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Solar Hot Water System Sizing 7.6.2010



Date: 07/06/2010

Well, good afternoon. I would like to welcome everybody to our 6th edition of the SunMaxx Solar product webinar series. Today I want to talk about system sizing. We’d like to do this once a week and we try to keep the conversations to a half an hour, so I will end this at 12:30, and after which, anybody would like to have any questions please feel free.

[okay, I’m going to have to start over with my audio, it doesn’t appear as if my audio is working, ok well, I got– thanks Tony, I appreciate the feedback. I’m going to continue to talk — Okay]

So, what I’d like to cover today is how to properly size a solar thermal system because as you know, a properly sized solar system –our main goal is to try to get the most out of the collectors, and by keeping that operating temperature low, we get a higher collector efficiency, which I will describe in more detail a little bit later on. As well as maximizing solar efficiency, we want to try to extend the solar day, and in cases where we can start the day earlier, or end the day later, we end up getting a considerable amount more BTUs over the course of the year.

I’d like to welcome everybody now that is now joining. And one of the most important things to system longevity is not overheating the system, preserving the glycol if its glycol, or if it’s a drainback system, we want to be able to not overheat the system, not over heat the pipes, not overheat the pump, not overheat the exchangers, and certainly not overheat the collectors. So, these can be accomplished with properly sized system. Additionally we want to try to meet the requirement. So if our temperature requirement is 120, we don’t really want to exceed that by more than 20%. The reason is, the higher the temperature of the storage tank, the lower the utilization of the system. So if we can target our temperature at 120, we might exceed that by 20-24, we might go up to 140-150, but we don’t really have any use for water at 170. So if we are — if our tank is at 170, then the storage volume is probably not been considered. And then the– probably the most important among all of these is that we’ve got to be able to present the best return to our clients, all right. So a large system that fills the roof isn’t always the best return. So we have to be realistic about what this system will produce, and try to present to our clients a system that gives them best return, and often times that is a smaller system, okay.

So I’ll go through a couple rules of thumb. I will be redundant because there are some points that are more important than others. If you have any questions, I’ll remind you to please feel free to type any questions in the chat box so I can see them, and I tend to respond immediately, okay.

All right, so a few things that we have to consider, first, is probably the roof space. Generally that’s going to be the limiting factor in system size. Probably 70% of the time, the roof space is never enough to get more than 40-50% solar fraction anyway. And also, when we are sizing systems, we size them based on the square foot of collector. Not how many collectors there are, rather it’s the square footage of collector surface, okay. And when you do this per–, you have to be clear whether you are talking about gross area, or aperture area. Some, for instance SRCC will give you the BTU output for collector based on gross area. So either way, just be sure you know what you are talking about in terms of absorber area or gross area, nevertheless we’re gonna size these systems based on square foot, not based on number of collectors, okay.

Another consideration, just to bring it up early, is, for those of you who have been through the training, you know that we try to size up our storage tanks at about 2 to 3 gallons per square foot. I wrote 2 there, but it’s really 1 square foot, whether it’s flat plate or evacuated tube. Now, I will talk a little more about the difference between 2 square foot vs. 3 square foot a little bit later, but generally it’s dependent on the temperature requirement. Storage space in terms of your solar thermal storage tank will be really dependent on the temperature requirement.

Another thing when it comes to sizing the system is that, for residential systems, we really don’t want to go more than 150 feet away, basically because of pressure drop and velocity. Another very important point, as you’ll understand as we move through the next half hour, is that the higher the solar fraction, the lower the total system efficiency. This is generally the case that as we approach 85 and 90% solar fraction, we end up using fewer of those total BTUs that we produced. So system efficiency is a function of the total number of BTU’s that strike our collector vs. those that we actually consume in our storage tank. And the higher that solar fraction is that we are trying to achieve the lower the total system efficiency. If we can get our system efficiency, total system efficiency, between 30-50%, that’s actually a pretty good efficiency and TSOL report which I will show you later will help us determine those efficiencies. So we’re shooting for the intercept between the highest solar fraction and the highest efficiency. And that goes right along the same lines as, if we’re– the higher solar fraction of 95% may have a system efficiency of 12%, but we can get double the efficiency with –or smaller solar fraction which means higher rate of return with a higher fraction.
Okay, another point that we need to know is the load profiles. We have to understand that peak consumptions during the day, the daily load profile. This will help us size up our storage tanks. So if I know that I’m consuming most of my BTUs at the same time that I’m producing most of my BTUs, then I’m gonna have a much, much smaller storage tank, and in some cases I won’t have a storage tank at all.

