programs for hybrid PV systems
Off-grid PV systems and, in particular, hybrid systems are characterised by a high degree of complexity at the dimensioning stage, where energy sources and energy storage systems must be sized to match estimated energy needs and the expected availability of solar radiation. For this reason, as in many similar fields, software design and simulation tools are an important aid. There is already a broad diversity of such programs on the market. Some are very comprehensive and perform their calculations down to a very detailed level, whereas others are rather more suited for fast »coarse dimensioning«. The various programs integrate different sets of technologies (PV, wind, additional generators, etc.) and some also include economic calculations. The costs of the software tools, in terms of initial purchase price, ongoing maintenance costs, and the time required to learn how to use the tool, also vary significantly. These differences often make it difficult to select the best package for the task at hand.
1. PV-SPS (Version 3.0)
PV-SPS (PV Stand-alone Power Systems) is a dimensioning program for PV-diesel off-grid systems based on Excel spreadsheets. It performs its calculations to the Australian industry standards and was developed by the Australian Business Council for Sustainable Energy (BCSE). It demands a certain level of prior knowledge for the specification of the loads and system components and is thus aimed above all at experienced users in the off-grid field. Furthermore, it is designed to be used in conjunction with the relevant Australian standards for off - grid systems (AS 4509 Parts 1, 2 & 3 and AS 4086 Part 2).
One special feature of PV-SPS is the distinction between summer and winter, for which two groups of loads (one for each season) need to be entered. In addition, regional factors are specified for each month for seasonal loads, alongside irradiation and temperature data for four different locations in Australia. The program outputs on the one hand three different PV generator sizes, based on the mean irradiation values of the best and poorest months, as well as the annual mean value, and on the other hand the size of a diesel generator corresponding to the selected system operating mode. Even though the layout of the individual input forms is not always optimal in terms of clarity, the two graphs presenting the monthly energy consumption and power generation give a good general impression of the system performance over the course of the year.
More information is available, and ordering of the software is possible, on the website: http://www.cleanenergycouncil.org.au/cec/accreditation/Solar-PVaccreditation/forms.html (accessed 11 January 2011).
2. RETScreen (Version 4)
This program, which is also based on Excel spreadsheets, was developed by the Canadian government (Ministry of Natural Resources) and supports basic dimensioning calculations for PV-diesel off-grid systems. RETScreen stands out by way of its fast and simple input, as databases are provided for PV modules and climate data for more than 6000 ground stations (month-by-month solar irradiation and temperature data for the year). Via a link to the NASA Internet site, climate data can also be retrieved for any chosen point on earth. Further components can only be defined with a limited scope of technical specifications, which means that the output configurations should best be treated as dimensioning guidelines. Economic viability and emissions calculations can be performed for the dimensioned system.
The program is offered in more than 30 languages and includes a wide variety of additional tools covering other renewable energy sources together with a comprehensive manual and a collection of case studies. It can be downloaded free of charge at the following website: http://www.retscreen.net/ang/home.php (accessed 11 January 2011).
3. PV*SOL Professional (Version 3)
PV*SOL, which was developed by Dr. Valentin, Energiesoftware GmbH in Berlin, Germany, is a time-step simulation program for off-grid and grid-coupled solar generation systems and is able to perform energy yield calculations, analysis of economic efficiency and analysis of the influences of shadowing. The program is available in different versions with varying capabilities.
Besides the full simulation over time, there is a »quick design« function for off-grid systems, though this is limited to dimensioning of the PV generator and storage battery (no additional diesel generator). The loads can be entered either in very fine detail (individual loads and their operating times) or by way of a general specification
of annual energy consumption and selection of a load curve. The »quick design« can be transferred into the simulation and supplemented accordingly (e.g. with an additional generator). Before the actual simulation is started, a plausibility check is performed and any inconsistencies are pointed out to the user. The simulation results are output in graph form (characteristic curves for specific parameters) or tables, with the possibility to output up to eight different variables at once. The system report (as a print-out or export file for further processing) comprises a system diagram, the most important technical data of the system and energy balances.
More information is available, and ordering of the latest version of the software (Version 4 as of January 2011) is possible, on the website: http://www.valentin.de/ (accessed 11 January 2011)
4. PVsyst (Version 4.33)
This time-step simulation program, developed at the University of Geneva, Switzerland, is able to simulate both grid-coupled and off-grid systems (energy flow, shadowing and economic viability). It provides dimensioning proposals for standalone installations (PV generator and battery size), and warns the user if the chosen component combinations are not technically feasible. The dimensioning proposals are calculated on the basis of load inputs, specification of the number of days for autonomous operation and an estimation of the so-called »loss-of-load probability«, i.e. the duration for which the load cannot be served by the PV and battery energy.
