COLLINSVILLE SOLAR THERMAL PROJECT - Australiaratchaustralia.com/collinsville/docs/Collinsville Solar Thermal... · Optimisation of Operational Regime Report R. Singh and H. Gurgenci

COLLINSVILLE SOLAR THERMAL PROJECT

OPTIMISATION OF OPERATIONAL REGIME

Report 4

Prepared for RATCH-Australia Corporation

PUBLIC VERSION

Optimisation of Operational Regime Report R. Singh and H. Gurgenci

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This project is supported by funding from the Australian Government under the Australian Renewable Energy Agency (ARENA).

Chief Investigators

Paul Meredith, Global Change Institute Craig Froome, Global Change Institute Hal Gurgenci, School of Mechanical and Mining Engineering John Foster, School of Economics Tapan Saha, School of Information Technology and Electrical Engineering

Authors

Rajinesh Singh, [email protected] Hal Gurgenci, [email protected]


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This report is one in a series of seven reports undertaken in relation to the Collinsville Solar Thermal Project. The series of reports included:

1. Yield forecasting

2. Dispatch forecasting

3. Solar mirror cleaning requirements

4. Optimisation of operational regime

5. Power system assessment

6. Energy economics

7. Fossil fuel boiler integration

Copies of all reports can be found at www.gci.uq.edu.au

These reports are part of a collaborative research agreement between RATCH Australia and the Global Change Institute at The University of Queensland (UQ) partially funded by the Australian Renewable Energy Agency (ARENA) The research was primarily undertaken through the Energy Economics and Management Group, School of Economics, School of Mechanical and Mining Engineering and the Power and Energy Systems Group, School of Information Technology and Electrical Engineering.


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Contents

1. Executive Summary ........................................................................................................................ 6

2. Introduction and Background ......................................................................................................... 6

2.1. Plant description ...................................................................................................................... 6

2.2. Features of the solar field ........................................................................................................ 6

2.3. Objectives and scope .............................................................................................................. 7

3. Literature Review ............................................................................................................................ 9

3.1. Two-phase flow instabilities ..................................................................................................... 9

4. Modelling Methodology ................................................................................................................ 11

4.1. Modelling of the solar field ..................................................................................................... 12

4.2. Constitutive equations for heat-transfer and pressure drop .................................................. 13

5. Results and Discussion –Steady State Performance .................................................................... 15

5.1. Two-phase heat transfer coefficients .................................................................................... 15

5.2. Pressure drop in the two-phase flow region .......................................................................... 17

5.3. Receiver heat-loss ................................................................................................................. 18

6. Results and Discussion –Dynamic Performance .......................................................................... 18

6.1. Dynamic performance of a single evaporator ........................................................................ 18

6.2. Dynamic performance of two evaporators operating in parallel ............................................ 20

6.3. Effect of operating pressure on two-phase flow stability ....................................................... 23

6.4 Effect of heat ramp on two-phase flow stability ..................................................................... 23

6.5 Effect of degree subcooling ................................................................................................... 24

6.6 Effect of (thick) tube-walls ...................................................................................................... 24

7. Conclusions ................................................................................................................................... 24

8. Suggestions for Further Research ................................................................................................ 24

9. References .................................................................................................................................... 26


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Figures

Figure 1. Schematic of the proposed gas-hybrid solar thermal power plant at Collinsville. .................. 8 Figure 2. Layout of two evaporator-superheater sets in the solar field connected in parallel ............... 9 Figure 3. Contributions of different pressure drop sources to the total pressure drop curve in

upward boiling flow (reproduced from Ruspini (2013)). ....................................................... 11 Figure 4. Schematic of the receiver in the solar collector used in modelling ....................................... 12 Figure 5. Different flow-regimes occurring during evaporation in horizontal tubes (reproduced

from Thome (2007)). ............................................................................................................. 13 Figure 6. Flow-regime based heat-transfer coefficient map as implemented in Dymola using the

Wojtan-Ursenbacher-Thome (2005) correlation at low heat-flux conditions (16.464 kW/m2). ................................................................................................................................. 16

Figure 7. Flow-regime based heat-transfer coefficient map as implemented in Dymola using the Wojtan-Ursenbacher-Thome (2005) correlation at high heat-flux conditions (32.928 kW/m2). ................................................................................................................................. 16

Figure 8. Variation of the Friedel pressure drop for adiabatic flow as a function of mass-flux in the evaporator calculated for different steam qualities in Dymola. ...................................... 18

Figure 9. Dynamic response of a single evaporator tube to a 2.5 MW step (from zero) in solar heat input at 500s. Total mass-flow, steam quality, steam and tube-wall temperature, heat transfer coefficient and pressure at the outlet of a single evaporator tube are plotted. ................................................................................................................................. 20

Figure 10. The occurrence of pressure drop oscillations in a set of two evaporators connected in parallel resulting from a 30s ramp (at 500s) and uneven distribution of solar heat input at low operating pressures (~2 MPa). ......................................................................... 22

Tables

Table 1. Design parameters and nominal conditions of the solar field and turbine ............................... 7 Table 2. Comparison of the pressure-drop in the evaporator tubes predicted using Thermoflex and

Dymola at steady-state conditions ......................................................................................... 17


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1. Executive Summary

This study investigates the dynamic characteristics and performance of the proposed 30 MWe hybrid direct-steam generation (DSG) solar thermal power plant to replace the pre-existing Collinsville coal-fired power station in North Queensland, Australia. A dynamic model of the solar field in the proposed power plant has been developed in Dymola® and simulations are conducted to understand steam generation characteristics during transient events affecting solar heat input such as passing cloud cover and heat ramps during startup and shutdown. Flow instability and maldistribution in the parallel evaporator tubes of the solar field is simulated and a control strategy for stabilising steam output quality in the evaporator tubes is also demonstrated using simulations.

