Potential and Challenges of ORC driven Heat Pumps Based on Gas Bearing Supported Turbomachinery

Electrically Driven Heat Pumps (EDHPs) have been identified as a key technology to reduce the energy consumption in the domestic space heating sector. However, since EDHPs require electrical power, they face issues related to network overload at peak-time, high operating costs, and increased carbon footprint. A promising alternative to address these shortcomings is the use of Thermally Driven Heat Pumps (TDHPs), which are powered by a heat source instead of electricity. TDHPs offer the possibility of running with numerous types of heat sources, even renewable ones. A promising TDHP technology is the ORC driven Heat Pump (HP-ORC). It consists in the combination of an Organic Rankine Cycle (ORC) and a Heat Pump (HP). This technology provides high flexibility in the heat source selection while offering the possibility of producing electricity. Furthermore, when combined with gas bearing supported turbomachinery, the HP-ORC technology offers an oil-free heating solution. The goal of this thesis is to identify the potential and challenges of the HP-ORC technology. Since HP-ORCs are complex systems, an integrated design and optimization procedure has been applied, aiming at objectives of performance, investment cost, and feasibility. While such integrated design procedures are attractive, they are complex and time consuming. Accurate reduced order models for the various system components are, therefore, highly beneficial for improving the design process. These models are, however, currently missing for small-scale turbomachinery, and hence are developed in a first step. These pre-design models are three orders of magnitude faster than mean-line analysis models while predicting isentropic efficiencies within a 4% error band. Moreover, the presented models provide updated design guidelines for radial turbomachinery and offer insights into the underlying phenomena that shape the efficiency contours. In a second step, the improved turbomachinery models have been used for the integrated optimization of the Compressor Turbine Unit (CTU). The results suggest that the performance trade-off is governed by the turbomachinery components. Further, the design robustness of the CTU is investigated, showing the importance of mitigating bearing manufacturing errors while having fluid leakage and turbomachinery tip clearances as small as possible. In a third step, the thermo-economic optimization of the HP-ORC is developed. For domestic heat pump applications, the optimum working fluid and heat exchanger design are retrieved. Using a hot source at 180°C, exergetic efficiencies over 50% and COPs above 1.8 are achieved, showing a 30% increase compared to the proof of concept. In addition, two configurations are compared, whether the ORC expander and HP compressor are mechanically coupled or not. Although the uncoupled HP-ORC offers more flexibility, it presents inferior thermo-economic trade-offs compared to the coupled configuration. The HP-ORC is compared to typical absorption systems, suggesting that single effect absorption heat pumps are competitive at low heat source temperatures (<120°C), whereas HP-ORCs outperform when the heat source temperature increases above 150°C. In a final step, the optimization tools developed in this thesis are applied to three case studies in which the HP-ORC may deploy its potential: domestic heating, greenhouse, and air conditioning in helicopters using the engine exhaust heat

ORC Driven Heat Pump Running on Gas Bearings for Domestic Applications: Proof of Concept and Thermo-Economic Improvement Potential

ORC driven Heat Pumps (HP-ORC) offer a promising technology of thermally driven heat pumps (TDHPs) for domestic applications. Recently, a 40 kW HP-ORC unit based on gas supported turbomachinery has been investigated in which the ORC radial turbine drives the HP centrifugal compressor, offering an oil free and high power density solution. The radial compressor and the radial turbine have tip diameters of the order of 20mm and the system has been tested at rotor speeds in excess of 200 krpm with shaft powers up to 2.4 kW, running with R134a. The compressor and the turbine were operated at pressure ratios of up to 2.8 and 4.4, while reaching thermal COP of the order of 1.5 and isentropic compressor and turbine efficiencies in excess of 70%, thus validating the HP-ORC concept based on small-scale turbomachinery. However, performance limitations were encountered due to suboptimal heat exchanger, working fluid and turbomachinery design. This article presents a full thermo-economic optimization of the HP-ORC enabling the identification of the best trade-off Investment Cost/ COP. Several working fluids have then been compared, revealing that a compromise is to be found between best cost/efficiency trade-off, highest performances achievable and technical feasibility. It also showed a thermal COP improvement potential up to 30% compared to the existing experimental system. Finally, the thermo-economic results of the HP-ORC have been compared with other typical TDHPs, such as absorption heat pump

