Lead Performer: Oak Ridge National Laboratory – Oak Ridge, TN
-- University of Maryland – College Park, MD
-- Sandia National Laboratory – Livermore, CA
DOE Funding: $895,977
Cost Share: $99,500
Project Term: October 1, 2014 to September 30, 2016
Funding Opportunity: Building Energy Efficiency Frontiers and Incubator Technologies (BENEFIT) ‐ 2014 (FOA DE‐FOA‐0001027)
In this project we will demonstrate the first refrigerator using the Sandia Cooler as an evaporator. This rotating heat exchanger has already won the Editor’s Choice R&D 100 Award in 2012. It is estimated that implementation of this novel HX can provide an estimated energy savings of 407 TBtu/year when implemented in both residential and commercial refrigeration. The first proof of concept will be in a residential refrigerator.
Refrigerator-freezers are an essential part of residential and commercial buildings, with a total annual consumption of approximately 3,128 TBtu/year. Any advancement in this mature technology is highly relevant to the Department of Energy Building Technologies Office (BTO) mission. Although there have been improvements in refrigerator compressor technologies in the past couple of decades, there has been little or no improvement in evaporators or defrost cycles during the past 40 years. The main outcome of this project will be a high-efficiency refrigerator with the Sandia rotating heat exchanger as an evaporator that can eliminate the defrost cycle, reduce fan power, and subcool the liquid refrigerant, as well as reduce the required cycle temperature lift. These benefits will ultimately lead to estimated energy savings for both residential and commercial refrigeration equipment. Additional applications could include HVAC and automotive, providing further energy savings.
The Sandia Cooler is a novel, motor-driven, rotating, finned heat exchanger that consists of three main components: an impeller (a rotating, finned heat sink), a baseplate, and an integrated brushless dc motor. The impeller is powered by the motor, allowing it to rotate on a thin hydrodynamic air bearing above the stationary baseplate. Originally developed for electronics cooling, the underside of the baseplate is mounted to a heat source. Heat flows through the baseplate, air bearing gap (0.01 mm), impeller base, and impeller fins and is ultimately transferred to surrounding airflow. Because the impeller fins rotate at up to several thousand rpm, the airflow experiences a continual radial acceleration that decreases the thickness of the boundary layer by up to a factor of 10. This thinning of the boundary layer significantly improves the air-side heat transfer coefficient of the rotating heat exchanger compared with traditional fan and fin devices.
Based on calculations using the BTO Market Definition Calculator, the projected residential refrigeration market for 2030 is 1,537 TBtu/year, plus an additional 1,590 for commercial, for a total of 3,128 TBtu/year. The proposed technology could produce energy savings of approximately 13%. This is achieved through elimination of the defrost cycle, refrigerant precooling, and higher UA compared with conventional heat exchanger technology.
The incremental unit cost is projected to be between $14 and $50. Assuming a 13% unit energy savings in residential refrigerators, the payback period is 1.0 to 3.5 years. The Sandia Cooler has a small number of parts, no exotic materials, and a simple part geometry that allows for low-cost, high-volume manufacturing. For example, Sandia Cooler impellers are intended to be cold-forged. The cold-forging process takes approximately 10 seconds and is carried out at room temperature. Each die set has a lifetime of several hundred thousand process cycles, and multiple cavity dies would be used for mass production. The hydraulic press used for cold forging costs are estimated at $250K, however, that cost could be amortized over millions of parts. Including the die set and the press, the manufacturing cost should only be a few cents per impeller.
Other benefits for the Sandia Cooler relative to current heat exchanger technology are additional refrigerated space, noise reduction, and fouling reduction. Additional refrigerated volume realized by the more compact rotating heat exchanger is of primary interest to consumers since volume is essentially what they are paying for.
Momen, A. M., Abdelaziz, O. A. and Rice, C. K., Novel Frost Handling Techniques Using Air Bearing Heat Exchangers for Household Refrigerators, to be published in ASHRAE Transactions, 2015.