NREL is a national laboratory of the U.S. Department of Energy, Office of Energy 

Efficiency & Renewable Energy, operated by the Alliance for Sustainable Energy, LLC.

Contract No. DE-AC36-08GO28308

Measurement of Hydrogen 

Purge Rates in Parabolic Trough 

Receiver Tubes

Preprint 

Greg C. Glatzmaier

National Renewable Energy Laboratory

Presented at SolarPACES 2010

Perpignan, France

September 21-24, 2010

Conference Paper

NREL/CP-5500-43966

October 2010
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1 

MEASUREMENT OF HYDROGEN PURGE RATES IN

PARABOLIC TROUGH RECEIVER TUBES

Greg C. Glatzmaier1

1 Ph.D., Chemical Engineering; CSP Thermal Storage Project Leader

National Renewable Energy Laboratory, 1617 Cole Blvd., Golden, CO 80401

(303) 384-7470, greg.glatzmaier@nrel.gov

Abstract

The purpose of this research is to investigate and develop methods to remove hydrogen centrally from 

commercial parabolic trough power plants. A mathematical model was developed that tracks the generation 

and transport of hydrogen within an operating plant. Modeling results predicted the steady-state partial 

pressure of hydrogen within the receiver annuli to be ~1 torr. This result agrees with measured values for the 

hydrogen partial pressure. The model also predicted the rate at which hydrogen must be actively removed

from the expansion tank to reduce the partial pressure of hydrogen within the receiver annuli to less than 

0.001 torr. Based on these results, mitigation strategies implemented at operating parabolic trough power 

plants can reduce hydrogen partial pressure to acceptable levels. Transient modeling predicted the time 

required to reduce the hydrogen partial pressures within receiver annuli to acceptable levels. The times were 

estimated as a function of bellows temperature, getter quantity, and getter temperature. This work also 

includes an experimental effort that will determine the time required to purge hydrogen from a receiver

annulus with no getter.

Keywords: parabolic trough, modeling, hydrogen, decomposition, permeation

1. Introduction

Parabolic trough power plants offer an attractive option for near-term, utility-scale, dispatchable, renewable 

electricity generation. More than 500 MW of capacity currently are in place in the United States and Spain, 

with many new installations planned for the next five years [1,2]. Key benefits of parabolic trough power 

plants are their reliable design and proven commercial performance. Essentially all of the currently operating 

plants use a heat-transfer fluid (HTF) that is a eutectic mixture of two organic compounds, biphenyl and 

diphenyl oxide. At elevated temperatures up to 393oC, the organic components undergo very slow 

degradation reactions that produce hydrogen as a byproduct [3]. Hydrogen is transported by the HTF to many

components within the power plant. Hydrogen that permeates through the receiver tube walls establishes an 

equilibrium partial pressure within the vacuum annuli of the receivers. Within the receiver annulus, the 

equilibrium partial pressure balances the net permeation rate into the annular volume from the receiver tube 

with the permeation rate out of the annular volume to ambient air. Permeation out of the annular volume 

occurs primarily through the metal bellows, which seal the receiver tube to the enclosing glass tube. 

Hydrogen within the annular volumes of the receivers may result in substantial degradation of the thermal 

performance of the receivers. This work is investigating methods to eliminate hydrogen from the receivers of 

a parabolic trough power plant by removing it from a central location within the plant.

________________________________________________________________________________________

The Alliance for Sustainable Energy, LLC (Alliance), is the manager and operator of the National Renewable 

Energy Laboratory (NREL). Employees of the Alliance, under Contract No. DE-AC36-08GO28308 with the

U.S. Dept. of Energy, have authored this work. The United States Government retains and the publisher, by 

accepting the article for publication, acknowledges that the United States Government retains a nonexclusive, paid-up, irrevocable, worldwide license to publish or reproduce the published form of this work, or 

allow others to do so, for United States Government purposes.
2 

Figure 1. Schematic of parabolic trough power plant.

The HTF within a power plant flows through several parallel loops that comprise the solar collector field (Fig. 

