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    • CommentAuthortoddm
    • CommentTimeMar 15th 2018
     
    Hi. American here -- a lonely advocate for multifuel central heat on this side of the Atlantic. At present I have a Stratford Ecoboiler heating a 750L vented storage tank. The storage heats underfloor radiant on thermostats. I also have a 24-tube pressurized solar system heating a 300-liter dual-coil tank, and a drainback loop that ties the two tanks together.

    My next project is to integrate an air-to-water heat pump. Because the solar tank coils are undersized for heat pumps, the ATW unit will heat the large tank through a flatplate heat exchanger.

    My question: how do I connect the solar system in series with the heat pump, using preheat to gain COP, while preserving its original loop for summer use when the heat pump is idle and the main storage tank is cold? The best I can think of is to wye solar to the entry water pipe adjacent to the heat pump, using a a back flow preventer in the flatplate pipe to prevent reversal when the pump isn't running, and wye it again on the hot side with another BF preventer blocking reversalon that side of the flatplate loop. The solar pump is downstream of the heat pump, drawing coolant through it. The solar system has a heat dump radiator that kicks on at 60 degrees Celsius so it isn't likely to harm an idle heat pump. (I think.)

    I might be overthinking this (my specialty.) In a parallel system, the heat pump still backs up domestic hot water because the tanks are linked. (That link also moots the question of whether the heat pump loop in operation would interfere with the the solar loop.) At any rate, your thoughts also on whether a series system is worth the trouble?
    • CommentAuthorTimSmall
    • CommentTimeMar 16th 2018
     
    Hmm. Maybe a diagram would be clearer? Also did you come across the Ecorenovator site, there are a few experienced heat pump hackers there...
    •  
      CommentAuthorSteamyTea
    • CommentTimeMar 17th 2018
     
    Posted By: toddmusing preheat to gain COP

    I am not sure what you mean here.
    To raise the CoP of a heat pump, you have to increase the input temperature i.e. the air, or reduce the output temperature i.e. the water.
    It is the temperature difference that sets the CoP (within the bounds of the HP design).

    Or I may have misunderstood what you want to do and
    Posted By: TimSmallMaybe a diagram would be clearer?
    is what is needed.

    We are also changing out refrigerant gasses to CO2, which I believe have a higher temperature output without relying on the inbuilt resistance heater.

    But, depending on what you are trying to achieve and how much you wish to spend on changing your system, you may be better off fitting more PV and using that generation to help run the HP and some simple resistance heating for your DHW.

    What part of the USA are you in?
    • CommentAuthortoddm
    • CommentTimeMar 17th 2018
     
    Thanks for your input. I am working up a schematic. But there's little point in plumbing niceties if there isn't much advantage. There are some academic papers on solar assist in series, albeit based on domestic hot water rather than central heat. https://uwspace.uwaterloo.ca/bitstream/handle/10012/12558/Chasse_Christopher.pdf?sequence=3&isAllowed=y Schematics begin on page 24.

    While the overall Delta T doesn't change with preheat, the amount of work does. Either that or the resulting output temperature is higher for the same kwh. To my (quite possibly naive) thinking, adding solar thermal output to the heat pump's entry water, multiplies solar output by the heat pump's COP. And because the point of multi-source is to choose the optimal heat of the day, abundant insolation is one of the reasons I would flip the switch on the heat pump rather than fire the ecoboiler.

    As I think about switch flipping, a motorized three-way valve is likely a simpler means of seasonal changes in solar thermal.

