A research group at the University of Nottingham in the United Kingdom has developed a heat pump system that uses water or ice as the heat source and utilizes existing gas pipelines for energy transfer.
The researchers explained that ice-source heat pump systems generate heat from ice or ice slurry derived from water delivered via underground gas pipelines or stored water provided by seasonal rainfall. “This process takes advantage of the high enthalpy of phase change, allowing significant energy absorption with minimal mass or volume,” they added. “This heat pump concept is effective with water as the heat source due to water's high latent heat of fusion, enabling substantial energy absorption with low mass.”
The proposed system, like geothermal heat pumps, can be connected to a central heat pump system, where numerous residential units are interconnected. This configuration is said to be particularly beneficial to newly developed areas or potential residential complexes, “where dimensions and pipeline layouts for new units can be meticulously planned.”
The system uses a compression refrigeration cycle that moves hot compressed gas from the compressor to the condenser, which then releases heat to the heat sink, where it undergoes a phase change and transforms into a liquid, resulting in a temperature decrease. The liquid refrigerant then enters the evaporator, where it absorbs heat from the cold source and evaporates. In the following stage, the evaporated refrigerant returns to the compressor, where it is compressed again to restart the cycle.
The academics acknowledged that the proposed heat pump has a lower coefficient of performance (COP) compared to conventional heat pumps, but they also stressed that these systems have reduced water circulation, which results in lower additional costs due to higher energy consumption and pipeline installation.
They also explained that the proposed solution is ideal for rainy countries like the United Kingdom, which is equipped to use wastewater from ice production in cities' sewer systems without significant impact.
“Importantly, the water used by the ice source heat pump in this scenario undergoes a closed cycle: A portion of the wastewater is treated and injected into the city gas pipes, absorbing heat from both the wastewater and the ground within sewer walls and gas pipes,” they further explained. “In this setup, the heat pump releases cold water or ice slurry into the city's drainage, which, after simple treatment, is partially returned to water storage tanks for reuse in the process and further powering the heat pumps.”
Through a series of simulations, the researchers assessed the performance of the ice source system and compared it with that of air-source and water-source heat pumps, with a more specific focus on a comparison with a 10 kW commercial air heat pump from German manufacturer Vaillant.
Their analysis showed that the ice-source system has a higher energy consumption by a factor of 2.7 compared to conventional gas boilers, with electric-resistance heating, air-source systems, and water-source systems having values of 7.5, 3.2, 2.5, and 2.7, respectively.
“Additionally, electricity consumption exceeds the current peak for 89 %, 47 %, 35 %, and 39 % of the hours in the year for each respective system,” they stressed. “These findings emphasize the proposed system's role in managing peak electricity demand and enhancing generation and transmission capacity post-natural gas phase-out.”
The system was introduced in the paper “Ice source heat pump system for energy supply via gas pipelines – Part1: Performance analysis in residential units,” which was recently published in Energy.
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