Teitel, M., Agricultural Engineering Institute, Agricultural Research Organization, Volcani Center, P.O.B. 6, Bet Dagan 58102, Israel
Tanny, J., Center for Technological Education Holon, P.O.B 305, Holon 50250, Israel

When a closed space is heated, thermal stratification can be established, such that the temperature at floor level is lower than near the roof. Under such conditions, in order to attain a desired temperature at the floor level, surplus of heat must be supplied, part of which is wasted on heating the upper region of the space. Therefore temperature stratification in heated buildings or greenhouses is usually undesired. Mixing the air by mechanical techniques, e.g. by fans, can eliminate the temperature stratification and produce a uniform temperature distribution. As compared to the stratified state, the temperature near the floor of the air-mixed space is increased while the temperature at the top region is decreased. This results in a more convenient micro climate for the humans, animals or crops which occupy the lower region of the space. In the present work we compare between two ways by which a desired temperature can be reached at the floor level, in a given thermally stratified space. The first is mixing mechanically the air to a uniform temperature and the second is heating the space without destroying the thermal stratification. Using a simple one-dimensional model we calculate the potential energy of each state. Then we compare between the amounts of energy required to reach from the initial stratified state, to (i) the uniform state or (ii) the new stratified state, both with the same higher temperature at floor level. The calculations are done for linear and exponential temperature profiles. The results show that air mixing requires much less energy than surplus heating and is therefore a more economical method. In both methods, heating and air-mixing, the energy needed increases with the initial temperature gradient existing in the heated enclosure and with the enclosures' height.