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Comment by PietdeVries | Hacker News Reader

While the idea is new to me - using friction in a windmill to heat your house - the practical use in Western Europe could be limited. Installing a 12 meter windmill in my backyard is not going to happen... And then efficiency. A heatpump has a COP of around 4 or 5, so every 1 kW into the system results in 4 or 5 kW out. A heatpump uses electricity, which can be generated using a windmill, but also via solar panels. And a heatpump is relatively easy to link to your general central heating system. Now if you'd want to link your water-brake windmill to the central heating I guess it would be a 'different biscuit'. And since the smaller systems generate 3.5 kW with a firm breeze, you need backup. Suppose you'd not use a water-brake but a generator and heatpump; wouldn't the overall system efficiency be better in that situation? Or at least more convenient if you add solar panels in case there's no wind?

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The article talks about this; it says the most efficient system is a mechanical heat pump (where the wind provides direct mechanical power to the compressor), but only for local generation, since heat transfer is much less efficient than electricity transfer.

If for some reason (I don't know, heating a blimp perhaps?) you were restricted to an air-sucking heat pump you could go wild and route the intake (or some fraction of it) of the heat pump through the generator and motor: this should recapture all the losses. A worthless optimization since the waste avoided would be free (just build a slightly larger turbine), but technology for low intensity heat is uniquely simple, but why not use three opportunity to make it amazing ("conversion losses? Yeah, they are taken care of")

> (I don't know, heating a blimp perhaps?)

Actually, given the recent interest in tethered blimps hosting high-altitude turbines, that's not as dumb an idea as it sounds. The prototypes shown so far generally use helium which is expensive and in limited supply.

Yes, my thought was to use the windmill to run the heat pump, then buffer the heat as hot water in an insulated tank to heat the house. Not really practical unless you live in a place with fairly regular wind in the winter. Where I live, winter air tends to be fairly still with occasional exceptions and I could be wrong but I doubt it would be a great way to heat the house.

Same with solar, in the dead of winter the sun is at quite a low angle in the sky, not even above the tree tops, and I doubt there's much to be gained either from photovoltaic solar or using it to directly heat water for the house.

Electricity production in winter can definitely be lower, but it depends on latitude. As you might guess, near the equator there's almost no difference in season.

According to the US Office of Energy Efficiency and Renewable Energy:

"Countries such as the United States, which lie in the middle latitudes, receive more solar energy in the summer not only because days are longer, but also because the sun is nearly overhead.

The sun's rays are far more slanted during the shorter days of the winter months. Cities such as Denver, Colorado, (near 40° latitude) receive nearly three times more solar energy in June than they do in December." [1]

Looking at these maps [2][3] for solar production in January and July, it seems like a lot of places drop by around 50% summer to winter. So it's a significant drop, but unless you're extremely far north, there's still energy to be harvested.

[1] https://www.energy.gov/eere/solar/articles/solar-radiation-b...

[2] January: https://www.nrel.gov/gis/images/map_pv_us_january_dec2008.jp...

[3] July: https://www.nrel.gov/gis/images/map_pv_us_july_dec2008.jpg

You could go all-out with an angled thermal collector panel (double-glazed or possibly vacuum insulated like some fancy solar hot water systems) which heats water to be piped back inside into a storage tank, then pump the heat from the storage tank into the air when required.

(For some scenarios this might even be a good solution without the heat pump, just run the water through a radiator. Is this something that people have tried?)

I'm inclined to agree. Domestic wind turbines never took off compared to solar. And solar thermal has been overtaken by solar electric. I don't think combining two technologies that failed in the marketplace are going to change that.

Solar thermal and solar electric have different uses. Id much rather use solar thermal to heat my water than solar electric.

My understanding is that currently the payoff for pv is quicker than thermal, which is why most people now go for it.

Now I don't know whether an equivalent pv system is cheaper in absolute terms than a thermal system, but from what I can work out, the marginal cost of adding extra pv to cover hot water is cheaper that adding a separate thermal system.

