Flat solar thermal panels or evacuated tube?

[MikeSolar]

In our area, Toronto, an installed thermal system is around $8k CDN or $6.5k US and there are no rebates of any kind. Gas is cheap here so people don't have a natural desire to install anything except for PV and that is only for the last 5-6 years. The PV thermal systems I use have a DC-DC converter to run a 6kw element and usually has a similar performance to a thermal system for the same or lesser cost and of course there is no PITA maintanence issues. Also, the power can switch over to batteries or go to the grid so that is gravy on top. I think a heat pump with PV might be a better use of panels but the jury is out on that.

[J.P.M.]

I could do that but the thermal manufacturers had to come out with something to counter the ever increaing talk about thermal not being cost effective vs PV. Do you have a particular product you like for the heat pump H2O?

I do not have a particular product in mind.

My opinion is based on the experience and knowledge that a well designed flat plate solar thermal system is able to operate with day long or average annual panel efficiencies of something approaching 40% or so and piping losses reducing that to a total system year long efficiency in the 30 - 35 % range (net to storage/P.O.A. insolation) and depending on climate - mostly ambient temp.

Assuming other, more suitable methods to heat water are not available, in my simple minded way of looking at such things, if PV operates at daylong/yearlong efficiencies of, say, 15 -17 % - also my experience and knowledge, I figure at least one of two things need to happen:

1.) With or without a heat pump, the LCOE of providing hot water with a PV system must be compared to the LCOE of a thermal flat plate system and the system with the lower LCOE deemed more cost effective.

2.) Even though not large, the likely added maintenance PITA factor for solar thermal must be considered.

I reinstalled a new pair of thermal flat plates and severely updated a DHW system about 8 yrs. ago. mat., including new everything except the tank and controller. the total cost after tax credit including labor was ~ $3,250. The panels were Sunearth, EC 32's, with potable water circulating through the panels and freeze protection provided by circulation when the controller senses the plate temps. < 38 F. The system meets ~ 90-95 % of the annual DHW demand, that demand, including tank/piping standby losses being ~ 10.7MM BTU/yr. or ~ 3,125 kWh/yr.

For my climate, admittedly a good one for solar, to provide, say 90 % of that load, and using 1,750 kWh/yr. per installed PV kW STC, would require adding ~ 1.8 kW to a system size, or a cost of ~ ($3.25/W) *(1,800 W) * (.70) = $4,095 after tax credit, or $5,850 before tax credit which looks like providing DHW with PV is more expensive for me, even in a mild climate.

The rest of the design/cost considerations get a bit involved. While I got good equipment and I did the redesign, which included some extra valving, branches and instrumentation, including several thermometers, pressure gauges and a couple of rotameters, and I used very good labor, I still got away fairly cheap, including about $680 each for the two EC-32 collectors. Even so, and without bad mouthing installers too much or at all, I have a hard time understanding how a system that's probably less complicated that I conjured up can cost something like $6K - 8K before tax credit - even when using indirect circulation and a heat exchanger or two. I suppose selling to the market may have something to do with it.

[MikeSolar]

I could do that but the thermal manufacturers had to come out with something to counter the ever increaing talk about thermal not being cost effective vs PV. Do you have a particular product you like for the heat pump H2O?

[J.P.M.]

It's been a while since I've been on here and since that time, I would say that 1/2 the thermal systems I install are now PV based. Although, I need more roof space (which isn't always available in the city) the PV is winning out. Just sayin, haha.

And hopefully using a heat pump H2O heater rather than electric resistance to heat the H2O.

[MikeSolar]

It's been a while since I've been on here and since that time, I would say that 1/2 the thermal systems I install are now PV based. Although, I need more roof space (which isn't always available in the city) the PV is winning out. Just sayin, haha.

[J.P.M.]

Flat Plates. Not just any flat plate - check out the SunEarth EC-32 or even better the TRB-32. You'll be happy.

I've owned/operated a pair of EC-32's plumbed in series for about 8+ years. They're fit for purpose with no problems whatsoever.

However, and as much as I live/breath solar thermal, if I was doing it again, I'd be doing solar hot H2O with PV and a DHW heat pump, or at least giving it a very hard look. Less plumbing and probably better overall annual solar fraction for what may be comparable $$.

If a very cold climate, I'd guess the PV/heat pump would still outperform, but particulars would need to be looked at with respect to temp./COP, etc. In any case, the need to address the thermal collector freeze condition would be gone.

I'd be somewhat skeptical of the aluminum/copper bond in the TRB-32. I understand what the spec sheets say, but it wouldn't take too many thermal excursions of cause bond failure from thermal expansion, especially under stagnation conditions of a panel with a selective surface.

[ithacaman]

Flat Plates. Not just any flat plate - check out the SunEarth EC-32 or even better the TRB-32. You'll be happy.

[LucMan]

Thanks for the responses people.
A few comments:

4) LucMan, sorry I only have the info I have supplied above. However I am looking to be able to serve at least 8 to 10 people on the regular and up to 15-16 people in a few unusual scenarios. That's why I think I would definitely need to add at least another solar collector (preferably the tube one for quicker heating), and then use the heaters that come in the tanks, for when that solar capacity falls short. What do you think?

The number of people is the only additional info required. A commercial type system for that amount of HW would work best.
That would include a large amount of storage ( > 500 gallons) with heat exchangers and pumps and of course a much larger array.
Sticking with flat plates would be best as they are more efficient heating large amounts of water. The EV tubes are better at heating smaller amounts of water to higher temps.

[AquaticQuests]

Thanks for the responses people.
A few comments:
1) Reducing the load (using less hot water): this wouldn't really work for me as it is a BnB type situation, so I will have little if any control on how much hot water the users use, save for some directions on conserving the environment by using less water, etc

2) Increasing Storage Capacity: this is something I will look into. Would this be storage distinct from the 300 liters containers that are already attached to the solar panels? So the hot water would be stored elsewhere?

3) Parallel or Serial: this is the issue that I would really need to know, so I've taken on board your comments that if combining flat panel and vacuum tube, serial may be the best way to go (Anyone else think otherwise?).
So if I went with serial,looks like I would need to include a pump in the set up? The current set up, I think does not involve any pumps.

4) LucMan, sorry I only have the info I have supplied above. However I am looking to be able to serve at least 8 to 10 people on the regular and up to 15-16 people in a few unusual scenarios. That's why I think I would definitely need to add at least another solar collector (preferably the tube one for quicker heating), and then use the heaters that come in the tanks, for when that solar capacity falls short. What do you think?

[LucMan]

Need more info. The system you describe should be good for 3-4 people.
How many sq. Ft of collector do you have? How many people in household, what is the load used for and the time of day of use, array orientation and tilt.
Is the system operating properly?

[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.

[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.]

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.

[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.

[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.

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