Designing a Solar Water Heating System

You won't get enough solar power for heating...unless you live somewhere so sunny that you don't need heating.

I built my own panels from this link They work very well, my only criticism would be to hand paint the panels rather than spray paint (V expensive) I built mine mostly from scrap. 2 panels each about 1m square. I used a central heating circulating pump, and simply T'd the solar circuit into the indirect hot water circuit with a couple of check valves to stop the CH hot water going through the panels...I built my own controller which turns on the pump when the panels are about 8degrees hotter than the tank.

I moved the CH hot water thermostat higher up the tank, and turned it down a bit over the summer so that I'd still get some hot water on dull days. I've drained them down for the winter as I cant afford sufficient antifreeze for the whole CH and Solar syatems (the water is common to the two systems).

Cheap, simple, effective.

Good luck

Del

Dear Del

would you please tell me how to get the design drawings from the link you provided;

www.bigginhill.co.uk/solar.htm

I have not been able to access any further than the excellent text it provided which by itself is a very good starting point even without the figures.

thanks

P.K.D.

Ha, yes I had that problem! The drawings don't seem to be there, but the text is excellent. I built my two panels using their text as a guid and they work fine!

I did a fair amount of research on solar water heating about 30 years ago, and I found out something very interesting: When someone gets a government grant to research the coating material which has the optimum abosrption versus re-radiation characteristic, and it costs more to build solar water heating panels using such a process than could ever be saved in several lifetimes, then that person has missed the point.

I built a couple of solar panels by soldering copper pipes to sheets of ordinary galvanized steel roofing panels, spray painted them flat black, put them on a 1" thick sheet of polystyrene insulation board, enclosed them in a 2 X 4 frame and covered them with polyethylene. The two panels cost about $50 (1975). People told me, "Hell, polyethylene won't last more'n a couple of years." I told them, "It's cheap -- who cares?"

I plumbed them into my domestic hot water with valves to put them in the circuit or bypass them or to drain them.

Got well over 2 GPM (in summer on bright day) plenty hot enough (had to mix cold water several times) to take a shower (I also installed a 2 GPM shower head). I also got well over 2 GPM in the dead of winter on bright, sunny days.

Similarly, I made a model concentrating solar collector (parabolic) out of ordinary aluminum flashing, and I boiled water in less than 2 minutes. Such would have required an heliostat, one of which I designed with simple photocells and tubes diverging radially from an imaginary point. I never did anything more with the thing, but I may pursue it again when I retire.

I made a model concentrating solar collector (parabolic) out of ordinary aluminum flashing, and I boiled water in less than 2 minutes.

I've been toying with building a solar kettle...(us brits like our tea!).... Have you any pics?

Del

Have you any pics? Del

Nah . . . that was a long time ago, but I'll whip out a drawing of the thing I built and post it. One of the "cute" parts was the concentric collector tube I made which gave me the supply and return on one end. The idea I had was to use the tubes as axles for the rotation of the parabolic reflectors to follow the sun, manifold several together and supply as much hot water or steam that the system could produce. One of the other features was to balance the collector frame and reflector so that the required torque would be mainly due to rotating friction, angular acceleration not being a factor, and eccentric loads minimized.

Life got busy around that time (we were coming out of a recession, trying to make enough money to feed the kids while I started back to school "seriously," etc.), so I put those things on the back burner. They're still there . . .

More later . . .

I would be very interested to see your designs too.....

Bill, you have a ton of common sense in what you say! A REAL engineer!!

I always said I would build my own system and add enough panels to make up for the missing efficiency!!!!

By the way, do not buy antifreeze for heating systems - too expensive, buy good car antifreeze by the barrel (in the summer?), it works just fine!!!!!

Just make sure you test it from time to time for correct strength as overstrength reduces the amounts of heat transfer slightly I am told (probably OK for Government work still!!) and too little, it will freeze at night.

A good system has built in "shade" when you need to "stop" the sun and repair something also......put in plenty of stop cocks to allow isolation and removal of each individual panel when needed.

I could go on, but commercial units miss out on some important details......and are far too expensive....

Andy, thanks for the compliment: "Bill, you have a ton of common sense in what you say! A REAL engineer!!"

"I always said I would build my own system and add enough panels to make up for the missing efficiency!!!!"

That was the whole idea. Sandia Labs solar collectors were about 3-5% more efficient than commercially-available solar panels which were so danged expensive that only a nut would not have noticed that the $8,000 worth of solar panels he bought from ACME Solar Panels and Buggy Whip Company weren't going to save money until the mortgage had been retired for 12 years.

Glad we're on the same page.

