View Full Version : Wait. Wat?!

08-08-2016, 02:59 PM
So, thinking about things I should have said during the interview for an HVAC design job last week has led to a much more detailed design of a calcium chloride dehumidification waterfall for the basement to eliminate the need for the refrigerant based dehumidifiers that keep dying every couple years and dumping their horrific smelling refrigerants into the basement. During the last few minutes, I was somehow thinking about something I don't recall ever seeing before: combined photovoltaic and solar thermal panels. PV panels are not super efficient and only turn certain wavelengths into useful energy with the rest of the absorbed light simply heating the panels which can decrease their useful life just like any other semiconductor. So, would attaching a heat exchanger to the back of the cells and placing everything in a vacuum insulated enclosure not produce the same amount of PV power and provide a not insignificant amount of additional thermal power (although not super high quality/temperature) in the footprint usually dedicated to only one of those technologies? Think I might have to experiment a bit, because that just sounds way too obvious to have not been implemented before.

08-08-2016, 06:18 PM
If you have a pool to heat, I suppose it would be useful!
The problem is that you really do have to keep cycling the heat out of there. Else you end up with less cooling than if your panels just had two sides of air.
Maybe with a heat pump you could do something more useful?

08-09-2016, 12:18 AM
I have both PV panels & Solar hot water, but as two separate systems. I think this is what jwatte is referring to.
The water pumps through pipes set in a glass hot-box on the roof, so we use almost zero electricity to heat our water. You couldn't combine the two systems due to the high temperature of the water coming back into the roof mounted box from the hot water reservoir.

The Thermoelectric generator may not be efficient enough at the temperature gradients you are looking at. The link above talks of 'low temp' gradients up to 400 kelvin, which is far above what I could find on my roof, & I'm in the tropics. I reckon I could get maybe 80-100 degrees Celsius between air temp (About 30 to 35) & inside the box. Could increase that by 10 to 15 degrees or so with water cooling, but then there's the 'cost' of pumping that water around. I've never seen any commercial Thermoelectric generators, but I guess I've never looked! :)

I know a few years ago they were looking at two-layered solar panels where the top layer was semi-transparent. The light was trapped between the layers - bounced around to get more energy efficiency. I don't know what happened about it, or if it was even cost effective for the extra complexity in construction.

08-09-2016, 12:54 AM
If you have a pool to heat, I suppose it would be useful!
The problem is that you really do have to keep cycling the heat out of there. Else you end up with less cooling than if your panels just had two sides of air.
Maybe with a heat pump you could do something more useful?
Thus why I said low quality/temperature thermal power. Pondering 3'x4' heat blocks made from 0.063" or 0.032" 6061 aluminum sheet brazed onto 'channels'/'hedge maze' created by lengths of 0.25"x0.5" 6061 bar. 3'x4' is quite well suited for an array of cheap 3"x6" ~1.8W PV cells. Six panels would likely produce enough heat for an occasionally supplemented hot water heater and to regenerate the calcium chloride brine used in the waterfall, as well as produce the better part of 1 kW peak of electricity. This started out as something of a joke project, but starting to take it way too seriously.

Big question is how much effort I might want to put into this experiment: Just a cheap (B-grade) PV system on heat blocks with 40~60C max water temperature at atmospheric pressure and simple heat exchangers to warm the potable water and brine? A DIY multi-stage flash distillation system for more efficient regeneration of the brine? Build a DIY liquid-ring vacuum pump to extract the condensate from the MSF? Decrease the operating pressure of the water loop through the heat blocks to reduce mechanical stresses on the heat blocks in their vacuum enclosures and to make the water boil at <60C and condense on the incoming brine lines at ~30C for better heat transfer in the brine heater? Add a heat engine or thermoelectric generator array (cheap semiconductor TECs that max at 60~80C, not high temperature metal thermocouple arrays like in radio-isotope thermoelectric generators) to attempt to extract some useful amount of electricity from any excess hot water? I've got several ~60W TECs that I bought a while ago for a robot mounted drink cooler and/or a DIY TEG, but just sitting in a box right now. Divert some of the waterfall's brine supply to misting chambers on the input and output of an evaporative cooler to make a water spray absorption cooler for the rest of the house?

Ah!!!! Why must you taunt me so, ebay and wikipedia?!

Overall efficiency is not so important if the input power is entirely solar irradiation that would otherwise just be making the living space miserably hot in summer and be no help at all in heating the living space during winter.

