DIY solar pre-heater

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Re: DIY solar pre-heater

Postby Tracker » Fri Jul 09, 2010 5:14 pm

Gordon-Loomberah wrote: ... not enough sleep, going to bed at 6am and getting up at 11am is never a good idea...

We have worked out just how much of a lunatic you are ! ;)

I think when cold water flows through the pipe, initally the pipe would gain a coating of solid wax within the surrounding liquid, further along where the water has warmed somewhat, some sinking of cooler wax would probably start to happen...if we were talking about a straight pipe through the wax, but with coils and or finned pipes/radiators etc, it would be a bit more complicated.


So, we are largely on the same wave-length -- As much heat uptake pipe/surface as possible, so that it never gets completely waxed over and hence becomes insulated, until the cold flow stops..


I think that effect would be minimised with distributed heating throughout the wax.


We normal mortals won't have that benefit ie. of WetBacks etc. We would be heating during the day ONLY..

It would need more than just the coil in the bottom, since there is no convection to distribute the heat once the wax solidifies, and the conductiviity is fairly low.


But does that matter, IF, we assume that we will eventually melt ALL the wax during the day's accumulation.
No difference to having the pot of wax on the stove.. You see NO EFFECT until the final layer melts and then you have it all hot.. So, in my case , I seek to build a heat-accumululator, as distinct from a transfer/accumulator.

Maybe I can find some used AC radiators for scrap value. I think they would be good for a bit of pressure too.


The only issue will be the TUBE SIZE.. Typically 8 -12mm, but all would need their pipes joined, to give the needed flow rates..

I can likely get my hands on NEW Condenser units, like the ones in Drink-Fridge compressor sleds, but they would be new and hence reasonably expensive, but ALL identical... 30mm square???

Food for thought ??

I have put out a feeler to see if we can buy them reliably via a refrigeration mob...
A square system would be easy to deal with, coz you could just use sheet Insulation..

Work-In-Progress ??? .. .. . :mrgreen:
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Re: DIY solar pre-heater

Postby 120ThingsIn20Years » Sun Jul 11, 2010 7:40 pm

Wow, so much info.

How much greater is the heat storage capacity of wax as compared to water?

I've arranged to collect a 180 litre hot water service (the electrics are toast but it still holds water) that I was intending to use as a heat storage tank to run some housewater connected copper through to pre-heat before the real hotwater service. After reading all those posts, I was thinking, depending on the amount of energy required to melt the wax, I might not ever see it melt.

Does the wax float? If so, might I be able to add wax a bit at a time down the track If I do indeed collect enough heat, and in fact collect more than I can store? If it floats then the extra capacity would all be at the top and there wouldn't be any conductivity issues. That way I would maintain a balance between conductivity within the system, and maximizing capacity. If I was ever at the point where I was collecting enough heat need all wax, there would be enough heat to no longer be worried about conductivity and the insulation (caused by that buildup of solid wax around the pipes as mentioned in a previous post)
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Re: DIY solar pre-heater

Postby Gordon-Loomberah » Sun Jul 11, 2010 8:54 pm

120ThingsIn20Years wrote:How much greater is the heat storage capacity of wax as compared to water?


that all depends on the temperature: a bit over half as much per degree, except around the fusion temperature of the wax, where it has tens of times as much, if you are working within a few degrees range.

For the range 0-100C, the wax only has slightly more heat storage capacity.

I've arranged to collect a 180 litre hot water service (the electrics are toast but it still holds water) that I was intending to use as a heat storage tank to run some housewater connected copper through to pre-heat before the real hotwater service. After reading all those posts, I was thinking, depending on the amount of energy required to melt the wax, I might not ever see it melt.


If you are not getting to the melting point of the wax, you are not making use of its latent heat of fusion, so there would be no point at all in using wax... unless you wanted LESS heat storage capacity!


Does the wax float?


Without looking it up, I'm pretty sure its density is less than that of water, so yes it will float. However, its going to get messy if you start mixing them together... not really a good idea.
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Re: DIY solar pre-heater

Postby Tracker » Mon Jul 12, 2010 1:11 pm

120ThingsIn20Years wrote:Does the wax float? If so, might I be able to add wax a bit at a time down the track If I do indeed collect enough heat,


I can't help the feeling that you are thinking of mixing technologies.. Are you talking about adding WAX to the HW tank and have it float on the pre-heated water.

In another discussion with Gordon, I came to the conclusion that part of the problem with using wax, is the efficient adding and removing of heat --- Heat from the entire mass, as if the wax cools onto a transfer pipe it then insulates the heat from getting where you want it to be..
If you had your 180Ltr tank - how would you connect it to your main system?
Are you talking of just a series connection..
If so, adding wax would just cause it to flow into the main tank..
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UNLESS - you remove the top siphon tubes and invert them so that you draw water from "Below the wax",
and then you will have a very inefficient system because the heat will be trapped in the WAX, unable to transmit down to the colder water..

