# Solar Heating Study



## begreen (Aug 31, 2015)

This will be a helpful study. Thanks to Alliance for Green Heat for the link.
http://phys.org/news/2015-08-sun.html


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## woodgeek (Sep 1, 2015)

Ugh, obsolete, expensive tech still under development?  Huge $$ thermal stores to achieve 60% heating needs in Germany's mild climate?

In the US, one would go for grid tie PV, mini-split HPs, and 'pretty good house' or better insulation to achieve 'net-zero' total energy....electricity, heat and AC.

Oh yeah, and the net-zero place will look like a 'conventional US house' rather than a Mars colony, and cost less than a German passivhaus!


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## Circus (Sep 3, 2015)

There's nothing wrong with thermal solar. It just doesn't lend itself well to a plug and play unitized approach. There's no dictatorial authority and it's perfect for DIY. SHWater & SHAir for 7 years.


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## EatenByLimestone (Sep 3, 2015)

If I was sure I could keep the panels free of snow I could see myself interested in solar panels heating a tub of water that my hydronic baseboards are plumbed to.  It would pay back faster than grid tying or batteries for me.  In the summer the panels could heat regular water for showers.


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## woodgeek (Sep 4, 2015)

EatenByLimestone said:


> If I was sure I could keep the panels free of snow I could see myself interested in solar panels heating a tub of water that my hydronic baseboards are plumbed to.  It would pay back faster than grid tying or batteries for me.  In the summer the panels could heat regular water for showers.



Until you computed the cost per BTU in the winter. And in the summer a HPWH would be a darned cheap alternative.


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## begreen (Sep 4, 2015)

Not every area or every homeowenr wants a high-tech solution, even if it is the most efficient. Technology wears out, often quickly as compared to the life of the house. Building design that incorporates good low tech solutions is laudable as long as it works well. That appears to be the focus of the research.


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## EatenByLimestone (Sep 5, 2015)

The best low tech approach that I've seen is insulation.  It isn't sexy or fun to talk about until the (lack of) heating bills come in.


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## woodgeek (Sep 5, 2015)

I would offer that the active systems and high-performance windows described in the article are not low-tech, and have an unknown lifetime.

I guess what bugs me is that this stuff was tried extensively in the 70s and 80s, and never worked that well....in the sense that it COULD work, it required continuous owner attention and tolerance for discomfort, and was inefficient, rendering the collectors really large, expensive and a PITA.  At the time, it was the only game in town, PV was very dear.

And then a much better, more versatile and cheaper solution comes along, powered by cheap PV, and now we are looking to invent reasons to keep doing things the old way?

Maybe those solar collectors are antiques.....I could be an antique collector collector!  It will be a nice addition to my collection of 21st century buggy-whips.


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## Circus (Sep 6, 2015)

woodgeek said:


> required continuous owner attention and tolerance for discomfort, and was inefficient, rendering the collectors really large, expensive and a PITA.



Couldn't be more wrong. I give it about as much attention as my windows. Once installed it quietly cut my heating utilities in half.
The solar water was a bit complex, worth it considering it's used year round. The solar air was a breeze, collecter materials costing $3 per ft2. Think of it as a very large window with no heat loss at night.
Obviously home insulation and infiltration is your first consideration.


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## woodgeek (Sep 7, 2015)

Ok, I retrofitted my heating BTU needs down 50% quietly without an active solar thermal system. And then cut my bill in half again by switching to a HP. And then C to zero by switching to utility scale PA wind.  After all that, I can't make solar thermal BTUs pencil out, either pro systems ($$$) or DIY systems (low performance averaged over the season, and durability concerns), or argue for it using comfort, convenience or eco reasoning either.

With current building science and energy costs, its simply not the 1970s anymore, at least for new construction.  With superinsulation and minisplits one can drive your heating (and cooling) bills way down (e.g. less than the cost of internet), and if one wants to be sustainable or low carbon, throw some cheap PV into the mix.  Voila, a net-zero home exceeding the wildest dreams of those pioneers in the 70s.


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## Circus (Sep 7, 2015)

woodgeek said:


> With current building science and energy costs, its simply not the 1970s anymore, at least for new construction. With superinsulation and minisplits one can drive your heating (and cooling) bills way down (e.g. less than the cost of internet), and if one wants to be sustainable or low carbon, throw some cheap PV into the mix. Voila, a net-zero home exceeding the wildest dreams of those pioneers in the 70s.




