Question: Water Heating Efficiency - Photovoltaic or Solar Thermal?

Does anyone have the energy comparisons between heating water using photovoltaic source to power a water heater (on demand) and solar thermal to heat water direct in the panel and using some form of high volume storage to extract using a heat exchanger?

Which is the more efficient and which is the more cost effective?

Thank you.

In lieu of anyone stepping in with some number crunching, I did a bit more digging myself into this topic, because we're all about the solar right guys and energy is energy.

Objective

My focus was on water heating and application for space heating and efficiency / cost for solar PV versus solar thermal.

I read a great deal of promotional literature online and all pretty much said the same thing:

Solar thermal wil convert 70-90% solar radiation into heat.
Solar PV will convert 15-20% solar radiation into energy.

There was even a generalised output comparison based on the area required, equating 10 sq. meters of PV to 2-3 sq. meters of Thermal panel, with a suggested ratio for mixed systems of 6 x PV for each thermal panel used.

Initially this would suggest to me that with less area and higher efficiency solar thermal would come top for water heating; however, as usual things are not quite as simple as that, cost and complexity of build being other factors.

Fortunately, I came across David Poz channel on YouTube who had set up a very practical experiment to demonstrate the difference between the two water heating methods. Links show his project's progression and outcome.

David Poz: Water Heating - Solar PV versus Solar Thermal

Part One - Intro
Part Two - The Build
Part Three - The Fifth Element
Part Four - Conclusion

My Own Preliminary Conclusion

With the rather interesting results and some variables mentioned such as ambient air temperature which I had not consideed, my own conclusion is that probably a mixed system would work best for water heating, using solar thermal to pre-heat the water and then adding in that little bit more energy from solar PV to bring up to the required temperature.

Tangents

Turning this on its head, I then then considered: If you already have guaranteed weather and lots of hot water can this then, without necessarily converting to steam, be used to generate electrical power? Well what do you know, they have tech for that ...

Thermoelectric tubes produce power from flow of and differential temperature between hot water and cold water:

1.3 Watts from the differential between 90C (194F) hot and 10C (50F) cold, for each 10cm long thermoelectric tube. With a power density 10kW per 1 meter cubed volume.

Definately something else to add to the reading list - fascinating.


References:

David Poz's channel did a test on the topic.


Also he has video on heat exchanger storage.

Those videos were a pretty poor example of PV heating. They did demonstrate that even with doing everything wrong, PV still performed reasonably. There was no attempt to even create a pseudo MPPT. As an example with direct connect and a two element heater as he used, you get more heat from using just a single element at 70% of current vs the two in parallel. That ideal resistance only valid at noon. By the time the panel current has dropped to 50% and lower, a single element produces twice the heat as two in parallel. I know I am quite happy when panels produce 70 and 50% current and these values will be very common throught the average day. Full MPPT captures even more power with 5X or more in bad weather. 5 times a low value isn't much but it all accumulates. I harvest excess power from my PV system that doesn't go to charging and heat water with it. This uses power direct from the panels and not a battery and inverter which reduces those conversion losses. Most days I am up to temp and shut off by 10:30. Frankly, most of the power goes to replacing heat loss over night. Typically you can loose 1-2KWH overnight. On grid, that much power is insignificant in cost. That is enormous to replace by PV. There are some commercial options for direct, I built my own. Pictures from a satelite don't even show my house and garage. I have panels not even on my property. All solar is local. It is a lot easier to move electricity some distance and few will tolerate those ugly thermal systems on a roof. Technically, PV is a very viable to heat water, maybe not practical with current controller costs. Supplemental PV heating with 500-750W in many cases is cost effective vs HPWH in low water use.

Water heating is something I've been looking into myself since I have excess solar capacity. I've also though about PV vs thermal. My current tank is mains pressure so that makes it more difficult to tap in a DIY thermal solution as at least part of it, say an external heat exchanger, would need to be pressure rated so PV is likely the answer for me.

I used to have a Saxon brand heater, made in Australia. It was an indirect water bath style heater where a copper coiled pipe sat in an open-to-the-atmosphere electrically heated water filled tank. Saxons were brilliant, but after 100 years they closed up shop. Mine lasted 39 years before the tank sprang a leak. Adapting them to externally heated was a doddle making them a favourite amongst off gridders. Attach hot side of external loop to the filler inlet, cold side of loop to drain at the bottom of the tank and, if needed, push the water slowly through the loop with an electric pump.

efficientPV said:

….. Full MPPT captures even more power with 5X or more in bad weather..... This uses power direct from the panels and not a battery and inverter which reduces those conversion losses...... There are some commercial options for direct, I built my own......

