Solar Thermal Piping 9.1.2010

Solar Thermal Piping 9.1.2010



Date: 09/01/2010

All right, well today’s topic is going to be solar thermal piping, and as many of you may know there’s a tremendous variability in the way you pipe a system, and piping really does influence performance in many ways. Not only performance, but also cost. So you’ve got to try to maximize performance and minimize cost, so there’s a few things you have to consider and what I’ve seen most is that guys are using pipe sizing that is too big. Too big in the sense that it works and there’s really no downside other than initial cost, and you would sacrifice a little bit of performance. So, we’ll move right along here.

When you’re designing a system, there are certain considerations that have to be made with regard to piping. Before I go too far, I should have done this – can someone confirm that you can hear me? I’m sure you can, but I like to find out ahead of time before I get too far into the meeting. All right, great, thanks.

Okay, so obviously piping is really going to be determined on flow rate, okay I don’t want to state the obvious, but flow rate is probably the most important factor in maximizing collector efficiency. So the flow rate is dependent on the collector, and the piping is dependent on the flow rate. So, I’m going to talk more about how to determine flow rate, and why it’s important in sizing up your system. Also, for those of you who aren’t hearing anything, it must be a local issue. Perhaps you can make some adjustments on your speaker volume, and if we go through this and unfortunately if you can’t still hear, then you might access the archives and you’ll be able to.

Okay, now your pipe sizing is also – you have to consider the temperature, not just the pipe sizing, but the type of pipe used. Many people are inclined to go with the least expensive, of which would be some pex tubing. Now as most of you know, pex tubing cannot be used for solar thermal systems. The rating at 180 degrees is still, if the system is running well, 180 is fine, but if Murphy has anything to do with it, there will be times when you might bring a 220 degree shot of steam down off of the roof, in which case it would melt and violently explode your pex. So, as a manufacturer, we recommend that you don’t use pex tubing at any part of the solar loop. Now pressure drop is another important consideration because it over the life of the system, the more pressure drop the more energy you’re going to have to consume with the pump and then that energy cost money. So you want to try to design the system that first accommodates the flow rate, but also reduces the pressure drop.

Now, obviously as I said before, you want to have the smallest diameter pipe possible, but the smaller diameter pipe also increases pressure drop, so there’s an intercept that we have to identify with the least pressure drop and the smallest diameter. Volume inside the piping is another important consideration because volume in some regards can be used as a buffer. Especially with larger systems, we can step up the size of the pipe because we want a larger volume of fluid in the pipe to act as a buffer because perhaps we’ve reduced the storage volume. And that can impact your system volume in your storage by 100 or 200 gallons by changing the diameter of the pipe, particularly for commercial jobs.

Installation and aesthetics, you know as you see this picture here, there’s probably about 18 feet of piping, but it’s been designed in a way that you see only about 3 feet of it. So you really want to consider how you’re going to pipe it, how it’s going to look on the roof, and then ultimately the cost. So, first of all, as I said, the most important thing is determining flow rate. So flow rate has to be determined by the total foot of the collector. If you have certain collectors in banks or in rows, then each row is going to have to have its own unique flow rate. Now if the layout of the system is not balanced, each array or each row may receive a different flow rate. The only real way to accomplish that unbalanced system is through the use of balancing valves, which I’ll talk about a little bit more later.

So for flat plates, the TitanPower, now this is unique to this manufacturer’s recommendations, obviously other manufacturers will have different recommendations, but we recommend a .022 gallons per minute per foot squared. Okay, so that is for all collectors in series. So one collector in series would be 21 square feet times .022, if you had 10 collectors in series, then it’s 10 times 21 times .022. Now you’re going to add the parallel arrays to get your total GPM, but each array will have a unique GPM. So, when you’re doing these calculations, you first have to add up the total square footage of each row, and then add the rows together. For vacuum tubes, the thermal power brand, we recommend .028. Now these flow rates are very closely connected, so if you were to build a hybrid system, which I know Pete Skinner of E2G Solar has been doing some studies on the use of hybrid systems, where we’re flowing flat plate into vacuum tube, and interestingly enough time and again those hybrid systems have been outperforming both the flat plate and the similar sized vacuum tubes. That’s a discussion in itself. But if you do, then knowing that the flow rate required for each of those are close enough that you would probably shoot for middle of the road, so with a hybrid system you would go with .025, okay.

Now, the flow rate then, once you’ve established your flow rate, then we have to determine what size diameter is going to be able to handle that pipe. Now generally you want to use the smallest diameter pipe. We have some systems out in California that are running with 3/8 copper tubing. Now the diameter of the pipe is really going to determine how many total BTUs can be transferred through there. And it always works out that the limiting factor of the flow rate for the collectors is usually less than what the BTU capacity that diameter pipe is anyway. So, there’s two ways to size pipe. One would be how many BTUs can you actually transfer through that diameter pipe, and the other is what’s the flow rate required for the collectors? And so we’ve always found that by using the flow rate to the collectors, you’re always going to be able to handle the minimum, or at least you’ll be producing less than the minimum BTUs that that diameter pipe can transfer, if you understand what I meant there.

So, total flow is really a function of velocity times volume. So, if we know we need a certain flow through the collectors, we have to understand what the volume in the collectors are, or in the piping, and then the velocity. Now the velocity, we can obviously increase the flow without increasing the volume simply by increasing the velocity, but there’s a threshold of velocity that we need to stay less than, and that’s 5 feet per second. One of the dangers is you get some particulate abrasion, the friction that’s created against the walls of the soft Type L copper, even the stainless steel, over time will erode the manifolds, the inside of the flat plates. So I’ll show you some charts in just a few moments. It looks at the function of velocity and volume and how it relates to flow rate. Another consideration for pipe size is the pressure drop. So the higher the velocity, the greater the pressure drop. The larger the diameter, the lower the pressure drop. So as I said, we want to find that intercept between having the smaller diameter pipe possible, keeping that velocity less than five feet per second, and reducing the pressure drop. Anybody have any questions?

