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SCIENCE CHINA Technological Sciences, Volume 59 , Issue 10 : 1524-1536(2016) https://doi.org/10.1007/s11431-016-0151-1

Entransy dissipation/loss-based optimization of two-stage organic Rankine cycle (TSORC) with R245fa for geothermal power generation

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  • ReceivedFeb 26, 2016
  • AcceptedJun 30, 2016
  • PublishedSep 18, 2016

Abstract

Based on organic Rankine cycle (ORC), the two-stage evaporation strategy is adopted to replace the single-stage evaporation to improve the system performance. In order to evaluate the temperature matching of the two-stage evaporation, a theoretical optimization model was established to optimize the two stage organic Rankine cycle (TSORC) based on the entransy theory and thermodynamics, with the ratio of the entransy dissipation rate of the TSORC to that of the ORC as the objective function. This paper aims to illuminate the improving degree of the system performance of the TSORC. The results show that the TSORC enhances the average evaporating temperature, thereby reducing the entransy dissipation rate in the evaporator and the total entransy dissipation rate. The maximal net power output is proportional to the entransy loss rate and inversely proportional to the entransy dissipation rate. However, compared with the ORC, the TSORC can output more power but requires a higher total thermal conductance. Moreover, there exists an optimal intermediate geothermal water temperature (IGWT) to maximize the net power output of the TSORC. The TSORC can be considered in engineering applications.


Acknowledgment

This work was supported by the National Natural Science Foundation of China (Grant No. 51406130).


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  • Figure 1

    Schematic diagram of a TSORC system.

  • Figure 2

    T-S schematic diagram of the TSORC.

  • Figure 3

    T-Q diagram for the heat transfer in the evaporators.

  • Figure 4

    Evaporating temperatures with the IGWT corresponding to the maximal net power output for mgw=1 kg/s.

  • Figure 5

    Entransy dissipation rate in the evaporator with the IGWT corresponding to the maximal net power output for mgw=1 kg/s. (a) Entransy dissipation rate in the evaporator 1; (b) entransy dissipation rate in the evaporator 2.

  • Figure 6

    Ratio of the entransy dissipation rate in the evaporator of the TSORC to the entransy dissipation rate in the evaporator of the ORC with the IGWT corresponding to the maximal net power output for mgw=1 kg/s.

  • Figure 7

    Entransy dissipation in the condenser with the IGWT corresponding to the maximal net power output for mgw=1 kg/s.

  • Figure 8

    Entransy dissipation of cycle components of the ORC and the TSORC corresponding to the maximal net power output for mgw=1 kg/s and IGWT=103.6°C.

  • Figure 9

    Entransy dissipation and entransy loss of ORC and TSORC with the IGWT corresponding to the maximal net power output for mgw=1 kg/s.

  • Figure 10

    Net power output and total thermal conductance with the IGWT for mgw=1 kg/s.

  • Table 1   Validation of the numerical model with previous published data for various fluids-based ORC

    Fluids

    Tgw,in (K)

    Tgw,out (K)

    mgw

    (kg/s)

    Tcw,inlet (K)

    Tcw,outlet (K)

    mcw

    (kg/s)

    VFR

    Wnet

    (kW)

    ηth

    (%)

    Sources

    R123

    364.25

    346.45

    69.44

    301.15

    311.15

    162.5

    2.62

    270

    3.96

    Experiment

    R123

    364.25

    346.45

    69.44

    301.15

    311.15

    162.5

    2.75

    288.1

    4.10

    Present

  • Table 2   Parameters used in this paper.

    Parameters

    Units

    Values

    Inlet temperature of geothermal water

    K

    378.15

    Outlet temperature of geothermal water

    K

    358.15

    Isentropic efficiency of the turbines

    %

    80

    Isentropic efficiency of the pumps

    %

    60

    Isentropic efficiency of the hot water pump

    %

    75

    Isentropic efficiency of the hot water pump

    %

    75

    Electrical generator efficiency

    %

    90

    Ambient pressure

    MPa

    0.101325

    Ambient temperature

    K

    298.15

    Temperature at the condenser outlet

    K

    303.15

  • Table 3   Thermodynamic properties of R245fa

    Substance

    Physical data

    Environmental data

    Source

    Type

    M

    (g/mol)

    Tb

    (K)

    Tcri(K) Pcri(MPa)

    ALT

    (yr)

    ODP

    GWP

    (100yr)

    R245fa

    134.05

    288.05

    427.20

    3.640

    7.6

    0

    1030

    [63]

    Dry

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