:thumbsup:Awesome site. All kinds of info.
solar-thermal panels
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:thumbsup:Awesome site. All kinds of info.
magicheater
Veteran Member
Quite the contrary, I was giving an example of what I have for a system. Any renewable energy system is good, however you can only expect so much out of them. To use air as a transfer medium and expect 135 degree water is a little unrealistic in my opinion and experience. Raising 40-50 degree water to 90-100 degrees lessens the amount of fossil fuel used to bring it the rest of the way. Just sharing my Data in this venue. I have an air system also but use it for space heating only and that is marginal at best when cold out. What kind of system do you have?
crazyal
Super Member
I've been toying with the idea of adding some myself. I've been looking at the evacuated tube style. They may cost more but from what I've been told the style with the copper tubes that run back and forth tend to expand and contract and leak. Here's a link:
http://www.rstreps.com/pdf/install-vacuum-tube.pdf
The water only goes across the header and if a tube breaks the system still works. Plus they are suppose to work better in shade.
http://www.rstreps.com/pdf/install-vacuum-tube.pdf
The water only goes across the header and if a tube breaks the system still works. Plus they are suppose to work better in shade.
magicheater
Veteran Member
These are supposed to be more efficient. I have not experienced any leaks in my panels but only have 5. Any more than 6 and you do need an expansion joint engineered in.
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Raspy
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The most important consideration when designing a hot water solar system is the freeze protection method. Sounds crazy, but it leads to more failures than anything else. Not just in the freeze damage, but also in other water related problems like corrosion and mineral buildup. Also, the collectors will be colder than the outside air at night, so even in temperate climates freeze damage can occur.
Next is collector "efficiency". This is another mistake people make. Evacuated tube collectors, for instance, can be expensive for their apperature are and also delicate. They can produce very high internal temps when stagnating. These are not characteristics wanted in a hot water system. Just stick with a good quality copper absorber plate with either black chrome (for marginal conditions) or black selective paint, a tempered glass cover and a good aluminum frame. This is the most cost effective design and the easiest to find. The riser tubes in the collector should be large and there should be minimal spacing between them. That's it.
Design the system with a relatively large storage and don't worry about getting the water up to temp. Design it as though the tank was being used to cool the collector. Do whatever you can to "cool" the collectors. Large storage, relatively high flow rate, small differential temps, etc. Remember that efficiency is based on temperatutre. The cooler the panel the less it's losing to the outside. This thinking will deliver the max to the storage tank.
Back to freeze protection and long system life. The best way to design the system is with a method to drain the collectors by gravity when there is no sun or any kind of failure in the system. Do not depend on power to protect the system! In other words, no recirc freeze protection, no electric valves that take power to drain the system, etc. This will make the system much more reliable and more efficient. My favotite is drain back. This is where there is a small holding tank, approximately 4 gallons. that holds the collector water at night. When the collectors warm up, the circulator pushes the water up to the collectors and the air that was in the collectors is stored in the small tank. Then, when the power fails or when the sun sets, the pump stops and the water drains back to the holding tank and the air moves back to the collector. Beautifully simple, no toxic or expensive antifreeze, no mineral buildup, no pressure in the collectors, etc.
Finally, and thanks for your patience, use a differential controller to run the collector pump. Not a timer or a photo switch. And not a photovoltaic collector dedicated to supply the power (spend your money on hot water collectors and use grid power). Place the collector sensor inside the collector against the fin, between the riser tubes, and near the top. The best position is against the back of the absorber where it is out of the sun but sensing plate temp. This can simply be attached with a small sheet metal screw. If you put it in the front it can be fished in near the piping outlet and just lay on the plate. The tank sensor can be slid in near the bottom of the tank, between the insulation and the tank itself, behind the lower cover or near the pipe. No mechanicaL connection is needed, just contact, and not where it is affected by outside conditions. So, try not to put the sensors on the piping near the collector or tank, but in the collector and against the tank itself.
