THERMOFLOW

☼ SPOTLIGHT ON SOLAR THERMAL MODELING ☼

2 THERMOFLOW

Thermoflow Software

Thermoflow provides software fordesign, simulation, and costestimation of power, cogeneration,process, and heating plants. Startingin 1987 with its flagship program GTPRO™, Thermoflow’s software suite has grown to include sevenpowerful, yet easy-to-use programsto analyze the spectrum of powergenerating technologies in usetoday, and under consideration tomeet tomorrow’s demanding challenges.

As of 2012, Thermoflow has soldover 7500 program licenses tocompanies in more than 75countries. This proven track recordmakes Thermoflow’s software suite the most widely-used, and well-respected in the power generationindustry.

This pamphlet focuses on solarthermal power and heating cycles, asmall subset of the full suite’s capabilities. You can learn moreabout the whole suite atthermoflow.com, or by contactingThermoflow directly.

THERMOFLEX™, together with PEACE™, provides design, simulation and cost estimation forsolar thermal power and heatingcycles. First released in 1995,THERMOFLEX has been undercontinuous development ever since.Today, THERMOFLEX is the mostwell-proven fully-flexible heatbalance program available.

PEACE (an acronym for Plant

Engineering And ConstructionEstimator) was introduced as acompanion to GT PRO in 1998.Today, PEACE is integrated intoThermoflow’s entire suite. PEACE provides physical equipment specs,capital cost, labor estimates, andestimated installed costs for“engineered” components, including many elements used in solarthermal power plants models.

THERMOFLEX & PEACE:

Solar Thermal Modeling

These powerful tools are used forheat balance design of thermalpower systems, and for simulationof off-design plant performance.THERMOFLEX is flexible. Itprovides the user full freedom toconstruct flowsheets usingcomponent models available in itstoolbox. THERMOFLEX has allthe components needed to modelcomplete power plants of virtuallyevery type, or to model only a smallsubsystem such as a pump andpipe.

THERMOFLEX Toolkit—

Component Models

The icon toolkit includes all thecomponent models and fluidproperties needed to build solarthermal power plant models. Themodel boundary can be all-inclusive; from solar irradianceinput to electric delivery on thehigh-voltage side of step-uptransformers. Alternatively, themodel can include only a steam

turbine and its feedwater heatingtrain, or anything in between.

Thermoflow Software

NaturalDraft CT

Water Cooled

MechanicalDraft CT

Direct Contact

Condensers & Cooling Towers

Air Cooled

Wet/DryMechanical Draft

These two pages show only a small subset of the full selection ofTHERMOFLEX / PEACE model icons - those typically used in solar thermalpower plants. As of 2012, THERMOFLEX / PEACE collectively include overone hundred and seventy-five (> 175) different icons. THERMOFLEXincludes built-in properties for seven (7) fluid types representinghundreds of specific fluids used in power and process applications.

User-defined Fields

Linear Fresnel Collectors

Parabolic Troughs

Solar Towers

Solar Fields & Solar Fields with Storage

Feedwater Heater Trains—shell & tube LP &

HP heaters, deaerators, flashtanks, general-purpose & general fluid heat exchangers

B

A

Deaerator (open heater)

FWH w/ flashback

General Fluid HX

Flashtank

FWH w/ pump forward

THERMOFLOW 3

Solar Boilers

Shell & Tube Superheater

Shell & Tube Economizer

Shell & Tube Evaporator

Solar Thermal Toolbox

Gas Turbines & Boilers—Supplemental steam,

backup heat input, parallel heating systems

HPIPLP

GT PRO Gas TurbineLibrary (>370 engine specs)

Heat Recovery SteamGenerators (HRSG) RH1 RH2

PARecirc Temp

`

SA

Fired Utility Boilers—coal, oil, gas

PackageBoilers

User-definedBoilers

Fluids—seven types with built-in properties to represent hundreds of

specific fluids

Water: subcooled, saturated, super-heated, & supercritical

Heat Transfer Fluids: DOW, Solutia,Paratherm, Duratherm Molten Salt,user-defined, etc.

Dry & humid air, combustion prod-ucts, pure gases such as N2, CO2, etc.

Fuels: solid, liquid,gaseous

Brine: seawater& brackish water

Refrigerants: sub-cooled to supercritical

Ammonia/Watermixtures

Pumps—multi-stage BFP, verti-cal turbine CW pumps, verticalcondensate forwarding, singlestage multi-purpose

Piping systems—physical models withstraight runs, headers, fittings, valves,branches, elevation changes, etc.

Pumps, Pipes, Headers, Valves, Processes—

Network fluid flow modeling

Steam Turbines

Condensing Reheat(single & multi-casing)

Condensing, Non-Reheat(single & multi-casing)Back Pressure

4 THERMOFLOW

Solar Field Component

Solar Field Model Options

In solar thermal plants, the solarfield supplies some or all of the heatneeded by the cycle. The field maydeliver hot thermal oil, hot water,saturated steam, or superheatedsteam.

THERMOFLEX has a completelyuser-defined solar field where theuser directly specifies solar field heatinput to the working fluid used inthe cycle. In this case, no detailedfield modeling is done byTHERMOFLEX, rather the user’s specified field performance isapplied directly. This simpleapproach makes includingmanufacturer-specified performancequick and easy.

THERMOFLEX also allows the userto model the solar field’s thermal-hydraulic-optical performancedirectly, in detail. THERMOFLEX

computes number and length ofeach collector row, the total solarfield size, fluid pressure drop, landuse requirements and estimatedfield cost based, on desired fieldperformance. At off-design the solarfield model estimates field heatingcapacity and fluid-side pressuredrop for given solar irradiance andfield operating conditions.

The THERMOFLEX solar fieldmodel is a general line collectormodel with options to pick specificparabolic trough and linear Fresnelcollector configurations, and abilityto specify user-defined collectorcharacteristics.

Design Point

The Main Inputs menu for designcalculations is shown here. TheCollector Hardware &Characteristics menu is shown atthe top of the next page. These two

menus allow the user to specify thedesired field thermal-hydraulicperformance and the physical andoptical characteristics of thecollector used. Default values aresupplied for all inputs, and the usercan always adjust the inputs to suittheir needs.

At design, THERMOFLEX usesthese inputs to compute the field’s thermal-hydraulic performance andestimate the collector size and landrequirements.

The solar field consists of a numberof flowpaths connecting cold supplyheader to the hot return header.Each flowpath spans one or morecollector rows. Large trough fieldstypically use two collector rows perflowpath so the hot and cold headersare at the same end of the rowbanks.

Some linear Fresnel collectors,especially with directsteam generation use oneflowpath per collector rowso cold fluid enters at oneend, and steam exits to asteam drum at theopposite end.

Smaller roof-top heatingcollectors often have manycollector rows perflowpath to accommodatethe desired temperaturerise in a limited footprint.

Main design-point model inputs. These are desired flowrate, exit temperature, pressure drop,tube velocity (mass flux), and optical efficiency for normal ray strikes. All inputs have defaultsettings that are easily reset as needed. The field model or the heat consumer can ultimatelydetermine fluid flowrate partly based on the ‘flow priority’ setting.

THERMOFLOW 5

The Flow Path Hardware menu(below) is used to specify hydraulicparameters affecting the pressuredrop from cold header to hot header.The fittings specified here togetherwith the straight run of receiver tubewith its specified roughness are used

to compute an equivalent length ofstraight piping. The pressure drop iscomputed using that length togetherwith flow conditions andtemperature/pressure-dependentfluid properties.

