Micro-inverters offer one solution for optimizing solar efficiency
Nov 2 2009 9:38AM | Permalink |Comments (12) |
Power-generating installations of solar photovoltaic panels are changing. Rather than the massive solar farms of the past10-20 years, smaller residential and industrial roof-top installations are gaining favor. These small-scale installations have different characteristics from the solar farms with their acres of arrays of solar panels that all face in the same direction and receive the same amount of sunlight. Solar farms have no obstacles such as neighbors’ trees or utility poles to shade a panel, causing a panel-to-panel variation in power output. On the other hand, residential and industrial installations have some features in common with solar farms: They are subject to dirt and require washing at regular intervals, and have panels that age at slightly different rates, causing variations in panel outputs.
S
olar panels, or modules, consist of arrays of individual solar cells and typically produce about 200W at around 25-30Vdc. These panels are themselves cascaded in series, forming solar panel strings with a string output voltage of 300-350Vdc. These strings in turn can be paralleled into large solar arrays. The output of these strings or an array feed into a central inverter (UPDATE: pictured to the left) that transforms the dc voltage to ac and syncs the ac voltage to the grid. Efficiencies for central inverters are as high as 98.5%.
However, because of variations due to shading, dirt, and aging of solar panels, individual panel voltages will vary, causing the output voltages of strings of panels to vary. To deal with these variations, the central inverter constantly performs maximum power point tracking (MPPT) to find the sweet spot of power generation from the panel string or array. A common technique is for the inverter’s power transformation circuitry to attempt to draw a little more current and see what the response is: Does the voltage drop or not? The algorithm searches for the point where it gets the maximum power from the string or array.
Because the central inverter is dealing with strings or an array of these varying panels, the chances are high that rather than finding a absolute maximum for the array, the inverter will find a local maximum: The algorithm will be dominated by the performance of one poorly performing panel. If all the panels are well-matched, the difference between the true maximum power point and the local won’t be significant – but you can’t count on well-matched panels. Thus, one poorly performing module can dictate the power that the other modules in series with it deliver.
I attended Solar Power International last week in Anaheim, CA, where my quest was to learn more about micro-inverters. Micro-inverters are pretty new on the solar scene – at least this time around. Rather than having an inverter that tries to find the MPPT for a string or array of panels, you have individual inverters capable of about 200W, called micro-inverters, dedicated to an individual solar panel. Each panel now produces at its maximum power point.
European solar companies tried this approach about 10 years ago, but the cost at that time of multiple individual micro-inverters far exceeded the cost of a single central inverter, and the concept died. However, over the years things have changed. A major difference now is the above-mentioned trend towards smaller installations. Within these mini-solar farms, architectural limitations may dictate different rooftop orientations. Utility poles and neighbors trees aren’t so easily removed. It’s more important now to optimize each panel.
In addition, central inverters are physically large and heavy, calling for cement pads and single, centralized connections to the grid, making the installation capital expensive and labor costs high. A 200W micro-inverter that only has to deal with 30Vdc input and no special installation investment becomes very attractive for small installations. Safety is also an issue: A solar panel string or array’s input to a central inverter can be as high as 600Vdc in the US and 1000Vdc in Europe -- hazardous voltage levels for installers, maintenance personnel, and emergency responders. Output from modules attached to a micro-inverter scheme will be at much lower levels between 200 -- 300Vac.
Ok, this is where the spooky fore-shadowing music comes in: Microinverters have to deal with quite a bit of ripple current and operate in elevated temperatures, constraints well-known for eating capacitors. At the same time, they are part of solar installations that typically require equipment guarantees of 20-25 years (typical for solar panels). How do micro-inverters meet the reliability challenge? Next up: Solar micro-inverter addresses capacitor reliability issues.
