Engineering topics

Patel, Viral K.

Publications and source records attributed to Patel, Viral K..

8 records

DOE OSTI2024

High-Efficiency Thermoelectric Clothes Dryer (CRADA Final Report)

A typical clothes dryer in the US accounts for 7% of the average residential customer’s electric bill. Nationwide, consumers pay about $\$$9 billion annually for clothes drying. While energy efficiency for most household appliances has improved by a factor of 2 or more in recent decades, today’s clothes dryers perform similarly to units from the 1970s. Dryer efficiency is measured by the combined energy factor (CEF), with today’s units typically drying 3.73 lb of cloth per kWh consumed. An ENERGY STAR qualified unit must achieve 3.93 lb/kWh (for standard size electric units) and dry in less than 80 minutes. ORNL and CRADA partner Samsung Electronics America have developed an efficient prototype clothes dryer that uses thermoelectric heat pumps instead of electric resistance to dry the clothes. The prototype fabricated at ORNL successfully demonstrated in the laboratory a CEF of 6.89 lb/kWh at standard conditions of 75°F and 50% Relative Humidity (RH), exceeding the original project target of 6.0 lb/kWh. Additional trials on the same prototype achieved faster dry time with slightly lower CEF, meeting all requirements for ENERGY STAR product qualification. Deploying dryers with energy factor of 6 nationwide represents a technical potential of 234 TBtu/yr primary energy savings. The modeling and prototype development activities for the thermoelectric clothes dryer under this CRADA are summarized in this final report.

32 ENERGY CONSERVATION, CONSUMPTION, AND UTILIZATI↗

DOE OSTI2023

Experimental Evaluation of Refrigerant Leak Characteristics for Different HVAC&R Equipment Types–Phase 2

The US Department of Energy (DOE) Building Technology Office (BTO) and the Air-conditioning, Heating & Refrigeration Technology Institute (AHRTI) collaborated to sponsor an experimental study of refrigerant leak characteristics at Oak Ridge National Laboratory (ORNL); AHRTI project 9012. The project has been conducted in two phases with objectives to conduct refrigerant leak tests on several heating, ventilating, air-conditioning and refrigeration (HVAC&R) systems under operating conditions representative of actual applications to document the pressure decay rate and the mass flow rate of leaked refrigerant as a function of time. Phase 1 of the project ran from late 2017 to late 2018 with refrigerant leaks imposed on five systems and the collection of data to meet the project objectives. Systems chosen covered three air-conditioning (AC) applications (packaged terminal AC or PTAC, 3-ton residential split system AC, and 5-ton packaged rooftop AC) and two refrigeration applications (split system unit cooler, and standalone display case). AHRTI members donated the test systems used for the project. The systems were tested at two different leak rates (catastrophic high rate simulating full line break and a lower flow rate), two leak locations (high-pressure side and low-pressure side), and two operating conditions (compressor ON or OFF). Phase 1 results were summarized in a project report by Baxter, et al. (2019)1. The leak rates imposed in Phase 1 led to much more rapid charge releases than the more typical much slower rates seen in practice that would take days, weeks, or months to empty a system of its entire charge. For Phase 2, the project sponsors asked that the “higher” leak rate target a total charge release time of about 4 min. This was intended to represent a “reasonable” worst case release scenario to match the refrigerant leak rate assumption used in the International Electrotechnical Commission (IEC) standard IEC 60335-2-40 (2016 version [IEC 2016]. A charge release time of about 20–40 min (leak release rate 5 to 10 times lower than higher rate) was targeted for the “lower” leak rate. This report provides a summary of the Phase 2 effort. Two additional HVAC systems were selected for testing in this phase. First was a multi-split air-conditioning (MS-AC) system with one outdoor section and two indoor units and a nameplate refrigerant charge of 1.75 kg (3.86 lb.) of R-410A. The MS-AC was tested in cooling only or AC operation. Second was a split system heat pump coupled to two different representative supply duct systems (HP-duct) and a nameplate charge of 3.83 kg (8.44 lb.) of R-410A. This system was tested in both space heating and AC modes. As in Phase 1 of the project, all tests were conducted using R-410A.

