Practical workshop 1Building a photovoltaic house

SecondarySchool

1. Conduct practical experiments to produce electricity using photovoltaic solar technology.

2. Handle an actual photovoltaic module (monocrystalline technology).

3. Understand energy storage challenges.

4. Complete a practical and technically successfulproject.

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Photovoltaic solar energy

Building a photovoltaic house Organize a technology workshop to build a photovoltaic house, and:

How does it work?General principle The small, monocrystalline module has two functions:

� It produces electricity using the photovoltaic effect (see"Solarama” Magazine p.8).

� It can be used as a light sensor (or twilight sensor) toswitch on an LED (Light-Emitting Diode).

The electricity is produced and then stored during the daywhen the roof of the photovoltaic house is exposed tolight. When night falls, the photovoltaic module transmitsthis information to the electronic system, triggering theLED. It is powered by energy accumulated in rechargeablebatteries (this will help students learn about energy storage).

To learn more: three functions of an electroniccircuit (see circuit plans, p. 4)

1. Energy storage (daytime)

� The photovoltaic mini-module has a higher voltage (3.5 V to 5 V) than batteries (2x1.2 V = 2.4 V) andtherefore supplies current to the NimH batteries andrecharges them.

� The D1 diode blocks the reverse current at night whenthe voltage of the mini-module (a few millivolts) isbelow the battery voltage (always 2x1.2 = 2.4 V).

� Battery sizing: the mini-module has a maximum voltagein full sun (at midday) of 5 V (open circuit) and a maximum current of 320 mA (short circuit), and duringbattery charging (2.4 V) of around 300 mA.

� The average charge current on a sunny day is 100 mA(0 mA at sunrise, increasing to 300 mA at midday anddecreasing to sunset). In summer, in France, there isenough sunlight for 14 hours, equivalent to a chargeof 14x100 = 1400 mAh; 2000 mAh batteries thereforeprovide a safety margin against overcharging.

2. LED input with voltage conversion

The minimum voltage needed for a white LED is roughlyunder 3.6 V; therefore, it cannot be activated with 2.4 Vbatteries. The system uses a voltage booster – an oscillator and a coil (or inductance), also called a “Joulethief” – to create an induction effect and drive the currentto around 20-40 mA. This boosts the LED voltage.The inductance is composed of two coils on a ferritetoroid (toroidal means donut-shaped) that oscillates andsupplies the current and voltage to power the LED. Theoscillation is created by transistor T1 and the coil. Whentransistor T2 is ON (see the twilight sensor), the resonance circuit is powered.

3. LED switch activation in the dark (at night!) or twilight activation

� In daylight, the photovoltaic mini-module applies voltage through potentiometer P1; connected to thebase of transistor T2 higher than its commutator, itis therefore OFF and will not power the resonator andLED. When the light is low, the photovoltaic module’svoltage and current is sufficiently low for the voltagein the base of T2 to be 0.6 V weaker than the voltageof the commutator powered by the battery. T2 istherefore ON and powers the circuit described in 2,which lights the LED.

� The potentiometer P1 is used to adjust the sensitivity of the trigger action by modifying the volt-age of the base of T2.

� The S1 switch cuts both the link between the mini-module and the batteries and the link betweenthe batteries and oscillator + LED. The function ofthis switch is to stop the lighting function. If thisfunction is deactivated, the batteries are no longerdischarged at night, but risk being recharged againevery day. However, it is not advisable to overchargebatteries since it can cause overheating, damage orleakage. This is why we also open the output circuitwith the S1 switch.

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Photovoltaic solar energy

Initially, transistor T1 is not activated by its base. The current from the coil begins supplying the base of transistor T1 via a winding; via the other winding, the commutator of T1 is powered up; T1 is now ON and allowsthe current to flow to earth. A current flows through T1and also the second winding, which is in series with T1. A negative voltage is induced in the first coil, resultingfrom the induction effect in the core; it is negative because the windings are inverted, reducing the voltagein the base of T1, which is closed as a result.

The current can no longer flow through T1, but the secondcoil has accumulated energy, creating a continuous current value from this coil (inductance effect); since thecurrent cannot flow through T1, the voltage rises andflows through the LED and lights it. The intensity of thecurrent drops continuously as the energy stored in thecoil is consumed by the LED. Once the current value isalmost zero, the voltage in the first coil has increasedagain, because the current in the second coil is weak andthe upstream voltage is always positive. The base of T1is therefore activated, T1 is ON, and a new cycle startsagain.

High-efficiency monocrystalline technology

There are several photovoltaic cells and module technologies: crystalline silicon (“blue” cells, the mostwidely used), thin-film, and organic solar. High-efficiency monocrystalline silicon cells have a distinctive architecture. Rather than transforming(doping) a solid silicon wafer to the front and rear withmore (+) above and less (-) below, the positive andnegative contacts are alternated on the rear of thecell. Since there are no metal contacts on the sun-side surface, more light enters the cell, raisingelectricity production. This technology is called IBC,Interdigitated Back Contact.

