SIA "S Plus" has developed a concept to improve the operation of biogas stations. Production of different types of energy within the Eco-Polygeneration cycle.

This concept involves integrating the heat recovery technology created by SIA "S Plus" (‘Device’) in the production cycle of biogas. Specifically, inside the biogas fermenter (Fig. 1), it is necessary maintain a temperature of 42oC, whereby the fermented matter heats up to approximately 40oC, becoming a warm, sludge-like substance (‘digestate’), which is then drained into the storage lagoon, and letter delivered to the field for soil fertilisation.

Figure 1. Conceptual diagram of the heat recovery systems.

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Figure 1 shows that once it leaves the biogas fermenter, the digestate enters the heat recovery Device, where it is cooled down, and separated from the part of its dry matter that is considered to be the most biologically valuable. The extracted heat energy is used to preheat the fermented matter (its initial temperature is 0–5 oC), thus significantly reducing the self-consumption of the biogas and, therefore, the costs. Biogas is an expensive end product, and spending it to maintain the production process is not economical.

Recovering heat from the digestate, the Device replaces the consumption of biogas, and the significance of this process will rise in the situation when the production of energy using biogas as part of mandatory procurement components is not there anymore in the future. Biogas will become the end product. The Device created by SIA "S Plus" will make it possible to cut energy costs. In accordance with the SIA "S Plus" concept, the Device will be partially fed by solar energy, receiving the electricity necessary for its operation from solar panels delivering green and cheap energy. This means that the station will produce biogas, excess heat from the production process for the station’s own heating and cooling needs, as well as electric power from the solar panels, SIA "S Plus" calls this combination of systems Eco-Polygeneration.

The purpose of the Device is to balance the demand for different types of energy, heating the system up in the one place, and cooling it down, in the other. The Device will not fully recover the heat energy given off by the digestate; it serves as a pre-heater for the fermented matter, and the lacking amount of energy will have to be supplied from another energy source (e.g., self-consumption cogeneration engine). The cooling cycle is economically feasible with a positive temperature curve; i.e., for cooling indoor premises or the production process. By preheating the fermented matter and satisfying the demand for cooling at the same time, the Device can make it possible to extract more energy at a lower cost, an advantage capable of reducing the prime cost of the company’s products.

Figure 2. Potential for recovered energy currently lost.

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Figure 3 shows a comparison in the cost of biogas fermenter preheating and production process cooling between biogas self-consumption and the options using the Device.

Figure 3. Reduction of up to 70% in CO2 emissions and energy costs.

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The comparison was made assuming that the biogas station incurs heat energy costs at a rate of EUR 0.06 per kWh, and the cost of self-consumed cogeneration electricity, at a rate of EUR 0.07 per kWh. The graph shows that heating the biogas fermenter with the digestate heat recovery Device offered by SIA "S Plus" is an economical and energy-efficient solution.

The cost of installing the Device depends on how complex the integration is. We have prepared approximate cost estimations, which in the current situation appear to be economically feasible, see figure 4.

Figure 4. Positive long-term investment plans.

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The cash flow shows that at a digestate rate of 10 m3 per hour, the cost of setting up the Device (up to EUR 350000) would pay off within 4 years of operation, WITHOUT A GRANT.