I don't understand why you are modifying the tank temps down at night "bringing it down to 135." Couldn't you use a mixing valve after the tank to regulate the desired 135 or whatever output temp and allow the storage tank to remain hotter?
Solar Evacuated Tube Domestic Hot Water System
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Night cool-down of the 1000 gallon storage tank sometimes is needed when hot water usage is low and solar hot water production is very high. This can result in the tank temperatures rising to 180F and higher, especially if two or more days of low usage and high production. So, by cooling the tank down at night, sufficient hot water is available to start the day and the tank has capacity to accept the high hot water production during the day without over-heating. On occasion in the past the tank had hot water over 200F. System redesign now implemented and night cooling should prevent this situation in the future.
The install does use a mixing valve to provide 120F water for domestic usage.
Still musing over a productive way to use excess hot water, but no good solution in sight.
The install does use a mixing valve to provide 120F water for domestic usage.
Still musing over a productive way to use excess hot water, but no good solution in sight.
Hi jebatty, if there were plans to install a ground source heat pump, using excess hot water to charge the ground in the summer to get better efficiencies would be an interesting experiment to run. I remember reading a few years back of a condo or co-housing development in Calgary that did just that.
I like the ground source heat pump recharge idea for excess domestic hot water. But a GSHP is not in the plans. A related idea that comes to mind would be spring/fall heating of the ground under a hoop building for growing vegetables, if it turns out that excess hot water is available at those times of the year.
Yesterday was a day of full sun, no clouds. The maximum btu hot water output of the system as shown by the analog temperature meters and flow meter was:
evacuated tube delta-T = 30F
gpm = 5
btuh = 30 x 5 x 500 = 75,000
75,000 btuh will provide 128 gallons of hot water per hour from 50-120F temperature rise.
Put another way, bottom of tank supply to the HX was 120F, and HX hot water return to the tank was 133F. If no hot water usage (no cold water being supplied to the tank), 75,000 btuh will raise the tank from 120F to 133F at the rate of 692 gal/hr.
evacuated tube delta-T = 30F
gpm = 5
btuh = 30 x 5 x 500 = 75,000
75,000 btuh will provide 128 gallons of hot water per hour from 50-120F temperature rise.
Put another way, bottom of tank supply to the HX was 120F, and HX hot water return to the tank was 133F. If no hot water usage (no cold water being supplied to the tank), 75,000 btuh will raise the tank from 120F to 133F at the rate of 692 gal/hr.
yrock87
Member
Would it not be possible (or perhaps not advisable) to segment your system. On low use day, rather than discharge your tank, simply disconnect a portion of the tube? You mentioned there are 10 heat exchangers. Would it cause oveheating issues on the tubes/exchangers to only use 5, or maybe only 3 on high solar, low usage days? It would obviously require he plumbing to support that kind of stepped or phased approach. But is a reason why this would not work?
I don't think this is advisable. Turning "off" some of the tubes would likely cause them to stagnate (absorb solar energy but not transfer the energy to the water/antifreeze transfer fluid, temperatures could rise to well-over 300F), and while the tubes are designed to stagnate without damage, repeated or prolonged periods of stagnation probably will damage the tubes or degrade the antifreeze solution and cause failure at some point.
It would be possible to cover some of the tubes so that they are shielded from the sun. This was done manually before the changes in the system. An automatic awning-type cover could work. So far since the system was activated after the plumbing changes, the system has operated safely without overheating. The night cool down seems to be the best way to assure that the 1000 gal water storage tank has sufficient capacity at the start of the day to accept the next day's system hot water/btu output. Also in place is the unit heater which currently is set to activate and blow off excess heat when the top of tank reaches 154 degrees.
With Fall arriving and reduced hours of sun, I doubt over-heating will be an issue until next Summer, and I now believe strategies are in place to prevent future over-heating, and if not, there will be a Plan B.
It would be possible to cover some of the tubes so that they are shielded from the sun. This was done manually before the changes in the system. An automatic awning-type cover could work. So far since the system was activated after the plumbing changes, the system has operated safely without overheating. The night cool down seems to be the best way to assure that the 1000 gal water storage tank has sufficient capacity at the start of the day to accept the next day's system hot water/btu output. Also in place is the unit heater which currently is set to activate and blow off excess heat when the top of tank reaches 154 degrees.
With Fall arriving and reduced hours of sun, I doubt over-heating will be an issue until next Summer, and I now believe strategies are in place to prevent future over-heating, and if not, there will be a Plan B.
Circus
Feeling the Heat
Your system is vastly larger and more efficient than my flat panels but fundamentally they face the same problems. My exchanger/storage tank is above the collectors. No air pockets no overheating no pumps. A simple thermo switch controls a small pump to another storage tank. That's everything. It's been in operation for 6 years without me doing anything except topping up the glycol once in a while.
Great to hear that your system is simple and works well. A major contributor to the overheat issue at DP is the wide variation in hot water demand. DP could have several clear, sunny days in summer with practically no demand, and then have an influx of more than 100 people with food service, showers, etc. for a major school program. At high occupancy there really is no issue, but a low occupancy ... that's another story.
The freeze protection regimen is mostly in place and the final implementation will be finished next week. For a quick review, the solar side is high temperature glycol rated to -28F. The array and the insulated supply/return piping is located about 40-100 feet from the structure with the pump station and hot water storage. Anyone knowing northern Minnesota knows that -28F protection is pretty good for most of the lower 48 + Hawaii, but that's the temp at which many Minnesotans first start wearing their winter coats. LOL. Draining the pressurized system was ruled out because winter sun (Nov-Feb), even at temps well below -20F, can produce 160F+ hot water to storage for about 4-6 hours, so the evacuated tube system can be quite productive in winter.
