How to Make Your Heat Pump Designs More Reliable, Safer, and with Higher Efficiency

June 27, 2022

Blog

Heat pumps perform both heating and air conditioning and are all-electric systems. Unlike a fossil fuel heating system, heat pumps do not convert combustible fuel into heat energy. Thus, heat pumps are another tool in the global effort to reduce carbon dioxide emissions and minimize the potentially catastrophic effects of climate change on the planet. Over 80 countries have endorsed the Net-Zero Emissions (NZE) initiative, which requires a 50% reduction in greenhouse gas emissions by 2030 and net-zero emissions by 2050.

Heat pump heating, ventilating, and air conditioning (HVAC) systems are a critical part of the emission reduction strategy. Governments encourage growth in heat pump usage by offering incentives, such as rebates on purchases. Also contributing to the growth are advances in inverter-driven motor technology and more efficient electronics, enabling the development of more efficient heat pumps. The improved efficiency allows heat pump use in a broader range of climates. As a result, the industry expects global annual heat pump installations to increase from 10M units in 2020 to 15M units in 2025, growing at an 8.5% CAGR.

Design teams need to ensure their heat pumps are reliable, safe, and efficient. The electromechanical and electronic systems require protection from hazards such as AC mains-induced overloads and transients and electrostatic discharge (ESD). The power components need protection from the destructive effects of overheating. Using the appropriate topologies and electronic components will contribute to optimized heat pump efficiency. This article will assist designers in achieving reliable, safe, and efficient heat pump designs.

Overview of heat pump operation

There are three types of heat pumps classified by power source: water-powered, geothermal, and air source. The most common heat pump is the air source heat pump which will be the focus of this article.

The air source heat pump is an all-electric HVAC system that moves heat, and unlike a fossil fuel heating system, does not convert fuel into heat energy. The elimination of fuel conversion enables a heat pump to be more efficient than a fossil fuel HVAC system, and a heat pump does not emit any CO2. Heat pumps are reversible evaporator-condenser systems with two fan motors. As a result, heat pumps are both heating and air conditioning systems. Heat pumps consist of an outdoor fan and coil unit, and an indoor fan and coil unit. The direction of refrigerant flow determines whether the heat pump is heating or cooling inside air. Figure 1 shows the two operating modes of a heat pump.

(Figure 1a: Heat pump in cooling mode)

(Figure 1b: Heat Pump in heating mode)

Sub-systems of a heat pump and the recommended components for protection, control, and sensing

Figure 2 shows a modern heat pump performing as an HVAC system and as a tank water heater for hot water. The diagram includes recommended protection, control, and sensing components for the major sub-systems of the heat pump.

(Figure 2: Example of heat pump HVAC and water heater systems in a commercial building. The text blocks around the diagram highlight the main sub-systems of the heat pump and the recommended protection, control, and sensing components for each sub-system.) 

Figures 3 and 4 show block diagrams of the outdoor compressor and indoor air-handling units' heat pump electronics. The table to the right of these diagrams list the recommended protection, control, and sensing technologies. These components provide robust protection of the circuit blocks from current overloads, high voltage transients, and ESD. Some components minimize power consumption to maximize efficiency, while others provide sensing for safety.

(Figure 3: Block diagrams of a heat pump outdoor unit and the indoor unit. The adjacent table highlights the outdoor unit's recommended protection, control, and sensing components.)

(Figure 4: Block diagrams of a heat pump outdoor unit and the indoor unit. The adjacent table highlights the indoor unit's recommended protection, control, and sensing components.)

Protection, efficiency, and safety for the outdoor unit

AC Input Protection circuit

The AC Input Protection block connects directly to the AC power line and is subject to hazards such as current overloads and voltage transients that propagate on the AC power line. Load faults on the power line can lead to large surges of current. At the input of the AC Input Protection block, use a fast-acting fuse to prevent a current overload from entering the outdoor unit circuitry. Consider these important fuse requirements:

• A current interrupting rating over 10 kA
• A low internal resistance below 100 mΩ to minimize power loss
• Shock-resistant and vibration-resistant construction 
• An Underwriters Lab (UL) recognized component compliant with UL standard 248-14 and International Electrotechnical Commission (IEC) standard 60127-1.

Ensure that users can easily replace the fuse after it has interrupted a current overload. Choose a fuseholder that allows for easy fuse installation and replacement, a minimum current rating of at least the fuse's rating, and a fuseholder compliant with UL and IEC standards.

Transients induced after lightning and voltage surges from activation and de-activation of high power-consuming devices can cause high voltage transients on an AC power line. Use a metal oxide varistor (MOV) to absorb transients to prevent them from reaching downstream circuitry. Select a MOV with:

• A transient clamping voltage that will prevent damage to downstream circuitry
• Capacity to absorb a few 100 J of transient energy
• An MOV that can withstand kV levels and absorb kA current pulses in compliance with IEC standard 60335-1.

Designers can utilize a surge protection device (SPD) as an alternative to a MOV. SPDs offer a higher level of protection by absorbing up to 50 kA of surge current from a voltage transient that can be as large as 3 kV. The SPD offers a status indicator to allow monitoring of its protection capability. The status indicator informs the control electronics whether a transient strike has damaged the SPD. The SPD trades off a higher level of protection and status monitoring than the smaller size and lower cost of a MOV. 

