Model predictive control of a free piston compressor/expander with an integrated linear motor/alternator

Linear positive displacement machines are becoming increasingly more attractive for applications that are normally known as unconquerable niches of rotary and scroll machines. Free-piston machines are characterized by the absence of a crank mechanism, since there is a direct transformation of electrical energy into the piston movement. From the point of view of manufacturing, these machines benefit from a higher robustness and reliability because of less mechanical components involved and reduced frictional losses associate with a conventional crank mechanism. However, the major challenge in replacing the rotary machines by linear ones is a lower efficiency at lower speeds which is unavoidable because of the nature of linear motion: continuous operation means a reciprocating movement within a stroke length with significantly long periods of acceleration and deceleration when the speed is far from its optimal value. However, the advantage of free-piston machines is the fact that the motion profile is freely configurable within physical constraints, which provides a possibility to optimize the speed given the efficiency map of particular linear motor. While the methods and results of the efficiency assessment for rotary machines are widely available, there is a lack of these analyses for linear machines. The current study provides in-depth analyses of a double-coil iron core linear motor also acting as a generator

Solar heat driven water circulation and aeration system for aquaculture

The proposed design concept of water aeration and updraft circulation in aquaculture is based on the Organic Rankine Cycle (ORC) technology and uses a solar energy absorbed by a floating collector. The pressure required for the aerator is created by evaporating a working fluid and optimized for an average depth of a pond. The working pressure is defined by the maximum achievable temperature of the working fluid. The condensing heat is rejected at a certain depth with the lowest temperature and drives the convective circulation. A prototype is designed by using common materials and off-the-shelf components to ensure maintenance-free and proper capacity to fulfil the needs of an average or a small aquaculture farm: the working fluid in the working chamber evaporates increasing in volume and pumping air out of the vessel as well as the expanded working fluid in the second working chamber. The working fluid is cooled down in the condenser which is submerged into the pond and it is condensed while decreasing in volume. The new design can perform multiple cycles per day increasing the volume of pumped air. In order to make the operation of this unit possible during the night, a heat buffer with a phase changing material (PCM) is used. A parametric study of suitable working fluids and PCMs has been performed in order to select the most appropriate combination for the target applications

Experimental characterization of single screw expander performance under different testing conditions and working fluids

During the last years, one of the most popular ways to recover low-grade waste heat is the organic Rankine cycle (ORC). This technology is widely studied and continuously optimized and, as a result, there are many efficient installations available on the market utilizing heat with stable parameters such as from geothermal sources or from the biomass combustion process. However, if a variable hot source in terms of either temperature or flow rate is introduced, the expansion devices have to work at non-optimal conditions, which decrease the global efficiency of ORC installations, e.g. in the case of waste heat recovery. In order to characterize the performance of a positive displacement expander close enough to the optimum, the influence of pressure ratios, filling factor, and working fluid properties on power output is studied. In this paper, experimental results obtained on a small-scale ORC test setup based on an 11 kWe single-screw expander are presented. Two working fluids are used during the tests, i.e. R245fa and SES36 (Solkatherm). These working fluids are common for ORC installations exploiting low-temperature waste heat. The waste heat source is simulated by an electrically heated thermal oil loop with adjustable temperature and flow rate. Various waste heat inlet flow rates are considered in order to find an optimal evaporation pressure and to maximize the power output with different heat source profiles. Based on the experimental data, the expander model is developed. For each working fluid, optimal working conditions are determined. In most cases, there is under-expansion due to a relatively small built-in volume ratio, causing certain losses. By means of the model, the ideal expansion process is simulated and compared with the real one obtained experimentally to quantify these losses and conclusions can be drawn whether significant benefits can be offered by using an optimized expander instead of an ”off-the-shelf” reversed compressor

Community Heat Pump Systems Utilizing Oil-Free Compressor Technology

The goal to decarbonize buildings is quickly driving growth in the adoption of heat pumps to replace fossil fuel-based heating equipment. The trend is significantly driven by the integration of renewables in the electric grid, also replacing fossil fuel-based sources, to in-turn drive decarbonization. The impact of this change is greater if also changing out end-use fossil fuel-based heating equipment to electric-driven. With the trend to heat pumps, there is a critical choice on both the scale and corresponding heat source. The heat pumps can be implemented with ambient air as the heat source or with other higher temperature/efficiency sources such as geothermal, process or district cooling heat recovery. Nevertheless, these sources are not always available in close proximity to the heat load. This raises the opportunity for larger-scale heating systems, serving multiple loads and with the corresponding opportunity to integrate one or more potential higher-temperature heat recovery heat sources. A related critical factor with the growth of heat pumps is resiliency. The term has historically mainly been associated with critical facilities and the ability to withstand critical events. Now it’s evolving because of that same integration of renewables into the power grid and their inconsistent availability. Now resiliency has more to do with preparing for this periodic unavailability – Ensuring that demand is met when supply is not necessarily available. This paper will present a concept for a community heating and cooling system utilizing oil-free turbo compressor technology, to address the electrification of heating while also taking advantage of multiple higher-temperature heat sources. The technology and heat sources ensure the most efficient system possible, resulting in minimum operating costs and maximum decarbonization, while the community configuration and multiple sources ensure resiliency, consistently meeting the demand requirements