In the mini-module used in this example, there are 14 small IBC cells of which the semiconductor materialis monocrystalline silicon (the silicon atoms are perfectly ordered and aligned). The voltage of the smallmodule is 5.0 V in standard sunlight.

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Photovoltaic solar energy

TOOLS

� Fine soldering iron� Thin screwdriver (00) for the terminal block� Mini drill + 2.5 mm drills (mounting hole) +

grinder to remove copper tracks� Saw or cutter to cut the copper

Name Quantity

Rechargeable batteries: 800 to 2000 mAh 2

Wire cross-section 0.01 mm2 ~ 1 m

Solder wire diameter < 1 mm

Battery support 2AAA for printed circuit 1

Schottky barrier diode – 1N5819 1

NPN bipolar transistor – 2N3904 1

PNP bipolar transistor – 2N3906 1

Resistance 1kΩ (0.25 W or 0.66 W) 1

LED 5-mm white 120 deg. 1

Copper strip card (80x60 mm mini) 1

Double bipolar switch 1

Potentiometer for CI 20 kΩ 1

Screw board Ci 2.54 mm 2cts or 3.5 mm 3

Ferrite core Ø ~18 mm 1

Telephone wire 2 colors 2 x 30 cm

Electronic circuit

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Photovoltaic solar energy

Components of the photovoltaic house

Item no. Room number Quantity

1 House rear 1

2 House side 2

3 House front 1

4 Monocrystalline module 1

5 Bar 73 6

6 Bar 55 2

7 Roof 1

8 Back 1

2

12

74

3

5 5 5

97

120

6.50

50

20

20

4

145

75

125°

85

125°22

45

87 2

97

70

75

97 92

2

73

2

2

55

3 House front 1 House rear 5 Bar 73 mm

8 House back 7 House roof 6 Bar 55 mm

2 House side

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Photovoltaic solar energy

2 3.50 25

7

5

22,5

2

564

54

View from above

Perspective view View from below

Facade

Detail D

Detail E

Detail C25

C

D

E

8

97

75

A

A

Cross-section A

120

97

6.5020

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Photovoltaic solar energy

Tips on assembling thephotovoltaic house

� Use boards at least 10-mm thick to ensure thehouse is solid. The house parts were designed to beassembled regardless of the board thickness. Itdoes not matter what type of board (pine, medium,plywood, etc.) you use.

� Adjust the width of the back “8” to the width (97mm) of the front “3” and rear “1” pieces for play-freeassembly.

� For the roof, however, to ensure the success of theproject, the thickness (3 mm) is important.

� To produce the small support bars (pieces 5 and 6)for the roof, you can glue long matches.

� The small bar no. 6 is intentionally shorter to leavespace for the photovoltaic-cell connection wires (detail E).

� You must solder the two connection wires of thesolar module on the top, on both sides.

� Depending on the type of material selected, you canassemble the house using screws, nails, pins, glue,etc.

� Do not forget to spot-face the switch hole. It hasbeen placed below the house so that it remains accessible but hidden.

� Fix the circuit to the inside of the house using ascrew to secure it in place.

� Remove (grind) the copper tracks around the screwholes to avoid short circuits.

� The terminal block makes the electrical assembly ofthe small photovoltaic module and the B1 switcheasier (see the electronic circuit diagram), and allows you to connect the photovoltaic module (note:respect the polarity). Use B2 to connect to the twopoles of the switch (S1). Lastly, use B3 to connectto the two other poles of the switch.

PHOTOVOLTAIC HOUSE CONSTRUCTION SAFETYWARNING

The photovoltaic house project involves building a model of asmall photovoltaic house by completing each stage with caution, in a technical workshop and supervised by a courseteacher.

Students should be reminded of the safety rules for technology lessons in schools at the beginning of the lesson,and they must be applied to the letter.

We recommend that the teacher has the wooden house wallpanels (or other material) pre-cut, to the dimensions shownon the plan, by a professional.

Since soldering is part of the lesson, students must be reminded of the safety rules before starting the project. Special safety equipment (protective glasses, gloves, ventila-tion, etc.) for soldering must be used and/or worn at all times.

The photovoltaic module should be handled with care to avoiddamage, thereby reducing the risk of injury.

The system’s voltage is low and does not pose a threat to thehuman body.

If the module breaks, put on gloves to gather the pieces carefully and place them in a container set aside for this purpose. Scraps should be disposed of using an appropriatesystem.

This module should not be used for anything other than its intended purpose.

Ownership:Total reserves all rights to the photovoltaic house. The total or partialuse, copying or distribution of its contents are prohibited without theexpress authorization of Total.All documents and data supplied in this document have been pro-duced with the utmost care. Total shall not be held responsible forthe timeliness, accuracy, completeness or quality of this information.Total does not accept any responsibility for damage caused, directlyor indirectly, by the use of this information, except in cases of grossnegligence or willful misconduct.This document is the property of Total and shall not be used or dis-closed without authorization.

Use:This document was produced exclusively for use in schools to whichit is assigned by an accredited establishment and shall not be usedfor any other purpose.

This document was printed onrecycled paper. The printer thatproduced this document is Imprim’Vert certified.

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