Trying to keep up w/ Chem Geek

[CarlD]

When water flows faster through your panel, it spends less time in the panel so gets heated up less. So while you push more water through the panel at higher flow rates, it gets heated up less and the net result is roughly the same amount of temperature rise in your pool. You should generally stay near the recommended flow rate for your panel and not exceed its maximum rate.

Richard, I am amazed at this. You are usually SO good at the science, but this is just flat-out wrong. It SEEMS to make sense that the water needs to heat up, and if it goes too fast it won't, but it's still wrong. It simply violates the laws of themodynamics. It's all about fluids.

Think about it: If your statement is true, then on a windy day, the BEST way to stay warm would be to open your coat...the air whistling past you wouldn't have time to get warm from your body and bleed off that heat.

But obviously, that is NOT true--the faster the wind moves past you, the more heat it sucks out--it's called "Wind Chill Factor" and you freeze your ka-loonies. This is JUST as true of the fluid in your solar panels--the faster it moves through the system the more heat it sucks out. Ideally, your panels on a hot, hot summer day should not be any hotter than your pool water.

Remember: Your solar panels are mechanically IDENTICAL to a car's radiator. In fact, if your pool is too hot, you run the panels at night to cool it. In a radiator, if you are idling in a traffic jam on a hot summer's day, your car overheats. You get moving, it cools down. The MORE fluid (air) passes accross the radiator, the more it cools down--faster is better. Do not mistake temperature for BTUs. It's BTU quantity that heats your pool. The more BTUs you can get into the water in a given amount of time, the faster the water heats up!

Why? Well a British Thermal Unit is the amount of heat energy necessary to heat one pound of water one degree farenheit. That's the WHOLE secret to why more flow is better. More heat energy, not more temperature. I'd MUCH rather have 10 gallons/minute of water 1 degree warmer than my pool than 1 gallon every 10 minutes that's 10 degrees warmer--my pool will heat up 10 times faster because it's moving 10x the BTUs.

My system DEPENDS on that--my panels are my deck and the water running through the deck simultaneously warms the pool while keeping the deck cool to walk on barefoot on the hottest days.

(BTW, in Metric it's called a KiloCalorie, or Calorie for short--though really a calorie is 1/1000 of a KiloCalorie. A Calorie is the amount of heat energy necessary to raise one kilogram of water one degree celsius--exactly the same as a BTU but with different measuring units.)

But there ARE limiting factors--I cannot run that high a pressure through my system or the panels start to spring leaks--that's the really, really big one. Too much pressure can damage the system...and I have some leaking panels to repair this winter... (anybody good at plastic welding?)

The other limiting factor is cavitation--bubbles in the system. When the water bubbles it cannot make complete contact with the panel's surface and cannot conduct heat away from the panel. This is VERY hard to achieve unless you have a lot of air in your system. Usually, it will start leaking first.

Both of these factors are result of the friction/resistance of the system. Lowering that resistance can increase efficiency immensely.

I increased the efficiency of my system by, I estimate, at least 40% by splitting my system into the two halves. I have 30 panels (I miscounted before) and originally they were all in sequence, from the first to the second, etc. The water had to pump through ALL 30 panels before it came out the return, nice and warm, but slowly. The resistance of the system meant any more water pressure and the early panels would leak.

So I split the system into 2 halves of 15 panels each. You can see the Tee in the picture. Each 15 panel half can flow FAR more water than the 30 panel whole because its resistance is half of what it was. And, with two halves, that higher flow is DOUBLED for the same 30 panels (now 15 and 15 in parallel to each other). The result? The last two seasons my pool heats up far, far faster than it used to.

Of course, since each 15 panel half has half the surface area it's maximum BTU capability is half. But I've doubled the flow rate for the same surface area and the result is my pool heats up far faster. 3 or 4 degrees a day is now 6 to 8 degrees. I'm getting more BTUs into my water.

Heat energy (BTUs) not temperature is the key to warming your pool.

The efficiency ratings of solar panels require a little different analysis--the loss of efficiency is due to back-pressure (friction), not the water moving "too fast to heat up".

As I was thinking about this, I remembered that I first came across Chem_Geek's natural mis-perception in a completely different context. Kevin Cameron is an utterly brilliant columnist for Cycle World Magazine. He specializes in technical issues. One article, several years ago, he took on this issue. The question he addressed was the same: If the water flowed too fast through the engine, wouldn't it not have time to heat up and bleed off the heat from metal? He explained why this perception was just plain wrong.

Well, the answer to this is FAR more critical to internal combustion engines, particularly performance-based engines like sports-cars and most motorcycles that are both highly stressed. An over-heating race engine, at speed, can seize and launch a poor rider as a human cannonball at 150mph or more! DEFINITELY far worse than a little loss of heating efficiency in our water. Plus, a leaking pressurized engine cooling system is deadly dangerous--that water can scald and cut like a knife--water at pressure has a FAR higher boiling point than 212 Deg! With motorcycle engines the penalty for being wrong is far more catastrophic!

Cameron's explanation was basically what I have offered above--the more water moves through the block's water jacket, the cooler the engine. Only cavitation sets it back. Otherwise, the more the better. It's exactly the same system, just in a different context with different constant values.

I remember reading somewhere here that it's quantity of water thru the panels that gives the most hot water. I sure did that last year with forcing all 1.5hp thru 2 panels that measure 3 x 9 each!

So, ShellyAnn, you keep doing what you were doing--that is, in fact correct! You want as much flow as you can without damaging your system.

The only other reason I had to lower my flow rate was on a different pool with a different set up. Too high a flow rate on too small a pump kept my returns from circulating my water properly--but that has NOTHING to do with the heat transfer issue!