Flat solar thermal panels or evacuated tube?

[AIT Solar]

I love solar thermal and solar hot water! I think its a huge part of what will be the cure for the global "Energy Crisis".

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[zeidan.omar]

Both are Better!

based on your requirements, flat plate collectors are mostly recommended for residential and commercial use. i both cold and hot regions the flat plate collectors can be the right choice to achieve the adequate domestic water temperature (say 60 oC). on the other hand, the vacuum tubes, besides being more expensive, can deliver temperatures as high as 120 oC for industrial use and manufacturing processes.

It doen't make sense to use a vacuum tube system for residential and relatively smaller scale installations if you have the flat plate collectors available.

Also, modern solar water heating systems with flat plate collectors are capable to avoid Frosting and Over Heating problems of the collectors field on your rooftop by utilizing certain technologies such as DRAINBACK technology.

eventually, as long as we are depending on the sun to heat up our water or spaces then we are definitely on the right track. makes both options better than other fossil fuel based solutions.

[russ]

Hi Omar - Welcome to Solar Panel Talk!

Russ

[manoharan]

based on your requirements, flat plate collectors are mostly recommended for residential and commercial use. i both cold and hot regions the flat plate collectors can be the right choice to achieve the adequate domestic water temperature (say 60 oC). on the other hand, the vacuum tubes, besides being more expensive, can deliver temperatures as high as 120 oC for industrial use and manufacturing processes.

It doen't make sense to use a vacuum tube system for residential and relatively smaller scale installations if you have the flat plate collectors available.

Also, modern solar water heating systems with flat plate collectors are capable to avoid Frosting and Over Heating problems of the collectors field on your rooftop by utilizing certain technologies such as DRAINBACK technology.

eventually, as long as we are depending on the sun to heat up our water or spaces then we are definitely on the right track. makes both options better than other fossil fuel based solutions.

Nice

[saniconenergy]

The application for ETC and PFC is now perceived to be different. ETC is efficient and effective wherein you have to heat up the water more than 70Deg C / 160F, in other words where the applicatioin relates to generation of steam or its near tempearature then ETC for sure would be the bet on any given day. However when it concerns to the heating up of the water to the safe limit of 60 Deg C and around for domestic consumption then Flate Plate is much more efficient. Plus there are sevaral othe advantages of FPC: 1. Much more strong and sturdy, much less issue of breakage and ultimate shutdown of the system. This holds true particularly for the areas which gets heavy hailstorm. 2. Snow would get melted easily if it gets accumalated on FPC rather than ETC. 3. In case of ETC, even if one tube gets broken then you have to shut down the entire system unlike FPC which due to its strength doesnt encounters so much of breakage issues.

[inetdog]

3. In case of ETC, even if one tube gets broken then you have to shut down the entire system unlike FPC which due to its strength doesnt encounters so much of breakage issues.

I do not see why you say this, since the working fluid does not pass through the tubes, just the manifold connected thermally to the top of each tube.

[saniconenergy]

I do not see why you say this, since the working fluid does not pass through the tubes, just the manifold connected thermally to the top of each tube.

"@inetdog
Correct, to be specific
If it's Indirect Heating in ETC then the system would continue to run, but if it's direct heating then it has to be shut down!"

[J.P.M.]

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.

Before you do solar for any DWH applications, your best off to first reduce your hot water load load as much as possible through water use reduction by shorter showers instead of baths, low flow shower heads, flow restrictor faucets and other measures.

Also, before solar thermal, know that in many but not all applications and locations, it is becoming more cost effective to combine PV with a heat pump H2O heater rather than solar thermal by itself. Probably less maintenance for the solar portion of such a system as well.

If solar thermal is still a choice, evac. tube is usually thermal overkill in many applications. That is, it can heat water way hotter than needed. such high temp. potential comes at a cost in both $$'s and maint. that can be avoided y using less sophisticated and simpler solar flat plate with glycol/H2O flowing in the collector loop transferring heat to the DHW via a heat exchanger. If in a completely frost free, and I mean never freeze climate, a direct system w/ the domestic H2O flowing directly through the collectors back to the main storage tank may be possible.

Either way, PV + a heat pump H2O heater may be the best way over a solar thermal collector system.

