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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, us

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

benj111 8y ago
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 :)
- droithomme 8y ago
  > 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.
  - benj111 8y ago
    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 https://en.m.wikipedia.org/wiki/Heat_pump
- caf 8y ago
  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.
- supermatt 8y ago
  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
adrianN 8y ago
Typical solar thermal installations are a bit more complicated that black buckets and require professionals to install. They usually don't work for free.
- supermatt 8y ago
  My PVC pipe solar shower begs to differ.
Johnythree 8y ago
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.
- supermatt 8y ago
  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
droithomme 8y ago
> (~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.

benj111 8y ago

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 :)

droithomme 8y ago
> 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.
- benj111 8y ago
  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 https://en.m.wikipedia.org/wiki/Heat_pump
caf 8y ago
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.
supermatt 8y ago
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

adrianN 8y ago

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

supermatt 8y ago
My PVC pipe solar shower begs to differ.

Johnythree 8y ago

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.

supermatt 8y ago
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

droithomme 8y ago

> (~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.