Question:
I live in Colorado, at about 6100 feet.solar pv panel for sale I would like to improve the efficiency of the collectors on my drainback solar thermal system. My collectors are constructed of sheet metal and glass. What should I use for insulation on the back of the collectors, and is this worth doing?solar pv panel for sale Are there other things I could do to improve efficiency?
Response:
The best thing that you can do is keep the panel as cool as possible… What I mean is, if there is more than one collector in the system, make sure that they are all at the same temperature ( one is not hotter than the others) and that the flow of water is good enough to pull the heat out fast enough to keep the panel as close to your storage tank water temp as possible.. Do a input temp and output temp water check ( delta T ) and see if you have less than about 10 deg of water temp difference…. any more than that and the flow of water needs to be increased to pull more heat from the panel ( that reduces the heat loss back to the air ) …Make sure that the panels are air tight ( ok .. not completely sealed, but no holes to let the hot air out)…keep the glass clean ANDuse a food grade corrosion inhibitor in the solar water loop to keep the corrosion down… the panels are copper but I’ll bet your storage tank uses a steel heat exchange that is not protected in any way… the zinc anode only covers the tank… and can’t protect what it can’t see ( zinc anodes only protect in a line of sight! )…. I found out the hard way about my heat exchanger… It’s a real bitch to cut open an 80 gallon tank, find the bad spots, fix them and weld the thing back closed…. I did it, Don’t recommend it …. CHANGE YOUR ANODE AND USE FOOD GRADE CORROSION INHIBITOR IN YOUR PANELS..solar pv panel for sale I live in Colorado, at about 6100 feet. I would like to improve the efficiency of the collectors on my drainback solar thermal system.solar pv panel for sale My collectors are constructed of sheet metal and glass. What should I use for insulation on the back of the collectors, and is this worth doing? Are there other things I could do to improve efficiency?
Response:
solar pv panel for sale. This goes counter to all the work on low flow systems. If you keep the panel cool by flooding it with water, then your storage tank stays cool. And then you’ll need to supplement with gas or electric heat to get that hot shower. Efficiency/(thermodynamic)effectiveness is all about temperature matching. If the system in question is for domestic hot water (i.e. showers, laundry, and dishes), then you want to get hot water in your storage tank. A study was done not to long ago that concluded most installed DHW panel flows were too high. Keep in mind that an important aspect of low flow systems is a stratified storage tank. Unfortunately, some drain-back systems do not allow this. Cap is correct that higher collector plate temperatures result in greater environmental losses. The trick is to reduce this loss. Your two big environmental losses are conduction/convection and radiation. So yes, greater insulation could help reduce your convective loss. Radiation loss would require more intrusive (expensive) remedies. These include multiple glazing (also reduces in convective losses, but also reduces absorption efficiency) and selective coatings (reduces low wavelength infra-red emittance). So, to sum it all up — if your collector walls are hot to the touch, then go ahead and insulate. Good luck,solar pv panel for sale The best thing that you can do is keep the panel as cool as possible… What I mean is, if there is more than one collector in the system, make sure that they are all at the same temperature ( one is not hotter than the others) and that the flow of water is good enough to pull the heat out fast enough to keep the panel as close to your storage tank water temp as possible.. Do a input temp and output temp water check ( delta T ) and see if you have less than about 10 deg of water temp difference…. any more than that and the flow of water needs to be increased to pull more heat from the panel ( tha t reduces the heat loss back to the air ) …Make sure that the panels are air tight ( ok .. not completely sealed, but no holes to let the hot air out)…keep the glass clean ANDuse a food grade corrosion inhibitor in the solar water loop to keep the corrosion down… the panels are copper but I’ll bet your storage tank uses a steel heat exchange that is not protected in any way… the zinc anode only covers the tank… and can’t protect what it can’t see ( zinc anodes only protect in a line of sight! )…. I found out the hard way about my heat exchanger… It’s a real bitch to cut open an 80 gallon tank, find the bad spots, fix them and weld the thing back closed…. I did it, Don’t recommend it …. CHANGE YOUR ANODE AND USE FOOD GRADE CORROSION INHIBITOR IN YOUR PANELS.. CAP I live in Colorado, at about 6100 feet. I would like to improve the efficiency of the collectors on my drainback solar thermal system. My collectors are constructed of sheet metal and glass. What should I use for insulation on the back of the collectors,solar pv panel for sale and is this worth doing? Are there other things I could do to improve efficiency?
Response:
solar pv panel for sale. This goes counter to all the work on low flow systems. If you keep the panel cool by flooding it with water, then your storage tank stays cool.
So are you saying that by increasing the water flow that the panel will stop supplying heat to the system?…. No, by increasing the water flow to the system you will reduce the temperature the panel is operating at ( the delta T of the heat exchangers ) by reducing the hot side temperature, and reducing the re-radiated losses to the atmosphere. The temperature of the water entering the panel will be the exit water on the storage heat exchanger ( the bottom tap) and the water flow of the system will determine the exit temperature of the panel ( all other things being equal ). The panel will just add heat to the water as it passes by, and it’s temperature will be determined by how long it’s in there. The water on it’s return path will enter the storage heat exchanger, dumping it’s heat to the stored water, causing the stored water to warm up,solar pv panel for sale causing the return water on the next trip to be a little warmer. and so on And then you’ll need to supplement with gas or electric heat to get that hot shower. Efficiency/(thermodynamic)effectiveness is all about temperature matching.
