Question: Water Heating Efficiency - Photovoltaic or Solar Thermal?

[efficientPV]

Radient is superb. Depending on location, it may be near impossible to heat the concrete floor. I'd like to hear what this electric bill was over what it would be normally. It is just a dream till you attach some numbers to it.

 

[DRLExpress]

…...Concrete floor is a pretty good thermal mass and can store a lot of energy...…. he switches in multiple resistive element to obtain near MPPT.....

Yes a LOT of energy, I figure once up to temp, a few hours of good sun a day would maintain it. But then that might be asking to much here in cloudy Oregon. Remember I am just playing, I always have the woodstove.

I have thought of trying that except with multiple heating elements. 3 or 4 of different resistance values could be combined for a lot of combinations.

 

[Supervstech]

Yeah, a poorly insulated slab will take a TON of energy to heat.
It has to be setup correctly from the start.
I have also built some portable bay heaters to slide under a vehicle to heat just the work area.

 

[DRLExpress]

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.

I will try also...
Open a 18'x8' garage door in a 30'x40' shop when it is 20* outside. By the time you move what ever and get the door closed again you have lost all your heat. With radiant the minute you close the door you are warm again.

 

[Capt Bill]

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.

I wonder if one could find some inexpensive option that does the same via Alibaba?

 

[Capt Bill]

I was impressed with David Poz lesson about parallel vs series wiring of PV watts wired directly to a water heating coil; and how parallel was way better than PVs wired in series. I was consider configuring an option for sending one of my 2000 watt PV array to a water heating dump task when my 24v 280ah LifePO4 set was near full; but lost wind in that sail seeing how my 2p/3S configured PV array would maybe get only half its' watts potential at a water heating coil(s). ... from studying this Direct to Water Heating thread: and reading: about one forum member finding reasonable success with a dc-to-dc boost converter.(taking 20-60 VDC, to 120 VDC, ... via pulsed DC/ and finding one for $25 on eBay) ... I am wondering if there way to get fuller watts power of one of my 2000 Watt PV arrays (in 2p/3s config) when wired (relay switched) directly to a water heating coil by wiring some inexpensive in between Pulsed or MPPT type hardware or controller???

The other Hot Water Dump options I am considering are: 1) I could send Battery Current to heat water only at range of near full battery voltage. or 2) I could configure to use AC through my LV2424 Inverter(s) to heat water at range of near full battery voltage.
I like to figure out the best way that make most efficient use of my electrical current options.

I am looking for specific hardware/ electronic options for sending full 2000 watt solar panel array to a water heating task, and load dump. an recommendation/ especially with references of using such/ are welcome. Of course, less expensive while dependable is better in my view. Happy Days :+) Bill

 

[RCinFLA]

Even if you live in cold climate and have to deal with heat exchanger direct solar thermal will be much more efficient.

Do not inject heated water into existing hot water heater input line. The constant temp shock on bottom enamel coating of tank will cause it to fracture and rust out tank within a year. There are 'J' port adaptors to replace drain spicket and point incoming water upward.

Another minor detail is to size the line from panel properly. You should always use a small circulating pump with thermistor sensors to controller. You don't want to waste a lot heated water sitting in a large pipe from panel when pump shuts off.

The best is to get as large a tank you can fit. Insulate it well and turn the solar thermal loose. I built one with 500 gallon tank and two 4x8 foot panels. Never ran out of hot water even with five days of rain.

Couple of times the tank water got a little over 200 degs F so I added a pressure relief value just to be safe.

 

[efficientPV]

Looking at the battery voltage will tell you when you can divert directly from PV. That doesn't mean it will divert efficiently. The PV voltage must be looked at. There are proportional phase triggered AC solid state relays or even a standard one which could look at PV voltage thru a string of zeners to create a voltage offset. When voltage exceeded a preset value, energy could be diverted to heating element from 120V. This would happen every half cycle. 240V elements on 120V would be 1/4 power.

Diverting from battery isn't that bad when you have lithium as voltage is a relatively good indication of battery state. That is not true with lead. It still ties up hardware that might be needed for other things.

With solar thermal, assets are tied up doing nothing when the water is hot. PV can still be used for other things like air conditioning.

 

[carlos1w]

Two things to consider: use a heat pump to increase the efficiency of the heating (at a cost of course). Check this: https://en.wikipedia.org/wiki/Coefficient_of_performance. For heating a room the COP can be as much as 4-5 which would make the PV+heat pump more efficient than direct heating. Heating water to a high temperature will reduce the COP (not sure the number but perhaps 2?). Another alternative is to have panels that do BOTH PV and heat (https://en.wikipedia.org/wiki/Photovoltaic_thermal_hybrid_solar_collector), like this instructable: https://www.instructables.com/id/Hybrid-solar-panel-photovoltaic-and-thermal/ (commercial solutions exis, here is one https://dualsun.com/en/product/hybrid-panel-spring/)

 

[efficientPV]

Typical HPWH do not work well over 120F. Sometimes the trick is to heat the water hotter, like 145 or higher as a way to store excess energy thru the night so the HPWH doesn't have to come on. I have a HPWH and divert excess PV to the resistive element. PV heating is dirt cheap if you know what you are doing, just $30 more than the panels themselves.