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ABSTRACT
The organic Rankine cycles (ORC) always operate under off-design conditions due fluctuations in renewable energy access and actual user demand. Therefore, this study considered geothermal ORC as the research object to obtain the optimal design. The off-design performance was investigated under varied load curves by selecting different design loads. First, the optimal ORC system with a constant-rate load was obtained through minimizing the specific investment cost. For this optimal system, the exergy efficiency initially increased and then decreased under off-design operation conditions. Moreover, the peak efficiency was improved by increasing the design pressure. Second, the load curves of different users were considered, including industry, business, and residents. To achieve the best operational performance of the ORC for these varied load curves, different rate loads and pressures were selected, and then the off-design performance was studied. The results demonstrate that the rate load and maximum demand load are not optimal and that the optimal load ratio (rate load divided by the highest demand load) is 60%. The operating evaporation pressure obtained through the design is 1.9 MPa. For the optimal ORC, compared with a system with a load ratio of 1.0 (rate load is equal to the highest demand load), the total investment cost decreased by approximately 30.7%, and the average exergy efficiency increased by 19.2%.
Nomenclature
| Latin and Greek symbols | = | |
| A | = | Surface Area [m2] |
| C | = | Cost |
| h | = | Enthalpy [kJ/kg] |
| H | = | Head [m] |
| k | = | Thermal Conductivity[W/(m·K)] |
| = | Mass Flow Rate [kg/S] | |
| P | = | Pressure [kPa] |
| Q | = | Heat [J] |
| R | = | Correction Factor [-] |
| T | = | Temperature [K] |
| U | = | Product of Overall Heat Transfer Coefficient [W/(m2·K)] |
| = | Volume Flow Rate | |
| μ | = | Dynamic Viscosity [N·s/m2] |
| η | = | Efficiency |
| Subscripts and superscripts | = | |
| 1 | = | Refer to ORC Location |
| cw | = | Cooling Water |
| crit | = | Critical |
| des | = | Design |
| eva | = | Evaporator |
| hw | = | Hot Water |
| In | = | Inlet |
| Off | = | Off-design |
| Out | = | Outlet |
| P | = | Pump |
| Pre | = | Preheater |
| T | = | Turbine |
| Wf | = | Work Flui |
| Abbreviations | = | |
| AFUDC | = | Allowance for Funds Used During Construction |
| CEPCI | = | Chemical Engineering Plant Cost Index |
| CGW | = | Cost of Geothermal Wells |
| FCI | = | Fixed-Capital Investment |
| GWP | = | Global Warming Potential |
| LCOE | = | Levelized Cost of Energy |
| LRD | = | Costs of Licensing, Research and Development |
| LMTD | = | Logarithmic Mean Temperature Difference |
| ODP | = | Ozone Depression Potential |
| ORC | = | Organic Rankine Cycle |
| PEC | = | Purchased-Equipment Cost |
| SIC | = | Specific Investment Cost |
| SUC | = | Startup Costs |
| TCI | = | Total Capital Investment |
| WC | = | Working Capital |
Disclosure statement
No potential conflict of interest was reported by the author(s).