Solar water heater

[Mattb4]

I’m wanting to put a couple of solar panels on to outbuilding roof and use output to heat immersion heater for domestic hot water ( I have boiler backup)
I don’t want a grid tied system - completely standalone
I want to keep the existing 3kW element as I don’t think it will come out of the cylinder!
Don’t want batteries- just want to feed the output via inverter direct to immersion
Is there an inverter that will do this?
Anyone done it?

Maybe I am missing something but you do not need solar panels to heat water from sunlight. There are several solar water heaters that just involve black pipes where water can pick up heat from being in the sun. You can then use a solar panel supplied pumping system to transfer the water.

Back when I first moved out to my remote location I simply used a black garden hose to get hot water for washing up. Had to be careful though because the water could be scalding hot after an hour in the sun.

 

[charliefleet]

Maybe I am missing something but you do not need solar panels to heat water from sunlight. There are several solar water heaters that just involve black pipes where water can pick up heat from being in the sun. You can then use a solar panel supplied pumping system to transfer the water.

Back when I first moved out to my remote location I simply used a black garden hose to get hot water for washing up. Had to be careful though because the water could be scalding hot after an hour in the sun.

I can't speak for the OP but my use-case is to site the PV panels on an external detatched garage... This is something i can install myself rather than hiring scaffolding to put on the house roof.

But i tend to agree with you that evaucated solar tubes, or even black radiator panels and some form of collector system appropriately fed into the H/W system would work great. However, there is a simplicity in using an existing immersion heater rather than significantly modifying an existing h/w system.

I'm sure others have done the calcs on which works best in say winter sun vs cloudy but warm...

 

[efficientPV]

A MPPT charge controller without a battery will not work. You need something like this https://allegro.actii.pl/files/AC7392-1-1380188.pdf which converts the DC to pulsed DC to match the panel current to the element at power point. I don't know why there isn't an entire category on this site devoted to efficient water heating. I built my own as these are simple and cheap devices.

 

[charliefleet]

A MPPT charge controller without a battery will not work. You need something like this https://allegro.actii.pl/files/AC7392-1-1380188.pdf which converts the DC to pulsed DC to match the panel current to the element at power point. I don't know why there isn't an entire category on this site devoted to efficient water heating. I built my own as these are simple and cheap devices.

View attachment 111966

That's exactly what i've been looking for!

And thanks for confirming my suspicion that MPPT (charge controllers) really need a battery to provide reference voltage etc. I guess the clue's in the name - charge controller. And with these you are also paying for all the additional control circuitry associated with monitoring and charging the battery system.

Incidentally, did you create your own PWM converter from scratch, or use an existing design?

 

[efficientPV]

There are dozens of circuits that can keep the panels at power point voltage. That board is one of my designs and it is full featured with arc interrupt so existing thermostats can be used as well as and external on off thru a low voltage dry contact. I have three control boards in parallel with individual priority. Battery is top priority, 4 gallon tank is #2 in water heating, makeup domestic water is #3, and laundry is #4. All automatic and don't have to think about it. Fridge turns on and lowest priority which is heating will be reduced or off. This is all known technology. What I have been pushing for years is the operation in parallel with an existing MPPT charge controller to use excess energy to heat water. It is done because panels are so cheap, but every panel should supply power where it is needed and not be dedicated to any one device. I didn't add any panels to heat water. Ever working system has about 25% excess panel capacity in order to recover. I design for reliable operation and overdesign costs only a couple bucks extra. You can read about the operation of these control boards in this paper.
https://www.mdpi.com/2071-1050/13/18/10270/htm download the pdf.

 

[camelCase]

Solar Water Heating Projects and Plans

Descriptions and plans for several types of solar water heating systems, including batch systems, drainback systems, and closed loop systems.

www.builditsolar.com

Some interesting reading about solar hot water heating

 

[charliefleet]

There are dozens of circuits that can keep the panels at power point voltage. That board is one of my designs and it is full featured with arc interrupt so existing thermostats can be used as well as and external on off thru a low voltage dry contact. I have three control boards in parallel with individual priority. Battery is top priority, 4 gallon tank is #2 in water heating, makeup domestic water is #3, and laundry is #4. All automatic and don't have to think about it. Fridge turns on and lowest priority which is heating will be reduced or off. This is all known technology. What I have been pushing for years is the operation in parallel with an existing MPPT charge controller to use excess energy to heat water. It is done because panels are so cheap, but every panel should supply power where it is needed and not be dedicated to any one device. I didn't add any panels to heat water. Ever working system has about 25% excess panel capacity in order to recover. I design for reliable operation and overdesign costs only a couple bucks extra. You can read about the operation of these control boards in this paper.
https://www.mdpi.com/2071-1050/13/18/10270/htm download the pdf.