As there are no inverter models for off-grid system simulation, it is only possible to model DC systems. The additional generator serves only to charge the batteries. The loads can be specified in various ways: individual inputs, load profile creation, probabilities of particular power values, data import and differentiation of individual periods. Comprehensive component databases are provided, permitting new components to be entered by way of typical technical data. The output options for the calculation results are similarly extensive and offer a wide range of presentation forms (e.g. characteristic curves of specific parameters, scatter plots, histograms, printed reports).
More information is available, and ordering of the latest version of the software (Version 5.31 as of January 2011) is possible, on the website: http://www.pvsyst.com/5.2/index.php (accessed 11 January 2011).
5. Hybrid2 (Version 1.3c R3)
This pure simulation program, developed by the National Renewable Energy Laboratory (NREL) of the U.S. Department of Energy in cooperation with the University of Massachusetts, is one of the pioneer programs in the field. It was conceived for analyses of hybrid systems with several energy generators (PV, wind and diesel generators) and loads (AC, DC and thermal loads) and offers not only a broad spectrum of energy management strategy options, but also an economic analysis function. While the input forms are well structured, the output options fall short somewhat in terms of user-friendliness and clarity. Hybrid2 is especially popular in universities and colleges for research because it permits very comprehensive system analyses even though it requires some time for learning the software.
DOE/NREL stopped funding development about 5 years ago and it is now primarily supported by the University of Massachusetts. The Alaska state energy office has a strong interest in off grid power systems and has negotiated a 3 year contract for a major upgrade of the software with the University of Massachusetts. This will greatly improve the modularity of the code, improve the interface, change the software from a power base to an electrical base (volt and amp) and add components such as flywheels, hydrogen fuel cells and expanded battery life calculation.
Hybrid2 can be downloaded free of charge at the following website: http://www.ceere.org/rerl/rerl_hybridpower.html.
6. PV-DesignPro (Version 6.0)
Developed by Maui Solar Energy Software Corporation in Hawaii, USA, this timestep simulation program is designed to simulate both grid-coupled and off-grid systems with PV and wind generators. The output of an additional generator (e.g. a diesel generator in a hybrid system) is constrained to match the shortfalls in
renewable energy, meaning that a realistic additional generator cannot be modelled. In addition to the energy balance calculations for dimensioning, the program also incorporates economic analysis, an optimisation tool, and a set of »sub-programs« which can be used to create load curves and to convert Meteonorm climate data into the PV-DesignPro data format.
Alongside a quite comprehensive internal climate database with temperature, irradiation and wind data, component databases are also provided, including over 400 PV modules. Further PV modules, however, can only be entered by specifying technical data based on the Sandia PV array performance model [7].
More information is available, and ordering of the software is possible, on the website: http://www.mauisolarsoftware.com/ (accessed 11 January 2011).
7. HOMER (Version 2.67 beta)
HOMER (Hybrid Optimisation Model for Electric Renewables) – another time-step simulation program developed by NREL – adds optimization to basic simulation capability. It simulates the annual performance of many different system configurations for a specified set of energy sources to find a configuration that satisfies technical constraints at the lowest life-cycle cost. It is also possible for the user to define sensitivities (e.g. different mean values for solar irradiation, wind or power consumption) to narrow the range of results. The outcome of the simulation is a list of the possible systems in order of life-cycle costs. A graph depicts the various ranges of the most cost-effective systems over the given operating period, based on the selected criteria. Detailed results can be output for each of the individual simulated systems (graphs, tables, scatter plot, print-out).
The program design is very user-friendly and incorporates not only PV, wind and small-scale hydro-generators, but also additional generators driven by a variety of fuels (e.g. diesel, bio mass, ethanol, hydrogen), and energy storage (both batteries and hydrogen). It is possible to connect the different sources and loads to either a DC or AC bus. However some expertise and attention is required to get valid results. For example, to get plausible values for the battery lifetime costs, a good understanding of battery lifetime behaviour and of HOMER’s battery lifetime model is
necessary. Also, no check is made to determine whether the entered component combinations are technically feasible. For example, the PV generator is modelled simply by its peak output and not, as in other programs, as the combination of individual PV modules. Energy conversion efficiency is not considered for power converters used in the system. For example, no efficiency can be defined for the PV charge controller in a DC coupled system or for the PV inverter in an AC coupled system.
