Building Efficient, Safe, and Reliable Smart Cities Begins with Robust Heat Pump Design

Ryan Sheahen, Littelfuse, Inc.


Ensuring safety and reliability with the right protection and control components

Click image to enlarge

Figure 1: Heat pump in cooling mode

The biggest megatrend for smart cities is improving energy efficiency and reducing carbon emissions. Developing smart electronics designs for heat pumps, water heaters, metering, and thermostats is mission-critical to achieving success. This article will help electronics design engineers improve their new heat pumps' energy efficiency, safety, convenience, and reliability so that tomorrow's smart cities can begin meeting their net-zero emission goals today.

Heat pumps are all-electric systems; air-to-air heat pumps can perform both air conditioning and heating. Unlike fossil fuel systems, heat pumps don't convert combustible fuel into heat energy. As a result, heat pumps are another tool in the worldwide effort to reduce carbon dioxide emissions and minimize the potentially catastrophic effects of climate change.

More than 80 countries have endorsed the Net-Zero Emissions (NZE) initiative. NZE requires a greenhouse gas emissions reduction of 50% by 2030, with net-zero emissions by 2050. Using heat pump technology are essential to this emission reduction strategy. Governments are encouraging the growth of heat pump usage by offering incentives like purchase rebates. Advances in inverter-driven motor technology are contributing to the increase. More efficient electronics also enable the development of more efficient heat pumps, which allows their use in a broader range of climates. As a result, the International Energy Agency (IEA) expects global heat pump deployment to reach 600 million units by 2030, covering approximately 20% of the annual heating requirements of buildings worldwide.

Heat pumps' electromechanical and electronic systems require protection from AC mains-induced overloads, electrostatic discharge (ESD), and transients. The power components need protection from the damaging effects of overheating. Using the appropriate electronic components and topologies contributes to optimizing heat pump efficiency. This article assists today's designers in achieving reliable, safe, and efficient heat pump design.

Heat pump operation overview

The three types of heat pumps are classified by power source: water-powered, geothermal, and air-source. The most common is the air source heat pump, which is the focus of this article. Unlike a fossil fuel heating system, air source heat pumps are an all-electric HVAC system that moves heat; it does not convert fuel into heat energy. Since they are electric, heat pumps do not emit any carbon dioxide (CO2) like a typical natural gas burning system. Air-to-air heatpumps are reversible evaporator-condenser systems with two fan motors, resulting in their ability to be a combined heating and air conditioning system. Heat pump systems consist of both outdoor and indoor fans with heat exchange coil assemblies. The refrigerant flow direction determines whether the heat pump is heating or cooling the inside air. The two heat pump operating modes are illustrated in Figure 1 and Figure 2.

Click image to enlarge

Figure 2: Heat pump in heating mode


Heat pump sub-systems: Recommended protection, control, and sensing

Figure 3 shows a modern heat pump performing as an HVAC system and a tank water heater for hot water. The diagram highlights the main heat pump sub-systems, including recommended protection, control, and sensing components.

Click image to enlarge

Figure 3: Heat pump HVAC and water heater systems in a commercial building


Figure 4 provides a block diagram of the outdoor compressor (top) and indoor air-handling units' (bottom) heat pump electronics. The table to the right of the diagram lists the recommended protection, control, and sensing technologies. These components provide robust circuit protection solutions from current overloads, ESD, and high voltage transients. Some components help minimize power consumption to maximize efficiency, while others offer sensing for safety.

Click image to enlarge

Figure 4: Heat pump block diagram of outdoor and indoor units


Outdoor Unit: Protection, efficiency, and safety design considerations

The AC Input Protection circuit connects directly to the AC power line. This circuit may experience overcurrent and overvoltage conditions. Large surges of current can result from load faults on the power line. At the circuit's input, use a fast-acting fuse to help prevent significant damage to the indoor and outdoor units if a short circuit occurs in the system.

Consider these essential fuse requirements:

·       Current rating sized for the circuit,

·       Voltage rating based on the power line voltage,

  • Current interrupting rating,
  • Low internal resistance to minimize power loss,
  • Shock- and vibration-resistant construction, and
  • Component compliant with regional safety standards.

Users should be able to easily replace the fuse after it has interrupted a current overload. Utilize a fuseholder that allows easy fuse installation and replacement. Select a fuseholder with a minimum current rating of at least the fuse's rating. Also, the fuseholder should be compliant with regional safety standards.

