Innovative Process Integrating Air Source Heat Pumps and Direct Air Capture Processes

Most integrated assessment models indicate a need for technological carbon dioxide removal from the atmosphere to achieve climate mitigation targets. Currently, direct air capture (DAC) appears to be one the "backstop" technologies suitable to provide this service. These technologies usually require low-carbon heat as part of their operation cycle. Here, we consider a way of providing this heat when no local heat source is available. Air source heat pump (ASHP) water heaters are a well-known technology that takes heat from the air to supply hot water. Variations on their operating conditions could provide water at 100 degrees C, when a trans-critical cycle is used. This level of temperature is required by several DAC adsorption processes as the thermal energy for the regeneration stage. For this reason, an innovative process integrating an ASHP and a DAC adsorption system is proposed here. The heat pump provides not only heating but also cooling, while three separate stages (adsorption, cooling, and regeneration) are considered for the DAC. In the integrated process, the air is sent to the adsorbent bed at first and after that to the evaporator of the heat pump and then used for the cooling stage. The hot water supplied by the heat pump is used for the desorption. Different working fluids (CO2, CO2-ethane, CO2-R41), with low ozone depletion and global warming potentials, are investigated. The results show that a high level of efficiency is possible for heat pumps supplying hot water at 100 degrees C. Moreover, energetic advantages are present with reference to the base case, where heat is provided by a municipal water incinerator and cooling by a cooling tower. Savings in the energy consumption of 55, 60, and 53% for the integrated process using CO2, CO2/R41, and CO2/ethane, respectively, are possible. Economic benefits are present when economic incentives are provided, ensuring lower costs up to 39 $/ton CO2, and the technology benefits from location flexibility as only a power supply (and not a heat source) is required.