The main idea of a circular economy (CE) is to separate economic growth from resource consumption and environmental impacts. The characteristic approach of a CE assumes the minimisation of the amount of waste generated at the design level and, as a standard, includes innovations throughout the value chain. From an agricultural point of view, agricultural biogas plants are particularly important because they enable the management of all waste biomass and its conversion into useful energy and agricultural fertiliser. This paper presents methods for assessing the economic effectiveness of an investment in an agricultural biogas plant. The research goal was to develop a financial model. The authors of this study used available examples of the profitability of commercial ventures. We considered the investment aspects of agricultural biogas plants. Exemplary solutions are discussed, allowing the reader to become acquainted with various methods and proposals for thus far estimated investments. It may seem chaotic, but this is how the biogas market is characterised in the context of the implementation of biogas projects. Guidance is given regarding how to understand investing in this sensitive private farming sector. It is admirable that the renewable energy market has been systematised, and we hypothesise that it is necessary to develop an investment model in Polish conditions.

This article concerns the method of material consumption assessment of the cylinder-piston group of diesel engines in the biodiesel environment. The obtained experimental dependences of the wear coefficients on the example of the tribounit cylinder liner and the piston ring can be used to forecast the resource use during operation under specific conditions of the engine and the environment as a whole. The article systematizes the types of biofuels, depending on the type of raw materials from which they were made, taking into account the process and application. The physical and chemical aspects of the catalysts used for biofuels were indicated. The applied experimental methods for tribological wear of the piston-cylinder pair were analyzed. B70 biodiesel was used in the research, i.e., 70% mineral diesel oil and 30% methyl esters of rapeseed oil. Experimental tribotechnical studies of the influence of biofuels on the behavior of various materials have shown that when using this type of fuel, it is necessary to replace the materials from which some parts of the cylinder-piston group are made. To solve this problem, research has been carried out on a specially designed friction machine. The novelty in the article concerns the association, based on the literature, of hydrogen consumption causing material wear in friction contacts. The mechanism of the interaction of various construction materials during such friction has been disclosed.

The article reviews selected methods and techniques of agricultural biogas desulphurization. Presented is the current state of technological and measurement systems as well as raw biogas purification methods in terms of control and measurement-socio-economic aspects were also pointed out. On the example of a pilot agricultural biogas with the use of pig slurry, the required technical and technological criteria for the production and processing of agricultural biogas were indicated. The article presents the preliminary results of experimental studies on the course of changes in the volumetric composition of biogas on the basis of the average daily production of agricultural biogas.The amount of H₂S in raw and purified biogas was analyzed with the proprietary biogas desulphurization method in terms of the process parameters. A novelty is the use of a developed carbon mixture (activated carbon) with turf ore (iron compounds), which allows for 100% desulfurization of raw agricultural biogas under process conditions for mesophilic fermentation. The measurement results show a clear influence of desulphurization using the proprietary adsorption-absorption technique-agricultural biogas.

The properties, types, and physical and chemical aspects of pig slurry used in the fermentation process were presented. Characterization of the pig slurry microflora for a controlled biogas production process was performed. A pilot biogas treatment installation was presented on the example of a farm with 1100 Dan Bred fatteners kept in a grate system. The research was carried out to measure the biogas flow rate resulting from the reference pressure in the fermentor. An independent assessment of the amount of biogas and the pressure drop in the skeletal deposit was carried out. The basis for assessing the hydrodynamics of gas flow through the adhesive bed is the flow characteristic, which results from the pressure that forces this flow. In each case, the determination of this characteristic consists in determining the influence of the biogas stream on the value of this overpressure, equivalent to the pressure drop (it is tantamount to determining the total biogas flow resistance through the adhesive bed). The results of the measurements indicate the practical application of pig slurry-a substrate in a polydisperse system for the production of agricultural biogas in the context of renewable energies. The article indicates that the ferment was periodically mixed during the day, together with the fermentation of the ferment with fresh substrate. The tests were conducted for 49 days, thus demonstrating that it is more advantageous to mix the ferment hydrodynamically, obtaining a CH₄ level of about 80%.

With regard to the problem of current solar irradiation of the tracking surface by the tracker, this can be rationally assessed using a modeled flux with parameters close to the natural direct in accordance with the maximum transparent atmosphere. As part of the experimental research, the values of the air optical mass index in the zenith angle ranging from 0° to 90° were presented for the surface at sea level. The conducted analysis made it possible to confront the experimental research with the models of the optical air transparency mass index. In order to maximize the energy of the controlled beam of the sun’s rays, the angle of their incidence on the tracking surface by the tracker should be minimized. The modeling results show the actual hourly dependencies, which are average values of the intensity of the multi-year measurement. It was found that seasonal dependencies can be considered as theoretically possible and then used to assess the energy efficiency of the tracking surface. Theoretically, they also correspond to the maximum possible energy efficiency of continuous tracking of the Sun with a two-axis rotary tracker. A polar-rotary tracker is a device where the axis of rotation is parallel to the axis of the earth and orients the tracking surface using the simplest synchronous algorithm with a constant angular speed ω = 15 grad/h. The novelty of the article is a method presenting the combination of a normal trajectory with direction towards the sun. Due to the deviation towards the polar axis, the normal touches the horizon, not at six hours after full sun, but earlier and simultaneously. In this way, an uniaxial pole-rotating tracker provides the same irradiation regime as a biaxial one in terms of the deviation of the tracking surface from the axis of rotation, which is equal to the tilt of the sun.

The widespread use of wind power plants can provide full or partial energy supply to the consumer, taking into account certain investments and the instability of energy production. Modern wind energy technology involves the conversion of mechanical energy of the wind flow into electrical energy with subsequent conversion, at the request of the consumer, into thermal energy. In addition, the unprocessed use of the low-potential part of the wind flow, characterized by non-uniformity and randomness of its reception for the purpose of supplying heat to the recipient, requires new approaches to solving this problem. Bench experimental studies of this heater confirmed the adequacy of the mathematical model: within an hour, the temperature increase of the heater core changed from 22 °C to 36 °C at a voltage of 66 V and the number of pulses entering the heater coil was (15–17) discharges, which corresponds to the values of the mathematical expectation of the wind speed of (4–5.2) m∙s⁻¹ in the range of wind speed (4–8) m∙s⁻¹. The scientific novelty of this work consists in the development of a mathematical model for the operation of an electric pulse heater, which made it possible to develop methodological provisions for determining its mode parameters and to estimate the temperature change of its elements at random wind speed.