Basic Solar Water heater design?

Markymark

But one has to admit this is worth setting up. It's damn dirt cheap, I can get like 100 foot of 1/2 irrigation line at lowes for like $10. A sheet of plywood or osb (if you build your tank, I'm sure some will be left over).

The one thing is about the irrigation line I think it tops out at 180F, so heating the water up too much can be bad. I've read of people doing the saran wrap thing and getting the water up to boiling. My system is going to be a CF and I am not gonna have a huge sump tank so I'm going to have to move water 24/7, this will also hurt my temperatures as moving water over cold rocks ain't a good idea. So I gotta figure out a easy way to close off the pipe during the cooler hours so I don't decrease the temperature anymore (the idea of returning the hot water to the ST is a good idea). I'm thinking something like a sprinter timer setup... There might be something cheaper, some kinda digital thing that's on batteries or something.

Sterfire

DuiNui wrote:

But if it's not linear, the faster you pump, the bigger the temp difference between the sun and ultimately the water will be, therefore transferring more heat (overall) to the water.
Bearing in mind that we are talking about a closed loop system, not a hot water heater for a shower etc.

Best case scenario, with some some help from the internet....

http://www.solar-rating.org/facts/collector_ratings.html#HowRated

LA gets 1777 BTU per sq foot each day.

1 BTU is enough heat to raise the temperature of one pound of water 1 degree Fahrenheit.

My 275 gallon FT weights 2200 lbs. so my tank would almost raise 1 degree per square foot of a "perfect solar collector"???

Its to late for me to really figure out what the max temp gain per square foot of solar collector would be realistically, but couldn't be more then 1 degree Fahrenheit per square foot (in LA)....

Sterfire

Markymark wrote:

But one has to admit this is worth setting up. It's damn dirt cheap, I can get like 100 foot of 1/2 irrigation line at lowes for like $10. A sheet of plywood or osb (if you build your tank, I'm sure some will be left over).

The one this is about the irrigation line I think it tops out at 180F, so heating the water up too much can be bad. I've read of people doing the saran wrap thing and getting the water up to boiling. My system is going to be a CF and I am not gonna have a huge sump tank so I'm going to have to move water 24/7, this will also hurt my temperatures as moving water over cold rocks ain't a good idea. So I gotta figure out a easy way to close off the pipe during the cooler hours so I don't decrease the temperature anymore (the idea of returning the hot water to the ST is a good idea). I'm thinking something like a sprinter timer setup... There might be something cheaper, some kinda digital thing that's on batteries or something.

if you used a thermal siphon, the water would only move when the water warms up, and if the exit to the FT is above the water line, it cant reverse...no need for a any thing fancy if you have a sump below the FT.

Markymark

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if you used a thermal siphon, the water would only move when the water warms up, and if the exit to the FT is above the water line, it cant reverse...no need for a any thing fancy if you have a sump below the FT.

Haha, you are the shit (pardon my french). I'm gonna have to look into that as I was kinda considering some type of siphon.

So I'm not 100% sure but I was considering getting an IBC and sinking it down about 1 foot (was planning on using this is the ST), so part of it sits above. The ST is going to be right next to the FT, so the FT will be 4 foot above the ground and the IBC I wanna swear doesn't sit up at 4 foot tall, so either way if I need to I'll bury it as to make sure it works properly.

Looks pretty simple:

The only hard part is picking a good place to locate it so it can get maximum sun.

fer9us

Perhaps you could use an old water heater for a tank? It'd be insulated and have pipe inlets attached... pretty heavy but ready to go...

Markymark

Well, in that picture the "tank" represents my Fish tank (if I understand correctly), and the collector will drop the hot water into my Sump Tank, so now the pump will then pump the warm water back into my Fish tank, this way it mixes with the water and doesn't allow 130+ degree water back into my FT. When it get's cold outside it won't circulate as it's not enough warmth to continue the siphon. So during cold hours it just wouldn't siphon. However I don't see why one couldn't use a water heater, but I don't wanna run gas too it etc, the holding/insulating properties wouldn't be a bad idea, but my backyard already looks kinda ghetto with all my scrap lumber and pvc, and other projects lol. If I added a water heater... hehe!

DuiNui

fer9us wrote:

Perhaps you could use an old water heater for a tank? It'd be insulated and have pipe inlets attached... pretty heavy but ready to go...