[organic farmer]

Since heat-pump systems are not capable of providing enough heat for 100% of home heating needs, they are best suited as a tertiary heat system, behind your primary home heating system and a secondary home heating system. Heat-pumps work great for supplemental heat during the limited span of when they work, to lessen demand on your primary heating system. But since heat-pump systems are so much more expensive then Solar-Thermal systems are, I am not sure I would recommend them over Solar-Thermal.

When you look at Net-Zero homes in my area, you see a lot of Solar-Thermal systems on application. You don't see so many heat-pumps being used in Net-Zero homes.

[J.P.M.]

Since heat-pump systems are not capable of providing enough heat for 100% of home heating needs, they are best suited as a tertiary heat system, behind your primary home heating system and a secondary home heating system. Heat-pumps work great for supplemental heat during the limited span of when they work, to lessen demand on your primary heating system. But since heat-pump systems are so much more expensive then Solar-Thermal systems are, I am not sure I would recommend them over Solar-Thermal.

When you look at Net-Zero homes in my area, you see a lot of Solar-Thermal systems on application. You don't see so many heat-pumps being used in Net-Zero homes.

How much heat pump systems can provide is a function of climate and system size. Supplying 100% of a space heating load is certainly possible in mild to moderate climates as my neighbors continue to prove every year. To a point and depending on application, heat pump systems become less, but perhaps still economically viable as the length and depth of the heating season grows.

Similarly, as with space heating applications, heat pump H2O heaters are less viable in winter for colder climates, but still possible, and competitive or more so in many/most applications w/ electric resistance heat. Additionally, in mild to moderate climates, there can be some additional benefit from additional reduction on the A/C equipment loading.

Since superinsulation is more/most common in cold climates, and despite recent improvements, Thermodynamics dictates that heat pumps are less efficient as the temp. drops, many well thought out low energy use dwellings use downsized fossil fuel fired equipment. for space heating.

Tertiary systems as you speak of are usually used in more severe climates such as yours and usually for space heating only, or space/DHW combos. Truly well designed (read well insulated/sealed) low energy dwellings usually do quite well with simple fossil fuel fired space heating systems. I've seen a built design in a cold climate that is so well insulated/sealed that a 40,000 BTU domestic water heater was used to supply heat to a rad. floor heating system.

In cold climate and cloudy winter combos, solar thermal DHW systems can get expensive to the point of getting less robust systems, operating them less than 12 months/yr. and buttoning them up for the winter. Buffalo and environs for example, are such a climate.

I haven't done a cost analysis at this time, but I suspect that a PV/heat pump system for domestic H2O heating can be life cycle cost competitive with a closed loop solar thermal flat plate collector system in moderate to cold climates.

[organic farmer]

Systems that work up North are different from what works down South. Here in Southern Maine many people have been putting in mini-split heat-pump systems, so far they just don't work stand-alone. You still need a primary heat source, and a back-up heat source that you can rely on, which makes heat-pumps tertiary. Obviously up North where it gets cold things are entirely different from here in more Southern climates.

Where I am Solar-Thermal can work great by itself. Thankfully it remains lower priced than heat-pumps.

:

[MikeSolar]

well, I have to say that I have been having a big re-think about thermal panels or PV for heating domestic water. Nearly 30 years of struggling to get solar thermal to be mainstream and the price of PV keeps getting lower. Now I can put in a PV DHW system for the cost of a traditional thermal system and have less headaches and maintenance down the road so for the first time, I don't have a thermal system on my own house. I'm using 2.5kw of PV to power the element and then it switches over to my net metered system. So far so good.

[J.P.M.]

well, I have to say that I have been having a big re-think about thermal panels or PV for heating domestic water. Nearly 30 years of struggling to get solar thermal to be mainstream and the price of PV keeps getting lower. Now I can put in a PV DHW system for the cost of a traditional thermal system and have less headaches and maintenance down the road so for the first time, I don't have a thermal system on my own house. I'm using 2.5kw of PV to power the element and then it switches over to my net metered system. So far so good.

I've not done as much homework on the practical aspects and efficiencies of heat pump H2O heaters in cold climates (say >~ 5,000 F. DD's/yr. or so) as for solar thermal, But I'd suspect that, while they're almost a no brainer in moderate climates, they may be less so in colder climates, even when the heat source is from the dwelling interior air. If available, CH4 is still cost effetiveness king where available.

