Original article
An experimental study on operation characteristics of the organic Rankine cycle system under the single-and multiple-variables regulation

https://doi.org/10.1016/j.seta.2020.100785Get rights and content

Highlights

  • Operation characteristics of the ORC system are analyzed on a kW-scale test bench.

  • The experiments included single-and multiple-variables regulation were conducted.

  • The effective regulation variables were verified according to the different regulation tests.

  • The appropriate regulation manner was explored to improve the system performance.

Abstract

This work aims to investigate the operation characteristics of an organic Rankine cycle (ORC) system under the variable mass flow rate of the working fluid (ṁwf), such as the flow excursion in the field of direct vapor generation (DVG) systems. Besides, the mass flow rate of cooling source (ṁcs) was verified as an effective manipulated variable to respond the variation of ṁwf, which was an easily available freedom in the ORC system. The operation characteristic was tested respectively for ṁwf and ṁcs, where the change form of the single variable was rectangular-wave steps with different amplitudes. To verify the effectiveness of regulating ṁcs, the operation characteristics under the changes of multiple variables including the ṁwf and ṁcs were tested. The results indicate that the ṁwf dominates the variation of operation parameters of the ORC apparently, especially the evaporation pressure. Specifically, the evaporation pressure increases about 26.10 kPa as the ṁwf increases about 0.01 kg/s. The ṁcs also affects the operation parameters, which shows the opposite trend in evaporation pressure and condensing pressure compared with the increase of ṁwf. The results show that when the ṁwf increases, the increase of pressure can be reduced and the thermal efficiency can be improved by regulating the ṁcs in the same direction simultaneously. The increase of the theoretical thermal efficiency of the system is about 11.21% when only the ṁwf is inclined 0.01 kg/s, while it is 18.00% when the ṁcs is inclined 0.12 kg/s at the same time. The experimental findings enable to give a reference on designing control strategies for the DVG ORC systems.

Introduction

Global environmental pollution and the greenhouse effect are getting worse. In order to reduce the use of carbon-containing energy, improving the secondary utilization of energy and developing renewable energy have attracted more attention. The organic Rankine cycle (ORC) system can not only effectively utilize the low and medium temperature heat sources, but also has the flexibility and stability compared with other heat-power conversion technologies [1]. Therefore, ORC system plays an important role in the fields of waste heat recovery (WHR) [2], heavy-duty diesel engine (HDDE) [3], internal combustion engine (ICE) [4], geothermal energy and solar energy [5].

The disturbances of energy often reflect in variables of temperature or mass flow rate of transport medium. Therefore, the manipulated variables in ORC operating process must be regulated when disturbances occur in temperature or mass flow rate of transport medium from supply side [6]. Solar energy, which is abundant and geographically independent, is vulnerable to environmental factors such as changes in the position of the sun and the presence of dark clouds. The disturbance caused by these factors usually makes solar ORC systems operate at off-design conditions.

In order to reduce equipment footprint and achieve economic benefits for the solar energy utilization system, the direct vapor generation (DVG) of ORC as shown in Fig. 1 was proposed by Li et al. [7]. It cuts down the extra cost of heat transfer fluid loop with parabolic trough collector as evaporator, as well as the land occupation. However, the change in phase of organic working fluid leads to the flow instability during the flow boiling, which is sensitive to the fluctuation of solar insolation and becomes an emerging challenge in DVG system. Among the phenomenon of the instability, Boure et al. [8] have considered flow excursion as a static instability. If the flow rate of the new steady state decreases, it may make the channels burn out [9], [10]. On the other hand, when the new steady state flow rate increases, it would affect the normal operation and safe for equipment of ORC system [11]. Therefore, it is very important to understand the operation characteristics of system under the variable mass flow rate of working fluid. In this way, effective regulation strategies in DVG system could be developed to address the negative impact of flow excursion.

There are a lot of researches that have focused on the influence of parameters on the flow excursion in the collector tube and the way to avoid the occurrence of the flow excursion. Since the cycle performance is mainly dominated by the fluid property, most of them concerned the specific working fluid, especially water as the working fluid. However, organic working fluid has been paid little attention. Odeh et al. used a hydrodynamic steady state model including a pressure drop model in DSG system and he proposed a more accurate correlation formula for calculating the friction pressure drop of two-phase flow by using the data obtained under different working pressures and solar radiation [12]. Nguyen et al. used a mathematical model to study the flow instability caused by the boiling of water in a channel and argued that pressure drop was the parameter most affected by flow instability [13]. The proposed robust controller could maintain the flow stability and improve the system operation efficiency effectively. It is obvious that the study of pressure is widely concerned in the phenomenon of flow excursion and flow instabilities.

Table 1 summarizes the existing research status on flow excursion and shows that the flow excursion phenomenon has caused widespread concern in the field of nuclear energy utilization. The ultimate purpose of studying the factors of flow excursion and its regulating strategies is to ensure the safe operation of the system, as well as continuous and efficient power output. Liu et al. demonstrated the drop of pressure caused by flow excursion in channel by decreasing the diaphragm pump’s frequency in an experimental system [22]. Results show that unstable flow excursion would cause the drop of pressure in tube to rise sharply in a short time and correspond well with the simulation results. It is obviously that although flow excursion was studied based on different systems, but only the collector tube was paid attention. Thus, the effect of the flow excursion, namely the flow rate, on the system operation level, as well as the corresponding regulation strategies, needs to be further explorated.