If you have a panel that is to small for the storage tank that you have then you will get the symptoms that you described.. If the system in question is for domestic hot water (i.e. showers, laundry, and dishes), then you want to get hot water in your storage tank.
Yes you want to get hot water into your tank, but you do not want to sacrifice heat to get temperature, or else you have a system that will run out of hot water to fast. A study was done not to long ago that concluded most installed DHW panel flows were too high.
Like to see the study.solar pv panel for sale Keep in mind that an important aspect of low flow systems is a stratified storage tank. Unfortunately, some drain-back systems do not allow this.
Again yes…. My drain back system uses a jacket wrapped around the storage tank.. allowing stratification… if your system uses an external heat exchanger and two pumps then, no it does not allow for stratification. – Hide quoted text — Show quoted text – Cap is correct that higher collector plate temperatures result in greater environmental losses. The trick is to reduce this loss. Your two big environmental losses are conduction/convection and radiation. So yes, greater insulation could help reduce your convective loss. Radiation loss would require more intrusive (expensive) remedies. These include multiple glazing (also reduces in convective losses, but also reduces absorption efficiency) and selective coatings (reduces low wavelength infra-red emittance). So, to sum it all up — if your collector walls are hot to the touch, then go ahead and insulate. Good luck, Tim The best thing that you can do is keep the panel as cool as possible… What I mean issolar pv panel for sale , if there is more than one collector in the system, make sure that they are all at the same temperature ( one is not hotter than the others) and that the flow of water is good enough to pull the heat out fast enough to keep the panel as close to your storage tank water temp as possible.. Do a input temp and output temp water check ( delta T ) and see if you have less than about 10 deg of water temp difference…. any more than that and the flow of water needs to be increased to pull more heat from the panel ( tha t reduces the heat loss back to the air ) …Make sure that the panels are air tight ( ok .. not completely sealed, but no holes to let the hot air out)…keep the glass clean ANDuse a food grade corrosion inhibitor in the solar water loop to keep the corrosion down… the panels are copper but I’ll bet your storage tank uses a steel heat exchange that is not protected in any way… the zinc anode only covers the tank… and can’t protect what it can’t see ( zinc anodes only protect in a line of sight! )…. I found out the hard way about my heat exchanger… It’s a real bitch to cut open an 80 gallon tank, find the bad spots, fix them and weld the thing back closed…. I did it, Don’t recommend it …. CHANGE YOUR ANODE AND USE FOOD GRADE CORROSION INHIBITOR IN YOUR PANELS.. CAP I live in Colorado, at about 6100 feet. I would like to improve the efficiency of the collectors on my drainback solar thermal system. My collectors are constructed of sheet metal and glass. What should I use for insulation on the back of the collectors, and is this worth doing?solar pv panel for sale Are there other things I could do to improve efficiency?
Response:
keep the panel cool by flooding it with water, then your storage tank stays cool. So are you saying that by increasing the water flow that the panel will stop supplying heat to the system?…. No, by increasing the water flow to the
I don’t think that is what he is saying, and my practical experience agrees with his sentiments. I used to live with a solar water heating system. The thermostat failed (the installer had soldered connections with acid flux solder!) and the pump was running continuously. The water in the tank closely tracked the outdoor temperature. It got a bit warmer than the air during the day, and a bit cooler than the air during the night, but was almost always within 5-10 degrees of air temp. I found what was going on and manually turned off the pump at night, and on in the morning for a few days. It didn’t help much. It did prevent the drop below air temp at night, but during the entire day it didn’t get above air + 10F. Finally I fixed the problem late one evening, so the pump only ran when the collector was about 10F above the tank. By the next afternoon the tank temperature was over 100F and the air temperature never got to 70F that day. What happens, is that the heat added by the panel is lost in circulating. Even with insulated lines, some heat is lost. If the panel exit temp is hotter, you lose more heat, but the net heat gain to the tank is still greater. If you slow the flow enough to get good heat gain from the collector, then you will lose more in the pipes.solar pv panel for sale If you speed the flow, you get less heat gain and soon the day is over. the re-radiated losses to the atmosphere. The temperature of the water entering the panel will be the exit water on the storage heat exchanger ( the bottom
Yup. tap) and the water flow of the system will determine the exit temperature of the panel ( all other things being equal ). The panel will just add heat to the
Yup. water as it passes by,solar pv panel for sale and it’s temperature will be determined by how long it’s in there. The water on it’s return path will enter the storage heat exchanger,
Yup. dumping it’s heat to the stored water, causing the stored water to warm up, causing the return water on the next trip to be a little warmer. solar pv panel for sale
Yup. But I don’t think the storage is ever going to get very warm. It never did for me. Keep in mind that an important aspect of low flow systems is a stratified storage tank. Unfortunately, some drain-back systems do not allow this.
That is an advantage with a thermostat control of the pump. sdb — More guns means less crime. ISBN:0-226-49363-6 *** Watch out for munged e-mail address. User should be sylvan and host is cyberhighway.net. Do NOT send me unsolicited commercial e-mail (UCE)!