Design Point Model Inputs

Incident Angle Modifiers (IAM) factors adjustnominal optical efficiency to account for non-incident ray strikes. IAM data may be editedto reflect the specific characteristics of any linecollector. The data is typically generated bycollector manufacturer using ray-tracingprograms. This is key to determining howmuch irradiance is incident on the receiver,which ultimately affects field efficiency andrequired size to achieve desired heating.

This menu is used to specify collector cross-section, receiver dimensions andheat transfer characteristics, and desired field arrangement. This data can beselected from a library of built-in collectors, and/or be edited directly.

Collector library includes layout and opticalproperties for various technologies. Thisbuilt-in data is easily adjusted to model aparticular collector.

Collector cross section data, as set by user, or automaticallyby THERMOFLEX, is displayed as one of the graphic outputreports.

Receiver tube roughness, and number/type of fittings installed ineach flowpath can be set automatically, or by user input. Theseparameters impact the computed field pressure drop, and hencepump size and power requirements.

6 THERMOFLOW

Solar Resource

Site irradiance levels and the relativesun-collector position are two keyparameters for the solar plantdesigner. Once a system is designed(sized), its heating ability at differenttimes of day, on different days of theyear is heavily dependent on thesite's solar characteristics.

THERMOFLEX provides four waysto input irradiance and relative sun-collector positioning. Each methodis designed to make it easy to useassumptions, actual measurements,or data from statistical analysis,such as TMY3 data.

1. Estimated from Site Data

This method is most useful for up-front scoping studies where the

plant designer has limited access todetailed site-specific irradiance data,yet still wants to compute a solarmodel. It’s a great way to “get started” or to compare relative site performance.

THERMOFLEX estimates the DNIand relative sun-collector anglesusing a model of relative sun-earthpositioning as a function of time ofday and day of year. Ground-basedirradiance is computed using anestimate of atmospherictransmissivity. This atmosphericrepresentation is most applicable forsites with a large number of sunnydays per year, those typically mostdesirable for solar thermal plantsiting.

This method makes it easy to pick a

time of day, and a day of year for aspecified site, and rely on theprogram to compute irradiance andsolar angles.

The input menu for this method isshown below. The EstimatedIrradiance panel along the topincludes the solar-specific inputsneeded to estimate irradiance. Sitealtitude is set elsewhere. The dailyvariation in DNI and ANI (ApertureNormal Irradiance) are shown as afunction of solar time as the greenand blue lines, respectively. Thegraph title shows a summary of theconditions used to estimate theirradiance together with the lengthof the solar day. Daily peak ANI anddaily average ANI values are shownto the right.

Solar Resource

Plot shows estimated variation in DNI and ANI throughout the day. Site-specific data and day of year used inthe estimate shown above the plot. Daily peak ANI and daily average ANI values are shown to the right.

THERMOFLOW 7

Irradiance and Solar Angles

2. User-defined DNI & Local

Time

More detailed plant design andsimulation often uses irradiancedata measured from ground orsatellite. This data is available froma number of sources. In the US,data for hundreds of sites isavailable in TMY3 datasets availablefrom National Renewable EnergyLaboratory (NREL). TMY3 datastatistically represent conditions at aspecific site by analyzingmeasurements made over decades.Data sources are available for otherlocations worldwide, some for freeand others on a commercial basis.Regardless of the source, the solardata is characterized by sitelongitude, latitude, altitude, localtime, day of year, and irradiance(DNI, diffuse, total).

THERMOFLEX includes thismethod of solar data input tofacilitate use of TMY3 (and similar)data sets. The Site location andcurrent time panel shown on theinput menu below lists the inputparameters needed. THERMOFLEXcomputes the solar time from thisdata using an equation of time. Therelative sun-collector positioning isused to compute azimuth and zenithangles associated with the specifiedday and time.

3. User-defined DNI & Solar

Angles

This method is used to directlyspecify DNI and location of the sunin the sky. THERMOFLEXcomputes Aperture NormalIrradiance (ANI) from this datausing inputs for collector orientationon the earth (N-S, E-W, or other)and tilt from horizontal.

This method is used to specify“typical” irradiance condition for collector design, or when scanningthrough a range of conditions foroff-design simulation.

4. User-defined ANI

This method is used to directlyspecify Aperture Normal Irradiance(ANI), that is how much beamirradiance falls normal to thecollector aperture. As such, it has asingle input value. In this case,THERMOFLEX simply applies thisvalue and ignores collectororientation, solar angles, and otherinputs that would be used toultimately compute this quantity.

This approach requires the leastamount of input to THERMOFLEXbut usually requires the largestamount of independent calculationoutside THERMOFLEX todetermine this input value.

Solar Angles

The diagram below shows thedefinition of solar angles relative tocollector midpoint. The collector isnot shown, but may be located withprimary axis along N-S, E-W, oranywhere in between. Largecollectors are typically installed withzero tilt, but the model allowsspecification of tilt away from thehorizontal if needed.

Sun

n (North)

e (East)

z (Zenith)

AzimuthAngle

Zenith Angle

AltitudeAngle

S

8 THERMOFLOW

Model Overview

The overall heat balance result froma THERMOFLEX model of theKramer Junction SEGS VI plant isshown below. The well-knownfacility is a single reheat indirectlyheated Rankine cycle with sixfeedwater heaters. The solar fieldheats Therminol VP-1 which flowsthrough the solar boiler to make andreheat steam. The steam turbineexhausts to a water cooledcondenser serviced by a wet coolingtower.

The model is a completerepresentation of the entire facilityincluding solar field, solar boilerelements, steam turbine, feedwaterheater train, condenser, coolingtower, and associated balance ofplant.

For a given model run, the minimumrequired inputs are (1) Ambientconditions, and (2) Solar irradiancedata. The program computes gross

power, net power, auxiliaryelectric loads, as well as flow,pressure, temperature, andenthalpy throughout the cycle.

The result below is for the100% solar loading case atdesign ambient conditions.The plant model produces 35MW gross electric power, consumes2.6 MW of auxiliary power, andproduces 32.4 MW net power. Inthe diagram only key state data aredisplayed for clarity. However, theuser can display the state data atevery node, and each icon includes aseries of text and graphic outputreports for each run.

Model predictions match designpoint data to a high level of fidelity.

Summary Report

Summary results for eachcomponent are available by double-clicking an icon from the overallheat balance view. The display

above is the summary display for thesolar field. THERMOFLEX includesa library of heat transfer fluids thatare commonly used in solarapplications. The fluid libraryincludes thermal and physical fluidproperties used in pressure drop andheat transfer calculations. In thismodel Therminol VP-1 circulateswithin the field and the solar boiler.The solar field diagram shows thestate of the Therminol (pink fluid)entering the field on the left, and thefield delivery condition on the right.

A performance summary is shown inblue in the lower left corner. In thismodel, DNI is 916 W/m2, total heat

Kramer Junction SEGS VI

Overall design point heat balance result from a THERMOFLEX model of the Kramer Junction SEGS VI plant.

THERMOFLOW 9

Select Model Results & Output Reports

transferred to fluid in the field is92.7 MW, and the fluid pressuredrop from receiver inlet to exit is 6.5bar, or about 27% of exit pressure.

Detailed Reports

A series of detailed text and graphicreports are presented to describe thecomputed heat balance, the physicalequipment description, and forPEACE components, estimatedequipment and installation costs.

The field size and layout report isdisplayed below. The bird’s-eyeview shows the collector rows areoriented North-South. Fifty (50) U-shaped flowpaths are arranged inone hundred (100) collector rows intwo row banks. Fluid enters fromand returns to headers in betweenrow banks. Major dimensions arelisted along with total field apertureand required land area.

