Simulation of a concentrating PV/thermal collector using TRNSYS

Transcription

1 Simulation o a concentrating PV/ermal lector using TRNSYS Abstract Centre or Sustainable Energy Systems Australian National University Canberra 0200 ACT AUSTRALIA Telephone: Facsimile: joe@aceng.anu.edu.au A TRNSYS component (Type 262) has been written to simulate a concentrating PV/Thermal lector. The component is based on a dynamic model o a concentrating PV/Thermal lector, which includes ermal capacitance eects, and detailed equations describing e temperature dependent energy low between e lector and surroundings. The CHAPS system, a 30x concentration parabolic trough PV/Thermal lector developed at e ANU, has been used to validate e accuracy o e Type 262 TRNSYS component. Results are presented comparing e annual output o a domestic CHAPS system at integrates e Type 262 lector, wi a lat plate solar hot water system and a PV array located side-by-side. 1 Introduction The Combined Heat and Power Solar System, or CHAPS system being developed at e Australian National University, is a concentrating parabolic trough system at combines photovoltaic (PV) to produce electricity, wi ermal energy absorption to produce hot water. The current status o e CHAPS project is described in Coventry et al. (2002) as well as details about e major components o e system and preliminary perormance data. This paper outlines a eoretical model o e lector, and shows some early validation results. Furer validation is necessary, particularly because electrical perormance data has not yet been available. Wi is proviso, e component has been integrated into a ull domestic hot water system model at includes a hot water tank, pump, controller, weaer data and water draw-o proile. Early results o simulations are presented. 2 The PV/T TRNSYS component TRNSYS is a transient simulation package used extensively to model solar systems, in particular heating, cooling and domestic hot water applications (Solar Energy Laboratory, 2000). TRNSYS relies on a modular approach to solve large systems o equations described by Fortran subroutines. Each Fortran subroutine (called a type) conta a model or a system component. A detailed analytical PV/Thermal TRNSYS component has been written (Type 262), based closely on e equations outlined in is paper. Furer technical detail about e component can be ound in e ANU reerence manual (Coventry et al., 2001). There have been oer PV/T models written, such as e Type 50 PV/Thermal lector available in e standard TRNSYS library. This model is based on modiications to e Hottel-Whillier-Bliss equations at are used or e standard Type 1 Flat Plate Solar Collector (Florschuetz, 1979) however it does not account or iation losses and has no ermal capacitance, and is ereore not considered to be detailed enough to simulate a CHAPS lector. It is also an empirical model and ereore it is diicult to model variations in physical subcomponents o e PV/T lector. A lat plate PV/T model is also under development at e Danish Teknologisk Institut (Bosanac, 2001). 3 Theoretical Formulation A dynamic model o a concentrating PV/T lector has been developed in order to simulate bo ermal and electrical perormance. The equations describe e ermal perormance wi reasonable precision However e electrical perormance is much simpliied, and is discussed in urer detail in Coventry et al. (2002). The CHAPS receivers are made up o solar mounted on an aluminium extrusion bonded to a copper pipe containing e heat transer luid, as shown in Figure 1. Figure 2 shows e reerence system or e dynamic model.

