PV Direct: Crazy or Crazy Good?

desert_sasquatch

PV Direct Heating is the use of solar panels without inverters or charge controllers to heat hot water. Instead, the panels are wired directly to a resistance element, which heats water.

For those who are wondering "but why would you do it that way when you can do grid tie and maybe use a heat pump?" I'll answer that--in long form--in a second post. For those of you wondering why I'd do this instead of a more traditional solar thermal system the answer is simple: I think it'll be cheaper, work just as well, and if solar thermal panels are on the way out then I'm also a bit concerned with finding people with the expertise to maintain such a system.

However mainly I'd like to know what people think of PV Direct. Is it crazy or crazy good? If it works as advertised--that is, if the panels put out about 80% of the power that a panel with an inverter or charge controller would--then I think the math pencils out and it has some important (to me) benefits over a grid tie system. But thus far it seems to be something only done by hobbyists and enthusiasts. And that makes me nervous. So I wanted to see if the brain trust here had some thoughts on it. Is there some obvious and problematic reason that--as far as I can tell--it's not used even by off-grid folks all that much? Or is it just that solar panels only became cheap enough for this to make sense in the last few years?

The best information I was able to find was from a kind of janky website someone put up. However he provides what look like good charts. I'm hoping folks here will take a peek and share their thoughts:

Here are the Solar Power Curves For PV Direct

And here are various wiring diagrams and hardware suggestions for such a system

desert_sasquatch

And as promised, here is the (long) post on why I'm interested in a system like this as opposed to a grid tie system. The short answer is that I'm concerned about transient voltage spikes ("dirty electricity") and also Net Metering 3.0 makes solar without storage a lot less economical.

Cost Of Grid Tie Solar

When last I looked a few months ago, enphase microinverters cost about $0.50 per watt. (I did look into using charge controllers instead but they only work with batteries and that becomes expensive quickly). I mention it because it's an added expense compared to PV Direct, which does not use inverters or charge controllers.

Good solar panels will put out a decent amount of power for 40+ years. Whereas good charge controllers will last maybe 25 years before requiring replacement. So there's a bit of funny dancing there in terms of whether you get cheaper panels that will fail at the same time as the charge controllers or whether you swap the charge controllers out after 25 years or 20 years or what.

Of course one advantage grid-tie solar has is that in times of surplus (say, in the summer) the excess electricity will be credited to your account, and can be "used" to pay for winter electrical needs. Since this is a comparison of heating systems I'm going to focus on electricity needed for heat: My overall heating needs are about 16.8 MWh per year. PVWatts tells me that 10,000 watts of solar panels will get me there in my location, if I get a 1:1 net metering credit for my power production.

However if my state went to something like California's new Net Metering 3.0 then I'd need more panels. In fact, since most heating happens at night I'd wind up sending most of my power to the grid and then buying that power back. Here's a rough guess of what that looks like:



The purple area is the only energy that is not going to or from the grid. The pink area is the energy coming from the grid. The blue is energy going to the grid.

The electricity credits I provided to the grid would, on average, be worth 1/4 of the power that they provided. There are reasons for this--I get that it costs money to shutter power plants during the day--but my rough guess is that the end result is that about 3/4 of my power (the unused portion during the day) would be worth 1/4 as much on a winter's day, resulting in a 56% drop in effective power output to my home from the panels in winter. In summer, it's probably more like 90% of the power output that's valued at 1/4 as much, which is a 66% drop.

*In California, there is an added expense which is the non-bypassable charges, which is basically a tax on any electricity sent to you from the grid (even if you "sent" an equivalent amount to the grid just that morning) but I'll ignore that since that may be solely a California thing.

So I'll estimate that in a Net Metering 3.0 type situation I'll need 1/0.37= 2.7 times as many solar panels.

The first solution that folks tend to offer is "get a heat pump". It would reduce power use for heating to about 1/3 of what I would use with resistance heaters. But for my home that's $8,000 for the unit, $6,500 for installation, plus at least $100 per year for maintenance, and it'll only last 12 years or so before requiring a replacement. Assuming reinstallation is a bit cheaper--$3,000 instead of $6,500--That's a prorated $46,500 for 40 years.

Plus of course it works less well when it gets cold until it eventually doesn't work at all and relies on electric heating elements. So that's only a solution with a robust and healthy electrical grid that can borrow power from out of state in a pinch. Whether our grid will be robust and healthy in 10 years is anyone's guess.

A cheaper and just-as-effective method would be to add a heat bank. If I recall correctly I believe I can do an external one for about $12,000 or so (I have quoted lower numbers before, but that was as part of the initial construction of the home--I'm guessing that excavating the plumbing for an external heat bank around an existing foundation et might cost a bit more). A solar array this size would achieve an approximate 90% solar fraction, which is what I'm shooting for. While I have hopes that heat banks will count as "batteries" for the sake of solar rebates, if I wait for 25 years before installing this...well who knows what the incentives would be.

Finally, and this won't be a concern everyone will share but it's a concern I have, grid tie solar would contribute transient voltage spikes to the AC wiring in my home (and my neighbors' homes). There are filters for this, but it would cost $6,600 and last 20 years or so before requiring replacement. Many people would skip this expense but I would not.

Cost Of PV Direct

Although PV Direct doesn't require inverters, it does suffer from about a 20% loss in efficiency as a result according to this website.

As mentioned earlier, since there are no inverters it's more economical to buy longer-lasting panels with, say, a 40 year lifespan.

The PV Direct system does require a heat bank. And that heat bank (because it is built into the foundation) costs maybe $9,000. Hopefully it counts as a battery and can receive a 30% rebate like the panels do but I'm not sure (I tried to ask but apparently the IRS is impossible to talk to unless you've pissed them off). The heat bank I have in mind would use PEX tubes--a lot of PEX tubes--so I'll guess that the lifespan is about 50 years. Hopefully more, but I won't count on it.

Since there's no grid to act as a bank for summer sun the system will more-or-less be sized for a 90% solar fraction (that is, 90% of the heat for domestic hot water and home heating should come from the solar panels). BEOpt tells me that means I need enough panels to produce about 56 kWh per day in the winter. That's about 1,700 kWh for the month of January. Which actually is about what a 10 kW solar array produces when it uses inverters or charge controllers. Of course, without inverters it would produce 20% less due to the inability to optimize the resistance for best solar gain. It would, however, negate the 4% drop in efficiency that an inverter produces. So all told it might produce 83% of the power that a rated panel with inverters would produce. Meaning that I would need 1/0.83=0.2 or 20% more panels which comes out to 12 kW of panels.