2. Introduction and Background

2.1. Plant description

The proposal for the replacement of the pre-existing Collinsville coal-fired power plant includes a 30 MWe gas-hybrid solar thermal plant with a gas-fired boiler supplementing steam generation during periods of low or no solar insolation. The turbines in the pre-existing Collinsville coal-fired power station were inspected during a site visit in March 2013 and were found to be in a condition unsuitable to be utilised in the proposed solar thermal power plant. A simplified schematic of the proposed solar thermal power plant layout is shown in Figure 1. The gas-fired boiler in the power plant supplies steam directly to the turbine. The location of steam feed from the gas-fired boiler is directly downstream of the superheating section of the solar field.

2.2. Features of the solar field

The proposed solar field of the solar power plant comprises of 20 evaporator and superheater sections connected in parallel to a common inlet and outlet header. The solar field comprises Novatec Solar collectors with linear Fresnel reflector (LFR) technology. Each set comprises of (in the direction of flow) an evaporator section, a phase separator, and a superheater section split by a desuperheater or spray attemperator that controls the temperature of the superheated steam entering the turbine. Water at conditions close to saturation is supplied to the solar field using a recirculating pump and is sourced from the feed heaters and liquid portion of the phase separators. The schematic in Figure 2 demonstrates the layout of two such evaporator-superheater sets of the solar field connected in parallel. Table 1 summarises the key design parameters and operational conditions for the solar field and turbine group.


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Table 1. Design parameters and nominal conditions of the solar field and turbine No. of parallel evaporator sections 20

No. of parallel superheater sections 20

Evaporator line length 493m

Superheater line length 269m

Evaporator absorber tube diameter ID 58mm,OD 70mm

Superheater absorber tube diameter ID 57mm,OD 70mm

Evaporator/superheater glass envelope diameter (Benz, Eck et al. 2006)

ID 119 mm, OD 125mm

Evaporator tube inlet mass flow (per tube) 5.875 kg/s

Superheater tube inlet mass flow (per tube) 1.96 kg/s

Steam quality 33%

Turbine inlet pressure 12 MPa

Turbine inlet temperature 500°C

Turbine mass flow rate 31 kg/s

Evaporator total solar heat input 43 MW

Superheater total solar heat input 34.5 MW

Turbine/generator power output 32.221 MW

No. of turbine stages 6

Gross electric efficiency (LHV) 41.57%

Proposed mode of plant operation The dynamic modelling and simulation study is conducted considering the boiler, steam field, and turbine operating in sliding-pressure mode. Operation in sliding-pressure mode allows the turbine to operate with variable steam temperature and pressure (as pressure and temperature at saturation conditions are related) depending on turbine demand and solar insolation.

2.3. Objectives and scope

Optimisation of the operational regime using dynamic modelling includes the investigation of flow stability and steam quality fluctuations during transient events including cloud passing and ramps in solar insolation. The dynamic modelling methodology utilised in this work has been primarily driven by the requirement for gaining a deeper understanding of steam generation characteristics unique to the design and arrangement of the solar field during direct normal irradiance (DNI) transients. The main objectives of this work are therefore to investigate the following issues in the evaporators which ultimately have a bearing on stability and power generated by the solar thermal power plant:

• The impact of DNI transients on two-phase flow instability between evaporator sections

connected in parallel (inter-tube)


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- The current arrangement of the solar field has evaporator sections connected in parallel.

Two-phase flow instabilities induced by transients and uneven distribution of solar heat-

flux amongst individual evaporator sections could result in undesirable fluctuations in

steam temperatures, pressures, and steam throughput to the turbine.

• The impact of DNI transients on intra-tube two-phase flow behaviour

- There is a need to understand two-phase flow regimes and stability in the evaporator

sections during operation. Fluctuations in DNI (solar heat input) could potentially cause

dry-out and negatively impact evaporator and superheater tube life in the solar field.

Figure 1. Schematic of the proposed gas-hybrid solar thermal power plant at

Collinsville.

TURBINE

CONDENSER/COOLING TOWER

RECIRCULATION PUMP

EVAPORATOR SOLAR FIELD

SUPERHEATER SOLAR FIELD

GAS FIRED BOILER

FEED PUMP FEEDWATER HEATERS

GENERATOR


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Figure 2. Layout of two evaporator-superheater sets in the solar field connected in

parallel

3. Literature Review

Few investigations have previously been conducted into dynamic modelling of direct-steam generation in solar fields including that for parabolic trough technology (Birnbaum, Feldhoff et al. 2011), as well as for linear Fresnel reflector (LFR) collectors (Pye, Morrison et al. 2007; Pye 2008). These studies have mainly investigated solar field configurations in which the collectors are connected in series only and most investigations lack the presentation of heat-transfer correlations as well as flow-stability issues in the evaporators of the solar field mainly due to confidentially reasons. To the best of our knowledge, there is no published literature currently available in the public domain discussing dynamic modelling and simulation of two-phase flow instabilities associated with operating a solar field with evaporator sections connected to a common inlet and outlet header (also referred to as being connected in parallel).

