Understanding Solar Energy

Thanks! That's about US$0.20-US$0.40 per watt, which is probably cheap enough to compete with overprovisioning solar panels. What's the usual duty cycle for such a heat pump? Can you buy smaller ones? 12kW seems like it could provide 8000 liters of hot water per day, which seems like a lot even for a mansion. Maybe a public bathhouse.

It is certainly true that energy-intensive buildings cannot be self-sufficient on solar, but perhaps you can put the solar panels near your house instead of on it.

You can buy smaller ones. Domestic heat pumps usually start around 6-8 kW for single phase ones and go up to ~20kw for 3-phase units. Keep in mind that these values are the peak power of the heat pump, which you would rarely use. Modern units have inverter motor driven compressors, which can modulate their power output in the range 25%-100%. 100% is not a happy place for the heat pump if it is cold (and humid) outside, because it will quickly accumulate ice on the outdoor unit and start doing many defrost cycles per hour, which lowers the COP (efficiency).

With respect to the duty cycle, obviously if you have solar power, you would prefer to use it predominantly and only add up some extra power from the grid when needed. This is the essence of the sizing problem, because that leads you to 2-3x power overprovisioning and the need for heat/cold storage. Heat storage can be two types - DHW and space heating. Space heating is the easiest to estimate. You need to know your house's heat loss (either by specification or just figure it out empirically if you have already lived in it). DHW storage is more difficult to estimate, because it depends on the usage (e.g. how many showers per day). Cold storage is the most problematic, because the fluid needs to be at least 16C or lower to do useful cooling work, however you cannot go much lower than 7C unless you are using propylene glycol (expensive) and even then your indoor units may start to freeze (I am not even mentioning indoor humidity management and dew points).

Lately, the industry has been exploring PCMs (phase change materials). The idea is to store heat/cold not as sensible heat, but as latent heat of the phase change. In practice the substances used are either salts (efficient, but corrosive to the storage tank) or paraffins (more expensive, less efficient, but still viable). These come rated at a specific temperature, but usually have some hysteresis/drift and other issues. I guess you are now feeling a bit frustrated from the engineering complexity :). If batteries were cheap, long lasting and environmentally friendly, this complexity would not be needed. However, I really doubt it that in the foreseeable future batteries will beat heat storage. Given that most of our domestic energy use is space heating/cooling and DHW, I think that PCMs may actually have some moat. There are already offerings on the market, but IMHO they are still not very compelling. What I see lacking is some integrated offering, that would take into account the PV schedule and also grid prices. One a side not, batteries still have an advantage if you can sell back to the grid at a high premium or if you need to e.g. charge your car in the night. So these technologies may be complementary, rather than competitive.

A very big factor is climate. Just to give you an example, I live in the mountain with a colder climate. Cold water from the faucet is around 10C. I rarely need cooling if at all, but I need space heating around 8-9 months during the year. Just 300 km south and by the sea (Greece), cold water from the faucet is around 20-25C, you need 4-5 months of cooling and only ~4 months of heating. Some countries, such as UK have very moderate climate without extremes and things are more predictable. Where I live, we get -15C in the winter and 38C in the summer.

Thank you!

It sounds like you might be interested in my notes and calculations on thermal energy storage in phase change materials, some of which are listed at https://dercuano.github.io/topics/phase-change-materials.htm.... But I think TCES systems are likely to be more significant because of their technical advantages, among other things for managing indoor humidity and possibly even for seasonal thermal stores; some of my notes on the topic are at https://derctuo.github.io/notes/desiccant-climate-control.ht... and https://dernocua.github.io/notes/shower-heating-tces.html. Various kinds of thermal energy storage do seem to beat batteries on cost by around three orders of magnitude; some of my relevant notes are listed at https://derctuo.github.io/topics/thermal-storage.html. I agree with you that there is no real prospect of batteries catching up with thermal energy storage in the foreseeable future.

With respect to the particular problem you mention with needing expensive propylene glycol in your heat transfer fluid to keep it from freezing, ice rinks commonly use brine systems instead, despite the corrosion problems you mention. Brines are very cheap, some like dipotassium phosphate are minimally corrosive, and the commonly used ones are pretty nontoxic.

Wow! Your notes look like a treasure trove on the topic(s)!. I will definitely read them. Ironically, my heat pump just failed yesterday and now I am going through the service manual, so the resistance heater proponents may have some merit :)

Hopefully they're correct!

Just FYI, it turned out to be a bad connection between the controler PCB and the NTC temperature sensor that measures the condensation temperature at the plate heat exchanger.

So it was easy to fix? What a relief!