Flat solar thermal panels or evacuated tube?

[MikeSolar]

Bob Nape, a solar installer in Ithaca NY, decided that the theoretical arguments were't satisfying and put up side-by-side systems. You can see the ongoing data comparisons and historical behavior at http://goo.gl/EMMM9. Here's a link to the previous 12 months of data of thermal output from the systems: http://goo.gl/2ZfX1. It shows the flat panel system producing slightly more BTUs in every month. The difference is rather small.

Will you get the same results? Maybe, maybe not -- depends on the particulars of your system and the particulars of your climate. Given that I'm in a similar climate (in mid-Vermont), this is convincing evidence that I should go with a flat panel system and plow my savings into other energy saving or generating efforts.

This is good data but what needs to be added to it is consumption data. It would be nice to know how much and at what time the water is drawn out.

Oops...I spoke too soon. The graph exists for consumption although I cannot tell if it is for both systems or just one.

[jtellerelsberg]

This is good data but what needs to be added to it is consumption data. It would be nice to know how much and at what time the water is drawn out.

Oops...I spoke too soon. The graph exists for consumption although I cannot tell if it is for both systems or just one.

Mike, how does it help to know the consumption data?
Jonathan

[russ]

Mike, how does it help to know the consumption data?
Jonathan

It shows the total amount of heat collected

[cssolar]

100% agreed Tony! Same for solar air heating.

Hi Russ,

Ive been busy building my solar air heater.

Or maybe I should say I have been busy drinking beer so I can use the empty tins for the collector.

Its been hard work, but I keep telling my self it all for a worth while cause.

Reminds me better have another beer.

Cheers

Tony

[russ]

Hi Russ, I have been busy drinking beer so I can use the empty tins for the collector.

Its been hard work, but I keep telling my self it all for a worth while cause.

Reminds me better have another beer. Tony

Salude Tony - Be sure to enjoy while working hard!

[MikeSolar]

Mike, how does it help to know the consumption data?
Jonathan

The system may be capable of providing X amount of heat but its efficiency is best when the panels run cooler. There is an international standard for measuring the effectiveness of a system and it includes X volume draw over a period of time in the morning plus X in the evening and the testing also notes losses between the panels and the tank, HX and pumping power (I am looking for the actual numbers but so far I have found my test requirements for panels only). But.... If you look at some test data like this one:

SHSF154ST5en.pdf

You can see that the performance changes not only for each city but the daily consumption. As you use more water, the efficiency and ROI for the system gets better. The efficiency of the flat panel will drop off when we try to heat to a high temp such as 100C where the vacuum tube efficiency will still be reasonable at that temp (for the same flow rate). As we usually heat to between 30-60C most of the time, the added efficiency of the tube is not needed.

[rainworm]

pressurised vx non pressurised

Ok tubes seems better for around freezing point in winter and hot in summer.
But there are pressurised and non pressurised systems.
??? Why????
What is the reason for pressurise?

[Naptown]

Ok tubes seems better for around freezing point in winter and hot in summer.
But there are pressurised and non pressurised systems.
??? Why????
What is the reason for pressurise?

What you are referring to is an open or closed loop. An open loop would be as in a drain back system. This will provide freeze protection as the water used to heat is drained out of the collectors when the system shuts down. Generally it is an open tank and a pump that pumps water from the drain back tank to the collectors back to a heat exchanger and then back to the tank. If the collectors are very high above the tank it will take a very large pump to pump the water up to the collectors.

the alternative is a pressurized close loop where anti freeze is used for freeze collection. This simplifies things a bit and allows for a much smaller pump. When a pressurized system no matter how high the collectors are above the tank or pump it is the same as if they are at the same level as the tank. there is no additional effort to pump the fluid to a higher level.

[rainworm]

confused

Ok now I am truly confused. I must apologise I am truly new to this tube invention.
I have a watertank. It runs on electricity from the grid. It is very expensive to use. I want to add to this a solar system. My watertank has not got any extra inlets or outlets. The solar goes maximum 1.5 metres above the tank. Here it can only be slight winter. Almost or just freezing. But very windy. Enough about the weather and chit chat. I read from a manufacturer the following.
Water is pure and drinkable
Heat insulation of water tanks adapts polyurethane foam
Silicone rings to connect and seal them
Tubes and supports are adjustable
Power bearing average and can assure vacuum tubes to use for long time
Whole glass vacuum tubes adapts super hard borax and silica glass
Absorption rate: 93%
Thermal radiation rate: 6% (100)
Resists MM hail
Lifespan: 15 years
Temperature when exposed to sun without water: 250°C
2.5 hours to boiling if using single tube
Used in electrical appliances to aid heating and switch off power supply before bathing
So I think there is tubes with own water system and then there is a collector with spiral that heats up from the tubes. The collector has water flowing through it and the spiral gives the heat to the water. Now this suplier states that water can be drinkable. ???
MayI ask advise to explain in easy words what it is I should go and look for on the market?
Thank you in advance.

