CO2 reverse cycles equipped with a bladeless turboexpander

Efficient and resilient chiller cycles at MW-scale are often needed in industry, and the growing interest in energy communities and electrification has increased attention to reverse cycle performance also in their utilisation as heat pumps. This work evaluates the optimal thermodynamic performance and economic features of CO2 refrigeration cycles comparing several layouts adopting a lamination valve (or expansion valve), an ejector and a bladeless (or Tesla) turboexpander, at various cold-source temperatures and refrigerant pressure levels. Carbon dioxide is a favourable substitute for traditional refrigerants, thanks to its low global warming potential (GWP) and adequate thermophysical properties. Tesla turboexpanders are a promising technology as small-scale machinery for energy harvesting, thanks to their low sensitivity to downscaling effects while retaining high rotor efficiency. The beneficial effect of introducing such an expansion device with respect to conventional base layout and state-of-the-art ejector layout was assessed. Simulations were conducted using and improving the existing WTEMP-EVO modelling tool. The results show that the use of a Tesla expander provides benefits in terms of augmented coefficient of performance (COP) in a range between 8% and 22%, depending on the cycle layout and cold-source temperature; moreover, thermo-economic analysis shows a lower specific cost for turboexpander layouts, while economic optimisation ensures lower specific costs for both simple compression and two-stage intercooled compression cycles, at the expense of a slightly reduced COP.