Case study: Kerkrade fixed structure project (I)

Introduction

The “Zonnepark Kerkrade” project consisted of production and commissioning of a solar photovoltaic installation, located in Kerkrade, with the objective of providing “clean” energy to the Dutch power grid. Once the project was completed, it was proven that the photovoltaic installation was capable of producing enough energy to cover the annual consumption of 4,000 average households.

As part of this project, Praxia Energy was responsible for the design, engineering, manufacturing and assembly of the structure which support the solar photovoltaic panels.

Client

German utility with 120 years of experience in the energy sector and world leader in the  supply of renewable energy. It has a group of installations related with energy production (wind generators, gas and coal power plants, lignite mines, hydraulic power station, etc.) in different countries all over the world, although its main market is Europe.

 

Starting data

The project is located in the south-eastern part of Netherlands, on the border with Germany. Specifically in Kerkrade province of Limburg (Figure 3).

The satellite image of the site (prior to the construction of the photovoltaic park) can be seen in Figure 4. The place of the installation is highlighted in yellow, while the transformer station “Terwinselen”, where the connection to the grid is made, is shown in green. Finally there is a blue line, which represents the wiring route to establish the electrical connection between the two points.

The details of the location are the following ones:

• It is divided into northern and southern plots by the public road “Gulperweg”, which gives access to both plots. The site has a total area of 12.6 hectares.
• There is a gas pipeline located close to the northern border (Figure 5).Figure 5: Gas pipeline
  There is a road used for maintenance work on the N299 highway along the south-eastern border (Figure 6).

 

 

• Along the south-western border is the N281 highway (Figure 7).

•  Concrete foundaments of two former wind mills in the northwest and southwest corners (Figure 8).

 

Floor structure, fixed, landscape configuration, modules of 6 rows and 13 columns, minimum height 0.70 m.

The structure of this project has a tilt of 8º, much lower than usual.

The decrease in the tilt causes a reduction on the shadow cast by one row of panels on the next, which allows to reduce the distance between rows. Reducing that distance allows to install a larger number of panels, and therefore more power. However, the decrease in tilt also reduces the efficiency of each panel, because they are not oriented perpendicular to the sun during the peak radiation hours, and therefore, panels produce less energy.

In this case, the client has come to the conclusion that installing a higher number of panels compensates for the loss of efficiency per panel. Therefore, for this case, tilt reduction is the optimal solution, mainly because the number of hectares available has been very limited due to the high cost of land in Netherlands.

Installation consisting of 36,036 Tier 1 photovoltaic modules of 400 Wp each manufactured by JA Solar.

The transformation from direct current to alternating current is carried out with 4 Tier 1 inverters manufactured by SMA.

The planned power of the project is 14.4 MWp (DC) / 10 MVA.

 

Figure 9 shows the layout of the photovoltaic park.

The requirements (RFQ) are as follows:

