Introduction

After a strong cup of coffee professor Maarten Krol, director of the BBOS research school, opened the symposium with an anecdote about solar panels. It has been claimed that these panels disturb the surface energy balance and as such contribute to global warming rather than mitigate it. Krol (with the audience) expressed some skepticism towards this claim, but the story served to illustrate the interplay between renewable energy and meteorology.

The energy sectors creates many jobs and poses new research questions: excellent opportunities for meteorologists.

The official programme started with a general introduction by the organisers. They briefly explained their own research projects. Arjan studies wind and turbulence in cities. His work is part of the “Windy city” project, which aims to improve understanding of the urban wind climate. Measuring and modelling urban wind is notoriously difficult due to the complex terrain. By studying how wind phenomena, such as the low level jet, transform from rural to urban landscapes can give insights in how cities shape the wind. This knowledge could later be applied to design ways to use urban wind for energy production. Peter works on North Sea wind fields. A realistic representation of the turbulent wind field is important not only for power forecasts, but also for load assessment studies. In this context, Peter’s study aims to quantify and reduce the uncertainty in the simulated wind fields. Better understanding of the marine atmospheric boundary layer is an essential part of the research. This work is part of the EUROS project which aims to reduce uncertainties in large offshore wind farm projects.

The EnergyScape

This talk was intended to give an overview of the ‘EnergyScape’: the ins and outs about renewable energy, the energy transition, energy policy, current status and connections to science/meteorology. As such, it set the scene for the following talks, which are more focused on science and meteorology. Unfortunately, the first speaker had to cancel his talk on short notice. Since it wasn’t possible to find a suitable replacement, the organizers decided to compile a presentation themselves.

Besides the climate change incentive, there are many other reasons why investments in renewable energy will pay off.

Renewable energy is almost a daily rubric in the news nowadays. With items related to record-low pricing, new project tenders, public and corporate opinions, ongoing elections, et cetera, the “energy transition” is hard to ignore. And why would we? Besides the climate change incentive, there are many other reasons why investments in renewable energy will pay off. For example improved air quality and energy independence. As technology matures and prices keep dropping, it will even become attractive from an economic point of view.

When we talk about renewable energy, we often hear the term “energy transition”. This term expresses the notion that new energy sources of energy by themselves are not enough. Additionally, a complete transformation of the energy infrastructure is required. Two important reasons for this are the fitful nature of wind and solar energy, and the shift from a centralized generators to a distributed power supply. A successful transition requires that the electricity grid will be bi-directional and energy demand and supply are calibrated. In the absence of a suitable storage solution, this reiterates the need for reliable weather and power forecasts.

The current status and prospects of renewables were presented on the basis of two reports:

Wind energy

Martin Dörenkämper is postdoctoral researcher at the Fraunhofer Institute for Wind Energy and Energy System Technology (IWES). After obtaining his master degree in meteorology he did a PhD on wind energy meteorology at Oldenburg University. As such, he has become a leading scientist in this field. The following summary of his presentation is written by the organizers.

A 6% error in wind speed can lead to an error of 30% in power

In his first slides, Dörenkämper explained why wind energy meteorology is so important. With a simple example he showed that a 6% error in wind speed can lead to an error of 30% in power. As the share of wind energy will only increase, every bit gained in forecast accuracy will pay off. When it comes decimal differences, small- to mesoscale effects that were perhaps deemed negligable in the past now become important. The main focus of the talk was on offshore wind energy, and it was based on measurements and simulations for the North and Baltic seas, which appeared to be very different. Four overarching research topics served as a common theme through the talk: wind and stability conditions in both seas; the influence of fetch on these conditions; the effect on the power of single wind turbines, and the influence on wind farm wakes.

Indeed, a clear coastal signature is visible in the offshore wind field

One of the differences between the North and Baltic seas is the diurnal cycle. Over the Baltic Sea, there is a pronounced diurnal cycle in e.g. potential temperature, which is shifted in time with respect to the onshore cycle. Over the North Sea the diurnal signature is much less pronounced. Also noteworthy is the predominantly stable stratification due to warm air advection, especially for the Baltic sea. Using a plot of fetch length versus wind direction, Dörenkämper illustrated that this coastal signature corresponds very well with the climatological wind shear. He further elaborated by introducing the concept of a wake rose. By comparing the power between two turbines in a wind farm, it can be used to assess the power loss for each turbine for different wind directions. Strikingly, a power excess was found for wind direction that was more or less parallel to a coastline and as such represented a strong gradient in fetch length.

Solar energy

Dr. Stefan Wilbert is team leader of the Solar Energy Meteorology team at the German Aerospace Centre (DLR) Institute for Solar Research. He is currently based in Spain, at the Plataforma Solar de Almería. This is a large solar power plant, where DLR runs a meteorological site for solar technologies. His presentation focused on these techniques: how they work, what meteorological information they require, and how we can measure these data at the site of the solar plant itself.

