Prototype of a thermally driven heat pump based on integrated Organic Rankine Cycles (ORC)

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    an oil-free high speed dynamic compressor-expander assemblyrotating on refrigerant vapor bearings. Unfortunately earlierattempts of the latter failed because of the lack of appropriatematerials and because of the problem of the depletion of the ozonelinked to the CFC refrigerants, which were, at the time, the onlynon-ammable and non toxic refrigerant candidates for such high

    technical feasibility of a thermally driven heat pump based ondouble Rankine cycle using a high speed oil-free Compressor-Turbine Unit (CTU).

    2. ORCeORC high speed concept

    An ORCeORC system includes an ORC power cycle drivinga reversed Rankine heat pump cycle (see Fig. 1). The condenser andthe subcooler, if introduced, are common to both cycles. This

    * Corresponding author. Tel.: 41 21 693 67 29; fax: 41 21 693 35 02.

    Contents lists available at

    Ener

    elsevier .com/locate/energy

    Energy 41 (2012) 10e17E-mail address: jonathan.demierre@ep.ch (J. Demierre).upgrade of the renewable heat from the environment. Thermallydriven heat pumps can use various fuels including wood pellets ornatural gas and are usually based on absorption heat pump cyclesor on a combination of a power cycle driving a compression heatpump cycle or a combination of both. One concept of the secondtype has been proposed by Strong [1] and is based on the use of anORC power cycle driving a reversed ORC heat pump cycle, bothusing the same uid. Such a concept can be made using a scrollexpander and a scroll compressor usually lubricated with oil or by

    one concept of miniature high speed centrifugal compressordirectly driven by a high speed electric motor rotating on refrig-erant gas bearings [2,3] opens the way to such devices with orwithout electric motor.

    The aim of this paper is to present the development of a ther-mally driven heat pump prototype based on a double ORC. Adescription of the principle is given at Section 2. The applicationthat is studied in this work is residential heating for small buildings.The goal of developing such a prototype is to demonstrate theSupercritical evaporatorGas bearingsResidential heatingOil-free

    1. Introduction

    Single combustion in boilers is a vThe higher cost of fuels and concernput more and more political pressurfor most building heating purposelinked to the use of heat pumps0360-5442/$ e see front matter 2011 Elsevier Ltd.doi:10.1016/j.energy.2011.08.049obtained with an in-house supercritical evaporator simulation program and measurements made on theDTC is presented. The design steps of the compressor-turbine are briey presented. The compressor-turbine unit has been balanced and tested, with air, at speeds up to 140,000 rpm.

    2011 Elsevier Ltd. All rights reserved.

    fcient heating process.t pollution are likely toevent the use of boilerster alternatives are allllow the recovery and

    temperature cycles. Progress in materials and the emergence ofnew uids, to substitute the CFCs with a reasonably high temper-ature chemical stability and/or acceptance, allow the reconsidera-tion of these ORCeORC concepts. At present, HFC-134a or R600 arekey working uid candidates, together with new low GlobalWarming Potential (GWP) refrigerants being in developmentwithin the same range of pressures. The recent demonstration ofThermally driven heat pumpORCheat exchanger for the supercritical evaporation is the double tube coil (DTC). A rst experimental setuphas been built to test the pump and the supercritical evaporator. A comparison between the resultsKeywords:system are the compressor-turbine unit, the supercritical evaporator and the pump. The selected type ofPrototype of a thermally driven heat puCycles (ORC)

    J. Demierre*, S. Henchoz, D. FavratEcole Polytechnique Fdrale de Lausanne, LENI-IGM-STI, Station 9, 1015 Lausanne, Swi

    a r t i c l e i n f o

    Article history:Received 7 October 2010Received in revised form24 August 2011Accepted 28 August 2011Available online 25 September 2011

    a b s t r a c t

    The concept studied in thiheat at the condenser) andcycle, both cycles using thdirectly coupled on the samadvantage of being oil-fredevelopment of an ORCeO

    journal homepage: www.All rights reserved.p based on integrated Organic Rankine

    land

    ork is a low power ORCeORC heat pump system (providing about 20 kWat is composed of an ORC power cycle driving a reversed ORC heat pumpame uid. The centrifugal compressor and the radial in-ow turbine arehaft rotating on self-acting refrigerant vapor bearings. The system has theully hermetic and with low maintenance costs. The paper presents theprototype, with HFC-134a as working uid. The main critical parts of theSciVerse ScienceDirect

    gy

  • Nomenclature

    Latin lettersDh0i fuel lower heating value (J/kg)_E mechanical power (W)_Q heat rate (W)_m mass ow rate (kg/s)

    Nu hfl, Nusselt number ()

    Pr cpml, Prandtl number ()