3.1. Two-phase flow instabilities

Two-phase flow instabilities are undesirable phenomena which are known to occur in a wide variety of boiling systems due to the potential for causing damage and reduction in plant life. Steam-generators employed in the industrial and nuclear sector are commonly natural-circulation boilers and therefore involve boiling in mainly vertical tubes. Two-phase flow instabilities and heat-transfer coefficients for flow-boiling in large-diameter vertical tubes are therefore comparatively well researched and validated to their horizontal counterparts due to their widespread application. The solar field of the proposed Collinsville power plant solar field however, involves boiling in long and large-diameter horizontal tubes. These tubes are also connected in parallel through common inlet and outlet headers which adds an additional source of instability through the potential for flow maldistribution.

Two-phase flow instabilities include both static and dynamic instabilities, namely

• Static instabilities: flow excursion or Ledinegg instability, flow transition instability,

boiling crisis

separator

separator

Evaporator 1

Evaporator 2

pump

Sinkp,h

from feed heaters/separators

VFD

Superheater 1

Superheater 2

Superheater 1

Superheater 2

desuperheater

desuperheater

(turbine)

Collinsville solar thermal project: Optimisation of Operational Regime

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• Dynamic instabilities: density wave oscillations, pressure drop oscillations, flow

maldistribution (parallel channel) instability, thermal oscillations

A thorough description and review of the different types of two-phase flow instabilities is given by Kakac (Kakac and Bon 2008) and by Ruspini (Ruspini 2013). Conditions required for triggering static and dynamic instabilities which commonly occur in boiling systems however are discussed briefly below.

The occurrence of Ledinegg instability (a static instability) and pressure drop oscillations (a dynamic instability) requires a negative gradient in the characteristic pressure drop versus mass flow rate curve of the boiling channel (Eq. 1). The gradient of the boiling channel must also be steeper than that of the system curve external to the boiling channel (e.g a pump) (Ruspini 2013).

𝑑𝑃𝑑𝑚

< 0 = 𝑢𝑛𝑠𝑡𝑎𝑏𝑙𝑒 (1) Pressure drop oscillations are a dynamic instability triggered by a static instability in the system. In the case of Ledinegg instability, the boiling system can settle at either of at least two new operating points if perturbed for a single boiling channel. The number of possible operating points in a parallel channel can increase exponentially in contrast, as the number of channels is increased. Pressure drop oscillations and Ledinegg instability are also influenced by fluid inertia and power density. Density wave oscillations are also dynamic instabilities and can result due to the interaction of flow inertia, compressibility, and pressure drop characteristics of the boiling system. Density wave oscillations therefore exhibit similar trends to pressure drop oscillations although occur at much high frequencies (Ruspini 2013). A demonstration of how the different pressure drop components of a boiling system can contribute to stabilising or destabilising the system in vertical and upward boiling channels is presented in Figure 3 as an example.


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Figure 3. Contributions of different pressure drop sources to the total

pressure drop curve in upward boiling flow (reproduced from Ruspini (2013)).

Each tube or boiling section in a bank of tubes connected to a common inlet and outlet header (or connected in parallel) will have the same pressure difference across inlet and outlet due to common inlet and output pressure boundary conditions. Flow through a standalone boiling section with two-phase flow (e.g steam-water) experiences a larger pressure drop than for liquid-only flow. In the case of two-phase flow through a bank of parallel tubes, the flow quantity through each tube gets redistributed in accordance with the relative vapour quality resulting in flow maldistribution. Flow maldistribution is therefore a major concern in the evaporators of the solar field due to the uneven distribution of solar heat amongst adjacent evaporator tubes.

Thermal oscillations result mainly due to two-phase flow-regime transition and therefore changes in heat-transfer coefficients as steam quality evolves along the length of evaporator tubes. Oscillations in evaporator tube-wall temperatures are subsequently induced potentially causing fatigue-related tube damage.

4. Modelling Methodology

The study has been conducted using pre-existing components adapted from the ThermalPower library in Dymola® [2014] (Dassault Systemes); a commercial multi-engineering modelling and simulation environment for complex physical systems based on the Modelica® language. The overall specifications and arrangement of the solar field have been adapted directly from that specified by Parsons Brinkerhoff (PB) for use in their Thermoflex steady-state model. The modelling methodology employs a finite-volume scheme in which the flow-volume through the absorber tubes of the solar field are axially discretised into sub-volumes. The conservation laws of mass and energy are solved at node points along the grid of sub-volumes with a staggered-grid utilised for solving the quasi-steady momentum equation used for determining pressures. A discretised approach


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is also used for modelling the wall of the absorber tube and glass envelope with only radial heat-losses being considered assuming negligible axial conduction. Flow is assumed to be homogenous, well-mixed, and fully developed both thermally and hydraulically for all heat-transfer and pressure drop calculations.

