**1. Introduction**

Supplying remote isolated installations with diesel generators into a Diesel Power System (DPS) has traditionally been one of the most common solutions for all sizes of systems, but mainly for low (kWs range) and medium (up to MWs) power ones. With the development of renewable energies, their incorporation into the existing diesel grids started with wind energy due to its lower generation cost in comparison with solar PV, constituting the so called Wind Diesel Power Systems (WDPS) [1]. However, the twenty first century brought a strong reduction in the generation costs for solar PV, which has opened the door to its presence in existing DPS and constitutes the solar PV Diesel Power Systems (PVDPS). Actual trends transitioning to very high percentages of renewable energies (RE) at all levels of power systems induces the need of the Renewable Energy Diesel Power Systems (REDPS), where wind and solar PV technologies might seem to be the most upfront ones and where the presence of medium and long term (usually electrochemical) storage is common in order to reduce the use of fuel consumption.

In recent years, research has shown a growing interest in the use of hybrid wind photovoltaic (PV) systems. Over the past twenty-five years, hundreds of articles have addressed the topic of hybrid systems considering different configurations and final uses (a good representation of these papers can be found in the impressive literature review of photovoltaic-wind hybrid renewable system research by considering the most relevant 550 articles [2]) and, over the past decades, many reviews have made a comprehensive summary of various results obtained, which include, for example, the impressive review on more than 150 recent articles (including review and research articles) on sizing methodologies of hybrid renewable energy systems [3].

On the other hand, REDPS market (which somehow may be associated with Hybrid Systems and Microgrids ones) is well established and has grown during the last years.

**Citation:** Arribas, L.; Bitenc, N.; Benech, A. Taking into Consideration the Inclusion of Wind Generation in Hybrid Microgrids: A Methodology and a Case Study. *Energies* **2021**, *14*, 4082. https://doi.org/10.3390/ en14144082

Academic Editors: Adrian Ilinca and Mohamed Benbouzid

Received: 29 April 2021 Accepted: 2 July 2021 Published: 6 July 2021

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**Copyright:** © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/).

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However, the microgrids installed in the last five years have tended to incorporate PV and battery storage with diesel generators as backup (i.e., PVDPS or solar hybrid microgrids), coping with around 63% of existing microgrids [4]. On the other hand, small scale wind is much less common for microgrids although some combine with diesel or solar PV [5]. Thus, the common approach has become the following: "Solar hybrid microgrids are currently the most viable and reliable solution for off-grid areas with sufficient population and load density" [6].

These figures from the real world REDPS installations are in contrast with the previous figures from the research world, where there is a maintained interest in REDPS publications. This fact evidences a gap between research and the implementation of REDPS, showing that existing research in hybrid PV/wind microgrids consists primarily of feasibility studies, focusing solely on techno-economic aspects of PV/wind solutions [7], however, this is not sufficient for the implementation of such solutions in real applications. The reason for this might be relative to wind technology, however, nowadays wind technology is highly technically and economically developed at the large scale where large companies and consultancies are available for any design because the large size permits the necessary (not small) budget for the design process. The gap comes neither from the design methodology for PV/Wind solutions as there are plenty of examples of different design methodologies available in the literature, nor from the wind technology itself where implementation is widely spread at the large scale.

The following question arises: why is wind generation nearly missing in the microgrid scale? The immediate answer would be that the dramatic cost reduction in the PV generation has taken it out of the playing field, but a pragmatic economic analysis shows that this is not always the case (that is, what the large number of related publications show). There must be other factors hindering the use of wind generation for this scale derived from the small size of the wind technology: Neither the technology nor the economic competitiveness are well achieved and the updated knowledge of this sector is not easy to maintain due to its constant impermanence. The impression coming from experience is that, in many cases, the small wind option is not even considered during the design process because of the lack and relative complexity of updated knowledge with respect to the technology ("implementation of the wind solution was discarded because the data analysis revealed a low energy production of the existing turbines ... ; the reason this is not working should be investigated" [8]), the wind resource ("Wind power is intermittent and its generation curve does not match the daytime load profile of communities" [4]), and its costs and main features ("A reduction in the cost of small-scale wind turbines at the same level as that seen with solar is not expected" [4]).

The main barriers identified in [7] for hybrid PV/wind microgrids in Kenya, which could easily be extended to most places in the world, are shown in Table 1, with some comments that will begin to show the scope of this work.

This publication is expected to provide some updated knowledge in order to facilitate the consideration of the inclusion of wind technology in the design of new or repowered REDPS of low power, which is not usually carried out by large consultancies and in which the budget for the design is usually limited by the size of the system.




**Table 1.** *Cont*.

## **2. Materials and Methods**

A methodology to cope with some of the barriers that designers might face when considering small wind turbines as an option during the design process is proposed in this section.