Contactor circuit

Use a contactor to safely control the power to the outdoor fan and compressor motor. Make sure that the contactor has a sufficient current rating to drive the full power requirement of the motor. Consider a contactor with long-life electrical contacts. Look for a contactor with at least a 2 kV dielectric strength between contacts and between contacts and coil material.

Rectifier Circuit

The Rectifier Circuit performs the AC-DC conversion. Investigate using, for rectification, MOSFETs with:

• Fast switching times to reduce switching losses
• Low gate capacitance to minimize gate charge time
• Small Rds(on) to reduce on-state power loss and contribute substantially to reducing power consumption.

Ensure efficient control of the MOSFETs with a gate driver integrated circuit.  Ensure that the gate driver can source and sink sufficient current for the selected MOSFETs and look for gate drivers that have a high dv/dt rating for fast switching of the MOSFETs.

As an alternative to discrete MOSFETs, consider rectifier modules. Look for modules with low forward voltage and low reverse leakage current. The fully integrated rectifier modules contribute to a reduced component count.

Power Factor Correction circuit

The Power Factor Correction Block minimizes the power drawn from the AC line by making the heat pump circuitry look as resistive as possible to minimize apparent power and maximize real power.

As an alternative to individual MOSFETs or low forward voltage diodes, consider a MOSFET-series diode integrated PFC boost assembly, reducing component count and saving printed circuit board space. The series diodes support enhanced dynamic operation for high-frequency operation, maximum efficiency, and fast switching.

Inverter circuit

Avoid damage due to overheating conditions. Use thermistor probes to monitor the temperatures of the motor windings, the refrigerant lines, and power semiconductors. Thermistors offer fast response and a small form factor. Select hermetically sealed thermistors for high reliability.

Investigate power IGBTs or IGBT modules to drive the compressor. These transistors have both excellent thermal characteristics and an ultra-low packaging profile. Like the power MOSFETs, gate driver modules conveniently control IGBTs.

Protection, efficiency, and safety for the indoor unit

Auxiliary Power Supply circuit

The Auxiliary Power Supply powers the control electronics and the DC fan motor, pushing conditioned air into the residential or commercial space. In addition to its primary function, this supply must capture and absorb any portions of overloads and transients propagated through the outdoor unit's AC Input Protection block. Use a fast-acting fuse and a MOV, as recommended for the AC Input Protection block. In addition, protect the sensitive downstream integrated circuits (ICs) with a transient voltage suppressor (TVS) diode. TVS diodes have these benefits for secondary transient protection:

• Ultra-fast, ps, response to a transient
• Low clamping voltages to protect sensitive electronic components
• Uni-directional or bi-directional configurations.

Look for a TVS diode that can absorb a minimum of a few hundred Watts of transient power.

Fan Motor Drive

As with the outdoor unit, use thermistor temperature sensors to prevent overheating of the DC fan motor coils, refrigerant lines, and the power semiconductors. In addition, protect the motor drive circuit and the motor from transients with a TVS diode.

Air Filter circuit

Ensure the air filter is properly positioned to remove particulates from the air supply. Consider a reed position sensor and a magnetic actuator to ensure the air filter is fully covering the air entrance duct. Look for hermetically sealed, magnetically operated contacts that require no standby power. Select the needed sensitivity for the design requirements.

Wireless Interface circuit

The Wireless Interface communicates system status and enables indoor temperature control using smartphones or notepad computers. The Wireless Interface circuit block is in contact with the external environment and is subject to ESD strikes. Use TVS diodes to protect the sensitive ICs. Investigate bi-directional TVS diodes that can safely absorb ESD strikes above 10 kV to protect against damage caused by human contact and through-the-air strikes. Look for a TVS diode with low capacitance below 1 pF to avoid corruption of transmission and reception.

In addition, protect the data lines from ESD with polymer ESD components. Polymer ESD components can provide a minimum of 8 kV of protection and minimal disturbance to signals with capacitances as low as 0.25 pF.

Ensure robust, efficient, and safe heat pump operation with wise protection, control, and sensing

A robust, efficient, and safe heat pump design does not require many components. However, the design project must have documented circuit protection, efficiency, and safety goals. Leaving these objectives as afterthoughts will result in additional re-design time and higher development costs. Application engineering assistance is available. Designers can utilize a manufacturer's expertise to save valuable design time. The application engineers can help with:

• Cost-effective component selection
• Guidance on meeting the required safety standards
• Pre-compliance testing (if the manufacturer offers this) to prepare designers for product certification testing.

Designers who achieve robustness, efficiency, and safety goals will have a substantial competitive advantage.

References:

1. Circuit Protection Products Selection Guide. 2020 Littelfuse, Inc.
2. Power Semiconductor Selection Guide. 2021 Littelfuse, Inc.
3. Sensing Products Selection Guide. 2019 Littelfuse, Inc.
4. Protection Relay & Controls Catalog. 2022 Littelfuse, Inc.