Response:
Hi Cap, An important aspect of system design/optimization is working with temperature. We must get away from looking solely at stored “heat”. For example, lets say I have 100 lbm of 150F water and 200 lbm of 90F water. They both contain the same amount of “heat” if we measured it in stored Btu or Joules (I used enthalpy for this comparison). Now let’s say our process requires 120F water? Well, the 90F storage is inadequate. So, you size the system to meet the requirements of your load. Let’s now consider a low flow panel and a high flow panel to produce 130F water (~55C) — a more common process temperature. Let’s consider the low flow panel first. Let us assume (because it is low flow) that our storage is allowed to stratify. The following text is from my graduate research proposal (which got funded): “Secondly, the reduced flow rate promotes temperature stratification in the primary solar storage tank. A stratified storage greatly improves system effectiveness because the very hot water at the top of the tank replaces the consumed water from the auxiliary heated tank during an energy draw. Thus, the auxiliary heaters may not be needed because the solar heated water is at, or exceeds, the set-point temperature. Furthermore, collector efficiency increases because the cold water at the bottom of the stratified solar tank is sent to the collector. Thus, a larger terminal temperature difference exists across the collector plate (which can be thought as a heat exchanger that transfers energy via radiative, conductive, and convective mechanisms), resulting in increased heat transfer — a larger portion of the sun’s energy is captured for useful purposes. In contrast, a constant high flow system may not even operate during periods of low irradiation because the outlet fluid temperature is less then the controller set-point value (causing the collector pumps to cycle).” A reasonable assumption for average plate temperature is the mean of inlet/outlet fluid temperature (only precise if your plate temperature gradient is truly linear). With a low flow system, we can assume that we have 70F inlet and 130F outlet. Thus, a mean plate temperature is 100F. We can then use this to approximate losses. Now, let’s consider a panel with a high fluid flow where there is only a 10F increase across the panel. Assuming our tank starts off isothermal at 70F, then the mean plate temperature would be 75F — nice and low. However, the collector outlet temperature is only 80F — far from our 130F target. Time goes by and the tank continues to charge. The bottom of the tank is now 120F to get the desired 130F water (assuming that we could still get a 10F rise). However, the mean plate temperature is now 125F which is 25F higher then the low flow system to get the same outlet temperature.solar pv panel for sale Do you see what I’m getting at? Cap, I’m not going to flood this note with a series of Nick Pine style calculations. I wouldn’t even consider a paper and calculator method to optimize collector flow. I’d use a simulation program like the TRNSYS code developed by Sandy Klein (the same guy who developed the F-Chart method) and base my optimization on maximizing the Solar Fraction. If higher collector losses also result in a higher Solar Fraction, so be it. We’re stilling maximizing our useful energy. Also, there are many many variables that have a significant impact on what’s best. You may be very well correct that shooting for a 10F collector rise is the best thing that the original poster should do. And I would never try and tell you what’s best for your system. You know it intimately, and I have no clue. There are, however, many merits to (very) low flow systems that the solar industry missed out on in the ’70s. I guess that’s my point. Please accept my apologies for being snooty in my original response to you. That was not my intent. Cheers, solar pv panel for sale
Response:
…We must get away from looking solely at stored “heat”. For example, lets say I have 100 lbm of 150F water and 200 lbm of 90F water. They both contain the same amount of “heat” if we measured it in stored Btu or Joules…
It seems reasonable to talk about “useful stored heat,” ie stored heat above a certain minimum usable temperature which depends on the application, eg 70 F for a solar house… It seems to me that 100 pounds of 150 F water contains 100(150-70) = 8K Btu of useful stored heat for this application, and 200 pounds of 90 F water contains 200(90-70) = 4K Btu. Let’s now consider a low flow panel and a high flow panel to produce 130F water (~55C)… A reasonable assumption for average plate temperature is the mean of inlet/outlet fluid temperature… With a low flow system, we can assume that we have 70F inlet and 130F outlet. Thus, a mean plate temperature is 100F.solar pv panel for sale We can then use this to approximate losses.
Losses to some lower outdoor temperature. For instance, on a 30 F day, a collector with a mean plate temp of 100 F and R1 glazing might lose (100F-30F)/R1 = 70 Btu/h to the outdoors. That’s 420 Btu over 6 hours. Now, let’s consider a panel with a high fluid flow where there is only a 10F increase across the panel. Assuming our tank starts off isothermal at 70F, then the mean plate temperature would be 75F — nice and low. However, the collector outlet temperature is only 80F — far from our 130F target. Time goes by and the tank continues to charge. The bottom of the tank is now 120F to get the desired 130F water (assuming that we could still get a 10F rise). However, the mean plate temperature is now 125F which is 25F higher then the low flow system to get the same outlet temperature.solar pv panel for sale Do you see what I’m getting at?
Not exactly, although I can appreciate that a partially-charged stratified store can deliver more useful heat than a well-mixed store, after gaining the same heat energy. In the second example, the mean plate temp might be 75 F for the first hour, with a 45 Btu/ft^2 loss 85 F for the 2nd 55 95 F for the 3rd 65 105 F for the 4th 75 115 F for the 5th 85 125 F for the 6th 95, ie a 100 F average temp, with a 420 Btu total loss…solar pv panel for sale
Response:
solar pv panel for sale”. For example, lets say I have 100 lbm of 150F water and 200 lbm of 90F water. They both contain the same amount of “heat” if we measured it in stored Btu or Joules… It seems reasonable to talk about “useful stored heat,” ie stored heat above a certain minimum usable temperature which depends on the application, eg 70 F for a solar house… It seems to me that 100 pounds of 150 F water contains 100(150-70) = 8K Btu of useful stored heat for this application, and 200 pounds of 90 F water contains 200(90-70) = 4K Btu.
