Matching different solar PV panels

[OtleySpark]

I've got myself a solar pv installation; I have a charger/controller feeding 2 100ahr batteries. Also a 1500w inverter. I started off with a single 250w panel (to get familiar with it all) and replaced it with 6 x 170w panels (arranged as 2 strings of three).

The 6 170 panels have been installed for 4 months now and all seems to be well.

Except I have the original 250w panel sat in the shed doing nothing. I was wondering if I got another 250w panel I could have a 3rd string. Each string being approx 500w.

The 250w panel has Vsc 60.5 (Vmp 50.6). The 170w 44.0 and 35.0
Each string of 3 170w is actually running in daylight at 118-120v.
Is it possible to get another 250w and pair it up in series with the original to give me a third string?
Is there some device available that could the used to make the new string match the other 2 (in terms of voltage)?

Your suggestions/ideas greatfully accepted.

 

[ericmark]

You do not explain how they are used but to get maximum needs some sort of Pulse Width Modulation (PWM) regulation. As to if this charges a battery or has a grid tie inverter to feed into the mains is not really the issue it's the ability to extract the maximum from the panel.

Charging a battery using two Pulse Width Modulation (PWM) regulators is not a problem but feeding into the mains using two units could defeat the power loss detection and stop the units failing safe in the event of a power cut.

So with mains units you will need to enquire from the manufacturer.

 

[OtleySpark]

You do not explain how they are used but to get maximum needs some sort of Pulse Width Modulation (PWM) regulation. As to if this charges a battery or has a grid tie inverter to feed into the mains is not really the issue it's the ability to extract the maximum from the panel.

Charging a battery using two Pulse Width Modulation (PWM) regulators is not a problem but feeding into the mains using two units could defeat the power loss detection and stop the units failing safe in the event of a power cut.

So with mains units you will need to enquire from the manufacturer.

ericmark - it's an off-grid system (at the moment just running a freezer and lights, computer in my shed/office) so no mains issues.

 

[Aragorn84]

Do you think your setup is actually cost effective to run? What are your reasons for it?

I'd have thaught that with the cost of the panels and inverter, plus batteries which will wear out and require replacement, you;ll probably just be cheaper using mains electricity?

Even the full size ~3kw grid-tie installations that most households have gone for recently are only cost effective because of the scam feed in tariff that everyone else is paying for.

 

[OtleySpark]

Do you think your setup is actually cost effective to run? What are your reasons for it?

I'd have thaught that with the cost of the panels and inverter, plus batteries which will wear out and require replacement, you;ll probably just be cheaper using mains electricity?

Even the full size ~3kw grid-tie installations that most households have gone for recently are only cost effective because of the scam feed in tariff that everyone else is paying for.

I was asking for possible technical solutions - not a discussion on the pros and cons of having a PV installation.

 

[Aragorn84]

A valid technical solution is to disconnect the PV installation and connect your shed to the mains instead!

Anyway, i wasnt asking about pros and cons, i was asking if you think your setup is cost effective.

You've chosen to spend more money on it (twice infact, if you include your original trial), which would indicate that it IS cost effective, so i was hoping you could give me some figures to disprove my preconception that its a waste of money.

 

[OtleySpark]

A valid technical solution is to disconnect the PV installation and connect your shed to the mains instead!

Anyway, i wasnt asking about pros and cons, i was asking if you think your setup is cost effective.

You've chosen to spend more money on it (twice infact, if you include your original trial), which would indicate that it IS cost effective, so i was hoping you could give me some figures to disprove my preconception that its a waste of money.

It all depends on what you call a waste of money. You could 'waste' even more on a holiday that's over in a week or two - this waste lasts for years

Now if the question was: 'Is it worth it?' I would say yes. There's several benefits:

Enjoy designing, building it.
Over its life it will produce less pollution, etc
Independence of supply
Do you think the price of electricity will go up or down in the coming years?

 

[ban-all-sheds]

Enjoy designing, building it.

Fair enough.

Over its life it will produce less pollution, etc

Possibly not as clear-cut as you might think: http://www.toledoblade.com/Energy/2...ncrease-in-hazardous-waste-sent-to-dumps.html

Plus there's the mining of materials needed.

Independence of supply

A small generator will give you that. Or batteries charged from your domestic supply if you only posit short outages.

Do you think the price of electricity will go up or down in the coming years?

Have you done any financial calculations with price change assumptions built in?

 

[DetlefSchmitz]

There seem to be some weird people around who have learned some automatic reactions to certain keywords, and are unable to examine actual facts.

To the OP. Most inverters deal with multiple strings by simply paralleling them up. So it is probably necessary for each string to contain the same number of the same type of panels, or you won't get an equal contribution of power from each string. (Think of a Thevenin equivalent of a voltage source with a series resistance for each string.)

There are some panels which have more than one input which can individually be converted. The inverter spec will tell you if this is the case, but such an inverter will be more expensive.

I am not an expert, but there are sites where you would get a more unbiased evaluation of what you have, so it is probably worth asking there.

I'll pm you a link if you make me a 'friend'.

 

[OtleySpark]

There seem to be some weird people around who have learned some automatic reactions to certain keywords, and are unable to examine actual facts.

To the OP. Most inverters deal with multiple strings by simply paralleling them up. So it is probably necessary for each string to contain the same number of the same type of panels, or you won't get an equal contribution of power from each string. (Think of a Thevenin equivalent of a voltage source with a series resistance for each string.)

