Novel Rainwater Storage System Designed to Reduce Electricity Demand

Physical Sciences : Mechanical

Available for licensing


  • Michael Webber , University of Texas at Austin
  • Charles Upshaw , University of Texas at Austin
  • Joshua Rhodes , University of Texas at Austin

Background/unmet need

Energy consumption continues to grow as the industrialized world moves deeper into the technological age. One of the major points of energy consumption in developed countries is air conditioning, which contributes not only to comfort but also to productivity and security.

Air conditioning systems are the primary driver of summer electricity use and peak power demand in residential homes in hot climates. Interestingly, the power consumption of HVAC units is mostly due to the refrigerant compressor in the condenser unit, creating a power demand on the order of kilowatts for each system.

Additionally, many areas in the world are experiencing persistent drought and are now facing severe water shortages. One way to help mitigate water use is to capture and store rainwater for irrigation. Historically low municipal water rates have made large-scale rainwater harvesting only marginally attractive. However, as drought depletes water supplies, water costs will increase, and many cities will expand strict water use standards, creating renewed commercial interest.

Invention Description

Researchers at The University of Texas at Austin have developed designs for a system that integrates thermal storage and water storage into a traditional direct-expansion air conditioning system to reduce peak electrical demand, while also reducing the consumption of treated municipal water for non-potable purposes. Computer modeling shows the system design saves consumers money for water and electricity, while improving reliability for water and electric utilities.

The system uses the thermal mass of a large volume of water by pre-cooling the water in the early morning hours, then using the cool water as a lower-temperature (more efficient) heat sink for the air conditioner condenser during the hottest peak hours of the day. The water used for the thermal mass is provided by rainwater capture and supplemented by air-conditioner condensate and grey water. The combined thermal/water storage system reduces peak electrical demand for air conditioning systems and reduces consumption of costly, energy-intensive treated water.


  • Reduces air conditioning load in hot climates during peak demand hours on the order of 50%
  • Peak load reduction financially beneficial to both utilities and consumers
  • Improves electricity grid reliability
  • Significantly reduces municipal water consumption by reducing or eliminating irrigation water use, saving both water and the energy associated with treating and pumping
  • Provides drought resiliency and flexibility to irrigation systems by allowing them to capture water and use it on days other than their designated watering day
  • Reduces costs associated with installing separate thermal storage and rainwater collection systems


  • Combines the benefits of thermal storage and water harvesting and storage into a single system.
  • Peak energy reduction duration scalable as a function of tank size
  • Scalable and adaptable to fit different house/building/property size
  • Capable of being retrofit to an existing AC system or integrated into new construction 

Market potential/applications

Electricity utilities, water utilities, grid operators, HVAC and appliance manufacturers, green builders, and homeowners are all potential beneficiaries of such designed systems. The market for air conditioners is projected to be $88 billion globally in 2014. 

IP Status