The so-called organic Rankine cycle (ORC) is an effective technology allowing heat recovery from lower temperature sources. In the present study, to improve its thermal efficiency, a preheated ejector using exhaust steam coming from the expander is integrated in the cycle (EPORC). Considering net power output, pump power, and thermal efficiency, the proposed system is compared with the basic ORC. The influence of the ejector ratio (ER) of the preheated ejector on the system performances is also investigated. Results show that the net power output of the EPORC is higher than that of the basic ORC due to the decreasing pump power. Under given working conditions, the average thermal efficiency of EPORC is 29% higher than that of ORC. The ER has a great impact on the performance of EPORC by adjusting the working fluid fed to the pump, leading to significant variations of the pump work Moreover, the ER has a remarkable effect on the working fluid temperature lift (TL) at the evaporator inlet, thus reducing the evaporator heat load. According to the results, the thermal efficiency of EPORC increases by 30%, when the ER increases from 0.05 to 0.4.
In the last two decades, the problems of fossil fuel shortages, environmental pollution, and carbon emissions caused by growing industrialization have become more and more prominent, and the exploration and utilization of waste and renewable energy have drawn a burgeoning number of attentions from both governments and several organizations. In order to gradually reduce the use of fossil fuels, it is expected that more electricity should be generated using renewable energy. A survey from the IEA indicated that electricity generation from renewables is expected to triple between 2010 and 2035, reaching 31% of total generation [
Commercial applications for different markets have been widely distributed in Europe and the US. Until January 21st, 2016, the total installed capacity of the ORC system was 2749.1 MWe in 563 power plants, and the new capacity planned is 523.6 MWe in 75 plants. In terms of market share, geothermal power plants contribute to 76.5% of all ORC installed capacity, and the US has the largest installed capacity. Some of the ORC equipment parameters are listed in
Manufacturer | Unit capacity |
Heat source type | Heat source temperature |
Working fluid | Thermal efficiency |
---|---|---|---|---|---|
ORMAT (USA) | 0.2~100 | Geothermal energy, Waste heat, Solar | 150~300 | Pentane | / |
Turboden (Italy) | 0.2~2 | Biomass, Waste heat, Geothermal energy | 100~300 | Solkatherm, R134a | 20~24 |
Exergy (Italy) | 0.1~22.5 | Geothermal energy, Biomass | 100~300 | R245fa | 16~24 |
TAS (USA) | 1~15 | Heat water | >88 | R134a, R245fa | / |
Kaishan (China) | 0.09~1.2 | Geothermal energy, Waste heat | 100~135 | R123, R245fa | / |
Purecycle (USA) | 0.28 | Heat water | 150 | R245fa | 8~9 |
Enertime (French) | 1~3 | Waste heat | 150~300 | R245fa | 17 |
GMK (Germany) | 0.05~2 | Geothermal energy, Waste heat | 120~350 | / | 9.1 |
Triogen (Netherland) | 0.06~0.165 | Waste heat | >350 | Toluene | 13~18.5 |
Because of the increased focus on low and medium temperature heat recovery, a large number of ORC studies have been completed since 2000 in order to improve ORC performance and maximize electricity production [
In ORC system, the working fluid collects heat from the evaporator’s heat source and releases it to the condenser’s heat sink. The temperature of the superheated vapor at the expander outlet is higher than that at the evaporator inlet. Some researchers proposed an ORC system with an internal heat exchanger (IHE) to recover the discharged heat that would otherwise be released in the condenser.
Li et al. [
To improve the efficiency of the ORC system, some researchers have tried to utilize the discharged heat from the expander to reduce the power consumed by the pump. The ejector preheater is a simple device in which high-pressure liquid and low-pressure gas mix and exchange heat directly. Li et al. [
In this paper, a novel ORC system integrated with an ejector preheater, called EPORC, is proposed. In this system, an ejector is selected as a mixer, where part of the working fluid from the expander is used to preheat the working fluid from the pump directly. The ejector leads to three advantages: (1) the average evaporating temperature is increased due to the fact that the working fluid is preheated to saturation temperature or near saturation temperature; (2) the heat load in the evaporator is reduced for recovering part of the discharged heat from the expander; and (3) the pump work is reduced since part of the working fluid is fed to the ejector directly without running though the pump. A detailed thermodynamic analysis is carried out in order to present the characteristics of the proposed system.
The schematic of the EPORC system is shown in
The liquid-vapor ejector preheater plays an important role in the EPORC system, although it has a simple structure. Its structure and working principle are shown in
The thermodynamic processes of the EPORC system are presented in the T-S diagram shown in
The flow mechanism in the ejector is complex. It is not easy to establish a model to simulate the detailed flow process in the system. For simplicity, the following hypotheses are adopted for the system modeling:
(1) Every component operates at a steady state.
(2) The efficiencies of the expander and pump are set as constants.
(3) The vapor and liquid exchange heat completely in the ejector, and the flow in the ejector is one dimension.
Considering the above hypotheses, the mathematical models of the EPORC system are defined as follows:
The ejector ratio (ER) is defined as below:
where, m
The total heat input in the evaporator is:
where, m
The power generated from the expander is:
where, h2,
The total heat released to the heat sink is:
where, h4 is the condenser outlet enthalpy.
The power consumed by the pump is:
where, h5 and
The process in the ejector is:
where, h6 is the second flow enthalpy of the ejector.
The net power output is:
The thermal efficiency is:
The temperature lift (TL) is expressed as the difference between the ejector outlet temperature and the primary flow temperature. It denotes the ability of the ejector preheater to preheat the working fluid.
where,
The basic ORC system numerical model is validated using previously published data from reference [
Fluid | Theat,inlet (°C) | Theat,outlet (°C) | mheat (kg/s) | Tcw,inlet (°C) | Tcw,outlet (°C) | mcw (kg/s) | Wnet (W) | η (%) | Sources |
---|---|---|---|---|---|---|---|---|---|
R245fa | 90 | 70 | 0.14 | 30 | 35 | 0.27 | 532 | 4.65 | Reference [ |
R245fa | 90 | 70 | 0.14 | 30 | 35 | 0.27 | 551 | 4.68 | Present |
The performances of ORC and EPORC are compared under the same operating conditions with R245fa as the working fluid. In order to highlight the characteristics of the EPORC system, the influences of the pump isentropic efficiency, evaporating temperature, and condensing temperature on the performances of these two systems are studied.
The ejector preheater ratio (ER) represents the performance of the ejector.
Temperature lift (TL) is an important indicator of the ejector preheater, showing the preheating effect of working fluid flowing into the evaporator. According to
In this paper, a novel ORC system integrated with an ejector preheater, called EPORC, is proposed. It is compared to the basic ORC system, considering the effect of pump efficiency and operating conditions. Moreover, the effects of ER on the system performance with two different working fluids are studied. The main conclusions drawn from the present study can be summarized as follows:
(1) Compared with the basic ORC system, EPORC has less pump power, resulting in higher net power generation.
(2) Under given operating conditions, EPORC has higher thermal efficiency than the basic ORC system. When the pump efficiency is 70%, the thermal efficiency of EPORC (6.96%) is 29% higher than that of basic ORC (5.38%).
(3) The preheating effect of the ejector is very good. Increasing ER can effectively improve the TL value. However, there is a maximum TL for the ejector, which is influenced by the saturation temperature corresponding to the saturation pressure in the evaporator. What is more, the thermal efficiency of EPORC increases by 30%, when the ejector ratio increases from 0.05 to 0.4.