Design and optimisation of a combination solar collector–thermal engine operating on Mars

A “dynamic” solar power plant (which consists of a solar collector–thermal engine combination) is proposed as an alternative for the more usual photovoltaic cells. A model for heat losses in a selective flat-plate solar collector operating on Mars is developed. An endoreversible Carnot cycle is used to describe heat engine operation. This provides upper limits for real performances. The output power is maximized. Meteorological and actinometric data provided by Viking Landers are used as inputs. Two strategies of collecting solar energy were considered: (i) horizontal collector; (ii) collector tilt and orientation are continuously adjusted to keep the receiving surface perpendicular on the Sun’s rays. The influences of climate and of various design parameters on solar collector heat losses, on engine output power and on the optimum sun-to-user efficiency are discussed.     

Thermoeconomic analysis of a solar driven heat engine

Thermoeconomic optimization has been carried out for an endoreversible solar driven heat engine using finite-time/finite-size thermodynamic theory. In the considered heat engine model, heat transfer from the hot reservoir is assumed to be radiation mode and the heat transfer to the cold reservoir is assumed to be convection mode. The power output per unit total cost is taken as objective function and the optimum performance and design parameters have been investigated. The effects of the technical and economical parameters on the thermoeconomic performances have been also discussed.     

Finite-time thermodynamic analysis of a solar driven heat engine

The collective role of radiation and convection modes of heat transfer in a solar driven heat engine is investigated through a finite time thermodynamics analysis. Heat transfer from hot reservoir is assumed to be radiation and/or convection dominated. The irreversibilities due to these finite rate heat transfers were considered in determining the limits of efficiency and power generation that were discussed through varying process parameters. Results were compared with Curzon–Ahlborn and Carnot analysis cases. It is found that the upper limit of efficiency is a function of both the functional temperature dependence of heat transfer and relevant system parameters.     

Optimum operating conditions of irreversible solar driven heat engines

An optimal performance analysis of an internally and externally irreversible solar driven heat engine has been carried out. A Carnot-type heat engine model for radiative and convective boundary conditions was used to consider the effects of the finite-rate heat transfer and internal irreversibilities. The power and power density functions have been derived and maximization of these functions has been carried out for various design parameters. The optimum design parameters have been derived and the obtained results for maximum power (MP) and maximum power density (MPD) conditions have been compared. The effects of the technical parameters on the performance have been investigated.     

Performance optimization of a solar driven heat engine with finite-rate heat transfer

An optimal performance analysis for an equivalent Carnot-like cycle heat engine of a parabolic-trough direct-steam-generation solar driven Rankine cycle power plant at maximum power and maximum power density conditions is performed. Simultaneous radiation-convection and only radiation heat transfer mechanisms from solar concentrating collector, which is the high temperature thermal reservoir, are considered separately. Heat rejection to the low temperature thermal reservoir is assumed to be convection dominated. Irreversibilities are taken into account through the finite-rate heat transfer between the fixed temperature thermal reservoirs and the internally reversible heat engine. Comparisons proved that the performance of a solar driven Carnot-like heat engine at maximum power density conditions, which receives thermal energy by either radiation-convection or only radiation heat transfer mechanism and rejects its unavailable portion to surroundings by convective heat transfer through heat exchangers, has the characteristics of (1) a solar driven Carnot heat engine at maximum power conditions, having radiation heat transfer at high and convective heat transfer at low temperature heat exchangers respectively, as the allocation parameter takes small values, and of (2) a Carnot heat engine at maximum power density conditions, having convective heat transfer at both heat exchangers, as the allocation parameter takes large values. Comprehensive discussions on the effect of heat transfer mechanisms are provided.     

An electrochemical heat engine for direct solar energy conversion

A system is described and tested which converts heat directly into electrical energy. It employs a solution electrochemical reaction with a small polarizability and a large molar entropy change . This is run in opposite directions in two cells: one at high temperature, where heat is absorbed, and one at low temperature, where heat is emitted. The difference in heat absorbed and heat emitted is available as electrical work; recirculation of the solutions between these cells gives a closed regenerative EMF system.The conversion efficiency of the system is high, varying from 50 to 75 per cent of the Carnot efficiency as the power output varies from maximum to 75 per cent of maximum. The power output depends strongly upon the reaction used. For the reaction tested here, the power output density was 6.4 W/m² of cell area for operation between 90° and 30°C. Design factors for improving power output density and minimizing costs are discussed, and basic requirements for successful cell reactions are given. The feasibility of obtaining power output on the order of 2 × 10² W/m² of cell area at 35 per cent conversion efficiency using 300°C input heat is discussed.     

