Active and passive solar strategy for so cal.

[Pappio]

A thought on vacuum tubes. The efficiency of one tube is much higher than a flat plate, the insulation value of the vacuum is so high it reduces the losses back to the outside.

Lining up a bunch of tubes side by side is where the heat output is more like a flat plate. I figure the spaces between the metal collector surface and the glass wall add up so you have 10% to 20% less exposure to the sun. This is then balanced by the increased efficiency, so as Russ said, the temperature is higher but the toal heat output is about equal. One 4' X 8' panel at 60% efficiency would be roughly equivalent to an array of 8' tubes 4' wide.

The efficiency of an evacuated tube where a single tube is used like a parabolic trough will yield much more in temperature and in BTUs than comparable area of a flat plate. In a parabolic reflector, the area collecting the sun's energy is equal to the size of the reflector, not the surface absorbing the heat.

[CADucatiman]

The way I am looking at things is that parabolic concentrator troughs are a good way to go for me. I could start out small before we build the new place with a bank of a few troughs using just copper pipe, maybe just use it for DHW or just have a tank of water and a heat sink. The goal at first is R&D. Then I could try a few of the vacuum tubes designed for parabolic trough concentrators. After monitoring the performance of both setups I will know how much it will cost as well as the performance of each setup.

The more heat I can collect and store them more things I will use it for. So Solar has worked with absorption chillers and has stated that their evacuated tube panels have a hard time keeping up with it.

I am looking into the vacuum tube concentrator setup because it seems to be the most efficient rig I can build myself to produce higher temperatures. A higher temperature will allow for more thermal storage at a given tank size as we have already covered. I if I have a storage tank at 100*c or even higher using a liquid to liquid exchanger I can send 40* to the floor. In the summer a different liquid to liquid exchanger can send 88*C or so to an aborption chiller.

[russ]

Hi Mike - You are correct about the misinformation out there!

When we decided to build homes (our own and others for sale) I had to start to study. Not too much in common with the iron ore processing plants I was involved with designing, constructing and operating over the years!

When you start to study residential equipment you soon learn that information falls into several categories - 1) self serving, 2) totally wrong, 3) the writer is confused and has no idea, 4) someone is just blowing smoke, 5) the smallest category is good information.

We try to give good, accurate and honest information and encourage others to try to share knowledge and experiences - it helps us all.

Russ

[So solar]

Cold floors are not comfortable. I like heated floors. I also love this forum. Its nice to read from intellgent peole who share the same passion for knowledge and solar as I. Thanks for the info. There can be so much misinformation out there. I tend to turn a blind eye and learn from experience. But I trust the info from the facilitators of this site.

[russ]

Hi Mike, One of the documents I have seen comparing vacuum tube collectors to flat plate.

http://homepower.com/view/?file=HP132_pg40_Mehalic

The document name title is Flat-Plate& Evacuated-tube Solar Thermal Collectors.

I found the document available for free from homepower.com. I have a couple of other references as well but have to look for them. My filing system is wonderful for filing but not so great for retrieving!

There are two problems with in floor radiant cooling - 1) condensation potential and 2) I personally do not prefer cold floors - warm floors OK but not cold floors.

I spent too much on the AC system here and have not used it the past two summers. A couple of nights it would have been nice but not worth the expense of operating the system.

The house is 4 story and normally you can create a draft through it. The location on the bay helps in that regard. In town it is boiling summer times due to the heat island effect.

Russ

[So solar]

Completely. Nothing wrong with solder. Commercially we will braze, welding id extreme. But I digress. You do have alot of studying to do to DIY an extensive system. Let me rephrase. "By adding extra evacuated tubesfor a large storage tank. Not circulate a direct loop into a floor." Flat plate collectors wouldn't do it. Id love to see the article that compare flat plate and evacuated tubes as the same. Commercially and residentially. Im a also a certified installer of Schucco. Recently I installed 450 for an unnamed tomato grower. Meant to keep the greenhouses a certain temp. Honestly, not very impressed with heat output and 1/2" headers. Im still a fan of forced air fan coil for cooling with a standard chiller for offpeak cooling and on peak use and radiant floor with same fluid, but bypassed to work with solar thermal and backup .Good luck in your venture.

[russ]

You have a lot of studying ahead of you.

The water temp to the floor loops should be 40

[CADucatiman]

[QUOTE=russ;16880]Look at it carefully - That system is designed to make steam for a turbine - much hotter than residential use. The document says 400

[russ]

Look at it carefully - That system is designed to make steam for a turbine - much hotter than residential use. The document says 400

[CADucatiman]

I could TIG a pop can cut in half back together with with a very pretty weld..

I guess I tend to work on even stationary projects with the same level of quality I apply to things that exceed mach 2.

Parabolic Trough Solar Thermal Collector-Solar Receiver Tube

is the page I linked to. It is indeed setup to pull a vacuum in the tube

[CADucatiman]

http://www.solar-evacuated-tube.com/solar_product_info_276.html

evacuated tube over a coated steel tube.

I think it is primarily budget and home made systems that use just a copper pipe. On the full scale power generation level they use evacuated tube.

I think I will take the time to really and I mean really figure these systems out. I have some absurdly intelligent friends in engineering that can help me if I need some help.

I think the best route may be to build my own tracking reflector rack setup and depending on the cost buy the evacuated tubes.

FYI I could not bring myself to use solder as a means of mechanical assembly on something like that. With a good TIG welder and the right filler rod (Copper or Si-Br) you can fuse parts together in a way that won't melt unless its also at the melting temperature of the copper itself.

[russ]

Parabolic troughs do not typically use vacuum tubes to collect the heat - the focal point of the concentrator will probably be a copper tube.

Vacuum tube panels do not collect more heat than a flat plate panel - they can provide a higher temperature but generally not more total heat.

The solder melting on a vacuum tube sounds strange - shouldn't see temps so high I thought.

[CADucatiman]

Well I have some ideas that will would make things stronger, more precise, lighter and easier to reproduce. The parabolic trough would be composite and reproduced on a mold. The plans also do not incorporate an evacuated tube.

I am still looking for parabolic trough evacuated tube concentrators so that I can compare the price of an off the shelf section vs rolling your own.

[russ]

Flat plate collectors will not provide adequate heat for an in floor radiant heating system - summer time they provide more but in the winter no.

My three Shuco panels provide enough hot water at 50

[So solar]

His ideas are good. But I question whether or not a system like that would be cost effective. Why not just add another panel. When it comes to hot water. The less active. The better. Besides. A system like that has too many unknown variables. For example: at what point can the glass melt on the tube. I've seen evacuated tubes heat solder to its melting point. And what if the accuuators fail. Then you have a serious condition in so many ways its tool lengthy to type. I've seen fireplaces with hydrocoils ran through them. It failed. And caused a home fire. Sometimes less is better. Its simple. Size the system according to needs. Without the hoopla and over-controling and it's fine. There's alot of products out there that are safety tested and work for everyones needs. Without unauthorized manipulation and unknown variables.