Heat transfer fluid

[dunay]

Hello,

Does anyone know what kind of heat transfer fluid is used for large scale solar thermal systems?  I believe it is some kind of oil, but I can't find the exact type.

Thanks,

Matthew

[Chiron]

The array owned by FPL in CA http://www.fplenergy.com/portfolio/solar/index.shtml uses a low viscosity synthetic hydraulic oil. The breakdown temp is a lot higher than mineral (petroleum) based oils and no parrifin type compounds that can turn into goo at high temps. Mobil 1 or one of the other full synthetics should do about as well I'd think.

[dunay]

Thanks for the link.

Aside from needing a double walled heat exchanger for domestic hot water, why isn't oil used in all thermal systems instead of propylene glycol?  Are the heat carrying qualities for oils poor at low temperatures (such as the temperatures generated by flat plates when compared to temps generated by parabolic troughs)?

[Chiron]

One reason for using water/glycol fluid for heat exchange is cost. If the system is designed to operate between 32F and 212F pure water can be used without pressurizing the system, water is very cheap, if it leaks there's no toxic or environmental problems and it's readily available.

 Glycol mixes with water can operate at higher and lower temps but still limited to about -40F and 250F. Pure glycol can be used at a bit higher temps but tends to generate flammable vapors as it breaks down. Cost is higher, last time I put antifreeze in my car it was US$8/gallon so a 50/50 mix would be US$4/gallon. Good choice for colder climates. If spilled glycol compounds have minimal environmental effects as long as they don't get to ground water, they are all toxic to some degree but soil bacteria will break them down.

Water or a glycol mix is ideal for domestic hot water and space heating applications. Water has a high specific heat, it can store and transfer more heat per unit of weight...

Synthetic oils can be used up to 700-800F depending on the type. They usually have detergents and stabilizers added that are toxic and pose environmental hazards if spilled. They have a much lower specific heat, I don't recall exactly but it's something on the order of 1/3 that of water. The hydraulic oil we used that was similar to the Thermoil® was about US$45 /gallon, and since it takes 3 times (or more) volume to transfer the same heat the pumps, plumbing, etc. will cost a lot more also.

For heating water either for domestic hot water or heating you don't need to operate the system near the boiling point of water so using a glycol mix, or pure water if climate permits, would be a lot more practical in many ways than any oil heat transfer.

If your looking to operate a steam turbine generating electricity at 300PSI then the Oil would be a better choice but unless your doing it on a utility scale you'd be better off putting your money in PVs or go to the people who host the website and buy a ready made wind turbine.

Parabolic vs flat collectors, if your looking to heat your house or water for a shower a flat collector should do, a parabolic collector doesn't collect more heat per unit area, it concentrates the heat to produce higher temps. Flat solar heat collectors if built right can boil water on a sunny day.

Maybe tell people here what your trying to do or search the site for the info your looking for. They have practical experience with systems to power a home, I work with systems that produce enough power for a small city. The basics are the same but the execution is very different.

[ghurd]

The X:01 / 'Top of the Hour' bug has taken a victim (who planned well).

Semi on topic.  Anyone know of 12V DC motorized valves?  24VDC?

G-

[ghurd]

(It made more sense when it was double posted)

[wooferhound]

It's a Feature

not a Bug

[dunay]

Chiron,

Thanks again.  Here is my problem:

I run a solar thermal company, and we install closed loop evacuated tube systems.  So far, we have done residential sized systems, and have had great success.  However, the problem with these system, if left unattended for a long period of time, is that they can over heat.  So, on the larger systems we have installed, we typically add an over-heat loop with a heat dissipater and motorized valves (kind of an expensive solution).

In all of our systems we run propylene glycol.  I have read in some cases people up-size their pressure tanks to deal with the extra pressure from boiling their antifreeze, but I would rather find a different way.

So thats why I'm interested in the oils, for the higher heat capacity.  The cost of the oil would be cheaper then running an overheat loop.

We are starting to install larger and larger systems.  Our next project will be 120 tubes.  At an insolation level of 4, we will produce an average of 91,000btu daily.  I have seen temps well above 300F in systems that have stagnated, which would kill the propylene glycol, so I was thinking the hydraulic oil would be a better bet?  Of course, I need to up-size all my pumps, but at some point hydraulic oil should make more sense then glycol in large evacuated tube systems, right?

In large evacuated tube systems, 300, 400, 500+ tubes, what kind of heat transfer fluid is used?  Do the systems always have overheat loops?

Chiron:  Do you work for FPL, or did you work on the SEGS in the Mojave Desert?

Thanks

Matthew Dunay

[fcfcfc]

Hi: You really want to stay with water based fluids. Rather than using the motorized valves and heat loop which I assume is some kind of ground or air based HX, you might want to try this. It costs less and works better. Just get a $10 snap disk in the temp differential that you want as your top collector output temp.. For the purposes of example, say 200-210DegF. Get a Red Hat HW solenoid valve and place in series with a standard SS ball valve and put it on the DHW hot out in the mech room. Pipe the output of the valve over to the PT relief valve dump pipe which hopefully goes outside, connect 110VAC (or valve coil voltage) and your done. Even if the person is on city water, the extra water cost for the few overheat times is nothing. The front end cost is low because piping runs are 3'or so since there is usually a hot out DHW pipe near the backup water heater (do you think!), snap disk is nothing and the only money spent is on the Red Hat. You can cut the flow down with the SS ball valve so it doesn't impact fixture pressure when open, etc, etc.. This is far better than trying the oil route in which the cure costs more than the problem. AT incoming 50 DegF and DHW tank 200 DegF, 40 gallons of DHW dump will cool 48,000Btu's. On a 200,000 Btu system that's almost 25% of your heat. You even get the added bonus of pulling some of that nice 200DegF water into the backup water heater to relieve the conventional heater's standing losses do to no system load. The savings there will offset the water cost easily. Piece of cake... cheap and effective..

