* Added an option to alter the capital cost of carriers by a factor via ``carrier+factor`` in the ``{opts}`` wildcard. This can be useful for exploring uncertain cost parameters. Example: ``solar+0.5`` reduces the capital cost of solar to 50% of original values (`#167 <https://github.com/PyPSA/pypsa-eur/pull/167>`_).
* Corrected HVDC link connections (a) between Norway and Denmark and (b) mainland Italy, Corsica (FR) and Sardinia (IT) (`#181 <https://github.com/PyPSA/pypsa-eur/pull/181>`_)
* Corrected setting of exogenous emission price (in config -> cost -> emission price). This was not weighted by the efficiency and effective emission of the generators. Fixed in `#171 <https://github.com/PyPSA/pypsa-eur/pull/171>`_.
* The optimization is now performed using the ``pyomo=False`` setting in the :func:`pypsa.lopf.network_lopf`. This speeds up the solving process significantly and consumes much less memory. The inclusion of additional constraints were adjusted to the new implementation. They are all passed to the :func:`network_lopf` function via the ``extra_functionality`` argument. The rule ``trace_solve_network`` was integrated into the rule :mod:`solve_network` and can be activated via configuration with ``solving: options: track_iterations: true``. The charging and discharging capacities of batteries modelled as store-link combination are now coupled (`#116 <https://github.com/PyPSA/pypsa-eur/pull/116>`_).
* An updated extract of the `ENTSO-E Transmission System Map <https://www.entsoe.eu/data/map/>`_ (including Malta) was added to the repository using the `GridKit <https://github.com/PyPSA/GridKit>`_ tool. This tool has been updated to retrieve up-to-date map extracts using a single `script <https://github.com/PyPSA/GridKit/blob/master/entsoe/runall_in_docker.sh>`_. The update extract features 5322 buses, 6574 lines, 46 links. (`#118 <https://github.com/PyPSA/pypsa-eur/pull/118>`_).
* Added `FSFE REUSE <https://reuse.software>`_ compliant license information. Documentation now licensed under CC-BY-4.0 (`#160 <https://github.com/PyPSA/pypsa-eur/pull/160>`_).
* Added a 30 minute `video introduction <https://pypsa-eur.readthedocs.io/en/latest/introduction.html>`_ and a 20 minute `video tutorial <https://pypsa-eur.readthedocs.io/en/latest/tutorial.html>`_
* Networks now store a color and a nicely formatted name for each carrier, accessible via ``n.carrier['color']`` and ``n.carrier['nice_name'] ``(networks after ``elec.nc``).
* Added an option to skip iterative solving usually performed to update the line impedances of expanded lines at ``solving: options: skip_iterations:``.
*``snakemake`` rules for retrieving cutouts and the natura raster can now be disabled independently from their respective rules to build them; via ``config.*yaml`` (`#136 <https://github.com/PyPSA/pypsa-eur/pull/136>`_).
* Removed the ``id`` column for custom power plants in ``data/custom_powerplants.csv`` to avoid custom power plants with conflicting ids getting attached to the wrong bus (`#131 <https://github.com/PyPSA/pypsa-eur/pull/131>`_).
* Add option ``renewables: {carrier}: keep_all_available_areas:`` to use all availabe weather cells for renewable profile and potential generation. The default ignores weather cells where only less than 1 MW can be installed (`#150 <https://github.com/PyPSA/pypsa-eur/pull/150>`_).
* Added a function ``_helpers.load_network()`` which loads a network with overridden components specified in ``snakemake.config['override_components']`` (`#128 <https://github.com/PyPSA/pypsa-eur/pull/128>`_).
* Bugfix in :mod:`base_network` which now finds all closest links, not only the first entry (`#143 <https://github.com/PyPSA/pypsa-eur/pull/143>`_).
* Bugfix in :mod:`cluster_network` which now skips recalculation of link parameters if there are no links (`#149 <https://github.com/PyPSA/pypsa-eur/pull/149>`_).
* Added information on pull requests to contribution guidelines (`#151 <https://github.com/PyPSA/pypsa-eur/pull/151>`_).
* Improved documentation on open-source solver setup and added usage warnings.
* Documentation on installation, workflows and configuration settings is now available online at `pypsa-eur.readthedocs.io <pypsa-eur.readthedocs.io>`_ (`#65 <https://github.com/PyPSA/pypsa-eur/pull/65>`_).
* The ``conda`` environment files were updated and extended (`#81 <https://github.com/PyPSA/pypsa-eur/pull/81>`_).
* The power plant database was updated with extensive filtering options via ``pandas.query`` functionality (`#84 <https://github.com/PyPSA/pypsa-eur/pull/84>`_ and `#94 <https://github.com/PyPSA/pypsa-eur/pull/94>`_).
* Continuous integration testing with `Travis CI <https://travis-ci.org>`_ is now included for Linux, Mac and Windows (`#82 <https://github.com/PyPSA/pypsa-eur/pull/82>`_).
* Data dependencies are now retrieved directly from within the snakemake workflow (`#86 <https://github.com/PyPSA/pypsa-eur/pull/86>`_).
* Emission prices can be added to marginal costs of generators through the keyworks ``Ep`` in the ``{opts}`` wildcard (`#100 <https://github.com/PyPSA/pypsa-eur/pull/100>`_).
* An option is introduced to add extendable nuclear power plants to the network (`#98 <https://github.com/PyPSA/pypsa-eur/pull/98>`_).
* Focus weights can now be specified for particular countries for the network clustering, which allows to set a proportion of the total number of clusters for particular countries (`#87 <https://github.com/PyPSA/pypsa-eur/pull/87>`_).
* A new rule :mod:`add_extra_components` allows to add additional components to the network only after clustering. It is thereby possible to model storage units (e.g. battery and hydrogen) in more detail via a combination of ``Store``, ``Link`` and ``Bus`` elements (`#97 <https://github.com/PyPSA/pypsa-eur/pull/97>`_).
* Hydrogen pipelines (including cost assumptions) can now be added alongside clustered network connections in the rule :mod:`add_extra_components` . Set ``electricity: extendable_carriers: Link: [H2 pipeline]`` and ensure hydrogen storage is modelled as a ``Store``. This is a first simplified stage (`#108 <https://github.com/PyPSA/pypsa-eur/pull/108>`_).
* Logfiles for all rules of the ``snakemake`` workflow are now written in the folder ``log/`` (`#102 <https://github.com/PyPSA/pypsa-eur/pull/102>`_).
* The new function ``_helpers.mock_snakemake`` creates a ``snakemake`` object which mimics the actual ``snakemake`` object produced by workflow by parsing the ``Snakefile`` and setting all paths for inputs, outputs, and logs. This allows running all scripts within a (I)python terminal (or just by calling ``python <script-name>``) and thereby facilitates developing and debugging scripts significantly (`#107 <https://github.com/PyPSA/pypsa-eur/pull/107>`_).
* Upload pre-built networks to `zenodo data repository <https://doi.org/10.5281/zenodo.3601882>`_ with `CC BY 4.0 <https://creativecommons.org/licenses/by/4.0/>`_ license.
* Send announcement on the `PyPSA and PyPSA-Eur mailing list <https://groups.google.com/forum/#!forum/pypsa>`_.
