combined PV and hot water

[tanuki.himself]

Having read this thread regarding stick on PV panels vs mounting with a clearance

Solar panel roof adhension

I have some flexable solar panels I want to adheres to the roof of my alu-cab without drilling and securing down. Anyone have suggestions? I ended up buying a roll of EPDM Quick Roof Seam Tape but wanted to seem if anyone had other recommendations. Royal Adhesives makes Heliobond PVA-600BT which...

www.expeditionportal.com

and seeing the comments about temperatures and panel efficiency, i've been thinking about combining a PV panel with a way of heating water for washing the dishes/taking the chill off a shower.

What would be the downside to taking some thin aluminium sheet, bending a series of washboard-style U channels in it, running a zig zag of 6mm high temp silicon hose in the channels, stick the whole assembly on the back of a PV panel with computer heat sink gel/adhesive, and then circulate a water supply through it using a small 12v pump to a tank in the floor of the camper, then roof mount the whole thing on a slight standoff with a rearward incline to encourage airflow underneath?

Worst case scenario you are just sticking a heat sink material on the back of the panel. Best case you are cooling the panel to make it more efficient, and by the end of the day you have a free tank of hot water ready to use. Doesn't add much weight on the roof to upset COG, probably won't cost much to add to the panels - similar to a couple of solar shower bags and i know in the Mexico sun they got plenty hot enough around camp to deal with greasy pans.

I'm still a few months away from being ready to buy and install my solar setup - still at the frame, jacks and tie downs build stage - but anyone think this is worth a try?

 

[dwh]

https://www.hindawi.com/journals/ijp/2009/732093/

I'd try to avoid actually gluing anything to the back of a good solar panel. The thermal collectors don't last as long as the PV modules. Just bolt the thermal collector close to the back and leave the air gap under the combined assembly instead of under the PV.

Since the water tank won't be on the roof, plan on a recirculating system.

 

[luthj]

Here is a off-the-wall idea.

Get some UV rated flexible tubing. Loop it across the back of the panel as many times as reasonable. Bond it in place with a good urethane that will stick to the plastic backer. Circulate water through to heat a reservoir. In direct sun you could get 200-500w of heat per square meter of panel.

The risks with this approach are the added weight, which could cause the panel to flex more on bumps. Or possibly cause the panel to delaminate.

 

[Trailwerx]

That could work but, because the PV panel has to already be using a lot of the solar energy hitting it (and turning it into electricity), it seems like constructing a separate panel just for domestic hot water (using your corrugated aluminum or copper idea painted flat black and with a clear glass cover) would be a lot more efficient at cranking out hot water if you have the roof space for one. But you bring up a good point: how come I never hear about solar DHW panels on campers? Has anyone else tried one? Are there any commercial models on the market? Obviously there would be issues in freezing weather unless you went with a closed glycol/heat exchanger or drain down system like they use on home setups.

 

[dwh]

There are solar hot water kits for RVs. I doubt they sell many.

The main reason I would do such a thing is the reverse...to pump water from the hot tank to heat up the solar panels to melt off snow and ice. But I'm not planning on wintering in the Great White North, so...meh.

 

[luthj]

One option is to design the system to self drain in cold weather, but a closed glycol loop would also work fine.

Solar panels are about 20% efficient at best. So the remaining 80% gets converted to heat. Now solar panels aren't insulated like a solar heater type system, so they won't be as efficient, and won't work at all in very cold weather. For the shoulder seasons you could get a nice warm shower though.

 

[tanuki.himself]

Get some UV rated flexible tubing. Loop it across the back of the panel as many times as reasonable. Bond it in place with a good urethane that will stick to the plastic backer. Circulate water through to heat a reservoir. In direct sun you could get 200-500w of heat per square meter of panel.

bonding pipe direct to the panel i would worry that it could act as an insulator if there was no water circulating, and could hotspots actually damage the panel? Using Au as a heatsink should avoid this inherently. You could probably thread new pipe through the channels if it leaks over time. You could even do it with 6mm Au pipe or copper heating/brake pipe, which may be more thermally efficient but costs and weighs more....

