radiant heat with solar panels

ecarutis

Hi everyone, I live in NYS where it does get cold in the winter. I have a radiant heat system running through the floor heated with a tankless water heater. Works just fine. I would like to reduce costs by having a tank water heater in line before the propane tankless heater. I would get some heat from the solar powered tank heater and the tankless would kick on when needed. just wondered what the economics would be. Wouldn't bother with batteries. Just direct line from the panels to the water heater. Only heat to 90 degrees or so. Could I hook directly up to the water heater. 

icarus

Resistance electric heat from solar is a very expensive option...You would be way better off using a heat pump, seasaonlly perhaps 2-4 times more efficicnt.  If you chose a air to water heat pump, it would feed your radiant at nearly ideal temps.  Even better would be a geothermal heat pump, drawing heat out of the ground.  If you are interested I can send you to a number of links abou the technology.

I am currently working with a client converting a gas boiler with cast iron radiators to heat pump.  The issue with the heat pump in that scenario is that the heat pump doesn’t like to put out heat above ~120f.

tony

icarus

To be clear...there are ~3412 btus in a kwh.  Assume you have 1000 watts of PV operating at 80% efficiency (pretty optimistic) system wide, and you had that in ideal sun for 6 hours a day, that would yield you 6kwh of power, or about  20,400 btus per day.    Question is, what are you sending through the demand?  From there you can “do the math”

Using a heat pump, assuming the same set of factors would yield you (depending on out door temp, but assuming a COP of 3.5 and and outdoor temp of ~30f... 71,000 btus.  Yes...a heat pump is going to cost more (a lot more) to install, but going forward, you could certainly do a properly unsalted grid tied system, couple it with existing hydro is loop, and reduce your total heating cost to near zero if you sized everything right.  Additionally you could then use the grid tie for “free electrcity in the non heating season, and/or use the heat pump, through an heat exchanger to cool the house as well.  What you really need is a good HVAC engineer to provide you some calcs, and then folks here can come up with solar plan to cover (or partially cover) that heating load.

As the costs of all fuels goes up even “clean” nat gas comes at a considerable environmental cost, and many places are moving away from nat gas as primary heat, as heat pumps are so much more efficient.  A study conducted in Maine (probably quite similar to your location?) figured that a heat pump was slightly cheaper to heat with net/net than any other fuel source, except wood.  Given the upward trajectory of fuel/energy prices, any solar installed now, serves to lock in that fuel/energy costs permanently.

a pretty good primer on heat pump stuff is here:

https://www.hpacmag.com/features/air-to-water-heat-pump-systems/

Later in the article you will find some specific info on low temperature radiant, some of the study is based in NYS

Good luck and keep in touch,

Tony

BB.

A gallon of gasoline contains roughly 120,000 BTU:

https://www.eia.gov/energyexplained/units-and-calculators/british-thermal-units.php

Sample Btu conversion factors

Energy source/fuel	Physical units and Btu1
Electricity	1 kilowatthour = 3,412 Btu
Natural gas	1 cubic foot = 1,036 Btu 1 therm = 100,000 Btu
Motor gasoline	1 gallon = 120,333 Btu2
Diesel fuel	1 gallon = 137,381 Btu
Heating oil	1 gallon = 138,500 Btu
Propane	1 gallon = 91,333 Btu
Wood	1 cord = 20,000,000 Btu3
1 Btu factors are for end-use consumption in 2018 from Monthly Energy Review, May 2019, excluding wood. 2 Finished motor gasoline sold at retail in the United States, including fuel ethanol content. 3 This conversion is an estimate. A cord of wood is a volume unit and does not take wood density or moisture content into account. Wood heat content varies significantly with moisture content.

-Bill

lucman2

ecarutis said:

Hi everyone, I live in NYS where it does get cold in the winter. I have a radiant heat system running through the floor heated with a tankless water heater. Works just fine. I would like to reduce costs by having a tank water heater in line before the propane tankless heater. I would get some heat from the solar powered tank heater and the tankless would kick on when needed. just wondered what the economics would be. Wouldn't bother with batteries. Just direct line from the panels to the water heater. Only heat to 90 degrees or so. Could I hook directly up to the water heater. 

Hi,
I also live in NY and I currently have Solar thermal, and PV. I can tell you from experience that installing anything solar for "just winter" use in NY is a total waste of money. The amount of sun available from December through March is negligible, the sun is too low in the sky even if we do get several days of clear skies.

NANOcontrol

I have 520W of panels mostly just for experiments. With a high efficiency heater control into resistance, I barely get a KWH of energy each day. This is supplemental heating to my HPWH..  I'd heat a metal plate aimed at me before I'd dump heat into concrete.

jonr

I ran some numbers and PV solar (say $1/W) direct to an electric  water heater (for year round use) has a pretty good ROI, even compared to some fossil fuels.  No batteries, no inverter, no net metering, no heat pump.

wellbuilt

I think you would need to use a evacuated tube system with a solar water tank to get any heat going at all .
i don’t think it would do much in the depths of winter , but it should work real good in the spring and fall 
 I don’t get much sun from thanks giving to the end of January things  are better in February but I need heat 10 months out of the year .
 This past year I was burning wood at night at the end of June and started again the last week of August
 I have 1400 ‘ of 1/2” tube in a 4” thick concrete slabs , the house stays at 67o all summer long . 

  

wellbuilt

I am in Ny 70 miles west of Kingston