# A Review of Worldwide Strategies for Promoting High-Temperature Heat Pumps
## 3 Heat Pumps: Technological Principles, Challenges, and Potential  
### Physical Principle  
An HP is a P2H technology that transfers heat from one or more low-temperature sources (T\({}_{\text{source}}\)) to one or more high-temperature sinks (T\({}_{\text{sink}}\)) with the aid of an external energy source [27]. The working principle of an HP is schematically illustrated in Figure 6.  
Essentially, they are designed to move thermal energy against the natural heat gradient, absorbing heat from a cold reservoir and discharging it to a hot one [37]. HPs are generally characterized by high performance because they operate based on energy transfer rather than energy generation or conversion. Unlike boilers, which convert the chemical energy of fuel through combustion to produce thermal energy, they move existing heat from one place to another, making them more efficient than conventional  
Figure 6: Operating principle of an HP. Self-elaboration based on [27; 37]. boilers. Different types of HPs are categorized based on their energy input. In this context, we primarily focus on electrically driven vapor compression HPs, which are the most common type and use electrical energy to transfer a lower-temperature thermal source heat to a higher-temperature environment. An electrically driven HP uses a compressor to move a refrigerant through a refrigeration cycle and heat exchangers, one to extract heat from the source and the other to deliver heat to the sink. In buildings, the heat is distributed using forced air or hydronic systems, such as radiators or under-floor heating. Additionally, many HPs can provide space cooling in the summer, in addition to heating in the winter. In industrial applications, these devices are used to supply hot air, water, or steam, or to directly heat materials. Large-scale HPs used in commercial or industrial settings, or in district heating networks, require higher input temperatures than residential units. These higher temperatures can be sourced from waste heat generated by industrial processes, data centers, or wastewater, representing a further advantage of the use of these devices [37; 62; 63]. An energy and exergetic analysis is available in the Appendix A at the end of the manuscript for readers who wish to deepen their understanding of this aspect related to HPs. HPs provide more energy in the form of heat than the amount of energy input they consume [49]. For instance, a typical household HP has a COP of around four, meaning the energy output Q\({}_{\rm H}\) is four times the electrical energy input W. This makes modern HPs up to five times more energy efficient than gas boilers [63]. The external energy required to operate an HP depends on the temperature increase (T\({}_{\rm lift}\)) needed for the low-quality heat. The higher the value of T\({}_{\rm lift}\), the greater the amount of energy input W needed to achieve it, resulting in a lower COP [62]. The efficiency of an HP largely depends on its heat source. In winter, the ground and external water sources are usually warmer than the ambient air, so ground-source and water-source HPs use less electricity and achieve a higher COP compared to air-source HPs. This is especially true in cold climates, where defrosting the outdoor components of air-source HPs can consume additional energy, therefore lowering the COP. However, ground-source HPs are more expensive to install due to the need for an underground heat exchanger, such as a deep vertical borehole or a large network of pipes buried at least one meter underground. Connecting a water-source HP to a nearby river, groundwater, or wastewater source can also be costly. As a result, ground-source and water-source HPs are generally less common than air-source pumps. Globally, almost 85% of all HPs sold for buildings are air-source, as they are easier to install. Many of these are air-to-air units, although air-to-water (or hydronic) units are becoming more common in regions where heating is a priority [63].