Liquid Desiccant AC for Grocery Stores

The liquid dessicant air conditioner (LDAC) report seeks to perform an assessment of LDAC in a supermarket application. The assessment involves reviewing and analyzing field-monitored data collected by a third-party contractor, as well as electrical and natural gas billing. The project focuses on evaluating the effects of LDAC on the indoor humidity and total building energy use. It is important to note that this assessment was initiated at a later stage of the project when system retrofit and data collection was already concluded. Therefore, the involvement of Southern California Edison (SCE) engineers was limited to the review and analysis of the contractors’ previously collected data. The electrical energy savings due to lowered store humidity ratio was expected to come from the reduction in latent load of refrigerated display cases. Space humidity ratio has a direct impact on the cooling load of refrigerated display cases, frost buildup on the evaporator coils, defrost duration and energy, and operation of anti-sweat heaters. The ultimate effect is on refrigeration system power and energy, as well as product quality and shelf life. Supermarkets are one of the largest electric energy-intensive buildings in the commercial sector. The main portion of their electrical energy consumption comes from the refrigeration system. The heating, ventilating, and air-conditioning (HVAC) system plays a key role in maintaining human comfort and efficient operation of refrigerated display cases. Industry-accepted supermarket design conditions are based on an indoor dry-bulb temperature (DBT) of 75 degrees Fahrenheit (°F) and a relative humidity (rh) of 55%. Many supermarkets, in reality, operate at slightly lower DBT and rh. Typical HVAC system in supermarkets consists of a constant volume direct-expansion (DX) with reheat. Air dehumidification, or moisture removal from air, occurs when air contacts a cooling coil with a surface temperature below temperature of the air. As a result, the discharge air leaves the coil at or near saturation. Under this condition, air is too cold to meet DBT set point. To compensate for overcooling, heaters downstream of the cooling coil will increase the DBT of dehumidified air before it is supplied to the conditioned space. Air dehumidification by continuous overcooling and reheating is not very efficient. Thus, under certain design conditions where low indoor rh is required, air dehumidification by DX system may not meet the desired set points efficiently. This challenge is even more pronounced in humid climatic regions. Consequently, it is not surprising to note maintaining a tight indoor rh set point is a common challenge in today’s supermarkets. Alternative HVAC system designs that involve integration of an independent latent cooling system, like a desiccant module, with conventional DX system are believed to provide better humidity control and efficiency. Desiccant materials can either be in solid or liquid form. All desiccants attract and hold moisture until they reach equilibrium with the surrounding air. Moisture is then removed from desiccants (also called regeneration) by exposing it to the airstreams with temperatures between 120 and 500°F. Compared to solid desiccants, liquid desiccants do not need complex dehumidifier geometries and offer greater design flexibility.