Overview

urbs consists of several model entities. These are commodities, processes, transmission and storage. Demand and intermittent commodity supply through are modelled through time series datasets.

Commodity

Commodities are goods that can be generated, stored, transmitted and consumed. By convention, they are represented by their energy content (in MWh), but can be changed (to J, kW, t, kg) by simply using different (consistent) units for all input data. Each commodity must be exactly one of the following four types:

• Stock: Buyable at any time for a given price. Supply can be limited per timestep or for a whole year. Examples are coal, gas, uranium or biomass.
• SupIm: Supply intermittent stands for fluctuating resources like solar radiation and wind energy, which are available according to a timeseries of values, which could be derived from weather data.
• Demand: These commodities have a timeseries for the requirement associated and must be provided by output from other process or from storage. Usually, there is only one demand commodity called electricity (abbreviated to Elec), but multiple (e.g. electricity, space heating, process heat, space cooling) demands can be specified.
• Env: The special commodity CO2 is of this type and represents the amount (in tons) of greenhouse gas emissions from processes. Its total amount can be limited, to investigate the effect of policies on the model.

Stock commodities have three numeric attributes that represent their price, total annual and per timestep supply. Environmental commodities (i.e. CO2) have a maximum allowed quantity that may be created.

Commodities are defined over the tuple (site, commodity, type), for example (Norway, wind, SupIm) for wind in Norway with a time series or (Iceland, electricity, Demand) for an electricity demand time series in Iceland.

Process

Processes describe conversion technologies from one commodity to another. They can be visualised like a black box with input(s) (commodity) and output(s) (commodity). Process input and output ratios are the main technical parameters for processes. Fixed costs for investment and maintenance (per capacity) and variable costs for operation (per output) are the economical parameters.

Processes are defined over two tuples. The first tuple (site, process) specifies the location of a given process e.g. (Iceland, turbine) would locate a process turbine at site Iceland. The second tuple (process, commodity, direction) then specifies the inputs and outputs for that process. For example, (turbine, geothermal, In) and (turbine, electricity, Out) describes that the process named turbine has a single input geothermal and the single output electricity.

Transmission

Transmission allows instantaneous transportation of commodities between sites. It is characterised by an efficiency and costs, just like processes. Transmission is defined over the tuple (site in, site out, transmission, commodity). For example, (Iceland, Norway, undersea cable, electricity) would represent an undersea cable for electricity between Iceland and Norway.

Storage

Storage describes the possibility to deposit a deliberate amount of energy in the form of one commodity at one time step; with the purpose of retrieving it later. Efficiencies for charging/discharging depict losses during input/output. A self-discharge term is not included at the moment, but could be added trivially (one column, one modification of the storage state equation). Storage is characterised by capacities both for energy content (in MWh) and charge/discharge power (in MW). Both capacities have independent sets of investment, fixed and variable cost parameters to allow for a very flexible parametrization of various storage technologies; ranging from batteries to hot water tanks.

Storage is defined over the tuple (site, storage, stored commodity). For example, (Norway, pump storage, electricity) represents a pump storage power plant in Norway that can store and retrieve energy in form of electricity.

Timeseries

Demand

Each combination (site, demand commidty) may have one timeseries, describing the (average) power demand (MWh/h) per timestep. They are a crucial input parameter, as the whole optimisation aims to satisfy these demands with minimal costs by the given technologies (process, storage, transmission).

Intermittent Supply

Each combination (site, supim commodity) must be supplied with one timeseries, normalised to a maximum value of 1 relative to the installed capacity of a process using this commodity as input. For example, a wind power timeseries should reach value 1, when the wind speed exceeds the modelled wind turbine’s design wind speed is exceeded. This implies that any non-linear behaviour of intermittent processes can already be incorporated while preparing this timeseries.