Photovoltaic Resistive Cost Effective

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Forhorse
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Re: Photovoltaic Resistive Cost Effective




by Forhorse » 11/03/18, 09:14

The project you quote is just the same as a PWM solar charger made in china found on all online stores. Personally I do not see the point of doing so even saw the price of this kind of device. But why not...
But D1 is still useless.

And the peculiarity of the PWM is that there is no impedance matching between the generator and the receiver.
That is to say that in case of low sunlight, the tension of the panels will rise rather quickly, but we will not be able to draw a lot of current on it.
A pwm does not make it possible to take advantage of this low current at higher voltage in order to convert it to higher current at a lower voltage (adapted to the receiver)
Chopping the current will limit the power dissipated by the resistor (or absorbed by the battery) but will not serve to exploit the production potential of the panel.
For that you have to implement a step-down controller and enslave it according to the maximum power that can be dissipated in the resistance. (The simplest algorithm, I think, is to gradually increase the setpoint until the dissipated power drops back a little and to try to increase the setpoint regularly and see what happens. and go back if necessary, and lower the setpoint if we see that the dissipated power drops without changing the setpoint.There are probably other algo MPPT more effective but I do not know them.
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Re: Photovoltaic Resistive Cost Effective




by izentrop » 11/03/18, 09:41

Yes, you're right for D1, it's just a security to avoid discharging the battery in the panel at night. In this case, the type P mosfet does not lead in principle.

What you explain for the DC / DC step up not down, it is valid to charge a battery to for example 28 V, while the optimal operating point is 25 V 2 A (case of a weak sunshine), the MPPT will convert it to 28 V 1.4 A and the energy is not lost, but in a resistance no problem.

With a PWM converter to charge a battery, you choose a PV rated voltage much higher than the battery and no problem either. The 30 V model chosen here would go very well to charge a 12 V battery in PWM and the overflow in a resistor
Chopping the current will limit the power dissipated by the resistor (or absorbed by the battery) but will not serve to exploit the production potential of the panel.
Yes !
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Re: Photovoltaic Resistive Cost Effective




by izentrop » 11/03/18, 10:06

A PV connected to a resistor, in simplified equivalent is a FEM (E), followed by its internal resistance (rI), the circuit being closed on the load (Rc), the power is optimal obtained when rI x I² = Rc x I².

rI varies with sunshine and E too, the only way to get the same power on the internal and external resistance is to play on the current and that is done by chopping it.
By cutting the current, one can measure E (empty PV), remains more than to adjust the duty cycle to obtain E / 2 across the resistor Rc. I will try to program in this way ... simple but not obvious.
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Re: Photovoltaic Resistive Cost Effective




by chatelot16 » 11/03/18, 10:18

to return to the simple chopper to the water heater, without battery, we must not forget that the photovoltaic does not store ... when the water heater is cut the power is lost

the solution is simple! an electrochemical capacitor to store current when the water heater is unplugged: the photovoltaic can provide the average current it can with just slightly variable voltage

when we want to make a cutting with winding, it is necessary that the frequency of cutting is well adapted to the winding ... with the decoupage in a resistance there is no constraint, one can cut out anyhow: so a simple amp op measuring the voltage of the capacitor is enough to control the decoupage ... tension too strong we turn on the water heater, too weak we put out ... with some hysteresis so that it oscillates
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Re: Photovoltaic Resistive Cost Effective




by Forhorse » 11/03/18, 10:42

I am not convinced that a simple PWM is enough to find the MPP of the panel.
A photovoltaic panel is more of a current generator than a voltage generator. Chopping a current under a strong voltage, does not allow to make a stronger current under a lower voltage.

And a step down is used to lower the voltage ... hence its name. https://en.wikipedia.org/wiki/Buck_converter
A step-up is used to increase the output voltage, that's not what we're looking for here.
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Re: Photovoltaic Resistive Cost Effective




by lilian07 » 11/03/18, 11:30

For the trunk I use wooden battens or half-rafters so as to have an air gap of about 1cm between EPDM and polycarbonate.
In the vertical it is much simpler it is necessary to position the cleats (in length of 2, 3 or 4 m) according to surface to be covered on the wall to the horizontal one in top and one in bottom (of the size of the whole polycarbonate ) then you position at the bottom (against the wall) an insulator (poly ... machin) that you glue (with mortar type MAP or type silicone glue (SIKAFLEX) then you can roll your length of EDPM that you can stick with the glue (SIKAFLEX) then you make your connections (plan a slope 3 to 4% which will be given to you by the position of the horizontal cleats to be able to empty or to protect the installation).
The polycarbonate will stick on the wood (SIKAFLEX) or sandwiche price by a small horizontal cleat per face.

You will have a sensor around 35 Euros of the m2 with a half-day setup for 10 m2 and a ladder without problem.

