A quantitative sustainable comparative study of two biogas systems based on energy, emergy and entropy methods in China

In China, as an important source of renewable energy, the standard biogas production module (SBPM) plays a key role and accounts for the majority of biogas production. Given its greater environmental pollution, it is particularly important to assess its quantitative sustainability status for optimizing system design and policy-making. This paper conducted the sustainability assessments of the open-loop standard biogas production module (OL-SBPM) and closed-loop standard biogas production module (CL-SBPM). Compared with the OL-SBPM system, the CL-SBPM system integrates a power generation subsystem. To evaluate two types of systems quantitatively, energy, emergy and entropy methods have been utilized in this study. The results illustrated that only partial indicators are more sustainable in the CL-SBP system rather than all indicators by comparing with the OL-SBPM system, which is contrary to the prevailing view. Taking the information entropy as an example, the equivalent information entropy flow values of the OL-SBPM and CL-SBPM systems are 2.04 × 1025 bits and 1.47 × 1025 bits, respectively, which also demonstrate that the OL-SBPM system has more useful information resulting in higher degree of sustainability. Therefore, these two systems need to be selected according to the existing conditions, rather than the subjective ideation.