In terms of the regulation strategies and dynamic behavior of ORC, some scholars have conducted the simulation and experimental study recently. Cao et al. proposed a control strategy for ORC system with R245fa as working fluid to achieve better performance when the heat source and cooling source parameters were changed, and verified the effectiveness of the control strategy when the volume flow and temperature of heat source and cooling source volume flow were step changed [23]. Wang et al. studied the influence of mass flow rate of cooling water in the Double loop ORC on inlet and outlet temperature, pressure of condenser, and output work by decreasing at a rate of 0.4 kg/s per 700 s [24]. The results showed that the decrease of the mass flow rate of cooling source led to the increase of the condenser pressure, and it was concluded that the increase of the mass flow rate of cooling source could slightly improve the output work of the Double loop ORC system. Obviously, the influence of the change of the mass flow rate of the cooling source on ORC system had been concerned. But the mass flow rate of cooling source was just studied as a disturbance variable.

Zhang et al. established an ORC system and performed the system-level dynamic simulation on the Modelica/Dymola software. The coupling correlations among the operation variables within the cycle were revealed by dynamic test and verified with simulation results [25]. Results showed that the operating parameters with the cycle were sensitive to the mass flow rate of working fluid. The mass flow rate of cooling source could also affect the operation variables in the cycle, which was potential to be an effective and efficient manipulated variable to regulate the ORC system. Hou et al. proposed a self-tuning generalized minimum variance controller for the ORC system used for waste heat recovery. In this system, the mass flow rate of cooling source was one of the four manipulated variables and played a good role in controlling the temperature of superheated steam and the pressure of expansion valve [26]. Zhang et al. [27] modeled a three-input three-out multivariable control system for the ORC. They adopted the mass flow rate of the cooling source to regulate the temperature at the outlet of the condenser, in order to improve the energy conversion efficiency. Luong and Tsao designed a three-input two-output ORC model to cope with the transient disturbances in evaporating and condensing pressure, which caused by heat source changes [28]. They found that the mass flow rate of cooling source and two throttle valve positions showed desired pressure regulation. A two-input two-output strategy was proposed by Hernandez et al. [29] for improving thermal efficiency in a ORC system with rotating speed of pump and expander as manipulated variables, and results showed the control strategy can achieve a higher average thermal efficiency. The current experimental researches mainly focused on the operation characteristics test under a single variable. The multivariate variables mainly existed in the simulation, and the existing data is not enough to support the multivariate simulation results. Therefore, it is necessary to explore the impact of multivariate variables on the performance of ORC [30].

In this paper, a kW-scale ORC system test bench with R245fa is established to explore the operation characteristics of the system with changing the mass flow rate of working fluid and the mass flow rate of cooling source, respectively. On this basis, the multi-variables regulation is implemented with changing both the mass flow rate of working fluid and the mass flow rate of cooling source, simultaneously.

The paper is organized as follows. The methodology of experiments and the accuracy and reliability of the experimental results are illustrated in section 2. The operation characteristics of system parameters with the changing of the mass flow rate of working fluid are analyzed in section 3. Meanwhile, it verifies the effectiveness of the proposed regulatory methods for practice in the experimental system with the mass flow rate of cooling source as a manipulated variable.

Section snippets

Description of the test bench

A kW-scale ORC system is established, with the working fluid of R245fa, Fig. 2 shows the schematic and test bench of the system, which is composed of a working fluid pump, a receiver, two heat exchangers mainly. The test bench lacks of expander due to the leakage of expander. As the range of theoretical thermal efficiency of all experiments based on this experimental bench is 151.16–988.75 W, and the maximum output work is less than 1 kW, two expansion valves are used to replace expander for

Operation characteristics under the change of the mass flow rate of working fluid

Fig. 5 indicates the operation characteristic when the change of ṁwf is rectangular-wave steps. Fig. 5 (a) shows the independent variables during the test related to the working condition of Case I in Table 4. The ṁwf is 0.04 kg/s initially and then is increased to 0.05 kg/s, 0.06 kg/s, and 0.08 kg/s, respectively. The heat source of hot water is kept the constant, i.e. 10 kW by an electric heater, with an initial heat source temperature of 80.2 °C and a mass flow rate of 0.16 kg/s. The

Conclusions

A platform of kW-scale ORC with self-made expansion valves is established, in order to test the operation characteristics of the system under the variable mass flow rate of working fluid and cooling source separately and simultaneously. The conclusions are as follows:

(1) The change of ṁwf mainly affects the pressure of ORC. For the increase of ṁwf with step change of 0.01 kg/s, the evaporation pressure changes significantly, with an average increase of 26.10 kPa, while the condensing pressure

CRediT authorship contribution statement

Mengjie Bai: Writing - original draft, Writing - review & editing. Ying Zhang: Methodology, Supervision. Shuai Deng: Methodology, Supervision. Li Zhao: Methodology, Funding acquisition, Project administration. Ruikai Zhao: . Yani Lu: Investigation.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

This work is sponsored by the National Natural Science Foundation of China (No. 51776138) and National Key Research and Development Plan under Grant No. 2018YFB0905103.

References (34)

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