electric heating

[normthehandyman]

as the cost of monocrystaline solar cells is so cheap on e-bay $1au/watt what do people think about using them to generate electricity to heat a mass(probably a pile of bricks fully insulated, to then produce warm air ducted heating.

doing some ballpark maths it seems to look good.

$3000 =  3Kw/hr

10hrs daylight = 30Kw/hr stored energy which should heat the house.  Because its brick I'm not limited by the max temp for water 100 C cos it boils bricks don't. also gives potential to use the electricity at a later date when flywheel energy storage becomes more of a reality. also solar cells produce useful energy all of daylight hours, any electricity can be converted to heat unlike solar heating panels.

Any thoughts welcome.

Normthehandyman

[jimovonz]

Hey Norm - First off the cost of the cells alone are only a portion of the cost of a panel. Manufacturers watt ratings are under 'ideal' conditions and are not achieveable in the real world. Even if you placed the panels on a tracker (more $$) you will not get the output you have calculated. If your aim is to heat your home you will be much better off to use dedicated heating panels which are much more efficient for this purpose. In most temperate climates you can easily achieve 3kWh/m2/day with your standard fixed heating panel. Panels here (New Zealand) can be had for around $US200/m2 new or cheaper second hand/DIY. Assuming you can get similar prices (probably better) you could get a reliable 45kWh per day for your $3000. Contrary to your statement regarding bricks, water is capable of storing more than 4 x as much heat per unit mass and for solar heating purposes, keeping under 100degC will vastly improve system efficiency (and you don't have to worry about boiling!). Obviously pannels in either case are only a portion of the overall system cost.

If you follow the directions/link in this post:

http://www.fieldlines.com/story/2004/10/6/164945/703

you can see how much solar energy is available in your specific region (as well as much more info) Solar heating panels are typically 50% efficient overall (can be as much as 80% in ideal situations - such as those refered to in manufacturers specs!) PV panels are typically 10-15% efficient (but to make any heating/PV comparisons you need to know the panel sizes)

[pyrocasto]

Jim is right, that might not be one of the best plans. First, you cost will be higher to build the panel as stated. You will also only get half or less of the power that you stated, because even though you can see for 10hours a day, the panels arent producing full power the whole time. Also, liquid would be an easier, and probably more efficient choice over air ducted brick.

I would probably go with the solar space heaters like Jim said, because it could cost you a heck of a lot less. One thing I have been thinking about, is cooling PV panels with either liquid or air, and using that to help heat stuff. That would help you out in both worlds, in making heat, and making the PVs even more efficiant.

Someone tell me if that's a bad plan or not.

[electrondady1]

twenty years ago ,after the first energy crisis i saw a film showing an experimental house . it was like half a greenhouse with heavy insulation on the north side . in the basement were tons of rocks. there was a single  large dia.  duct that sucked hot air all day fron the peak . and a fan was used to blow it through these rocks. at nite this same fan was used in reverse. warm air from the warm rocks was redirected into the lower living area.  the "thermal mass" idea works but your using highgrade energy (electricity) and turning it into low grade energy(warm air at room temerature) you need two seperate systems. low grade  for low grade jobs , high grade for high higrade jobs.

[picmacmillan]

i know of a guy up near barrie ontario, who uses rocks similar to electrondaddy's post...the rocks are under his house...under all that is some building blocks...you know the type with the holes in them...we call them foundation blocks here...anyhow, he lined all the holes up, and his piping runs to the house from these heated blocks and the rocks on them..he said when it gets warm it stays warm for three days or more...i can't remember how he heats the rock, but maybe external wood stove?..pickster

[JeroenH]

It think heating stuff with PV is very inefficient, both in terms of $$$ and space. For a given amount of heat it will be more expensive and take up more space on your roof.

Use PV for electricity and thermal solar (either concentrated or flat-plate) for heat. And of course build/adapt your house for maximum winter passive solar gain.

