Voltage Problems on Farms with Agricultural Biogas Plants—A Case Study

Abstract

Constructing agricultural microbial gasification plants near livestock farms is essential for technical, economic, and environmental reasons. Utilising substrates from these farms allows for producing electricity, heat, and environmentally friendly manure. However, biogas plants often face technical challenges. This study evaluates the power quality of an agricultural biogas plant on a dairy farm. It was found that the plant was connected via a cable with an insufficient conductor cross-section, leading to significant voltage overshoots exceeding 14.6%, which prevented the activation of the second generator. Both generators could operate after replacing the feed-in cable, but considerable fluctuations in the feed-in voltage persisted. Further measurements indicated the need for changes in the digester design. Specifically, replacing the current two mixers with more lower-powered mixers operating alternately was proposed. Sharing these solutions more broadly can help prevent similar issues in future microbial gas plant constructions and optimise electricity production.

1. Introduction

In recent years, renewable energy generation has become crucial for the sustainable development of countries, regions, and individuals. The use of feedstocks to produce biogas and biomethane holds significant potential, potentially meeting about 20% of the current global gas demand. Agricultural and industrial waste negatively impacts health and the environment, emitting significant greenhouse gases such as methane and nitrous oxide [1,2]. Methane fermentation is one of the most effective technologies for converting organic waste into renewable energy. Agricultural biogas plants help reduce agriculture’s environmental impact, and the digestate obtained from the processed substrates can be effectively used as an organic fertiliser [3,4].

Additionally, biogas can mitigate the harmful effects of fossil fuel use [5], significantly increasing atmospheric CO₂ concentrations [6]. Biogas is an alternative energy source that generates electricity, heat, and cooling, and it can replace natural gas derived from fossil fuels [7]. Biogas produced in biogas plants mainly contains methane (CH₄) and carbon dioxide (CO₂) [8]. It also contains smaller proportions of water, nitrogen, hydrogen sulfide, ammonia, oxygen, siloxanes, and other particles [9]. The composition of biogas depends primarily on the substrates used and the technology employed [10].

When building a biogas plant, the location plays a key role, which mainly depends on the availability of input materials (type of farm), the availability of the electricity grid, and the distance from urbanised areas (due to the potential impact of the biogas plant on noise and odours) [11,12,13]. From the point of view of sustainable agricultural development, the most important are micro biogas plants, built directly on farms and fed with waste (e.g., slurry) generated on the farm. These measures allow for efficient and rational use of resources at an optimal cost level. A farm with a micro biogas plant is seen as modern and environmentally friendly. The profitability of agricultural biogas plants located at cattle farms depends mainly on the price of electricity and on having the capital to purchase and maintain an electrical generator [14,15]. In addition, suitable technical conditions for connecting such an energy source at the location are necessary [16]. As herd size increases, there is a noticeable decrease in the cost of building a biogas plant per cow [17]. When calculating the profitability of an investment in an agricultural biogas plant, the equipment’s maintenance and servicing costs should also be considered [18,19]. Various numerical methods, such as AHP [20], Vikor [21], and the numerical taxonomy method [22], can be used to assess the profitability of the investment. The payback period for the construction of an agricultural microbial gas plant is 6–7 years [23] (investments with a payback of up to 8 years are considered financially beneficial [24,25]). In comparison, the payback for a P.V. plant is within 5–6 years [26].

Agricultural biogas plants enable the generation of electricity heat and cooling and have several other advantages, among which are:

• the possibility of using biogas as a fuel for internal combustion engines [27];
• an increase in regional/national energy security through the development of distributed energy [28];
• a reduction in the cost of disposing of organic waste [29];
• mitigation of adverse climate change [30];
• use of local energy resources [31];
• production of wholesome manure [32].

