Abstract
Ocean Thermal Energy Conversion (OTEC) has garnered international interest as a clean and sustainable source of secondary power generation, without impacting the environment and regardless of seasonal weather conditions, for offshore Oil & Gas (O&G) processing facilities. This is considering the suitability of waste heat extraction from existing offshore deepwater Oil & Gas platforms as a complementary application for integrated OTEC modules (hereafter abbreviated as i-OTEC). Such an application is possible with the availability of cold seawater drawn from extended seawater lift caissons or deepwater intake risers. This study investigates, in detail, the various refrigerants that are feasible alternatives to Ammonia (NH3) for a 1MWe i-OTEC scheme.
Anhydrous NH3 or R717 was initially selected as the refrigerant for the aforementioned i-OTEC scheme which is modelled based on an Organic Rankine Cycle (ORC). It is widely accepted that R717 is the ideal refrigerant for this application due to R717's high enthalpy change during expansion. Although R717 is classified toxic, R717 emissions generally do not create environmental problems, and is detrimental to health only upon presence of R717 at a Short Term Exposure Limit (STEL) of 35ppm. For comparison purposes, other alternative refrigerants were also studied; focusing particularly the risks associated with the refrigerants including toxicity, flammability, asphyxiation, reactivity and physical hazards. Hence, the selection of the refrigerant was a major factor in the design as it provides the basis for further investigation. Shortlisted alternatives, which include R1234yf, R410a, R134a, R290 and R32, were additionally assessed on multiple factors including expected enthalpy gain from the Rankine cycle, circulation flowrate, equipment sizing, hazard classification of refrigerants, compliance to international/local exposure standards, handling and storage of refrigerants.
It is identified that the aforementioned refrigerants are possible alternatives to R717 - but with some drawbacks from a thermodynamic performance perspective, and the fact that some of these refrigerants would be rendered obsolete in various industries. Consideration of the next generation of refrigerants must balance out factors such as public awareness, manufacturers availability, safety aspects and environmental concerns. The latest refrigerant with minimal environmental impact is R1234yf and it has been slowly accepted in the HVAC and motor industry.
Additionally, the turboexpander-generator shaft sealing was identified as the major source of refrigerant emissions during normal operation. Hence, various shaft sealing solutions were also scrutinized to assess the viability of utilizing R717 as a refrigerant whilst still fulfilling safety and emission requirements.