Natural cooling systems and an economic feasibility study of long-term storage of coolness

Abstract

Natural cooling systems are described and classified according to sources of coolness ‡ storage materials, and modes of fluid flow and heat transfer, as well as the duration of storage. Ambient air, water vapor in the air, and clear sky provide the major sources of coolness, for both short-term (a few hours) and long-term (a few months) storage. Materials suitable for short-term storage of coolness are the building mass, water, rocks and phase-changing salt hydrates. For long-term storage, water (stored in aquifers and in the form of ice and frozen soil), ground, rocks, and phase-changing materials are possible candidates. Seasonal storage of coolness in rocks, ground, water (stored in aquifers), and in the form of ice (produced in a covered underground pond) and frozen soil were considered to meet the cooling demands of three commercial buildings. The characteristics of each system were determined, and an economic analysis was performed for these systems, as well as for a conventional vapor compresssion refrigeration machine. It was found that the cost of land and the interest to be paid on it constituted the major initial and annual costs, respectively. For the aquifer storage, where the land used for the system accessories is very small, and for the covered underground ice pond, where the land above the pond can be used for parking, landscaping, etc., the cost of one GJ of coolness delivered by these systems is lower than that delivered by the conventional unit. Both initial and unit coolness costs of the ground and rock-bed storage systems, even without considering the cost of land, are higher than those of the conventional unit. If the cost of land is excluded, the cost of one GJ of coolness stored in the form of frozen soil is competitive with the unit costs incurred using aquifer and ice pond systems.