32 ENERGY CONSERVATION, CONSUMPTION, AND UTILIZATI↗

DOE OSTI2023

A thermodynamic model of integrated liquid-to-liquid thermoelectric heat pump systems

Thermoelectric (TE) heat pumps (TEHPs) are advantageous for heating and cooling in various applications because of their modularity and simple design. A TEHP system includes the TE modules with p- and n-type materials bonded to substrates, plus heat exchangers, thermal interfaces to the heat exchangers, and heat transfer fluids. Although modeling an individual TE module has been extensively studied, limited studies have reported performance at the larger system-level. Furthermore, no prior study has addressed the impact of temperature-dependent TE material properties (e.g., electric resistivity, thermal conductivity, and Seebeck coefficient) on overall heat-pump-system-level performance. Here, this work presents a mathematical model for TEHP system performance based on Goldsmid's approach for TE material performance, “effective” TE material properties, Gnielinski's correlation for convective heat transfer, and thermal balance theory for a heat exchange network. This combined approach provides an accurate model of the liquid-to-liquid TEHP system. Three different approaches—one empirical, one based on the manufacturer's specifications, and one drawn from the literature—were then used to determine values for TE material properties. The first two methods treated properties as constants, while the last approach treated properties as surface-temperature-based functions. Finally, experimental TEHP data was used to validate the models, all with relative absolute deviations of approximately 10% when predicting heating capacity and 10%–25% when forecasting cooling capacity up to a 30 K surface temperature lift. The results demonstrated that, at the TEHP system level, the TE material properties could be treated as constants, avoiding solver iterations and reducing the performance uncertainty by up to 95%.

42 ENGINEERING↗

DOE OSTI2021

Fabric properties and electric efficiency limits of mechanical moisture extraction from fabrics

Residential clothes drying consume about 650 TBtu of primary energy per year in the United States, equivalent to 3% of primary residential energy consumption. There is a strong impetus to reduce the energy consumption of clothes dryers by both improving existing technology and developing alternatives that use fundamentally different drying mechanisms. Clothes drying technologies are broadly classified into either evaporative or mechanical drying. The focus of this paper is on mechanical drying, including vibrational, centrifugal, and press-based methods. In this work, the physical processes involved in these mechanical fabric drying processes were analyzed to develop general theories of mechanical cloth drying energy efficiency. Quantitative evaluation of the theories requires measured fabric properties. To accomplish this, a set of experiments was conducted on samples of a standard test fabric. The fabric was a cotton-polyester blend specified by the US Department of Energy to evaluate the standardized efficiency of all residential clothes dryers in the US. Mercury porosimetry experiments were conducted to determine the fabric pore size distribution, apparent density, and porosity. Elasticity experiments were conducted to determine the fabric’s Young’s modulus. Isostatic press experiments were conducted to establish a relationship between compression force and fabric moisture content. The data resulting from these experiments were combined with mathematical models developed in this work to calculate the theoretical maximum performance limits for mechanical drying of the standard fabric. The results of the analysis are used to make recommendations for the most promising technologies that offer the greatest potential energy savings for residential clothes drying.

36 MATERIALS SCIENCE↗

DOE OSTI2021

Ultrasonic oscillatory two-phase flow in microchannels

Experimental and numerical investigations are performed to provide an assessment of the transport behavior of an ultrasonic oscillatory two-phase flow in a microchannel. The work is inspired by the flow observed in an innovative ultrasonic fabric drying device using a piezoelectric bimorph transducer with microchannels, where a water-air two-phase flow is transported by harmonically oscillating microchannels. The flow exhibits highly unsteady behavior as the water and air interact with each other during the vibration cycles, making it significantly different from the well-studied steady flow in microchannels. Here, the computational fluid dynamics (CFD) modeling is realized by combing the turbulence Reynolds-averaged Navier-Stokes (RANS) k – ω model with the phase-field method to resolve the dynamics of the two-phase flow. The numerical results are qualitatively validated by the experiment. Through parametric studies, we specifically examined the effects of vibration conditions (i.e., frequency and amplitude), microchannel taper angle, and wall surface contact angle (i.e., wettability) on the flow rate through the microchannel. The results will advance the potential applications where oscillatory or general unsteady microchannel two-phase flows may be present.

