This paper deals with the modelling and optimisat ion of a series of typical hydrogen systems. The systems models have been developed on the TRNSYS 1 platform. The mathematical models and the methodology used for analysis are de scribed in this paper, as well as the validation of the models. An application on an island in Norway is also presented

Ete, Arnaud

Hoffheinz, Geoff

Kelly, Nick

Ulleberg, Øystein

University of Strathclyde Institutional Repository

Ete, Arnaud and Hoffheinz, Geoff and Kelly, Nick and Ulleberg, Øystein (2008) Hydrogen energy systems : development and application of modelling tools. In: 10th World Renewable Energy Congress, 2008-07-19 - 2008-07-25. , This version is available at https://strathprints.strath.ac.uk/9301/Strathprints is  designed  to  allow  users  to  access  the  research  output  of  the  University  of Strathclyde. Unless otherwise explicitly stated on the manuscript, Copyright © and Moral Rights for the papers on this site are retained by the individual authors and/or other copyright owners. Please check the manuscript for details of any other licences that may have been applied. You may  not  engage  in  further  distribution  of  the  material  for  any  profitmaking  activities  or  any commercial gain. You may freely distribute both the url (https://strathprints.strath.ac.uk/) and the content of this paper for research or private study, educational, or not-for-profit purposes without prior permission or charge. Any correspondence concerning this service should be sent to the Strathprints administrator: strathprints@strath.ac.ukThe Strathprints institutional repository (https://strathprints.strath.ac.uk) is a digital archive of University of Strathclyde research outputs. It has been developed to disseminate open access research outputs, expose data about those outputs, and enable the management and persistent access to Strathclyde's intellectual output. 1										 Arnaud Eté1, Geoff Hoffheinz2, Nick Kelly1, Øystein Ulleberg3 (Authors in alphabetical order) 1 ESRU, Dept. of Mechanical Engineering, University of Strathclyde, Glasgow, UK, Tel: +440141054802854, Email: arnaud.ete@strath.ac.uk; nick@esru.strath.ac.uk, 2 SgurrEnergy Ltd., Glasgow +4401410 43304668, 3  Dept. of Energy Systems, Institute for Energy Technology, Kjeller, Norway, Tel: +47063080063084, oystein.ulleberg@ife.no   : This paper deals with the modelling and optimisation of a series of typical hydrogen systems. The systems models have been developed on the TRNSYS1 platform. The mathematical models and the methodology used for analysis are described in this paper, as well as the validation of the models. An application on an island in Norway is also presented.     	The so0called hydrogen economy and the energy systems associated with it are often presented as the means to solve both global warming and depletion of fossil fuel resources. However, the technology is still immature and the performance of prototype hydrogen systems is often disappointing when compared to conventional alternatives. This poor performance can often be attributed to poor design of the system and controls, coupled with ad0hoc selection of components. However, analysis using computer models can help to improve the situation, leading to more robust hydrogen demonstration system designs, and move what is a research field towards technical and commercial viability.   The University of Strathclyde in Glasgow and SgurrEnergy, an engineering consultancy specialising in renewable energy in Glasgow, have been working in collaboration for two years on developing generic computer models suitable for the analysis and optimisation of hydrogen system designs. These systems models make use of a suite of hydrogen component models developed by Institute for Energy Technology in Norway. 	  			 PV generator The model of the PV generator is based on an equivalent circuit of a one0diode as shown in Fig. 1.  The relationship between the operation current I and the operation voltage U of the equivalent circuit can be expressed by:  shSSLshDLRIRUaIRUIIIIII+−− +−=−−= 1exp0   with IL the light current (A), ID the diode current (A), Ish the shunt current (A), I0 the diode reverse current (A), Rs and Rsh the series and shunt resistance (B), a the curve fitting parameter.  Fig. 2. shows typical current0voltage and power0voltages characteristics for a PV generator.                                                        1 TRaNsient SYstem Simulation program: http://sel.me.wisc.edu/trnsys/  2 Fig. 1. The equivalent electrical circuit for the PV cell    Fig. 2. Typical I0U and P0U characteristics for a PV generator  Power conditioning equipments Power conditioning equipments are used to operate the electrolyser and the fuel cell. The model is based on empirical efficiency curves and an expression for the power loss is: 220 outoutioutoutSoutinloss PURPUUPPPP ++=−=   where US is the set point voltage (V) and Ri is the internal resistance(B). With respect to the nominal power Pnom of the power conditioner, the previous equation can be expressed as: 220 1 ⋅⋅+⋅++=nomoutnomoutipnnomoutoutSnomnominPPPURPPUUPPPP  Electrolyser The model for the high pressure alkaline electrolyser shown in Fig. 3 is based on a combination of thermodynamics, heat transfer and empirical electrochemical equations. The electrochemical model is based on a temperature dependent I0U curve for a given pressure (Fig. 4) and a Faraday efficiency curve independent of temperature and pressure.   The electrode kinetics of an electrolyser cell is modelled using empirical I0U relationships. Overvoltages and ohmic resistance play an important role in the modelling of the I0U curve:  ( ) +++×+++++= 1//log2321232121 IATtTttTsTssIATrrUU rev   being U the operation cell voltage (V), Urev the reversible cell voltage (V), ri the parameters for the ohmic resistance of electrolyte (B), si and ti the parameters for the overvoltage on the electrodes, A the electrode area (m2), T the temperature of the electrolyte (°C) and I the current through the cell (A).   Fig. 3. Scheme of a high pressure alkaline electrolyser  Fig. 4. Typical I0U curve for an electrolyser   3Proton0Exchange Membrane Fuel Cell The PEMFC model shown in Fig. 5 is largely mechanistic, with very theoretical basis.  