A successful project always starts with a good solar power plant design. A good design ensures an optimum balance between the costs of the system and the anticipated output. However, a well-thought-out design also ensures lower maintenance costs. We regularly receive questions relating to the design of solar parks. In this article, we use these questions to explain a number of choices and considerations that need to be made when designing a park.
Where do I start when designing a solar park?
To produce a good design, we must explore the extremes. Firstly, we establish what the maximum capacity that can be installed in a location is. Account is thereby only taken of the ‘physical’ restrictions of a location. These restrictions might, for example, be imposed by the permit granted (height restrictions, positioning direction, environmental constraints, etc.), landscaping to be created, minimum row distances for maintenance, ditches present, etc. Narrow row distances will mean that this variant leads to an array with a lot of internal shading (shadow from one row of panels on another row, called near shadow) and therefore relatively low production per panel. Set against this is the fact that the most kWh are produced in absolute terms. The filling factor for this positioning variant is typically at around 0.6 to 0.7, which means that 60-70% of the surface is used for panels.
At the other end of the spectrum, a positioning variant is calculated whereby the panels produce optimally. The row distance will thereby have to be greater in order to prevent inter-row shading. The production per panel is optimal, but fewer panels can be placed at the location and so the absolute quantity of kWh will be significantly lower. With this variant, the filling factor will be around 0.2 – 0.3.
When the extremes are known, the optimum positioning can be determined on the basis of number of iterations. What is optimal will depend heavily on the location’s general fixed costs, such as the costs of using land, fees, network feed-in, etc. If fixed costs are high, more kWh will have to be generated in order to achieve the lowest cost price per kWh.
What is the best approach with regard to inverters?
The inverter is one of the more expensive and maintenance-sensitive components within a solar park. Since the solar panels will only produce at full capacity a few instances a year, the design will seek to install as few inverters as possible. This will raise the overload factor. The overload factor is the relationship between the capacity of the inverter (AC side) and the PV modules installed capacity (DC side). Undersizing the inverters can save costs on inverters. The consequence of this is that at times of peak production a small proportion of the production is lost (approx. 1-3%). Depending of the exact cost of the inverters and the location-specific energy production, the design can be further optimised in this way.
Another reason for undersizing inverters
The peak capacity of a solar panel (kWp) is the capacity that is achieved during the factory tests under Standard Test Conditions (STC). These test conditions (including an irradiance of 1,000 W/m2 (at vertical irradiation), a panel temperature of 25°C and an air mass of 1.5 spectrum) are not achieved in practice. For example, sunlight in the UK is virtually always less than 1,000 W/m2. A higher panel temperature results in a reduction in the yield. The panel temperature can easily be 20⁰-30⁰C higher (depending on the amount of irradiation, cooling by air flow and the external temperature). A panel temperature of 45⁰C can lead to loss of capacity of 8-9 %. A lower panel temperature has the opposite effect. We see overload factors between 20% and 40%. At a 20% overload factor in the UK, account must be taken of small loss of production during high irradiation. However, this does not outweigh the gain during lower radiation, since an undersized inverter is more heavily loaded and therefore has a higher efficiency.
Relationship between Design and Maintenance
Maintenance costs are deeply affected by two issues: 1) the choice of the material used, and 2) the design of the solar park. The fact that the choice of material affects the maintenance costs goes without saying (see our previous articles). The design must also take careful account of the maintenance of the operational park, such as the construction of maintenance roads, the accessibility of panels and the inverters, the layout of the site, the routing of cables, the choice of planting, the use of vegetation species, positioning of gates/fencing, distances to ditches, etc. The choices made here have a major impact on the number of maintenance hours required on site.
An oft overlooked area for optimising the park is the distance between panels. This is often set at around 2 metres in order to allow sufficient space for machinery to conduct maintenance. This distance between rows can often be reduced if the machines can manoeuvre under the high side of the panels on one side. With an array of 3 panels in portrait at an angle of 20°, the highest point will readily reach 2.4 metres, which offers around 0.5 metre of clearance for machinery. One or two rows of extra panels can then be installed on a large plot. It is therefore worth thinking carefully about the maintenance and taking account of it at the design stage.
Is it a good idea to use trackers?
We are often also asked about the benefit of solar trackers (single or multi-axis). In the UK, solar panels perform optimally at an angle of 37°-41°⁰ and an azimuth of 0⁰ (due south). If the panels are not 100% oriented to the south, but deviate by say 5°, that leads to reduced performance. That effect can be prevented by turning the panels with the sun and therefore keeping them continuously optimally aimed at the sun. Tracking the sun can, in theory, result in approximately 25% more production in the UK without taking account any additional losses (such as shadow). However, trackers significantly increase the system costs and lead to higher maintenance costs due to more parts in motion.
The installed capacity is also reduced by the space required for the tracker tables. Account must be taken of 10-12% higher purchase costs and 3-4% higher operational costs. In practice, in the UK, where the climate has a lot of diffuse light (diffuse light normally represents between 30 to 50% of the global light in the UK), we have not yet seen any examples whereby the benefit of using trackers outweighs the higher costs.
East-West: a worthwhile alternative design?
An east-west orientation means that the panels are not oriented to the south, but to the east and west. The panels are at a shallower angle (8⁰-15⁰). A solar panel with a due south orientation has a typical energy yield with a sharp peak around noon. There is little or no energy yield in the morning and evening. An east-west orientation, on the other hand, has a less sharp peak around noon but more yield in the morning and the evening, which also has consequences for the number of inverters and the overload factor.
The shallow angle of the panels means that there is less shadow cast between the panels themselves. The installed capacity (kWp/m2) is therefore higher (filling factors of 0.9 are possible) in a park with an east-west orientation. Depending on the PPA and the development of the electricity price, this could be lucrative over the coming years in view of the fact that the electricity price is higher on average in the morning and evening than around noon. The more solar power is installed, the more interesting this will become. Particularly in the regions in the UK with relevant high solar irradiation and projects with high general costs, we recommend that an east-west orientation should also be carefully costed.