Max water pressure allowable on solar panels

[ethanesh]

TL;DR Summary: Trying to figure out how much water pressure from a pressure washer is allowable on your typical solar panel.

Hi, I'm an engineering student and I'm trying to figure out the maximum pressure from the spray of a pressure washer that would be allowable on a solar panel. (I know pressure washing solar panels isn't typically recommended, but I have my reasons, possibly even good ones.) Once I know the max allowable pressure, I can determine the pressure setting and distance of the pressure washer, which is my ultimate goal. I have found that the typical wind loading resistance of a solar panel is around 2400 pa or .35 PSI, but that seems rather low to my untrained eye. I do not know if the wind loading is a reasonable approximation for the pressure exerted by the water jet of a pressure washer (I feel like it isn't, but I can't think of any way to prove it one way or another.) Any advice or guidance would be invaluable. Thank you!

 

[Baluncore]

Welcome to PF.

The density of water is a thousand times greater than air, so for the same velocity as the wind, water will be a real problem.

Panels fail by hailstone damage or ground leakage between the solar cells and frame. Pressure washing is sure to open the seals, wet, and ground out the internal electrical bits. Remember that the voltage between the panel and the frame is often 600 VDC. Any moisture that enters will destroy the panel.

 

[ethanesh]

Welcome to PF.

The density of water is a thousand times greater than air, so for the same velocity as the wind, water will be a real problem.

Panels fail by hailstone damage or ground leakage between the solar cells and frame. Pressure washing is sure to open the seals, wet, and ground out the internal electrical bits. Remember that the voltage between the panel and the frame is often 600 VDC. Any moisture that enters will destroy the panel.

Thank you for the response! I understand that water will have a greater impact than air if it is going at the same velocity (due to the principle of momentum) but I thought that the damage caused by a given pressure from wind loading might be comparable to the same pressure caused by sprayed water. ie, if 2400 pa of wind loading is the max a panel can take, it can also take 2400 pa of pressure caused by high velocity water (ignoring the effects the water would have on the internal electronics.) Am I misunderstanding this?

 

[DaveE]

ignoring the effects the water would have on the internal electronics.

Which would be a huge mistake. That's how electronics often fail. Go ahead with your "spherical cow" analytical approach if you like. But, That's not the real engineering problem. The real problem is keeping water out, which depends on a bunch of things related to the seal materials and construction. The key parameters for that analysis are specific to the panel(s) in question, their environment, and history. We won't have the answers in a real world sense.

My unjustified practical advice is just don't pressure wash the seals on your solar panels. My more normal engineering approach would be to ask the manufacturer of the panels. Don't be surprised if they don't know because they never tested that sort of behavior.

 

[ethanesh]

Which would be a huge mistake. That's how electronics often fail. Go ahead with your "spherical cow" analytical approach if you like. But, That's not the real engineering problem. The real problem is keeping water out, which depends on a bunch of things related to the seal materials and construction. The key parameters for that analysis are specific to the panel(s) in question, their environment, and history. We won't have the answers in a real world sense.

My unjustified practical advice is just don't pressure wash the seals on your solar panels. My more normal engineering approach would be to ask the manufacturer of the panels. Don't be surprised if they don't know because they never tested that sort of behavior.

I know it's not a great idea, however, it is the task that my team and I have been assigned by a sponsor, so I have to do my best at it. Right now, I'm just concerned about trying to find the pressure that isn't going to crack the panel (and I'm genuinely interested in the results because it will be good for my understanding regardless- whether I'm right or wrong about the wind load pressure and equivalent pressure from the spray having the same effect, I mean.)

