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Anonymous Poster #1

Solar Roofs

01/13/2012 1:24 PM

How do PV solar panels (with their batteries and inverter systems), compare with "hot water thermal" panels (with 1 pump and 1 storrage container)? PV panels can not be used to heat water for "thermal radiant in-floor or under floor heating systems. Which one is more efficient? Do you need both kinds of systems for a "solar home"?

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#1

Re: solar roofs

01/13/2012 1:31 PM

While I can't speak for thermal heating, I have compared solar hot water heaters to electric solar panels for cost and efficiency and the solar hot water is about an order of magnitude cheaper in cost.

One solar panel will give us hot water most every day of the year, but you will need many, many more PV panels to run an electric hot water heater.

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#2

Re: solar roofs

01/13/2012 1:37 PM

Generally speaking thermal is going to be more efficient, because of direct conversion...That's only good if your using it for a heat source though, for electric, that's not usually the case on a small scale...

http://www.green-buildings.com/content/781991-solar-thermal-vs-photovoltaic-which-should-i-choose

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#3

Re: Solar Roofs

01/13/2012 2:35 PM

We have solar hot water for our 5 unit condominium building. It has a fairly large tank, at least 75 gallons capacity. The sun can heat the tank up in no time flat, the rest of the sun's energy is lost when the circulating pump shuts off. I wish there was some way to use all that power for something, some stirling electric generator or something.

Ideally, the hot water heater would not use electricity, but on cloudy days or very high usage after sundown the electric kicks in. If everybody bathed during sunlight we probably would not use electricity to heat any water. Not likely, folks are usually gone during the day.

Solar electric is great, but complicated and expensive. Storage still requires batteries, and that technology still has problems.

For heating a house, I only know of using air heated in the attic with solar roof panels, and a rock heat storage room. Worked fine for a friend of mine.

Using water would require a huge tank, and careful water quality monitoring. I would want to monitor the ph and hardness of the water, and control germs and such.

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#6
In reply to #3

Re: Solar Roofs

01/13/2012 11:44 PM

I'd allow that 75 gallons is not enough capacity. For five apartments, 180 would be more effective, as well as a tempering valve to prevent scalding. You don't run a solar water heater tank at faucet temperature, but much higher, 180 degrees is easily obtained. Then you use a tempering valve to mix cold water with the very hot water. For the 180 gallon tank you'd use maybe three panels, 4x8 feet. It might give you, depending on your location, 100,000 BTU or more per day..

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#19
In reply to #6

Re: Solar Roofs

01/14/2012 9:35 PM

Just checked, the tank is 110 gallons. It has a mixing valve, I have not been using this system to it's capabilities! I don't know how to set the mixing valve, it's a Tuco (see pic). I can increase the temp from the panel with the electric control. I would like to install the thermo-wells for the thermometers on the inlet from the roof and the outlet to the condos, if that's the best place for them. Installing will be a hassle, the copper is old. (20 years at least)

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#4

Re: Solar Roofs

01/13/2012 3:05 PM

I asked a similar question some time ago. It seemed that the answers given tended to be biased toward the expertise of the respondent. Pretty soon, I realized that it really broke down into a couple of basic questions.

The first question to answer is, "How much money do you want to spend?"

The alternative first question is, "Which do you do best? Plumbing or electrical work?"

By producing electricity, you can sell the excess back to the utility company to the point at which your net usage is zero. But you will still have some residual charges just to stay connected. You can't make any money by selling back more than you use.

Batteries are expensive, and full of corrosives. They don't like temperature changes but should not be kept inside unless adequately ventilated. Super-capacitors are very attractive as an alternative.

As for water heating, you can gather heat all day and then by using heat exchangers and some form of distribution system, you can preheat cold water going into the water heater, or radiate heat into a cool air return, or any number of ideas where you need heat. It really just amounts to what you are comfortable maintaining over time. Water leaks can be as much of a pain as good electrical connections.

That, of course, begs the last question which is, "You will be doing some of the work required to maintain the system, won't you?"

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#5

Re: Solar Roofs

01/13/2012 7:03 PM

Yes

Solar thermal is about as efficient as one can get for direct low grade heating.

