Traditional ASHPs and Climate Differences
Traditional ASHPs have been common in the warmest parts of the United States for many years. In warmer climates, these ASHPs are typically sized for cooling and offer a portion of the heating capacity needed on colder days, often assisted by back up heat if needed. These warmer climate ASHPs have typically been equipped with single speed, single stage compressors. For colder climates, these ASHPs were often sized to meet heating needs, while being oversized for cooling, resulting in moisture issues and inefficient operation. With the introduction of single-speed, two-stage modulating compressors, two stage ASHPs found increased use in colder climates, as they could operate at full load during winter and part load during summer. Most ASHPs are split systems with one coil and fan inside the structure (indoor unit - IDU) and another coil, compressor, and fan outside (outdoor unit - ODU), or systems with all coils, compressor, and fans outdoors in a single unit (package heat pump). Bosch ASHPs are available as ducted or ductless, depending on the model, providing air distribution options for consumers.
Inverter Technology
Applied to Bosch ASHPs –The Solution for Different Climates
Bosch inverter ASHPs are now being utilized in colder climates as they run longer while meeting the structure heating needs while providing better comfort than traditional single, and two stage modulating, ASHPs. They also easily meet the lower cooling needs in colder climates as well. The opposite occurs in mild and warmer climates with the unit running longer to meet cooling needs, and less to meet heating needs. Bosch inverter ASHPs include all the typical components found in traditional ASHPs with the addition of several more components which enhance operational ability, efficiency, and comfort. These are the Inverter, the Inverter Variable Speed Compressor, the Electronic Expansion Valve (EXV), the Pressure Transducer, and the accumulator to store the extra refrigerant when not running at maximum speed.
Bosch Inverter ASHP Components & Operation
Inverter
An inverter is an electronic component that can turn Direct Current (DC) to Alternating Current (AC). Our system first converts the 240V AC to a ~300 V DC (with a rectifier), then converts back to a 3 phase AC (inverter) at different frequencies to control speed. Many applications exist for inverter technology. These include portable lights, televisions, kitchen appliances, and power tools. Renewable energy such as solar generation systems also use inverter technology, as well as battery powered mobile and marine vehicles, and electric cars. Stationary appliances such as window unit air conditioners, refrigerators, freezers, washing machines, dryers, and dishwashers also utilize inverter technology for improved operation. For heating and cooling applications, inverter ASHPs utilize this energy saving technology to eliminate wasted operation by efficiently controlling motor speed and providing continual and maximum compressor efficiency.
Inverter Variable Speed Compressor
Inverter ASHPs are equipped with variable speed compressors which are controlled digitally with DC voltage and operate almost continuously with less off and on than non-inverter (fixed speed and modulating 2-stage) compressors. An inverter ASHP with inverter technology can self-analyze structure load conditions while running and can self-adjust. Typically, inverter ASHPs turn on at slow speed to maintain temperature and speed up if the temperature changes due to load. The variable speed compressor can ramp up and down to provide load matching and set point matching. Minimal power is used compared to noninverter, traditional ASHPs, and improved comfort levels can be achieved. Additionally, inverter ASHPs allow system pressure changes in the refrigerant circuit, and compressor, and provide more precise control of the operating evaporator.
Two types of compressors are typically utilized for current inverter ASHPs in the heating, ventilating, and air-conditioning (HVAC) industry, rotary and scroll. Rotary compressors are utilized in lower capacity units in lieu of scroll compressors. These compressors use rotating impellers and are very compact with the working mechanism being simpler compared to scroll compressors. Scroll compressors are used in larger capacity units. They have two interleaving spiral-
shaped scrolls to compress refrigerant progressively, operating smoothly, quietly, and efficiently, with less moving parts than prior reciprocating compressors. The scroll design minimizes wear for long compressor life. Scroll compressors tend to be very energy efficient and have typically low load leading to alleviated noise and vibration issues. Both rotary and scroll compressors with inverter technology provide supremely efficient comfort by adjusting compressor capacity rather than a simple on/off control like traditional units, thereby reducing operating costs.
Inverter Compressor Background
A Bosch inverter ASHP compressor is variable speed while energized, always closely matching the heating or cooling structure load at all outdoor ambient temperatures during normal operation in either cooling or heating. The inverter system does not have a compressor contactor but instead converts 230V AC voltage to ~310V DC using a diode bridge rectifier. The DC voltage then passes through a filter, smoothing the voltage and sending it to an inverter. It is then “inverted”, or reformed, back into a 3-phase sine wave that is sent in small increments called “hertz” (one cycle per second = one revolution per second) of compressor crankshaft speed to the compressor at variable frequency. The compressor motor speed is then controlled by the AC hertz delivered to it by high-speed bipolar transistors, depending upon the heating or cooling load conditions, and can ramp up or down as needed.
The system first detects reference temperatures with sensors (return air temperature and set point, outdoor air temperature). The system calculates the corresponding capacity and controls the compressor. It then detects the operating pressure at the suction line every 20 seconds and calculates the deflection from the target pressure. Adjustment of compressor frequency (hertz) is then ramped up or down as necessary to match load requirements.