The ideal for me with EPDM is to work in a "drain back" tank 'plastic with cover' at atmospheric pressure isolated by DIY. Then all you have to do is immerse a small balloon (10, 20, 30 or 50l) in this tank which can be (a recovery exchanger, a mixer, a bare stainless steel tank ... without insulation (to make exchange and ensure the pressure water intake).
The temperature protection can be done by circulating water, drilling a hole in the horizontal timber frame or by a simple, reliable, energy-saving and indestructible automatic system.
https://www.tectake.fr/systeme-d-ouvert ... 8sQAvD_BwE
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Re: Photovoltaic Resistive Cost Effective




by PVresistif » 11/03/18, 12:12

link on nuclear power other than current; note the sentence "States that have opted for atomic energy have long sought to stifle wind and solar power, and none have been willing to take into account the well-known problems of burial of fissile material."

https://www.latribune.fr/entreprises-fi ... 02366.html
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Re: Photovoltaic Resistive Cost Effective




by Forhorse » 11/03/18, 12:34

To continue on this pwm and MPPT story ...

If we look at the curves of the panel in question in the original post of this wire, we clearly see that a PV panel is mainly a current generator.
For example for the irradiation max at 1000w / m² it leaves about 8.5A whatever the voltage up to the point of maximum power where the voltage always increases but the current drops.
This curve is dependent on the impedance of the load connected to it:
At low impedance, it is the current imposed by the panel that determines the output voltage and it is the internal resistance of the panel that absorbs the rest.
High impedance (beyond the point of maximum power) ben it is the load that imposes are current depending on the voltage (the panel becomes more a voltage generator than a current generator)

If we stay on the example of this subject, we see that the resistance of the water heater of about 4 ohms sticks rather well to the panel chooses for the 7.5A max irradiation under 30V it gives a power of 225W for a panel of 250W not bad...

Now, if we go under a very average irradiation of 400w / m² we see that the panel can only output 3.5A
Under these conditions, under a load of 4 ohms the voltage will be only 14V (11V if we consider the cold resistance)
that's basically a power dissipated between 38 and 49 watts depending on the temperature of the resistor.
This is balo, because in these irradiation conditions the panel is still able to provide 98w; at best we still lose 50% of production.

Well, so far nothing new, that's what dirk-pitt had shown on page 3 I think.

Now, under these conditions, we put a chopper between the panel and the resistance (PWM)
let's apply it a cyclic ratio of 50% (it's for the example, the value does not matter)
we see that the panel can only provide 3.5A at ​​best with a Voc about 36V

When the chopper switch (transistor off) is open, then there is a voltage of 36V on the panel and 0A
When the chopper switch is closed, the current can not exceed 3.5A (this is the limit of the panel under these sun conditions) as the resistor is fixed, so the voltage is between 11V (cold resistance) and 14V ( hot resistance)
Either an instantaneous power dissipated from 49W at best.
Then the chopper switch opens, and the current drops back to 0 for the other half of the cycle time.

On average, we have a power dissipated in the resistance of 24.5W ... well, we are moving away from the 98W MPP panel!

A PWM can only be used to regulate the power received by the receiver ... not to adapt the impedance of a receiver from the point of view of a source of current or voltage, that this source is a photovoltaic panel does not change nothing.
At the limit if the receiver is inductive and we add a diode wheel-free it can work ... ah ben thin, it is precisely the principle of a regulator buck step-down say so!
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Re: Photovoltaic Resistive Cost Effective




by chatelot16 » 11/03/18, 12:42

Forhorse wrote:I am not convinced that a simple PWM is enough to find the MPP of the panel.
A photovoltaic panel is more of a current generator than a voltage generator. Chopping a current under a strong voltage, does not allow to make a stronger current under a lower voltage.

And a step down is used to lower the voltage ... hence its name. https://en.wikipedia.org/wiki/Buck_converter
A step-up is used to increase the output voltage, that's not what we're looking for here.


when the lighting varies, the optimum tension is constant pressure ... it is with the temperature that the optimum tension varies and that the mppt is justified ... with a regulator with constant tension it will be necessary to adjust it for the maximum temperature, and when the temperature drops we will not take advantage of the additional voltage at low temperature: constant power received

if we adapt the voltage to the low temperature we have a slightly stronger power, but a complete collapse when the temperature rises and makes a voltage lower than what the regulator wants

the goal is not to lower the voltage, but to adapt the power consumed by the radiator to the power provided by the panel

when the illumination is reduced and makes a power 4 times lower, it would be necessary to divide the voltage by 2 if it was a real voltage reducer with a decoupage

but here it is a simple decoupage without self ... to divide the power by 4 we divide the time of operation by 4, but it does not change the instantaneous current in the resistance, which remains equal to the current at full power

therefore when the power is divided by 4, the voltage remains constant in the photovoltaic, the produced current is divided by 4 and charges the capacitor which is drained by pulses at rated current during 1 / 4 time

reason that remains valid for any other figure than 4

if we also want to charge a battery, and send the rab to a water heater when the battery is full, it is better to connect the water heater directly on the photovoltaic panel, it prevents the battery is alternately chrger and unload for a zero average but it uses it almost as much as loading and unloading with longer cycle

to average between the current supplied and the current consumed by impulse must absolutely be made by capacitors that do this work without wearing out, and do not use the battery for that
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Re: Photovoltaic Resistive Cost Effective




by izentrop » 11/03/18, 13:23

I could finally do some tests between 2 clouds, which sport : Wink:
I confirm: the hash does not bring anything.
The simple solution is to switch resistances as discussed previously, if you do not want to invest in a MPPT.

This kind of 400W 15A DC Step-up Constant Current should be suitable for the panel 250 W (must be wide with the Chinese stuff) and for a low price : Wink:

Lilian thank you, how do you order the opening of a faucet with this automatic greenhouse system?
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