Hot water is by far the biggest energy cost a household can readily and economically covert to 'alternate energy'. Given a suitable aspect, the ROI* is the highest of any and all 'water heater' technologies - (bar having a thermal spring in your yard).

Surplus PV output can be diverted to electric water heaters equipped with a low voltage element. You would not do this via an inverter, as it's highly intermittent and would roughly double the size of inverter required - which would be very poor ROI.

Air-conditioning is the next highest power consumer. This "need" should be 'designed out' of a "solar home".

Cooking is next greatest and may be designed in via a methane digester

This leaves PV for electrical gadgets.

Though PV capture is falling in price/W; storage cost and service life remain poor ROI - (short of having a reasonable scale hydro re-pumping facility to hook into).

Overall; one technology will best suit any given application. It's rather silly/inefficient, to try and do everything with one energy form. I.e. the "all electric house" is roughly as smart as the "gas powered TV"

*ROI = return on investment

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Anonymous Poster #1
#22
In reply to #5

Re: Solar Roofs

01/29/2012 1:44 PM

It seems to me that "solar roofs" and "subdivisions" do not mix. The homes are built within the lot lines and don't pay much attention to the "solar south" side of the house, and rarely do you see a good solar green house. (vertical double hung windows with screens, and an "overhanging roof"). The designers/owners should get a tax credit for a solar re-design of the south side of the house, and it should/could be the most important part of the design. Once installed these types of homes really do put a smile on your face, and are very energy efficient.

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#7

Re: Solar Roofs

01/14/2012 4:39 AM

You can make and install your own hot water panels very cheaply.
Here and here .
You can't really compare the efficience very conveniently. Solar hot water is V efficient for what it does but 0% efficient for powering lighting and a PC.
Both would be desirable for a 'solar home'.
The financial return on solar PV is pretty long unless you get government subsidies or you are forced to be off grid.
If you really need electricity then solar and wind is good, but a waterwheel is prob best if you happen to have a convenient head of water to hand.
Del

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#8

Re: Solar Roofs

01/14/2012 9:55 AM

Production efficiency is not relevant in this context. If you cannot replace purchased energy with solar, (either thermal or PV), it is not useful, nor efficient.

My point is, while thermal collectors undoubtedbly convert insolation to BTU's more efficiently than PV does to KW's, (3147 BTU's = 1 KWH), heat is much less useful, except at baseline year around levels. (Domestic Hot Water) Space heat from thermal may appeal to people based on it's efficient production curve, but it's seasonal and hourly demand fluctuations render it difficult to consume. If perhaps you could offset a pool heater that you were paying for propane or electricity to heat it, it could improve the profile of production versus consumption, but even then, you must recognize that a structure must still have the ability to provide heating when solar gain is insufficient. A massive storage complex can overcome some of these "inefficiencies," but they are seldom any where close to cost effective.

A net metered PV system is bar far the least maintenance, highest efficiency system out there. !00% of the energy produced is consumed. That makes the system 100% efficient. There are virtually no moving parts, and virtually no maintenance requirements. Alternative energy schemes (wind, geothermal, solar thermal, biogas, ethanol, biodiesel, wave, hydro cannot make that claim.)

Finally, transmission convenience and efficiency is the answer. Like Great Britain in the 18th century commanded the worlds shipping lanes, and projected it's power to influence politics, that country that distributes electricity globally will dominate the energy world, which in my opinion, will be the primary geopolitical driver of the 21 century. We should invest in an ultra high voltage (efficiency) electricity transportation grid.

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#9
In reply to #8

Re: Solar Roofs

01/14/2012 10:39 AM

For home residential use paid by the homeowner, solar hot water beats PV electrical hands down when you consider upfront cost, operational cost, and return on investment.

In the case of the original poster, he is trying to generate heat, and solar hot water wins over PV by a wide margin.

