Spark Up Your Solar Power: Impedance Heating vs. MI Heat Trace for CSP Towers

Solar thermal power towers are a high-temperature energy renewable energy source that can provide clean and reliable power. These towers use directed mirrors to focus the sun onto a central point, a receiver, which concentrates the solar energy into incredible heat.

The heat generates steam, which drives a big turbine to generate electricity.

For a complied dataset of Current and Future CSP projects worldwide see SolarPaces "CSP.guru 2022-07-01.zip"

One of the challenges of concentrated solar power towers is getting a stable desired temperature in the heat transfer fluid piping inlet and discharge to the central receiver. High temperature heating systems, such as mineral insulated (MI) heat trace, have been used for this application.

However, these systems can be challenging at high temperatures, in both energized and non-energized states, due to junction temperature exposure.

Impedance heating uses electric current flowing directly through the pipe material to generate heat, as a byproduct of resistance, and maintain a desired temperature.

This technical blog, presents a comparative analysis of impedance heating and traditional high temperature heating systems such as MI heat trace for concentrated solar power (CSP) tower applications.

Discusses the advantages and disadvantages of each system based on a current and past literature review. The findings suggest that impedance heating has advantages over traditional heating systems, in reliability and rapid response times, but not necessarily in flexibility and retrofit conditions.

IMPEDANCE HEATING

Impedance heating is created by sending electric current directly through the pipe material which generates heat. The pipe essentially becomes the resistor, which generates heat without the use of cabling.

The main component of the impedance heating system is a scott-t transformer or similar, which has high reliability. This system can be sized based on pipe material resistance, length and transformer secondary voltage to rapidly heat up the piping for quick response time necessary in cyclic CSP plants.

Because of the direct current generated in the piping material, heating in very quick, and therefore advantageous to the CSP transfer systems. Chavez et al. (1995) reported that impedance heating has been successfully tested at Sandia National Laboratories for solar thermal power tower applications.

MINERAL INSULATED HEAT TRACE

MI heat trace is a traditional heating system, that originated in the 1930s, and became commercially viable during the 1950's. Mineral insulated cables are used to maintain the process temperature in the piping system to maintain the viscosity of the heat transfer medium.

The cables are made of a solid conductor, an inorganic insulation(usually MgO-magnesium oxide or SiO2-Silicone Dioxide)and overall outer metal sheath. MI cable incorporates a copper or nickel conductor, and a copper, Inconel or stainless steel sheath material.

The metal sheath provides a high level of thermal conductivity and the inorganic insulation is an excellent dielectric at high temperatures. MI heat trace is often used in harshly caustic and classified environments because it is resistant to corrosion (depending on sheath material), moisture (when atmospherically sealed), and physical damage.

However, MI heat trace has temperature limitation above 537 C (1000 F) on the joints and hot/cold section connections. MI heat trace is subject to thermal shock and fatigue at extremely high temperatures, which can cause failure in the system.

COMPARATIVE ANALYSIS

The reviewed literature on impedance heating and MI heat trace for CSP power tower applications suggest that impedance heating has some advantages over MI heat trace like increased reliability and faster response times.

Impedance heating does not require joint points, other than those required to provide contiguous connection. The volume of material involved tends reduce the material fatigue under load conditions. Impedance heating can be oversized to reduce the heat up cycle, reducing the pre-heating and thaw times.

MI heat trace is a reliable and highly flexible heating system in use in industrial and commercial applications for many years.

It’s resistance to corrosion, moisture, and physical damage, making it a good product for harsh environments. However, it can be subject to thermal shock and fatigue at extreme temperatures, primary as a result of the cable material, weak conduction contact with the metal pipe and cable profile in relation to power output per meter. (contact with the piping is critical at CSP temperatures)

The small circular profile of MI cable limits the power output potential of the cable, limiting how many BTU’s can be transferred to the CSP thermal process transfer medium in the process pipe.

That being said, MI cable also has the benefit of retrofit, and ease of adding cable to pipe branches which have not been sized into the impedance heating calculations.

CONCLUSION

Impedance heating is an excellent primary heating system vs. traditional heating systems such as MI heat trace for CSP power tower applications.

The literature review suggest that impedance heating has many advantages over MI heat trace, including increased reliability, faster response times. Impedance heating can be sized to rapidly preheat or thaw cycle the heat transfer medium material in the piping system.

MI cable has it’s place for CSP process branch line additions and system retrofits. Its lower application cost, and flexibility are excellent for the smaller thermal medium process lines requiring lower wattages to maintain a minimum CSP Molten Salt process transfer temperature.

When designing a CSP process the use of impedance heating is ideal as the primary thermal engine.

REFERENCES

"Power Tower System Concentrating Solar-Thermal Power Basics" US Department of Energy - Solar Energy Technologies Office

“Today’s Solar Power Towers”, Sandia National Laboratories’ brochure.

Pacheco, J.E. and Kolb, W.J, (1997). "Comparison of an Impedance Heating System to Mineral Insulated Heat Trace for Power Tower Applications" Solar Thermal Technology and Test Departments Sandia National Laboratories Albuquerque, NM - 1997 ASME International Solar Energy Conference

Tyner, C.E, Sutherland, J.P and Gould, W.R Jr. (1995) "Solar Two: A Molten Salt Power Tower Demonstrator" Solar Thermal Technology and Test Departments Sandia National Laboratories Albuquerque, NM

Pacheco, J. E., and Dunkin, S. R. (1996). “Assessment of Molten-Salt Solar Central-Receiver Freeze-up and Recovery Events,” Solar Engineering 1996, Proceedings of the 1996 American Society of Mechanical Engineers International Solar Energy Conference, April 1996, pp. 85–90

Kenneth M. Armijo, Matthew D. Carlson, Dwight S. Dorsey, Joshua M. Christian, Craig S. Turchi (2020) "System Design of a 2.0 MWth Sodium/Molten Salt Pilot System" ASME 2020 14th International Conference on Energy Sustainability, 17-18 June 2020 7 pp.

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