 

[renormalize]

Right now, I'm just concerned about trying to find the pressure that isn't going to crack the panel

Perhaps you can get a handle on this by comparing the maximum possible impulse $\Delta p$ delivered by a pressure-washer stream to that due to a hailstone impact. Solar panels are designed and tested to survive storms of large hailstones falling at significant speeds. From https://www.velosolar.com/can-hail-damage-solar-panels/:
"Most monocrystalline and polycrystalline solar panels are rated to withstand 25 millimeter (0.98 inches) diameter hail falling at 50 miles per hour. Flexible thin film panels have a lower hail rating due to the nature of their construction.
Underwriters’ Laboratories (UL) and the International Electrotechnical Commission (IEC) have each come up with a hail ratings system based on impact tests they’ve conducted.
If your business is in a hail-prone region, you’ll want to look for top tier solar panels that have either a UL 61730 rating or an IEC 61730 rating, as these have been deemed those most resistant to hail damage. In testing, panels with these markings have withstood 1 to 3 inch diameter hail traveling as fast as 88 miles per hour. Storms that produce larger hail traveling at faster speeds are extremely rare."

 

[Baluncore]

... ie, if 2400 pa of wind loading is the max a panel can take, it can also take 2400 pa of pressure caused by high velocity water ...

The wind loading is a pressure applied to the entire area of the panel. The total force is going to distort the panel or break the glass and seals.

The pressure washing is applied to a very small area, so it will get past the seals.

Meanwhile, we can compute the water velocity needed to apply that pressure.
Pressure = 0.5 * density * v² ; Pa
V = √( 2 * pressure / density); m/s
For water, density = 1000 kg/m3 ;
Vmax = √( 2 * 2400 / 1000 )
Vmax = 2.2 m/s ; That is not pressure washing, it is a soft soapy sponge.

 

[berkeman]

Do these solar panels have an IPX-n rating? If they do, that tells you what level of waterproofing they have, and how they were tested using standard tests:

https://www.hyper-gear.com/pages/ratings

 

[ethanesh]

Perhaps you can get a handle on this by comparing the maximum possible impulse $\Delta p$ delivered by a pressure-washer stream to that due to a hailstone impact. Solar panels are designed and tested to survive storms of large hailstones falling at significant speeds. From https://www.velosolar.com/can-hail-damage-solar-panels/:
"Most monocrystalline and polycrystalline solar panels are rated to withstand 25 millimeter (0.98 inches) diameter hail falling at 50 miles per hour. Flexible thin film panels have a lower hail rating due to the nature of their construction.
Underwriters’ Laboratories (UL) and the International Electrotechnical Commission (IEC) have each come up with a hail ratings system based on impact tests they’ve conducted.
If your business is in a hail-prone region, you’ll want to look for top tier solar panels that have either a UL 61730 rating or an IEC 61730 rating, as these have been deemed those most resistant to hail damage. In testing, panels with these markings have withstood 1 to 3 inch diameter hail traveling as fast as 88 miles per hour. Storms that produce larger hail traveling at faster speeds are extremely rare."

I will look into that, thank you!

 

[ethanesh]

The wind loading is a pressure applied to the entire area of the panel. The total force is going to distort the panel or break the glass and seals.

The pressure washing is applied to a very small area, so it will get past the seals.

Meanwhile, we can compute the water velocity needed to apply that pressure.
Pressure = 0.5 * density * v² ; Pa
V = √( 2 * pressure / density); m/s
For water, density = 1000 kg/m3 ;
Vmax = √( 2 * 2400 / 1000 )
Vmax = 2.2 m/s ; That is not pressure washing, it is a soft soapy sponge.

Thank you! I think my mistake might have been looking at the pressure as the effect that causes the damage, while I think you're saying it is the force (at least for the case of wind load.) Just to be clear; are you saying a wind loading of x pressure is likely to cause more damage in the form of breaks/cracks than a loading of the same pressure in a small area, since the total force experienced is greater?

And another question to help my understanding, if you don't mind; it is typically advised not to walk on solar panels as that can cause damage. The pressure exerted by a human standing on their feet is around 2.5 psi. Would it then be a reasonable approximation to say that the damage caused by water striking the panel in a small area and causing it to experience 2.5 psi would be similar to that of a human walking on it? (I promise I'm not going to walk on solar panels, I am just trying to figure out when/if I can draw equivalencies between different pressures like this, or if there is some important concept I am overlooking that makes doing so impossible.) Thank you for your time!