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#10
In reply to #9

Re: Solar Roofs

01/14/2012 2:05 PM

it seems to me then, that if you use the proper distribution piping and pump system to harvest all of the energy gathered up (efficiently) in the storage system, then you can get the most "BANG FOR YOUR BUCK". Using three 4x8 flat plate thermal collectors, 1 pump, 1 heat exchanger, and 1 500 gal "open" storage container, and a radiant in-floor system, or under floor, the temperature can be maintained and distributed at a good rate for most of the heating year,on average (especially here in the S.W.) for the cost of 2 pumps and electricity for the year (not much). Excess temp. in summer, can be stored in a hot tub or pool, or just "dumped". Maintenance and cleaning not much. This is basically "free" hot water all year long, and if you incorporate the solar roof into a south side green house it makes it a whole lot simpler. (storage in greenhouse), and the "growing" of the south side of your home, adds so much. The costs for the collectors and storage tank, the pumps and piping and elect, are approx. ($5,000-$7,000), for materials. Labor costs are pretty straight forward if done over a greenhouse. and once functioning, tends to pay for itself very quickly. The costs of a good solar home with mechanical, should be paid for (80% up front and 20% (and much less) from then on to maintain it, instead of 20% to start then 80% to run in today's building market) If you build the efficiency (passivness) of the house and it's mechanical system, right into the design, and build it right away, it is the best that it can be. (you can use it all during construction). The back up systems can be whatever you want as long as you pay for them "after" the SIMPLE SOLAR SYSTEMS, and greenhouse, are up and running. You can really "Bust" your budget on "back-up" systems and most of the time, hi efficiently heating equipment, are not user friendly.

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#12
In reply to #10

Re: Solar Roofs

01/14/2012 4:39 PM

I agree in general but do find your equipment estimates a bit thin.

The underfloor radiant system if it is to be responsive to things like solar gain from windows, loss on north sides etc; not only needs to be sized correctly, but also needs to be broken into useful zones for control.

Now there are a lot of inexpensive options in here, but not quite free. So thermostatic control layers, valves and switching, some kind of backup for the 100 year day or power outage.

Many folks run these systems on small PV panels since the draw is low, but then you risk night time problems during power outage if it is the only source.

So we are tying multiple electrical (supply, signal) systems in here too.

Not trying to discourage you because it is VERY doable, but gotta be realistic too.

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#11
In reply to #9

Re: Solar Roofs

01/14/2012 3:26 PM

That is a blanket statement that is far too simple. It depends on many factors, including hot water demand, lifestyle, alternate energy choices and pricing, size of system, intended uses of hot water, etc. You seem intent on providing a simple answer to a complex question. If you are talking about domestic hot water only for a large consumer, with relatively high priced alternate fuel costs, you may be right. But people get obsessed with the concept, and start making space heat, or putting systems in that provide more than they can consume, and wonder why feasibility is so dismal.

Is a diesel motor better than gasoline?

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#13
In reply to #11

Re: Solar Roofs

01/14/2012 4:40 PM

I coulda shut up if I'd read through your post

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#14
In reply to #11

Re: Solar Roofs

01/14/2012 4:50 PM

For heating water if the choice is between photovoltaic and solar hot water panels, there is no competition for personal residential home owners.

On a dollar per dollar cost you get at least 4 times and as much as 10 times more hot water out of a thermal solar panel than a PV panel. Even if the panels cost the same, the PV still needs additional expensive components to make it work.

I believe it is simpler than you are trying to make it out.

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#15
In reply to #14

Re: Solar Roofs

01/14/2012 6:04 PM

Don't think there was a dispute about efficiency at this point, but I concur with PFR that while thermal solar is clearly more efficient, the APs post lent itself to misinterpretation as "One size fits all" or underestimating the cost and complexity of a system like radiant subfloor heating.

If all ya want to do is heat some water - no question.

If you want to heat your house with that water, either pay for good engineering or mistakes.

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#20
In reply to #15

Re: Solar Roofs

01/15/2012 1:14 PM

you can pay for good engineering, but you must design and pay for a good HIGH EFFICIENCY HI MASS PRIMARY HEATING SYSTEM. for you and your family. There are some really good user friendly systems out there that are PROVEN, but they do not include "hi-tec" gas boilers" and tankless burners because (especially at high altitude), they are just to much trouble, not to mention pocket busting. A well designed primary loop heating system will do around 75% of your needs,(depending on size), for low monthly bills. The other 25% comes from a small back up system, and some times you must use it. (whatever the cost).

