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Hybridization of wind farms: an opportunity for the sector

07 March 2021
Article by Bruno Silva and Miguel Marques and José Carlos Matos, members of INEGI's wind energy consulting team

Renewable energy sources are today an unquestionable part in the definition of energy policy, which ranges from macro planning, at national and community level, to the domestic level.

When we talk about electricity, specifically, the importance of exploring the diversity, and possible complementarity, of different renewable resources is increasingly evident. Full diversity ensures a balance, in terms of sources, in the production of electricity which, on the one hand, does not accentuate dependencies on specific sources or geographic regions and, on the other hand, maximizes the use of transport and distribution infrastructure without, however, the overload.

The vision of the operationalization of this diversification of sources, currently, involves the construction and operation of specific plants - the wind farm, the photovoltaic plant, the small hydro plant. However, the increase in power of existing electric power plants, with complementary technologies, can be an interesting opportunity for the National Electric System and all its stakeholders, from production to consumption.

Solar energy and wind energy, in particular, have characteristics that, depending on the intraday and intranual cycles, enhance their complementarity, and may have advantages in comparison to their individual and separate use. Let's see why.

The wind-photovoltaic energy potential

Each renewable energy source, whether wind, solar, water or biomass, has a different generation profile. There are variations over time, in geographic space, and in terms of availability and intensity.

There are, however, places that, due to their own meteorological conditions, allow a greater degree of complementarity.

Consider the example of places where there is more wind at night, than in places where there is more wind during the day. Here, the complementarity between wind and photovoltaic production would make it possible to boost energy generation with less volatility for 24 hours, making the most of each natural resource. A greater degree of complementarity allows for a greater smoothing of the generation profile of a hybrid power plant.

Hybridization, understood here as the combination of different resources, is not limited to the potential for smoothing the electricity generation profile. It also allows extracting more value from an asset with a potential limited to the productivity associated with the plant's technology: the grid connection infrastructure itself.

Capacity, location and intraday cycles are key factors

Wind farms in Portugal have an average productivity of 2300 to 2400 hours equivalent to full load. The connection of these farms to the network has, of course, the same degree of use, which in turn means that more kWh delivered to the network by this route results in the use of the idle capacity of this asset. In other words, if the cost of producing additional electricity requires additional investment, the cost of delivering that electricity to the grid is residual, since that delivery is made through an existing asset. Something that ultimately results in a reduction in electricity costs.

And why is this question posed to wind farms?

From a conceptual point of view, the hybridization hypothesis can be placed for any power plant. However, in view of the dimension supported by wind energy in the National Electric System, the opportunity is more obvious here.

Let us remember that, in mainland Portugal, in the last 5 years, electricity from wind sources corresponded, on average, to 24% of electricity consumption.

In addition to the level of intensity of the wind and solar sources, however, it is necessary to verify the degree of complementarity between the two, project by project. A high simultaneity of resources can have a significant impact in terms of the limitation imposed on the hybrid power plant, assuming the lack of storage.

Practical cases

We illustrate here three conceptual cases based on concrete cases studied by INEGI, designated by "North", "Center" and "South", with the designations corresponding to the approximate geographical location, the greater or lesser degree of complementarity.

In both cases, the existence of wind farms for the three examples with identical generating capacity and similar annual capacity factors is taken as a starting point1. It is assumed for both cases the additional installation of a photovoltaic base generating capacity identical to the existing one, the wind power, maintaining, in any case, the connection power of the power plant. It is also assumed that the power limitation, if it occurs, affects exclusively the additional power, that is, the photovoltaic capacity. In any case, the "North" project is located on a terrain with a much higher altitude than the other projects, which has some implications regarding the comparability of the intensity of the solar resource in these places.

Figure 2 shows the intraday variation of the expected output of the different facilities, both in annual average terms and for the summer and winter periods2. The variation is expressed separately for the technologies that make up the plant and for the whole, affected by losses due to limitations.

