Small-Scale and Oil-Free Turbocompressor for Refrigeration Applications

The evolution of the coefficient of performance of domestic heat pumps has been stagnating since the introduction of scroll compressors in the early nineties. Compact oil-free direct driven radial compressors represent a promising alternative to the current state of the art refrigeration compressors. The operation characteristics of dynamic compressors naturally fits the heat pump load and the absence of oil allows the full deployment of the potential of enhanced surface evaporators and the implementation of advanced multi-stage cycles both proven means to increase the heat pump performance. This article presents the experimental investigation of a hermetic single-stage and direct-driven radial compressor supported on gas-lubricated bearings designed for domestic heat pump applications. The 20 mm tip diameter compressor was tested at rotor speeds of up to 210 krpm reaching pressure ratios in excess of 3.3 and measured isentropic efficiencies above 78% while processing R134a. Rigidly mounted herringbone grooved journal and spiral groove thrust bearings lubricated with vapor phase refrigerant fluid have been used to support the rotor. The electric motor is a permanent magnet machine delivering a mechanical power of up to 2 kW. Further theoretical investigation based on the experimentally validated compressor design tool identifies tip clearance and relative surface roughness as the main drivers for losses in small-scale turbomachinery for refrigeration applications. The research therefore suggests that both system design to achieve small tip clearances and impeller manufacturing process require particular attention at small scale

Experimental investigation of a two-stage oil-free domestic Air/Water heat pump prototype powered by an oil-free high-speed twin-stage radial compressor rotating on gas bearings

Domestic heat pumps have been identified as a key-technology to decrease the energy consumption of the domestic sector, which represents 29% of the world final energy consumption. Space & water heating represent more than two third of the domestic sector energy consumption in many countries. Next generation domestic heat pumps powered by oil-free high-speed radial compressors are expected to (a) be more efficient at rating operating points and in partial loads, (b) to match better the energy demand characteristics, (c) to be more compact and lighter, (d) more silent, and (e) to need lower refrigerant charges. As a first step towards the development of such advanced heat pumps, an oil-free twin-stage Air/Water domestic heat pump prototype powered by an oil-free high-speed twin-stage radial compressor has been developed and tested. The heat pump layout corresponds to a two-stage heat pump cycle with an open economizer. The two main functions of the economizer are (a) to separate the liquid from the gas before the second stage compressor inlet and offer a proper mixing process, and (b) to store the liquid refrigerant not located in the heat exchangers at a given time. This key-component has been coupled with the compression unit and improved through incremental experimental steps. The heat pump driven by an oil-free twin-stage radial compressor has been successfully tested at the rating operating point A-7/W35. The performance reached with the prototype is considered very promising and constitutes a breakthrough in the domain. This article describes the experimental setup, the Operating Point (OP) A-7/W35, the methodology applied for data analysis, followed by the results. The issues limiting the performance of the prototype are also identified and briefly documented

Multi-Temperature Heat Pumps - A Literature Review

Reducing primary energy consumption by utilizing heat recovery systems has become increasingly important in industry. In many sectors, heating and cooling is required at different temperature levels at the same time. For this purpose, heat pumps are highly attractive energy conversion devices. Heat pumps are widely used for refrigeration, air-conditioning, space heating, hot water production, heat upgrading, or waste heat recovery. The aim of this paper is to review the literature for mechanical driven heat pumps and refrigeration systems with focus on multi-temperature applications. Different design strategies are presented, including cycles with multi-stage compressors, (multiple) ejectors, expansion valves, cascades (with secondary loops), and separated gas coolers. This review highlights the major advantages, challenges, and industrial applications of each multi-temperature heat pump cycle family. Schematics and pressure-enthalpy diagrams illustrate the most promising cycles. The performance of the cycles is compared in terms of First Law efficiency (COP) and Second Law efficiency (exergy) using simplified thermodynamic simulations. The literature reveals that the major part (approximately 70%) of multi-temperature heat pump applications are found in refrigeration, i.e. supermarket food cooling, household fridges/freezers, and cooling/air-conditioning/storage during transportation. In contrast, studies on multi-temperature heating applications are rather rare with the exception of space floor heating and hot water production. Most multi-temperature cycle designs use two heat sources or two heat sinks. Heat pumps with more than three stages are not common, except for natural gas liquefaction. In supermarket applications, multiple compressors with transcritical CO2 are an established key technology. Cascades with secondary loops are another frequently applied system, mostly in the USA. Cycles with multiple ejectors are ready to market and seem to be a promising modification for system performance improvement. Ejector cycles in refrigeration and air-conditioning systems are still under development. Expansion valve cycles are an established technology in household refrigeration. Separated gas coolers for space and hot water heating have recently attracted attention due to the possible combination with supercritical CO2 cycles. Overall, this review paper serves to select the most appropriate multi-temperature heat pump cycle for a specific application