    I feel compelled to explain that I am not completely mad. A Frank Lloyd Wright buff, I built a passive solar house where it doesn't belong -- in the woods in south central Pennsylvania. On the plus side, fuel falls from the sky here, and FLW was right that homes should be designed from the inside looking out.
    •  
      CommentAuthorSteamyTea
    • CommentTimeMar 17th 2018
     
    Posted By: toddmsouth central Pennsylvania. On the plus side, fuel falls from the sky here
    Yes, I lived in Midtown, PA, fuel fell from the sky there in 79, some is still lurking I think :sad:
  1.  
    Posted By: toddmmultiplies solar output by the heat pump's COP


    No!!! It does definitely NOT multiply the solar output - it just uses it at a maximum COP of 1. The COP is a measure of the output heat versus the input electrical energy - the difference coming from either the air, in an ASHP or the ground in a GSHP. It's not a multiplier, it's just a measure of what heat comes from where. If you have a certain amount of input flux from the sun, you cannot multiply this! ASHP and GSHP systems appear to do this multiplication because the input source is essentially boundless (you have as much air as you want to move over the coil, or, in a GSHP case, the stored heat in the ground is much much more than you extract in a season - so it's not a multiplication, it's just a measure of how much you move from the source).

    Paul in Montreal.
    • CommentAuthortoddm
    • CommentTimeMar 17th 2018
     
    Thanks, Paul. The advantage of the Internet is no one can see my red face.

    So.... Hacking a water-to-air heat exchanger in front of the evaporator?

    SteamyTea, Three Mile Island's owner is threatening to decommission the remaining reactor there if it can't get rate relief from the state. Thanks to fracking, Pennsylvania is now number two to Texas in natural gas production. Propane and electricity should stay cheap as long as the Marcellus shale boom lasts, but we have already established that I am not entirely rational.
    • CommentAuthorTimSmall
    • CommentTimeMar 18th 2018
     
    The solar + HP systems that I have seen are:

    1. Systems which use both, but they're configured as independent heat sources going into the same thermal store or water cylinder.

    2. Systems which use a large solar thermal collector to produce a lot of low grade heat, and store this in water, and then use a heat pump (via a water to refrigerant heat exchanger) - you could hack an ASHP into this configuration by removing the fan and refridgerant-air exchanger and replacing it with a pump and fan.

    3. Systems which have an over-sized solar thermal collector and dump excess summer solar thermal into a vertical GSHP ground loop, to use as a seasonal store. This can work OK, as long as you don't have a lot of ground water movement (you'll need to find a local geotechnical engineer to confirm this).

    4. Systems which use PV and heat pumps, and intelligent controls to try and make the HP mostly use the PV-sourced electricity.

    The energy transition show has some interesting coverage of the shale boom, and pressures on nuclear and coal generation from shale gas in the US. https://xenetwork.org/ets/
    •  
      CommentAuthordjh
    • CommentTimeMar 18th 2018
     
    Posted By: TimSmall3. Systems which have an over-sized solar thermal collector and dump excess summer solar thermal into a vertical GSHP ground loop, to use as a seasonal store. This can work OK, as long as you don't have a lot of ground water movement (you'll need to find a local geotechnical engineer to confirm this).

    This is the only one that makes sense to me, in terms of using solar thermal (or PV for that matter) in conjunction with space heating. At times when you want space heating there is no power in the sun. That's why you need the space heating!
    • CommentAuthorTimSmall
    • CommentTimeMar 19th 2018
     
    Re:

    4. Systems which use PV and heat pumps, and intelligent controls to try and make the HP mostly use the PV-sourced electricity.

    I think this makes sense too and is a better idea than the "traditional" "dump-to-immersion" waste-of-energy solutions which are incentivised by the way that the UK solar subsidies work.
    • CommentAuthorringi
    • CommentTimeApr 2nd 2018
     
    Posted By: TimSmall
    3. Systems which have an over-sized solar thermal collector and dump excess summer solar thermal into a vertical GSHP ground loop, to use as a seasonal store. This can work OK, as long as you don't have a lot of ground water movement (you'll need to find a local geotechnical engineer to confirm this).


