The recent international focus on the value of increasing renewable energy supply highlights the need for revaluating all alternatives, particularly those that are large and well-distributed. In this context, low enthalpy geothermal water dominant fields (with a brine temperature ranging between 130°C and 170°C) represent a high potential energy source for the electricity generation. The most efficient and cost-effective way to exploit this type of reservoir is based on the use of binary ORC cycles. In 2007 ENEL acquired the rights for the exploitation of four low enthalpy geothermal fields located in Western United States, which are expected to add about 150MW of electric capacity in the next years. Currently, two ORC cycles are already in operation. The purpose of this work was to select, design and demonstrate an advanced ORC technology to be applied to the exploitation of water dominant geothermal resources acquired by ENEL. The main targets to be reached via this new technology were: (i) high performance in terms of maximum power, in order to maximize the annual energy production for a certain geothermal source and (ii) high operating flexibility in terms of negligible performance decline due to external condition variability, such as brine temperature and mass flow rate, as well as environmental air temperature (flexibility is also a key-point for a future integration of binary plants with a solar field). During the preliminary selection of the working fluid and cycle configuration, carried out in collaboration with Massachusetts Institute of Technology and Politecnico di Milano, different Rankine cycles using hydrocarbons or refrigerant as working fluid were extensively studied and optimized in order to maximize the overall conversion efficiency. This analysis allowed the assessment of both subcritical and supercritical cycles and showed that in a supercritical cycle the possibility to operate outside the fluid saturation curve during the heat adduction phase guarantees a higher operational flexibility compared to subcritical cycles. Results pointed out that, in the considered range of geofluid temperature, the best ORC configuration is represented by the supercritical ORC cycle using a refrigerant as working fluid. Based on the theoretical analysis results, it was decided to demonstrate the identified advanced, high efficiency binary cycle at the pilot scale (500 kWe). This activity was developed in the framework of a collaboration agreement between ENEL, Politecnico di Milano and Turboden. The power plant was built at ENEL's experimental area in Livorno (Italy), and an extensive experimental activity, which lasted more than 700 hours, was carried out in 2012. Due to the flexibility of the experimental facility, a wide range of brine temperature and mass flow rate was simulated, thus testing the application of the supercritical ORC technology to different types of low enthalpy geothermal resources. Moreover, experimental activities covered the Winter and Summer period, thus also the influence of the ambient conditions on the ORC performance was assessed. The experimental data obtained were analyzed in order to: (i) validate the design criteria of the main components of the cycle and thus reduce the risks related to the evaluation of the studied technology on the full scale, (ii) check the nominal cycle performance, (iii) assess the reliability of the thermodynamic database available for the working fluid, (iv) optimize the cycle performance and verify the optimal operating parameters for different operating conditions, (v) verify the thermal stability of the working fluid, (vi) verify the control system stability for different operating conditions, (vii) assess the reliability of the most critical components of the cycle, (viii) carry out a sensitivity analysis of the supercritical ORC performance as a function of the geothermal brine characteristics. Expected performance were experimentally confirmed. A detailed model of the power plant was developed via Aspen Plus® and Aspen EDR®. The model was validated on the basis of experimental data, and resulted to be consistent with them, thus demonstrating the reliability of the codes and thermodynamics libraries used. The experimental results showed a high performance level in terms of power production (a specific work higher than 44kJ/kgBRINE for the full scale application, e.g. for the 10MW scale) and a high operating flexibility (good performance for all the considered values of brine temperature and mass flow rate). Thus, the developed technology was demonstrated to be commercially competitive.