I have no argument with your suggestions. When I investigated thermal solar I found nothing but exploitation so I focused on materials for green houses. There's a wealth of cheap, durable, cutting edge materials that simply weren't available 40 years ago.



woodgeek said:


> "quietly without an active solar thermal system


My collectors need no pumps, fans, controls, electricity, monitoring or attention.
Do need to top up the propylene glycol a couple times a year because of my climate and a design oversight (needs a radiator cap). If I fail to maintain it, nothing bad can happen, just less free hot water til it's topped off. 130ish water/antifreeze doesn't boil but does slowly evaporate without the low pressure a radiator cap provides.


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## woodgeek (Sep 7, 2015)

How many BTUs per year for HW and space heat?


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## Circus (Sep 8, 2015)

woodgeek said:


> How many BTUs per year for HW and space heat?



Your request is unreasonable, at least unreasonable to think I would have the means to measure it. Data so far is pocketbook and a clock wired into the furnace. There are realistic tables available to predict  BTUs/sq ft for specific factors.

Don't get me wrong, solar air has wild temperature swings. I solved it by installing the ducts from the larger collector into a downstairs bedroom with 2 liter water bottles lining a wall. The room isn't used @ 3pm and cools down enough by midnight.
Negligible heat in the summer.


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## woodgeek (Sep 8, 2015)

Ok.  I understand measurement is hard.  Looking just at heating bills is tricky...you didn't do any other airsealing or insulation projects while you were installing DIY solar units?

Again, if you want me to be impressed re space heat, you have to tell me BTUs/yr.  When I use those tables predictively even for large collectors, I get a pretty meager BTU return.  So, look up tables for BTU/h in full sun * thermal eff (prob 40%, or likely even lower if you are relying on thermosiphons) at your mean outdoor temp during heating season, then multiply that by your solar thermal resource (likely a bit lower than the PV resource, i.e. PVWatts, due to the steep dropoff in eff in single-glazed thermal systems at lower incidence angle).


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## Circus (Sep 8, 2015)

woodgeek said:


> if you want me to be impressed re space heat, you have to tell me BTUs/yr.


 
You nailed the problem, lack of quantifiable evidence. That's what the article addresses.


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## Circus (Sep 9, 2015)

What Solar hot air really needs is a 70ishF PCTM (phase change thermal mass) material that acts like water at 32F. Thousands of gallon of water could be replaced with fifty gallons in radiator like containers.
A PCTM would also buffer the temperature swings of wood stoves and lower air conditioning by storing the cool of the night for the heat of the day.


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## woodgeek (Sep 9, 2015)

Agreed.  Passive solar and solar hot air has too big swings and enormous mass.   IF sized to cover peak winter needs (including cloudy day carryover) its hopelessly oversized for all the rest of the season, and useless in the summer.

In other words, simple solar air heaters, sun spaces and passive features are not heating systems...they are heating system helpers.  Can they provide 20% of needed seasonal BTUs...sure, almost easy.  Can they provide 50%? much harder and running into comfort issues, and needing to provide some control ventilation (which becomes an owner PITA).

In contrast a large solar active system (with water storage) can have automatic control, and provides an excess of DHW during shoulder seasons and summer.  Can such a system provide close to 100% of winter heat, sure.  At 1970s insulation and airsealing levels, it would require a monstrous amount of collection and storage, but at a Passivhaus or Pretty Good house (1 BTU/sqft*HDD or 2 BTU/sqft*HDD, respectively) level, it would be doable with a large reservoir.

The problem is that the conventional heating bill on those properties is only ~$200/year, with a fossil or mini-split heat source, and DHW can be about $200/yr also, with a HPWH and low-flow fixtures.  While the hypothetical solar thermal system offsets say 80% of this amount, $320/year, it does *nothing* for appliance loads, AC loads, plug loads, or EV loads.  A non-DIY system might look like a conventional solar DHW system roughly times 3 (6 panels, 300-500 gallons of storage).  In my area, that would cost >$20k installed, with a resulting 60 year simple payback.  It also largely misses out on the largest part of the solar resource (being 3x too large in the summer).