[Norm]

Hi Norm,

   Always use the most direct route to accomp-

lish the end result...in your case a couple of

homebrew solar heating panels...plenty of good

ideas posted here...a difference between a big

sliding glass door letting in sunlight and the

same glass door used to make a solar heating panel.

    If your dwelling is insulated good enough,

plus solar heating panels, will cut down on the

heating bill.

  Wasting energy and smoking are both bad habits,

 we can do without both!

                ( :>),(the other), Norm.

   

[ghurd]

Well, I can do without the wasting energy...

G-

[normthehandyman]

thanks for the advice guys however my plan was slightly more warped.

Water has 10 times the storage capacity of rock but can probably only be used for direct heating of air down to 50 C correct me if i'm going wrong. rock can be heated electrically to at least 500 c and contained at that temp fairly "easily".  So we have 50c/kg for water and 450c/kg rock but rock is 3 times the density. this means the heat reservoir is a 1/3 of the volume ish. also i now have a reservoir of high temp heat which could be used in a cooling system as the hot side as well.  plus enough generating power to start to worry Energy Australia!!!!! hopefully.

[troy]

One problem is that you will have great difficulty heating the rock mass to the temperatures you envision.  Storage losses would be very high (nice toasty basement or crawlspace?).  Some have also had problems with dust, molds, etc that polute the rock bed and pretty much can't be removed.

Do a little exercise and figure out how many BTU's it takes to boil 1,000 pounds of water (which doesn't take up that much space.  8.3#/gal.  Your solar array would take a very long time to boil that water...

Good luck and have fun!

troy

[wdyasq]

I'm not sure how well solar cells burn.  And, they will be expensive even if you can buy them by the cord.

Ron

[Norm]

  ( :>) Norm.

[Aelric]

saw an interesting thing while browsing one day called "Earth tubes" uses the thermal mass of the ground around your house to heat or cool your home.  Not sure how efficient it is but might be worth looking into.  As I understand it, this is simply taking a 12" pipe running it underground for a length and blowing air thru it, the heat or cool from the ground (depending on if its north side shaded or south side sunny) Anyway just a thought

[richhagen]

Add me to the list of those who think that using solar electricity to heat the rocks would be inefficient.  I use heating elements (resistors) as dump loads, but the primary use of the panels is to charge batteries.  The cost per BTU (or joule or other unit you use to measure the heat energy) is most likely significantly more for event the cheapest solar electric panel than it would be for a solar water heating panel able to deliver the same thermal energy.  Also, I would lean towards a water (or water ethylene glycol mixture) tank to store the heat for the following reasons.  I can pipe the warm water to other locations and run it through a radiator to distribute heat, The water pipes being smaller and generally easier for me to run than the heated air.  Water has a large heat capacity and can store a large amount of thermal energy in a relatively compact area.  

With a 8000Kg tank (roughly 900 gallons, 0r 120 cubic feet), it would take approximately 16000 BTU of energy to change the temperature by about a degree celsius.  This means that if you had the water/glycol mixture heated to 90 degrees celsius and could get acceptable heat transfer through your radiators down to 50 degrees celsius, you would have roughly 640000 BTU of thermal energy minus any losses to use.  Which if my estimations are correct would be more than enough to heat a 2500 square foot home in Chicago where I am at for a day, although I would research much more thoroughly and over size a system like that if I were building one.  Assuming you have space, you could adjust the size of your tank to fit your needs.   Also, if you were using solar water heating panels, they would likely be designed to heat the water to a lower temperature which would of course require a still larger tank.  

While I like the idea of being able to heat your home with solar or wind, the amount of panels or size of mill(s) required makes the idea impractical for me.  Rich Hagen

[jimovonz]

If storage volume is an issue you can look at utilizing phase change materials to increase your energy density. Consider paraffin which is available with a variety of melting points. You can store approx 60kWh/m3 buy melting parafin.