Il recente focus internazionale sull’importanza di accrescere il contributo delle energie rinnovabili al fabbisogno energetico globale, impone di valutare attentamente tutte le alternative disponibili, specialmente quelle ben distribuite sul pianeta. In tale contesto, le risorse geotermiche ad acqua dominante (caratterizzate da una temperatura del geofluido compresa tra 130°C e 170°C), altrimenti definite come risorse geotermiche a bassa entalpia, rappresentano una fonte energetica ad alto potenziale ai fini della produzione elettrica. La tecnologia più efficiente ed economicamente sostenibile per lo sfruttamento di tali risorse è quella dei cicli binari ORC (Organic Rankine Cycle). Nel 2007 ENEL ha acquistato i diritti per lo sfruttamento di 4 campi geotermici a bassa entalpia in Nord America, dai quali ci si attende un apporto di circa 150MW di capacità installata nei prossimi anni (attualmente, due impianti ORC sono già operativi su tali risorse). Lo scopo del presente lavoro è quello di selezionare, progettare e validare una tecnologia ORC ad alta efficienza, per futura applicazione ai campi geotermici detenuti da ENEL. I principali obiettivi di tale lavoro sono: (i) elevate performance in termini di potenza massima prodotta, al fine di massimizzare l’energia elettrica annua prodotta, e (ii) elevata flessibilità operativa, tale da garantire all’impianto la possibilità di operare con buone performance anche in condizioni di off-design (in termini di temperatura ambiente e di caratteristiche della georisorsa). Durante la fase preliminare di selezione del ciclo e del fluido di lavoro ottimali, condotta in collaborazione col Massachusetts Institute of Technology e col Politecnico di Milano, differenti configurazioni impiantistiche e differenti tipologie di fluidi di lavoro sono state analizzate. In tale fase sono stati considerati sia cicli subcritici che cicli supercritici ed è emerso come, in quest’ultimo caso, la possibilità di operare al di fuori della curva di saturazione del fluido di lavoro durante la fase di adduzione del calore permette una flessibilità operativa superiore rispetto ad un impianto subcritico. I risultati hanno evidenziato che, all’interno del range di temperatura del geofluido di interesse per l’applicazione in esame, la configurazione impiantistica più promettente consiste nel ciclo ORC supercritico operante con un fluido frigorifero quale fluido di lavoro. Sulla base dell’analisi teorica effettuata, è stato deciso di validare sperimentalmente i risultati ottenuti attraverso l’applicazione della tecnologia supercritica su scala pilota (500kWe). Questa attività è stata svolta nel contesto di un Accordo di Collaborazione tra ENEL, Politecnico di Milano e Turboden. L’impianto sperimentale è stato installato nell’Area Sperimentale ENEL di Livorno. Un estesa e dettagliata indagine sperimentale è stata svolta su tale impianto, attraverso oltre 700 ore di sperimentazione condotte interamente nel 2012. Grazie all’elevata flessibilità dell’impianto sperimentale è stato possibile riprodurre valori estremamente variabili, in termini di portata e temperatura, nelle caratteristiche del fluido termovettore che è andato a simulare il geofluido. Inoltre, l’attività sperimentale è stata condotta sia durante il periodo invernale che durante quello estivo, permettendo così di individuare l’influenza delle condizioni ambientali sulle performance e sulla regolazione del ciclo. I dati sperimentali ottenuti sono stati analizzati al fine di: (i) validare i criteri progettuali adottati per i componenti principali dell’impianto pilota, (ii) verificare il raggiungimento delle performance nominali, (iii) valutare l’affidabilità dei database termodinamici impiegati nella fase di modellazione, (iv) ottimizzare le prestazioni del ciclo ed identificare i parametri operativi ottimali al variare delle condizioni operative, (v) verificare la stabilità termica del fluido di lavoro, (vi) verificare la stabilità del sistema di controllo al variare delle condizioni operative, (vii) verificare l’affidabilità dei componenti più critici dell’impianto, (viii) sviluppare un’analisi di sensitività delle performance del ciclo studiato rispetto alle caratteristiche (portata, temperatura) della georisorsa. Le performance attese sono state confermate sperimentalmente. Un modello di dettaglio del ciclo è stato sviluppato tramite Aspen Plus® and Aspen EDR®. Tale modello è stato validato tramite i dati sperimentali. I risultati modellistici sono emersi essere in linea con quelli sperimentali, dimostrando l’affidabilità delle librerie termodinamiche impiegate. I risultati sperimentali hanno dimostrato elevate performance in termini di potenza massima (un lavoro specifico maggiore di 44kJ/kg_H2O per l’applicazione full scale, i.e. 10MWe) e la capacità del ciclo di operare in condizioni estremamente variabili in termini di portata e temperatura della georisorsa. Applicazioni full-scale della tecnologia studiata sono quindi emerse essere competitive anche sotto il profilo commerciale.