Higher quality panels that last for 40 years cost $1 per watt.

desert_sasquatch

Cost Comparison

Grid Tie (Best Case)

Assumptions:

Panels: $0.75 per watt ($7,500 per 10,000 watt array) [lasts 25 years]
Enphase Microinverters: $0.50 per watt ($5,000 per 10,000 watt array) [lasts 25 years]
Installation: $1.25 per watt ($12,500 per 10,000 watt array)
Dirty Electricity Filter: $6,700 [lasts 20 years]
Heat Bank: $11,000 [lasts 50 years]
Propane Backup: $437 per year for hardware, installation, maintenance, and fuel.
Solar Rebates: 30% for panels, microinverters, and installation. 10% for reinstallation of the same after 25 years, and for the heat bank.
Prorating: Heat Bank won't be installed for 25 years. Propane Backup will be installed initially but I'll assume half expenses until the 25 year mark because it won't be used much.

40 Year Total: ($7,500+$5,000+$12,500)*0.7+($6,700*2)+($7,500+$5,000+$12,500)*0.9*15/25+($11,000)*0.9*15/50+($437)*0.5*25+($437)*15=$59,387.5

Grid Tie (Better Panels & Microinverters Swapped out After 20 Years)

Panels: $1 per watt ($10,000 per 10,000 watt array) [lasts 40 years]
Enphase Microinverters: $0.50 per watt ($5,000 per 10,000 watt array) [lasts 20 years]
Initial Installation: $1.25 per watt ($12,500 per 10,000 watt array)
Microinverter Swap Labor: $0.10 per watt ($1,000 per 10,000 watt array)
Dirty Electricity Filter: $6,700 [lasts 20 years]
Heat Bank: $11,000 [lasts 50 years]
Propane Backup: $437 per year for hardware, installation, maintenance, and fuel.
Solar Rebates: 30% for panels, microinverters, and installation. 10% for reinstallation of the same after 20 years, and for the heat bank.
Prorating: Heat Bank won't be installed for 25 years. Propane Backup will be installed initially but I'll assume half expenses until the 25 year mark because it won't be used much.

40 Year Total: ($10,000+$5,000+$12,000)*0.7+($5,000+$1,000)*0.9+($6,700)*2+($11,000)*0.9*15/50+($437)*0.5*25+($437)*15=$52,687.5

Grid Tie (Net Metering 3.0 In 5 Years)

Assumptions:

Panels: $0.75 per watt ($7,500 per 10,000 watt array) [lasts 25 years]
Enphase Microinverters: $0.50 per watt ($5,000 per 10,000 watt array) [lasts 25 years]
Installation: $1.25 per watt ($12,500 per 10,000 watt array)
Dirty Electricity Filter: $6,700 [lasts 20 years]
Heat Bank: $11,000 [lasts 50 years]
Propane Backup: $437 per year for hardware, installation, maintenance, and fuel.
Solar Rebates: 30% for panels, microinverters, installation, and the heat bank. 10% for anything reinstalled after 25 years.
Prorating: Heat Bank won't be installed for 5 years. Propane Backup will be installed initially but I'll assume half expenses until the 5 year mark because it won't be used much.

40 Year Total: ($7,500+$5,000+$12,500)*0.7)+($6,700*2+($7,500+$5,000+$12,500)*0.9*15/25+($11,000)*0.7*35/50+($437)*0.5*5+($437)*35=$66,177.5

PV Direct

Assumptions:

Panels: $1 per watt ($12,000 per 12,000 watt array) [lasts 40 years]
Installation: $1.25 per watt ($15,000 per 12,000 watt array)
Heat Bank: $9,000 [lasts 50 years]
Propane Backup: $437 per year for hardware, installation, maintenance, and fuel.
Solar Rebates: 30% for panels, installation, and the heat bank.
Prorating: Everything is installed and used immediately.

40 Year Total: ($12,000+$15,000)*0.7+($9,000)*0.7*40/50+$437*40=$41,420

Short Heat Pump Discussion

For my home if the solar part of the equation cost $69,750 then the heat pump breaks even. Of course, that only works with grid tie, since a heat pump that was sized to operate only when the solar panels were on would be much larger.

But it doesn't matter because even in a best case scenario with current 1:1 credits being locked in for the next 25 years and no use of the dirty electricity filters the cost of the solar part of the system (ignoring the propane backup) is much lower than the $69,750 break even point. In this case that means that the energy savings from a heat pump doesn't justify the cost.

Summary

The least expensive option is grid tie with long-lasting panels. However it only beats out PV direct by $2,000, and only when I assume that the current 1:1 credits are locked in for the next 25 years and when I leave out the dirty electricity filters. A lot of people will shrug and say that that's good enough for them, but for me the price difference is negligible and the benefits in terms of health and insulation from uncertain government policies leaves me wanting to go with PV direct.

Please do let me know if you think I've left something out or made a mistake. Better to know now than after the project has begun!

WMno57

DC vs AC. Switching DC requires much more expensive switches, relays, etc because DC arcs. AC is easily switched because voltage goes to zero 60 times per second. Not so with DC, and that is why arcing will destroy a switch not specifically designed for DC.

Jamie Hall

If you don't mind operating at no more than 24 percent efficiency, well.. I suppose. Solar thermal is close to 100 percent, so you'll need four times the solar collector array to do the same job.

Also, as @WMno57 said high voltage (120 or 240) DCcontrol is a very different animal (and a good deal more expensive) than AC. And you'll still need voltage staiblisation -- the open circuit voltage of your solar panels is a good bit higher than the loaded voltage...

desert_sasquatch

DC vs AC. Switching DC requires much more expensive switches, relays, etc because DC arcs. AC is easily switched because voltage goes to zero 60 times per second. Not so with DC, and that is why arcing will destroy a switch not specifically designed for DC.

Thanks @WMno57, My grasp of electrical engineering is thin enough that I'd be nervous laying Romex without someone looking over my shoulder and telling me what to do every step of the way so I may ask some dumb questions here...

