Hmm. Maybe a diagram would be clearer? Also did you come across the Ecorenovator site, there are a few experienced heat pump hackers there...

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.

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.

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/

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.

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.

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

In Canada for space heating (say 30c into UFH) how does PV with a heap pump compare to solar thermal?

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.