System Advisor Model

271 Pages·2006·4.09 MB·English

Simulation and Performance Evaluation of
Parabolic Trough Solar Power Plants

by

ANGELA M. PATNODE

A thesis submitted in partial fulfillment of
the requirements for the degree of

MASTER OF SCIENCE
(MECHANICAL ENGINEERING)

at the

UNIVERSITY OF WISCONSIN-MADISON

2006

Approved by

Professor Sanford A. Klein

January 10, 2006

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Abstract

Nine Solar Electric Generation Systems (SEGS) built in southern California between 1984
and 1990 continue to produce 14-80 [MWe] of utility-scale electric power each from solar
thermal energy input. The systems collect energy using a synthetic heat transfer fluid
pumped through absorber tubes in the focal line of parabolic trough collectors. The heated
fluid provides the thermal resource to drive a Rankine steam power cycle.

A model for the solar field was developed using the TRNSYS simulation program. The
Rankine power cycle was separately modeled with a simultaneous equation solving software
(EES). The steady-state power cycle performance was regressed in terms of the heat transfer
fluid temperature, heat transfer fluid mass flow rate, and condensing pressure, and
implemented in TRNSYS. TRNSYS component models for the steam condenser and cooling
tower were implemented in the simulation as well. Both the solar field and power cycle
models were validated with measured temperature and flow rate data from the SEGS VI plant
from 1998 and 2005. The combined solar field and power cycle models have been used to
evaluate effects of solar field collector degradation, flow rate control strategies, and
alternative condenser designs on plant performance.

Comparisons of measured solar field outlet temperatures between 1998 and 2005 indicate
some degradation in field performance. The degradation in performance over time may be
attributed, in part, to loss of vacuum in the annulus surrounding the absorber tube. Another
potential contributor to solar field degradation is hydrogen accumulation in the annular
space; hydrogen may dissociate from the synthetic heat transfer fluid and permeate through
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the absorber tube into the annulus. The thermal losses and resultant outlet temperatures are
modeled assuming 50% of collectors experience some loss of vacuum and/or hydrogen
permeation. The loss in electric power from the cycle is quantified as a function of the
prevalence of vacuum loss and hydrogen accumulation in the field.

The electric power output from the system at a given incident radiation depends on the
system efficiency, defined as the product of the solar field efficiency and the power cycle
efficiency. The solar field efficiency will decrease with increasing outlet temperature, while
the power cycle efficiency will increase with increasing outlet temperature. The magnitude
of these competing trends is such that the net change in system efficiency with outlet
temperature is small.

The SEGS plants use induced draft cooling towers for heat rejection. Cooling towers provide
an effective means of heat rejection, but require makeup water to compensate for evaporative
losses. The use of air cooled condensers can reduce plant water consumption; however,
system efficiency suffers with the higher condensing pressure. The optimal size of an air
cooled condenser unit is evaluated, and its performance assessed and compared to that of the
current condenser/cooling tower system.
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Acknowledgements

I owe thanks first and foremost to my three advisors:

Bill Beckman – Your experience and wisdom have benefited me a great deal. Thank
you for all of your counsel and advice.

Doug Reindl – Thank you for all of your enthusiasm for this project, as well as your
supportive attitude. Maybe some day I’ll learn how to speak in English units.

Sandy Klein – It’s an honor to be your advisee and student. It’s a pleasure to call you
my friend. Thanks for being my rabbit.

I am especially grateful for the feedback and support of all of the Concentrating Solar Power
staff at the National Renewable Energy Laboratory: Hank Price, Nate Blair, Mary Jane Hale,
and Mark Mehos. Special thanks as well to Carin Gummo, Nick Potrovitza, Scott Cawein,
Dan Brake, Harvey Stephens, and all of the SEGS plant personnel that have hosted myself
and other members of the Solar Energy Laboratory for site visits. These trips were an
incredibly valuable part of my research experience, and I am indebted to everyone at SEGS
and NREL that helped make them possible.

To my husband David, for being my sounding board several times: Thanks for letting me
vent my frustrations … and occasionally for solving my problems in doing so.
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Thanks to all of the friends and fellow graduate students in the Solar Lab, especially Kate
Edwards, who can always make me smile.