I'm vaguely aware that arcing happens when the lowest resistance pathway through a circuit involves vapor which turns into plasma, I guess. Basically when part of the "best" circuit available goes through the air. And that this is particularly an issue with switches, because if you have a mechanical contact that you're separating or connecting there's a point at which the two pieces of metal haven't yet touched (or haven't yet separated a whole lot) where the current will flow through the air between those contacts, creating arcing.

Er, probably there's A LOT more to it than that, including fancy ways people have come up with to avoid arcing, but that's about as well as I understand it right now. Is that about right? Or are there other circumstances where arcing is an issue? Say, between the "outbound" and "return" legs of a heating element? Is that the issue here?

But if that's not the issue then I'm a bit confused. The current from the panels doesn't appear to go through anything but the wires, the "business end" of the single phase solid state switches, a fuse/circuit breaker, and the resistance heating elements. If these are all meant for DC...would they really be damaged?

I did spend some time staring at the guy's wiring diagrams--it's mostly Greek to me, and I don't read Greek--but I did manage to find some DC-DC relays that appear to be similar to the ones he was suggesting (Solid State Single Phase DC-DC) and they don't appear to be all that expensive. One that's rated for 100A costs about $16. One REC Alpha panel has a short circuit current of a little over 10 A so in theory one of these could cover...I'm not sure what the margin of safety here is but just ballparking I'd say at least 9 panels? It doesn't seem too expensive unless I'd be buying a new one every year. Even then it wouldn't necessarily break the bank..

Again, sorry if I'm asking dumb questions here because my understanding of electricity is pretty limited. Thanks for putting up with that

desert_sasquatch

Jamie Hall said:

If you don't mind operating at no more than 24 percent efficiency, well.. I suppose. Solar thermal is close to 100 percent, so you'll need four times the solar collector array to do the same job.

Also, as @WMno57 said high voltage (120 or 240) DCcontrol is a very different animal (and a good deal more expensive) than AC. And you'll still need voltage staiblisation -- the open circuit voltage of your solar panels is a good bit higher than the loaded voltage...

Thanks @Jamie Hall . Can you say more about why you think it will operate at 24 percent efficiency? Looking at the solar power curves for PV direct that he provides it looks a lot more like 80% efficiency to me. And that's without the optional business with the "photo-eye" that tells the system whether to include a second resistance heater in the series in order to optimize for cloudy days.

Regarding high voltage DC: I do get that lower voltage DC would require more wire. Just to ballpark that for a second: 100 feet of 1/0 cable costs about $500. If each run from the panel to the heating element is about 33 feet on average, then that's $333 per run (there and back). 1/0 cable at 33 feet of length gives me maybe 90 A of capacity if I want to only lose about 3% of the heat to resistance. That should work for about 8-9 strings of 400 watt solar panels. If I do strings of 1 (not sure if I'm saying that right, but basically if they're all in parallel) and if I need 12,000 watts of panels then I might need something like $333*3.5=$1,165 for wiring. A higher voltage system--120 V or 240 V AC with string inverters--would cost less in this respect. But in the context of a $40,000 price tag over 40 years I'm not sure this is a dealbreaker...

But that said...lets say I did go with high voltage DC: Can you say more about why high voltage DC is more expensive than high voltage AC?

And can you say more about voltage stabilization? I do understand that the open circuit voltage of the panels is higher than the ideal voltage if one is trying to generate power/heat from the solar panels. But I was under the impression that hooking the panels up to a resistance heater was the solution to this, insofar as it provides a constant resistance for the circuit. Maybe it's an imperfect solution, as it doesn't change with changing weather conditions but at least it should stop the panels from putting out their open circuit voltage, and should get the power (and thus the voltage and current) output of the panels into the right neighborhood...right? At least, that's what those charts seem to say.

Hot_water_fan

Solar PV is about low to mid 20s efficiency, in terms of watts generated / watts potential. Solar thermal is higher, but it varies based on what water temperature you’re aiming for. High is less efficient and the collectors are pricer. 

Jamie Hall

OK. @Hot_water_fan gave you the answer on efficiency.

One fairly typical solar PV array which I am looking at has a full load rating of 7 KW. It has a rated full load current of 25 amperes at its DC working voltage of 300 volts DC (which, in this case is downconverted to 240 volts single phase AC at 30 amps). However, from the standpoint of protective devices, it requires a 30 ampere FUSE rated at the peak open circuit voltage, which is 1000 volts DC. This fuse is not cheap. Any other control device such as a switch must also be capable of breaking the current at that voltage.

Now you could probably home brew a system -- with a good bit of electrical knowledge -- which had a lower working voltage and hence lower breaking voltage (it's a matter of how you arrange the individual panels in series/parallel). However, to make any stock water heater element work, you need its rated voltage -- whether AC or DC -- which is typically 240 volts.

You are right that hooking the panels up to a resistance will stabilise the voltage at whatever they can put out with that much sunshine, which will be -- in the example I cited -- around 300 volts at 25 amps. However. If the circuit is opened with light on the panels the voltage will rise immediately to the open circuit voltage of the particular panel design -- again, in the example I cited, 1000 volts. I might also observe that the working voltage -- not the open circuit voltage -- drops linearly with reduced light intensity, and thus the power output, which varies with the square of the voltage, drops more rapidly.

You have a problem. The equipment which you are looking at simply isn't suitable for the job.

Now you enquire a little about why DC is harder to switch than AC. In AC power, the current reverses at a frequency of 60 hz. Among other implications, that means that there are 120 times per second when the current and voltage are zero. While an arc will form if the switch is opened that arc requires current to be maintained -- and less than 1/120th of a second after the arc forms, the current will pass through zero and the arc will extinguish. Not so with DC. When the switch opens, as before, an arc forms -- but the current never passes through zero, so that won't extinguish the arc. Instead, the contacts have to open far enough that the arc can't be maintained. The arc length at the working voltage of the panel cited above would be somewhere around an inch, This is the minimum distance the contacts must open to extinguish the arc.

I hope this brief sketch helps.

