Solar-Heated Hot Water System

For this project, I decided to have a go at using Thermonuclear Power - to see if I could harness the most powerful exothermic reaction in the universe - Hydrogen Fusion!!

So I built the following **Thermonuclear Energy** Capture Device:

This hot water system uses a large solar panel collecter (6m x 5m = 30 sq m), which heats water running through a 400m length of polyethylene pipe zig-zagging across the surface of the collector.

(You'll have realised by now that the "Thermonuclear" reaction I'm talking about is the one which burns constantly in the SUN!! *Smile*)

This is not very efficient, I know, but it's cheap and simple to build, and because it's so large, the efficiency doesn't need to be high for it to collect a useful amount of heat from the Sun.

The collector is made of corrugated roofing iron, painted black on the top surface. The corrugations are laid HORIZONTALLY, not up-and-down as in a normal roof, so that the pipe can be laid horizontally to lie flat in the corrugations. The whole thing is mounted as a roof on an open-sided pole shed as shown in the diagram below:

Building paper is laid down under the iron, (on chicken netting) to reduce the amount of heat lost by radiation and air convection beneath the roof.

25mm x 25mm battens are laid down on top of the roof to support a layer of clear greenhouse plastic film. This traps the solar radiation in the air layer between the iron and the plastic, and prevents the wind from cooling the iron too much.

Here's a picture of the partially completed roof...

The diagram above shows an enlarged section of the roof, looking from the side. The water flows inside the 15mm pipe laid along the channels of the corrugated iron, picking up heat on the way.

It took some doing to lay the pipe, because I had to hold it in place with pairs of nails driven though the iron into the rafters below. There are about 80 corrugations, each with 5m of pipe lying in it, and with 4 pairs of nails holding the pipe down in each section. That's over 300 nails, and a lot of swearing and sore thumbs from missing the nails!

The pipe is easy to bend at the ends of the runs, but I had to skip a corrugation to avoid kinking. So I made the pipe zigzag UP the roof, and then zigzag DOWN again, to use up all the corrugations.

Basically it works like this:

When the sun is shining, a relay turns on a low-power pump which pumps water from the bottom of the hot water cylinder, up the zig-zag polypipe to the top of the solar panel, then down the second zigzag polypipe to the bottom again. The hot water then goes to the top of the hot water cylinder, and so the circulation continues. See the diagram below.

On days when I want to clean out our spa pool, I can re-fill the pool again with hot water, by opening tap A and closing tap B. This bypasses the hot water cylinder, and allows hot water to flow straight from the solar panel to the spa pool.

The hot water cylinder holds around 180 litres, and is surrounded by a thick insulating cladding of pink batts (glass fibre ceiling insulation) in addition to the normal insulation and galvanized steel surround.

I did some measurements the other day, and it seems that I'm getting around 7kW of heating power from the sun. It heats up 180 litres of water in around one and a half hours, to about 55 degrees centigrade.

Here's how I did the calculation, if you're interested:
First I set the taps to allow water to run to the spa pool, and measured the flow with a bucket. It took 43 seconds to fill a 2 litre bucket.
That's 170 l/h, or 47 ml/sec.

The temperature of the water going in to the solar panel was 19.5 degrees centigrade, and the temperature of the water coming out was 55.3 degrees.
That's a rise of 36 degrees.

Now we know that if we heat up one gram (= 1ml) of water through 1 degree it takes one Calorie of energy.

So, to heat up 47 grams of water through 36 degrees takes 47 x 36 calories of energy, which = 1692 calories.
Converting to Joules (1 calorie = 4.2 joules), we get 1692 x 4.2 joules of energy, which = 7100 Joules (near enough)

Ok, so it's producing 7100 Joules EVERY SECOND - that's 7.1 kW of power. (Because one Joule per second = 1 Watt)

Compare that with an ordinary jug or kettle for boiling water, which has an element of 2.4 kW, or an ordinary hot water cylinder, which also has an element of 2.4 kW. My panel is three times as powerful. (Although it's also a whole lot bigger!)

So, if we assume that the sun's radiation is falling at 1kW per sq. meter, then the overall efficiency of the system is around 23% (because 30 sq.m should theoretically receive 30 kW of solar energy, and 7 divided by 30 is 0.23) - fairly impressive, really, and much better than I expected.

Here's a picture of the completed panel alongside the house, to give you some idea of scale. (The Spa Pool Solar heater is in the foreground)

I've modified the flow diagram a little - instead of using a pump, I now use a small washing machine relay (5 watts) to let cold water into the panel from our main cold water system for the farm (gravity-fed from a spring in the hills behind), and allow it to flow to waste after going through the hot water cylinder....

As you can work out from the diagram, when the sun shines and the valve OPENS, water comes in from the main supply, runs through the panel (and comes out of the other end at 55 degrees!), into the top of the hot water cylinder. Then it pushes the cold water out of the HWC, and up into the Header tank, which overflows to a toy water-wheel in the garden!

STOP PRESS: Energy Savings Calculation

I've just worked out what our energy savings will be for this panel, based only on the savings on HOT WATER heating. (No space heating, even tho' I do intend to use the excess heat from the panel for heating the house - see the next article below...)

I'm assuming that we will NEVER have to turn on our hot water heater again (well, ALMOST never, as long as we don't get more than 3 or 4 days of total cloud cover in a row - that's how much storage we have)

Hot water use over the past 24 month period average = 9.6kWH per day
We pay 11c per kWH, so that's $1.06 savings per day.

Doesn't sound like much, but.... That's $30 per month, or $360 per year.
Assuming that the total cost of all the materials was $1,000, that's a payback period of just 3 Years!

(Compared with 10-20 years for most conventional off-the-shelf solar hot water systems, that's not too bad, is it?)

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