This work presents the development and validation of a numerical model that represents the performance of a gamma Stirling engine prototype. The model follows a modular approach considering ideal adiabatic working spaces; limited internal and external heat transfer through the heat exchangers; and mechanical and thermal losses during the cycle. In addition, it includes the calculation of the mechanical efficiency taking into account the crank mechanism effectiveness and the forced work during the cycle. Consequently, the model aims to predict the work that can be effectively taken from the shaft. The model was compared with experimental data obtained in an experimental rig built for the engine prototype. The results showed an acceptable degree of accuracy when comparing with the experimental data, with errors ranging from +/- 1% to +/- 8% for the temperature in the heater side, less than +/- 1% error for the cooler temperatures, and +/- 1 to +/- 8% for the brake power calculations. Therefore, the model was probed adequate for study of the prototype performance. In addition, the results of the simulation reflected the limited performance obtained during the prototype experiments, and a first analysis of the results attributed this to the forced work during the cycle. The implemented model is the basis for a subsequent parametric analysis that will complement the results presented.

The Stirling engine is a closed-cycle regenerative system that presents good theoretical properties. These include a high thermodynamic efficiency, low emissions levels thanks to a controlled external heat source, and multi-fuel capability among others. However, the performance of actual prototypes largely differs from the mentioned theoretical potential. Actual engine prototypes present low electrical power outputs and high energy losses. These are mainly attributed to the complex interaction between the different components of the engine, and the challenging heat transfer and fluid dynamics requirements. Furthermore, the integration of the engine into decentralized energy systems such as the Combined Heat and Power systems (CHP) entails additional complications. These has increased the need for engineering tools that could assess design improvements, considering a broader range of parameters that would influence the engine performance when integrated within overall systems. Following this trend, the current work aimed to implement an analysis that could integrate the thermodynamics, and the thermal and mechanical interactions that influence the performance of kinematic Stirling engines. In particular for their use in Combined Heat and Power systems. The mentioned analysis was applied for the study of an engine prototype that presented very low experimental performance. The numerical methodology was selected for the identification of possible causes that limited the performance. This analysis is based on a second order Stirling engine model that was previously developed and validated. The simulation allowed to evaluate the effect that different design and operational parameters have on the engine performance, and consequently different performance curves were obtained. These curves allowed to identify ranges for the charged pressure, temperature ratio, heat exchangers dimensions, crank phase angle and crank mechanical effectiveness, where the engine performance was improved. In addition, the curves also permitted to recognise ranges were the design parameters could drastically reduce the brake power and efficiency. The results also showed that the design of the engine is affected by the conditions imposed by the CHP interactions, and that the engine could reach a brake power closer to 832 W with a corresponding brake efficiency of 26% when the adequate design parameters were considered. On the other hand, the performance could also be very low; as the reported in experimental tests, with brake power measurements ranging 52-120W.

The reliability of modelling and simulation of energy systems strongly depends on the prediction accuracy of each system component. This is the case of Stirling engine-based systems, where an accurate modelling of the engine performance is very important to understand the overall system behaviour. In this sense, many Stirling engine analyses with different approaches have been already developed. However, there is a lack of Stirling engine models suitable for the integration into overall system simulations. In this context, this paper aims to develop a rigorous Stirling engine model that could be easily integrated into combined heat and power schemes for the overall techno-economic analysis of these systems. The model developed considers a Stirling engine with adiabatic working spaces, isothermal heat exchangers, dead volumes, and imperfect regeneration. Additionally, it considers mechanical pumping losses due to friction, limited heat transfer and thermal losses on the heat exchangers. The predicted efficiency and power output were compared with the numerical model and the experimental work reported by the NASA Lewis Research Centre for the GPU-3 Stirling engine. This showed average absolute errors around ±4% for the brake power, and ±5% for the brake efficiency at different frequencies. However, the model also showed large errors (±15%) for these calculations at higher frequencies and low pressures. Additional results include the calculation of the cyclic expansion and compression work; the pressure drop and heat flow through the heat exchangers; the conductive, shuttle effect and regenerator thermal losses; the temperature and mass flow distribution along the system; and the power output and efficiency of the engine.

A desired feature of solar power systems would be to continue producing at high output a few hours after sunset in order to cover local peak loads. An energy storage system would be able to help with solving this problem. The simplest and most cost-effective energy storage method is a thermal accumulator, where hot water or another fluid is stored at a given temperature higher than the surroundings. Conversion of thermal energy into mechanical power when compared to photovoltaic systems, however, is limited in efficiency and requires comparatively complex equipment, which might not be as cost-effective as desired, suffer from low reliability and require frequent maintenance.

