heating water with PV

[electrofelon]

In the olden days of off grid PV it was pretty common to have a diversion load controller to shift the PV output to water heating when the batteries were full. These came out of small wind and hydropower where a diversion load was necessary when power from these types of systems could not be curtailed as easily as PV. Most people in grid tied PV have not been exposed to these systems so the expertise is being lost.

Yeah diversion controllers are great for off grid. I had one for my wind turbine because they need to stay loaded. But even with solar, most off grids are over paneled a lot, so when rh sun is out u can get quite a bit of energy for water heating. The two main diversion controllers are the xantrex and the tri star. The midnight classic has some nice functions such as a programmable PWM output that can control a SSR.

 

[electrofelon]

If the modules are free hook them all up thru a DC contactor to the heating elements and switch the contactor on off via the thermostat.

No need to worry about efficiency if they are free.

A better option is to use solar directly to heat the water. No need to convert to electricity first.

Well I know I said "free", but if the modules are not free but just cheap, maybe one would want something more efficient than a direct connection to a resistor. Also I didn't say the racking was free, so there is that. Of course one would have to weigh all this out for a given set of circumstances.

 

[electrofelon]

I just found a published paper where someone delves into using diode chains for heating from solar panels:

Hot diodes!: Dirt cheap cooking and electricity for the global poor?

Direct DC Solar (DDS) electricity can inexpensively cook food and charge appliances. Insulating the cooking chamber allows the food to cook with a low…

www.sciencedirect.com

Very clever!

 

[pv_n00b]

Do you know which microinverters they are?

I don't remember if they ever said. This was many years ago, who knows if these systems are still being sold. I did a quick internet search and the results were all solar thermal systems.

 

[ggunn]

Unless your POCO has a policy unfavorable to PV with regard to exporting power and if a calculation shows that all the electrical energy generated by the array of microinverters will eventually be used by the water heater, by far the simplest idea would be to connect them grid tied to offset your electric bill.

 

[electrofelon]

Unless your POCO has a policy unfavorable to PV with regard to exporting power and if a calculation shows that all the electrical energy generated by the array of microinverters will eventually be used by the water heater, by far the simplest idea would be to connect them grid tied to offset your electric bill.

But that's too simple and boring... .

 

[pv_n00b]

Unless your POCO has a policy unfavorable to PV with regard to exporting power and if a calculation shows that all the electrical energy generated by the array of microinverters will eventually be used by the water heater, by far the simplest idea would be to connect them grid tied to offset your electric bill.

I think these were popular in Hawaii because of the residential non-export tariff. Other than that it's just another storage option, export to the utility, store in a BESS, store in thermal storage. It's good to have options.

 

[electrofelon]

Unless your POCO has a policy unfavorable to PV with regard to exporting power and if a calculation shows that all the electrical energy generated by the array of microinverters will eventually be used by the water heater, by far the simplest idea would be to connect them grid tied to offset your electric bill.

But that's too simple and boring... .

Well , the premise was exactly that, that you can't backfeed. Even if you could, I think it's a fair discussion to talk about if there is a way to avoid spending money on inverters.

 

[winnie]

It is an interesting question. If your goal is to go from solar energy to hot water, what is the cheapest way to get there?

As I see it, some options are:
solar thermal, eg. the nifty vacuum insulated tubes that absorb sunlight but don't leak much heat
solar PV running a heat pump water heater
solar PV running a heat pump water heater where the evaporator is thermally integrated with the solar panels
solar PV running an inverter running a resistance water heater
solar PV running a resistance water heater directly
solar PV running a non-linear water heater
concentrated solar thermal

The solar thermal approach is probably the most efficient, though the the solar PV/heat pump/thermally integrated solar panels might be more efficient because you reduce thermal losses in your collection area.

But when the sun is your energy source, cheap is usually better than efficient.

-Jonathan

 

[ggunn]

It might be possible to string the modules directly up to a heating element in a water heater with a thermostatically energized contactor to shut off the DC circuit when the water reaches the desired temperature. A similar thing is often done with a pump feeding water to a stock tank with a float switch that disconnects the PV when the tank is full.

