Effect of partial shading of solar panel

Partial shading of solar panels causes the output power of solar panels to be drop more than one would expect. Even waiving a hand in front of a panel shows up in the output voltage of a 6'x4' panel. The reason for larger than expected loss is that a panel is made up of multiple electrical circuits, which in turn is made of multiple small solar cells. Removing sunlight from even 1 cell in the circuit causes the entire sub circuit to switch off. This is similar to Christmas lights that are connected in series. 1 bulb goes out and entire string is switched off. Note that even though 1 sub circuit goes off, others on the panels are still generating power.
     

Standards governing solar installations - NEC 2008

National Electric Code's (NEC) 2008 section 690, Solar Photovoltaic Systems, defines guidelines and recommendations specific to solar installations. Reading the Code itself is like reading any standard where accuracy is more important than ease of reading. There are two other sources of NEC's PV related code that are user friendly.

1. Photovoltaic Power Systems and the 2005 National Electric Code: Suggested Practices - free paper that is easy to read and explains the NEC code with respect to PV systems.

2. NEC 2008 - handbook - The handbook is easier to read and has illustrations to make reading interesting.

Standards associated with Solar inverters

The main motivation for complying with standards is to get approval from local govt authority and to get financing from banks for the installation. The most widely adopted accepted standards include

1. UL-1741 - These slides present a good overview of the standard
2. IEEE-1547 - UL standard refers to IEEE code for Interconnecting Distributed Resources with Electric Power Systems
3. NEMA6 - Standard for external enclosure specification and performance.

There are several government and independent testing facilities that can give certification for compliance to these standards.

So how can I use solar power at night ?

Solar power is great for the environment and available in abundance however it is available only when the sun shines. Thus arises the need to store the generated power so that it can be used even during nights and evenings. 
There are two solutions to energy storage. First is to charge batteries with the generated power. The advantage is that the charged batteries can provide power even during night time. This is simpler to design and most components to charge the battery and convert the power back to AC-electricity are readily available. The main issue is cost of buying and maintaining batteries specially for large storage systems. Also the big battery banks require significant storage space.

Second solution is to feed the generated power back to Grid. Grid is the electric network, like that of PG&E, that supplies power to our homes and offices. This reduces the installation and maintenance cost associated with storage batteries. However it adds to complexity of system design and associated components.
   This solution relies on the concept of "net meeting". In the USA, as part of the Energy Policy Act of 2005, al lpublic electric utilities are required to provide net metering to it's customers.

Keeping tab on your solar power-house

The monitoring solution for PV installations provides the following advantages

1. Provide data to calculate energy produced per panel per hr/day. This can help estimate the cost saving for standalone system as well as grid tied systems.
2. Total energy produced/saved data by itself is not as valuable as it is when compared with expected energy production for given location. There are several applications available that can give the annual expected energy production based on irradiation levels. This when compared for you system can help identify system performance issues. 
3. The above data is closely monitored by PPA providers as several of them provide minimum power production guarantee. The PPA provider would have to pay a fee to consumer in case minimum
   

Micro-inverter vs. DC-DC boost

There are two technologies that are garnering both interest and investment in the emerging solar-PV market.

• DC-DC boost - Boosting the DC voltage reduces the transmission losses due to lower current. Power loss is I*I*R. That is a 5% reduction in current leads to 25% reduction in power loss. Also the copper requirement reduces, thus reducing cost. Above all, the target market is existing centralized inverter installations. Besides the BOS cost saving, this technology also increases the total power generated by existing installation by providing per panel MPPT rather than per string optimization. 

• Micro-inverter - Unlike DC-DC boost, the target is new installations. The higher AC voltage makes installation faster and cheaper. And like dc-dc boost, it also reduces the cost of BOS components while providing higher efficiency. 

 There are several new companies in micro-inverter and DC-DC boost segments. However, it is interesting to note that Solar-magic choose to target the existing installations with dc-dc boost. National seems to play it safe given that the micro-inverter market is still nascent. There is third up-coming technology, optimizing per-cell power by making the panel smarter. The cost-benefit analysis is still against this technology, but this might change with new designs and manufacturing processes.  

Market Leader in micro-inverters

Currently enphase energy is the market leader since it was first to market the micro-inverter in June-2008. Since then it has not only launched next generation products but has also added several models to it's product line. Looking at their career openings, it seems that they are now moving to a custom ASIC from the current FPGA based implementation. As the competition heats up with multiple new entrants, reducing the BOM is a great strategy to stay ahead of the coming pricing war.