Flat solar thermal panels or evacuated tube?

[Jbergess]

Flat Plates vs. Evacuated Tubes

There's a lot of good information contained in this thread. I work for a manufacture where we distribute both flat plates and evacuated tubes.

It all comes down to value - which system produces the most in your area for what you spend. However, there are a few general pros and cons for each technology to consider.

Evacuated tubes produce more on cloudy / overcast days than flat plates. They also produce higher internal temperatures (our manifolds generally stay around 160-170F during the day), and are easier to maintain.

The internal temperatures will not burn anyone or cause a hazard - the heat is inside the glass tubes, and the vacuum insulation makes the outside the same temperature as it is outside.

If damage occurs - such as in a storm - a tube can be replaced easily and with minimal expense, and without draining the system or messing with any piping or components. Most manufactures will pack in a few extra spare tubes with each collector - however, replacement tubes cost around $25/each.

Evacuated tubes are also modular - they come in separate pieces that are assembled on the roof. Therefore, it's generally easier to install from a labor standpoint.

The cons, are that evacuated tubes are generally about 20% more expensive than their flat plate counterparts. They also can get very hot - and generate too much heat, so a form of heat dissipation is needed, especially in the summer months. This adds about 10% extra cost in components.

In a closed loop system (or a drain back), this is the only real difference in cost versus a flat plate. Flat plates have the same general components - controls, tanks, pumps, glycol, etc - other than dissipation and the valve that controls them.

The real point to consider is it it worth the extra $300 to $500 in system costs for an evacuated tube versus a flat plate for what you receive in return?

The answer to the question is that it depends on really where you live. In warmer climates where the outside air temperature stays fairly warm (70F+), and the weather is fairly consistently sunny, the flat plates can be a better value overall, as they will perform as well as the evacuated tubes. So in areas like Florida, Texas, Arizona, etc, most contractors and homeowners tend to opt for flat plates.

In cooler and/or cloudy climates, the evacuated tubes out perform the flat plates. So if your temperature in the winter and shoulder months stays 50F or lower, or you have rainy/cloudy seasons, most contractors and homeowners tend to opt for evacuated tubes.

The other thing to keep in mind and consider is that some flat plates can be very inexpensive and cheaply made - and some evacuated tubes can be very expensively priced. I have seen a flat plate sell for $450 or so, and an evacuated tube collector sell for $3500. Therefore, at times, when people compare costs, they run across prices like these and get an idea that tubes are vastly more expensive.

My recommendation would be to shop around and ensure you get the best value for your money. Look at the SRCC certificates and see what kind of production you get for the price you pay, and make sure you get nothing less than a 10 year warranty on either technology.

Our flat plates generally sell to a contractor for around $950 or so and the evacuated tubes that we manufacture in the states generally sell for around $1250 or so - and other manufactures will generally be around the same price or so.

Hope this information helps!

[PSS]

Don't forget about the maintenance and overheating

Before purchasing either the Evacuated Tubes or Flat Panels, you should consider the costs for system maintenance and any overheating concerns especially with closed loop glycol systems. Flat panels are a better value for the money but they are also a lot easier on the glycol during stagnation and the flat panel system can last up to 10 times longer on the glycol.

System maintenance and repair can distroy any chance of payback if you do not have the right system.

[Eherst98]

Flat solar heating panels - Have had them for 10 years

Hi All
I put 3 flat solar hot air panels on the south side of my home at least 10 years ago. I live in the northern lower part of Michigan.
They have worked well and work good with snow on the ground.
There is a fan to blow the hot air in too my basement.
In the beginning I figured that the 3 panels would carry 30 % of my heating load. That is when the sun was out. I just foget they are there and only when the fan comes on do I think of them. Some what care free.
I hope to get my solar electric panels some what care free too.
Ernie

[russ]

Perfect Solution Mountain!

Your temperature loss problem can be solved by insulation.

The quantity of water you need to recirculate you are not going to get from solar panels unless you have a lot of them.

Vacuum tube only has the capability to produce hotter water - not more total heat. Your location - winter time you are never going to get 200 liters per panel per day from solar panels.

[mountain]

It's cold enough you can not be bare-foot during the winter. I have tried everything I know to eliminate the cold floor.

http://www.mec.ca/Products/product_detail.jsp?FOLDER%3C%3Efolder_id=2534374302734361&PRODUCT%3C%3Eprd_id=845524442619876&bmUID=1261700478477

made in an ethical sweat-shop

[netttech]

Sundug, that was my original plan. Install Pex tubing under the floor, pump solar heated water thru it. My panel design (flat) just couldn't heat water hot/fast enough.

It could heat water to 150 degrees (in about 10-15 minutes), but could only muster 90-100 sustained pumping 1 gpm thru it. A loss in heat will occur between the panel to the pex tubing. That's why I was hoping to get 130 water leaving the panel. My house to panel location is about 15' apart so it will lose heat. If the panel can only muster 90-100 hot water @ the panel, the water temp would surely be 80 (or less) by the time it enter the pex.