4.1. Modelling of the solar field

Dynamic models of the absorber-tube walls for the evaporator and superheater sections, and the wall of the glass evacuated-tube (Figure 4), have been integrated into the dynamic model of the solar field to account for heat loss from the collector receivers to the environment. Optical performance of the solar collector is not considered in this study due to unavailability of actual optical performance data of the Novatec Solar collectors.

Figure 4. Schematic of the receiver in the solar collector used in modelling

Heat-loss mechanisms from the absorber tube and glass envelope are based on that in (Forristall 2003) and include:

• Wind-related convective heat-losses from the evacuated-tube (glass envelope) wall

• Radiative heat-losses from the absorber-tube wall to the evacuated-tube wall

• Radiative heat-losses from the evacuated-tube wall to the environment and,

• Convective heat-transfer from the wall of the absorber tube to the bulk-fluid (water-

steam) flowing through the absorber-tube

The emissivity of the absorber tube implemented in the dynamic model is based on data for a Schott PTR®70 receiver coating reported in (Burkholder and Kutscher 2009).

Water - Steam

Absorber tube

Evacuated tube (Vacuum)

Glass envelope


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4.2. Constitutive equations for heat-transfer and pressure drop

Heat-transfer A diabatic, flow-regime based, heat-transfer coefficient for two-phase flow has been implemented in Dymola and used in dynamic modelling of the evaporator tubes in the solar field. The main objective in resolving the heat-transfer coefficient in the two-phase flow regime is to avoid the inception of dryout and eventually mist flow which can result in tube burnout and a significant reduction in tube life.

The two-phase heat-transfer coefficient is influenced by the stage of boiling (e.g nucleate, convective), flow-regime, tube diameter, tube-orientation, gravitational effects, as well as whether the flow is diabatic or adiabatic. Dynamic modelling of the heat-transfer coefficient during evaporation therefore requires utilising appropriate correlations which account for these different factors which are mainly pre-defined, except for the different flow-regimes occurring during convective boiling in the evaporator tubes. The respective flow-regimes are generally determined as a function of mass-flux, flow quality, temperature, and heat-flux in the tube and defined using flow transition boundaries in a flow-pattern map.

Figure 5. Different flow-regimes occurring during evaporation in horizontal tubes

(reproduced from Thome (2007)). Many different two-phase flow-pattern maps exist, mainly for evaporation of refrigerants in micro-channels and small diameter tubes. However, the Kattan-Thome-Favrat (1998) (Kattan, Thome et al. 1998) flow-pattern map is considered the most comprehensive and accounts for vapour qualities from 0-100% as well as both adiabatic and diabatic flows in horizontal tubes. Several improvements have been made to the Kattan-Thome-Favrat (Kattan, Thome et al. 1998; Kattan, Thome et al. 1998) map including that by Thome-El Hajal (2003) (Thome and Hajal 2003) and more recently by Wojtan-Ursenbacher-Thome (2005) (Wojtan, Ursenbacher et al. 2005; Wojtan, Ursenbacher et al. 2005). The Wojtan-Ursenbacher-Thome flow-pattern has been validated for diabatic refrigerant flow in 13.84 mm diameter tubes and maximum mass-flux and heat-flux values of 500 kg/m2.s and 57.5 kW/m2, respectively (Wojtan, Ursenbacher et al. 2005). Hence, the Wojtan-Ursenbacher-Thome correlation is considered the most suitable in terms of the current availability of flow-regime based heat-transfer coefficient correlations for diabatic flow in large diameter tubes and has therefore been implemented in Dymola for steam generation simulations in


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the solar field. It must be noted though that the validity of this correlation can only be confirmed for steam generation in the evaporators of the solar field through an experimental campaign.

The general implementation of the heat-transfer correlations in Dymola is based on the whether subcooled, saturated boiling, or superheating is taking place in the tubes with a smoothing function applied for switching between the three flow states to aid the solver. In the case of subcooled and superheating during which single-phase flow exists, the general heat-transfer correlations commonly used for laminar and turbulent flow are utilised.

Pressure drop The quasi-steady form of the momentum equation takes the following form:

∆𝑃!"!#$ = ∆𝑃!"#$%#&' + ∆𝑃!"#"$% + ∆𝑃!"!#$%&! (2)

The total pressure drop for internal flow in a tube consists of pressure drop due to wall friction in the pipe, pressure drop due to acceleration associated with the change of fluid density with quality (negligible for single phase flow), and that due to static elevation between the pipe inlet and outlet. The determination of frictional pressure drops for horizontal two-phase flows also poses a challenge in dynamic modelling of the evaporator tubes mainly due to the lack of availability and published data on validated correlations for diabatic two-phase water-steam flows in large diameter horizontal tubes.