Now, in terms of the seasonal load profile, this will help us determine system size. So over the course of the year, does that BTU load change/vary from one season to the next, and how does that seasonal load profile compare with our seasonal production profile. As often times these two points, a daily and seasonal load profiles are not readily available. But when you do have access to the load profiles, particularly the daily load profile, it really helps us understand the total system size — storage tank size.
Another consideration is, the summer time– the amount of solar radiation often is 2 to 3 times greater than it is in the wintertime. So if we have a low load in the summer and a high load in the winter, then what’s gonna happen in the summer time, we’re gonna produce — we might produce 4 times than what we need. So we really have to look at our summer time insolation, to determine the maximum system size, we’ll do that a little bit later. Now we can also affect that system efficiency for summertime by increasing the incident angle. So by standing our collectors upright, we decrease the total production of the collectors in the summertime, which gives us a greater amount of BTUs in the wintertime, and fewer–. So basically we can compensate for that higher insolation level by increasing our incident angle.

I’ve got a question here, what do you mean by efficiency? Well efficiency is really a function of the total number of BTUs from the solar insolation that strikes the collectors, relative to the number of BTU’s that we produce that we actually consume, okay. So, its efficiency of the system is not only efficiency of the collectors but it’s also the utilization of the system. Utilization being how many BTUs we product vs. how many BTUs we consume. And that’s– collector efficiency is a function of the insolation that strikes the collectors relative to the temperature of the collectors and the ambient temperature. I’ll get into more of that in just a bit.

Okay, Some guidelines for domestic hot water. I’ll try to keep it very simple. For small residential systems, which is the largest market in the United States right now, we recommend 10 sq. ft. of collector per person, okay. Now I wrote in parenthesis there 52% solar fraction (SF) solar fraction. That is really designed for Syracuse, New York, one of the places in the country that gives us the least amount of insolation. So following that guideline, 10 sq. ft. of collector per person for Syracuse New York — if you live anywhere outside of the Southern tier, or central New York, you’re probably gonna get more insolation than us which will give you a higher solar fraction. Okay, to be specific, in terms of your solar fraction, your gonna look to your SunMaxx representative, and they’ll give you a specific program– analysis– TSOL report, that will give you your solar fraction for your particular area. But you can guarantee that the federal incentive, which requires 50% solar fraction, if you go with this 10 foot of collector per person, then that’s designed for Syracuse, New York which has a very low insolation level. We can also look at the total hot water bill– I’m sorry, the utilities bill, for heat and hot water, and typically domestic hot water is 30% of that, so we can indirectly determine the number of BTUs consumed by those individuals for hot water if we understand that they’re household fuel built. Another way to look at is that generally the American average is 20 gallons per person. Now that’s only for the first 2 people, okay. The third person, they consume 15 gallons, and the forth person is 10 gallons. So we have a D rating of the usage per person per day. This is the American average, all right.

Now for large commercial domestic hot water systems, or even large residential systems, we can calculate the BTU load per day if we know the gallons. So the delta G that I am referring to is the difference between your max and the min. What is the required temperature minus the temperature of the incoming ground water? That gives us our gradient, our delta g. How much do we want to see the temperature rise by? Every degree requires 8.3 BTUs per gallon, okay. So I can calculate the BTU load by looking at the delta G, which is the difference between max and min, multiply that by total gallons and I multiply that again by 8.3, that gives me the BTUs per day. 8.3 incidently, is the number of BTUs that it takes to heat one gallon, one degree. Or, it’s the amount of 1 BTU is the amount of energy it takes to raise 1 pound of water, 1 degree, and there’s 8.33 pounds to a gallon.