Data-Driven Predesign Tool for Small-Scale Centrifugal Compressor in Refrigeration

Domestic scale heat pumps and air conditioners are mainly driven by volumetric com-pressors. Yet the use of reduced scale centrifugal compressors is reconsidered due to their high efficiency and power density. The design procedure of centrifugal compressors starts with predesign tools based on the Cordier line. However, the optimality of the obtained predesign, which is the starting point of a complex and iterative process, is not guaranteed, especially for small-scale compressors operating with refrigerants. This paper proposes a data-driven predesign tool tailored for small-scale centrifugal com-pressors used in refrigeration applications. The predesign model is generated using an experimentally validated one-dimensional (1D) code which evaluates the compressor performance as a function of its detailed geometry and operating conditions. Using a symbolic regression tool, a reduced order model that predicts the performance of a given compressor geometry has been built. The proposed predesign model offers an alternative to the existing tools by providing a higher level of detail and flexibility. Particularly, the model includes the effect of the pressure ratio, the blade height ratio, and the shroud to tip radius ratio. The analysis of the centrifugal compressor losses allows identifying the underlying phenomena that shape the new isentropic efficiency contours. Compared to the validated 1D code, the new predesign model yields deviations below 4%on the isen-tropic efficiency, while running 1500 times faster. The new predesign model is, therefore, of significant interest when the compressor is part of an integrated system design process

Small scale radial inflow turbine performance and pre-design maps for Organic Rankine Cycles

While small scale ORCs are currently dominated by volumetric expanders, the use of turbomachines is reconsidered due to their high efficiency and power density. Yet, suitable performance maps, which ensure an accurate starting point for the turbine design, are still missing for small scale turbines. This paper proposes a new non-dimensional performance map tailored for small scale turbines. The map is generated using an experimentally validated 1D code and adapted to small scale ORCs applications. A new polynomial fit is proposed, which accounts for the pressure ratio, since it is suggested to have a strong influence on the shape of the map. Through the analysis of the turbine losses, the underlying phenomena shaping the efficiency map are explained. A sensitivity analysis of the geometrical dependencies on the map shows a strong impact of the shroud to tip radius ratio, and explains why the Ns-Ds surface of the presented map is smaller than the original one. Compared to the experimentally validated 1D model the new map yields prediction errors below 4%

Multi-objective optimization of turbo-ORC systems for waste heat recovery on passenger car engines

Waste heat recovery from passenger car internal combustion engines by means of an organic Rankine cycle (ORC) system is promising for reducing CO2 emissions. In this study, different cycle configurations capable of converting waste heat from both coolant and exhaust gases are investigated based on different working fluid categories. Radial-inflow turbines are considered as expansion devices and corresponding isentropic efficiencies are evaluated based on a preliminary design map accounting for the effect of the pressure ratio. Mechanical losses resulting from the use of a gas-bearing-supported rotor driving a permanent magnet generator are also evaluated. In order to identify the turbo-ORC system design tradeoffs, constrained multi-variable and multi-objective optimizations are performed using an evolutionary algorithm. It is found that the optimal cycle configuration and working fluid depend on the available space in the vehicle and that the condenser is the most critical component for the ORC system integration. In addition, the most suitable working fluids for this application are characterized by (1) a boiling point close to the heat sink temperature, (2) a high critical pressure, and (3) a high molecular weight. The resulting optimal radial-inflow turbines are 10e33 mm in tip diameter and operate at 80 e330 krpm

Thermo-economic optimization of an ORC driven heat pump based on small scale turbomachinery and comparison with absorption heat pumps

