Power Output of Renewable Energy Systems

Solar panels on a roofNot very long ago, renewable energy was the realm of a small core of quirky enthusiasts. As one outpost operator put it “I had to buy my solar panels from a hippie looking guy who drove an old VW bus”. Now, you can walk into your local Canadian Tire store and see wind turbines and photovoltaic or PV panels on display. Unfortunately, this new found availability has often resulted in even more confusion over what is reasonable and practical in the world of renewable energy.

By far the biggest item of confusion is over the ratings of PV panels and wind turbines. Advertised ratings, by themselves, tell you very little about how much power you can actually produce. Life is so much simpler in the world of generators that most of you are familiar with.

I have a 5kW generator as a backup for my farm. I chose the size so that it could handle my essential power requirements during occasional power outages due to storms. I looked at the things I needed to keep running, like the refrigerator and freezer, as well as my water pump and oil fired water heater. Adding up all the loads that might be in use at one time, I determined that the 5kW rating would keep me going, and experience has proven this to be correct.

Generator ratings are easy to understand because a generator is capable of its full rated output all of the time. My standby generator will continue to provide up to 5kW of power 24 hours a day, as long as I keep filling it with gas. Life is not nearly so simple with wind turbines or PV panels. Although these devices have a published power output, that power output is only available when certain conditions exist. PVs only produce power when the sun shines, and wind turbines need the wind to blow.

Let’s look mainly at PV panels, since they are generally acknowledged to be a much more likely choice for most tourist lodge or outpost applications. If I look in a catalogue, I may find a panel that is rated at 200 watts. At today’s prices, that 200W PV will likely cost a bit more than my 5kW generator – around $1000. So how much power can I produce with that 200W panel?

As you can guess, it will not produce power 24 hours a day like a generator. Furthermore, it will only produce the full 200W rated output during the part of the day when the sunlight is brightest. In addition, your system involves charging batteries and converting the DC in the batteries to AC with inverters. This process is not 100% efficient, so we will need to account for these losses.

There are websites available where you can find how much solar energy is available for your location, and some even feature calculators that will do the math for you. One very easy to use site is http://rredc.nrel.gov/solar/calculators/PVWATTS/. Although it is designed for grid interconnected systems rather than off grid, you can enter a different system efficiency value to account for the additional battery charging losses.

If I calculate values for a .8kW array – that’s 4 of our 200W panels and use an efficiency value of 75% (this number is probably a bit high, 65% might be more realistic), lets see how much energy we can produce in a day in Thunder Bay in July.

The calculator shows us producing 94 kWh of energy for the month of July, or just over 3kWh per day. As a point of comparison, that standby generator at my farm can produce 5kW x 24hours = 120kWh per day. Of course, it does so at a considerable fuel cost and at very low efficiency during times of low load. What this illustrates, however, is that it takes a rather large and expensive PV array to generate even a modest amount of power.

A bit of fine tuning will improve our results slightly. Since most tourist lodges are seasonal, we can install the solar array so that it is angled to maximize efficiency in the summer. That will increase our July energy output to 105kWh per month, or about 3.4kWh per day. A motorized array that tracks the path of the sun across the sky would increase our output further, but at added cost and complexity.

These calculations underscore the need for energy efficiency. Our hypothetical solar array would barely operate a single ten year old refrigerator or freezer, but it could easily operate as many as three modern, energy efficient ones. Basic energy efficiency is much cheaper and easier than adding solar panels.

The issues for wind power are similar. Wind turbine outputs are referenced to a certain wind velocity, and outputs drop dramatically at lower wind speeds. Without a detailed look at the specifications and accurate wind data for your location, it is impossible to estimate how much power you could generate with wind. For most northern Ontario locations however, particularly in the summer, it is very unlikely that wind turbines will be a better choice than photovoltaic panels.

Is this article an attempt to discourage adoption of renewable energy? Absolutely not! It is, I hope, a bit of a reality check, however. The planning required for a renewable energy system is much more complex than the process of choosing a generator, and the systems are much more expensive. However, fuel costs continue to rise, and there are numerous practical and environmental advantages to solar or wind power. Renewable energy is no longer uncharted territory – there is a solid knowledge base and numerous success stories. If you do your homework and plan carefully, it is possible to greatly reduce both your energy cost and your environmental footprint.

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