Small, High-Speed, Oil-Free Radial Turbo-compressors for Cooling Applications: Refrigerant Selection

Demand for heating and cooling continues to grow as populations and living standards continue to increase around the globe. At the same time, persistent uncertainty in energy prices and increasing awareness of the environmental impacts associated with the use of fossil fuels - including climate change and poor air quality - are motivating an interest in reducing energy use, in general, and fossil energy, in particular. Heat pumps with scroll or reciprocating compressors have been gaining ground for residential, commercial and industrial heating and cooling applications on a scale up to several tenths of kWmech (over 100 kWtherm). They usually reduce energy use relative to conventional approaches (e.g. fossil-fuel heating in conjunction with electrically-driven cooling) but require higher first costs. Micro-centrifugal compressors with impeller diameters as small as 10-20 mm operating at rotational speeds exceeding 100,000 rpm are now feasible. They are assembled with precisely machined components and they are driven directly (i.e. without gears) by efficient high-speed motors. They have impellers suspended without contact with solid surfaces through bearings lubricated by refrigerant vapor. Thus, they realize high efficiencies and eliminate well-known disadvantages resulting from the use and management of conventional lubricants in positive-displacement systems such as having to: i) ensure adequate lubricant circulation (especially for two stage systems), so as to prevent lubricant depletion at the compressor and associated increased wear and lubricant accumulation in heat exchangers (especially cold evaporators) and associated reduced rates of heat transfer (especially from enhanced surfaces); and ii) limit the maximum operating temperature of high-temperature heat pumps, so as to maintain long-term lubricant chemical stability. HFO-1336mzz(Z) (CF3CH=CHCF3) is a hydro-fluorolefin with a normal boiling point of 33.4 oC and an A1 safety classification (non-flammable, lower toxicity) according to ASHRAE standard 34. It has an ultra-low global warming potential over a 100 years of 2, which virtually eliminates business risk from climate protection regulations emerging around the world and allows R&D investments for technology development. It has been commercialized for use as a component in R-514A, a replacement for HCFC-123 in centrifugal chillers, as a working fluid for high temperature heat pumps and Rankine power cycles and as a foam expansion agent. This paper proposes novel heat pump systems based on micro-centrifugal compressors for selected heating and cooling applications, discusses refrigerant selection, evaluates the performance of systems with HFO-1336mzz(Z) as the refrigerant and identifies promising applications with potentially attractive economics for further development

Two-Stage Heat Pump using Oil-Free Turbocompressors - System Design and Simulation

The combination of multi-stage heat pump cycles with small-scale oil-free turbocompressor technology running on gas bearings could be a promising way to increase performance in domestic and commercial heat pumps. This paper presents a novel two-stage heat pump system with two heat sources at two different temperature levels using two separate turbocompressors rotating on gas bearings optimized for R134a. The system allows integration of unused heat sources, e.g. solar thermal or waste heat, into heat production with a minimal loss of exergy. The cycle comprises an evaporator for the first heat source, a condenser as heat sink, an open economizer with integrated heat exchanger for the second heat source, and a tube-in-tube suction line heat exchanger (SHX) in the high-pressure for superheating and subcooling. The aim of this study is to evaluate theoretically the performance of this heat pump cycle using a system model programmed in the software EES (Engineering Equations Solver). The simulation assumes steady-state, negligible pressure drops and heat losses, and adiabatic expansion processes. The superheating in the evaporator and the SHX is 5°C, and there is no subcooling in the condenser. The heat exchangers are modeled using effectiveness-NTU models. At the design point, the heating capacity of the condenser is set to 6.5 kW and provides hot water of 55°C. The first heat source is brine of 5°C. The second heat source is water of 30°C and has been designed to provide up to 30% of the total condenser heat capacity. The two turbocompressors are designed specifically to meet the heat pump design point. Presently, one-dimensional (1D) compressor maps are used in the heat pump model. Simulation results show that coefficient of performance (COP) improvements of 20% to 30% are achievable, depending on the source temperature levels of the heat pump cycle and the amount of second heat source added to the system. The COP increases with higher source temperatures, higher second heat source capacity, and lower sink temperature. The pressure ratios are defined by the imposed temperature levels. The mass flow rate of the refrigerant in the first stage is mainly determined by the second heat source capacity, and in the second stage by the heat capacity of the condenser. In future work, this novel heat pump concept will be tested experimentally