Steam Turbine

THERMOFLEX output reportsinclude text and graphics to describesteam turbine performance,configuration, and cost. Reportsinclude detailed heat balance resultsin and around the turbine, sectionefficiencies, turbine casingconfiguration, leakage schematics,estimated turbine generator size,weight, capital cost, and installationlabor.

The steam turbine expansion path,Mollier diagram, is shown in the topright corner above. Extraction

pressures forfeedwaterheaters areshown along the

path which consist of an HP sectionand an IP/LP section with steamreheat in between. Steam exhaustsat 80 mbar with a quality of about90%.

PEACE cost and installationestimates are based on equipmentsize, weight, and configurationdetails. A series of reports presentthis data. The estimated elevationview for the steam turbine is shownbelow along with a summary ofoverall dimensions for the turbineand its generator. The steamturbine design model is entirelydynamic, so any changes to designparameters are reflected in thesereports, and in the cost andinstallation labor estimates.

Solar field bird’s-eye view output graphic showing field arrange-ment and computed land area, aperture area, flowpaths, etc.

10 THERMOFLOW

The Storage Issue

On “good” days, in “sunny” locations, pure solar plants (seepreceding pages) produce electricityfor seven to thirteen hours of theday. As the sun rises, sets, and isblocked by clouds, the energycapture rate changes; sometimesvery rapidly. The steam powerplant’s electricity output follows solar capture rate up to a point, butcannot always follow withouttripping off-line. In these situations,the solar field must “dump” energy, or defocus to prevent overheatingthe fluid and field components.

During startup, or following a trip,the solar field and power plant arerestarted, and resynched to the grid.This process takes time, and is notalways possible given the time ofday, or prevailing and expectedweather conditions.

One way to mitigate some problemsassociated with varying solaravailability is to include a thermalstorage system to store heatcaptured by the solar field. Thestorage system decouples the solarfield’s ability to capture heat from the power plant’s demand for heat to achieve a desired power productionlevel. This capacitance effect isuseful to ride out transients, and fortime-shifting the power productionrelative to the sun. Storage systemspotentially help plants dispatchpower in a more predictable andreliable fashion. So, rather thanproducing a continuously variablepower level throughout the day, aplant can deliver a fixed baseloadlevel for a more predictable period.Or, a plant could be dispatched tomeet morning and evening peakdemands experienced by manyutilities.

Storage Systems

Various types of thermal storagesystems have been tried in pilotprojects and in commercial powerplants. A number of advanced andnovel concepts are currently thefocus of research efforts. The basicforms of storage are (1) direct oilstorage, (2) indirect storage using asecond liquid such as salt, (3)indirect using a solid such asconcrete, and (4) indirect using aphase change material to capitalizeon relatively high apparent heatcapacity from melting and freezingsuitable materials.

Early storage projects used direct oilstorage. Such a system wasimplemented for a period of time atKramer Junction, but is no longeroperational. In this system, somethermal oil from the field is divertedto insulated storage tanks instead of

Solar Power with Thermal Storage

THERMOFLOW 11

Andasol 1 Power Plant

used to makesteam. When theheat is needed lateron, hot oil ispumped from thestorage tanks to thesolar boiler whereit is used to makesteam. This systemincreases theamount of thermaloil onsite, which isproblematic forpermitting reasons,and economicreasons becausethermal oils arerelativelyexpensive.

Currently, commercial-scale solarthermal storage is being designedand built using indirect storage. Thestorage medium is molten salt, notthermal oil. Molten salts areadvantageous because they havehigh volumetric heat capacity, canbe stored in atmospheric tanksbecause of salt’s low vapor pressure, and are relatively inexpensive.

Two-tank Molten Salt

The Andasol plants located inGranada Spain, and other similarfacilities, use a two-tank molten saltstorage system. The system iscomprised of separate “hot” and “cold” insulated storage tanks of roughly equal volume. Theschematic above shows such asystem operating in charging mode.

Molten salt is pumped from tank totank through the salt-to-oil storageHX as the storage system chargesand discharges. When the solar fieldcaptures more heat than is neededby the power plant, some thermal oilflows through the storage HX to heatthe salt flowing from the cold tank tothe hot tank. This charges thestorage system with the excess fieldheat. In contrast, when the solarfield absorbs less heat than isneeded to run the power plant, coolthermal oil flows in the otherdirection through the heatexchanger where it is heated by saltflowing from the hot tank to the coldtank. Thermal oil heated by thestorage system is combined with anyoil heated by the field and used tomake steam for the power plant.Discharging the storage allows the

power plant to make power whenneeded or desired, as opposed tobeing held entirely captive by thesun’s availability.

Solar fields with two tank storagesystems have been designed toprovide between six and twelvehours of full load power withoutsunshine.

Andasol 1 Plant Model

THERMOFLEX was used to createthe model shown to the left. It is a50 MW reheat Rankine cycle similarto the Andasol 1 solar thermal powerplant located in Granada, Spain. Ithas five feedwater heaters and steamis condensed in a water cooledcondenser serviced by a wet coolingtower. A solar boiler with parallelreheater produces and reheats steamin shell-tube heat exchangers. The

Solar FieldDP = 5.378barAperture defocused = 0 %Heat from field = 100 %156 of 156 flowpaths in use

Heat ExchangerUA = 25785 kW/CDTLM = 4.952 C

DP = 3.201 bar

DP = 3.24 bar

DTc = 4.019 C DTh = 6.019 C

Cold TankVol = 14524 m^3

Level = 38 %

Hot TankVol = 14524 m^3

Level = 62 %

Stored fluid: Nitrate Salt 60% NaNO3 - 40% KNO3 by wtStorage system: charging mode

36.88 p288.9 T963.9 h535 m

From networkTHERMINOL VP-1

36.88 p292.5 T972.1 h1079.4 m

26.05 p391 T1212 h1079.4 m

34.15 p291.8 T970.6 h1079.4 m

DP=

2.7

35bar

DP=

2.7

22bar

28.77 p392.2 T1215 h1079.4 m

26.05 p391 T1212 h535 m

To network

36.88 p296 T980.1 h544.4 m

40.09 p392 T1214.7 h544.4 m

5.582 p291.9 T842.4 h892.2 m

2.341 p386 T984.1 h892.2 m

1.854 p291.8 T842.1 h892.2 m

2.341 p386 T984.1 h892.2 m

274.1 kW

1696.3 kW

bar - pC - TkJ/kg - hkg/s - m

12 THERMOFLOW

heaters use hot thermal oil(Therminol VP-1) provided by thesolar field. A two-tank molten saltstorage system is used to storeexcess heat generated by theoversized solar field, when possible.

Steam conditions are lower thanwith fossil fired plants because theyare limited by thermal oil operatingtemperatures. Nominal steamturbine conditions are 100 bar, 381C at HP admission, and 18 bar, 381C at reheat admission. At nominalconditions, approximately 535 kg/sof hot thermal oil is required to runthe power plant.

The solar field consists of a largenumber of parabolic troughcollectors that focus solar energy ona receiver tube carrying the thermaloil. The oil is heated as it passesthrough the field.

The solar field is sized at noon onthe Vernal equinox with a solarmultiple of 1.5. The storage systemcapacity is 1045 MWhr, and thestorage heat exchanger is rated atjust under 140 MW. Operating onstorage alone, this plant can operatefor about 7.5 hours at rateddischarge capacity. The model wasdeveloped using ambient conditionstypical of the Andasol 1 location insouthern Spain (37.1 north latitude,1100 masl).