Okay, now we have to determine what the BTU output is per square foot of collector. And I’ll go through a little exercise is a minute to show you how we do that. And typically we look at the efficiency, average efficiency of our SunMaxx collectors is 73%, that’s at a very low or 0 delta T, that efficiency is gonna change periodically throughout the year, so it’s very difficult for us to calculate exactly the production per day without using software or taking our time and methodically going through the logarithms to determine the change in efficiency relative to the insolation. And then what we want to do is fine what solar fraction is most suitable. And by most suitable I mean, which one is gonna accommodate the roof space that’s available, the storage space that’s available, and the budget that’s available. So we– even though they might have 100,000 dollars for a solar thermal system, a smaller system might give them a better return. Okay, so you have to remember that your business is going to be referral driven, and you want your customers to reap the reward financially as much as possible. So sometimes a smaller system is better.
Okay, one rule of thumb, and remember, I told you I was gonna be a little redundant because some are very important, the higher the temperature requirement, the lower the total utilization, okay. So whenever we can lower the temperature requirement, you get more out of the solar. For instance, if I have a solar thermal system and my hot water tank is set at 120, and I’ve got two flat– I’ve got a 42 square feet of collector. Well I can increase the utilization of my solar thermal by lowering the temperature requirement of that storage tank down to 115. Most people take showers at 104 to 107 anyway, and in many cases, hot water, dish water – dishwashers have their own little heating element to bring the temperature of the water up. So if we can lower the temperature requirement, you’re gonna get more out of the solar thermal systems. Okay, and again, a high solar fraction equals a lower system efficiency, and we never want it to exceed 100%. So if we look at the June’s data, I wanna look at June’s load, and it might be low, but the insolation is very high, so I really have to set my maximum number for June. If I can meet 100% of June, knowing that I’m not going to go over 100%, then I can determine the total solar fraction by looking at that maximum square footage that I’ve determined to be less than 100% in June, and find the most suitable solar fraction. And we’ll do that in just a few minutes.

Okay, we have to a also understand the consumption vs. production, okay. So insolation varies from day to day, but we have to try to accommodate the varying insolation levels so that we can meet a pretty good load each day. Whether we meet 30% of the load or 90% of the load, we have to take into consideration the changes in insolation not only seasonally, but daily. And from our experience, the solar fractions that are giving us the greatest return are those that are falling between 30 to 60%. So one of the biggest mistakes you can make is to go tell the client you’re gonna replace their existing fuel source, or you’re going to reduce their hot water bill by 95-100%. There’s very rarely does that happen, and when it does happen, they’ve spent more money than they needed to, so we want– have to play it safe this early in the industry and we wanna make sure that our clients are getting their money back from these systems and those that are getting the most money back are the ones that have the high hot water load, low temperature requirement, and have had systems designed for 30 to 60% of their solar fraction.

Okay, and again, we can determine the storage volume required, by looking at the amount of BTUs that can actually be stored in one gallon of water. So if I have a useable BTU temperature of 120 degrees, anything above that, I can very easily calculate how many gallons it’s gonna take to store a certain number of BTUs, because I know that each gallon per degree can store 8.3 BTUs .
Okay, for sizing and design, first we have to determine total BTU load per day. And this may change daily, it may change seasonally, but we have to get an average. We have to know what that total BTU load is. Once we do that, then it’s important for us to understand the load profile seasonally and daily. If you remember from earlier, not quite 15 minutes ago, I described the importance of seasonal load profile and that is for system size, and the importance of daily load profile is for storage tank size, okay. Now you can– one of the biggest mistakes you can make is not understanding the solar potential, okay. The solar potential — what we really need to know is available roof space, and the insolation that’s gonna strike that roof. So when it comes to using a, or Google earth, and zooming down onto a roof top, you’re getting a picture of that roof. You might now see any shading, it looks like its got full sun, but it can be really misleading. So you really have to use some type of a tool, an instrument, particularly a solar instrument to determine solar potential. Solar Pathfinder, Solmetric SunEye, those are two very easy readily available instruments. The Pathfinder is about 300 dollars. You can get that right from your SunMaxx rep, or a Solmetric SunEye, those I’ve heard are in the 1600 – 1700 dollar range, but you’ve gotta determine the solar potential on the roof, and that’s our starting point.

RETScreen is a free software, I’ll show you a little bit later, that will help us determine the insolation values for different regions of the world, that might now get very specific to a small town in America, but for most states, there are several different cities to choose from that will be very close. So we have to understand, the roof potential and the insolation, both of which you can find from either going on site using a Solar Pathfinder, and then online using a RETScreen for your insolation data. From that we have to calculate instantaneous collector efficiency, although you may know that the thermal collectors operate at 73 to 75% efficiency, that changes relative to the incoming temperature, it also changes relative to the ambient temperature. So in order for us to determine total system output, instantaneous collector efficiency really has to be calculated on a day by day basis. It can be very belaboring. Luckily we use some software that does all the calculations for us, and then we need to know production vs. consumption and from that then we’re gonna determine the flow rate and storage size, and what type of heat exchange that we’re gonna use. Heat exchangers, by the way, will be another webinar in the very near future, but there’s a lot of things we can do to increase the system performance by choosing the right and most appropriate heat exchanger.

