• Energy Research
• 2021-2025close

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.

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.

Abstract The cost-effectiveness of turbomachinery is a key aspect within the small-size compressor market. For this reason, Tesla turbomachinery, invented by Nikola Tesla in 1913, could be a good solution, particularly for low volumetric flow applications, where volumetric compressors are usually used. It consists of a bladeless rotor which stands out for its ease of construction and its ability to maintain almost the same performance as size decreases. One of its advantages is that it can run either as a turbine or as a compressor with minor modifications at the stator. The objective of this paper is to investigate a 3kW Tesla compressor, which design was derived from an analogous Tesla expander prototype (59% isentropic efficiency from the numerical study), by conducting a computational fluid dynamic analysis for different disk gaps and diffuser configurations. The potential of the Tesla compressor is shown to be quite promising, with a peak isentropic efficiency estimated at 53%. Although bladeless compressor is a simple turbomachinery device, different parts i.e., diffuser, tip clearance, and volute need to be optimized. Utilizing computational fluid dynamics algorithms, different disk gaps and different diffusers are simulated in order to increase the overall performance of the compressor and understand the flow dynamic behavior behind this technology. The dimensionless Ekman number is used to express the optimum disk space of the compressor rotor. Thus, the overall performance of the Tesla compressor is improved by 5–10% points compared to the initial model. Simultaneously, diffuser optimization strategies are applied and proved that there is a direct impact on the optimum design conditions, improving the pressure ratio at high mass flow rates.

Abstract The cost-effectiveness of turbomachinery is a key aspect within the small-size compressor market. For this reason, Tesla turbomachinery, invented by Nikola Tesla in 1913, could be a good solution, particularly for low volumetric flow applications, where volumetric compressors are usually used. It consists of a bladeless rotor which stands out for its ease of construction and its ability to maintain almost the same performance as size decreases. One of its advantages is that it can run either as a turbine or as a compressor with minor modifications at the stator. The objective of this paper is to investigate a 3kW Tesla compressor, which design was derived from an analogous Tesla expander prototype (59% isentropic efficiency from the numerical study), by conducting a computational fluid dynamic analysis for different disk gaps and diffuser configurations. The potential of the Tesla compressor is shown to be quite promising, with a peak isentropic efficiency estimated at 53%. Although bladeless compressor is a simple turbomachinery device, different parts i.e., diffuser, tip clearance, and volute need to be optimized. Utilizing computational fluid dynamics algorithms, different disk gaps and different diffusers are simulated in order to increase the overall performance of the compressor and understand the flow dynamic behavior behind this technology. The dimensionless Ekman number is used to express the optimum disk space of the compressor rotor. Thus, the overall performance of the Tesla compressor is improved by 5–10% points compared to the initial model. Simultaneously, diffuser optimization strategies are applied and proved that there is a direct impact on the optimum design conditions, improving the pressure ratio at high mass flow rates.

Abstract In the present energy scenario, heat pumps (HPs) are assuming an important role to improve energy efficiency in reversible cooling systems. In particular, in case of industrial size plants, centrifugal compressors are preferred with respect to other solutions; nonetheless, they are often subjected to variable operation and therefore they must withstand off-design conditions. For studying unstable operation of refrigerant closed-loop systems, Carrier provided the University of Genoa with a small size chiller rig equipped with an innovative high speed centrifugal compressor driven by a variable speed motor. This paper presents the vibro-acoustic signature analysis of such a closed-loop rig from a system perspective, mainly focusing on mechanical response of compressor casing. First, its vibro-acoustic characterization is conducted at on-design and stable operation. Afterwards, some surge transients are obtained by progressively closing some valves in plant feeding lines, and meanwhile vibro-acoustic signals are acquired at relevant plant locations to characterize system response just before instability onset. In this way, suitable surge precursors can be defined in incipient surge conditions for early surge detection in this kind of complex plants. Indeed, system dynamics is significantly affected by interposed volumes, and therefore the effect of heat exchangers in system response may be relevant. To this aim, system dynamics is analyzed in detail both in subsynchronous frequency range and in high frequency region to assess how its vibro-acoustic response varies when moving from stable conditions toward surge. Surge precursors are obtained by relying only on vibro-acoustic signals both in low and high frequency ranges in order to perform early surge detection in such chiller pilot system. The main advantage of the proposed approach is to exploit nonintrusive probes, which allow to define diagnostic indicators without interacting directly with the working fluid, therefore preserving system integrity and reliability.

Abstract In the present energy scenario, heat pumps (HPs) are assuming an important role to improve energy efficiency in reversible cooling systems. In particular, in case of industrial size plants, centrifugal compressors are preferred with respect to other solutions; nonetheless, they are often subjected to variable operation and therefore they must withstand off-design conditions. For studying unstable operation of refrigerant closed-loop systems, Carrier provided the University of Genoa with a small size chiller rig equipped with an innovative high speed centrifugal compressor driven by a variable speed motor. This paper presents the vibro-acoustic signature analysis of such a closed-loop rig from a system perspective, mainly focusing on mechanical response of compressor casing. First, its vibro-acoustic characterization is conducted at on-design and stable operation. Afterwards, some surge transients are obtained by progressively closing some valves in plant feeding lines, and meanwhile vibro-acoustic signals are acquired at relevant plant locations to characterize system response just before instability onset. In this way, suitable surge precursors can be defined in incipient surge conditions for early surge detection in this kind of complex plants. Indeed, system dynamics is significantly affected by interposed volumes, and therefore the effect of heat exchangers in system response may be relevant. To this aim, system dynamics is analyzed in detail both in subsynchronous frequency range and in high frequency region to assess how its vibro-acoustic response varies when moving from stable conditions toward surge. Surge precursors are obtained by relying only on vibro-acoustic signals both in low and high frequency ranges in order to perform early surge detection in such chiller pilot system. The main advantage of the proposed approach is to exploit nonintrusive probes, which allow to define diagnostic indicators without interacting directly with the working fluid, therefore preserving system integrity and reliability.

Abstract The SMART PIPING project aims to design, create and test innovative solutions for resilience of water and energy networks, aiming to develop technologies for the energy autonomy of stations monitoring of infrastructures suitable for the transport of fluids (natural gas and biogas, hydrocarbons, water). In this paper, an innovative bladeless turbine is designed and manufactured in relevant environment to demonstrate the feasibility of harvesting significant amount of energy in the urban pipeline distribution network to power local devices to monitor the piping health and enable leakage early-warning. Results show that bladeless turbine can constitute a cost-effective and highly reliable device to enable the digital and energy-efficient transition of urban communities.

Abstract The SMART PIPING project aims to design, create and test innovative solutions for resilience of water and energy networks, aiming to develop technologies for the energy autonomy of stations monitoring of infrastructures suitable for the transport of fluids (natural gas and biogas, hydrocarbons, water). In this paper, an innovative bladeless turbine is designed and manufactured in relevant environment to demonstrate the feasibility of harvesting significant amount of energy in the urban pipeline distribution network to power local devices to monitor the piping health and enable leakage early-warning. Results show that bladeless turbine can constitute a cost-effective and highly reliable device to enable the digital and energy-efficient transition of urban communities.