Exactly my point. The lower mass — higher temperature storage has twice the available energy. That was my goal in setting up this example. I’m glad someone pushed the numbers to “discover” this :^). – Hide quoted text — Show quoted text – Let’s now consider a low flow panel and a high flow panel to produce 130F water (~55C)… A reasonable assumption for average plate temperature is the mean of inlet/outlet fluid temperature… With a low flow system, we can assume that we have 70F inlet and 130F outlet. Thus, a mean plate temperature is 100F. We can then use this to approximate losses. Losses to some lower outdoor temperature. For instance, on a 30 F day, a collector with a mean plate temp of 100 F and R1 glazing might lose (100F-30F)/R1 = 70 Btu/h to the outdoors. That’s 420 Btu over 6 hours. Now, let’s consider a panel with a high fluid flow where there is only a 10F increase across the panel. Assuming our tank starts off isothermal at 70F, then the mean plate temperature would be 75F — nice and low. However, the collector outlet temperature is only 80F — far from our 130F target. Time goes by and the tank continues to charge. The bottom of the tank is now 120F to get the desired 130F water (assuming that we could still get a 10F rise). However, the mean plate temperature is now 125F which is 25F higher then the low flow system to get the same outlet temperature. Do you see what I’m getting at? Not exactly, although I can appreciate that a partially-charged stratified store can deliver more useful heat than a well-mixed store, after gaining the same heat energy. In the second example, the mean plate temp might be 75 F for the first hour, with a 45 Btu/ft^2 loss 85 F for the 2nd 55 95 F for the 3rd 65 105 F for the 4th 75 115 F for the 5th 85 125 F for the 6th 95, ie a 100 F average temp, with a 420 Btu total loss…
Ahhh, I disagree with your assumption that it will take only an hour to ratchet the storage up 10F for each time period. The time constant will be an exponential decay with the mean storage temperature looking something like (I hope this comes of with proportional font): Mean Storage Temperature ^ | * | * | * | * | * | * | * | * | * | * | * |* So the question is, how long do we operate with an elevated plate temperature to achieve a goal of 130F water? This really requires an integrated system simulation of some time to determine the time constant. I’m not a betting man, but I will state that FOR MOST “TYPICAL” INSTALLATIONS a low flow system will out perform a high flow system if the storage is allowed to stratify. TTFN, Tim
Response:
On any system for producing hot water, it is absolutely essential to reduce heat losses as much as is economically viable. Conduction and convection heat losses for any system are proportional to the difference in temperature between the inside and the outside of the system, the materials and insulation techniques used set the constant by which this figure is multiplied. Radiation losses are rather harder to control, but fortunately are relatively small at solar panel temperatures. In a conventional system such as yours, you could add extra layers of glass, and use special coatings to reduce radiation loss, but this reduces collection efficiency, so losing some of the advantages gained by reducing heat losses. I would think that at your altitude, with high sunshine levels, that you would not gain a lot from extra glazing, and may in fact lose out. You are right to identify the back of the panel as the most likely site for reducing heat loss, as metal is an extreemly good conductor of heat. You do not say what type of roof you have, or how the panel is mounted on the roof, ie with or without a gap under it. In the second case, you will already be getting some insulation effect from your roof insulation. In the first case, heat losses from the metal back of a panel could be very high. Possibly the easiest way to insulate the back of the panel, would be to get some insulating foam board from a building supplier, and glue it to the back and sides of the panel. You would then refix the panel to the roof, and add a frame to the sides and or back of the panel to prevent destruction of the insulation by birds, vermin, and weather. Without any form of insulation behind the panel, I would guess that your heat losses would be approximately 5-10 watts per degree per square meter. This means that if the back of the panel is 40 centigrade warmer than its surroundings, you would get a heat loss of about 200-400 watts/sqm, which I think would be 20-40% of solar gains. Heat losses for the insulated panel would be about 0.5w/sqm from the back of the panel. These improvements would of course make no difference to the front of the panel, so that overall heat loss from the panel would roughly halve. If you wish to optimise other aspects of the system, then insulate all pipe work as thoroughly as possible, and use a photovoltaic panel to drive your pump. This allows the rate of pumping to adjust according to the intensity of solar radiation falling on the panel. The more the water is heated by the sun, the faster the flow rate. You could also install a thermostatic cut out to stop the pump if the water from the panel is less than10 centigrade hotter than the water in the tank. I also do not know what type of heating you use, if you are using a system based on water filled radiators, then you could divert your solar hot water feed to the inlet of your boiler, and so use any spare solar capacity for space heating. By feeding into a bigger system, ie. hot water supply + space heating rather than just hot water, the use of solar heated water is optimised, and even a small temperature rise is beneficial. In the hot water only system, solar heated water must get hotter than the water in the tank, or the whole system can end up radiating heat into the atmosphere rather than the reverse. Water inlet temperature for a panel would be around 5-20 C when used to supply preheated water to a boiler, but could be 40-50 C or more when recirculating water from a hot water tank. This first case gives a far greater thermal efficiency for the solar panel. Assuming that you have a panel of 2 m x 1m, putting my insulation suggestions into practice should give you at liest 2 kw hours, or about 40 litres of extra hot water per day, and with well insulated pipes and good control systems, you could be looking at 100 litres.
