This tutorial presents the sizing and construction of an electrical system for a solar generator (1 kWp or ‘kilowatt peak’) which can be moved by bicycle. This structure was designed to fit on the CHARRETTE, an assisted trailer designed by the Véloma association whose plans are freely available.
This trailer is a functional demonstration designed as a part of the Scholar Grid project.
Supported and piloted by the Fondation Schneider Electric in partnership with these associations Low-tech Lab,Énergies sans Frontières and Atelier 21, this project intends to investigate innovative solutions to provide affordable and clean electric energy to training centers that train future electricians. The energy systems created by the technical experts and the teachers of the training centers, will be implemented by students and serve as a pedagogical base.
The fields of investigation of this project were the following:
To test these techniques in real conditions, Low-tech Lab constructed a mobile generator trailer. With the power of 1kW, it combines the repaired second hand solar panels and the regenerated lead batteries. This was designed on the basis of concrete needs: to provide electricity for the Festival Low-tech organized in Concarneau in July of 2022.
Beginning with this concrete case, "the tutorial details the general steps of sizing a photovoltaic installation in self-consumption". The context, the preliminary evaluation of needs and the choice of adapted energy sources are explained in detail in the document "An energy-autonomous Festival?’ In the ‘Files’ section".
|Quantity||Designation||New Material Price||Project Material Price (donation, second-hand, reclaimed)|
|3||Peimar 330W Solar Pannel||500||105|
|1||Victor Energy Phoenix 24V/1200VA Inverter||503||200|
|1||Schneider Electric Connext 60A/150V MPPT charge controller||650||0|
|12||Hawker 4PzS240 traction battery - 2V 240Ah||1980||650|
|1||Victron Energy BMV700 battery controller + crimped cable + wall bracket + VE Direct Bluetooth Smart Dongle||250||250|
|1||Fuse MEGA-Fuse 125A / 32V (x5) + Fuse holder MEGA Victron Energy||33||33|
|1||Schneider Electric C120N 125A Circuit Breaker||320||0|
|2||Schneider Electric iC60N 63A Circuit Breaker||25||0|
|1||Schneider Electric Resi9 10A Circuit Breaker||15||0|
|1||Schneider Electric 30mA/40A differential Circuit Breaker||90||0|
|16m||1x4mm² red/black solar cable with MC4 connector||24||0|
|1||Male MC4 Connector||4||0|
|1||Female MC4 Connector||4||0|
|6m||Red / blue / green-yellow cable 1x16mm²||16||16|
|6||Tubular battery terminal 16mm²-12||4||4|
|2||Tubular battery terminal 35mm² - 10||2,6||2,6|
|6||Gray PG16 cable gland + Washer||11,5||11,5|
|6||Gray PG11 cable gland + Washer||9,5||9,5|
|1||Bloc multiprise extérieur IP44||14||14|
|TOTAL||4442 € (~4,764.22 $USD)||1282 € (~1,375.00 $USD)|
The cheapest and cleanest energy is that which we do not produce/consume !
In line with this principle, this approachnégaWatt proposes to rethink our vision of energy by applying a three step approach: Sobriety, Efficiency, then renewable energy. Before sizing a photovoltaic electrical installation, it is necessary to consider several questions:
Detail on these questions applied to the case of our Festival is available in the "Files" tab located above.
To materialize the electricity consumption of everyday equipment and possibly be able to eventually prioritize, the open-source game REVOLT translates these consumptions in pedaling time.
Me: This step is the most important of an autonomous photovoltaic sizing. It will have a big influence on the price, autonomy, and the durability of the installation.
This step will permit you to see where your large electrical consumption items are located and to make choices accordingly. (Ex: An electric oven demands 5000W of power. Is electrical energy the most pertinent to respond to this need?)
Several softwares can help you in sizing a solar installation. We have used the free software CalcPvAutonome developed by David Mercereau. A interface dédiée lets you calculate your daily electrical needs. We came up with what we call an Energy audit (simplified).
The principle is simple:
There is also the option to enter the energy consumption manually. For that, you have to measure it! Connect a Wattmeter-Consometer to the equipment for 1 to 7 days to have the actual daily consumption (see image).}}
In the case of our Festival, we have thus evaluated our electrical needs (see image) :
We have deliberately chosen to not fuel the power hungry devices in energy by electricity !
>The kitchen (for 100 people) was made using a wood-burning stove.