Related entries in: Power supplies | Solar/Photovoltaics |
Nov 2 2009 9:38AM | Permalink |Comments (12) |
Power-generating installations of solar photovoltaic panels are changing. Rather than the massive solar farms of the past10-20 years, smaller residential and industrial roof-top installations are gaining favor. These small-scale installations have different characteristics from the solar farms with their acres of arrays of solar panels that all face in the same direction and receive the same amount of sunlight. Solar farms have no obstacles such as neighbors’ trees or utility poles to shade a panel, causing a panel-to-panel variation in power output. On the other hand, residential and industrial installations have some features in common with solar farms: They are subject to dirt and require washing at regular intervals, and have panels that age at slightly different rates, causing variations in panel outputs.
Solar panels, or modules, consist of arrays of individual solar cells and typically produce about 200W at around 25-30Vdc. These panels are themselves cascaded in series, forming solar panel strings with a string output voltage of 300-350Vdc. These strings in turn can be paralleled into large solar arrays. The output of these strings or an array feed into a central inverter (UPDATE: pictured to the left) that transforms the dc voltage to ac and syncs the ac voltage to the grid. Efficiencies for central inverters are as high as 98.5%.
However, because of variations due to shading, dirt, and aging of solar panels, individual panel voltages will vary, causing the output voltages of strings of panels to vary. To deal with these variations, the central inverter constantly performs maximum power point tracking (MPPT) to find the sweet spot of power generation from the panel string or array. A common technique is for the inverter’s power transformation circuitry to attempt to draw a little more current and see what the response is: Does the voltage drop or not? The algorithm searches for the point where it gets the maximum power from the string or array.
Because the central inverter is dealing with strings or an array of these varying panels, the chances are high that rather than finding a absolute maximum for the array, the inverter will find a local maximum: The algorithm will be dominated by the performance of one poorly performing panel. If all the panels are well-matched, the difference between the true maximum power point and the local won’t be significant – but you can’t count on well-matched panels. Thus, one poorly performing module can dictate the power that the other modules in series with it deliver.
I attended Solar Power International last week in Anaheim, CA, where my quest was to learn more about micro-inverters. Micro-inverters are pretty new on the solar scene – at least this time around. Rather than having an inverter that tries to find the MPPT for a string or array of panels, you have individual inverters capable of about 200W, called micro-inverters, dedicated to an individual solar panel. Each panel now produces at its maximum power point.
European solar companies tried this approach about 10 years ago, but the cost at that time of multiple individual micro-inverters far exceeded the cost of a single central inverter, and the concept died. However, over the years things have changed. A major difference now is the above-mentioned trend towards smaller installations. Within these mini-solar farms, architectural limitations may dictate different rooftop orientations. Utility poles and neighbors trees aren’t so easily removed. It’s more important now to optimize each panel.
In addition, central inverters are physically large and heavy, calling for cement pads and single, centralized connections to the grid, making the installation capital expensive and labor costs high. A 200W micro-inverter that only has to deal with 30Vdc input and no special installation investment becomes very attractive for small installations. Safety is also an issue: A solar panel string or array’s input to a central inverter can be as high as 600Vdc in the US and 1000Vdc in Europe -- hazardous voltage levels for installers, maintenance personnel, and emergency responders. Output from modules attached to a micro-inverter scheme will be at much lower levels between 200 -- 300Vac.
Ok, this is where the spooky fore-shadowing music comes in: Microinverters have to deal with quite a bit of ripple current and operate in elevated temperatures, constraints well-known for eating capacitors. At the same time, they are part of solar installations that typically require equipment guarantees of 20-25 years (typical for solar panels). How do micro-inverters meet the reliability challenge? Next up: Solar micro-inverter addresses capacitor reliability issues.
