1. Introduction
In order to achieve the goal of carbon peaking and carbon integration, besides vigorously developing the renewable energy, a large amount of medium and low temperature heat has to be recovered [
1]. As a classical thermal power generation technology, the organic Rankine cycle (ORC), has been widely employed to convert the thermal energy of medium and low temperature [
2] due to the low boiling temperature of organic working fluid and the relatively high energy conversion efficiency. In the past two decades, a significant global effort has sought to address the engineering challenges of ORC so as to promote its commercialization. The application fields of ORC include biomass energy, solar energy, geothermal energy, ocean thermal energy, waste heat recovery from internal combustion engines, and gas turbine cycle [
3]. Nowadays, numerous studies have been published to theoretically and experimentally investigate the ORC [
4,
5].
In terms of the theoretical studies on ORC, much of the attention has focused on the selection of the working fluid, cycle configuration, design and optimization, dynamics, and control [
4]. As an energy transfer fluid, the used working fluid significantly influences the thermodynamic performances and operation safety of ORC. Thus, by comprehensively considering the cycle performances and environmental properties, researchers have proposed the enumerative method and molecular design to screen the optimal fluids from numerous organics. For the enumerative method, the candidate fluids are first predetermined and then various performances are compared for these fluids so that the optimal fluid can be determined [
6]. As for the molecular design, the fluids are generated by combining groups. By comparing the performances of these generated fluids, the best fluid can be found [
7]. In addition to the working fluid, the system performance is also closely related to the cycle configurations. Nowadays, on the basis of a basic ORC, researchers have developed various configurations, such as the regenerative cycle, the split regenerative cycle, the cascade cycle, and the dual-pressure cycle [
8,
9]. These cycles generally have different performances at different application scenarios. Thus, aiming at the characteristics of the given heat sources, suitable structures can be identified by comparing performances of different cycles. In addition, for given working fluid and cycle configuration, operation parameters have to be optimized to obtain the highest net work and the lowest cost. In the literature, the optimization consists of single- and multi-objective optimizations. The widely used optimization algorithms are the genetic algorithm [
10] and particle swarm algorithm [
11]. Based on the above studies, system design parameters can be obtained for a given heat source. However, in practical engineering, with the change in heat source condition and ambient temperature, the system will operate dynamically under non-rated conditions. Thus, dynamic behaviors of ORC have been investigated by researchers [
12,
13]. Furthermore, to ensure the high efficiency and safety of system operation, the corresponding controls were also explored [
14,
15].
Even though fruitful results have been obtained in the theoretical studies on ORC, these are underpinned by a large number of assumptions. Thus, before performing an engineering application of ORC, experiments are still needed to test the performance of ORC. Compared with rich theoretical studies, experimental investigations of ORC are relatively smaller due to the high experimental cost. Furthermore, although various cycle configurations have been theoretically proposed, the experiments mainly focused on the basic and regenerative ORCs. For example, Feng et al. [
16] established an experimental setup of 3 kW ORC with the working fluid R123. The used expansion machine was scroll expander. With the varying of working fluid mass flow rates in the range of 0.124~0.222 kg/s and heat source temperatures in the range of 383.15~413.15 K, the behaviors of components and corresponding system performance were discussed in detail. Thereafter, the research group selected R245fa as working fluid and explored the effects of the pressure drop, degree of superheating, and condenser temperature on the system’s overall performance. The maximum expander shaft power and thermal efficiency are 2.64 kW and 5.92%, respectively [
17]. Based on this experimental apparatus, Li et al. [
18] further examined the effects of heat source temperature on the system performances. The experimental results show that, under a temperature in the range of 353.15~373.15 K, the system efficiency is in the range of 4.9~5.6%. Furthermore, an experimental performance comparison of the basic and regenerative ORC systems was also conducted by this group. The results indicate that the basic ORC system has a higher heat source temperature utilization than the regenerative ORC system [
19]. With R1233zd(E) as working fluid, Li et al. [
20] also constructed another experimental test facility based on a conventional recuperative ORC system. A single-screw expander was employed as the expansion device. Under the design condition, the system can output work 11 kW. In the experiments, the best thermal efficiency can reach up to 3.6% with dry expansion. In addition, Eyerer et al. [
21] further compared the experimental cycle performances between R1233zd(E) and R245fa. The results show that the maximum thermal efficiency of R1233zd(E) is 6.92% higher than that of R245fa, but the output power generated by R245fa is 12.17% higher than that of R1233zd(E). For more experiments, the interest readers can refer to the studies [
5,
22]. In fact, there are yet hundreds of experimental platforms to test the performance of ORC from the energetic point of view. In these experiments, the commonly considered working fluids are R123, R245fa, R134a, R600a, R601a, and R1233zd(E). As for the expansion machine, scroll, screw, vane, turbomachines, and other volumetric types are employed. Among these expanders, the scroll machine has been the most popular choice and the output power is located in the range of 0.35–7.5 kW, followed by the screw expander. However, although these experiments have explored a large number of system operating characteristics, they mainly focus on the heat source conditions, and few experimental studies have been conducted that have specifically focused on the effects of mass flow rates of working fluid and cooling water.
The above review indicates that, for the influence of key parameters on ORC performances, experimental data are still scarce in the published literature. In view of this, an experimental test facility of basic ORC was established. The used working fluid and expansion device were R123 and scroll expander. In the experiments, the system performances of ORC were first investigated under the design conditions. After that, by varying the heat source temperature as well as the mass flow rates of working fluid and cooling water, the corresponding effects on system performances were revealed.
4. Conclusions
By establishing the experimental setup of the organic Rankine cycle (ORC), this paper experimentally investigated the cycle performances of R123. Under the experimental conditions, the effects of the heat source temperature as well as the mass flow rates of working fluid and cooling water on the ORC performances were clarified. The relevant conclusions can be drawn as follows:
(1) Under the design conditions, the heat source temperature as well as the mass flow rates of working fluid and cooling water are 160 °C, 25 g/s, and 0.83 kg/s, respectively. The experimental system of the ORC can output a net work of 0.55 kW and reach a cycle efficiency of up to 8.7%.
(2) For the effects of the operation parameters, as the heat source temperature increases from 130 °C to 160 °C, the evaporated heat and condensed heat show small increases, while the net work and cycle efficiency increased significantly. For the cooling water rate, it has no obvious effects on the involved heat. However, the larger the mass flow rate of the cooling water, the higher the net work and cycle efficiency. Furthermore, except for the cycle efficiency, all the performances are proportional to the mass flow rate of working fluid. Under various conditions, the maximum net work is 0.55 kW and the highest efficiency is 9.40%.
Due to the limitations of the experimental setup, this work only preliminary explores the performances of basic ORC with the working fluid R123. Thus, in work to be undertaken in the near future, an internal heat exchanger will be added to the experimental apparatus, and more experimental studies will be conducted to further understand the operation characteristics of this system. Besides R123, commonly used working fluids such as R245fa, R1234yf, and R600 will be considered. Furthermore, according to the experimental data under various conditions, the non-design performance model of ORC will be developed. Meanwhile, a dynamic model is planned to be built for ORC. The simulation results will be checked by the existing experimental system and a simulation library is expected to be developed.