The solar field is oversized forsummer operation and can heatalmost twice as much fluid at noonthan is needed to run the powerplant at its rated load. The solar

field and storage system aremodeled together in a single icon(#16) which has built-in logic tocharge and discharge the storagesystem as the field heating abilitychanges throughout the day.

This system is 100% solar; there isno backup thermal oil heater toprovide heat when the solar field isout of service, or unavailable due toa lack of sunshine.

Simulation of Daily Plant

Operation

Storage system operation isnaturally a time-dependent process.THERMOFLEX calculates steady-state models, so modeling plantoperation with storage is done with aquasi-steady approach using a seriesof runs, each representing a singleslice of time where the plant isassumed to operate in steady-state.E-LINK is the tool used to carry outthese runs where the inputs areentered in Excel, andthe outputs arepresented in Excel.

In Excel, the storagesystem’s state at the end of the currenttime step is fedforward as the initialcondition for the nexttime step. Quasi-steady modeling is auseful tool forapproximating plantoperation over longtime periods wherethe transientsencountered are fast

relative to the time step.

The model shown in preceding pagewas run from midnight to midnighton day 173, the summer solstice.The initial condition for hot tankstorage level is the value computedby the previous day’s simulation. THERMOFLEX’s built-in solarirradiance model was used toestimate DNI and solar anglesthroughout the day. The latitude is37.1 degrees north, the location ofAndasol 1 in Granada, Spain.

The following series of charts showvariation in selected parametersthroughout the day. In each case,the x-axis is solar hour of day.

Daily Plant Operation

0

100

200

300

400

500

600

700

800

900

1000

0 3 6 9 12 15 18 21 24

DN

I,W

/sq

.m.

DNI

These calculations use irradiance from the built-in sun model.This plot shows computed variation in Direct Normal Irradiancefor Day 173 at elevation of 1100m at 37.1° North. This is the keydriver governing heat input to the plant.

THERMOFLOW 13

Thermal Storage

0

10

20

30

40

50

60

0 3 6 9 12 15 18 21 24

Mai

nst

eam

flo

w,k

g/s

370

375

380

385

390

Tem

per

atu

re,C

Main steam flow

Main steam temperature

Plant loading in this model is established by setting a desired hot oilflow on the solar field with storage icon, #16 in diagram on previouspage. The delivered oil temperature depends on the heating source.When the field heats all the oil, it is available at its designtemperature. The oil temperature drops whenever some of the heatcomes from storage.

The result is variable steam flow to steam turbine, and variable HPand RHT steam temperatures. This plot shows main steam flow ofjust below 50 kg/s at 373 C when the storage system is the only heatsource. During the day, the steam flow rises by about 4% and isavailable at 382 C due to higher oil temperature from the field.

This plot shows Therminol flows from the solar field, from the storage system, and to the storage system as a percent of flow delivered tothe solar boiler. The solid line shows field flow. The long dashed line shows storage discharge used to makeup field shortfall. The shortdashes represent flow used to charge the storage system.

The early summer sunrise allows the field to begin to produce hot oil just after 5AM. Field oil heating continues until almost 7PM. Theflowrate increases until just before 9AM when the storage system charge rate hits its maximum due to maximum salt flowrate. Thiscapacity limit is in place until just before 3PMwhen the storage tank becomes filled tocapacity. At this time, the storage system shutsdown and mirrors are defocused to restrict fieldheating to that needed to just meet demandflow. Starting around 5:30PM, the field flowbegins to drop because of waning sunlight.

The discharge flow mixes with any field-heatedfluid and is delivered at the mixturetemperature. During the evening and overnightperiod the storage system provides allTherminol. During the daytime, the field aloneis capable of satisfying flow demand and thestorage system is operated in charge mode or isshutdown if it’s full.

The field flowrate is large enough to begin to charge the storage system at about 6AM. The charging period that lasts until just before3PM. The cap in charging flowrate occurs when the storage system charge rate hits its maximum, as implied by maximum salt flowratefrom tank to tank. The charging flow quickly drops to zero at about 2:45PM when the hot tank is full.

0

50

100

150

200

250

0 3 6 9 12 15 18 21 24

Flo

was

per

cen

to

fd

eliv

ery

flo

w,%

Therminol from field

Therminol from storage

Therminol to storage

0

10

20

30

40

50

60

70

80

90

100

0 3 6 9 12 15 18 21 24

Ho

tst

ora

ge

tan

kle

vel,

%

Hot tank level

Hot and cold tank levels change throughout the day. In thismodel, the hot tank is about 30% full at midnight, the startingtime for this quasi-steady analysis. The hot level drops steadilyas the demand for hot oil remains constant. Once the hot tankis empty, the storage system shuts down, and no oil isdelivered to the network, so the power plant shuts down.

At about 6:30AM, the field is capable of heating more oil thanrequired by the network. At this time, the storage system goesinto charging mode and the hot tank level begins to rise. Therate of rise increases initially as the DNI level rises. Justbefore 9AM the storage system charge rate hits its limit, andthe hot tank level begins to rise at a constant rate. At about2:45PM the hot tank is full and the storage system cannotabsorb anymore heat. At about 5:30PM the storage systembegins to discharge because the solar field cannot produceenough hot oil to satisfy demand by itself.

14 THERMOFLOW

Integrated Solar Combined

Cycle (ISCC)

Integrated Solar Combined Cycleplants are a gas turbine combinedcycle with a solar thermal plant toadd heat to the combined cycle.While solar-captured heat may beincorporated in many ways; it istypically introduced as high pressuresaturated steam, mixed with HRSGHP steam, and superheated foradmission to the steam turbine.THERMOFLEX can readily modelthis, or any other arrangementunder consideration.

Medium to large scale (100 to 500MW) ISCC plant designs have beenproposed where the solar

contribution is used to augmentplant capacity, or to replace gas-fired duct burners to generate extrasteam during peak power demandperiods. In many warm locations,power demand peaks in the mid-dayhours of the summer whensignificant air conditioning loadsoccur. This demand profile is well-matched to a solar field’s capacity profile.

THERMOFLEX together with GTPRO deliver rapid plant scopingcapability together with flexibleplant modeling features. Here, GTPRO was used to create the initialplant model, where solar heat inputwas modeled as an “external heat addition”. Afterwards, the GT PRO

design was imported toTHERMOFLEX and the solar fieldand solar boiler were added togenerate steam in the model shownbelow.

The plant design is derived from aheat balance provided courtesy ofSiemens Industrial Turbomachinery.It is a 2x1 ISCC with two SiemensSGT-800 gas turbines exhaustinginto fired single pressure HRSGsmaking steam at 83 bar / 565 C foradmission to a condensing steamturbine. Steam is condensed in a dryair-cooled condenser. The parabolictrough solar field nominally adds 50MWth to augment HP steamgenerated in the HRSG.

Under desert-likeambient conditions,35 C, 35% RH, 928mbar, and with 49.3MWth heat inputfrom the solar field,the plant generates157.6 MW grosselectric output using276.3 MWth LHVfuel input.Considering thefuel-free solarcontribution, theplant operates witha 57.1% gross LHVelectric efficiency,considerably higherthan typicallyachieved with onepressure non-reheatGTCC plants.

Integrated Solar Combined Cycle (ISCC)

THERMOFLEX model of a heat balance provided courtesy of Siemens Industrial Turbomachinery.It is a 2x1 ISCC with two Siemens SGT-800 gas turbines exhausting into fired single pressureHRSGs making steam for admission to a condensing non-reheat steam turbine. Model includes aparabolic trough solar field that adds just over 49 MWth to the plant as saturated HP steam. Thesolar-generated steam is about 80% of duct burner heat input at this condition, and representsabout 32% of the steam flow to the steam turbine.