[MikeSolar]

Ok now I am truly confused. I must apologise I am truly new to this tube invention.
I have a watertank. It runs on electricity from the grid. It is very expensive to use. I want to add to this a solar system. My watertank has not got any extra inlets or outlets. The solar goes maximum 1.5 metres above the tank. Here it can only be slight winter. Almost or just freezing. But very windy. Enough about the weather and chit chat. I read from a manufacturer the following.
Water is pure and drinkable
Heat insulation of water tanks adapts polyurethane foam
Silicone rings to connect and seal them
Tubes and supports are adjustable
Power bearing average and can assure vacuum tubes to use for long time
Whole glass vacuum tubes adapts super hard borax and silica glass
Absorption rate: 93%
Thermal radiation rate: 6% (100)
Resists MM hail
Lifespan: 15 years
Temperature when exposed to sun without water: 250°C
2.5 hours to boiling if using single tube
Used in electrical appliances to aid heating and switch off power supply before bathing
So I think there is tubes with own water system and then there is a collector with spiral that heats up from the tubes. The collector has water flowing through it and the spiral gives the heat to the water. Now this suplier states that water can be drinkable. ???
MayI ask advise to explain in easy words what it is I should go and look for on the market?
Thank you in advance.

If you are at all familiar with flat hot water collectors, there is no difference in how the systems are piped and the specs you posted above are a very small part of the story so I would not focus on them.

First, a solar system to add to your electric tank would include another tank that is used to store the solar hot water (SDHW), no matter what type of panel you use. The cold water from the street goes into the tank,, gets heated by the solar, and then goes to your electric tank. This is the simplest way to explain that part. The rest is a bit more complicated.

Forget what the manufacturer says above. Why would any one want to drink the hot water from the solar is beyond me, boil water if you want it hot, solar is for showers and possibly heating etc. ANY solar panel will give you warmer temperatures when the sun is out, you will not see 250C in your tank (nor would you want to). A properly sized solar system will give you around 50-60% of your hot water annually, most of it in the spring/summer/fall and it doesn't matter much if it is a vacuum tube or a flat panel. A flat panel system will have a longer lifespan than a vacuum tube panel (most of the time) and needs less maintenance (typically).

As said above, a glycol (antifreeze) system pumps the antifreeze from the panels to a heat exchanger that transfers the heat to your solar storage tank then back to the panels to get hot again. It needs a small pump to do this, perhaps as little as 10w or as much as 90w which only runs when the panels are warmer than the storage tank.

a drainback system takes water which is stored in a smaller tank above your solar storage tank and pumps it (along with the air in the piping) up to the panels where they grab the heat in the same way as the glycol system, then it transfers the heat to the tank in the same way as the glycol system. The difference is that the pumped water drains back to the little tank after the heating cycle is finished. The pump needed is usually a minimum of 140w or more which must be accounted for in the cost to run the system.

The system you are looking at has a tank on the roof which heats the shower water directly. It is viable only if you live in a temperate climate that seldom sees freezing temps. In Canada and the Northern US, most of these system need an electric trace cable to keep the lines from freezing when the cold comes.

The Canadian govt tested this type of system verses a typical flat panel system. Typical performance of the above system is 600kwh of heat annually vs 2200- 2400kwh from a 2 flat panel solar system. You decide which one is more worth it.

