How Efficient is a Solar Collector?

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As you should already know, choosing the most efficient collector for your solar thermal installation is more about your situation, location and climate than it is about just picking the top-rated one off a list.

A great example of this can be drawn from the graph below. By taking sample collectors from ten different manufacturers and testing them in accordance with ASHRAE's (at www.ashrae.org) guidelines, this graph from the SRCC (at www.solar-rating.org) shows how the different types compare, on average.

Average collector ratings

From the graph, you can see that when the temperature at the inlet is around ambient, a pool collector is the most efficient. But simply going out and buying one on that premise would almost certainly be a terrible mistake: you'd be basing that decision on a single piece of information and ignoring everything else of importance. Similarly, you can't use the graph to make the decision to buy a flat plate or evacuated tube collector.

Remember that the graph was created in controlled circumstances, measuring each collector equally so that the ratings can be compared. Although it's useful information, it's only one part of the equation, and your location, climate and particular installation choices can change the results dramatically.

So what's the right choice for you? In most cases, residential solar thermal systems operate with the goal of heating the solar fluid to between 120°F and 140°F (49°C to 60°C). Let's say it's around 60°F (15.5°C) outside. At that point on our graph, flat plate collectors are still a bit more efficient than evacuated tubes – not by much, but even a couple of percent can be significant when considered over an entire year (or a decade).

As long as your solar thermal installation has been correctly built, most residential setups run at a maximum of around 100°F to 110°F (38°C to 43°C) at the inlet, so there isn't too much of a gap between that and your goal level. In most cases, a flat plate collector will suit you best.

However, if you're going for temperatures a bit higher – say, around the 160°F (70°C) range – you're heading into the area of the graph where evacuated tube collectors come into their own and will prove more efficient.

Those calculations assume a mix of weather conditions, so there's another step to this comparison. If you live in an area where it's often cloudy and you're dealing with diffused light a lot more frequently than direct radiation, you need to consider the graph below, which charts the SRCC's "cloudy" results as well:

Average collector ratings in all weather conditions

As you can see, evacuated tube collectors start to outperform flat plates much quicker than for direct sunlight and could well be a better option, despite increased expense.

Of course, it's still not that simple! Another step in your comparison should take into account the overall amount of solar radiation in your location. If it's frequently cloudy and gray, installing a far more efficient but much more expensive system might not be worth it: although it will convert a bigger percentage of the available sunlight into power,