THERMOFLOW 15

Solar Thermal Desalination

Solar Thermal Desalination

Desalination of sea and brackishwater to produce fresh water fordomestic and industrial uses isbecoming increasingly important inmany places around the world.Presently, the major desalinationprocesses in use are ReverseOsmosis (RO), Multi-EffectDistillation (MED), and Multi-StageFlash (MSF).

Electrically-driven RO is typicallyused in small to medium-scale waterproduction facilities. Thermally-driven MED and MSF use low tomoderate grade heat, and are usedin most of the medium to large-scalesystems installed today.

THERMOFLEX together withPEACE can model these desalinationprocesses at the design-point and atoff-design conditions. Model resultsinclude detailed thermodynamicstates, performance metrics, andinstalled system cost estimate.

The THERMOFLEX model atright was supplied courtesyof Solar Power Group,GmbH. It models water &power producing plant in thecoastal Surt region of Libya,owned by the LibyanMinistry of Energy.Nominally, this plantproduces 15,000 m3 per day(3 MIGD) of potable waterand has 15 MWnet electriccapacity.

Heat input is provided by thecombination of a solar field andnatural gas-fired boiler, installed inparallel. This solar-fossil hybriddesign leverages the site’s sunny climate and its proximity to naturalgas fields. The hybrid design canproduce power and water 24 hoursper day without a solar storagesystem. The desalination systemcondenses steam turbine exhaustand produces potable water fromseawater. It takes the place of whatwould be a steam condenser in apure power plant.

The solar field uses Linear FresnelCollectors (LFC) with direct steamgeneration (DSG). It consists ofthree sections; one to preheat water,one to evaporate water, and the finalsection to superheat steam. Theevaporator is designed to produce30% quality steam. A steam drumseparates the phases; liquid recircu-lates to evaporator inlet, and drysteam flows to the superheater field.

A single-casing steam turbineoperates at high speed with areduction gear coupled to asynchronous generator. Nominalturbine inlet steam conditions are 55bar, 400 C. Three steam extractionsprovide steam to a deaerator andtwo higher-pressure feedwaterheaters. Water is preheated to about190 C before the economizer sectionof the solar field. Feedwater to thegas-fired boiler comes directly fromthe feedpump.

The desalination system iscomprised of two parallel units, eachhaving a nominal capacity of 7,500m3 per day (1.5 MIGD). The MEDdesign has a nominal performanceratio of 10.4 (gain output ratio of9.4). This high level of efficiency isachieved by using twelve effects overa temperature range of 30 C.

16 THERMOFLOW

Solar Tower Fields

Solar Tower Field

The Solar Tower and Solar Towerwith Direct Storage models wereintroduced in Thermoflow 22,February 2012. These modelsprovide design and simulation forboth external and cavity receiverswith surround and directional(wedge-shaped) fields. The receivercan be used with water/steam,molten salt, thermal oils, and air andother gases. Storage is availablewhen using molten salt or thermaloils only.

The tower field can be integratedinto power and heat cycles using thefull feature set available inTHERMOFLEX.

Tower field optical performance iscomputed outside THERMOFLEXusing any of the available tools, suchas Solar Advisory Model (SAM),HFLCAL, etc. and specified as inputto the tower icon. The opticalperformance efficiencies arespecified as a two-dimensionalmatrix of data parameterized bysolar zenith and azimuth angles.

The tower model computes thethermal-hydraulic performance ofthe tower supply pipes, receiver, andreturn pipes. For models withstorage, the system pump withoptional energy recovery turbine andtank system is automaticallyhandled by the model. The storagesystem is sized in design, and used

at off-design in either charging,discharging, or off-line modes. Themodel provides for ability to limitheat input, and logic for shutdownunder low DNI conditions.

Design Point

The Main Inputs menu for designcalculations is shown here. TheCollector Hardware &Characteristics menu is shown atthe top of the next page. These twomenus allow the user to specify thedesired field thermal-hydraulicperformance and the physical andoptical characteristics of thecollector used. Default values aresupplied for all inputs, and the usercan always adjust the inputs to suittheir needs.

At design,THERMOFLEX usesthese inputs to computethe field’s thermal-hydraulic performanceand estimate thecollector size and landrequirements.

With external receivers,the reflector fieldconsists of a largenumber of heliostatssurrounding the tower,perhaps biased to oneside to match the tower’s output to the utilityneeds. With cavityreceivers, the reflectorfield is biased towardsthe northern side of thetower (above theequator) so they can

Main design-point thermal-hydraulic model inputs include desired flowrate, exit temperature,pressure drops, and heat losses from pipes. The solar multiple and desired number of fields areused for sizing. All model inputs have default settings that are easily reset as needed. The fieldmodel icon itself, or the heat consumer (a solar boiler in many cases) can ultimately determinefluid flowrate partly based on the ‘flow priority’ setting.

THERMOFLOW 17

“see” the cavity. The size of the reflector field, the height of thetower structure, and the cavity areaare output from design based onthermal demand required of thetower given a solar condition andthe tower’s optical characteristics.

The fluid flows up to the receiverthrough the cold pipe, and returnsthrough the hot pipe. The overallpressure drop from cold inlet to hotoutlet is comprised of the frictionallosses in the pipes and receiver, andthe net gravity head between fluidcolumn in the cold and hot pipes.

Model Features

This menu is used to specify collector cross-section, receiver dimensions and heattransfer characteristics, and desired field arrangement. This data can be selectedfrom a library of built-in collectors, and/or be edited directly.

Collector library includes layout and opticalproperties for various technologies. This built-indata is easily adjusted to model particular plants.

Tabulated optical efficiency as a function of solarzenith (1-altitude) and azimuth angles. Efficiencymaps produced by any optical design program suchas HFCAL, and even SAM can be entered torepresent a particular field. Several built-in towersare available if no data are available.

Heliostat field layout showing location of the tower is available onoutput. Land area required, number of heliostats, and towerstructure height are included in the output reports.

So lar T o wer Eff iciency850 M Wt Surro und F ield - External R eceiver

0

10

20

30

40

50

60

0 15 30 45 60 75 90A lt i t ud e A ngle, d eg

0

60

120

180

18 THERMOFLOW

Solar Tower Power Plants

Nine solar thermal power plantprojects were approved by theCalifornia Energy Commision in2010 alone. As of mid-2011 anumber of these are in constructionin the southeastern Californiadesert. Some of these projectsutilize solar towers with molten saltand some use towers with directsteam generation and reheat in thereceiver.

When molten salt is used as theprimary heat absorption fluid, it canbe stored in large atmospheric-pressure tanks to provide capacity toride-out cloudy periods, for loadmanagement, and power productionafter sunset.

Rice Solar Energy Project

The Rice Solar Energy Project is onethat uses molten salt with direct

storage. This project’s plan was submitted to the CEC bySolarReserve LLC and was approvedby the CEC in December 2010. Themodel described here was createdusing publicly available informationfrom the California EnergyCommission website (http://www.energy.ca.gov/sitingcases/ricesolar/index.html).

A THERMOFLEX model of Rice isshown below. The solar tower andboiler are on the left in the greenbox. The reheat steam turbine,feedwater heater train, and air-cooled condenser system are shownon the right. Nominal performanceis shown; the turbine generatesabout 150 MWe and the plantexports 142 MWe, consuming about8 MWe of auxiliary load to run theACC, pumps, and other loads.

