Dynamic effects in a salinity-gradient solar-pond heating system

Numerical computer models have been developed to study the dynamics of a salt-gradient solar-pond heating system in a northern cold climate. The models are applicable for predicting the temperature and salinity profiles in a pond. Special emphasis is laid on the behaviour of the upper convective layer. In the calculations, the solar pond is considered as a part of a community-scale residential heating system and the effects of the pond's dynamics on the overall system performance are assessed. All calculations were made with 1-h time steps for a hypothetical pond in Helsinki (60° N). The results indicate that the consideration of the dynamics of the salinity profile may reduce the pond's bottom temperature by 10°C in comparison with a static salt distribution. The maintenance of the salinity gradient would allow a maximum surface washing interval of 5 weeks without severely affecting the pond's performance. Then the daily salt consumption would be about 40 g per square metre. For regions with cold winters, the surface should be washed with fresh water, just before surface freezing takes place, to prevent shrinking of the non-convective stabilizing gradient zone. It was also observed that a solar-pond heating system may reach considerable solar fractions in a northern climate.