Underground storage of solar energy for greenhouse heating. II. Comparison of seasonal and daily storage systems

ABSTRACT Performance of solar energy storage systems which utilize the soil layer under a greenhouse as a thermal storage medium was studied using a numerical simulation model under various conditions. Several modes of system operation, including seasonal and daily storages, were investigated under each condition. In almost every case studied, seasonal storage operation mode did not give positive net energy savings, because large amounts of electric energy were required for heat transfer from solar collectors to the underground (17 to 26 MJ/m2 (greenhouse floor) during July and February of the following year). On the other hand, daily storage modes always gave positive net energy savings. The solar energy storage system was most effective for middle cultivating temperature crops such as cucumber (about three times larger net energy saving than that for tomato cultivation in a daily storage mode) but was useless for high temperature cultivating crops such as paprika. When the greenhouse scale became larger, a larger portion of the required energy for greenhouse heating could be covered by the seasonal storage system but the net energy savings was still negative (about -18 MJ/m^). In cold cites such as Niigata or Sapporo, effect of the heat storage system was negligible. The more effective insulation of the greenhouse surface by thermal screens in the cold season reduced scope for functioning of the storage system.