Transients induced from lightning or voltage surges from activating and de-activating of high power-consuming devices can cause high voltage transients on an AC power line. A metal oxide varistor (MOV) is used to absorb transients, preventing them from reaching downstream circuitry. Select a MOV with:

  • A transient clamping voltage that prevents damage to downstream circuitry, 
  • Capacity to absorb a few 100 J of energy transients, and
  • Ability to absorb kA current pulses in compliance with IEC 61051. 

The Contactor circuit provides on/off control to the power supply of the outdoor fan and compressor motor. Ensure 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.

The Rectifier circuit performs the AC-DC conversion. High power rectifier diode modules with low forward voltage, low leakage current, and high reverse breakdown voltage can provide efficient rectification. A single two-diode module provides half-wave rectification, and two 2-diode modules can provide full-wave rectification.  

The Power Factor Correction circuit minimizes the total power drawn from the AC line by making the heat pump circuitry look as resistive as possible to reduce apparent power. One option to consider is a MOSFET and series diode integrated PFC boost device. This approach reduces component count and saves valuable printed circuit board (PCB) space compared with use of multiple discrete components.

The Inverter converts DC power to AC power to drive the variable-speed compressor and fan motors. Investigate using power IGBTs or IGBT modules to drive the compressor and gate drivers to control the IGBTs. These devices have both excellent thermal characteristics and an ultra-low package profile. Thermistor probes can be used to monitor the temperatures of the power semiconductors, motor windings, and refrigerant lines. Thermistors offer a small form factor and fast response time. To ensure high reliability, use thermistors that are hermetically sealed.

Due to the complexity and diversity of options, it is recommended to seek the assistance of knowledgeable power semiconductor applications engineers. For help with selecting the right components for the Rectifier, Power Factor Correction, and Inverter circuits, consult with the applications team at Littelfuse.

Indoor Unit: Protection, efficiency, and safety design considerations

The Auxiliary Power Supply provides power to the control electronics and the DC fan motor, pushing the conditioned air into the building. Use a fast-acting fuse and a MOV. Be sure to protect the sensitive downstream integrated circuits (ICs) with a transient voltage suppressor (TVS) diode. TVS diodes include these benefits for absorbing portions of transients that pass through the AC Input Protection circuit:

  • Protect sensitive electronic components with low clamping voltages,
  • Uni-directional or bi-directional TVS diode configurations, and 
  • Ultra-fast response to transients.

The Fan Motor Drive, like the outdoor unit, uses thermistor temperature sensors to monitor temperature of the DC fan motor coils, power semiconductors, and to sense the temperature of the refrigerant lines. Additionally, protect the motor drive circuit and the motor from transients with a TVS diode. 

For the Air Filter circuit, ensure the air filter is properly positioned to remove particulates from the air supply. Consider a reed-based position sensor with a magnetic actuator to ensure the air filter fully covers the air entrance duct. Look for hermetically sealed, magnetically operated contacts that require no standby power. Select the needed sensitivity for the design requirements.

The Wireless Interface circuit communicates system status and enables indoor temperature control using smartphones or tablet computers. Use TVS diodes to protect the sensitive wireless protocol ICs. Consider selecting 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. Find a TVS diode with low capacitance below 1 pF to avoid corruption of transmission and reception. 

In addition, use polymer ESD components to protect the data lines from ESD. 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.

Robust, efficient, and safe heat pump design should be a forethought, not an afterthought.

Designing a robust and reliable heat pump doesn't require a lot of components. However, it is essential to have well-documented circuit protection, power efficiency, and operational safety goals. When these objectives remain an afterthought in the design process, it results in significant re-design time and increased development and certification expenses. Be aware that some component manufacturers offer application engineering assistance. Consider utilizing a manufacturer's expertise to save valuable research, development, and design time. For example, a manufacturer's application engineers can assist with the following:

  • Selecting cost-effective components from a broad portfolio of protection, power control, and sensing technologies,
  • Guidance on meetingregulatory safety standards, while improving the design’s safety, reliability, and energy efficiency, and
  • Pre-compliance testing to prepare designers for product certification testing.

Heat pump designers achieving robustness, efficiency, and safety goals will have a competitive advantage.