Despite the apparent increase in biogas production, there is often a lack of relevant knowledge and technology in agriculture [33]. Therefore, an essential action is to broker information from agriculture to the technology industry, covering all legal, environmental, administrative, organisational, and logistical aspects, to implement as many biogas plant projects as possible successfully [34]. Many adverse effects, including financial losses, result from investors’ ignorance, who often violate the basic principles of biotechnology when selecting substrates for their installations [35]. Existing installations are constantly being improved in terms of their production efficiency and environmental impact [6]. The local use of the generated energy avoids the losses associated with the transmission of energy over longer distances [36].

Biogas power plants, being renewable energy sources, are characterised by the stability of the generated power over time (concerning wind or P.V. power plants). Previous authors’ studies on agricultural biogas plants connected to the Polish medium-voltage grid confirm this characteristic [8]. However, there is a lack of analysis in the literature of the results of studies that would present an in-depth analysis of the impact of generation in biogas plants on the voltage of the electrical system supplying consumers installed on farms. Power plants connected to the electricity grid can cause an increase in voltage at their point of connection [37]. This increase in network voltage stresses the insulation of electrical equipment, reducing its service life and, in extreme cases, can damage it [38]. Voltage variability depends on the structure of the grid and the power of the connected power plants [39]. In the case of P.V. plants, as the generation level increases, the number of voltage oscillations increases, which can cause instability—short-term voltage stability is strongly influenced by the disconnection of P.V. generation and the resulting voltage drop [36]. The main factors affecting static voltage stability are the power and location of the sources and how they are controlled [40]. Better voltage stability occurs near the transformer station. The variation in the voltage value depends not only on the actual power generated but also on the quality of the current connections, the length and cross-section of the wires connecting the micro installations to the main switchgear and the resistive parameters of the low-voltage network [41]. When the line is not loaded, and the distributed energy sources operate at total capacity, the voltage at each point in the system is higher than the voltage at the transformer terminals. This situation worsens as one moves away from the transformer. The effect of generation on mains voltage values is described in detail for P.V. systems. It is not easy to find publications that describe the impact of a biogas plant (as a stable energy source) on the voltage parameters of the power grid.

The impact of microbial biogas plants on the parameters of the supply voltage is all the more relevant because, in Poland, farmers with small agricultural biogas plants have reported problems with the operation of specific consumers, including milking clusters. The authors have filled this research gap by conducting a detailed study of one Polish microbial gas plant. The main objective of the research was to analyse the causes of voltage problems on the farm and to propose and apply a possible solution to this issue. Particular analysis was carried out on the parameters determining the quality of electricity generated in the agricultural micro biogas plant. Presenting the proposed solutions to a broader audience may make it possible in the future to avoid mistakes made in the construction of micro gas plants and even influence the companies building their components to optimise the amount and quality of energy fed into the grid.

2. Materials and Methods

The study was conducted at a Polish agricultural biogas plant directly connected to a farm installation. The primary substrate used in this biogas plant was manure from approximately 120 dairy cows. The biogas plant comprises three main facilities: a digester, an engine house, and a digestate tank. The digester had a diameter of 15.5 m and a wall height of 3.7 m, with a total height of 8.7 m, including the biogas tank. During regular operation, the digester held approximately 590 cubic meters of substrate, mixed by two agitators with a rated power of 9 kW. The engine room was constructed as a container housing an internal combustion engine connected to a generator and other technical and control equipment. Biogas purification was achieved using a gas filter and an activated carbon filter. The biogas produced was combusted in two WG1605 combustion engines, each with a power output of 20 kW and an efficiency of 97%. A type 4P/IE2 asynchronous generator, operating at 1500 rpm, was coupled to the engines, which were also equipped with a water-cooling circuit.

