Solar panel questions

[tomzo]

Greetings - first post here so bear with me!

I built my own pool last year and recently came into possession of 5 solar panels (fafco). Even though these are probably not quite enough for the ultimate system I am going to hook them up and start using them and add more panels as my budget allows. I have a Pentair easytouch controller that will manage the system.

I have a couple of questions:

- I will be installing the panels on my patio cover which is basically flat. (My roof slopes the wrong way and I don't want to mess with the tiles). I was planning on installing the vacuum relief at the panel farthest from where the supply and return come onto the patio cover. Since there will be no "top" of the system will this work?
- Does anyone have a recommendation on the best make and model of these valves?
- Is it customary to install valves between sets of panels to control flow to the panels and balance the system? I will have two sets of panels (one with two and the other with three tied together)
- I need to get the solar temperature sensor (10K Ohm thermistor) and these are crazy expensive. I have seen 10K Ohm thermistors for 1/5th the cost from vendors other than Pentair - does anyone know if these will work? It would seem to me that the physics involved is pretty basic and since I don't need this sensor to go into a pipeline I don't need the configuration that Pentair is selling.
- I was looking at a 2" check valve at my local hardware supplier and it seems decent but I worry about head loss. Do the more expensive Jandy check valves have better head loss performance than the ones you can buy at the local hardware store?

Thanks in advance for your help!

Tom

 

[solarboy]

The vacuum release is normally on the input, not the highest point. This set up can use a check valve after the filter, and one after the panels. That way, when the solar goes "off" or the pump stops, the water can only drain back into the pool and not back through the filter dragging sand in to the pool. The release valve before the panels allows air in to replace the water in the panels. Any water before the release valve stays in the pipes unless you have a "leaky" 3 way valve.
With 2 sets of panels, one larger than the other balancing is tricky. A technique I've used recently is an infrared thermometer pointed at the output pipes of both panels, adjusting valves to equalise temperature.

 

[JasonLion]

I'm a little concerned when you say there is no "top" to the system. If you mount the panels horizontally, they will never completely drain, and you will need to take them down and manually drain them before any freezing weather. If you are going to be doing that anyway, you defeat some of the reasons for having a vacuum breaker valve in the first place.

Good installs try to avoid using valves to balance the flow, but sometimes it is unavoidable. Ideally you want to setup the plumbing so the flow is naturally balanced. Valves are another thing to worry about that is best avoided if practical.

The thermistor has to be the correct resistance and be weather proof. The Pentair ones are $30-$40 on the Internet and you only need one, so I'm not sure why this is an issue.

The Jandy check valves are extremely good, but there are several others in the same quality range. Avoid inexpensive check valves with a heavy spring. I'm not sure what your hardware store carries, some stores carry good ones, some don't.

 

[tomzo]

Thanks for the replies!

I live in San Diego and it never freezes here, so freezing is not an issue. Is that the primary purpose for draining the system?

I will go ahead and get the Pentair thermistor - there are units for PV solar systems that are only $10 so I was just curious whether anyone else has used them.

With respect to balancing the flow I will strive to have the same length of inlet piping but I may put a couple of ball valves in there just in case.

I am wondering that if I do not have the system set up to drain when the panels are not in use will I need the second check valve? I planned to have a three way valve direct the water up to the panels when in use and bypass them when they are not.

Thanks again!

 

[JasonLion]

There are two reasons to drain. The main one is to avoid freeze damage. But it is also nice to get the water out of the panels most of the time to reduce wear and tear on both the panels and whatever they are mounted on.

There are many different ways to plumb solar panels and they all have pros and cons. Typically a check valve is used to prevent water from ever running backwards through the filter when the pump is off. Often another check valve is used to prevent the solar panels from pressurizing through the return line when water is set to bypass the panels. With good quality check valves, their extra flow resistance isn't a big deal. But there are other approaches that avoid the check valves. Partly it depends on what kind of filter you have. Sand filters don't care so much about water running backwards, but that is a real problem for DE or cartridge filters.

 

[tomzo]

I have a cartridge filter and I think I will add that second check valve - found them at Westside Wholesale at a decent price.

Thanks again!

 

[JohnT]

Another reason to drain is that some panels can be damaged by water sitting in the panel. It can heat up above the rated temperature of the panels and cause them to deform.

 

[wiscousonian]

Another reason to drain is that some panels can be damaged by water sitting in the panel. It can heat up above the rated temperature of the panels and cause them to deform.

That does not make any sense...Isn't the water going to be heated to the same temp of the plastic? If the plastic is 100 degrees, the water cant be 120 degrees, it's not possible. Whether there is water in the panel or not it's still going to be heated to as hot as the temp of the sun allows.


I just put in a heater on my FLAT roof, No relief valve, Manual operated without sensors. I needed to drain mine last week for a frost warning, used a leaf blower to blow out the panels and it worked great. Simply removed both supply and return line and blew it out.

 

[bobby1017]

Another reason to drain is that some panels can be damaged by water sitting in the panel. It can heat up above the rated temperature of the panels and cause them to deform.

That does not make any sense...Isn't the water going to be heated to the same temp of the plastic? If the plastic is 100 degrees, the water cant be 120 degrees, it's not possible. Whether there is water in the panel or not it's still going to be heated to as hot as the temp of the sun allows.

