Proper sizing of energy systems is a key aspect that allows avoiding overestimated installation costs or failures in operation and dispatch. However, most of the available sizing tools focus on systems dedicated only to electrical loads, omitting combined energy systems with simultaneous supply of various thermal demands. This study presents an adaptation of an existing open access techno-economic optimization model for broadening the design tool for small-scale energy systems supplying both, electrical and thermal needs. For this, a new typology of an energy system was proposed considering the use of biogas, solar energy and adding thermal components. This was followed by modifying the model framework, constraints equations and objective function, which is the net present cost of the system. Once the design tool was verified a model was constructed to analyse the feasibility of a polygeneration plant for an association of 30 small dairy farms. The developed model was able to optimize the sizing of the main system components for different proposed scenarios, encompassing supply of electricity, refrigeration, biogas for cooking and fertilizers. For the selected application it was found that the aggregated cost of producing electricity and heat ranges from 0.044 to 0.070 USD/kWh; the penetration of solar energy can reach up to 32%; while the annual potential savings of CO2 emissions of applying the solution ranges from 109 to 127 ton of CO2.

The transition towards sustainable economies with improved resource efficiency is today’s challenge for all productive sectors. The dairy sector in Latin America is growing without considering a clear path for sustainable energy and waste management solutions. This study proposes integrated solutions through a waste-to-energy approach. The solutions consider biogas production (via cow manure) as the main energy conversion pathway; technology solutions include biodigesters, power generators, and combined heat and power systems that supply not only the energy services demanded by dairy farms (for cooking gas, electricity, refrigeration and hot water) but also provide organic fertilizers. Biogas’ potential was estimated to verify whether it can cover the energy demands of the farms, while the levelized costs of producing biogas and electricity were the indicators for the techno-economic evaluation of the solutions. Greenhouse gas emission reductions were estimated by following IPCC guidelines. Specifically, the proposed solutions lead to energy self-sufficiency in most dairy farms with relevant biogas and electricity costs in the range of 1.7–3.7 and 6–12 USD cents/kWh, respectively. In addition, implementing the proposed solutions in Latin American dairy farms would allow annual greenhouse gas emission reductions of 32.8 Mton CO2 eq. with an additional 17 Mton if widespread use of the supplied organic fertilizers is achieved.

This study presents a comparison of combustion performance, losses, and efficiency at steady-state and transient conditions for different biomass types in a residential boiler. The types of biomass used were 6 and 8 mm wood pellets, 6 mm sugarcane bagasse pellets, 6 mm sunflower husk pellets, and Brazil nut shells. The comparison in the development of the temperature in the combustion chamber during the start-up revealed that the 6 mm wood pellets ignite and propagate faster than the rest of the biomass fuels due to their smaller size compared with 8 mm wood pellets and lower ash content compared with the rest of the biomass sorts. Thermal power output and efficiencies under steady-state and transient conditions were calculated by the direct method, i.e., by measuring the heat recovery by the water boiler, and the indirect method, i.e., by measuring the heat losses. By using the indirect method, the availability of the flue gas thermal power during the start-up was seen more in detail than when the other method was applied. When comparing both methods as tools for boiler efficiency evaluation for different fuel types, the discrepancy of the resulting efficiencies between is larger when there are higher amounts of chemical losses in the boiler. Therefore, this method shows good agreement also for bagasse pellets but is, without modification, proposed not to be valid for fuels emitting higher amount of carbon monoxide (CO). Boiler efficiencies reached class 3 boilers according to EN 303-5 (> 74.8 %) for all biomass sorts.

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.