Molten salt is pumped from the cold

tank through thereceiver and returned tothe hot tank. The solarboiler uses salt from thehot tank to producesuperheated steam fromfeedwater coming fromthe feedwater heatingtrain, and to reheat steam from theturbine. Five feedheaters and adeaerator preheat the feedwaterbefore it enters the solar boiler. Theplant uses a dry cooling system(ACC) to condense low pressuresteam to produce condensate. Useof an ACC minimizes waterconsumption, but penalizes thepower cycle because the exhaustpressure is necessarily higher than itwould be with a wet cooling system.

The heliostat field is roughly circularwith a 9000 foot (2750 m) diameter.At this size, the plant uses about 9 to10 acre per nominal MWe capacity.

Rice Solar Energy Project

Output flow diagram shows tower and storage system operation at 3:00 PM ona clear summer afternoon. Here, the oversized solar collector absorbs about30% more heat than required to run the turbine at full load. The excess energyis used to heat salt taken from the cold tank and stored in the hot tank.

Rice Solar Energy Project—design point heat balance. All data extracted from publically available sources, California En-ergy Commission. Solar tower with molten salt storage system and solar boiler shown on left in green shaded region. Re-heat steam turbine with six feedwater heaters and an air cooled condenser (ACC) shown on the right side. Turbine capacity

Rice SolarElectric Plant

Ambient temp 85 FAmbient RH 50 %Gross power 150047 kWNet power 142458 kWNet electric eff. 39.82 %

296.6 p839.7 T257.3 m1442.2 h

176.3 p710.3 T245.7 m1380.8 h

37.65 p386.3 T217.9 m1230 h

11.4 p199.5 T203.1 m1141.5 h

1.65 p119.1 T203.3 m1034.3 h

243.6 p117.1 T203.3 m

11.01 p197.8 T14.85 m1140.5 h

11.4 p199.5 T14.85 m1141.5 h

230.3 p193.8 T247.5 m

36.73 p383.9 T14.07 m1229 h

37.65 p386.3 T14.07 m1230 h

215.5 p257.1 T247.5 m

95.24 p578.5 T14.54 m1319.2 h

100 p581.2 T14.54 m1320.2 h

200.4 p319.3 T247.5 m

167.9 p707.5 T11.56 m1379.8 h

176.3 p710.3 T11.56 m1380.8 h

282.4 p836.8 T11.43 m1441.2 h

296.6 p839.7 T11.43 m1442.2 h

1895.4 p416.9 T285 m

460.1 p611.8 T14.48 m1309.6 h

81.34 1049.41182.9

81.54 1049.4680

78.84 851.31182.9

78.84 851.5680

78.84 851.41862.9

53.42 633.41862.9

29.15 5481862.9

29.15 5481862.9

1844.7 p615.4 T285 m

1844.7 p624.4 T285 m

473.9 p615.3 T14.48 m

5.838 p115.7 T203.4 m

1.5 p115.7 T203.3 m

1903.7 p373.6 T285 m

29.15 10491862.9

1837.1 p950 T285 m

1800 p946.3 T285 m

460.9 p950 T264 m

450 p947.5 T264 m

473.9 p615.3 T264 m

466.4 p612.6 T264 m

1887.9 p459 T285 m

1886.4 p458.6 T285 m

G1

150047 kW

5

HPT

6

7

IPT-1

8

9

IPT-2

10

11

IPT-312

13

LPT-1

1 4

15

LPT-2

16

17

LPT-3

18

19

20

2 1c ondens at...

22

23AS

YP

ac

kin

ge

xh

24

25

2 6

27

28

29

3 0

31

32feedpump

34

35

3 6

37

3 8

3 9

AS

YE

xc

es

ss

tea

m

40

42

12

43 44

45

46

47

48

1salt pump

52

52

2HP Steam

3Hot Reheat

4Cold Reheat

33

296.6 p839.7 T257.3 m1442.2 h

176.3 p710.3 T245.7 m1380.8 h

37.65 p386.3 T217.9 m1230 h

11.4 p199.5 T203.1 m1141.5 h

1.65 p119.1 T203.3 m1034.3 h

243.6 p117.1 T203.3 m

11.01 p197.8 T14.85 m1140.5 h

11.4 p199.5 T14.85 m1141.5 h

230.3 p193.8 T247.5 m

36.73 p383.9 T14.07 m1229 h

37.65 p386.3 T14.07 m1230 h

215.5 p257.1 T247.5 m

95.24 p578.5 T14.54 m1319.2 h

100 p581.2 T14.54 m1320.2 h

200.4 p319.3 T247.5 m

167.9 p707.5 T11.56 m1379.8 h

176.3 p710.3 T11.56 m1380.8 h

282.4 p836.8 T11.43 m1441.2 h

296.6 p839.7 T11.43 m1442.2 h

1895.4 p416.9 T285 m

460.1 p611.8 T14.48 m1309.6 h

81.34 1049.41182.9

81.54 1049.4680

78.84 851.31182.9

78.84 851.5680

78.84 851.41862.9

53.42 633.41862.9

29.15 5481862.9

29.15 5481862.9

1844.7 p615.4 T285 m

1844.7 p624.4 T285 m

473.9 p615.3 T14.48 m

5.838 p115.7 T203.4 m

1.5 p115.7 T203.3 m

1903.7 p373.6 T285 m

29.15 10491862.9

1837.1 p950 T285 m

1800 p946.3 T285 m

460.9 p950 T264 m

450 p947.5 T264 m

473.9 p615.3 T264 m

466.4 p612.6 T264 m

1887.9 p459 T285 m

1886.4 p458.6 T285 m

THERMOFLOW 19

Ivanpah 1 SEGS

Ivanpah SEGS

Ivanpah SEGS will consist of two100MWe blocks and one 200MWeblock. The 100MWe blocks will usethree towers to generate HP steamand one to reheat the steam. The200MWe block will use four towerfields to generate HP steam and oneto reheat the steam.

This design generates steam directlyin the receiver. It thereby eliminates

the cost and complexity of themolten salt loop, the storage tanks,the salt-to-water heat exchangesystem, and the charge of salt.However, it has no ability to storeheat, and as such cannot makepower unless the sun shines. Thedesign does include a small gas firedboiler aid in plant startup and allowthe plant to ride out transients dueto temporary cloud cover. The useof gas backup is anticipated to resultin a small amount of gas usage overthe course of the year.

A THERMOFLEX model of thisplant (shown below) was built using

publicly available information fromthe California Energy Commissionwebsite (www.energy.ca.gov/sitingcases/ivanpah/index.html).

In this model, the three HP towerfields are represented by a singleicon on the left, and the singlereheat tower is modeled by its ownicon to the right. The tower blocksand relatively long piping runs areshown in the green area.

Output flow diagram shows tower and storage system operation at 3:00 PM ona clear summer afternoon. Here, the oversized solar collector absorbs about30% more heat than requiredto run the turbine at full load. The excess energyis used to heat salt taken fromthe cold tank and stored in the hot tank.

Ivanpah 1 SEGS—design point heat balance. All data extracted from the publically available California En-ergy Commission website. Solar tower field consists of three tower fields to generate high pressure steamfrom feedwater, and a separate reheat tower to heat cold steam exhausting from the HPT before readmissionat the reheat turbine inlet. No storage system is included, but the plant includes a gas fired boiler to aid inplant startup and transient mitigation from passing clouds.

20 THERMOFLOW

Model Overview

The overall heat balance result froma THERMOFLEX model of aproposed hybrid solar-fossil powerplant is shown below.