I meant every word as a compliment to your brains and sagacity, even for CR4 you are obviously one of the few really "clever" ones.....

I hope we meet often on CR4!!

I would avoid the auto anti-freeze (ethylene glycol) as most local codes prohibit toxic substances used in a system unless there is double-barrier separation with the heat exchangers. Much better would be the RV anti-freeze which is propylene glycol which is not poisonous.

I'm looking to add solar hot water panels to my roof for domestic hot water pre-heat(geyser for you folks on the other side of the pond) and supplemental hydronic heating of the floors when the conditions are favorable. The natural gas fired hot air system would still be the basis for the heat when solar is not providing enough BTU's.

You could also use Black PVC pipe on the roof to heat water. We used 60' of 3/4" and 20' of 2" with a connection to the outside tap from a hose to heat the pool. Works great and even cools the house a fraction as it pulls heat from the black shingles. I see no reason why a fully piped roof wouldn't be able to supply more hot water. Heck add some of those new fangled solar cell shingles and save power too.

Given that the specific heat of water remains near constant up to the boiling point couldn't significant energy be saved by just preheating the supply water for the hot water heater?

Back in the mid 70's a friend had taken advantage of some tax credits given for solar water heating. His system used a diversion valve that opened the solar supply line to the water heater once the temperature of the water in the solar line exceeded that of the metered supply line. It didn't work, and I believe the reason it didn't work is because the valve was operating on the temperature gradient between the metered supply and solar supply at the valve and not the panels. The panels were on the roof and the valve was near the hot water heater. Because the thermal conductivity of the switching valve was so high, what ever was controlling the switching function never read a large enough gradient to activate the valve. The panels were on the roof and the comparative temperature valve was near the hot water heater.

I don't see any kits or valves at Lowes or Home Depot for either new construction or retrofit. It must be that energy is still too cheap.

Combining space heating and water heating: Thinking about this as I type, why couldn't solar heat exchangers be used for space heating as well as water heating?

If the water and space heating was supplied by the same panels, could prioritization be accomplished simply by the order the two diversion valves were placed in the line? Or perhaps putting them in parallel with different temperature triggers?

http://www.solar4power.com/solar-power-insolation-window.html

Given a yearly average solar insolation rate for Ft Worth, TX of 5.43 KWhr/day per square meter; a 5 x 5 meter panel would provide the potential for 135 KWhr/per day. Given a guess of applied thermal conversion efficiency of about 20 percent, (I haven't a clue) would give a potential energy savings of about 27 KWhr/per day. The problem being is our use is "on demand" so, unless it is used as a constant on source for space heating, for us to able to "use" this energy we have to store it.

Looking at the coldest winter days and reducing the insolation rate to the minimum value given. 4.8 KWhr/day/sq meter X 25 sqm = 120 kwhr/day and the same thermal conversion rate of 20 percent gives a value of 24 Kwhr/day.

Since I will use a solar panel to drive my small pump I do not need to include that power as being parasitic to the system so I get 24 KWhrs/day of thermal heating to use for water preheating and then switched to assist in space heating.

Could I heat the water on the coldest days to a temperature of 85 degrees? Guessing that would be the temperature I would need in order to get a decent exchange for space heating.

My co-op charges me about $.073 per KWhr. My savings would be about $1.75 per day on days I used the thermal energy to reduce the electrical load needed for water and space heating IF my use of the applied electrical power was 100 percent, which of course it isn't. Because of this my actual savings would be greater than the 1.75 per day.

Given its use only in water and space heating applications that 1.75 per day is much higher than my average daily savings throughout the year. So I will quess that value down to about half that, or .875 a day. That amounts to $1596.87 that I could invest expecting a pay back in five years.

Now, here in East Texas if I could shade my roof with the panels I could reduce my heat gain through the roof thus saving on my cooling costs also, off setting the fact that I can not use all of the thermal energy produced.

In any case, when you engineers can make the dollar numbers crunch, design a product that works, and somebody to market it; the rest of us will be standing in line to buy it.

Also, the grocery store is charging me 79 cents a gallon for drinking water, could I make my own by modifiying a dehumidifier? Also, why is condensate from air conditioning units not being reclaimed? I don't see any products on the market.

Gavilan

the grocery store is charging me 79 cents a gallon for drinking water.

Isn't your tap water drinkable? Or are you waaaay out in the countryside and off the water main?

Del

<Given that the specific heat of water remains near constant up to the boiling point couldn't significant energy be saved by just preheating the supply water for the hot water heater?>

This is exactly what is done. It is provided to reduce expenditure on fuels for space water heating.