Water heater is likely to contain metal, probably copper, which is a no no in AP.

fer9us

Markymark wrote:

Water heater is likely to contain metal, probably copper, which is a no no in AP.

oh. OK. bummer that. There was a guy at the Bellingen markets the other day selling solar hot water heater systems that use black glass tubes within a vacuum -- kinda like a thermos. Apparently the design originally came from a team in NSW (Australia). Pretty amazing. Super hot, but probably quite expensive too. Second hand???
I'm keen to set up a hot water heater in Jakarta too because we have a flat roof there that gets lots of sun throughout the day all year around, and we don't have hot water, crazy huh!?!
I've seen commercial branded solar hot water heaters there already but surely they can't be too hard to make... surely?

bunson

There's a family D'outh which have made a "small" modification to their rooftop domestic solar storage hot water system; they simply run a pipe from the (in their case, spa, in our case, fish tank) up to the roof and through the HW tank. They use an 8000lph pump which at the head only delivers about 500lph. Their "local" hot-water specialist did the job for a carton of beer and cost of parts, used two pressure-tight bulkhead fittings and some sort of ceramic tubing inside the tank itself. Their supply and return pipes are lagged copper, but there is no reason that a suitable alternative couldn't be used in the AP scenario.

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The rooftop storage tank doubles as a heat exchanger so now the sun heats their spa right through summer and most of the other seasons. In the dead of winter when they have to electrically boost their shower water, the spa water temperature only needs to be "topped up" using the standard gas-powered spa heater. The pump is manually switched, but there is no reason an electronics buff couldn't install a (semi)automated controller.

Anyway, this might give someone an idea or two?

Scott

P.S. Don't try cutting into your hot water service yourself -- ALWAYS use a professional who knows what they're doing! Hot water under pressure, electricity and amateurs are not a good mix!

RupertofOZ

bunson wrote:

There's a family D'outh which have made a "small" modification to their rooftop domestic solar storage hot water system; they simply run a pipe from the (in their case, spa, in our case, fish tank) up to the roof and through the HW tank. They use an 8000lph pump which at the head only delivers about 500lph. Their "local" hot-water specialist did the job for a carton of beer and cost of parts, used two pressure-tight bulkhead fittings and some sort of ceramic tubing inside the tank itself.

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The rooftop storage tank doubles as a heat exchanger

Well unless the exchanger inside the header tank is copper, which it usually is... then the tank itself, with a ceramic insert wont exchange a lot of heat...

It might do so during the day... but will lose heat during the night....

Of course you wouldn't pump through the night for that reason....

But, as has been said before... it's no problem getting heat into the system during the day... it's keeping it there during the night.....

But it can help.... as my purpose built unit showed... some 5 years ago.... :wink:

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Search the "aquaheat" thread...

fer9us

RupertofOZ wrote:

purpose built unit

Looks good Rupe. Are you still using it?

How much did that cost??? looks kinda schmick i.e. pricey :/

RupertofOZ

Can't remember.. but the stainless steel header tank cost more than the tubes would these days...

Haven't got it hooked to my current system, as I took it down to Bamarang for a project that never got off the ground...

(i'll have to go down and get it... winters almost here..)

rsevs3

A quick run down from a tafe module i have just completed about Heat Exchangers. This is dealing with liquid - liquid exchangers mostly, but i think we can think of the air as a liquid for our purposes.

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The rate at which heat transfer takes place is due mainly to the difference in temperature of the two related bodies...

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Heat transfer occurs when the temperature of a substance is raised or lowered. Generally the greater the temperature difference between the connecting masses, the greater will be the rate of heat transfer.

There are four main factors affecting heat transfer:


The surface area

Temperature difference

Transfer coefficient

Velocity of flow

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Surface area
A Heat Exchanger with 10 tubes will transfer twice as much heat as one with 5 tubes. The area of an exchanger is important and by area we mean the total external surface area that is available for exchange.
Provided that the metal is the same thickness, doubling the area doubles the heat flow.

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Temperature difference
The temperature difference or t affects heat transfer rate. The greater the temperature difference the greater the exchange rate.
There are two types of flow in heat exchangers or a combination of both:
 Co-current
 Counter current
With co-current flows (flows going in the same direction), the temperature differential at the inlet may be high but the overall differential low, giving a poor exchange efficiency compared with counter current flow. In counter current flows, the average temperature differential is higher throughout the exchanger, therefore achieving greater efficiency.