A case study in the form of a feasibility study will then be presented as an example of the application of the proposed. The bulk of this example is not the techno-economic analysis of PV/wind solutions itself but the methods to cope with small wind technology issues within a more or less standard design procedure.

#### *2.1. Methods: Methodology to Account for Small Wind Turbines Barriers during the Design Process*

The reference design process will be briefly described (Section 2.1.2) once the configuration of the REDPS to be studied has been selected and justified (Section 2.1.1). Then, the main differences arising from the presence of wind generation in REDPS will be identified and the possible existing solutions for each identified factor will be analyzed (Section 2.1.3). Finally, the proposed methodology to account for small wind turbines barriers during the design process will be presented (Section 2.1.4).

#### 2.1.1. Selection of the Configuration of REDPS to Be Designed

It is important to select the system configuration at the beginning because different configurations results in different design procedures and, thus, different considerations. The common feature of all the systems covered in this work is the presence of diesel generation. However, REDPS existing today can be classified from a technical point of view into diesel-dominated and inverter-dominated hybrid systems [9] depending on which component is in charge of maintaining grid stability.

•Diesel dominated hybrid systems.

Diesel engines have traditionally been one of the main options when electrifying rural and remote areas. However, the important drawbacks of this option (such as the rising cost of diesel fuel and carbon emissions concerns) when compared to renewable energy opens the door to the inclusion of such renewable energies in diesel dominated grids.

In these systems, the grid is formed by diesel generator(s). In fact, the common case is that there is an already existing grid supplied by the diesel engine(s) to which a renewable energy generating system is connected for retrofitting. Larger systems usually contain more and larger equipment that allows for an economy of scale and thus lowers power costs.

The system design is strongly related to the amount of energy that is expected from the renewable sources (system penetration), which will define the methods used to control the power system. System penetration can be defined either as Instantaneous penetration (renewable power output divided by the load power) or as Average penetration (renewable energy output divided by the total load energy over a given time period, typically a month or year). Instantaneous penetration relates to the power system complexity to maintain acceptable power quality. Average penetration relates to the steady state general system operation characteristics.

#### • Inverter dominated hybrid systems.

Most of the existing REDPS systems can be included in this group, which is characterized mainly by the inverters as the grid forming unit, and the use of long-term storage (in the past, usually lead acid batteries) as the main source to stabilize the grid. Fossil fuel generators are present but function as a back-up unit and usually with the options of supplying the load directly and bypassing the inverter if needed. From a configuration point of view, size is not such a grea<sup>t</sup> issue for this configuration but it is a fact that size is a very important issue for component availability, cost, and design.

The target configuration for this paper will be the inverter dominated REDPS, with one main diesel generator (there may be two for back up, but only one working at the same time) and with long term (several hours at least) electrochemical storage that is either AC or DC coupling.

#### 2.1.2. Description of the Design Process

It may be worth remembering at this point that the methodology presented in this publication is not a design methodology, for which many other proposals have been published [3], but a methodology to cope with small wind turbine barriers during the design process. However, as this methodology is applied together with a design process, it will be briefly presented here.

From a design point of view, PV-Wind REDPS can be roughly considered as PVDPS where wind generation is added, as depicted in Figure 1. This approach has the advantage of allowing the use of all knowledge coming from existing solar PV standalone systems, which is quite extentsive.

**Figure 1.** From a basic point of view, the design of Wind-PV-Diesel Power Systems may be seen as PVDPS where wind generation is added (adapted from [10]).

The research in REDPS might have started at the time when the IEA's Task 8 (1985– 1994) "Study of Decentralized Applications for Wind Energy" aimed both to define costeffective models and techniques suitable for obtaining wind and load data necessary for planning and specifying decentralized wind energy conversion installations and to apply and further develop models suitable for analyzing the performance of wind-diesel systems. The results of this collaborative research were summarized in the book "Wind-Diesel Systems", published by the Cambridge University Press [11]. At that time, the inclusion of

solar PV generation was so expensive that it was not even considered. Since then, some implementation guidelines have already been proposed for both PV-Hybrid Systems [12] and Wind Hybrid systems [13,14], which both have many elements in common with PV-Wind Hybrid systems or even for small wind turbines (SWT) [15]. Some call for tenders for REDPS and offer their own design methodology [5].

From these guidelines, an adaptation has been made to update those particular issues that arise in PV-Wind DPS systems in three design stages: data collection, sizing study, and implementation Project.

#### 2.1.3. Differences Arising from the Presence of Wind Generation in PV-Wind REDPS

It is a fact that the inclusion of wind generation results in different uncertainties in the design process of the system which needs to be taken into account. Some of the most important ones are addressed in the following paragraphs.