Performance characteristics of a low heat rejection diesel engine operating with biodiesel

Vegetable oils are a promising alternative among the different diesel fuel alternatives. However, the high viscosity, poor volatility and cold flow characteristics of vegetable oils can cause some problems such as injector coking, severe engine deposits, filter gumming, piston ring sticking and thickening of lubrication oil from long-term use in diesel engines. These problems can be eliminated or minimized by transesterification of the vegetable oils to form monoesters. These monoesters are known as biodiesel. The important advantages of biodiesel are lower exhaust gas emissions and its biodegradability and renewability compared with petroleum-based diesel fuel. Although the transesterification improves the fuel properties of vegetable oil, the viscosity and volatility of biodiesel are still worse than that of petroleum diesel fuel. The energy of the biodiesel can be released more efficiently with the concept of low heat rejection (LHR) engine. The aim of this study is to apply LHR engine for improving engine performance when biodiesel is used as an alternative fuel. For this purpose, a turbocharged direct injection (DI) diesel engine was converted to a LHR engine and the effects of biodiesel (produced from sunflower oil) usage in the LHR engine on its performance characteristics have been investigated experimentally. The results showed that specific fuel consumption and the brake thermal efficiency were improved and exhaust gas temperature before the turbine inlet was increased for both fuels in the LHR engine.     

Model calculations on a flat-plate solar heat collector with integrated solar cells

A detailed physical model of a hybrid photovoltaic/thermal system is proposed, and algorithms for making quantitative predictions regarding the performance of the system are presented. The motivation for the present work is that solar cells act as good heat collectors and are fairly good selective absorbers. Additionally, most solar cells increase their efficiency when heat is drawn from the cells. The model is based on an analysis of energy transfers due to conduction, convection and radiation and predicts the amount of heat that can be drawn from the system as well as the (temperature-dependent) power output. Special emphasis is laid on the dependence of the fin width to tube diameter ratio. We attribute values to the model parameters, and show that hybrid devices are interesting concerning system efficiency as is also confirmed by previous experiments. Possible applications of such systems are also proposed.     

Methodology for optimized operation strategies of solar thermal power plants with integrated heat storage

The entry into the Spanish market of solar thermal power plants with integrated heat storage offers operators the opportunity to increase their yearly revenues. If maximum revenues are to be obtained, the actual electricity price and weather forecast must be taken into account and the power plant must consequently be run with a price-driven strategy. To achieve this, it is necessary to solve a complex optimization problem for scheduling the power selling at the day-ahead market. The derivation of the optimization problem is presented and its adaption to dynamic programming will be shown in the present paper.     

Theory and performance analysis of a new heat engine for concentrating solar power

The objectives of this paper are to introduce a new heat engine and evaluate its performance. The new heat engine uses a gas, such as air, nitrogen, or argon, as the working fluid and extracts thermal energy from a heat source as the energy input. The new heat engine may find extensive applications in renewable energy industries, such as concentrating solar power (CSP). Additionally, the heat engine may be employed to recover energy from exhaust streams of internal combustion engines, gas turbine engines, and various industrial processes. It may also work as a thermal‐to‐mechanical conversion system in a nuclear power plant and function as an external combustion engine in which the heat source is the combustion gas from an external combustion chamber. The heat engine is to mimic the performance of an air‐standard Otto cycle. This is achieved by drastically increasing the time duration of heat acquisition from the heat source in conjunction with the timing of the heat acquisition and a large heat transfer surface area. Performance simulations show that the new heat engine can potentially attain a thermal efficiency above 50% and a power output above 100 kW under open‐cycle operation. Additionally, the heat engine could significantly reduce CSP costs and operate in open cycles, effectively removing the difficulties of dry cooling requirement for CSP applications. Copyright © 2014 John Wiley & Sons, Ltd.     

Performance, emissions and heat release characteristics of direct injection diesel engine operating on diesel oil emulsion

Emulsions of diesel and water are often promoted as being able to overcome the difficulty of simultaneously reducing emissions of both oxidises of nitrogen (NO_x) and particulate matter from diesel engines. In this paper we present measurements of the performance and NO_x and hydrocarbon emissions of a diesel engine operating on a typical diesel oil emulsion and examine through the use of heat release analysis differences found during its combustion relative to standard diesel in the same engine. While producing similar or greater thermal efficiency and improved NO_x and hydrocarbon emission outcomes, use of the emulsion also results in an increase in brake specific fuel consumption. Use of the emulsion is also shown to result in a retarded fuel injection, but smaller ignition delay for the same engine timing. As a result of these changes, cylinder pressures and temperatures are lower.     

