Heat pumps – how do they work?

Heat pumps are gaining popularity in Poland due to increasing gas bills, prompting individuals to seek alternative methods for home heating. This raises the question: What exactly are heat pumps, and how can they be effectively utilized?

What is a heat pump?

A heat pump, in physical terms, operates as a heat machine transferring heat from a lower temperature area to one with a higher temperature. Employing a thermodynamic cycle opposite to that of a heat engine, the device facilitates the transfer of heat through a working medium. This medium absorbs heat at a low pressure in the evaporator, subsequently experiencing a pressure increase leading to a rise in temperature. The heat is released through condensation in the heat exchanger (condenser). Following this, the working medium undergoes expansion and returns to the evaporator to continue the cycle. The majority of these devices rely on electricity to perform the necessary work.

Heat pump operation cycle.

The main benefits of using heat pumps

As heat pumps are designed not to generate heat but to transfer it, their efficiency is measured by the ratio of energy obtained to the energy supplied, known as the Coefficient of Performance (COP). This efficiency can surpass 1, often ranging between 200% and 600%, and in some instances even reaching 1000%. This characteristic positions heat pumps as highly energy-efficient, cost-effective, and environmentally friendly alternatives compared to traditional electric heating systems.

Heat pumps find versatile applications, including heating and cooling spaces. In many cases, a single device can serve both functions by altering the circulation direction. This reversible feature is evident in certain air conditioning systems, showcasing the adaptability of heat pumps for diverse thermal needs.

Types of heat pumps

Air (ASHP – air source heat pump)

Air heat pumps function by transferring heat between exchangers—one located outside and the other inside the building. A fan draws in air, which is then utilized to directly warm the indoor air or heat water circulating through the building’s heating system. To reduce the indoor air temperature, the device’s circulation can be reversed as needed.

These heat pumps excel in cooling efficiency, boasting the highest Coefficient of Performance (COP) with a small temperature difference (COP=3÷4). Consequently, they are most effective in temperate climates. However, in extremely cold conditions, below -30°C, traditional electric heating with heating resistors proves more efficient.

For exhaust air (EAHP – exhaust air heat pump)

Certain air source heat pumps utilize exhaust air from heating systems, enabling the recovery of some heat before releasing the air into the atmosphere. However, these devices are marked by a low heat recovery coefficient and elevated operating costs.

Ground-source heat pump (GSHP)

Ground heat pumps, also known as geothermal heat pumps, harness heat from the soil or groundwater. Below a depth of 9 meters, the temperature remains relatively constant throughout the year. This feature contributes to the high efficiency of ground-source systems, typically ranging between 300-600%. However, a drawback is the higher installation cost compared to other types, primarily due to the need for drilling.

Hybrid

Hybrid heat pumps are devices that employ multiple techniques, such as extracting heat from both the air and the ground, to enhance their efficiency over the course of the day or the entire year. This category also encompasses heat pumps working in tandem with another heating device.

Integrated with solar installations

A portion of the energy required to operate heat pumps can be sourced from photovoltaic panels, rendering the system more environmentally friendly and decreasing home heating costs. However, optimizing the system’s operating conditions poses a challenge, as heat pumps and photovoltaic panels exhibit opposing efficiency trends, often influenced by factors such as the temperature of the working medium.

Absorbent

Absorption heat pumps operate without electricity; instead, they rely on energy from an alternative source, such as natural gas, propane, or water heated by solar or geothermal energy. In this system, ammonia serves as the working medium, being absorbed by water. The resulting mixture undergoes pressurization through pumping. To separate the mixture, it is heated, causing the ammonia to evaporate. This initiates the absorption cycle once again.

Primarily employed in industrial settings, these devices also find utility in sizable residential buildings. Their key advantage lies in their independence from electricity, making them adaptable to various heat sources.