Solar cells and shade

One critical element to understanding photovoltaic performance is that solar cells are current sources. This means that when you connect them in series, the weakest cell will determine the total current. In many (most?) solar panels, the individual solar cells are connected in series. For example, our 28V panels consist of

60 cells, each producing about 0.48V, connected in series. One shaded cell on a panel will determine the power output of the whole panel.

For example, our 28V, 200W panels produce about 7A per panel under full sun. However, 50% shading of a single cell causes its output to fall to 3.5A, which drags the output of the whole panel down to 3.5A. This is why you often see descriptions of 10% shading costing 50% output. Basically, shading causes the current-voltage (I-V) curve of the solar cell to drop (lower current for same voltage output). This is why all

modern solar cells should have bypass diodes. The resistive nature of the underperforming cell produces a reverse bias on that cell, and the bypass diode conducts, providing a shunt for the excess current produced by the fully performing cells. (That said, although Canadian Solar does say our panels have bypass diodes, they do not say whether each cell has a bypass diode, or if there is a single bypass diode for each panel.) A nice graphic description of how this works can be found on this web site.

Still, partial shading is quite costly. For this reason, there are devices that will adjust the maximum power point (MPP)¹ of a panel to up the overall output in the face of partial shading or under-performance. These are called "Power Optimizers". Some examples of these are the SolarMagic, which is produced by National Semiconductor, and SunMizer from Xandex Solar. Unfortunately, these are pretty expensive ($200 or more).

(For those more technically oriented, here is a solar cell equivalent circuit, which I found on Wikipedia.)

It is a good question whether, with bypass diodes, Power Optimizers are helpful. As I mention in the previous paragraph, a bypass diode will not kick in unless the voltage across a cell is reversed. At MPP, a cell is performing optimally for the quantity of sunlight it receives and may still be boosting the system voltage, which removes the diode.

¹Maximum Power Point (MPP) is the location on a photocell I-V curve at which maximum power is produced. There is a nice explanation of MPP tracking on the National Instruments website.

Some resources on shade and photovoltaic performance

Shading and Solar Panel Efficiency

Shade Happens, by Ralf J. Muenster

My Solar Home

Web resources:
English:
Puerto Rico Incentives for Renewables and Efficiency

Español
Ley 248 del 10 de agosto de 2008
Carta Circular Rentas Internas 08-13
Resumen de ley 248

Introduction
In August of this year (2008) the government of Puerto Rico announced tax incentives for the installation of solar energy systems in homes and businesses. These include both electricity and water heaters. They pay up to 75% of the cost through June 30, 2009, 50% during the following year, and 25% thereafter. Costs that exceed the taxpayer's tax liability can be carried over to following years, up to 10 years.

My goal is to install a grid-tied solar electricity system to provide for most of my house most of the time. I hope to reduce my electricity use by 1/2, and I hope to sell a little back to the utility (at 10¢/kW-h). Here in Puerto Rico we have adequate to good sun all year, so this should work well.

My home has a solar water heater, and I have been considering solar panels since we began building in 2002. This tax incentive has inspired me to move forward, though I have been studying the options for some time. An internet search identified a handful of solar energy retailers in Puerto Rico, and I send out several e-mail and webform inquiries. I received a response from PRGreenTech.com, and most of what follows came from that communication. I also have visited Casa Solar, PR in Bayamón.

Julio Correa, of PR Green Tech, visited my home on November 21. I was concerned by the fact that there is a tree-covered hill to the east and many afternoons are cloudy, both of which may limit sun exposure, but Correa did not feel these were such important concerns. Sun will be on the panels by between 8:30 and 9:00, even in mid-winter, when the shading is worst, and we should typically have 4-6 hours of full sun. The fact that Puerto Rico is 18N latitude also helps.

Correa works with 48 V DC systems, and the panels are 195 W, 24 V. Rather than connecting to the charging system in pairs, producing 48 V in full sun, he uses the panels in sets of 3, which will produce 48 V even in some degree of shade. The panels are polycrystalline, which, though they are less efficient, have two advantages: 1. they are more durable, and 2. (I understand that) their "turn-on" light level is lower than for monocrystalline (I have to confirm this).

My house has a few items that produce heavy loads. We will exclude the one air conditioner, which will continue to connect directly to the grid. It is a 18,000 BTU/h unit, which is rated for 1950 W. That excluded, we still have the following:
1. A water (deep well) pump. We always use water from a 600 Ga reservoir, which is supplied from the aqueduct authority. This is because water supplies here are quite unreliable (we recently had 2+ weeks with no water during the day, and only a trickle at night). This pump is rated for 1500 W.
2. A microwave oven. Correa says these are among the biggest problems for solar inverters.
3. A toaster oven.
4. Blowdrier
Happily, our oven and cloths drier are both connected to gas. The solar water heater has an electrical backup, but if we are using that, it means there hasn't been sun for a while, so we will be running off the grid in any case.

In future blog posts, I will describe the process by which we choose our solar energy system. These are divided up as follows:
1. Total power output of the panels.
2. Specifics of the charging system (voltage regulation)
3. Battery capacity (in A-h), costs etc. (batteries are not covered by the tax incentive law, so cost is critical)
4. Inverter capacity
5. Grid-tie system
6. Questions:
- At what point in the battery depletion do we switch off the solar powered system and go to grid?
- How much solar panel capacity is needed to provide sufficient charge to the batteries to run until charging re-commences?
- How much energy consumption (A-h) will we need, and how to we convert that throughout the system?
- How do we make sure the system is expandable, so we can add capacity later for running the air conditioner?