The Case for Microchannel Technology in Air Source Hybrid Electric Water Heaters (HEWH) White Paper
Executive Summary
Hybrid electric water heaters are also referred to as Heat Pump Water Heaters (HPWH) and are a combination of an air source heat pump and an electric water heater storage tank that can be used to heat water in domestic applications. HPWHs are a promising technology and use the same mechanical principles as refrigerators and air conditioners, but in reverse, taking heat from the environment and concentrating it to heat water for service needs. Many HPWHs are designed and fabricated with wrapped-tank D-shape coils, or microchannel condenser coils. Now that electrification is the goal, what is the most efficient condenser coil solution for HPWHs? Which HPWH coil technology is best and provides better efficiency, lower refrigerant charge, and superior operational ability for consumers? This Tech Tip answers those question and details the advantages of utilizing micro channeling in HPWHs and their benefits to contractors and their customers.
Introduction
HPWH Design
The use of microchannel wrapped tank condenser coils offers reduced refrigerant charge, high efficiency while maintaining adequate heat transfer effectiveness, =improved Uniform Energy Factors (UEF), and improved recovery time over HPWHs with D-shape condenser coils.
HPWH Wrapped-tank D-shape Condenser Coils
Wrapped-tank D-shape coils are shaped like a “D” and wrapped around the storage tank. The tubing is rolled to produce this D-shape cross-section before being attached to the tank, resulting in an increase in the contact area between the coil and the tank wall. The coil is typically wrapped eighteen or more times around the tank. Heat is transferred through the tank wall and directed inward towards the water by employing thermal insulation to minimize heat loss (1)
Microchannel Condenser Coils
D-shape Tube, wrapped tank coil
A microchannel condenser coil is a type of heat exchanger typically made from aluminum. These coils typically provide 20% to 40% higher heat transfer performance, excellent flexibility, lower internal volume, 10% to 30% smaller coil surface, and up to 60% less weight than D-shape condenser coils. They also offer improved performance by increasing air-side heat transfer between tubes and ambient air with lower air-side pressure drops, higher corrosion resistance, robust construction, less refrigerant charge – up to 60% higher UEF values, and optimized raw material cost (2)
HPWHs with microchannel condenser coils typically offer a higher maximum heating energy rating and shorter heating time than HPWHs with wrapped-tank D-shape condenser coils. Even though D-shape coils typically have larger inner volume than microchannel coils, they often have less surface area in contact with the water through the tank causing increased heat loss to the surrounding air. Thus, the microchannel condenser coil shows very promising features in a HPWH. An earlier example of a microchannel wrapped tank coil is shown (3)
Coil Comparison of D-shape versus Microchannel
Comparing HPWHs with D-shape coils to HPWHs with microchannel coils considers similar capacity tanks, Uniform Energy Factor (UEF), and refrigerant charge level in ounces. In a comparison at the Oak Ridge National Laboratories, a test unit HPWH with a D-shape condenser coil was compared to a HPWH with a microchannel condenser coil. The test focused on a baseline UEF value for each HPWH. The result shows that with 50-gallon tanks, the HPWH with a microchannel coil achieved a UEF within 10% of the baseline value while reducing the refrigerant charge by 40%, clearly indicating the microchannel coil as the more effective design. And further, by optimizing the condenser design higher UEF values could be achieved (4) .
Microchannel, wrapped coil
Initial concept microchannel achieves approximately 40% refrigerant charge reduction with a UEF value within 10% of the D-shape coil.
The Solution ‒ HPWHs with Optimized Microchannel Condenser Coils
Optimized Microchannel Condenser Coil Superiority
Optimized microchannel condenser coils are made with aluminum and have a flat shape laid on the tank. They are also coated with thermal paste to enhance heat exchange efficiency. Their rectangular channel design provides greater surface area in contact with the medium and offers more efficient heat transfer. They are also more compact than D-shape coils and use less refrigerant. Hot gas diffuses through a manifold to microchannel tubes in parallel across the entire tank area and with the larger surface area provides better first hour ratings than wrapped D-shape condenser coils.
Microchannel heat exchanger counter flow design diffuses refrigerant flow evenly creating a more uniform heat transfer across the entire tank.
Coil Comparison of D-Channel versus Optimized Microchannel
Comparing HPWHs with a D-shape coil to HPWHs with optimized microchannel condenser coils considers similar capacity tanks, Uniform Energy Factor (UEF), and refrigerant charge level in ounces. In the comparison of 50-gallon tanks, the Bosch HPWH with an optimized microchannel coil has a refrigerant charge that is about 20% less than the HPWH with D-shape coils. However, its UEF value is approximately 10% higher, indicating greater efficiency (6)
Optimized microchannel achieves approximately 20% refrigerant charge reduction with approximately 10% higher than D-shape.
Conclusions
a. Utilizing microchanneling in HPWHs offers benefits to contractors and their customers such as a higher maximum efficiency rating, and shorter heating time than HPWHs with wrapped-tank D-shaped coils.
b. State-of-the-art HPWHs with optimized microchannel technology will offer 3-4 times more efficiency than electric water heaters and offer hybrid technology with both heat pump and electric heating elements combined.
c. Bosch GreenTronic 7000 T HPWHs (referred to as Hybrid Electric Water Heaters) have the optimized microchannel coil design.
Bosch Home Comfort Group is a leading source of high quality water heating and comfort systems. The company offers gas tankless, electric whole house and point-of-use water heaters, Bosch and Buderus floor-standing and wall mounted boilers, Bosch and FHP geothermal, water-source and air-source systems as well as controls and accessories for all product lines. Bosch Home Comfort is committed to being Simply Smart by offering products that work together as integrated systems that enhance quality of life in an ultra-efficient and environmentally friendly manner. For more information, visit bosch-homecomfort.us.
Bosch Group
The Bosch Group is a leading global supplier of technology and services in the areas of Automotive, Industrial Technology, Consumer Goods and Building Technology. The company was founded in Stuttgart, Germany, in 1886 and presently has more than 440 subsidiaries and is represented in over 150 countries.
In the U.S., Canada and Mexico, the Bosch Group manufactures and markets automotive original equipment and aftermarket solutions, industrial drives and control technology, power tools, security and communication systems, packaging technology, home comfort solutions, household appliances and software solutions. The Bosch Group’s products and services are designed to improving quality of life by providing innovative and beneficial solutions. In this way, the company offers technology worldwide that is Invented for life. Additional information is available online at www.bosch.com
Bosch Home Comfort Group
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