The performance of a fuel cell is represented by its output voltage. The voltage of the single cell is calculated as: ohmicactcell EU ηη ++=   where E is the thermodynamic potential, ηact is the anode and cathode activation overvoltage (the voltage loss associated with the electrodes), and ηohmc is the ohmic overvoltage (the losses associated with the proton conductivity of the solid polymer electrolyte and electronic internal resistances).   E is defined through the Nernst equation:  ( ) ( )5.022ln0000431.029800085.023.1 OHstackstack ppTTE ⋅⋅⋅+−⋅−=   ηact is based on the theoretical equations from kinetic, thermodynamic and electrochemistry fundamentals: ( ) ( ) ( )2ln000076.0ln000192.0ln000192.000243.095.0 OstackFCstackPEMstackstackact cTITATT ⋅⋅+⋅⋅−⋅⋅+⋅+−=η Finally, ηohmc is totally empirical, based on temperature and current experimental data: ⋅+⋅+⋅ −⋅⋅⋅−=364.113536.3exp8PEMFCPEMFCstackstackPEMPEMFCohmicAIAITTAtIγη   Fig. 6 shows the cell voltage variation as a function of the current.   Fig. 5. Scheme of an PEM fuel cell  Fig. 6. Typical current0voltage curve for a PEMFC  Compressor The compressor model is based on a two0stage polytropic compression process with intercooling. The total compression work for the process is given by:  ( )compgascompWWnWη21. +=  with −−= −nnxPpnnRTW111 11 and −−= −nnxPpnnRTW122 11 being gasn.the gas flow (mol.s01), W1 and W2 the polytropic works (J.mol01), compη  the compressor efficiency, n the polytropic coefficient, R the universal gas constant (8.314 J.K01.mol01) , T the inlet gas temperature (K) and pi the pressure values (Pa).   4Gas storage The compressed gas storage model calculates the pressure in the tank based on either the ideal gas law or van der Waals equation of state for real gases. According to the ideal gas law, the pressure of a gas storage tank is given by:  VTRnpgas⋅⋅=   For real gases, the Van der Waals equation of state indicates that the pressure of gas stored in a tank is:  22VnabnVTRnpgas ⋅−⋅−⋅⋅=   with crcrpTRa⋅⋅⋅=6427 22and crcrpTRb⋅⋅=8  where Tcr is the critical temperature (K) and pcr the critical pressure (Pa) of the gas.  ! "		#		The models described previously have been successfully validated using a combination of analytical tests and empirical comparisons. The analytic tests included the use of typical curves found in the literature and other energy systems modelling tools.  Fig. 7 and 8 show the current0voltage curve and the power curve obtained from the simulation of the Flagsol (KFA) 150 cell module at different temperatures. Fig. 9 presents the current0voltage curve of the same module at different solar radiation levels. At standard test conditions (25°C and 1000 W/m2 of solar radiation), the simulation indicates a 173.18 W maximum power point. Fig. 10 presents the effect of temperature on the simulated performance of the PHOEBUS alkaline electrolyser. Finally, the effect of temperature on the performance of a typical PEM fuel cell is simulated on Fig. 11 and 12.   Fig. 7. PV current at different temperatures  Fig. 8. PV power at different temperatures  Fig. 9. PV current at different irradiation levels  Fig. 10. Temperature dependence of an electrolyser  5 Fig. 11. PEMFC voltage at different temperatures   Fig. 12. PEMFC power at different temperatures  All these results are typical and verify the results available in the literature. They prove that the components models are accurate and that simulating hydrogen systems with TRNSYS provides reliable results.  $ 			#	Models of four generic hydrogen systems based on the components models previously presented have been developed on the TRNSYS platform. The models cover the broad spectrum of prototype hydrogen demonstration systems emerging around the world. These are:  0 a stand0alone power system 0 low power application 0 filling station 0 energy buffering system for large scale renewables  TRNSYS can be particularly tricky to use. To make the models easier to use, TRNSED applications have also been designed. TRNSED is a customised interface that presents a simplified view of the TRNSYS file with only a few selected parameters available to users.  A cost0benefit analysis model has been included to analyse the economic performance of the systems. The total net present cost of the system and the levelized cost of energy are calculated.   The model also includes an optimisation method to optimise the technical and economic performance of the systems modelled.    Fig. 13. TRNSYS model of a Stand0Alone Power System   6 Fig. 14. Flow chart of the optimisation process  % 	The models have also been successfully applied to the analysis of two real systems: the HARI Project in Loughborough, UK and the Utsira Project in Norway.  Utsira is a small island community located approximately 18 kilometres off the west coast of Norway. The Utsira project is the world’s first full0scale stand0alone renewable energy system where the energy balance is provided by wind and stored hydrogen.  To be completed…. The application of the models improved confidence in their predictions and capabilities and also highlighted problems in the design and operation of the real and prospective systems including: poor control strategies, poor sizing of system components and mismatches of energy supply and demand.   & '	 TRNSYS 16, Volume 5, Mathematical Reference. University of Wisconsin0Madison, 2006  Rank the feasible solutions by total net present cost and select the cheapest one Create a parametric table with all the system configurations to be simulated Run the simulation with the optimal sizes of components Analyse the technical and economic performance of the optimal system Each feasible solution is saved Non0feasible ones are ignored Repeat the optimisation process until a refined optimal system is found The optimal configuration is found (Start) (End) 

Hydrogen energy systems : development and application of modelling tools

This paper discusses hydrogen energy systems with reference to development and application of modelling tools

Hydrogen energy systems : development and application of modelling tools

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