2 Simulation o a concentrating PV/ermal lector using TRNSYS Copper pipe Insulation Water Aluminium cell tray Silicone rubber Glass cover Figure 1. Cross-section o a CHAPS receiver. Solar cell Fluid in Receiver Fluid out Figure 2. Reerence system or e ermal model o a CHAPS receiver. Equation 1 describes e change in temperature o e luid in e receiver wi respect to time: dt ( m C m C ) + m& C ( T - T ) = & ( T) p- + p- p- inlet (1) m, m, mass low, T and dt C p - and p inlet x-direction C - are e mass and speciic heat terms or e lector and e luid, m& is e luid & is e energy low. T are e outlet and inlet luid temperatures, and ( T) The temperature and illumination gients perpendicular to e low, and e conductive heat transer parallel to e low are neglected. The lector is divided up into a series o elements along its leng. In e case o a concentrating lector, it is enient to divide e lector up by elements o leng equal to at o a solar cell. It is assumed at each element can be characterized by a single temperature T. Equation 1 is derived rom e energy balance o a control element, taking into account: The change in energy content o e element The energy transer by e luid low The temperature dependent energy low between e element and surrounding, A line heat source. Alough e ermal energy low ( T) to e solution o e equation is to base e calculation o ( T) & is a unction o temperature, which changes wi time, a numerical approach & on e temperature o e element a short time earlier. Thereore, equation 1 can be rearranged to orm a irst order dierential equation. This meod o solution o e energy balance equation is suggested in e TRNSYS reerence manual (Solar Energy Laboratory, 2000) or components wi a temperature response dependent on time. T t + AT = B A = ( m C + m C ) m& p - C p- p - and B = & ( m C + m C ) + m& p - C p- T inlet p - (2) Solving or T wi respect to time t allows an average outlet temperature T over some small time interval o t to t+_t, to be calculated by integrating e outlet temperature: T = 1 ADt Ê Á Ë B A + T ˆ ADt B ( e -1) A initial - (3) 2 Proceedings o Solar Australian and New Zealand Solar Energy Society Paper 1

3 Simulation o a concentrating PV/ermal lector using TRNSYS T initial is e outlet temperature at a starting time t. The value ( T) & o can be calculated by solving a set o non-linear equations at physically describe e temperature dependent energy low between e element and e surroundings. The ermal network describing is arrangement is shown in igure 3. Surroundings Surroundings elec Glass cover Solar Absorber plate Fluid Collector mass Absorber tube Figure 3: Thermal network describing a PV/T concentrating lector. The main terms o e energy balance are described below: Sun input : total direct iation I d absorbed by e solar as relected by e mirror area A mirror, including t, and absorptivity o e solar a. It also includes scaling actors or e transmissibility o e cover accuracy F shape and relectivity mirrors F dirt. = I A r F F 1 d mirror mirror r mirror o e concentrator, e shading o e mirrors F shade, and e dirtiness o e shape dirt ( - Fshade ) t a & (4) Thermal output : e energy transerred into e water (not including e eect o ermal capacitance). U pt is e heat transer coeicient between plate and tube, and A pt e area o contact. T tube and T are e temperatures o e tube and luid respectively, and A t e surace area o e ide o e tube. The ection coeicient h cw is deined below. ( T - T ) = h A ( T T ) & (5) - = U pt A pt plate tube c - w Electrical output elec t tube : derived rom e simpliied maximum power output expression given in Wenham et al. (1994). Sometimes a simpler linearised relationship is used, however is becomes more inaccurate at e higher temperatures possible wi a concentrating PV/T lector. The reerence eiciency h is measured at a reerence temperature T re, usually 25 C. b is e temperature coeicient giving e relationship between solar cell eiciency and temperature (around or silicon solar ), and T e temperature o e solar. & ( ( T T ) & (6) elec = hre exp b - a Radiation loss re ; is opaque to iation emitted rom e, and ereore e cover becomes e emitting surace. s is e Stean-Boltzmann constant, _, A are e emissivity and area o e cover, T e temperature o e outer surace o e, and T amb is e ambient temperature. It is assumed at e surroundings are at ambient temperature. 4 4 ( T T ) = s e A - (7) amb re Proceedings o Solar Australian and New Zealand Solar Energy Society Paper 1 3

