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  • Solar power for Heat Pump Water Heaters more efficient?

    Hi Forum,
    Heat Pump water Heaters (HPWHs) are more efficient then a standard electric water heater. Some HPWHs, 75% of the heating comes from the air it uses.
    The other 25% can come from a conventional electrical heating element if needed.. It depends on the outside air temperature.
    For those not familiar with Heat pumps,...... If sucks in the warm air around it, sometimes using an external system, uses the heat it extracts from the air to heat the tank containing water. Cold air is expelled. It very similar to an air conditioning unit, but working in reverse.
    The cost of these in Europe are under 800 Euros for a 100 litre model. Such as made by Ariston, and others. They replace the standard type of boiler easily.
    With the cost of Solar power installed, now a lot lower than years past, it would make financial sense to install solar power and a HPWH.
    I believe its also possible to utilise the cold air coming out of the Hot water heat pump for use to cool an area.
    Solar power,on a percentage basis, is much more reliable than solar hot water systems.( a lot less moving parts in solar power!). The HPWH does have moving parts, but is a lot less complex than a solar water heating system.
    HPWHs do take longer to heat up when in standard mode. They all have a boost mode which uses the electrical heating element.
    With solar power installed, this will take the edge off the cost of the boost. Also when not using the boost heating element, your electric bill comes down as well. So a double benefit.
    I am not suggesting installing a solar power system to power the boost mode, just a system that recoups the kw/h used by the HPWH over the year.
    (Probably only need a 1-1.5kw system, but solar power experts would have a better idea than I, regarding size required.)

    C.R.J.

  • #2
    There is what can be done and what you can do. Grid tie is the most sensible option for most. As you said, every all in one HPWH has a heating element that could be used for solar PV. At home I have a NYLE external HPWH added on to a conventional tank. I heat water in that tank with solar PV at power point. Just 500-700W of panels used as supplemental heating gives as good a payback as a HPWH. A small PV wattage insures that 100% of the panels production is used to heat water. Heat pumps have minimal temperatures and are not very useful in storing heat over night. And in colder climates you are paying twice to heat that water. At my summer residence I am 100% PV hot water. People tend to focus on one little part of the picture and ignore the rest.

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    • #3
      The answer is not clear cut or universal. A heat pump water heater is good if a
      geo thermal source is used. Running with surrounding air, it would try to freeze
      my basement and my house heat pump would have to make up the loss. I have
      been too busy re arranging other systems to address this here. But propane and
      a generator are my backup to surviving an ice storm, etc. I am afraid if I switch
      over water heating entirely to PV supplied KWHs, my propane supplier will
      disown me for never buying propane. But house heat and electric are entirely
      net metering PV supplied. Bruce Roe

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      • #4
        Wait till the HPWH fails. They are un-repairable. Early models had lots of failures. Later models, yes they are efficient, but at what initial cost and still very complex
        Powerfab top of pole PV mount (2) | Listeroid 6/1 w/st5 gen head | XW6048 inverter/chgr | Iota 48V/15A charger | Morningstar 60A MPPT | 48V, 800A NiFe Battery (in series)| 15, Evergreen 205w "12V" PV array on pole | Midnight ePanel | Grundfos 10 SO5-9 with 3 wire Franklin Electric motor (1/2hp 240V 1ph ) on a timer for 3 hr noontime run - Runs off PV ||
        || Midnight Classic 200 | 10, Evergreen 200w in a 160VOC array ||
        || VEC1093 12V Charger | Maha C401 aa/aaa Charger | SureSine | Sunsaver MPPT 15A

        solar: http://tinyurl.com/LMR-Solar
        gen: http://tinyurl.com/LMR-Lister

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        • #5
          Natural gas is by far the cheapest, easiest, and fastest way to heat water. Very few moving parts, very efficient, excellent recovery times, works when the sun isn't shining. But not all homes / areas have natural gas available. And the CO2 emissions contribute to global warming. But my family sure loves their hot showers!

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          • #6
            HPWH look nice on paper and may actually be fit for purpose under some, but by no means all conditions.
            - They'll work better and be more cost effective in warm climates. In cold(er) climates like Bruce's, or where I came from (Buffalo) they're probably not cost effective.
            - They're a lot more complicated than just about any other method of domestic water heating. More to go wrong violates the KISS rule.
            - They're a lot more expensive up front than a tank heater using electric resistance heat. Careful analysis is called for to compare long term costs of operation, particularly in a cold(er) climate and especially when compared to using nat. gas as a heat source.
            - The recovery time for nat. gas heat is much better. That's important for large draws over extended time periods, such as for lots of, or long, showers.

            Get a plan to use less hot water and see how that reduces use. Then, using that reduced DHW load, compare cost effectiveness for various DWH methods vs. a HPWH.

            My guess is nat. gas is a no brainer if available.