The digestate, the post-production biogas material, was stored in an open concrete tank. A schematic of the biogas plant is shown in Figure 1. The biogas plant was connected to the farm’s main meter switchgear via a 198-meter-long YKY 4 × 25 mm² cable. The farm’s connection power was 40 kW, limited by overcurrent protection C63. The meter switchgear was supplied from the 63 kVA 15/0.4 kV transformer station by a 413 m long overhead line with 50 mm² conductors. The overhead transformer station was supplied from the main power point of the electricity system by a 6000 m long overhead line made of steel and aluminium conductors with a cross-section of 50 mm². According to information obtained from the Distribution System Operator, the power system had a short-circuit capacity of 102 MVA.

The owner of the biogas plant reported the following problems with its operation:

Due to voltage exceedances at the agricultural biogas plant’s connection point, it is impossible to run the second generator.

Increasingly frequent cluster malfunction—voltage error displayed.

The slurry stayed in the fermenter for an average of two to three weeks but no less than 12 days. Daily, a portion of the digested slurry (regulated by the biogas pressure sensor) was pumped out of the digester into the digestion tank. When the lower biogas pressure threshold value was exceeded, a new slurry was pumped into the digester (from the collection pit) to replenish the digestate.

Power quality is defined as a set of parameters describing the characteristics of the power supply process to the user under normal operating conditions, characterising the supply voltage, and determining the continuity of the power supply to the consumer. The following parameters describing power quality were analysed in the study [42,43]:

• Mains frequency is the number of repetitions in the time waveform of the fundamental component of the supply voltage measured over a specified time interval. The frequency deviation is the difference between a given value and the rated frequency value exhibited during regular power system operation over at least a few seconds. The frequency deviation should, in most cases, not exceed +/− 1% of the rated grid frequency.
• Voltage deviation (slow voltage variation) is the difference between the actual and rated mains voltage values. In most cases, the free voltage variation should not exceed ± 10% of the rated mains voltage value.
• Voltage fluctuations (rapid changes in voltage). Indicators that characterise voltage fluctuations include:

  -

  The amplitude of voltage fluctuations is expressed as the ratio of the voltage variation value to the rated voltage. In most cases, this value should not exceed 3%,

  -

  frequency of voltage fluctuation amplitudes or, in the case of periodic fluctuations, the frequency of voltage fluctuations;

  Short-term flicker index P_st (index indicating the annoyance of flickering light over a few minutes). P_st = 1 is the conventional threshold for the annoyance of light flicker;

  -

  the long-term flicker index P_lt (an index indicating the annoyance of flickering light over a long period, of the order of a few hours). In most cases, the index P_lt should not exceed 1. The value of the index is determined from successive values of P_st, according to the relation:

  $${\mathrm{P}}_{\mathrm{l}\mathrm{t}}=\sqrt[3]{{\displaystyle \frac{{\sum }_{\mathrm{i}=1}^{12}{\mathrm{P}}_{{\mathrm{s}\mathrm{t}}_{\mathrm{i}}}^3}{12}}}$$

  (1)

  in which P_sti (i = 1, 2, 3, ... 12) are successive values of the short-term flicker indices P_st.

• Voltage asymmetry—unequal voltage values and/or unequal angles between successive phase voltages. The asymmetry of the system of supply voltages results, among other things, in the appearance of symmetrical components of the opposite order. The parameter describing this condition is the voltage asymmetry factor α_U% (opposite voltage asymmetry) [36,44]:

  $${\mathsf{\alpha }}_{\mathrm{U}\mathrm{\%}}=\left|{\displaystyle \frac{{\underset{\_}{\mathrm{U}}}_2}{{\underset{\_}{\mathrm{U}}}_1}}\right|·100\mathrm{\%}$$

  (2)

  where: U₁, U₂—the composite values of the symmetrical components of the consensual and opposite order of the voltage.
  In most cases, the voltage asymmetry factor should not exceed 2%.
• The distortion of the voltage waveform, defined by the total harmonic distortion factor (THD_U) [45]:

  $${\mathrm{T}\mathrm{H}\mathrm{D}}_{\mathrm{U}}={\displaystyle \frac{\sqrt{{\sum }_{\mathrm{h}=2}^{\mathrm{\infty }}{\mathrm{U}}_{\mathrm{h}}^2}}{{\mathrm{U}}_1}}·100\mathrm{\%}$$

  (3)

  where: U₁—rms value of voltage for the first harmonic, U_h—rms value of voltage for the h-th harmonic, h—order of harmonic.

