Author Topic: Augmenting heat with PV solar.  (Read 1826 times)

BDWW

  • Pencil Stache
  • ****
  • Posts: 584
  • Location: MT
Augmenting heat with PV solar.
« on: January 31, 2020, 01:05:18 PM »
Heating is our biggest source of utility expenditure. We already turn the heat down during the day and night via programmable thermostats.

Rooftop grid-tie definitely doesn't paper out if I pay for installation. Our utility is very anti-pv, and so there's other issues that make it very marginal even if I had the time to DIY it.

Optimal solution would be solar hot water system. Either with PV resistance heating elements or a collector system. The hangup is the system work in the house needed to make use of it.

So I'm wondering if anyone has or has thought about making a simple switched PV auxiliary heater?

My idea is to have some solar panels wired directly into a resistance heating element (electric baseboard with the control module removed). My assumptions are 3 panels would generate generate ~3000w/hr or roughly 10000 btus of heating on a good day.
I'm still working on the math for payback, but the main upside is the simplicity. It would be fairly simple to install requiring only a minimum amount of wiring compared to a solar water heat system.  It would simply displace some traditional heating cost during the day, and could be disabled in the summer by a simple switch or thermostat.



BobTheBuilder

  • Stubble
  • **
  • Posts: 204
Re: Augmenting heat with PV solar.
« Reply #1 on: February 01, 2020, 03:51:54 AM »
What's your primary source of heating? Is it natural gas? What's the cost per kwh / BTU?

BDWW

  • Pencil Stache
  • ****
  • Posts: 584
  • Location: MT
Re: Augmenting heat with PV solar.
« Reply #2 on: February 01, 2020, 11:55:03 AM »
The napkin math (it's ugly):

Current cost per therm: .6693c
Current cost per kwh: .1301c

4 panels at 315w = 1.25kw ~= $1000
110v 1500w electric baseboard* ~= $80
Wiring,thermostat,misc ~= $120
Approx system cost $1200

Approx avg winter monthly production at my latitude 115kwh
115kwh * 3412 = 394,000 btus ~= 4 therms

Offset 4 therms per month * 6 months of the year =
.67*4*6 = $16.08 gas saved per year
1200/16.08 ~=  75 year payback.

If I substract Summer KWH - Winter KWH and estimate 75% is furnance/heating costs, I get
2.3 Kwh usage per therm usage.
2.3*4*6 *.1301 = $7.18 electric saved per year

$1200/(16.08+7.18) = 51 year payback.

*I have some old 240v baseboards, but they would quarter the output and wouldn't save that much (1100/23.26 ~= 47 year payback).

So much for that idea...



Ripple4

  • 5 O'Clock Shadow
  • *
  • Posts: 15
Re: Augmenting heat with PV solar.
« Reply #3 on: February 01, 2020, 06:31:35 PM »
This guy found out a way to run a mini-split heat pump on ~1kw solar power. Heat pumps are great because you get 3-4 times the heating per kilowatt as resistance heat. the drawback is that in cold Montana winters the evaporator is trying to extract heat from very cold air and it will just fail to work at subzero temps. itís hard to find one that will even work into -10F temps. In the efficiency vs capability tug-of-war, below -10F, capability is all that is important. Anyway, the trick this guy has is using the all of the suns energy with his novel air ducting system. How he has it setup is the PV panels convert 20% of the energy into electrical power to run the motors, but he preheats the evaporator air with the 80% of the PV waste heat by drawing air in behind the panels. Itís close to genius.

https://renewablesystemstechnology.com/solar-heat-pump-system.html

BobTheBuilder

  • Stubble
  • **
  • Posts: 204
Re: Augmenting heat with PV solar.
« Reply #4 on: February 04, 2020, 11:21:54 AM »
Oh that is impressive! Will tell my parents about that too. A bit smaller though at their house :-)

lutorm

  • Stubble
  • **
  • Posts: 166
  • Location: A large island in the middle of the Pacific Ocean
Re: Augmenting heat with PV solar.
« Reply #5 on: February 09, 2020, 10:26:34 PM »
My idea is to have some solar panels wired directly into a resistance heating element (electric baseboard with the control module removed).
I don't think you can simply connect a PV panel to a resistive heater and get very good efficiency. To extract maximum power from a PV panel, you need to operate it at a specific voltage (roughly constant but depends on amount on sun and temperature). You could try to match the resistance to what the panels provide at full sun but power will quickly drop off at lower insolation levels when the panels provide less current.

Some heat pump arrangement is probably your best bet, either air source or ground source. The system mentioned that extracts heat from the PV panels is clever, although I don't understand how they run a compressor directly from the PV without a battery or some intermediate source since PV panels can't handle motor startup currents.


BudgetSlasher

  • Pencil Stache
  • ****
  • Posts: 839
Re: Augmenting heat with PV solar.
« Reply #6 on: February 12, 2020, 01:58:10 PM »
If you are looking for a heating solution that uses PV independent of the grid, do some research on off-grid DC powered heat pumps (mini-splits). You should get more BTU per watt and you would have some cooling and (maybe) dehumidification in the none heating season.

If you were to do solar you need to use the power year round, not just the heating season to have any hope of making the payback look somewhat reasonable.

zolotiyeruki

  • Magnum Stache
  • ******
  • Posts: 3550
  • Location: State: Denial
Re: Augmenting heat with PV solar.
« Reply #7 on: February 12, 2020, 03:53:12 PM »
For the kind of money you're thinking of spending, there's a good chance you'd get far better ROI by improving your insulation and air sealing.  Seal all penetrations and top plates in your attic with expanding foam, add another foot of blown-in insulation, and rent a thermal camera to chase down all the leaky and/or uninsulated spots in your youse.

Katabatic

  • 5 O'Clock Shadow
  • *
  • Posts: 1
Re: Augmenting heat with PV solar.
« Reply #8 on: February 17, 2020, 08:08:15 AM »
PV heating is definitely an idea whose time has come. But the problem here is it's only useful in winter, darn. I just got used solar panels for around 20 cents per watt on ebay, so you could look around and find a good deal on solar panels and update your math. Here's some for 10 cents a watt, minus shipping. https://sunelec.com/shop/solar-panels/used-70-watt-thin-film-modules/ It is true you need to match your load to your panels to get maximum power. Usually MPPT controllers are used for that, so you can get a high voltage one, or if you know what you're doing you could probably design it without one.
« Last Edit: February 17, 2020, 08:11:40 AM by Katabatic »

GodlessCommie

  • Stubble
  • **
  • Posts: 133
  • Location: Sterling, VA
Re: Augmenting heat with PV solar.
« Reply #9 on: February 17, 2020, 03:35:05 PM »
Here's a guy that built his own solar hot water/space heating setup, as well as a bunch of other similar projects:

https://builditsolar.com/

BDWW

  • Pencil Stache
  • ****
  • Posts: 584
  • Location: MT
Re: Augmenting heat with PV solar.
« Reply #10 on: February 17, 2020, 03:46:57 PM »
Here's a guy that built his own solar hot water/space heating setup, as well as a bunch of other similar projects:

https://builditsolar.com/

Yeah but he's like 5 miles from me so...