Energy Engineering - Power Production from Renewable Energy

Ex 2 - GEOTHERMAL BINARY POWER PLANT - Text

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GEOTHERMAL BINARY POWER PLANT Power Production from Renewable Energy – A.Y. 20 20 /21 Prof. Paolo Silva – Ing. Dario Alfani A geothermal well produces a flow rate of 1000 t/h of liquid water (vapor quality x=0) at a temperature of 170° C. The proposed energy conversion system is a saturated cycle using iso -pentane as working fluid (chemical formula C5H12; in FluidProp 3.0 use “ freeStanMix; isopentane ”). 1) First, consider a closed Rankine c ycle, co ndensed with water from a cooling tower , given the following data: - Inlet t emperature of the cooling water to the condenser Twat ,in = 18°C - Temperature i ncre ase of the water in the condenser Tcond = 8°C - Sub -cooling ∆T of the fluid at economizer outlet Tsub-cool = 3°C - Evaporator exchange area Aeva = 1300 m2 - Overall heat transfer coefficient of the evaporator Ueva = 900 Wm -2K-1 - Overall heat transfer coefficient of the economizer Ueco = 800 Wm -2K-1 - Overall heat transfer coefficient of the condenser Ucond = 500 Wm -2K-1 - Condensation temperature Tcond = 30°C - Min imum reinjection temperature to the geothermal well Treinj  70°C - Turbine isentropic efficiency ηis-T = 80% - Electrical generator mechanical -electric al efficiency ηmel-g = 95% - Pump hydraulic efficiency ηhyd -p = 70% - Pump mechanical -electric al efficiency ηmel-p = 90% - Relative pressure loss (∆p/p ) in the economizer (p/pin)eco = 0.2 - Absolute pressure loss in the condenser ( cooling water side) pcond = 2 bar - Other cycle pressure losses negligible It is required to: a) Determine the evaporation pressure that optimizes the plant net power output . b) Calculate the cycle electrical efficiency , the overall plant efficiency and the second law efficiency for the optimal pressure value found in a) . c) Report in a table the values of mass flow rate , temperature, pressure, enthalpy, entropy, density, and vapor quality for all the streams reported in the plant scheme . d) Draw the T-Q (temperature vs. thermal power) diagram of each heat exchanger . e) Calculate the heat transfer surface of the condenser and the amount of cooling water needed. 2) Consider now the possibility to include heat cogeneration for a district heating network through a water/water heat exchanger ( overall heat transfer coefficient U= 400 Wm -2K-1), located along the geothermal fluid circuit at the economizer outlet . The temperature of the water returning from the district heating network is equal to 65 °C and the temperature increment obtained in the heat exchanger is 20 °C. Assuming to cool the geothermal fluid down to 7 0°C in the cogenerati on heat exchanger, calculate the excha nge area required and the first -principle overall efficiency of the plant. 3) Finally, consider a two pressure levels Rankine cycle . Assume for the lower level (LP) the optimal evaporation pressure determined in point 1 a). For the higher level (HP), consider as evap oration temperature the average between the geothermal source temperature (170 °C) and the evaporation t emperature at low pressure . For the HP and LP evaporators, assume pinch -point ∆T equal to 5° C. For each heat exchanger, assume values of the overall heat transfer coefficients equal to the ones given in point 1). It is required to: a) Calculate the cycle electrical efficiency , the overall plant efficiency, the second law efficiency and all the stream thermodynamic conditions ( temperature, pressure, enthalpy, …) . b) Optional: draw the heat exchange diagrams T-Q and c alculate the overall increment of exchange surfaces compared to the plant with one pressure level. c) Optional: optimize both LP and HP evaporation pressures . GEOTHERMAL BINARY CYCLE Power Production from Renewable Energy Prof. Paolo Silva – Ing. Dario Alfani