    Due to heat loses, this works better on a large scale where you have many vertical GSHP ground loop near to each other.
  2.  
    See http://dlsc.ca/ - this is an array of boreholes used to store solar energy for an entire community

    DLSC Highlights

    The largest subdivision of R-2000 single family homes in Canada, each 30% more efficient than conventionally built homes.
    A first in the world, with over 90% of residential space heating needs being met by solar thermal energy.
    A reduction of approximately 5 tonnes of greenhouse gas (GHG) emissions per home per year.
    Won the 2011 World Energy Globe Award for Sustainability.


    Project Status

    10th year of reliable operation with no unscheduled interruptions in heating delivery operations;
    100% solar fraction in the 2015-2016 heating season, meaning all the heat required by the houses for space heating was supplied by solar energy;
    Consistent solar fractions above 90% over the last 5 years, with an average of 96% for the period 2012-2016;
    High solar fraction of 92% even during the very cold winter of 2013-2014;
    Continuous monitoring and improvements during the last 10 years, leading to system over performing solar fraction design estimates;
    Very low electricity usage, with coefficient of performance above 30. This means that for every kWh of electricity, the system delivers more than 30 kWh of heat;
    Throughout the years, the electricity used for pumps was reduced by energy efficiency measures and on-site solar PV generation was increased with the addition of 18 kW in 2011. The operation is now net-zero electricity for pumping power, which includes pumping through the solar collectors, district heating loop and borehole thermal energy storage (BTES);
    Strong educational component with frequent site visits by professionals and students. The technology is currently being considered by several communities in Canada as well as internationally by China, United States and several European countries.

    Paul in Montreal.
    •  
      CommentAuthorSteamyTea
    • CommentTimeApr 3rd 2018
     
    A quick look at the Drake's Landing installation shows that it has 800 ST modules. If laid flat these would take up 2,300 m^2, they are at 45° so will take a bit less.
    If that was a PV installation it would be a little bit over 4 MW.
    Has anyone does a comparison with a 4 MW PV installation at a similar latitude and climate regime?

    I am not sure if this sort of thing would work well in the UK as we have a lot more cloud cover (I think, or it could just be where I am, the best yielding PV area of the country).
  3.  
    Don't forget that the installation is now 10 years old which means the concept and planning is perhaps 5 years older than that. At that time PV was not what it is today so at the time ST was probably 'the choice'. Today I would expect a similar construction to be totally PV
  4.  
    <blockquote><cite>Posted By: Peter_in_Hungary</cite>Don't forget that the installation is now 10 years old which means the concept and planning is perhaps 5 years older than that. At that time PV was not what it is today so at the time ST was probably 'the choice'. Today I would expect a similar construction to be totally PV</blockquote>

    Given the Alberta climate, heating completely dominates energy requirements and Okotoks is one of the sunniest locales in Canada, despite the location:

    Location: Okotoks, Alberta. 51.1 deg N, 114 deg W, 1084 m elevation
    Weather: Winter -33 C; Summer 28.3 C DB/15.6 C WB
    https://www.worldweatheronline.com/lang/en-ca/okotoks-weather-averages/alberta/ca.aspx

    Paul in Montreal.
    • CommentAuthorringi
    • CommentTimeApr 13th 2018
     
    In Canada for space heating (say 30c into UFH) how does PV with a heap pump compare to solar thermal?
  5.  
    Posted By: ringiIn Canada for space heating (say 30c into UFH) how does PV with a heap pump compare to solar thermal?


    Where in Canada? The range of climate (and insolation) here is huge, as is the local price of electricity.

    Paul in Montreal.
    •  
      CommentAuthorSteamyTea
    • CommentTime7 days ago edited
     
    Posted By: Paul in MontrealWhere in Canada?
    I was in Halifax, NS end of last year. I thought of 'popping' down to Montreal, it was a 770 mile drive, and I would have had to go though New Brunswick.
    Flights and car rental are high compared to the UK, as are mobile phone charges (not that they work much anyway, long story). It was cheaper to fly from London to Toronto than Halifax to Toronto. Cheap coffee though.
    And it snows, which cripples PV/ST performance.
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