In most cases, the voltage distortion coefficient THD_U in low-voltage networks must not exceed 8%.

At the agricultural biogas plant under study, electricity was the primary end product, and its production was maintained at the highest possible level. The tests were carried out using a SONEL PQM-701 portable power quality parameter analyser, performing measurements in accordance with Class A of the EN 61000-4-30 standard [46]. The recorder has a legalisation certificate issued by the Research and Calibration Laboratory in Świdnica. The recording of electricity parameters was carried out with 1-minute averaging and recording of the measurement results. The measurement of voltage/current was carried out with an accuracy of ±0.1% of the rated voltage/current, and the measurement of voltage unbalance with an accuracy of ±0.15% (absolute error). The maximum error of the harmonic measurement was ±0.05% U_n for voltage and ±0.15% I_n for current.

3. Results and Discussion

3.1. Analysis of the Supply Network Parameters of the Agricultural Biogas Plant under Study

The recorded grid frequency values are shown in Figure 2. Although frequent changes in frequency were noted, the maximum deviation from the rated value was slight, not exceeding 0.18%. However, a particular recurring pattern is worth noting: the lowest recorded frequency values occurred in the afternoon during peak electricity network load and increased as the load decreased.

The waveforms of the variation in voltage levels and the percentage deviation of the voltage from the rated voltage recorded during the tests are shown in Figure 3 and Figure 4.

The analysis of the recorded voltage waveforms reveals a cyclic pattern of changes similar to those observed in frequency. Voltage fluctuates with changes in the energy system’s load, decreasing as the load increases. Increases in voltage deviation values accompany these voltage decreases. The voltage varied from approximately 200 V to nearly 260 V. Such significant fluctuations can cause interference with electronic equipment.

The recorded values of the long-term flicker annoyance factor (P_lt) are included in Figure 5.

The values of the long-term flicker annoyance index also vary cyclically. The lowest P_lt index occurs at night, and the highest in the afternoon. It is usually related to the presence of so-called unstable loads (loads that consume electricity that are significant in value and variable in time) in the power system.

The voltage asymmetry factor also varies cyclically (Figure 6). The recorded waveform usually reaches a maximum in the afternoon, while the minimum occurs at night.

The recorded THD_U values for each phase are shown in Figure 7.

The values of the total voltage distortion coefficient recorded at the biogas plant also vary in the diurnal system, mainly due to the cyclic operation of equipment drawing current distorted from the sinusoidal waveform from the grid. The highest values were recorded mainly in the afternoon.

The results of the analysis of the active power levels generated by the agricultural biogas plant are summarised in Figure 8.

Figure 8 and information from the owner indicate that only one turbogenerator is operational at the agricultural biogas plant. The data also show long periods when the biogas plant is not generating electricity (the power is negative). The times when the agitator is running are visible on the waveform, with the recorded power dropping close to zero when the generator is running or around −17 kW when no energy production occurs. Therefore, it can be inferred that one 20 kW generator is necessary to meet the peak needs of the agricultural biogas plant. Without the agitator running, the biogas plant consumes about 1 kW (for control systems, monitoring, etc.).

The inability to switch on the second generator is due to the mains voltage exceeding the permissible value of 253 V. Even with one generator running, voltage exceedances were recorded (Figure 3), causing the generator unit to disconnect from the grid to protect other electrical equipment sensitive to voltage spikes. These voltage overshoots are also evident in the voltage deviation waveform (Figure 4), which repeatedly exceeds the regulatory limit of 10%. To enable the operation of the second generator, an analysis of the power supply system should be conducted to prevent voltage overshoots.