1). Each loop consists of multiple solar collector assemblies (SCAs), which include the parabolic reflector, 

receiver, and accompanying support structure. Piping sections connect the SCAs within each loop. A header 

supplies HTF from the plant power block to the SCA loops. A second header returns HTF to the power block 

from the loops. Components within the power block that contain HTF are the steam generator and expansion 

tank. HTF flows from the power block to the solar collector field and back to form a closed loop.

The HTF that is currently used in essentially all of the operating plants is a eutectic mixture of two organic 

compounds, biphenyl oxide and diphenyl ether. The fluid is thermally stable to a maximum temperature of 

390oC. Its vapor pressure at 393oC is about 10 atmospheres, and its freezing point is 12oC. These thermal 

properties allow parabolic trough power plants to operate within a temperature range of 293o–393oC. Its low 

freezing point minimizes the risk of freezing in the solar collector field when the plant is off-sun.

Alternate HTFs with higher operating temperatures are being considered for use in future power plants [4]. 

These fluids will generate higher-temperature steam and will allow the turbine and plant to operate with 

greater thermodynamic efficiencies. Molten nitrate salt mixtures offer higher operating temperatures with 

low vapor pressure, but their freezing points are typically too high to prevent freezing during off-sun periods. 

Because there is currently no demonstrated alternative HTF, near-term installations are being designed to 

operate with the organic eutectic. 

At temperatures near 393oC, the organic components undergo a variety of degradation reactions. These 

reactions include ring closure of the ether linkage in diphenyl oxide and free-radical polymerization of 

biphenyl to form products with higher molecular weight. Both reactions produce hydrogen as a byproduct. 

The hydrogen is transported throughout the power plant to all components that contain HTF. Its high 

permeability in metals allows it to permeate through the steel walls of the expansion tank (Fig. 2), steam 

Expansion tank

Steam

generator

Hot header

Piping section

Solar collector

assembly (SCA)

Power block

Cold header

Solar field loop

n 1st th
3 

Figure 2. Hydrogen generation and transport in the HTF expansion tank.

generator, piping, and receiver tubes. Hydrogen that permeates through the receiver-tube walls establishes an 

equilibrium partial pressure within the annular volumes of the receivers (Fig. 3). 

The equilibrium partial pressure balances the net permeation rate into the annular volume from the receiver

tube with the permeation rate out of the annular volume to ambient air. Permeation out of the annular volume 

occurs primarily through the metal bellows, which seal the receiver tube to the enclosing glass tube. All of 

the hydrogen ultimately passes from the plant to ambient air.

The occurrence of hydrogen within the annular volumes of the receivers may result in substantial degradation 

of their thermal performance. Decreased thermal performance (Fig. 4) is caused by the relatively high thermal 

conductivity of hydrogen at low partial pressures of about 1 torr [5]. Receivers that contain hydrogen in their 

annular volumes retain less of the absorbed thermal energy than those without hydrogen. They operate with 

lower thermal efficiency and cause a decrease in the overall plant efficiency. The resulting decrease in power 

output from the plant can be substantial. Current receivers contain getter materials that absorb hydrogen 

within the annulus. New receivers contain getters with more capacity to absorb hydrogen and maintain 

vacuum for many years. A method that removes hydrogen from a central location in the power plant would 

complement the receiver getters.

Figure 3. Hydrogen generation and transport within the receiver.

Expansion Tank

Head-space gas

HTF liquid

H2 efflux

H2 influx within HTF

within HTF

H2 permeation from 

tank to ambient

H2 gas-liquid 

exchange

H2 generation

Receiver tube Glass tube

Annulus

Bellows

H2 permeation from 

receiver tube to annulus

H2 permeation from 

annulus to ambient

H2 influx

within HTF

H2 efflux

within HTF

H2 generation
4 

Figure 4. Receiver heat loss as a function of hydrogen partial pressure.

2. Objective

The objectives for this work were to determine the rate at which hydrogen diffuses out of a receiver annulus 

as a function of receiver tube and bellows temperatures and compare results to the model predictions.

3. Approach

Previous work modeled hydrogen generation and transport within a parabolic trough power plant. Modeling 

was accomplished using a steady-state hydrogen material balance on the components of the power plant that 

contain HTF. Several mechanisms for hydrogen generation and transport were included in the model. 