Response:
The collectors are mounted at an angle above the roof. My system has three loops, one for the collectors/storage, one to the water heater, and one for a fan coil unit. Pipes are insulated already, although the insulation is old. Controller is “differential” with temperature readout. Pumps are AC induction motors, in the solar closet with the storage. “Insulating foam board” sounds great, but I am a little concerned about possible melting. The collector temperature hits 175F and above now during the summer, and for safety margin I would expect it to reach 200F (almost boiling at my altitude). What kind of foam material can safely handle those contact temperatures without danger of melting or burning ? What glue would be good for metal/foam bonding at these temps? Separate question: Shortly after the main collector pumps start, and hot water returns back to the storage (drainback system), I get horrible vibration and noise. After about 30 seconds it stops. This only happens when the storage tank has warmed up some (say, above 130-140F), but I don’t think it is simple boiling in the tank, because it stops after the pumps are running for 30 seconds or so, but the hot water circulating down from the collectors is still very hot (150F+). Thoughts ? – Hide quoted text — Show quoted text – On any system for producing hot water, it is absolutely essential to reduce heat losses as much as is economically viable. Conduction and convection heat losses for any system are proportional to the difference in temperature between the inside and the outside of the system, the materials and insulation techniques used set the constant by which this figure is multiplied. Radiation losses are rather harder to control, but fortunately are relatively small at solar panel temperatures. In a conventional system such as yours, you could add extra layers of glass, and use special coatings to reduce radiation loss, but this reduces collection efficiency, so losing some of the advantages gained by reducing heat losses. I would think that at your altitude, with high sunshine levels, that you would not gain a lot from extra glazing, and may in fact lose out. You are right to identify the back of the panel as the most likely site for reducing heat loss, as metal is an extreemly good conductor of heat. You do not say what type of roof you have, or how the panel is mounted on the roof, ie with or without a gap under it. In the second case, you will already be getting some insulation effect from your roof insulation. In the first case, heat losses from the metal back of a panel could be very high. Possibly the easiest way to insulate the back of the panel, would be to get some insulating foam board from a building supplier, and glue it to the back and sides of the panel. You would then refix the panel to the roof, and add a frame to the sides and or back of the panel to prevent destruction of the insulation by birds, vermin, and weather. Without any form of insulation behind the panel, I would guess that your heat losses would be approximately 5-10 watts per degree per square meter. This means that if the back of the panel is 40 centigrade warmer than its surroundings, you would get a heat loss of about 200-400 watts/sqm, which I think would be 20-40% of solar gains. Heat losses for the insulated panel would be about 0.5w/sqm from the back of the panel. These improvements would of course make no difference to the front of the panel, so that overall heat loss from the panel would roughly halve. If you wish to optimise other aspects of the system, then insulate all pipe work as thoroughly as possible, and use a photovoltaic panel to drive your pump. This allows the rate of pumping to adjust according to the intensity of solar radiation falling on the panel. The more the water is heated by the sun, the faster the flow rate. You could also install a thermostatic cut out to stop the pump if the water from the panel is less than10 centigrade hotter than the water in the tank. I also do not know what type of heating you use, if you are using a system based on water filled radiators, then you could divert your solar hot water feed to the inlet of your boiler, and so use any spare solar capacity for space heating. By feeding into a bigger system, ie. hot water supply + space heating rather than just hot water, the use of solar heated water is optimised, and even a small temperature rise is beneficial. In the hot water only system, solar heated water must get hotter than the water in the tank, or the whole system can end up radiating heat into the atmosphere rather than the reverse. Water inlet temperature for a panel would be around 5-20 C when used to supply preheated water to a boiler, but could be 40-50 C or more when recirculating water from a hot water tank. This first case gives a far greater thermal efficiency for the solar panel. Assuming that you have a panel of 2 m x 1m, putting my insulation suggestions into practice should give you at liest 2 kw hours, or about 40 litres of extra hot water per day, and with well insulated pipes and good control systems, you could be looking at 100 litres.
Response:
“Insulating foam board” sounds great, but I am a little concerned about possible melting. The collector temperature hits 175F and above now during the summer, and for safety margin I would expect it to reach 200F (almost boiling at my altitude).