Finally, a special thank you goes to all of my family in Omaha – my parents, Mary and Jerry
Jacobson, and my siblings, Becky, Steve, and Stacy. You are the most important people in
my life.

This research work was sponsored by the National Renewable Energy Laboratory under
Contract 144-MQ55.
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Table of Contents
Abstract.........................................................................................................................................i
Acknowledgements..................................................................................................................iii
Table of Contents.......................................................................................................................v
List of Tables..............................................................................................................................ix
List of Figures............................................................................................................................xi
1 Introduction.............................................................................................................................1
1.1 Background for Solar Electric Generation Systems (SEGS)..........................................1
1.2 Literature review.............................................................................................................6
1.3 Objectives of current work..............................................................................................8
2 Solar Field Model.................................................................................................................11
2.1 Introduction...................................................................................................................11
2.2 Solar Irradiation Absorption.........................................................................................16
2.2.1 Direct Normal Insolation......................................................................................16
2.2.2 Angle of incidence................................................................................................17
2.2.3 Incidence Angle Modifier (IAM)..........................................................................26
2.2.4 Row Shadowing and End Losses..........................................................................28
2.2.5 Field Efficiency and HCE Efficiency...................................................................32
2.3 Receiver Heat Loss.......................................................................................................34
2.3.1 Analytical Heat Loss Derivation...........................................................................34
2.3.2 Linear Regression Heat Loss Model.....................................................................36
2.3.3 Solar Field Piping Heat Losses.............................................................................42
2.4 HTF Energy Gain and Temperature Rise.....................................................................42
2.5 Heat Transfer Fluid Pumps...........................................................................................44
2.6 Model Capabilities and Limitations..............................................................................46
3 Power Cycle Model...............................................................................................................47
3.1 Introduction...................................................................................................................47
3.2 Temperature – Entropy Diagram..................................................................................51
3.3 Assumptions..................................................................................................................52
3.4 Component Models.......................................................................................................54
3.4.1 Superheater / Reheater..........................................................................................54
3.4.2 Steam Generator (Boiler)......................................................................................62
3.4.3 Preheater...............................................................................................................64
3.4.4 Turbine..................................................................................................................66
3.4.5 Condenser.............................................................................................................70
3.4.6 Pump.....................................................................................................................71
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3.4.7 Closed Feedwater Heater......................................................................................73
3.4.8 Open Feedwater Heater (Deaerator).....................................................................77
3.4.9 Mixer.....................................................................................................................79
3.5 Power Generation and Cycle Efficiency.......................................................................81
3.6 Linear Regression Power Cycle Model........................................................................84
3.7 Conclusions...................................................................................................................86
4 Balance of System Models....................................................................................................88
4.1 Introduction...................................................................................................................88
4.2 Storage Tank.................................................................................................................89
4.3 Water-Cooled Condenser..............................................................................................92
4.4 Cooling Tower..............................................................................................................95
4.5 Conclusions...................................................................................................................98
5 Model Validation..................................................................................................................99
5.1 Introduction...................................................................................................................99
5.2 Data Consistency Analysis and Validation.................................................................102
5.2.1 HTF mass flow rate.............................................................................................103
5.2.2 HTF inlet temperature.........................................................................................106
5.2.3 Steam mass flow rate..........................................................................................112
5.2.4 Energy Balances..................................................................................................114
5.3 Solar Field Model Validation......................................................................................117
5.4 Power Cycle Model Validation...................................................................................132
5.5 Condensing Pressure Predictions versus Plant Data...................................................145
5.6 System Gross Electric Power Predictions versus Plant Data......................................153
5.7 Conclusions.................................................................................................................161
6 Solar Field Performance Analysis......................................................................................162
6.1 Introduction.................................................................................................................162
6.2 Assumptions................................................................................................................163
6.3 Change in Solar Field Heat Retention and Outlet Temperature.................................165
6.4 Effect on Electric Power Output.................................................................................172
6.5 Conclusions.................................................................................................................175
7 Optimized Control of Solar Field Flow Rate......................................................................178
7.1 Introduction.................................................................................................................178
7.2 Solar Field, Power Cycle Efficiency with Flow Rate.................................................179