Selection principle of working fluid for organic Rankine cycle based on environmental benefits and economic performance

Highlights

• Analyses of greenhouse gas emissions from different phases and sources were conducted.

• Fluid selection based on environmental and economic criteria was carried out.

• Effects of heat source temperature on greenhouse gas emission reduction were discussed.

• Correlations between the emission reductions and heat source temperature were given out.

Abstract

The utilization of renewable energy and waste heat recovery can effectively alleviate the ongoing energy crisis and environmental pollution. The organic Rankine cycle has been proven to be reliable in converting low-to-medium-grade waste heat to power. The fluid selection is a crucial factor in the organic Rankine cycle design procedure, because the cycle performance depends mainly on the thermophysical properties of the working fluid. In this study, the optimal selection principle, based on environmental and economic criteria, for 14 different working fluids is proposed, considering a heat-source temperature range of from 90 to 230 °C. Electricity production cost and reduction of greenhouse gas emissions were selected as the objective functions. Through carbon footprint analysis, the greenhouse gas emissions generated by the organic Rankine cycle were investigated. Then, parametric studies were performed to analyze the matching relationship between the heat-source temperatures and corresponding fluids. The results demonstrate that for organic Rankine cycles with low-global warming potential (GWP) fluids, the construction phase generates the majority of the total emissions, approximately 66.24–90.21%. For high-GWP fluids, such as R134a and R245fa, the majority of the emissions are generated during the operation phase and include approximately 481.17 tons and 374.47 tons CO_2,eq, respectively. From the viewpoint of environmental benefits, R600a exhibited the highest emission reduction, approximately 9531.06 tons CO_2,eq, followed by R152a, R600, and R245fa, at a heat-source temperature of 150 °C. The matching relationship study indicated that the optimal temperature ranges for R601 are 363–384 K and 481–503 K, based on the maximum emission reductions. Regarding the economic analysis, the suitable temperature range corresponding to the best economic performance for R245fa was 363–468 K. Finally, correlations based on the best environmental benefits between the heat-source temperatures and optimal fluids were provided.

Introduction

Today, the increasing energy demand and growing concerns regarding environmental pollution have become the leading challenges for society. To address these issues, renewable energy utilization and the harnessing of waste heat are potential solutions. Among the different waste heat recovery technologies, such as the osmotic heat engine (OHE), Kalina cycle, and supercritical carbon dioxide (CO₂) Brayton cycle, the organic Rankine cycle (ORC) is the most popular for converting low-to-medium-grade waste heat to power, owing to its simple maintenance and structure and the relatively low investment required. Moreover, the ORC can be driven by different types of energy sources, including industrial waste heat [1], [2], biomass energy [3], solar energy [4], engine waste flue gas [5], and geothermal energy [6]. In recent years, ORC studies have focused mainly on the following aspects: system structure improvement, economic and environmental assessments, multi-criteria/objective optimization, and dynamic characteristics analysis.

The selection principle of the working fluid is another active topic because thermal efficiency, economic viability, component size, and security issues are all influenced by the properties of the organic fluids. During the selection process, both the physical properties (for example, low specific volume and high thermal stability) and environmental influence of the fluids should be discussed. According to the stringent requirements regarding emission control, i.e., “Mobile Air Conditioning (MAC) Directive” and “F-gas Regulation” adopted in European Union [7], the latest environmental requirements for organic fluids are a zero ozone depletion potential (ODP) and a low global warming potential (GWP). Thus, chlorofluorocarbons (CFCs) and hydrofluorocarbons (HFCs), for example, must be phased out and replaced by hydrofluoroolefins (HFOs) and hydrocarbons (HCs) in heating, ventilation, and air-conditioning (HVAC) systems, over the next decade. To date, there have been numerous reports on fluid selection for the ORC. For example, Ustaoglu et al. [8] established a regenerative ORC model, and 12 different fluids were selected as candidates in their study. They concluded that the larger scale of the heat source would provide improved system performance and that R-141b exhibited the highest overall system efficiency of approximately 16.7% among the chosen fluids. A mathematical model was created by He et al. [9] to determine the best fluids in subcritical ORC. They chose net power output as the objective function and considered 22 pure fluids including wet, isentropic, and dry types. The results suggested that ORC could generate a higher net power output when the fluid critical temperature was similar to the inlet temperature of the heat source. Bellos and Tzivanidis [10] studied an ORC system driven by solar energy and waste heat, considering four working fluids. Their results indicated that toluene generated the greatest power output, followed by cyclohexane, MDM and n-pentane. Zhang et al. [11] conducted a thermo-economic evaluation of a subcritical ORC consisting of different heat exchanger types, and analyzed different economic parameters such as electricity production cost (EPC), dynamic payback period, and capital recovery factor (CRF). RC270, R600a, R600, R601b, R601a, and R601 were recommended for their superior economic performance, from among the 11 candidates. Mohammadkhani and Yari [12] discussed the economic performance of a trans-critical dual-loop ORC with four working fluids. It was found that an ORC with toluene and R143a outperformed the others.