3.2. Analysis of the Feeding System of the Biogas Plant under Study

To carry out the analyses, a computer model of the power supply system for the agricultural biogas plant was created using the Neplan program, a global standard in power system analysis. The system data described in Section 2 were used for the calculations.

The original power supply system for the biogas plant consisted of a 198-meter-long cable line using YKY 4 × 25 mm². As shown in Figure 9, this cross-section does not ensure that the voltage remains within legal limits. The cable cross-section should be no smaller than 61 mm² to meet the voltage requirements, with the closest standardised cross-section being 70 mm². Changing the cross-section of the cable reduces the power loss from 1.82 kW to 0.65 kW. Assuming the biogas plant operates 8400 h annually, this change reduces annual energy losses by 8 MWh. An electricity price of €95/MWh results in an annual cost saving of €760. A cable with a more considerable cross-section costs €23 per meter. Considering only the reduction in power losses on the power cable, the upgrade from a 25 mm² to a 70 mm² conductor will pay for itself in 6 years.

3.3. Analysis of the Main Supply Parameters of the Studied Agricultural Biogas Plant after Changing the Supply Cable

The recorded values of the mains frequency after the power system conversion are shown in Figure 10.

Analysing the course of the variation in the frequency value recorded after the power cable change, repeatability analogous to that before the conversion can be observed.

Analysing the recorded voltage waveforms (Figure 11) and voltage deviation (Figure 12), as in the previous case, it is possible to notice a cyclic variation. However, the variations are noticeably more minor—it is much less frequent for the voltage to take values above 250 V, and only one measurement was recorded in which the voltage dropped below 210 V. This is reflected in the voltage deviation values (Figure 12), which were within ±10% of the rated voltage throughout the recorded period.

The unstable operation of the agricultural biogas plant is reflected in the recorded values of the flicker nuisance index (Figure 13). These are nearly three times lower than the values found in the system before the power cable change (Figure 5).

As with the original system, a cyclic variation in the voltage asymmetry factor was also recorded after changing the power cable (Figure 14). This does not show any significant differences in the values achieved, but comparing the two waveforms (Figure 6 and Figure 14), it is clear that the waveform recorded after the power cable change is less ‘jagged’.

As in the original system, the maximum values of the voltage distortion factor occur in the afternoon and evening (Figure 15). However, these values are almost twice as low as the first biogas plant studied.

The maximum changes in the recorded active power values of the agricultural biogas plant after changing the feed cable (Figure 16) were due to the operation of the two digester mixers. Notably, at no point during the measurement did the active power reach the generator’s rated power. This discrepancy was caused by the energy consumption of the equipment installed in the biogas plant.

3.4. Analysis of the Main Supply Parameters of the Studied Agricultural Biogas Plant after Changing the Supply Cable

The value standardised in Polish regulations [42,43] is the 95% quantile. It is defined in the standard [42] as the highest value obtained from 95% of the recording time during a week. A summary of the recorded supply voltage frequency values is shown in

Table 1. Results of statistical frequency analysis.

Parameter	Before the Changes		After Changes
	Frequency	Deviation from Rated Value	Frequency	Deviation from Rated Value
	[Hz]	[%]	[Hz]	[%]
Average value	49.995	−0.010	49.996	−0.008
Minimum value	49.910	−0.180	49.900	−0.200
Maximum value	50.080	0.160	50.070	0.140
Quantile 95	49.970	0.040	49.970	0.040

.

A summary of the measured voltage levels in each phase of the system supplying the agricultural biogas plant is shown in

Table 2. Results of statistical analysis of tension levels.

