Thermally-Activated Technologies

Heat & Mass Transfer Based Technologies

Energy recovery systems have taken the form of auxiliary equipment and are generally the purview of installers and not part of a well thought out integration design or strategy. These critical elements of integrated energy efficiency have not undergone rigorous research and development. New technologies, like microturbines and fuel cells provide low temperature waste heat (~ 150 - 550°F) which requires a rethinking of heat transfer approaches. “Out-of-the-box” approaches are required to develop low cost systems approaches.

Significant advancements in heat and mass transfer have been made in the past two decades that allow for more efficient recycling of thermal energy. One such innovation is the micro-channel heat exchanger that was initially designed to reduce the size, weight and cost of automobile radiators. Another potential innovation is the use of centrifugal force to significantly increase the rate of heat and mass transfer.

Microturbine Heat Recovery Heat Exchanger
Presently, heat recovery is accomplished from the microturbine waste stream through means of a separate heat exchange unit that requires considerable floor space, ductwork, and cost. A new design will utilize a compact coil design that mounts on the top of the turbine exhaust flange. The design will feature lightweight tubing that is able to withstand the high temperatures (600ºF) in the micro-turbine exhaust stream. The heat exchanger will be designed with a low pressure differential to minimize back pressure which reduces the turbine efficiency. The advanced design will be tested to compare the performance and installed cost against a conventional heat exchange unit.

Micro-Channel Heat Exchanger Development
Plate and frame micro-channel heat exchangers can provide space savings ranging from 50 to 90%, and improved efficiency. ORNL plans to develop and test a first generation micro-channel heat exchanger for a 5 to 10 ton refrigeration unit (scalable to 100 tons). Partners in the effort include United Technologies Research Center and Southwest Gas.

Heat Exchanger Maldistribution
Maldistribution of the fluid flowing through heat exchanger circuits results in capacity losses on the order of 15 to 30%. Efforts to reduce maldistribution are hampered by the ability to effectively measure the temperature in different areas of the heat exchanger. It is proposed to develop a measurement technique based on previous work at ORNL which uses phosphor paint and fiber optics to measure temperatures throughout the heat exchanger. The phosphor method has been applied with a precision of about 0.01 degree and would be a significant improvement over present methods. A test apparatus, using this technique to determine the flow maldistribution throughout the heat exchanger, is planned.

If you have any questions or comments regarding this section or the CHP Technologies Program in general, please contact us.