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The advantage of using renewable multi-source heat pumps

Heat pumps are considered the 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 (via refrigerant evaporation) and transferring it to a high-temperature sink (via condensation), using electricity to power a compressor. For more details, see the previous blog post: “Heat pumps and refrigerant circuits“. For the sake of 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 coil heat exchanger to draw heat from outdoors. 

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

 

The performance of an air-source heat pump is highly affected by fluctuations in outdoor air temperature and humidity, both across seasons and within daily cycles. 

In particular, performance tends to decline during periods of low outdoor temperatures, as these conditions lower the refrigerant evaporation temperature. This is especially critical during winter mornings, when outdoor temperatures are typically at their lowest and heating demand is at its highest.

Furthermore, a combination of cold outdoor temperatures and high relative humidity can lead to frost formation between the evaporator fins. Frost acts as a thermal insulator, hindering heat transfer and reducing evaporator efficiency, and subsequently 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 supply to the user.

To mitigate the limitations imposed by outdoor air conditions on air-source heat pump performance, one possible solution is to exploit alternative renewable heat sources, such as the ground or solar irradiance. 

2. Ground-source heat pumps

 

Ground-source heat pumps use the earth as a low-temperature heat source, through ground heat exchangers. As a result, they are much less affected by variations in outdoor air temperature, as soil temperature remains nearly constant throughout day-night cycles and across seasons, especially at greater depths. Therefore, compared to air-source heat pumps, ground-source systems can provide more stable heating and higher performance. The ground heat exchanger can be installed either in vertical boreholes or in horizontal trenches. Horizontal installations require a larger ground surface area but only a few metres of depth, while vertical installations require less surface area but involve drilling to greater depths. 

Pros

  • Higher performance than air-source heat pumps, especially in colder climates.
  • Lower operating cost.
  • No defrosting problems in the heat exchanger.
  • No noisy outdoor unit.
  • Can provide cooling in summer in reverse-cycle operation.

Cons

  • Requires a large surface area for the ground heat exchanger.
  • Complex and time-consuming installation.
  • High installation cost.
  • Long payback period, typically more than 15 years compared to air-source heat pumps.

 

3. Solar-source heat pump

 

Solar-source heat pumps exploit heat from solar irradiance and to a lesser extent from air temperature, through solar thermal collectors. An innovative alternative to solar thermal collectors is the use of 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.

Pros

  • Higher performance than air-source heat pumps, especially on sunny days.
  • Lower operating cost.
  • No defrosting problems in the heat exchanger.
  • No noisy outdoor unit.

Cons

  • Reduced performance during cloudy days or at night, or at high latitudes.
  • Large surface area required for solar panels.
  • Complex and time-consuming installation.
  • High installation cost.
  • In summer, temperature management is necessary to avoid overheating of the collectors.

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

 

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

a) Indirect expansion

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

b) Direct expansion heat pump

The refrigerant flows directly through 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 heat exchange losses, lowers the risk of corrosion and freezing due to the secondary fluid, and decreases 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

 

One 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 distinct evaporators, thereby enhancing overall system performance. The most common dual-source configurations are air-ground or solar-air systems, where the sources are used alternately. However, to effectively switch between sources and always operate using the most advantageous one, the system must include a smart controller capable of continuously monitoring and predicting heat pump performance in response to dynamic environmental and operational conditions. Despite this, alternating operation does not always achieve the maximum potential performance of the two sources. An alternative approach is the simultaneous use of both sources by integrating two evaporators into the refrigerant circuit. This setup can be implemented in two main configurations based on the evaporator arrangement: 

  • parallel configuration: the refrigerant mass flow rate is split between the two evaporators. 
  • series configuration: the refrigerant passes sequentially through both evaporators, one after the other.

Simultaneous use of two heat sources increases system flexibility and design versatility. Even a limited ground or solar collector area (and thus a lower cost of investment) can significantly improve performance when combined with an air-source system, making multi-source heat pumps a viable solution for boosting efficiency in space-constrained or variable environments.

6. Conclusions

 

The main challenge in developing increasingly high-performance heat pumps will be harnessing renewable heat sources such as solar and geothermal energy, while seeking a balance between system complexity and versatility, installation and maintenance costs, and control strategies. Research is ongoing and hopefully we will see more and more heat pumps with renewable sources on the market.
 

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: https://doi.org/10.1016/j.apenergy.2024.123478 

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: https://doi.org/10.1016/j.applthermaleng.2025.126533

 

topic: ComPubBuilding topic: HVAC topic: Residential
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