• EN 10025 structural steel or national equivalent.
• All steel products must be delivered with inspection documents type 3.1 according to EN 10204:2005.
• Galvanised steel products according to EN 10346 or EN ISO 1461.
• Deliveries accompanied by certificate of conformity according to EN ISO/EC 17050.
• Galvanised steel or aluminium structures according to EN 1991-1-4:2010, 1991-1-6:2010, 1993-1-1:2010, 1993-1-3:2010, 1993-1-5:2019, 1993-1-8:2010, 1993-1-9:2010, 1993-1-10:2010, 1993-5:2019, 1997-1:2014,1997-2:2010, 1992 – 2011-01, 206-1: 2017-1, 1999-1-1: 2014-03 and local equivalents.
• The mounting structures shall guarantee the prescribed tilt in the module plane and shall maintain this condition even after extreme loading events foreseen in the structural calculation.
• The engineering of the module support structure shall take into account the selection of permanent, wind and seismic loads, the structural dimensioning and the control of the foundation, the sizing of members, the control of connections, and the effect of temperature changes in accordance with all applicable Codes and Standards.
• The metallic supporting bases for Modules shall be of steel components hot dip galvanized as per EN 10346 and/or EN ISO1461 or equivalent or by an appropriate anodized aluminium of heavy duty type alloy according to the standard EN 485, 755, 1559-1, 1559-4 and 1706 or equivalent for the better anti-corrosion protection of the construction.
• The installation shall not require any welding to be performed at the Site.
• All M6 an M8 connections including bolts, nuts, shall be of stainless steel or compliant with other industry standard practices appropriate for the application defined ensuring no corrosion risk. Connections other than M6 and M8 shall be at least coated electro-galvanic or any other that the manufacturer considers appropriate.
• In case of installation by the manufacturer, the manufacturer shall undertake a pull-out test and deliver the results of such testing to the Employer in order to enable the Employer to satisfy itself as to the sufficiency of fixing of the metallic structures. Manufacturer will check the pull-out test to the minimum required in the calculations.
• All structures shall be continuously bonded and grounded to the earthing system.
• Module clamping system shall comply with the module manufacturer´s requirements. Sufficient space shall be left between module frames to allow for thermal expansion. Generally, the installation guidelines of the Module manufacturer shall be respected.
• Manufacturer shall demonstrate that modules will remain attached to the structure under all environmental conditions reasonably expected at Site and that the maximum loading condition prescribed by the module manufacturer is not reached. No stepping of modules will be tolerated.
• Modules shall be mounted either by means of aluminum / steel clamps or by direct fixing through screws or rivets to the structure and the configuration may depend upon the type of structure. It is compulsory that, whatever the clamping / fixing technique; this is accepted by the module manufacturer and does not invalidate the module warranty.
• Configuration of module tables may vary depending on the supplier of the structure system; however, the following aspects shall be taken into account when selecting the structure and the table configuration: i i. Lay-out for project as delivered by request; ii ii. required minimum distance from the ground: >60 cm; iii iii. Meets Site Conditions and calculations regarding wind, snow, etc.).
• Installation restrictions in terms of ground levelling and maximum slope shall allow a certain degree of flexibility and tolerances shall be suitable for the Site.
• Manufacturer will set the tolerance in degrees + and -.

In this case the client did not provide a geotechnical report, so Praxia Energy carried out pile driving tests.

Design of the installation

 

 

 

 

 

 

 

Based on the data provided by the client, the design of the installation was carried out, including the 3D model (figure 10), stress calculation (figure 11), drawing up of drawings (figure 12), selection of materials, etc.

 

 

 

 

 

 

 

The results obtained are as follows:

• Material. Hot dip galvanised S 355 steel for the piles and Magnelis® (Zn – Mg alloy) for the rest of the components, except for the clamps, which are made of aluminium. When assembling, the aluminium clamps are placed on Magnelis® purlins, which brings two different metals into contact, producing a galvanic coupling and resulting corrosion. To prevent this from happening, a polyamide component is added (figure 13).

• Structure. It consists of 2,015 x 996 x 40 mm modules, each one has 7 purlins per beam spaced 1 m, 78 photovoltaic panels, 20 piles and 157 m2 of panel area. In order to reduce the weight, and therefore the cost of the installation, while ensuring all requirements, the structure was tested in a wind tunnel. Using the model shown in figure 14, the wind loads assumed in the stress calculation were simulated to ensure the correct operation of the structure with as little material as possible.

 

• Pile driving depth. Pile driving tests were carried out with the objective of calculating the driving depth of the piles. It must be verified that at this depth it is guaranteed that the piles are able to support the maximum load of the installation (combination of the weight of the structure, panels, wires and other elements together with wind loads, snow loads, etc.) with a reasonable safety margin and without displacement. These tests consisted of measuring the forces supported by the piles with a dynamometer (figure 15) before displacement for different depths and at various locations (figure 16). The forces measured were the shear force and the pull-out force, the push-in force was not necessary as it was not going to be the critical force in any case.

The maximum loads on the piles (obtained in the calculation of the structure by combining self-weight with external actions such as wind, earthquake, etc.) are:

•    Push-in: 10.3 kN
•    Pull-out: 6.0 kN
•    Shear: 1.5 kN
•    Moment: 0.74 kNm

These loads must be lower than the loads measured in the driving tests at the time of displacement. Table 1 shows the results of one of the tests carried out.

Figure 17 shows a graphic with the results of the worst case. It shows the maximum pull-out loads that the structure can bear without displacement according to the driving depth. The red line is the maximum load on the piles (minimum that the structure must bear), while the blue line represents the forces measured in the ground.

In the above graphic, it can be seen that a depth of 1.75 m will be enough to support the maximum loads required by the structure.