Hydrogen (H2) is initially contained in the HTF and is generated by the thermal decomposition of the HTF 

components. Therefore, transport of hydrogen within the plant occurs via the normal flow of HTF between 

the plant power block and the solar collector field. In addition, permeation through the walls of the expansion 

vessel, steam generator, piping, headers, and receivers results in transport of hydrogen out of the plant to 

ambient air. At steady state, the model predicted a hydrogen partial pressure of 1 torr in the receiver annuli 

when hydrogen is present in the plant HTF. Steady-state modeling results showed that if the HTF within the 

expansion tank of the plant is kept free of hydrogen, then the HTF that is in circulation within the solar 

collector field will also remain free of hydrogen. This result is due primarily to the fact that the time required 

to generate significant concentrations of hydrogen in the HTF is much greater than the time required to 

circulate HTF from the expansion tank to the collector field and back again. Under these conditions, the 

model predicted that the steady-state partial pressure of hydrogen in the annuli of the receivers will reduce to 

acceptable levels of less than 0.001 torr. At these partial pressures, heat loss from the receivers is minimized 

and the plant will operate with maximum thermal efficiency. 

Acceptable limit = 0.002 torr
5 

Our latest work included transient modeling of a receiver annulus to predict the times required for hydrogen 

to permeate out of the receiver annulus. We modeled conditions in which the receiver tube is at the normal 

plant operating temperature and there is no hydrogen present in the HTF. The estimated times depended on 

the quantity of getter material in the receiver and the getter temperature and ranged from 2 to 10 months. We 

also modeled cases in which the receiver contained no getter material as a function of bellows temperature. 

The results were used to design laboratory experiments that determine the times required to purge hydrogen 

from the receiver annulus by permeation through the receiver tube wall and bellows. 

4. Experimental Design

To substantiate the modeling results, we designed and constructed a laboratory experiment that determines 

the time required to purge hydrogen from a receiver annulus as a function of receiver tube and bellows 

temperatures. The tests use a receiver that contains no getter material and has a glass extension welded to the 

receiver glass. The extension provides access to the receiver annulus and allows the annulus to be evacuated 

and backfilled with hydrogen to a specific partial pressure. A schematic of the gas-handling equipment is 

shown in Figure 5. 

Figure 5. Gas-handling hardware and controls for experimental test setup. 

The receiver is equipped with multiple electric heaters that are located within the receiver tube (Fig. 6). Two 

linear-cartridge heaters provide heating to the receiver tube and are able to maintain the tube at 400o

C. Four 

coil heaters (two on each end) maintain the temperature of the receiver tube ends and prevent axial 

temperature gradients. Thermocouples provide temperature measurements of the heaters, receiver tube, glass 

tube, and bellows. Computer-based feedback control maintains the receiver tube at the desired temperature 

(400o

C). The bellows are insulated in a manner that replicates the bellows insulation at the SEGS power 

plants in Kramer Junction, California.

Control box

Hydrogen cylinder

Roughing pump

Pressure transducer

Bellows

Vacuum-gas stand

Metal/glass o-ring seal

Power supply

Remote controller Computer

Data

Work station

A/D converter
6 

Figure 6. Heater hardware and controls for experimental test setup

Standard tests consist of evacuating the receiver annulus and backfilling with hydrogen to a pressure of 1 torr. 

The receiver annulus is sealed and isolated. The receiver tube is heated to 400o

C for 8 hours each day to 

simulate operation at a commercial power plant. The temperature of the glass tube is monitored when the 

receiver tube is at 400o

C. Initially, when hydrogen is present in the receiver annulus, the glass temperature 

will be elevated to about 150o

C. As hydrogen permeates out of the annulus, the glass temperature will

decrease to about 50o

C. These tests will provide valuable information on the expected response of the 

receivers in an operating power plant when hydrogen is purged from the expansion tank.

5. Current Results

Figure 7 shows annulus pressure at various active purge rates in mol/s from a single vent located on the 

expansion vessel. These results indicate that actively purging hydrogen from a centralized location within the 

power plant will reduce the receiver hydrogen pressures within the annuli to acceptable levels.

Figure 7. Steady-state annulus hydrogen partial pressure versus expansion tank purge rate.