Meteorological data is a tool for cost reduction

Dr. Wilbert introduced various forms of solar energy technology. The most dominant type are the the well-known PV panels, that can be found on rooftops or in fields. They differ among each other in cell type, and can either have a fixed tilt or tracker. Concentrated Solar Power (CSP) and PV (CPV) make use of directed mirrors to focus solar radiation to a central point. Solar heating & cooling were also briefly mentioned. The efficiency of these techniques depends on site-specific information about irradiance, but also temperature, wind, humidity, dust, and so on. Measurements at the ground are used to make predictions of the current plant yield, but also as model input to calculate future yields. Meteorological data and models are vital to support operational decisions, leading to higher yields, and also to reduce costs and select optimal technologies for specific sites.

Interaction of climate and wind power

Dr. Robert Vautard is senior scientist at the laboratory of climate and environmental science in France. He has extensive experience in the field of climate modelling and extreme events. Also he contributed to the 5th IPCC report. In this symposium he will place renewable energy in the context of climate change and long-term climate variability. He will mainly focus on wind energy aspects, and address three important questions. First of all, how is does climate change impact wind power resources? Secondly, does wind power development have in impact on regional climate. And finally, combining the two: does wind power development have an impact on wind resource?

To answer the first question, Vautard starts with a brief introduction to regional climate modelling. Because long-term climate models run at a relatively course resolution, regional climate models are used to refine these simulations onto a smaller domain. This is important to assess the influence on climate change on regional circulation, especially for accurate projection of extreme events. It can also be used to assess the impact of regional climate policy. As such, it is suitable to investigate the effects of large wind farms. The EURO-CORDEX regional climate model is used in this study. To assess uncertainties, it is run in an ensemble configuration.

Overall changes in wind power potential are within 5%, but with clear spatial differences.

Vautard describes an experiment in which the power yield from wind turbines was computed, based on the EURO-CORDEX wind projections. A simple extrapolation from the 10m wind to hub height is used in conjunction with a reference power curve. This method is quite simple, but in the context of this study it is enough to give reasonable estimates of the changes in wind power. A complete dataset with turbine locations and characteristics is available here (not free). To account for future expansion of wind energy, several scenarios have been defined for the future wind turbine fleet. To construct these scenarios, the “CLIMIX” approach was used. Turbines are added over the model domain, with higher density in areas with better wind resources and taking care to avoid unsuitable land-use categories, until the projected wind energy production has been reached. This gives a seemingly realistic distribution of wind turbine locations over Europe.

The downscaled RCP8.5 climate scenario shows that 10m winds decrease in a very broad “coastal” zone along the Atlantic and the Mediterranean. Further inland and at the Baltic Sea, however, 10m winds increase. As a result, the overall changes in wind power potential are within 5%, but with clear spatial differences. The decrease of wind speed over the Mediterranean in the RCP8.5 scenario has been linked to a poleward shift of the Hadley circulation. This might give rise to an increased frequency of “wind droughts”: extended episodes of low wind speeds.

Results suggest that as more wind turbines are added to a windfarm, electricity generation initially increases, but levels off around 1 W/m²

Energy-meteo in cities

The final presentation of the day was given by Dr. Sanda Lenzholzer, who works at the chairgroup of Landscape Architecture at Wageningen University. Her presentation provided a nice contrast with the strictly scientific talks heard before, by focussing on the aesthetic and practical side of renewable energy. The previous talks discussed several techniques and innovations, but how do we implement those in our current landscape? What is the best way to create acceptance for renewables? Using student projects, Sanda gave several examples of how we can transform the cities of today into sustainable cities of the future.

Without making a personal connection to the people involved, renewables projects are doomed to fail

The challenge behind sustainable urban design is to make use of local fluxes of sun and heat to meet the city’s energy demand, without having to redesign the entire area from scratch. The students in Sanda’s course made qualitative analyses of the local available sun and wind to decide where they would place their design structures. What sort of buildings do you implement to make sure an area is sunny in winter, but doesn’t become a source of heat stress in winter? Design plans for two very different neighbourhoods in the Netherlands were presented, to show the challenges and solutions in creating the future city.

The neighbourhood of Oosterburg in Amsterdam currently consists mainly of empty buildings and parking lots, which allows for a complete overhaul of the area into a green neighbourhood. Several sunny spots emerged from the students’ analysis, as well as the realisation that the building height needs to increase towards the North to reduce wind nuisance. The aptly named “wind wall” project constructed a series of buildings with sloping roofs, to guide the wind over the rooftops away from street level. By guiding the wind through several urban wind turbines on top of their “wind wall” it produces energy as well!

Debate and interaction

Without such an engaged audience, the day would not have been as great. In addition to scheduled elements, they participated actively with questions and feedback throughout the day.

Before lunch, an interactive element was added to the programme to stimulate discussion. Using an online questionnaire tool a number of propositions was presented and the participants could vote for one of the answers via their laptops or smartphones. This resulted in interesting discussions. Additionally, the audience was encouraged to contribute to a tangible outcome of the symposium by pasting post-its to what was introduced as an ‘idea-tree’. On these post-its they could write questions, propositions, interesting facts, etc. The original idea was that these post-its would be used to guide a discussion at the end of the day, but it turned out this wasn’t even necessary. Instead, the organizers requested that each participant write down a personal take-home message that they would like to share. Here, we have summarized some highlights of the discussion.