So, before we begin to size, we have to know what the functionality of the collectors are that we’ve chosen, so I’m gonna show you a picture of an SRCC certificate, and I guess I need to apologize for those of you if this is unreadable. You can visit Solar Rating is the location where all the SRCC certificates are located online through SRCC. So if you do a web search SRCC, you’ll come to this website. And you can pull up the certificates, and I encourage you to do that. And a couple of things you want to pay particular attention to, again I see something a little different on my screen, it’s not visible, but you might be able to see this.

Things you want to look at when you’re sizing up the collectors are the slope. Okay, one is the slope. So BTUs per hour per foot per degree basically tells the losses in efficiency relative to the Ti minus Ta which is this category right over here. Categories A, B, C, D, and E are 5 different categories basically representing climactic conditions, so for instance, the Ti minus Ta of 144 degrees would be in category E. Okay, under those conditions it says mildly cloudy, these collectors are going to produce 1.3– basically 1300 BTUs per panel per day. Okay, so the two most important things to look at when you see these–when you’re looking at the certificates, are the Y intercept, this Y intercept is our collector efficiency at 0 Delta T, and I’ll show you a graph in just a moment, and also the slope. The slope is the degree to which the collector loses efficiency based on the Ti minus Ta, or the ambient temperature vs. inlet temperature. Okay, so these different categories are climactic categories, and clear, mildly and cloudy represent 3 different levels of insolation, 2000, 1500 and 1000 BTUs per foot squared per day.

Okay, so when the collectors are measured– analyzed through SRCC and performance tested, they are– determining the collector efficiency, which on the certificate is called the Y intercept, and they do that by looking at the inlet fluid parameter. The inlet fluid parameter is basically the inlet temperature, the temperature coming into the collector minus the ambient temperature. Okay, so whatever the ambient temperature is subtract that from the inlet temperature, and then it’s divided by i. i is the total solar insolation that strikes the collectors, okay, and that’s per foot. So we don’t need to know the total for the collector, because it’s divided by– per square foot of the collector, all right. So if we look at the graph here on the right hand side, the collector efficiency is the function of the inlet fluid parameter. The inlet fluid parameter, Ti minus Ti divided by i times this– multiply it here, this is the slope from SRCC, so we multiply the inlet fluid parameter times the slope, and it gives us the efficiency. If we work our way over to the Y axis, this will tell us the Y intercept, okay. So for example, of an inlet parameter of .6, the efficiency of the flat plate is 24%. If we are looking at the inlet fluid parameter of .4, the efficiency of both flat plate and evacuated tubes are the same.
So it’s very important to understand what determines this inlet fluid parameter, because inlet fluid parameter is gonna affect our performance more than anything. Well, there is two things that determine inlet fluid parameter. The ambient temperature, and the collector inlet temperature, all right. Which of those two do you have anything to do with? Well, the only way you can affect your ambient temperature is to move south, which in most cases can’t happen. The second thing you can do is lower your temperature requirement so that inlet temperature is basically a function of your storage temperature of your requirement. So the lower that is, the lower your inlet fluid parameter, which gives you a higher efficiency, okay. So these are things that we need to consider before we can determine the system size.

Okay, so this is another graph showing you how we determine the inlet fluid parameter. We’ve got the Y intercept is the location where this slope crosses the Y. So if your inlet fluid parameter for example is .7, .7 will give us an efficiency of 20%. So obviously as I said before, the best way that we can increase the efficiency of the collectors is to lower that inlet temperature. Lowering that inlet temperature by — by lowering the temperature requirement, will give us a higher efficiency, which will ultimately give us a better payback.

Okay, so to determine the number of square foot of collector. First thing we need to do is find that maximum square footage of collector is, and remember we don’t wanna exceed 100%. Then we have to determine what the total BTU load per day is and divide that by our peak production, okay. I’m going to run through an example in just a moment that illustrates this. So once we have our total BTU load per day, okay, which we used our delta G times 8.3 times a gallons. I have to divide that by their peak production per day that’ll give me the maximum number of square feet. Once I have the maximum number of square feet I have to find out what the best solar fraction is. The best solar fraction is the one that never exceeds 100%, because, remember the more BTUs I produce that I don’t consume, the more that system is gonna cost, okay. So I want to try to bring the cost down as much as possible, and I do that by consuming every BTU. So for step 4, I have to find the maximum number of square foot, which I did in Step 2, and multiply that by the average output. Okay, that is using insolation values, and the average efficiency of the collectors which will give me the average total production. Once I have the average total production, then I divide the average total production by the total BTU load per day and that will give me the recommended solar fraction so that I do not exceed 100% in June, okay.
Now, this is something that you can potentially go to your client with to give them a preliminary proposal, and say, yeah it looks like we can do about 60% of your solar fraction, whatever. To be much more professional, this would be a good chance for your to turn to your SunMaxx rep so they can print out a– or actually I send you a copy of a Tsol report that then you can present to your client which gives them a much more specific solar fraction in detailed analysis.