Electronic Expansion Valve (EEV or EXV)
All Bosch inverter ASHPs will have an “Electronic Expansion Valve” (EXV) metering device designed to regulate and ensure proper refrigerant flow during all conditions, with the primary function relating to the heating mode. This helps optimize the unit’s operation to the highest efficiency possible (1) . The EXV has a “Pressure Transducer” (PT) (currently IDS/IDP only) and temperature sensor (IDS/IDP/MS) at the evaporator outlet that constantly measure the refrigerant. The controller reads these signals and uses refrigerant data stored in the controller to convert the pressure into the refrigerant’s saturation temperature. The controller compares the actual temperature to the converted temperature to calculate the operating superheat. The controller then sends a signal to the EXV’s stepper motor to energize the coils and adjusts the valve’s position as necessary. This allows the compressor to have a much greater range of operation than with a traditional Thermostatic Expansion Valve (TXV), allowing more precise control at lower capacities and greater refrigerant flow when higher capacities are needed. For example, if the cooling load increases, the refrigerant in the operating evaporator boils off faster, which increases the suction pressure and temperature. The EXV senses this and opens to allow more refrigerant into the operating evaporator. Conversely, if the cooling load decreases, the EXV closes, limiting refrigerant into the operating evaporator. These EXVs are fully open when the compressor is on (normal operation) during cooling mode and adjusted during heating mode. They are controlled in heating mode via compressor discharge superheat (2) . In the heating mode the superheat is calculated from the low-pressure equivalent saturation temperature converted from the pressure detected by a low-pressure sensor in the ODU and the temperature detected by a suction line thermistor. The EXV valve opening is regulated so the superheat becomes close to the targeted superheat value.
Bosch IDS/IDP ASHPs also have a traditional TXV positioned in the indoor air handler (like all traditional split system ASHPs) to control superheat during the cooling mode. The TXV at the cooling coil auto adjusts the amount of restriction against refrigerant flow to ensure that the refrigerant is completely gas after leaving the cooling coil to protect the compressor from slugging, and controls operating superheat. Bosch MSs will not have a TXV. Single zone MSs will have ODU EXVs and temperature sensors. Multizone MSs will have ODU EXVs, temperature sensors, and a capillary tube metering device.
Pressure Transducer (PT)
Bosch inverter IDS/IDP ASHP compressor speed is controlled based on coil pressures monitored by the unit’s PT located on the suction line in the ODU and compared to a desired value or set point. The PT then converts that pressure into a proportional electrical output signal, typically in the form of frequency. The PT provides compressor frequency control, EXV control (in heating mode only), high pressure protection (in heating mode), and low-pressure protection (in cooling mode) (3) . The Low GWP IDP, and IDS will have the same control logic. To ensure stable and adequate capacity, the compressor speed will modulate relative to evaporator pressure (indoor coil) during cooling operation and relative to condensing pressure (indoor coil) during heating operation. The target pressure can automatically adjust based on compressor operation so optimal capacity can be achieved.
Indoor Coil Target Temperatures
In a Bosch inverter ASHP system, the “Electronic Control Unit” (ECU), or control board, adjusts the compressor speed/input according to ambient environment (temperature) based on system pressure and temperature readings (occurring constantly during run), designed to consistently maintain a target indoor coil temperature in cooling or heating. Target coil setpoint varies based on model.
The Bosch inverter IDS/IDP ASHP target cooling mode evaporator temperature is currently 46ºF (or reduced by 4°F in cooling to 42°F if selected for Accelerated Cooling on the unit’s control board), that is constantly being sensed and maintained below dew point. This process assures that dehumidification will continually be occurring during cooling without having to change blower speed or air flow volume Cubic Feet per Minute (CFM). There will be no need to adjust indoor air flow to maintain the indoor conditions at the desired “Sensible Heat Ratio” (SHR). The Bosch inverter IDS/IDP ASHP target heating mode coil temperature is currently 108°F (or increased by 4°F in heating to 112°F if selected for provide Accelerated Heating, by increasing unit capacity on the ECU). Bosch IDS/IDP ASHPs have enhanced humidity control features and can be more efficient that traditional ASHPs and features an “adaptive capacity” function. This feature is “self-learning” which allows a range of target coil temperatures to adapt for better unit operation and reduced short cycling (4)
Electrical Limitations (In-rush Current)
Traditional ASHPs have a high start-up current when energized in either heating or cooling which can result in voltage fluctuations. This frequent on and off for traditional units creates an in-rush current surge which uses more energy and causes unneeded wear and tear on the system (5) .
The inverter system smoothly ramps up operation, avoiding spikes in energy use and protecting the system’s life cycle (6) . Bosch inverter ASHP compressors will provide true soft start with no in-rush current as compared to traditional ASHPs.
Through the flexible start-up control technology of the compressor, the inverter compressor can achieve low-frequency step-by-step start-up. Taking the AHRI210/240 A2 condition as an example, the instantaneous start-up current of a 5-ton inverter compressor is about 1A, while the start-up current of the same fixed-speed compressor will reach more than 20A. On the one hand, this process can reduce the load on the compressor and other electronic components during start-up, improve the reliability of the heat pump equipment itself, and on the other hand, the smaller start-up current can also reduce the impact on the power grid (7)
About Bosch
Bosch Home Comfort Group in North America
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
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