A simple residential heating system is the key to "efficiency". Let the 1 or 2 t-stats run the pumps and let an aquastat run the tank(s) temp. You don't want to turn on a flame every time the air temperature fluctuates a bit. (wastes energy, and you could put a coat on) All of the gas flames in the home MUST be adjusted to the altitude of house. For example, a flame on a boiler at 7,000 feet, is 27% less efficient. (unless your plumber de-rated the burner and adjusted the pressure).

"one size (does) fit all" smaller, well designed, user friendly homes.

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#16
In reply to #14

Re: Solar Roofs

01/14/2012 7:02 PM

The question of whether it makes sense to heat water with thermal panels versus PV is almost a nonstarter. The question of deploying capital to obtain the best ROI with solar systems will very rarely point to thermal systems for space heat. The OP is implying that Solar thermal for DHW and Space heat and PV are choices A and B, and that if he asks which is better, someone like you will tell him thermal is better.

I engineer and install systems, both large commercial and residential, thermal and PV. It isn't clear or simple, because there are many variables. If you have consistent demand, solar domestic hot water is almost always a good choice. Solar space heat rarely is, looking at ROI, but is common. With Heat Pump hot water heaters, often a good argument can be made for a net metered PV system, especially in high cost electricity areas, for powering a radiant floor. They don't work well for forced air heat x coils. Robbing the DHW Peter to pay the Space heat Paul is simply building a fragile system that doesn't perform in season with demand. Upsizing solar thermal for space heat only worsens your utilization of total energy capture, which is directly related to equipment quantity and system complexity. (Cost) That's why at the beginning, I said that efficiency is not really relevant to this specific question.

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#17
In reply to #16

Re: Solar Roofs

01/14/2012 7:54 PM

"The OP is implying that Solar thermal for DHW and Space heat and PV are choices A and B"

The OP actually states PV is NOT suitable for space and radiant floor heating.

Further, there is no indication of a grid tie being available.

And I'm not sure why you feel laying pipes in a slab is much more complex than elements.

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#18
In reply to #16

Re: Solar Roofs

01/14/2012 8:02 PM

After a winter where I spent $4.50 for propane and $250 a cord for fire wood I was willing to spend about whatever it took to not repeat.

I ran solar thermal every which way I could for the obvious benefits. I bought plumbing books and learned to size systems; I got the latest info on solar tubes and read up on storage methods and finally ended up with this:

You put a 25Kw unit in the backyard and sell electricity to your utility.

Then you buy all the propane you want

But they wouldn't sell me one

Anything else I was going to build was going to cost about as much and I'd have to engineer so much I could never sell the house.

If you want you can do what Del (the Cat) did, a little solar air heating during the day, a solar 'booster' to unload your water heating bill.

Almost anything else and you are building a new house because the infrastructure need to be built in - and then you will probably need to remain connected anyway.

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#21
In reply to #16

Re: Solar Roofs

01/15/2012 3:46 PM

I will always say that thermal is better only if it contains enough "mass" to make up for "off" periods. A certain amount of energy is used to keep a comfort zone, but it is always there in the mass. Not to mention that the southern solar greenhouse adds an amazing "laughing place"( Lord knows we need that) to the whole house.

A southern solar greenhouse has it's own costs, but dollar for dollar, this space adds so much value to a home. (heat loss can be dealt with using good design, and excess heat can be used )

Let's face it, any heating system has a certain amount of complexity to it, but simpler systems can be maintained by the home owners. If the heating system is forced air, then the DHW is a different 130 to 140 degree system, and the high efficiency furnaces and ducting cannot heat DHW. If the heating system is Hydronic baseboard heaters, then you need 180-190 degree water flowing through these baseboards. Since the actual amount of hot water needed for DHW is fairly low and can be conserved, adding a complex solar system on the roof, just to do that, can get pretty pricey. A good in-floor heating system has only 115-120 degree water flowing through the tubes.

A good solar roof on the south side, adds a "future" to the home.

A good solar design is worth it's weight in "smiles", and education.

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