The animation shows the refrigeration cycle of a heat pump. The four components (evaporator, compressor, condenser and expansion valve) are displayed with color coded temperature visualization and animated mass flow. The thermodynamic state is shown in a log p-h diagram or a T-s diagram.
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Description of the Animation
The left side of the animation shows the piping system of the heat pump. The refrigerant circulates through four components: the evaporator (at −5 °C), the compressor, the condenser (at 45 °C) and the expansion valve. The pipe color indicates the refrigerant temperature. Animated markers show the direction of the mass flow.
The right side displays the thermodynamic cycle in either a log p-h diagram (pressure vs. enthalpy) or a T-s diagram (temperature vs. entropy). The saturation dome of propane is drawn in both diagrams. The current state point moves along the cycle as the slider advances.
The four process steps are color coded and labeled in the legend: compression (1 → 2), condensation (2 → 3), throttling (3 → 4) and evaporation (4 → 1). The operating parameters are: superheating 6 K, subcooling 4 K and isentropic compressor efficiency 0.7.
Interactive Controls
The following controls are available:
- Cycle position (0–100 %): Moves the state point through the four steps of the refrigeration cycle
- Diagram selection: Switches between log p-h diagram and T-s diagram
- Pause / Play: Stops or resumes the cycle progression
How a Heat Pump Works
A heat pump transfers thermal energy from a cold source (e.g. outside air or ground) to a warm sink (e.g. a heating system). This transfer goes against the natural direction of heat flow and requires mechanical work, supplied by the compressor.
The process exploits the phase changes of a refrigerant. Evaporation at low pressure absorbs heat from the source. Condensation at high pressure releases heat to the sink. The compressor provides the pressure difference that makes these phase changes occur at different temperature levels.
The energy balance follows the first law of thermodynamics:
\[ Q_H = Q_C + W \]
The heat delivered to the sink (\( Q_H \)) equals the heat absorbed from the source (\( Q_C \)) plus the compressor work (\( W \)). A heat pump therefore delivers more heat than the electrical energy it consumes.
The Four Process Steps
1 → 2 Compression
The compressor draws in low pressure gas from the evaporator and compresses it to the condensation pressure. Both pressure and temperature rise. In an ideal (isentropic) compression, entropy remains constant. The isentropic efficiency \( \eta_s = 0.7 \) in this animation accounts for friction and internal losses.
In the log p-h diagram, the compression appears as a line from state 1 to state 2, moving upward and to the right. In the T-s diagram, an ideal compression would be a vertical line; the real compression curves to the right because of the entropy increase.
2 → 3 Condensation
The hot gas enters the condenser and releases heat to the heating system. The refrigerant first cools as a superheated gas (desuperheating), then condenses at constant pressure and temperature, and finally subcools by 4 K below the saturation temperature.
In the log p-h diagram, this step runs horizontally from right to left at the condensation pressure \( p_c \). In the T-s diagram, the condensation section lies along the upper part of the saturation dome.
3 → 4 Throttling
The liquid refrigerant passes through the expansion valve. The pressure drops from condensation pressure to evaporation pressure without any work being done. This is an isenthalpic process: the enthalpy before and after the valve is identical.
Part of the liquid evaporates, and the temperature drops to the evaporation temperature. In the log p-h diagram, throttling appears as a vertical line downward at constant enthalpy. In the T-s diagram, the entropy increases because the process is irreversible.
4 → 1 Evaporation
The cold, partially evaporated refrigerant enters the evaporator and absorbs heat from the environment at constant pressure. The temperature remains constant during the phase change. After complete evaporation, the gas is superheated by 6 K to protect the compressor from liquid droplets.
In the log p-h diagram, evaporation runs horizontally at the evaporation pressure \( p_0 \) from left to right. In the T-s diagram, the process follows the lower part of the saturation dome and curves upward during superheating.
Coefficient of Performance
The efficiency of a heat pump is expressed as the coefficient of performance (COP). It is the ratio of useful heat delivered to the work input:
\[ \text{COP} = \frac{Q_H}{W} \]
Since \( W = Q_H – Q_C \) (first law of thermodynamics), the COP is always greater than 1. A COP of 4 means that for every 1 kWh of electrical energy consumed, 4 kWh of heat are delivered.
For an ideal (Carnot) heat pump operating between temperatures \( T_C \) and \( T_H \), the maximum COP is:
\[ \text{COP}_{\text{Carnot}} = \frac{T_H}{T_H – T_C} \]
With the temperatures in this animation (\( T_C = 268.15 \) K, \( T_H = 318.15 \) K):
\[ \text{COP}_{\text{Carnot}} = \frac{318.15}{318.15 – 268.15} = \frac{318.15}{50} \approx 6.4 \]
Real heat pumps achieve lower values. The isentropic efficiency of 0.7 and the irreversibility of the throttling reduce the COP to values between 3 and 5 for typical operating conditions.
The COP depends on the temperature difference between source and sink. The smaller the difference, the higher the COP. Floor heating (low supply temperature) is therefore more efficient with a heat pump than radiators (high supply temperature).
The Refrigerant: Propane (R290)
The animation uses propane (R290) as the refrigerant. Propane is a natural refrigerant with a global warming potential (GWP) of 3. Synthetic refrigerants such as R134a or R410A have GWP values above 1000. The European F-Gas Regulation progressively restricts the use of high GWP refrigerants, making natural alternatives increasingly important.
Propane has a low boiling point (−42 °C at atmospheric pressure) and a critical temperature of 96.7 °C. These properties make it well suited for heat pump applications. The main disadvantage is its flammability (safety class A3), which requires specific safety measures in system design.
Heat Pumps in Practice
- Residential heating: Air source heat pumps extract heat from outside air, even at temperatures below 0 °C. Ground source heat pumps use the stable temperature of the soil through buried pipes or boreholes.
- Hot water production: Heat pumps can heat domestic water to 55–60 °C. Higher temperatures reduce the COP.
- Industrial processes: Large heat pumps provide process heat in food production, paper mills and chemical plants at temperatures up to 150 °C.
- Reversible operation: Many heat pumps can reverse the cycle and function as air conditioners in summer. The condenser becomes the evaporator and vice versa.
- Refrigeration: Every refrigerator uses the same cycle. The useful effect is the heat absorbed at the evaporator (cooling), not the heat rejected at the condenser.
Historical Background
The theoretical basis for the heat pump was laid by Sadi Carnot in 1824. His analysis of ideal heat engines implied that the process could be reversed: instead of converting heat into work, work could be used to transfer heat from cold to warm.
William Thomson (Lord Kelvin) made this idea explicit in 1852. He proposed using a reversed heat engine to heat buildings and calculated that such a device would deliver more heat than the energy consumed.
The first heat pump for building heating was built by Heinrich Zoelly in Zurich in 1938. It extracted heat from the Limmat river. During the oil crises of the 1970s, heat pumps gained renewed interest as an alternative to fossil fuels.
Today, heat pumps are a key technology for reducing emissions in the building sector. The European Union aims to install 60 million heat pumps by 2030. In Norway and Sweden, heat pumps already cover a large share of residential heating.
Further Resources
- Heat Pump – Wikipedia – Overview of heat pump technology and applications
- Coefficient of Performance – Wikipedia – Definition, derivation and typical values
- IEA Heat Pumps Programme – heatpumpingtechnologies.org – International research collaboration on heat pump technology
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Overview
| Title | Heat Pump |
| Target Audience | Teachers and Lecturers |
| Features | Fullscreen mode Lossless scaling Large screens and projectors supported |
| License | MIT |