My system is currently (2) 4X8 collectors with painted absorbers set up this way. It has 120 gallons of storage in series with a 50 gallon water heater. My solar tank is simply a 120 gallon electric water heater and my holding tank is a 4 gallon electric water heater. There is a recirc pump on a separate differential controller that recircs the hot water between the gas water heater and the 120 gallon solar tank whenever the solar is warmer than the water heater, giving me 170 gallons of storage and instant hot water at the faucets. With three of us here we can turn off the water heater for about 6 months with no shortage of hot water. For a few months the water heater runs some as needed and, in the stormy winter, it carries most of the load. This is in the Central Valley of CA at near sea level and with poluted air. Higher and cleaner locations can see about 30% to 50% better performance.
Next is collector "efficiency". This is another mistake people make. Evacuated tube collectors, for instance, can be expensive for their apperature are and also delicate. They can produce very high internal temps when stagnating. These are not characteristics wanted in a hot water system. Just stick with a good quality copper absorber plate with either black chrome (for marginal conditions) or black selective paint, a tempered glass cover and a good aluminum frame. This is the most cost effective design and the easiest to find. The riser tubes in the collector should be large and there should be minimal spacing between them. That's it.
Design the system with a relatively large storage and don't worry about getting the water up to temp. Design it as though the tank was being used to cool the collector. Do whatever you can to "cool" the collectors. Large storage, relatively high flow rate, small differential temps, etc. Remember that efficiency is based on temperatutre. The cooler the panel the less it's losing to the outside. This thinking will deliver the max to the storage tank.
Back to freeze protection and long system life. The best way to design the system is with a method to drain the collectors by gravity when there is no sun or any kind of failure in the system. Do not depend on power to protect the system! In other words, no recirc freeze protection, no electric valves that take power to drain the system, etc. This will make the system much more reliable and more efficient. My favotite is drain back. This is where there is a small holding tank, approximately 4 gallons. that holds the collector water at night. When the collectors warm up, the circulator pushes the water up to the collectors and the air that was in the collectors is stored in the small tank. Then, when the power fails or when the sun sets, the pump stops and the water drains back to the holding tank and the air moves back to the collector. Beautifully simple, no toxic or expensive antifreeze, no mineral buildup, no pressure in the collectors, etc.
Finally, and thanks for your patience, use a differential controller to run the collector pump. Not a timer or a photo switch. And not a photovoltaic collector dedicated to supply the power (spend your money on hot water collectors and use grid power). Place the collector sensor inside the collector against the fin, between the riser tubes, and near the top. The best position is against the back of the absorber where it is out of the sun but sensing plate temp. This can simply be attached with a small sheet metal screw. If you put it in the front it can be fished in near the piping outlet and just lay on the plate. The tank sensor can be slid in near the bottom of the tank, between the insulation and the tank itself, behind the lower cover or near the pipe. No mechanicaL connection is needed, just contact, and not where it is affected by outside conditions. So, try not to put the sensors on the piping near the collector or tank, but in the collector and against the tank itself.
My system is currently (2) 4X8 collectors with painted absorbers set up this way. It has 120 gallons of storage in series with a 50 gallon water heater. My solar tank is simply a 120 gallon electric water heater and my holding tank is a 4 gallon electric water heater. There is a recirc pump on a separate differential controller that recircs the hot water between the gas water heater and the 120 gallon solar tank whenever the solar is warmer than the water heater, giving me 170 gallons of storage and instant hot water at the faucets. With three of us here we can turn off the water heater for about 6 months with no shortage of hot water. For a few months the water heater runs some as needed and, in the stormy winter, it carries most of the load. This is in the Central Valley of CA at near sea level and with poluted air. Higher and cleaner locations can see about 30% to 50% better performance.
Last edited:
magicheater
Veteran Member
Wetspirit, Interesting take on solar water heating. Glycol systems are fairly straight forward and simple. Heat exchanger for domestic hot water and then routed to a heat dump, floor or outside for the summer. 12 volt pump and photo voltaic panel for it. No need to drain or have complicated differential controllers, etc. for the system. Less control items to purchase. I cross connected a boiler system with my solar system to use the sand beds in the winter off the boiler with very satisfactory results. Here in Wisconsin I believe we have a little more of a winter than you do in Northern California. Perhaps this is where the design of systems can vary. Ultimately the goal is to use the sun more and non renewable fuel less.:thumbsup: One question I have for you when you drain the system at night is: How do you automatically repressurize the system to eliminate your head losses?:confused2: Glycol systems are automatic and worst case is change a few valves seasonally.