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Are you saying I can get the advantage of MPPT without battery's and a charger. If so, can you explain. I understand the sizing on David Poz videos and like the simplicity but would like more efficiency of MPPT

Not a recommendation to buy, and quite expensive at that, but here's a MPPT device that will run an unmodified electric water heater tank. It chops the output so that the standard AC rated thermostat will be able to safely open the circuit.

The Techluk unit only works stand alone and can not be used in parallel with an existing PV
system to divert excess power.

Tanks never have enough ports for heating elements. I have seen external thermal siphons
to add extra heating element. Not much more than a pipe from drain to an upper port typically
where the pressure relief valve is. This prevents pulling water when demanded. Often hot water
outlet has a check valve to prevent heat loss. This prevents siphon from working.

Recently I read a university paper that reported MPPT had a 20-35% advantage
heating water over DIRECT CONNECT. That may seem reasonable, but it isn't the
real story. You need hot water and don't want to wait all day or wait for a good day.
Average daily total isn't valid if you don't have high storage capacity. Report did stress
how location was key given different weather conditions. Why bother with that extra
electronics when direct connect is so simple. The study however was not done by solar
people reflecting normal use. Total heating over a year really doesn't matter. You
want hot water every day regardless of weather conditions. Here is a real
comparison between MPPT and direct connect of heater elements. For simplicity,
a 60V 10A array and element to perfectly match that. Solar panels are current
sources and the voltage of them is based on the load resistance. The fixed load
resistance is 6 ohms.

MPPT 10A 60V 600W
DIRECT 10A 60V 600W

MPPT 9A 60V 540W 1.1X
DIRECT 9A 54V 486W

MPPT 8A 60V 480W 1.3
DIRECT 8A 48V 384W

MPPT 7A 60V 420W 1.4X
DIRECT 7A 42V 294W
DIRECT 2R 5.4A 65V 350W 1.2X estimate

MPPT 6A 60V 360W 1.7X
DIRECT 6A 36V 216W
DIRECT 2R 5.2A 62V 324W 1.1X estimate

MPPT 5A 60V 300W 2.0X
DIRECT 5A 30V 150W
DIRECT 2R 5A 60V 300W EVEN

MPPT 4A 60V 240W 2.5X
DIRECT 4A 24V 96W
DIRECT 2R 4A 48V 192W 1.25

MPPT 3A 60V 180W 3.3X
DIRECT 3A 18V 54W
DIRECT 2R 3A 36V 108W 1.6X

MPPT 2A 60V 120W 5.0X
DIRECT 2A 12V 24W
DIRECT 2R 2A 24V 48W 2.5X

MPPT 1A 60V 60W 10X
DIRECT 1A 6V 6W
DIRECT 2R 1A 12V 12W 5X

As you can see, MPPT greatly outperforms direct connect in less than
ideal conditions which happens most of the day.

From physics perspective it will always be more efficient to heat air or water directly from Sun, rather than converting radiation to electricity and then back to heat. PV efficiency around 20% means you just threw away 80% of heat, which is the thing you needed in this application. It is crazy to do that.
Storage is another problem, much cheaper to store heat than electricity. Using battery to store energy to later burn in heat is insanely inefficient.
However, given limited surface area and the fact that you need electricity for other things, you may decide to over-scale your PV system and use some of the energy as heat, instead of building 2 systems, one for PV and one for water heating. Need to run specific numbers to see how it plays out.

Thanks for the replies, guys. I guess I should have said a little more about what I was trying to do. I have a shop/mancave at the house. Since I like to experiment I put pex in the slab and have tried a couple ways to heat the water so far. One of the things I have tried was I welded up an aluminum holder for a water heater element and a in and out port for pex. Worked fine but the electric bill did not like it. Everything is already in place, just want to power the heating element with PV direct.
I understand the efficiency of thermal panels but with so much of the system already in place, and I like to play....
The Techluk website talks about just the basic board but in their Ebay store they only have it mounted in a box with other stuff that does not look like $250 worth to me.

YOU put in the pex? Did you pour the slab? How much insulation did you put under the slab? Radiant floors need a foil covered 2”foam barrier to earth...

Yes, I set everything up (foam, pex and rebar) and have someone just pour the slab. 2.5" of foam. This is the 3rd place I have built for us with floor heat and would not put any other heat.

