A heat pump serves as an alternative to gas or electric boilers, relying on the production of heat. Unlike boilers, a heat pump doesn’t generate heat but extracts energy from the air, water, or ground.
Heat pumps and electric boilers both draw power from the mains electricity supply, yet heat pumps exhibit higher efficiency. This efficiency is contingent upon the conversion efficiency, measured by the Coefficient of Performance (COP), of a specific heat pump. The COP represents the ratio of heat energy received to the electricity consumed, particularly in the operation of the pump’s compressor unit. Notably, a heat pump consumes 3-6 times less electricity than an electric boiler with the same output.
Even in challenging conditions, such as an outside air temperature of -25°C, heat pumps excel in providing heating. Simultaneously, they achieve a high COP – generating 2-5 kW of heat or cold (depending on the type of heat pump) per 1 kW of electricity. This starkly contrasts the lower efficiency of gas and electric boilers.
Heat Pump Use Potential
The economic (rising energy costs) and environmental (effects of climate change) aspects of heat pumps should also be noted when discussing heat pumps. Heat pumps make it possible to utilize renewable heat resources such as geothermal, solar thermal energy and recovered heat from the urban environment. In addition, heat pumps maximize the decarbonization potential of renewable electricity sources (such as wind and solar) by converting them into renewable heat. In combination with thermal storage and electric boilers, heat pumps provide flexibility and security to the building life-support system, offering daily, weekly, and seasonal flexibility.
Today, the capacity of large heat pumps in heating and cooling networks within the EU alone stands at up to 2.5 GW, representing about 1% of the total capacity. This modest percentage conceals significant growth potential, primarily driven by the rapid deployment of renewable electricity in the EU (Figure 2). Based on outlined investment plans, there is an anticipated surge in the installed capacity of large heat pumps by at least 80% by 2030, leading to profound changes in the generation portfolio and network growth.
A number of realized projects confirm the advantages of using heat pumps. For instance, consider the joint project between Fjernvarme Fyn and Meta, which harnesses waste heat from a data center. In this innovative initiative, the surplus heat from a large heat pump is incorporated into the district heating system, resulting in the production of 160,000 MWh. This output is equivalent to heating 11,000 households. The electrical power consumed for this operation is 42 MW.
Thermodynamic Cycle of a Heat Pump
Now that we have discussed what heat pumps are and their potential impact, let’s dive into some theory. The main components of a heat pump are the evaporator, compressor, condenser, and expansion valve. The cycle uses refrigerant, a special substance that extracts heat (air, ground, water, etc.) in the evaporator.
It is then compressed in the compressor, causing its temperature to rise. According to the Second Law of Thermodynamics, “Heat cannot transfer by itself from a colder body to a warmer one” (Rudolf Clausius, 1850). However, in refrigeration machines, such as heat pumps, heat is taken from less heated bodies and transferred to more heated ones, like the environment. Importantly, there is no violation of the Second Law of Thermodynamics in this process because it doesn’t occur spontaneously. Instead, it requires the expenditure of mechanical energy consumed by the electric motor to drive the compressor, which compresses the working fluid after the evaporator. That is, the process of “taking” heat from the cold source is accompanied by the compensating action of the compressor. As working fluids that aid in realizing the necessary processes, substances such as air, water, and so-called refrigerants—substances with a low boiling point under normal conditions—can be employed. The following factors are considered when selecting a refrigerant for the designed system:
- Thermodynamic cycle efficiency
- System size
- Hazards posed by the refrigerant (flammability and toxicity)
- Environmental friendliness
For example, a good alternative for small and medium-sized boiler plants is a high-temperature heat pump (air to water) using the natural refrigerant CO2. It is based on the CO2 transcritical cycle. This heat pump is characterized by low operating costs and high energy efficiency (COP reaches 5 or more), which is 4.1 times higher than that of an electric boiler, 3.1 times higher than that of an oil boiler, and 1.9 times higher than that of a coal and gas boiler.
CO2, being a natural refrigerant, poses no harm to the environment or the ozone layer, with a global warming potential (GWP) of 1. Recognized by the United Nations Environment Programme (UNEP), CO2 is recommended for HVAC equipment. Carbon dioxide emissions from a heat pump are approximately 34% lower than those from a coal-fired boiler.
Heat pumps are classified into various groups based on energy transfer methods:
- Compression Heat Pumps: Employ the compression-expansion cycle of the coolant, ensuring efficient thermal energy consumption/dissipation. These pumps are known for their operational ease and high efficiency, contributing to their widespread popularity.
- Absorption Heat Pumps: Utilize a combination of absorber-cooler, representing a new generation of high-performance devices.
Calculation of Heat Pump Cycles
Now, let’s explore the vapor compression cycle, widely utilized in various applications. The preliminary calculation of heat pump cycles is usually performed with a number of assumptions. Common mistakes at this initial stage include incorrectly considering the hydraulic resistance of the heat exchange equipment and using a constant compressor capacity (independent of the operating mode), etc. These assumptions critically affect the change of parameters important for the consumer and lead to significant errors.
Therefore, it is prudent to adopt a comprehensive, integrated approach to engineering design problems. This approach enables engineers to evaluate the impact of each component and accurately analyze the interaction of these components. Cycle evaluation should encompass the precise design of compressor units, the design and engineering of heat exchangers with consideration of heat transfer, and an evaluation of the performance of the components under consideration throughout the cycle. This approach to heat pump design represents one possible method of reducing development time, costs, and engineering errors.
An integrated heat pump design approach can be realized in the AxSTREAM Platform:
- Cycle and heat exchanger simulation can be conducted through system-level modeling, utilizing 0D thermodynamic and 1D thermal-fluid network software. This enables the design and analysis of cycles for various energy systems, both new and existing, taking into account operating conditions
- Detailed modeling of heat exchangers involves obtaining boundary conditions (temperatures, flow rates, and pressures) for the fluids on each side from a preliminary thermodynamic cycle calculation. Further obtained geometrical parameters allow their use in modeling hydraulic resistances on the pipe and shell sides, assessing pressure drop between inlet and outlet flow, and estimating thermal parameters such as wall temperature, heat fluxes, and heat transfer coefficients using the 1D thermal-fluid network approach.
- The design, selection, and calculation of compressors can be performed in AxSTREAM using its flow path modules. AxSTREAM is a fully integrated and optimized solution that covers the entire turbomachinery design process along with the development of related systems.
In the pre-system design process, it is necessary to integrate the various tools for the design/modeling of the thermodynamics and thermal-hydraulic networks of each specific heat pump component or subsystem into a single iterative process.
Successful completion of the design of a heat pump requires a complex and accurate integrated approach. This approach enables the evaluation of the parameters of the main thermodynamic cycle and its components in off-design modes. An analysis can be conducted as a function of ambient temperature to ensure a comfortable temperature in the building.
It is important to note that utilizing a design approach that incorporates 1D and 0D solvers provides the ability to perform customized installations and quickly evaluate all necessary parameters within a single design framework. This approach impacts the installation by allowing for accurate heat exchanger thermal areas, consideration of pressure drop values, and optimal performance of the heat exchange equipment.
If you’re interested in learning more about heat pump design strategies, join us for our next webinar on Holistic Heat Pump Design. Learn more and register here: https://www.softinway.com/education/webinars/holistic-heat-pump-design/