I doubt if any benefit would occur with water only in the 80's range.

I haven't 'totally' given up on solar water. I just don't expect to get it working for this winter. I can easily open a duct-vent to my crawl space to eliminate the cold floor, then close it 'IF' I can get a solar water panel to perform.

I have been avoiding heating the crawl space because it's also where I store my wine collection. The wine needs the natural 50-65 temp range (winter/summer) to ensure it doesn't go bad. It's a 'catch-22' isn't it.

It's one of the reasons I joined this forum, to get ideas.
Thanks

[sundug]

Mtmtntop is using the same idea I am working towards. Solar water during the day, boiler in the evening. I am trying to convert an old wood/coal buring unit into a boiler to supply hot water at night (still at it).
Besides trying to lower my winter heating expense as much as possible, I am also trying to correct a cold floor on the lower level on my house. It's a bi-level design & the floor of the main level is 'cold'. It is over a crawl space (4' tall) but the floor temp is about 55 degrees (regardless of room temp).

It's cold enough you can not be bare-foot during the winter. I have tried everything I know to eliminate the cold floor. The floor is insulated with 6" of insulation. I have done extensive sealing/insulating in the crawl space.

I have considered installing an air-duct from my furnace in the crawl space to heat the space (700 sq ft) to eliminate the cold floor. However, that doesn't lower my overall heating bills....thus my concentration on Solar thermal.

I may have to concede the solar idea & open a furnace ductwork, until I can get the wood-burner/boiler to work.

Thanks for the input.
Jeff

Pull the insulation down, and tack up PEX with aluminum heat spreaders, reinstall insulation, pump solar heated water thru. This will put heat up into the room thru the floor, and not heat the crawl space.

[netttech]

Mtmtntop is using the same idea I am working towards. Solar water during the day, boiler in the evening. I am trying to convert an old wood/coal buring unit into a boiler to supply hot water at night (still at it).
Besides trying to lower my winter heating expense as much as possible, I am also trying to correct a cold floor on the lower level on my house. It's a bi-level design & the floor of the main level is 'cold'. It is over a crawl space (4' tall) but the floor temp is about 55 degrees (regardless of room temp).

It's cold enough you can not be bare-foot during the winter. I have tried everything I know to eliminate the cold floor. The floor is insulated with 6" of insulation. I have done extensive sealing/insulating in the crawl space.

I have considered installing an air-duct from my furnace in the crawl space to heat the space (700 sq ft) to eliminate the cold floor. However, that doesn't lower my overall heating bills....thus my concentration on Solar thermal.

I may have to concede the solar idea & open a furnace ductwork, until I can get the wood-burner/boiler to work.

Thanks for the input.
Jeff

[russ]

Hydronic heat temp

İ am using an air source heat pump (air to water) to supply the hydronic 'in floor' heating system. İt is a 5 ton unit and produces 3.5 m3/hour of water at set point - right now it is set at 30 deg C min & 35 deg C max.

To supply that quantity of hot water would take more solar panels than İ have roof and this is a big house! İ have 3 Schuco collectors - Flat plate panels (good ones) may make roughly 200 liters per day - summer time very hot and winter time İ have electric backup come on at 50 deg C.

İ didn't find a solar thermal connection to the hydronic 'in floor' heat system of interest.

[sundug]

Many in the two best solar heating groups I know of use under floor radiant tubing at temps down to near room temperature, but that's when it's circulating from a solar heated storage tank. Join- solarheat and simplysolar Yahoo groups

[mtmtntop]

re evacuated tubes and radiant floor and my prior post. my 75 tube array shares a boiler loop with a backup boiler. on this loop is a radiant loop for 450 sq ft, a heat exchanger for a 2700 gallon endless pool (indoors) and heat exchanger for dhw. the tubes run during the day. the boiler runs after the sun sets to catch up if there is a need. I only let the pool take boiler heat for an hour max via a wiring setup and relay i have with the pool circ pump.

the boiler turns off at 8am and cools to prevent thermal shock from the tubes that start circulating about 10 am this time of year.

the solar gain(south facing, mostly glass) in this room along with the floor heat keep this room at 66. i have turned off the boiler on a really cold night (-20) to see what happened and the room temp fell to 59 so the floor holds alot of heat .(The floor has a 2" "mudpack" base under ceramic tile.)

i am running glycol as this is a closed system, not a drainback.

once the loop is warmed, even now when the sun is low the panel loop will continuously run about 120-125, and heats the pool and the floor as long as they are calling for heat.

although i have run the array at 170, it does not run that hot continuously except in the long days of the summer. I find that i can harvest the most heat running it with a differential setpoint controller set at 135/115.

i am still working on the dhw as i have a standalone hw heater for now. it has been problematic getting the aquastat to call correctly as there is no port for it on the conventional tanks. this may have to wait for the how water heater to be replaced.

finally, i have my array at 70 degrees to maximize the low winter sun. the array runs less efficiently in the summer. I rarely dump heat even in the summer with this setup.

