Modeling a Combined Energy-Water Storage System for Residential Homes and Analyzing Water Storage Tank Size
Authors: Upshaw Charles R. , Rhodes Joshua D. , Webber Michael E. 
Published: April 2014

Air conditioning systems (AC systems) are the primary driver of summer electricity use and peak power demand in residential homes in Texas, mostly due to the refrigerant compressor in the condenser unit. The power demand for a residential AC compressor is on the order of kilowatts. Peak power demand from residential AC systems could be reduced by means of pre-cooling a thermal storage reservoir, which can be used as a heat sink instead of the air-cooled outdoor condenser that is subject to ambient conditions. The concept of thermal storage is not new, and is in widespread use in large-scale HVAC systems for the commercial and industrial sectors. However, residential thermal storage systems, while available, are not widespread due to high costs relative to the costs of the AC system.

This paper discusses the development of a simplified thermodynamic model of a water-based sensible thermal storage reservoir for reducing peak AC compressor loads, and determines optimal tank sizing based on a few key design parameters. The motivation behind this project is the idea of utilizing a large water reservoir that could be on-site for other purposes already, specifically large rainwater collection systems. Such a combined energy/water storage configuration might increase the cost effectiveness of both a thermal storage system and a rainwater collection system by means of shared costs and avoided energy and water expenses.

The system configuration consists of a typical direct expansion residential air conditioning system with a typical air-cooled condenser unit, but with an additional thermal storage condenser/evaporator heat exchanger connected into the refrigerant lines with reconfigurable flow paths and solenoid valves to control the discharging and recharging of the thermal reservoir. The large volume of stored water acts as a lower temperature thermal reservoir for the secondary condenser. The lower temperature and better heat transfer capabilities of water improve operating efficiency and reduce power consumption when used instead of the air-cooled condenser during the hottest hours of the day.

The system model was evaluated using cooling load outputs for a simulated 1800 square foot home in Austin, Texas based on weather data from summer 2011, which was a record hot summer that stressed the Texas electricity grid to its limits. Preliminary analysis based on a simplified model of the system, along with the specified model parameters, suggests that thermal storage systems would be on the order of several thousand gallons, which corresponds to that of a large rainwater collection system. Additionally, the analysis suggests that power demand reduction during peak is likely the primary benefit of the system, with an average reduction on the order of 30–70% less than the system without storage, depending on operating parameters. However, total energy consumption could be either slightly higher or lower than the baseline, depending on a variety of factors such as diurnal temperature swing, discharge/recharge control, compressor efficiency, and tank size.