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            • #7
              I was only really comparing HPWH with solar water heating panels, not with gas powered systems. I was also taking into account that the climate at the HPWH's location would be conducive to its operation. (I can't see Iceland or Greenland using them!).
              As has been said, the cold air from the HPWH will make the surrounding area cold in the winter, therefore increasing your heating bill. In a previous house I had the cold air ducted with a diversion flap. In the summer it fed into the house area, in the colder months, it fed to outside. I do live in Spain, where people assume it is hot all year round, it's not! In the central and northern areas of Spain we get minus temperatures and snow in the winter months. I concede that it's not as cold as many places within parts of the U.S, where a HPWH would not be viable of course. Reliability is always an issue. I cannot contend that the system I had was reliable in the long term, It was only installed for about 10 months before i moved house. I did see a reduction in my electric bill during that time, but again more data is needed to fully evaluate it. As yet, i haven't found a source with long term data to compare HPWH's with other systems for a climate such as mine. For now I will remain content with solar power reducing my electric bill, and continue using my standard electric water heater. (Gas not available)

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              • #8
                There are a number of studies (at least four) done by northeast utilities in the hotshot program that have lots of good engineering data.

                https://aceee.org/files/pdf/conferen...2a_johnson.pdf is just one of them. You might have to slog thru a lot to get them. There is a coffee maker by the same name. I bought one of their new HPWH left over from the program dirt cheap.

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                • #9
                  Originally posted by Mike90250 View Post
                  Wait till the HPWH fails. They are un-repairable. Early models had lots of failures. Later models, yes they are efficient, but at what initial cost and still very complex
                  ??? It's just a refrigerator running backwards. There were some poorly build HPWHs initially specifically the GeoSpring but the newer ones from Rheem seem to be doing really well. There's nothing overly complicated about them. In most of the country air-source heat pumps do just fine. They're well worth the energy savings, in terms of typical annual energy savings it's equivalent to ~2kW of PV; for ~$1500 that's a pretty good deal.

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                  • #10
                    Originally posted by PNPmacnab View Post
                    ....... Heat pumps have minimal temperatures and are not very useful in storing heat over night. And in colder climates you are paying twice to heat that water. At my summer residence I am 100% PV hot water.
                    I have heat pump water heaters and run them at 125 F with anti scalding mixer valves. I would not call 125F minimal.. They are in two different residences and they are both very well insulated and store hot water overnight with minimal temperature drop. Both locations are in California but one location sometimes gets into the 30s at night. Even then I dont pay twice to heat that water. I have not experienced any of the issues you mentioned. I don't use the heating element because it is too inefficient.

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                    • #11
                      I would call it minimal. That is barely enough to keep bacteria from growing. Turn off your HPWH at 6pm and see what you have in the morning for temp. Off grid solar people have to get up around 145 for the system to be viable.And running it at 125F is at the high end and not as efficient as you think. I have one at home. It is just not a good idea for off grid. I've done my testing and read many engineering studies on HPWH. Resistance heating for off grid is fine because PV solar systems wast so much energy anyway. People just don't know it.

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                      • #12
                        I am gridtied which explains our differing opinions. I still have hot showers in the morning after setting back the temp on the HPWH at 4PM. Even at 125 my HPWH is still.more efficient on heat pump mode than using the resistance element.. There is a financial cost to me if I waste energy.

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                        • #13
                          That is the bottom line. I have an external NYLE left over from one of those heat pump studies. A new unit at $350 shipped was a good deal. I would never be able to justify retail with low use. I have it on a 30 gallon tank and the minimum recommendation is 50 gallons. I'm low useage. If you are low use it doesn't make sense to run over 120F. Some here are off grid and without city water and low temps can be an issue. Tanks can be very stratified

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                          • #14
                            Keep the cart before the horse and keep the goals in mind: Safe, reliable hot water in sufficient quantity with as little maintenance as possible for as little long term cost of equipment and fuel as possible.

                            First, use less hot water. Low flow shower heads, etc. You don't pay to heat what you don't use. Also, equipment can be smaller for a smaller duty = probably less initial cost.

                            Second, Insulate the crap out of everything that's warmer than ambient, tanks, lines and such stuff as won't be harmed non exposure to any required cooling from ambient exposure ' = fewer standby losses which can be quite significant for storage tanks and lines.

                            Then, if you have nat. gas available, using it with a tank type heater and a lot of (added) insulation to meet the remaining and now smaller DHW load seems to hit the goals with well developed tech. that's simple, cheap, reliable and fit for purpose.

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                            • #15
                              Living with PV, with frequent clouds and overcast sky and no NG, have added large insulated storage tank inline with electric 240 VAC DHW tank. Any excess PV that could be sold to grid is able to be diverted/dumped into both tanks by 4 stages...2 independent elements for each tank. Summer time rarely needs 1 and 2, and winter time 3 and 4 might see little use...but never are opportunities missed. It is a good sight to see meter stand still during daylight hours...

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