PV direct to water heat
 

I have an unused area in the middle of my heated basement that should be ideal for four IBC's. I also have a place near the house to put about 3KW in PV. I do not, at present, have the time or money to build a complete off-grid system with heat pumps.

Since most of my winter electric bill is for heating and DHW I was wondering about the possibilities of using the PV's to provide DHW and thermal storage in an IBC array. I have used IBC's for mineral oils at 100-110F without any problems.

Are there any known complications in using the DC output from the PV's to directly power a DHW tank or resistive elements?

This system would help me through the winter, by which time I will be in a better position to install an inverter for next summer's air conditioning.

Grid tie is not an option due to local policies.

The only real issue in this type of system is the switch. Common relay contactors are rated for alternating current and won't live long trying to switch a DC power source. Make sure that you get power contactors rated for direct current.

Switching DC did occur to me as a problem. I got around this in the past by using substantially oversized force guided contactors according to their de-rating schedule. It may not work in this case. The contactors I have here don't even specify a de-rating for DC applications.

Use MOSFETs or IGBTs for switching. Or for the simplest temporary solution for space heating, leave the heaters connected at all times during the winter and open a window as needed to vent excess heat.

If you're running low voltage (24vdc or less), contact arcing is less of a problem and regular AC rated contactors can work. Once the voltage goes up above about 50vdc, the contactors with magnets inside must be used. When the switch contacts open, the magnets repel the arc and blow it out. AC contacts tend to either weld to each other or vaporize in short order trying to do their job with high voltage DC. The more current run through the switch, the faster the arc welding process occurs. I try to get a magnet contactor rated about 50 percent over the circuit breaker amps.

Then again you could use a solid state device. They operate silently and for many more cycles than a mechanical contactor. I try to get double the voltage and current ratings in the specs on the label. The things tend to run cooler and handle blips and spikes without failing immediately.

I was involved in the design/build of Chicago's 3200 series transit cars which operate on 600VDC. We wanted to redesign a disconnect due to space considerations. GE said sorry, all the engineers that did that sort of thing were dead. We got a 50 year old design and made it fit.

Your comments immediately made sense. Those disconnects had huge blast chutes.

You and Mike are probably right about using solid state devices. As for Mike's suggestion to vent any excess heat through the window, it just occurred to me the garage could always take the unused heat. That would keep the condensation down.

Thank you both for the help.

I found a MPPT water heater control that has been on the market about five years.

Solar Hybrid Hot Water Solution - No Plumbing Mods Required - At TechLuck

The manufacturer is based in Las Vegas. He sells them on ebay and has good feedback. I've only seen one independent review on the device but it was positive. I'm probably going to order one. The price seems a little steep but if it works as advertized I don't mind someone making a buck off a good idea.

Comments?

I'm curious about this myself. Please post anything you learn. Thanks.

Here is a follow up on the above mentioned device. First note that I haven't had the chance to test it yet.

It came well packaged but without documentation. I was about to contact the seller when an email arrived with the PDF manual. It is well written and clearly explains how to use the board. I was a little disappointed that it did not contain the technical data I was after. Since none of this data would be useful to the average buyer I can't blame the seller for omitting it.

I was able to extract enough details from the seller's website to extrapolate the rest. A PIC16F690 provides the MPPT algorithm. There is a MIC 10 blocking diode and a rather large zinc oxide varistor for spike protection. The double sided PCB is thoughtfully layed out and all components are screenprinted on the top side.

Being insensibly curious I wanted to know how this thing worked. So I started following traces and looking up component datasheets. Nothing made sense because because I really didn't know the first thing about an IGBT. It also didn't seem to have any means of monitoring current.

Re-reading the website I finally got it. This device doesn't need to monitor current to perform MPPT because the load is a simple resistance.

P=V**2/R.

The maximum power point therefore occurs at maximum voltage. There is no requirement for a buck/boost circuit. The PV voltage simply passes through the IGBT to the water tank heating element. The PIC modulates the on-time and a few electrolytics act as a buffer.

At 100% duty cycle the standard 240V, 4400W element would run at about 1100W if three PV panels in series were operating at a knee voltage of 120V and a little over 9A.

Purists might quibble over the use of the term MPPT. The end result is the same. It's just a special case made possible by the fact that the load is entirely predictable.

There is what appears to be a MOSFET next to the IGBT, both of which are heat-sinked to the aluminum mounting plate. I'm guessing it is used to drive the IGBT - a wild guess as I still don't know much about IGBT fundamentals.

Best of all, no batteries!

Thanks for those details Doug. I've got one on the way as well.

Very cool product. I bet you could get decent payback on this.

How is it working for you?

