The issues of energy high-speed consumption and safe supply have aroused widespread concern in society. In order to effectively solve the energy problem, countries around the world have proposed measures that focus on both energy development and conservation. According to statistics, 50% of the energy used by humans is directly emitted by low-temperature waste heat. If this energy can be used, it will not only solve some energy problems but also reduce environmental pollution. Among much of the low-grade thermal energy conversion and utilization technologies, the organic Rankine cycle is extensively applied on account of its wide temperature range and moderate power [
1,
2,
3]. In addition, the organic Rankine cycle is often used in conjunction with other systems to achieve higher efficiency. Peris et al. [
4] were interested in using ORC for combined heat and power (CHP) applications, and Capata et al. [
5] recovered vehicle waste heat with small-scale ORC. In addition, solar energy, wind energy, and ocean temperature difference energy are of particular interest in small-scale ORC systems [
6,
7,
8]. Therefore, extensive research studies have been conducted on the ORC.
For the industrial ORC prototype, the back work ratio is as high as 25%, indicating that improving the pump performance is a key for ORC commercial application. Numerous studies devoted main efforts on the pump improvement, including piston pumps, gear pumps, and centrifugal pumps. The vane pump delivers energy by the impeller, while the positive displacement pump is dependent on the periodic variation of the volume. Mathias et al. [
9] conducted a comparison between the piston pump and gear pump, representing that the piston pump was preferred for the ORC system. Lei et al. [
10] tested an ORC system using a roto-jet pump, stating that the pump efficiency of 11–23% was obtained. Bianchi et al. [
11] conducted a test on an ORC system using a sliding vane pump, reporting that the shaft power has a significant influence on mass flow rate and pressure. Villani et al. [
12] proposed two different ORC systems combined with the heavy diesel engines. One is that the pump and the expander are connected to achieve a fixed speed, and the other is that the pump and expander are separated, the pump optimal speed and expander optimal speed are selected by adjusting parameters. Zeleny et al. [
13] applied a gear pump on the ORC system to evaluate the pump mechanical losses. Xu et al. [
14] tested the operation characteristics of a piston pump on an ORC system, demonstrating that the low pump frequency was applicable to all expander torques. Carraro et al. [
15] integrated a multi-diaphragm positive displacement pump into a 4-kW experimental prototype and found that the pump global efficiency was about 45–48%. Bianchi et al. [
16] changed the pump speed to measure the performance of the pump and the overall system in a micro-ORC and found that the pump has a back-work ratio of 50–75% and causes a lot of power consumption. Therefore, special attention should be paid to the design of the pump in micro-ORC. Zhang et al. [
17] experimentally studied the change in pump characteristics with evaporation and condensation temperatures. The pump consumption decreases with increasing condensation temperature and presents a non-linear relationship with increasing evaporation temperature. Xi et al. [
18] experimentally tested the transient process for the sudden stop of the working fluid pumps in the ORC and regenerative ORC (RORC) systems and found that the expander showed good performance if the pumps were closed when the working fluid was overcharged. Abam et al. [
19] analyzed the exergy performance for each component of the four different ORCs and showed that the exergy destruction of evaporators was the largest and that of the pumps was the smallest. Aleksandra et al. [
20] integrated that the use of different refrigerants can produce different pump work. Wu et al. [
21] utilized a booster pump instead of a common working pump to optimize system characteristics. Meng et al. [
22] investigated the performance of the centrifugal pump applied to the engine exhaust recovery ORC device and found that the total efficiency of the pump was 15–65.7%. In addition, Yang et al. [
23] and Sun et al. [
24] compared a variety of pumps that can be applied to the ORC in order to find the most suitable pump for ORC systems, indicating that the hydraulic diaphragm metering pump was suitable for low heat capacity, whereas the multistage centrifugal pump was preferred for higher heat capacity. Feng et al. [
25,
26,
27] experimentally compared the system behaviors on a 3kW ORC between pure working fluids and mixture working fluids using a scroll expander. In addition, when the working fluids are condensed in a condenser, the working fluids’ temperature and pressure decrease, resulting in the working fluid pump cavitation. Cavitation affects the stability of the system, resulting in a decrease of system efficiency. D’amico et al. [
28] introduced the thermodynamic model of a piston pump in the ORC and proposed the prediction of available head margin to avoid cavitation. Liu et al. [
29] found that cavitation can occur when the working fluid was insufficient and proposed overcharging working fluid to avoid cavitation. Yang et al. [
30] stated that a subcooling of 20 °C was needed to prevent the cavitation of the piston pump. Pei et al. [
31] emphasized the use of a bypass tube to balance the pressure of the pump and the tank to solve the cavitation. Galindoe et al. [
32] and Dumont et al. [
33] used a liquid sub-cooler to prohibit the cavitation.
To lessen the influence of the pump on the ORC property, a novel concept was proposed—pumpless ORC. Gao et al. [
34] raised the gravity-type pumpless ORC to ensure the stability and continuity of the system shaft power output. Bao et al. [
35] believed that an ORC system without a pump can achieve a more compact and efficient arrangement, which can improve the system’s net efficiency. Jiang et al. [
36] raised a cascade cycle of power and refrigeration, applying the pumpless ORC to the upper cycle and the adsorption refrigeration cycle to the lower cycle. Within the range of experimental parameters, the maximum power is 232 W and the maximum cooling capacity is 4.94 kW.
As mentioned above, it is evident that several experimental investigations using different pumps have been performed. However, limited studies fulfilled the work on the experimental comparison on an ORC operation characteristic using different pumps. Simultaneously, micro- ORC is still in infancy, and more effort should be focused on the components’ design and test. For the micro-ORC prototype, the considerably low efficiency of the pump may heavily affect the overall performance. Therefore, comparing the operation characteristics of small-scale ORC prototypes using different pumps is certainly of great interest. In the present study, a 10 kW R245fa-based ORC experimental prototype is used to study the operation characteristics. A plunger pump and centrifugal pump are adopted, which were widely used in previous experimental tests. The basic operation characteristics for the plunger pump and centrifugal pump are first analyzed. The components’ behaviors are addressed, and the overall performance is examined.