4 ms·

Why are people even considering an electric heating element, when you can get at least 2-3 times the efficiency of a DHW heat pump that would probably cost you

by kavalg 2y ago

Why are people even considering an electric heating element, when you can get at least 2-3 times the efficiency of a DHW heat pump that would probably cost you ~ $4000. In my experience, I have found that for PV panels it is often the roof area / orientation that limits the energy capacity that you can install. Installing a heat pump instead of resistive heater can effectively reduce this 2-3 times. Yes, heating DHW with a heat pump is not that trivial. There could be problems when the tap water is hard (limescale problems in heat exchangers), you often need 2-3 times larger tank in order to cover the daily cycle, but still looks more efficient than a big battery and an electric heater. PS: I've accumulated lots of knowledge on the topic. DM me if you are interested in exchanging on this.

kragen 2y ago
What size of DHW heat pump costs you ≈$4000? Which currency are we talking about here? If you aren't limited by roof and other outdoor area for PV panels, US$4000 buys you about 50000 watts of "low cost" solar panels at current wholesale prices: https://www.solarserver.de/photovoltaik-preis-pv-modul-preisindex/ https://www.solarserver.de/photovoltaik-preis-pv-modul-preis... At a nominal capacity factor of 15%, that works out to about 5000 liters per day of domestic hot water: ~ $ units -t '50000W 15%/(30K 1kcal/kg/K)' kg/day 5162.5239 Even in countries like the US with aggressive anti-renewable-energy regulation, it's hard to see how the heat pump comes out cheaper.
- kavalg 2y ago
  USD 4000 will buy you something like 12 - 16Kw R32 heat pump. If you have enough roof area to install 50kW PVs then you must live in a mansion :). Another thing is that heat pump installations are usually dual purpose (space heating/cooling and DHW).
  - kragen 2y ago
    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.
    - kavalg 2y ago
      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.
jajko 2y ago
Heat pumps come with a lot of restrictions. What about constant noise? Plus stating that they are cheap ain't correct. Our housing unit in Switzerland recently needed to replace older oil heater and one of the options was heat pump... which was by far the worst choice based on various criteria and we at the end voted for the oil again.
- kragen 2y ago
  How much was the cost, for how much heating capacity?
  - jajko 2y ago
    Sorry don't remember exactly, it was for 17 rather large apartments/semi houses (120-190m2), IIRC to the tune of 200k. Heat pumps would also require additional space to be put in, something we didn't have easily (not without some remodeling of whole area). They would be in the face of everybody's windows, another drawback combined with noise that was unavoidable as per proposition from vendor. Another issue was that they were not available for a long time (around 6 months delivery time with no guarantee), something not relevant here but it also affected decision of owners.
    - kragen 2y ago
      I appreciate the information!
    - kavalg 2y ago
      Thanks for the info! Now it is much easier to understand your concerns. It is a rather big installation that you have here, in the range around 100kW (if both space heating and DHW are considered). Such systems (aka chillers) are usually installed away from buildings or on the rooftop if local regulations allow it.
- kavalg 2y ago
  Noise could be a problem, but it is often the result of a bad installation. I'd be curious how much was the quote for the heat pump vs the oil heater.
megaman821 2y ago
How about lifetime costs? A resistive water heater is going to last longer and you can get non-metallic water heaters for extremely long life. If you don't have net metering (or just a terrible power purchase rate), why not just sink that extra solar energy into a water heater?
- kavalg 2y ago
  That is a good question. The usable life of a combined heat pump (space heating/cooling + DHW) is somewhere between 10-15 years. For the ones, dedicated to DHW, I cannot really tell, but I would expect it to be longer, because they are usually less heavily loaded. Then the answer of your question will depend on the price of installation (e.g. heat pump vs 3 x extra PVs), local climate (affects efficiency) and price of electricity. Heat pumps don't work well in very cold climates, but for most US/EU cities they will work fine. For very hot climates it may be better to use solar water heaters, although for commercial installations there is the option to use the exhaust heat from cooling to heat DHW (it is essentially free energy).

by kavalg 2y ago

kragen 2y ago

What size of DHW heat pump costs you ≈$4000? Which currency are we talking about here? If you aren't limited by roof and other outdoor area for PV panels, US$4000 buys you about 50000 watts of "low cost" solar panels at current wholesale prices: https://www.solarserver.de/photovoltaik-preis-pv-modul-preisindex/ https://www.solarserver.de/photovoltaik-preis-pv-modul-preis... At a nominal capacity factor of 15%, that works out to about 5000 liters per day of domestic hot water: ~ $ units -t '50000W 15%/(30K 1kcal/kg/K)' kg/day 5162.5239 Even in countries like the US with aggressive anti-renewable-energy regulation, it's hard to see how the heat pump comes out cheaper.

kavalg 2y ago
USD 4000 will buy you something like 12 - 16Kw R32 heat pump. If you have enough roof area to install 50kW PVs then you must live in a mansion :). Another thing is that heat pump installations are usually dual purpose (space heating/cooling and DHW).
- kragen 2y ago
  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.
  - kavalg 2y ago
    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.

jajko 2y ago

Heat pumps come with a lot of restrictions. What about constant noise? Plus stating that they are cheap ain't correct. Our housing unit in Switzerland recently needed to replace older oil heater and one of the options was heat pump... which was by far the worst choice based on various criteria and we at the end voted for the oil again.

kragen 2y ago
How much was the cost, for how much heating capacity?
- jajko 2y ago
  Sorry don't remember exactly, it was for 17 rather large apartments/semi houses (120-190m2), IIRC to the tune of 200k. Heat pumps would also require additional space to be put in, something we didn't have easily (not without some remodeling of whole area). They would be in the face of everybody's windows, another drawback combined with noise that was unavoidable as per proposition from vendor. Another issue was that they were not available for a long time (around 6 months delivery time with no guarantee), something not relevant here but it also affected decision of owners.
  - kragen 2y ago
    I appreciate the information!
  - kavalg 2y ago
    Thanks for the info! Now it is much easier to understand your concerns. It is a rather big installation that you have here, in the range around 100kW (if both space heating and DHW are considered). Such systems (aka chillers) are usually installed away from buildings or on the rooftop if local regulations allow it.
kavalg 2y ago
Noise could be a problem, but it is often the result of a bad installation. I'd be curious how much was the quote for the heat pump vs the oil heater.

megaman821 2y ago

How about lifetime costs? A resistive water heater is going to last longer and you can get non-metallic water heaters for extremely long life. If you don't have net metering (or just a terrible power purchase rate), why not just sink that extra solar energy into a water heater?

kavalg 2y ago
That is a good question. The usable life of a combined heat pump (space heating/cooling + DHW) is somewhere between 10-15 years. For the ones, dedicated to DHW, I cannot really tell, but I would expect it to be longer, because they are usually less heavily loaded. Then the answer of your question will depend on the price of installation (e.g. heat pump vs 3 x extra PVs), local climate (affects efficiency) and price of electricity. Heat pumps don't work well in very cold climates, but for most US/EU cities they will work fine. For very hot climates it may be better to use solar water heaters, although for commercial installations there is the option to use the exhaust heat from cooling to heat DHW (it is essentially free energy).