Parameter	L1 Phase		L2 Phase		L3 Phase
	Phase Voltage	Deviation from the Rated Voltage	Phase Voltage	Deviation from the Rated Voltage	Phase Voltage	Deviation from the Rated Voltage
	[V]	[%]	[V]	[%]	[V]	[%]
Before the changes
Average value	238.913	3.452	238.120	3.109	233.096	0.934
Minimum value	203.260	−11.986	200.130	−13.341	197.070	−14.666
Maximum value	258.650	11.999	257.120	11.336	248.810	7.738
Quantile 95	253.480	9.760	252.110	10.167	244.450	10.850
After changes
Average value	242.655	5.073	241.368	4.515	240.101	3.967
Minimum value	214.320	−7.197	212.130	−8.145	208.580	−9.682
Maximum value	254.120	10.037	254.280	10.106	251.760	9.015
Quantile 95	250.980	8.678	251.737	9.005	247.690	7.253

. Several relationships emerge from analysing these values. Noticeable voltage asymmetry exists in the individual phases. The lowest voltage occurred in the third phase, likely due to the asymmetrical connection of single-phase loads to the grid. However, in each phase, the voltage recorded after changing the power cable was lower than that of the primary circuit, translating into voltage deviation values. After the power cable change, although momentary exceedances of the permissible voltage deviation values were registered, the regulatory normal quantile of 95% slightly exceeded 9%. The operation of the biogas plant causes an increase in voltage at its point of connection, which, as seen from the measurements, is kept close to the upper limit allowed by regulations. Switching on the digester mixer reduces generation by almost half, resulting in a drop in voltage values. Turning it off causes the voltage to rise again; this cycle repeats every 30 minutes. This cyclic, step-by-step change in voltage is not beneficial for both the generated electricity’s quality and consumer equipment’s durability. It is mainly noticeable as a change in the intensity of the lighting installed on the farm and can lead to malfunctioning of voltage-sensitive equipment, potentially causing deactivation or even damage.

Table 2 provides the answer to the malfunction of the milking apparatus—its shutdown. Before the power cable was replaced, the lowest recorded voltage value was only 197 V, below the usually accepted voltage limit of 200 V for milking clusters. Consequently, the clusters reported a voltage error and went into emergency mode. After replacing the power cable, the lowest recorded voltage value was 208 V, higher than the minimum value allowed by regulations. As a result, there were no more emergency shutdowns of the consumers during the test period.

A summary of the results of the statistical analysis of the values of the voltage asymmetry coefficient in the power supply system of the biogas plant under study is presented in

Table 3. Results of statistical analysis of voltage asymmetry coefficients.

Parameter	Before the Changes	After Changes
	αU	αU
	[%]	[%]
Average value	0.315	0.350
Minimum value	0.010	0.020
Maximum value	1.230	1.120
Quantile 95	0.610	0.620

. As was shown during the analysis of the recorded waveforms of variation in the asymmetry coefficients, the analysis of the values presented in Table 3 did not reveal any significant differences in the values of this parameter before and after the change of the power supply cable.

A summary of the results of the statistical analysis of the long-term flicker nuisance value (P_lt) at the connection point of the agricultural biogas plant during the recording period is presented in

Table 4. Results of statistical analysis of the long-term flicker annoyance index values (P_lt).

	Before the Changes			After Changes
Parameter	PltL1	PltL2	PltL3	PltL1	PltL2	PltL3
Average value	2.688	2.762	2.887	0.993	1.141	1.074
Minimum value	1.540	1.620	1.690	0.720	0.830	0.790
Maximum value	3.840	3.960	4.160	2.180	1.830	1.860
Quantile 95	3.500	3.580	3.767	1.384	1.691	1.440

. The values presented there show that the P_lt factor in each analysed system exceeds the permissible value (1). However, replacing the power cable reduced the value of the 95% quantile by almost three times.

In the primary system, non-compliance with the voltage distortion requirements from the sinusoidal waveform was observed (

Table 5. Results of statistical analysis of the coefficients of higher harmonic content of voltage—THD_U.

