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 http://www.modelon.com/news/news-display/artikel/modelica-conference/

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. • 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. PARABOLIC TROUGH AND OTHER CSP
• Increase energy density by mirrors -
Concentrating Solar Power plant concepts

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. • 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. 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. SYSTEM MODEL PRINCIPLES
THF model
Sun model
Collector model
Rankine cycle model
Storage model
System and controls

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. COLLECTOR MODEL
Discretization along the pipe

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. 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. 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. SOLAR POWER SYSTEM IN DYMOLA
THF control
discharge control
charge control
rankine contro
dump control

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. • Partly clouded summer
day
SIMULATION RESULTS
• Typical clear summer day

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