Solar VO heating

Collecting, filtering and dewatering of WVO SVO vegetable oil. For Biodiesel producers too.

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Solar VO heating

Postby WyoSam » Sat Mar 08, 2008 11:09 am

I'm looking at moving my vegoil cleaning and storage into a large calving barn. This change has several challenges, one large one is that there is no electricity. I have a diesel generator, and am looking at doing a lister conversion to VO, regardless, this changes the cost of production via electricity significantly from the very low cost of production right now.

One of the options is to employ a solar heating system to use from the post collection stages up to the centrifuge cleaning. I know SunWizard has used solar, and anyone else please feel free to pass on your knowledge.

Is it best to set up a passive, hot box containing black barrels arrangement, or passive convection through black pipe in a collector, or actively pump the vegoil through black pipe, or heat a propylene glycol mixture and pump that through the oil? What kind of temps can I expect out of each system? How much collector space, sq foot, per gallon of oil?

Sam
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Postby John Galt » Sat Mar 08, 2008 12:48 pm

I would go with the passive hot box with barrels against the south side of the building, augmented by a coil of 1.5" black poly 'pipe' on the building roof. The coil can be plumbed to circulate passively or perhaps with a pump on very hot days.
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Postby SunWizard » Sat Mar 08, 2008 4:43 pm

Passive is the cheapest and simplest. But it probably won't get it hot enough (160F) for optimum CF action. So I agree with John, a combo of passive pre-heat and then using the CF pump to run through an on the roof active collector while you CF.

A coil on the roof won't give much gain unless its big and sealed behind glass. Here is a rough way to figure it, I used a 3x8' concentrating collector on my CF, and was getting about a 20F gain with a 2gpm flow rate through it. This is about what you need for a CF. There's a pic of it in my dieselcraft thread.

To compare, sunlight is 1000watts per square meter, and then cut it in half for conversion efficiency. So my 3x8' panel is about equal or slightly more than 1 of my 1125w electric heating elements.
YVORMV - Your veg. oil results may vary.
95 Dodge Cummins 4x4 SVO WVO conversion.
81 Mercedes 300D- stock and happy on V80/D20 blend.
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Postby WyoSam » Sat Mar 08, 2008 10:17 pm

Thanks to both of you.

I'm figuring to use my electric heat while the CF is running, since after all, I'll have the electric pump needing juice anyway. But otherwise I could make good use of passive solar, especially since I have oil sitting during several day periods.

In a hot box, how much is there to gain using flat sided tanks, as opposed to curved barrels?

So could I figure 500 watts/hour/sq. meter from about 10 am to 2 pm. Then figuring some heat lost during the rest of the day and night it would 4-5 days to bring 100 gallons to 120 degrees?

Sam
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Postby SunWizard » Sun Mar 09, 2008 12:12 am

The answers to both questions depend entirely on how well you insulate it, and how cold it is outside. You will get large heat loss at night and cloudy days through the glass unless you have an insulated door or blanket you can close.

An advantage of an active panel on the roof, if you design it right any VO you had circulating there could drain back into your insulated tank at night avoiding the large loss at night. My house uses a combo passive/ active drain back system with a delta-temp thermostat that automatically shuts off the solar pump whenever the temp at the top of the panels is not higher than the storage tank. Then all the water in the panels drains back. Probably too much expense for your VO setup, I think those controls are about $200 including the 2 sensors.

Another option if you can put the panel below your tank, you can use thermo-siphon action, the hot VO rises, no pump needed. But you need a special low restriction check valve to prevent the opposite flow at night.
YVORMV - Your veg. oil results may vary.
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81 Mercedes 300D- stock and happy on V80/D20 blend.
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Postby canolafunola » Sun Mar 09, 2008 11:54 am

I have heard people painting white poly 55 gal barels to improve on solar gain. Paint don't stick too well to polyethelene so I've never tried it. I use 55 gal black garbage bags that can be stretched tightly over the barrels and it helps in the solar gain.
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Postby Burbarian » Sun Mar 09, 2008 2:12 pm

Quick and easy flat plate solar collector:
Two metal roofing panels stacked on top of each other with a 1/4" gap and the edges sealed with high temperature RTV forming an inch-wide sealing strip around the perimeter. Foam board insulation behind, greenhouse corrugated polycarbonate in front with a 1" static air gap. Paint the corrugated metal roofing panel behind the polycarbonate a flat black. Fittings connect from behind, upper left to lower right. Use large diameter low restriction insulated piping to provide good flow through the collector. Restricted flow causes overheating and could cause the greenhouse polycarb to sag.

