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Solar Heat Collector

Every tropical fish room needs some sort of heating - at least if you are not living near the equator. My fish room (even so it is highly insulated) needs some form of heating for about nine month of the year, which is pretty much autumn, winter and spring - especially on clear days which therefore are usually cold. On those days the heat demand is great, but so is the solar radiation! So the obvious thing to do is to build some sort of collector that use the solar radiation, transform it into warm air and heats the fish room with it.

The idea for the design of the solar heat collector came from the Aluminium Window Screen Solar Air Heating Collector on the Build-It-Solar website. I changed the designe a little bit to accomodate my requirements and to suit our climate here in New Zealand. The collector is pretty much a wooden black box that contains a frame with one layer of black aluminium fly screen on both sides and corrugated polycarbonate glazing to seal the collector. Cold air enters the bottom of the collector and needs to pass at some stage through the fly screen to exit at the top back. While traveling through the fly screen the heat exchange takes place and the air heats up. Once hot and risen to the top, a 12V blower pushes the air through a plastic pipe inside the fish room. That's the principal, let's get a bit more technical now.

The back of the collector box is made out of a treated 10mm sheet of plywood with the measurements of 2.4m x 1.2m. The top, sides and bottom are made out of treated boards that are 25mm thick and 150mm wide and are glued and screwed to the plywood back. The top is slightly tilted forward to ensure rain can run off. The bottom has eight 50mm holes which are covered with fly screen. They allow cold air to enter the collector but keep out insects. Everything on the inside is painted flat black to help a better heat absorption. I didn't insulate the collector since the temperatures in the Waikato are not as extreme as in America where the original designe comes from. Inside the collector is a frame made out of 20mm x 20mm wood that is also painted black. Both sides of the frame are covered with black aluminium fly screen. The frame will act as absorber and heat exchanger for the air while traveling through the collector. The frame is mounted at the very back at the bottom and at the front up the top which means it sits diagonal inside the collector which helps forcing the air through the screens before exiting at the top back. Here is a 78mm hole cut into the collector. This lines up perfectly with a hole I cut through the outside wall of the fish room. A plastic pipe is inserted into this hole and siliconed into place to prevent water entering the fish room and collector. Inside the collector a 12V blower fan is mounted at the end of the pipe with the following specifications:
Air Volume: 26(CFM), Input: 10.32W, Static Pressure: 25mm

I decided against a normal fan due to the (nearly) perfectly sealed fish room. Normal fans have very low static pressure and wouldn't be able to push the air inside the sealed fish room. But with 25mm static pressure the blower manages to push air inside the room with 2.7m/s or 531ft/min measured at the end of the pipe inside the fish room. The collector is covered with corrugated polycarbonate sheets. Foam profiles seal everything perfect at the top and bottom and rubber profiles seal both sides so there is no gap between the sheets and the wooden frame for hot air to escape or cold air to enter the collector. I didn't use Twinwall-Polycarbonate sheets because they are very expensive and again we don't have the extrem temperatures here. It was more important to me to get the maximum radiation onto the fly screen.

My fish room is perfectly aligned East-West so one side wall gets sun all day long. Here I mounted the collector with brackets and stainless steel screws directly onto the fish room wall and stained the outside the same way as the rest of the fish room. That way the collector blends in nicely and doesn't stick out too much.

The fan gets controlled by two cheap digital temperature controllers from Shenzhen Meihang Electronic Co, Ltd. This is necessary because not always is the air in the collector warmer than the inside of the fish room. For example at night and on a very cloudy day the temperature will be below the required 23C. So the fan needs to get turned on only when the temperature in the collector is above 23C but needs to be turned off when the maximum temperature of 28C inside the fish room is reached.

Therefore the controller, which sensor is inside the collector, is operated in cooling mode (HC = C) with the temperature (HC temp) set to 23C and the temperature range (CP) set to 2C. When the temperature inside the collector reaches 25C the relaise gets closed. When the temperature in the collector reaches 23C, the relaise opens again.
The controller with the sensor inside the fish room is operated in heating mode (HC = H) with the temperature (HC temp) set to 28C and the temperature range (CP) set to 5C. When the temperature inside the fish room goes under 23C the relaise gets closed and opens up again once the temperature reaches 28C.
The blower is wired in series to both controller relaise and only turns on when both are closed. This happens when the collector temperature is above 25C AND the inside temperature of the fish room is between 23C and 28C.

So far so good - but how is the collector performing? How much energy is he creating?
According to the Build-It-Solar website, we can determine the heat output of a collector by measuring just two things:

  1. The temperature rise of the air from the collector inlet to the collector outlet and
  2. The quantity of air flowing through the collector.

 

The heat output is directly proportional to the product of these two quantities.  Any collector design change that increases this product increases the heat output of the collector. The actual heat output is:

Qout = (Vair)*(Aduct)*(Dair)*(Toutlet - Tinlet)*(Cair)

 

Where:
Vair = average air velocity out the outlet duct (ft/min)
Aduct = the total area of the outlet duct(s)  (ft^2)
Dair = density of air  (lb/ft^3)
Toutlet = average temperature of the air exiting the outlet duct (deg F)
Tinlet = average temperature of the air entering the inlet duct (deg F)
Cair = specific heat of air (BTU/lb-F)

 

Dair is 0.075 lb/ft^3 at sea level and 60F, but the air density decreases as it is heated, so for normal collector temperatures, 0.065 lb/ft^3 is pretty good. Cair is 0.24 BTU/lb-F

 

For my specific collector at 10am on a sunny winter day in New Zealand we have:
Aduct = 0.051sqft (the area of the outlet pipe)
Toutlet = 119.84F (the average air temperature of the outlet air)
Tinlet = 41F (the average air temperature of the inlet air)
Vair = 531ft/min (the average velocity of the air in the outlet pipe)

 

Then, the collector output would be:
Qout = (Vair)*(Aduct)*(Dair)*(Toutlet - Tinlet)*(Cair)
Qout = (531 ft/min)*(0.051 ft^2)*(0.065 lb/ft^3)*(119.84 F - 41 F)*(0.24 BTU/lb-F)
Qout = 33.31 BTU/min,  or 1,998 BTU/hr or 0.585 kW/hr or 585W/hr

 

The fan uses 10W and the two controllers use 3W each so effectively the collector creates 569W of free energy in form of warm air that heats my fish room!