It is a condensing steam turbinepower plant with an air-cooledcondenser (ACC), a low pressurefeedwater heater, and a deaerator.Steam is directly generated in aLinear Fresnel Collector (LFC) solarfield and/or by a gas-fired packageboiler installed in parallel. The solarfield consists of three sections, oneto preheat water, one to evaporatewater, and the final section tosuperheat steam. The evaporator isdesigned to produce 30% qualitysteam. A steam drum separates thephases; liquid recirculates toevaporator inlet, and dry steamflows to the superheater field.Nominal turbine inlet conditions are65 bar, 450 C, 13.6 kg/s. NominalACC pressure is 125 mbar in a 32 Cambient. This plant designminimizes plant makeup waterrequirements, consistent withdesert-like site conditionspresent at many solar sites.

This design includes anatural-gas fired backupboiler, in parallel with thesolar field, to generatesteam when the field isunavailable due tomaintenance, weather, ortime-of-day. The backupboiler facilitates firmelectric dispatch, withoutstorage.

The steam cycle is small, does notinclude reheat and has few heaters.Therefore the base cycle efficiency isrelatively low. However, this plant isalso relatively simple, inexpensive,and easily capable of operation infull solar mode, full gas-fired mode,or in hybrid mode when some steamis generated in the field and thebalance is provided by the firedboiler. So, it is flexible.

This model was used to simulateoperation over a year using ambientand irradiance conditions typical ofDaggett California, USA. The plantwas run on a 24 hour schedule for8000 hours per year. The annualaverage net LHV (lower heatingvalue) efficiency was computed fromthe sums of net power produced andnet fuel consumed; (GWhr electricexport / GWhr LHV fuelconsumption). Results of the yearlysimulation show this relatively lowefficiency steam cycle operates at41% effective net LHV electricefficiency, a high value by Rankinecycle standards. This efficiencywould be far higher if the plant were

shut down overnight, and would belower in locations with poorer solarcharacteristics.

Direct Steam Generation

THERMOFLEX can computepressure drop and heat transfer forreceiver tubes carrying single phasethermal oils, single phase water,two-phase water, and superheatedsteam. It includes a detailedphysical model of thermal-hydraulicbehavior of solar fields using DirectSteam Generation (DSG).

Estimates of pressure gradient andheat transfer coefficient in two-phase flow situations is morecomplicated than for single-phasesituations. THERMOFLEX uses aone dimensional model where theflow path is discretized into anumber of steps. The modelestimates step-wise local values forinternal heat transfer coefficient andpressure gradient based onprevailing flow conditions andphysical characteristics of theflowpath including length,roughness and fittings.

Hybrid Solar-Fossil Power Plant

BACKUPBOILER

Condensing Steam Turbine

Ov erf low by pass

SOLAR FIELD

Gross power 10954 kWNet power 10260 kWNet fuel input(LHV) 0.0002 kWPlant auxiliary 693.7 kW

2

66.12 45050.46 3294

23

66.12 4501.713 3294

130.8428 50.2244.98 210.2

11

66.12 459.650 3318

1269.65 285.5

50 2706

1679.38 281.1

50 1241.8

39

66.12 4500 3294

4184.94 181

50 771.1

4276.04 181.1

0 771.1

500.125 50.2738.22 2332.7

7

66.12 45048.75 3294

G1

10954 kW

Pac k age Boil er[20] - Bac k up Bl r: Aux

0 kW

M

BFP CND FWD

Superheater

Ev aporator

Economiser

HPTIPT

LPT

AS

YS

TLeak

AS

YS

TLe

ak

35desup

35

desup

Recirc

2

66.12 45050.46 3294

23

66.12 4501.713 3294

130.8428 50.2244.98 210.2

11

66.12 459.650 3318

1269.65 285.5

50 2706

1679.38 281.1

50 1241.8

39

66.12 4500 3294

4184.94 181

50 771.1

4276.04 181.1

0 771.1

500.125 50.2738.22 2332.7

7

66.12 45048.75 3294

THERMOFLOW 21

Direct Steam Generation (DSG)

This series of three graphs showdistributions of computed pressure,temperature, pressure gradient, heattransfer coefficient, mass flux, andbulk velocity from economizer inletto superheater exit for this plantmodel operatingat design heatbalanceconditions.Three distinctregionscorrespond tothe separatefields forheating water,making steam,andsuperheatingsteam.

Pressure distribution (below) isdiscontinuous because of pressurelosses in piping systems betweenfields. The temperature plot isdiscontinuous between economizerand evaporator because subcooledeconomizer exit water mixes withsaturated liquid recirculated backfrom the steam drum. The finalsteam temperature exceeds theturbine inlet by 10 C, requiring use

of a desuperheaterbetween the solar fieldand turbine.

The pressure gradientand heat transfer coefficient

distributions (above) arediscontinuous in valuebecause the mass flux ineach field is different, toensure reasonablevelocities in each section.The slope of pressuregradient in evaporator isdiscontinuous becauseinlet water is slightlysubcooled. The sharpdiscontinuity in value ofheat transfer coefficient

between evaporator exit wheresteam quality is 30%, andsuperheater inlet illustrates howdramatically this differs between wetlow quality steam and dry vapor.

The number of paths in each fieldsection is different, although thereceiver tube diameters are the samethroughout (70 mm OD). Therefore,the mass flux in each section isdifferent, and the velocities arediscontinuous at field boundaries.Velocity varies inversely with densityalong the flow path.

65

67

69

71

73

75

77

0 200 400 600 800 1000Position, m

Pre

ssur

eba

r.

150

200

250

300

350

400

450

500

Tem

pera

ture

C.

EVAP SUPECO

2

6

10

14

18

22

0 200 400 600 800 1000Position, m

Pre

ssur

eG

radi

ent

mba

r/m

.

0

4

8

12

16

20

Hea

tTra

nsC

oeff

kW/m

2-K

.

EVAP SUPECO

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

0.8

0 200 400 600 800 1000Position, m

Mas

sF

lux

kg/m

2-h

.

0

5

10

15

20

25

Bul

kV

eloc

itym

/s.

EVAP

SUP

ECO

THERMOFLEX outputs include a bird’s-eye view anda cross-section of the collector. Starting with Version20, THERMOFLEX provides a 3D view of the solarfield as shown here. This helps the user visualizeeffects of changing collector design, spacing, fieldarrangement, etc.

22 THERMOFLOW

Off-design Modeling

Once a plant design is established,off-design simulations are used tocompute expected plantperformance at site and operatingconditions expected during the year.Typically simulations are done atdifferent ambients, solar conditions,load levels, etc. Results are used tomap expected plant performancethroughout the operating envelope,and to compute yearly totals forpower production, fuelconsumption, water consumption,etc. Sometimes off-designsimulations identify ways to fine-tune the original design so it moreeffectively satisfies expected dutycycle.

THERMOFLEX models can run indesign mode, in off-design mode, orin mixed mode where somecomponents are in design and somein off-design.

With Thermoflow software, “design”

mode means the user specifies (orTHERMOFLEX automaticallydetermines) equipment physicalcharacteristics, generalconfiguration data, and desiredthermodynamic constraints.THERMOFLEX computes the heatand mass balance and alsodetermines the equipment sizeneeded to realize the heat balanceresult. In contrast, “off-design” mode means the equipment size isalready established by a designcalculation (subject to user edits),and the model computes howequipment of that size operates atuser-specified loading, ambient, andsolar conditions.

In both modes the computed heatand mass balance parameters arethe same, but the method ofcomputing them is different.