#### *(a) Characterization of Wind Resource*

Firstly, wind resource is much more variable (both temporarily and spatially) than solar resource and there is no geometrical method to predict it (in the case of solar resource, calculations of solar–Earth geometry provide a good estimation from latitude). This means that, on one hand, it is more difficult to evaluate and, on the other hand, small variations in the site may bring important variations in the wind resource.

#### - *Wind Resource Assessment Methods*

There are a variety of methods to assess wind resource and they range from lower to higher costs. The methods of assessing the wind resource usually stem from using a general wind map and using that wind speed information to form the basis of a production estimate. Another approach is to use a commercial wind resource model to identify, more "precisely", the annual wind speed range. A third approach is to assess wind measurements from nearby projects, wind resource towers, airports, or other weather stations. Having equipment and tools that provide wind rose information are invaluable for the estimation of wind turbine production. Wind maps typically provide a basis for the wind speed, which is the other important factor in understanding production.

These three approaches do not take into account the impact of local micro-siting and the dramatic effect that it has on a small wind turbine. The only method where the wind resource is truly quantifiable and accounts for obstacle, terrain, wind direction, and blockages is to measure the wind at the exact location and exact hub height of the proposed small wind installation. Even though on-site wind measurements are the most reliable method of assessing the local wind resource, it is expensive and time-consuming and the cost is not justified for SWTs [16]. Historically the cost of wind measurement equipment and analysis has been prohibitive for small wind turbines. Recently new wind resource measurement approaches have been developed, including lower cost wind measurement equipment and towers and new drone technology [17].

Methods to cope with these cost restrictions are listed as follows: Using regional wind maps specifically for small wind turbines implementation, reanalysis data, or nearby Met office statistics; and choosing a site for the wind turbine as free from obstacles as possible.

## - *Using Reanalysis Data*

Given that the temporal variability of the wind, it important to know the time distribution over a period. Meteorological stations may be an option; however, they are not always close to the site or it is not possible to access their information. In that case, it is feasible to use reanalysis databases. Reanalysis data uses assimilation processes to combine observed (or measured) data obtained from satellites, ships, sensors, and weather stations with numerical models.

Since the observed data are unevenly distributed over the Earth, numerical meteorological models allow the estimation of the state of different layers of the atmosphere for a certain place and time period using a regular grid. With this approach, it is possible to

generate a time series of gridded atmospheric parameters, such as air temperature, pressure, and wind at different altitudes; and surface parameters such as rainfall, soil moisture content, ocean-wave height, and sea-surface temperature. The three leading global data sets and its most recent bases are described below [18]:


Using reanalysis data also allows the easy performance of long term analysis on the viability of the inclusion of wind generation in hybrid diesel microgrid as data for decades.

#### - *Small Wind Resource Assessment*

These databases have the advantage of having a high percentage of availability; however, they are not influenced by local effects of orography and roughness. Since roughness measures the decrease in wind speed due to friction with the surface and orography generating alterations in the wind flow, it is necessary to take them into account so that the energy assessment of the winds is as representative as possible of the site.

Using these global datasets to generate initial and boundary conditions for the simulations, some mesoscale and/or microscale models may produce higher resolution grids, such as WAsP ([21]). The most common mesoscale model is WRF [22], which is a weather prediction system used to generate meteorological forecasts or hindcasts. WRF downscales the global datasets and the results can be used to generate spatial wind maps and as a valuable source of long-term time series wind data. The WRF grid and output resolution is typically a few kilometers and ca. 3 km is often the preferred choice. It is possible to downscale the data from the global level to the mesoscale level and, further, to the microscale level, which is usually offered by commercial tools (as those from EMD or UL) with proprietary microscale models [23].

However, small wind turbines are often installed under wind conditions far from the conditions specified in standards and this is expected to result in large power curve uncertainties [17].

Complex terrain sites are typical small wind turbine sites and pose another challenge in accurately estimating wind turbine production. Underproduction was originally believed to be dominantly due to uncertified turbines and inconsistent turbine rating approaches; but as more turbines have become certified and wind turbine ratings are more globally consistent, underproduction is believed to be a strong function of the local micro wind conditions [16].

Over time, new modeling tools, site assessment technology, and study methodologies will evolve. Without streamlined customer-friendly approaches, the small wind turbine market will continue to be a smaller niche market. Better site assessment can be a step toward easing owner purchase decisions [16].

This paper is proposed for that aim.

*(b) Existing Technology*

There does not exist a unique definition for what a SWT is, in terms of size, but a more or less universal convention is that it refers to wind turbines smaller than 100kW. Within this range, some classification can be made according to Table 2.