    Ref Cfm=r

    , Reynolds number based on ()A heat transfer area (m2)

    l thermal conductivity (W/(m K))m dynamic viscosity (Pa s)

    4cross sectional areaperimeter

    , hydraulic diameter (m)

    r density (kg/m3)

    Indices1 condenser (PHX) outlet, refrigerant2 evaporator (DTC) inlet, refrigerant3 evaporator (DTC) outlet, refrigerant4 condenser (PHX) inlet, refrigerantC1 condenser (PHX) inlet, cooling waterC2 condenser (PHX) outlet, cooling waterF fuelfumes fumes resulting from the combustion

    J. Demierre et al. / Energy 41 (2012) 10e17 11C ow velocity (m/s)cp isobaric specic heat (J/(kg K))COP Coefcient of Performance, ()

    f dPdL

    f

    12rC2

    , friction factor ()

    h heat transfer coefcient (W/(m2 K))system works between three main temperature levels and istherefore similar to an absorption heat pump. The thermally drivenheat pump dealt with in this paper uses a supercritical evaporatorat the high temperature of the topping ORC and a geothermal probeat the low temperature evaporator of the heat pump cycle. Heat issupplied to the house heating system at medium temperature bya condenser. The studied concept is a relatively low heat rate

    k roughness (m)L tube length (m)nsc compressor specic speed ()nst turbine specic speed ()P pressure (Pa)T temperature (C)U overall heat transfer coefcient (W/(m2 K))u standard uncertainty

    Greek lettersU error on the overall heat transfer coefcient between

    simulation and measurements ()hc compressor isentropic efciency ()ht turbine isentropic efciency ()

    condenser

    supercritical evaporator

    evaporator

    compressoturbine uni(CTU)

    Fig. 1. Schematic owsheet and T-s diagramH1 evaporator (DTC) inlet, hot thermal oilH2 evaporator (DTC) outlet, hot thermal oillm logarithmic meanmeas measuredpump power cycle pumpR refrigerantsimu predicted by simulationW watersystem (about 20 kW thermal at the condenser in the case treatedhere) with a one-stage centrifugal compressor and a one-stageradial in-ow turbine. The compressor and the turbine aredirectly coupled on the same shaft rotating on self-acting refrig-erant vapor bearings. This allows the system to be oil-free, fullyhermetic and with low maintenance costs in spite of the morecomplex circuitry. Because of the characteristics of dynamic

    Superscipts the entity is received by the system from the outsidee the entity is delivered by the system to the outside

    AcronymsCTU Compressor-Turbine UnitDTC Double Tube Coil heat exchangerEU European UnionGWP Global Warming PotentialORC Organic Rankine CyclePHX Plate Heat eXchanger

    r-t

    Entropy

    Tem

    pera

    ture

    of a simple ORCeORC heat pump unit.

  • is the heat supplied by the fumes resulting from the methanecombustion and _E

    pump is the electrical power consumed by the

    4. Prototype layout

    The layout of the prototype is shown in Fig. 4. For this rstprototype, instead of having a unique condenser for both cycles, itwas decided to have one condenser for each cycle for an easiercontrol of the system. This gives the possibility to test the turbineand the compressor more independently, since, with this layout,the turbine outlet pressure and the compressor outlet pressure canbe different. Two on/off valves enable the connexion of bothcondensers whenwewish to simulate a unique condenser. The CTUhousing is connected to the low pressure of the heat pump cycleand a valve enables to regulate the pressure in the gas bearings. Theturbine bypass and the valves at the turbine inlet and outlet allowto start and heat up the cycle without having the working uid toow through the turbine. This allows to start the turbine only whenthe refrigerant vapor at the inlet is entirely dry. In fact, at highspeed, any droplet would damage the CTU rotor. A vertical tubewith a large section (about 150 mm diameter) after the heat pumpcycle evaporator acts as a separator to avoid droplets at the inlet of

    nergORCeORC pump (which is the only electrical power consumed bythe ORCeORC). _E

    pump is divided by 0.56 to take into account for the

    efciency of a modern combined cycle power plant that producesthis electrical power, in order to dene a COPwhich is strictly basedon the conversion of the fuel (here, methane) into heat. For a rstcompressors and turbines a low density uid is preferred andrefrigerant HFC-134a has been selected for the rst prototype. It ischemically stable at relatively high temperatures (at least up to180 C).

    3. Preliminary design

    A preliminary designwas made based on the results of previousstudies [4,5]. An ORCeORC system design and optimization toolhad been developed. This computer tool consists of a modeldeveloped on a commercial owsheeting software, Belsim-Vali [6]that is linked to an in-house energy integration tool [7] and anin-house multiobjective optimization tool [8].

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