Design of Oil-Free Turbocompressors for a Two-Stage Industrial Heat Pump under Variable Operating Conditions

Pair of mechanically driven turbocompressors running on gas lubricated bearings have been designed for a two-stage heat pump application functioning under variable operating conditions. Novelty in the present two-stage heat pump system lies in the application of oil-free turbocompressor technology and the introduction of unused secondary heat from various sources. Managing the operational deviations and the secondary heat during off-design heat pump operation is challenging for the turbocompressors. The turbocompressors can potentially exceed their operating range defined by the surge and choke margins, and the maximum rotational speed limit set by the structural and rotordynamic considerations. A wide operating range is, therefore, a prerequisite design condition for the turbocompressors. The present paper will guide the readers through different stages of the design process of such turbocompressors subjected to various constraints. Moreover, a stochastic evaluation on the influence of variable operating conditions on the heat pump and turbocompressor performance will be detailed

Experimental Investigation of Water Injection in an Oil-Free Co-rotating Scroll Machinery for Compressed Air Energy Storage

The high efficient isothermal reversible machine creates the opportunity to be used as a very efficient small-scale compressed air energy storage (CAES). This new type of CAES links the large-scale smart grid to the decentralized electricity production from renewable sources. In this article is presented an experimental study about a novel oil-free co-rotating scroll machine currently in a prototyping stage. This co-rotating scroll unit does not have a discharge check valve that gives it the possibility to operate as compressor and also in expander mode without any hardware modifications. The distinctive feature of this machine in comparison with an orbiting scroll machine is two mobile involutes working in synchronized co-rotation, one relative to another. The prototype was tested in two experimental test rigs (compressor and expander test rig mode) to determine the viability of the working principle, preliminary performance and also the effect of the water injection on the mechanical power and efficiencies. The experimental results demonstrate that the principle of co-rotating could operate without damage up to a rotational speed of 83.3 Hz in compressor mode (without lubrication). The maximum overall isentropic efficiency obtained from the experimentation in compressor and expander modes were 39 % and 32 %, respectively. The maximum compressor’s volumetric efficiency and expander’s filling factor were 45 % and 2, respectively. The water injection has a positive effect on the performance because it improves the polytropic coefficient and this in turn decreases/increases the power consumption/production of the unit

High Temperature Heat Pumps: Market Overview, State of the Art, Research Status, Refrigerants, and Application Potentials