[cssolar]

vacuum tubes for solar water heaters

Ok now I am truly confused. I must apologise I am truly new to this tube invention.
I have a watertank. It runs on electricity from the grid. It is very expensive to use. I want to add to this a solar system. My watertank has not got any extra inlets or outlets. The solar goes maximum 1.5 metres above the tank. Here it can only be slight winter. Almost or just freezing. But very windy. Enough about the weather and chit chat. I read from a manufacturer the following.
Water is pure and drinkable
Heat insulation of water tanks adapts polyurethane foam
Silicone rings to connect and seal them
Tubes and supports are adjustable
Power bearing average and can assure vacuum tubes to use for long time
Whole glass vacuum tubes adapts super hard borax and silica glass
Absorption rate: 93%
Thermal radiation rate: 6% (100)
Resists MM hail
Lifespan: 15 years
Temperature when exposed to sun without water: 250°C
2.5 hours to boiling if using single tube
Used in electrical appliances to aid heating and switch off power supply before bathing
So I think there is tubes with own water system and then there is a collector with spiral that heats up from the tubes. The collector has water flowing through it and the spiral gives the heat to the water. Now this suplier states that water can be drinkable. ???
MayI ask advise to explain in easy words what it is I should go and look for on the market?
Thank you in advance.

Hi Rain worm,

I have quite a lot of experience working with the type of Chinese swh you are describing,

I am in Spain and that type of system works great, climate is important, mainly clear sky with sunshine is best , out side temperature is not to important as long as it dose not go below freezing for long periods

The unit you describe is a direct non pressure unit with gravity feed, I would recommend you use a pre heat unit, it uses the same non pressure tank but has a mains pressure copper coil heat exchanger inside for high pressure water.

These are self contained compact ( plug and play ) systems very simple, low cost and work very well.

you can feed your existing electrical cylinder with pre heated water via the swhs heat exchanger, also recommend fitting a by pass so you can use the solar heated water direct if you choose.

The vacuum tubes can reach 250 degrees Celsius DRY so be very careful not fill them with cold water when hot as the thermal shock may cause them to shatter.

It is true that a single vacuum tube ( in Spain with sun ) will boil in under 3 hours

Best Regards Tony

[rainworm]

thanks

water solar system 2.gifThank you for this. Can I ask you for pointers in assembling when i try to put this toghether? This will be mid April. I will probably need someone to keep the head cool haha. I will try to keep the tubes inside the house untill filling up time. I adjusted an existing drawing. Is this correct? The small cannister then should be the heat exchanger?

[cssolar]

vacuum tube swh assembly

[ATTACH=CONFIG]2592[/ATTACH]Thank you for this. Can I ask you for pointers in assembling when i try to put this toghether? This will be mid April. I will probably need someone to keep the head cool haha. I will try to keep the tubes inside the house untill filling up time. I adjusted an existing drawing. Is this correct? The small cannister then should be the heat exchanger?

Hi Rainworm,

Assembly is quite easy,

1.First build the frame , it is normally in kit form and can be assembled at the point of use

2. drop the solar tank onto the frame , dont do the bolts up yet

3. if assembling in the sun, take the vacuum tubes out of the box one at a time ( close the box each time )
prime the tubes with water ( fill them with a hose ) slide the dust cap over the open end of the tube, lubricate the end of the tube and the silicone rings in the tank with some cooking oil,

4. push the tube into the tank with a slight twisting action, seat the bottom of the tube into the frame bottom rail ( some units have plastic cups to support the tube ends )

5. fit all the tubes in the same way

6. if your unit is a pre heat type ( with copper coil heat exchanger ) feed cold water to the coil and the assistant tank ( small tank that sits on top of the main tank with float valve to maintain water level )
remember to fit a safety valve on the cold feed to the coil

7 return the hot out of the coil to feed your existing electrical cylinder, or feed in direct to your hot water system. ( optional by pass is good )

8 if your swh has a back up heater , normally they do not have thermostat in built so it is best to fit a temp sensor to the main tank, with an electronic controller to control the heater.

9 also as the main tank is not a pressure vessel it is very important to make an open ended vent ( normally up to just below the level of the assistant tank over flow.

These are for guidance only, for pre heat model ( with a non pressure tank )

Good luck Tony

[Kobayashi]

flat plate

First off, great forum..been reading the past hour and now I have a question. We want to put solar panels on our roof to heat our water. Is it better to go with the flat panels or the evacuated tubes? We want the most of it because it is so expensive to get it set up! Thanks in advance.

Flat plates will perform better in most climate zones and they are cheaper.

[Twexcom]

Evacuated tubes are better, but if you live in a snowy area, then flat panel maybe better. The evacuated tubes do not melt off the snow as good as the flat panel does.

Thank you for pointing this out.

I have no experience with flat-plate thermal collectors. However, in the tropical climate I live, I use an evacuated tube solar water heater, and it paid for itself in about a year. It worked reliably year-round, despite the fact that the backup electric heater was not plugged in. That backup heater was never used.

Mod note = forget the links