PNNL research on CO2 sequestration leads to geothermal heat extraction breakthrough

Nanostructured Metal-organic Heat Carrier (MOHC)

In Washington, research on nanomaterials at the Department of Energy’s Pacific Northwest National Laboratory (PNNL) intended to help capture carbon dioxide has led to a material that can boost the power generation of relatively cool geothermal resources. In fact, the nanostructured metal-organic heat carriers (MOHCs) didn’t work at all with CO2.

PNNL research fellow Pete McGrail said that the MOHCs are added directly as a suspension to the liquid used in a system’s heat exchanger creating what they have termed a biphasic fluid. At any given time, there exists in concert either solid (the MOHC) and vapor phases or solid (MOHC again) and liquid phases. The MOHCs have been tested with organics such as isobutene, pentane, hexane, and propane with similar results. The choice of organic would depend on the starting temperature of the water extracted from a geothermal well.

Organics have lower boiling points, meaning that less heat is needed to flash to gas to drive a turbine. The MOHCs improves efficiency by holding onto gas molecules at a much higher temperature, preventing the fluid from flashing to gas in the heat exchanger until it gets to a higher temperature and pressure.

When they realized their material would not perform as they had hoped with CO2, they decided to see if there were other used for the MOHCs. Knowing the properties of what they had produced, McGrail said that alkenes and alkanes were some of the first materials tested, so it wasn’t long before they had found another use for the nanomaterials. And while they will continue searching for additional uses, their work now is being driven by the response to PNNL’s announcement.

In the interim, they intend to build a functioning bench-top prototype – a heat exchanger, mini turbine, condenser, and pump – and have it generating electricity by the end of the year. The researchers need to know if the MOHCs will remain in suspension or begin to adhere to components such as blades, valves, etc.