Tunnel temperatures

Understanding a little of how we maintain optimum water temperatures for our Tilapia in our tunnels is important. In a well designed tunnel the air temperatures can reach or exceed 65C on warmer days. This is vital to achieve because it is the hot air that warms the ponds that warms the water in the ponds via process called convection or convective heating. Convection occurs simply by heat being transmitted from a hot source to a cold one via physical contact.

Water generally is an excellent heat reservoir – it may take a while to heat the water but once warm it takes a long time for it to cool off.

Hence inside the tunnels unless your water temperatures are reaching above 30C you will not need to vent the hot air out of the tunnel in order to prevent your fish from boiling alive. It is a myth to think that Tilapia will grow faster at temperatures above 28C. They will not. All that happens with over warm water is that the oxygen carrying capacity reduces as the temperature increases and you run the risk of asphyxiating your fish. In our tunnels when the temperatures hit 30C we open the flaps at either end and allow the tunnel to cool down a little by allowing air flow through the tunnel.

In engineering school some time ago we were taught about the importance of “temperature driving forces”. What this means in essence is a simple way of explaining where heat goes inside our tunnels. For instance if the water temperature is 10C and the air temperature is 50C the temperature driving force is the difference between the two – i.e. 50 minus 10 = 40C. A 40 degree difference in temperature is a big driving force and we will see the water increase in temperature more rapidly than if the water temperature was already at say 30C (where the driving force would be only 20C).

Of course, in the ideal world we’d keep the air temperature at exactly 28C all day long and then the pond water temperature would never change from 28C.

However there are other things that cause water to lose heat. Evaporation is one of these – as evaporation takes place the water body will cool slightly. Also at night instead of radiation from the sun heating the water directly (radiation bypasses the air) you have your ponds radiating heat into the night sky (called radiative cooling). So your ponds will typically lose heat overnight by both of these processes, all the more so if the air temperature drops considerably inside the tunnel.

Radiation losses can be countered by simply covering your ponds at night with a U/V and infrared impenetrable plastic. Do not place these directly on the surface of the water – remember your fish need plenty of oxygen and your surface is a vital ‘lung’ for them. Rather lift it 100 or 150mm clear of the water surface.

Convective heat losses are a little harder to deal with, other than heating the air in the tunnel you are pretty much stuck with them. On average in our systems we experience about a 2C variation in water temperatures from morning to evening. Radiation shielding reduces this to about 1C.

Each site is specific. You will find that your tunnel has its own thermal profile characteristics that you can adjust and improve over time. Our model allows for a 300 day per annum grow out period so that gives you 65 days to play with in terms of allowing for colder spells. You can also start to do the math on the costs of heating over winter vs the returns on capital so deployed.