Solar hot water + TEG questions...

Heyo!

Long time lurker, first time poster.  I've been following the forums for years - long before I would even consider myself a permie - and I can't tell you how much I appreciate all the wisdom and insight I've gleaned from here while just collecting information to use *one day*.

The day(s) is(/are) now...

We've recently bought a rural acre in New Zealand complete with house bus (and 5x 100W solar panels)  We've been using an LPG powered califont for showers, and a little butane powered camping single burner to heat water to wash dishes (it and all the butane canisters were already in the bus so we might as well use them).  Now that we're heading into Autumn, none of this is going to be enough to keep us warm or powered up long enough to ride out what can be quite long stretches of cold, grey days where we live.  We don't have access to a stream, and the wind out here averages about 3 m/s so while I will get around to building a turbine at some point it's not at the top of my priority list as it's likely only to give us decent bursts of power for a day or two at any given time and then not give us anything for awhile.

I've decided to build an oversized solar hot water system with a decent sized storage tank and use it for underfloor heating as well.  I'll mostly follow the plans at builditsolar.com for the $2k water + underfloor heater, except because of the cost of copper (well, everything) here I'll use PEX (we don't really have cpvc) instead and build slightly larger to make up for small efficiency loss.  I've estimated the size of the system I need, then gone through the calculator and other tools provided there and came to approximately the same conclusion (my estimate was a bit larger, so I'll stick with that as they only had weather station info for a town about an hour away in a different microclimate with marginally more sun).

Here are my questions:

1 - In a solar thermal panel, is there any reason that I wouldn't fill the panel with a mass instead of leaving the air space?  Based on my understanding, silicon carbide might be the best filler material as theoretically excellent heat conductive properties and relative ease of access since sand blasting media...?

1a - if I do that, is it likely that I'll overheat the PEX and should I, in fact, buck up and buy the copper?

2 - If that was a thing that made sense, wouldn't it make sense to also cut holes in the back of the panel just large enough to put TEGs underneath the silicon carbide filler and use that resultant electricity to help charge my batteries, especially on cloudy, grey days?

I understand I'm potentially adding a lot of money for my build.  I'm ok with that if it's logical and keeps me from having to upsize later and/or continue to use fossil fuels.

Thoughts?  Ideas?  Questions?

Thanks team!

Thomas, welcome to Permies!

Interesting design challenges you have for yourself.  I think that solar heating water is a very doable idea.  The TEG’s get tricky though.  The efficiency rating for TEG’s is worse than miserable.  Please don’t misunderstand, I love the idea of TEG’s and I do wish you the best of luck with them.  I think they will require constant heating in order to work.

This is a concept with good merit.  Using water as a heating mass is great thanks to its high specific heat capacity, allowing the water to continue heating for some time.  I just don’t know how many watt hours you can get out.  Please don’t get me wrong, I would love to be proven wrong.  I have looked into TEG’s before and they seem best adapted to making use of waste heat.  If you have this extra heat then I would say you have a doable idea.

If you have not checked already, there is an interesting site called tegmart that specializes in exactly this type of system.  Their most efficient units are designed to sit against a hot object while being cooled with water.  It could be interesting to see how this works for your proposed system.

Thomas, I am intrigued by your idea despite the challenges and I would love to see how things work out.  If I can offer any more specific help, I would gladly do so.

Eric

Thomas,

I just checked tegmart.com and their top unit produced a respectable 100 watts.

Eric

Thomas, welcome to Permies!
The solar thermal panel is a collector, not a storage device, and therefore does not need/want mass. Your heat transfer fluid (water, glycol?) is the mass, as is your radiant floor slab, and any storage tank you add.
Granular material is not a good conductor, even if the solid it is made of is, since there is a lot of tiny air space (insulation) and tiny contact areas between particles. So, a panel filled with silicon carbide grit, would only get hot at the surface, and not transfer that heat to the heat transfer fluid in the piping.

Gary's plans at Build It Solar for the $2k water heater are neat! and his tests of the performance seem well done.

I can't speak to the TEG, but wonder if it compares at all to photovoltaic panels for output and cost? My understanding was that if heat/fuel was free or was all you had such as a remote oil/gas site, that it was a good way to have limited, but maintenance free power, or a refrigerant free heater/cooler.

OK, what are TEG's please?

Thermoelectric generators, something like a Stirling engine, or bimetal devices that generate a current when heated at one end and cooled at the other.

How cold does your area get? Is freezing a regular occurrence, or rare, or pretty much nonexistent? One good freeze can potentially destroy copper, while pex may be able to cope with an occasional freeze. If you use pex in the panel, you will have to ensure that it doesn't overheat - boiling temp is probably bad for it, especially if the system is pressurized. I would overbuild the amount of pex tubing in the collector relative to plans based on copper. Tilting the collector so it gets maximum solar exposure at midwinter and considerably less in summer would be wise too. If you can get enough energy for winter heating, summer hot water use should be easy.

Eric - thanks for that!  Definitely I understand that TEGs are pretty inefficient.  I'm just trying to utilize what we've got (or will have soon) to make the most of our situation.  We're right in the middle of paradise but, because of our climate and the lack of mass in our bus, we're relatively limited in the most common forms of alternative energy/heating.  My long-term goal for our infrastructure (and my soon to be food forest) is no external inputs so that also weeds out some other low hanging fruit.  So *if* I did add TEGs to the system, it seems my best bet will be to actually preheat cold water through a heat exchanger on the cold side of the TEGs and either pump it into the storage tank if it needs heat or into an external tank to cool back down again if need be (probably routed via the anaerobic digester to keep it warmer than ambient outdoor temperature).  Doing that might provide an (at least) marginal boost to both the hot water system as well as the electrical generation...?  I'll definitely check out TEGmart for some inspo, thanks!

