After almost a year of operation of the demonstration prototype and measurements on different areas of the slurry pond, the results obtained by the project “Alternatives in the use and control of the potential of slurry ponds” are presented, within the 2020 call for subsidies to support collaborative actions of agents in the agricultural sector.
The prototype of the floating photovoltaic system was installed last November 2022 to cover the slurry pond of a mother pig farm located in the municipality of Tauste.
The performance of the prototype has been evaluated in three ways:
1.- On the one hand, the correct functioning of the photovoltaic system has been checked, i.e. that it produces energy.
2.- Secondly, the degradation of the materials has been visually evaluated. In addition, since the floating photovoltaic prototype also incorporates the combination with a common solution for the covering of ponds (hexagonal pieces of plastic), the correct interaction between both systems has been evaluated.
3.- Finally, measurements were taken of the quantities of ammonia emitted by the pond, comparing the emission levels of the free surface of the pond with those of the covered surface.
The economic profitability of the system has also been studied, taking into account the investment and savings produced. With the results obtained, the suitability to fulfil the function of reducing emissions from slurry ponds in a sustainable and cost-effective way has been assessed.
Main features of the prototype:
Photovoltaic production
Data has been taken from the start of the system functioning at the end of November 2022 until the dismantling of the prototype at the end of August 2023. However, in order to make the data comparable with each other, only the full months are presented: December 2022 to July 2023.
In total, 34.6% of the farm’s electricity consumption has been covered directly.
During its operation, the demonstration prototype has produced a total of 23,371.25 kWh, of which 83.5% has been used to directly cover the farm’s demand, while the remaining 16.5% has been injected into the grid. These surpluses, if the farm were to use self-consumption with compensation, could produce additional savings on its electricity bill.
Degradation of materials and interaction with each other
In view of the results obtained during the operation of the prototype, it was decided to remove it from the pond to facilitate visual inspection and sampling. The degradation of the materials has been checked by visual inspections during operation and after disassembly of the prototype.
The following conclusions are noted:
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There is oxidation of some of the metal components of the system, which react with the ammonia environment. These components (screws, nuts and metal terminals) are therefore not suitable for use in this type of application.
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The panels get dirty quickly in the pond environment. A thorough and frequent cleaning schedule will be necessary, using water at low pressure, or cleaning with soapy water by climbing onto the system platform.
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The floats and other components of the flotation system become fouled with slurry. Dirt has been found to be embedded in the float plastic, in the plastic connecting parts (long and short joints, bolts and nuts) and in the mooring ropes. In addition, due to the shape of these floats, insects accumulate in the “bathtub” underneath the panels, although, in principle, they have not been found to affect the materials or the functioning of the installation in any negative way.
In addition, the interaction between the floats of the system and the plastic hexagonal elements has been evaluated. It is found that they are placed in a honeycomb arrangement around the PV system without any problems. However, they are not able to completely slip through the gaps in the floats. A problem for the correct interaction between the two elements is the crusts that form on the semi-liquid slurry, which stick to the walls of the floats and the hexagons. This makes it difficult for them to enter the gaps between the floats. It is therefore recommended that the hexagons are thrown into the pond after the slurry has been churned.
Emission reductions
Measurements have been carried out both in the area covered by the photovoltaic system and on the free surface of the pond, with a frequency of 15 days. It was found that there is more than a 40% reduction in NH3 emissions in the covered area compared to the uncovered area, which would be sufficient for an existing farm to comply with emission reduction regulations.
However, some limitations are noted with regard to the measurement method used: during the project, difficulties have been encountered with the ammonia sensors due to the high measurement sensitivity required; furthermore, measurements should be taken over a longer period of time (during a warm summer and a cold winter period) in order to adequately assess the emission reductions. This has not been possible due to limitations in the project schedule.
Economic profitability study
Taking into account the variables of the initial investment in the system and photovoltaic production, which generates savings both in terms of reducing the energy demanded from the grid (direct self-consumption) and in terms of compensating the photovoltaic surpluses fed into the grid, the profitability of the proposed solution is studied: in total, the photovoltaic system, under the self-consumption with surplus compensation modality, represents a total saving of 41% on the farm’s electricity bill.
Taking these savings into account, the initial investment would pay for itself in about 6 years.
Conclusions
The project has raised new issues that could not be resolved during it’s duration:
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The need arises to study different floating elements that can be compatible with the ammonia environment and suitable for application in the covering of slurry ponds, including the possible modification to adapt to this. On the one hand, the system must ensure maximum coverage of the surface of the pond and, on the other hand, it must allow the methane generated to be evacuated without causing problems.
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It is necessary to replicate the experiment taking into account a more accurate method of measuring ammonia emissions, both in terms of measurement technology used and duration of measurements. In this way the emission reduction results can be validated.
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There is a need to study the feasibility and cost-effectiveness of the solution on different farms (with different slurry characteristics). For example, on fattening pig farms, which have less liquid slurry.
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It is necessary to analyse methane emissions for at least one full year in order to properly assess the risk of ignition or explosion from the installation of electrical components on the basin.
These questions give rise to the possibility of continuing with the study in the future, extending it to other types of situations and taking a more precise measurement protocol, having access to a larger budget. It is therefore intended to address them in future projects.