Accordingly, it can be concluded that the optimal fluids are different when different selection principles are applied. There is no single fluid that is appropriate for different heat-source temperatures while being utilized in different application backgrounds. As summarized in Table 1, the reports regarding this topic are diverse because the fluid selection principle is influenced by different factors including the type, inlet temperature, mass flow rate of the heat source, component type, and specific layout of the system.

Furthermore, the environmental impact of an ORC has become another active topic in recent years. The life-cycle assessment and carbon footprint (CFP) methods are two commonly used tools for quantifying the environmental impact of a product or system throughout its life-cycle. CFP is focused on the quantification of greenhouse gas (GHG) emissions, which are classified into direct emissions and indirect emissions. The environmental impacts of different ORC systems have been discussed in the literature. Hickenbottom et al. [33] estimated the environmental impacts, especially the GHG emission reduction, of OHE and ORC systems via life-cycle assessment. The results indicated that the environmental impacts of ORC during the construction and operation phases were less than those produced by OHE. Liu et al. [34] established the life-cycle framework for the ORC and found that GHG emissions from the construction phase contributed the most to GWP. Mago and Luck [35] studied the potential GHG emission reduction for a combined power generation unit including an ORC system. They concluded that the potential emission saving would be influenced by the location of the system. Moreover, GHG emission was also selected as an environmental indicator in multi-criteria/objective optimization. Zhang et al. [36] performed a 4E (energetic, exergetic, economic, and environmental) study on a trans-critical ORC with five organic fluids. They observed that R134a demonstrated the best economic performance and that R290 yielded the maximum GHG emission reduction. Intaniwet and Chaiyat [37] investigated a 20 kW_e ORC system using the 3E (energy, economic, and environmental) model. They determined that the electricity cost of their system was approximately 0.052 USD·kg CO₂-eq/kWh².

At present, the research indicates ongoing concern regarding the environmental impact of ORC systems. The fluid selection must also be seriously considered. Because pollutant emissions had been extensively discussed in previous reports, this study focuses on the environmental benefits and electricity costs of the ORC over its lifetime, especially considering the matching relationship between different heat-source inlet temperatures and working fluids. The main purpose is to determine the optimal working fluids and corresponding suitable temperature ranges of the heat source. The novel contribution of this work is the selection principle results from the concept of environmental benefits, i.e., CO_2,eq emission reduction. This is different from the previous studies regarding fluid selection for an ORC. The GHG emission reduction is a comprehensive index that integrates the total electricity production and GHG emissions of an ORC during its lifetime instead of the traditional net power output or thermal efficiency. Two kinds of optimization objectives, GHG emission reduction and EPC, are employed according to their environmental and economic aspects. The present study can provide valuable insight regarding the fluid selection for an ORC at different heat-source inlet temperatures, especially when the environmental and economic benefits are emphasized by the decision makers.

In this study, 14 different working fluids are chosen as candidates to analyze the GHG-emission-saving potential and economic performance of an ORC. Section 2 presents the detailed description and conditions of the ORC system. Section 3 describes the methodologies and models including the thermodynamic, heat exchanger, and thermo-economic and CFP models. Section 4 discusses the solution strategy including decision variables, objective function, and solution method. Section 5 discusses the matching relationship and determines the optimal working fluids as well as the suitable temperature ranges. Finally, the conclusions are summarized in Section 6.