WMno57

Jamie explained it much better than I could have. Switching DC becomes harder as voltage rises. Arcing switches was why auto manufacturers gave up on transitioning from 12 volt to 36 volt (would have charged at 42 volts).
https://www.se.com/us/en/product/VH222AWKGL/safety-switch-heavy-duty-fused-viewing-window-nema-12-240v-60a-2-pole-ground-lugs/
$647 for just the switch. It is rated for DC.
I'm glad you posted the links to Waterheatertimer. Interesting site. But I wouldn't want any of that stuff in my home. I don't like how he suggests using the three poles on a three phase AC contactor to spread the arc over multiple contacts. If any one of those 3 poles arc welds closed, that switch is now permanently on.
The DC contactors he posted scare me. Just because some third world electrical gizmo is for sale on Amazon, doesn't mean it's safe. Is it UL listed?
Direct DC PV, when the electrical is done to standards that won't burn your house down and/or electrocute someone will be costly because 250 volt DC electrical switch gear is uncommon and expensive. Compare apples to apples. I think many other ways to heat your home would be less expensive.

desert_sasquatch

Hot_water_fan said:

Solar PV is about low to mid 20s efficiency, in terms of watts generated / watts potential. Solar thermal is higher, but it varies based on what water temperature you’re aiming for. High is less efficient and the collectors are pricer. 

Ah, gotcha. Efficiency compared to total solar irradiance. Sorry, I should have figured that out.

No, I'm not fussed about that. Space isn't that much of a concern, I can accommodate 12,000 watts of PV panels.

And when I said that PV direct appears to achieve 80% efficiency I just meant compared to how much power the panel could produce using MPPT uh...hardware.

Thanks for setting me straight on that @Jamie Hall and @Hot_water_fan.

WMno57

We were talking about solar here a year ago.
https://forum.heatinghelp.com/discussion/comment/1706734/#Comment_1706734
@JakeCK shared with us some details of his grid tied rooftop solar installation. I believe his system has Enphase IQ7+ micro inverters on each panel. I think he also said that the Enphase IQ8 did not require grid electricity and could therefore provide backup power when the grid is down.
That was a light bulb moment for me. I realized that micro inverters on each panel are the way to go for most people because it avoids the problems of dealing with high voltage direct current.

WMno57

desert_sasquatch said:

Dirty Electricity Filter: $6,700 [lasts 20 years]

I'll guess that most rooftop solar installations don't have one of these. Why do you think you need one? Poor electrical infrastructure in your neighborhood? Maybe the power company should fix that.

mattmia2

Without a charge controller I don't think you are going to get the most power out of the panel at most lighting conditions. The job of the charge controller is to match the impedance of the panel to the impedance of the load. It is unlikely the geometric voltage/power relationship of a resistance element is going to match the relationship of the panel voltage to power output.

desert_sasquatch

Jamie Hall said:

OK. @Hot_water_fan gave you the answer on efficiency.

One fairly typical solar PV array which I am looking at has a full load rating of 7 KW. It has a rated full load current of 25 amperes at its DC working voltage of 300 volts DC (which, in this case is downconverted to 240 volts single phase AC at 30 amps). However, from the standpoint of protective devices, it requires a 30 ampere FUSE rated at the peak open circuit voltage, which is 1000 volts DC. This fuse is not cheap. Any other control device such as a switch must also be capable of breaking the current at that voltage.

Hmm... OK, sorry for the dumb question but... I looked on Amazon for a circuit breaker for a solar system. I found a lot of stuff but here's one example: DC Miniature Circuit Breaker, 2 Pole 1000V 63 Amp Isolator for Solar PV System, Thermal Magnetic Trip, DIN Rail Mount, Chtaixi DC Disconnect Switch C63. Is this the expensive fuse you were talking about? It's rated for 62 amps and 1000 volts. So it should work for two of your 240 volt 30 amp arrays. And it costs $16. But you say that these fuses are not cheap so...am I looking at the wrong product?

Jamie Hall said:

OK. @Hot_water_fan gave you the answer on efficiency.

Now you could probably home brew a system -- with a good bit of electrical knowledge -- which had a lower working voltage and hence lower breaking voltage (it's a matter of how you arrange the individual panels in series/parallel). However, to make any stock water heater element work, you need its rated voltage -- whether AC or DC -- which is typically 240 volts.

I see what you're saying. I did find one element that supposedly will work for the panel I'd like to use (nominally 48 volts with an open circuit voltage of 59 volts) but it seems to be a specialty heating element (it's 120 V 2000 watts and looks like it's probably one of those ones they use for heating livestock feeding tanks) I admit that relying on what may or may not be specialty hardware to make the system work (will that size of heating element be available in 20 years?) does make me slightly nervous. So probably I would be operating at a higher voltage, or at least the possibility exists that I would be forced to do so in the future.

Jamie Hall said:

You have a problem. The equipment which you are looking at simply isn't suitable for the job.

I'll follow up on this in a separate post.

Jamie Hall said:

Now you enquire a little about why DC is harder to switch than AC. In AC power, the current reverses at a frequency of 60 hz. Among other implications, that means that there are 120 times per second when the current and voltage are zero. While an arc will form if the switch is opened that arc requires current to be maintained -- and less than 1/120th of a second after the arc forms, the current will pass through zero and the arc will extinguish. Not so with DC. When the switch opens, as before, an arc forms -- but the current never passes through zero, so that won't extinguish the arc. Instead, the contacts have to open far enough that the arc can't be maintained. The arc length at the working voltage of the panel cited above would be somewhere around an inch, This is the minimum distance the contacts must open to extinguish the arc.

That's very helpful. And makes sense. I guess my question is (if no one has answered it already--I'm going through responses one at a time): How do we prevent arcing? Would not the relays that cut the system off when the water is too hot be built to prevent this at the voltages for which they are rated? So what's the thing that fails? Don't a lot of off-grid systems keep the wiring all DC from the panels to the battery? Would this PV Direct system pose any more of a fire hazard, then, than those systems?

desert_sasquatch

Jamie Hall said:

You are right that hooking the panels up to a resistance will stabilise the voltage at whatever they can put out with that much sunshine, which will be -- in the example I cited -- around 300 volts at 25 amps. However. If the circuit is opened with light on the panels the voltage will rise immediately to the open circuit voltage of the particular panel design -- again, in the example I cited, 1000 volts. I might also observe that the working voltage -- not the open circuit voltage -- drops linearly with reduced light intensity, and thus the power output, which varies with the square of the voltage, drops more rapidly.

So I think I have a basic understanding of how the resistance of the circuit determines the power point of the PV panel and how that, then, determines the total amount of power the panel produces. At the open circuit voltage the panel would generate no current. At the short circuit current there's no voltage. Somewhere in between is the maximum power point.



As I understand it, a chart like this the power produced is the area under the box formed by vertical and horizontal lines drawn from the x axis and y axis to the point on the curve, which is determined by the resistance of the circuit.

