11:10
System Design and Optimization I
Chair: Prof. Ennio Macchi
11:10
20 mins
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DESIGN AND DELIVER GEOTHERMAL POWER PLANTS PERFORMANCE WITH CONFIDENCE
Guofu Chen
Abstract: The design and the actual performance of a geothermal air-cooled power plant utilizing a supercritical refrigerant of R134a as the working fluid are discussed.
A supercritical Organic Rankine Cycle (“ORC”) in many cases outperforms a sub-critical cycle, from the net kilowatt (kW) generated point of view. Additionally, the plant configuration is simpler to design and easier to operate. An additional advantage of using non-flammable working fluid in the cycle, such as R134a, eliminates the risk of fires.
During the design stage, a preliminary process flow diagram is established based on the standard process engineering practices in HYSYS, a simulation software from ASPENTECH. Based on the preliminary process requirement, the components of the cycle, including the shell and tube heat exchanger(s), expansion turbine, air-cooled condenser, and working fluid feed pump are sized and selected. A true simulation model is built to analyze the off design performance of a “virtual plant”. Given the geothermal heat source information and the ambient conditions, the power output is maximized and committed to the customer (Model 1 with geometries).
After the plant is successfully commissioned, by measuring the flow rate, temperature and pressure, a plant reality model is built to reflect the actual plant operating conditions (Model 2 without geometries).
Normally the process conditions of Model 2 are different from Model 1. To validate Model 1, developed in the design stage, the process conditions of Model 2 are extracted and input into Model 1, thus Model 3 with actual process conditions and actual geometries is established.
Model 2 is the reality, while Model 3 is used to predict the reality with actual process conditions and actual equipment selection. By comparing these two models, Model 3 accurately predicts the gross power and net power generated at various operating conditions.
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11:30
20 mins
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MODELLING OF THE PART-LOAD PERFORMANCE OF A COMBINED GAS TURBINE - ORGANIC RANKINE CYCLE FOR OFF-SHORE APPLICATIONS
Leonardo Pierobon, Ulrik Larsen, Fredrik Haglind
Abstract: In off-shore platforms gas turbines are employed to provide the load required in processes such as crude oil separation, gas compression, seawater injection and oil and gas export. Off-shore gas turbines are designed to ensure high system performance, fuel flexibility and compactness. Furthermore, one or more redundant gas turbines are installed to improve the reliability during maintenance periods. In order to enhance the performance of the power generation system, an organic Rankine cycle can be utilized to recuperate the gas turbine exhaust heat. Benefits of this technology are high thermal efficiency in full and part-load, low weight, high compactness and little complexity. Furthermore, the organic Rankine cycle eliminates the problem of turbine blade erosion due to liquid droplet formation by utilizing a “dry” fluid as working fluid. On the Draugen off-shore oil and gas platform (North Sea, Norway) three SGT-500 gas turbines are installed. The normal load is normally shared the two engines while the third is on stand-by for maintenance. Hence, the analysis of off-shore combined gas turbine – organic Rankine cycle requires an accurate part-load model.
The present paper aims at deriving the off-design performance of the combined SGT-500 gas turbine - organic Rankine cycle. First, we model the part-load performance of the gas turbine by means of the stage stacking approach. Subsequently, we evaluate the organic Rankine cycle at off-design by introducing specific equations for the pump, heat exchangers and expander. The results indicate that the mass flow and the exhaust temperature of the SGT-500 gas turbine are assessed with an error lower than 2.5% at 50% load and 9.0% at 10% load. The simulations indicate that the thermal efficiency of combined SGT-500 - organic Rankine cycle drops down from 43.3% to 20.7% when the load is decreased from 100% to 15%. The performance of the SGT-500 engine declines substantially, i.e. from 31.3% to 9.8%, while the thermal efficiency of the organic Rankine cycle lies within a smaller range (between 27.4% and 23.3%). As a practical consequence, the methodology can be applied to simulate the real time operation of the combined gas turbine - organic Rankine cycle on the Draugen platform and to evaluate the fuel savings and the reduction of CO2 and pollutants emission during the year.
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11:50
20 mins
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A PROGRAM FOR FIRST ESTIMATION OF POWER OUTPUT, COSTS AND PROFIT OF GEOTHERMAL HEAT AND POWER PLANTS
Stefan Lecheler
Abstract: A program was developed to estimate the payback period for geothermal heat and power plants including district heating grid with only a few input parameters, which are usually available for a desired location. With this program, investors and communities can easily estimate, if a new geothermal heat and power plant will be profitable at the intended place.
In a first step all thermodynamic quantities are calculated. Volume flow and temperature of the thermal water give the available heat. From the number of inhabitants and the length and shape of the waste water grid, the needed power for district heating of a nearby village is estimated. The remaining heat is then used for an ORC- or Kalina-power plant. The thermodynamic cycle is automatically adapted to the actual temperature levels. Different working fluid can be chosen, where the thermodynamic properties are automatically calculated from fluid databases. Also the coupling between heat and power station can be changed from serial to parallel or combined. Finally the power output and the thermal efficiencies are calculated.
In a second step all costs of the geothermal project are estimated via cost functions. This includes drilling, components like heat exchangers, turbine, pumps, piping and measurement equipment, the build-up of the geothermal heat and power plant and of the district heating grid and finally management. The inflation rate is also taken into account. The cost functions are a function of power and calibrated by values from literature and existing plants and district heating grids.
In the third step the profit is estimated. With known prices for thermal and electrical power and typical operating hours, the yearly earnings can be estimated for the next years. Taking also interest rates for credits and inflation rates into account, typical amortization maps show, after how many years the investment is profitable. By varying input parameters, their influence can be easily investigated.
The program is available in MS-EXCEL and uses currently fluid property libraries for iso-butane, n-butane, propane and ammoniac-water-mixture from FluidEXLGraphics [1]. It could be easily extended to other fluids, if libraries are available. It is already used for several geothermal projects around Munich, where a large aquifer is available in a depth between 3000 and 5000 meters. The validation of the program with existing data is still ongoing, because not many financial data are available.
[1] FluidEXL Graphics, H.-J. Kretzschmar, Hochschule Zittau/Görlitz, Germany, http://thermodynamik.hs-zigr.de
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