Practically speaking, there is only one section of roof on the house that actually has the slope mostly facing south for solar and it is part of the ~16'x16' porch sitting on a whopping FIVE thin-walled steel tubes on concrete footings spaced 8' on the perimeter opposite the side hanging off the rest of the house. The chimney exterior that shoots up through the porch roof has already been destroyed by woodpeckers hunting insects, the chimney and fireplace have never worked properly, the porch roof is kinda pulling away from the rest of the house along one interface line with the roof of the living room, and it is shaded in the morning by the rest of the house and in the evening by a line of trees. So not really wanting to add anything onto that if I am likely to demolish it within the next couple years when the rest of the roof is replaced. The current structurally integrated garage would be a much better candidate if I ever get to tear it down and replace it with an independent steel structure in mostly the same footprint but without being mechanically linked to the house (part of existing garage slab becomes ~4' sidewalk between house exterior and new structure). Want a two-ish story steel framed building for garage, machine shop, utility management, and storage space with a single sloped roof facing mostly south like the current porch to line up with existing foundation and slabs.

08-09-2016, 01:18 PM
You couldn't combine the two systems due to the high temperature of the water coming back

That's the problem I was fearing. Which is why perhaps a heat pump/exchanger system might work.
But the cost of building and installing would be higher than it would cost just buying the saved energy from coal.
(I hear coal has been defined as clean now, and global warming is still not a thing, so that's great!)

If you want to get energy from solar, it's hard to beat the cost of import panels. About $200 for 220-250 Watt panels.
You'd need 130 of those super-cheap small 1.8W cells to compare, and that'll probably cost more than $200 :-)
(Costco sells them in singles, even.)

08-09-2016, 10:44 PM
I also hear that evolution is a lie straight from the pit of hell, the democrats are all marxist-communist-atheist-gay-islamic-jihadis hellbent on destroying 'western' civilization, cupping and homeopathy totes work for realz cuz there just aren't any scientists that want to be awarded the most important Nobel prize in all of history for completely upending all of physics, and vaccines totally ended so many illnesses but don'cha know you can't trust the FDA or CDC claims that they are safe because big pharma! Oh, and permanently storing nuclear waste in above ground sites and banning research is soo much safer than secure, semi-permanent, underground storage while continuing research into ways to eliminate it completely.


Water at 40~60C is essentially 'waste heat' quality that can be used for many things like desalination and heating, but not really capable of super efficient electricity generation. I knew this from the start, as the original plan was simply solar thermal regeneration of the CaCl brine at a higher temperature with evaporated water exiting the solar heaters through a condensate line at normal atmospheric pressure. The thought of combining PV with lower temperature solar thermal was that it might permit extraction of more useful power from the same surface area than either one alone.

Remembered shortly after my last post that we have a wooden powerline-like pole (slightly hidden at the back of the porch beside the AC condenser) that once held a big TV antenna that never worked well and was ripped off in a storm many, many years ago. Would be in full sun during much of the day after the porch roof is demolished (still blocked by rest of house in early morning, but far enough from trees to not be shaded so much in evening) and the top couple feet appear to even be in full sun during much of the afternoon as-is...

Currently thinking the testbed will be two 12"x12" heat blocks mounted on small insulated spacers ~1.5" above the reflective back surface in a vacuum enclosure ~6" deep and with a TSL2561 luminosity sensor mounted between the two blocks to get a rough idea of how much light that location really gets. Both blocks get painted flat black all over and one gets an additional covering of eight 3"x6" PV cells with heat transfer tape/adhesive. Each block is supplied with water by its own peristaltic pump driven by a common motor; reservoir is large plastic drum sitting in shade on the porch, possibly with a radiator, fan, and additional pump; identical flow rate to blocks is adjusted to maintain the PV block at either 50C or 60C. PV panels connected in series with a hall-effect current sensor and a low ohm power resistor to provide continuous loading. Temperature sensors on front and back of heat blocks to measure block/cell temperature, more temperature sensors in the input and output ports of the heat blocks to measure water temperatures, and another to measure ambient temperature by the reservoir. Combination of temperature sensors and water flow rate should give decent approximation of heat input to compare thermal-only to thermal+PV to figure out how much less heat is getting absorbed because of the PV cells.

mlsolar on ebay - polycrystalline cells direct from <arnie voice>california</arnie voice>. 40 A-grade 1.8W cells with tabbing wire, diode, and solder flux pen for $25. 84 B-grade 1.8W cells (90% area intact for ~1.6W) with tabbing wire, diode, and solder flux pen for $32. 500 A-grade 1.8W cells for $200 or 550 A-grade 1.8W cells with tabbing wire, buss wire, diodes, and solder flux pens for $260. The process of assembling and encapsulating them can be rather costly, unless you have way too much free time and already intended to make your own overly complicated enclosures integral to the roof structure to provide PV, hot water, and possibly heat driven ventilation of the living space...