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Thank you for all the help. What do you all consider as the best heat Sink/Dump system/setup for a four panel solar thermal system in New Mexico?
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Always room for a hot tub.
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By heat sink I refer to , as an example a commercial baseboard heater unit attached to upper back area of panels which dissipates extra heat into atmosphere or alternative heat sink/dump, so as not to overheat glycol and cause excess acidity. I am trying to determine details of the best system for a heat sink/dump. Thank YouComment
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Solar thermal design is not that complicated but a few or more of the design details/parameters are different than conventional HVAC design.
Because the heat collected via solar thermal is usually much more expensive to acquire because of the greater capital cost of the equipment, solar thermal systems for domestic heating, be it for space heat or domestic hot water are designed in such a way as to make intentional heat dumping unnecessary for most situations. For most solar thermal applications, well designed systems will not, under most design conditions require heat to be rejected.
One mantra of system design is to aim for the best match between required use temp. (kept as low as possible) and max. generation temp. in the collectors. Any over temp. in the collectors is much more costly than an overtemp. There are tricks to keeping the use temp. and the collector temp. as low and as close to one another as possible. One common way is to have a large thermal storage capacity. There are others.
What you need is a decent text on solar thermal system design. There's more than can be explained on a forum, at least by me.
BTW, one other reason for keeping system design temps. as low as the application will allow is, as you have pointed out, to keep the collector loop working fluid from breaking down. In any case, working fluid chemistry and transport properties need to be monitored on a regular basis. There will be times - like in the summer, when collector temps. and thus collector working fluid temps will be quite high, such as during stagnation or pump failure. For HVAC, other tricks are to drain the collector loop during the warm, non heating season, and/or simply covering the collectors. A lot easier, cheaper and less hassle than effectively having a mechanical cooling setup. It's easier to not make heat in the first place than getting rid of it. For DHW applications, one trick is to set the collector tilt for max. winter production. That may tend to increase the collector area/cost a bit, but will also help avoid summer overheating. I cover about half of my solar DHW array during summer, but that's because my HOA wouldn't let me tilt the array at 60 deg. in an 18.75 deg. tilt roof. Lots of other tricks are available.Comment
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Many manufacturers are now recommending "Steam Back" to control overheating in pressurized systems. The system consists of an oversized expansion tank sufficient to store all the heat transfer fluid in the panels.
When the the system reaches max temp the pump turns off, steam is created in the panels pushing all the liquid down into the expansion tank. This type of system requires proper design as high pressures are generated that can be dangerous to equipement and people.
As stated above why dump excess energy, design the system to utilize all the energy produced without dumping excess.
Another option is to use a drainback back system to eliminate glycol and all of the maintainance that comes with it.Comment
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My climate [Portugal] is similar to yours in New Mexico I think, and I have the same problem.
Even when I tilt my collectors for maximum winter efficiency, simulations show the array will gain 20kwh per day in summer. With domestic hot water the only load and 8" of insulation, there is the strong possibility that bad things could happen.
Emptying the collectors would cause them to go to extremely high temperatures every day, with possibility of damage.
A heat dump is fairly simple; you can use a domestic home radiator, or a car radiator. I plan to have one outside, in the shade below the collectors. A fan would increase the heat loss a lot if you need to do that.
You could also back pump through your collectors at night, to cool your buffer tank and allow it to absorb the next day's heat.
I don't like that much, as it requires weather prediction as part of the safety system; I think a heat dump is important for a large array.
Of course you can always dump hot water, and add fresh cold water, if your system is not susceptible to corrosion. That would be simple, but I understand that in New Mexico water is precious.
Another idea is a little roof / shade above the array at the angle of winter sun, that would partially shade the collectors in summer but not in winter. That would be superior design, but depends on your physical situation there.
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Mark , the summer shade is excellent thought, thank you. I cannot dump fluid since it is a glycol mix, due to common freezing temperatures.Comment
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I'd suggest that's a workable scheme. I cover one of two water heating collectors at my house for several months/yr. Just be sure to keep the collector sensor(s) out of any shade, and probably shade at the inlet to the collector array as much as possible rather than the outlets. That may well mean shading the bottom half of a collector or array which can get tricky if the shade mechanism is a roof or is located some vertical distance from the solar collector(s). Also, partially shading a single collector in a direction parallel to the fluid flow direction can cause thermal expansion problems, particularly on stagnation or no flow under sun conditions.Comment
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