A low temperature latent heat storage based on a cheap salt hydrate has been developed to support a dry cooler on hot days and ensure a constant low cooling water temperature for a sorption machine. The technology helps to overcome problems commonly encountered with the performance of solar thermal cooling and heating systems.

Reliable solar thermal cooling and heating systems feature high solar fraction and ahigh seasonal energy efficiency ratio (SEER). Currently, bulky buffer tanks areused to improve the solar fraction for heating purposes, and during the summer, sorption chillers are used to convert the solar heat into useful cold, The dissipation of waste heat is a critical factor affecting system efficiency.

Why replace wet cooling towers, dry air coolers, or hybrid coolers?

In most sorption cooling installations, wet cooling towers are applied for waste heat dissemination. While this has a beneficial effect on the chilling capacity and Coefficient of Performance (COP) of the sorption chiller, the open water system of wet cooling towers increases the risk of legionella and bacterial growth aswell as calcification and fouling which require a costly water treatment system and thus high maintenance and inspection costs. Furthermore, legal restrictions related to these risks and limited availability of wet cooling towers forsmall-scale sorption cooling systems hinder the use of such equipment. 

A dry aircooler is an alternative technology to wet cooling towers, but the cooling water return temperature significantly increases. This results in a decrease in both COP and chilling capacity, if the driving heat temperature is not increased accordingly. Also, a large specific heat exchanger and a high airflow through the cooler is needed.

Hybrid coolers offer a third option, but combine the advantages and disadvantages of wet cooling towers and dry air coolers. What is more, due to the high investment costs and the open water systems, a comprehensive implementation, especially in small to medium sized solar heating and cooling systems may be problematic. 

Latent heat allows for low operating temperature of solar thermal systems

A favourable solution is to apply low temperature heating and cooling facilities, such as floor or wall heating systems, activated ceilings or fan coils. A latent heat storage system allows for a low operating temperature of the solar thermal system, which yields efficient operation with solar gain thanks to constant low storage temperatures, especially for low exergy (available energy) space heating systems. 

In summer,when chilled water is provided by the sorption chiller, the storage system may be used in combination with a low maintenance dry cooling tower to avoid theuse of a wet cooling tower. The latent heat storage supports the dry cooler by partly absorbing the waste heat of the chiller during the day and ensures constant low cooling water temperatures, even on hot days. The accumulated waste heat is then dissipated to the ambient during the night, partly shifting heat dissipation of the chiller to periods with lower temperatures. 

Combination dry re-cooled sorption chiller testing results in lower operational and maintenance costs

This novel system concept for a solar heating and cooling installation comprised of a dry re-cooled sorption chiller and new low temperature latent heat storage has been developed and tested throughout the course of a three-year research project. 

During its cooling mode, the latent heat storage supports the dry air cooler, especially on hot days, ensuring a constant low cooling water return temperature to the sorption chiller.

In comparison to a wet cooling towers, maintenance and operational costs of the new system are considerably less. Furthermore, the unloading process that occurs during the night at favourable ambient conditions allows for efficient heat dissipation. When in heating mode, the latent heat storage serves as additional heat storage if a low exergy heating system is installed in the building.

Measurement data as well as in-situ measurements show the general functionality and feasibility of the concept under different climactic conditions in Europe. These measurements identify a significant positive effect on the key figures of seasonal energy efficiency ratio and solar fraction. Furthermore, the latent heat storage system has passed extensive cycle tests concerning thermal performance and long-term durability.More than 800 completed cycles in several storage samples have revealed no degradation or separation.

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