Tienda Hernandez photovoltaic feasibility study
 Photovoltaic Feasibility Study for Local Grocer
Jose Leandro Hernandez, a local grocer in Parras, Coahuila MX, is interested in harnessing the ample year-round sunshine that Parras receives to power his store. The grocer is willing to invest in a photovoltaic system, but first needs to determine the economic feasibility of the project. Our goal was to design a system suited to the grocer´s needs and conduct a thorough study to determine the feasibility of the project.
We first designed a stand-alone system and found that the payback time is more than 25 years (the expected life of the system). We then designed a grid intertie system with a payback of 11 years. The stand alone system has more components and is far more expensive than the grid intertie system. With the intertie system, the grocer could produce more energy than he uses during the day and feed that energy into the grid (saving a lot of money on batteries) and then draw a similar amount of energy back from the grid at night when the panels aren't producing.
Unfortunately, grid intertie systems aren't legal in Mexico. Connecting to the grid to draw power when the PV system isn´t meeting the demand is legal, but feeding excess energy into the grid is not. Since the grocer can't afford the stand alone system, and since the system won't be built until next summer (when another troupe of Lonny Grafman's students from HSU bombard this city), we put our efforts into a feasibility comparison between the two systems. With the help of Jose Leandro, we sent a petition complete with the two feasibility studies to the Federal Energy Commission requesting that such systems be legalized. The Federal Energy Commission is currently considering legalizing intertie systems and giving credit for energy produced and fed into the grid, so that at the end of the month the owner of a PV system will pay only for energy used that his system didn't produce. Our hope is that the feasibility studies and petition will help sway their decision. There is at least one high-profile pilot project feeding energy back into the grid in Coahuila, and the most recent revision of energy laws in Mexico included mandates to encourage local renewable energy generation. And if it all fails, next year's program can easily modify our design to a "peak shaving" system, where the panels produce only enough energy for daytime loads and the grid supplies the nightime demand.
 Methods and Resources
Our first task was to determine the energy demand of the business. We used the highest energy bill of the past year, which also showed a graph of energy use over the past two years. We also completed an energy audit of the store to verify the information. The grocer's house is connected to the same utility meter as the tienda. Commercial businesses pay a higher tax on energy than do residences, so the grocer hopes to put the tienda and house on separate meters and use the PV system to power only the tienda. The wiring is already in place to do this (see the gallery). It was difficult determining exactly how much energy is used in the house and how much in the tienda, but we compared the bill to the energy audit and feel confident that our estimate of 9.45KWh/day is close to the true demand.
We then had to search for the peak sun hours of Parras. We used RETscreen, a free system design/cost analysis program, to find the average peak sun hours per month in Parras. RETscreen also calculates the average insolation at various slopes.
We used both RETscreen and the sizing worksheets from Sandia Labs to size the two systems. We put a crude version of the Sandia Labs worksheets into Excel (Media:Crude_worksheets.xls) and used these together with the directions and default values from the Sandia Labs sheets. The Appropedia Photovoltaics site provided a lot of useful information. Both windsun.com and noutage.com helped with choosing an inverter and abcsolar.com helped with sizing wires.
 The Systems
We sourced the materials for the system from Mexico when possible to avoid greater shipping costs and tariffs. Grid intertie inverters and bi-directional utility meters are not yet available in Mexico, so we sourced the inverter from the US and left the tariff out of the cost analysis, assuming that if/when grid intertie systems are legalized then these inverters will be sold in Mexico.
The components of the off-grid system:
|Trojan L-16P 6V Deep-Cycle Battery||trojan-battery.com||12||$146|
|Sharp NE-175U1 175W PV Module||wholesalesolar.com||16||$945|
|OutBack MX60 MMPT Charge Controller||thesustainablevillage.com||1||$595|
|Xantrex DR 2424 DC-AC Inverter||xantrex.com||1||$1210|
The components of the grid intertie system:
|Sharp NE-175U1 175W PV Module||wholesalesolar.com||16||$945|
|OutBack GTFX2524 2500W Grid-Intertie Inverter||altenergystore.com||1||$1481|
See Media:Offgridsheets.xls for the complete off-grid system sized in Excel and Media:Intertiesheets.xls for the intertie system. Module support structure, battery housing, etc. are not included in these simplified price tables, but are incorporated into the cost analysis in RETscreen. Wires, fuses, and switches were sized and priced in Parras and are also included in the cost analysis.
The complete cost analysis for the off-grid system (Media:Offgrid.xls) shows a total cost of $19,885 with a payback time of greater than 25 years, which is longer than the 25-year expected life of the system. The cost analysis for the intertie system (Intertie Cost Analysis (xls) and Intertie Cost Analysis (pdf)) shows a total cost of $17,595 and a payback time of 11 years. The off-grid system has more components that will likely need replacing throughout the 25-year expected life of the system; this explains why the payback time is so much longer for the off-grid despite the similar up-front costs of the two systems.
It is unfortunate that the grid intertie system is illegal, as this one is clearly more cost-effective than an off-grid system. Hopefully the Federal Government will legalize it before the next summer program in Parras. The Federal Energy Commission's official website should show if it has. If not, the tools we present here can be used and the worksheet entries modified to design a system that provides for the daytime usage of the system and allows the grocer to use the utility's energy when the panels are not actively producing electricity. If this proves more cost effective than the off-grid system, the grocer may still be interested in implementing it. We gave him both cost analyses and a list of the system components, and he is looking into finding them more locally and at better prices. Our search for components was rushed and confined to the internet; it's very likely that they can be found cheaper. Meanwhile we are still petitioning the government and Jose Leandro is waiting for next summer's program to tell him more...