Raspy
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magicheater,
Exactly, the ultimate goal is to use the sun more and non-renewable less. And also to provide what is needed in the most efficient and trouble free way. That's the rub. More parts and more maintenence means the systems are more for enthusiasts and cost more to build. You mentioned that a few valves may need to be replaced each season. Who pays for that and who decides when to do it? How much does that cost offset the gain from the system? Simple and invisible are the ultimate. With ground storage, for example, a lot of energy get's used to run pumps to store the energy but it may be hard to really know how much is actually recovered later. A good example of the ultimate radiant heating system is one where they don't even know they have a thermostat or they have never needed to adjust it. They are out there, but few and far between. I was told by one person (with an enormous solar system that did not work) that it was the best kind of system because he never had to do anything to it. Well, he never had to do anything to it because it had failed years before and there was no noise or leaking water to give it away. No problem.
The system I mentioned in my previous post is never re-pressurized. It's a drain back heat exchanger system. The collector loop is closed but there is no need for glycol. It's like a glycol system with a huge bubble in it. At night the air is up in the panels and, while running, the air is down in the holding tank. In this case the holding tank is an air separator and collector water storage tank, that's all. If the system high limits there are no stagnation temps trying to boil the glycol because the collectors drain back and are empty. A power failure or pump failure drains them too. And no need for a double wall exchanger, expensive fluid, fluid changes, pressure releif valve, pressure gauge, air vent or expansion tank. Never test or replace the fluid. There cannot be any mineral buildup and the heat exchanger gets reverse flushed every time the system shuts off. If the system ever does leak it looses a small amount of drinking water and that's it, with no affect on the house water supply and no contamination or health hazard. Propylene glycol is an excellent material, but it doesn't last forever, it's expensive and requires equipment to fill the system to the required pressure and make sure it stays there after all the air has been bled out. If the exchanger does leak, the water is contaminated. I know, it's not poisonous, but would you like to drink it? Especially after is has degraded from excessive temperatures for a few years and now contains copper and other materials from the system.
I don't like the PV panel to run the pump (if you are on the grid) because it's an additional expense that produces no heat and it runs the pump very slowly at times so it may be leaving heat in the panels reducing their efficiency. Fast even flows with a small differential is what produces the most stored energy. A PV pump cannot work well on a non pressurized drainback system because it must run a full speed to overcome the initial head needed to fill the collectors. This means full sun and lost energy before it can function.
The best way to design a solar system is to think of it as a cooling system. A cooling system that is trying to cool the panels as efficiently as possible so no heat gets lost to the environment. The cooling media is the stored water. Don't think of the system as one trying to heat the stored water. It will, and that's what we ultimately want, but getting the BTUs out of the collector is where it's at for efficiency. You must also keep freeze protection in mind as a top priority. And then consider what you will do for backup energy and how you will deal with excessive heat. That's the game in a nutshell.
It sounds like in your case you are dumping a lot of heat in the ground. But, in most DHW systems there is no infinite heat sink and excessive temps must be dealt with some how. With radiant heating you can't just dump extra heat in the floor because when you have the most potential is when you need heat the least. Radiant floors need bursts of heat at appropriate times rather than to be used as a storage system. A certain threshhold continuous temp of around 70 degrees is fine, but that's about it. More and you're heating too much in the summer. The summer is when you have excess energy from a large system and must put it somewhere.
Solar and radiant heating systems are interesting and fun heating problems. There's a lot of room for experimentation. In the end it boils down to common physics and human comfort.