4 Simulation o a concentrating PV/ermal lector using TRNSYS Convection loss : can be calculated analytically (see or example Duie and Beckman (1974)). However, a simpler empirical approach is used, hc - a is e ection coeicient, u wind is e wind speed and c 0, c 1, and c 3 are coeicients derived by parameterisation studies described in Section 4. The signiicance o wind speed dominates oer actors such as e tilt o e receiver. = hc - a A ( T T ) & (8) - amb h = c + c u + c u 2 c- a 0 1 wind 2 wind (9) Loss rough e ulation : it is assumed at e heat transer coeicient U rema reasonably constant wiin e range o operating temperature and at e ulation ickness is uniorm. A is e area o ulation, and T plate is e temperature o e absorber plate surrounded by e ulation. It is assumed at e temperature dierence between e outer cover and ambient is very small, and ereore detailed calculations o ection and iation losses rom e cover are unnecessary. & (10) = U A - ( Tplate Tamb ) Convection coeicient between e tube and e luid h c-w : is changes signiicantly, depending on wheer or not e luid low in e pipe is laminar or turbulent. Empirical relations describing how e ection coeicient can be calculated can be ound in Holman and White (1992), Incropera and DeWitt (1990), and similar texts. The mass low is reasonably constant or e application described in is paper, and ereore to simpliy e calculation in e model, e ection coeicient is entered as a parameter in e TRNSYS model and assumed to be constant along e receiver. Oer relationships required to determine ( T - Tplate) = & & U A + are: = (11) ( T - T ) = & & U A + = (12) & + & + & = (13) elec and are e energy transers rom solar to e plate and cover respectively. U, U, A and A are e heat transer coeicients and areas o e cell-plate and cell- interaces respectively. 4 Validation Experimental validation o e PV/T component has commenced, but is yet to be inalised. The validation work us ar has ocused on ermal output, as experimental data or electrical output power has not been available. Data rom a number o ny days wi a range o input conditions measured at ree-minute intervals were selected, and system parameters at were easy to measure (such as mirror dimensions, solar cell area, etc) were ixed in e model. Oer parameters, such as e coeicients determining e relationship between wind speed and ection losses, were ound by parameterisation techniques using GenOpt, which is a generic optimisation program developed at e Lawrence Berkeley National Laboratory (Wetter, 2000). An objective unction was deined in TRNSYS based on e dierence between e predicted and measured ermal output. GenOpt was en used to minimise is unction by running e Nelder-Mead-O Neill algorim to vary e selected parameters. Figure 4 shows e plot o predicted and measured ermal energy and outlet temperature or e compiled data. The large spikes are not real as ey relate to e changeover time steps between dierent data sets. These spikes and oer anomalies are iltered out or e purposes o e parameterisation wi GenOpt. The results or is seem accurate when e outlet temperatures are compared, however e ermal energy outputs show at a small dierence in output 4 Proceedings o Solar Australian and New Zealand Solar Energy Society Paper 1

5 Simulation o a concentrating PV/ermal lector using TRNSYS temperature results in a signiicant dierence in ermal energy low. Measured ermal energy output Simulated ermal energy output Figure 4. TRNSYS output window showing simulated and measured ermal energy output. The concurrent lines show e measured and simulated output temperature. Furer work will be carried out to gaer more data rom a range o weaer and inlet conditions and ine-tune e PV/T component necessary to improve e accuracy o e model. 4.1 Domestic CHAPS system model A simulation has been built at models e irst domestic CHAPS system prototype located on a mock-up roo at e Faculties Teaching Centre at e ANU (described in detail in Coventry et al. (2002)). The model includes two CHAPS lectors, a tank, a demand proile, and shading losses. The simulation compares e CHAPS system to a solar hot water system o an equivalent size mounted on e same roo, using two Solahart Type K black chrome lectors modeled wi e Type 1 TRNSYS component. Data about e eiciency o ese lectors is obtained rom e Solar Rating and Certiication Corporation (2000). The CHAPS system is also compared to an equivalent sized photovoltaic array, based on ree high-eiciency BP5170 modules, and modeled using e TRNSYS Type 94 component. Weaer data or Canberra compiled by e University o NSW (Morrison and Litvak, 1988) was used or all ree systems. The data is a compilation o mons rom dierent years, chosen such at ey relect long-term averages or e particular mon. The hot water demand proile or e CHAPS and Solahart systems was based on an energy draw proile set out in e Australian Standards AS A Solahart Streamline tank is modeled using a Type 140 tank wi 10 ermal zones, a ermostat set to 60 C, and a 3.6kW auxiliary heating element, in e second and ird zone rom e top o e tank respectively. The UA-value was 2.27 W/K or e 300L tank. The tank employs simple delta T control, wi e over-temperature cuto set to 95 C. Shading o e rear mirror is calculated using equations particular to e geometry o e CHAPS system. Solar energy raction is used as a measure o annual ermal perormance, deined as 1 aux / dem, aux is e auxiliary energy use in heating water and dem is e annual hot water demand. As indicated in table 2, e perormance o e CHAPS system annually compares very well wi bo e lat plate hot water lector and e PV array. System Hot water demand Auxiliary energy use Electrical energy generated Incident iation kwh/year KWh/year KWh/year kwh/year Solar hot water energy raction Annual electrical eiciency Photovoltaic array 842 7, % Solar hot water system 3,486 1,434 7, % CHAPS system 3,486 1, , % 9.9% Table 2. Comparison o e CHAPS system wi entional solar hot water and PV systems. Proceedings o Solar Australian and New Zealand Solar Energy Society Paper 1 5