I'm looking at this for when I'm in north and southern Africa and islands around the med - if i can get away without the weight of a fixed water heater and the fuel supply to it i've got more capacity and flexibility for carrying water and fuel.....definitely won't need to worry about melting the snow off the panels or draining down for winter

I suspect a backyard trial is in order next spring when I start buying the PV build components....

 

[shade]

Interesting concept. @tanuki.himself , what type of vehicle are you using? For my meager needs, the complexity of such an arrangement wouldn't be worth the trouble, but a larger vehicle with more infrastructure might see some benefits.

 

[luthj]

If you drive ever other day, its fairly easy to make up an engine coolant heating system. Either on demand, or for a fixed tank.

 

[Alloy]

When there is no demand for HW the system will cause the panel(s) to overheat....or you'll need a way to dump the excess heat. It's an issue with all solar hydro systems.

 

[shade]

If you drive ever other day, its fairly easy to make up an engine coolant heating system. Either on demand, or for a fixed tank.

With my current vehicle, I'm staying away from anything even that complicated. I still use it for enough other things that I don't have it built out as a camper full time, so for hot water, I use a pressurized Corny keg that I can heat on my stove or campfire. That's as simple as I can get for a pressurized hot water source.

When there is no demand for HW the system will cause the panel(s) to overheat....or you'll need a way to dump the excess heat. It's an issue with all solar hydro systems.

Add a radiator loop for on demand use, I guess. Starts getting complicated pretty fast. It might be better to use a dedicated solar water heating system and leave the PV panels air cooled.

 

[dreadlocks]

Keep in mind, most DC pumps dont have the head to even get 6ft up in the air, most only trickle a little bit out with only a few feet of head pressure.. only ones I know of are the high pressure pumps and that would make it a maintenance nightmare, I dont think those pumps are rated for continuous duty unless you spend a small fortune, and they'd use a ton of electricity if ran all the time.. Finding a low power pump w/enough head pumping capabilities is likely going to require violations to the laws of physics.

May just be better to put black tank on roof, or pipes hooked together, and use a transfer pump to move water from down low to up high.. let it passively heat the water up, then gravity feed it for your needs.. if you travel with it empty it would not hurt yer CG too bad.. yeh know what, I may have to do something like that.

 

[Joe917]

Engine waste heat is your best water heater. A loop off the engine is pretty simple. if you want real simple get a sun shower, basically a black water bladder you hang in the sun. your combined solar PV water heater is asking for problems with two systems at once.

 

[luthj]

A bit OT, but plate heat exchanges combined with a coolant circulation pump (either idling engine, or an electric pump) can make hot shower water easily. You will need a thermostatic mixing valve and pump for the fresh water feed. On my previous van, I would make hot shower water for an hour or two after the engine was shut off, and hot water in 15 minutes of idling. Worked quite well.

https://amzn.to/328scVO

For the solar system, once you prime the lines, there will be no gravity head losses, as the downward side cancels the upward side.

 

[dwh]

Keep in mind, most DC pumps dont have the head to even get 6ft up in the air, most only trickle a little bit out with only a few feet of head pressure.. only ones I know of are the high pressure pumps and that would make it a maintenance nightmare, I dont think those pumps are rated for continuous duty unless you spend a small fortune, and they'd use a ton of electricity if ran all the time.. Finding a low power pump w/enough head pumping capabilities is likely going to require violations to the laws of physics.

May just be better to put black tank on roof, or pipes hooked together, and use a transfer pump to move water from down low to up high.. let it passively heat the water up, then gravity feed it for your needs.. if you travel with it empty it would not hurt yer CG too bad.. yeh know what, I may have to do something like that.

On a closed loop with the air bled out it wouldn't matter...gravity assist on the downleg.