Okay. Now here’s a sizing example I can run through you real quick. For example we have 100 gallons per day water consumption, okay. Actually 1000 gallons per day. Water consumption is 1000 per day. Our delta G, our rise in temperature is the difference between max and min., and that’s 65 degrees. And our BTU load per day is 65 degrees times 8.33 times 1000 which is 541,450 okay. And then I need to find out what the average insolation per day is so I use RETScreen, and I found 4.62 kilowatt hours per meters squared per day. In order to convert that to BTUs I multiply by 317 which gives me 1465 BTU’s of average insolation per day. Now to determine my BTU output, I got to multiply the average insolation which is 1465 times the collector efficiency. So using the parameters that I showed you on the previous slide, in determining the collector efficiency, I’ve calculated the average efficiency of 66%. So I multiply 1465 by .66 and it gives me 966 BTUs per day per foot squared of collector. Now I need to find out the peak insolation. And I found that to be 6.3 Kilowatt hours, using RETScreen. Multiply that by the converting number of 317.1, gives me 1997.73 BTUs per foot square per day. Finding the peak output, I looked at the collector efficiency, which I determined to be .66, multiply that by the peak insolation, and I get an output of 1318 BTU’s. So now I take 1318, and I divide that into my total load, which is 541,450. And that tells me that in June, if I’m producing 1300 BTU’s per foot squared, I can meet that load with 410 square foot of collector. All right, that gives me 100% of my June load, and ideally not a BTU more. So if I take my 410 square foot maximum, and I multiply that by the average output per tube, which is 966, that gives me an average production of 396,844 BTU’s. So then I take 396,000 my average BTU production and I divide that by my average BTU load and it gives me a 73% solar fraction. So this solar fraction is pretty high. The heat demand is low. The lower the load, the lower the temperature requirement, the higher the solar fraction can be, okay.

Now RETScreen is a free download. I strongly recommend using this as a tool. You can go to and download RETScreen, or call on me or any of the sales reps to give you some simple advice on how to use this, but it’s pretty user friendly, and it’s a good tool to look at insolation data. It does a lot more than what you really need it to do. So for starters, if you haven’t used it yet, give it a shot, it’s a good encyclopedia if anything else. And then, to give you a quick little glimpse of some of the reports that you’ll get from TSOL, we’ll get our system schematic which will show us the collectors, the square foot of collectors, the azimuth angle, the inclination angle, give us the volume of storage, give us our traditional heat source, the load, 40 gallons per day at 120 and any heat transfer that’s associated with that. It’ll also give us in the course of the year, our production profile relative to our consumption profile, which is exactly what we’ll need for system sizing, okay. And then it breaks down all these little components very specifically, ultimately giving us the total system solar fraction, which in the case is 45%.

Okay, and lastly storage tank sizing. We really have to understand the daily load profile, so that we know how many extra BTUs are we going to consume during the day, that we won’t consume– how many will we produce that we won’t consume, the time of use, what time will be peaking our consumption. Also the temperature requirement. What is the necessary temperature that we are trying to achieve? And remember that we don’t want to achieve or reach more than 20% of the requirement. This allows us to increase the efficiency of our collectors. If you remember the temperature requirement is basically going to determine the Ti, the inlet temperature of the collector, and Ti is the only variable that you have any control over. So the only way that you can really increase the collector efficiency is to lower the temperature requirement which properly sized storage tanks can do that for you. And then obviously we have to look at the output of the collectors which changes throughout the year, based on the efficiency and the ambient temperature, and azimuth orientation.

What I try to do, is I try to be prompt when I start, I have run 4 minutes over, I want to keep these to a half an hour so I don’t take up too much of your time. I’m always more than happy to help you out with any further questions. If you have any other questions and you’d like to have them answered now, go for it and type a question right in. Otherwise I would formally end the presentation now and hopefully I can see everybody back next week. We do have a pretty good schedule set up for the next– I think we’re already set in stone through September, and– but we’re always open for suggestions. If anybody has a particular topic that they’d like us to cover. We have been saving these webinars and you can revisit the website where you are registered and you can see the past presentations. It takes a few days to get them online, but it’s really my responsibility to record as I start, and so far I have recorded 3 out of 5 successfully. So go ahead and visit and you can find a copy of this presentation and others, okay. Well, I’m gonna stop recording and if anybody has any questions, I will be glad to stick around for a few minutes.

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