11.2 Heat pump systems

Heat pump systems: Working principle and applications

A heat pump transfers thermal energy from a low-temperature source to a higher-temperature sink by running a refrigeration cycle. Instead of generating heat directly (like a furnace or electric resistance heater), it moves existing heat, which is why it can deliver several units of thermal energy for every unit of electrical work consumed. That core distinction is what makes heat pumps so important in modern thermodynamics and building energy systems.

System components and their functions

A heat pump uses the same four core components as a standard vapor-compression refrigeration cycle:

• Compressor: Raises the pressure and temperature of the refrigerant vapor. This is where the work input enters the cycle.
• Condenser: The high-pressure, high-temperature refrigerant rejects heat to the heated space (or hot water supply). The refrigerant condenses from vapor to liquid as it releases energy.
• Expansion valve (throttling device): Drops the refrigerant pressure and temperature without extracting work. The refrigerant becomes a low-quality, two-phase mixture.
• Evaporator: The low-pressure refrigerant absorbs heat from the low-temperature source (outdoor air, ground, or water body) and evaporates back into a vapor, completing the cycle.

The key difference from a refrigeration cycle is which heat exchanger you care about. In refrigeration, the useful effect is heat absorption at the evaporator. In a heat pump, the useful effect is heat rejection at the condenser.

Versatility in heating and cooling applications

Heat pumps can switch between heating and cooling by reversing the refrigerant flow direction (using a reversing valve). In heating mode, the outdoor coil acts as the evaporator and the indoor coil acts as the condenser. In cooling mode, those roles flip, and the system operates like a conventional air conditioner.

Common heat source classifications:

• Air-source heat pumps (ASHP): Extract heat from ambient outdoor air. Most common and least expensive to install, but performance drops as outdoor temperature falls.
• Ground-source (geothermal) heat pumps (GSHP): Extract heat from the ground or groundwater via buried loops. Ground temperatures are more stable year-round, so these maintain higher efficiency in cold climates.
• Water-source heat pumps: Extract heat from a nearby lake, river, or ocean. Performance depends on water temperature and availability.

Typical applications include residential and commercial space heating/cooling, domestic hot water production, and industrial processes that need simultaneous heating and cooling (e.g., food processing, chemical plants).

Heat pump vs traditional systems: Performance comparison

Energy efficiency advantages

A conventional electric resistance heater converts 1 kW of electricity into 1 kW of heat (COP = 1). A fuel-fired furnace is even lower when you account for combustion losses. A heat pump, by contrast, moves heat from a low-temperature reservoir rather than creating it, so it can deliver 3–5 kW of heat per 1 kW of electricity consumed under favorable conditions.

This advantage depends heavily on the temperature difference between source and sink:

• Smaller temperature lift (mild climate, moderate indoor setpoint) → higher COP
• Larger temperature lift (very cold outdoor air, high supply temperature) → lower COP

In cooling mode, a heat pump operates identically to a standard air conditioning system, so you get both functions from a single piece of equipment.

Cost considerations and system performance factors

Heat pumps typically cost more upfront than a furnace or baseboard heater, but lower operating costs can offset that over time. Payback period depends on local climate, electricity and fuel prices, and how many hours per year the system runs.

Several factors affect real-world performance:

• Climate: In very cold regions (below about $-10\,°\text{C}$), air-source heat pump capacity and COP drop significantly. Auxiliary or backup heating (electric resistance or fuel-fired) may be needed during extreme cold snaps.
• Building insulation: A well-insulated building has a smaller heating load, so the heat pump runs at part load more often and maintains a higher seasonal average COP.
• System sizing: An oversized heat pump short-cycles (turns on and off frequently), reducing efficiency and equipment life. An undersized unit can't meet the load on peak days.

Heat pump efficiency: COP and heating capacity

Coefficient of Performance (COP)

The COP of a heat pump is defined as the ratio of useful heating output to the required work input:

$\text{COP}_{\text{HP}} = \frac{Q_H}{W_{\text{net,in}}}$

where $Q_H$ is the heat delivered to the warm space and $W_{\text{net,in}}$ is the net work input (compressor power). A COP of 3 means the system delivers 3 kJ of heat for every 1 kJ of work consumed. COP is always greater than 1 for a functioning heat pump, which is the whole reason heat pumps are attractive compared to direct electric heating.