Exactly, the ultimate goal is to use the sun more and non-renewable less. And also to provide what is needed in the most efficient and trouble free way. That's the rub. More parts and more maintenence means the systems are more for enthusiasts and cost more to build. You mentioned that a few valves may need to be replaced each season. Who pays for that and who decides when to do it? How much does that cost offset the gain from the system? Simple and invisible are the ultimate. With ground storage, for example, a lot of energy get's used to run pumps to store the energy but it may be hard to really know how much is actually recovered later. A good example of the ultimate radiant heating system is one where they don't even know they have a thermostat or they have never needed to adjust it. They are out there, but few and far between. I was told by one person (with an enormous solar system that did not work) that it was the best kind of system because he never had to do anything to it. Well, he never had to do anything to it because it had failed years before and there was no noise or leaking water to give it away. No problem.
The system I mentioned in my previous post is never re-pressurized. It's a drain back heat exchanger system. The collector loop is closed but there is no need for glycol. It's like a glycol system with a huge bubble in it. At night the air is up in the panels and, while running, the air is down in the holding tank. In this case the holding tank is an air separator and collector water storage tank, that's all. If the system high limits there are no stagnation temps trying to boil the glycol because the collectors drain back and are empty. A power failure or pump failure drains them too. And no need for a double wall exchanger, expensive fluid, fluid changes, pressure releif valve, pressure gauge, air vent or expansion tank. Never test or replace the fluid. There cannot be any mineral buildup and the heat exchanger gets reverse flushed every time the system shuts off. If the system ever does leak it looses a small amount of drinking water and that's it, with no affect on the house water supply and no contamination or health hazard. Propylene glycol is an excellent material, but it doesn't last forever, it's expensive and requires equipment to fill the system to the required pressure and make sure it stays there after all the air has been bled out. If the exchanger does leak, the water is contaminated. I know, it's not poisonous, but would you like to drink it? Especially after is has degraded from excessive temperatures for a few years and now contains copper and other materials from the system.
I don't like the PV panel to run the pump (if you are on the grid) because it's an additional expense that produces no heat and it runs the pump very slowly at times so it may be leaving heat in the panels reducing their efficiency. Fast even flows with a small differential is what produces the most stored energy. A PV pump cannot work well on a non pressurized drainback system because it must run a full speed to overcome the initial head needed to fill the collectors. This means full sun and lost energy before it can function.
The best way to design a solar system is to think of it as a cooling system. A cooling system that is trying to cool the panels as efficiently as possible so no heat gets lost to the environment. The cooling media is the stored water. Don't think of the system as one trying to heat the stored water. It will, and that's what we ultimately want, but getting the BTUs out of the collector is where it's at for efficiency. You must also keep freeze protection in mind as a top priority. And then consider what you will do for backup energy and how you will deal with excessive heat. That's the game in a nutshell.
It sounds like in your case you are dumping a lot of heat in the ground. But, in most DHW systems there is no infinite heat sink and excessive temps must be dealt with some how. With radiant heating you can't just dump extra heat in the floor because when you have the most potential is when you need heat the least. Radiant floors need bursts of heat at appropriate times rather than to be used as a storage system. A certain threshhold continuous temp of around 70 degrees is fine, but that's about it. More and you're heating too much in the summer. The summer is when you have excess energy from a large system and must put it somewhere.
Solar and radiant heating systems are interesting and fun heating problems. There's a lot of room for experimentation. In the end it boils down to common physics and human comfort.
Raspy
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- Joined
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crazyal,
Be careful with the sales pitch from the evacuated tube companies. Really look at the cost of your finished system and the BTUs it delivers. Evacuated tubes have been seriously discredited and will not work in shade. Around here the supplier only stocks them so they can say they do, but no one is very interested. And for good reason.
There are many ways to do things, but I have seen the most outlandish claims over the years by everyone who seems to be able to invent a new problem that they want to exploit a cure for. Many, so called breakthroughs, come and go.
All materials expand and contract when heated and cooled. So does copper. This no particular problem with copper collectors. And, either way, both types of systems are plumbed with copper piping that also heats and cools. The system shown in the link you posted is a very complicated system that has all the components of a normal system with copper plate absorbers. The only difference is the fragile, expensive and inneficient evacuated tubes. Be careful.