Experimental study on optimized composition of mixed carbonate salt for sensible heat storage in solar thermal power plant

Thirty six kinds of mixed carbonate molten salts were prepared by mixing potassium carbonate, lithium carbonate, sodium carbonate in accordance with different proportions. The data of melting point, decomposition temperature, specific heat are measured by the analysis of the TGA and DSC curves of 36 kinds of salts, which show that melting points of major ternary carbonate are close at around 400 °C and decomposition temperatures of most ternary carbonate are between 800 and 850 °C. Twleve kinds of eutectic molten salts with low melting point were selected among 36 kinds of salts. The correlations of the eutectic salts between specific heat and temperature were obtained by fitting the experimental data of specific heat. The costs for sensible heat storage of 12 kinds of carbonate salts are obtained. The optimum preparation ratio is recommended for sensible heat storage.     

The maximum power,heat demand and efficiency of a heat engine operating in steady state at less than Carnot efficiency

An imperfect, Carnot-like engine operating in steady state and receiving heat through conductance k₁ from a source at T₁ and discharging heat only through a conductance k₂ to a sink at T₄ has an efficiency at maximum net power output of $\text{η}{}_{\mathrm{m}}\text{= (}\text{?}\text{g9}\text{){1 ? √(1 ? ?(1 ?}\text{T}{}_4\text{T}{}_1$)}, where ? is a non-Carnot efficiency and $\text{? =}\text{(?k}{}_1\text{+ k}{}_2\text{)}\text{(k}{}_1\text{+ k}{}_2\text{)}$.     

Homogeneous charge compression ignition engine operating on synthesis gas

Mixtures of hydrogen and carbon monoxide were used to simulate the fuel component of synthesis gas and operate a single cylinder engine in homogeneous charge compression ignition (HCCI) mode. The engine was originally an air-cooled direct injection (DI) compression ignition (CI) engine. The original diesel fuel injection system was removed and a port fuel injection (PFI) system with intake air heating was added. The engine speed was maintained at a constant 1800 RPM.     

Harnessing low-grade waste heat by operating a hybrid piezoelectric-electromagnetic energy harvester combined with a thermomagnetic engine

This work deals with the development of a thermomagnetic engine driven by using a magnetocaloric material (gadolinium), coupled with both a disk-shaped electromagnetic generator (EMG) and a flexible piezoelectric generator (PENG). The thermomagnetic engine generated the highest rotational speed with an average of 226 ± 1.76% rpm when the temperature differed by 45°C between the hot and cold water jets that operate the engine. At this rotational speed, the EMG and TENG deliver output powers of 8.4 mW (corresponding to the power per unit/volume of 81 W/m³) at a loading resistance of 100 Ω and 74.4 nW (corresponding to the power per unit/volume of 7.2 mW/m³) at a loading resistance of 20 kΩ, respectively. Moreover, the hybrid (EMG-PENG) generator exhibited outstanding performance whose respective short circuit current and open circuit voltage was recorded to be3.6 mA and 7.5 V. It showed a combined power output of 7.8 ± 2.1% mW. The proposed energy harvester demonstrated its capability to charge a 1000 μF capacitor which can be used as an effective power source for diverse smart applications. It was found that the operation of the hybrid (EMG-TENG) generator in line with the thermomagnetic engine holds great potential in harnessing low-grade thermal energy with high added value as compared to common practices in its utilization.     

On optimized solar-driven heat engines

Heat engines will usually be designed somewhere between the two limits of (1) maximum efficiency, which corresponds to “Carnot” or reversible operation, albeit at zero power, and (2) maximum power point. Each of these limits implies a specific dependence of heat engine efficiency on the temperatures of the hot and cold reservoirs between which the heat engine operates. We illustrate that the energetically optimal operating temperature for solar-driven heat engines is relatively insensitive to the engine design point. This also pertains to solar collectors whose heat loss can range from predominantly linear (conductive/convective) to primarily radiative. Potential misconceptions are also discussed regarding the maximum power point and the Curzon-Ahlborn efficiency of “finite-time thermodynamics.”     

Emission characteristics of diesel engine operating on rapeseed methyl ester

Biofuels are being investigated as potential substitutes for current high pollutant fuels obtained from the conventional sources. In the past, fuels were generally selected on the basis of lowest cost relating to the requirements of the engine, and no attention was given to the possible effects of their use on the environment. Recent concerns over the environment, increasing liquid fuel prices and scarcity of supply in the last decade have promoted interest in the development of alternative sources of liquid energy. The purpose of this research was to evaluate the potential of rapeseed methyl ester (RME) as a liquid fuel for diesel engines in relation to meeting emission requirements. The test results showed that RME and its blends with diesel fuel emitted high CO₂ compared to test results on diesel fuel. A very significant reduction in emissions of hydrocarbon (HC) were recorded when running on RME and the blends. HC emissions were noted to increase with increased amount of diesel fuel in the blend. The fuel economy was a little worse when running on RME due to its low energy content. There were no marked difference noted for the exhaust temperatures of the blends, RME and diesel fuel at high speed operation. However, the diesel fuel operation produced high exhaust temperatures at low engine speed. Lubricating oil analysis showed reduction in viscosity indicating oil dilution.