Photovoltaic panels

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Aboard the Gato Verde, we use electricity to power the propulsion system and house systems such as navigation equipment, lighting, refrigeration, scientific recording equipment and personal computers. Our average daily power consumption is 29.8 Kwh (1), with the majority of this power going towards “silent” motoring. Using the Kill A Watt meter, Elise determined that three hours of typical computer use consumes 0.28 Kwh, and I doubled this figure to represent a full afternoon and evening of computer use. I then multiplied the above figures by 30 to determine monthly electrical energy consumption, with 900 Kwh/month total consumption and 16.8 Kwh/month consumed for evening computer use. The average household in the United States uses 741.7 Kwh of electricity each month (2), though directly comparing our energy use aboard the Gato Verde to the electricity usage of a US household is problematic because we use consume the greatest amount of energy when running the propulsion system to travel to different locations, something that the typical house does not do. In US households, refrigerators and central air conditioning are the biggest energy users, accounting for up to 14% of the total Kwh consumed (3).

At this time, the vast majority of the electrical power used aboard the Gato Verde is generated when the biodiesel generator is running. Secondary sources include plugging into shore power and power generated from the propellers spinning while the boat is traveling under sail (very small source). Running off of biodiesel seems like a good alternative to petroleum based diesel as soybean biodiesel produces 41% less greenhouse gas emissions than diesel fuel, and soybean biodiesel returns 93% more energy than is used to produce it (4). Alternative studies have created a more complicated picture of biodiesel production and usage. Analysts at SRI Consulting found that biodiesel derived from rapeseed grown on dedicated farmland emits nearly the same amount of greenhouse gases as petroleum diesel due to high CO2 equivalents output during production of the biodiesel, and that if the farmland was instead used to grow trees, petroleum diesel would emit only a third of the CO2 equivalents as biodiesel (5).

An alternative option for electricity generation, that could potentially free the Gato Verde from biodiesel dependence, would be to invest in photovoltaic (from photo, derived from the Greek word for light, and volt, relating to electricity pioneer Alessandro Volta (6)) cells, also known as solar cells to convert sunlight into electricity. See the Wikipedia website for a good description of how PV cells work (7). The biggest challenge consumers face when considering PV systems, is a large initial monetary investment, though rebates and tax-cuts can be obtained from the state and federal governments. The technology involved in making a PV system is complex, and I had a difficult time finding information on the environmental impact of PV cell production. Some of the benefits of a PV system include independence from fossil fuels and an associated reduction in air pollution and a nearly free source of power following the initial investment (8).

Once a consumer decides that solar is the way they want to go, there are some convenient on-line calculators to help them determine the size of PV system necessary to meet their electricity needs. I used the Off Grid Calculator from The Alternative Energy Store’s webpage (9) because this calculator was simple and it incorporated energy losses due to system inefficiencies. To use this calculator, I needed to know the Gato Verde’s monthly Kwh usage, and the average daily insolation (from INcoming SOLar radiaTION, measured in Kwh/m2/day) at the boat’s location. I used my calculations above for the Gato Verde’s monthly Kwh usage, with 900 Kwh/month for total usage and 16.8 Kwh/month for typical computer use. I found insolation maps at the National Renewable Energy webpage (10). The insolation values on the map I selected are for a flat plate PV cell that is facing south and tilted to match the local latitude. The average monthly insolation for Puget Sound during the month of September is 4.5-5.0 Kwh/m2/day and is 3.0-3.5 Kwh/m2/day during the month of October. I used the average of these two figures, 4.0 Kwh/m2/day for my calculations to give the average insolation for the period of time Beam Reach is aboard the Gato Verde.

To generate all of the power necessary to run all of the Gato Verde’s systems during the months of September and October, the PV system would require a total wattage of 9.75 Kw. This translates into 63 155 w PV cells. Using the SunWize SW 150/155 Solar Module (dimensions 66.61 in x 30.27 in) (11) as a model for PV panel size, the calculated surface area of 63 155 w PV cells is 127,026 in2 (882 ft2, 81.9 m2). To generate the power necessary for an evening of computing during the months of September and October, the PV system would require a total wattage of 0.19 Kw. Which translates into 2 100 w PV cells. Using the SunWise SW90C/SW100C solar module (dimensions 56.89 in x 22.80 in) (12) as a model for PV panel size, the necessary PV cells would have a surface area of 2594 in2 (18 ft2, 1.67 m2).

So is it reasonable to power all of the Gato Verde’s systems using solar power and PV cells? No. But is it reasonable to use solar power and PV cells to generate the electricity necessary to power an average evening of computing aboard the boat? Yes, it certainly is. Two SunWise SW90C/SW100C PV modules could easily by mounted above the cockpit. Before leaping ahead with this technology aboard the Gato Verde, additional boat-specific considerations should be made, such as the effects of shading by the sails, and the PV panels’ resistance to salt water. PV cells and solar power look promising, now does anyone have a couple thousand extra dollars lying around?

1. Average daily power consumption calculated from ten daily readings of the Xantrex Link 10 amp hour meter.
2. 10/20/07
3. 10/20/07
4. 10/20/07
5. 10/20/07
6. 10/20/07
7. 10/20/07
8. 10/20/07
9. 10/20/07
10. 10/20/07
11. 10/20/07
12. 10/20/07

Anne M. Harmann, Beam Reach 071