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Comment by jrmg | Hacker News Reader

Before PV became efficient and cheap enough, it was not uncommon to see ‘solar panels’ on roofs in California that actually contained water pipes rather than silicon. Water circulated through them and was used either as regular hot water for the house, or in a ‘heated’ pool.

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You see them far more than Solar PV in Denmark as well, because IIRC you get taxed per kWh consumed, no matter where it's generated, so it's more economical to use the roof space for domestic hot water instead.

I wonder how many semi/complete guérilla setups there are.

The output of your own panels that you use yourself is taxed per kWh? That's insane. And unenforceable, surely?

Welcome to Scandinavia. Everything is taxed. It's the price of "free" healthcare and all the other social benefits.

I'm not against taxes, and the Scandinavian model seems to lead to decent societies, but this seems unnecessarily 'invasive'.

Seems pretty easy to enforce (at a binary level) with sat maps.

Not sure about Denmark, but in the UK part of the MCS signoff to allow you to export to the grid and receive payments is fitting of an generation meter. It's a holdover from the old FIT days (because you were paid per kWh generated), and has never been deprecated - your actual export (and any payment per unit) is recorded on your smart meter anyway.

In Sweden you only get taxed if you exceed half a megawatt.

My Dad did something like this for our pool in Australia in the '80s. He ran a bunch of black PVC pipe in a zigzag pattern on the garage roof and ran water through it from the pool filter. It was pretty effective at heating the pool!

We have the same, and it works great. Was (relatively) cheap, durable, and also helps a lot to keep the upper level of the house cool.

While it’s a lot less energy efficient, once saw a video of someone that replaced one of his tank-based residential water heater elements with a combo 240v/24v element and attached a panel to the 24v element with a temperature-driven cutoff relay. Set the tank to 50C or whatever and the element to a higher point to dump extra heat when possible without getting too hot.

A very cost efficient approach that probably gets you some double digit reduction without a lot of risky plumbing or freezing issues.

Roof-mounted solar water heaters were very common in South India too. Our childhood home had two solar panels heating a tank of water. Worked very well in summer, and there was a backup electric heater for the winter months. We also got some rebate from the electricity supplier, if I remember correctly.

The entire system was quite simple, and hasn't needed any maintenance in close to 20 years, I think?

What are the benefits of solar electric panels compared to water heating panels?

No moving parts, nothing to leak, and a joule of electricity is more valuable than a joule of low grade heat.

It's my understanding that if you want solar hot water these days you just get PV and a heat pump water heater. Sure, the heat pump has moving parts, but it's not on the roof.

And in a hot climate, the heat pump water heater provides a little cooling (when you dump the hot water outside).

Those were likely evacuated tube solar collector systems. They can boil water in direct sunlight when there's snow on the ground.

The ROI on a solar water heater is terrible. Cost us $10k before rebates (half that with) and saved us less than $50 a month in electricity.

If I stuck around for a decade it might have paid off but given a family member's unit didn't last that long (the part that held water went bad and a replacement was $4k, not exactly economical on a ⁴ year old system)

Ended up it was a pop up company designed to appropriate the green money by "improving your A/C efficiency" which of course probably only lasted a year or two given it was just blasting foam into the vents...

There are a lot of grifters out there wasting peoples money spent with good intentions, which is really frustrating. Sometimes it's not even a grift, just incompetence will lead to poor ROI, because contractors lack the skills to be efficient or do quality work.

I'm somewhat convinced I'm going to have to become a home builder myself to end up with something that's long-lasting, comfortable, healthy, and energy efficient, all without being robbed blind.

It's really starting to get that way isn't it? You basically have to build your own house to get something that isn't made of paper.

You still see new buildings in California with rooftop solar water heaters. I can't make it pencil out but builders seem to disagree. Here's one I happen to know of which is especially baffling because they have no rooftop PV despite an overabundance of space.

https://www.google.com/maps/@37.8027685,-122.2898842,118a,35...

Maybe a LEED credit (scam) thing?

Part of it is licensing/permits/requirements - you can often add rooftop solar water with no additional permits or a minor one, whereas solar PV is an electrical permit at minimum, and likely plans and licensed electricians, too.

This is doubly true in California, where adding PV means dealing with PG&E. Also, if you're grandfathered in on a NEM 2 electricity bill rate, then they apparently penalize you for upgrading your installation (by forcing you onto NEM 3).

Australia has had houses with tubes on the roof to heat the pool for a long time already.

I don't see the benefit here. In the summer you want a cool pool, and in the winter you need that heat to hit your house to reduce heating costs. Unless you live somewhere so warm in the winter that you still have to cool your house, in which case a pool would be pretty temperate and need no heating. If you've lived in a house like this, what's the energy benefit?

In most cases the average temperature is a bit lower than what your comfortable swimming temperature is. Sure it gets hotter than that some days, but pools are very slow to heat up/cool down and so they generally track the average temperature not the peak temperature. (you are probably swimming when it is peak temperature, and not at night when it is colder outside)

Yeah I suppose. I enjoy the unpreditctability outside, it's a bit of a philosophical point for me I guess, I don't want the whole world finely tuned to keep me in my comfort zone. A nice cool dip, even a little uncomfortably cool, is exilirating to me.

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