I think what he means is that when the water is circulating the panes would be kept at nearly the same temp as the pool water, with out they could get much hotter.

 

[JasonLion]

When the panels get really hot they get softer. If they are also full of water they start to stretch. If they get hot enough while full of water the weight of the water can stretch them out of shape. That should never happen if the panels are made correctly, but it has none the less happened to some people.

 

[wiscousonian]

I think what he means is that when the water is circulating the panes would be kept at nearly the same temp as the pool water, with out they could get much hotter.

well yes obviously, So you tell me that all solar heaters must have water being circulated in them in order to keep them from breaking? So if someones pool is hotter than what they would like they still must run the heater to prevent it from breaking? Doubtful, no one would use them then...It's not very efficient to have the pump running whenever it's sunny, If so they would have light sensors rather than temperature sensors. Regardless if there is water within the panel or not, it's still going to heat to the same temp, the water would just take longer to heat and then retain the heat a little longer while cooling.

When the panels get really hot they get softer. If they are also full of water they start to stretch. If they get hot enough while full of water the weight of the water can stretch them out of shape. That should never happen if the panels are made correctly, but it has none the less happened to some people.

Ok this i can see, especially if mounted on a steep roof where the weight makes the panels sag, But on the OP users roof (and mine) where it's flat, the panels are fully supported and not at any slope as to provoke stretching, other than flattening due to gravity. I'm sure this type instillation is the easiest on the panels as far as stress goes.

 

[JohnT]

Stagnant water in the panels can heat up to a much higher temperature than the empty panels. Maybe even boiling the water according to some panel manufacturers. The relatively low thermal mass of the panels makes it difficult for them to retain heat without water in them.

Extreme heat and the vacuum caused by gravity pulling the water can result in panel deformation in some materials.

 

[solarboy]

Solar panels for water heating (Glycol filled) also have warnings and systems to prevent "stagnation" as the liquid can become superheated and under much higher pressure than normal.

 

[bobby1017]

well yes obviously, So you tell me that all solar heaters must have water being circulated in them in order to keep them from breaking? So if someones pool is hotter than what they would like they still must run the heater to prevent it from breaking? Doubtful, no one would use them then...It's not very efficient to have the pump running whenever it's sunny, If so they would have light sensors rather than temperature sensors. Regardless if there is water within the panel or not, it's still going to heat to the same temp, the water would just take longer to heat and then retain the heat a little longer while cooling.

All I said was the water would get much hotter if it was'nt circulating.

Many modern systems have light sensors so even when the pump turns on no water will circulate through the solar system unless the sun is out. If it used temperature only it could be circulating when there is no sun on the panels, trying to heat the pool, but really cooling it.

 

[chem geek]

My panels use a temperature sensor and it works fine. Sensing the temperature of the panel is very reasonable since even a bright cloudy day can still heat the water if the air temperature isn't too cool. If the temperature of the panel is warmer than the temperature of the pool water, then heating will occur so the valve to have water flow to the panels is usually triggered when this temperature difference exceeds a certain small amount of a couple of degrees which is also enough to ensure that water going through the panels doesn't shut off this valve too early.

 

[wiscousonian]

ok, so how should my system be changed to remove the water from the panels if the system is not being used? It's laying flat on my roof, and I'm unable to puncture my flat roof to mount any type of rack. I cant be the only one with the problem...there are many more flat roofed homes in warmer climates.

 

[solarboy]

I've used multi-silane polymer (MSP) glues to glue rack mounting pads to roofs with perfect results, even after 5 years.

 

[bobby1017]

My panels use a temperature sensor and it works fine. Sensing the temperature of the panel is very reasonable since even a bright cloudy day can still heat the water if the air temperature isn't too cool. If the temperature of the panel is warmer than the temperature of the pool water, then heating will occur so the valve to have water flow to the panels is usually triggered when this temperature difference exceeds a certain small amount of a couple of degrees which is also enough to ensure that water going through the panels doesn't shut off this valve too early.

Good point, I wonder if the temp sensor is a better way to go that the light type.

 

[tomzo]

I guess I am having a hard time figuring out the relative concerns about flat mounted solar panels. Sure, most roof mounted systems are not flat by nature and perhaps there is some efficiency gain by orientation of the panels to the sun, but my system is being controlled by a Pentair Easytouch controller that monitors the water temp, the air temp, and has a solar sensor, so the only way the water will just sit in there during daylight is if my pool has already met its high temp limit or it is a cloudy day.

Water will expand when it freezes but it will not expand when it is heated. Therefore it seems to me that the panel materials could not get any hotter with water in them than they would without water. I would be interested in seeing some documentation from a solar panel manufacturer that indicates that water can get to the boiling point.

My panels are supported on wooden slats that are 2" wide with 2" gaps, so I don't think that deformation will be an issue. Stagnation could be a water quality issue if the water was allowed to sit for long periods (many days) but my filter runs every day for 10+ hours (I run at a very low flow rate to save electricity).

The only reason this is important to me is that I don't want to go through the expense of constructing some sort of rack on top of my patio cover for no reason. I have about 1500 SF of patio cover and while it is a very substantial structure, the extra weight will need to be dealt with somehow. The solar panels are very light on a per square foot basis so they are no concern to me.

This has been an interesting discussion - thanks for all your help!