The thermal path of converting solar energy into electricity is certainly promising but has largely been underestimated and underutilized. Several thermal-to-electricity energy conversion technologies already exist in either conventional form or at close-to-commercialization phase and can be further optimized and adapted to low-cost low-temperature solutions. Combined heat and power (cogeneration) facilities at small scales can be attractive for a quicker and wider deployment in solar-rich locations.

This study evaluates and compares several candidates for the conversion of low-temperature solar thermal energy into power and examines their technical feasibility and thermodynamic performance, as well as their potential for low-investment strategies and integration with thermal energy storage. With temperatures in the solar collectors limited to 150

oC (300 oF), the suggested energy conversion techniques include flat plate and evacuated tube solar collectors combined with low-parameter steam Rankine cycles or turbocharger derivative Brayton cycles, organic Rankine cycles and novel thermoelectric solutions.

Results show that common steam, organic, or air expansion cycles optimized for low parameter applications are feasible for further development and deployment in the near future, based on established components featuring turbines derived from commercial products. Thermal-to-electricity efficiency of around 5% - 12% and solar-to-electricity efficiency of around 4 – 8% can be achieved by some of the cycle alternatives at their best operational conditions.

This paper aims at evaluating the possible upgrading of an existing district heating plant for production of electricity and pellets. The evaluation is carried out by optimizing the alternatives from the economic, thermodynamic and environmental point of view. In order to examine how the design can be optimized, a detailed model of the process has been elaborated using ASPEN Utilities and Matlab optimization toolbox. The parameters of the polygeneration plant have then been varied in order to examine how optimal economic benefit can be extracted from the biomass streams whilst still meeting the fundamental process demands of the industries and heat demand of the community. A multi-objective optimization has been used to investigate the Pareto-optimal trade-offs that exist between low electricity costs and investment cost. The resulting polygeneration plant designs conclude that it is feasible toproduce 18 and 25 MW of power while at the same time supplying the process steam required by the nearby industries and district heating for the community. The results also shown that it is feasible to operate the plant more hours per year by producing pellets and it could be possible to generate additional district heating (up to 25 ton/h of hot water) to cover the demands of a growing community.

Biomass-based fuels have attracted worldwide interest due to their plentiful supply and their environmentally friendly characteristics. In many cases they are still considered waste but for most industries in Sweden, biomass has changed from being simply a disposal problem to become an important part of the energy supply, thanks to the long-term efforts made by the government, researchers and industry, where energy policies have played an important role. However, the amount of power that could be generated from biomass resources is much greater than that which is currently used. To effectively capture this resource requires a new generation of biomass power plants and their effective integration into already existing industrial processes.The implementation of an integrated polygeneration scheme requires the simultaneous consideration of technical, economic and environmental factors to find optimum solutions. With this in mind, a unified modeling approach that takes into account thermodynamic as well as economic and environmental aspects was used. The analysis was done using ASPEN Utilitiesand the MATLAB optimization toolbox. A specific case of a sawmill in Sweden, with an annual capacity of 130’000 m3 of sawn wood, has been analyzed and different options for generating electricity and process heat (for the sawmill and fora district heating network) as well as densified biofuels was analyzed. Optimization was then applied for different configurations and operational parameters. The results show that the sawmill has the capability to not only supply its own energy needs, but also to export from 0.4 to 1MW of electricity to the grid, contribute 5 to 6 MWth of district heating and 20 000 ton/y of biomass pellets. The production of pellets helps to maintain the electricity production throughout the year when the district heating demand is lower. However, the levelized electricity cost is higher than the usual electricity price in the Nordic electricity market and may have difficulty to competing with low-cost electricity sources, such as nuclear energy and hydropower. Inspite of this, polygeneration remains attractive for covering the energy demands of the sawmill and pelletization plant.

Agricultural residues continue to attract interest for energy recovery purposes as a renewable, CO₂ neutral and increasingly cost-competitive alternative to traditional fossil fuels. Furthermore, some of these residues, like palm oil residues, represent a disposal problem for the processing industries, or they are not used efficiently. Several palm oil mills (POM) lack efficient energy systems and thus there is a considerable potential for improvement. These factors represent a strong driving force for the development of innovative polygeneration plants with combined electricity, heat and refined fuel production based on conversion of solid residues. This paper aims at analyzing the use of agro-industrial residues as fuel. For that, we propose different technology configurations based on the case of a small-scale palm oil mill in Colombia processing 30 tons of fresh fruit bunch per hour. The technology configurations include steam cycles using backpressure turbines, condensing-extraction turbines and also gasification-gas engine cycles in hybrid configurations. The possibilities to produce pellets from the residues from palm oil were also analyzed. The steam cycle base operational parameters were 20 bar and 350 °C. However, more advanced steam conditions (40 bar) were also considered and evaluated. All the analyses performed included a maximum of 60 % of the empty fruit bunch (EFB) produced in the POM for energy purposes due to its value as natural fertilizer in the palm oil plantations. The results show that the POM under study and other POMs that use electricity from the national grid have the capacity of being self-sufficient to cover of all their energy needs using the solid residues available. This means that POMs that currently only generate the required heat for the process can generate the electricity required and in some cases even an excess of energy that could be sold to other users with an adequate use of the residues available. Furthermore, based on the modeling done in Aspen Utilities Planner® it is shown that it is possible to cover the demand of the POM, the required energy demand for EFB preparation included possible pelletization of these residues and even generate an excess of electricity. In several of the configurations, excess electricity generation could be achieved in the range of 0.5–8 MW.

