Know what, I decided to address how the output of individual collectors drops as the solar fraction increases for solar hot water systems, as I was working on a recent FChart analysis for an upcoming project.
The energy output per collector, often referred to as the collector yield, is measured in million British thermal units over the course of the year. This metric is very important as the best designed systems should have every collector working as hard as it can. Owner of the system has invested initial capital in every and any collector, right after all. Solar thermal collectors should behave as wellpaid employees! Now regarding the aforementioned fact… Not lazy bums.
The solar fraction defined as the percentage of the overall load that is supplied by the system over a specific time period.
If a building uses 1000 hot gallons water per day and a solar hot water system supplies 500 gallons of that, the solar fraction equals 50percentage for that day. Do you know an answer to a following question. It’s an interesting fact that the question is often asked, Why not choose a 100 solar fraction quite often therefore, this is the case right? If X number of solar collectors will supply half the load, 2X must supply the entire load, after all. Not exactly. Of all, unlike conventional energy sources, we can’t just flip a switch to make the sun shine when we need it. It can be cloudy or rainy for a few consecutive days. Oftentimes there’s nothing we can do about the weather. This means the contribution from a solar water heater could have been zero for many days out of the year. Getting an annual solar fraction of 100 isn’t practical.
Well, So in case you can’t get 100, shouldn’t you take as much as you can get, is that the case? That’s not what a graph would show us. On top of this, we would see that as the overall number of collectors in a system increases, the solar fraction does so it’s a classic example of the Law of Diminishing Returns! The graph should also show that as the total amount of collectors goes up, the yield per collector steadily decreases. Now pay attention please. Each additional collector makes all the others slightly lazier. Adding collectors adds cost. On top of that, we don’t look for to be spending more money and have the performance per collector going down. In any diminishing return situation, the question is where to stop.
Folks who design solar hot water systems above the sweet spot end up throwing away money.
The cost of their system increases but the energy output per unit drops off. In the analysis above, I used identical hot water demand throughout, and changed the general amount of collectors. Then, the storage tank is always sized to match this place of the collectors.
Note that the monthly solar fraction doesn’t reach 100 until look, there’re 120 collectors, and the annual fraction is 89percentage. Each collector produces 11 MBtu/yr which is 37 less energy than a system with a fraction of 35percentage. That’s not an ideal deal for the owner. Don’t make this mistake when designing! Note that the monthly solar fraction doesn’t reach 100percentage until look, there’re 120 collectors, and the annual fraction is 89. Each collector produces 11 MBtu/yr which is 37 less energy than a system with a fraction of 35percentage. That’s not a decent deal for the owner. Don’t make this mistake when designing!
Dr. Ben Gravely is widely recognized as the country’s foremost authorities on solar hot water systems. With more than 1500 residential and commercial systems to his credit, Ben has an unique encyclopedia specific to solar thermal design, installation, and energy output which he shares through