Mixed AC/DC off-grid photovoltaic power at "El sitgetí" organic farm (Catalonia, Spain)

El Sitgetí organic farm is located about 2 km away from the town of Bonastre (Catalonia, Spain) in a little valley away from civilization! The farm is located too far from the village to be connected to the electricity grid. And although his owner, Charlie, has relied on a truck battery to power his computer for the last 10 years it was about time to install a decent size micro photovoltaic power plant there! The whole project started by the acquisition of second hand solar panels (believed to be 3 years old, bought from a refurbished photovoltaic power plant, 80€/panel, a real bargain!). The system consists of 28 monocrystalline Silicon panels of 220W power (Voc=48V; Isc=5.3A, FF=72%), for a total of over 6 KW (though this is a theoretical value, which is never matched in practice)  

The 4 lines of 7 panels connected in series. 

So I haven't talked all that long about DC power just for the sake of it! Here is how we plan on taking full advantage of the DC power generated. The 28 panels are connected as follows: 4 lines of 7 panels in series are connected in parallel through 4 blocking diodes. Each line produces a theoretical maximum power point voltage (VMPP) of 336V (at 1 SUN) which corresponds approximately to the maximum point of the sine wave of the mains supply in European countries (230 x sqrt(2) = 325V) (see previous post for an explanation on that). 

Theoretical I-V curve of individual panels and of the 7 panel array

The DC source produced from the 4 lines of panels can therefore be used directly to power equipment running on DC (battery chargers, variable frequency drives, boiler, etc...). In practice the VMPP varies between 250 V and up to 400V (at 1 SUN), depending on various factors, such as  the external temperature, and the cloud conditions. We will refer to it as DCV350, or DC BUS from now on. Despite this variation in VMPP our system will still work perfectly as a DC source to power various equipment. 

Map of the installation

Map depicting the electrification of the farm

The total land area of the farm covers some 28 ha, which is pretty big. Luckily most it is forest and only about 2 ha are actually farmed. Given the location of the panels (placed at one of the highest accessible and most cleared point on the farm) with respect to where the electricity is to be distributed, some calculation were necessary to minimize voltage losses across the wires. First the location of the ACV230 inverter was chosen to be placed at middle distance between the most remote places where the AC has to be distributed. This leaves about a 100m distance to be covered by the current both ways before  being consumed. Taking into account a maximum power of 4000W (which is the maximum power of our inverter) this leaves us with 10 mm2 wire diameter to limit the voltage drop to less than 3% across the lines as is the norm in most European countries (click here for a voltage drop calculator). For the DC lines, the cross section of the wire can be downsized a bit because the voltage is higher that on the AC line. The losses in this case will not interfere with appliances operation as in AC in the case of a large voltage drop. But it should still be minimized to avoid power generated being lost to heat through the cables. A cross section of 6 mm2 was then used. 

There are 7 points of connections and distribution, which are all marked on the scheme. Note that there is a set of battery in 4 locations. Battery sets of 12V are only used for illumination. All the light bulbs used in the farm are LEDs, so consumption is ridiculously low. All 12V sets are recycled batteries: one 20 years old NiCd , one  > 30 old NiFe, two truck batteries, but they still offer a minimum autonomy of 15 days in our condition. The 24V set is a refurbished (9 years old) Lead/acid set, which has only a small fraction of its original capacity, but as explained in my previous post, the all point of the mixed DC/AC system is not to rely on batteries as in conventional systems.
All the batteries are charged via the DC BUS using switched mode power supplies (SMPS). The 24V set is charged with a modified welder/inverter (working on the same principle of SMPS) and is connected to the ACV 230 pure sine wave inverter.

Panels location survey, tilt angle and shading

Location

First of all it is imperative to know where the south lies respect to where we want to install our solar panels, there are different way to determine it, as explained here. Then the way we proceeded was to print the sun path chart for our location by simply entering the exact latitude and longitude of the site on this web site. We then built a solar elevation gauge by following the instruction of this very useful link. Given that the farm is located in a valley with hills on both sides, the hours of sunshine are slightly limited by the fact that the sun only goes over the east lying mountain about one hour after sunshine, and similarly for sunset.

Solar path map. the green line at 11° elevation represent the hills located  eastward  and westward of the panels site

Tilt angle 

Given that we installed our panels on a flat surface it made sense to take advantage of it to mount our panels on variable titling angle mounts. Unfortunately given the height of the mount holding 3 lines of panels (see below to understand why we chose such type of structure)  it could only be manufactured with 2 tilting options. Although the angles of maximum energy production (for a 2 tilting angle system, with angle adjustment twice a year) are 16° and 54° in our latitude, we finally chose our system to be 17° (summer) and 34° (autumn, winter, spring). This was, on the one hand, to avoid the structure from being too tall and risking damage from strong winds. On the other hand these 2 angular positions are optimum for summer (17°) and spring-autumn (34°) which are the period of higher activity and thus higher energy demand on the farm (low temperature vegetable storage being the most import energetic demand in summer-spring-autumn), winter being a very quiet season and sufficiently cold not to require any additional energy to preserve vegetables. The price of the two mount was about 1200 € (bought from Alusinsolar, Spain) .

Shading

Shading is one of the most important parameter to be taken into account when placing the arrays of panels as even the slightest area of shade over a panel is likely to shut down the energy production of that entire panel (here is a nice study of the effect of shading on output power). In our case shading is even more critical since our system will be very dependent on the DC voltage of the arrays, therefore shading over one panel would reduce the DC voltage of the entire array and could limit or impede the operation of certain appliances. Given the area of flat surface available to us, two options were envisaged. the first one consisted of having four individual lines of panels placed on the ground spaced by 1.60 m distance, or a multiple array stand. After some trigonometry calculations it turned out that the shadow on the arrays resulting from the front row of panels could have been substantial especially in winter (morning and noon) when the sun's elevation angle is below 20° (see figures below, but rather that bothering with tedious, though not complicated, algebraic trigonometry, there is a better option to simulate shade using a free software SketchUp as shown here). To remedy this problem, one should have elevated the 2nd, 3rd and 4th row from the ground as depicted in the scheme below, in order to have the shadow passing bellow the panel's array of the line to its back .

Calculation of the panels height to avoid shading on the back row (system of individual panel arrays, not chosen due to the complexity related to elevating the panels from the ground and having a central pivot point)

 Finally a multiple array mount was chosen for its simplicity, though only mounts holding a maximum of 3 panels lines could be manufactured by our supplier, meaning that the first row would lie on an individual mount at ground level, spaced 2.10 m from the 3 panels lines mount. The shadow produced by the first row only affects the second row (the first row of the 3 panel mount), and only early in the morning and late in the afternoon the shortest days of winter (for roughly a month, from mid December to mid January). The loss on power generation is therefore not very significant, since at the time of shadowing the power generation is low.

Lateral view of the two mounts arrays adjusted to their autumn-spring angular position (34°). The individual mounts were connected together with twined copper wire (approx. 7 mm diameter each) and connected to the ground through a 1.5m copper coated steel rod to protect the system against lightnings.