Theoretical Model of Solar Photovoltaic Air Conditioning with Ice Thermal Storage

An investigation is undertaken of a prototype building-integrated solar photovoltaic-powered thermal storage system and air conditioning unit. The study verifies previous thermodynamic and economic conclusions and provides a more thorough analysis. A parameterized model was created for optimization of the system under various conditions. The model was used to evaluate energy and cost savings to determine viability of the system in several circumstances, such as a residence in Phoenix with typical cooling demand. The proposed design involves a modified chest freezer as a thermal storage tank with coils acting as the evaporator for the refrigeration cycle. Surrounding the coils, the tank contains small containers of water for high-density energy storage submerged in a low freezing-point solution of propylene glycol. The cooling power of excess photovoltaic and off-peak grid power that is generated by the air conditioning compressor is stored in the thermal storage tank by freezing the pure water. It is extracted by pumping the glycol across the ice containers and into an air handler to cool the building. Featured results of the modeling include the determination of an optimized system for a super-peak rate plan, grid-connected Phoenix house that has a 4-ton cooling load and requires a corresponding new air conditioner at 4.5 kW of power draw. Optimized for the highest payback over a ten year period, the system should consist of a thermal storage tank containing 454 liters (120 gallons) of water, a 3-ton rated air conditioning unit, requiring 2.7 kW, which is smaller than conventionally needed, and no solar photovoltaic array. The monthly summer savings would be $45.The upfront cost would be $5489, compared to a conventional system upfront cost of $5400, for a payback period of 0.33 years. Over ten years, this system will provide $2600 of savings. To optimize the system for the highest savings over a twenty year period, a thermal storage tank containing 272 liters (72 gallons) of water, a 40-m2 photovoltaic array with 15% efficiency, and a 3.5-ton, 3.1-kW rated air conditioning unit should be installed for an upfront cost of $19,900. This would provide monthly summer savings of $225 and 1062 kWh grid electricity, with a payback period of only 11 years and a total cost savings of $12,300 over twenty years. In comparison, a system with the same size photovoltaic array but without storage would result in a payback period of 16 years. Results are also determined for other cooling requirements and installation sizes, such that the viability of this type of system in different conditions can be discussed. The use of this model for determining the optimized system configuration given different constraints is also described.