – based on a single scale and a single quantitative accounting confuses the three semantic categories and entails an important loss of information. Finally, we illustrate an innovative scheme for energy accounting across hierarchical levels that: (i) addresses the autocatalytic nature of energy transformations; (ii) provides a multi-scale quantitative representation; and (iii) preserves the semantic distinction between relevant energy forms (PES, EC, and EU).

Three quite different scales for measuring the amount of primary energy the world uses annually are currently in use. The Engineering Information Administration, the International Energy Agency and the Working Group III (WG III) of the Intergovernmental Panel on Climate Change (IPCC), each has its own scale for measuring and recording primary energy. The purpose of this paper is to clarify the relationship between these scales so the reader can avoid introducing errors into work involving primary energy. An example is presented to show how mixing of these scales, and not identifying them, constitutes an error in 16 of the 40 energy scenarios in the Special Report on Emissions Scenarios prepared by WG III of the IPCC. These three scales arise mainly because of legitimate differences of opinion about how to measure and record primary energy, and especially for generating electricity from nuclear and renewable energies. All three scales use “Joules” as the unit of energy measurement, but these “Joules” are arbitrary units and are not the precise Joule as defined and used in physics and chemistry.

The performance of a thermoelectric device used as a heat pump or refrigerator is mainly affected by the Peltier, Fourier, Joule and Thomson effects. The differential equations governing the temperature field inside the device operated between two heat reservoirs are established by using nonequilibrium thermodynamics. New expressions are derived for the COP and the rate of heat pumping. Effects of the Thomson heat on these parameters are analyzed.

A thermoelectric power cycle (TPC) has the capability of converting a part of the Joule losses to utilizable electrical energy. The effect of this capability on the total efficiency of a TPC is examined by comparing it with conventional d.c. power cycle (DCPC). For reversible heat exchange and zero heat leakage, the total efficiency of the TPC is always greater than that of the DCPC. With heat leakage, there remains a power interval in which this same ordering holds when suitable materials are used. Efficiencies are also compared for the maximum electrical power output.

With rising concern about the increasing atmospheric concentration of CO 2 and its potential impact on global climate, there have been suggestions that the CO 2 emission rates be considered in selecting among fuel-producing technologies. Many previous comparisons have been inadequately drawn. We discuss criteria for carefully drawn comparisons and compute CO 2 emission rates in kgC/10 9 joules for a variety of conventional or synthetic fuel processes. Although the total CO 2 release per joule is on the order of 1.8 (±0.2) times as much for burning liquid fuels from coal as for liquid fuels from crude oil, useful comparisons among synthetic fuel processes are not easy and the results are subject to changes in boundary conditions of resource availability, plant location, environmental requirements, output slate desired, economics, etc.