Most Styrofoam begins to degrade at 165 F. Foamglas and fiberglass can work at higher temps. Shortly after the main collector pumps start, and hot water returns back to the storage (drainback system), I get horrible vibration and noise. After about 30 seconds it stops. This only happens when the storage tank has warmed up some (say, above 130-140F)…
Might be cavitation if the downpipe is unpressurized. Nick
Response:
Brian: It sounds like you should use some of the fiberglass boards for insulation.. If you have a combo ( space heat and hot water ) then you have quite a few panels in your system ( quite an investment!) … The system is probably of the 70’s design ( according to Tim Dierauf ) , and using the 10 differential control, and should be using a flow per panel of 1-3 gph per sq. ft… If you have a lot of panels that can be a big pump. My panels are for domestic hot water only, and it is a drain back system ( two loops ), and the panels are using the urethane foam insulation… not the best choice but so far they have not melted through the housings anyway…. In the afternoon when my system restarts ( the panels are empty when the pump is off ) I get 15 PSI steam coming from them ( that’s about 300 deg F ) , and it melts the cheep foam hot water pipe insulation that is covering the return lines… and at the time the steam is coming through the lines ( the first 30 seconds or so ) the lines kinda shake and rattle… maybe that is your noise? CAP
Response:
Hi Nick,
Hi Tim, – Hide quoted text — Show quoted text – …With a low flow system, we can assume that we have 70F inlet and 130F outlet. Thus, a mean plate temperature is 100F. We can then use this to approximate losses. Losses to some lower outdoor temperature. For instance, on a 30 F day, a collector with a mean plate temp of 100 F and R1 glazing might lose (100F-30F)/R1 = 70 Btu/h to the outdoors. That’s 420 Btu over 6 hours. Now, let’s consider a panel with a high fluid flow where there is only a 10F increase across the panel. Assuming our tank starts off isothermal at 70F, then the mean plate temperature would be 75F — nice and low… …In the second example, the mean plate temp might be 75 F for the first hour, with a 45 Btu/ft^2 loss 85 F for the 2nd 55 95 F for the 3rd 65 105 F for the 4th 75 115 F for the 5th 85 125 F for the 6th 95, ie a 100 F average temp, with a 420 Btu total loss… Ahhh, I disagree with your assumption that it will take only an hour to ratchet the storage up 10F for each time period. The time constant will be an exponential decay with the mean storage temperature looking something like (I hope this comes of with proportional font): Mean Storage Temperature ^ | * | * | * | * | * | * | * | * | * | * | * |*
This collector loss would be higher than in the linear case, since it spends less time at the lower temps and more at the higher temps. I’m not a betting man, but I will state that FOR MOST “TYPICAL” INSTALLATIONS a low flow system will out perform a high flow system if the storage is allowed to stratify.
Well, what do we mean by “outperform”? Fig. 12.5.1 on page 498 of Duffie and Beckman’s 1991 Solar Engineering shows a nice gain in solar fraction for low flow systems, based on Wuestling’s 1985 simulation studies. I had the impression that this gain had more to do with the stratified store (backup heat less frequently needed) than increased collector efficiency. Now I’m not so sure. I’m also not sure why the low-flow system has such a pronounced peak at a certain flow. Lower than that and the output temp is high enough that collector efficiency suffers? Higher than that and the output temp is low enough that the upper tank temp is too low, and backup heat is required more often? And does this apply to systems in which the solar fraction is 100% by design, eg solar houses with no backup heating systems, in which performance (rarely) suffers, but backup fuel is never used? Norman Saunders calculates that backup heat (or a sweater) would only be needed over short times every 35 years or so in his solar houses. He uses Gaussian stats to show that these events would be almost as rare as 100 year floods 
The usual concept of “solar fraction” seems to have little meaning here. Nick
Response:
Hi Nick,
[snip] – Hide quoted text — Show quoted text – Well, what do we mean by “outperform”? Fig. 12.5.1 on page 498 of Duffie and Beckman’s 1991 Solar Engineering shows a nice gain in solar fraction for low flow systems, based on Wuestling’s 1985 simulation studies. I had the impression that this gain had more to do with the stratified store (backup heat less frequently needed) than increased collector efficiency. Now I’m not so sure. I’m also not sure why the low-flow system has such a pronounced peak at a certain flow. Lower than that and the output temp is high enough that collector efficiency suffers? Higher than that and the output temp is low enough that the upper tank temp is too low, and backup heat is required more often? And does this apply to systems in which the solar fraction is 100% by design, eg solar houses with no backup heating systems, in which performance (rarely) suffers, but backup fuel is never used? Norman Saunders calculates that backup heat (or a sweater) would only be needed over short times every 35 years or so in his solar houses. He uses Gaussian stats to show that these events would be almost as rare as 100 year floods The usual concept of “solar fraction” seems to have little meaning here.
As you point out (and as I qualified for all my statements) — It is a stratified storage that gives low-flow systems the advantage. If there was a paddle mixing the low-flow storage making it isothermal, well it wouldn’t perform well. You are also correct about finding the optimum collector flow. Too low, and your losses are too high. To high, and you are leaving some money on the collector. The best solution is variable collector flow with a “smart” controller. This is an expensive option though. There is, however, another option that is very elegant — use a PV-panel to drive a DC pump for the collector loop. Size the PV-panel and pump such that the collector flow vs. insolation relation approaches optimum flow. FWIW, this was my original research proposal (and many others have done great work on this application). My test bed, however, was a natural convection system (Thermodynamics LTD), and I soon discovered that our models of Natural Convection Heat Exchangers (NCHXs) were not yet sufficient for my needs. Thus, I changed my topic to getting a better handle on the NCHX. As for stratified storage, the 2nd Law of thermo provides a very simple explanation to it’s benefits. A stratified storage is ordered, where a mixed (isothermal) storage has lost that order. This is esoteric though, but the 2nd Law does provide some slick methods of looking at energy cycles that go beyond considering the conservation of mass and energy. Temperature is a very important component — and this goes back to my original post on this subject. Some texts refer to 2nd Law studies as “Exergy Analysis”. I apply 2nd law considerations frequently in my work of optimizing cycles. As for your America’s Cup solar house, I cannot think of any example where a stratified storage would detract from maintaining comfort levels (when compared to an isothermal storage). Cheers, Tim p.s. Did you know that America’s Cup boats get towed to the race course? ;^)
Response:
Hi Nick,
Hello again Tim… It seems reasonable to talk about “useful stored heat,” ie stored heat above a certain minimum usable temperature which depends on the application, eg 70 F for a solar house… It seems to me that 100 pounds of 150 F water contains 100(150-70) = 8K Btu of useful stored heat for this application, and 200 pounds of 90 F water contains 200(90-70) = 4K Btu. Exactly my point. The lower mass — higher temperature storage has twice the available energy. That was my goal in setting up this example. I’m glad someone pushed the numbers to “discover” this :^).