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Transfer coefficient
In an exchanger, heat must travel through the hot fluid to the wall of the exchanger tube, then move through the tube and enter the cold fluid. The rate at which this occurs depends on the properties of the fluid and also on the properties of the material from which the tube is manufactured.
Some materials conduct heat better than others, that is, they have a high thermal conductivity rate. Also, the thinner the tube is, the better or quicker the rate of conduction across the tube.
The properties of the material from which the heat exchanger is made, are used to calculate the coefficiency of the exchanger.
The coefficiency is a measure of heat that can transfer in one hour across one square foot of exchanger area for each degree Fahrenheit temperature difference between the fluids.
It must be noted that the choice of tube size and material is greatly dependent upon the service for which the equipment is to be used. Most often, materials such as carbon steel; stainless steel; admiralty brass; titanium; or alloys are used.

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Velocity of flow
There are basically two types of flow in liquids “Laminar flow” and “Turbulent flow”.
Laminar flow comes from the idea of one lamination, or a thin cylindrical layer of liquid flowing inside another.
Other names for this type of flow are streamline and viscous flow. The flow is more or less in straight, unbroken lines.
As liquid velocity increases, the cylindrical shape of the concentric laminations is subject to increasingly disruptive forces. At some critical velocity, the flow pattern begins to break down and liquid motion becomes more turbulent.
The turbulence increases until the liquid particles cease to flow directly along the pipe or tube. Instead the movement is more random although the overall direction is still along the pipe.
At this and higher velocities, the liquid is said to be in turbulent flow.
If the flow of each liquid is slow, the temperature difference will be about the same and heat transfer slow. If we increase the velocity so that the flow is turbulent we have the maximum temperature difference and greater heat transfer.
The amount of turbulence that occurs depends upon the velocity of the fluid.
The greater the velocity, the more turbulent the flow, and more heat is transferred.
Exchangers are designed so that the flow will be turbulent.
In shell and tube heat exchangers baffles are used on the shell side to direct the flow of fluid. If there were no baffles, the shell side fluid would move slowly through the exchanger.
The baffles cause the fluid to move up and down as it flows through the exchanger, thereby increasing the velocity. Enough baffles are included in the shell side to assure a turbulent flow.
Other factors that affect heat exchange are the pattern and number of tubes.

With respect to the velocity, i dont think 1000L/h is required. Just enough so that if you look at the water exiting the pipe, you can see that it is a turbulent flow rather than a nice round stream.

I also wonder if having the poly pipe in a black box that is actually filled with water would give you a greater transfer of heat as well. So say if you had 1/2" pipe, you had a black box 1" deep filled with water that was heating up. Even if it didnt give you a greater temp rise it might shift the hours you are warming your AP water up. (ie instead of pumping warm water into your AP tank from 9am-4pm it might shift to 11am - 6pm giving you less hours of temp drop. :dontknow:

) If you were confident that your poly was water tight in the box of water you could give it a strong dose of chlorine/bleach to prevent any algae growth etc.

dbird

I made a system like markymark's 30 years ago and it still works.I put the hot water tank on the roof and a coil of poly pipe alongside of it and let it thermo siphon.On a hot day with no wind it worked well but if there was any wind it was useless.When I put it in a glass case it gave me all the hot water I needed even with a teenage daughter having showers.there are two simple ways to solve the problems that you are all concerned about.
You can get a swinger valve from a plummer which will stop the water flowing backwards at night,these are made for hot water systems for this perpose.You could use a solar pump which would only run when the sun was out.If this was too feeble to pump enough water you could use it to switch on a bigger pump.A washing machine has a pressure switch in it to switch the machine on when the bowl has enough water in it.You could connect one of these into the pressure line from your solar pump and to whatever size pump you want.I am working on what I think will be a much better solar panel now,just have to get around to it.
My origanal hot water system had 36 metres of 19mm poly pipe and it still works,the copper ones corrode out every few years.

dbird

This is my solar heater which i made to worm the water up,one raised the water up a fair bit ,I will make up a couple more next year.The plants don't do any good in the winter when the temperature is down to six c .The carp thought it was good,they huddled around the return pipe like people standing around a fire.

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