Parameter	Before the Changes			After Changes
	THDUL1	THDUL2	THDUL3	THDUL1	THDUL2	THDUL3
	[%]	[%]	[%]	[%]	[%]	[%]
Average value	4.366	3.641	4.693	2.114	1.821	1.925
Minimum value	2.360	2.020	2.820	1.220	1.010	1.010
Maximum value	13.380	7.190	10.750	7.330	6.050	6.290
Quantile 95	8.557	5.470	7.550	2.760	2.530	2.700

). The maximum THD_U value is almost 1.7 times higher than the required (8%) [42,43]. The higher harmonics cause additional voltage and power losses (increased cable heating) in the biogas plant’s power supply system. These overshoots are correlated with voltage drops occurring in the electricity network. Changing the power cable reduced the value of the voltage distortion factor by almost half.

The issue of supply voltage is particularly evident in photovoltaic (P.V.) plants, where increased generation often causes the voltage at the plant’s connection point to exceed current regulatory limits. This challenge arises due to the high prevalence of P.V. systems. On the other hand, agricultural biogas plants, including micro gas plants, are generally considered stable energy sources with minimal voltage issues. However, the authors’ research indicates that these plants can also face difficulties maintaining voltage within regulatory limits. The analysis presented in this article applies to other facilities connected to the low-voltage network.

To determine the maximum connectable power of a source, it is essential to understand the impedance of the supply system and the voltage level maintained at the transformer station and to apply well-known power and voltage loss relationships. The technical solutions proposed in this paper aim to mitigate voltage level issues in micro gas plants. For biogas plants fed by longer lines, more significant, cross-sectional area cables are recommended to reduce voltage drops and power losses.

4. Conclusions

The authors’ investigations revealed that the power supply voltage issues the owner reported were technical. Specifically, the problem with starting the second generator stemmed from an incorrect cable cross-sectional area used to supply the biogas plant. The inadequate cable size led to significant voltage drops, resulting in voltage levels at the connection point that significantly exceeded permissible limits. Replacing the power cable allowed both generators to operate continuously but did not fully resolve the voltage issues.

The operation of the digester mixers caused the most substantial voltage fluctuations. When both mixers ran simultaneously, the power input to the electrical system dropped by an average of approximately 14.2 kW. Increasing the number of mixers while reducing their power ratings is recommended to address this. Additionally, operating the mixers in cycles—where only one mixer runs at a time—would help stabilise power consumption and reduce voltage fluctuations at the biogas plant’s connection point. Lower-powered motors would also minimise inrush currents and reduce voltage variations during startup. The exact number and power rating of the mixers will depend on the size and design of each mixer.

A well-sized agitator should be large enough to mix the fermentation mass thoroughly but small enough to be driven by a less powerful motor. Alternating between lower-powered mixers should mitigate voltage fluctuations caused by the activation and deactivation of individual devices. With appropriately selected mixing components, no significant changes in biogas production efficiency are expected. The authors are researching the optimal parameters for devices (agitator and motor) to maximise efficiency while minimising electricity consumption.

Oversizing the generator capacity (relative to the connection capacity) would cover auto consumption and enable power close to the connection capacity to be fed into the grid. The costs of oversizing the generator, already at the design stage of the biogas plant, are not high compared to the costs incurred when replacing the already installed device. Instead of two generators with a capacity of 20 kW each, it is recommended to install units with a capacity of 22 kW, which follow the manufacturer’s range of biogas generators. At the same time, it is necessary to simulate voltage changes caused by the operation of an agricultural biogas plant already at the design stage. It is also possible to post-measure the network parameters at the planned biogas plant connection point. These measurements allow well-known formulas to determine the maximum connectable power to meet the system’s voltage requirements. It will enable the proper operation of not only the biogas plant itself but also other equipment installed on the farm. Their smooth functioning will be essential regarding sound management’s organisational, financial, and environmental aspects.