[AquaticQuests]

Ok so I've read through this thread and picked up the various points about cost of Evacuated Tubes vis a vis Flat Panel.
I have also noted the climate considerations. I have two questions, and for perspective, I should mention that:
a) I am in a climate with tropical temperatures, generally warm all year round, which would suggest flat panel may have an edge!
b) In practice, there is now NO significant 'up front' (ie at point of purchase) price difference between the 2 technologies where I am. Whichever route I take, up front cost will be the same.
c) The house in question is a new home which comes with an existing flat panel system with 300 liter capacity, however due to demands for hot water capacity, there is need to increase the capacity significantly.
d) Due to the above, my options are to get another flat panel system to add to the existing panel, or add an evacuated tube system to add to the flat panel system.

NOW TO MY QUESTIONS:
1) I understand that evacuated tubes generate much hotter water temperatures. Is it therefore fair to say that if I install evacuated tube system, I will be able to generate hot water for more users?
My reasoning is that if the water is hotter, the users mix in more cold water and less of the hot, to bring it to a comfortable temperature, and so overall, the hot water will be used by more people than the same amount of hot water from a flat panel?
If indeed hotter water, means the water goes a longer way for many users, that is definitely a plus point for me!

2) If I chose to install an evacuated tube system in combination with the existing flat panel system, do you think I should connect the system in series or in parallel, and what other issues should I keep in mind especially given this would be a combination of two different collectors pumping out water of different temperatures? (I have scoured the internet for some guidance on this but found no help)!

[J.P.M.]

Ok so I've read through this thread and picked up the various points about cost of Evacuated Tubes vis a vis Flat Panel.
I have also noted the climate considerations. I have two questions, and for perspective, I should mention that:
a) I am in a climate with tropical temperatures, generally warm all year round, which would suggest flat panel may have an edge!
b) In practice, there is now significant 'up front' (ie at point of purchase) price difference between the 2 technologies where I am. Whichever route I take, up front cost will be the same.
c) The house in question is a new home which comes with an existing flat panel system with 300 liter capacity, however due to demands for hot water capacity, there is need to increase the capacity significantly.
d) Due to the above, my options are to get another flat panel system to add to the existing panel, or add an evacuated tube system to add to the flat panel system.

NOW TO MY QUESTIONS:
1) I understand that evacuated tubes generate much hotter water temperatures. Is it therefore fair to say that if I install evacuated tube system, I will be able to generate hot water for more users?
My reasoning is that if the water is hotter, the users mix in more cold water and less of the hot, to bring it to a comfortable temperature, and so overall, the hot water will be used by more people than the same amount of hot water from a flat panel?
If indeed hotter water, means the water goes a longer way for many users, that is definitely a plus point for me!

2) If I chose to install an evacuated tube system in combination with the existing flat panel system, do you think I should connect the system in series or in parallel, and what other issues should I keep in mind especially given this would be a combination of two different collectors pumping out water of different temperatures? (I have scoured the internet for some guidance on this but found no help)!

The goal is enough hot water to meet demand and do so in the safest, most cost effective ways possible. Right ?

1.) Before anything else: One very cost effective way to increase system capacity as a % of the load is to reduce the load. Use less hot water. Many common ways to reduce a DHW load are in the popular literature. Next: Have the current system checked out to ensure it's running as designed. Many systems are not.

You may not have an undersized system as much as an oversized demand, and a system that is not performing as designed. Don't knee jerk the situation.

2.) Hotter water will mean more heated water available at the end use - essentially a larger capacity to store hot water. However, the storage and collector piping system losses will increase in proportion to the water temperature and work against you. One way to increase storage capacity, besides hotter temps. is larger tank capacity. Lower temps. will mean fewer system losses and more energy to the load.

3.) If a evac. tube system is chosen, without knowing the particulars of the application, and even though not my choice, I'd consider putting the evac. tube system in series downstream of the existing flat plate system. paralleling different type systems can cause problems and flow irregularities for a lot of reasons. Plumbing in series will cost more in terms of pressure drop and probably a bigger pump but if done as suggested will be the best use of the strengths of both systems.