Heiß – Kalt

Johannes Köbler

Ulrike Myrzik

Heat Pump in the Audi Q7 e-tron

Audi presents the world’s first heat pump in a plug-in hybrid model.
It delivers a new kind of efficiency in interior climate control.

Main component – A so-called scroll
compressorserves as an electrical compressor.
The compactcomponent forms the heart 
of the heat pump.

In the shadow of the large Technical Development building complex, technical building N 16 appears small and unimposing. It serves as one of five refrigeration centers that form the refrigeration network at the Audi plant in Ingolstadt, which extends across the entire northern section of the factory site. Per hour, up to 4,000 cubic meters of cold water flow through the network’s extensively insulated pipe work. It supplies the cooling ceiling and ventilation systems in many offices as well as the Paint Shop, its biggest consumer.

We have an appointment in N 16 with Dr. Klaus Straßer, Head of Simulation, Flow Technology and Development Thermal Management Electrified Vehicles. Straßer and his team developed the heat pump in the Audi Q7 e-tron 3.0 TDI quattro, which arrives at German dealerships at the start of 2016. The new technical components in the plug-in hybrid model and the large-scale refrigeration equipment make use of the same physical principle – mechanical power can be converted into a usable flow of heat via a circuit process incorporating the phases of compression, heat extraction, expansion and heat absorption.

In refrigeration center N 16, four refrigeration machines generate a combined output of 10 MW, for which its electric-drive compressors require around just 1.8 MW of power. When the network’s water reaches the return line in N 16, it is at a temperature of 12 degrees Celsius; when it flows back into the network after the cooling process, it has been cooled down to 6 degrees. “The principle in the car is similar, albeit somewhat more complex,” says Dr. Straßer. “Our heat pump gathers waste heat from the components in the electric drivetrain and raises it to a higher temperature level. This enables us to generate up to three kilowatts of heating power from one kilowatt of electrical power.”

The multi-source heat pump for the new Audi Q7 e-tron quattro offers a multitude of impressive capabilities. Function Team Leader Michael Schuster explains: “There were two main guidelines during the development process. Firstly, we want to use the heat pump to increase the range under electric power. Secondly, the customer has to enjoy at least the same comfort as in a conventional vehicle.”

We’re generating up to three kilowatts of
heating power from one kilowatt of electrical power.

Dr. Klaus Straßer
Head of Development Thermal Management Electrified Vehicles

Audi achieved both targets. Compared with a conventional air-conditioning system with an electric heater, the heat pump saves a significant amount of energy. In everyday use, it extends the electric range by around 15 percent, which is roughly seven kilometers in the Audi Q7 e-tron 3.0 TDI quattro. The customer gains even more range if he pre-conditions the interior prior to the start of the journey, while the car is still charging at the outlet.

In winter, the heat pump and the electric heater integrated into the system quickly heat the interior of the Audi Q7 e-tron 3.0 TDI quattro up to the temperature set by the driver. Afterward, the heat pump alone can maintain a comfortable interior temperature of around 22 degrees at an outside temperature of down to zero degrees Celsius. The heat pump demonstrates another of its strengths when it comes to keeping the windows clear of condensation during damp weather. First, it cools the air to remove the moisture and then heats it back up again without the need for an electric heater. Conventional heating systems in electric-drive vehicles have to use energy twice to do this.

“I’m proud of what my team has achieved,’ says Dr. Klaus Straßer. “With the heat pump in the Q7 e-tron quattro, we are presenting a key technology for electric driving and a world first at the same time, because this technology has never existed before in a plug-in hybrid.”

The Team

The first Audi with a heat pump was the R8 e-tron – the experience garnered by Audi with the electric high-performance sports car flowed into the new Q7 e-tron quattro project. Its development work was carried out within an interdisciplinary team in which specialists from Simulation, Test Control and Engineering worked closely together.

The core components – the electric air-conditioning compressor, the indirect condenser, the refrigeration circuit valve, the pressure temperature sensors and the TME control unit are conceived as a Group module. This means that these parts, which were developed under the leadership of Martin Kronbichler, are suitable for application across multiple brands and models. The software, too, which was created by the team under Thomas Kobs, is an in-house development. The specific expertise acquired here is fully protected.

From left:
Dr. Klaus Straßer (Manager), Christian Stippler (Test Engineer), Bert Brandes (Coordination Component Development), Erwin Sander (Engineering Refrigerant Circuit), Michael Schuster (Function Team Leader), Martin Kronbichler (Head of System and Component Development), Andreas Djermester (Control and Application), Frank Meller (Cooling Layout, Simulation).

Not pictured here:
Thomas Kobs (Head of Function Development), Thomas Wegele (Test Engineer), Stephen Rost (Development Engineer Air-Conditioning Compressor) and Tim Groke (Engineering Refrigerant Circuit).

200 Switching States

As an integral part of the thermal management in the Audi Q7 e-tron quattro, the heat pump is connected to three refrigerant circuits in which temperatures can vary from -30 to +110 degrees Celsius. The high-temperature circuit supplies the 3.0 TDI, its ancillaries and the automatic transmission. The low-temperature circuit cools the electric motor and its power electronics. Another circuit cools the battery and the charging device and can be coupled to the air-conditioning system and the electric motor’s LT circuit.

The system uses a wide variety of sensors, valves and pumps to link the circuits with one another in the best manner possible for comfort, vehicle performance and efficiency. The TME control unit communicates with other control units (see image) to select the best from more than 200 possible switching states in accordance with requirements.

Control unit
thermal management
Control unit battery
Control unit
power electronics
Control unit
air-conditioning controls
Engine control unit

Extensively networked – Important control units interact with the heat pump.

Electric motor
Power electronics
High-voltage battery
Charging device

The cooling system – The main elements of the electric drivetrain.

Heating and Drying

The three most important operating states for the heat pump are heat mode, cool mode and reheat mode for dehumidifying the air.

Heat mode: To heat the interior, the electric air-conditioning compressor compresses gaseous refrigerant, substantially raising its temperature. The indirect condenser, a compact plate heat exchanger, transfers the heat from the hot gas to the interior heating circuit, causing the gas to cool down and condense.

The now liquid refrigerant is expanded via an electric expansion valve, before evaporating in the chiller, which is another plate heat exchanger. In the process, the chiller draws heat from the low-temperature circuit of the electric drivetrain, lowering its temperature by around three to five degrees Celsius. Via this process, the heat pump makes the waste heat from the electric drivetrain usable for heating the interior.

Reheat mode: The air is initially cooled and thereby dehumidified, before then being heated up again. The heat absorbed by the refrigerant when the air is cooled is then made available for the interior by the indirect condenser. If required, the system can also access the waste heat from the electric drivetrain via the chiller.

Cool mode: Unlike in heat mode, the hot, gaseous refrigerant is not liquefied in the indirect heat exchanger, but instead in the large condenser at the front of the vehicle. It is expanded and evaporated in the airconditioning system’s evaporator, thus cooling the interior.

Reheat mode

Heat mode

High-voltage PTC device serves as
electric heater
Heat exchanger in the air-conditioning
device for interior heating
Chiller as a heat exchanger between
LT circuit and heat pump
Valve block
Heat pump’s indirect condenser
Electric-drive scroll compressor
Condenser in front of the main water cooler