Okay, it appears on my screens that you may not be able to read this chart. And I can make this available to you. This comes out of the U.S. solar handbook that was written back in the ’80s. And I’ll reference it for everybody at the end, if you like. But basically this lays out for us examples of different pipe sizes and flow rates, using a 50/50 mix of propylene glycol and 150 per foot is going to give us, if we’re using half-inch, about a 14 psi drop in pressure. And as you can see, if we increase the velocity from 4 feet per second to 6 feet per second, the pressure drop doubles. So an increase of 30% in velocity, the pressure drop doubles. So that’s a pretty consistent relationship that you have to remember. It’s a disproportionate increase. And that’s true with any diameter pipe. So we really have to be careful of increasing the velocity too much.

Another consideration is balancing the system. So once we’ve determined our pipe diameter, for example, just to back up one second. If we typically install ours for residential system, we’ll go with 3/4″ pipe. Without really making any other considerations, 3/4″ pipe would work and it makes your decision easy. But the difference of 3/4″ pipe, perhaps you can use 1/2″. For a one-collector or two-collector job, your flow rates only going to be about 1 to 2 gallons a minute. And at 2 gallons a minute, we can use the 1/2″ pipe. So you’re going to be able to save, if you have 200 feet of piping, you can save a considerable amount of money by reducing that pipe volume, or pipe diameter. So I want to point your attention to the fact that many times, and I say that with a lot of experience, most systems that I’ve been involved with selling or designing, the initial design always has pipes that are oversized. Now, that’s not a 100% “you’ve got to reduce the pipe,” because, remember, the larger volume means lower pressure drop. And a lot of the pumps that come into our pump stations and other manufacturers’ similar pump stations, they’re really designed for optimum operation at 3 to 4 gallons per minute. So when we get up to 5 or 6 collectors, that is to say, 150 square feet a collector, then it’s time to go to a larger pump. But anything less than that, you should consider reducing the volume or the diameter of the pipe.

Now onto balancing systems. Now, as I mentioned flow rate being most important, you have to make sure that each array has the appropriate flow rate. Putting collectors up on the roof, piping them according to manufacturer specs, and then walking away and looking at your flow rate down at the pump station, you may know you’re sending 2 gallons a minute up to the roof. But if you have multiple arrays or several rows or even two rows or two collectors in parallel, you have to be sure that each collector is receiving the right flow. And there’s really two ways to do this. The most traditional is reverse return piping. By making sure that each loop has the same pressure drop, the pressure drop in that case would be a function of the pipe length. So by using reverse return, you balance the pressure drop. By balancing the pressure drop, you indirectly balance the flow rates.

Now flat plates, most of them on the market now use internal reverse return piping. So I’ll show you some diagrams here momentarily. Even though you may assemble the collectors in series, internally they’re really piped in parallel.

Yeah, I just got a reminder. All of our webinars are archived at And I think we’re up to 11 now, maybe 12. So you can always access these. If you have to leave, I certainly wouldn’t be offended. But please remember that if there’s questions, feel free to ask them now. I’ll just give you my e-mail later. You can e-mail me. And as always, you can access these archives at

One of the considerations with reverse return, especially in colder climates, is you do suffer from a greater amount of heat loss, because you will have more external piping. The other option for balancing systems is to use balancing valves. One thing I’ve noticed with talking to 3,000 or more installers across the country is, generally, there’s a division between reverse return piping has generally been accepted by plumbers. Plumbers tend to use reverse return piping. And it guarantees that a system will be balanced inherently based on the pressure drop. Yes, there is audio recording as well, Eric. Yup. Balancing valves have been most often adopted by HVAC professionals. Now balancing valves will cost a little bit more, but they can be adjusted. So if you add onto an array or you reduce an array, you can always adjust it. So it’s much easier, I think, over the life of the system to ensure the proper flow rate through the use of balancing valves. A drawback, obviously, is it will cost you a little bit more. And it does increase the pressure drop, which means that you will need a larger pump. And lastly, the balancing valves will go bad. Everything has a life, a shelf life. A valve will go bad before a pipe goes bad. So if long-term maintenance is an issue and a concern of yours, then you probably would consider doing reverse return.

Now as I mentioned, I just want to point out, these half-hour webinars are intended to peak you interest, to help answer some questions you have. But it’s very difficult to cover everything that I’d like to say about piping in 30 minutes. So I just want to remind you to e-mail me questions. We can help answer anything else, beyond what I can cover in just 30 minutes.

Piping series vs. parallel. Many of you may understand the efficiency and how efficiency is determined, whether it’s TI – TA and the fact that the colder the temperature coming in, ultimately the higher the efficiency will be for the collector. So by putting collectors in parallel, each collector will have the same Delta-T, and thus each collector will exhibit not only the same efficiency, but you’ll have higher efficiencies in each of those collectors. Now that efficiency increase is really proportionate to the size of the system. So if you only have two collectors, you’re not going to see much difference, whether you put those collectors in series vs. parallel. But in a larger system, in the end, parallel banks will yield a higher efficiency. It also does lower the pressure drop, which will cost you less money or cost the client less money in the end. Putting banks in parallel does require more external piping as well. So external piping means more heat loss. So these are considerations. It’s not an end-all. It’s not going to answer everything. But you do have to consider how much external piping will I need if I put these banks in parallel. Now in series, it’s a trade-off, although you will lower the efficiency, the result will be a higher temperature. So we’ve been doing this a lot with systems where we’re tying into heating systems that have a high temperature load. The more–

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