I've ordered the 2kw version of the PWM MPPT device as that will be sufficient given my array size. It also uses an external 12v PSU so I assume is less affected by fluctuating PV input voltage.

Will post an update once installed and commissioned!

There's a video about its operation here:

 

[efficientPV]

For a long time, both units were out of stock. While I have not worked with these, the two module unit appeared to have the option of using a fixed voltage point or MPPT tracking. That would allow it to be used in parallel with a charge controller. The one in the preferable box appears to be only used with independent panels. Neither can use existing thermostats because it doesn't have arc interrupt capability and can't switch between upper and lower tank elements. Every control has advantages and disadvantages. Someone else here has the other module and was going to give a report on its operation. That has been more than a year ago. Hope you will update us on your results.

 

[camelCase]

I don't understand using PV solar to heat water. PV is typically about ~15% efficient at converting sunlight to electricity, while a well designed solar thermal water heating system can reach 90% efficiency. In terms of cost and space required, thermal is vastly superior.

 

[efficientPV]

It is easy to take a very narrow view of a problem. Like it or not, solar thermal has never had much success in the marketplace. PV is easy and adaptable. I only heat with excess PV, power that would just be lost otherwise. There is no place on my roof that gets more than an hour of sun. I can combine the output of multiple panels to heat water even in poor conditions. PV can easily be directed to heat a small quantity of water instead of a full tank. I know a South African who starts his day dumping 4KW of electricity into his thermal tank to get it started each day. When a tank reaches temperature, thermal panels are just wasted space on the roof. Let's face it, success is having the tank reach temperature early. PV works better in cold climates. Thermal is hard to install and a maintenance issue. They used to say when panels dropped below $1.50 a watt PV was more cost effective. It's done that. I have a super system. Almost every PV system here is pretty inefficient. Solar thermal makes sense some places. It is still a maturing technology. Solar thermal has already lost.

 

[charliefleet]

For a long time, both units were out of stock. While I have not worked with these, the two module unit appeared to have the option of using a fixed voltage point or MPPT tracking. That would allow it to be used in parallel with a charge controller. The one in the preferable box appears to be only used with independent panels. Neither can use existing thermostats because it doesn't have arc interrupt capability and can't switch between upper and lower tank elements. Every control has advantages and disadvantages. Someone else here has the other module and was going to give a report on its operation. That has been more than a year ago. Hope you will update us on your results.

I'll definitely update on how well it works.

It appears to incorporate a wired temperature sensor that is used to monitor the hot water cylinder's temperature (independently to the existing bimetallic thermostat).

Once the set temperature is reached, the controller shuts off power to the element. And triggers a relay output that can be used to 'signal' or alarm that the cylinder has reached temperature.

This could then be used to trigger secondary-priority such as electric towel rails.

 

[charliefleet]

I don't understand using PV solar to heat water. PV is typically about ~15% efficient at converting sunlight to electricity, while a well designed solar thermal water heating system can reach 90% efficiency. In terms of cost and space reqed, thermal is vastly superior.

For my case, the garage was an obvious location for DIY PV installation. And being 10m from the house wouldn't suit thermal solar. But I generally agree that solar thermal is more efficient - just the logistics and payback period would not work for me.

 

[efficientPV]

Another new entry into the PV water heating market from another Polish company, Green Boost 3000 by a larger company VOLT POLSKA. I've been told it is about $300. It has two outputs with one having priority. Output is a H bridge like a MSW inverter file:///C:/Users/option2/Downloads/Karta%20produktu%20-%20przetwornica%20solarna%20Green%20Boost%203000.pdf

 

[wattmatters]

I don't understand using PV solar to heat water. PV is typically about ~15% efficient at converting sunlight to electricity, while a well designed solar thermal water heating system can reach 90% efficiency. In terms of cost and space required, thermal is vastly superior.