From an energy balance on the cycle:

$Q_H = Q_L + W_{\text{net,in}}$

so the COP can also be written as:

$\text{COP}_{\text{HP}} = \frac{Q_H}{Q_H - Q_L}$

The Carnot COP gives the theoretical upper limit for a heat pump operating between two thermal reservoirs:

$\text{COP}_{\text{Carnot,HP}} = \frac{T_H}{T_H - T_L}$

where $T_H$ and $T_L$ are the absolute temperatures (in Kelvin or Rankine) of the high- and low-temperature reservoirs, respectively. Notice that as $T_L$ approaches $T_H$, the Carnot COP goes to infinity, confirming that smaller temperature lifts yield higher theoretical efficiency.

Also note the useful relationship between heat pump and refrigerator Carnot COPs:

$\text{COP}_{\text{HP}} = \text{COP}_{\text{R}} + 1$

This follows directly from the energy balance and is a handy check on exam problems.

Heating capacity and influencing factors

Heating capacity is the maximum rate of heat delivery ($\dot{Q}_H$) under specified operating conditions, typically expressed in kW or BTU/h. It depends on compressor displacement, heat exchanger surface area, refrigerant mass flow rate, and the thermodynamic properties of the refrigerant.

Factors that reduce both COP and heating capacity in practice:

• Larger source-to-sink temperature difference: Forces the compressor to work harder across a bigger pressure ratio.
• Refrigerant choice: Different refrigerants have different saturation pressures, latent heats, and specific volumes, all of which affect cycle performance.
• Component irreversibilities: Real compressors have isentropic efficiencies well below 100%, there are pressure drops through heat exchangers and piping, and heat transfer across finite temperature differences adds entropy. All of these push the actual COP below the Carnot value.

Proper component selection, regular maintenance (clean coils, correct refrigerant charge), and good system design help minimize these losses.

Heat pumps: Environmental benefits and challenges

Reduced greenhouse gas emissions and decarbonization

Because heat pumps run on electricity rather than burning fuel on-site, they can significantly cut greenhouse gas emissions, especially when the electricity grid includes a large share of renewables (solar, wind, hydro). As grids continue to decarbonize, the emissions advantage of heat pumps grows automatically without any change to the equipment itself.

Many countries with aggressive climate targets view heat pumps as a critical technology for reducing building-sector emissions. Policy tools like rebates, carbon taxes, and building codes that restrict new fossil-fuel heating installations are accelerating adoption.

Indoor air quality improvement and infrastructure challenges

Heat pumps eliminate on-site combustion, which removes the risk of indoor pollutants like carbon monoxide, nitrogen oxides, and particulate matter that come from gas furnaces or wood stoves.

However, the environmental benefit depends on the carbon intensity of the local grid. In a region powered mostly by coal, the upstream emissions from electricity generation can offset some of the gains. A full lifecycle comparison (sometimes called a "source energy" analysis) is needed to evaluate net benefit in any specific location.

Widespread heat pump adoption also raises infrastructure concerns. Large-scale electrification of heating increases peak electricity demand, potentially straining grid capacity. This may require investment in grid reinforcement, demand-response programs, and energy storage.

Refrigerant management and end-of-life considerations

Most heat pumps use synthetic refrigerants, and some of these (particularly hydrofluorocarbons, or HFCs) have very high global warming potential (GWP). Even small leaks during operation or disposal can contribute meaningfully to climate change.

International regulations are driving a transition to lower-GWP alternatives:

• The Kigali Amendment to the Montreal Protocol commits signatory countries to phasing down HFC production and consumption.
• Newer systems increasingly use refrigerants like $\text{R-32}$, $\text{R-290}$ (propane), or $\text{CO}_2$ ($\text{R-744}$), which have much lower GWP values.

At end of life, proper refrigerant recovery and recycling are essential. Regulations require technicians to capture refrigerant before decommissioning equipment. Designing heat pumps for easier disassembly also helps recover valuable materials and reduces environmental impact from disposal.