Technical and economic optimization of subcritical, wet expansion and transcritical Organic Rankine Cycle (ORC) systems coupled with a biogas power plant

Abstract

A B S T R A C TGenerally, > 40% of the useful energy (cooling engine and exhaust gases) are wasted by a biogas power plant through the cooling radiator and the exhaust gases. An efficient way to convert this waste heat into work and eventually electricity is the use of an organic Rankine cycle (ORC) power system. Over the last few years, different architectures have been widely investigated (subcritical, wet expansion and trans-critical). Despite the promising performances, realistic economic and technical constraints, also related to the application, are required for a meaningful comparison between ORC technologies and architectures. Starting from the limited literature available, the aim of the present paper is to provide a methodology to compare sub-critical, transcritical and wet expansion cycles and different types of expanders (both volumetric and turbomachinery) from both technical and economic point of view, which represent one of the main novel aspects of the present work. In particular, the paper focuses on the thermo-economic optimization of an ORC waste heat recovery unit for a 500 kWe biogas power plant located in a detailed regional market, which was not investigated yet. By means of a genetic algorithm, the adopted methodology optimizes a given economic criteria (Pay-Back Period, Net Present Value, Profitability Index and Internal Rate of Return) while respecting technical constraints (expander limitations) and thermodynamic constraints (positive pinch points in heat exchangers, etc.).The results show that optimal ORC solutions with a potential of energy savings up to 600 MWh a year and with a pay-back period lower than 3 years are achievable in the regional market analysed.

Biogas from Anaerobic Digestion: Power Generation or Biomethane Production?

“…Organic Rankine cycles (ORCs) are another opportunity to improve the efficiency of the cogeneration system by increasing the electric power output. Many authors in the literature have analyzed the feasibility of adopting an ORC in a biogas plant, and have mainly focused on the optimization of the cycle parameters such as expander geometry [16], type of architecture [16], and cycle parameters [17]. In a previous work, the authors showed that simple modifications to the digestion plant can lead to efficiency improvements over one year of operation [18].…”

Exergy Analysis and Performance Improvement of a Subcritical/Supercritical Organic Rankine Cycle (ORC) for Exhaust Gas Waste Heat Recovery in a Biogas Fuelled Combined Heat and Power (CHP) Engine Through the Use of Regeneration

“…In other respects, as a result of the calculations made for simple ORC in the previous paper, the net power production, thermal and exergetic efficiencies are calculated as 79.23 kW, 15.51% and 27.20% for the subcritical ORC and 81.52 kW, 15.93% and 27.76% for the Different types of ORC configurations (simple, regenerative, subcritical or supercritical) has many advantageous and disadvantageous to each other. For instance, the investment cost of a supercritical ORC is higher than that of a subcritical one [12,68] Moreover, compared with the subcritical ORC, the supercritical ORC has a higher turbine inlet pressure which affects the safety as well as investment cost [69,70]. When the ORC systems are compared in term of the regenerative and simple (non-regenerative) cycle, there are many studies which prove the superiority of the performance of rORC over simple ORC [71][72][73][74][75][76].…”

“…Consequently, nuclear energy and renewable energy sources such as solar [5], wind [6,7], geothermal [8] and biomass [9], which are sustainable and environmentally friendly, draw the attention of most governments, technology companies and scientists. Biogas, one of the anaerobic digestion products, is also among the most attractive alternative energy sources [10][11][12][13]. There are various biogas utilisation systems like stoves, fuel cells, gas turbines and hydrogen production systems [14][15][16][17][18][19].…”

“…The use of a regenerator and operating in supercritical conditions is also often applied to improve the system performance [46][47][48][49][50][51]. When the previous studies are examined in detail, it becomes obvious that there are many studies that reveal the capacity of ORCs to recover waste heat from different sources, but only some of them analyse the probability of using organic Rankine cycles to assist small and medium Energies 2019, 12, 575 3 of 22 scale CHP engines. Moreover, only a few studies examine the parametric optimisation of the ORC and almost all of them examine the ORC with regard to only one of the subcritical and supercritical operating conditions.…”

Turbine Design for Low Heat Organic Rankine Cycle Power Generation using Renewable Energy Sources

“…ORC has at least two advantages, namely as a power generation system, ORC has simple components and the availability of components that are quite a lot in the market. Furthermore, ORC also uses organic working fluids that perform better than water as a working fluid at low temperature and pressure [12][13][14][15][16][17][18]. Various ORC applications have been reported in previous studies.…”