Build your own flat panel solar thermal collector

Step 7 Results

See the image below for a plot of temperature vs time.

Thermo-Siphon Flow Rate
The hoses are setup such that the bottom hose draws cold water from the bottom of the tank and the top hose delivers hot water to the top of the tank. The water in the tank does not mix much due to the low flow rate. Therefore the water drawn into the bottom hose stays at almost a constant temperature (the original water temperature) until all the water in the tank has been drawn out and been replaced by warm water that has passed through the collector. Dividing the tank volume by the time till the temperature starts to rise gives a rough approximation of the flow rate through the collector.

Tank volume: 12.8 litres
(Note: I filled it this much so the total volume in the system including the panel and hoses would be an even 20 litres)
Time to empty: 25 minutes
Calculated thermo-siphon flow rate: 0.8 litres per minute

Note that the thermo-siphon flow rate decreases as all the water heats up and the temperature difference between the tank and the panel is less.

Power Calculation
The temperature change I was able to achieve was about 23 degreesC over a period of 1 hour. The heat capacity of water is 4.18 kJ/kg/degreeC. There were 20kg of water in the system. Given this information it is possible to calculate the average power that was actually input into the water:

Power = 4.18 kJ/kg/degreesC * 20 kg * 23 degreesC / 3600 seconds = 0.53 kW or 530 Watts.

Efficiency Calculation
The collector area is about 1.4 m². Energy available from sunlight is about 1000 W/m². Therefore the panel receives about 1400 W of incoming power when aimed directly towards the sun. The efficiency is simply the power actually extracted divided by the power available.

Efficiency = 530 Watts / 1400 Watts = 0.378 or 38%.

This is quite comparable to commercially available solar collectors. However, I'm doing this in my back yard with uninsulated hoses, a non-air tight panel, a single plastic pane that's slightly opaque, an open topped tank and no pump. The fact that I can achieve commercial level efficiencies with this setup is a testament to the design and indicates there is plenty of room for improvement in the industry.

Comments

[paborralho]

I forgot the important comment, I think it is better if the "in" and "out" of the panel are opposed, to avoid water circulating by the shortest path. Paulo Borralho "There are lots of Yesterdays and Tomorrows"

[paborralho]

paulo_do_campo match
p b x p t o @ g m a i l . c o m
sin espacios

[iwilltry]

Agreed. My bad. Someone else pointed it out too. It's especially important for a pump operated system. For a thermosyphoning system it is not as important since there is a negative feedback in that faster flowing channels heat up less, causing the flow rate to decrease, while slower moving channels heat up more causing the flow rate to increase.

[jhearty]

OK, so my wife and I finally got one of these built and tested.  We built it as a drainback system and used plain water dyed black using pond dye.  We did not paint the panel.  We got frosted tempered glass panes from Craigslist to build this and the next ones.  We used an old hot water circulator pump also from Craigslist.

We did a 4 hour test on a clear day, readjusting the panel angle a few times during the test.  The full spreadsheet is available but I was not sure if it could be posted here, so I just posted a screen shot of it.  It is a 1.814 square meter panel with 37.85 liters (10 gal) of water in the system.  We used a 55 gallon plastic drum for the tank.  The tank and hoses were not insulated.

At the 1 hour mark average power was 951 watts, 52% efficient.  At 2 hours, 768 watts, 42%.  At 3 hours, 593 watts, 33%.  At 4 hours, 464 watts, 26%.  We also did a stagnation test with no water in it, and it got up to 152 degrees F on a 45 degree day.  We are looking forward to mounting it permanently and testing reliability/longevity.  One thing we still need to do is get UV clear paint to help protect the panels from UV breakdown, and see if that affects the efficiency much.

[paborralho]

Congratulations! I Hope it lasts more than mine. Sure it will "boil", once you put it behind a glass. 2 Years ago I tried a similar one with policarbonate (supports up to 180°C), and PVC (not suitable for hot water). With glass glazing I had nearly 1000W/m2. It is really great, because the plastic is very thin and heats the water fast. Although, I had problems with the PVC and the silicone adhesive. Finally I got back to Air to Water Heating, Not so efficient but more reliable. Keep Going Paulo Borralho "There are lots of Yesterdays and Tomorrows"