Solar PV. Question

[Mike2007]

Does 4kw DC = 4kw AC when put through an inverter?

 

[ricicle]

If the AC value is RMS then it will be more or less the same as the DC value but there will be some losses in the invertor which only the manufacturer can tell you.

 

[Adam_151]

The losses in a modern inverter are quite low, no more than a few percent.

You will, however get much greater reductions in output from a poorly setup or sub-optimal PV array. The panels work best on a 35 degree roof pointing due South. Anything upto either due East or due West can be viable but will not be ideal. North or north east/west is not worth bothering with!. Shading is a big issue and most small (domestic) arrays have a single string with all modules wired in series, so shading just a small part can result in a drastic reduction, although solar edge do have a solution which claims to solve this issue. The parameters of the array need to match the inverter and it should ideally run at a voltage towards the top end of what the inverter can accept to minimise volt drop losses. DC cables should be as short as possible.

In summary. Don't worry about inverter losses. *Do* worry about how well the PV array is set up. Your MCS registered contractor will be able to advise, select them upon recommendations from others or get 3 quotes and compare. You must use a MCS registered contractor to get FIT payments

 

[flameport]

Also note that PV arrays are rated as kWp - the important p being Peak.
Therefore that is the expected output on a sunny day in June.
At other times of the year, the output will be considerably less.

 

[ban-all-sheds]

At other times of the year, the output will be considerably less.

Like when your demand for electricity goes up, you mean?

 

[bernardgreen]

The losses in a modern inverter are quite low, no more than a few percent

When the input voltage from the solar array is the optimum for the invertor.

You need to look at the graphs of efficiency versus input voltage and efficiency versus load. You will find the efficiency given in the sales blurb will often be the highest from those graphs. The system may never reach that figure in reality.

 

[Adam_151]

Again, thats an issue addressed at the design time of the system, the voltage of the array should be picked to be towards the top end (most efficent) end of the allowed range of the inverter. The voltage on the system is largely indepandant of the irradiance (which affects current rather than voltage) and will only vary very slightly with temperature

 

[bernardgreen]

The voltage on the system is largely indepandant of the irradiance (which affects current rather than voltage)

I think you will find that applies to the over all system at the output of the invertor.

Unless I am very out of touch with latest developments the voltage on the terminals of the solar cells is dependent on the amount of energy falling on the cell. The voltage at the output of the array is also affected by current drawn through the impedances internal to the array. More current = increased voltage drop between solar cells and array output

 

[jj4091]

Shading is a big issue

So I guess that means that in an area like ours, where we have little sunshine, (few days/year & lots of cloud) it is probably not viable. There seems to have been a few locals who have recently had them installed, that will probably be disappointed in the future performance of them. Is it possible to get a survey of an area, as they do for wind generation, so that you know what the likely output of an instalation will be?

 

[Stoday]

Also note that PV arrays are rated as kWp - the important p being Peak. True
Therefore that is the expected output on a sunny day in June. False
At other times of the year, the output will be considerably less. False

For radiation on the ground, the sun is strongest on the 21 June, when it reaches the altitude of 61° in the middle of the England. That's not the case for pv because roofs are at an angle. The sun's energy peaks when the sun is normal to the array, i.e. at right angles to the array and roof. The time and date is normally different from midday on 21 June

For a typical roof angle of 40° the sun would have to be at 50° to be at right angles to it. Thus you'd expect peak output to be on 20 April midday GMT because the sun would be at 90° to the array and not on 21 June, when the sun would be at 78° to the array.

That assumes the roof is "pointing" due south. If it's not, the times and dates of the peak change. If the roof "points" south east, the sun is at right angles at 10 am on May 18. That's when peak occurs. The expected peak power is the same as that for the south roof on 20 April; both are higher than at midday on 21 June.

This is all a matter of geometry. You can take the easy route by downloading a sun chart for your location here

 

[Stoday]

Shading is a big issue

So I guess that means that in an area like ours, where we have little sunshine, (few days/year & lots of cloud) it is probably not viable. There seems to have been a few locals who have recently had them installed, that will probably be disappointed in the future performance of them. Is it possible to get a survey of an area, as they do for wind generation, so that you know what the likely output of an instalation will be?

Jj's right. Cloud is the second most important factor.

With an array at the optimum angle to the sun, the same peak energy will be generated no matter if the location is in England, Spain, North Africa or even the North Pole. Northern latitudes would generate less over the year, of course, because there are fewer hours when the sun shines at a usable height above the horizon. That's the first most important factor.

You can download sunshine factors for weather stations. At Royston (Herts) for example http://www.iceni.org.uk/

 

[simond]

The story isn't purely limited to the angle of the sun.

Places such as Spain struggle to get the efficiency we can manage, because their solar arrays lose efficiency when it gets hot.

Places like Germany, UK etc are ideal because we get a fair bit of infra red without the high temperatures to go with it. The panels deliver more current in this scenario.

 

[martinxxxxxx]

At other times of the year, the output will be considerably less.

Like when your demand for electricity goes up, you mean?

They might co-incide in other countries.

 

[ban-all-sheds]

So we're going to put little PV arrays on houses a long way away, invert to AC, step it up to 100's or 1000's of kV, send it 10,000's of miles to feed into the grid, and that's going to be a sensible and economic idea, is it?

 

[bernardgreen]

For me the answer about private PV installations comes from a local mature eco warrior who serves on a couple of committees and NGOs promoting alternative energy sources. He evaluated private PV systems and the "benefits" of linking them into the grid. He found them non economically viable. He has instead solar water heating which he considers is cost effective, simple and more reliable than the equipment necessary for PV.