Related entries in: Power supplies | Solar/Photovoltaics |
Reader Comments
at 11/2/2009 3:51:33 PM, Andy T said:
Unless it was snapped in Lilliput, there's nothing 10E-6 about the inverter size in that accompanying picture. A deciverter? Perhaps. "Semiverter"? Better. "Micro"? Don't think so. And if a "micro" is indeed that big, clearly they are using good old, reliabe, Leyden Jars for caps.
at 11/2/2009 4:26:28 PM, Micro Inverters said:
Margery,
As I did not attend Solar Power Intl but am more than curious about Micro Inverters - who is manufacturing them?
jimk@omnipro.net
at 11/2/2009 8:29:16 PM, Larry said:
I really think that the "answer" to residential solar is to make the individual panels closer to a standard 4'x8' in size and equip each one with its own micro ~500W inverter. The larger size means less labor and makes the inverter more cost effective.
at 11/3/2009 1:27:54 PM, Curt said:
Andy -- that's a central inverter pictured. Look at the next post for the micro-inverter -- several of which are supposed to replace that central inverter.
Larry -- the whole point of the micro-inverter mentioned here is to do exactly what you are proposing.
at 11/3/2009 8:02:25 PM, Marc said:
Typo in the article:
Quote:
A common technique is for the inverter’s power transformation circuitry to attempt to draw a little more current and see what the response is: Does the voltage drop or not?
(end quote)
That should read: "...Does the POWER drop or not?"
Of course the voltage is going to drop if you draw more current. The question is: When the voltage drops under higher current draw, is voltage x power greater or less than it was before?
at 11/3/2009 10:52:32 PM, Robert Godes said:
Margery, The best ones on the market are at
www.xetenergy.com/solar/solutions/
at 11/4/2009 3:32:31 AM, colin dore said:
A new British startup company called Enecsys are also entering the market. www.enecsys.com
at 11/5/2009 11:24:52 AM, Chuck said:
Rather than a micro-inverter, a boost-buck charger chip for each individual panel would be a better combination to keep the DC output voltage of the panel constant and relieve the need for the MPPT function... only the current would vary, not the voltage.
at 11/5/2009 2:16:38 PM, PV_Jon said:
Actually, you are one step behind in the progression of PV power technology. Micro-inverters do not provide a complete solution because one still needs to procure and match a PV module to the inverter - both electrically and mechanically. There are also needs for additional system architecture, such as a DC disconnect switch, an AC disconnect switch, hardware, separate ground connections. etc. In contrast, an "AC module" is a true complete PV-based solar energy generation system, self-contained and ready to deploy. Petra Solar is currently the only manufacturer in the US with such a system. Petra Solar was also at the Solar Power show.
at 11/5/2009 9:51:45 PM, Akhilesh Bahuguna said:
As is known to all of us, that the biggest challenge in popularizing the solar energy is to increase the conversion efficiency. But as that has to do with the band gap, and hence becomes a matter of pure research, I think the next challenge lies in energy storage. The batteries have been good, but have not been cost effective, and the durability is biggest issue.
I am just wondering if there is a method of generating Hydrogen from the week energy that we get from sun, It can solve the problem of storage. And then lots of cost related to electricity transmission as well. There we can beat the other available electricity sources.
Any body has any idea, of generating Hydrogen from electricity. Any Chemical Guru out there?
at 11/6/2009 7:42:40 AM, Gary Lynch said:
I worked for a PV inverter manufacturer in the 90's.
We had product lines going down to 2 kW and some
installations on rooftops. They were bought out by a
UPS manufacturer who converted their factory over to
make UPSs.
I had no direct marketing info, but my sense was this
industry was never profitable without the government
subsidies, and even then it was marginal.
Hope you're right about an improvement.
at 11/9/2009 4:51:03 PM, BeamMeUp said:
One of the reasons Europe went away from micro-inverters was when a large manufacturer went out of business because a single grid power surge took out over 10,000 units at once. Ouch. All of the string inverters intermixed in the same area survived. The manufacturers who have the experience with many units in the field for many years will tell you it's a harsh world out there and the larger components tend to survive surges and lightning much better than smaller ones.
My 15+ years in various power industries agrees.