Study and development of new power generation technologies for geothermal low enthalpy resources
TIZZANINI, ALESSIO
Abstract
The recent international focus on the value of increasing renewable energy supply highlights the need for revaluating all alternatives, particularly those that are large and well-distributed. In this context, low enthalpy geothermal water dominant fields (with a brine temperature ranging between 130°C and 170°C) represent a high potential energy source for the electricity generation. The most efficient and cost-effective way to exploit this type of reservoir is based on the use of binary ORC cycles. In 2007 ENEL acquired the rights for the exploitation of four low enthalpy geothermal fields located in Western United States, which are expected to add about 150MW of electric capacity in the next years. Currently, two ORC cycles are already in operation. The purpose of this work was to select, design and demonstrate an advanced ORC technology to be applied to the exploitation of water dominant geothermal resources acquired by ENEL. The main targets to be reached via this new technology were: (i) high performance in terms of maximum power, in order to maximize the annual energy production for a certain geothermal source and (ii) high operating flexibility in terms of negligible performance decline due to external condition variability, such as brine temperature and mass flow rate, as well as environmental air temperature (flexibility is also a key-point for a future integration of binary plants with a solar field). During the preliminary selection of the working fluid and cycle configuration, carried out in collaboration with Massachusetts Institute of Technology and Politecnico di Milano, different Rankine cycles using hydrocarbons or refrigerant as working fluid were extensively studied and optimized in order to maximize the overall conversion efficiency. This analysis allowed the assessment of both subcritical and supercritical cycles and showed that in a supercritical cycle the possibility to operate outside the fluid saturation curve during the heat adduction phase guarantees a higher operational flexibility compared to subcritical cycles. Results pointed out that, in the considered range of geofluid temperature, the best ORC configuration is represented by the supercritical ORC cycle using a refrigerant as working fluid. Based on the theoretical analysis results, it was decided to demonstrate the identified advanced, high efficiency binary cycle at the pilot scale (500 kWe). This activity was developed in the framework of a collaboration agreement between ENEL, Politecnico di Milano and Turboden. The power plant was built at ENEL's experimental area in Livorno (Italy), and an extensive experimental activity, which lasted more than 700 hours, was carried out in 2012. Due to the flexibility of the experimental facility, a wide range of brine temperature and mass flow rate was simulated, thus testing the application of the supercritical ORC technology to different types of low enthalpy geothermal resources. Moreover, experimental activities covered the Winter and Summer period, thus also the influence of the ambient conditions on the ORC performance was assessed. The experimental data obtained were analyzed in order to: (i) validate the design criteria of the main components of the cycle and thus reduce the risks related to the evaluation of the studied technology on the full scale, (ii) check the nominal cycle performance, (iii) assess the reliability of the thermodynamic database available for the working fluid, (iv) optimize the cycle performance and verify the optimal operating parameters for different operating conditions, (v) verify the thermal stability of the working fluid, (vi) verify the control system stability for different operating conditions, (vii) assess the reliability of the most critical components of the cycle, (viii) carry out a sensitivity analysis of the supercritical ORC performance as a function of the geothermal brine characteristics. Expected performance were experimentally confirmed. A detailed model of the power plant was developed via Aspen Plus® and Aspen EDR®. The model was validated on the basis of experimental data, and resulted to be consistent with them, thus demonstrating the reliability of the codes and thermodynamics libraries used. The experimental results showed a high performance level in terms of power production (a specific work higher than 44kJ/kgBRINE for the full scale application, e.g. for the 10MW scale) and a high operating flexibility (good performance for all the considered values of brine temperature and mass flow rate). Thus, the developed technology was demonstrated to be commercially competitive.File | Dimensione | Formato | |
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https://hdl.handle.net/10589/74902