In the case of the subcooled and superheated flows, single-phase laminar and turbulent pressure drop equations are utilised. For evaporation, several correlations are available including those that consider flow-regime using flow-pattern maps such as the Quiben and Thome (Moreno Quibén and Thome 2007) correlation, while others use two-phase multipliers. However, these correlations are generally developed using refrigerants and generally valid for small tube diameters. The Friedel (Friedel 1979) correlation developed for horizontal and vertical upward flows has shown promising results and has been the most widely utilised for direct-steam generation pressure-drop determination in parabolic troughs (Schenk and Hirsch 2009), and compact linear Fresnel reflector (CLFR) (Pye, Morrison et al. 2007) plant investigations. The utilisation of the Friedel (Friedel 1979) correlation is also attractive as it is relatively easy to implement computationally and has previously been shown to deliver more accurate estimates when compared to the more complex correlations such as that of Quiben and Thome (Moreno Quibén and Thome 2007). A comprehensive discussion on the available correlations is presented in (Ingenieure 2010). A comparison between experimental data from the DISS project at Plataforma Solar de Almeria in Spain and that estimated using Friedel’s method overestimated by 5% “consistently” with a maximum deviation of 15% as reported in (Pye 2008). These comparisons were however for relatively lower operational pressures in the order of 60 bar when compared to the Collinsville power plant operating pressure of 120 bar in this study.

It is clear in the literature that results from currently available two-phase pressure-drop correlations can produce results which vary by 50% or more. The results of a calculation for the operating conditions of the Collinsville solar thermal power plant conducted in this study showed that the pressure drop prediction using the Friedel method (Friedel 1979) is


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double that predicted by the Quiben and Thome (Moreno Quibén and Thome 2007) correlation. The Friedel (Friedel 1979) correlation is implemented in Dymola for determination of the dynamic performance of the evaporators in the solar field for this study however as highlighted earlier, the accuracy of the predicted results can only be ascertained using an experimental setup in the future.

During the evaporation process, the flow through the tube is accelerated from the slower moving liquid phase to the faster moving gas phase resulting in a pressure drop due to a change in momentum of the fluid. Several methods are available for estimating the momentum pressure drop which requires evaluation of the void fraction including homogenous, heterogeneous, and the drift-flux methods. The Steiner (1993) modification of Rouhani and Axelsson correlation is validated against experimental data for evaporation of refrigerants in Kærn, Elmegaard et al. (2011) and shown to be most accurate for dynamic modelling of refrigerant evaporation. The contribution of the momentum pressure drop determined using the Steiner (1993) modification of Rouhani and Axelsson correlation for void fraction was determined to contribute less than 2% of the overall pressure drop. The overall pressure drop in the evaporator tubes is dominated by the friction term at peak operating conditions and hence the contribution of the momentum pressure drop can be considered negligible for dynamic modelling calculations.

5. Results and Discussion –Steady State Performance

5.1. Two-phase heat transfer coefficients

The flow pattern-maps and flow-regime based heat transfer coefficients for steam at low heat-flux conditions (16.464 kW/m2) and high-heat-flux conditions (32.928 kW/m2) are shown in Figure 6 and Figure 7, respectively. The variation in heat transfer coefficient (green) is plotted versus process mass-flux (red) as the vapour quality of flow increases in a horizontal evaporator tube. The figures are generated in Dymola using the Wojtan-Ursenbacher-Thome (2005) correlation and show heat transfer coefficients and flow-transition boundaries in the evaporator tubes for boiling at a pressure of 12 MPa with a total mass-flux of 1760 kg/m2.s through a tube with an internal diameter of 58mm.

The sudden drop in heat transfer coefficient in Figure 6 and Figure 7 at high vapour qualities as flow enters the mist and subsequently dry-out flow regime testifies that designing a solar field for operation at high steam qualities is risky. This is due to the high possibility for mist-flow or dry-out occurring in the tubes causing excessive tube-wall temperatures in the solar field. This risk is encountered earlier at lower steam qualities at higher heat-flux values (as indicated by the difference in the incline of the mist-flow transition boundary in Figure 6 and Figure 7).


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Figure 6. Flow-regime based heat-transfer coefficient map as implemented in Dymola using the Wojtan-Ursenbacher-Thome (2005) correlation at low heat-flux conditions

(16.464 kW/m2).

Figure 7. Flow-regime based heat-transfer coefficient map as implemented in Dymola using the Wojtan-Ursenbacher-Thome (2005) correlation at high heat-flux conditions

(32.928 kW/m2).

0

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Hea

t Tra

nsfe

r C

oeffi

cien

t (W

/m2.

K)

Mas

s Flu

x (k

g/m

2.s)

steam quality (x)

STEAM Flow Pattern Map (Implemented in Dymola) : Wojtan-Ursenbacher-Thome (2005)

process value (kg/m2.s)heat transfer coefficient

AnnularIntermittent

Mist

Stratified-Wavy

Conditions:

STEAMmass flux: 1760 kg/m2.stube diameter = 58 mmheat flux = 16.464 kW/m2saturation temperature = 324 degCsaturation pressure = 12 MPa

Slug dryoutSlug+Stratified-Wavy

Stratified

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t Tra

nsfe

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oeffi

cien

t (W

/m2.

