HTHP Symposium 2024, Picture credit: Organizer of HTHP Symposium
HTHP Symposium 2024, Picture credit: Organizer of HTHP Symposium

Transcritical CO2 Heat Pumps Could Make Thermal Desalination More Energy- and Cost-Efficient, According to New Study

CO2 heat pumps could provide both potable water and cooling for district cooling systems.

Hybrid thermal desalination systems with transcritical CO(R744) heat pumps could make thermal desalination more energy- and cost-efficient by providing both potable water and cooling, according to a new study.

“A commercially available 19MWel transcritical CO2-based heat pump system could produce up to 16,000 m³/d [16,000,000l/4,226,752gal per day] of potable water and 60MW (17,060TR) of cooling energy,” the study said. “This highlights the potential use of heat pumps as a sustainable solution for addressing water scarcity and space cooling.”

The study, “Techno-Economic Analysis of Large-Scale Heat Pump Integration to Assist Sustainable Water Desalination and District Cooling,” was presented by Nils Hendrik Petersen at the High-Temperature Heat Pump (HTHP) Symposium 2024, held in Copenhagen from January 23–24. Petersen is a research leader at RWTH Aachen University’s Institute for Power Plant Technology, Steam and Gas Turbines.

Petersen noted that thermal desalination, which relies on high heat that is often generated by non-renewable sources, has given ground to reverse osmosis (RO), which relies solely on electricity, “making it more adaptable for decarbonization.”

But transcritical CO2 heat pumps present an opportunity to decarbonize thermal desalination systems while also providing cooling to a district cooling network, eliminating the need for additional cooling equipment. The study’s authors subtracted this “saved” electricity, referred to as a “cooling credit,” from the energy demand of the modeled system.

“When the desalination process solely relies on heat, the specific energy consumption is approximately 30kWh per cubic meter of water, which is less efficient compared to reverse osmosis plants with an energy consumption of 9kWh for the same volume of water,” said Petersen.

“However, when factoring in the cooling aspect, the system’s overall energy consumption drops significantly,” he added. “This adjustment matches the efficiency of state-of-the-art RO processes, requiring only about 6kWh per cubic meter of water.”

According to the study, the results show that only considering heat supply to thermal desalination processes by the HTHP leads to a significant increase in the levelized cost of heat (LCOH) compared with state-of-the-art thermal desalination systems. The LCOH was calculated based on prices in the Middle East and North Africa region, which has a high demand for desalination and cooling.

“If only potable water is produced, the LCOH are significantly higher than the state-of-art for thermal-based desalination processes, but with the inclusion of this cooling credit, the expenses for heat supply align with the existing costs for thermal-based desalination, making it economically competitive,” explained Petersen.

Model development

The study’s authors note that the heat pump in their model was based on the 50MW (14,217TR) MAN Energy Solutions CO2 heat pump system installed in the Port of Esbjerg, Denmark, to power the local district heating network. That system consisted of two Electro-Thermal Energy Storage CO2 heat pumps that drew their heat from seawater.

The authors arrived at an estimated power demand of 19MW with a COP of 2.6 based on assumptions they made in a previous study on heat pumps and thermal desalination. That previous study had the heat source provided by a district cooling network, with the potable water used to superheat the CO2 before it entered the compressor, both assumptions that were carried over into the study presented at the HTHP Symposium.

Another assumption made is that the heat pump will operate at temperatures of up to 180°C (356°F). HTHPs capable of reaching such temperatures are currently under development but not commercially available.

The study’s authors acknowledged the limitations of their model, writing that “further research is indispensable in unlocking this potential.”

“In a world grappling with the intensifying impacts of climate change, the global demand for potable water faces unprecedented stress,” Petersen said.

“Over two billion people continue to struggle with limited access to secure sources of clean drinking water, hindering progress in arid regions and underscoring the urgency of cost-effective seawater desalination,” he continued.

Maximilian Arras of State Key Laboratory, Ma Linwei from Tsinghua University, and Manfred Wirsum from RWTH Aachen University collaborated on this research.

“When factoring in the cooling aspect, the system’s overall energy consumption drops significantly, for thermal desalination with CO2 heat pumps. This adjustment matches the efficiency of state-of-the-art RO processes, requiring only about 6kWh per cubic meter of water.”

Nils Hendrik Petersen, Research leader at RWTH Aachen University’s Institute for Power Plant Technology, Steam and Gas Turbines

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