The current paper presents the thermo-economic optimization and comparison, through a multi-objective algorithm, between HP-ORC and the competing single effect absorption heat pump (SEAHP), with the aim to find the best trade-off between investment cost and exergetic efficiency. Heat exchanger (HX) cost, which is the dominant cost driver, is predicted on catalogue-based correlations as a function of heat exchanger areas. The dominating HP-ORC working fluid is R-245fa for all operating conditions, except for hot source temperatures of 180 °C, where R-600a is a better candidate. The resulting Pareto frontiers suggest that, for hot source temperatures up to 120 °C the SEAHP and HP-ORC are highly competitive by yielding similar thermo-economic tradeoffs. For higher hot source temperatures, however, the HP-ORC outperforms the SEAHP both in terms of performance and cost while providing a significantly wider performance range (0.3 < ηexe < 0.55) with respect to the SEAHP (0.4 < ηexe < 0.49)

Potential of Small-Scale Turbomachinery for Waste Heat Recovery on Automotive Internal Combustion Engines

This paper investigates the waste heat recovery potential of internal combustion engines, using organic Rankine cycles running on small-scale radial turbomachinery. ORC are promising candidates for low-grade thermal sources and the use of dynamic expanders yields very compact systems, which is advantageous for automotive applications. As engine coolant and exhaust gases are the major available heat sources, different cycle configurations and working fluids have been investigated to capture them, in both urban and highway car operation. Pareto fronts showing the compromise between net power output and total heat exchange area have been identified for a set of cycle’s variables including turbine inlet conditions and heat exchanger pinches. A preliminary optimization, including only R-1234yf working fluid, shows that a single-source regenerative cycle harvesting the high temperature exhaust gas stream performs averagely better than coolant-driven and dual-source cycles. A more in-depth optimization including eight working fluids as well as aerodynamic and conceptual limitations related to radial turbomachinery and automotive design constraints, finally shows that an ICE exhaust heat recovery ORC could improve the first law efficiency of the driving system by up to 10% when implemented with fluid R-1233zd

Dimensionless correlations and performance maps of scroll expanders for micro-scale Organic Rankine Cycles

In the endeavor towards distributed power systems, not seasonal-dependent micro-power generation technologies are expected to integrate the energy scenario in the years to come. In this context, the Organic Rankine Cycle (ORC) in the (1e10) KWe scale has chronically lacked a suitable expansion device, hindering its market attractiveness. As scroll expanders have been pointed out as strong potential candidates, performance correlations and pre-design maps based on a review and analysis of published experimental data are presented. A dimensionless approach based on the traditional Ns, Ds dimen-sionless numbers stemming from turbomachinery has been chosen for greater generality. In addition, the lubricating oil mass fraction effect on the scroll expander performance has been included. The generated maps contribute to accelerating the pre-design phases at the system and component level with beneficial effects for the overall development process. Basic geometry and size characteristics are considered as well, acknowledging their importance in micro-power embedded applications; these considerations are illustrated in a passenger car waste-heat recovery case study. Findings suggest that optimized scroll expanders may potentially reach very interesting nominal electric isentropic efficiencies (up to 80% for an oil lubricated scroll expander)

SPR imaging based electronic tongue via landscape images for complex mixture analysis

International audienceElectronic noses/tongues (eN/eT) have emerged as promising alternatives for analysis of complex mixtures in the domain of food and beverage quality control. We have recently developed an electronic tongue by combining surface plasmon resonance imaging (SPRi) with an array of non-specific and cross-reactive receptors prepared by simply mixing two small molecules in varying and controlled proportions and allowing the mixtures to self-assemble on the SPRi prism surface. The obtained eT generated novel and unique 2D continuous evolution profiles (CEPs) and 3D continuous evolution landscapes (CELs) based on which the differentiation of complex mixtures such as red wine, beer and milk were successful. The preliminary experiments performed for monitoring the deterioration of UHT milk demonstrated its potential for quality control applications. Furthermore, the eT exhibited good repeatability and stability, capable of operating after a minimum storage period of 5 months