Flat solar thermal panels or evacuated tube?

[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?

[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]

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.

[ithacaman]

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

[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.

[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.]

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]

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.]

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]

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.]

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.

Understood. Thanx. A Heat pump for DHW in your area will likely be less cost effective than in a milder climate.

[Captron]

From my other post:
There are so many crazy claims and counter claims about ST that I find it hard to talk to people that already have their minds made up. Here is reality for 2020.

To be clear I am in the business of Solar Thermal and have been for almost 2 decades. We have a new system preferably for new buildings that uses interseasonal storage. We can store MWt in this area and its huge, so this is the best way to prevent overheating, hands down, and it extends the solar heating into dark times and winter.

Retrofitting in low caloric heating systems (ie radian heat) is possible. If you have an existing hydronic heating system, and enough room adjacent, the storage system can be installed there and integrated.

No moving on to solar collectors (not panels) for hot water that overheats. Yes you Can cover the collectors, but it inhibits their performance on poor days of insolation. So how do you keep them cool?
The easiest solution is to divert fluid when it reaches sub 100C let’s say 93C (200F) and run it through a cooler like a baseboard pipe with fins behind the header box on EvT collectors or anywhere behind flat plate, or run to a pool or some other heat dissipating location. Then you can walk away and not worry about overheating. There are several ways to do this, so I won’t go into them here.

Efficiency: An EvT collector typically exceeds 80c efficiency. A PV panel rarely exceeds 20% efficiency.

EvT vs Flat Plate: In cold areas EvT collectors are the option of choice, they even run fairly well even at -30c, so for heating and even hot water they rule. In locations where winters are rarely colder than say -6c (20f), there a new flat plate collectors that will do, if less costly. In really warm winter locations it doesn’t matter. Therefore it all depends on where you are as to which is best. If it freezes near you or you have lots of overcast conditions - you need EvT. See attached.

EvT issues: If your solar EvT tubes are older than 5 years old there can be design issues for some EVT units. There are 3 main types of EvT’s:
Basic Heat Pipe collectors, the typical heat pipe installation: Typically double wall glass with vacuum between and no vacuum in the center. Older collectors had issues with cold weather. The interior of the heat pipe is a narrow heat absorbing pipe typically copper, where a moderate high vacuum is pulled and a water fluid injected. The new fluids have a copper content to stop them freezing up and warping the pipe. They work even in very cold weather. Heat pipe EvT’s are great for Drainback systems as the header allows the Drainback. If you have old Heat Pipes, the good news is you can USUALLY replace them, fairly inexpensively.
High temp heat pipes: These operate as above but are sealed through the glass, usually single glass with vacuum inside. Not recommended for residential use.
U tube EvT’s: Less common but These are the most efficient, often as high as 90%, but lack any control of heating except by increasing heat transfer fluid flow. They don’t drainback due to the network of headers and pipes. Only the glass double envelope can slide off during service.

So there is a primer on these collectors.

[paul.ketcham]

I have a question regarding the number of water collector panels on my house. I have six 7’ and one 9’ panels. I have one 119 gallon solar hot water tank. I have a two story house with full basement. I have five bathrooms. The solar company to installed my solar panels on my roof told me I will only need one 7’ panel on my roof to take care of my hot water. I believe this is wrong and I believe there should be four. I am traveling to Florida this weekend to pick up another 119 gallon tank. I am going to sell the collectors and hot water tank I do not use.

[Mike90250]

We have an efficient house, off grid, and a single rooftop solar collector. About a 4'x4' panel with a 40 gal storage tank on top ( glycol in the panel and heat exchanger loop ).
With 2 adults, showers, dishes and washing machine - we have 130F water from that all summer long. That's preheat for our tankless. Summers, the tankless does not do much, Winter we feed it preheated water from a 80gal tank that is plumbed into our masonry heater. That preheat water is 80-90F and the tankless just loafs along.

So depending on your usage and timing, one panel might be enough

[bcroe]

From my other post:
There are so many crazy claims and counter claims about ST that I find it hard to talk to people that already have their minds made up. Here is reality for 2020.

To be clear I am in the business of Solar Thermal and have been for almost 2 decades. We have a new system preferably for new buildings that uses interseasonal storage. We can store MWt in this area and its huge, so this is the best way to prevent overheating, hands down, and it extends the solar heating into dark times and winter.


Efficiency: An EvT collector typically exceeds 80c efficiency. A PV panel rarely exceeds 20% efficiency.

I see this type of system as the only currently available technology that could compete
with my net metering system, with season to season storage. Yes my panels may be
only 20% efficient, but to compensate my heat pumps are 400% efficient. Bruce Roe