Despite these issues, which are important primarily when doing detailed system design, HOMER is a very convenient and widely used tool, especially where the economic aspects of a system are to be considered. HOMER is maintained by Homer Energy as of 2009 and version 2.68 of the program is available for free download at: http://www.homerenergy.com/ (accessed 11 January 2011). The latest version is V2.80 and must be purchased.
8. ViPOR
ViPOR (Village Power Optimization for Renewables) is an optimization tool for designing village electrification systems which is developed by NREL. ViPOR decides which houses should be powered by isolated power systems (e.g. standalone solar home systems [SHS]) and which should be included in the village minigrid. The mini-grid distribution network is optimally designed with consideration of local terrain. To analyze costs and the proper layout of the distribution grid (e.g. length and diameter of cable), it is necessary to know the costs of cables, the relative costs of generating the electric energy by a centralized PV System and a solar home system and to have some data about the local geography. ViPOR requires a geographical description of the local terrain including the load points such as houses, schools, stores and health posts. It is possible to import geographic data from GIS database, a GPS survey, or a digitized map or simply enter the data by clicking with the mouse. To calculate the generating costs of PV electricity it is also possible to use HOMER as additional software that provides input for ViPOR.
The graphical user interface is user-friendly, but it is necessary to be skilled in village electrification, especially for setup costs. It is much easier to modify an existing example.
ViPOR is the only readily available tool for evaluating trade-offs between solar home systems or centralized PV mini-grids. It can be very useful for designing village electrification systems, but good information about local conditions is needed for good results. A limit of ViPOR is that it does not take power losses in the distribution network into account.
ViPOR is not as fully developed as HOMER, so it is recommended to use ViPOR closely with NREL. A pre-release version is available for free download from http://analysis.nrel.gov/vipor (accessed 11 January 2011)
9. TRNSYS
Available commercially since 1975, TRNSYS (abbreviation of “TRaNsient SYstems Simulation, pronounced "tran-sis" ) was initially developed for modelling of thermal energy flows in multiple zone systems. It is a transient system simulation program based on a modular architecture of Fortran code blocks. Hundreds of components are available, including PV arrays, charge controllers and weather generators. TRNSYS’s extensive use of Fortran generally requires that users be familiar with this programming language, at least at the development level. More information is available, and ordering is possible, at http://www.trnsys.com (accessed 12 January 2011)
10. INSEL
The program INSEL, which was initially developed for the simulation of hybrid PV systems by the University of Oldenburg, Germany, and is today supported by the South German company Doppelintegral GmbH is similar to TRNSYS and both include a comfortable graphical user interface. This means the users need not be familiar with the internal programming language and code, but they still have the option to develop and add their own models in Fortran or C. More information is available, and ordering is possible, at http://www.inseldi.com (accessed 12 January 2011).
11. MATLAB/Simulink and other general system simulators
MATLAB, developed by the MathWorks (www.Mathworks.com) in 1984 and subsequently upgraded several times, is a technical computing environment offering advanced mathematical manipulation tools with a powerful and intuitive scripting language. Coupled with Simulink, a graphical modular simulation environment, it provides an easy to use modelling and simulation tool. Many industrial companies and universities use MATLAB/Simulink as a standard commercial system simulator for simulation of technical systems. Other, similar, software packages are available, including VisSim (http://www.vissim.com), Dymola/Modelica (http://www.dynasim.se), Simplorer (http://www.ansoft.com/), and the free, open source Scilab/Scicos (http://www.scicos.org/) , MATLAB provides multiple toolboxes for optimisation and systems analysis in addition to data acquisition hardware that can be linked to the simulation. While the routines are powerful and varied, development of hybrid system simulations is not trivial. The Simulink interface is easy-to-use, but the platform assumes that the system is readily modelled as a system of differential equations. The solvers can be sensitive with closed-loop, non-linear systems, including hybrid power systems with battery storage. Experience and knowledge of the system behaviour is essential to ensure stable and universal models of different components of PV hybrid systems.
For MATLAB no commercial toolboxes for PV components and systems are available, but several research institutes [8], [9] have created their own toolboxes, see Table 1. . The system simulator Simplorer (www.ansoft.com) has commercially available toolboxes for PV and renewable energy components and systems.
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