Glycol in a solar system

CountryTodd

We have an evacuated tube system that was professionally installed in 2008. I believe we were the company's first evacuated tube customers, possibly their first solar customers! Not good. We recently moved, brought the system with us, and are designing how the system will be set up at our new home.

After seeing the plumbing diagram of how it was plumbed before, a friend brought up an interesting point/possible plumbing error.

Our storage tank has two coils. The glycol from the collectors flows into the tank through the first coil, exists the tank, goes back into the tank through coil #2, exits the tank and goes back to the collectors. Our domestic water is the water inside of the tank (not in a coil), cold into the bottom, hot out of the top of the tank.

Is this setup ok or is another layer of protection required between the glycol and domestic water?

hot_rod

Sounds like you have a dual coil tank? Do you have a brand name and model #?
They are sometimes used with solar input in the lower coil, and a boiler backup in the second , upper coil. Eva tube collectors often run a low flow rates, the manufacturer spec sheet will give you that floret info.

Depending on the type of coil, and pressure drop, the two coils could be put in series like that to increase HX surface area.

Evac tube collectors should use the best high temperature solar glycol you can find, and I would flush out the system with a cleaner, like Rhomar when you re-install it.

Evac tube systems without a consistent load can overheat the glycol and cause it to breakdown, then it drops in ph level and becomes aggressive to the metals in the system.

kcopp

That's one way to do it.
I would be curious to see what the temp going in is and the temp going back to the collectors is...

The reason for two coils is to have the solar on one and a backup on the other... like a boiler.
As long as the glycol is non toxic a single wall coil will be fine by most codes.

Some do require a double wall Heat exchanger...especially is the heat transfer fluid/ glycol is toxic.

This is also a good time to flush and clean your system & piping this will improve your system efficiency.

GreenGene

A better way to do it is to have someone convert to a drain back type, if you're pump dies the glycol will overheat and cook into candy in your panels and then they are scrap.

With a drain back the panels sit empty when not used.

hot_rod

GreenGene said:

A better way to do it is to have someone convert to a drain back type, if you're pump dies the glycol will overheat and cook into candy in your panels and then they are scrap.

With a drain back the panels sit empty when not used.

I am a big proponent of drainback for this and other reasons.

No many of the evac tube systems can be drained back successfully. I tried a few mounted on their side and still had enough fluid trapped to cause a freeze up. I would still use glycol in a DB vac tube system for extra protection,as much as I dislike solar and glycol.

Apricus had a DB system on the market for a bit, I haven't seen them in a few years.

CountryTodd

Keep in mind that the system is currently idle, in pieces, in storage, waiting for a game plan to reinstall. I'm trying to understand why the original installers installed it the way that they did. We had issues with some of their work, so I'm not convinced that things were correct even if the system worked when we dismantled it.

Our system:
-two 25 tube collectors, in series
-one 80 Gallon (we think) storage tank with 2 coils in it, "Tank 1"
-one 80 Gallon (we think) storage tank with 1 coil in it, "Tank 2"
-ground mounted

Initially, the installers thought that Tank 1 was going to be adequate. They quickly determined that it was not going to be enough storage, so Tank 2 was added. We never over produced and the site was better for solar than our new site is.

Prior to dismantling, hot HTF from the collectors flowed to Tank 1 upper coil, Tank 1 lower coil, Tank 2 coil (in the bottom), and back to the collectors.

Domestic water from the well flowed into the bottom of Tank 1, out of the top of Tank 1, into the bottom of Tank 2, out of the top of Tank 2, and then into the electric water heater.

The System was installed in the fall of 2008.

The collectors and tanks came from Silicon Solar in Bainbridge, NY.

The tanks have a stainless steel skin, but no name or model number. They are 20" in diameter and stand 86" high. Each tank has a port for an electric heat element (or two?).

I believe each 25-tube collector is rated at a flow of 0.7 gpm.

As for the temp difference coming in vs out of the coils, I do not recall.


Our new home requires roof mounting, cocked or tipped running up the roof. They won't be as easy to tarp during a power outage, so I was planning a steamback system. I honestly don't know the details of a drainback system.

hot_rod

Really the bottom line with any SDHW solar domestic hot water system is matching the collector array to the tank and then to the load. you can run a solar simulation with your actual data, location, mounting of the equipment and estimated DHW use per day.

Any idea on how much DHW you will use on a typical day? Some suggest 15 gallons per person per day. It really depends on your family usage.

When I see extra tanks and doubled up HX it may have been modified to lessen an overheating condition. I think one 25 tube array would be a good match for an 80, maybe even a 120 gallon tank.

With SDHW you either need to use it or lose it some how. in summer months with low DHW use, plan on needing over-heat protection. same for when you leave the home for a vacation, you need to have some means to limit the overheating of the glycol.