The other side of the old tank is how far you would go to convert it to a DIY pre-heater.
The tank will likely only last 7 to 10 years tops before rust etc. condemns it, and how old is it now? :(
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Re: DIY solar pre-heater

Postby 120ThingsIn20Years » Tue Jul 13, 2010 7:26 pm

my plan was to have a closed loop using convection to move heat from the solar collector to a storage tank - all isolated from my house water. Then run the house water through a copper coil in the storage tank to preheat water before it enters the working hotwater service connected to my house. All joins would be outside the storage tank and thus not subject to any toxic crud that may exist, or grow within that tank.

collector -> storage tank (old hotwater service) -> collector etc

I think my confusion was due to some overflow into this thread, or just general confusion on my part. On rereading this thread I'm still confused, so who knows.

and all this off the back of an alcohol free week. It simply doesn't pay.
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Re: DIY solar pre-heater

Postby friendlyfisho » Tue Sep 14, 2010 11:30 am

In another discussion with Gordon, I came to the conclusion that part of the problem with using wax, is the efficient adding and removing of heat --- Heat from the entire mass, as if the wax cools onto a transfer pipe it then insulates the heat from getting where you want it to be.

Sounds like what you need is an aqua based pre heater to warm the incoming freezing cold water BEFORE routing it through your wax based pre heater.....

This way your wax should never solidify, so the heat recovery pipe work in the wax based pre heater can be simpler and therefore cheaper.

...I Think?
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Re: DIY solar pre-heater

Postby Tracker » Tue Sep 14, 2010 4:02 pm

friendlyfisho wrote:This way your wax should never solidify, so the heat recovery pipe work in the wax based pre heater can be simpler and therefore cheaper.


Only problem there is that IF you cannot get the wax to go through the transition phase, then you never get the thermal benefit of phase-change thermodynamics, so you might as well just use water...
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Re: DIY solar pre-heater

Postby dhaslam » Wed Sep 29, 2010 9:37 pm

I am using a slightly different approach in using hot air and clay storage/heat exchange instead of trying to put water through the sheets. The purpose is to produce heat and hot water in winter and also dull days in summer to supplement normal solar panels. The project is still in progress and needs a few days dry weather to continue.

The idea is to blow hot air from the panels through a large pile of clay, insulated with straw bales, and to collect the heat when needed by circulating water through the clay. The air pipes need to be large. I am using 40 metres of 4" drainage pipe and a 70watt duct fan for each pair of 8'X4' Correx sheets. There is 36000 litres of clay and rock per 8'X8' collector It will take a year to tell whether the sizing is correct. Hot air comes through at about 20C less than the surface temperature of the Correx in full sun so it probably can heat air to almost 100C in midsummer, about 70C today and perhaps 50C in mid winter. An estimated 50% of the winter heat will be from stored seasonal heat. It is easy to set the thermostats on the fans manually because the store temperature will only change slowly.
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Re: DIY solar pre-heater

Postby 120ThingsIn20Years » Thu Sep 30, 2010 6:03 pm

What are you using for thermostats?

Are there fans available that have thermostats built in?

My solar heating experiments are now focusing on heating a1800 liter aquaponics fish tank.
I was looking at a thermostat controlled power point that I found in a homebrew shop. It just consists of a probe and a single 240 volt plug. There are two models. One that turns on if _below_ a given temperature, and one that turns on if _above_ a given temperature.

Using just the one that tuns on when below a given temperature, and putting the probe in the fishtank, would allow me to store heat when there is too much, and grab some when needed to transfer to the fishtank. In my system I think I'll use passive convection to move heat to an old hot water service, then a small fishtank pump at the cool side of a heat transfer plate on the side of the fishtank, to suck water from top of the hot water service, and return it to the bottom. By locating the pump close to the hotwater service inlet, I should be able to protect it from excess heat.


I believe there is also some phase change advantage with your system when used to regulate glass houses. From what I've read water condensing and evaporating within a system such as yours has some beneficial effects.

I understand water can store up to 5 times as much heat as rocks. But I guess a hole in the ground is as inexpensive as things get. It will be interesting to see the data results after you've had a full range of seasons.

Do you have a drawing or plan of what you are doing?
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Re: DIY solar pre-heater

Postby dhaslam » Thu Sep 30, 2010 9:31 pm

The thermostats use external probes have a setting range up to 90C. The fans have to be external to the collectors with free air movement around them to avoid overheating them.

I haven't any plans but should have photographs soon. There will be a depth three straw bales (wrapped) underground and five bales overground plus the top layer. There will be drainage pipe all around to keep the area as dry as possible. Because rock has higher density than water it partly makes up for lower specific heat. Limestone specific heat is about .84 compared to 4.2 for water but limestone and heavy clay are about 2.6 times more dense so the storage capacity is closer to half that of water. The clay fills the space around the rocks so the capacity is better than rock on its own. I
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