That leads to a different optimization problem involving the choice of operating temperature for a solar heat store: is it cheaper to use a) a large tank of warm water with fewer collectors or, b) a smaller higher temp tank with more collectors, all other things being equal, including the useful energy stored. That depends on the price of tanks (including floorspace) and collectors. Suppose we want to store 1 million Btu at 70 F min to keep a solar house warm for 5 cloudy days in a row, in that unlikely event. What’s the optimum steady-state water storage temp T, after a long string of average days? Say a sunspace keeps the house warm on average days, with no load on the store. We need 1MBtu/(8(T-80)) = 125K/(T-70) gallons of water. A 1500 gallon poly tank costs about $400. Say cheap floorspace and insulation makes that $1/gallon. Indoors, it loses about 24h(T-70)5×64ft^2/R20=384(T-70) Btu/day, about (T-70)/4 Btu/gallon. If a square foot of vertical south collector with R1 glazing with 90% transmission gathers 900 Btu on an average 30 F January day and loses 6h(T-30)1ft^2/R1, for a net gain of 1080-6T Btu/day, keeping G gallons of water at T (F) requires A ft^2 of collector, ie (1080-6T)A = (T-70)G/4, so A = (T-70)G/(4(1080-6T)). With $4/ft^2 collectors, the tank+collector cost is G+(T-70)G/(1080-6T) = 125K/(T-70)+125K/(1080-6T) = 20.8K/(180-T)+125K/(T-70). The collector cost increases with T to 180 F, and the tank cost increases as T drops to 70 F. Maximizing A/(U-T)+B/(T-L), T^2(B-A)+T(2AL-2BU)+BU^2-AL^2 = 0, and a = 104166.7 and b = -42083K and c = 3947918K in this quadratic formula, so T = (-b-sqrt(b^2-4ac))/(2a) = 148.1 F, but the collector plus tank cost isn’t very sensitive to the choice of temperature… Nick 100 FOR T =130 TO 170 STEP 10 110 CC=125000!/(1080-6*T) 120 TC=125000!/(T-70) 130 PRINT T,CC+TC 140 NEXT RUN 130 2500 140 2306.548 150 2256.944 160 2430.556 170 3333.333
Response:
Hi Nick, I don’t have the time to expand and collect the various terms in your equations below. What did you conclude? If you spell things out a little more, I could jam them into a few of the solver packages on my computer here (i.e. define the variables, provide the independent equations/constraints, an the objective function). Economics aside, the 2nd Law teaches us to match temperatures in our cycles. Thus, if you need a process temperature of ~75F, then maintain a storage of 75F plus whatever temperature drops occur to get the heat from the storage to the process (i.e. living spaces). I’m sure that you would agree that heating something to 300F and then attemperating it down to 75F before it could be used would not be the most effective thing to do. Cheers, Tim
- Hide quoted text — Show quoted text – Hi Nick, Hello again Tim… It seems reasonable to talk about “useful stored heat,” ie stored heat above a certain minimum usable temperature which depends on the application, eg 70 F for a solar house… It seems to me that 100 pounds of 150 F water contains 100(150-70) = 8K Btu of useful stored heat for this application, and 200 pounds of 90 F water contains 200(90-70) = 4K Btu. Exactly my point. The lower mass — higher temperature storage has twice the available energy. That was my goal in setting up this example. I’m glad someone pushed the numbers to “discover” this :^). That leads to a different optimization problem involving the choice of operating temperature for a solar heat store: is it cheaper to use a) a large tank of warm water with fewer collectors or, b) a smaller higher temp tank with more collectors, all other things being equal, including the useful energy stored. That depends on the price of tanks (including floorspace) and collectors. Suppose we want to store 1 million Btu at 70 F min to keep a solar house warm for 5 cloudy days in a row, in that unlikely event. What’s the optimum steady-state water storage temp T, after a long string of average days? Say a sunspace keeps the house warm on average days, with no load on the store. We need 1MBtu/(8(T-80)) = 125K/(T-70) gallons of water. A 1500 gallon poly tank costs about $400. Say cheap floorspace and insulation makes that $1/gallon. Indoors, it loses about 24h(T-70)5×64ft^2/R20=384(T-70) Btu/day, about (T-70)/4 Btu/gallon. If a square foot of vertical south collector with R1 glazing with 90% transmission gathers 900 Btu on an average 30 F January day and loses 6h(T-30)1ft^2/R1, for a net gain of 1080-6T Btu/day, keeping G gallons of water at T (F) requires A ft^2 of collector, ie (1080-6T)A = (T-70)G/4, so A = (T-70)G/(4(1080-6T)). With $4/ft^2 collectors, the tank+collector cost is G+(T-70)G/(1080-6T) = 125K/(T-70)+125K/(1080-6T) = 20.8K/(180-T)+125K/(T-70). The collector cost increases with T to 180 F, and the tank cost increases as T drops to 70 F. Maximizing A/(U-T)+B/(T-L), T^2(B-A)+T(2AL-2BU)+BU^2-AL^2 = 0, and a = 104166.7 and b = -42083K and c = 3947918K in this quadratic formula, so T = (-b-sqrt(b^2-4ac))/(2a) = 148.1 F, but the collector plus tank cost isn’t very sensitive to the choice of temperature… Nick 100 FOR T =130 TO 170 STEP 10 110 CC=125000!/(1080-6*T) 120 TC=125000!/(T-70) 130 PRINT T,CC+TC 140 NEXT RUN 130 2500 140 2306.548 150 2256.944 160 2430.556 170 3333.333