Here is an image from the website I linked in the first post:



As you can see, it shows the power output of a system where the solar panel circuit has a set resistance. And if the chart is accurate, it looks to me like the sunny day efficiency is 90-98%, while in light cloud it drops to 73% and in dark cloud it drops to 40%. (Again this is efficiency vs a panel with a device with MPPT on it).

Since cloudy days provide less sunlight generally, in a system with storage that lasts for multiple days the sunny days are the more important ones. So they'd be weighted more in this calculation. Which is how I came up with the 80% figure I cited earlier. I think on average it might produce 80% or so of the power that it would produce if it had an MPPT device on it.

But that's what I'm asking about: Is this crazy, to suggest that this setup would provide 80% of the power that a panel with an MPPT controller would produce?

desert_sasquatch

WMno57 said:

$647 for just the switch. It is rated for DC.
I'm glad you posted the links to Waterheatertimer. Interesting site. But I wouldn't want any of that stuff in my home. I don't like how he suggests using the three poles on a three phase AC contactor to spread the arc over multiple contacts. If any one of those 3 poles arc welds closed, that switch is now permanently on.
The DC contactors he posted scare me. Just because some third world electrical gizmo is for sale on Amazon, doesn't mean it's safe. Is it UL listed?
Direct DC PV, when the electrical is done to standards that won't burn your house down and/or electrocute someone will be costly because 250 volt DC electrical switch gear is uncommon and expensive. Compare apples to apples. I think many other ways to heat your home would be less expensive.

Thanks, this makes sense. The arc welding thing especially--that's what I've been wondering, what horrible thing could happen and cause a problem. Not that I knew that was a possibility! And the thing I posted on amazon before is not UL listed.

But while his thoughts on the AC/DC contactor do make me nervous (if he thought that was safe what other dangerous thing did he think was safe?) I do notice that the AC/DC contactor seems to be an option, rather than the primary suggestion. At least if I'm understanding it correctly.

So I guess the follow up is: If this was done with UL-listed contactors would it still present an increased fire risk? Is the simple fact that the DC voltage is high a danger in itself, even when the right hardware is used? Because that would be enough of a reason for me to avoid this.

And the other question is: What other huge expenses am I not seeing here? It seems like a pretty hardware-light system. There are "ordinary" temperature controllers, an ordinary DC power supply, and then the switches and a fuse/breaker. I get that I need good quality things for each of those last items but even if they're all $650 that's still not a dealbreaker for a $41,000 system, I'd think?

(BTW I do plan on taking another look at solar thermal. The internet seems to be saying that installation costs--that is, all non-panel costs--are much lower for solar thermal panels than they are for PV. But I can't quite figure out why this is, so I'm not sure if I should trust it. If true, though, that would make solar thermal much more attractive, and worth looking at again. And with Net Metering 3.0 and the need to store energy, maybe solar thermal isn't as dead as folks thought.)

desert_sasquatch

WMno57 said:

We were talking about solar here a year ago.
https://forum.heatinghelp.com/discussion/comment/1706734/#Comment_1706734
@JakeCK shared with us some details of his grid tied rooftop solar installation. I believe his system has Enphase IQ7+ micro inverters on each panel. I think he also said that the Enphase IQ8 did not require grid electricity and could therefore provide backup power when the grid is down.
That was a light bulb moment for me. I realized that micro inverters on each panel are the way to go for most people because it avoids the problems of dealing with high voltage direct current.

I actually talked with Enphase about the off-grid mode, and it sounds like it wouldn't work for what I want, which is to use the panels in "off grid" mode all the time and push all that power into water heating. The way they work, if the load exceeds capacity multiple times in a row then it will just disconnect that load until the system is connected to the grid again. I think it disconnects loads in a particular order but nonetheless, connecting to the grid is an essential part of this, which for me means I'd need that expensive dirty electricity filter, which bumps up the price for me.

desert_sasquatch

WMno57 said:

desert_sasquatch said:

Dirty Electricity Filter: $6,700 [lasts 20 years]

I'll guess that most rooftop solar installations don't have one of these. Why do you think you need one? Poor electrical infrastructure in your neighborhood? Maybe the power company should fix that.

I know a lot of folks with electrosensitivity, and this is one of the things that bothers them. It's not something I've had a problem with at this point--not to my knowledge--but I do have other health problems and am willing to pay some money to stay on the safe side of all that. There's actually a different, somewhat cheaper filter that's used to protect one's home from the dirty electricity from other people's solar panels. But as I understand it, that other filter isn't enough to fix the problem coming from your own solar panels.

I'm unclear exactly what causes solar panels to put out transient voltage spikes--something to do with the way that the cells sort of fill up with power and then release it in one burst? Or maybe it's the power point tracking that does that? Honestly I'm not clear. I'm curious (but also don't know) if this is an issue that could be fixed by makers of solar panels or inverters or something but it would just cost extra money and no one thinks there's a market for that, or if the issue is kind of inherent in the technology. I just know that homes with PV panels--and then other homes that...use the same transformer?--will test as having a lot more dirty electricity than they did before the panels went up.

Jamie Hall

On the circuit breaker. I can't seem to find any reference to the reported manufacturer, which makes me exceedingly wary. It may be a perfectly good product, and indeed a well designed magnetic system can -- usually -- extinguish the arc before the system is damaged. Would I use it? No. But I'm a very conservative type in that regard. I would use a fuse -- as there is no possibility of a fuse failing to break the current either from a continuing arc or from a welded contact. (I might add that your local friendly power company protects its devices -- and you -- with both fuses and circuit breakers. The circuit breakers are designed to trip first, since they can be reset (usually...) and the fuses are the last resort)

On the efficiency. It isn't really efficiency that you are talking about, it is the way the power output varies with light intensity. The solar cell itself will convert pretty much every photon in the right energy range that it intercepts to a free electron, and that's what produces the current. There is a slight loss when the cell is oriented directly at the sun, due to reflection on the surface, but in a well designed cell that should be very small. However, as the angle of incidence changes, that reflected loss increases radically, as does the smaller projected area of the cell -- in fact, the power available power drops off very nearly as the square of the cosine of the angle of incidence (this is why tracking panels can be much smaller than flat panels for a similar total energy output). You see this in the power variation depending on time of day on your graph. Further, of course, clouds diffuse the incoming light, sharply reducing the peak output. You see that in the curve for a cloudy days (curiously -- though not if you think about it -- time of day has less of an influence under cloudy or hazy conditions than it does under clear skies, although the effect is almost impossible to quantify).

