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Hydrogen Sensor/Separation Module for Parabolic Trough Technology

Concentrating Solar Power (CSP) plants have been in use for energy production for a long time. Among the commonly used designs are parabolic trough systems, which employ heat absorber tubes along curved concentrating mirrors to absorb solar energy. During this process the organic-based fluid that delivers heat from the parabolic trough receivers to the electricity-generating steam turbine undergoes a small but constant thermal breakdown. The resulting hydrogen off-gassing can cause thermal losses that reduce overall plant efficiency and revenue.

Now (2020), scientists at NREL have found a solution to avoid hydrogen build up in CSP plants and harness the energy produced more effectively. They took infrared images of the receivers to monitor the temperature of glass sleeves that enclose the tubes transporting heated fluid. The receivers had been designed to operate with a glass surface temperature of 60°C. The infrared images, however, measured glass surface temperatures of 140°C to 160°C, which meant that the insulating value of the glass sleeves had been reduced, and heat loss was taking place. Analysing the problem, the researchers found that the heat-transfer fluids were off-gassing hydrogen and hydrogen was leaking through the steel tubes. To solve the problem, the scientists first designed a computational model to calculate the hydrogen extraction rate necessary to maintain suitable concentrations of hydrogen in the circulating heat-transfer fluid. Next, they built a hydrogen sensor that could make gas-concentration measurements every one or two minutes. The third task was to develop a simple means of extracting hydrogen from the headspace gas in the expansion tanks. A palladium membrane was used to separate the hydrogen from the gas, which was then extracted via vacuum pumps into a catalytic oxidizer to be discharged as water vapour. The final task was to combine hydrogen sensing and extraction and integrate them into a physical prototype that could be used as a model for a CSP-plant-sized module.

The concept of hydrogen removal from parabolic trough systems has been discussed in scientific literature for quite some time. In 2016, scientists used a simple model to calculate the balance of hydrogen in a CSP system. As input data for the simulation, extrapolated hydrogen generation rates were used. Hourly weather data, surface temperatures of the tubing system and hydrogen permeation rates for stainless steel and carbon steel grades were also incorporated into the model. In the first step the effect of heat transfer fluid ageing, build-up of hydrogen pressure in the HTF and hydrogen loss rates through piping and receiver components were modelled. In the second step a selective hydrogen removal process was added to the model. The simulation results suggested that active monitoring and controlling the effective hydrogen partial pressure in parabolic trough solar thermal power plants with diphenyl oxide/biphenyl heat transfer fluid was needed.

In 2017, a novel process for the reduction of the H2 concentration in the heat transfer fluid via stripping and gas separation was simulated for the operation in parabolic trough CSP plants. Applying the proposed process, the concentration of H2 could be reduced to 1 ppb. The simulation conditions were applied to a 50MW PT CSP plant with 7.5 hours storage. 2550 hours of solar field operation were simulated. The thermal power for these conditions was assumed to be 270 MW, which resulted in a power block heat to electricity efficiency of 37%. A cost comparison between the H2 separation process and frequent replacement of parabolic trough receivers was carried out which yielded the result that proposed H2 removal process was more economic.

The new sensor/separation module has added several improvements to existing CSP plants and the ongoing evaluations suggest that it is operating successfully. The module not only prevents future efficiency loss in new receivers but can also clean existing contaminated receivers by removing the hydrogen from the glass sleeves, thus restoring them to their original operating efficiency. If you consider that the cost of replacing the degraded solar receivers until the fields are decommissioned is nearly $20 million, the integrated hydrogen sensor/separator module is cheap as well as game changing. Also, the installation can be done without any costly interruptions to the operation of the plant, thus increasing the attractiveness of this technology to a broader audience.

The sensor/separation module has shown to have clear benefits in terms of efficiency and economy. It is also an excellent example of technology transfer from laboratory to industry. It is small wonder, therefore, that NREL is currently exploring opportunities to install integrated hydrogen sensor and separator modules at other CSP plants.