E-LINK—Running

THERMOFLEX from Excel

E-LINK allows Thermoflow modelsto be run frominside MicrosoftExcel. E-LINK is afeature includedwith anyThermoflowsoftware license.E-LINK is a greattool for parametricstudies, performingbatch runs, andmaking automatedcalculations.Values for user-selected modelinputs are entered

in normal Excel cells, and computedresults are stored in associated cells.The inputs and outputs are treatedlike any other Excel cell so they canbe used in formulae, as source datafor charts and tables, or linked toother Excel-aware applications.With E-LINK, any number of modelruns can be made in a workbook.So, E-LINK is the tool to use formaking annual yield calculationswhere some users make 8760simulations to map out the year.

Daily Operation

This example uses the hybrid solar-fossil plant with DSG described onthe previous page. During the daythe solar and ambient conditionschange. Prevailing values for thesekey model inputs are used to predicthourly plant operation. In thismodel, automated plant loading isaccomplished using a steam flowcontroller icon. This logicalcomponent is connected upstream ofthe steam turbine and regulatessteam flow to the turbine so it staysin a specified range. When the solarfield makes less than the minimumsteam turbine admission flow, thecontroller automatically drawssteam from the backup boiler tomakeup the shortfall. If the solarfield makes more than the maximumadmission flow, the controller shutsdown the auxiliary boiler and dumpsexcess steam to the condenserthrough a letdown station. Thecontroller’s limits maintain steam turbine power between roughly 8and 11 MW.

Daily Plant Operation & Annual Yield

E-LINK workbook used to simulate hour-by-hour perform-ance throughout the year. Each column (case) representsone hour. Model inputs are in the yellow region, and com-puted model results are stored in the blue region. Other cellsare normal Excel cells, available for use as needed.

THERMOFLOW 23

Off-design Modeling

Annual Yield Simulations

Hour-by-hour simulations are usedto compute annualized totals andaverages. In this example the plantmodel is the hybrid solar-fossilpower plant described on theprevious page. It is operated on a24-hour schedule with power levelsranging from just over 8 MW to amaximum of 11 MW. Power limitsare established by limiting steamturbine admission flow in a range of40 to 53.4 t/h.

Here, the hourly inputs and outputsfor three particular days are shownto demonstrate how the plantoperates under different conditions.Estimated ambient temperature andDNI for particular summer, winter,and shoulder season days are shownin the plot above. The ambient datais from a model of the site insouthwestern US, near the KramerJunction SEGS plants. The DNI isestimated by the program using itstheoretical sun model.

Summer conditions are shown inred, winter in blue, and shoulderseason in green. The solid linesshow ambient temperature whichhas a strong impact on air cooled

condenser performance, and hencesteam turbine power production.

DNI (dashed lines) and solar anglesvary with solar hour and day of year.These parameters strongly influencesteam production in the solar field.The solar angles, not shown, arecomputed by the program for thislocation based on the day and thesolar hour.

Hour-by-hour simulation results forwinter, spring/fall, and summerdays are plotted below. Steamturbine flow from the solar field isshown with bright green bars. Flowfrom the backup fossil-boiler isshown with light green bars. Steamflows are plotted on the left axis intonne/hour (t/h). Net plant power(MW) is plotted as a solid black lineon the right-hand axis.

The solar field cannot make theminimum admission flow at anyhour of the winter day. For sixhours in the middle of the day thesolar field can make about 45% ofthe steam needed to load the turbineat the minimum power. Throughoutthe winter day the plant generates aroughly constant power level

associated with minimum steamflow to the turbine.

On the summer day the solar fieldmakes more steam than the turbinecan swallow for six hours, andmakes all needed steam for eighthours in the middle of the day.During early morning and lateafternoon the field can still generatea significant fraction of maximumsteam for the turbine.

Plant net power varies throughoutthe day. The variation is mostpronounced in the summer andshoulder season. This variation isthe result of two effects. First, theadmission steam flow variesthroughout the day. Increasedsteam flow to the turbine raises itsoutput power. Second, the aircooled condenser’s capacity varies throughout the day. During thehottest parts of the day, the capacityis reduced which in turn reducessteam turbine gross power. Themodel accounts for these effectsautomatically, consistent with plantequipment capacity.

Daily & Seasonal Variation of Ambient Temperature & DNI

-5

5

15

25

35

45

0 3 6 9 12 15 18 21 24Solar Hour of Day

Am

bien

tTem

pera

ture

,C.

0

150

300

450

600

750

900

DN

I,W

/sq.

m.

- Summer -

- Spring / Fall -

- Winter -

W inter Day

0

10

20

30

40

50

60

0.5 4.5 8.5 12.5 16.5 20.5Solar Hour

Ste

amF

low

toT

urbi

ne,t

/h.

Shoulder Season Day

0.5 4.5 8.5 12.5 16.5 20.5Solar Hour

Backup BoilerSolar FieldNet Power

Summer Day

0.5 4.5 8.5 12.5 16.5 20.5Solar Hour

4

6

8

10

12

Pla

ntN

etP

ower

,MW

24 THERMOFLOW

Thermoflow, Inc.2 Willow Street—Suite 100 · Southborough, MA 01745 · USA · info@thermoflow.com · +1 508-303-5033

Thermoflow Europe GmbHGartenstrasse 18, D-35469 Allendorf · GERMANY · huschka@thermoflow.com · +49 640-790-6991

www.thermoflow.comAugust 2012

Starting in 1987 with its flagship program GT PRO™, Thermoflow’s software suite has grown to include seven

powerful, yet easy-to-use tools to analyze the spectrum of power generating technologies in use today, and

under consideration to meet tomorrow’s demanding challenges.

As of 2012, Thermoflow has sold over 7500 program licenses to companies in more than 80 countries. This

proven track record makes Thermoflow’s software suite the most widely-used, and well-respected in the power

generation industry.

A complete list of Thermoflow customers is available at www.thermoflow.com. A small sampling is listed below.

☼ MAN Solar Power Group ☼ eSolar ☼ E.ON Engineering ☼ NEM ☼ NOVATEC Solar ☼ Siemens ☼

☼ Lahmeyer International ☼ Electricite de France ☼ SUEZ Tractebel ☼ Fraunhofer ISE ☼

☼ Siemens CSP (Solel) ☼ Office National de l'Electricite (ONE) ☼ Renewable Energy Systems (RES) ☼

☼ Lockheed Martin ☼ Ecolaire Espana ☼ Bechtel Power ☼ Black & Veatch ☼

☼ Kraftanlagen Muenchen ☼ Chiyoda ☼ Duke ☼ Fluor ☼ Hyundai ☼ PB Power ☼ Stone & Webster ☼

☼ Toyo ☼ TransCanada Power ☼ ALSTOM ☼ CMI Boilers ☼ Hangzhou Boiler ☼ Foster Wheeler ☼

☼ General Electric ☼ Kawasaki ☼ Harbin Turbine ☼ Mitsubishi ☼ Rolls-Royce ☼ Solar Turbines ☼

☼ Dong Fang Turbines ☼ Akzo ☼ BP ☼ British Energy ☼ Chevron ☼ ConocoPhillips ☼ ExxonMobil ☼

☼ Tokyo Gas ☼ Total S.A. ☼ TransAlta Energy ☼ MIT ☼ China TPRI ☼ Korea EPRI ☼ Taiwan Power ☼

Univ. di Bologna ☼ Univ. of Calgary ☼ Univ. di Firenze ☼ Cobra Energia ☼ Iberdrola ☼

☼ FERA Srl ☼ Veolia Energy ☼ Fotowatio ☼ TSK-INGEMAS ☼ ...

...and hundreds more at www.thermoflow.com