.....Bill www.WeAreSolar.com

[healerenergy]

In a popular mechanics issue that I read a very long time ago they talked about using cotton seed oil because it can handle temps of around 600 degrees and more if in a pressurized system. How well cotton seed oil stands up without breaking down is one thing I have no idea about.  But it should be something to look into.

George  

[g reif]

are there any good sites to get the snap disk switches from?

[fcfcfc]

Hi: Lots..you can start with WW Grainger and Google the web for others...

[Chiron]

I see there's already been some suggestions about how to deal with your problem..

If you wanted to switch to an oil based fluid running in the 300-400F range than plain old transmission fluid since it's designed for heat transfer anyway. I forget if it's Dextron or the "Ford" type thats thinner (Mercon?). Maybe build a small experimental system, let it overheat and see what happens. Are you relying on grid power for pumps, etc.? Why not install PVs to run the pumps as long as your going solar, as long as you got Sun, you got pumps, no sun, no overheating problems...... If the transmission fluid breaks down you might try petroleum based hydraulic oil, it has a higher breakdown temp and is lower viscosity. If that fails try Mobil 1® 0W50... Experiment

I used to work for FPL at a wind farm in MN, http://www.fplenergy.com/portfolio/contents/lake_benton.shtml  Note, the old waterpumper in the pic was still in operation as of last August and you can see my ex in the lower right corner

Right now I work part time for a company that services small windfarms and independent turbine owners, I'm their "electronics wizard" and do checkouts/optimizations on new turbines and fix stuff the wind techs. can't handle. Don't ask for a link, they don't have a website....

Solar I've played with in the past and wired a solar house, PV and heating panels, as far back as '93.

HTH

[dunay]

I like the idea about dumping hot water when my storage reaches its max temperature.  I have considered this option before, but for some it seems to me this would not pass plumbing code?  Does anyone know if this is a common solution?  On very large systems, is this setup used to regulate storage tank temperatures?

Right now, I'm designing a solar thermal system for a restaurant.  They have very regular DHW usage patterns.  With solar hot water, you are always trying to match the collector size to the storage size to the usage patterns.  Because water storage is expensive, I think it make the most sense to have a large collector array and small storage.  Everyday, the restaurant uses a lot of hot water, so large water storage is not important.  However, if the restaurant were to close for a long weekend during the summer, overheating would likely occur.  

I am using grid power for my pumps.  As of right now, I'm more worried about overheating occurring due to lack of storage, rather then the power going out.

I think using an overheat loop, or dumping DHW down a drain are both good ideas, I'm just wondering what other solutions are out there to regulate temperatures on large solar thermal systems.

A drain back system would be another option, but I don't think anyone uses that type of system on a large scale.

Thanks for the input.

[elvin1949]

g reif

 Try this

http://www.strombergschickens.com

 I have the 2006 catalog. Page 8 top left

of the page.$10.50       22 amp micro snap switch.

later

Elvin

[fcfcfc]

Hi Dunhay: I tend to be a rather blunt person with the way I say things, so I am pondering how to say the following the right way. I think it might help to step back and think about what is happening during the whole day cycle at the restaurant and why storage is used in the first place with solar thermal. Lets say we have a test system with an array matched to the load based on a PSD at the restaurant but ZERO storage. To keep this text somewhat short, the end result would be an extreme up and down temperature output from the array as the system was loaded and not loaded. So, (#1) storage by nature changes an un even system load to an even system load by "holding" energy until it is needed. It also allows peak energy load to exceed peak generation for short periods which would other wise not be possible. Second (#2) and perhaps more importantly with solar, storage allows energy to be available on days where there is no sun. You mention in your response that, "Everyday, the restaurant uses a lot of hot water, so large water storage is not important.". Saying, I will design the array size based on the usage per day and have just enough storage to keep the temps "smooth" throughout the day (reason for storage #1) is great but based on what energy input? A perfect solar day, a ½ PSD, a 1/3 PSD. I believe you are in VA, so PSD's 90% of the time like Las Vegas are not in the cards. If you say I will size based on 1 PSD = load, you will end up with a dismal solar fraction. So to get the customer a decent solar fraction, you have to size two to three times that, ergo storage. The rest should be self evident...

Drain backs are great on any system size but using that as a control to keep from overheating the storage is not such a good idea especially with evacs because it abuses your collectors. Flat plates not so bad due to the lower stag temps..

As far as dumping hot for heat control, you could always just attach to the normal DHW drains, instead of the PT relief pipe, which get all water temps anyway, so what's the code problem? Does not seem to be much of an issue there....