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Understanding Solar Energy

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261 points chmaynard | 6 comments | 20 Mar 25 12:09 UTC | HN request time: 0.001s | source | bottom
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bryanlarsen ◴[20 Mar 25 14:23 UTC] No.43423941[source]▶
Great article. Unfortunately his California duck curve graph only shows 2023. A graph including 2024 shows how batteries are dramatically flattening the duck curve:

https://cdn-ilcjnih.nitrocdn.com/BVTDJPZTUnfCKRkDQJDEvQcUwtA...

https://reneweconomy.com.au/battery-storage-is-dramatically-...

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Calwestjobs ◴[20 Mar 25 17:00 UTC] No.43425755[source]▶
Hot water tank heated by electricity and powering on at noon is flattening curve. You can say hot water tanks are cheapest, simplest and fastest deployed energy storage device.

Solar + hot water tank can provide any house in US with 100% solar hot water (from PV!) for 80% of time, remaining 20 % of time you can have 10-99% solar heated water.

So we should focus on saying to people that if they buy solar and add electric heating element to hot water tank, then PV system will pay itself much sooner and their batteries will last longer. Becasue it is known and predictable load, you need hot water every day. And hot water is order of magnitude more energy then TV, lighting...

By lowering household usage like this we can make energy transition faster, cheaper.

Also proper construction - house heated only 10 days in a year - https://www.youtube.com/watch?v=5KHScgjTJtE

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epistasis ◴[20 Mar 25 17:26 UTC] No.43426144[source]▶
Converting a gas water heater to electric and/or solar is one of the best bang for the buck on decarbonization too. Something that should be done before buying an electric car or swapping out your gas furnace for a heat pump. Though I'm terrible at following my own advice, I still have a gas water heater, just because I needed to replace my car and furnace before I needed to replace my water heater. That said, the sunk cost fallacy applies to carbon emissions just as hard as it does to dollars so I have little excuse for not replacing it except laziness (and space on the breaker panel...)
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opwieurposiu ◴[20 Mar 25 20:19 UTC] No.43428408[source]▶
If you want to DIY a solar PV water heater I made a whole website about it with instructions and a simulator to estimate what your payback period could be.

https://www.pvh2o.com/

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PaulDavisThe1st ◴[20 Mar 25 21:54 UTC] No.43429420[source]▶
Resistance heating is so 20th century. Granted, you likely cannot do a DIY air source heat pump build, but the COP is so high for such systems, that it's probably worth it to just buy it.
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opwieurposiu ◴[20 Mar 25 22:12 UTC] No.43429594{3}[source]▶
My mom had a Heat Pump water heater at her house and I was always having to go and fix it or clean the filter. It would start beeping loudly in the middle of the night when it wanted attention. The hot water was frequently not very hot.

Hopefully the new heat pump water heaters are better. The advantage of resistance heating is simplicity and cost, with no moving parts. Solar panels are so cheap now they make it hard to justify the expense of the heat pump, assuming you have room to mount the panels.

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PaulDavisThe1st ◴[20 Mar 25 22:16 UTC] No.43429646{4}[source]▶
a COP of 4 can certainly justify having to install 3-4x less panels.

it disappoints me (but thrills me) that improvements in PV efficiency and cost have made solar thermal hot water more or less pointless.

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1. NullPrefix ◴[21 Mar 25 07:28 UTC] No.43432714{5}[source]▶
Heat pumps have COP of 4 when reaching temperatures needed for room heating. Hot water is way hotter than that. The more the difference between outside air and heated water, the less the COP becomes. I don't expect COP of 4 at 200 fahrenheit
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2. kavalg ◴[21 Mar 25 09:04 UTC] No.43433370[source]▶
True, but you never need 200F for DHW. This is even dangerous (scalding) and harmful to the piping. Typical temperatures for heat pump derived DHW are around 120F (45-50C), with some (bi)weekly cycles to avoid legionella build up at 150F(65C). Your statement is generally true, because a heating installation that is optimized for heat pump works at 85-100F, however in practice not many installations in old buildings are like that.
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3. nimos ◴[21 Mar 25 11:12 UTC] No.43434269[source]▶
If your concern is storage you want as hot as possible, less boiling. Thermostatic mixing valves can bring the temp down to a safe temperature for use.

Losses are higher but you store more energy per L, which is often the limiting factor.

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4. hnaccount_rng ◴[21 Mar 25 12:06 UTC] No.43434643{3}[source]▶
But you only need to store enough for a day or two. So you can alternatively just go bigger
5. kragen ◴[21 Mar 25 12:37 UTC] No.43434899{3}[source]▶
Is energy per liter often the limiting factor? Suppose a 70m² apartment with 3-meter ceilings costs US$3000 a month because it's in an expensive city center like San Francisco or Manhattan. That's still only 17¢/liter/year. Expanding your hot water storage space from 45 liters to 200 liters consumes space worth US$27 per year of your rent. Even that cost seems far too low to be a limiting factor, and the vast majority of people live somewhere much cheaper than that.

I think that what actually costs money is not the space but the tank. Higher temperatures mean not only more expensive materials and shorter lifetimes for tanks and piping but also higher conductive losses.

6. kavalg ◴[21 Mar 25 15:59 UTC] No.43437211{3}[source]▶
As hot as possible is not the way to go. Heat pump COP will degrade dramatically if you try to boil the water. It is not even possible with the popular refrigerants (R32, R410a and even R143a), because they will exceed their critical temperature. If you cannot afford the space for a bigger DHW tank, then there are two options:

1. Consider PCM heat storage (still relatively new technology, but works well with heat pumps)

2. Maybe the problem shall be solved at the building level, not individual apartments.