There are some panels which have more than one input which can individually be converted. The inverter spec will tell you if this is the case, but such an inverter will be more expensive.

I am not an expert, but there are sites where you would get a more unbiased evaluation of what you have, so it is probably worth asking there.

I'll pm you a link if you make me a 'friend'.

I'd appreciate that, thanks.

 

[ericmark]

looking at this regulator it states 160W maximum I don't know if you can get larger ones.

However there is no real reason for not using the simple heat sink type something is better than nothing but the HRSi I think is a three stage charger so you would need to supply through diodes or the simple charger will try to sink the power from all the solar panels.

We had to be very careful with this on the Falklands with wind chargers where if one used a standard battery charger without diodes between the regulator for wind charger and batteries it would try to sink too much power.

On the build of Sizewell power station where many of us lived in caravans we had problems with standard car battery chargers over charging the caravan leisure batteries so I designed and built a sink regulator using a voltage regulator to give sample voltage and a power opamp to switch a 2N3055 power transistor to reduce the power dissipated by transistor it switched on a headlamp bulb which also limited the energy it could sink and gave indication as to battery state.

But although this did work should your existing charger be a stage charger then it could be counter productive.

Again at Sizewell we had an old forklift with a faulty cell so running on 22 volt not 24 and I made a simple two stage charger for it swapping from constant current to constant voltage at a set voltage. It was a long time ago but clearly can be done.

However may be easier to use separate batteries and supply through a diode depends how you use the power.

With the narrow boat the 3kW inverter was using around 300A and clearly 300A diodes are not cheap.

We did consider solar panels but when we worked out cost of running engine with two 120A alternators fitted and cost of buying solar panels remembering only required when not at the marina it was just not cost effective. However if like you we already had panels it would have been different.

The HRSi with three stage charging can nearly double the output of the panels the idea is the DC from panels is varied in voltage to get maximum from panels then turned into AC and transformed to best voltage to charge batteries. The three stage charging at first give maximum current to batteries until the voltage reaches a set level when it changes to a fixed voltage. At this point it measures the current and as the current drops to a pre-set level it then drops to third stage a float charge at a lower voltage.

However this is not the only method used to pump extra power into the battery at the early stage of recharging. Sterling Power Products use a pulse method to monitor battery condition and with this method any other charger would mess it up. However there is a multi-alternator system which allows more power. The idea is it tricks the standard alternator into thinking the battery wants more power and again turns DC to AC and back to DC to pump in the extra power.

Also there are battery to battery chargers these again turn DC to AC and back to DC designed to charge a bow thruster battery from the main battery bank at stern of boat.

However non of this is cheap. And of course first question has to be what system is installed already.

This solar regulator at £219.90 is not cheap. 24 volt 50A output with 0- 60 volt input however you have not stated the voltage of the units you have.

 

[SimonH2]

Not my field, but ...

What I do know is that it's not uncommon to have multiple inverters on solar PV installations. Situations that might call for it would include panel groups with different orientations (eg on east and west facing roofs) where the output from the panel groups will vary independently. Thus, as the sun comes up, the east facing panels will produce more power then the west facing ones - and the operating regime to get the most effective extraction of power will be different.
The same applies if one group can be shaded (eg by a tree at certain times).

Another situation would be if different voltages or types of panels are used.

Loss of grid detection is far, far more complicated than just detecting externally applied voltage. I believe rate-of-change of frequency is one commonly used technique. Not just loss of local supply either - it could be loss of an upstream connection leaving an embedded generator trying to power an island, though that's probably not a practical consideration for a domestic PV installation of max 4kW. It is a consideration for an 89kW turbine as one of our customers at work has. "G59" is the magic string to throw at your favourite search engine for larger generators. I assume different rules apply to the smaller domestic systems.

If you think about it, you cannot detect loss of applies external voltage just by looking for lack of voltage - after all, your inverter would keep a low voltage relay held in as long as it could supply the loads. I assume some combination of low/high voltage and/or rate of change of frequency is normally used.


If all the panels are the same chemistry and same nominal voltage and same orientation/view of the sky then I can't see why you couldn't parallel them.
Looking back at the OP, I don't think this is the case. I'm not familiar with the specs, but the 250W panel is given as having Vsc (should that be Voc ?) = 60.5 (Vmp = 50.6), the 170w panels are stated as having 44.0 and 35.0.
That means a string of 2 x 250W panels would figures of 121 and 101.2, while the strings of 3 x 170W would have 132 and 105. That means that in a system setup to maximise output from the 170W panels, you'll load them down to 105V for max power - which means you'll under-load the 250W panels by running them at 105V instead of 101.2. So that's losing 3.8V which is 19% of 19.8V (the difference between 121 and 101.2).

My understanding is that you can consider a PV panel as a perfect voltage source (the generating diode junction) in series with internal resistance. So Voc tells you the output voltage from the diode junctions, while Vmp tells you the voltage where you maximise output power - run at a higher voltage than Vmp and current drops off reducing power (Ohms law across the internal resistance), pull the voltage lower and you dissipate more power in the internal resistance of the panel and thus lose output power.

So if my understanding is correct, by just paralleling the panels, you'll only get 200W each out of each 250W panel - max.

 

[SimonH2]

As an addition ...
I would not be too surprised to find system that use DC-DC converters between individual panel strings and a DC bus; followed by a single inverter/converter to supply the load/export power.