Session: 06-02: Heat Transfer in CSP Applications 1

Paper Number: 142458

142458 - Evaluating Heat Transfer Coefficients in a Particle/sco2 Heat Exchanger With Non-Contact Temperature Measurements 

Abstract: 

Particle heat exchangers are a critical component of a particle-based CSP system. However, particles are known to have low heat transfer coefficients relative to heat transfer fluids used in state-of-the-art CSP systems like molten salt and synthetic oil. This is due to the discrete nature of the particles which reduces contact area with the heat exchanger surface and results in a low bulk thermal conductivity. In addition, measuring particle temperatures poses a challenge for devices like thermocouples because the probe is not in continuous contact with the solid phase. Wall temperatures are often measured for particle flows, and particle temperatures are estimated from these values. However, this approach requires knowledge of typically unknown parameters, such as the near-wall air gap thickness and local particle-wall heat transfer coefficient, to calculate particle temperatures from a model of the flow. Thus, direct non-contact temperature measurements offer an alternative method for measuring particle temperatures in a heat exchanger and assessing the heat transfer performance of the system. These measurements come with their own set of challenges, including a need for optical access into the channel of the heat exchanger and knowledge of the radiative properties of the particles (and also how the properties change with temperature). This work is a demonstration of two different non-contact temperature measurement techniques—thermal imaging and pyrometry—applied to a fully-integrated particle/sCO₂ moving packed-bed heat exchanger with optical viewports on both external walls. The heat exchanger assembly, located at the National Solar Thermal Test Facility (NSTTF) at Sandia National Labs, is capable of providing controlled inlet temperatures of particles and sCO₂ up to 550 °C as well as controlled mass flow rates up to ~200 g/s. The particles used in the test are CARBOHSP 40/70, a sintered-bauxite proppant with a mean diameter of 350 µm. The experiments cover a parameter sweep of particle temperatures (200 °C to 550 °C) and particle flow rates (50 g/s to 175 g/s), while the sCO₂ temperatures were controlled to maintain a temperature difference between the inlets of 200 °C, and mass flow rate was held constant at 100 g/s. Temperature data for the particles and sCO₂ was collected at both the inlet and outlet of the heat exchanger with thermocouples and RTDs, while temperature measurements along the height of the heat exchanger were performed with a thermal imaging camera (FLIR, E60sc) on one side and pyrometers (Micro-Epsilon, CTL-CF2) on the other. Wall temperatures at each viewport were also recorded with thermocouples. Heat transfer coefficient calculations for each technique are compared, and the effects of inlet temperature and mass flow rate are quantified. Advantages and disadvantages of non-contact temperature measurements are discussed in the context of how they can be applied in systems like a particle heat exchanger to study and potentially optimize heat transfer performance.

Presenting Author: Mike Mayer University of Michigan

Presenting Author Biography: Mike is a PhD candidate in mechanical engineering at the University of Michigan with a research focus in radiation heat transfer for sustainable energy systems.

Authors:

Mike Mayer University of Michigan
Dimitri Madden Sandia National Laboratories
Kevin Albrecht Vacuum Process Engineering
Noah Walters University of Michigan
Rohini Bala Chandran University of Michigan