Introduction: The Building Blocks of the Heat Pump

A heat pump consists of four main components that work together in a cycle:

1. Evaporator (heat exchanger for heat absorption)
2. Compressor (the heart)
3. Condenser (heat exchanger for heat release)
4. Expansion valve (pressure reduction)

In addition, there is the refrigerant, which circulates through all components. In this article, we take a closer look at each component.

The Heat Exchanger: Giving and Taking Heat

Without the heat exchanger, modern heating and cooling technology would be hard to imagine. This component fulfils the core task of heat absorption and release.

Operating Principle

In a heat exchanger, heat is exchanged between two media without the media directly touching.

Important: "Warm" always moves to "cold" – this is the second law of thermodynamics in action.

Heat transfer occurs mainly through:

• Conduction: Heat conduction through materials
• Convection: Heat transport by flowing media

Everyday Examples

Heat exchangers are encountered daily in many devices:

Device	Heat Absorption	Heat Release
Car radiator	Hot coolant	Passing air
Refrigerator	Interior	Back (grille)
Heat pump	Environment (air/ground)	Heating water

Types of Heat Exchangers

Tube Heat Exchangers

Structure:

• A tube with circulating medium
• Defined space around the tube with another medium
• Heat is transferred through the tube wall

Advantages:

Tube heat exchangers score particularly well for their simplicity:

Advantage	Explanation
Simple construction	Few components
Robust	Insensitive to pressure fluctuations
Easy to maintain	Easy to clean
Cost-effective	Inexpensive manufacture

Disadvantages:

The simplicity also brings limitations:

Disadvantage	Explanation
Lower efficiency	Smaller transfer area
More space required	More installation space needed

Plate Heat Exchangers

Structure:

• Multiple plates with small gaps
• Alternating warm and cold medium
• Counter-flow direction for maximum efficiency

Advantages:

The compact design offers decisive advantages:

Advantage	Explanation
High efficiency	Large surface area
Compact	Little space required
Flexibly expandable	Plates can be added

Disadvantages:

The higher complexity also has disadvantages:

Disadvantage	Explanation
Higher costs	More elaborate manufacturing
Pressure sensitive	Seals susceptible
Harder to clean	Many small channels

Use in Heat Pumps

Different heat exchanger types are used in heat pumps depending on position:

Position	Designation	Heat Exchanger Type
Input	Evaporator	Fin-tube or plate
Output	Condenser	Brazed plate heat exchanger

The Compressor: The Heart

This component is responsible for compressing the refrigerant. Through compression, the temperature rises to a usable level.

Operating Principle

1. Gaseous refrigerant is drawn in from the evaporator
2. The compressor mechanically compresses the gas
3. Pressure rises → temperature rises
4. The hot gas is passed on to the condenser

The compressor is the actual "pumping" in the heat pump!

Structure

A compressor consists of:

• Drive unit: Usually an electric motor
• Compression area: Impellers or pistons

Compressor Types

Scroll Compressor (Standard for Heat Pumps)

Operating principle:

• Two spiral-shaped elements
• One fixed, one movable
• Eccentric movement compresses the gas

Advantages:

Scroll compressors have become the standard for good reasons:

Advantage	Explanation
Very quiet	No jerky movements
High efficiency	Efficient compression
Long lifespan	Little wear
Constant delivery	Smooth operation

Inverter Compressor (Modern)

Combines the scroll compressor with an inverter:

• Motor speed is variable
• Adjusts output to demand
• No constant on/off switching → less wear

Advantages:

Variable speed brings decisive advantages:

Advantage	Explanation
Energy efficient	Only as much power as needed
Quiet	No start-up noises
Durable	Reduced stress
Precise control	Constant temperature

Other Compressor Types

Besides those mentioned, there are other compressor designs for special applications:

Type	Application
Reciprocating compressor	Large refrigeration systems
Rotary compressor	Small air conditioning units
Turbo compressor	Industrial systems

The Expansion Valve: The Counterpart

The expansion valve is the counterpart to the compressor. It regulates the return flow of refrigerant from the condenser to the evaporator.

Operating Principle

After heat release in the condenser, the refrigerant still has:

• Elevated pressure
• Elevated temperature

The expansion valve:

1. Reduces pressure through controlled release
2. Thereby temperature drops
3. The refrigerant is ready for the next cycle

Valve Types

Unregulated Expansion Valves

• Simple construction
• Fixed opening width
• For simple systems (e.g. refrigerators)

Regulated Expansion Valves

• Adjusts flow rate automatically
• Reacts to temperature and pressure
• Standard for heat pumps

Through regulation, the heating output can be precisely adjusted.

The Refrigerant: The Magic Fluid

Without the special properties of the refrigerant, heat pumps could not function.

What Makes a Refrigerant Special?

Refrigerants have special physical properties that make them ideal for this purpose:

Property	Significance
Low boiling point	Evaporates even at low temperatures
High heat capacity	Absorbs a lot of heat
Phase change	Changes efficiently between liquid and gaseous

Refrigerant vs. Coolant

Caution: These terms are often confused! A look at the differences shows clear distinctions:

	Refrigerant	Coolant
State	Changes (liquid ↔ gaseous)	Stays the same
Heat transfer	Through phase change	Only through flow
Application	Heat pumps, air conditioning	Engine cooling

Refrigerants Used

Natural Refrigerants

Natural refrigerants are more environmentally friendly but each has specific advantages and disadvantages:

Designation	Properties
Propane (R290)	Environmentally friendly, slightly flammable
CO2 (R744)	Non-flammable, high pressure
Ammonia (R717)	Very efficient, toxic

Synthetic Refrigerants

Synthetic refrigerants are increasingly being replaced by more environmentally friendly alternatives:

Designation	Status
R410A	Still permitted but phasing out
R32	Modern standard
R1234yf	Future technology

Banned Refrigerants

Due to environmental and safety regulations, the following are banned:

• CFCs (e.g. R11) – ozone layer destroyers
• HCFCs (e.g. R22) – greenhouse gas
• From 2025: Further HFCs with high GWP

GWP (Global Warming Potential): Measure of a substance's greenhouse potential.

The Interplay of All Components

        Evaporator (outside)
              │
              │ Gas (cold)
              ▼
        Compressor ←── Electricity
              │
              │ Gas (hot, high pressure)
              ▼
        Condenser (inside)
              │
              │ Liquid (warm)
              ▼
        Expansion valve
              │
              │ Liquid (cold, low pressure)
              ▼
        back to evaporator

The Energy Flow

1. Ambient heat (free) → Evaporator
2. Electricity → Compressor
3. Useful heat → Heating

The special feature: For 1 kWh of electricity, you get 3–5 kWh of heat!

Conclusion

Continue reading: In the next article Heat Pump Key Figures and Sizing, you will learn how to select the right heat pump for your house.

---

The Complete Article Series "Heat Pumps"

1. The Anti-Refrigerator: How Does a Heat Pump Work? – Fundamentals
2. The Components: Heat Exchanger, Compressor and Expansion Valve – You are here
3. Heat Pump Key Figures and Sizing – COP, SPF and more
4. Operating Modes: Monovalent, Bivalent and Hybrid – Operating modes
5. Heat Pump Types and the Dream Team with Solar Systems – Air-water, ground-source & solar

Sources

• Ökoloco: Heat Exchangers
• Wolf: Heat Pump Heat Exchanger
• Heizung.de: Scroll Compressor
• Energie-Experten: Compressor
• Heizung.de: Expansion Valve
• Wolf: Heat Pump Refrigerant