Thermodynamic performance comparison of ORC between mixtures and pure fluids under closed heat source

doi:10.11949/0438-1157.20190882

Abstract

Abstract:

In order to compare the performance of zeotropic mixtures and pure fluids in organic Rankine cycle (ORC), a numerical simulation model is established for basic ORC and ORC with internal heat exchanger (IHE) under the closed heat source with given heat supply and inlet and outlet temperatures. Mixtures R600a/R601a at different mass fractions are employed, and a certain range of mass flow rate of cooling water is considered as the condition of heat sink. In the simulation, the cycle evaporation and condensation temperatures are simultaneously optimized to obtain the maximum net output power, and the performance parameters of mixtures and pure fluids are obtained. Based on the performance comparison, it can be concluded that zeotropic mixtures are not certain to have higher cycle performance than pure fluids. Under the given condition of closed heat source, mixtures generally have better temperature matches in the evaporator and condenser, so that exergy losses of these heat exchangers are reduced. Regarding the ORC with IHE, IHE will affect the temperature distribution of the working medium in the phase change heat exchanger, but it has a small effect on the plutonium loss in the phase change heat exchanger.

Key words: zeotropic mixture, organic Rankine cycle, internal heat exchanger, closed heat source, thermodynamic performance comparison

摘要：

为了全面比较有机朗肯循环(ORC)中混合工质与纯工质的性能优劣，在给定供热量及进出口温度的闭式热源条件下，建立了基本ORC和回热ORC的数值仿真模型，所用工质为R600a/R601a，所用冷源为一定流量范围的冷却水。在模拟中，以最大净输出功为目标，同步优化循环的蒸发及冷凝温度，得到了不同组分工质的性能参数。比较结果表明，混合工质的热力性能不一定优于纯工质。在闭式热源下，相变换热器中具有更好温度匹配的混合工质一般也具有更小的换热损失。针对回热ORC，回热器将影响工质在相变换热器中的温度分布，但对相变换热器中的损失影响较小。

关键词: 混合工质, 有机朗肯循环, 回热器, 闭式热源, 热力性能比较

CLC Number:

TK 123

Yong MING, Yannan PENG, Wen SU, Guolong WEI, Qiang WANG, Naijun ZHOU, Li ZHAO. Thermodynamic performance comparison of ORC between mixtures and pure fluids under closed heat source[J]. CIESC Journal, 2020, 71(4): 1570-1579.

明勇, 彭艳楠, 苏文, 魏国龙, 王强, 周乃君, 赵力. 闭式热源下混合工质与纯工质的ORC性能比较[J]. 化工学报, 2020, 71(4): 1570-1579.

Figures/Tables 13

Fig.1 T-s diagram of ORC with zeotropic mixture

Fig.2 Temperature glides of R600a/R601a at different bubble temperatures

Table 1 Cycle conditions for closed heat source

系统稳定参数	值
加压水进口温度/K	413.15
加压水出口温度/K	353.15
供热量/kW	300, 350, 400
蒸发过热度范围/K	0~15
冷凝过冷度/K	0
系统压力范围/MPa	0.1~2.5
冷却水进口温度/K	293.15
冷却水流量范围/(kg/s)	4~12
蒸发器窄点温差/K	15
冷凝器窄点温差/K	10
回热器窄点温差/K	10
工质泵等熵效率 $η p u m p$ /%	60
膨胀机等熵效率 $η e x p$ /%	75

Table 1 Cycle conditions for closed heat source

系统稳定参数	值
加压水进口温度/K	413.15
加压水出口温度/K	353.15
供热量/kW	300, 350, 400
蒸发过热度范围/K	0~15
冷凝过冷度/K	0
系统压力范围/MPa	0.1~2.5
冷却水进口温度/K	293.15
冷却水流量范围/(kg/s)	4~12
蒸发器窄点温差/K	15
冷凝器窄点温差/K	10
回热器窄点温差/K	10
工质泵等熵效率 $η p u m p$ /%	60
膨胀机等熵效率 $η e x p$ /%	75

Fig.3 Mass flow rate of working fluids at different mass fractions

Fig.4 Mass flow rate of cooling water at different mass fractions

Fig.5 Regenerative heat in IHE at different mass fractions

Fig.6 Output work at different mass fractions

Fig.7 Cycle efficiency at different mass fractions

Fig.8 Temperature distribution and temperature difference in evaporator

Fig.9 Temperature distribution and temperature difference in condenser

Fig.10 Temperature distribution and temperature difference in IHE

Fig.11 Exergy loss of ORC system at different mass fractions

Fig.12 Exergy efficiency at different mass fractions

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