 

[erobz]

As far as I can tell finding the pressure distribution of the impinging fluid jet on the vane is not trivial, otherwise it would be in my text book. If we take the normal force acting over the area of the incoming jet you'll find that the pressure is:

$$ p = \frac{F}{A} = \rho v^2 $$

This falls out of impulse/momentum via Reynold Transport Theorem (from my understanding of the equation).

Bernoulli's, which is a special result from Euler's Equation, claims $\frac{1}{2} \rho v^2$.

The two are used in concert with each other to solve these problems, but the outcomes appear to disagree for this type of problem.

I don't know which is the more accurate model. I'd take the first on grounds of increased factor of safety.

 

[vela]

I do not know if the wind loading is a reasonable approximation for the pressure exerted by the water jet of a pressure washer (I feel like it isn't, but I can't think of any way to prove it one way or another.) Any advice or guidance would be invaluable. Thank you!

The impression I get regarding wind load is about maintaining the structural integrity of the solar panel installation, not necessarily about what the solar panel itself can withstand. A 2400 Pa pressure on, say, a 1 square meter panel would result in over 500 lb of force. The structure that's holding the panel needs to be able to withstand that additional force.

Searching the internet, I found out that some people do pressure wash solar panels. The typical advice is to go with a gentler method if you can, but if you insist on pressure washing, you need to be very careful and to not exceed the manufacturer's recommendation. So it sounds like if you were to contact the manufacturer of the panel, they actually might be able to help you out.

 

[Baluncore]

I don't know which is the more accurate model. I'd take the first on grounds of increased factor of safety.

The difference between 2.2 m/s and 4.8 m/s is unimportant,
it is still the speed of a bird, or a soapy sponge being used to wash the panels.

Just to be clear; are you saying a wind loading of x pressure is likely to cause more damage in the form of breaks/cracks than a loading of the same pressure in a small area, since the total force experienced is greater?

Yes. The total force is greater, because the area is greater.

The panel usually clips onto a couple of rails at the edges, so it is those four points that will carry all the force during a wind gust. That will distort the frame near the points of attachment.

On the other hand, raindrops or hail falling at a few metres per second will distribute the small point forces in space and time, which will sum to less total force on the frame and mounts than would a gust of wind.

It is clear that the panels are specified at installation, and that time will see an accumulation of UV damage to sealant materials. As a contract "pressure washing" business, you will be finding all the old panels, just before they reach the end of their life, and replacing them with new panels, courtesy of your insurance company. Your insurance premium will be very high, and you will build a reputation as a business that damages panels.

Unfortunately, finding a couple of damaged panels in a string will necessitate replacing the entire string since the specifications of the original panels cannot be matched, and the disturbance of partial replacement will stress the remaining good panels, making them bad.

Maybe you could market the business as a panel testing service, one that washes the panels for free, as part of the inspection and testing procedure. You would then make your money by selling strings of new and improved panels to your customer victims.

 

[erobz]

The difference between 2.2 m/s and 4.8 m/s is unimportant,
it is still the speed of a bird, or a soapy sponge being used to wash the panels.

It's 2.2 m/s and 1.5 m/s (both less than an average household faucet). While it might not be important for this application, I still find the discrepancy worth noting in general.

But I agree that the wind load is designed on deflection of the panel/structure. It's not relevant for the water pressure it takes to get through a seal.

 

[berkeman]

I'm not very familiar with current solar panel construction, but if the pressure wash is only directed at the flat glass part of the panels, and away from any edge seams, would that minimize/eliminate the concerns about water intrusion into the panels?

 

[Tom.G]

I'm not very familiar with current solar panel construction, but if the pressure wash is only directed at the flat glass part of the panels, and away from any edge seams, would that minimize/eliminate the concerns about water intrusion into the panels?

Yup.
Right up to the time that someone shouts something at you and you turn to answer. Bye bye weather seal.

 

[erobz]

Yup.
Right up to the time that someone shouts something at you and you turn to answer. Bye bye weather seal.

I’ve done this to my hand on a ladder while power washing my home… took about a 1 inch swath 2inch long to the white meat! I was fine all day until the wife got home and was at the ladder…one “you missed a spot”…