> To serve fresh beverages, we have agreed to a partnership with the Concarneau auction that permanently produces ice and stored it in an unplugged freezer (vs.4800Wh/d). In the same way, the beer taps are frozen directly with the ice and stay disconnected (vs 5600Wh/d).
Once the energy needs are entered in CalcPVAutonome, several important parameters must be entered for the sizing.
Once the parameters are entered, we begin the calculation!
CalcPVAutonome Proposes an estimate of necessary material in accordance with these parameters. This is given for information only and must be analyzed. In our case, we have readjusted accordingly with the material already available to us that we used.
To satisfy our daily needs of 4080Wh/d, a minimal SP power of 937W is required (detailed calculations done by the software). The software tells us that 5 monocrystalline panels of 190W would work.
But 5 panels were too cumbersome for our trailer + we already had 1 solar panel of 330W. We have chosen 3 panels of 330W for a total power of 990W.
Generally, the voltage of a battery pack is determined in accordance with the power of the SP:
In our case, we have 990W of SP, therefore the final voltage of our battery pack will be 24V.
To allow an autonomy of 8 hours (~0.3d), the software calculates that the nominal capacity of batteries of 170Ah en C10. (See detailed calculation in image)
However, to ensure the longevity of the battery packs, their charging current must not exceed 20% of their normal capacity. (See Operation, maintenance and regeneration of lead acid batteries)
That is: 170 x 20% = 34A.
Or, with 990W of SP the charging current is 990 / 24 = 41,25A.
You can choose to restrict the production of SP thanks to the charge regulator, but generally we advise you to increase the capacity of the battery packs in consequence. So here, 41,25 x 100 / 20 = 206Ah.
Having found a good opportunity, we have finally opted for the purchase of a set of battery packs Operation, maintenance and regeneration of lead acid batteries l regenerated traction batteries of 240Ah. It consists of 12 240Ah-2V batteries assembled in a series to have a voltage of 24V. This will raise our autonomy to 10 hours.
The characteristics of the charge regulator are determined in accordance with the characteristics of the maximum current leaving the SP. We therefore need the characteristics of SP given by their technical file or on the back of the panels. It is necessary to know the " Open Circuit Voltage " (Voc) and the "Intensity of Short-Circuit Current " (Isc) of SP.
In our case, for each panel: Voc= 40,49V and Isc= 10,25A
When adding the panels in series: Voc_tot= 121,5V et Isc_tot= 10,25A
In taking safety margins of 20%, a 150V 20A MPPT regulator might have been suitable.
Having had the opportunity to recover a "'Conext MPPT 150V/60A regulator"', we opted for this model.
The choice of converter is decided in accordance with the power that the installation must deliver (in AC) and in accordance with the voltage of the battery pack
We wanted to be able to occasionally power devices up to 1000W.
We have opted for a "'Victron 24V/1200VA"' converter that increases the maximum output power of 1200W with possible peaks up to 2400W.
To know the charging state of our batteries and to prolong their lifespan, we have chosen to use a battery monitor (strongly recommended).
The devices are placed on the 2 polarities (+ and -)
Between the SP and the charge controller:
Between the charge regulator, the inverter and the batteries:
Iop Being the current in operation. It varies if you are in battery charging or discharging mode :
"'We chose the maximum value so"'Ifuse>104 A'. We have chosen a MEGA 125A 32V fuse from Victron Energy.
To avoid the losses due to overheating, or even fire risks, it is important to size all the installation cables correctly. That is to say, calculate the minimum section (in mm²) of the cable
The formula is the following :
S > ρ*2*L*I / ε*U
For this you need to know:
|Between the SP and the charge regulator||'Between the charge regulator and the batteries||Between the batteries"' and the inverter|
|S > 0,023*2*4* 10,25 / 121,5*0,01||S > 0,023*2*1* 1000 / 24*24*0,01||S > 0,023*2*0,5* 2400 / 24*24*0,01|
From these calculated sections, it is then necessary to choose the upper commercial section:
|Between the SP and the charge regulator||Between the charge regulator and the batteries||Between the batteries and the inverter|
|S = 2,5mm²||S= 10mm²||S=10 ou 16 mm²|
We can verify the maximum allowable intensity values corresponding to these sections in the counting frame (see image).
Document by Guénolé Conrad within the framework of the Scholar Grid project. A project initiative of the Schneider Electric Foundation with the technical support of Energie Sans Frontières, Atelier 21 and the Low-tech Lab
English Translation by Suzanne Kane