6 Simulation o a concentrating PV/ermal lector using TRNSYS The primary motivation or a CHAPS system is to bring down e cost o bo renewable electricity and hot water. Thereore, given at e annual output o e CHAPS system is similar to separate lat plate PV and solar hot water systems *, savings could be achieved i e cost o e system is lower an e sum o cost o e separate systems. Early cost estimates or e CHAPS system are positive in is regard, however urer cost inormation will not be available until e irst 30 systems are talled in e coming 12 mons as part o a broader trial supported by e Sustainable Energy Development Auority o NSW. Signiicant roo space savings are also achieved, wi e CHAPS system occupying around hal e area o equivalent sized separate PV and solar hot water systems. 5 Conclusion A simulation model o e CHAPS lector has been developed based on analytical equations describing e ermal and electrical perormance o a concentrating PV/Thermal lector. The lector model has been integrated into a ull domestic CHAPS system model to show at e annual ermal energy output is similar one o e best available lat plate solar hot water lectors, and e annual electrical energy output is similar to one o e highest eiciency available PV modules. 6 Acknowledgements The work described in is paper has been supported by e Australian Cooperative Research Centre or Renewable Energy (ACRE). ACRE's activities are unded by e Commonweal's Cooperative Research Centres Program. Joe Coventry has been supported by ACRE Postguate Research Scholarships. Thanks to Chris Bales or his help and experience wi TRNSYS, and to Mike Dennis or his shading calculations. 7 Reerences Bosanac, M., Teknologisk Institut, Denmark, Personal communication Coventry, J., Kreetz, H., and Dennis, M., 2001: Technical manual - TRNSYS Components developed at e ANU. Coventry, J., Franklin, E., and Blakers, A., 2002: Thermal and electrical perormance o a concentrating PV/Thermal lector: results rom e ANU CHAPS lector. ANZSES Solar Energy Conerence, Newcastle. Duie, J. A. and Beckman, W. A., 1974: Solar energy ermal processes. 1st Edition ed. John Wiley & Sons Inc Florschuetz, L. W., 1979: Extenssion o e Hottel-Whillier Model to e Analysis o Combined Photovoltaic/Thermal Flat Plate Collectors. Solar Energy, 22, Holman, J. P. and White, P. R. S., 1992: Heat Transer. 7 ed. McGraw-Hill Inc. Incropera, F. P. and DeWitt, D. P., 1990: Fundamentals o heat and mass transer. 3rd ed. John Wiley & Sons. Solar Energy Laboratory, 2000: TRNSYS Reerence Manual, University o Wisconsin-Madison Morrison, G. and Litvak, A., 1988: Condensed Solar Radiation Data Base or Australia Report 1, Solar Thermal Energy Laboratory, University o NSW, 13 pp. Solar Rating and Certiication Corporation, 2000: Directory o SRCC Certiied Solar Collector and Water Heating System Ratings. Wenham, S. R., Green, M. A., and Watt, M. E., 1994: Applied Photovoltaics. Centre or Photovoltaic Devices and Systems, University o NSW, 290 pp. Wetter, M., 2000: GenOpt Generic Optimisation Program. Lawrence Berkeley National Laboratory. * or e simulated tallation and location. 6 Proceedings o Solar Australian and New Zealand Solar Energy Society Paper 1