 

[electrofelon]

Another option (under the limited / already maxed out back feed scenario). Is to just over panel the crap out of everything, running a super high DC/AC ratio.

 

[ggunn]

Another option (under the limited / already maxed out back feed scenario). Is to just over panel the crap out of everything, running a super high DC/AC ratio.

Some (most? all?) inverter manufacturers publish a maximum allowable amount of DC that can be connected to an inverter; I usually see it being 150% of the rated AC power.

 

[tallgirl]

I've seen systems with modified microinverters connected directly to water heater elements. The utility is not involved, no interconnection requirements. They are dead simple. First time I saw it was in Hawaii many years ago.
I've also seen some designs that combine a PV module with a heat exchanger on the backside that water is pumped through to provide low temperature preheating for water.

Modified MICROinverters? I've never met a microinverter which didn't require a grid connection to un-island itself.

 

[tallgirl]

I think these were popular in Hawaii because of the residential non-export tariff. Other than that it's just another storage option, export to the utility, store in a BESS, store in thermal storage. It's good to have options.

The problem in Hawaii, and I know this because I worked on solving it for my then-employer, was positive voltage gradiant and SEVERELY overloaded (in the wrong direction) feeders.

 

[electrofelon]

Some (most? all?) inverter manufacturers publish a maximum allowable amount of DC that can be connected to an inverter; I usually see it being 150% of the rated AC power.

Yes for sure, there is some limit. But what exactly happens? The inverter will try to shed the power by shifting the I/V curve, and when it cant anymore I assume it will shut down. Then what, does it try again after a certain time?

 

[ggunn]

Yes for sure, there is some limit. But what exactly happens? The inverter will try to shed the power by shifting the I/V curve, and when it cant anymore I assume it will shut down. Then what, does it try again after a certain time?

If you connect more than the published maximum DC power to an inverter and something goes wrong, it is likely that the inverter company will not honor the warranty even if the problem is unrelated to the DC:AC ratio. As far as what would physically happen, there may be a limit to how much an inverter can safely clip the output of a DC array, but that is just a guess.

 

[winnie]

It likely depends upon how smart the inverter is when exposed to excessive string voltage. I could easily imagine a system smart enough to short its input, which drops the string voltage to zero and simply has Isc flow through the components. Since Isc is set by the individual panel, and is not much greater than normal operating current, the inverter should tolerate this with no problem. In this case the failure mode is 'if the string is too long you simply get reduced or zero output'.

On the other hand I could imagine that if the voltage gets too high, the inverter might try to operate and subject components to excessive voltage stress. Then the magic smoke comes out and Isc flows through the resulting arc....

 

[electrofelon]

It likely depends upon how smart the inverter is when exposed to excessive string voltage. I could easily imagine a system smart enough to short its input, which drops the string voltage to zero and simply has Isc flow through the components. Since Isc is set by the individual panel, and is not much greater than normal operating current, the inverter should tolerate this with no problem. In this case the failure mode is 'if the string is too long you simply get reduced or zero output'.

On the other hand I could imagine that if the voltage gets too high, the inverter might try to operate and subject components to excessive voltage stress. Then the magic smoke comes out and Isc flows through the resulting arc....

Why would there be excessive voltage? I am assuming the system is designed with the temperature corrected VOC not exceeding 600V or whatever.

 

[electrofelon]

If you connect more than the published maximum DC power to an inverter and something goes wrong, it is likely that the inverter company will not honor the warranty even if the problem is unrelated to the DC:AC ratio. As far as what would physically happen, there may be a limit to how much an inverter can safely clip the output of a DC array, but that is just a guess.

How would they have any idea what the DC/AC ratio was?

 

[ggunn]

How would they have any idea what the DC/AC ratio was?

Years ago an inverter rep confirmed to me that an internal register of input voltage in their inverters would tell them if the DC array ever exceeded the maximum voltage allowed, and that it would cause the warranty to be voided. I don't know if there is any way for the inverter to record if there had been a similar problem with DC system current (although if they were to send a rep to inspect the system, that is how they would know), but that is beside the point.

If you want to take on a situation where preservation of the warranty depends on the inverter company never finding out that you did something that they expressly prohibit, that is of course up to you, but I would never do it, especially if the system were for a customer instead of for my own use.