SIMULATION OF PARABOLIC TROUGH POWER PLANTS WITH MOLTEN SALT AS HEAT TRANSFER FLUID

Florian Böss1, Thomas P. Fluri

2, Raymond Branke

3, and Werner J. Platzer

4

1 MSc, formerly with Division Solar Thermal and Optics, Fraunhofer ISE

2 Dr., Senior Research Engineer, Division Solar Thermal and Optics, Fraunhofer ISE,

Heidenhofstr. 2, 79110 Freiburg, Germany, phone: +49 (0) 761 4588 5994, e-mail: thomas.fluri@ise.fraunhofer.de

3 Dipl.-Ing., Research Engineer, Division Solar Thermal and Optics, Fraunhofer ISE

4 Dr., Director Division Solar Thermal and Optics, Fraunhofer ISE

Abstract

Recently several new heat transfer fluids (HTF) have been proposed and analyzed with the aim of reducing

the levelized costs of electricity (LCOE) of parabolic trough power plants [1] [2]. An existing simulation

model at Fraunhofer ISE has been adapted to the use Solar Salt and HITEC® as HTF in parabolic trough

power plants. This model uses steady-state and semi-transitional methods to calculate the annual electrical

energy yield of solar power plants at different locations including thermal energy storage. Finally rough cost-

estimations in order to compare the LCOEs of the novel media and the conventionally applied thermal oil –

Therminol VP-1® [3] are drawn. It turns out, that the freeze protection has significant impact on the net

energy production of systems with molten salt. For a reference plant with a configuration similar to the

Andasol 3 power plant, but located in Daggett/USA the net energy yield of Solar Salt varies between +5.6 %

and −4.3 % compared to thermal oil, depending on the implemented freeze-protection strategy. Systems

using Solar Salt as HTF are allowed to have up to 14.4 % higher investment costs to achieve the same LCOE

as the conventional systems with thermal oil Therminol VP-1, but simulation results indicate that for an

economic use of molten salt as HTF alternative freeze protection strategies should be investigated. Another

key factor is the geographic location of the site, as sites with better solar resource seem more suited for use of

molten salt as heat transfer fluid.

Keywords: Molten Salt, Parabolic Trough Power Plant, Yield Analysis, System Comparison

1. Introduction

Recently the use of molten salts, which consist of eutectic mixtures of nitrate und nitrite-salts as heat transfer

fluid in parabolic trough receivers, has been discussed and initial testing with these fluids have proven their

feasibility.

Besides a high temperature stability of up to 600 °C, higher densities and higher heat conductivities the

nontoxic and nonflammable liquid salts are also broadly available at lower prices than thermal oil [4] [5].

This is why molten salts are already used in central receiver systems and as sensible heat storage media in

parabolic trough power plants where they have proven their effectiveness [6]. The main advantage of molten

salts is the higher achievable operation operating temperature of the heat transfer fluid in the solar field

(Table 1), since, with these higher temperatures, the conversion from thermal to electrical power in the

connected steam cycle can reach higher efficiencies.

Therminol-VP1 Solar Salt HITEC

HTF Operating Temperatures 20°C – 390°C 290°C – 550°C 200°C – 450°C

Table 1: HTF Operating Temperatures in Parabolic Trough Power Plants (data from [3], [7], [8])

Furthermore, the combination of higher densities and higher heat conductivities of molten salts lead to higher

heat capacities per transported volume unit of heat transfer fluid (as shown in Table 2), so storage costs can

be reduced due to smaller tanks and heat exchangers [9]. The main drawbacks of these fluids are the required

fail-safe freeze protection caused by the high melting point of the molten salts and the higher heat losses –

mainly through radiation– in the solar field caused by higher operating temperatures. Whether these

drawbacks compensate the given advantages and whether there are optimization parameters that can increase

energy yields for molten salts will be discussed in this paper.

Solar Salt /

Therminol VP-1

HITEC /

Therminol VP-1

Heat Capacity 0.62 0.67

Density 2.36 2.40

Heat Capacity per Volume 1.47 1.61

Kinematic Viscosity 3.71 5.72

Heat Conductivity 5.93 4.16

Thermal Expansion Coefficient 0.22 0.23

Table 2: Relation of Mean Fluid Property Values in respective HTF Temperature Range

(data from [3], [7], [8])

2. Simulation Environment

For all simulations in this study, the simulation environment ColSim is being used [10] – enhanced by an in-

house library for concentrated solar thermal (CSP) applications [11].