Nor does a solar cell convert all the photons, as implied above. This is why the peak output of the cell noted (around 180 watts) is significantly less than the 1 kilowatt per square meter of the solar radiation. This, too, isn't really an efficiency problem, but rather simply reflects the fact that the cell junction can only respond to a rather narrow range of photon energy. The only way to change that is to change the junction chemistry. Folks are working on that...

Neither of these "efficiency" differences have anything to do with the controlling device *such as an MPPT), or lack of one. They are intrinsic to the physics of the process. From the controller standpoint, the MPPT type is vastly superior to the PWM type (much more efficient)-- though more expensive. In any case, you do need some type of controller -- either PWM or MPPT -- to avoid overvoltage problems, which will destroy batteries if they are used in your system, and can, if resistive heating devices are in play, can create a real fire hazard. Reported efficiencies (and here efficiency is the correct term) seem to run around 70% for PWMs and 95% for MPPTs.

This also has nothing to do with DC to AC inverters, which are very different devices, and have widely varying efficiencies depending on the quality of the AC power output and the device design. It is possible to get very high conversion efficiencies -- if you are willing to have a square wave output rather than a sine wave. Very few devices, even motors never mind electronic devices, will be happy with a square wave...!

I applaud what you are trying to do, at least up to a point, but I urge you to remember that you are dealing with power electricity at both high voltage and power and low source impedance, not with an electronic device, and honestly unless you are well qualified in the design of these things, you may find it more rewarding to optimise the installation of the panels and any batteries etc., and leave the control and safety devices to others...

To answer your specific question, by the way, (would this produce 80% of the power etc.) it might, but only if you had a way to keep it operating somewhere close to the maximum power point. Without a controller of some kind, your output voltage is going to vary radically with load.

desert_sasquatch

@Jamie Hall

OK, thanks, perhaps we're getting to the meat of it.

But I'll start with a bit of confusion: You use "efficiency" both to talk about the efficiency of a panel in harvesting energy from the sun (in other words the efficiency relative to the total irradiance) but you also use it to compare types of charge controllers, where the meaning is "compared to if the circuit magically had the perfect resistance to put the panel at the maximum power point for whatever this circumstance is (irradiance, angle of incidence, perhaps temperature)". The latter is how I meant to use "efficiency" when talking about PV Direct. But you seem to be saying I shouldn't use it in that context. What am I misunderstanding?

But back to the main thing: Are you saying that overvoltage issues are basically unavoidable (or at least impossible to prevent reliably) with this kind of system, and then when hooked up to a heating element, there would be a risk that arcing would occur if an overvoltage situation occurred, and that would create a fire hazard? That would definitely be concerning...

FWIW I wouldn't be doing this myself. If I did this it would be something I'd take to a solar installer who hopefully would know how to do it right. But if there are safety concerns that can't be mitigated entirely then I'll just chalk it up to another crazy idea that didn't pan out.

Jamie Hall

Perhaps I didn't put it well. Strictly speaking, the solar cell itself has an efficiency -- that is, conversion of photons within the correct energy range to electrons -- of very very close to 100%. Where I think it gets confusing -- to anyone -- is that one also needs to consider the effect of the panel overall, particularly orientation, on the power output. There is a proper use of efficiency there, as if the panel isn't oriented directly at the sun some of the photons will be reflected either by any protective cover or by the surface of the cell itself, rather than being absorbed. The other aspect is that the apparent cross section area -- the area of the bundle of rays intercepted by the panel, if you will, is less than the actual physical area of the panel unless the panel is oriented directly at the sun. That second effect isn't, in the strictest sense, efficiency at all -- but it is very significant for static arrays (it is not a factor for tracking arrays which, as the name implies, move so that they are always directed towards the sun).

There really isn't much harm to using the term "efficiency" to describe the relationship of the power output of an array to the hypothetical 1 KW per square meter of the solar irradiance under various conditions -- except for accuracy.

The overvoltage problem is inherent in the semiconductor junction in the cell. Each single cell has an open circuit of about 0.6 volts. At optimum voltage it will be about 0.45 volts. The difference is because the cell itself has an internal resistance, so the more current you draw from the cell the lower the voltage at the cell terminals will be. Nothing one can do about either of these -- it's a characteristic of the semiconductor junction which does the conversion. However, the individual cells within an array can be wired in quite a variety of different connections, to produce various output voltages and, corrospondingly, output currents at optimum external load. Ideally, then, the maximum output voltage at no load will be about a third again as great as the output voltage for optimum load, whatever that may be for the particular connection scheme used.

If one has a constant load of some kind, it is fairly simple to match the overall array design to the voltage and current requirements of the load (particularly for resistive loads) and, in principle, not need a controller of any kind. Where the voltage problem arises is if the load is reduced below that design point, and the panel as a result produces a higher voltage. For instance, suppose you have an optimum load of 1200 watts, and you are feeding it with a panel which produces 120 volts -- 10 amps -- at that load. Now if you reduce the load to, let's say, a 300 watt element, the output voltage may rise to around 160 volts. Now we have to look at those two elements, not in terms of nominal power (which varies with voltage) but with regard to resistance, which doesn't. The 1200 watt load -- say four of our 300 watt elements in parallel -- has a resistance of about 12 ohms. Thus each element has a resistance of about 48 ohms. At 120 volts, that gives us our 300 watts. But what happens at 160 volts? Now the power output of that element will be about 533 watts -- and the element will get very hot indeed, if it doesn't burn out very quickly. Depending on what the element and where it's located, it may just burn out the wires of the element. Or the excess heat may light nearby flammables on fire... (batteries, by the way, do present a varying load as they charge -- and have a nasty habit of melting their cases if subject to overvoltage, or in some types -- such as lithium ion -- simply exploding).

In sort of an answer to your question above, yes, the overvoltage problem is unavoidable, but if you have a specific, constant load you can design around it.