K)

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s Flu

x (k

g/m

2.s)

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STEAM Flow Pattern Map (Implmented in Dymola) : Wojtan-Ursenbacher-Thome (2005)

process value (kg/m2.s)heat transfer coefficient

dryout

Annular

Intermittent

Mist

Stratified-Wavy

Stratified

Conditions:STEAMmass flux: 1760 kg/m2.stube diameter = 58 mmheat flux = 32.928 kW/m2saturation temperature = 324 degCsaturation pressure = 12 MPa

Slug

Slug+Stratified-Wavy


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5.2. Pressure drop in the two-phase flow region

Comparison of steady-state results from Thermoflex and Dymola® during normal operation A comparison of the steady-state pressure-drop at design solar heat-input conditions using the PB Thermoflex® model and that predicted using steady-state simulations with the Dymola® dynamic model is presented in Table 2. The evaporator pressure drop predicted by Dymola is less than half that predicted by Thermoflex. The modelling in Dymola employs a discretised finite volume method in which the length of the evaporator tube is divided into several sub-volumes. The evolution of the vapour generation is consequently tracked along the length of the tube resulting in a more accurate calculation of the pressure drop.

Table 2. Comparison of the pressure-drop in the evaporator tubes predicted using Thermoflex and Dymola at steady-state conditions

Thermoflex estimation Dymola estimation Pressure drop (bar)

18.8 8.6

Initial Conditions

Mass-flow rate per tube: 5.875

kg/s (117.5 kg/s for 20 tubes in parallel) Inlet temperature: 326.9°C

Inlet pressure: 134.1 bar Heat input per tube: 2.5 MW

In the Dymola model, the steam quality only reaches 34% close to the outlet of the tube and the total pressure drop for the evaporator tubes is determined to be 8.6 bar using the Friedel correlation. This pressure drop prediction using a discretised modelling approach is much lower than that predicted using a non-discretised approach which relies on the assumption of a constant steam quality along the entire length of the tube. This was confirmed by an estimation conducted using the Dymola model with steam quality along the entire length of the tube set at a constant 34% (adiabatic flow) and is shown in Figure 8 for different mass-fluxes and vapour qualities.

It is known that the Thermoflex model also uses a discretised approach and therefore tracks the evolution of vapour generation along the length of the heated channel however the specific two-phase pressure drop correlation used in the software remains unknown. Hence, it is a possibility that the cause of the difference between the pressure drop estimated by the Thermoflex model and the Dymola model is due to a difference in the specific two-phase pressure drop correlation used in modelling as there are suggestions in the literature of significant differences in pressure drop estimations when using different two-phase flow correlations.


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Figure 8. Variation of the Friedel pressure drop for adiabatic flow as a function of

mass-flux in the evaporator calculated for different steam qualities in Dymola.

5.3. Receiver heat-loss

The calculated steady-state heat loss from the Schott PTR®70 evaporator receiver for an average absorber temperature of 327°C is determined to be 133 W/m using the heat loss model implemented in Dymola. The environmental conditions used for this heat loss calculation includes an average ambient air temperature of 25°C and a wind speed of 1 m/s blowing in the axial direction relative to the absorber tube. The predicted steady-state receiver heat loss value using the model implemented in Dymola compares well (within +5%) to results from tests conducted at the National Renewable Energy Laboratory (NREL) reported in (Burkholder and Kutscher 2009) which had an uncertainty in heat loss measurements of ±10 W/m. Using the Dymola model, the average wall temperature of the glass envelope is determined to be 46°C and the absorber wall temperature to be 333°C for the stated conditions.

6. Results and Discussion –Dynamic Performance

6.1. Dynamic performance of a single evaporator

This section presents the dynamic response of a single and individual evaporator tube to a step increase in solar heat input of 2.5 MW from zero, at 500s in Figure 9. The simulations are conducted with a fixed outlet pressure boundary of 11.5 MPa and inlet mass-flow rate of 5.875 kg/s while inlet pressure is allowed to vary. The plots in Figure 9 show total flow (water-steam), outlet steam quality, outlet steam temperature, inlet pressure, and the heat transfer coefficient at the outlet of the evaporator tube.

0.0E+00

5.0E+05

1.0E+06

1.5E+06

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0 500 1000 1500 2000 2500

ΔP (P

ress

ure

drop

in P

asca

ls)

Mass flux (kg/m2.s)

quality=30%quality=60%quality=90%


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As solar heat to the evaporator tube increases from subcooled initial conditions, the resulting density change in the evaporator causes the evacuation of mass from the tube resulting in a large increase in mass-flow rate at the outlet of the evaporator. The evolution of steam quality and therefore steam flow is however relatively smooth and gradual. The peak in the total mass-flow rate leaving the tube is sensitive to the rate of increase in solar heat-flux to the evaporator. The results demonstrate that there will be a momentary spike in water (or condensate) returning from the separators to the condensate drum with a rise in solar heat input which if not considered in the design stage can cause a sudden and significant rise in drum water levels during dynamic operation of the power plant.


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Figure 9. Dynamic response of a single evaporator tube to a 2.5 MW step (from zero)

in solar heat input at 500s. Total mass-flow, steam quality, steam and tube-wall temperature, heat transfer coefficient and pressure at the outlet of a single

evaporator tube are plotted.