Fig. 1: Simplified Sketch of the ColSim Simulation Model for Solar Thermal Power Plants

The main components of the modeled plant are the solar field solar field, consisting of several loops of solar

collector assemblies (SCA) with connecting headers, the thermal energy storage and the power block. Each

Power BlockSolar Field High Temperature Energy Storage(dependent on the concept in question)

Simulation Management

Controller

HTF hot

HTF cold PumpHTF

Temperature

PumpPower

StorageState

StorageControl

Results àß System Parameters

Plant Net Power

Heat-TracePower

Weather Data

of these units again comprises several analytical models for the description of the optical and thermo-

dynamical processes and loss mechanisms of the system components and the connecting pipework. Each unit

has its own parameter dialog, so boundary conditions, configurations and locations can be varied easily. A

central control unit controls the HTF mass flows, taking into account the current state of the system as well

meteorological conditions. The solar field outlet temperature as the main control parameter is kept constant,

when the solar field is in operation.

3. Detailed Modeling

The available solar energy input is calculated by an implemented algorithm from the given direct normal

insolation (DNI) in the considered time step, with an approach similar to that of Lippke [12]:

( ) ,

with being the effective aperture area of the considered solar concentrator assembly (SCA), ( ) accounts for the cosine effect due to the incidence angle of the solar vector, considers end losses of

the heat collecting elements (HCE) and considers mutual shading of parallel collector rows. The

incident angle modifier (IAM) used for the characterization of the assessed SCAs (Schott PTR 70 alike) is

generated with an independent ray-tracing procedure from given geometry and parameters and provided as

input to the simulation model.

A rough synopsis of further modeling which directly influences the plant behavior dependent on the HTF can

be found in the following.

3.1 Solar Field Heat Losses

Assuming the use of modern receivers (Schott PTR 70) for all analyzed HTFs the characteristics of the

receiver’s heat losses (per meter) as a function of the absorber inner wall temperature can be

written as follows [13]:

Using this relation in a one-dimensional discretization and assuming a constant irradiation over the whole

solar field, the receiver heat-losses and therefore the solar field heat losses can be determined. The fluid flow

at each node1 is characterized by the local Nusselt number which leads to a local heat transfer coefficient.

The heat flux between the HTF and the absorber wall at each node with the heat transfer coefficient α

can therefore be described as:

( )

With this simple balance the receiver heat losses at each node can be found analytically.

Furthermore heat losses in headers are taken into account. Assuming a sufficient insolation that leads to the

same heat transfer coefficient for all header pipes, heat conduction outweighs convection at the piping inner

wall which can therefore be neglected. Hence length dependent heat losses can be calculated with

the ambient temperature , the header length header-length dependent thermal conductivity

.

( )

1 mathematical representation of a fluid segment within a pipe

3.2 Solar field auxiliaries

The auxiliary power consumption based on pressure losses in the solar field piping is dependent on the fluid

velocity in the pipes. Minor and major pressure losses that occur in headers, receivers and further

components as elbows and joints are considered. In order to maintain reasonable pressure losses the header

diameter is adapted for each HTF to maintain a maximum fluid velocity of 3 m/s. For the pumps variable

speed control and a constant efficiency is assumed [14]. In all operating cases mass flow iteration is carried

out.

The freeze protection for molten salts as HTF in the solar field is crucial. For Solar Salt the temperature

should not fall below 290 °C to prevent freezing and possible pipe bursts [2]. A simplified assumption is that

all loops have the medium temperature between in and outlet as soon as pumping is stopped and that the

heating efficiency is 100 %. Further it is assumed that the loops cool out homogeneously until a lower

boarder temperature is reached. Joule resistance heating will then hold the fluid temperature constant until

solar radiation starts reheating the HTF. Those assumptions avoid complex modeling and calculation time, as

annual simulations have to be completed.

3.3 Power block characteristics

The power block characteristics are evaluated assuming a dry cooled condenser, one reheater and five

preheaters for the oil-driven steam cycle based on the SEGS-VI layout [13] using the commercial software

tool Thermoflex. For the salt system besides the superheater pressure and temperature mainly the preheating,

the reheating and the condenser pressures are adapted. Also the arrangement of the preheating section is

being rearranged in order to receive on optimized efficiency for the molten salt driven steam cycles. The

power block characteristics, which contain a variation of the ambient temperature and the thermal load cases,

are then included in the simulation model. Due to higher steam temperatures the net-efficiency for nominal

thermal load using Solar Salt (39.7 %) is higher than for Therminol VP-1(35.3 %).