Theoretical and experimental characterization of single-screw expanders for ORC applications

Medium to low grade waste heat recovery from industrial processes is of particular interest to address both environmental concerns and the need of more efficient thermal systems. Organic Rankine cycles (ORCs) have been demonstrated to be a viable solution to convert waste heat into valuable electric power. In an ORC, the conversion of thermal energy into mechanical energy and, successively, into electrical energy occurs by means of an expander. In this work, a single-screw expander is investigated as a potential technology in the medium to low power output range due to its symmetric rotor configuration. At first, a generalized framework for the simulation of positive dispalcement compressors and expanders (PDSim) has been enhanced and applied to model both conventional and novel compressor types. A Z-compressor has been used as example to show how to handle different numerical challenges. Furthermore, such tool has been employed to develop a comprehensive mechanistic model of a single-screw machine. Secondly, a small scale ORC test rig has been used to characterized the performance of the expander with two working fluids, i.e. R245fa and SES36. This work resulted in numerical and experimental assessments of such machine to improve its design

Techno-Economic Analysis for Two-Stage Vapor Injected System for Heating Applications

The rise of energy consumption and environmental concerns necessitate research efforts on optimizing HVAC systems. Air source heat pump systems are widely used as space conditioning systems because of their low cost of installation and the possibility of achieving both heating and cooling from the same device. However, under extreme conditions especially in heating mode, conventional heat pumping systems face challenges when operating in cold climate at ambient temperatures that fall below 0°C (32°F). In this paper, a two-stage vapor injection heat pump system with R-32, R-290 and R-410A as the working fluids, was investigated by considering both single objective (heating COP and unit cost of heating (UCH), as the thermodynamic and thermo-economic criteria, respectively) and multi-objective (maximum heating COP and minimum UCH) optimizations at low ambient conditions. The system model was developed by using Engineering Equation Solver (EES) and the optimizations have been carried out with the available genetic algorithm (GA) method. From a multi-objective standpoint, the Pareto frontier decision-making process was used for the selection of final solution. The results revealed that R-32 and R-290 were the best selections for the investigated system based on exergo-economic and thermodynamic criteria, respectively. The system with R-32 and R-290 had a minimum UCH of 265 $/kWh and a maximum heating COP of and 3.94. Whereas, for the baseline system with R-410A, the heating COP, exergy efficiency and the UCH were estimated to be 3.75, 30.31$/kWh, respectively

Modeling of a Variable-BVR Rotary Valve Free Piston Expander/Compressor

The concept of a free-piston expansion/compression unit with a variable Built-in Volume Ratio (BVR) is proposed. This device has no crankshaft mechanism which provides a possibility to optimize the expansion process free of mechanical limitations. An additional degree of freedom is used, namely the rotation to control the in- and the outlet ports timing. Further, the operation in the expander mode will be described. In most of the existing linear expanders/compressors, bouncing chambers or devices are used to reverse the piston movement at extreme positions. This approach is characterized by relatively high energy losses due to irreversibility of such a process. As an alternative, a fully controlled movement of the piston is proposed. This paper is focused on the control algorithm based on rules, which have been obtained and based on the insight in the system. Including the rotation timing, resulting in an optimal expansion process with an outlet pressure matching with the required one

Theoretical Analysis of Noise and Vibration Generated by Gas Pressure Pulsation in Hermetic Compressors

Due to the intermittent nature of discharge flow in hermetic compressors, gas pulsation is a major source of noise, vibration and harshness (NVH) effects. In the presented study on gas pulsation noise of hermetic compressors, a model was developed where the thermodynamics aspects of the compression process and the vibro-acoustics aspects are coupled together. The vibro-acoustics submodel is based on finite element method, and it describes the two-way coupled interaction of compressor cavity acoustics and shell vibration. The thermodynamics submodel is based on compressor model developed in previous studies, PDSim. It is used to compute the discharge gas pulsations that excite the cavity. Meanwhile, the acoustics response in the cavity is coupled with the thermodynamics submodel though proper modeling of discharge valves. Simulations were conducted to validate the models developed in the current work as well as the to demonstrate how these modeling tools can help compressor manufacturers to gain better understanding of the physical reasons behind NVH effects of compressors. These tools can also provide guidelines on NVH oriented design optimization of compressors