Schuco from Germany introduced the steamback concept to the US, they have since pulled out. With their system a serpentine type flat collector was used to allowed flashing of the fluid to empty the collector. The intent again was to lessen stagnation of the glycol fluid in the collector. They did suggest using their blend of fluid in the systems.

I have not seen a steamback concept used in evac tube style, it would depend on the construction of the header.

I'm assuming Silicone Solar is no longer around? Did they propose a setback type protection concept?

I'd be willing to bet all the equipment was sourced in China, tubes and tank.

If it was a listed system you can find test and performance data at www.solar-rating.org

At the SRCC, OG-100 was the collector rating. OG-300 was the entire system listing, tank and collector, etc.

kcopp

Wouldn't a small DC operated circulator hooked up a small PV panel be a good choice if power outages are a concern?

CountryTodd

kcopp said:

Wouldn't a small DC operated circulator hooked up a small PV panel be a good choice if power outages are a concern?

The PV panel idea sounds great, but it didn't work for us. Since the evacuated tubes are round, they make heat from almost any sun angle. The PV panel is flat and only makes electricity when the sun is more straight on it. The DC pump also didn't have enough flow. It's been suggested that two DC pumps might be up for the task, but that doesn't solve the round vs flat problem.

CountryTodd

Silicon Solar is still in business. Customer service is poor, to say the least. The people that own the business also own SunMaxx Solar, if that name rings a bell.

Yes, I assume these components are generic Chinese made.

The over-production system proposed and installed was an internal space heater/radiator. Dumping heat into the house during the summer sounds really practical, doesn't it? The backup protection for power outages was a black tarp. More practical than the heater, but no good if we were away from home. Now that the collectors will be roof-mounted, tarping is even less practical.

I doubt that the system was listed, but I'll have a look.

hot_rod

I don't see any OG-300 system ratings, this may be close to what you have, square footage-wise. Sumacs has a few different models listed.




The data shows around 36,000 BTU/day at typical DHW conditions. That is about what a gas fired DHW is rated per hour.

It is sort of counter productive to harvest the solar energy, then pay pumping power to dump excess. At some point you may go backwards

That is another advantage of the drawback, when the tank hits setpoint, just power down the system, instead of dumping excess. via a pump running

The steamback is an attempt to do the same, power down the pump and allow the fluid to leave the collector.

Remember IF, the collector fires back up after a dry drainback or steam back, condition, you have the fluid at maybe 100F, hitting a collector with the potential temperature in excess of 300F. That will make some noise.

Most of the new controls have an EM, emergency temperature number to prevent the system from firing up, typically set at 275F, just below the glycol high temperature number.

Modern controllers have a few different overheat setting onboard as well a tube specific functions.

Maybe the best advice it try not to install more array than you can use. One of those tube arrays with an 80 should be plenty.

Give me your location and family size and I run a quick sim.

CountryTodd

That collector is a U-pipe.

I believe our's is called ThermoPower VHP25. There isn't a rating on solar-rating.org for VHP25, only for the VHP20 and VHP30 collectors.

I did locate a "Product Data Sheet" on SunMaxx's site for the VHP25.

http://www.sunmaxxsolar.com/data-thermopower-vhp-collectors

That link may be helpful.

I agree, why make more DHW than we need and then pump it away/dump it to make room for tomorrow's heat.

2 adults, occasionally 3 kids visiting.
Waverly, NY 14892
Site location is not horrible, but not great either

hot_rod

Here it is
facing south, 45° pitch
25 sq. ft array
80 gallon tank
40 gallons per day use
Elmira weather data

It should provide about 52% of your DHW at 30% efficiency

The color chart shows your usage and amount solar will contribute.

When you over size the array you would see the solar off the top of the chart in some months. That is when you get to over heat conditions.

Use one array, one tank, sell the rest

This look pretty good for your application, not knowing your exact mounting detail.

CountryTodd

wow, that's impressive!

Does this take into consideration how good the site is for solar?

Thank you for your time

hot_rod

This assumes it is facing solar south, 45° pitch, no shading. really about 9:00- 3:00 in direct sun is a good solar "window"

The program allows you to change any of the above parameters, add shading, etc.

As you know, solar is not 100% predictable, the simulation are based on a 20- 30 year weather record for the closest weather station in your area.

Certainly installation details, like flow rates, piping insulation, solar tank HX condition, etc need to be considered.

The number you want to look at is SF, solar fraction. We try to hit a 50% fraction in SDHW designs, indication the system should provide that portion of your DHW load, which I entered at 40 gallons, 130°F per day. The color graph shows typical production month by month.

In some areas you need to furnish a report like this to qualify for rebate and incentives.