6.2. Dynamic performance of two evaporators operating in parallel

An uneven distribution in solar heat-flux amongst adjacent evaporators in the solar field has the potential to trigger undesirable two-phase flow instabilities in the tubes as discussed earlier in Section 3.1. A bank of parallel evaporator tubes such as that in the solar field at

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the proposed Collinsville power plant will possess an inherent system pressure drop characteristic and individual evaporator tubes in the bank will possess individual characteristics as functions of vapour which determine flow throughput. Simulations have been conducted in Dymola to demonstrate and investigate the occurrence of two-phase flow instabilities in parallel tubes with geometrical parameters of the evaporators representative of those at the Collinsville solar thermal power plant (Table 1). Simulations have been conducted with the assumption of thin tube walls for a set of two evaporators connected in parallel. The results for operation at relatively low pressures of approximately 2 MPa are presented in Figure 10.

The following subsections discuss the influence of operating pressure, subcooling, and thick tube-walls on two-phase flow stability in the context of the effect of uneven solar heat input distribution amongst parallel evaporators. The discussion is presented in the context of the configuration and operating pressure range of the solar field at the proposed Collinsville power plant.


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Figure 10. The occurrence of pressure drop oscillations in a set of two evaporators connected in parallel resulting from a 30s ramp (at 500s) and uneven distribution of

solar heat input at low operating pressures (~2 MPa).

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6.3. Effect of operating pressure on two-phase flow stability When operating at pressures less than 6 MPa, a very small difference in solar heat distribution amongst the two evaporators is significant enough to render one tube entirely subcooled while the other has two-phase flow. This flow maldistribution effect amongst the connected tubes is the occurrence of Ledinegg instability. An evaporator tube with the lower heat-flux will experience higher mass-flow and therefore entirely subcooled flow, while an adjacent tube with the higher heat-flux will experience lower mass-flow and therefore entirely superheated flow. The occurrence of this situation in the solar field would lead to excessively high temperatures and dry operation of the superheater sections the turbine. Pressure drop oscillation in addition to flow maldistribution instability is also observed when operating with an uneven distribution of solar heat-flux amongst parallel evaporator sections at operating pressures below 6 MPa. From the simulations performed, it appears that the general tendency especially at low pressures is for flow in one tube to stay or tend towards a subcooled condition when a heat-flux ramp is applied to the tubes from an initial state of water entering the evaporators at conditions close to saturation (bubble point). This effect triggers pressure drop oscillations in the solar field when the tubes are operating at low pressures with high heat-flux and high mass-fluxes.

The occurrence of pressure drop oscillations and flow maldistribution instabilities are shown in a set of two parallel evaporator tubes operating with a nominal pressure of 2 MPa, in Figure 10. The oscillations are initiated upon flow entering the two-phase region during a 30s heat-ramp from 0 to 0.9 MW, and 0 to1 MW (0.1 MW difference between the evaporators) applied at 500s in the figure. The simulations are conducted with a fixed outlet pressure and inlet mass-flow rate boundary condition and in the absence of any thermal inertia due to walls. Pressure-drop oscillations induce oscillations in steam flow, quality, and in steam and tube wall temperatures (steam - shown at 20% length and at 100% length (outlet) of the evaporators in Figure 10). These instabilities could result in thermal fatigue of the tubes and are therefore undesirable. Whether this situation is to occur in the Collinsville solar field which has thick tube-walls in the solar field due to the capability for high pressure (12 MPa) operation will be determined in later sections of this report.

For higher operating pressures (greater than 6 MPa and less than 12 MPa), the occurrence of Ledinegg instability and associated flow maldistribution only is observed using the simulations. At high operating pressures, a difference in solar heat input with a ratio greater than approximately 2.5 between adjacent tubes is required to cause one tube to experience dry-out and therefore become partially superheated with excessively high temperatures and temperature gradients. This occurrence is also influenced by the rate of increase in heat-flux applied to the tube which has a destabilising effect.

6.4 Effect of heat ramp on two-phase flow stability The rate of increase in the heat-flux also influences the amplitude of pressure drop oscillations along with the nature of the oscillations. A larger rate of increase in heat-flux results in a higher degree of instability by increasing the frequency of oscillations as well as causing oscillation amplitudes to progressively increase exponentially.


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6.5 Effect of degree of subcooling Increasing the degree of subcooling of the water entering the evaporators is observed to have a stabilising effect on pressure drop oscillations. A higher degree of subcooling dampens pressure drop oscillations and a continual increase eventually results in only flow maldistribution being observed.

6.6 Effect of (thick) tube-walls Walls of the evaporator tubes provide a dampening effect due to thermal inertia and therefore serve to stabilise flow in the tubes. It is therefore expected that the thick walls of the evaporator tubes in the Collinsville solar thermal power plant will contribute towards eliminating the occurrence of pressure drop oscillations.

A detailed analysis on the time constants of direct steam generation in the solar field and required response time of the backup fossil-fuel fired boiler is presented in a separate report titled Fossil Fuel Boiler Integration.

7. Conclusions

Preliminary investigations into optimisation of the operational regime at the proposed Collinsville solar thermal power plant have included the development of a dynamic model of the solar field in Dymola. The identification and implementation of appropriate flow-regime based heat-transfer correlations in the dynamic model along with pressure drop and heat-loss models for the evaporator and superheater solar fields has also been conducted. The need for consideration and resolution of two-phase flow instabilities in the parallel evaporators of the solar field is highlighted using steady-state analyses and dynamic simulations.