This study reviews the current state of the art of high temperature heat pumps (HTHPs) with heat sink temperatures of 90 to 160°C. The focus is on the analysis of heat pump cycles, suitable refrigerants, and the operating ranges of commercially available HTHPs and heat pumps at the research status. More than 20 HTHP models from 13 manufacturers have been identified on the market that are able to provide heat sink temperatures of at least 90°C. Only a few heat pump suppliers have already managed to exceed 120°C. Large application potentials have been recognized particularly in the food, paper, metal, and chemical industries, especially in drying, pasteurizing, sterilizing, evaporation, and distillation processes. The heating capacities range from about 20 kW to 20 MW. The refrigerants used are mainly R245fa, R717, R744, R134a, and R1234ze(E). Most circuits are single-stage and differ primarily in the applied refrigerant and compressor type. Internal heat exchangers (IHX) are used to ensure sufficient superheating. Process optimization is achieved with economizer cycles or two-stage turbo compressors with intermediate vapor injection. Two-stage cascade cycles or open flash economizers are also used in commercial HTHPs. The COP values range from about 1.6 to 5.8 at temperature lifts of 130 to 40 K, respectively. Several research projects push the limits of the achievable COPs and heat sink temperatures to higher levels. Groups in Austria, Germany, France, Norway, the Netherlands, Switzerland, Japan, Korea, and China are active in the experimental research of HTHPs. Several laboratory scale HTHPs have been built to demonstrate the technical feasibility of sink temperatures above 120°C. The heat pump cycles examined are mainly single-stage and in some cases contain an IHX for superheating or an economizer for vapor injection into the compressor. The investigated refrigerants are R1336mzz(Z), R718, R245fa, R1234ze(Z), R600, and R601. R1336mzz(Z) enables exceptionally high heat sink temperatures of up to 160°C. The experimentally obtained COPs at 120°C heat sink temperature vary between about 5.7 and 6.5 at 30 K temperature lift and 2.2 and 2.8 at 70 K lift. New environmental friendly refrigerants with low GWP and improved components lead to a need for research on optimized cycles. The high level of research activity and the large number of demonstration R&D projects indicate that HTHPs with a heat sink temperature of 160°C will reach market maturity in the next few years. However, despite the great application potential, other competing heating technologies and most importantly low prices for fossil fuels are still hindering the wider spread of HTHPs in industry

High temperature heat pump using HFO and HCFO refrigerants - System design, simulation, and first experimental results

High temperature heat pumps (HTHPs) with heat sink temperatures in the range of 100 to 160°C are expected to become increasingly commercialized in the coming years. Major applications have been identified, particularly in the food, paper, metal and chemical industries, especially in drying, sterilization, evaporation, and steam generation processes. With the intensification of the F-gas regulations, only refrigerants with low GWP may be used in the near future. Replacement fluids for the currently applied hydrofluorocarbons (HFCs) R245fa and R365mfc are required. The actual research gap in the field of HTHPs is to extend the limits of efficiency and heat sink temperature to higher values, while using environmentally friendly refrigerants. Natural refrigerants such as water (R718) or hydrocarbons (e.g. R601 or R600) are promising candidates. However, special heat pump cycle designs with multi-stage recompression or sophisticated safety measures against flammability are needed, which can increase system costs. Various hydrofluoroolefins (HFOs) and hydrochlorofluoroolefins (HCFOs) have recently been developed, which exhibit very low GWPs, are non-flammable and show potential for use at high temperatures (i.e. their critical temperatures are above 150°C). The thermodynamic properties of these fluids allow subcritical heat pump operation at condensation temperatures in the range of 100 to 160°C. This paper investigates the environmentally friendly HFOs R1336mzz(Z) and R1234ze(Z) and the HCFOs R1233zd(E) and R1224yd(Z) and compares the coefficient of performance (COP) and the volumetric heating capacity (VHC) with the refrigerants R365mfc and R245fa at different condensation temperatures and temperature lifts. Based on simulations and literature findings, a single-stage HTHP with internal heat exchanger (IHX) has been designed and built to test the performance of various refrigerants and high-viscosity oils. The established laboratory scale HTHP provides 10 kW heating capacity and heat sink temperatures of 80 to 150°C. The system operates with a variable-speed reciprocating compressor and has an oil separator installed on the discharge side of the compressor. An IHX is used to ensure adequate superheating control. The system design, theoretical simulations and first experimental test results with R1233zd(E) are presented

Towards the Next-Generation of Solid Oxide Fuel Cell Systems

To improve the industry benchmark of solid oxide fuel cell systems (SOFC), we consider anode off-gas recirculation using a blower as an add-on to our next-generation SOFC system. Evolutionary algorithms compare the different design alternatives, i.e. co-flow or counter-flow stack operation with hot or cold recirculation. The system performance is evaluated through multiobjective optimization criteria, i.e. maximization of electrical efficiency and cogeneration efficiency. The results obtained suggest that improvements to the best SOFC systems, in terms of net electrical efficiency, are achievable