Hi Pinball;

The controller is still sitting on my desk. I've got the panels, wire, lightning arrestors, breaker, switches, conduit, etc. Just waiting for a minor health setback to go away so I can put it all together. The final installation with two 340W panels should be about $800.

That is a lot less money than a HPWH and it has no mechanical components to fail. Whether or not this system will outlast a HPWH is yet to be determined. Having read so many negative reviews on box store HPWH systems I'm betting in favor of this device. If it does go poof I can probably repair/improve it at very little expense.

Hope that health issue resolves quickly. I've been down that road before. :(

Looking forward to how this device works for you and Sunspot. I've seen how well a solar thermal works in South Florida, but this is much less complicated. I've got a segment of roof I could devote to a series of PV panels, if it works out as planned. I've already reduced the wattage of the elements in my 50 gallon water heater, to allow me to heat water on a generator if needed.

Where2 - Thank you for your concern.

The manufacturer recommends using the standard 4500W element. This provides optimal power transfer since the circuit does not have a buck/boost stage like a normal PV controller. The controller provides power to the lower element. Leaving the upper element connected to mains power allows partial heating on cloudy days. The panels are wired in series. As the sum voltage goes up so does the power. Maximum working values are as follows:

current - 10A
Voltage - 200V
Panel nameplate watts - 1280W

He lists various approved combinations without going into detail as to why, though again, I think he is trying to prevent "Tim the Toolman" mentality and the resulting failures. The mounting plate doubles as a heat sink and is inadequate for much more than 700W. The manufacturer recommends a bigger box for higher wattages. He is adamant about what box to use, probably the result of bad prior experience with unknowing customers. I think a real heat sink or even a bigger slab of aluminum would be a better approach. He certainly deserves credit for avoiding cooling fans.

I was discussing this device with a tinkerer friend and fellow dumpster diver. He offered me two large solar collectors as an alternative. After considering the plumbing and various safety mechanisms that would be required I decided the PV approach would be cheaper and simpler. We get a lot of freezing weather here.

The igbt devices aren't a mystical mystery device at all. If you know the characteristics of the old school bipolar power transistors, that's how igbt transistors handle the high power switching side. High maximum voltage and current handling, they have a minimum "on-state" impedance, and have a negative temperature coefficient. They can experience thermal runaway and self destruct. They're pretty impervious to voltage spikes on the power lines as well.

On the control side, they're pretty much exactly the same as a n-channel mosfet. Super high impedance, defined and narrow threshold voltage for switching, really stable over a wide temperature and power range. That's because an igbt is basically the same as a mosfet on the control side. In fact, an igbt is basically a power mosfet with an extra slice of material bonded to the substrate.

The only real significant difference kicks in at high switching frequency. The extra slice of material is p-type material, which creates a drain-collector pn junction. At high frequencies, the junction adds some turn-off lag to the switching action that a mosfet does not. This "collector current tail off" affects switching losses. The higher the switching frequency, the higher the losses.

Recent improvements in manufacturing have blurred the selection process as far as which device is better for doing the job at hand. Below about 25 khz, either one is fine. Above that, it becomes an exercise in choosing between conduction losses versus switching losses. Above about 100 khz, the switch time lag of the igbt is too slow, and the switching losses pile up against it.

Hmm negative thermal coefficient, That probably explains why IGBT's can be put in parallel without instabilities. This water heater board was designed for two in parallel but they were too close together for proper heat dissipation. I think that is why he left the second one out of the final product.

The vast majority of the parts on this board are for the MPPT. It derives its logic voltage from the PV voltage so there are also some parts for the low voltage power supply. The high side is just an IGBT, nothing else. There isn't even a dedicated gate driver, just a resistor fed by two transistors (in push-pull I think) for turning the gate off and on. The collector goes straight to the output lug. The seller advises keeping the wire between the board and heater under five feet. I'm guessing this is to reduce radiated noise.

Looks like a perfect application for an Arduino. I think I would splurge an extra three bucks for an optically coupled gate driver though, like the HCPL3120. Logic and high voltage/power should live in separate rooms.

Depends on the design. A lot of igbt stuff uses the igbt connected like a relay. A bipolar drive is inherently less noisy than a mosfet due to the lower capacitance and the pn junction lag, so lots of circuits don't have any emi suppression components. Maybe a damper diode to help with inductive loads. They are more forgiving than a power mosfet.

Some of the literature I read says IGBT turn-off time (and thus the frequency) depends on the driver's ability to pull the gate down quickly. This is because of gate-emitter (I think it was the emitter) capacitance.

My PV's are stored 30 miles from here so I haven't been able to fire this thing up. I have no idea what the switching frequency is.