42 ENGINEERING↗

DOE OSTI2020

Validated model of a thermoelectric heat pump clothes dryer using secondary pumped loops

The energy associated with clothes drying makes up 4% of the total residential electrical energy consumption in the US. The cost is high since most clothes dryers use electric resistance heating which is inexpensive but also energy inefficient. State-of-the-art vapor-compression heat pump clothes dryers offer significant energy savings but have limited market penetration in the US. Here, a new configuration of clothes dryer was studied that utilizes a solid-state thermoelectric heat pump with pumped secondary water loops and conventional fin-and-tube water-to-air heat exchangers. Experimental results are presented for the thermoelectric dryer prototype with a combined energy factor of 6.89 lb BDW /kWh (specific moisture extraction rate of 1.71 kg w /kWh) and a dry time of 84 min for a standard load size of 8.45 lb (3.83 kg) with 57.5% starting moisture content. This is a 85% energy efficiency improvement over baseline electric resistance dryers. The efficiency and drying time results achieved are similar to vapor compression heat pump dryers. Compared to previous air-based thermoelectric clothes dryers, the energy efficiency has been increased by 6%, but drying time has been reduced by as much as 47% using secondary pumped loops and separate heat exchangers, which represents a significant advancement in the technology. In addition to the experimental prototype, a model was also developed to study the performance. The model was validated against experimental prototype data and used to identify the range of performance (dry time and efficiency) that could be expected of the design.

42 ENGINEERING↗

DOE OSTI2020

Measurement and analysis of clothes dryer air leakage

Each year clothes dryer appliances are sold globally in the tens of millions. Both vented and ventless types are common and are heated by combustion, electric resistance, or electric heat pumps. In the dyer air path, segments can be defined between components such as the drum, blower, filter, screens or grills, and heat exchangers (where applicable). In this work, a technique was developed to experimentally measure air leakage into and from the segments of a clothes dryer. Detailed leakage measurements were taken on two vented and one ventless residential clothes dryer. The measurements were quantified as a leakage flow coefficient for each segment. For two dryers (one vented and one ventless), these flow coefficients were combined with in situ operating pressure measurements to determine leakage flow rates for each segment. For these two units under normal operation with air pressures within 0.5 kPa (50 mm water column) of ambient pressure, volumetric system air leakage was found to be about 20–60% of the blower airflow. Furthermore, a quasi–steady state psychrometric analysis was conducted on vented dryers with negatively pressurized drums. The analysis revealed that leakage quantity, location, and direction are essential to achieving an acceptable energy balance and accurate modeling results for a vented heat pump clothes dryer but are of limited significance for vented electric resistance clothes dryers.

42 ENGINEERING↗

DOE OSTI2020

Coupling of electroelastic dynamics and direct contact ultrasonic drying formulation for annular piezoelectric bimorph transducers

A newly developed technique for drying clothes without thermal energy has been developed through the utilization of ultrasonic vibrations from piezoelectric transducers. The novel technique incorporates the actuation of a thin stainless steel disk in contact with wet fabric via annular piezoelectric rings, where water in the liquid form is atomized, transported through microchannels in the disk, and ejected as a mist. In such a system, resonance matching between the actuation portion of the transducer and the portion contacting fabric must be realized, with theoretical results from the developed electromechanical model showing a reduction in energy consumption by 50% when resonance matching is achieved. The electrically coupled distributed parameter model for an annular bimorph piezoelectric transducer is developed for optimization of ultrasonic drying technology. The thickness mode vibrations are shown to dominate the behavior of the system, where the analytically developed model can be optimized to increase the output acceleration of the transducer, thus increasing drying performance. Here, the electromechanical equation developed will be connected to the drying rates of fabrics in contact with said vibrations, where the novelty of the coupled equations and its description of the physics of ultrasonic drying will be discussed.

36 MATERIALS SCIENCE↗