The winerys energy needs: the use of pruning residues and the environmental sustainability

The management of the vine shoots can be done with two different methods to be assessed in relation to the environmental impact calculated with the life cycle analysis (Life Cycle Assessment). A research activity carried out at a social winery in the Oltrepò Pavese area, which works the grapes produced in an area of about 600 hectares, has allowed comparing the different methods and results

Besides some positive aspects and unquestionable opportunities, such as the virtuous management of “secondary” matrices, the diversification of income and the reduction in dependence on fossil fuels, the development of agro-energy supply chains still involves some problems to be solved, such as regulatory authorizations, the need for specific skills, etc.

In the case of viticulture, vine pruning residues are usually a problem, if only for the cost that their management represents within the economic budget of the winery. Moreover, if the energy produced can be exploited directly in the winemaking process their exploitation for energy purposes represents an opportunity to diversify the sources of income and reduce the costs of grape production.

The traditional management of the residues provides for their shredding and their subsequent burying, in order to increase the organic matter content of the soil; alternatively, the biomass is accumulated on the headland of the plot and then burned, this last solution is forbidden almost everywhere to reduce the highly polluting impact on air quality. On the other hand, the shredding and the burying can create problems from the phytosanitary point of view, because in the pruning residues some pathogens/parasites winter and they can become a source of infection.

In this scenario, in recent years many studies have addressed the topic of energy enhancement of grape pruning residues, especially from a technical and economical point of view, considering direct combustion or gasification from the product packaged in small cylindrical bales, or from wood chips or pellets.

Generally, the collected biomass has a medium-low quality, above all because of the non-negligible content of residue and/ or ashes, and it is burned in devices with a thermal power lower than 150 kW, for heat production for domestic use, or in gasifiers or larger plants, for cogeneration of electricity and heat. To date, the experiences of trigeneration and/or combined generation of hot and “cold” are few, although in the course of winemaking the need for “cold” is not negligible, notably the temperature reduction of the white grapes must, the tartaric stabilization, etc.

In any case, together with the assessments of technical and economic feasibility, the environmental aspects must also be examined carefully, because of the consumers growing attention (and, consequently, of the public decision-makers) on the sustainability of the various solutions.

 

The management scenarios

With respect to a social winery of the Oltrepò Pavese area, which produces grapes in an area of about 600 ha, two alternative scenarios for the management of vine pruning residues have been studied.

Currently, the winery consumes 56.400 kWh/year of thermal energy and satisfies its “cold needs” with a compression refrigerating unit, which works 1100 h/year and is characterized by a COP (Coefficient Of Performance) of 3, with an electric power of 50 kW (150 kW cooling capacity).

The first scenario examined is the traditional one, with the shredding and subsequent incorporation of the pruning residues in the ground. Since there is no biomass energy valorization, all the winery needs are satisfied with the combustion of natural gas for the need for thermal energy, and as far as refrigeration is concerned, with electricity coming from the national grid, guaranteed through a conventional refrigerating unit.

Conversely, in the second scenario, the residues are collected with a forage harvester, and the produced wood chips feed a biomass boiler located near the winery. The produced heat is used in part for the thermal needs of the cellar and in part to feed an absorption refrigerator unit, in which the dissolution heat of a solute (ammonia) is used in a solvent (water), cyclically concentrated and diluted. In this case, even with a more detailed management of the pruning residues, it is possible to meet part of the energy needs of the winery, avoiding the fossil fuels consumption.

Table 1 shows the characteristics of the operating machinery used in the two scenarios: in addition to their mass, for the environmental assessment, the “virtually” value consumed for each of them was calculated on the basis of their service-life and annual use. Table 2 shows the values ​​of the main parameters considered to estimate the thermal energy produced by the combustion of the pruning residues in the biomass boiler and the subtracted thermal energy (the “product cold”) from the absorption refrigeration unit (GFA). The thermal power necessary for the GFA to generate the same cooling capacity of the classic compression refrigerator is about 215 kW and can be calculated considering the required power (150 kW) and the refrigeration efficiency index of the device (a parameter indicating how much energy thermal is removed by consuming 1 kWh of heat). To generate the heat necessary for the operation of the GFA, therefore, 82 t/year of wood chips will be needed, which can be obtained from 43.2 ha of vineyards, while the by-product that can be collected on an additional 10.3 ha is needed to meet the thermal requirements. The total area required, therefore, becomes about 54 ha, corresponding to 9% of the vineyards that give the grapes to the cellar.

 

The environmental impact assessment

The environmental impact assessment was carried out according to the life cycle analysis method (LCA, Life Cycle Assessment). Codified by specific standards (ISO, 14040 and 14044), LCA is the most used method for environmental assessments and, although originally developed for the study of industrial processes, it is increasingly applied to agricultural and forestry systems. The LCA approach makes it possible to evaluate different environmental aspects: in this case, global warming, the formation of fine dust, soil acidification, eutrophication of fresh water and the consumption of mineral and fossil resources.

Considering that the different environmental effects are quantifiable with different units of measure, the comparison is usually made by setting the highest environmental impact or benefit at 100, with the other calculated in proportion. If the values ​​are positive, there is a negative effect on the environment because the impact linked to the emission of pollutants and consumption of inputs exceeds the benefits deriving from the replacement of energy consumption of fossil origin of the winery; if conversely, we have values ​​below zero, there is a benefit for the environment.

For example, for global warming, the baseline scenario causes an impact on the environment while the alternative scenario involves a considerable benefit. Because of the formation of fine dust, both scenarios damage the environment (the values ​​are positive), but the impact of the base scenario, compared to the alternative, is less than 97%.

The alternative scenario presents the best results for global warming, soil acidification and eutrophication of fresh water while, mainly due to the emissions of pollutants present in the biomass boiler fumes, it implies a higher impact on the formation of fine dust and smog. As regards the consumption of mineral and fossil resources, the greatest impact of the alternative scenario is due to the construction of the biomass boiler and of the absorption cooling unit.

The valorization of the pruning residues is certainly an interesting solution able to diversify the sources of income and/or reduce the production costs, enhancing a by-product in a circular economy scheme. Nevertheless, from an environmental point of view the solution, which foresees their combustion in small devices not equipped with advanced emissions abatement systems, can lead to an increase in impacts, especially in relation to the emission of fine dust.