Response:
Re Foam insulation, I should have considered the temperature involved and the possibility of melting. You would probably be better off with fibreglass board. Regarding the use of the system in combo mode, I was not suggesting that the system provide ALL heating and hot water, rather that it may be more efficient to provide 50% of combined water and space heating than 150% of required water heating on those days when the sun is strong, and the air is cold – a situation which I would expect to be common at high altitude, particularly in spring time
Response:
Hi Nick,
Hello again Tim, I don’t have the time to expand and collect the various terms in your equations below. What did you conclude?
I explored the choice of operating temperature for a multiple-day solar heat store: which is cheaper, a) a large tank of warm water with fewer collectors, or b) a smaller higher temp tank with more collectors, all other things being equal, eg the useful energy stored. My conclusion was, “That depends on the price of tanks and collectors.” If you spell things out a little more, I could jam them into a few of the solver packages on my computer here…
We doan need no steenkeeng com, urm, oh well, if you like… The sum of storage and collector costs boiled down to this: 20.8K/(180-T)+125K/(T-70). The (first) collector cost rises as the chosen storage temp T rises, until we need umpteen skillion square feet of collectors as their efficiency hits zero when T = 180 F. The (second) tank cost grows until its volume becomes infinite as the same chosen (steady-state, trickle-charged) storage temp T drops to 70 F. Economics aside, the 2nd Law teaches us to match temperatures in our cycles. Thus, if you need a process temperature of ~75F, then maintain a storage of 75F plus whatever temperature drops occur to get the heat from the storage to the process (i.e. living spaces)…
Plus the storage temperature drop over 5 cloudy days in a row… I’m sure that you would agree that heating something to 300F and then tempering it down to 75F before it could be used would not be the most effective thing to do.
Yes, especially since 300 F essentially precludes water, for which “tempering” may not be a big deal, eg controlling a hydronic floor circulation pump with a heating thermostat. Nick – Hide quoted text — Show quoted text – Suppose we want to store 1 million Btu at 70 F min to keep a solar house warm for 5 cloudy days in a row, in that unlikely event… We need 1MBtu/(8(T-80)) = 125K/(T-70) gallons of water. A 1500 gallon poly tank costs about $400. Say cheap floorspace and insulation makes that $1/gallon. Indoors, it loses about 24h(T-70)5×64ft^2/R20=384(T-70) Btu/day, about (T-70)/4 Btu/gallon. If a square foot of vertical south collector with R1 glazing with 90% transmission gathers 900 Btu on an average 30 F January day and loses 6h(T-30)1ft^2/R1, for a net gain of 1080-6T Btu/day, keeping G gallons of water at T (F) requires A ft^2 of collector, ie (1080-6T)A = (T-70)G/4, so A = (T-70)G/(4(1080-6T)). With $4/ft^2 collectors, the tank+collector cost is G+(T-70)G/(1080-6T) = 125K/(T-70)+125K/(1080-6T) = 20.8K/(180-T)+125K/(T-70). The collector cost increases with T to 180 F, and the tank cost increases as T drops to 70 F. Maximizing A/(U-T)+B/(T-L), T^2(B-A)+T(2AL-2BU)+BU^2-AL^2 = 0, and a = 104166.7 and b = -42083K and c = 3947918K in this quadratic formula, so T = (-b-sqrt(b^2-4ac))/(2a) = 148.1 F, but the collector plus tank cost isn’t very sensitive to the choice of temperature… 100 FOR T =130 TO 170 STEP 10 110 CC=125000!/(1080-6*T) 120 TC=125000!/(T-70) 130 PRINT T,CC+TC 140 NEXT RUN 130 2500 140 2306.548 150 2256.944 160 2430.556 170 3333.333
Response:
Hi Nick, Hello again Tim, I don’t have the time to expand and collect the various terms in your equations below. What did you conclude? I explored the choice of operating temperature for a multiple-day solar heat store: which is cheaper, a) a large tank of warm water with fewer collectors, or b) a smaller higher temp tank with more collectors, all other things being equal, eg the useful energy stored. My conclusion was, “That depends on the price of tanks and collectors.”
Well that’s just the hard truth about it all — it comes down to cold cash. Cheers, Tim
Response:
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