Thermosiphon:
If both the inlet and outlet fittings of the panel are a few feet below the storage tank then - solar gain heats the water in collector which rises, exits from the upper left hand fitting, goes up the hose to the top of the insulated storage tank. This displaces cooler water from the bottom of the stratified tank which goes down and enters the lower right of the solar collector to be reheated. This automatically regulates thermal flow, without the need for any additional controls. Connecting the collector hot side (top fitting) to the top of the tank provides for highest temperature gain as it stratifies the water in the tank. Hot water enters from the top. However, like Sun says, you will need low restriction check valves, unless you have sufficient vertical displacement. I've found that having the top fitting of the collector 10ft below the bottom fitting of the tank is adequate to stop reverse thermosiphon. As the collector temperature drops below ambient, the heat exchange fluid becomes denser in the collector than at any point within storage and will stop flow.
Connecting the collector hot side to the bottom of the tank will cause storage medium agitation via convection. It will not produce as much gain but with the tank fittings reversed, you will not need the low flow check valves even when the collector top fitting is at the same level as the tank lower fitting. Circulation will also cease if the tank temperature approaches the collector temperature.
For oil heating you could have a spiral coil of tubing in the insulated oil tank and have the heated transfer medium circulate through that tubing instead, transferring its heat to the oil.

Using solar collectors as chillers:
A more efficient use of your solar collectors is to use them as radiant bi-level exchangers.
Nighttime - Switch the lines via a pair of 3-way valves so that the collector is now connected to an insulated storage tank below it. With a clear night sky, the collector now becomes a thermal radiator, and the thermosiphon loop runs in reverse, chilling the lower storage tank to well below ambient. You can use this to discharge thermal capacitors (drums of brine solution) in a cold storage (root cellar/meat locker), which then slowly chill their surroundings. This is particularly handy in the summer. Free refrigeration and possibly air conditioning depending on your storage mass. For air conditioning just run a fan to blow through a radiator in the living quarters with the radiator fed through a small water pump and connected to a loop of tubing in the cold storage. If you're not storing slaughtered deer in the cold storage, you should be fine for tapping into it.

With adequate spacing you can even forego the 3-way valves.
Properly engineered with large diameter low restriction insulated pipes between the hot and cold storage and the radiant exchanger, the system works in a passive manner requiring no external controls, no sensors, pumps or anything else. Solar energy hitting the collectors during the daytime heats the upper level hot storage, and night time radiant emission chills the cold storage. Radiant chilling works with flat plate heat exchangers, but not with solar concentrators due to lack of radiant surface area.

Sun: This may probably go into an off-topic section.
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Postby WyoSam » Sun Mar 09, 2008 3:52 pm

Burbarian wrote:For oil heating you could have a spiral coil of tubing in the insulated oil tank and have the heated transfer medium circulate through that tubing instead, transferring its heat to the oil.


Is there a problem with heating the vegoil directly in the spiral coil tubing? Why would water or "heated transfer medium" be needed at all?

Thanks,

Sam
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Postby SunWizard » Sun Mar 09, 2008 5:22 pm

WyoSam wrote:Why would water or "heated transfer medium" be needed at all?

Its not needed and would make the heat transfer less efficient. And it makes it more complex.
YVORMV - Your veg. oil results may vary.
95 Dodge Cummins 4x4 SVO WVO conversion.
81 Mercedes 300D- stock and happy on V80/D20 blend.
Low fossil net zero house- 100% solar power and heat.
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Postby Burbarian » Sun Mar 09, 2008 5:47 pm

Agreed. For a dedicated oil heating system, water is not needed. If you want the flexibility to be able to switch heat sinks with simple valves, like redirecting the heat to your domestic hot water tank, then water is useful. Depends on what you want to do, how large your collector is, how sunny it is, and how much oil you want to process.