Agricultural residues offer the possibility of reducing fossil fuel consumption, increasing energy security, and lowering  greenhouse gas emissions. However, certain residues, like palm oil residues, either represent a disposal problem for the processing industries or they are not used and thus, there is a considerable potential for improvement. These factors represent a strong driving force for the development of innovative polygeneration plants based on solid residues. This paper considers an energy analysis of a Palm Oil Mill (POM) in Colombia processing 30 ton of Fresh Fruit Bunch per hour (FFB/h).  Different heat and power generation options were considered with solid residues as feedstock. These configurations included steam cycles using backpressure or condensing-extraction turbines.  The possibilities to produce pellets from the residues and biodiesel from palm oil were also analyzed.  The steam cycle base operational parameters were 20 bar and 350 °C. More advanced steam conditions (40 bar) were also considered. The results show that it is possible to cover the demand of the POM and the required energy demand for residues preparation including possible pelletization and also biodiesel production. It is possible to obtain an excess of electricity between 0.4 and 3 MW if only residues are used.

Solar thermal energy and biomass fuels are often available at locations where they can benefit from combined hybrid energy utilization methods for the generation of electricity, representing suitable and advantageous integration alternatives. The feasibility of concentrating solar power (CSP) systems depends on cost limitations, desired installed power capacity and direct solar insolation, where smaller scales and low-cost solutions can often be preferred to large-scale investmentintensive installations. Biomass residues of various types, on the other hand, can be considered as proven fuels for small-to-midscale utility or industry based power or cogen arrangements and utilized through various technologies. The thermodynamic integration between a biomass fired power plant and a CSP unit can help to significantly increase the availability of the plant, improve its partial load characteristics, compensate for the intermittency of the solar energy resource while preserving the purely renewable profile of the generated electricity, and at the same time showing better overall performance when compared to two separate plants while avoiding the need for costly energy storage solutions. Biomass fuels can help reach better steam conditions in a steam plant based on CSP-generated steam, and thus improve the efficiency of energy conversion for the integrated hybrid system if compared with two individual single-fuel power units. In this study, an overview of feasible solar-biomass integration concepts is presented. A deeper thermoeconomic analysis of a selected integrated utility-scale biomass and CSP electricity generation plant is attempted, with certain simplifications. Furthermore, a multiobjective optimization strategy is regarded as very necessary and thus included in the analysis, where several major environmental aspects plus the cost of electricity are involved and defined in terms of desired parameters and conditions representative to Central Europe and Southeastern United States. The results are compared with conventional power generation alternatives. On that basis, a low-parameter CSP solution integrated with conventional biomass-fired combustion unit, where solar-generated steam is being superheated by the biomass fuel, has been chosen as the focus of the analysis in this study.

This paper summarises the authors’ own teaching experiences from two large MSc-level courses taught as part of several international Master programmes related to Sustainable Energy Engineering, organized by the Department of Energy Technology at the Royal Institute of Technology (KTH) in Stockholm, Sweden.  

Some important hinders and obstacles to effective learning are presented and discussed, addressing especially certain challenges for the students and their effect on student performance. Observations have been made throughout several years of increasing demand for energy- and sustainability-related knowledge by ever larger student groups. The growing number of international students and the fact that many students are aiming at expanding their abilities by specialising in energy engineering without having the necessary background, as well as the fact that many students following certain non-engineering programmes focusing on environmental or sustainability issues need nevertheless to study also purely engineering courses, brings many positive characteristics to the blended student team but also displays serious challenges to the practical optimisation of the learning activities, the intended learning outcomes, the speed of advancement in knowledge, and the general quality of education for such a diverse group of students.

Possible improvements and augmentations of the learning activities with the goal of finding solutions to these challenges on both a programme level and course level are proposed and subjected to testing in recent student batches. The expected results in terms of improved student performance, and the plausible further extension of this work, are introduced and analysed.