3.4 Financial Model

Two characteristic values are being considered to estimate the economic feasibility of molten salts as heat

transfer fluid. The first one is the LCOE assuming the same solar field costs for each transfer fluid. The

second one is the marginal cost of the solar field that is possible for the use of molten salts assuming the

same LCOE as for thermal oil. The calculation of the LCOEs is determined by the annual energy yield of a

plant design weighted by plant availability factor and the constant annuity of the investment costs

(investment costs times annuity factor), the annual O&M and insurance costs annual costs, with an assumed

plant availability of 96% and an annuity factor of 8.88%, following the approach outlined by Morin [11]:

Fig. 2. Visualization of the Simplified Receiver Heat-Balance

5. Results and Discussion

5.1 Reference Case

For the direct comparison of molten salts and thermal oil as heat transfer medium, a solar power plant

configuration according to Andasol 3 is chosen [15]. Along with the receiver field with nearly 500,000

square meter of aperture area all modeled systems include thermal two-tank storage for 8 hours of full plant

operation.

For the comparison as many system parameters as possible are kept constant, such as configuration and

aperture area of the solar field, header lengths and all components of the steam cycle. Piping and insulation

materials as well as needed storage volume were adapted to higher operating temperatures and higher

volume-related heat capacity of the molten salts. As in the molten salt configurations only one medium for is

both, heat transfer and storage is needed, one of the heat exchangers of the original configuration is obsolete,

as shown in figure 4. In all configurations conventional heat-tracing strategy using electrical Joule resistance

heating is applied.

Quarter-hourly weather data from the Meteonorm meteorological database [16] is used for the location

Daggett / USA with an annual insolation of 2700 kWh/m². A solar field availability of 100 % during the

whole year and no soiling of the mirrors are considered in the comparison.

A closer look at the course of a summer day shows all significant factors that influence the altered electrical

net energy yields of the different HTFs. According to higher operating temperatures and thus increased heat

losses the available thermal energy from an identical solar field is less for molten salt than for thermal oil

during irradiation times. Therefore the thermal storage empties faster for the molten salts in the evening as

less energy is stored. Yet the electrical net energy production during plant operation is higher than for oil, due

to enhanced steam cycle efficiencies. Most disadvantageous for the salt is the necessity of a freeze protection,

which has to be activated during several hours every day. Regarding a whole year the use of this energy –

which nowadays is provided electrically and therefore decreases the net electrical energy output – is decisive

for possible advantages towards thermal oil as it accounts for up to 10 % of the annual electrical energy

output.

Fig. 3. Investigated Plant Layouts. Left side: conventional Andasol-3 layout for thermal oil as HTF;

Right side: adopted system layout for molten salt as HTF with one less heat exchanger

[source of picture: [15]]

The following figure shows the higher annual gross electrical output for Solar Salt (2.1 %) and HITEC

(0.8 %) compared to Therminol VP-1. The available solar energy at solar field aperture is identical for all

configurations with 1350 GWh.

Fig. 5. Annual Yields in Electrical Power with relative deviation to Therminol VP-1 results

(for reference case at location Dagget/USA)

Disregarding the electrical joule resistance heating as freeze protection this advantage is even more visible

for the net energy production of the plants (+ 5.7 % for Solar Salt and + 2.7 % for HITEC). If joule resistance

heating is applied in the solar field the energy yield of Solar Salt is 4.3 % less (HITEC + 0.4 %) than for

Fig. 4. Results of Comparison Solar Salt & HITEC with Therminol VP-1

(50 MWel reference case, Daggett/USA; for 21th

June)

thermal oil. For this case – which is the most likely at the moment – the annual efficiency for Therminol

VP-1 is 16.4 % while for Solar Salt it lies around 15.7 %.

For the given simulation case a rough economic viability comparison can be performed. The following table

summarizes the results. For the most likely case of joule resistance pipe heating and increased solar field

costs, the maximum additional costs in order to achieve an advantage compared to thermal oil for Solar Salt

are 14.4 % (HITEC 13.7 %). In the short term this may not achievable, as the additional cost, e.g. for the heat

tracing equipment, may be high. The positive marginal solar field costs are positive, even though the

electrical energy yield for molten salt is lower than for thermal oil, because storage costs can be reduced

vastly for molten salts [17]. Assuming that the thermal power for the freeze protection could be provided by a

fossil-fired source the solar field cost can further be increased as the following table shows.

Calculation Case

LCOE for Constant Solar Field Costs Marginal Solar Field Costs

Solar Salt HITEC Solar Salt HITEC

Joule Resistance Heating -8.7 % -7.9 % 14.4 % 13.7 %

Alternative Heating -14.9 % -9.4 % 27.1 % 16.6 %

Table 3. Economic viability comparison

6. Sensitivity Analysis

Various parameters have been varied while keeping all but one parameter constant regarding the described

reference case. If the simulation time is to be reduced it is possible to run annual simulations on a 30 minute

or hourly basis. Through averaging the weather data, the annual energy yield show a maximum deviation of

2 % compared to the reference case. Main differences can be seen in the course of day. Therefore also the

annual operating hours vary by up to 4.6 %.

