Hydronic Solar Collection Ideals

Mike T., Swampeast MO

Am likely making poor assumptions. Please correct or embelish.

When using plain (as opposed to evacuated tube collectors) I want to:

1) Keep the fluid entering the collector as low in temp as possible.

2) Move the fluid slowly enough through the collector that it approaches the highest practical temperature that can be produced by the panels given the air temp surrounding the collector.

3) Transfer the heat to the collection area as rapidly as possible by keeping the collection area as cool as possible and the transfer mechanism as hot as possible.

4) Use as much of the transferred heat <I>as it is being produced</I> as possible to ensure that transfer stays as efficient as possible.

5) When production exceeds utilization, let the heat collect while accepting less efficient heat collection and transfer.

George Peteya_2

Assumption #2

Mike -
You want to keep the average temperature of the collector as low as possible, by keeping the flow rate as high as possible without causing solar loop circulator cycling. I'm assuming that the solar loop circ is running off a differential control, just like in the old days. If so, the control will have an "on" and "off" differential ... they used to be either 20°F on, 5°F off, or 8 and 3 for glazed collector systems.

The collector efficiency at a given condition is determined by three things: The operating temperature, the ambient temperature, and the solar intensity (insolation) in Btu per hour per °F. These are used to determine the Fluid Parameter. Once you know that, you can calculate the collector efficiency.
Unless the collectors are homebrewed, there will be a label which will show a performance equation which uses the Fluid Parameter to determine collector efficiency. The performance equation will be something like: N=0.783-.893P, where P is the Fluid Parameter. You calculate the Fluid Parameter as follows: Fluid parameter = (Operating temp minus ambient temp)/insolation.
The literature will show "operating temp" as "inlet temperature." If the temperature rise through the collector is only a few degrees, this is pretty accurate. BUT, if you slow the flow way down, the average temperature of the collector goes way up and the efficiency goes way down.
Hope this helps, Mike. If this raises more questions than it answers, email me directly at gpeteya@slantfin.com.

Carl PE

How about this..

1. Yes.

2. Not Necessarily. The collector efficiency (loss) is dependant on the average fluid temperature. You want to adjust your return water temperature to the lowest practical temperature that you can use in the rest of the system. This will give you the best collector efficiency for your particular system.

3. I'm not sure what you mean by "keeping the transfer mechanism as hot as possible". Keeping the collectors cool will increase your efficiency, per comments 1 & 2.

4. Yes. If you're transferring heat to storage, you need the storage to be at a slightly higher temperature. You'll have an extra step of heat transfer when you want to use it later.

5. See 4.

Does that help?

Mike T., Swampeast MO

Thanks George!

Makes very good sense.

Was hoping for more sophisticated control with a small variable speed DC circulator. The drop in efficiency with high rise could actually be a good thing in some circumstance in the system I'm trying to build.

Mike T., Swampeast MO

Thanks Carl.

> 1. Yes.

>

> 2. Not Necessarily. The collector

> efficiency (loss) is dependant on the average

> fluid temperature. You want to adjust your return

> water temperature to the lowest practical

> temperature that you can use in the rest of the

> system. This will give you the best collector

> efficiency for your particular system.

>

> 3. I'm

> not sure what you mean by "keeping the transfer

> mechanism as hot as possible". Keeping the

> collectors cool will increase your efficiency,

> per comments 1 & 2.

>

> 4. Yes. If you're

> transferring heat to storage, you need the

> storage to be at a slightly higher temperature.

> You'll have an extra step of heat transfer when

> you want to use it later.

>

> 5. See 4.

>

> Does

> that help?

Mike T., Swampeast MO

Thanks Carl

Yes, it definitely helps.

Regarding 3: "keeping the transfer mechanism as hot as possible". Was referring to a HX coil in a tank as the "transfer mechanism". Thought that ensured good, efficient transfer from the coil to the fluid. A bit at odds with 2) huh? Suppose that's one of the advantages of evacuated tubes as their efficiency doesn't drop off as rapidly with increasing average temp, right?

Any recommendations for good books? Prefer deep theory/math regarding heat transfer in various types of collectors.

Carl PE

You're welcome.

I like the idea of variable-speed pumping tied to outlet temperature, but you're getting into some fairly sophisticated controls. Maybe a 3-phase pump with a small (i.e. cheap) vfd? Use the differential controller to enable it, then tie an extra collector sensor to the resistance input to determine speed?

Please note that this will require a lot of fiddling and tuning to get it dialed-in. Maybe a 3-speed brute tied to multiple diff controllers would be simpler..

Regarding the evac tubes.. yes, they're nice, but a lot of $/sf. You'll get a lot more bang for your buck using flat plates. Hot Rod can probably fix you up with some used collectors if you want to save some $. It's a lot easier to fix them than to build them from scratch.

For the books.. I've got the original f-chart book if you want to borrow it. I personally like the old "homeowner-how-to" books from the 70's.. lots of installation details and practical advice. Those hippies were pretty smart, and most of those books got donated to the local library in the 80's. Also, you might call up your local college and offer to take a professor to lunch. They love to talk, and may even loan you stuff from their private collections.

Mike T., Swampeast MO

Thanks for your suggestions! Our local library is rather archaic in the building science department and I do know some professors fairly well...

Had a big collection of solar articles, phamplets, etc. from the 70s in my old room, but dad (Mr. Clean) got tired of the stuff and threw it out without asking if I wanted it.

Larry Weingarten

Variable speed...

...pumping is done simply by hooking up dedicated photovoltaics to a DC pump. I know this ignores storage temperature, but has applications. Keeping the flow slow as you propose will help maintain tank stratification, helping tha case for DC pumping. Dat's good!

Yours, Larry

Mike T., Swampeast MO

We seem to think along the same lines Larry.

Larry Weingarten

Well...

...that means either we're doing something right or you're crazy too. I've been doing solar since 1978 so have had lots of opportunity to learn what not to do again ;~)

Yours, Larry