Be careful with the sales pitch from the evacuated tube companies. Really look at the cost of your finished system and the BTUs it delivers. Evacuated tubes have been seriously discredited and will not work in shade. Around here the supplier only stocks them so they can say they do, but no one is very interested. And for good reason.
There are many ways to do things, but I have seen the most outlandish claims over the years by everyone who seems to be able to invent a new problem that they want to exploit a cure for. Many, so called breakthroughs, come and go.
All materials expand and contract when heated and cooled. So does copper. This no particular problem with copper collectors. And, either way, both types of systems are plumbed with copper piping that also heats and cools. The system shown in the link you posted is a very complicated system that has all the components of a normal system with copper plate absorbers. The only difference is the fragile, expensive and inneficient evacuated tubes. Be careful.
magicheater
Veteran Member
Wetspirit:
You mentioned that a few valves may need to be replaced each season. Not replaced but opened or closed depending on where you want the heat.It sounds like in your case you are dumping a lot of heat in the ground. The summer is when you have excess energy from a large system and must put it somewhere. The heat dump is only used in the summer.Radiant floors need bursts of heat at appropriate times rather than to be used as a storage system. A certain threshhold continuous temp of around 70 degrees is fine, but that's about it.When using sand beds beneath your floors they respond a little differently, too large with an inadequate heat source and they do little, too small with an adequate heat source and you are too warm.With radiant heating you can't just dump extra heat in the floor because when you have the most potential is when you need heat the least. This is where sand beds can help with the storage. However it is not a "movable storage medium" as a fluid is. I have sand beds with varying depths and they all respond differently. They were common here in the 80's but once you have them.....Solar and radiant heating systems are interesting and fun heating problems. There's a lot of room for experimentation. In the end it boils down to common physics and human comfort. Hence the experimentation with cross connecting the boiler with the floor loops. Although the system is not a completely automatic system I have achieved the comfort I desired and the added bonus of having warm garages all winter long. Exactly, the ultimate goal is to use the sun more and non-renewable less. And also to provide what is needed in the most efficient and trouble free way. That's the rub. Both of us have systems that work, comparatively which is better? Not important for me. Do I want to learn more about yours? YES Will I continue to tweak mine? YES. I have effectively removed 2-3 months from the "heating season" with my system. Since I heat strictly with wood this is 2-3 months I do not need to start fires. A house designed with solar/radiant heating likely will work more efficiently than a retrofit to an existing older home. Insulation packages need to be considered as well. Let the experiments continue in all their varying degrees of success!
You mentioned that a few valves may need to be replaced each season. Not replaced but opened or closed depending on where you want the heat.It sounds like in your case you are dumping a lot of heat in the ground. The summer is when you have excess energy from a large system and must put it somewhere. The heat dump is only used in the summer.Radiant floors need bursts of heat at appropriate times rather than to be used as a storage system. A certain threshhold continuous temp of around 70 degrees is fine, but that's about it.When using sand beds beneath your floors they respond a little differently, too large with an inadequate heat source and they do little, too small with an adequate heat source and you are too warm.With radiant heating you can't just dump extra heat in the floor because when you have the most potential is when you need heat the least. This is where sand beds can help with the storage. However it is not a "movable storage medium" as a fluid is. I have sand beds with varying depths and they all respond differently. They were common here in the 80's but once you have them.....Solar and radiant heating systems are interesting and fun heating problems. There's a lot of room for experimentation. In the end it boils down to common physics and human comfort. Hence the experimentation with cross connecting the boiler with the floor loops. Although the system is not a completely automatic system I have achieved the comfort I desired and the added bonus of having warm garages all winter long. Exactly, the ultimate goal is to use the sun more and non-renewable less. And also to provide what is needed in the most efficient and trouble free way. That's the rub. Both of us have systems that work, comparatively which is better? Not important for me. Do I want to learn more about yours? YES Will I continue to tweak mine? YES. I have effectively removed 2-3 months from the "heating season" with my system. Since I heat strictly with wood this is 2-3 months I do not need to start fires. A house designed with solar/radiant heating likely will work more efficiently than a retrofit to an existing older home. Insulation packages need to be considered as well. Let the experiments continue in all their varying degrees of success!