For the location in the American Mojave desert in the reference case the annual insolation is high with

2700 kWh/m². At other locations those values cannot be achieved, which especially affects the performance

of Solar Salt driven plants as the relative heat losses and the need of energy for the freeze protection rise.

Therefore three other locations with lower annual insolation in Spain, India and Egypt are examined (also

based on Meteonorm data). The example of Bikaner in India shows that neither the gross nor the net energy

disregarding joule resistance heating for the molten salts perform better than the conventionally used thermal

oil as the thermal efficiency drops by around 8 % due to the high temperature level which has to be

maintained in the solar field while optical efficiency changes affect all systems in the same way.

Fig. 6. Annual Yields in Electrical Power with relative deviation to Therminol VP-1 results

(for reference case at location Bikaner/India)

Daggett/

USA

Borg el

Arab/ Egypt

Alméria/

Spain

Bikaner/

India

Annual Insolation [kWh/m²] 2700 2080 2000 1840

Ratio of the net energy yields of Solar

Salt to Therminol VP-1,

not considering heat-tracing

+5.6% -2.1% -2.5% -4.1%

Ratio of the net energy yields of Solar

Salt to Therminol VP-1,

considering heat-tracing

-4.3 % -13.6 % -16.5 % -18.0%

Table 4: Comparison of Achievable Annual Net Electrical Power Yields at Locations with Different

Annual Insolation

Furthermore several parameters have been investigated to estimate the optimizing potential and further

essential parameters for the molten salt plants. Possible uncertainties are the receiver heat losses. As heat loss

characteristics for molten salt receivers were not available, the characteristic of the Solel UVAC3 [18] has

been assumed for the use of salt, which leads to higher heat losses. Regarding a whole year the yields can

drop for up to 6.3 %.

Another interesting parameter is the solar field layout. For the reference case, the same layout is assumed for

all HTFs. While keeping the same aperture area the collector loop length for molten salt can be increased

which leads to higher fluid velocities and better heat transfer in the absorber piping but also to an increased

pumping power consumption. Initial variations show a potential of an increase in yield of 1.8 % for molten

salt plants compared to the reference case.

As the reference case shows, the freeze protection consumes vast amounts of electrical energy if joule

resistance heating is applied. Therefore an alternative operating strategy during solar field halt is investigated.

For the reference case it was assumed that the loops have one medium temperature as soon as the pumps

stop, which then continuously decreases until heating is activated. For the headers it was assumed that the hot

header never cools out and the cold header is heat-traced as soon as the receiver pipes are. The alternative

investigation assumes that all receiver and header pipes have a homogeneous temperature as soon as

radiation is too low for solar field operation. That would require a continuous circulation of the HTF in the

field, because header pipes do not cool down as fast as the receivers. If this strategy is followed the annual

yield of the whole plant can increase by up to 2.7 % while the heat-trace energy demand is reduced by 25 %.

Another interesting parameter which is expected to have an influence on the plant performance for molten

salt driven cycles is the solar field outlet temperature. An increased solar field operating temperature leads to

higher heat losses, while the efficiency of the steam cycle rises and the need for freeze protection is lowered

thanks to longer cooling cycles. Furthermore the need of pumping power rises with a decreased temperature

span owing to higher required mass flows.

Fig. 7. Influence of Solar Field Outlet Temperature on Plant Performance

The impact of a variation in solar field outlet temperature on annual yield is small due to the opposing

effects. The optimum is at an outlet temperature of about 530 °C with a plus of 0.7 % compared to the case of

550 °C. Further investigation and testing will have to prove whether the use of high temperature salts does

lead to higher electricity production.

Conclusions

A comprehensive C-Code model to run annual simulations of parabolic trough plants has been built. For the

comparison of molten salt with focus on Solar Salt with thermal oil (Therminol VP-1) a modern parabolic

trough plant layout at a location with high solar insolation has been chosen. As the simulations show, the

performance of molten salts as HTF in parabolic trough power plants is mainly dependent on further cost

development for components that can handle salts as HTFs. In the short run LCOEs will not be lower than for

oil-based systems.

The required heat-trace energy significantly hampers the performance of Solar Salt. Finding operation

strategies and non-electrical heat-trace options is crucial to make molten salts competitive. Reducing the

freezing point can be another possibility. Multi-component salts may be used for that purpose [19]. As could

be shown for the example of the solar field layout the use of molten salt has an optimization potential that can

be extended. Especially efficient control algorithms for plant startup, shutdown and halt modes should be

investigated. Moreover the optimal solar field temperature is not necessarily the highest possible. Further

studies and testing will be necessary to find the optimal temperature range.

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