Possible hybrid PV/hot water panel product?

niel

Re: Possible hybrid PV/hot water panel product?

ok, seeing this is going on without end i'll address this in a way i think most of you haven't thought of, but that have been addressed on the forum before. yes a pv would benefit by being cooler, but you won't gain very much in power. the voltage rises, but the current falls too and it just doesn't quite equalize so there is a slight gain in power when cooling.
great, now we've established a slight gain, but how do you get it? water? ok, so does the water just sit there behind the pv? no, it has to move and that takes power. you might say, "but so what we've gained solar heat". have you? the pv is cooled and the water temperature recovered is on the low side as it was on;y used to keep a pv lower in temp and so the water just slightly raised its temp to cool the pv. this is not real usable heat. so how do you gain from doing this?

Gooserider

Re: Possible hybrid PV/hot water panel product?

Niel, I would tend to agree with you in a couple of respects...

1. Without running numbers, I'd suspect that you are probably about right that the energy output increase on the solar PV side is probably going to be pretty much canceled by the energy consumption needed to circulate panel cooling water. I'd be quite willing to say that part of it is a wash...

2. A hybrid panel is NOT going to produce as much total output as two comparably sized conventional panels, one PV, and one Solar heating. However, this also assumes that one has the available mounting "real estate" to put up double the number of panels in a system.

3. I think you are possibly correct if you are attempting ONLY to maximize the PV output about not getting much useable heat from the thermal side.

However I do think that in those cases where a user has reasonable needs for BOTH hot water and electric, and a finite amount of mounting area available, that a hybrid panel could be used that might be tuned to optimize the TOTAL power output from both sides, possibly even running the panel hotter than air cooling (admittedly at some sacrifice to PV output) in order to get the requisite solar water heating gain...

As a hypothetical, our house w/ swimming pool and hopefully retrofitted radiant infloor heating system has a definitely limited amount of roof space, and I don't see any other potential mounting locations in the yard.

The PV side has the same sort of demands as any such grid tied home install...

We would have three potential uses for hot water:

1. The radiant heating system - Max. design temperature would be 120*F circulating water, assuming a decent heat exchanger between the glycol loop for the solar setup, probably maximum 130*F ouput needed from the solar panels to drive the radiant to 120* - but this is only needed on "design days" - Using an outdoor reset controller, most days would presumably need far lower temps.

2. Domestic Hot Water - Again, 120* ouput from a DHW system (code maximum) would call for 130* at the panels, though there would be advantage to charging the DHW tank to 170*F or higher and using a mixdown valve to get the 120* maximum out at the taps

3. The swimming pool - Target pool temps around 80-90, panel temp could be anything over the present pool temp...

Note that all of the above numbers assume all the heat would be coming from the panels as a worst case - it may well be that the panels are only supplementing the stored output from the wood boiler that was cranking the 1Kgal storage tank up to around 160-180*F peak... In that case the panel output would only need to be hotter than the return temp from the load plus the heat exchanger losses...

If we target a 10-20*F differential for the water side (which seems to be optimal), and a a 130*F output, then most of the panels will be effectively running at 120*F or less. From what I've seen about panel operating temps, that is probably less than typical temps in the summer, and somewhat higher in the winter, but well within the good performance range.

Thus the PV output on the hybrid should be about comparable to a standalone system. In addition you now get the thermal output that would otherwise not be collected - According to the numbers in the link Stephendv gave, it looked to me like their hybrid panel did about 4-5% less output than a standalone hot water panel - not great but it's still a big improvement over the "nothing" you'd get from a bare PV panel....

A typical PV panel is supposedly about 17-18% efficient - a typical solar water panel is about 60% from what I've seen (depending on design, delta T, etc) Discounting the solar hot water performance by the amount used by the PV panel still leaves you in the 65+% TOTAL efficiency range, which is an improvement on turning those sunbeams into human useable energy over either panel type by itself...

Thus I stick to my position that I haven't seen ANYTHING that changes it - namely that given no mounting space limits, separate systems are better, but in a finite space situation, where both outputs can be used, a hybrid should be beneficial...

Gooserider

niel

Re: Possible hybrid PV/hot water panel product?

that might do well for preheating water as it won't reach real high temps while generating electric by pv. a wash? not hardly as it will be a loss as it will take far more power to move that water than you could possibly gain. look on the charts for many pvs that show the curve of power at various temps and see you might be getting a few watts or so for the larger pvs. sure a large array might come up with enough extra power to run a modest pump, but then the larger array will also introduce more resistance to that water flow and up the pump requirements.
as said, it may work for preheating and it will be at the use of m,ore than you would've reaped otherwise and few circumstances will find this advantageous.

RCinFLA

Re: Possible hybrid PV/hot water panel product?

A closed loop solar hot water heater normally operates at a much higher temperature then you want PV panels to be. I had two 4' x 9' panels that generated a lot of hot water with closed loop circulation pump that activated whenever the output port temp was higher then input port temp into a 200 gallon insulated tank. Water temp would commonly get to 180 deg F with box temp >220 deg F. Water would boil in the panel if not under normal house pressure and circulated through the tank. I did not have to worry about freezing panel in south Florida winter so did not employ a heat exchanger system. Having to deal with potentially freezing temps requiring anti-freeze liquid heat exchanger system really adds complexity.

If only going for a one-pass pre-heater system thru the panels with new cold water entering should be okay but storage quantity of this 'preheated water' can be a problem.

Sounds like a viable solution for a pool heater where input water is always relatively cool. I would still only do it if I did not have the area to add relatively low cost but dedicated solar hot water panels. Wiring of PV panels is enough without having to also deal with fluid plumbing rat-race on them. IMHO...

icarus

Re: Possible hybrid PV/hot water panel product?

