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.

Modeling and Experimental Investigation of an Oil-Free Microcompressor-Turbine Unit for an Organic Rankine Cycle Driven Heat Pump

Jonathan Demierre, Antonio Rubino, Jürg Alexander Schiffmann

Domestic heating and cooling will more and more have to rely on heat pumps (HPs) in order to support a more rational use of primary energy consumption. The HP market is mainly dominated by electrically driven vapor compression cycles and by thermally driven sorption processes. The drawback of electrically driven vapor compression cycle is their dependence on an electrical grid and the fact that they increase the winter or summer electricity peak demands. Hence, a thermally driven vapor compression cycle would offer substantial advantages and flexibility to the end user for heating and cooling applications. This paper presents the investigation of an oil-free compressor-turbine unit (CTU) used for a thermally driven HP (TDHP) based on the combination of a HP compression cycle and an organic Rankine cycle (ORC). The CTU consists of a radial inflow turbine and a centrifugal compressor of the order of 2 kW each, directly coupled through a shaft supported on gas lubricated bearings. The CTU has been tested at rotor speeds in excess of 200 krpm, reaching compressor and turbine pressure ratios up to 2.8 and 4.4, respectively, and isentropic efficiencies around 70%. Comparisons between the experimental data and predictions of models, that are briefly described here, have been carried out. A sensitivity analysis based on the experimentally validated models shows that tip clearance, for both compressor and turbine, and surface roughness of the compressor are key parameters for further improving performance.

American Society of Mechanical Engineers2015

Prototype of a thermally driven heat pump based on integrated organic Rankine cycles (ORC)

Jonathan Demierre, Daniel Favrat, Samuel Henchoz

The concept studied in this work is a low power ORC-ORC system (about 20 kW heat at the condenser) which is composed of an ORC engine cycle driving a reversed ORC heat pump cycle, both using the same fluid. The radial compressor and turbine are directly coupled on the same shaft rotating on refrigerant gas bearings. This gives the system the advantage of being oil-free, fully hermetic and with low maintenance costs. The paper presents the development of an ORC-ORC prototype, with HFC-134a as working fluid. The main critical parts of the system are the compressor-turbine unit, the supercritical evaporator and the pump. The selected type of heat exchanger for the supercritical evaporation is the double tube coil (DTC). A first experimental setup has been built to test the pump and the supercritical evaporator. A comparison has been done between the results obtained with an in-house supercritical evaporator simulation tool and the measurements made on the DTC. The design steps of the compressor-turbine are briefly presented. The compressor-turbine unit has been balanced and tested, with air, at speeds up to 140'000 rpm.

2010

Theoretical and Experimental Study of a Thermally Driven Heat Pump Based on a Double Organic Rankine Cycle

Jonathan Demierre

Nowadays, one of the main alternatives for a more rational use of energy in heating applications is the heat pumping technologies. The market is dominated by two kinds of heat pump systems: the electrically driven vapor compression heat pumps (EHP), which are the most widely used in residential heating applications and the thermally driven heat pumps (TDHP), that are usually based on a sorption process. In this thesis, theoretical and experimental investigations of a concept of thermally driven heat pump, based on the coupling of a vapor compression heat pump cycle and an organic Rankine cycle, are presented. The system is named here as ORC-ORC. The studied concept uses a single stage centrifugal compressor directly coupled to a single stage radial inflow turbine. The shaft is rotating on gas bearings, which allows the system to be oil-free. Like most of the other TDHP's, this system has the advantage to work with a variety of fuels or heat sources like wood pellets, natural gas, solar heat, geothermal heat or waste heat. The concept studied in this work is a gas red system for space heating and domestic hot water production in small residential buildings. A systematic approach has been used to evaluate, in term of energy efficiency, the potential of ORC-ORC systems and to propose optimal design specifications at different levels of the design process. The method is based on the optimization which allows to identify the best configurations at each design step with respect to the designer choices. This approach is divided in three steps. In the first step, a model of the complete system has been developed based on a process integration approach. This step allows to quickly determine whether the system is potentially attractive or not, for given conditions, before going deeper in the design process. The results show that, in general, it is advantageous for the ORC evaporation to be supercritical. In the second step, based on the process integration results and heuristic rules, a suitable system heat exchanger network has been generated and modeled. The predicted values of the coefficient of performance (COP) at design conditions, for the different situations, are in the range 1.30 and 2.19. In a last step, the off-design characteristics of the compressor-turbine unit are considered, in order to evaluate the COP as well as the heating capacity range that can be covered with a specific compressor-turbine unit design. For this purpose, a model of the compressor-turbine unit has been developed. Experimental investigations have been carried out with R134a as working fluid. A first simple test rig has been developed to test the selected ORC evaporator in supercritical conditions. The measurements have allowed to calibrate and to validate an in-house supercritical evaporator simulation tool. An ORC-ORC prototype has been developed and built. Commercially available equipment has been used, except for the compressor-turbine unit that has been designed especially for this application. The targeted operating point, with an HP evaporation temperature of 0 °C and a condensing temperature of 35 °C, has not been reached, because of experimental difficulties which caused some delay. A maximum temperature difference of 20 °C between the HP evaporation and the condensation has been achieved, which corresponds to a compressor pressure ratio of 1.9. Measurements have been performed with compressor-turbine unit rotational speeds up to 171 krpm. The theoretical investigations shows that the proposed ORC-ORC concept is an interesting alternative of thermally actuated heating device for small residential buildings. Concerning the experimental investigations, although a number of problems have been encountered during the tests, it has been demonstrated that the proposed concept is feasible with today's technology.

EPFL2012

Theoretical and Experimental Study of a Thermally Driven Heat Pump Based on a Double Organic Rankine Cycle

Jonathan Demierre

EPFL2012