Dynamic modelling of a parabolic trough solar power plant


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Models for dynamic simulation of a parabolic trough concentrating solar power (CSP) plant were developed in Modelica for the simulation software tool Dymola. The parabolic trough power plant has a two-tank indirect thermal storage with solar salt for the ability to dispatch electric power during hours when little or no solar irradiation is present. The complete system consists of models for incoming solar irradiation, a parabolic trough collector field, thermal storage and a simplified Rankine cycle.

In this work, a parabolic trough power plant named Andasol located in Aldeire y La Calahorra, Spain is chosen as a reference system. The system model is later compared against performance data from this reference system in order to verify model implementation. Test cases with variation in solar insolation reflecting different seasons is set up and simulated.

The tests show that the system model works as expected but lack some of the dynamics present in a real thermal power plant. This is due to the use of a simplified Rankine cycle. The collector and solar models are also verified against literature regarding performance and show good agreement.

Full text at: http://www.ep.liu.se/ecp/096/110/ecp14096110.pdf


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Dynamic modelling of a parabolic trough solar power plant

  1. 1. 1 DYNAMIC MODELLING OF A PARABOLIC TROUGH SOLAR POWER PLANT Robert Östholm1, Jens Pålsson2 1 Lund University, Faculty of Engineering, Energy Sciences, Lund, Sweden 2 Modelon AB, Ideon Science Park, SE-223 70 Lund, Sweden 2014-03-12 © Modelon
  2. 2. • Solar Power = renewable energy source • ”By 2050 with appropriate support, CSP could provide 11.3 % of global electricity [IEA, 2010] • Constantly varying solar irradiation prevail  Integrated thermal storage,  Fuel power back up  Dynamics/controls • More challenges  Transmission, cooling, cost MOTIVATION
  3. 3. PARABOLIC TROUGH AND OTHER CSP • Increase energy density by mirrors - Concentrating Solar Power plant concepts
  4. 4. SOLAR IRRADIATION • Beam radiation and diffuse radiation. • Design basis for CSP is the direct irradiation • Direct normal irradiance (DNI) - amount of solar radiation received per unit area by a surface perpendicular to the rays • Total radiation received is DNI scaled with the cosine of the incidence angle DNI with 2500-3000 kWh /year /sqm
  5. 5. • the collector • Mirrors (type determines the CSP technology) • Tracking system, follows the movement of sun, • one axis systems (east-west) • two axis systems (additionally north-south) • Receiver, heat absorbing device THF reaching 400-600 C the thermal heating fluid (THF) • Circulates and provides power cycle with heat source • Oil based THF reaches normally 400 C, molten salt 600 C • the thermal storage • key to cost efficient and flexible CSP plant operation. • Allows dispatch of power and stable power output • Heat can be stored in different media (molten nitrate, rock, sand and oil). • the power cycle • Most CSP use a Rankine (steam turbine) cycle for electricity production. • steam data of around 350 to 550 C and100 bar • Some CSP use a heat engine (such as Stirling motor or Brayton cycle) • Normally air cooled condenser instead of water as cooling medium CSP PLANT SUBSYSTEMS
  6. 6. Plant name Andasol-I and Andasol-II Plant location Aldeire y La Calahorra, Spain Plant type Parabolic trough Start date June 1, 2009 Receiver type Schott PRT-70, pipe length appr. 90’000m Sun tracking One axis in north- south direction Collector type Flabeg RP-3, 6 m width Thermal heating fluid type Dowtherm A Turbine type Siemens SST-700 50MW steam turbine Thermal heat storage Two-tank indirect with molten solar salt (36x14 m, 28500 tn) ANDASOL I-II THE REFERENCE PLANT
  7. 7. SYSTEM MODEL PRINCIPLES THF model Sun model Collector model Rankine cycle model Storage model System and controls
  8. 8. SUN MODEL clock startTime=0 (day - ? time_offset add +1 +1 add + +1 +1 combiTable offset=offset DNI Parameters: • Day • startTime • Longitude • Latitude • timeZone Weather data on File Incidence =f(azimuth, declination, hour angle)
  9. 9. COLLECTOR MODEL Discretization along the pipe
  10. 10. MEDIA MODELS HEAT TRANSFER FLUID (THF) • Transport of heat between collector to the power cycle • Therminol VP-1 (instead of Dowtherm A) • Organic fluid with high thermal stability (12-400 C) • Table based media template from Liquid Cooling Library THERMAL HEAT STORAGE FLUID • Solar Salt (60 % NaNO3, 40 % KNO3) • High Cp, high density, low vapor pressure, low cost • High temperature stability, liquid up to 560 C, but rather high smelting point (238 C) • Table based media template from Liquid Cooling Library
  11. 11. THERMAL HEAT STORAGE • 10 C pinch when charging/discharging (THF limit 373 C) • Collector field oversized by appr. 40 % to be able to  charge at the same time as providing heat to power cycle  After full charge collector field need dump certain zones • Charge and discharge module based on heat exchanger from MBL POWER CYCLE • Simplified approach: The Rankine power cycle model consists of a single heat exchanger (base model from MBL) • Rankine cycle dynamics and thermal inertia of boiler not considered MORE SUBSYSTEMS
  12. 12. The control system consist of four automatic control regulators: • Thermal heating fluid pump control – keeps THF temperature to 393 C by controlling mass flow. Feed forward type. • Thermal storage control - two PI- regulators controlling the mass flow rate of solar salt.  The charging control unit - controls “cold” solar salt temperature (383 C).  The discharge control unit – rate of hot salt when delivering heat to THF (373 C) • Dump control - in the case of thermal storage being fully loaded and too much heat absorbed (defocus part of the collector field) • Rankine cycle control - a PI-regulator controlling mass flow rate of boiler MAIN CONTROLS
  13. 13. SOLAR POWER SYSTEM IN DYMOLA THF control discharge control charge control rankine contro dump control
  14. 14. VERIFICATION – COMPARISON ANDASOL• Nominal operating point • Median value of incoming solar irradiation for typical weather year (day 92 at 10 am) • Rankine cycle not verified (Andasol data used as input) Efficiency Andasol System model Solar field – solar irradiance to steam 43 % 42.6 % Rankine – steam to electricity 38.8 % 38.8 % System – solar irradiance to electricity 16 % 16.5 %
  15. 15. • Partly clouded summer day SIMULATION RESULTS • Typical clear summer day
  16. 16. • Solar power technology increasingly important and dynamic tool well suited • Modelica models of CSP parabolic trough components and system achieved • Compared against reference power plant performance • Improvements: Controls, rankine cycle, component verificiation, address solar industry issues SUMMARY