It can be seen in the North project that, on the one hand, the energy potential of the wind resource during the day is slightly lower than that seen at night and, on the other hand, the potential during the summer is significantly lower than that seen in winter. It appears that the wind resource is in a remarkable counter cycle with the solar resource, available only during the day and with greater intensity in the summer.
As for the Center project, it is observed that not only is there some more intensity of the wind during the day, but there are no notable differences in the intensity of the wind from summer to winter. There is, therefore, a greater temporal coincidence in terms of resource intensity, wind and solar, aggravating the congestion of the connection point in comparison with the previous case.

It is observed in the South project that the intranual variability of the wind resource is not significantly marked from summer to winter. But given that the intraday variability of this resource is remarkable, with much less wind in the daytime, very interesting conditions are met for complementarity with the solar resource.

Figure 2: daily cycle of the average capacity factor, the Annual, Summer and Winter periods of three conceptual power plants, wind and solar, with identical generating capacity and a hybrid plant composed of the two in three regions of the country (top in the North, middle in the Center region and low in the South region)

The impact of the degree of simultaneity of resources for the two projects is shown in Figure 3, which illustrates the losses in electricity production due to limitations in photovoltaic installations, expressed in terms of the capacity factor related to photovoltaic power. As would be expected, the losses due to limitation are more significant in the "Center" project, being globally 50% higher than the expected losses in the "North" and "South" projects.

Figure 3: daily cycle of losses due to limitation of the photovoltaic electroproductive installation, in terms of the average capacity factor for the cases presented here.

And the location?

Thus, we conclude that one of the main inputs for the definition of the optimum point in the exploration of a hybrid plant is found in the degree of complementarity between capacity factors. However, this is not the only relevant aspect: location is also a key aspect, with the particularity of being located in the vicinity of the wind farm, if not in the area of the parks itself.

In this sense and in addition to basic aspects such as the characteristics of the surrounding terrain, the classification of the soil and the existing resource levels, it should be necessary to take into account the impact of the wind turbines in the generation of photovoltaic base (the effect of shading) or in what regards the safety of people and equipment (for example at the level of the projection of ice accumulated on the blades onto the panels).

Figure 4: classification of areas of interest in the vicinity of a photovoltaic park.

Aware of this, INEGI uses a methodology based on GIS tools (Geographic Information System) to evaluate the surrounding terrains that allow to equate the levels of incident solar irradiation, the orographic characteristics of the terrain (orientation, inclination, etc.) and the construction solutions to classify and pre-select the areas of greatest interest, in view of the basic criteria, in addition to local verification and assessment of environmental aspects.

Final notes

The hybridization of wind farms, using photovoltaic generating capacity, constitutes a very interesting opportunity for promoters who own the former. For the typical productivity of wind farms in the country, it is expected that there will be significant idle capacity on the part of the grid connection infrastructures, which can be used with renewable sources as long as they are out of sync with wind energy.

In any case, the size of the photovoltaic plant added to the wind farm can be optimized according to the potential and its characteristics, aiming to maximize electricity production for the whole year or only for a specific period of the year if, in the overall balance sheet of the hybrid plant, this proves to be more interesting. In fact, gauging the cycles, both intraday and interannual, of electricity production from different sources, and determining the greater the degree of desynchronization (i.e., the lesser the degree of simultaneity) is critical to optimize the sizing of the photovoltaic plant.

Given the reduction in the degrees of geographic freedom of a hybridization project, aspects related to the equipment occupation area are also of particular relevance.

In any case, these are just some of the other aspects to be considered in studies to increase the power of wind farms with photovoltaic generation. The potential replacement of wind turbines with more efficient machines, the so-called repowering, the conclusion of periods with administratively determined tariffs, and the entry of wind farms into the market or any profound changes in the profile of electricity demand - all can lead to profound changes in the models of business.

INEGI, whose competencies in the use of renewable sources for the production of electricity are widely recognized, is already working with several companies in the development of hybridization projects for wind farms with photovoltaic equipment, a market that is believed to be promising.

1 The capacity factor of any electro-producing system is defined as the electricity produced during a given period divided by the electricity produced in the same period, assuming the continuous operation of the generator at full load.
2 It is assumed here summer as the months from April to September and winter the rest.

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