Remote controller

Computer

Set points Data

Work station Power electronics box

Digital I/O

Control box

Thermocouple

module

SSC 1

SSC 3

SSC 2

208 V 3 phase

TC temperatures

SSR 1 SSR 2 SSR 3 SSR 4 SSR 5 SSR 6

1.E-04

1.E-03

1.E-02

1.E-01

1.E+00

0.0E+00 1.0E-05 2.0E-05 3.0E-05

Hydrogen Partial Pressure (torr)

Purge rate (moles/s)

1st SCA in loop

nth SCA in loop

acceptable limit (0.002)
7 

Figure 8 shows the estimated purge time to reduce the partial pressure of hydrogen within the receiver 

annulus by two orders of magnitude. At bellows temperatures of less than 150o

C, essentially all of the 

permeation is occurring through the receiver tube wall. Under this condition, the required purge time is 17 

days (8 hours per day). As the bellows temperature increases from 150o

C, a greater portion of the permeation 

is occurring through the bellows walls. At 400o

C, the time required to purge hydrogen has decreased to about 

1 day. These results show that if the bellows can be maintained at temperatures close to 400o

C, then they will 

account for most of the permeation. The relatively high permeation rate through the bellows is due to the fact 

that the bellows wall is quite thin (0.254 mm) compared to that of the receiver tube (3 mm).

Figure 8. Hydrogen purge time versus receiver bellows temperature. 

The experimental hardware shown in Figures 5 and 6 has been fabricated and assembled. The heaters, 

computer software, and safety controls are going through their final checks to ensure that they function 

properly when testing begins. In the next few months, we will conduct a series of tests to experimentally 

determine the hydrogen purge rates from the receiver annulus. Those results will be compared to the results 

that were predicted by the modeling work.

6. Future Work

Our intention is to demonstrate a method to remove hydrogen centrally from an operating, commercial 

parabolic trough power plant. We have identified a novel method to remove hydrogen from the HTF 

expansion tank of a parabolic trough plant. In March of 2010, a utility patent application for the method was 

submitted to the U. S. Patent & Trade Office. That application is currently patent pending. We have had 

discussions with several power plant owner/operators who have expressed interest in hosting a demonstration. 

Detailed discussions will not occur until the experimental testing described in this paper is concluded.

0

2

4

6

8

10

12

14

16

18

0 50 100 150 200 250 300 350 400 450

Purge time (days)

Bellows temp (C) (no getter)
8 

Acknowledgements

This work was done under U.S. Department of Energy Contract No. DE-AC36-08GO28308 to the National 

Renewable Energy Laboratory. 

References

[1] DOE/GO-102007-2400, February 2007. 

[2] K. Zweibel et al., Scientific American (January 2008) 64–73. 

[3] L. Moens et al., NREL/TP-510-42468. Golden, CO: National Renewable Energy Laboratory, 2008. 

[4] D. Kearney et al., Journal of Solar Energy Engineering 125 (2003) 172. 

[5] F. Burkholder et al., Proceedings of the ASME 2nd International Conference on Energy Sustainability, 

ES2008-54176, August 2008. 
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Measurement of Hydrogen Purge Rates in Parabolic Trough 

Receiver Tubes: Preprint

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14. ABSTRACT (Maximum 200 Words)

The purpose of this research is to investigate and develop methods to remove hydrogen centrally from commercial 

parabolic trough power plants. A mathematical model was developed that tracks the generation and transport of 

hydrogen within an operating plant. Modeling results predicted the steady-state partial pressure of hydrogen within 

the receiver annuli to be ~1 torr. This result agrees with measured values for the hydrogen partial pressure. The 

model also predicted the rate at which hydrogen must be actively removed from the expansion tank to reduce the 

partial pressure of hydrogen within the receiver annuli to less than 0.001 torr. Based on these results, mitigation 

strategies implemented at operating parabolic trough power plants can reduce hydrogen partial pressure to 

acceptable levels. Transient modeling predicted the time required to reduce the hydrogen partial pressures within 

receiver annuli to acceptable levels. The times were estimated as a function of bellows temperature, getter quantity, 

and getter temperature. This work also includes an experimental effort that will determine the time required to purge 

hydrogen from a receiver annulus with no getter.

15. SUBJECT TERMS

parabolic trough; modeling; hydrogen; decomposition; permeation

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