[normthehandyman]

 I must have missed something. According to the info on the E-bay cells on http://www.internetfred.com/ under solar construction the 3" * 6" cells produce 0.5v at 3 amps ie 1.5 watts per cell. lets call a m^2 40" * 40" =1600 sq inches

divide by solar cell area gives 88 cells.  multiply by 1.5 gives 132 watts/m^2.

according to nasa (thanks for the link jimovonz) i should get 3kw/m^2 ish  at my location (lets keep the numbers ball park.)  these monocrystaline cells are supposed to be 25% ish (26)efficient so I should be able to generate 1/4 of 3Kw ie 750Watts per m^2.  Where does the difference come from?

I checked my bills for last winter and my usage came out at 27Kwh/day this was obviously for heating and lighting and I was at home for a large part of it so it's higher than I would expect. So I'll probably need at least 20Kwh/day for heating in winter That's July here.  My wife has given me tentative permission to put up 40m^2 of panels.  If I can get this by her the council should be no problem.  The basic advantage I see going electric is that I am producing energy which although it is NOT efficient it is available to me all year.

Anyone know the best price for a flywheel energy storage system say 50Kwh? Is one actually available that is affordable?

To me, the cost of the cells is the majority cost.  I realise this may not be the case for anyone else.

Thanks again to everyone who is helping me get to grips with this many flavoured can of worms.

normthehandyman

[ghurd]

There is a problem with a couple numbers.

Efficiency is listed above as 25%. The 'best' sun is 1000w/m^2. The panels are rated at 132w/m^2, that is 13.2% eff.  13-15% is more real than 25%.

So, 13.2% X 1000w X 3 hours = 396 watts for 1 hour (or 1w for 396 hours).

Same as a large light bulb for a few hours.  Not many BTUs.

With 40m^2, that is 15,840 watts for 1 hour. Still not that much.

(about 6- 1/2 of 100w light bulbs for a day)

With a electric cost of $0.20/kw-h, it would only save $3.20 a day.

A heating system only saves money in the winter. Maybe it would save $275 a year?

I can not imagine the system would ever save enough to pay for itself.

A solar heating panel is going to take up much less space, cost much less money, and work better.

A 50kwh flywheel would store about all the power a 3HP engine can make in 3 days!

I doubt there is one available at all.

If you are set on solar electric panels, use them for electricity all year instead of heat.

G-

[monte350c]

Hi Norm -

One thing to remember about the solar insolation charts (like at NASA) is that they give the ratings in kwh per day.

So if your location shows 3kwh per square meter, that's per day (not per hour). Which you can work backwards through - perhaps 6 productive hours of sunlight or so in the winter - means around .5 kwh per hour or 500 watts per hour per sqaure meter potential.

If the cells are 25% efficient then you'll end up with 125 watts per hour per sqaure meter which, coincidentally works out pretty close to the observed output of 132 watts.

Ted.

[wooferhound]

What are you going to do with the power when you don't need the heat ?

[ghurd]

Lights?

Computer?

TV?

G-

[jimovonz]

Like Gurd said, your figures still don't stack up. The only 25+% efficient cells I've heard of are hideously expensive and/or exist only in laboratories. Your own figures give ~13% efficiency which I would say are much more realistic. If your daily solar insolation is 3kWh/m2 then you are looking at ~15kWh/day from your 40m2 of cells (thats cells not pannels). This is still 33% off your own estimation of usage of 20kWh/day. Realistically you would require 55m2 of cells to meet your requirements. If low grade heat is your ownly requirement, you could probably get the same thermal output from 10-11m2 of heating panels (panels are much more efficient when the difference between the target and ambient temps is minimised. I base my calcs on a target temp of 30degC, 60% efficiency) As far as storage goes, there is a reason most folk use batteries (if the end goal is electricity) - they are simply the most cost effective. I would forget any exotic storage mechanisms - stick with the tried and true. I heat my entire 2800ft2 home using 16m2 pannels and 1500 litres (1.5m3) water for storage. The entire system is mostly made from second hand/salvaged parts and cost around $US1000. Works great. I have never even come close to considering PV cells for heating anything - its counterintuitive.