Just to reignite this thread:http://www.sundrumsolar.com/Home.html

Sounds like a solution in search of a problem.

Tony

Cariboocoot

Re: Possible hybrid PV/hot water panel product?

I'd think the paradox here is obvious:

PV's want to be as cold as possible; water heater wants to be as hot as possible. If you take the PV's down to their 'desired' operating temp, you don't get hot water. If you take the water up to its 'desired' temp the PV's are sweltering. The optimum temperatures for each are too far apart.

You'd need to run refrigeration and dump the heat to a water tank to get this to work. The energy used to do that most likely wouldn't be regained in extra electricity or hot water.

Just my opinion.

Gooserider

Re: Possible hybrid PV/hot water panel product?

I am still of the opinion that it could work in a very narrow set of circumstances, though it definitely wouldn't be a tactic everyone should be doing...

Antifreeze is IMHO a total non-issue. It doesn't add that much complication, and what it does add is going to be down below in the equipment room. It certainly would be no more of an issue with a hybrid panel setup than it would be with a dedicated hot water panel. If you live in a place where freezing temperatures are a factor, some sort of freeze protection for the system is MANDATORY, and it is arguable about whether anti-freeze or drain-back is more complex, as each has it's own set of issues.

Water temps are going to be dependent on what you are doing with the water.

Radiant floor when done properly uses water in the 120* F or less range, and only hits that on "design day"; if using outdoor reset the operating temperature goes lower most of the time.

Domestic hot water can be heated higher in the storage tank, but is prohibited by code from being delivered at more than 120*F. Using a large tank and / or a small array for pre-heating, w/ a "top up" heater to bring things up to 120* would keep the array temps low and efficient...

Swimming pools are obviously going to be low temperature load.

As long as one designs the hybrid panel so that the electric and plumbing connections aren't in the same place, it shouldn't be that difficult to deal with both, though obviously separate panels would be easier...

All that said, I would repeat my earlier comments about the constraints of who should look at such a setup...

1. Limited "real-estate" - not enough room to install BOTH the desired size PV and hot water arrays.

2. A suitable set of loads for year round use of the hot water system output, at low temperatures. DHW would work, but I would see more need if one had a suitable low-temp heating system, and a swimming pool...

In my own case, I've had a little bit of an evaluation done, and it appears that I don't meet constraint number 1. My roof faces roughly SE, and has a large chimney that sticks up in the middle of the west end, and does a "sundial" shadow across about 1/2 the roof area. This makes that area unsuitable for PV as the chimney shadow would kill the panel performance severely. However, it would work for solar hot water, since a shadow only kills a hot water panel performance to the area it covers... Thus my roof has about a 50-50 split in areas that would be suitable for discrete PV and SHW panels - I would have liked a bigger PV area, but can live with that. The other area would probably be enough for SHW.

Gooserider

icarus

Re: Possible hybrid PV/hot water panel product?

Goose,

There is no question that it can work. The bigger question is does it make sense. Go to the link, and decide for your self. Not knowing the cost, for me I don't think it does.

T

Ps. this is from Jim several years ago on the technical aspects of the same subject: http://forum.solar-electric.com/showthread.php?t=621&highlight=effect+temperature+output Post # 33

"The relationships between power, voltage, current and PV cell temperature are “linear enough”, in my view, for our applications. For example, a typical temperature coefficient of power might be expressed as “-0.5%/degree C (or K)”. That’s a straight line with a negative slope.

Check the graphs in the linked Mitsubishi cut sheet; they’re pretty telling. See: http://lib.store.yahoo.net/lib/wind-...ubishi-125.pdf

The application key is to adjust cell operating temperature for ambient conditions. Specifically, cell operating temperature of a well-aligned PV module under full insolation can be ~34 C above ambient, depending on wind conditions. So, at 15 C (59 F) ambient, cell temp could well be 49 C (120 F), or 24 K (43 R) above the STC reference of 25 C (77 F). At -35 C (-31 F; brrrrr!), cell temperature could be ~-1 C (30 F), or 26 K (47 R) below STC ref.

Radio Shack sells an inexpensive IR thermometer that you might find useful.

Finally, note that very low ambient temperature dramatically increases both STC Voc and Vmp. In fact, the standard US National Electrical Code (NEC) temperature correction factor for PV Voc in a -35 C environment is 125%. But, this is where the MPPT benefit can really shine!

This is a great site, isn’t it? I find it wonderful that so many people are able to usefully exchange and debate technical information, and almost always with the intent of finding practical solutions. A few of us go off on tangents on occasion, but I do believe it’s invariably with the best of intentions.

As a result, we’ve collectively created a technically competent site with virtually no flaming, political messages, or spam. I gather that’s a rarity, and I’m therefore glad to be a part of this community.

Best of luck with your calculations!

Regards,
Jim / crewzer"

Gooserider

Re: Possible hybrid PV/hot water panel product?

icarus wrote: »

Goose,

There is no question that it can work. The bigger question is does it make sense. Go to the link, and decide for your self. Not knowing the cost, for me I don't think it does.

T

I am familiar with the product, and my understanding is that it is roughly comparable in cost to a similar size dedicated panel.

As I said, and keep saying, if one has the space, separate dedicated panels make more sense. If one doesn't, then the Sundrum product seems like a reasonable approach.

Note that it is NOT necessary to put their panels under all the PV panels in an array, or even in a single string. Sundrum seems to primarily target folks wanting to do DHW preheating, and suggests only putting the product under 2-4 of the PV panels depending on demand, and so forth...

Gooserider

JeffreyDV

Re: Possible hybrid PV/hot water panel product?

I agree with Gooserider here. While a hybrid system is by no means ideal it does have its place. For example, my roof is completely covered by pv panels. If I want solar hot water the only way is to use a hybrid system as I have no available roof space.