net metering

Our net metering system was installed on October 18. We can now see when the meter is going backwards, how much we have consumed and how much we have produced in excess of our consumption (that is, when the meter goes backwards). That is nice. Our 2 kW system has averaged about 2 kWh per day over consumption for the first 5 weeks. That is in contrast with an average daily consumption of just under 12 kWh. The way the billing here works, we will get charged the 22.9¢ per kWh for about 12 kWh per day and get back 10¢ x 2 kWh per day.

One issue that just came up with the first bill, however. That gives a consumption of 901 kWh during late September and October, more than double our actual consumption. It is clearly a mistaken reading that requires correction, and I will lose a day trying to get it fixed.

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

Some details on our installation

Here is a circuit diagram of our solar installation.

It consists of 10 panels connected in series, each producing about 28 V. These consist of 60 cells also connected in series, so we have 600 solar cells in a series connection, producing over 280 V DC. This is then inverted in the SMA Sunny Boy 4000US grid-tie inverter.

In my next post, I will talk about some of the headaches of having the cells connected in series, namely, the problem of shade.

Dealing with Hacienda

Solar Blog Entry - Dealing with Hacienda

Dealing with the Puerto Rico tax office Hacienda is an important element of an alternative energy installation. The tax credit for tax year 2009 is 50%, which we expect to qualify for.

It was important for us for our file to be opened in 2009, as many of the required documents took time to obtain and were not finally submitted until early January 2010. All of these documents need to be submitted in person to office 624B of the Divisíon de Opiniones Administrativas y Legislación del Negociado de Asistencia Contributiva y Consultas Especializada (that mouthful translates roughly to the Division of Administrative Opinions and Legislation of the Office of Tax Assistance and Specialized Consultations). Some are submitted by the installer and others by the customer, as I summarize below.

1. First, they require demonstration that the taxpayer/customer has no debts to any government office or agency. For this, they require Certificaciones Negativas de Deuda, or Negative Debt Certificates from ...

- Department of Hacienda (tax office)

This can be obtained from the local Hacienda office or online from http://pr.gov.

- Centro de Recaudaciín de Ingresos Municipales, or CRIM (Municiple Property Tax Office), which refer to property taxes.

This must be obtained from the local CRIM office (have fun :-)).

- Departamento de Trabajo y Recursos Humanos (Department of Labor and Human Resources), which requires communicating with the central office in San Juan but can be carried out via FAX. This is automatic for a home installation, if you are not a business owner and have no relationship with this department.

We discovered that our local office in Arecibo could not provide this, and we called the main office in San Juan at 787-754-5818 who asked us to FAX the required information (photocopy of my drivers license and social security number along with an address of where to send the certification) to 787-281-5649.

- Corporación del Fondo del Seguro del Estado (State Insurance Fund Corporation), which, for a modest annual fee, provides accident and injury coverage for home and business owners who contract work on their property.

We went to our local Fondo office for this certification.

- ASUME, which is the office that monitors legal obligations such as child support and alimony payments.

We received this certification at the local ASUME office, which was the most efficient of all the offices we visited.

- and if the installation is for a business, such certifications must be obtained from its investors and partners who own over 25% of the company's shares or represent over 25% of its operations.

Happily, we did not have to do this one.

2. Second, you have to deliver certification of tax filings, which are obtained from Hacienda. If there is a partnership involved, such as for a business with investors and/or partners, they must also obtain these certifications.

3. The Installer should be responsible to submit the Photovoltaic Equipment Certifications (Certificados de de Equipos Fotovoltaicos) filed by the Administración de Asuntos Energéticos (Energy Issues Administration of Hacienda) (AAE).

4. The Installer Certificate must be provided, same as that issued by the AAE. This is also responsibility of the Installer.

5. The Installer should submit a "Design and Installation of Photovoltaic System" certificate, again issued by the AAE.

6. A copy of the Commercial Invoice from the Installer (i.e. the receipt for the installation, including both parts and labor, from the Installer to the Customer).

7. Any other document the petitioner considers relevant.

All of these documents must be delivered within a period of 60 days. Our process began December 14 and ended on January 25. Once delivered, Hacienda evaluates the case and submits a determination of whether the project qualifies for the tax credit. We are currently awaiting that determination, which we hope to have in time for filing our taxes in April.

My next post will summarize the first month of performance of our photovoltaic system.