Andreasen, Jesper Graa

Elmegaard, Brian

Haglind, Fredrik

Online Research Database In Technology

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Downloaded from orbit.dtu.dk on: Dec 20, 2017Utilization of low temperature heat for environmentally friendly electricity productionAndreasen, Jesper Graa; Elmegaard, Brian; Haglind, FredrikPublication date:2014Link back to DTU OrbitCitation (APA):Andreasen, J. G., Elmegaard, B., & Haglind, F. (2014). Utilization of low temperature heat for environmentallyfriendly electricity production. Poster session presented at Conference on Energy and Environment for theFuture, Copenhagen, Denmark.ii “poster” — 2014/11/21 — 16:01 — page 1 — #1 iiiiiiUtilization of low temperature heat forenvironmentally friendly electricity productionJesper Graa Andreasen1,*, Brian Elmegaard1, Fredrik Haglind11 Technical University of Denmark, Department of Mechanical Engineering, Nils Koppels Allé, Building 403, 2800 Kgs. Lyngby, Denmark* Corresponding author, Tel.: +45 4525 4123, Email: jgan@mek.dtu.dk1 MotivationUtilization of low grade heat is not feasiblewith conventional steam Rankine cycles dueto undesirable properties of steam. Insteadthe organic Rankine cycle (ORC) is typicallyused, since it enables the choice of a work-ing fluid, e.g. hydrocarbons, with desirableproperties.3421ExpanderCondenserPump Cooling waterinletHeat sourceinletSuperheaterGeneratorEvaporatorPreheaterFigure 1: A sketch of an organic Rankine cycleOne of the key issues for improving the per-formance of ORCs is to optimize the heattransfer processes for adding and removingheat from the cycle, which can be achievedby employing mixed working fluids. A sketchof the ORC is shown in Fig. 1.2 Optimization of working fluidsThe aim of this project is to evaluate the fea-sibility of using mixtures as working fluids inORCs considering both thermodynamics andeconomics.The hydrocarbon mixture propane/isobutanehas shown high thermodynamic performancein an ORC utilizing a low temperature heatsource at T=120 ◦C [1]. The result of an op-timization of propane/isobutane for differentmole compositions is shown in Fig. 2.0 0.2 0.4 0.6 0.8 110601080110011201140116011801200Mole compositionNet power [kW]isobutane propaneFigure 2: Net power maximization for a propane/isobutane mixtureThe 0.8/0.2 propane/isobutane mixtureachieves the highest net power output, whichis 6.1 % higher than pure propane. The netpower is generally higher for the mixtures,since the non-constant temperature duringphase change enables a reduction in the tem-perature difference in the condenser and theevaporator resulting in a reduction of heattransfer losses.3 Heat transfer analysis (condenser)The heat (Q˙) transferred in a heat exchangercan be expressed as:Q˙ = UA∆Tm (1)where ∆Tm is the mean temperature differ-ence between the heat exchanging streams,U is the overall heat transfer coefficient andA the heat transfer area.The required heat transfer areas for mixturesare generally higher than those of pure flu-ids, due to the reduced temperature differ-ence and a lower heat transfer coefficient.Table 1 shows a comparison of the net powerand heat transfer parameters for isobutane,propane and two mixtures.Table 1: Net power and heat transfer parameters for isobutane, propane andtwo mixturesW˙net [kW] ∆Tm [◦C] U [W/m2-K] A [m2]isobutane 1076 7.6 994 15260.5/0.5mole 1157 6.5 948 19470.8/0.2mole 1192 5.2 1040 2196propane 1123 7.5 1394 1134Figure 3 shows how the temperature differ-ence between the working fluid and the cool-ing water is lower for the mixture comparedto the pure fluid, resulting in higher perfor-mance and larger and more expensive heatexchangers.Heatltransferl[d]0 20 40 60 80 100Temperaturel[°C]1618202224262830323436 Purelisobutane0.8/0.2lpropane/isobutaneCoolinglwater4411FlowloflcoolinglwaterloutsideltheltubeHeatlflow41Condensationloflworkinglfluidlin-tubeFigure 3: A Q˙T -diagram of condensation for isobutane and 0.8/0.2propane/isobutane4 ConclusionThe use of mixed working fluids in ORCs hasthe potential of increasing the net power out-put when utilizing low temperature heat. Onthe other hand, the use of mixtures requireslarger and more expensive heat exchangerscompared to pure fluids. It is therefore neces-sary to carry out economic analyses in orderto assess the feasibility of using mixed work-ing fluids in ORCs.5 References[1] Andreasen J.G., Larsen U., Knudsen T.,Pierobon L., Haglind F., Selection and op-timization of pure and mixed working fluidsfor low grade heat utilization using organicRankine cycles. Energy 2014;73:204-13.

Utilization of low temperature heat for environmentally friendly electricity production

The focus on reduction of fossil fuelled electricity generation has increased the attention on exploitation of low grade heat as the energy source for electricity producing power plants. Low grade heat is heat, which isavailable at a low temperature, e.g. from waste heat from marine diesel engines and industrial processes orfrom geothermal and solar heat sources. Utilization of such heat sources makes it possible to produce electricity with no additional burning of fossil fuel, and does therefore represent an environmentally friendly alternative to fossil fuel based electricity production. Utilization of low grade heat is not feasible with conventional steam Rankine cycles (steam engines) due to undesirable properties of steam. Instead the organic Rankine cycle is typically used, since it enables thechoice of a working fluid, e.g. hydrocarbons or refrigerants, with desirable properties. One of the key issues for improving the performance of organic Rankine cycles is to optimize the heat transfer processes for adding and removing heat from the cycle, which can be achieved by employing a working fluid consisting of a mixture of two or a number of pure fluids.This project is aimed at quantifying the benefits of using mixtures compared to pure fluids as working fluids in organic Rankine cycles. In order to do so, thermodynamic and economic analyses are carried out, first on an overall cycle level, and next on component level including detailed modelling of heat exchangers, pumps and expanders involving project collaborators with expertise in these areas. In addition to this, novel innovative cycle layouts are developed with the aim of increasing the economic feasibility of utilizing low temperature heat. As an example, this can be achieved by implementing separators in the power cycle to create optimal variations in mixture composition throughout the cycle (equivalent of combining a power cycle with a distillation process). In collaboration with DTU Chemical Engineering, a search for novel pure fluids and mixtures are initiated in order to develop working fluids that are tailored for maximizing the profitability of the power cycles; both the organic Rankine cycle and the novel power cycles

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Utilization of low temperature heat for environmentally friendly electricity production

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