Peak load reduction and water savings potential of integrated thermal energy and auxiliary water storage systems for residential buildings in Austin, Texas




Upshaw, Charles Roberts

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Long-term climatic shift is projected to lead to a hotter and dryer Texas. At the same time, the population is expected to grow by approximately 80% over the next 50 years. Both of these trends will increase demand for both water and electricity for air conditioning. As a result, cities and consumers are looking for ways to reduce stress on local water supplies, and reduce on-peak electricity consumption. Utilizing auxiliary water sources such as rainwater, graywater, and air conditioner condensate could be a way to reduce consumption from municipal water supplies, and thermal storage is on viable method to significantly reduce on-peak electricity demand from cooling systems. This research is aimed at reducing municipally-supplied water consumption and on-peak electricity load using integrated thermal energy and auxiliary water storage systems, referred to herein as `ITHERST' systems. This dissertation describes the design and operation of these systems, and develops semi-empirical thermodynamic, heat transfer, and fluid flow system operation models to estimate peak load reduction and municipal water savings from residential ITHERST systems. Two system configurations are discussed: the first scenario (referred to as ITHERST-DX) uses condenser-side thermal storage for a typical residential house with a direct expansion air conditioning system, and auxiliary water collection for reducing irrigation water consumption; the second scenario (referred to as the ITHERST-Hydro) uses indirect chilled water thermal storage in a hydronic cooling system, and a potable rainwater collection system to reduce household water consumption, as part of a prototype next-generation sustainable house. Analysis of the ITHERST-DX thermal storage system, showed average peak load reductions on the order of 40%, and average energy consumption increased by approximately 5-10+% over the base case depending on tank size and weather data input. Irrigation water savings from the ITHERST-DX system were approximately 20-90% depending on auxiliary water storage volume. Similarly, the ITHERST-Hydro analysis showed peak load reductions on the order of 75%, energy consumption increased by approximately 7-9%, and household water savings on the order of 60-90%. Lastly, net annual utility bill savings were calculated by incorporating electricity and water billing logic and pricing information into the two ITHERST system models. Annual savings were on the order of approximately $500-1000 per year for the ITHERST-DX system, and $200-500 per year for the ITHERST-Hydro system, depending on the weather year and specified utility rates. The biggest savings were from water and wastewater, since both systems substantially reduced water demand at larger volumes. Electric bill savings from thermal storage were only substantial when the on-peak price of electricity was very high, and the off-peak price was comparatively low. This economic analysis suggests these systems could have economic value to the homeowner, but savings are highly rate-dependent. Future work will focus on developing low-cost system designs, as well as further evaluate other potential configurations and markets for other economic opportunities.


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