Arcing really shouldn't be a problem if the loads are all resistive and the switches are rated for the use -- a conservative value would be twice the open circuit voltage. If there are inductive or capacitive loads, such as a motor -- or the power supply of a sound system! -- the voltage when the circuit starts to break may easily rise to at least twice the open circuit voltage which, in a DC system, will light the arc -- and then the open circuit voltage will maintain it (electric arc welding, if you have ever done it, is a superb illustration of this -- you have to get your stick close to strike the arc, but you can then withdraw the stick quite a distance and maintain the arc). Not good. Which is why any switches or protective devices need to be rated at least 3 times the working voltage.

Larry Weingarten

Hi, I may have shared this with you before, but you'll find here: https://www.heatinghelp.com/systems-help-center/another-solar-myth-bites-the-dust/ an article about solar thermal not necessarily being dead. Rather, it can be a low-tech, do it yourself sort of thing. I'm thinking it's far less complex and probably less expensive than what you're considering with PVs. Any solar thermal design is climate dependent, of course.

Yours, Larry

desert_sasquatch

@Jamie Hall

Thanks for all this. That makes sense about changes in the resistance of the circuit. And--if I understand you correctly--there isn't something like a circuit breaker that can shut the whole thing down if the voltage rises above a certain level. And also, if I understand you correctly a ... I don't know if this is the right word but... a partial short circuit (?) that bypasses only part of the resistance in the circuit (maybe something bypasses one of the resistance elements but not the others) would lead to the other elements getting quite hot and whatever problems might arise from that like fires or melting stuff or something. And I'm not clear if that would actually trigger a fuse because I'm not sure how much more current we'd be dealing with in that case. Hell, it sounds like we'd be dealing with less current because the lowered resistance would increase the voltage and lower the current coming from the panels?

So while in theory a system could be designed to be safe, in practice the amount of engineering required to make it truly safe would be more than I could do, and more than most people could do. It'd be like the Fukishima power plant where in theory it should be fine but in practice a natural disaster (earthquake, for instance) could cause all sorts of unforeseen problems, resulting (possibly) in a catastrophe. Is that about right?

Because the other thing you said was that

If one has a constant load of some kind, it is fairly simple to match the overall array design to the voltage and current requirements of the load (particularly for resistive loads) and, in principle, not need a controller of any kind.

and

the overvoltage problem is unavoidable, but if you have a specific, constant load you can design around it.

And I did think that that's what was being proposed here--a constant resistive load only, not connected to batteries or motors or anything else. I want to make sure I'm understanding your skepticism correctly!

desert_sasquatch

@Jamie Hall oops, I tagged Larry by mistake in the last post instead of you. I edited it but I'm not sure if it will show up, so I'm tagging you here.

OK, on to a reply to @Larry Weingarten:

Hah I think you did share it; I've definitely read it. The issue for me is that I'm in climate zone 6, and I gather that means I need an insulated system...right?

Jamie Hall

Solar thermal is anything but dead, but it isn't ... um... sexy enough for a lot of people. Nor, for that matter is truly passive solar for space heating. Again, not sexy -- in fact, many passive houses and buildings don't look much different from any other house or building.

Look into it. Quite feasible (both) in Zone 6.

desert_sasquatch

@Jamie Hall I've asked for a referral for a local solar thermal installer, so I'll definitely be looking into that. For what I want to do (store heat in a big heat bank) it really might be the best way. FWIW I do view it as more "sexy" than PV if only because electricity remains a bit mysterious and dangerous-seeming to me whereas hydronic heat seems, on the surface, to be straightforward (though I know from reading Idronics that it's more complicated than I would have assumed).

Passive solar is, I think, trickier. As far as I can tell in my climate zone in the winter solar panels are more efficient, from a cost perspective, than south facing windows because a cold solar panel doesn't make the house cold too. A hundred and fifty miles south it might be a different story. Shrug. I will of course have windows but in terms of roof orientation I'll probably prioritize the panels facing south over windows facing south.

But in any case I do want to do a full comparison, and that means understanding why folks say that high voltage DC systems are dangerous. Is it, like I said, sort of like Fukishima where no matter what I do, a high voltage DC system is subject to catastrophic failure in the event of an earthquake or perhaps another unforeseen disaster? Or is the extent of the concern really just "I need to make sure to get good quality, UL-listed switches and fuses?

Edit: OK, I looked again at passive solar. I see I didn't factor in savings from not having to pay for the wall where the window is. It's still more expensive than panels but not nearly as noticeably. I'll have to think more about how I can integrate passive solar in the design. Hmm...

Larry Weingarten

Hi @desert_sasquatch , The system I described has undergone freezing multiple times without damage. My temptation in your climate would be to install collectors with enough slope to allow snow to slide off or do a ground mount system that could easily be reached for cleaning. It's seldom done, but tilting the collectors at latitude plus around ten degrees should get you fairly even output year round. I've known people who put collectors vertically on a south facing wall to get better winter performance and avoid the need for any cleaning.

You'll still want some sort of backup heating, but even using a 120 VAC element powered from your inverter shouldn't be a problem if you have adequate battery storage. I know I'm making assumptions in this reply...

Maybe test a piece of poly pipe by filling it with water and freezing it to see if it can be easily damaged this way. I'm sure the conversations generated on finding a black snake-like piece of pipe in the freezer would be interesting

I'm a real fan of elegant simplicity, having learned it from Steve Baer of Zomeworks in New Mexico. Things just don't seem to hold up well if they are very complex. From what I can tell, a big reason to go with PV in your case is that the freezing concern is dealt with. Ploly pipe deals with this by stretching a little.

Yours, Larry

Jamie Hall

You might want to look up some of the work done by Bill Shurcliff and Norman Saunders on passive solar in New England -- some 50 plus years ago now...

desert_sasquatch

@Larry Weingarten

Hmm, very interesting. What kind of plastic pipe are you using? And how long is it supposed to last if it has direct UV exposure (or are you protecting it in some way)?

Hot_water_fan

@Jamie Hall sexy is the wrong adjective, expensive is the right one ha

desert_sasquatch

@Jamie Hall I did read probably half of Super Solar Houses. I loved the creativity, but the solar roof (roof made of glass) made me nervous about mold--all those connections seem like potential places where leaks can happen. Plus the use of air to pull in heat from the greenhouse also made me nervous, as that would be pretty wet air and generally I'd like to avoid pulling wet air into the home.

I did think the solar attic was a pretty interesting idea. But my conclusion at least for now is that my heat bank will be cheaper if I put it inside the foundation.

desert_sasquatch

@Hot_water_fan I'm surprised to see you say that, as I thought that "everyone" was saying that solar thermal was more expensive than a basic grid tie PV system. Of course, if Net Metering 3.0 comes to my state then the math changes, I think (because storage becomes incentivized, which it's not with most net metering programs right now). But it seems like you're saying that solar thermal should be price competitive without all that?