Kenneth - This is exactly what I needed to know about adding mass.  I'm capable enough to work out a system, but incapable enough to often follow random ideas and spend too much to get there.  I've been hemming and hawing for a couple of weeks now because I want to do it right the first time, for our needs in our climate, but haven't really been able to verify random ideas that I *think* will add efficiency.  Thank you!  While we can get used PVs for less than 1NZD/W here, but to get us through the winter we'd have to either seriously oversize our array, or add another couple of panels and then spend another 1,500 on batteries that we'll one day have to replace.  We've got about 500 usable amp hours and that lasts us for about two days without sun (I think I'm losing some electricity somewhere, but I haven't chased it down yet - too many conflicting priorities).  Wind is generally negligible here and when we do get it it's super gusty so not good for wind turbines.  Watt for watt, I know the TEG is less efficient than the PV (and more costly), it just seems like something that could fit nicely into our soon to exist systems without taking up more space.

John - Here's a pretty good, short-ish video about TEGs.  https://youtu.be/zzGnNkOxdpI

Glenn - it will get to freezing here, and we'll get hard frosts, but it won't stay frozen during the day.  The Build It Solar plans are for a drain back system, so at least there won't be water in the pipes overnight.  Thankfully it was designed in/for MUCH harsher conditions than we have here so some good safety mechanisms built in.  I'll go about vertical for the angle - as you said, best for mid-winter when we need it the most.  Thanks for that!

Any TEG you add to the solar thermal panels is going to depend on taking some of the heat that would otherwise go into the water. I think you will get more energy out of just getting the water hotter. If electricity specifically is important, then I would size the panels a bit bigger to give you enough incoming energy for water plus TEG powering. The hotter you can get the hot side, the more return you will get from the initial investment in TEGs. If you want to harvest waste heat bleeding out the back of the panel, I think you will get better return on investment by buying more insulation to eliminate waste heat loss.

Thomas my experience with tegs would say they wont perform well for you. If you were using a vacuum tube arrangement with high temp exchange fluid maybe but a pex hot water collector will be a higher volume of fluid with a lower upper temperature just due to the thermal limitations of pex. Without that high differential of hot to cold the tegs dont generate much power. At their best only about 4 percent of the heat is converted to ele trinity which isn't horrible since that heat is transferred to fluid but you wont come near that if the differential is not there. I found even having the hot side on my stove and cold floor fluid going through the other side it was hit and miss.
Sont want to be a downer the solar thermal side sounds cool.
Cheers,  David

Thomas, have you seen the solar power experiments at Tamera, in Portugal? (there is a lot going on...but here's just one piece..)
They are using evacuated tube collectors, and hot oil as the transfer/storage fluid, and can therefore store heat at 200C/~375F at atmospheric pressure. If the TEGs work better with a higher temperature difference, then a hot oil system/storage just to run the TEGs might be another way to approach having a "battery" to get you electricity through overnights and cloudy days. At higher temps such as these for the "hot" side, the "cold" side might be able to be your radiant heat return line on its way to the solar hot water panels to be reheated, or back to storage while running at night, or to make DHW!?!? I'm just following my intuition here, maybe Glenn or Eric could back this up with maths?

<edit> Cross-posted with David Baillie! and he answered some of what I was thinking about...

Glenn - It is specifically electricity that's important.  We've already got a califont so have a backup for hot water, and I'll add an additional backup for heating the water for the tank without fossil fuels, but definitely I'm trying to figure out how to add some oomph to my batteries when it gets grey for long stretches of time.  

Currently my plans are about 10-15% larger than than what was recommended based on our situation.  I did a general estimate, and then followed that up by using some tools that took into account our particular situation.  I went through it and input size of roof, floor, walls, and windows and the estimated R value of each.  I based the R values off of an estimate for an older, uninsulated home because that's the closest I could get.   We do have ceiling and floor insulation, but it was already a house bus when we bought it, so I haven't had cause to go tearing anything up and learning about the quality of the actual insulation.  Plus I suspect the metal exterior will add some volatility to heat loss/gain than an old timber/brick home.  They also recommended adding a value to the solar gain from the windows (of which we have plenty) but I decided not to do that because I'd rather oversize than under.  As far as I can tell I'm safely in the oversized range, but I don't know and that does a bit of a number on me when starting a new project.  

David - it's good to get some feedback from someone who has used TEGs.  I appreciate that, even if it doesn't back up my plans.  I reckon there's always a reason I get no results when I Google something.  I'm bright, but don't reckon I'm bright enough to be the only person to have thought of this, so logic would dictate that it hasn't been massively successful in other people's attempts.  :0/

Kenneth - This may be just the rabbit hole I was looking for, I'll do some research!  Thank you!  If that's a thing that makes sense, there's always the possibility of running two concurrent systems (copper is about $5/metre here, vs pex or other high temp plastics at $0.60 - $1/metre) - one with oil for underfloor heating and possibly powering the TEGs, the other for DHW.

While I'm thrilled to be in New Zealand in general (especially for the last year), I do miss the predictable weather(read: sun) of Central Texas and the cost of goods!