The water-to-water heat pump has become a cornerstone technology for operators seeking to decarbonize heating and cooling systems while maintaining high efficiency, reliability and long-term cost stability.
As governments, utilities and private developers accelerate their transition toward net-zero carbon targets, demand is growing for technologies capable of harnessing renewable and low-grade energy sources. Among them, water-to-water heat pumps—including seawater, groundwater and wastewater-based systems—stand out as one of the most mature and effective solutions available today.
Their relevance is particularly strong in district heating and cooling networks, industrial facilities, data centers and large commercial buildings, especially in regions with access to stable water sources.
What is a Water-to-Water Heat Pump?
A water-to-water heat pump is a type of heat pump that transfers thermal energy between two water circuits:
-
A water-based heat source
-
A water-based heat sink or distribution system
Unlike air-source heat pumps, which exchange heat with ambient air, water-to-water heat pumps rely on the stable and predictable temperatures of water bodies, resulting in higher efficiency and more consistent performance throughout the year.
Typical water sources include:
-
Groundwater
-
Rivers, lakes and seawater
-
Industrial process water
-
Cooling water from industrial facilities
-
Sewage and wastewater
Because water temperatures fluctuate far less than air temperatures, these systems achieve higher seasonal efficiency and are particularly suitable for large-scale and mission-critical applications.
How Does a Water-to-Water Heat Pump Work?
A water-to-water heat pump operates on the same thermodynamic principles as other vapor-compression heat pumps, but with water acting as both the heat source and the heat sink.
Main components
The system consists of three core components:
1. Evaporator
Heat is extracted from the source water as a refrigerant circulates through the evaporator coils. The refrigerant absorbs thermal energy and evaporates, cooling the source water in the process.
2. Compressor
The vaporized refrigerant is compressed, increasing its temperature and pressure.
3. Condenser
The high-temperature refrigerant releases its heat to a second water loop, which distributes the heat for space heating, industrial processes or district heating networks.
The refrigerant then condenses back into a liquid state and the cycle repeats.
Cooling mode
When cooling is required, the cycle is reversed: heat is removed from the building or process and rejected into the water source.
What Types of Water-to-Water Heat Pumps Exist?
There are two main system configurations, each suited to different site conditions and regulatory environments.
Open-Loop Water-to-Water Heat Pumps
Open-loop systems extract water directly from a natural source (such as groundwater or seawater), pass it through a heat exchanger, and return it to the environment.
Key characteristics
-
No antifreeze required
-
High heat transfer efficiency
-
Large water volumes handled
Considerations
-
Requires reliable water availability
-
Higher operational and maintenance requirements
-
Careful design needed to avoid freezing or environmental impact
Open-loop systems are common in coastal regions, river-adjacent developments and large district energy projects.
Closed-Loop Water-to-Water Heat Pumps
Closed-loop systems circulate a sealed fluid (often water with antifreeze) through submerged or buried piping.
Key characteristics
-
No direct contact with the water source
-
Lower environmental and permitting constraints
-
Slightly lower efficiency than open-loop systems
These systems are often preferred for lakes, ponds or regulated environments where direct water extraction is limited.
How efficient are Water-to-Water Heat Pumps?
Water-to-water heat pumps are among the most efficient heating and cooling technologies available today.
Efficiency is measured using the Coefficient of Performance (COP), which indicates how much thermal energy is produced per unit of electricity consumed.
-
Typical COP values range from 2.8 to 4.5
-
Seasonal performance often exceeds air-source heat pumps
-
Efficiency increases as the temperature difference between source and sink decreases
Research on seawater heat pumps by Petteri Aalto shows heating coverage of up to 77% with seasonal COPs between 2.8 and 4.6, depending on climate and operating conditions. These findings align with performance ranges published by ASHRAE, which reports seasonal COP values between 2 and 4 for water-based systems.
Why are Water-to-Water Heat Pumps important for decarbonization?
Water-to-water heat pumps play a critical role in electrification and decarbonization strategies because they:
-
Reduce reliance on fossil fuels
-
Enable large-scale renewable energy integration
-
Utilize naturally available or waste heat sources
-
Significantly cut greenhouse gas emissions
When powered by renewable electricity, these systems can deliver near-zero-carbon heating and cooling, making them a strong candidate for long-term climate strategies.
Organizations such as Euroheat & Power consistently identify water-based heat pumps as a foundational technology for next-generation district heating networks.
Where are Water-to-Water Heat Pumps commonly used?
Water-to-water heat pumps are increasingly deployed in:
-
District heating and cooling systems
-
Industrial facilities with process heat demand
-
Data centers, especially where waste heat recovery is possible
-
Large commercial and institutional buildings
-
Urban developments near rivers, lakes or coastlines
Their ability to deliver both heating and cooling makes them especially valuable in mixed-use developments and smart city energy systems.
Key Benefits of Water-to-Water Heat Pumps
High Energy Efficiency
Stable water temperatures lead to consistently high COP and lower electricity consumption.
Renewable Energy Integration
They harness renewable thermal energy stored in water bodies and waste streams.
Heating and Cooling in One System
A single installation can provide year-round thermal comfort.
Scalability
Suitable for applications ranging from individual buildings to large district networks.
Reduced Emissions
Lower carbon footprint compared to boiler-based systems.
Water-to-Water Heat Pumps in District Heating Systems
When combined with district heating, water-to-water heat pumps amplify their benefits by centralizing heat production and maximizing efficiency at scale.
They are particularly effective when paired with:
-
Thermal Energy Storage (TES)
-
Low-temperature district heating (4th and 5th generation DH)
-
Waste heat recovery from data centers or industry
This combination enables flexible, resilient and low-carbon energy networks.
Conclusion
Water-to-water heat pumps represent one of the most effective technologies available today for decarbonizing heating and cooling at scale. By leveraging stable water temperatures and renewable thermal energy, they deliver high efficiency, flexibility and long-term sustainability.
At Araner, we help operators devise cost-efficient and sustainable heating structures. As such, we’ve been involved in the design and implementation of water-to-water heat pump initiatives that combine state-of-the-art technology and cutting-edge heating engineering to maximize results. Get in touch with our team and find out how we can help you.
LINKEDIN
YOUTUBE