Hot_water_fan

@Hot_water_fan I'm surprised to see you say that, as I thought that "everyone" was saying that solar thermal was more expensive than a basic grid tie PV system. Of course, if Net Metering 3.0 comes to my state then the math changes, I think (because storage becomes incentivized, which it's not with most net metering programs right now). But it seems like you're saying that solar thermal should be price competitive without all that?

No, I mean solar thermal is too expensive. That's why it's unpopular, not sex appeal.

Larry Weingarten

Hi @desert_sasquatch , Tubing used in the collector is NSF rated, 200 psi (SDR-9), black polyethylene pipe. The black comes from carbon black. If the composition of the tube has 2% carbon black in it, the tube is protected from UV degradation. That bit of info is hard to find, but I can say that systems using this material have lived over twenty years now.

Yours, Larry

desert_sasquatch

@Larry Weingarten Interesting. Is this polyethylene tubing the kind of thing you'd find a garden store for irrigation? That is enticingly cheap. $150 for 38 square feet of coverage. Plus, perhaps, a bit more to insulate the back. Maybe a sheet of foamed glass? That would bring the price up to something like $287 per 38 square foot panel. Or more, since there will be waste. And of course there will be labor. Call it ... I don't know, $600? Even if it fails after 20 years and has to be replaced that's still cheaper than the flat plate collectors.

But that said: If I understand correctly, the glass plate on the flat plate collectors protects the metal fins from some heat loss due to cold air blowing over them. What you're proposing is basically an unglazed solar hot water heater. And they lose efficiency quickly when the ambient temperature gets cold:



(I swear I saw a better graph in an Idronics but damn if I know which one now...)

This would definitely work in the summer, but I'm looking for something that will work--at least when there's direct sunlight--in the winter. So I guess my question is: Would insulation on the back be sufficient to make these panels work well when delta T is something like 60 degrees F? Or is this meant as basically a cheap way to get to maybe 30% solar fraction (mostly during summer and early fall)?

Larry Weingarten

Hi, This requires somewhat twisted thinking. We want more square feet of inefficient collector. Insulation and air sealing could cause overheating in stagnant summer conditions. By having a large collector and plenty of storage, you get quick reheating and the ability to coast longer through periods of little sun. Having a steeper tilt also will work against overheating. Here's a link to the pipe at HD: https://www.homedepot.com/p/Advanced-Drainage-Systems-3-4-in-x-300-ft-IPS-200-psi-NSF-Poly-Pipe-X2-75200300/205909033 No insulation on the back, just plywood, painted black. On the front, some single or twinwall polycarbonate sheet, UV protected. Turns out that twinwall is half the price of 1/8" polycarbonate, and might work better in your climate. Also, this glazing makes it far less affected by windwashing. What I don't know is how well this can perform in sunny, cold conditions. In a climate that generally ranges from 45F to 90F, this has about a 90% solar fraction.

Yours, Larry

Jamie Hall

The trick with a solar thermal collector, as @Larry Weingarten implies, is to make sure it collects well when the sun is shining and doesn't lose heat when it isn't -- like at night. This is one reason why drainback type installations, despite their additional complexity and needing larger pumps, are attractive in cold climates. Combined with adequate heat storage, well insulated, operating at as low a temperature as possible, somewhere else they can be made to work well even in the coldest climates. Yes, even the best types of collectors lose efficiency when the collector is well above ambient. You can't help that. But they can work.

The low temperature, by the way, is one reason why passive systems are more attractive in cold climates -- there is no need for active heat transfer and the storage is in the structure itself -- and at "room" temperature rather than say the 120 or more needed for a hydronic type active system.

hot_rod

Idronics 3 and 6 are he solar issues
Unglazed collectors are great when ambient and fluid temperature are close, like pool heating. Performance drops quickly as the air temperature drops. Here is the math to calculate that.

A 1990 vintage SolaRoll system on a roof.

Drainbacks can be super simple with a single tank.

I have used a dual pump system. Run both until the siphon establishs and then use an ECM for the upper pump, the running pump. 37W of pumping power will run a fairly large residential ST system.

If the drainback tank is up high below the collectors, the pump size can be very small, low lift head.

desert_sasquatch

Thanks everyone.

@hot_rod so if I understand that chart correctly (thanks for finding it; I knew I'd seen it but couldn't remember which Idronics it was in) it seems like when delta T is about 60 degrees F and irradiance is about 200 btu.hr.ft^2 that's an inlet fluid parameter of .3, which means most unglazed flat plate collectors won't produce anything. They might produce a little during the peak of the day--11:00 am to 1:00 pm or so--but that's a good deal less than the glazed flat plate which should produce useful energy for maybe 6-7 hours of the day.

Here's the charts I'm referring to, for anyone interested:





I'm close enough to 40 degrees latitude that that should be roughly accurate, I think.

desert_sasquatch

But it seems like @Larry Weingarten Is suggesting at least some amount of "glazing" in the form of UV-resistant polycarbonate sheets. And while a wood backer isn't as insulating as an inch of rockwool, I suspect it's better than nothing. It's not like the traditional glazed collectors had a ton of insulation, right?

OK, onto the duel (and related?) issues of overheating in the summer and the drainback design that @Jamie Hall mentioned. I have some vague ideas about how those pieces fit together but I want to make sure I'm getting it right:

I do get why drainback is useful in the winter when the pipes are made of metal--it stops the water from freezing in them and bursting the pipes. Larry says this won't be an issue with polyethylene, though. Also I assume there is some benefit to only having to heat the water up from "room temperature" or whatever vs having to heat the water up from "frozen overnight" temperatures.

I guess my overarching thought/question is: If I leave the back of the panel sufficiently insulated so that excess heat dissipates in the summer before causing damage (however that happens) then I'd expect the panel to operate more like an unglazed collector than a glazed flate plate collector. Which means worse efficiency on an average 28 degree F December day. On the other hand it sounds like if I DO insulate the collector in a manner similar to glazed flap plate collectors then overheating is a concern in the summer. And so my question then is whether designing the system to drain back would fix this, or whether it would not because the polyethylene tubing would still be damaged by the heat even if it had no water in it.