A similar investment in PV instead could get the same house to net-zero, covering 100% of space heat, DHW and all other electrical needs, including a lot of EV miles.  Since the sum of the electrical loads are likely $1500-2000/year, saving that amount per year (rather than the $300 portion of DHW and space heat) looks like a much more sensible investment, with a ~10-15 year simple payback.


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## Circus (Sep 9, 2015)

Winters here are sunny at <0F, cloudy at 30F. That one windy sunny <0F day would have consumed the same fuel as five 30F days.
 Here they're starting to retroactively eliminate net metering, raise fixed fees and tax PV.  


woodgeek said:


> heatig bill on those properties is only -$200/year, with a fossil or mini-split heat source, and DHW can be about $200/yr also with HPWH


$400 for heat and hot water! Have you been reading my bills?


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## begreen (Sep 9, 2015)

EatenByLimestone said:


> The best low tech approach that I've seen is insulation.  It isn't sexy or fun to talk about until the (lack of) heating bills come in.


That's correct. A lot of homes can't take advantage of solar heating due to siting, climate zone, etc.. This can be fully compensated for by a good thermal-break envelop, sealing and insulation. The home could be designed so that it is mostly heated by lighting, appliances and body heat. If some passive solar can be worked into this design the house will almost heat itself. 

My sister's house is pretty close to this and they built in in the early 80s. They use very little fuel for heating. Since they added a very well insulated attached greenhouse (R10 glass) about 10 yrs back, the house heats itself on all but the coldest and cloudiest days. If the house is already warm they just close the greenhouse door and open a vent in there. As woodgeek noted, this is a passive supplement that only works on sunny days. They have a built-in wood pizza oven that does most of the heating and a wood boiler for extended very cold spells like last winter had. The nice thing is that for 90-100% of their heating needs it is relatively simple and low tech to heat the house. Proper insulation and good sealing make a big difference for the life of the house.


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## woodgeek (Sep 9, 2015)

Circus said:


> Winters here are sunny at <0F, cloudy at 30F. That one windy sunny <0F day would have consumed the same fuel as five 30F days. Here they're starting to retroactively eliminate net metering, raise fixed fees and tax PV. $400 for heat and hot water! Have you been reading my bills?



Hmm.  If you were airsealed, that 0F day should use ~2x the heat of the 30F day (based on HDDs), not 5x.  If you are that sensitive to wind, then more than half of your heating bill can be eliminated by airsealing, no solar required.

I have two teen daughters, and 15 cent elec, and I think I spend $300/yr on hot water, total.  I could switch to 1.3 gpm heads from the current (luxurious) 2.0 gpm and get it down to $200.

A passive house of 2500 sq ft in a southern New England climate (5000 HDDs) only needs 12 million BTUs per season for heat.  A pretty good house would need double that (24 MMBTU).  Appliances and body heat provide about 8 MMBTU during the heating season, bringing the passive house down to almost nothing, 4 MMBTU, and the pretty good house down to 16 MMBTU.

Nat gas or minisplit BTUs run about $15/MMBTU, so $60/yr for a passive house, $240/yr for a pretty good house.

My 1960 retrofit needs ~55 MMBTU (net) and my crappy heat pump is more like $22/MMBTU, so my bill is $1200 per season.


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## Circus (Sep 9, 2015)

woodgeek said:


> Hmm. If you were airsealed, that 0F day should use ~2x the heat of the 30F day (based on HDDs), not 5x. If you are that sensitive to wind, then more than half of your heating bill can be eliminated by airsealing,


 
All houses are affected by wind.


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## EatenByLimestone (Sep 12, 2015)

That is true, but air sealing stops drafts of cold air into the house and convection currents out of the house.  This will lower bills substantially for a very small cash outlay.  Unfortunately it takes time to do.  Unfortunately it's even less sexy than insulation.  "Last night I stood on a step ladder with my caulk gun for 3 hours sealing every light fixture in my ceiling.  Then I started pulling the covers off the wall plugs!"  "My family was so excited!"  Probably never follows the first statement.  I can tell you that wives and Aunts appreciate the warmth though.


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## woodgeek (Sep 12, 2015)

Circus said:


> All houses are affected by wind.



Properly airsealed houses much less so.


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