Note that efficiency is a product of temperature difference and flow rate. The cooler the collector, the more efficient it is at collecting heat. Water is a better thermal conductor and much less viscous than oil, certainly cold water is far less viscous than cold oil and will flow faster. At any given pipe sizing, the heated water will flow faster, dump the heat sooner into the cold oil, and go around for another pass. You don't want the fluid to necessarily get hot in the collector so much as to absorb the most heat to transfer to the insulated tank. Warmed water at a faster circulation rate transfers more heat energy overall at a better efficiency as opposed to hotter water with a slower circulation rate going through the same collector. Glycol/water as thermal transfer fluid will also get your heating up and running much sooner without danger of plugging from thick or solidified oil in your pipes. This will prevent collector overheating and possible pressure rupturing. Lastly, if the purpose is to heat WVO for a centrifuge, then by definition the oil is filthy and/contaminated. Not the best thing to have circulating in your solar heater's plates and pipes. Better to have a clean sealed easy flowing thermal transfer fluidic circuit.
Last edited by Burbarian on Mon Mar 10, 2008 4:36 am, edited 1 time in total.
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Postby Johnno » Mon Mar 10, 2008 2:32 am

SunWizard wrote:
WyoSam wrote:Why would water or "heated transfer medium" be needed at all?

Its not needed and would make the heat transfer less efficient. And it makes it more complex.


Hi Sunwiz, congratulations on the new forum, I like the layout,
But to get back on topic here is a photo of my solar heated Vege oil setup. No pump needed but panels are on the ground as you can see. http://static.flickr.com/105/260565713_0f8f155daa_m.jpg
I use water in the panels, cause if I used oil it would be just one more oil spill.
Not complex either :)

Johnno
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Postby SunWizard » Mon Mar 10, 2008 9:04 am

Good points about efficiency, if you were running a thermo-siphon system is where that would apply most. I don't think you could ever rupture a panel since veg doesn't boil. It would melt any solid VO in the pipes after the veg in the panels got hot due to heat conductance along the VO in the pipes. But a blockage would cause there to be no thermo-siphon action while it slowly melted the block.
YVORMV - Your veg. oil results may vary.
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81 Mercedes 300D- stock and happy on V80/D20 blend.
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Postby WyoSam » Mon Mar 10, 2008 9:45 am

I'm thinking of doing a combination of hot box and thermo siphon panel with one inch black pipe wrapping horizontally through the panel. What would be the minimum spacing between one inch pipe in a thermo siphon system?

Ideally, this stage of vegoil will be primarily settling and slow particle separation through screen gates into different barrels as the oil warms enough to pass. I'll then pump into my CF barrel with the electric element for final cleaning.

I'm still in the concept stage for all this, so things can change, likely to make for fun summer project.

Sam
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Postby SunWizard » Mon Mar 10, 2008 10:07 am

In order for to thermo to work, they need to have no "heat traps" where the heat can't easily flow upwards. Horizontal could trap. Its best to have a header at the bottom and top, and vertical pipes. And have the pipes attached to a black absorbing metal. My commercially made panels have 6" spaced vertical 1/2" copper pipes, soldered to a black copper absorber that also wraps around each pipe about half way. Like a pipe that fits tight behind an inverted U shape.

You can find used commercial panels for cheap in CO. Often cheaper than the scrap value for the copper and aluminum. And they are made with special low iron tempered glass with better heat gain qualities.
YVORMV - Your veg. oil results may vary.
95 Dodge Cummins 4x4 SVO WVO conversion.
81 Mercedes 300D- stock and happy on V80/D20 blend.
Low fossil net zero house- 100% solar power and heat.
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Postby Burbarian » Mon Mar 10, 2008 11:04 pm

SunWizard wrote:I don't think you could ever rupture a panel since veg doesn't boil. It would melt any solid VO in the pipes after the veg in the panels got hot due to heat conductance along the VO in the pipes. But a blockage would cause there to be no thermo-siphon action while it slowly melted the block.


I'm worried more about thermal expansion. For the oil to flow by thermosiphon, it must by needs expand when heated, which reduces its density and allows the thermosiphon to work. However if the outlets are plugged, there will be no room to expand. Pressure exerted by thermal expansion of a volumetrically constrained incompressible fluid can easily reach damaging levels. If the magnitude of forces does not cause an immediate rupture, it will still subject the joints and seals to daily cyclic tension forces, which will accelerate eventual failure.
Paradoxically, if there is significant gas entrainment such as from a small amount of boil-off prior to shutdown, this would provide an air space for which the expanding oil could occupy. As the oil expanded it would collapse the bubbles, compressing them and hence reducing hydrostatic pressure.
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