The economics of home-brew installations work out well for UK latitudes. It's a long-shot for a commercial installation for the home.

One of the simplest solar hot water heating designs I ever saw was nothing more than 100 feet of 1" coiled black poyethylene pipe set at the average solar angle with a clear hemispherical sky-light-turned-collector-dome and a little insulation on the bottom. The guy who built it lived in Texas where the temperature inside the enclosure got up to 150 degrees or more.

Lessee . . . 100 feet at 1" diameter yields about 4 gallons of very hot water. Certainly enough for a quick shower.

My wife and kids got weary of my water heating project, so I eventually plumbed it into the supply side of the hot water heater and left it there. Never bothered to compare one months electric bills with any prior to the great energy saving project, but as an engineer, I knew I was saving something.

Never bothered to compare one months electric bills with any prior to the great energy saving project, but as an engineer, I knew I was saving something.

Ah!...that's exactly the same accounting principles I use!

Del

That accounting method has served me well. If I actually ever did a real engineering economic analysis of some of my harebrained ideas, I'd probably be very disappointed.

I also built my own "instant hot water heater" using a 2" pipe + tee + reducer + 4000 Watt screw-in water heater element which heated water about 25^oF at 2 GPM, if I remember correctly. The wife wouldn't let me install it in the shower, however, 'cause it had wires on it.

The wife wouldn't let me install it in the shower, however, 'cause it had wires on it. .

Yeh. Mrs Cat isn't very keen on my bows 'cos the arrows are a bit too pointy!

Given that the specific heat of water remains near constant up to the boiling point couldn't significant energy be saved by just preheating the supply water for the hot water heater?

That's what I plan on doing. Using the solar panels as a "Pre-heater" and run the warmed water into either the boiler or water heater.

Combining space heating and water heating: Thinking about this as I type, why couldn't solar heat exchangers be used for space heating as well as water heating?

Again what I had in mind. What I was actually looking at was something like THIS. Heated water from a closed system solar panel exchanges heat at the exchanger. The warmed water of the exchanger is used for preheated water for the hot water heater and preheats water for the boiler.

The problem being is our use is "on demand" so, unless it is used as a constant on source for space heating, for us to able to "use" this energy we have to store it.

I was looking at the volume of the heat exchanger as a source of storage. This is part of the equation I still need to figure out, sizing the storage tank.

Could I heat the water on the coldest days to a temperature of 85 degrees?

85 degrees F or C. Talking to a person that has a commercial system installed they get from 95-72C and live in a similar climate as I do. I run my radiant heat at around 150-160F/65-71C. Of course this is the part I'm trying to figure out. Given the constants, Kwhr/sq meter, water temps and usage, efficiencies, what do I need for , number of panels, size of panels, size of tubes, flow, storage tank volume etc.

~Matt

Has anyone had experience of home made heat pumps? From fridges or similar? Results?

Years ago, I remember investigating heat pipes to use in some research project, but it turned out to not have the characteristics I needed, but I found out a fair amount about them. Here's a fairly good explanation of heat pipe technology. That was also around the time when solar energy research was also being researched around here 'cause the gas prices went up to $0.50/gal.

The pseudo-neat thing about heat pipes is that there's no moving parts, so to speak.

See if you can come up with a good way to utilize the things as heat pumps using a combination of waste heat from other processes or solar, or . . .

Andy; I have played alot with Refer systems. I got out of the solar biz after I discovered I could accomplish the same level of energy saving with a refer based system as I could with a solar system. I was living in the cloudy Pacific Northwest at the time and had great luck with air to water heat pumps built from surplus water cooled condensing units. I dabbled with refrigerant based solar evaporators, ground coupling and water to water using a lake water as the heat sink. I even designed and built a refrigerant based commercial swimming pool room dehumidifier. My wife thought I was a bit crazy when I built a small water chiller for the house out of an old carrier heat pump I picked up in a salvage yard.

Do you know of anywhere that shows designs and theory/practice for making them yourself?

I could take the innards out of a fridge, but would it be as efficient when using it "the other way around"?

If any efficiency loss, significant or can it be ignored?

The University of New Mexico has several booklets on solar design criteria which you will probably help you with the calculations needed to design your system. Start with www.unm.edu

Go find a copy of Solar Heating Design by Beckman, Klien and Duffie. my copy was printed in 1977. It was the bible of solar for many years and contains all the basic info you need to design a DHW system. I was a solar contractor back in the days of the 40% tax rebate. I found that home built panels made from copper tubing and flat flashing sheet work almost as well as commercial panels. Also try back issues of Mother Earth News.