I totally agree. Especially for a workshop floor... NOTHING else compares to the comfort and efficiency of a heated slab floor.
Heating with electric can certainly rack up the bills on grid.

With a simple pumping system it is easy just to mount several heating elements in 1 1/2 inch pipe with 3/4 fittings for the heaters. With four heating elements a fairly efficient pseudo MPPT control system could be made from off the shelf components. Years ago they claimed that when PV cost per watt dropped below $1.50/W that PV would be more cost effective. Think we are there. Yea, I'd rater buy 500W more of panels than spend $250 for that thing. I have a PEX tool and have done lots of plumbing. I would never consider thermal due to so much support hardware for locations that freeze. It is easy to focus on only one criteria. What can you do with that thermal energy when up to temp. Electricity can be moved around. Thermal is a false economy.

WE KNOW that a properly designed, passive, solar hot water system with pv assisted automatic circulation and temperature control, can make beaucoup quantities of hot water, in the winter, on full bright sunny days in most places on oith where most human beans live. ~ The only questions are realistically how much dinero are you willing to spend to build a hot water system, (including emergency back up water heating method's), and how much of the system design and build are you willing to do yourself?

efficientPV said:

With a simple pumping system it is easy just to mount several heating elements in 1 1/2 inch pipe with 3/4 fittings for the heaters. With four heating elements a fairly efficient pseudo MPPT control system could be made from off the shelf components. Years ago they claimed that when PV cost per watt dropped below $1.50/W that PV would be more cost effective. Think we are there. Yea, I'd rater buy 500W more of panels than spend $250 for that thing. I have a PEX tool and have done lots of plumbing. I would never consider thermal due to so much support hardware for locations that freeze. It is easy to focus on only one criteria. What can you do with that thermal energy when up to temp. Electricity can be moved around. Thermal is a false economy.

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The systems I build are not in use every day, so a tank to store heat is installed. I like the 15gallon office tanks, and 115v elements. Wrapp in 2” fiberglass, and fill with glycol. Tie into the loop, and boom. Simple on demand thermal battery.

Supervstech said:

The systems I build are not in use every day, so a tank to store heat is installed. I like the 15gallon office tanks, and 115v elements. Wrapp in 2” fiberglass, and fill with glycol. Tie into the loop, and boom. Simple on demand thermal battery.

Click to expand...

THAT is a truly brilliant idea. If you have a PV system that can provide the power necessary to heat enough water to assist truly assist in maintaining the warmth of living spaces, that is of course practical. I wonder which is more efficient: A sunny day when the PV panels are producing enough electricity to power 1,500 watt hours of heating elements, or, a sunny day when the PV panels run a 200 watt pump circulating the glycol through passive solar panels?

I picked up one of those oil finned space heaters. They have two elements for about 1400W total. Putting them both in parallel and running on a 60V array is about 375W if I remember. At that small wattage I wouldn't call them touch safe. Space would have to be pretty cold to take over a KW. Concrete floor is a pretty good thermal mass and can store a lot of energy. electrodacus has a home in Canada he heats with PV. It is specially designed with wires in the concrete and he switches in multiple resistive element to obtain near MPPT. He has a wife he should hold onto like Grim Death to live like that. Doing anything with any kind of solar in winter is difficult. Even harder without a sophisticated control system. It all depends on what you can tolerate, wait all day for things to warm up just a little. With the grid bill being a shock, the bill for hardware necessary for any solar will be even more. Get a mini split and power that with grid tie.

efficientPV said:

I picked up one of those oil finned space heaters. They have two elements for about 1400W total. Putting them both in parallel and running on a 60V array is about 375W if I remember. At that small wattage I wouldn't call them touch safe. Space would have to be pretty cold to take over a KW. Concrete floor is a pretty good thermal mass and can store a lot of energy. electrodacus has a home in Canada he heats with PV. It is specially designed with wires in the concrete and he switches in multiple resistive element to obtain near MPPT. He has a wife he should hold onto like Grim Death to live like that. Doing anything with any kind of solar in winter is difficult. Even harder without a sophisticated control system. It all depends on what you can tolerate, wait all day for things to warm up just a little. With the grid bill being a shock, the bill for hardware necessary for any solar will be even more. Get a mini split and power that with grid tie.

Click to expand...

In a wood frame home, sure. But in a working shop with concrete floors... it is impossible to convey the comfort of radiant heated slab.

I will try...
Imagine opening the garage walking into a 40degree air temp room, and having all your tools, the vehicle, and your body completely comfortable...

That is slab heating.