Energy services could be greatly improved by using of residues from local food industries in small-scale combustion units. Wood pellets are a reliant and proven fuel to be used in small-scale combustion units. However, these units should preferably be able to use different types of biomass depending what it is locally available. Therefore, studies have been focused on exploring the suitability of using agricultural residues for small-scale heat and power generation using direct combustion. This study targets to compare the combustion of different agricultural residues in a single unit designed for wood pellets. The different biomass fuels used are circle divide 6 mm and circle divide 8 mmwood pellets, circle divide 6 mmbagasse pellets, circle divide 6 mmsunflower husk (SFH) pellets and Brazil nut (BN) shells. The results reveal a decrease in the fuel power input, higher oxygen levels in the flue gases and shorter cycles for ash removal when using the agricultural residues. The excess air ratio was calculated based on a mass balance and compared with a standard equation showing a good agreement. CO and NO emission levels as well as the relative conversion of fuel-C to CO were higher for the BN shells and SFH pellets in comparison to the other biomass types. SO2 emission was estimated based on the analysis of unburned sulfur in ash and mass balances; the higher estimated levels corresponded to the BN shells and SFH pellets. All the biomass sorts presented over 95% relative conversion of fuel-C to CO2. Wood pellets and BN shells presented the highest amount of unburned carbon in ash relative to the fuel-C. The relative conversion of fuel-N to NO and fuel-S to SO2 were higher for wood pellets. Bagasse pellets showed similar emission levels and relative conversion efficiency to wood pellets.

Access to energy services is crucial for the development of countries. Therefore, in developing countries, the access to modern conversion technologies would contribute to reduce the poverty, improve health services and promote the economy especially in rural areas. Around 2.5 billion people in these countries use biomass for cooking. However, major concerns are due to the unsustainable use of biomass and the inefficient conversion technologies employed in rural areas. Therefore, the use of locally available biomass in modern biomass conversion technologies would significantly reduce emissions and improve the energy efficiency. These modern technologies may include residential pellet stoves and boilers which at the moment only are used for heating appliances in industrialized countries. Their combination with a prime mover like a Stirling engine could a very attractive solution to produce combined heat and power (CHP) though still in prototype stage. In this context, this study is mainly focused on the development of an energy system fuelled by locally available biomass to produce heat and electricity based on a Stirling engine. The main objective is to perform experiments to find relevant parameters that characterize the energy system proposed.

In the first stage of this work, the suitability of using agricultural residues in a pellet boiler was evaluated in comparison to commercial wood pellets. The agricultural residues used during the tests were: sugar cane bagasse, sunflower husks and Brazil nut shells. The first two residues were pelletized and the last one was reduced to a uniform size. Parameters and energy used during the pelletizing were found. Emission levels and boiler efficiency under steady-state and transient conditions were also presented for the different biomass sorts. In the second stage, the integration of the same pellet burner and the Stirling engine was characterized in terms of losses and efficiency calculations. Finally, the integration of the pelletizing, combustion, and heat and power generation was discussed based on experimental and predicted results.

The integration a Stirling engine with a pellet burner is a promising alternative to produce heat and power for residential use. In this context, this study is focused on the experimental evaluation of the integration of a 20 kW_th wood pellet burner and a 1 kW_e Stirling engine. The thermal power not absorbed by the engine is used to produce hot water. The evaluation highlights the effects of pellet type, combustion chamber length and cycling operation on the Stirling engine temperatures and thermal power absorbed. The results show that the position of the Stirling engine is highly relevant in order to utilize as much as possible of the radiative heat from the burner. Within this study, only a 5 cm distance change between the Stirling engine and the pellet burner could result in an increase of almost 100 °C in the hot side of the engine. However, at a larger distance, the temperature of the hot side is almost unchanged suggesting dominating convective heat transfer from the hot flue gas. Ash accumulation decreases the temperature of the hot side of the engine after some cycles of operation when a commercial pellet burner is integrated. The temperature ratio, which is the relation between the minimum and maximum temperatures of the engine, decreases when using Ø8 mm wood pellets in comparison to Ø6 mm pellets due to higher measured temperatures on the hot side of the engine. Therefore, the amount of heat supplied to the engine is increased for Ø8 mm wood pellets. The effectiveness of the engine regenerator is increased at higher pressures. The relation between temperature of the hot side end and thermal power absorbed by the Stirling engine is nearly linear between 500 °C and 660 °C. Higher pressure inside the Stirling engine has a positive effect on the thermal power output. Both the chemical and thermal losses increase somewhat when integrating a Stirling engine in comparison to a stand-alone boiler for only heat production. The overall efficiency of the pellets fired Stirling engine system reached 72%.