Here is where I could use some advice and even better some expert knowledge. I know that the viscosity/reduced flow capacity of 50% propylene glycol in water increases rapidly as temperature falls below freezing. Therefore, I believe that it is important to establish and maintain flow through the piping/array well before the solution reaches -28F. I somewhat arbitrarily picked 0F as the lowest temperature that should be permitted in the glycol, and that at and below this temperature heat must be added to the glycol to maintain flow. Is this a good temperature point and if not, what should it be? The freeze protection regimen must never permit piping/array temperature to fall below the point that flow cannot be maintained in the solar side of the system.
The freeze protection regimen now implemented is:
1) When temperature at the array falls to 0F, the pump station circulators are cycled to circulate heated glycol (transfer from DHW side storage via hx to solar side) until the temperature rises to +4F.
2) During daytime the array can be well above 0F for several hours but not above 160F which is the "on" set point for solar to heat DHW. Therefore, the insulated piping between the pump station and the array can be much colder and may be as low as outside air temperature. Consequently, if the array is above 0F but air temp is 0F or below, the solar circulator only is set to "on" to circulate warmer array glycol through the solar side of the system and maintain the piping/array at the temperature being produced by the array (above 0F).
3) If the temperature at a sensor point in the piping is at __???__ or below, then heat tape in the piping is cycled until the sensor temperature rises to __???__, The heat tape sensing system is the last step to complete and should be in place within a week. What should these temperatures be?
Freeze protection 1) has been active since Fall, Protection 2) has been active only for a couple of days, and Protection 3) will be active within a week.
So far this year low air temperature has fallen to -16F and only Protection 1) was active. The data shows that the array temperature was maintained between 0F and +4F, which also means that flow was maintained through the piping. The flow rate is unknown.
Thanks in advance for your input.
Here is where I could use some advice and even better some expert knowledge. I know that the viscosity/reduced flow capacity of 50% propylene glycol in water increases rapidly as temperature falls below freezing. Therefore, I believe that it is important to establish and maintain flow through the piping/array well before the solution reaches -28F. I somewhat arbitrarily picked 0F as the lowest temperature that should be permitted in the glycol, and that at and below this temperature heat must be added to the glycol to maintain flow. Is this a good temperature point and if not, what should it be? The freeze protection regimen must never permit piping/array temperature to fall below the point that flow cannot be maintained in the solar side of the system.
The freeze protection regimen now implemented is:
1) When temperature at the array falls to 0F, the pump station circulators are cycled to circulate heated glycol (transfer from DHW side storage via hx to solar side) until the temperature rises to +4F.
2) During daytime the array can be well above 0F for several hours but not above 160F which is the "on" set point for solar to heat DHW. Therefore, the insulated piping between the pump station and the array can be much colder and may be as low as outside air temperature. Consequently, if the array is above 0F but air temp is 0F or below, the solar circulator only is set to "on" to circulate warmer array glycol through the solar side of the system and maintain the piping/array at the temperature being produced by the array (above 0F).
3) If the temperature at a sensor point in the piping is at __???__ or below, then heat tape in the piping is cycled until the sensor temperature rises to __???__, The heat tape sensing system is the last step to complete and should be in place within a week. What should these temperatures be?
Freeze protection 1) has been active since Fall, Protection 2) has been active only for a couple of days, and Protection 3) will be active within a week.
So far this year low air temperature has fallen to -16F and only Protection 1) was active. The data shows that the array temperature was maintained between 0F and +4F, which also means that flow was maintained through the piping. The flow rate is unknown.
Thanks in advance for your input.
While verifying that freeze protection was operating as expected, I pulled some data showing results plus a glimpse at hot water production on a cold winter day. The system has antifreeze rated to -18F. First level freeze protection is on at 0F and off at +4F; 2nd level is on at -6F with continuous flow, off at -2F. A 3rd level with heat tape is on when interior of the outside piping is at -10F and off at -5F. Third level is deactivated pending determination as to whether the 2nd level provides adequate protection.
December 28-29
3:00 pm: outside temp (OS) 10F, array temp (AT) 167 - producing hot water
6:00 pm: OS 7F, AT 65F
9:00 pm: OS 1F, AT 25F
midnight: OS 0F, AT 6F
3:00 am: OS -4F, AT 1F
6:00 am: OS -7F, AT 109F
9:00 am: OS -9F, AT 107F
10:00 am: OS -8F, AT 131F (sun shining, solar heating in progress, hot water flow begins at 160F)
10:30 am: OS -5F, AT164F - producing hot water
1:00 pm: OS 1F, AT 168F - producing hot water
2:00 pm: OS 3F, AT 168F (this is when I pulled the data) - producing hot water
hot water production likely continued until about 4 pm
The insulating performance of the evacuated tubes is evidenced by frost being on the outside of the tubes until about noon and yet the fluid in the tubes was +100F since 6:00 am.
December 28-29
3:00 pm: outside temp (OS) 10F, array temp (AT) 167 - producing hot water
6:00 pm: OS 7F, AT 65F
9:00 pm: OS 1F, AT 25F
midnight: OS 0F, AT 6F
3:00 am: OS -4F, AT 1F
6:00 am: OS -7F, AT 109F
9:00 am: OS -9F, AT 107F
10:00 am: OS -8F, AT 131F (sun shining, solar heating in progress, hot water flow begins at 160F)
10:30 am: OS -5F, AT164F - producing hot water
1:00 pm: OS 1F, AT 168F - producing hot water
2:00 pm: OS 3F, AT 168F (this is when I pulled the data) - producing hot water
hot water production likely continued until about 4 pm
The insulating performance of the evacuated tubes is evidenced by frost being on the outside of the tubes until about noon and yet the fluid in the tubes was +100F since 6:00 am.
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