Heat pumps are considered a key technology in the transition to sustainable heating and domestic hot water production in buildings. They work by extracting heat from a low-temperature source (through the evaporation of a refrigerant) and transferring it to a high-temperature reservoir (through condensation), using electricity to power a compressor. For more details, see the previous blog post: "Heat Pumps and Cooling Circuits." For simplicity, reversible heat pumps are not considered in this discussion.

One of the most widely adopted systems in buildings is the air source heat pump, which uses a finned heat exchanger to capture heat from outside.

1. What is the main problem with air source heat pumps?

The performance of an air source heat pump is highly influenced by variations in outdoor air temperature and humidity, both seasonally and daily.

In particular, performance tends to decline during periods of low outside temperatures, as these conditions reduce the evaporation temperature of the refrigerant. This is especially critical during winter mornings, when outside temperatures are typically lower and heating demand is higher.

Furthermore, a combination of cold outside temperatures and high relative humidity can lead to ice formation between the evaporator fins. Ice acts as a thermal insulator, hindering heat transfer and reducing evaporator efficiency and, consequently, the performance of the air-source heat pump. To maintain high evaporator efficiency, defrost cycles are necessary, which increase energy consumption and temporarily interrupt heat delivery to the user.

To mitigate the limitations imposed by outside air conditions on the performance of air source heat pumps, one possible solution is to use alternative renewable heat sources, such as ground or solar radiation.

2. Ground Source Heat Pumps

Geothermal heat pumps use the ground as a low-temperature heat source through buried heat exchangers. As a result, they are much less affected by variations in outside air temperature, as the ground temperature remains virtually constant throughout the day and seasons, especially at greater depths. Thus, compared to air-source heat pumps, geothermal systems can provide more stable and higher-performance heating. The heat exchanger can be installed in vertical boreholes or horizontal trenches. Horizontal installations require a larger area of ​​land but are only a few meters deep, while vertical installations require less surface area but involve drilling to greater depths.

Advantages:

• Superior performance over air source heat pumps, especially in cold climates.

• Lower operating costs.

• No ice formation problems on the heat exchanger.

• No noisy outdoor unit.

• It can provide cooling in summer with reverse cycle operation.

Disadvantages:

• Requires a large area for the heat exchanger.

• Complex and time-consuming installation.

• High installation cost.

• Long payback period, typically exceeding 15 years compared to air source heat pumps.

3. Solar Source Heat Pumps

Solar-source heat pumps harness heat from solar radiation and, to a lesser extent, from air temperature, through solar thermal collectors. An innovative alternative to thermal collectors are photovoltaic-thermal (PV-T) collectors, which can simultaneously generate electrical and thermal energy in the same space. Although PV-T collectors have lower thermal efficiency than solar thermal collectors, the cooling effect of the working fluid on the PV cells helps improve their electrical output.

Advantages:

• Better performance than air source heat pumps, especially on sunny days.

• Lower operating costs.

• No ice formation problems on the heat exchanger.

• No noisy outdoor unit.

Disadvantages:

• Reduced performance on cloudy days, at night, or at high latitudes.

• Need for a large area to install solar panels.

• Complex and time-consuming installation.

• High installation cost.

• In summer, it is necessary to manage the temperature to avoid overheating of the collectors.

4. How to design a heat pump that uses a renewable source?

When designing a geothermal or solar heat pump, two main system configurations are possible: indirect expansion and direct expansion.

a) Indirect expansion

The refrigerant does not circulate directly in the heat exchanger that absorbs heat from the renewable source. Instead, a secondary fluid (typically a mixture of water and glycol) circulates in the heat exchanger, absorbing the heat and transferring it to the refrigerant through an intermediate heat exchanger. This configuration is widely used and facilitates system management—for example, it allows the integration of thermal storage tanks to improve flexibility. However, it causes heat losses and entails higher installation costs due to additional components.

b) Direct expansion heat pump

The refrigerant circulates directly in the heat exchanger in contact with the renewable source, absorbing heat and evaporating. Compared to indirect systems, this configuration reduces installation costs, improves energy performance by avoiding intermediate losses, reduces the risk of corrosion and freezing due to the secondary fluid, and reduces operating costs. However, direct expansion systems require precise refrigerant charge management and careful design to ensure safe and efficient operation.

5. Multi-source heat pump

A promising solution to overcome the limitations of single-source systems is the use of multi-source heat pumps, which can exploit different low-temperature energy sources through separate evaporators, thus improving overall system performance. The most common dual configurations are air-to-ground or solar-to-air, where the sources are used alternately. However, to effectively switch between sources and always operate with the most advantageous one, the system must include an intelligent controller capable of continuously monitoring and predicting heat pump performance in response to dynamic environmental and operating conditions. Even so, alternating operation does not always achieve the maximum potential performance of both sources. An alternative approach is the simultaneous use of both sources, integrating two evaporators into the refrigerant circuit. This configuration can be implemented in two main ways, based on the evaporator arrangement:

• Parallel configuration: the refrigerant mass flow rate is divided between the two evaporators.

• Series configuration: the refrigerant passes sequentially through both evaporators, one after the other.

The simultaneous use of two heat sources increases the system's flexibility and versatility. Even a limited area of ​​geothermal or solar collectors (and therefore a reduced investment) can significantly improve performance when combined with an air-source system, making multi-source heat pumps a viable solution for increasing efficiency in environments with limited or variable space.

6 Conclusions

The main challenge in developing increasingly high-performance heat pumps will be harnessing renewable heat sources such as solar and geothermal energy, seeking a balance between system complexity and versatility, installation and maintenance costs, and control strategies. Research continues, and hopefully, we'll see an increasing number of renewable heat pumps on the market.

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References:

• Riccardo Conte, Emanuele Zanetti, Marco Tancon, Marco Azzolin, Sergio Girotto, Davide Del Col. The advantage of running a direct expansion CO₂ heat pump with solar-and-air simultaneous heat sources: experimental and numerical investigation . Applied Energy, 2024. Vol. 369, page 123478. DOI

• Riccardo Conte, Marco Tancon, Mohammad Mozafarivanani, Emanuele Zanetti, Marco Azzolin, Davide Del Col. Investigation on a direct expansion multisource carbon dioxide heat pump to maximize the use of renewable energy sources . Applied Thermal Engineering, 2025. Vol. 274, page 126533. DOI