Thermal energy storage for solar heating and off-peak air conditioning

Abstract

Latent heat thermal energy storage materials suitable for solar heating and off-peak air conditioning were investigated and evaluated in terms of criteria developed to judge their usefulness. The often recommended sodium sulfate decahydrate and its mixtures were shown to have undesirable melting properties, causing a reduction in storage capacity upon repeated cycling. Materials more suitable at present are paraffin waxes for solar heating and paraffin waxes and tetrahydrofuran hydrate for off-peak air conditioning. Sensible heat storage has the economic advantage of not requiring heat transfer surfaces between storage and collection and between storage and delivery. Latent heat storage provides operational advantages of smaller temperature swings and lower, more efficient solar collector temperature, smaller size and lower weight per unit of storage capacity. The optimization of the thermal energy storage subsystem is dependent on the collection and delivery characteristics of the system. Parametric designs for two latent heat materials (sodium thiosulfate pentahydrate and a paraffin wax) and for a sensible heat material (a 1:1 mixture of water and ethylene glycol) were compared as to cost, performance, and space requirements. The conditions of equal cost for latent heat and sensible heat storage systems were determined as functions of latent heat capacity, thermal conductivity, and the temperature swing allowed in the sensible heat storage tank. The comparative designs include the cost of the heat exchanger required for latent heat storage and the operating penalty due to storage temperature swings inherent in sensible heat storage.