H2RES is a balancing tool that simulates the integration of renewable energy into energy systems. The model is developed by the Instituto Superior Técnico, Lisbon and the Faculty of Mechanical Engineering and Naval Architecture at University of Zagreb, Croatia  in 2000. Ten versions of the model have been released to date but the number of users is not monitored. The model is not yet sold to external users, instead it is supplied to internal users to complete their research and the training period required to use the model is up to two months.
The H2RES model balances the hourly time series of water, electricity, heat, and hydrogen demand, appropriate storages, and supply over any user-defined period. The model has been specifically designed to increase the integration of renewable sources and hydrogen into island energy-systems which operate as stand-alone systems. It can also serve as a planning tool for single wind, hydro or solar power producers or it can be used for planning of larger power-systems. The model considers all forms of thermal generation except nuclear power, and all renewable technologies except tidal power. Also, all storage and conversion technologies are considered by H2RES except compressed-air energy-storage, but only hydrogen vehicles are simulated in the transport sector. The desalination of water is also considered by H2RES as this is often used on island systems. To simulate wind, solar and hydro, wind velocities, solar radiation and precipitation data are obtained from the nearest meteorological station. This raw meteorological data is inputted into the H2RES program and the output is simulated from the appropriate renewable-energy technology. The biomass module of H2RES takes into account the feedstock information, the desired mix of feedstocks, conversion processes (combustion, gasification and digestion) and desired output production (power, heat or combined heat and power). Biomass module is set to follow the heat load and it generates electricity as by-product. The biomass module is designed to utilise the available resource so that there is enough biomass in storage to supply demand. This is a major factor when dealing with isolated systems as they cannot afford to run out fuel. The geothermal module functions as base load, where the installed power generates electricity for the system continuously, except when it is in maintenance. To simulate the electricity sector, the load module is used. It is based on a given criteria for the maximum acceptable intermittent and renewable electricity in the power system, integrates a part or all of the available renewables output into the system and either stores or discards the rest of the renewable/intermittent output. The sequence of sources in supplying of demand could be easily set up according to criteria. Excess renewable-electricity can be stored in a pumped-hydro facility, batteries or as hydrogen, used for some non-time critical loads (deferrable loads), and used for desalination. If there is still unsatisfied electricity load it is covered by fossil fuels blocks or by the mainland grid if such a connection exists. Financial calculations are not completed in H2RES but this area is currently under development.
The H2RES model has previously been used to create a methodology for the assessment of alternative scenarios in energy and resource planning on island energy-systems , to analyse different energy scenarios in Malta , to investigate the role of hydrogen in future island energy-systems  specifically by aiding the integration of renewable energy , and to analyse the potential energy production from biomass for a wood processing factory . Finally, H2RES has previously simulated a 100% renewable energy-system for both the island of Mljet in Croatia  (where the results obtained were compared to those obtained in EnergyPLAN ) and the island of Porto Santo, Portugal .
- H2RES, 23rd April 2009, http://www.powerlab.fsb.hr/h2RES/
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