[normthehandyman]

I completely agree it is counterintuitive to use solar cells for heating.  However I'm trying to consider a whole system here not just heating.  Lets make a couple of assumptions. These are my assumptions I'm not asking anyone to agree or disagree.

1. Methods for energy storage will change.  Flywheel storage technology for electric cars will give us access to a polution free energy storage system.  The prototypes have already been built. Lets say that is in the next 10 years at a price we can all afford. A couple of 50Kwhr units gives 4 days storage and dual redundancy.
   
2. Electricity is more useful than heat. I can create heat from electricity but not easily the other way round.
   
3. I have a finite amount of space and money available. Lets call it 50m^2 and $5k au to spend on a project to save me some cash 10 years down the track.
   
   

If I go with what is being generally suggested scavange together a water storage heating system and just heat the house I spend $1000US $1300Au I now don't have to pay for heating saving 20Kwh for 3 months plus my hot water costs for the rest of the year. say 90 days by 22 Kwh and 275 by 2Kwh I save about $250/yr giving a payback period of 5 yrs not bad.  We wont look at the life of a pump or panel or servicing the system.

If I go with solar cells spend 5k on cells I save 15Kw by 90days and 7Kw by 275 days this gives about $330/yr saving giving a payback of 15 yrs This is probably much better since I am now being a bit more conservative in power estimates.

However this forgets about my massive surplus of electricity available during the summer months. Maybe run an airconditioner to chill a water mass. I have of course forgotten about battery/ flywheel costs. Not cheap!

Am I crazy or a genius?  See, even I don't know!

The main thing is it keeps me off the streets at weekends and makes the world a safer place.

[jimovonz]

Is there any reason that you can't use both systems?

[nothing to lose]

Am I crazy or a genius?

Hmmm, since you asked

I would put up solar for electric if you wanted that, then go solar for heat as a seperate unit. No matter how you figure it, your not gonna come out ahead making electric with panels and creating heat from electric you made. If you could do that the best way would probably be to make electric and store it in batteries then run electric heaters right where you want the heat anyway. I mean your gonna lose heat in many places heating bricks and storing heat anyway, then trying to move that heat would be more loses. If you need fans to move heated air, more losses.

 Though I hate the thought, I do think electric heaters would be better than  heating bricks with electric then moving the heat!

Put up a solar system to make the electric for your house. Build a large greenhouse type building beside your house, insulate 3 sides, the roof and floor really well, 4rth wall use plate glass facing directly into the sun. This heats the rocks all day and stores lots of heat. Even better would probably be black steel barrels full of water. Pipe the heat into the house, easiest to do with water. Though for many of us all the rocks we want are free like in my yard and only need moving

Now as for summer use, use the solar heat unit to keep your swimming pool nice and warm. Maybe add a couple benches to sit on in the greenhouse type room and use it for a sonna. If built right and lots of thought put into it, you could actaully use it as a greenhouse for growing plants durring spring fall and winter while also heating the house as needed.

Years ago I had a friend that cut his heating bills alot with the heat room and rocks/water barrels. He had a 2 story house, outside he heated the rocks and water barrels in a room I described and that held heat very well. Then he just ran an insulated duct from the back of the building to the second floor of the house and let the heat rise as it wanted. Since heat rises and using a large duct he kept the upstairs pretty warm this way and it also helped to keep downstairs warm since the hot air there had no reason to rise to the already warm upstairs. Before installing his system that was always a problem he had in that house. There was a cold draft comming down from the second floor as the heat from the first floor went up. Once his second floor was solar heated the cold draft stopped as there was no cold air up there. I forget where he ducted the cold air return from into the building, I think he brought it out from the basement to the bottom of the building. This provided a complete circle for the air to flow freely always warming the coldest air from the house and returning it back into the house heated. Durring summer he just removed a section of duct to stop the heat from entering the house at all.