So yes they have different uses, pv generates electricity which can be used for a variety of things, thermal just generates heat. So if pv is cheaper or has a quicker payback, it seems like a no brainer to me.

The efficiency is MUCH higher for thermal over PV (~80 vs ~25). Its not an either/or thing. If you want heat, you use solar thermal, if you want electricity, use PV. Theres nothing stopping you having both.

Edit: Not sure what you mean by 'pay-off'. Solar thermal can be as simple as a black bucket sitting in the sun. It doesnt get much cheaper than that :D

A number of recent studies have shown that PV used to heat water is actually cheaper and more efficient.

It's counter-intuitive, but the reality is that PV has become much cheaper, while solar hot-water panels plus plumbing are expensive.

Plus you can use any surplus PV for multiple purposes.

I think it depends on the requirement and configuration.

A simple passive open-loop system is very cheap, and adequate for a hot water during warmer seasons (if you dont need hot water in the morning, i may add :D)

During colder seasons, you would need a closed loop system with a heat exchanger (often a heat pump, which you can power by PV) and insulated storage. Thats when it starts to get expensive, and the effective efficiency can be decreased if you are pumping more heat than you consume. This is an alternative to the air-source heat pump that your research probably used for comparison.

That said, I can totally see how PV could fulfill a hot water need right through the year, its just WAY more expensive than a low-tech solar thermal system (and likely less efficient in the summer).

Again, no harm in using both if you have the funds (and space) :D

A pv at 25% efficiency attached to a heat pump with a COP above 4 is still above 100% so still beats thermal. And yes you can have both, but the cost to have the installers add a few extra KW of pv is less than getting other installers out to do an entire solar thermal system, so what's the point?

Payoff as in if you buy a pv system it will pay for itself quicker than a thermal system. Electricity is more expensive and valuable than 'heat' (read gas substitute) so even if the pv system is more expensive, the savings are greater, which offsets the higher purchase price.

Hey if you want to shower under a black bucket, go for it :)

> A pv at 25% efficiency attached to a heat pump with a COP above 4 is still above 100%...

Could you name the particular brands and models of PVs you are using that produce 25% in the field? 15% under ideal conditions is more typical and your 25% is above current state of the art available for home installation. Also interested in your heat pumps above 3. Are these geothermal? If not, who makes them?

Normal set up is going to be 15% max on the PV and 3 on the heatpump, for 45% max, but more typically 25% as the PV is seldom set up to capture ideally.

I was quoting my parent on the 25% efficiency figure.

"The COP for heat pumps range from 3.2 to 4.5 for air source heat pumps to 4.2 to 5.2 for ground source heat pumps"

https://en.m.wikipedia.org/wiki/Heat_pump

The COP of a heat pump hot water heater is strongly dependent on the ambient and current water temperature. You get the best COP heating from cold, but it's not often that you have to reheat an entire tank of cold water. The actual achieved COP in normal use is more like 2-2.5.

Sometimes all you need is a black bucket. If you want to throw an 8KW solar array and a $3000+ heat pump at a shower then go for it! :D

> (~80 vs ~25)

The most efficient panels are at 22% right now, and that's with no overcast conditions or rain, sun directly overhead, and full tracking. The 22% are significantly more expensive. Most panels have around 15% efficiency, when new, under ideal seasonal, weather, tracking and lighting conditions. Quite a bit less when not.

I have a self built solar water heater and a house designed around passive solar. This handles heating and cooling sufficiently without grid inputs. My house stays at temperate conditions year round and my monthly energy bill, consisting of non-heating uses, is around $25/month on average.

Typical solar thermal installations are a bit more complicated that black buckets and require professionals to install. They usually don't work for free.

My PVC pipe solar shower begs to differ.

Why not both!? It increases electric output by cooling PV panel and makes you some hot water!

Using solar electric to run a heat pump is more efficient in most climates.

Agreed. Plus in moderate climates you can use a standard reverse cycle split system for cooling as well as heating. Granted you could do the same mechanically but it would be a custom design as opposed to commodity AC.

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