**Table 2.** Classification of SWT (Source: CIEMAT).

The values that define the ranges for this classification have been chosen from the norms and legislation affecting SWTs. The value of 40 m<sup>2</sup> was the limit established in the first edition of the IEC-61400-2 standard and is the range intended at the present time for the integration of SWT into the built environment; the 200 m<sup>2</sup> limit was established in the second edition of the above mentioned IEC-61400-2 standard in 2006 and includes most SWT applications. Finally, the limit of 100 kW is defined in many countries as the maximum power that can be connected directly to the low voltage grid. The pico-wind range is commonly accepted as those SWTs smaller than 1 kW [24].

Despite being more uncertain, models and topographical background data are at a level of quality that makes the calculated wind resources valuable for SWT projects. Having determined the wind resources, the second source of uncertainty in energy yield calculations is the wind turbine type or more specifically the power curve of the wind turbine. Many countries today have standards for how wind turbine manufacturers should collect and process data to produce certified power curves; this improves the accuracy of the power curves that could otherwise be too "optimistic" [18].

Quality assurance has proven to be indispensable for establishing an enabling environment for a rapid uptake of renewable energy technologies. Quality assurance of standards are intended to ensure that products and services perform as expected and also includes the mechanisms to verify that such requirements are fulfilled, e.g., testing and certification [25]. This is of particular importance for SWT: A grea<sup>t</sup> effort has been conducted during the last two decades to increase the SWT quality and there are many reliable models in the market, but it is also possible to find many commercial models that have not been certified nor tested. A description of norms and standards affecting SWT can be found in [25] but, in general, an effort should be made to work with, at least, independently tested SWT. In [26], some guidelines can be found to assure quality for buying SWTs safely, which covers the manufacturer, the product reviews, the warranty, testing and certification and the installer.

Another important issue related to SWTs is the availability of wind turbines, which refers to both the available sizes and the available manufacturers. Before choosing a small wind turbine, it is advisable to be informed about sizes and maintenance support service in the area. This limits the number of available wind turbines for the design. An added difficulty is the highly changing characteristic of the SWT market (as an example, from the around 20 small wind turbines manufacturers in Spain in 2014, only eight are still active in this field in 2020 [27]). Information on available manufacturers and models should be updated frequently in order to be aware of the present situation. Both myWindTurbine [28] and HOMER Pro [29] include SWT databases, which are useful as a reference, but both of them are neither completely updated nor exhaustive. The reference, [30], may be a good starting point.

The last issue that will be highlighted here is the cost of the SWT. On one hand, there is a grea<sup>t</sup> variety in the cost of similar size commercial SWT: Cost should not be the only criterion to choose a SWT because, as it was mentioned before, not all of them will be of the same quality. On the other hand, the information on the cost of a particular SWT is not always easily accessible: Of course, the best method is always to have a particularized (the cost may be different according to the site) quote from the manufacturer/installer but some general references can be found in literature for a first approach [31–33].

*(c) Availability of Design Tools*

In general, when talking about available power systems design tools, there are different approaches depending on the following factors:


**Figure 2.** Example of tools used for a high RE penetration REDPS in the different design stages [34].

In the case of the selected configuration for this paper, dynamic analysis is not necessary for inverter dominated systems with one main diesel generator and with long term (several hours at least) electrochemical storage as the battery and the battery converter are able to maintain short term stability.

There is a grea<sup>t</sup> variety of sizing tools for PV Hybrid systems [35] but the number is limited when including wind generation in the system. Fortunately, even though the number of tools for PV-Wind REDPS design is limited, there are some high-quality available tools. According to a comparative study of 68 computer tools for the integration of renewable resource in various energy systems, HOMER was evaluated as one of the most applicable for optimization, feasibility, and sensitivity analysis of both off-grid and grid connected micro power systems and also pointed out as the most used and best known of all the software tools developed so far [8].

In particular, HOMER Pro has become a standard in the design of REDPS, as it merges most of the capabilities for a feasibility study in a single tool and it also includes wind technology (other well-known tools for PV systems design, such as PVSYST, does not); thus, it is the recommended tool for the configurations under study in this work. However, nowadays HOMER Pro software is no longer free (previously, there was a Legacy free version) and some amount must be paid depending on the desired use.

#### 2.1.4. Methodology to Account for Small Wind Turbines Barriers during the Design Process

In the first place, the methodology described here deals only with some technical considerations of the design related to the consideration of wind technology; the development and managemen<sup>t</sup> of a REDPS system is a relatively long and complicated process and involves other key aspects such as social, environmental, management, contractual, quality assurance, training, and some other aspects. These aspects obviously have to be taken into account through the project development, but are out of the scope of this work due to time and space limitations. Some useful references may be in [36,37].

As presented in Section 2.1.2, an adaptation has been made to cope with those particular issues that arises in REDPS systems during the consideration of wind technology, concluding with some suggestions that basically include the following stages.