Don't know about where you are but solar thermal HW in Australia has had its lunch eaten by solar PV on cost effective basis for many many years. Solar PV for resistive element HW is now simpler, cheaper and more reliable here.

As to efficiency, here in Australia you can expect a COP of ~5 if you use solar PV to power a heat pump HW system. It's WAY more efficient than solar thermal for a similar cost.

The best thing about using PV is once the PV has finished water heating duties the energy can be used for other things. It's a far more flexible use of valuable roof real estate.

The main thing with off-grid water heating is to ensure the water reaches at least 60°C on a regular basis, else there is a risk of legionella bacteria taking hold in the water.

 

[biglebowski]

I would recommend using a vented cylinder or thermal store for a diy solar pv project instead of an unvented system. No danger of cylinders exploding even if the temperature control fails. The problem is finding a reliable 24 or 48v immersion element. Most of these don't appear to have a thermostat although there are a wide variety of other controllers available. I did find a retailer who interestingly suggests you can directly connect a 12v pv panel to a 12v element. This would be nice in avoiding the cost of interim controllers etc.

 

[Daddy Tanuki]

the more i look at this the more i am inclined to get a heat pump style water heater and just expand its tank size for what i want. I guess I will have to reconsider all of my calculations for tank size and heating ability.

 

[Vigo]

I would recommend using a vented cylinder or thermal store for a diy solar pv project instead of an unvented system. No danger of cylinders exploding even if the temperature control fails.

I haven't really seen people talking about DIY'ing the storage vessel, since it is usually a commercial water heater tank that already exists in a home (of course exceptions exist). Those are all mandated to have safety relief valves, although whether they are ever tested after installation is.. an almost definite no.

I have cleaned out and continued to use my water heater tank from 1994. Usually when water heater tanks are replaced it has to do with the economics for the installer rather than whether the old one could be repaired and stay in use. So discarded water heater tanks are usually perfectly good storage vessels and i've come across several for free. I would recommend anyone considering DIY'ing a tank for hot water storage to just find a cast-off water heater and DIY the cleaning it out and putting it back into use rather than DIY the entire creation of a tank!

 

[efficientPV]

I got a free tank because the plastic dip tube broke off inside the tank. That caused cold water to go from the top of the tank into the hot outlet. The down side was I filled a full 5 gallon bucket with lime sediment which isn't easy out of a 3/4 inch hole. I did get 2 new heating elements in their attempt to repair it. Tall tanks are the way to go because of stratification.

I just got a brand new BOSCH ES4 point of use for less than $50 shipped FEDEX on ebay, open box from a liquidator that had more than a half dozen. This only takes an hour to heat up in the morning from excess solar.

A 12V heating element on a 12V panel, that is a solar guy you don't want to listen to. That is never a good match. Go with a 24V element.

Solar thermal systems are only about 45-65% efficient when all the heat losses are figured in.

 

[Vigo]

A 12V heating element on a 12V panel, that is a solar guy you don't want to listen to. That is never a good match. Go with a 24V element.

I feel like im maybe just outing myself for something obvious im overlooking, but wouldn't it depend what wattage the element was rated at? The panels will only flow what they will flow as far as current, and the actual wattage of heat would depend on what voltage the panels could make when hooked to that resistance. I would think the only time hooking "12v" panels to a 12v element is bad is if your panel wattage is close to or over the wattage of the element, at which point the panels might be able to sustain over 12v when hooked to it and push the element 'beyond its rating'. I feel like if you are way below the wattage of the element you will just get something close to your panel wattage and they won't overheat/overpower the element no matter what their open circuit voltage is... Is the 24v element idea that the circuit voltage won't drop as much as it would when hooked to a 12v element?

 

[efficientPV]

A 12V panel will put out 17-18V at power point. You would want a lower wattage 12V element than the panel wattage.

element 300W /12V = 25A 12V/ 25A = .48 ohms
panel 300W /17V = 17.6A 17V/ 17.6A = .96 ohms 8.44v x 17.6A = 148W with 300W 12V element
element 300W /24V = 12.5A 24V/ 12.5A = 1.92 ohms
element 600W /24V = 12.5A 24V/ 25A = .96ohms 300W