Results from the study at both low and high operating pressures have indicated that the thick-walls of the solar collectors have a stabilising effect on steam generation and flow during solar heat fluctuations and uneven distribution of solar heat across the solar field. The solar field is however highly susceptible to flow maldistribution and therefore potential resulting damage. The possibility for certain evaporator tubes to be entirely superheated and others to be entirely subcooled due to uneven heat distribution are highlighted in the simulation results. A control strategy has been proposed to mitigate potential boiling instabilities identified using the simulations. This proposed control strategy has shown promising results in the simulations to meet the objective of delivering predictable steam quality and flow through the solar field during fluctuations in solar heat input and for operation with uneven solar heat distribution across the solar field.

8. Suggestions for Further Research

1. Dynamic simulation of the integrated evaporator/superheater/turbine group with and without proposed control strategy for both low and high operating pressures.

2. Experimental campaign to validate predictions of flow transition and pressure drop in parallel horizontal solar collectors


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3. Modelling dynamics of solar field with the integration of solar collector optical performance data from the manufacturer (Novatec Solar)

4. Improvement of the control strategy through investigation of adaptive controllers suitable for application in low and high solar heat operation and sliding mode operation


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9. References

Benz, N., M. Eck, et al. (2006). Development of receivers for the DSG process. SolarPACES 2006. Seville, Spain.

Birnbaum, J., J. F. Feldhoff, et al. (2011). "Steam temperature stability in a direct steam generation solar power plant." Solar Energy 85(4): 660-668.

Burkholder, F. and C. Kutscher (2009). Heat loss testing of Schott's 2008 PTR70 parabolic trough receiver (NREL/TP-550-45633). Golden, CO, USA, , National Renewable Energy Laboratory.

Forristall, R. (2003). Heat transfer analysis and modeling of a parabolic trough solar receiver implemented in Engineering Equation Solver (NREL/TP-550-34169). Golden, CO, USA, , National Renewable Energy Laboratory.

Friedel, L. (1979). "Improved friction pressure drop correlations for horizontal and vertical two phase pipe flow." 3R International 18(7): 485-492.

Ingenieure, V. D. (2010). VDI heat atlas. Dusseldorf, Germany, Springer-Verlag. Kærn, M. R., B. Elmegaard, et al. (2011). Experimental comparison of the dynamic

evaporator response using homogeneous and slip flow modeling. International Modelica Conference. Dresden, Germany, March 20-22.

Kakac, S. and B. Bon (2008). "A Review of two-phase flow dynamic instabilities in tube boiling systems." International Journal of Heat and Mass Transfer 51(3–4): 399-433.

Kattan, N., J. R. Thome, et al. (1998). "Flow boiling in horizontal tubes: part 1—development of a diabatic two-phase flow pattern map." Journal of Heat Transfer 120(1): 140-147.

Kattan, N., J. R. Thome, et al. (1998). "Flow boiling in horizontal tubes: part 3—development of a new heat transfer model based on flow pattern." Journal of Heat Transfer 120(1): 156-165.

Moreno Quibén, J. and J. R. Thome (2007). "Flow pattern based two-phase frictional pressure drop model for horizontal tubes, Part II: New phenomenological model." International Journal of Heat and Fluid Flow 28(5): 1060-1072.

Pye, J. D. (2008). System modelling of the compact linear Fresnel reflector. Doctor of Philosophy Doctoral Thesis, The University of New South Wales.

Pye, J. D., G. L. Morrison, et al. (2007). Unsteady effects in direct steam generation in the CLFR. Solar 2007, Annual Conference of the Australian and New Zealand Solar Energy Society. Alice Springs.

Ruspini, L. C. (2013). Experimental and numerical investigation on two-phase flow instabilities. Philosophiae Doctor Doctoral Thesis, Norwegian University of Science and Technology.


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Schenk, H. and T. Hirsch (2009). Pressure drop analysis of steam generation parabolic trough plants. Solar Paces 2009, Berlin, Germany.

Thome, J. R. (2007). Engineering Data Book III, Wolverine Tube Inc. Thome, J. R. and J. E. Hajal (2003). "Two-phase flow pattern map for evaporation in

horizontal tubes: latest version." Heat Transfer Engineering 24(6): 3-10.

Wojtan, L., T. Ursenbacher, et al. (2005). "Investigation of flow boiling in horizontal tubes: Part I—A new diabatic two-phase flow pattern map." International Journal of Heat and Mass Transfer 48(14): 2955-2969.

Wojtan, L., T. Ursenbacher, et al. (2005). "Investigation of flow boiling in horizontal tubes: Part II—Development of a new heat transfer model for stratified-wavy, dryout and mist flow regimes." International Journal of Heat and Mass Transfer 48(14): 2970-

2985.


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About the Global Change Institute The Global Change Institute at The University of Queensland, Australia, is an independent source of game- changing research, ideas and advice for addressing the challenges of global change. The Global Change Institute advances discovery, creates solutions and advocates responses that meet the challenges presented by climate change, technological innovation and population change. This technical report is published by the Global Change Institute at The University of Queensland. T: (+61 7) 3443 3100 / E: [email protected] Global Change Institute (Bldg. 20) Staff House Road University of Queensland St Lucia QLD 4072, Australia www.gci.uq.edu.au

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