[mountain]

Interesting. As mentioned before my attempt at solar water heating failed (copper in a flat panel). I'm wondering...you mention flowing glycol @120. Are you using the heated anti-freeze for heat or just hot water? Curious about the GPM of heated water (AV).

I'm wanting to install Pex radiant heat, but using solar to heat it. The array would need to sustain a constant 110-130 temp while pump is running.

I'm wanting constant pumping of hot water (AV) for the 6 hours of good sunlight. WHen the fluid heats to 120, pump comes on, stays on as long as sun shines.

Do you think this type (evacuates tubes) can produce that much hot water?

I'll be looking into it.

Thanks Jeff

Hello Jeff

Here is another way to look at it.
The ultimate power you can harvest from any type of panel, before losses, is approx 950W/m2, or about 88W per sq ft of collector area. Useful winter sun ~6 Hrs so ultimate daily energy harvested = 6 Hrs x 88W = 528 WH per sq ft

In Canada, typical winter heat requirement for an average dwelling averages out around 2.5 W/sq-ft, so I'm thinking that in Illinois, a reasonably well insulated house could maybe get by with 1.5W/sq ft. So a 1000 sq ft house would require 1500W x 24Hrs = 36,000 WH per day, for heat.

36,000 WH / 528 WH per sq-ft = 68.2 sq ft of solar-thermal collector panel area, per 1000 sq-ft floor-space.

This is your minimum panel area, for any solar energy harvesting system before losses, and with every day a blue-bird day.
Of course any system has less than 100% efficiency, so you must factor that in. My direct-air panel has a measured efficiency of ~74%, so, using that number, 68.2 / .74 = 92 sq-ft of collector per 1000 ft floor-space. Round that up to 100 & you should be good.

The argument over flat-panel vs evacuated-tube vs direct-air is 100% irrelevant. The sun simply does not care... 950W/sq-m is all we get... period.
The best system is simply the one with the lowest overall losses & the quickest payback. The efficiency differences between solar-thermal panel types is minimal, and more directly related to common-sense, understanding the losses (& mitigating) & care in construction.

Vacuum tube elements (expensive) have low radiation losses in themselves, but a well insulated collector box has the same effect, perhaps for lower overall cost. Removing the harvested energy to house-inside quickly is key in minimizing radiation loss.

Vacuum tube collectors have their real place at the focus of parabolic troughs. Google "parabolic trough".

Heating the air in the space below a floor (if there is such a space) has the same net effect as directly heating the floor itself. My basement has pipes & things that must not freeze, so the basement needs heat anyways. Our main-floor surface temperature is exactly the same as the basement air below. I chose a direct-air collector, because my mission was to solar-contribute to heating my basement air, & thus the house.

M

[russ]

How many liters per hour (GPM) are you talking about? The panels (flat or vac tube) have very limited output.

İ have been interested in this due to having radiant floor heat installed and have tried my best to figure out how to make it work at a sensible cost with no success.

The 110 to 130 degree temp will limit the output of the panels even farther. They work best at the lowest temperature you can use. My floor is designed for 35 deg (95F) water temperature.

Typically you use glycol in a pressurized system - especially if there are any freeze concerns.

[netttech]

Interesting. As mentioned before my attempt at solar water heating failed (copper in a flat panel). I'm wondering...you mention flowing glycol @120. Are you using the heated anti-freeze for heat or just hot water? Curious about the GPM of heated water (AV).

I'm wanting to install Pex radiant heat, but using solar to heat it. The array would need to sustain a constant 110-130 temp while pump is running.

I'm wanting constant pumping of hot water (AV) for the 6 hours of good sunlight. WHen the fluid heats to 120, pump comes on, stays on as long as sun shines.

Do you think this type (evacuates tubes) can produce that much hot water?

I'll be looking into it.

Thanks Jeff

[mtmtntop]

2 things to think about
re snow on tubes, consider your latitude. the farther north, the higher the pitch of the array. i live at 46 degrees but have my array at 70 degrees to maximize the low winter sun. i can assure you no snow stays on the tubes at that angle. they are not as efficient in the summer, but i have alot more daylight for them to work in. i made my rack with the angle adjustable but i have not seen the need to change the angle.

if it is cold alot where you are, tubes may be better. mine were happily flowing 125 degree glycol when it was 20 below last week. they also work in cloudy weather, though not as good as when the sun is shining, of course.

these tubes will run alot hotter if you need that. i have had my array running at 170 but at the moment they are at 135.

other posts were correct. these are tubes inside of tubes, thus the vacuum. they are air temperature to the touch.

i would never put these at ground level only on a roof where errant rocks such as from a lawnmower could find them.